6th International Conference on Nanotechnologies and Biomedical Engineering: Proceedings of ICNBME-2023, September 20–23, 2023, Chisinau, Moldova - ... in Medicine (IFMBE Proceedings, 91) 3031427742, 9783031427749

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IFMBE Proceedings 91

Victor Sontea Ion Tiginyanu Serghei Railean Editors

6th International Conference on Nanotechnologies and Biomedical Engineering Proceedings of ICNBME-2023, September 20–23, 2023, Chisinau, Moldova - Volume 1: Nanotechnologies and Nano-biomaterials for Applications in Medicine

IFMBE Proceedings Series Editor Ratko Magjarevi´c, Faculty of Electrical Engineering and Computing, ZESOI, University of Zagreb, Zagreb, Croatia

Associate Editors Piotr Łady˙zy´nski, Warsaw, Poland Fatimah Ibrahim, Department of Biomedical Engineering, Faculty of Engineering, Universiti Malaya, Kuala Lumpur, Malaysia Igor Lackovic, Faculty of Electrical Engineering and Computing, University of Zagreb, Zagreb, Croatia Emilio Sacristan Rock, Mexico DF, Mexico

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The IFMBE Proceedings Book Series is an official publication of the International Federation for Medical and Biological Engineering (IFMBE). The series gathers the proceedings of various international conferences, which are either organized or endorsed by the Federation. Books published in this series report on cutting-edge findings and provide an informative survey on the most challenging topics and advances in the fields of medicine, biology, clinical engineering, and biophysics. The series aims at disseminating high quality scientific information, encouraging both basic and applied research, and promoting world-wide collaboration between researchers and practitioners in the field of Medical and Biological Engineering. Topics include, but are not limited to: • • • • • • • •

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Victor Sontea · Ion Tiginyanu · Serghei Railean Editors

6th International Conference on Nanotechnologies and Biomedical Engineering Proceedings of ICNBME-2023, September 20–23, 2023, Chisinau, Moldova - Volume 1: Nanotechnologies and Nano-biomaterials for Applications in Medicine

Editors Victor Sontea Department of Microelectronics and Biomedical Engineering Technical University of Moldova Chisinau, Moldova

Ion Tiginyanu Academy of Sciences of Moldova Chisinau, Moldova

Serghei Railean Department of Microelectronics and Biomedical Engineering Technical University of Moldova Chisinau, Moldova

ISSN 1680-0737 ISSN 1433-9277 (electronic) IFMBE Proceedings ISBN 978-3-031-42774-9 ISBN 978-3-031-42775-6 (eBook) https://doi.org/10.1007/978-3-031-42775-6 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2024, corrected publication 2024 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Paper in this product is recyclable.

Preface

This volume presents the Proceedings of the 6th International Conference on Nanotechnologies and Biomedical Engineering (ICNBME), which was held on September 20–23, 2023, in Chisinau, Republic of Moldova. ICNBME-2023 continued the series of international conferences in the field of nanotechnologies and biomedical engineering with the main goal focused at bringing together scientists and engineers dealing with fundamental and applied research for reporting on the latest theoretical developments and applications in the fields involved. The conference covered a wide range of subjects of primary importance for research and development such as nanotechnologies and nanomaterials, biomicro/nanotechnologies and devices, biomaterials for medical applications, biosensors and bioinstrumentation, biomedical signal and image processing, bioinformatics and computational biology, medical physics and biophysics, molecular, cellular and tissue engineering, clinical engineering, health technology management and assessment, innovation, development and interdisciplinary research, nuclear and radiation safety and security, medical physics and radiation protection, new technologies for diagnosis, treatment and rehabilitation and personalized approaches in medicine. The papers included in the Proceedings reflect the results of multidisciplinary research undertaken by about one hundred of groups worldwide. Special attention is paid to the development of novel nanotechnologies and nanomaterials, in particular of bio-nanotechnologies and bio-nanomaterials. New biocompatible materials are proposed for use in regenerative medicine, cellular and tissue engineering. Interesting data on novel chemical and biosensors are reported which are based on nanostructured metal oxides and hybrid nanocomposite materials. A wide range of new technologies for diagnosis, treatment and rehabilitation and personalized approaches in medicine are also presented. Considerable progress has been achieved at the intersection of nanotechnologies, information technologies and biomedicine as, for example, in health informatics, ehealth, telemedicine, biomedical instrumentation and signal processing. New theoretical and experimental results are highlighted in such fields as meta-materials, aeromaterials, micro-opto-electronic and photonic materials, photovoltaic structures, quantum dots, one- and two-dimensional nanomaterials, 3D nanoarchitectures and multifunctional hybrid materials like sandwich and core–shell structures, etc. The Proceedings reflect the state of the art in controlling the properties of several classes of nanocomposite materials for important future applications in various fields.

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Preface

We hope that the papers included in the ICNBME-2023 Proceedings will be of interest for established researchers working in multidisciplinary fields of science and technology, young scientists, students and broad community wishing to get up-to-date information on progress in the fast-developing areas of nanotechnology and biomedical engineering. Victor Sontea Ion Tiginyanu Serghei Railean

Organization

6th International Conference on Nanotechnologies and Biomedical Engineering ICNBME-2023, September 20–23, 2023, Chisinau, Republic of Moldova

Committees Conference Chairs Victor Sontea Ion Tiginyanu

President of the Moldavian Society of Biomedical Engineering, Republic of Moldova Academy of Sciences of Moldova, Republic of Moldova

International Advisory Committee Emil Cebanu

Rainer Adelung Viorel Bostan Pascal Colpo Yury Dekhtyar

Vladimir Fomin Hans Hartnagel

Nicolae Jula S, eref Komurcu Ratko Magjarevi´c Hidenori Mimura Ala Nimerenco Nicolas Pallikarakis

Nicolae Testemi¸tanu State Medical and Pharmaceutical University, Republic of Moldova Institute for Materials Science; University of Kiel, Germany Technical University of Moldova, Republic of Moldova Joint Research Center, Italy Institute of Biomedical Engineering and Nanotechnologies, Riga Technical University, Latvia Leibniz Institute for Solid State and Materials Research, Dresden Technical University Darmstadt, Institute of Microwave Engineering and Photonics, Germany Military Technical Academy, Romania Anadolu Medical Center, Turkey University of Zagreb, Croatia Research Institute of Electronics, Shizuoka University, Japan “Ministry of Health of the Republic of Moldova” University of Patras, Greece

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Organization

Masakazu Kimura Alexander Pogrebnjak Vladimir Krasnov Bogdan Simionescu Sokol Yevgen I. Ashok Vaseashta

Research Institute of Electronics, Shizuoka University, Japan Sumy State University, Ukraine Stockholm University, Sweden Romanian Academy, Romania National Technical University, Kharkiv, Ukraine International Clean Water Institute, Manassas, USA

International Program Committee S, ontea Victor

Tighineanu Ion Lupan Oleg Buzdugan Artur Ciorba Dumitru Corciova C˘alin Crudu Oleg Curocichin Ghenadie

Korotcencov Ghenadii Dinescu Adrian

Dragoman Mircea

Grigor Tatisvili

Groppa Stanislav

Kolisnyk Kostiantyn V.

President of the Moldovan Society of Biomedical Engineering, Technical University of Moldova, Republic of Moldova President of the Academy of Sciences of Moldova, Republic of Moldova Technical University of Moldova, Republic of Moldova Technical University of Moldova, Republic of Moldova Technical University of Moldova, Republic of Moldova Grigore T. Popa University of Medicine and Pharmacy, Romania Clinical municipal hospital Saint Trinity, Republic of Moldova Nicolae Testemi¸tanu State Medical and Pharmaceutical University, Republic of Moldova Moldova State University, Republic of Moldova National Institute for Research and Development in Microtechnology – IMT Bucharest, Romania National Institute for Research and Development in Microtechnology – IMT Bucharest, Romania Institute of Inorganic Chemistry and Electrochemistry of I. Javakhishvili Tbilisi State, Georgia Nicolae Testemi¸tanu State Medical and Pharmaceutical University, Republic of Moldova National Technical University, Kharkiv, Ukraine

Organization

Kulyuk Leonid Macovei Mihai Nacu Viorel

Sidorenko Anatolie Toma Ian Tronciu Vasile Tsiulyanu Dumitru Ursaki Veaceslav Verestiuc Liliana Vovc Victor

Moldova State University, Republic of Moldova Moldova State University, Republic of Moldova Nicolae Testemi¸tanu State Medical and Pharmaceutical University, Republic of Moldova Technical University of Moldova, Republic of Moldova The George Washington University, USA Technical University of Moldova, Republic of Moldova Technical University of Moldova, Republic of Moldova Academy of Sciences of Moldova, Republic of Moldova University of Medicine and Pharmacy, Romania Nicolae Testemi¸tanu State Medical and Pharmaceutical University, Republic of Moldova

Organizing Committee Sergey Railean Sontea Victor Brinza Mihai Galea-Abdusa Daniela

Gorceag Gheorghe Litra Dinu Matcovschi Sanda Monaico Eduard Pocaznoi Ion Sereacov Alexandr Surlari Ilie

Technical University of Moldova Moldovan Society of Biomedical Engineering, Technical University of Moldova Moldovan Society of Biomedical Engineering, Technical University of Moldova Nicolae Testemi¸tanu State Medical and Pharmaceutical University, Republic of Moldova Moldovan Society of Biomedical Engineering Technical University of Moldova Moldovan Society of Biomedical Engineering Moldovan Society of Biomedical Engineering, Technical University of Moldova Moldovan Society of Biomedical Engineering, Technical University of Moldova Technical University of Moldova Moldovan Society of Biomedical Engineering

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Organization

Reviewers List Adrian Dinescu Anatolie Sidorenko Artur Buzdugan C˘alin Corciova Corneliu Druga Dumitru Tsiuleanu Eduard Monaico Ghenadie Curocichin Ian Toma Ionel Sanduleac Konstantin Kolesnik Leonid Kulyuk Liliana Verestiuc

Mihai Macovei Mircea Dragoman Nistor Grozavu Oleg Lupan Tudor Branis, te Vasile Tronciu Veaceslav Ursaki Victor Sontea Victor Vovc Victor Zalamai Viorel Nacu Vladimir Fomin

Organizers

Moldovan Society of Biomedical Engineering

Technical University of Moldova

In Collaboration with

Nicolae Testemitanu State University of Medicine and Pharmacy

The International Federation for Medical and Biological Engineering (IFMBE)

Organization

European Alliance for Medical and Biological Engineering & Science

Academy of Sciences of Moldova

Supported by

National Agency for Research and Development of Moldova, project 20.80009.8007.26

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Plenary Speakers, Abstracts

Genes, Cells and Discovery in Basic Science and Disease

Randy Schekman Department of Molecular and Cell Biology, Howard Hughes Medical Institute, University of California, Berkeley, USA [email protected] Our understanding of the basic processes of life at the cellular and molecular level has substantially changed the outlook for the treatment of the greatest diseases of mankind. As a result of the development of tools to explore genes and chromosomes and the protein molecules they encode, therapies to treat heart disease and cancer have been designed with a level of precision that has saved countless lives. Beginning with the discovery of the structure of DNA and continuing with the elucidation of the path taken to express the genes in our genome, we are now able to modify genes that show promise of curing genetic diseases such as sickle cell anemia. These breakthroughs will surely lead to treatments for cancers and neurodegenerative diseases where heritable mutations are the source of illness. My interest began with a toy microscope that I received as a gift which stimulated a fascination with the microbial world. That interest matured at university and in my PhD work where I learned the powerful tools of biochemistry from Arthur Kornberg, a Nobelist who discovered an enzyme that copies DNA stands. For my independent career, I took the lessons from Kornberg and from broader readings on modern approaches to the elucidation of complex cellular processes and applied them to a molecular genetic dissection of the process of protein secretion in a simple eukaryotic organism, Baker’s yeast. Using simple genetics to discover essential genes required for protein secretion, my research team elucidated a pathway similar to that discovered in pancreatic tissue by the great Romanian Nobelist, George Palade. The genes we discovered are evolutionarily conserved and employed in mammals to execute the diverse processes in secretion essential to normal physiology. This conservation allowed the biotechnology industry to harness yeast cells as a secretion platform for the production of clinically important proteins such as human recombinant insulin. Following on the genetics, we developed biochemical approaches to identify the functions of a number of the secretion genes in yeast and their equivalents in human cells. Several of the genes encode subunits of the channel in the endoplasmic reticulum (ER) membrane responsible for the first step in the transfer of newly synthesized secretory proteins from their site of synthesis on ribosomes in the cytoplasm across the ER membrane into the interior luminal space. Another set of the genes encodes subunits of a coat protein complex that pinches transport vesicles carrying secretory cargo proteins for traffic from the ER to the Golgi apparatus. Some of these genes have been found to be the basis of human genetic diseases of protein secretion. Knowledge of these precise mechanisms contributes directly to the development of novel therapeutic interventions.

Tetrapods and Aeromaterials for Antiviral and Antibacterial Treatment and Therapy

Rainer Adelung Functional Nanomaterials, Department for Material Science, Kiel University, Germany [email protected] This talk will give an overview of recent advances in the field of antiviral and antibacterial treatments either for personal therapy or in air filtration systems from the functional nanomaterials group at Kiel University. In contrast to molecular drugs or similar medical agents, the main effect here is on the physical interaction or interaction with a nanostructured surface and not on a chemical effect influenced, for example, by the solubility of a drug. The entire system, i.e., in the case of a technical system its entire environment or in the case of personal therapy not only the disease, but also the surrounding network such as the immune system is exploited. Two main examples are followed: tetrapodal microcrystals of zinc oxide and the graphene-based aeromaterials created from them by a templating process. While the pharmaceutical effects of zinc oxide nanoparticles, which are simply based on an excess of nanoparticles, have been widely used for a long time in various products such as creams and ointments due to their weak antiseptic and drying effect, e.g., to support the healing of herpes blisters, tetrapodal zinc oxide, which actually showed a curative effect based on an immunization only in 2016 [1] in animal models, has only recently been translated from the research group into the pharmaceutical market (see Fig. 1). For this purpose, the tetrapodal zinc oxide enables the immune system to detect the virus at an early stage via the CD4 and CD8 signaling pathway with the help of antigen presenting cells, which can easily internalize the virus enabled by an immobilization on specially adjusted nanoscale surface structures on the zinc oxide crystal. Besides the first product experiences of “Afinovir”, a cream that contains GMPcertified tetrapodal zinc oxide in herpes therapy, further antibacterial effects are now in focus. These might be utilized in 3D-printed skin patches and their specific antibacterial effects and their ability to deliver proteins, like the VEGF for wound healing assistance, as shown by Leonard Siebert [2]. Compared to the effects in personal therapeutic medicine, the effects of sterilization provided by the aeromaterials in air filtration systems are much less sophisticated. The combination of the structural features of aeromaterials, the interconnected large free volume and the low weight are employed for a pyrolysis [3] of pathogens. Low mass means low heat capacity, which results in reaching high temperatures with relative low power. High free volume in connection with a hierarchical micro–nanostructure means high filter efficiency. However, the example given in the framework of the Graphene Spearhead Project AEROGrAFT, a passenger jet air filter system that is developed together with the aviation company Lufthansa Technik. It will be shown and discussed that beside technological obstacles, the aviation certification procedure provides a similar challenge.

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Fig. 1. Micrographs of tetrapodal zinc oxide. A. Scanning electron microscopy image, the arm diameters of the ZnO microcrystals are in the order of ~1–3 μm SEM. B. Fluorescence microscopy of a tetrapod with GFP labeled herpes virus bound to a tetrapod

References 1. J. Immunol. 196(11), 4566–4575 (2016) 2. Adv. Funct.Mater. 31(22), 2170154 (2021) 3. Mater. Today 48, 7–17 (2021)

Single-Crystal Diamond Radiation Detector

Toru Aoki Shizuoka University, Hamamatsu, Japan [email protected] We have reported on the growth of single-crystal diamond and its application to radiation detectors. This time, we report on the measurement of an imager with a photon-charge counting readout integrated circuit (ROIC) connected to a single-crystal diamond. The single-crystal diamond was 3.0 × 3.0 mm × 0.5 mm thick. The bump connection area of our test ROIC is a 3.2 mm × 3.2 mm area with 40 × 40 pixels pads at 80 μm pitch. Silver-based bumps were formed on short-crystal diamond using a super inkjet printer and bumped to the ROIC using flip chip bonders. The imaging experiment was conducted by irradiating X-rays at 90 kV and 0.2 mA at 30 cm distance. The results showed that the X-ray transmission image with a clear contrast between the area shielded by 2 mm thick lead and the unshielded area was captured, demonstrating a prototype single-crystal diamond-type X-ray imager. CVD growth single-crystal diamond was used for the semiconductor detector. The crystal was 3 mm × 3 mm × 0.5 mm in size. The plate electrode was formed by sputtering a thin indium film. The other side had no electrode formed before stacking, but was connected to the ROIC by contact with the silver paste formed on the ROIC side during stacking. The 40 × 40 electrodes were arranged at a pitch of 80 μm for the ROIC, and the maximum crystal size was 3.2 mm × 3.2 mm. However, due to the small crystal size, the detection area is 3.0 mm × 3.0 mm and 38 × 38 pixels. For the connection to the crystal, silver paste bumps were formed on the electrodes of the ROIC using a SIJ-S050 super inkjet printer manufactured by SIJ-Technology, Inc. The readout architecture of the ROIC was designed by our group [3, 4], and the transistorlevel circuitry was designed by Brookman Technology, Inc. (now TOPPAN Inc.) and fabricated in a Taiwan Semiconductor Manufacturing Company Limited (TSMC) with 0.13-μm standard CMOS process. Figure 1 shows the assembled diamond imager and its stacked structure. The ROIC was operated in electron collection mode. A bias voltage of −500 V was applied to the plate electrodes. The frame rate of the ROIC was 200 Hz. About 10 s (2,000 frames) was taken, and the counts were summed to obtain one image. An X-ray tube with a tube voltage of 100 kV was used as the X-ray source. The distance from the tube to the detector was 20 cm. A 3 mm × 3 mm lead plate with a thickness of 1 mm was used as the X-ray shield. The shielding body was shaped like a Japanese character. The shielding was placed between the X-ray tube and the detector, 5 mm away from the detector. It was confirmed that X-rays were detected as expected when X-rays were irradiated with and without the shielding. Next, a shielding was placed in front of the detector, and

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the shielding was imaged. As shown in Fig. 2, the shape of the shielding was obtained as an image.

Fig. 1. Schematic diagram of diamond detector (L) and picture (R)

Fig. 2. X-ray imaging result with a lead object of イ (Japanese character)

References 1. Takagi, K., et al.: Readout architecture based on a novel photon-counting and energy integrating processing for X-ray imaging. IEEE Trans. Radiat. Plasma Med. Sci. (2020) 2. Takagi, K., et al.: Study of an X-ray/Gamma ray photon counting circuit based on charge injection. Sens. Mater. 30(7), 1611–1616 (2018)

Strategic Integration of Electrospinning and Additive Processing for Smart and Sustainable Nanostructures

Ashok Vaseashta1,2 1 International

Clean Water Institute, Manassas, VA, USA [email protected] 2 Institute of Electronic Engineering and Nanotechnologies “D. Ghitu”, Chisinau, Moldova Electrospinning is an effective and versatile technique applied to fabricate porous structures ranging from submicron to nanometer dimensions. Using a variety of highperformance polymers and blends, several porous structure configurations have become possible for applications in tactile sensing, energy harvesting, filtration and biomedical applications; however, the structures lack mechanical complexity, conformity and desired three-dimensional single/multi-material constructs necessary to mimic desired structures. A simple, yet versatile, strategy is through employing digitally controlled fabrication of shape morphing by combining two promising technologies, viz., combinatorial electrospinning and 3D printing/additive processing. Using synergistic integration of configurations, elaborate shapes and patterns are printed with mesostructured stimuli-responsive electrospun membranes, allowing for in-plane modulations and internal interlayer stresses induced by swelling/shrinkage or mismatch, thus guiding morphing behaviors of electrospun membranes to adapt to the changes of the environment. Recent progress in 3D/4D printing/additive processing includes materials and scaffold constructs for tactile and wearable sensors, filtration structures and sensors for structural health monitoring, biomedical scaffolds, tissue engineering and optical patterning, among many other applications to support the vision of synthetically prepared smart material designs that mimic the structural aspects with digital precision. A novel technology called 3D jet writing was recently reported that propels electrospinning to adaptive technologies for the manufacturing of scaffolds according to user-defined specifications of the shape and size of both the pores and the overall geometric footprint. This presentation reviews the hierarchical synergy between electrospinning and 3D printing as part of the precision and rapid prototyping of smart, sustainable and biomedical structures that are likely to evolve next-generation structures into reality.

Superconducting Order Parameter in Inhomogeneous Superconductors

Balázs Újfalussy1 , Gábor Csire2 and Bendegúz Nyári3 1 Wigner

Research Centre for Physics, Budapest, Hungary [email protected] 2 Materials Center Leoben Forschung GmbH, Leoben, Austria [email protected] 3 Budapest University of Technology and Economics, Budapest, Hungary [email protected] Superconductivity is the state of matter in which the electronic wave function spontaneously takes on a definite complex phase. The most fundamental ingredient in the theoretical description of this phenomenon is the superconducting order parameter. Modern computational methods and the corresponding computer codes allow the quantitative predictions of superconducting properties for realistic experimental settings involving not only bulk superconductors but also heterostructures. One major consequence is that it enables the accurate calculation of the superconducting order parameter in inhomogeneous systems, which has been limited so far due to the lack of an appropriate theoretical tool set, even though the superconducting order parameter is one of the central quantities of superconductivity. This is because the BCS theory of superconductivity, in its original formulation, is not easily generalized for heterostructures, especially in the relativistic domain. In this talk, we attempt to provide a quantitative theory that takes into account the ab initio band structure with its full microscopic complexity together with magnetism, effects from relativity, spatial inhomogeneity of the lattice and different orbital symmetries on the same footing. This can be achieved within the framework of multiple scattering theory (MST), this time combined with the Bogolubov-deGennes (BdG) reformulation of the BCS theory. Similarly to the normal state, the central quantity of such an MST is the quasiparticle Green’s function, which now carries information about the scattering of quasiparticles [1]. The main extra ingredient of such scattering events compared to the normal state Korringa-Kohn-Rostoker (KKR) formalism is the so-called Andreev reflection. It occurs when an electron, with energy lying in the superconducting gap, arriving from the normal metal to the superconductor–normal metal (S/N) interface is retro-reflected as a hole and a Cooper pair is formed in the superconductor. This formalism allows vast applications, and we shall focus on the behavior of superconducting order parameter revealing its complexity and physical meaning in realistic systems. As an example, I will revisit the well-known proximity effect, which is part of the standard textbook physics vocabulary, and it is mostly understood within the quasiclassical picture. However, the real microscopic mechanism behind the proximity effect is the

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Andreev reflection. If one considers this microscopic picture for some artificial materials, often referred to as heterostructures, it reveals the difference in important concepts, like the superconducting order parameter, pairing potential and superconducting gap, which are not well separated in the conventional BCS theory of bulk superconductors. We will use the example of Nb/Au heterostructures in the talk. Then we will apply the approach to the complimentary problem, namely when a superconducting impurity cluster is embedded into a non-superconducting material. By doing so, we shed light on the build-up of the superconducting phase and its connection to the order parameter. As a testbed for our calculations, we consider superconducting Nb atoms surrounded by non-superconducting bulk Nb [2]. For a relatively small cluster of material with nonzero pair interaction parameter embedded in a normal metal, the superconductivity will be suppressed and the superconducting gap will be forced to close. However, the interesting question is what happens when the size of the cluster reaches the same order as the corresponding superconducting coherence length. Here one should remember that the Cooper pairs are extended in real space, since Cooper pairs are formed in momentum space and not in real space. In fact our local pairing model is the analog of conventional BCS theory, where Cooper pairs are formed by electrons with different quantum numbers k,↑ and –k,↓ in which states from the region of the gap around the Fermi level are mixed. The coherence length is the extension of these wave packets in real space given by the BCS theory. When a magnetic impurity or a cluster of magnetic impurities is introduced into a singlet s-wave time-reversal invariant superconductor, there is a pair-breaking effect due to the spin-dependent scattering, and the superconducting transition temperature decreases as the impurity concentration increases. Yu, Shiba and Rusinov revealed that local states (referred as YSR states) appear within the BCS energy gap due to multiple scattering between conduction electrons and magnetic impurities. Later, it was realized theoretically that one can engineer nanostructures, where hybridized YSR states form a topological fully gapped quasiparticle spectrum. For magnetic impurities, an especially rich internal structure of the superconducting order parameter can be calculated from the local Green’s function as nonzero elements in the electron–hole off-diagonal block. It will be demonstrated that the complexity of the ab initio electronic structure allows the appearance of an exotic local triplet state in magnetic s-wave time-reversal symmetry breaking superconductors. Furthermore, we also show how topological superconductivity is manifested in the structure of the superconducting order parameter.

References 1. Csire, G., et al.: Phys. Rev. B 91, 165142 (2015). https://doi.org/10.1103/PhysRevB. 91.165142 2. Saunderson, T.G., et al.: Phys. Rev. B 102, 245106 (2020). https://doi.org/10.1103/ PhysRevB.104.235426

Rehabilitation Using a Data Glove for Moving the Paralyzed Fingers

Hidenori Mimura, Soich Takigawa, Katsunori Suzuki, Kamen Kanev, Toru Aoki and Masakazu Kimura Research Institute of Electronics, Shizuoka University, Japan We have synthesized spinnable carbon nanotube (CNT) [1] and have developed the CNT strain sensors as components of a textile-based, wearable sensing system for real-time motion detection [2]. Well-aligned CNT sheets are fabricated by stacking and shrinking the CNT webs. Experimental CNT strain sensors are manufactured by placing the CNT sheet on a flat and smooth substrate in a direction parallel to the stretching direction and impregnating it with elastomeric resin. The sensor resistance is proportional to the applied tensile strain that increases with the applied force. The temporal strain changes are closely followed by the variation of the strain sensor resistance that can exceed 200%. We have developed a data glove using the CNT strain sensors [2]. The data glove is a wearable device with incorporated strain sensors that allow for motion and posture tracking of the user’s hands and fingers. Since direct sensing is employed, there are no environment restrictions, and timely, highly reliable data can be collected. Motion interactions are then implemented through real-time analysis and recognition of the user’s hand and finger posture and gesture. In this study, we have applied the data glove to functional electrical stimulation (FES) training [3]. FES is used to restore motor function in paralyzed patients because of stroke or spinal injury. In FES, electrical stimulations activate nerve tissue connected to muscle groups to contract muscles to induce movement of the hands. Its restoration improves the quality of life of a paralyzed patient, because hand function is crucial in daily life. We have proposed an FES training method triggered by motions of the opposite hand. Symmetrical motions in the target hand are triggered by the response to multiple motions of the opposite hand. The posture of the opposite hand is recognized by a data glove, and the electrical stimulation points of the multi-pad electrodes of the target hand are dynamically selected on the basis of the recognized posture. The patient can make the target hand produce postures symmetrical to the multiple grasping postures of the opposite hand without being aware of a special device. Electrical stimulation points that elicit the desired grasping motions are explored in advance with reference to the postures of the opposite hand. The proposed method can be applied to contralaterally controlled functional electrical stimulation (CCFES) and a combined method of mirror therapy and FES to train paralyzed patients to recover their grasping function more effectively. Figure 1 shows the experimental flow (upper) and the target grasping postures and a relaxed open posture recorded with the opposite hand (lower). The experiment was conducted as follows: in step 1, multiple grasping postures were recorded with the subject’s opposite (right) hand using the data glove to create a data table for identifying hand

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posture. In step 2, the stimulation electrodes of the subject’s target hand were scanned, and the optimal stimulation electrodes to produce the pre-recorded target grasping posture were determined by detecting the evoked postures with the data glove. In step 3, symmetrical postures were produced in the target (left) hand. Specifically, the grasping posture of the opposite hand was detected with the data glove. Then the optimal electrical stimulation pattern identified in the search was applied to the target hand according to the discriminated grasping posture. This procedure produced symmetrical postures in the target hand in response to the grasping postures of the opposite hand. In summary, we have developed a system that combines multichannel FES and a data glove with CNT strain sensors. Experiments on four healthy subjects demonstrated that the system selectively activates the muscles of the target hand in response to the grasping postures of the opposite hand. Additionally, the method produces postures symmetrical to those of the opposite hand. By this method, patients can intuitively train multiple grasping motions using FES without operating special instruments. Applying this method to CCFES or its combination with mirror therapy should improve the efficacy of training with FES.

Fig. 1. Flow of the three steps of the experiment (upper) and target grasping postures and a relaxed open posture recorded with the opposite hand (lower)

References 1. Inoue, Y., et al.: Appl. Phys. Lett. 92, 213113 (2008) 2. Suzuki, K., et al.: ACS Sens. 817 (2016) 3. Takigawa, S., Mimura, H.: Sens. Mater. 33, 3645 (2021)

Importance of Training Healthcare Professionals in Medical Technology

Nicolas Pallikarakis University of Patras, Patras, Greece [email protected] The progress of medical technology during the past 60 years is the driving force of the radical change in the way health care is delivered nowadays. Many simple devices like syringes, needles, personal protection and many other sterilizable products in the 1950s have been replaced by single-use devices that are produced in quantities of tens of billion items each year with an accelerated pace. Implantable devices, like knee and hip prostheses or dental implants, are common practice with a huge variety of products on the market. The same holds for active implantable medical devices (AIMDs) like pacemakers, defibrillators or infusion pumps. The progress in the in vitro diagnostics (IVDs) is also impressive with thousands of tests available today. It is estimated that over a million products available on the world market today are classified as medical devices. A clear indicator of this progress is the number of patents released every year for medical devices. Just as an example, according to MedTech Europe, more than 15,000 patents applications are deposited at the European patent office (EPO) that represents 41% of the worldwide total medical technology patent applications. The medical device sector together with digital communication is in the top of the list overpassing all other sectors, like pharmaceuticals or computer technology in this aspect. In response to these developments and to assure the quality and safety of the product that are reaching the market, in most countries there have been established regulatory mechanisms for medical devices approval before been placed on the market. In the EU, for instance, after the introduction of the Medical Devices Directives (MDDs) for AIMDs (90/385/EEC), MDs 93/42/EEC and IVDs 98/79/EC, these directives have been accompanied by several guidelines on classification, nomenclature and vigilance, just to name some. A certification system has been established through the involvement of notified bodies assigned for this task by the Competent Authorities of each member stage, or other linked non-EU countries to this system. Hundreds of harmonized technical standards or European norms (EN) have been and are continuously developed and updated by CEN/CENELEC to face the needs. These directives of the 1990s have been replaced in 2017 by two regulations: one for IVDs (2017/746) and one (2017/745) for all other MDs including AIMDs. These regulations are stricter than the previous directives aiming to improve safety and better protect the patients in a balanced way to avoid big obstacles for innovation. Therefore, control of the EU market seems well regulated, and it is similar in other parts of the world.

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Additionally, the regulatory frameworks for MDs over the world contain a provision of a vigilance system that follows them after they have entered the market with an associated adverse event reporting system and in some cases an obligatory parallel post-market surveillance system. These systems aim to increase patient safety by preventing the recurrence of adverse events, like already reported ones. This is achieved by mandating users and manufacturers of medical devices to report to the health authorities, incidents where a medical device has potentially contributed in death or injury of a patient or user, and where appropriate, dissemination of information, which could be used to prevent such repetitions, or to alleviate the consequences of such incidents. Health technology assessment (HTA) for medical devices has also attracted the interest of regulators, and a new Regulation (EU) 2021/2282 has been voted recently aiming to coordinate the way assessment is done for medicines and certain medical devices groups, by the respective national HTA agencies. This regulation is planned to be applied by January 2025. For medical devices, it also is recently recognized that they need a different approach of assessment, considering their big differences with medicines, in their way of application, action and use. It is therefore clear that both placing MDs on the market and their assessment are well regulated. However, the third pillar of medical technology safety, that is management of MDs during use, remains non-regulated. In fact, the way the medical devices are maintained, repaired and used is left to the healthcare units they belong. Quality control, preventive maintenance, repair or withdraw do not necessarily imply certified involved parties or users. This last issue is very critical, since correct application of medical technology, as intended by the manufacturer, is of prime importance for diagnosis, treatment and overall safety of the patients. According to data extracted by the Manufacturer and User Facility Device Experience (MAUDE) database of the US Food and Drug Administration (FDA), almost 3 million adverse event reports, involving MDs, are submitted each year. A large proportion of them are due with use errors. There are several reasons behind the large increase in the number of adverse events attributed to non-appropriate use of medical devices. Among these can be mentioned the variety of devices available today, their non-uniform instructions for use, the nonself-evident user interfaces of the medical equipment that are often computer driven, etc. For instance, in a medium-sized modern hospital of less than one thousand beds, one could find more than 15 different types/models of infusion pumps, 17 different types/models of portable ventilators, 10 different types/models of ICU ventilators, 20 different types/models of multiple vital physiological parameters monitoring systems and 35 different types/models of bedside monitoring systems. This situation creates a burden for the medical and paramedical staff that must pass from one device to the next, with completely different user handling requirements, thus increasing the risks for an adverse event due to use error. Therefore, it is very important to establish regular and continuous user training programs to maintain the knowledge and skills of medical and paramedical personnel, in the principles of operation and the practical use of medical devices. There are various ways to implement such programs nowadays. Apart from the traditional face-to-face

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or distance learning courses, there are numerous alternatives for synchronous or asynchronous teaching approaches that have been developed to respond on the needs imposed by the COVID-19 pandemic. Additionally, new simulation means are available for virtual training, and finally, the use of AI is expected to greatly increase the way courses will be prepared and presented in the future.

Nanocomposites and Polymer Thin Films: From Gas Phase Synthesis to Functional Applications

Stefan Schröder1 , Alexander Vahl1 , Salih Veziroglu1 , Oleg Lupan1,2 Cenk Aktas1 , Thomas Strunskus1 and Franz Faupel1

,

1 Chair

for Multicomponent Materials, Department of Materials Science, Faculty of Engineering, Kiel University, Kaiserstraße 2, 24143 Kiel, Germany [email protected] 2 Center for Nanotechnology and Nanosensors, Department of Microelectronics and Biomedical Engineering, Technical University of Moldova, 168 Stefan cel Mare Av., 2004 Chisinau, Moldova Among the functional nanocomposites, our group has focused on highly filled particulate metal-dielectric nanocomposites films due to their unique functional properties with hosts of applications. To explore collective interactions between the particles, we control the particle separation on the nanoscale by employing vapor phase deposition, which is a scalable approach permitting, inter alia, excellent control of the filling factor. For deposition of functional polymer thin films, we have recently used initiated chemical vapor deposition (iCVD) to avoid decomposition of the functional groups [1]. Examples include highly stable electrets for electret microphones and magnetoelectric sensors [2], 3D superhydrophobic coatings [3], nanoscale gradient copolymers and strain-invariant conductors for soft robotics [4]. For the fabrication of the nanocomposites, the nanoparticles can form during gas phase co-deposition via self-organization or by means of high-rate gas aggregation cluster sources, which provide independent control of filling factor and size as well as in situ monitoring and control of the composition of alloy nanoparticles. Recent examples of nanocomposites range from plasmonic metamaterials through photoswitchable [5] molecular plasmonic systems to memristors and memsensors for neuromorphic electronics [6]. We also explored nanoscale synergetic effects of plasmonics and photocatalysis [7], e.g., for photoinduced enhanced Raman spectroscopy (PIERS) [8]. In cooperation with the group of Oleg Lupan, we have also used alloy nanoparticles with tailored composition to enhance the sensitivity and selectivity of chemical sensors made up of micro- and nanostructured wide bandgap semiconductors [9]. Cross-sensitivity against moisture could be successfully eliminated with fluoropolymer coatings deposited by iCVD [10]. In addition to particulate nanocomposites, we are also concerned with multilayer nanocomposites. Here, emphasis is put on magnetoelectric sensors consisting of magnetostrictive and piezoelectric components on a vibrating beam. We also use electrets for readout and for obtaining a well-defined nonlinear restoring force [11].

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References 1. Schröder, S., Polonskyi, O., Strunskus, T., Faupel, F.: Nanoscale gradient copolymer films via single-step deposition from the vapor phase. Mater. Today 37, 35–42 (2020). https://doi.org/10.1016/j.mattod.2020.02.004 2. Schröder, S., Strunskus, T., Rehders, S., Gleason, K., Faupel, F.: Tunable polytetrafluoroethylene electret films with extraordinary charge stability synthesized by initiated chemical vapor deposition for organic electronics applications. Sci Rep 9, 2237 (2019). https://doi.org/10.1038/s41598-018-38390-w 3. Aktas, O.C., et al.: Superhydrophobic surfaces: superhydrophobic 3D porous PTFE/TiO2 hybrid structures. Adv. Mater. Interfaces 6, 1970029 (2019). https:// doi.org/10.1002/admi.201801967 4. Barg, I., et al.: Strain-invariant, highly water stable all-organic soft conductors based on ultralight multi-layered foam-like framework structures. Adv. Funct. Mater. 221268 (2023). https://doi.org/10.1002/adfm.202212688 5. Burk, M.H.,et al.: Fabrication of diazocine-based photochromic organic thin films via initiated chemical vapor deposition. Macromolecules 53, 1164 (2020). https:// doi.org/10.1021/acs.macromol.9b02443 6. Vahl, A., Carstens, N., Strunskus, T., Faupel, F., Hassanien, A.: Diffusive memristive switching on the nanoscale, from individual nanoparticles towards scalable nanocomposite devices. Sci. Rep. 9, 1 (2020). https://doi.org/10.1038/s41598-01953720-2 7. Veziroglu, S., et al.: Photodeposition of Au nanoclusters for enhanced photocatalytic dye degradation over TiO2 thin film. ACS Appl. Mater. Interfaces 12, 149 (2020). https://doi.org/10.1021/acsami.9b18817 8. Shondo, J., et al.: Nanoscale synergetic effects on Ag–TiO2 hybrid substrate for photoinduced enhanced Raman spectroscopy (PIERS) with ultra-sensitivity and reusability. Small 18, 2203861 (2022). https://doi.org/10.1002/smll.202203861 9. Vahl, A., et al.: Surface functionalization of ZnO:Ag columnar thin films with AgAu and AgPt bimetallic alloy nanoparticles as an efficient pathway for highly sensitive gas discrimination and early hazard detection in batteries, J. Mater. Chem. A 8, 16246–16264 (2020). https://doi.org/10.1039/D0TA03224G 10. Schröder, S., et al.: Sensing performance of CuO/Cu2O/ZnO:Fe heterostructure coated with thermally stable ultrathin hydrophobic PV3D3 polymer layer for battery application. Mater. Today Chem. 23, 100642, 2468–5194 (2022). https://doi.org/10. 1016/j.mtchem.2021.100642 11. Mintken, M., et al.: Nanogenerator and piezotronic inspired concepts for energy efficient magnetic field sensors. Nano Energy 70, 104420 (2020). https://doi.org/10. 1016/j.nanoen.2018.11.031

3D Nanoarchitectures—A Novel Class of Materials: Perspective for Sustainable Development

Vladimir M. Fomin1,2 1 Leibniz

Institute for Solid State and Materials Research (IFW) Dresden, Dresden, Germany [email protected] 2 Moldova State University, Chi¸sin˘ au, Moldova

Extending nanostructures into the third dimension has become a vibrant research avenue in condensed-matter physics, because of geometry- and topology-induced phenomena. Modern advances of high-tech fabrication techniques have allowed for generating geometrically and topologically nontrivial manifolds at the nanoscale, which determine novel, sometimes counterintuitive, electronic, magnetic, optical and transport properties of such objects and unprecedented potentialities for design, functionalization and integration of nanodevices due to their complex geometry and nontrivial topology [1]. I will focus on three directions of key importance for sustainable development of technologies. Firstly, recently suggested Möbius-strip microcavities [2] as integrable and Berryphase-programmable optical systems are of great interest in topological physics and emerging classical and quantum photonic applications. For photons resonating in a Möbius-strip cavity, the occurrence of an extra phase—known as the Berry phase— with purely topological origin is expected due to its nontrivial evolution in parameter space. However, despite numerous theoretical investigations, characterizing the optical Berry phase in a Möbius-strip cavity has remained elusive. Here we report the experimental observation of the Berry phase generated in optical Möbius-strip microcavities. In contrast to theoretical predictions in optical, electronic and magnetic Möbius-topology systems where only Berry phase π occurs, we demonstrate that a variable Berry phase smaller than π can be acquired by generating elliptical polarization of resonating light. Secondly, an efficient tailoring of acoustic phonon energy spectrum in rolled-up multishell tubular structures [3] opens up prospective applications in microelectronics, in cases when low heat conduction is required. The phonon energy spectra in the Si/SiO2 multishell nanotubes are obtained using the atomistic lattice dynamics approach. Thermal conductivity is calculated using the Boltzmann transport equation within the relaxation time approximation. Redistribution of the vibrational spectra in multishell nanotubes leads to a decrease of the phonon group velocity and the thermal conductivity as compared to homogeneous Si nanowires. Phonon scattering on the Si/SiO2 interfaces is another key factor of strong reduction of the thermal conductivity in these structures (down to 0.2 Wm−1 K−1 at room temperature). We demonstrate that phonon

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thermal transport in Si/SiO2 nanotubes can be efficiently suppressed by a proper choice of nanotube geometrical parameters: lateral cross section, thickness and number of shells. Thirdly, prospect directions and challenges in the domain of superconductivity and vortex matter in curved 3D nanoarchitectures and their great potential for magnetic field sensing, bolometry and information technology have been demonstrated [4]. A topological transition between the vortices and phase slips under a strong transport current is found in open superconductor Nb nanotubes with a submicron-scale inhomogeneity of the normal-to-the-surface component of the applied magnetic field [5]. This transition determines the magnetic field−voltage and current−voltage characteristics, which imply a possibility to efficiently tailor the superconducting properties of nanostructured materials by inducing a nontrivial topology of superconducting screening currents. A transition between the vortex and phase-slip regimes depends on the magnetic field only weakly if the magnetic field and/or transport current are switched on gradually. In the case of an abrupt switch-on of the magnetic field or transport current, the system can be triggered to the stable phase-slip regime within a certain window of parameters. A hysteresis effect in the current–voltage characteristics is predicted in superconductor open nanotubes [6]. Dynamic topological transitions in open superconductor nanotubes occur under a combined DC+AC transport current [7]. The key effect is a transition between two regimes of superconducting dynamics. The first regime is characterized by a pronounced first harmonic in the Fast Fourier Transform (FFT) spectrum of the induced voltage at the AC frequency. It is typical of two cases, when the dominant area of the open tube is superconducting at relatively low magnetic fields and/or weak DC currents or normal at relatively high magnetic fields and/or strong DC currents. The second regime is represented by a rich FFT spectrum of the induced voltage with pronounced low-frequency components due to an interplay between the dynamics of superconducting vortices or phase slips and those driven by the AC.

References 1. Fomin, V.M.: Self-rolled Micro- and Nanoarchitectures: Topological and Geometrical Effects. De Gruyter, Berlin, Boston (2021) 2. Wang, J., et al.: Experimental observation of berry phases in optical möbius-strip microcavities. Nat. Photonics 17, 120–125 (2022). https://doi.org/10.1038/s41566022-01107-7 3. Isacova, C., Cocemasov, A., Nika, D.L., Fomin, V.M.: Phonons and thermal transport in Si/SiO2 multishell nano-tubes: atomistic study. Appl. Sci. 11(8), 3419, 1–12 (2021). https://doi.org/10.3390/app11083419 4. Fomin, V.M., Dobrovolskiy, O.V.: A Perspective on superconductivity in curved 3D nanoarchitectures. Appl. Phys. Lett. 120(9), 090501, 1–12 (2022). 5. Rezaev, R.O., Smirnova, E.I., Schmidt, O.G., Fomin, V.M.: Topological transitions in superconductor nanomembranes in a magnetic field with submicron inhomogeneity under a strong transport current. Commun. Phys. 3, 144, 1–8 (2020). https://doi.org/ 10.1038/s42005-020-00411-4

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6. Bogush, I., Fomin, V. M.: Topological defects in superconductor open nanotubes after gradual and abrupt switch-on of the transport current and magnetic field, Phys. Rev. B 105(9), 094511, 1–11 (2022). https://journals.aps.org/prb/abstract/10.1103/ PhysRevB.105.094511 7. Fomin, V.M., Rezaev, R.O., Dobrovolskiy, O.V.: Topological transitions in ac/dcdriven superconductor nanotubes. Sci. Rep. 12, 10069, 1–10 (2022). https://www.nat ure.com/articles/s41598-022-13543-0

Engineering Heterostructured Nanomaterials for Nanoelectronic and Biomedical Applications

Oleg Lupan1,2 1 Technical

University of Moldova, 168 Stefan cel Mare Av., 2004 Chisinau, Moldova [email protected] 2 Kiel University, Kaiserstraße 2, 24143 Kiel, Germany

Engineering heterostructured nanomaterials for nanoelectronics, as well as for biomedical applications, have attracted huge attention in the past decade. It is because heterostructured nanomaterials are constructed by two or more single-component nanoparticles with certain structure, order of nanolayers and synergistically enhanced functional properties. Heterostructures made of nanoparticles and nanostructured thin films or metalorganic frameworks are integrating advantages of porosity, nanosize, structure, optical and electrical performances. Recently, diverse nano-heterostructured materials are engineered and grown through various approaches and strategies and have proved promising potential for applications in battery safety sensors (BAS), gas, vapor and UV sensors, as well as biosensors for biomedical applications [1–7]. Novel two-in-one battery safety sensors have been developed based on the CuO/Cu2 O and TiO2 /CuO/Cu2 O heterostructures, as an example of real application [1–4]. These sensors enable early detection of solvents or the vapors of their degassing products, which are produced by Li-ion batteries at the onset of runaway [1–5]. Coating ZnO nanocolumns using Al2 O3 and thermal annealing offers the resulting Al2 O3 /ZnO heterostructure that enhances the gas sensing properties toward the detection of the components in the electrolytes of the lithium-ion batteries. Columnar films of Al2 O3 /ZnO with a thickness of 10 nm for the top-coating layer exhibit the highest sensitivity and selectivity toward the vapors of C3 H4 O10 . Experimental and computed results indicate that relative humidity will not affect the sensing properties of the such heterostructures toward the volatile organic compounds (VOCs) and degassing products used in the electrolytes of lithium-ion batteries [1–6]. As well as, new two-in-one sensor for NH3 and H2 detection is discussed, which ensures stable, precise and very selective characteristics for the tracking of these vapors at low concentrations. The fabricated TiO2 layers, which were annealed at 610 °C, formed two crystal phases, anatase and rutile, and after coverage with a thin PV4D4 polymer nanolayer via initiated chemical vapor deposition (iCVD), show response to ammonia at room temperature and exclusive hydrogen detection at elevated operating temperatures. These results open new possibilities for applications, e.g., like biomedical diagnosis, biosensors and the development of non-invasive technology [7]. Compared to unprotected CuO/Cu2 O/ZnO:Fe, the coated CuO/Cu2 O/ZnO:Fe exhibits a much better sensing performance at higher relative humidity and tunability of the gas selectivity [3].

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The higher responses to specific volatile organic compounds, VOCs, are controlled and tailored for the samples synergistically enhanced with dopants and nanoparticles simultaneously. In addition, the recovery times are reduced for the developed nanocolumnar layers for a range of operating temperatures. The response of the synergistically enhanced sensors to gas molecules containing certain functional groups is in excellent agreement with density functional theory calculations performed in our work too [8]. This new fabrication strategy can underpin the next generation of advanced materials for photocatalytic, VOC, and gas sensing applications and prevent levels that are hazardous to human health and can cause environmental damage. As well as, it can be used for detecting gases used as traces for specific molecules that act as biomarkers in exhaled breath or outgassing VOCs of various biological systems. Acknowledgments.This research was funded by the SulfurSilicon Batteries (SuSiBaBy) Project (LPW-E/3.1.1/1801), which was funded by the EUSH and EFRE in SH.

References 1. Lupan O., et al.: Heterostructure-based devices with enhanced humidity stability for H2 gas sensing applications in breath tests and portable batteries. Sens. Actuators A Phys. 329, 112804 (2021). https://doi.org/10.1016/j.sna.2021.112804 2. Schröder S., et al.: Tuning the selectivity of metal oxide gas sensors with vapor phase deposited ultrathin polymer thin films. Polymers (Basel) 15, 524 (2023). https://doi. org/10.3390/polym15030524 3. Schröder S., et al.: Sensing performance of CuO/Cu2 O/ZnO:Fe heterostructure coated with thermally stable ultrathin hydrophobic PV3D3 polymer layer for battery application. Mater. Today Chem. 23, 100642 (2022). https://doi.org/10.1016/j.mtchem. 2021.100642 4. Lupan O., et al.: TiO2 /Cu2 O/CuO multi-nanolayers as sensors for H2 and volatile organic compounds: an experimental and theoretical investigation. ACS Appl. Mater. Interfaces 13, 32363–32380 (2021). https://doi.org/10.1021/acsami.1c04379 5. Lupan O., et al.: Development of 2-in-1 sensors for the safety assessment of lithiumion batteries via early detection of vapors produced by electrolyte solvents. ACS Appl. Mater. Interfaces 13, 32363–32380 (2023). https://doi.org/10.1021/acsami.3c0 3564 6. Santos-Carballal, D., et al.: Al2 O3 /ZnO composite-based sensors for battery safety applications: an experimental and theoretical investigation. Nano Energy 109, 108301 (2023). https://doi.org/10.1016/j.nanoen.2023.108301 7. Brinza M., et al.: Biosensors 13(5), 538 (2023). https://doi.org/10.3390/bios13050538 8. Postica V., et al.: ACS Appl. Mater. Interfaces 1131452−31466 (2019). https://doi. org/10.1021/acsami.9b07275

Brain-Like Artificial Neural Network Based on Superconducting Neurons and Synapses

Anatolie Sidorenko1,2 1 Institute

of Electronic Engineering and Nanotechnologies “D.GHITU”, Technical University of Moldova, Str. Academiei 3/3, 2028 Chisinau, Moldova [email protected] 2 Skobeltsyn Institute of Nuclear Physics, Moscow State University, 119991 Moscow, Russia Energy efficiency and the radically reduction of the power consumption level become a crucial parameter constraining the advancement of supercomputers. The most promising solution is design and development of the brain-like systems with non-von Neumann architectures, first of all— the artificial neural networks (ANNs) based on superconducting elements. Superconducting ANN needs elaboration of two main elements—nonlinear switch, neuron [1], and linear connecting element, synapse [2]. We present results of our design and investigation of artificial neurons, based on superconducting spin valves—S/F/S Josephson Junctions with weak-link F fabricated from magnetic material (Ni or alloy CuNi) and superconducting synapse based on layered hybrid structures superconductor/ferromagnet. We obtained and analyzed results of experimental study of the proximity effect in a stack-like superconductor/ferromagnet (S/F) superlattices Nb/Co with F = Co ferromagnetic layers of different thicknesses and coercive fields and S = Nb superconducting layers of constant thickness equal to coherence length of niobium which can serve as an artificial synapse. The superlattices Nb/Co demonstrate change of the superconducting order parameter in thin niobium films due to switching from the parallel to the antiparallel alignment of neighboring ferromagnetic layers magnetization. We argue that such superlattices can be used as tunable kinetic inductors for ANN synapses engineering. As the result of design of the ANN, using that two elaborated base elements, artificial neurons and artificial synapses, allows construction of the computer with 6–7 orders of magnitude lower energy consumption in comparison with the traditional computer designed from semiconducting base elements. The study was supported by Grant RSF No. 22-79-10018 “Controlled kinetic inductance based on superconducting hybrid structures with magnetic materials” (theory development, samples measurements, results evaluation) and by the Moldova State Program Project «Functional nanostructures and nanomaterials for industry and agriculture» No. 20.80009.5007.11 (samples fabrication and characterization).

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Fig. 1. a) TEM image of the artificial neuron—Nb/F/Nb Josephson Junction, b) sketch of the Nb/F/N Josephson Junction with F-magnetic material (Ni, or CuNi), c) I–V switching curve of the Nb/CuNi/Nb Josephson Junction at fixed temperature T = 0,5 K, d) c) I–V switching curves of the Nb/Ni/Nb Josephson Junction at number of fixed temperatures listed in the inset, e) temperature dependence of the critical current of Nb/CuNi/Nb Josephson Junction and f) temperature dependence of the critical current of the set of Nb/Ni/Nb Josephson Junctions with various thickness of the Ni layer

References 1. Klenov, N., et al.: Periodic Co/Nb pseudo spin valve for cryogenic memory. Beilstein J. Nanotechnol. 10, 833–839 (2019). https://doi.org/10.3762/bjnano.10.83 2. Bakurskiy, S., et al.: Controlling the proximity effect in a Co/Nb multilayer: the properties of electronic transport. Beilstein J. Nanotechnol. 11, 1336–1345 (2020). https://doi.org/10.3762/bjnano.11.118

Evaluation of Health Technology in Republic Moldova

Victor Sontea1

and Artur Buzdugan2

1 President

of Moldovan Biomedical Engineering Society; Head of National Center of Biomedical Engineering, Technical University of Moldova, Republic of Moldova [email protected] 2 Technical University of Moldova, Republic of Moldova Medical devices (MDs) are indispensable in performing the medical act, and their importance has become a priority at the medical institution level as well as at the national level. To ensure the efficient functioning of a health system, it is necessary to equip it with medical devices, in accordance with the progress of medical technologies. However, the use of quality, safe and effective medical devices also requires qualified human resources, as well as the implementation of an effective management of medical devices [1]. The degree of endowment with high-performance medical devices and ensuring an appropriate level of professionalism of the medical staff are the key tools in ensuring the good functioning of the health system and exert a direct impact on the functional effectiveness of the system, on the quality of the service and the degree of satisfaction of the beneficiary .The effective use of them presupposes, as a matter of priority, the increase in the number of cost-effective and qualitative investigations and treatment. For these reasons, the WHO recommends and it is essential to have a policy at the national level regarding the Management of Medical Devices (MMD), which includes the provision of DM, ensuring the maintenance, verification and correct use of medical technologies, the training of specialists in the field, the creation of a system of their continuous training, etc. In the Republic of Moldova, Law no. 102 of June 9, 2017, regarding medical devices with the aim of adjusting the legal framework of the Republic of Moldova to the community, acquired for the implementation of European technical regulations in the field of MD and consumer protection of medical services, offered through the application of MD. The signed law stipulates the definitions according to the European Directives with application to DM and establishes that they can be introduced on the market, put into operation or used only if they are certified and registered, so that they do not affect the safety and health of patients, users and other people and the environment. With the support of the Swiss Development and Cooperation Agency through the PERINAT and REPEMOL projects, all existing procedures related to MDD at the hospital level were analyzed, the necessary set of procedures was defined, and the model of MD management and administration procedures was developed, which were initially implemented in five medical institutions and were developed with the support of JICA, the Guide regarding the establishment criteria, roles and responsibilities of biomedical engineering departments/sections within medical institutions. The results of the MDM evaluation at the medical institution level demonstrated the positive impact in the efficient use of the DM, the reduction of the maintenance expenses

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of the DM, through appropriate internal services in a timely manner, the increase in the cost-efficiency and safety of the medical act. In the same context, the Mother and Child Institute jointly with the Technical University of Moldova established the “Management of Medical Technologies” Pilot Center, which aims to develop procedure models for MMD to European and international standards, the development of procedures for maintenance, diagnosis and repair of medical devices, with the support of JICA, 5 departments of Biomedical Engineering were organized. According to the Statistical Yearbook of the Republic of Moldova for 2020, 12,552 doctors and 23,584 medical personnel worked in the health system, of which 18,514 were nurses—all users of medical devices. One of the most important roles in the management of medical devices belongs, of course, to the personnel responsible for the maintenance of medical devices (medical bioengineers, technical engineers, technicians, mechanics, etc.). Starting from 2016 and until now, annual evaluations have been carried out regarding the endowment of the health system with human resources (medical bioengineers, engineers and technicians). Recent evaluations have shown that their total number is about 150, of which 50 are medical bioengineers. At the same time, the real need of the health system being over 300 medical bioengineers. For this purpose, the National Biomedical Engineering Center within the Technical University of Moldova [2], authorized by law with such functions, plays a special role in the periodic training and improvement of staff. The expected result following the implementation of the MMD is ensuring the quality of medical devices and optimizing the use of public financial means through a proposed rational and efficient management. Acknowledgments. This work was supported by Project 20.80009.8007.26 “Piloting the application of the principles of personalized medicine in the conduct of patients with chronic non-communicable diseases”, contracting authority: National Agency for Research and Development, Republic of Moldova.

References 1. Sontea V., Buzdugan A.: Management of medical technologies in the Republic of Moldova – the basic component of safety, efficiency, and quality of medical services. J. Med. Health Sci. Educ. East. Europe Cent. Asia (MEDH-EECA) 1(2), 34–44 (2022). https://lsmu.lt/wp-content/uploads/2022/10/MEDH-EECA-2022-Volume-1No-2-web.pdf. ISSN 2783-6797 2. The Law on Medical Devices no. 102, no. 244–251, 389. Monitorul oficial of the Republic of Moldova (2017). https://www.legis.md/cautare/getResults?doc_id=105 695&lang=ro. Accessed 9 June 2017

New Areas of Research and Applications for GaN

Ion M. Tiginyanu

and Tudor Braniste

Academy of Sciences and Technical University of Moldova, Chisinau, Republic of Moldova [email protected] The properties of semiconductor materials can be modified in a controlled fashion, or even new characteristics may be brought to light by building hybrid 3D nanoarchitectures. The goal of this presentation is to review the research efforts undertaken over the last years to develop novel bio-inspired hybrid 3D nanoarchitectures based on binary compounds such as GaN, ZnS, ZnO and TiO2 . One of the most promising 3D nanoarchitectures proves to be the so-called aero-GaN or Aerogalnite, which represents the first artificial material exhibiting dual hydrophobic–hydrophilic behavior, its characteristics being close to those inherent to a biological cell membrane. The Aerogalnite consists of gallium nitride hollow micro-tetrapodal structures with nanoscopic thin walls, the inner surface being covered by an ultrathin film of zinc oxide [1]. The lateral faces of GaN tetrapods were found to show hydrophobic properties, while the free end of the arms—hydrophilic ones. This new result was achieved in close collaboration with other research groups (see [1] and https://physicsworld.com/a/hydrophobic-or-hydrophilicaero-gallium-nitride-is-both/). Approaching each other hollow GaN tetrapods floating on the water surface leads to the formation of waterproof rafts showing impressive stretching and cargo performances. The interaction between tetrapods resembles the interaction of fire ants forming live rafts on the water surface which enable the insects to survive during floods [2, 3]. The elasticity and stretching performances of self-assembled aero-GaN membranes were studied using communicating vessels. It was found that the aero-tetrapods of gallium nitride interact with each other on the water surface until a consolidated membrane forms. The membrane is elastic and can be used as a separation barrier between liquids, avoiding direct contact and mixing, but keeping the gas exchange due to a very high degree of porosity. We found that the membranes can withstand liquid droplets hundreds of times heavier than the membrane [1]. It was found that aero-GaN platelets with the thickness of 1–2 mm exhibit impressive shielding capabilities against electromagnetic radiation in a wide range of frequencies including Gigahertz and Terahertz ones [4, 5]. The shielding effectiveness in the frequency range from 0.25 to 1.37 THz exceeds 40 dB, which places Aerogalnite among the known best Terahertz shields. We succeeded in preparing liquid marbles using GaN aero-tetrapods. In order to explore the aero-GaN liquid marble properties, different deviations from the spherical symmetry were induced during the fabrication process. It was found that aero-GaN-based liquid marbles exhibit energy-efficient long-term translational movement for several

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hours and fast velocity of rotation up to 750 rpm [1]. The rotation speed and the time decay of spinning liquid marbles are highly dependent on their weight. The lighter liquid marbles show higher rotation speed, while the heavier ones are characterized by a much higher inertia keeping the spinning for a longer time [6]. The rotation of liquid marbles is highly dependent on the specific architecture of the enveloping shell consisting of GaN hollow microtetrapods with dual hydrophilic–hydrophobic properties, and the deviations from the spherical shape lead to behavioral changes of the marbles. It was found that elongated liquid marbles exhibit pulsed rotation, attaining the same maximum speed of rotation at each pulse, after which the speed of rotation drops down sharply. This phenomenon was described by using a simple analytical model which takes into account the uplift of the marble and formation of water columns underneath during the spinning process. When the rotation speed increases, the marble tends to detach from the water surface, which leads to interruption of the propulsion mechanism, and consequently, the marble drops on the water surface and continues the rotation at much lower speed [6]. In the plenary report, results of investigation of architectures based on Ga2 O3 , ZnS, TiO2 will be presented as well and the areas of possible applications of these aeromaterials will be discussed. The research was funded by National Agency for Research and Development of Moldova under Grant #20.80009.5007.20 “Nanoarhitecturi în baz˘a de GaN s, i matrici tridimensionale din materiale biologice pentru aplicat, ii în microfluidic˘a s, i inginerie tisular˘a” and by the European Commission under Grant #810652 “NanoMedTwin”.

References 1. Tiginyanu, I., et al.: Self-organized and self-propelled aero-GaN with dual hydrophilic-hydrophobic behaviour. Nano Energy 56, 759–769 (2019). https://doi. org/10.1016/j.nanoen.2018.11.049 2. Hu, D.L., Chan, B., Bush, J.W.M.: The hydrodynamics of water strider locomotion. Nature 424, 663–666 (2003). https://doi.org/10.1038/nature01793 3. Mlot, N.J., Tovey, C.A., Hu, D.L.: Fire ants self-assemble into waterproof rafts to survive floods. Proc. Natl. Acad. Sci. U.S.A. 108, 7669–73 (2011). https://doi.org/ 10.1073/pnas.1016658108 4. Braniste, T., et al.: Terahertz shielding properties of aero-GaN. Semicond. Sci. Technol. 34(12), 12LT02 (2019). https://doi.org/10.1088/1361-6641/ab4e58 5. Dragoman, M., et al.: Electromagnetic interference shielding in X-band with aeroGaN. Nanotechnology 30(34), 34LT01 (2019). https://doi.org/10.1088/1361-6528/ ab2023 6. Braniste, T., et al.: Self-propelled Aero-GaN based liquid marbles exhibiting pulsed rotation on the water surface. Materials 14(17), 5086 (2021). https://doi.org/10.3390/ ma14175086

Manifestations of Unconventional Pairing Symmetry in Superconducting Hybrids

A. A. Golubov1,2 1 Faculty

of Science and Technology, University of Twente, The Netherlands 2 Moscow Institute of Physics and Technology, Russia

Superconducting states with broken time-reversal symmetry may arise in structures with nontrivial topological properties and are currently of high interest from fundamental and applied points of view. We will discuss theoretical basis for the description of interfaces between unconventional superconductors (S) and normal metals (N). We will present the results for electronic and spin transport in multiterminal S/N hybrid structures [1–3]. Technically, the derivation of boundary condition for the Nambu-Keldysh quasiclassical Green’s functions at the S-N interfaces will be outlined. Of particular interest is application to superconductors with mixed s + p-wave superconducting pairing symmetry, including the cases of chiral and helical p-wave state in two dimensions, as well as the so-called Balian-Werthamer state in three dimensions. The local density of states, charge and spin conductance will be discussed. The cases will be identified when the proximity-induced pairing in N has odd-frequency spin-triplet s-wave symmetry. This state is characterized by the existence of a robust zero-energy Andreev bound state. Within the developed approach, three- and four-terminal S/N structures are investigated where the superconducting potential is a mixture between s-wave and p-wave potentials. The ways are proposed to determine whether S has a mixed pair potential and to distinguish between chiral and helical p-wave superconductivity. In this case, a difference in conductance for electrons with opposite spins arises if both an s-wave and a p-wave components are present, even in the absence of a magnetic field. It is shown that a setup containing two S-N junctions provides a clear difference in spin conductance between the s + chiral p-wave and s + helical p-wave symmetries. Further, we propose new approach to distinguish p-wave from s-wave symmetry by measuring conductance in a four-terminal junction consisting of S and N terminals. The N terminals are used to manipulate the energy distribution functions of electrons in the junction in order to control the charge transport. It is shown that the differential conductance of junctions containing p-wave and s-wave superconductors is distinctly different, thus providing experimental test to detect potential p-wave superconductivity. Acknowledgement. The work is partially supported by Grant 23-72-30004 from Russian Science Foundation.

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References 1. Tanaka, Y., Kokkeler, T.H., Golubov, A.A.: Phys. Rev. B 105, 214512 (2022) 2. Kokkeler, T.H., Golubov, A.A., Geurts, B.J.: SUST 35, 084005 (2022) 3. Kokkeler, T.H., Tanaka, Y., Golubov, A.A.: Phys. Rev. Res. 5, L012022 (2023)

Perspective of Electronics at Atomic Scale—2D SnS—A Ferroelectric for Low-Power Applications

Mircea Dragoman1 , Daniela Dragoman2,3 , Adrian Dinescu1 , Andrei Avram1 , Silviu Vulpe1 , Martino Aldrigo1 , Tudor Braniste4 , Victor Suman5 , Emil Rusu5 and Ion Tiginyanu4,6 1 National

Institute for Research and Development in Microtechnologies, Erou Iancu Nicolae Street 126A, Voluntari (Ilfov), Romania 2 Physics Faculty, University of Bucharest, PO Box MG-11, 077125 Bucharest, Romania 3 Academy of Romanian Scientists,, Str. Ilfov 3, 050044 Bucharest, Romania 4 National Center for Materials Study and Testing, Technical University of Moldova, 168 Stefan cel Mare Av., 2004 Chisinau, Moldova 5 Institute of Electronic Engineering and Nanotechnologies, Academiei Street 3/3, 2028 Chisinau, Moldova 6 Academy of Sciences of Moldova, 1 Stefan cel Mare Av., 2004 Chisinau, Moldova

In this work, we show that 2D tin sulfide (SnS) is an in-plane ferroelectric with important microwave properties. Further, we will show that field-effect transistors based on 2D SnS have a subthreshold below 60 mV/decade. Further, we demonstrate experimentally that a 10 nm thick SnS thin-film-based back-gate transistors which in the absence of the gate voltage, the drain current versus drain voltage (I D – V D ) dependence, are characterized by a rather weak ambipolar drain current. Applying a gate voltage as low as 1 μV, the current increases by several orders of magnitude and the I D – V D dependence changes drastically, with the SnS behaving as a p-type semiconductor. This happens because the current flows from the source (S) to the drain (D) electrode through a discontinuous superficial region of the SnS film when no gate voltage is applied. On the contrary, when minute gate voltages are applied, the vertical electric field applied to the multilayer SnS induces a change in the flow path of the charge carriers, involving the inner and continuous SnS layer in the electrical conduction. Thus, we demonstrate that we can switch reversible the channel of a FET in on/off states with 1 μV gate voltage, the lowest known to date.

Contents

Nanotechnologies and Nanomaterials Properties of Vacuum-Evaporated CH3 NH3 PbCl3−x Ix Perovskite Layers . . . . . . Gagik Ayvazyan, Surik Khudaverdyan, Lenrik Matevosyan, Harutyun Dashtoyan, and Ashok Vaseashta

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Functional Capabilities of Two-Barrier Semiconductor Structures . . . . . . . . . . . . Surik Khudaverdyan, Ashok Vaseashta, Gagik Ayvazyan, Mane Khachatryan, and Ashot Khudaverdyan

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Gamma Radiation Sensitization of ZnO/Al2 O3 Sensors Based on Nanoheterostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cristian Lupan, Adrian Bîrnaz, Artur Buzdugan, Nicolae Magariu, and Oleg Lupan Electrical Properties of the (Copper, Dysprosium)-Containing Complex Compound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Andriy Semenov, Volodymyr Martyniuk, Maria Evseeva, Oleksandr Osadchuk, Olena Semenova, and Tetyana Yushchenko Morphological and Sensing Properties of the ZnO-Zn2 SnO4 Ternary Phase Nanorod Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dinu Litra, Cristian Lupan, Tim Tjardts, Haoyi Qiu, Tudor Zadorojneac, Dominic Malai, Alexandr Sereacov, Cenk Aktas, Leonard Siebert, and Oleg Lupan Characterization of Films Prepared by Aerosol Spray Deposition in the (MgO)x (In2 O3 )(1−x) System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vadim Morari, Daniela Rusu, Emil V. Rusu, Veaceslav V. Ursaki, and Ion M. Tiginyanu Nanocomposite Films Based on Photosensitive Azopolymer with Gold Nanoparticles: Synthesis, Film Deposition, Diffractive Optical Elements Recording and Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elena Achimova, Vladimir Abashkin, Alexei Meshalkin, Constantin Losmanschii, Vladislav Botnari, and Giancarlo Pedrini

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MOF-Coated 3D-Printed ZnO Tetrapods as a Two-in-One Sensor for H2 Sensing and UV Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Barnika Chakraborty, Philipp Schadte, Mirjam P. M. Poschmann, Cristian Lupan, Tudor Zadorojneac, Nicolae Magariu, Ajay Padunnappattu, Fabian Schütt, Oleg Lupan, Leonard Siebert, Norbert Stock, and Rainer Adelung A Nanosized Heteronuclear {Fe18 Tb6 } Coordination Wheel Based on Pivalate and Triethanolamine Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Daniel Podgornii, Sergiu Shova, Victor Ch. Kravtsov, and Svetlana G. Baca

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Organic Nanostructured Crystals for Thermoelectric Cooling in Medical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ionel Sanduleac, Silvia Andronic, and Ion Balmus

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General Nature of Serration Effect in Crystals and Other Materials Under Indentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Daria Grabco, Constantin Pyrtsac, and Olga Shikimaka

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Trends in Evolution of the Energy Band Structure of Chalcopyrite CuBIII XVI 2 Compounds with Variation of the B and X Compositions . . . . . . . . . 106 Alisa Ma¸snic, Victor Zalamai, and Veaceslav Ursaki Optical and Photoelectric Properties of Cadmium Diarsenide and Surface-Barrier Structures Based on It . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Ivan Stamov and Dmitry Tkachenko Preliminary Study on Silver Nanoparticle Synthesis Through Chemical and Biological Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 Ramona Mirela Plesnicute, Anamaria Vacariu, Iuliana Motrescu, and Dorina Creanga Advanced Nanotechnology-Based Approaches to Waste Water Purification from Organic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Tatiana Datsko, Veacheslav Zelentsov, and Dmitri Dvornikov Micro-Raman Analysis of Some As-S-S-Te Nanostructured Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Oxana Iaseniuc and Mihail Iovu Optical Properties and Photoinduced Anisotropy of PEPC-co-SY3 Nanocomposite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Constantin Los, manschii, Elena Achimova, Vladimir Abaskin, Alexei Mesalchin, Alexandr Prisacar, and Vladislav Botnari

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Ground and Excited States of Excitons in GaSe Single Crystals . . . . . . . . . . . . . . 166 Ecaterina Cristea, Ivan Stamov, and Victor Zalamai New Characteristics of Blue Self-pulsating InGaN Lasers . . . . . . . . . . . . . . . . . . . 174 Eugeniu Grigoriev, Spiridon Rusu, and Vasile Tronciu Parametric Anomaly of the Phonon Spectrum of a Thin Free-Standing Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 Sergiu Cojocaru Photoluminescence and Cathodoluminescence of Layered ZnIn2 S4 and Zn2 In2 S5 Compounds Thermally Processed in Sulfur Vapor and Vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Efim Arama, Valentina Pîntea, and Tatiana Shemyakova ZnO Microtetrapods Covered by Au Nanodots as a Platform for the Preparation of Complex Micro-nano-structures . . . . . . . . . . . . . . . . . . . . . . 197 Eduard V. Monaico, Armin Reimers, Vladimir Ciobanu, Victor V. Zalamai, Veaceslav V. Ursaki, Rainer Adelung, and Ion M. Tiginyanu Synthesis Technology for CdSe/CdTe Heterojunctions and Characterization of Their Photoelectric Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 Ludmila Gagara, Ion Lungu, Lidia Ghimpu, and Tamara Potlog Illumination-Dependent Photovoltaic Parameters of CdS/ZnTe Solar Cells . . . . . 214 Ion Lungu, Lidia Ghimpu, Victor Suman, Dumitru Untila, and Tamara Potlog Fine Dispersion and Intensification of Heat Transfer at Boiling in Electric Field on the Modified Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Ion Chernica and Mircea Bologa Photodetector Based on β-Ga2 O3 Nanowires on GaSx Se1-X Solid Solution Substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 Veaceslav Sprincean, Mihail Caraman, Haoyi Qiu, Tim Tjardts, Alexandr Sereacov, Cenk Aktas, Rainer Adelung, and Oleg Lupan Aero-Materials Based on Wide-Band-Gap Semiconductor Compounds for Multifunctional Applications: A Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Ion Tiginyanu and Tudor Braniste

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Technological Features of Creating Hole Structures on the Base of MoS2 and the Electrochemical Behavior of MXene/Holey MoS2 Hybrids in Oxygen Reduction Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Havva Nur Gurbuz, Hasan H. Ipekci, Vladimir Goremichin, Nikita Siminel, Leonid Kulyuk, and Aytekin Uzunoglu The Water-Soluble Zinc Phthalocyanine Substituted with Sulfur-Containing Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Iacob Gutu, Victor Suman, Alic Barba, and Tamara Potlog Effect of Particle Size and Roughness on Contact Angle of ZnTe Thin Films . . . 268 Ion Lungu, Simon Busuioc, Elena I. Monaico, and Tamara Potlog Patterning Nanoelectronic Devices Using Field Emission Scanning Electron Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 Adrian Dinescu, Mircea Dragoman, Andrei Avram, and Daniela Dragoman Controlling Hydrophobic/Hydrophilic Properties of ZnO Microtetrapods Structures by Means of Thermal Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 Vladimir Ciobanu, Veaceslav V. Ursaki, Armin Reimers, Geanina Mihai, Victor V. Zalamai, Eduard V. Monaico, Rainer Adelung, Marius Enachescu, and Ion M. Tiginyanu Quantum Oscillations in Topological Insulator Bi2 Te2 Se Microwires Contacted with Superconducting In2 Bi Leads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Leonid Konopko, Albina Nikolaeva, and Tito Huber Biomaterials for Medical Applications Design and Simulation of a Biocompatible Prosthesis Ti-15Mo-XTa Alloy: An Analysis of Mechanical Integrity Using Finite Element Modeling . . . 305 A. Najah Saud, Hasan Sh. Majdi, Erkan Koç, and Mohammed Al Maamori Modification of Acrylic Paint by Acetamide to Be Antibacterial Used for Medical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 Mohammed Al Maamori, Hasan Sh. Majdi, Ali Kareem, and A. Najah Saud Interaction Between Thin Layers of Polysaccharides Studied by Quartz Crystal Microbalance with Dissipation (QCM-D) . . . . . . . . . . . . . . . . . . . . . . . . . . 324 Sergiu Coseri, Gabriela Biliuta, and Andreea Laura Chibac-Scutaru

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The Critical Size Bone Defects - In-Vivo Experimental Method of the Treatment with the Decellularized Vascularized Bone Allografts . . . . . . . . 332 Elena Pavlovschi, Alina Stoian, Grigore Verega, and Viorel Nacu Antigenic and Biodegradable Characteristics of the Extracellular Matrices from the Pig Dermis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348 Olga Macagonova, Adrian Cociug, Tatiana T, aralunga, Vladimir Ciobanu, and Viorel Nacu Effectiveness of Tissue Engineering in Obtaining the Extracellular Composite Vascularized Bone Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 Alina Stoian, Elena Pavlovschi, Nicolae Capro¸s, Grigore Verega, and Viorel Nacu Effect of Gold Nanoparticles Functionalized by Arthrospira Platensis on Rats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366 Liliana Cepoi, Ludmila Rudi, Tatiana Chiriac, Inga Zinicovscaia, Dmitrii Grozdov, and Valeriu Rudic The Significance of Computed Tomography in Diagnosing Pediatric Tuberculosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 Constantin Iavorschi, Stela Kulcitkaia, Igor Ivanes, and Nadejda Pisarenco Mechanical Characterization of Decellularized Blood Vessels: A Valuable Tool to Provide Comprehensive Information About the Scaffold . . . . . . . . . . . . . . 386 Tatiana Malcova, Gheorghe Rojnoveanu, Anatol Ciubotaru, and Viorel Nacu Comparative Assessment of In Vitro Effects on the Human Lymphocytes in Tuberculosis Patients of the Zinc Oxide Nanoparticles Biofunctionalized by Sulfated Polysaccharides from Spirulina . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 Tatiana Chiriac, Evelina Lesnic, Serghei Ghinda, Ludmila Rudi, and Liliana Cepoi Synthesis and Characterization of Self-assembled Hydrogels Based on Amphiphilic Derivates of Chitosan and Gelatin . . . . . . . . . . . . . . . . . . . . . . . . . 407 Andreea Simion, Andreea Luca, Florina-Daniela Cojocaru, Liliana Verestiuc, and Vera Balan Ionic Crosslinked Biopolymer-Ceramic Beads for Bone Tissue Engineering . . . . 417 Florina Daniela Cojocaru, Claudia Valentina Toader, Gianina Dodi, Ioannis Gardikiotis, Anca Elena Calistru, Aurelian Rotaru, Vera Balan, and Liliana Verestiuc

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Composites Based on Biopolymers and Ag Nanoparticles as Potential Wound Dressing Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 Maria-Gabriela Sibechi, Simina-Andreea Lasl˘au, Iustina-Petronela Dit, u, Isabella Nacu, Florina-Daniela Cojocaru, Maria Butnaru, and Liliana Verestiuc Synthesis and Study of Dextran:Zinc Aminomethylphthalocyanine Copolymers for Medicinal Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437 Stefan Robu, Petru Bulmaga, Ana Popusoi, Ion Bulimestru, Ion Lungu, and Tamara Potlog Effects of Nickel, Molybdenum, and Cobalt Nanoparticles on Photosynthetic Pigments Content in Cyanobacterium Arthrospira Platensis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447 Ludmila Rudi, Tatiana Chiriac, Liliana Cepoi, and Vera Miscu Preservation of Microorganisms of Biotechnological Interest Involving Fe2 O3 , Fe2 ZnO4 , and ZnO Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457 Tamara Sirbu, Cristina Moldovan, and Olga Turcan Clinical and Cost Effectiveness of Telerehabilitation System in Balance Disorder Patients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 Karla Mothejlova, Gleb Donin, and Romana Svobodova Innovation, Development, and Interdisciplinary Research Design of the Hardware Subsystem of a Proposed Autonomous Drone . . . . . . . . 479 Ionel S, erban, Corneliu-Nicolae Drug˘a, Barbu Cristian Braun, and Alexandru-Constantin Tulic˘a Thyroid Hormones Interpretation in Children with Juvenile Idiopathic Arthritis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 Rodica Eremciuc, Olga Gaidarji, Irina Nikitina, and Ninel Revenco Nanotechnology, Counterproliferation and Proliferation . . . . . . . . . . . . . . . . . . . . . 496 Artur Buzdugan Nutritional Quality of Bread and Bakery Products: Case Study: Republic of Moldova . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505 Rodica Siminiuc, Dinu T, urcanu, and Sergiu Siminiuc The Recovery of Alpha-Lactalbumin at the Electroactivation of Whey . . . . . . . . . 514 Elvira Vrabie, Irina Paladii, Mircea Bologa, Natalia T, islinscaia, Valeria Vrabie, Albert Policarpov, Tatiana Stepurina, and Catalina Sprincean

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Influence of CYP2C19*2 Polymorphism on Clinical Outcomes in Moldova’s Patients Treated with Clopidogrel After Percutaneous Coronary Intervention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 528 Marta Dogot, Daniela Galea-Abdusa, Anastasia Buza, Andrei Grib, Ghenadie Curocichin, Eleonora Vataman, and Natalia Capros Antibacterial Activity of “Green” Silver Nanoparticles (AgNPs) in Combination with Benzylpenicillin and Kanamycin . . . . . . . . . . . . . . . . . . . . . . 537 Seda Ohanyan, Lilit Rshtuni, and Ashkhen Hovhannisyan The Impact of Biogenic Silver Nanoparticles on the Enzymatic Antioxidant System of Wistar Rats’ Kidney . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 546 Juleta Tumoyan, Shushanik Kazaryan, and Ashkhen Hovhannisyan The Sentinel Surveillance System of Severe Acute Respiratory Infections Associated with Influenza in Children from Republic of Moldova . . . . . . . . . . . . 554 Ala Donos and Albina-Mihaela Iliev Neural Circuits-Adjusted Diagnostic Approach to Predict Recurrence of Atrial Fibrillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 564 Ludmila Sidorenko, Irina Sidorenko, Roman Chornopyshchuk, Igor Cemortan, Svetlana Capcelea, Fliur Macaev, Ludmila Rotaru, Liliana Badan, and Niels Wessel Combination Thermostated Vacuum Gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574 Igori Belotercovschii, Anatolie Sidorenko, Elena Condrea, and Vladimir Smyslov Predicting Pain Scores Using Personality Trait Facets and Personality Trait Domains Assessed by Personality Inventory for DSM-5 . . . . . . . . . . . . . . . . 582 Ina Timotin, Svetlana Lozovanu, Andrei Ganenco, Ion Moldovanu, Oleg Arnaut, Ion Grabovschi, Eugeniu Coretchi, Tudor Besleaga, and Victor Ojog Towards Improved Assistive Inertial Positioning Solutions by Using Finely Tuned Wavelet Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 Ioana-Raluca Adochiei, Teodor Lucian Grigorie, Felix-Constantin Adochiei, Petre Negrea, Vidan Cristian, and Nicolae Jula

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Correction to: Towards Improved Assistive Inertial Positioning Solutions by Using Finely Tuned Wavelet Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ioana-Raluca Adochiei, Teodor Lucian Grigorie, Felix-Constantin Adochiei, Petre Negrea, Vidan Cristian, and Nicolae Jula

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Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 607

Nanotechnologies and Nanomaterials

Properties of Vacuum-Evaporated CH3 NH3 PbCl3−x Ix Perovskite Layers Gagik Ayvazyan1(B) , Surik Khudaverdyan1 , Lenrik Matevosyan2 , Harutyun Dashtoyan1 , and Ashok Vaseashta3 1 National Polytechnic University of Armenia, Yerevan, Armenia

[email protected]

2 Institute of Radiophysics and Electronics, Ashtarak, Armenia 3 International Clean Water Institute, Manassas, USA

Abstract. Perovskite layers as a photo absorber material are widely used in singlejunction and tandem solar cells. The characteristics of these solar cells have been greatly improved in recent years. This study prepared the CH3 NH3 PbI3−x Clx metal halide perovskite layers on glass substrates covered with a thin indium tin oxide film by vacuum thermal evaporation method. Inorganic lead iodide (PbI2 ) and organic methylammonium chloride (CH3 NH3 Cl) were used as precursors. The layers were characterized by scanning electron microscope, X-ray diffraction, and C–V measurements. The transmission and absorption of the obtained layers were studied within the wavelength range of 400 to 1100 nm. It was found that the structural and optoelectronic properties of sequentially (layer-by-layer) evaporated (after annealing at the temperature of 100 °C for 30 min) and co-evaporated (jointly) perovskite layers are similar. The perovskite layers had a tetragonal crystal structure. They densely, without pinholes and cracks covered the surface of the substrates. The layers show a favorable band gap of 1.57 eV. The low-temperature optical studies were carried out to reveal the temperature dependence of the band gap energy. The possibility of increasing the layers’ thermal stability by adding 2.3% cesium iodide to the PbI2 precursor during the evaporation process was also shown. Keywords: Solar Cells · Vacuum Evaporation · Perovskite · Properties · Thermal Stability

1 Introduction Perovskite (PVK) layers as a photo absorber material are widely used in single-junction and tandem solar cells [1, 2]. The characteristics of these solar cells have been greatly improved in recent years. In 2009, when PVK-based single-junction solar cells were first reported, their power conversion efficiency was 3.9%. Currently, this value has increased to 25.2% [3]. PVK/silicon tandem solar cells showed even higher power conversion efficiency values (31.3%) [3]. For solar cells, the most representative is CH3 NH3 PbI3−x Clx metal halide PVK, which is characterized by low production cost, good stability, and relatively low toxicity [4, 5]. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 3–11, 2024. https://doi.org/10.1007/978-3-031-42775-6_1

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Of the numerous methods for direct deposition of PVK layers on dielectric or silicon substrates, thermal vacuum evaporation is the most promising. Using this method, it is possible to obtain evenly and uniformly PVK layers that compactly cover even nano or microtextured surfaces of the substrates [6, 7]. In addition, this process avoids the use of toxic solvents. Vacuum evaporation of PVK layers is carried out in two ways, namely, sequentially (layer-by-layer) and jointly (co-evaporated) [8, 9]. In the case of coevaporation, the sublimation of the initial organic and inorganic materials (precursors) of PVK is carried out in a vacuum chamber simultaneously. In sequential evaporation, the inorganic precursor is deposited first, followed by the organic precursor. It should be noted that the first method does not require a post-annealing step, but in the second case, post-annealing is required to achieve complete conversion of precursors into PVK. To date, the main attention of researchers is focused on PVK synthesized by the simplest method – solution processing, for example, the spin-coating method [8, 10, 11]. The number of reports on vacuum-evaporated PVK is very limited and further research is required in this area. This work is aimed at studying the optoelectronic and structural properties of the CH3 NH3 PbI3−x Clx metal halide PVK layers prepared by vacuum thermal evaporation.

2 Experimental Details The PVK layers were deposited by vacuum sequential evaporation and co-evaporation methods in a high vacuum chamber using inorganic lead iodide (PbI2 ) and organic methylammonium chloride (CH3 NH3 Cl or MACl) raw precursors on glass substrates covered with a thin indium tin oxide (ITO) film. PVK stoichiometry was varied by the composition and evaporation rates of precursors, the temperature of quartz crucibles, and the process pressure in the chamber. The thickness and evaporation rate of the PVK layers were controlled with a quartz crystal oscillator. Since MACl and Pbl2 sublimate well, the temperatures of the evaporators were chosen below the melting points of both components to carry out sequential evaporation and co-evaporation without any changes in the process modes. The following optimal mode of the deposition process was chosen during the experiments. The structures were placed on a rotating holder (speed was 30 rpm) at a distance of about 12–15 cm from the evaporators. At a precursor composition of 4: 1 (MACl: PbI2 ) and a process pressure of 1.3 × 10–5 mbar, the evaporation rate of the MACl was maintained at a level of 1.2 Å/s, while that of the PbI2 was kept at 0.4 Å/s. The temperatures of the crucibles of these precursors were maintained at 190 °C and 345 °C, respectively. During sequential evaporation PbI2 was first deposited (step I), followed by MACl (step II). After sequential evaporation, the obtained layers were annealed outside the vacuum chamber in the air at the temperature of 100 °C for 10, 20, and 30 min (step III). Details of the evaporation process parameters are given in [12]. Figure 1 shows schematic diagrams of the coevaporation and sequential evaporation processes. The structural properties of the PVK layers were studied using a Hitachi S4800 Scanning Electron Microscope (SEM). The optical properties of the samples were studied on a FILMETRICS F20 spectrometer in the wavelength range λ = 400–1100 nm. Crystal structure PVK layers were measured by X-ray diffraction

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(XRD) using a Panalytical Empyrean diffractometer equipped with Cu–Kα radiation of 1.54178 Å. The capacitance-voltage (C–V ) characteristics were measured at frequencies of 10 kHz–1 MHz using an E7-25 immittance meter.

Fig. 1. Schematic diagrams of the co-evaporation (a) and sequential evaporation (b) processes.

3 Results and Discussion Figure 2 indicates the transmission curves of the sequentially evaporated and coevaporated PVK layers. The research showed that, after annealing, the optical transmittance spectrum of successively as-evaporated PVK layers (Fig. 2, curve 1) changes significantly, especially in the short-wavelength region of the spectrum (Fig. 2, curves 2–4). After long-term annealing, the transmittance spectra of sequentially evaporated and co-evaporated layers almost coincide (Fig. 2, curves 4 and 5). The transmittance of the PVK layers is less than 0.03 (in a.u.) when λ < 580 nm. In the region of 580–740 nm, the transmittance increases from 0.03 to 0.18. In the region of 740–950 nm, the transmittance rapidly increases, approaching 0.8. We also note that, as further studies showed, sequentially evaporated and co-evaporated layers had practically the same structural, electronic, and other optical properties. Therefore, the results of the study are presented below only for co-evaporated PVK layers. It is known that the problem with the large-scale use of PVK-base solar cells is instability as their parameters gradually change when exposed to the environment (humidity, temperature, UV radiation). The appropriate solar cell encapsulation may prevent the damaging effects of humidity and UV radiation, but the heat emitted during the operation of solar cells gradually reduces their performance. One of the ways to increase the thermal stability of metal halide PVK is the implantation of cesium (Cs) atoms into the PVK composition [13, 14]. We tested this method by adding up to 2.3% cesium iodide to the PbI2 precursor during the evaporation process. Figure 3 shows the transmission spectra of the CH3 NH3 PbI3−x Clx PVK layers with and without Cs atoms after storage for 60 min at 120 °C in a dry and dark place. Optical images of the same layers are also presented there. It can be seen that the PVK layer without Cs atoms decomposes after thermal storage (the organic component evaporates) with the formation of PbI2 of a specific yellow color. The layer containing Cs practically does not change – it retains both the original black color and the transmission spectrum. Under normal storage conditions, the properties of the PVK layers do not change for a long time.

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Fig. 2. Transmission spectra of the sequentially evaporated (curves 1–4) and co-evaporated (curve 5) PVK layers.

Fig. 3. Transmission spectra of the PVK layers with (curve 2) and without (curves 1 and 3) Cs atoms before (curve 3) and after (curves 1 and 2) storage for 60 min at 120 °C.

The band gap energy Eg of obtained PVK layers was estimated from the Tauc   plot of the absorption spectrum. This parameter can be calculated as (Dhυ)2 = C hυ − Eg , where D is the absorption coefficient, C is the constant, h is Planck’s constant, and υ is the frequency of the incident photon. The energy gap was determined via linear absorption edge part of the (Dhυ)2 vs photon energy hυ curve (Fig. 4). The obtained PVK layers show a favorable for solar cells Eg 1.57eV . To determine the temperature dependence of the band gap energy, we studied the optical absorption of PVK layers in the temperature range of 77–300 K (Fig. 5). The calculated Eg temperature dependence is shown in the inset of Fig. 5. It can be seen that, as the temperature decreases, the absorption edge of the layer shifts toward higher energies, and at temperatures < 140 K the absorption peak of the first exciton level gradually

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Fig. 4. (Dhυ)2 vs photon energy for the PVK layers.

begins to appear. The Eg temperature dependence can be divided into three characteristic regions: high-temperature, low-temperature, and intermediate. These changes are due to certain changes in the PVK crystal lattice structure from tetragonal symmetry at room temperature to orthorhombic symmetry at low temperatures. The transition from one symmetry to another occurs within the temperature range of 170–140 K. These changes in the crystal structure of organometal halogen PVK within the above-mentioned temperature range were also reported in [15]. The X-ray diffraction studies showed that the PVK layers had distinct peaks in the XRD at the reflection angles of 14.1°, 28.4° and 31.9° (Fig. 6) These peaks are assigned respectively to the planes (110), (220), and (310) for CH3 NH3 PbI3−x Clx PVK with a tetragonal crystal structure. The observed weak diffraction peak at 12.5° (001) indicates that the inorganic precursor was not completely converted into PVK during the evaporation process, as often observed for similar MACLPbI3 PVK. Compared with the similar peaks of the single-halogen CH3 NH3 PbI3 PVK [5, 15], a slight shift of the peaks was observed, which was due to the partial replacement of the iodine atoms by the chlorine atoms. Figure 7 shows the typical cross-sectional and top SEM images of the PVK layers. It can be seen that the evaporated PVK layers have a granular structure. The layers are dense, without pinholes, cracks, or other serious aggregations. The grain size in the plane is 80–160 nm and in the perpendicular direction – 200–400 nm. The total thickness of the PVK layer is about 600 nm, i.e. 2–3 grains are located along the thickness of the layer from bottom to top. This close-packed structure with large grains is favorable for charge transport since more photogenerated charges can successfully reach the electrodes instead of recombining at grain boundaries. The observed structural features are very similar to previously reported vacuum-deposited PVK layers on silicon substrates [12, 14]. The exciton binding energy is the most important parameter of PVK-based solar cells. For the efficient operation of the solar cell, most of the excitons, at the operating

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Fig. 5. Absorption spectra of the PVK layers at different temperatures. The inset shows the temperature dependence of the band gap energy.

Fig. 6. XRD pattern of the PVK layers.

Fig. 7. Top (a) and cross-sectional (b) SEM images of the PVK layers.

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temperature, need to dissociate into free electrons and holes [16] which could participate in the processes of free charge transport. It is assumed that excitons in PVK are the excitons with weak binding energy known as Wannier-Mott excitons. The binding energy for the n exciton level is described by En = Eg − R∗ /n2 ,

(1)

where Eg is the energy gap and R∗ is the Rydberg effective constant: R∗ = R0 m∗ /m0 ε2 ,

(2)

where R0 = 13.6eV is the Rydberg constant, ε is the dielectric constant of the layer, m∗ is the reduced mass of the exciton, m∗ = me mh /(me + mh ), me and mh are the effective masses of electrons and holes, respectively. For the organometal halide PVK m∗ = 0.1m0 [17]), where m0 is the free electron mass. The values of the exciton binding energy in PVK calculated by Eqs. (1) and (2) have wide spacing (from several meV up to almost 50 meV), which is due to the big difference between the static and high-frequency dielectric constants. The experimental values of the exciton binding energy determined from the absorption spectra at low temperatures also have wide spacing (5–100 meV). Based on the C–V characteristics measurements, we determined the frequency dependence of the dielectric constant of the obtained PVK layers, which is shown in Fig. 8. The inset also shows a diagram of measurements using indium point contacts. It can be seen that as the frequency increases, the dielectric constant first decreases rapidly, and then begins to decrease slowly and reaches the value ε = 6.55 at f = 1000kHz.

Fig. 8. Frequency dependence of the dielectric constant of the PVK layers. The inset figure shows the diagram of measurements using indium point contacts.

4 Conclusions In this study, we have systematically investigated the structural and optoelectronic properties of the CH3 NH3 PbI3−x Clx metal halide perovskite layers prepared by vacuum thermal evaporation. Two evaporation methods were used: sequential and co-evaporation.

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The properties of the layers were characterized by SEM, XRD, C–V measurements, absorption and transmittance spectra. It was found that, after long-term annealing, successively evaporated layers had essentially the same properties as co-evaporated layers. The addition of a small amount of cesium to the composition of the perovskite layer significantly increases its thermal stability. The absorption, band gap energy, and dielectric constant of the obtained perovskites have been illustrated in the wavelength range of 400–1100 nm. Acknowledgments. This work was supported by the Science Committee of the Republic of Armenia under the frameworks of the research projects № 21AG-2B011 and 21T-2B028.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Wang, R., Huang, T., Xue, J., Tong, J., Zhu, K., Yang, Y.: Prospects for metal halide perovskitebased tandem solar cells. Nat. Photon. 15, 411–425 (2021). https://doi.org/10.1038/s41566021-00809-8 2. Marangi, F., Lombardo, M., Villa, A., Scotognella, F.: New strategies for solar cells beyond the visible spectral range. Opt. Mater. X 11, 100083 (2021). https://doi.org/10.1016/j.omx. 2021.100083 3. Green, M., et al.: Solar cell efficiency tables (version 61). Prog. Photovolt. Res. Appl. 31, 3–16 (2023). https://doi.org/10.1002/pip.3646 4. Gevorkian, Z., Gasparian, V., Lozovik, Y.: Large diffusion lengths of excitons in perovskite and TiO2 heterojunction. Appl. Phys. Let. 108, 051109 (2016). https://doi.org/10.1063/1.494 1242 5. Al-Asbahi, B.A., et al.: Effect of deposition method on the structural and optical properties of CH3 NH3 PbI3 perovskite thin films. Opt. Mat. 103, 109836 (2020). https://doi.org/10.1016/ j.optmat.2020.109836 6. Ayvazyan, G., Vaseashta, A., Gasparyan, F., Khudaverdyan, S.: Effect of thermal annealing on the structural and optical properties of black silicon. J. Mater Sci. Mater. Electron. 33, 17001–17010 (2022). https://doi.org/10.1007/s10854-022-08578-y 7. Cojocaru, L., et al.: Detailed investigation of evaporated perovskite absorbers with high crystal quality on different substrates. ACS Appl. Mater. Interfaces 10, 26293–26302 (2018). https:// doi.org/10.1021/acsami.8b07999 8. Guesnay, Q., Sahli, F., Ballif, C., Jeangros, Q.: Vapor deposition of metal halide perovskite thin films: process control strategies to shape layer properties. APL Mater. 9, 100703 (2021). https://doi.org/10.1063/5.0060642 9. Gevorkian, Z., Matevosyan, L., Avjyan, K., Harutyunyan, V., Aleksanyan, E., Manukyan, K.: Determination of the complete set of optical parameters of micron-sized polycrystalline CH3 NH3 PbI3−x Clx films from the oscillating transmittance and reflectance spectra. Mater. Res. Express 7, 016408 (2019). https://doi.org/10.1088/2053-1591/ab5c46 10. Salhi, B., Wudil, Y.S., Hossain, M.K., Al-Ahmed, A., Al-Sulaiman, F.A.: Review of recent developments and persistent challenges in stability of perovskite solar cells. Renew. Sustain. Energy Rev. 90, 210 (2018). https://doi.org/10.1016/j.rser.2018.03.058 11. Zhou, D., Zhou, T., Tian, Y., Zhu, X., Tu, Y.: Perovskite-based solar cells: Materials, methods, and future perspectives. J. Nanomater. 2018, 1 (2018). https://doi.org/10.1155/2018/8148072

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12. Vaseashta, A., Ayvazyan, G., Khudaverdyan, S., Matevosyan, L.: Structural and optical properties of vacuum-evaporated mixed-halide perovskite layers on nanotextured black silicon. Phys. Status Solidi RRL 17, 2200482 (2023). https://doi.org/10.1002/pssr.202200482 13. Choi, H., et al.: Cesium-doped methylammonium lead halide perovskite light absorber for hybrid solar cells. Nano Energy 7, 80–85 (2014). https://doi.org/10.1016/j.nanoen.2014. 04.017 14. Ayvazyan, G.Y., Kovalenko, D.L., Lebedev, M.S., Matevosyan, L.A., Semchenko, A.V.: Investigation of the structural and optical properties of silicon-perovskite structures with a black silicon layer, J. Contemp. Phys. 57, 274 (2022). https://doi.org/10.1134/S1068337222030069 15. Milot, R.L., Eperon, G.E., Snaith, H.J., Johnson, M.B., Herz, L.M.: Temperature dependent charge-carrier dinamics in CH3 NH3 Pbl3 perovskite thin films. Adv. Func. Mater. 25(39), 6218–6227 (2017). https://doi.org/10.1002/adfm.201502340 16. Baranowski, M., Plochcka, P.: Excitons in metal-halide perovskites. Adv. Energy Mater. 10, 1903659 (2020). https://doi.org/10.1002/aenm.201903659 17. Miyata, A., Mitioglu, A., Plochocka, P., Portugall, O., Snaiteet, H.: Direct measurement of the exciton binding energy and effective masses for charge carriers in organic-inorganic tri-halide perovskites. Nat. Phys. 11(7), 582–587 (2015). https://doi.org/10.1038/nphys3357

Functional Capabilities of Two-Barrier Semiconductor Structures Surik Khudaverdyan1 , Ashok Vaseashta2(B) , Gagik Ayvazyan1 Mane Khachatryan1 , and Ashot Khudaverdyan1

,

1 Armenian National Polytechnic University, 105 Teryan Str., Yerevan, Armenia 2 Office of Applied Research, International Clean Water Institute, Manassas, VA, USA

[email protected]

Abstract. Two-barrier semiconductor structures with a high-resistance sublayer and longitudinal illumination, using certain design and technological parameters, have several unique functionalities, such as injection amplification of the photocurrent, and spectral selective sensitivity. This investigation considers the possibility of creating highly sensitive devices in the optical (CdTe, Si) and X-ray (CdTe) ranges of electromagnetic waves. The process of mutual compensation of photocurrents arising in opposite potential barriers overlapping the sublayer, with longitudinal absorption of radiation, leads to pronounced short-wavelength and long-wavelength maxima in the spectral distribution of intensity or photocurrent. Using structures based on cadmium and silicon telluride, as examples, the phenomenon of the sign reversal of the spectral photocurrent and the possibilities of measuring wavelengths are demonstrated. To study the photoelectronic processes occurring in these structures, the obtained mathematical expressions are used, which relate the parameters of the structure and optical radiation. The algorithm developed using these expressions is based on a new spectral analysis mechanism, which makes it possible to implement it as affordable, small-sized, low-material, and low-power devices. All this is considered in the context of solving urgent problems of quantitative remote identification of the components of an optically transparent medium suitable for solving environmental issues. Keywords: Photodetector · Spectral Analysis · Injection Amplification · Si · CdTe

1 Introduction Advances in the field of creating solid-state photodetectors have reached a high level of development. Further progress in this area, the researchers associated with the search for new materials and physical and technical principles that can provide high photosensitivity in the optical and X-ray regions of the spectrum, radiation resistance, as well as new spectrophotometric capabilities of photodetectors. Previously, we proposed a new physical principle and, based on it, a photodetector, according to which, using a semiconductor structure with a two potential barrier, it is possible to record the spectral composition of information radiation coming from an optically transparent material in © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 12–21, 2024. https://doi.org/10.1007/978-3-031-42775-6_2

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the wavelength range of 350–900 nm [1, 2]. This structure allows observing the spatially separate absorption depths of individual informative wavelengths in the conditions of longitudinal absorption of electromagnetic radiation, to separate their relative proportions in the total photocurrent induced by the integrated radiation flux. Thus, the content of the integral photo signal from the examined substance is selectively registered. The most suitable material for photodetectors is silicon (Si) [3, 4]. The process of their preparation is incomparably simple, and the material used – Si, is the most technologically and financially affordable semiconductor. In addition, nanostructured modifications of Si, in particular, black Si, have excellent spectrometric properties [5, 6]. To ensure high photosensitivity and radiation resistance, structures based on cadmium telluride (CdTe) [7, 8]. The large band gap ensures the low thermal generation of charge carriers and low noise levels at room temperature. CdTe has a large average atomic number and a high concentration of intrinsic defects. These properties provide effective absorption of penetrating radiation and high radiation resistance. What matters is that it is easy to obtain CdTe with both n- and p-type conductivity and create an n-p junction in them [9]. In the present work, a comparative analysis of experimental studies of photodetector structures with a high-resistance layer based on Si and CdTe is carried out, in which injection amplification of the photocurrent occurs, while in others, photocurrent changes sign, and exhibits spectrophotometric properties.

2 Two-Barrier Structures 2.1 Cadmium Telluride-Based Structure The CdTe-based diodes with a high-resistance layer were made from relatively lowresistance virgin n-type crystals with a resistivity of ~102 *cm, by 5 min thermal diffusion of platinum (Pt) at the temperature of 823 K under an argon atmosphere. A semi-transparent silver (Ag) layer served as a contact from the side of the high-resistance layer and an aluminum (Al) layer served as a contact from the opposite side. The diodes thus obtained had the structure Ag–CdTe:Pt–n-CdTe–Al (Fig. 1) and, at the total length of 400 μm, the high-resistance layer was 2 μm (the penetration depth of Pt, in the specified technology mode). When measuring the short-circuit current of the diode, the illumination was carried out through a semi-transparent Ag contact.

Fig. 1. Cross-section of the CdTe-based photodetector structure.

Figure 2 presents the spectral dependencies of the photocurrent at a constant light intensity but at different bias voltages (positive potential on the Ag contact). Two peculiarities are observed. Firstly, with the increase of bias voltage, the wavelength range

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of positive pho-to-sensitivity significantly increases. Secondly, the point of change in the sign of the photocurrent shifts to long waves (in the case of reverse bias voltage, to short waves). The study showed that the reversal of the sign of the spectral photocurrent occurs when the difference between the heights of the potential barriers is available. In this case, according to calculations considering technological parameters, it was 0.1 eV.

Fig. 2. The spectral dependence of the photocurrent at a constant power of incident radiation, but at different bias voltages, 1 – 0.04 V; 2 – 0 V; 3 – 0.02 V; 4 – 0.04 V; 5 – 0.06 V.

2.2 Silicon-Based Structure Figure 3 shows a cross-section n+ -p-n+ of the Si structure. Similar to the samples based on CdTe, there is a difference in the height of the potential barrier, which in this case, is 0.04 eV.

Fig. 3. The distribution of the potential energy of holes in the valence band and the direction of the photocurrents.

The spectral dependence of the photocurrent of the samples is shown in Fig. 4. The following features are observed on the spectral characteristic: • With increasing wavelength, in the structure absorption depth increases the generated photo-current of the rear transition begins to increase and counteract the photocurrent of the surface barrier. After the short-wavelength maximum (490 nm), the photocurrent decreases.

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• At a wavelength of 600 nm, there is a maximum compensation of oppositely directed photocurrents. With a further increase in the wavelength, the photocurrent is determined mainly by the rear barrier, and as it increases, it passes through a maximum (830 nm – the Si intrinsic absorption region). • The presence of the change of the spectral photo-current sign within the bias voltage range of −0.010 – + 0.040 V.

Fig. 4. Spectral dependence of sample photocurrent on bias voltages.

A selective sensitivity algorithm has been developed, which consists of the following: depending on the bias voltage, the docking point xm is changing. There is a change in the photocurrent due to a change in the fraction of absorption of radiation in both barriers. The difference in photocurrents corresponding to the nearest voltage values is mainly due to the wave that has the predominant intensity at a given depth. This will allow you to calculate the wavelength and its intensity by changing the bias voltage step by step using the previously obtained mathematical model [1]. This algorithm was used to obtain the distribution spectra of green and blue LEDs (Fig. 5).

Fig. 5. The spectral dependence of the intensity of the blue (a) and green (b) LEDs.

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3 Functional Capabilities of Two-Barrier Cadmium Telluride-Based Structures 3.1 The Injection Amplification of the Photocurrent Below, an experimental study of photodetector structures with a high-resistance layer based on CdTe, in which the injection amplification of the photocurrent occurs, is carried out. In the case of a sharp asymmetric p-n junction, when generation and recombination in the space charge region can be neglected, the solution of the system of diffusion-drift equations leads to the following expression for the I-V characteristic of the diode in the high injection level mode [1]:      d qV /(b + 1) (1) c = 2 ∗ b + ch I = Ic exp ckT Lb where d is the thickness of the base region (semiconductor thickness); L p is the length of the diffusion bias of minority carriers; V is the total voltage drop on the diode:  1/2  2b Ip = Dp τp b+1

(2)

When deriving Eq. (1), the rear contact is considered ohmic. Equation (1) shows that if d/L p > 1, there is a very strong dependence of the current on the change of the lifetime of the mobility of nonequilibrium carriers. These parameters can change with the change of the current flowing through the photodiode or under external influences. 3.2 Mechanism of the Photosensitivity of Diodes at Forward Bias The principle of injection amplification photocurrent is the following. When absorbing radiation, the photocarriers generated into the base of the photodetector modulate its conductivity. With the constant voltage applied to the diode, the increase in the base conductivity leads to the redistribution of voltage between the base and the p-n junction. The voltage drops on the p-n junction increase, which leads to the increase in the injection of minority carriers through the junction. The increase in the injection, in its turn, further modulates the base conductivity. This can lead to an increase in the current through the diode by several orders of magnitude. Figure 6 shows a direct branch of I–V characteristics. The presence of nonlinearity in the current-voltage characteristics under illumination can be explained by the theory of a long diode. The semi-logarithmic I–V characteristics of the diode in the case of direct bias in the voltage range 0.3–3.8 V are approximated by an exponential dependence (see Eq. (1)). The exponent c and the current Ic (see Eq. (1)) are determined by the slope of the exponential site of the direct branch of the I–V characteristics. The obtained photosensitivity was about 50 A/V, which is a good indicator. Especially when you consider that without injection amplification, the photosensitivity does not exceed 1 A/W.

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Fig. 6. Direct branches of the I–V characteristics of diodes with a beam power of 0.1 μW at a wavelength of λ ~ 0.8 μm.

3.3 Possibility of the Creation of Injection Detectors of Ionizing Radiation 1. The principle of the creation of photodiode structures with high sensitivity, based on the injection amplification, may as well be realized in the X-ray region of the spectrum. It is known that, in cadmium telluride, the depth of the effective radiation absorption does not exceed 100 μm up to the quantum energy of 40–50 keV [7], and the intrinsic light is absorbed in the depth of ~2 μm [10]. Consequently, the optimum size of the detector (the length of the diode structure and the relation of the thicknesses of its different regions) may be somewhat different, depending on the energy of the registered quanta. However, the registration principle will remain the same, with some distinctive features. 2. Due to the higher penetrability of the X-rays, the electron-hole pairs may be generated not only in the front of the high-resistance layer and the p-n junction region but also in the base region. Therefore, to effectively register the deeply penetrated radiation, it is necessary to create a sufficiently “thick” high-resistance layer. The conductivity modulation of the base region occurs behind the p-n junction, which remained in the dark when illuminated with its light. 3. The generated charge carrier is “hot”. Thus, the probability of Auger recombination increases. In the longitudinal irradiation mode, the change of the recombination mechanism is significant for the distribution of the nonequilibrium carriers along the sample length. 3.4 Mechanism of Free Carrier Generation When Irradiated by High Energy Photons When the diode structure is irradiated by high energy quantum, photo- and Compton electrons appear in the bulk of the semiconductor. The generated electrons spend their energy on ionization, excitation of the atoms of the semiconductor, and bremsstrahlung. As a result, charge carriers – electrons and holes, are generated in the bulk of the diode. For homogeneous semiconductors, knowing the average energy required for the generation of one pair of carriers in CdTe (4.65 eV) [7, 8], it is possible to find the energy absorbed in the bulk of the semiconductor.

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3.5 Characteristics and Parameters Under X-ray Irradiation The diodes thus obtained had the structure Ag–CdTe:Ag–n-CdTe–In. The thickness of this sample is ~800 μm and ρ ~ 107 –108  cm. The high-resistance p-type region with a thickness of ~3.5 μm is created by the thermal diffusion of Ag at 850 K in the argon atmosphere for 1.5 min. The n+ -type rear contact to diode structures is created in the process of annealing the film of indium (In). To measure the parameters of the obtained structures, the facility used consists of X-ray generators – molybdenum, aluminum, copper, steel tubes, electrometer TR-1501, and a DC voltage source. Figure 7 shows the results of the studies of the properties of diode structures exposed to X-radiation (molybdenum tube). The measurements of the X-radiation dose were made by the dosimeter, which was placed at the same distance from the source as the diodes when their properties were studied. It allowed ignoring the loss of X-ray quanta in the air gap between the source and the receiving surface of diodes. When measuring, the 0,1 μm Ag contact was believed to completely transmit the X-ray quanta.

Fig. 7. The direct branch of the diode at power dose (R/s). 1 – 1.1 * 10–4 ; 2 – 5.1 * 10–2 ; 3 – 0.

The I–V characteristics of straight-biased diodes, when irradiating the sample with X-rays, starting from a voltage of V = 0.5 V, are superlinear. As the voltage increases, it gradually becomes linear. The analysis of the I–V characteristics in the semi-logarithmic scale revealed the initial exponential section corresponding to the range of the current changes by an order of magnitude and more, depending on the irradiation rate (Fig. 7, curves 1 and 2). The calculated values of the constant from Eqs. (1) and (2), describing the exponential law, are 16 and 12 for the powers 1.1 * 10–4 R/s and 5.1 * 10–2 R/s, and the corresponding lifetimes of minority carriers turn to be equal to 8 * 10–5 s and 10–4 s. That is, similarly to the mechanism of the operation of diodes, when illuminated by its light, the conductivity modulation of the n-base is conducted by an additional mechanism. It should be noted that taking into account the relationship of the quantum energy of X radiation (17 keV) and intrinsic radiation (1.5 eV), and the energy of electron-hole pair generation (4.65 eV), the quantum efficiency under X radiation at 1.8 * 10–4 R/s turns to be equal to ~3 * 106 R/s, which is considerably more than under the illumination by its light. These results can be explained if it is assumed that, as opposed to the radiation from the self-absorption region when the light modulates only the conductivity of the

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interlayer, most X-ray quanta is absorbed in the depth of the semiconductor (in the nregion). This brings to the reduction of the resistance of the part of the n-base and the additional redistribution of voltage between the p-n junction and the high resistivity pand n-regions. As a result, the amplification possibilities in the device increase due to the more intensive realization of the positive feedback, especially at low dose rates, since at high dose rates there is the probability of the increase of the absorption of X-ray quanta near the contact opposite the surface. Consequently, there is a probability of an increase in the losses of secondary electrons and a decrease in the registration efficiency.

4 Functional Capabilities of Two-Barrier Silicon-Based Structures 4.1 High Photosensitivity of Photodetectors The opportunity to obtain highly sensitive photodetectors with low noise and dark currents has led to the creation of a structure with two oppositely directed barriers with heights ϕ1 and ϕ2, respectively [1]. The studied samples have high spectral photosensitivity with two spectral maxima (Fig. 8).

Fig. 8. The spectral distribution of the photosensitivity of the samples upon annealing at 950 °C at different bias voltages.

The monochromatic radiation and different light sources with the wavelengths of λ = 300–1100 nm were used for this research. The radiation power incident on the photodetector was measured with the help of Si sensors, Model 3A-IS having a spectral range of 350–1100 nm, and power range 1 μW – 3 W (error wavelength 5%). The opencircuit voltage of the sample is about 5 mV and indicates that the calculated difference in the heights of the barriers (ϕ1–ϕ2 = 0.4 eV, Fig. 4), after annealing the guard ring at 950 °C, decreases. 4.2 Mechanism of High Photosensitivity Longitudinal absorption of the radiation takes place through the photosensitive surface. The spectral distribution of the photocurrent has marked short-wave (560 nm) and

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long-wave (830 nm) maximums with unusually high photosensitivity (Fig. 8), and the long-wave maximum is in the intrinsic absorption region of Si. The generated photocarriers partially reduce the heights of both barriers and create favorable conditions for the electrons to penetrate (to get injected) from the near-surface layer into the rear layer through the base (where they significantly increase the concentration of minority carriers). The decrease in ϕ1 is especially noticeable when the waves are short since most of them are absorbed in the region of the surface barrier. The photosensitivity is higher with the forward-biased near-surface barrier than with the forward-biased rear barrier. This may be the result of the greater injection through the near-surface layer where the impurity density and the absorption share radiation are higher. The photocurrent sign is determined by the photocurrent of the reverse-biased barrier.

5 Conclusions For efficient injection amplification, the base region in X-ray sensors must be sufficiently highly resistive. This makes the essential difference between the X-ray sensors and photodiodes. In photodiode structures the base region remains “non-exposed”, and its resistivity must be small, so that the decrease of the resistance of the interlayer under illumination as a result of the redistribution of the voltage in the structure, leads to the increase of the voltage drop on the p-n junction and, eventually, to the additional injection modulation of the resistance of the interlayer. The semiconductor structure based on Si with two potential barriers and with a junction point of the depleted regions has the spectral distribution of photosensitivity with the short-wave (560 nm) and longwave (830 nm) maxima under the longitudinal illumination. The numerical values of the short-wave photosensitivity are unusually high (up to 4 A/W). The difference in the heights of the potential barriers leads to the change in the spectral photocurrent for the voltages that increase the height of the rear (reverse-biased) barrier up to the level of its comparison with the height of the high barrier. There is no change in the sign beyond this voltage range. Within the bias voltage range where the sign of the spectral photocurrent is inverted by the step-by-step change in the voltage, it is possible to separate individual waves and their intensities and, thus, obtain the spectral distribution of the intensities. Acknowledgments. The work was supported by the Science Committee of the Republic of Armenia in the frames of research projects No. 21AG-2B011 and No. 21T-2B028.

Conflict of Interest. The authors declare that there is no conflict of interest.

References 1. Khudaverdyan, S., Vaseashta, A., Ayvazyan, G., Khachatryan, M., Atvars, A., Lapkis, M., Rudenko, S.: On the semiconductor spectroscopy for identification of emergent contaminants in transparent mediums. In: Vaseashta, A., Maftei, C. (eds.) Water Safety, Security and Sustainability. ASTSA, pp. 663–689. Springer, Cham (2021). https://doi.org/10.1007/978-3-03076008-3_29

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2. Khudaverdyan, S., et al.: On the selective spectral sensitivity of oppositely placed doublebarrier structures. Photonics 9, 558–568 (2022). https://doi.org/10.3390/photonics9080558 3. Fang, Y., Armin, A., Meredith, P.: Accurate characterization of next-generation thin-film photodetectors. Nat. Photon. 13, 1–4 (2019). https://doi.org/10.1038/s41566-018-0288-z 4. Wu, Q., Cen, G., Liu, Y., Ji, Z., Mai, W.: A simple-structured silicon photodetector possessing asymmetric Schottky junction for NIR imaging. Phys. Lett. 412, 127586 (2021). https://doi. org/10.1016/j.physleta.2021.127586 5. Ayvazyan, G., Vaseashta, A., Gasparyan, F., Khudaverdyan, S.: Effect of thermal annealing on the structural and optical properties of black silicon. J. Mater Sci: Mater. Electron. 33, 17001–17010 (2022). https://doi.org/10.1007/s10854-022-08578-y 6. Vaseashta, A., Ayvazyan, G., Khudaverdyan, S., Matevosyan, L.: Structural and optical properties of vacuum-evaporated mixed-halide perovskite layers on nanotextured black silicon. Phys. Stat. Sol. RRL 17, 2200482 (2023). https://doi.org/10.1002/pssr.202200482 7. Sordo, S.D., Abbene, L., Caroli, E., Mancini, A.M., Zappettini, A., Ubertini, P.: Progress in the development of CdTe and CdZnTe semiconductor radiation detectors for astrophysical and medical applications. Sensors (Basel) 9, 3491–3526 (2009). https://doi.org/10.3390/s90 503491 8. Yang, G., Kim, D., Kim, J.: Photosensitive cadmium telluride thin-film field-effect transistors. Opt. Express 24, 3607–3612 (2016). https://doi.org/10.1364/OE.24.003607 9. Valmik, B.G., et al.: Investigation and fabrication of Cadmium Telluride (CdTe) single crystal as a photodetector. J. Phys. B: Cond. Matter. 614, 4130271 (2021). https://doi.org/10.1016/j. physb.2021.413027 10. Sze, S.M.: Physics of Semiconductor Devices, 2nd edn. John Wiley & Sons, NY (1981)

Gamma Radiation Sensitization of ZnO/Al2 O3 Sensors Based on Nanoheterostructures Cristian Lupan1

, Adrian Bîrnaz1 , Artur Buzdugan1(B) and Oleg Lupan1,2

, Nicolae Magariu1

,

1 Technical University of Moldova, Chisinau, Moldova

[email protected] 2 Kiel University, Kiel, Germany

Abstract. Reliable detection of dangerous gases by using devices based on semiconductor materials in environments, with different influencing factors, such as gamma radiation, is a challenge for a medical facility or space program. A study of the influence of gamma radiation on the electrical and sensing properties of ZnO/Al2 O3 core@shell heterostructure has been carried out in this work. Using as radiation source Cs-137, a low level of ionizing radiation was applied. It was observed that gamma irradiation did not affect the electrical resistance in real time measurements, but changes have been observed once comparing I-V characteristics before and after measurements. Initial gas tests showed that ZnO/Al2 O3 heterostructure does not detect volatile organic compounds (VOC) gases in the operating temperature range between 150–200 °C and gas concentration up to 100 ppm. Further gas sensing tests, after irradiation, showed that the experimental results are of interest for the gas sensors development based on the ZnO/Al2 O3 heterostructure, showing an increase in response value by more than 100% and 200% for 100 ppm 2-propanol and n-butanol VOC gases at operating temperature of 200 and 250 °C, respectively. These findings can be used for further development of gas sensors in environments with gamma radiation field and for biomedical applications too. Keywords: Zinc Oxide · Aluminum Oxide · heterostructure · Gas Sensors · Gamma Irradiation

1 Introduction Multilayer systems of various semiconductors have always attracted the attention of researchers due to the possibilities of joint use of the properties of separate nanostructures. Regardless, that it is referred to their optical, photoelectric, electric, magnetic, chemical, biological, and other properties. A particular interest for such layered structures is the exploration of the properties of metal oxides or semiconductors in the form of core@shell nanostructures and nanocrystals. Nanoparticles (NPs) have a much larger surface area per unit volume compared to their macroforms, and the increase in NP surface area is non-linear with decreasing NP volume, but also depends on its shape. This generates an increased chemical reactivity © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 22–30, 2024. https://doi.org/10.1007/978-3-031-42775-6_3

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and adsorption of the NP. Some theories describe that the role of the NP surface is dominated by the surface energy which in turn dictates the formation mechanisms, 3D morphology, reactivity in various media, and surface segregation of NPs [1]. Other authors consider that this interpretation requires taking into account the real form of the single NP [2]. However, interest in the nanoform of different structures is increasing, due to the new properties manifested, which differ from the properties of the same materials as macro-structures. Similarly, the combination of nanomultilayers brings out other characteristics through the synergism of conjugated nanomonolayers in a single multilayer nanostructure. ZnO is one of the most attractive semiconductor oxide materials for applications in micro(opto)electronics, photonics, acoustics and extensively in sensors. The extensive use [3] of ZnO nanoparticles also caused the study of their influence on biotics [4]. The wide use of ZnO is also due to its high photocatalysis [5, 6], as well as to the increase of photocatalytic activity in the visible region of the spectrum when combining ZnO with other elements, for example with Al. In many respects, ZnO is considered as a potential alternative to GaN for device applications due to its lower production cost, multiple technological approaches and superior optical properties. However, performances, such as thermal conductivity, relatively low charge carrier mobility in ZnO compared to GaN, and strong electron-phonon interaction are serious deficiencies of the ZnO macroform. But, the manifestation of new characteristics in nanoforms inspires optimism regarding the competitiveness of ZnO-based nanostructures for various uses in nanoelectronics, optoelectronics and sensors. Depending on the technological method of obtaining, the technological parameters such as temperature, humidity, treatment with UV, IR, the formula of the initial chemical substance, and the type of solvent (for obtaining from the solution), the physico-chemical properties of ZnO differ strongly [7–10]. The influence of ionizing radiation on the properties of semiconductor materials is varied and can also affect ZnO structures [11]. As an example, the experimental results show that the formation of X-ray-induced radicals in aqueous medium is caused by the presence of NPs from elements with high atomic number (Z), their oxides or sulfides. Contrary to expectations, NPs of low Z element oxides also exhibit characteristics of efficient radiosensitizers. These results denote the important role of the physico-chemical identity of the NP in the physico-chemical and chemical processes upon interaction with the environment and contribute to the total radicals induced by the action of X-rays on material properties. Another result indicating the role of the NP shape (ZnO single crystal nanowires) irradiated up to 10 h with 662 keV gamma rays (Cs-137 source) is the PL increase with relaxation to the initial state for several days due to the migration of vacancies on the surface of the nanowires, which indicates the restoration of the nanowires to their original state [12]. The recent paper [13] shows the possibility of developing alpha radiation detectors based on the scintillation of ZnO:Ga nanorods made by an original hydrothermal method at low temperature. The ones exposed, as well as the example of ZnO/Al2 O3 nanostructures as efficient gas sensors [14–16] based on the change of electrical resistivity, made in the Center for Nanotechnology and Nanosensors from the Technical University of Moldova suggest the possibility of some changes in the properties of ZnO/Al2 O3 nanostructures and interface

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after being treated in gamma radiation fields. Our results, in this work, demonstrate that gas sensors based on ZnO/Al2 O3 nanoheterostructure treated in gamma radiation fields possess an increased response to 2-propanol and n-butanol vapor at relative low operating temperature.

2 Experimental Section 2.1 Technology of ZnO/Al2 O3 Heterostructures ZnO thin films were synthesized on glass substrates from zinc sulfate and sodium hydroxide as precursors, using chemical synthesis from solutions method (SCS) [17]. Afterwards, thermal annealing at different temperatures and duration took place for these sample sets. Thermal treatment in an electric furnace at 650 °C for 2 h was used as a post-growth annealing process, which allowed to improve the crystallinity of the produced ZnO layers. The Al2 O3 nanofilm deposition on the surface of ZnO films was performed by using the thermal atomic layer deposition method. As the aluminum source the trimethylaluminum Al2 (C2 H5 )6 , was used which was oxidized by using H2 O after chemisorption. N2 gas was used to carry precursor vapors into the pre-reaction chamber and to remove by-products from the reactor. In addition, the obtained ZnO/Al2 O3 thin film heterostructures were thermally treated at 550 °C and 650 °C for 40 min in an oven to improve crystallinity and stability during sensing measurements. The details of the technological procedure are represented in the previous paper [14]. 2.2 Irradiation Process For investigating the effects of ionizing gamma radiation on the properties of ZnO/Al2 O3 nanostructures, the study in the electrical resistivity changes and the sensitivity to a wide spectrum of gases, after the action of a gamma radiation field, was selected. The method of determining the effects of gas sensitivity was based on the fact that these sensors demonstrated a high sensitivity to different gases hydrogen, volatile organic compounds (2-propanol, n-butanol, acetone, etc.) [14]. A source of Cs-137 was used as a source of ionizing radiation in these experiments. General scheme of the irradiation process is represented in Fig. 1. The irradiation source was placed at a distance of 1 m from the samples. Variation of irradiation dose was done using attenuator of 1000, 100 and 10, positioned in the way of gamma irradiation in front of these samples. At the maximum attenuation of the radiation, no changes in the electrical resistance of the sensor were observed. Changes in sensor resistance were identified at the minimal attenuation of 10 times that provides irradiation about 63 µGy/min [18]. The measurements took place at a temperature of about 300 K. The multimeter was placed outside the gamma ray flux cone. The nanostructures were irradiated under Cs-137 for 60 s, with the in-situ measurements of the electrical resistance of the samples in real time, during and after irradiation.

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Fig. 1. General scheme of sample measurement in gamma field.

3 Results and Discussion Initially, electrical properties and then sensing performances of ZnO/Al2 O3 structures have been studied before and after irradiation. Sensors have been measured to a series of gases at different operating temperatures from room temperature up to 350 °C. During irradiation process, electrical resistance was measured in real-time, as presented in Fig. 2. It was observed that low levels of applied gamma radiation do not have an immediate effect on the electrical resistance of the sensors. This indicates that radiation stimulated structural changes do not occur at this level.

Fig. 2. Variation of ZnO/Al2 O3 sensor’s resistance during irradiation under Cs-137 for 60s in ambient.

Influence of irradiation process on the current voltage (I-V) characteristics of ZnO/Al2 O3 structures has also been studied in the range from −10 V to +10 V at room temperature, as presented in Fig. 3. It was observed that after irradiation current (in absolute value) decreases and I-V characteristic showed linear and symmetrical dependence indicating ohmic contact properties. Ohmic contact resistance measured or determined (dV/dI) from the I-V characteristic is about 50–100 kOhms.

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Fig. 3. I-V characteristics before and after irradiation of ZnO/Al2 O3 sensor.

Gas response (S) has been determined using the ratio of currents in air (Iair ) and during gas exposure (Igas ): S=

Igas Iair

(1)

For comparing the change in gas response (CS(%)) value before (Sbefore ) and after irradiation (Safter ), following formula has been used: CS(%) =

Sbefore ∗ 100% − 100% Safter

(2)

This formula has been used because in case of no response (S = 1), the change will be 0%: CS(%) =

1 ∗ 100% − 100% = 0% 1

(3)

In Fig. 4 are presented results for the change in gas response at different operating temperatures for a series of gases. No response was detected at room temperature, observing response only at operating temperatures from 150 °C to 350 °C. No response for methane and ammonia gas was observed at any operating temperatures. It was observed that after irradiation response value increased for VOC and hydrogen gas. For tests with 100 ppm n-butanol and 2-propanol, sensor response value increased by about 100% and 200% after irradiation, when samples were measured at operating temperatures of 200 °C and 250 °C, respectively. Increase in response value can be attributed to generation of oxygen defect level on the surface of ZnO/Al2 O3 due to gamma irradiation, leading to a change in carrier density, as reported before in [19]. In Fig. 5 are presented dynamic responses to 100 ppm n-butanol, ethanol and 2propanol vapor at operating temperatures of 150 and 200 °C, respectively. It was observed

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Fig. 4. Change in gas response value of sensors based on ZnO/Al2 O3 at low operating temperatures (150–250 °C) before and after irradiation.

that initially, before gamma irradiation, no response was detected at these operating temperature (150–200 °C). After irradiation, response value increased from 1.0 to ~1.3 (Fig. 5a) and from 1.0 to ~2.1 (Fig. 5b) for 100 ppm n-butanol gas at 150 and 200 °C temperatures, respectively. For other test gases (ethanol and 2-propanol) similar results have been observed ((see Fig. 5(c, d) and (e, f) respectively)). Further studies about the effect of gamma irradiation on structural properties are needed, to explain the sensing mechanism after irradiation. These results confirm that irradiation in gamma field influences the sensitivity of sensors based on ZnO/Al2 O3 heterostructure by lowering the operating temperature and increasing response value. Further studies are needed in order to: a) increase gas sensitivity; b) reduce the working/operating temperature of the sensors; c) increase the sensitivity of heterostructures sensors to gamma radiation with the aim of using them as detectors; d) identify the solution for ensuring the selectivity of the sensors.

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Fig. 5. Dynamic gas response for 100 ppm n-butanol (a, b), ethanol (c, d) and 2-propanol (e, f) at 150 °C (a, c, e) and 200 °C (b, d, f).

4 Conclusions The effects of the irradiation in gamma field on the properties of ZnO/Al2 O3 -based sensors has been studied. It was observed that during irradiation, electrical resistance value was rising continuously, in the range of 87.5–88.5 k. By comparing I-V characteristics, it was observed that the contact characteristics were ohmic, and that after irradiation the value of the electric current decreased. Sensing results measured before irradiation showed that ZnO/Al2 O3 -based sensors can detect a series of gases, particularly VOC at

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operating temperatures higher than 250 °C. An increase in gas response value for 100 ppm n-butanol and 2-propanol by about 100% and 200% was observed after irradiation at operating temperatures of 200 and 250 °C, correspondingly. For ZnO/Al2 O3 -based sensors a decrease of optimal working temperatures from 300–350 °C to about 150–250 °C was observed after irradiation in gamma field. The obtained results are important for development new detectors for biomedical applications and gas sensors that work in gamma radiation field. Acknowledgments. The results of the work were obtained in the framework of the cooperation between the National Nuclear Security Support Center, the Nanotechnology and Nanosensors Center of the Technical University of Moldova, the National Institute of Metrology and is part of the PFA funded in 2022 by the Swedish Radiation Safety Authority.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. He, L.-B., Zhang, L., Tang, L.-P., et al.: Novel behaviors/properties of nanometals induced by surface effects. Mater. Today Nano 1, 8–21 (2018). https://doi.org/10.1016/j.mtnano.2018. 04.006 2. Amara, H., Nelayah, J., Creuze, J., et al.: Is There Really a Size effect on the Surface Energy of Nanoparticles? https://doi.org/10.13140/RG.2.2.26218.85446 3. Franco, M.-A., Conti, P.P., Andre, R.S., Correa, D.S.: A review on chemiresistive ZnO gas sensors. Sens. Actuators Rep. 4, 100100 (2022). https://doi.org/10.1016/j.snr.2022.100100 4. Pullaguralaac, V.L.R., Adisaae, I.O., Rawatac, S., et al.: Finding the conditions for the beneficial use of ZnO nanoparticles towards plants – a review. Environ. Pollut. 241, 1175–1181 (2018). https://doi.org/10.1016/j.envpol.2018.06.036 5. Moradipour, P., Dabirian, F., Moradipour, M.: Ternary ZnO/ZnAl2 O4 /Al2 O3 composite nanofiber as photocatalyst for conversion of CO2 and CH4 . Ceram. Int. 46(5), 5566–5574 (2020). https://doi.org/10.1016/j.ceramint.2019.09.009 6. Nguyen, N.T., Nguyen, V.A.: Synthesis, characterization, and photocatalytic activity of ZnO. Nanomaterials prepared by a green, nonchemical route. J. Nanomater. (2020) .https://doi.org/ 10.1155/2020/1768371 7. Khan, M.I., Imran, S., Saleem, M., Rehman, S.U.: Annealing effect on the structural, morphological and electrical properties of TiO2 /ZnO bilayer thin films. Results Phys. 8, 249–252 (2018). https://doi.org/10.1016/j.rinp.2017.12.030 8. Garnier, J., Bouteville, A., Hamilton, J., et al.: A comparison of different spray chemical vapour deposition methods for the production of undoped ZnO thin films. Thin Solid Films 518(4), 1129–1135 (2009). https://doi.org/10.1016/j.tsf.2009.01.157 9. Bacaksiz, E., Parlak, M., Tomakin, et al.: The effects of zinc nitrate, zinc acetate and zinc chloride precursors on investigation of structural and optical properties of ZnO thin films. J. Alloys Compd. 466(1–2), 447–450 (2008). https://doi.org/10.1016/j.jallcom.2007.11.061 10. Jakschik, S., Schroeder, U., Hecht, T., et al.: Crystallization behavior of thin ALDAl2 O3 films. Thin Solid Films 425(1–2), 216–220 (2003). https://doi.org/10.1016/S00406090(02)01262-2

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11. Higgins, M.M., Banu, A., Pendleton, S., Rojas, J.V.: Radiocatalytic performance of oxidebased nanoparticles for targeted therapy and water remediation. Radiat. Phys. Chem. 173, 108871 (202). https://doi.org/10.1016/j.radphyschem.2020.108871 12. Mayo, D.C., Nolenb, J.R., Cookc, A., et al.: Zinc oxide nanowire gamma ray detector with high spatiotemporal resolution. In: Proceedings Volume 9737, Synthesis and Photonics of Nanoscale Materials XIII, SPIE, San Francisco (2016). https://doi.org/10.1117/12.2214229 13. Sahani, R.M., Kumari, C., Pandya, A., et al.: Efficient alpha radiation detector using low temperature hydrothermally grown ZnO:Ga nanorod scintillator. Sci. Rep. 9, 11354 (2019). https://doi.org/10.1038/s41598-019-47732-1 14. Lupan, O., Santos-Carballal, D., Magariu, N., et al.: Heterostructure-based sensors for volatile organic compounds in safety application. Appl. Mater. Interfaces 14(25), 29331–29344 (2022). https://doi.org/10.1021/acsami.2c03704 15. Lupan, O., Chow, L., Shishiyanu, S., et al.: Nanostructured zinc oxide films synthesized by successive chemical solution deposition for gas sensor applications. Mater. Res. Bull. 44, 63–69 (2009). https://doi.org/10.1016/j.materresbull.2008.04.006 16. Santos-Carballal, D., Lupan, O., Magariu, N.: Al2 O3 /ZnO composite-based sensors for battery safety applications: an experimental and theoretical investigation. Nano Energy 109, 108301 (2023). https://doi.org/10.1016/j.nanoen.2023.108301 17. Lupan, C., Khaledialidusti, R., Mishra, A.K., et al.: Pd-Functionalized ZnO:Eu columnar films for room-temperature hydrogen gas sensing: a combined experimental and computational approach. ACS Appl. Mater. Interfaces 12(22), 24951–24964 (2020). https://doi.org/10.1021/ acsami.0c02103 18. Lupan, C., Bîrnaz, A., Buzdugan, A., et al.: The reliability to gamma radiation of gas sensors based on nanostructured ZnO:Eu. In: Electronics, Communications and Computing, 12th Edition, pp. 69–72. Tehnica-UTM, Chis, in˘au (2023). https://doi.org/10.52326/ic-ecco.2022/ EL.01 19. Van Duy, N., Huu Toan, T., Duc Hoa, N., et al.: Effects of gamma irradiation on hydrogen gas-sensing characteristics of Pd–SnO2 thin film sensors. Int. J. Hydrogen Energy 40(36), 12572–12580 (2015). https://doi.org/10.1016/j.ijhydene.2015.07.070

Electrical Properties of the (Copper, Dysprosium)-Containing Complex Compound Andriy Semenov1(B) , Volodymyr Martyniuk1 , Maria Evseeva2 , Oleksandr Osadchuk1 , Olena Semenova1 , and Tetyana Yushchenko2 1 Vinnytsia National Technical University, Vinnytsia, Ukraine

[email protected] 2 National Pirogov Memorial Medical University, Vinnytsia, Ukraine

Abstract. A new semiconductor material tetrakis-μ3-(methoxy)(methanol)pentakis(acetylacetonato) (tricopper(II), dysprosium(III)) (I) was synthesized, with the following composition: [Cu3 Dy(AA)5 (OCH3)4 CH3 OH], where HAA=H3 C–C(O)–CH2 –C(O)–CH3 . By data of the elemental analysis and physicchemical research methods, the obtained complex compound (I) was established to contain atoms of copper (II) and dysprosium (III) in a ratio Cu:Dy = 3:1, and its composition was established to correspond to a gross formula: Cu3 DyO15 C30 H51 . The electrical conductivity of the obtained material was measured in compressed form. The following parameters were calculated for the complex compound (I): the number of valence electrons in one molecule was 276; the mass of one molecule was 166.777·10–20 kg; the total number of molecules in a cylindrical sample with a 0.138 g mass and a 19.72·10–9 m3 volume was 8.274·1013 molecules. The resistivity of the pressed sample decreases from 9·1010 to 7·104 ·cm in a 303–413 K temperature range. This confirms that the synthesized compound is a semiconductor with a bandgap of 1.38 eV. The conductive properties of the complex compound as a heat-sensitive element were studied. An experimental sample of compressed material with geometric sizes of 1:10−3 m × 0.5:10−3 m × 0.5:10−3 m was employed for investigations. Keywords: Temperature · Magnetic Field · Concentration · Semiconductor · Conductive Properties · Complex Compound

1 Introduction Modern industry requires the improvement of existing means of managing technological processes as well as the development of the new ones [1]. Nowadays, composite semiconductor materials are widely used to solve various technical tasks in automation, telemechanics, thermometry, electronics, electrical engineering, and telephony [2, 3]. Synthesis of new semiconductor complex compounds, which physical parameters change under the influence of temperature and magnetic field, is also a relevant task [4, 5]. Sensors created on such synthesized semiconductor materials promote developing new, more sensitive thermo- and magneto-sensitive secondary sensors. Semiconductor © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 31–40, 2024. https://doi.org/10.1007/978-3-031-42775-6_4

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thermistors are ones of the most prominent among them. Their advantages are high temperature sensitivity, stability of characteristics over time, small dimensions and almost complete absence of special maintenance during operating. These characteristics largely depend on the quality of semiconductor material used during their manufacture. This is because impurities in the semiconductor material, methods of its heating treatment, and contact patterning methods significantly affect the sensitivity and accuracy of thermistor indicators [6]. Therefore, searching for new materials and studying the nature of their electrical conductivity, which will provide developing elements with high sensitivity to changes in temperature and magnetic field on their basis, are important issues [2, 4]. Heterometallic complex compounds containing two or more metal atoms of different nature joined together by ligands of different nature are known to be used for various practical applications due to their unique electrical, magnetic, optical, luminescent and other properties [7–9]. Among such compounds, heterometallic alkoxy complexes containing β-diketone as a ligand and having semiconducting properties are of particular interest [10].

2 Methods The purpose of this study is the synthesis and creation of a (copper, dysprosium)containing heterometallic complex compound in solid state and an experimental study of its electrical properties. A method was developed to synthesize a heterometallic complex compound – tetrakis-μ3-(methoxy)(methanol)-pentakis(acetylacetonato)(tricopper(II), dysprosium(III)) (I) with the following composition: [Cu3 Dy(AA)5 (OCH3 )4 CH3 OH], where HAA=H3 C–C(O)–CH2 –C(O)–CH3 . The heterometallic (copper, dysprosium)-containing complex compound was synthesized in the following way: portions of dysprosium(III) nitrate hexahydrate with a 6.85 g (15 mmol) mass and copper (II) nitrate trihydrate with a 10.87 g (45 mmol) mass were dissolved in 200 ml of anhydrous methyl alcohol, then 80 ml of orthoformic ether, which served as a water-absorbing reagent, was added to the resulting mixture. The obtained mixture of reagents was kept for 3 h at room temperature in a hermetically sealed flask and stirred constantly. After that, 8.2 ml (75 mmol) of acetylacetone was added to the reaction mass, heated to (~50 °C) while constantly stirred in a hermetically closed conical flask with a reversible water cooler. In 10 min, piperidine was added in small portions to the reaction mass to create an alkaline environment (pH = 9), the color of the solution being changed and a light blue sediment formed. Next, the reaction mixture was continuously stirred on a magnetic stirrer and the temperature of the reaction mass (~50 °C) was maintained for 1 h. After that, the flask with the reaction mass was cooled and a blue fine-crystalline sediment was formed, its homogeneity was confirmed by magnifying on a glass slide under a microscope. After complete settling, the sediment was filtered on a glass filter, washed first with a small amount of methyl alcohol and then with diethyl ether, dried in a vacuum desiccator over calcium chloride. The practical yield of the obtained complex compound was equal to 11.14 g, which was 74% of the theoretically calculated one. The extracted complex compound (I) is a fine crystalline blue-colored powder, which dissolves in a mixture of chloroform and

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dimethylformamide taken in 1:1 ratio, is poorly soluble in aliphatic alcohols, diethyl ether, much better soluble in dimethylsulfoxide or dimethylformamide, hydrolyzed in water. To determine the quantitative content of metals in the extracted compound (I), a weighted amount of dry powder of this compound was treated with a mixture of concentrated sulfuric and nitric acids, then a quantitative determination of the metal content was carried out by the gravimetric method. The obtained data confirm that the extracted compound includes atoms of copper and dysprosium in a 3:1 ratio. Also, a carbon and hydrogen content in the extracted compound was determined experimentally. The performed quantitative elemental analysis of the compound (I) showed that in a dry powdery state it contains: Cu – 18.97%; Dy – 9.96%; C – 35.86%; H - 5.08%, and its composition corresponds to the gross formula of Cu3 DyO15 C30 H51 . X-ray phase analysis, magnetochemical, IR-spectroscopic and thermogravimetric tests were carried out to establish a structure, a probable arrangement of chemical bonds in this compound and a manner of ligand coordination with atoms of copper and dysprosium [10]. Considering the obtained data of physicochemical tests, a tetrameric cubane scheme for the arrangement of chemical bonds was proposed for the extracted compound (I), where three cations of copper(2+) and one cation of dysprosium(3+) are connected by bridging methoxy groups, each cation of copper(2+) forms bonds with one monodeprotonated anion of acetylacetone, the cation of dysprosium(3+) is coordinated by two anions of acetylacetone and one molecule of methyl alcohol. The extracted heterometallic complex compound (I) was established to correspond to the following composition [Cu3 Dy(AA)5 (OCH3 )4 CH3 OH], where HAA=H3 C–C(O)–CH2 –C(O)–CH3 and is named tetrakis-μ3-(Methoxo)(methanol)-pentakis(acetylacetonato)(tricopper(II), dysprosium(III)) (I) [10]. Conductive properties of the selected complex compound [Cu3 Dy(AA)5 (OCH3 )4 CH3 OH] (I) were investigated in solid state. To perform measurements, the fine-crystalline powder (copper, dysprosium) containing acetylacetonate was kept in a drying cabinet at a 105 °C temperature up to constant mass forming, and pressed using a specially developed pressing device [11]. A cylindrical sample of compressed material with a 0.138 g mass of and a 19.72·10–9 m3 volume was utilized for further experimental investigations. The density of the cylindrical sample of the pressed test material was calculated by formula (1): ρ = m/v = 6.998 · 103 kg/m3 ,

(1)

where ρ is the substance density; m is the experimental sample mass; V is the experimental sample volume. For the extracted compound [Cu3 Dy(AA)5 (OCH3 )4 CH3 OH] (I), the molar mass was calculated to be 1004.0 g/mol, and the number of valence electrons in one molecule was calculated to be 276. The mass of one molecule is calculated by formula (2) m0 = M /NA = 166.777 · 10−20 kg,

(2)

where M is the molar mass of compound (I); m0 is the mass of one molecule of compound (I); N A is the Avogadro number.

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The total number of molecules inside the cylindrical sample of a 0.138 g mass and a 19.72·10–9 m3 volume was calculated by formula (3): Nmol = m/m0 = 8.274 · 1013 molecules,

(3)

where N mol is the total number of molecules; m0 is the mass of one molecule of the compound (I); m is the mass of the experimental sample. The total number of valence electrons is: N = 272 · Nmol = 22.5053 · 1015 .

(4)

3 Results of an Experimental Study The experiment showed that in the 303–413 K temperature range, the specific resistance of the pressed sample of investigated compound (I) decreases from 9·108 to 7·102 ·m, this confirms that the extracted compound is a semiconductor. Considering experimental data, the specific conductivity of the compound at these temperatures was calculated. σ1 = 1.1·10–9 (·m)−1 for T1 = 303 K and σ2 = 14.14·10–4 (·m)−1 for T2 = 413 K. The bandgap was determined considering these calculations [4, 12] E = 

k ln 1 T2

−

σ1 σ2 1 T1

 = 2.208 · 10−19 J = 1.38 eV ,

(5)

where E is the bandgap width of the complex compound [Cu3 Dy(AA)5 (OCH3 )4 CH3 OH] (I); k is the Boltzmann constant; T is the absolute temperature; σ is the specific conductivity of the material. To use this compound as a sensitive element, the test sample was pressed as an SMD resistor with geometric dimensions of 1·10−3 m × 0.5·10−3 m × 0.5·10−3 m. When the bandgap width for the complex compound is known, the temperature dependence of specific conductivity in a 273–493 K range can be obtained considering the performed investigations (see Fig. 1). The graph in Fig. 1 indicates that the specific conductivity of the studied structure grows from 3.3·10–12 (·m)−1 to 0.76 (·m)−1 when the temperature rises from 273 K to 493 K. The resistivity changed from 3.01·1011 ·m to 1.31 ·m in the temperature range from 273 K to 493 K. To examine changes in resistance, current density, concentration versus temperature, and the magnetic field dependences of Hall voltage and voltage inside the conductor for tetrakis-μ3-(methoxy)(methanol)-pentakis(acetylacetonato) (tricuprum(II), dysprosium(III)) (I) an experimental sample of compressed material with geometric dimensions of 1·10−3 m × 0.5·10−3 m × 0.5·10−3 m was used. Figure 2 shows the temperature dependence of the complex compound resistance. As can be seen in Fig. 2, the resistance of the sample drops rapidly: for example, at a temperature of 273 K it is 1.2·1015 , at 303 K it is 3.63·1012 , while at 493 K it is 5257.9 . A step by 12 orders confirms that this material can be used for creation of thermosensitive resistors.

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Fig. 1. Temperature dependence of specific conductivity of the semiconductor material.

Fig. 2. Temperature dependence for resistance of the material with geometric dimensions of 1·10−3 m × 0.5·10−3 m × 0.5·10−3 m.

The logarithmic temperature dependence of the current density at a supply voltage of 10 V is shown in Fig. 3. The given graph (Fig. 3) illustrates that the curve crosses the temperature axis at 333 K. The current density value changes from 3.31·10–8 A/m2 to 7607.57 A/m2 when the temperature changes from 273 K to 493 K.

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The temperature dependence of the charge carrier concentration of the heterometallic complex compound for the material with geometric dimensions of 1·10−3 m × 0.5·10−3 m × 0.5·10−3 m. The charge carrier concentration increases from 2.09·1014 m−3 to 4.8·1025 m−3 in the 273–493 K temperature range.

Fig. 3. Temperature dependence of the current density at the U = 10 V voltage.

The obtained dependencies confirm that the synthesized material can be used for creation of thermosensitive elements [13]. Calculating the Hall constant at 30 °C yields the following results: RH = 1/nq = 90 m3 · C −1 ,

(6)

where n is the charge carrier concentration; q is the electron charge. The Hall quantum constant is calculated by formula (7): Rqu H = −3π/8nq = −105, 98 m3 · C −1

(7)

Formula (8) was obtained by using formula (7) and the equation for the temperature dependence of the charge carrier concentration. This formula shows the temperature dependence of the Hall constant: Rqu H = −

E 3π · e kT 8qn0

(8)

According to formula (8), the temperature logarithmic dependence of the Hall quantum constant was drawn as shown in Fig. 4. As can be seen from Fig. 4, the value of the Hall quantum constant for such material decreases from 35.14·103 m3 ·C−1 to 1.53·10–7 m3 ·C−1 when the temperature increases from 273 K to 493 K.

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Fig. 4. Temperature dependence of the Hall quantum constant.

To find the mobility of charge carriers from the experimental data of specific resistance, at 303 K, ρ = 9·108 ·m, the specific conductivity σ = 1.1·10–9 cm/m was calculated. μn = RH · σ.

(9)

The mobility of charge carriers for the quantum case is μn = Rqu H · σ = 1, 16 · 10−7 v3 · (V · s)−1

(10)

After having substituted the temperature dependences of the Hall constant and specific conductivity, the charge carrier mobility was determined to be a constant μ = 1.165 × 10–7 m3 /(V·s) and not to depend on temperature. The logarithmic magnetic field induction dependence of the Hall electric field inside the semiconductor at various temperatures is given in Fig. 5. The plot in Fig. 5 shows that the electric field increases inside the material from 1.1 10–2 V/m to 0.46 V/m within the 10–400 mT range, and there is a linear dependence with a slow change from 0.46 V/m to 1.16 V/m within the 400 mT–1 T range. A similar dependence is observed for the Hall voltage. The logarithmic magnetic field induction dependence of the Hall voltage is shown in Fig. 6. It can be seen in the Fig. 6 that the Hall voltage increases from 5.83·10–6 V to 1.16·10–4 V within the 10–200 mT range, from 1.16·10–4 V to 3.49·10–4 V within the 200–600 mT range, and from 3.49·10–4 V to 5.82·10–4 V within the 600–1000 mT range. The dependencies of the intensity of the Hall field inside a semiconductor on the magnetic field induction at various temperatures (Fig. 5) and the Hall voltages (Fig. 6) show that these values are independent of temperature and their curves are coincident.

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Fig. 5. Logarithmic magnetic induction dependence of the electric field strength inside a semiconductor.

Fig. 6. The logarithmic dependence of the Hall voltage on the magnetic field induction.

4 Conclusions The dependence of electrophysical parameters of the newly synthesized complex compound tetrakis-μ3-(methoxy)(methanol)-pentakis(acetylacetonato) (tricopper(II), dysprosium(III)) (I) with the following composition: [Cu3 Dy(AA)5 (OCH3 )4 CH3 OH] was

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investigated. The method of synthesizing such material was presented and the effects of temperature and magnetic field on properties of such semiconductor were investigated. Investigation of conductive properties of the synthesized complex compound in compressed form in the 273 K–493 K temperature range showed that with temperature increasing, its resistivity decreases sharply from 3.01·1011 ·m to 1.31 ·m, and the width of forbidden zone is 1.38 eV, which is typical for semiconductor materials. The chemical compound starts to decompose at 513 K, the charge carrier concentration increases from 2.09·1014 m−3 at 273 K to 4.8·1025 m−3 at 493 K, meanwhile the quantum Hall constant decreases from 3.51·104 m3 ·C−1 to 1.53·10–7 m3 ·C−1 as the temperature rises from 273 K to 493 K, the Hall voltage changes from 5.83·10–6 to 5.83·10–4 V in the magnetic field range from 0 to 1000 mT.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Koksharova, T., Ptashchenko, A., Masleeva, N., Fel’dman, S., Pasternak, N., Stukalov, S.: Solid state conductivity and catalytic activity of hexacyanoferrate(II)–thiosemicarbazide complexes of 3d-metals. Theor. Exp. Chem. 38, 263–267 (2002). https://doi.org/10.1023/A:102 0524100707 2. Osadchuk, O., Martyniuk, V., Osadchuk, I., Semenova, O., Sydoruk, T., Evseeva, M.: The impact of temperature and magnetic field on physical field on physical parameters of the strontium-containing heterometallic coordination compound of copper (II). In: 2020 IEEE 15th International Conference on Advanced Trends in Radioelectronics, Telecommunications and Computer Engineering (TCSET), pp. 511–515. IEEE, 25–29 February 2020, Lviv-Slavske, Ukraine (2020). https://doi.org/10.1109/TCSET49122.2020.235485 3. Blasdel, N.J., Monty, C.N.: Temperature sensitive fabric for monitoring dermal temperature variations. In: Mukhopadhyay, S.C. (ed.) Wearable Electronics Sensors. SSMI, vol. 15, pp. 193–220. Springer, Cham (2015). https://doi.org/10.1007/978-3-319-18191-2_8 4. Osadchuk, O., Martyniuk, V., Semenova, O., Semenov, A., Martyniuk, H., Sydoruk, T. Physical parameters of the synthesized semiconductor material based on a heterometallic complex compound of copper (II) with N,N -Bis(Salicylidene)Semicarbazide. In: 2022 IEEE 16th International Conference on Advanced Trends in Radioelectronics, Telecommunications and Computer Engineering (TCSET), pp. 432–435. IEEE, 22–26 February 2022, Lviv-Slavske, Ukraine (2022). https://doi.org/10.1109/TCSET55632.2022.9766980 5. Candra, R., Flaminggo, N., Natalia, A., Yuliza, E., Khairurrijal, K.: Making counter clockwise analog thermometer under project-based learning method. J. Phys. Conf. Ser. 1204, 012116 (2019). https://doi.org/10.1088/1742-6596/1204/1/012116 6. Panchenko, T., Evseeva, V., Ranskiy, A.: Copper(II) and nickel(II) with N,N bis(salicylidene)thiosemicarbazide heterometal complex compounds. Ch&ChT 8(3), 243– 248 (2014) 7. Ranskyi, A., Yevseeva, M., Panchenko, T., Gordienko, O.: Synthesis and properties of heterometallic coordination compounds of copper(II), nickel(II) or cobalt(II) and alkaline earth elements with N,N -bis(salicylidene)semicarbazide. Ukrainian Chem. J. 79(2), 74–80 (2013). http://nbuv.gov.ua/UJRN/UKhJh_2013_79_1-2_16 8. Samus, N., Khoroshun, I., Sinitsa, I., Gandziy, M.: Heterometallic (lanthanide or yttrium, p- or d-element) containing N,N -ethylene-bis-salicylideneimates. Coord. Chem. 19(9), 729–732 (1993)

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9. Samus, N., Tsapkov, V., Gandziy, M.: Heterometallic μ-alkoxo (copper, bismuth) containing acetylacetonates. J. Gen. Chem. 63(1), 177–182 (1993) 10. Samus, N., Gandziy, M., Tsapkov, V.: Heteronuclear μ-methoxo(copper, yttrium or lanthanide) acetylacetonates. J. Gen. Chem. 62(3), 510–515 (1992) 11. Semenov, A., Baraban, S., Semenova, O., Voznyak, O., Vydmysh, A., Yaroshenko, L.: Statistical express control of the peak values of the differential-thermal analysis of solid materials. Solid State Phenom. 291, 28–41 (2019). https://doi.org/10.4028/www.scientific.net/SSP. 291.28 12. Animalu, A.: Intermediate Quantum Theory of Crystalline Solids. Massachusetts Institute of Technology, Prentice-Hall Inc., Englewood Cliffs, New Jersey (1977) 13. Semenov, A., Baraban, S., Osadchuk, O., Semenova, O., Koval, K., Savytskyi, A.: Microelectronic pyroelectric measuring transducers. IFMBE Proc. 77, 393–397 (2019). https://doi. org/10.1007/978-3-030-31866-6_72

Morphological and Sensing Properties of the ZnO-Zn2 SnO4 Ternary Phase Nanorod Arrays Dinu Litra1 , Cristian Lupan1 , Tim Tjardts2 , Haoyi Qiu3 , Tudor Zadorojneac1 , Dominic Malai1,2,3 , Alexandr Sereacov1(B) Cenk Aktas2 , Leonard Siebert3 , and Oleg Lupan1,2,3

,

1 Center for Nanotechnology and Nanosensors, Department of Microelectronics and

Biomedical Engineering, Technical University of Moldova, 168, Stefan cel Mare Av., MD-2004 Chisinau, Republic of Moldova [email protected] 2 Department of Materials Science, Chair for Multicomponent Materials, Faculty of Engineering, Kiel University, Kaiserstraße 2, 24143 Kiel, Germany 3 Department of Materials Science, Faculty of Engineering, Chair for Functional Nanomaterials, Kiel University, Kaiserstraße 2, 24143 Kiel, Germany

Abstract. In this paper, the morphological and sensing properties of the Sn-doped ZnO-Zn2 SnO4 nanorods obtained by the hydrothermal method are presented. The developed methodology exhibits high levels of efficiency and cost-effectiveness, making it particularly suitable for implementation in the field of nanoelectronics and biomedical applications. Scanning electron microscopy was used to analyze the morphology of the Sn-doped ZnO-Zn2 SnO4 nanostructures showing nanorod arrays formation. Energy dispersive X-ray spectroscopy was involved to determine the chemical composition and shows uniform distribution of Sn. Structural analysis by X-ray diffraction shows high crystallinity of Sn-doped ZnO-Zn2 SnO4 samples with (0002) main orientation and formation of a ternary phase Zn2 SnO4 . These nanostructures obtained by the hydrothermal method were tested as sensor materials for ethanol and carbon dioxide. A high response of about 130% to 100 ppm ethanol vapor with a very fast response time of 1s at an operating temperature of 250 °C was observed. This factor is very important for the detection of harmful or explosive gases. Sn-doping in ZnO and the formation of Zn2 SnO4 is considered to be the key factor that changes the morphological and sensing properties for application use in miniaturized photodetectors, light emitting diodes, laser light source, and gas sensors. Keywords: Zinc oxide · Zn2 SnO4 · Doped · Sensors · Hydrothermal

1 Introduction Currently, nanotechnology and nanomaterials including metal oxides attracted enormous interest in the field of science and contribute to almost all aspects. One of the most studied metal oxides is SnO2 . It possesses both sensory [1, 2] and optical properties, being © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 41–51, 2024. https://doi.org/10.1007/978-3-031-42775-6_5

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applied in photodetectors, luminescent diodes and laser light sources [3, 4]. One of the investigated crystalline phases of this material is Zn2 SnO4 . Recently, significant progress has been made in the synthesis of various axial, radial, octahedrons, and branched 1D nano-heterostructures [4–6]. Zhou and co-authors reported on shape control with the decoration of Zn2 SnO4 nanostructures on 1D nanowires for improving response characteristics and stability [5]. However, reports on the synthesis of nano-heterostructures composed of two types of wide-gap direct semiconductor Zn2 SnO4 with a bandgap of about 3.6 eV and ZnO (~3.6 eV) with a high exciton binding energy of 60 meV are relatively limited [7–9]. It has been reported to have high electron mobility, high electrical conductivity, and low visible absorption, making it suitable for applications in thin-film photovoltaic devices, solar cells, and sensors for combustible gas and moisture detection [10]. It is known that the sensory properties of Zn2 SnO4 largely depend on its composition, morphological structure and other properties [5]. Doping semiconductors with noble metals has proven to be an effective method of improving sensory properties such as changing selectivity, increasing sensitivity, improving response and recovery time, and others [11]. One of the proposed applications of the obtained nanostructures would be their use as a gas sensor for volatile organic compounds (VOCs), as they proved to be effective in this field [12–14]. Other fairly current applications of these sensors would be electric batteries with the purpose of detecting gases that can be emitted by Li-ion batteries or in food as detectors of spoiled food as reported [15–17]. In this paper, the results of the research carried out on Sn-doped ZnO-Zn2 SnO4 nanorod arrays obtained by the hydrothermal method are presented. In turn, high levels of doping lead to the appearance of changes in the nanomaterial, which can be observed in the SEM images. SEM, XRD, XPS and Raman studies have been carried out in order to investigate the crystalline properties of the obtained nanostructures. Depending on the synthesis method, high dopant concentrations can essentially contribute to the sensory properties of the nanostructures based on such materials. Our technological approach is rapid and cost-effective, which is significant for various applications in nanoelectronics and nanotechnologies.

2 Materials and Methods Sn-doped ZnO-Zn2 SnO4 nanorod arrays and nanostructures have been obtained using the hydrothermal method, [18] on a glass and SiO2 /Si substrates, as presented previously in our work [19]. Reagents were used in the received form without further purification. In our approach, SiO2 /Si wafers and glass substrates were cleaned in a diluted HCl for 5 min, then ultrasonically in acetone, followed by ethanol and rinsing with deionized water. Next, cleaned substrates were inserted in the reactor and heated at a temperature 94–97 °C for 10 min on a hot plate, then it was allowed to cool down to room temperature. Samples were rinsed with deionized water and annealed at 380 °C for 10 min. SEM, XRD and micro-Raman studies have been carried out as previously reported [20, 21]. The phase structures of nanorod arrays were investigated by a X–ray diffraction (XRD, 3000 diffractometer, Cu Kalpha radiation, λ = 1.5403 Å). To verify the tin (Sn) content on the Sn-doped ZnO-Zn2 SnO4 samples surface, X-ray photoelectron spectroscopy (UHV XPS from Prevac sp. z.o.o., Al-anode, 300W) was

Morphological and Sensing Properties of the ZnO-Zn2 SnO4

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utilized. The charge correction was done with the C 1s line of advantageous carbon at 284.8 eV. For the analysis of XPS spectra the software CasaXPS (version 2.3.23) was utilized. As a sample a ZnO: Sn doped sample marked as #(13b) was selected. Upon the XPS analysis the sample was iteratively cleaned by an IS 40 Ar ion etching system (Prevac sp. Z.o.o.) with a 5 mA, 4 kV Ar ion beam and 600 V focus voltage at a pressure of ≈ 2.010−6 mbar in the respective buffer chamber. This cleaning process was conducted in between two survey scans in order to clean a major part of the residual carbon and other contaminations from the sample surface and maximize the signal coming from the desired sample. With such an approach, it is expected to increase the chances of verifying the Sn doping. The end of the total Ar cleaning processes was estimated by observing changes in the Zn LMM/Sn 3d interval of the respective survey scan carried out after each cleaning step. The cleaning was considered to be complete when a signal which can be related to Sn 3d could be seen in the Zn LMM interval of the spectrum. This was achieved after a total cleaning time of 270 min. To further investigate the formation of Sn-doped ZnO-Zn2 SnO4 nanorod arrays, micro-Raman spectroscopy was performed using the WITec alpha300 RA system (WITec GmbH, Ulm, Germany) equipped with a triple grating spectrometer and a CCD detector. The grating parameters were configured to 600 g/mm with a blaze wavelength of 500 nm. A green laser with a wavelength of 532 nm served as the excitation source. Before conducting the investigations, the spectrometer was calibrated using a Si wafer. Sensors based on Sn-doped ZnO have been studied to a series of gases at different operating temperatures, as reported before in paper [20].

3 Results and Discussion 3.1 SEM and EDX Characterization Figure 1 shows SEM images of continuous layers of Sn-doped ZnO-Zn2 SnO4 nanorod arrays on glass substrate, where it was observed that nanostructures are distributed uniformly on the substrate’s surface for both doping concentrations. At a lower doping concentration (Fig. 1a), the majority of nanostructures have a well-defined hexagonal form. The nanorods are interpenetrating forming a continuous layer and an electrical conduction path with potential energy barriers favorable for sensors. At higher Snconcentration (Fig. 1b) it was observed that the size of nanorods increased and their hexagonal structure was deformed. Figure 2 presents the SEM images for ZnO-Zn2 SnO4 with 2× higher Sn-doping concentration grown by hydrothermal method on glass substrate, observing their predominantly hexagonal form (Fig. 2a). The average radius of nanorod was ~100 nm in Sn-doped ZnO-Zn2 SnO4 nanorod arrays. On the surface of the nanorods, unevenness was observed which was due to the layers of material during the deposition process. The chemical composition of the nanostructures was studied using EDX. EDX mapping is shown in Fig. 3. Distribution of the individual elements Zn, O and Sn are presented in Fig. 3b. It can be observed that Sn is well distributed on the surface, meaning that nanorod arrays are uniformly doped. Next, it will investigated elements and their corresponding atomic percentage.

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Fig. 1. SEM images of Sn-doped ZnO-Zn2 SnO4 nanorod arrays grown by the hydrothermal method: (a) – lower Sn doping concentration, 0.27 at%; (b) – 2× higher doping concentration.

Fig. 2. SEM images of ZnO-Zn2 SnO4 nanorod arrays with 2× higher Sn doping concentration grown by hydrothermal method at higher magnification: (a) scale 200 nm; (b) scale 100 nm.

Table 1 shows the concentration of the elements in at% that were detected during the EDX study. The concentration for Sn is 0.27 at% at a lower doping concentration. The atomic percentage of oxygen is slightly higher than that of zinc. 3.2 XPS Results Figure 4a) shows the survey scans from 1300 eV to 0 eV for the heat-treated Sn-doped ZnO-Zn2 SnO4 sample on SiO2 /Si substrate before and after the Ar ion cleaning process. A distinct Zn-2p signal at about 1045.4 eV (Zn-2p 1/2) and 1022.4 eV (Zn-2p 3/2) can be seen in both scans. Furthermore, the survey scan show contributions of carbon and oxygen on the sample surface. The carbon signal can be associated with adsorbed hydrocarbons from the atmosphere and is most likely not part of the sample itself. The presence of oxygen is expected due to the fact that ZnO is the main part of the sample itself and significant surface oxidation should be present on the sample as well. When considering the potential presence of Sn, the expected main signal Sn 3d is not present in the survey scan before the Ar cleaning process, indicating that Sn is may not present at the sample surface. After the Ar cleaning process, a small increase at the expected Sn 3d position according to [1] is observable which is shown in the inset of Fig. 4a). In order to investigate the signal associated with Sn 3d in detail, a highresolution scan has been performed after the Ar cleaning process. The resulting signal

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Fig. 3. (a) EDX mapping of all elements in Sn-doped ZnO-Zn2 SnO4 nanorod arrays and nanostructures. (b) EDX mapping of the individual elements.

Table 1. Elements and their corresponding atomic percentage detected during EDX mapping of Sn-doped ZnO-Zn2 SnO4 nanorod arrays Element

Atomic percentage (at%)

O

50.69

Zn

49.05

Sn

0.27

Total

100.00

can be found in Fig. 4b). In addition to the pronounced Zn LMM peaks, small steps in the spectrum can be seen in the range of 495 eV to 480 eV. These steps are near to the reference lines for Sn 3d3/2 (493 eV for metallic Sn) and Sn 3d5/2 (485 eV for metallic Sn) according to [2]. The relative shift of approximately 2 to 3 eV of the small peak at 486.5 eV from the respective metallic Sn 3d5/2 reference position can be associated with the possibility of oxidized Sn at the sample surface which would cause such a shift in binding energy according to previous paper [2]. However, the overall intensity of the related peak is relatively small and the doping concentration might be out of the range of detection for the XPS set-up used in our experiments.

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Fig. 4. a) XPS survey spectra of the heat-treated Sn-doped ZnO-Zn2 SnO4 nanorod arrays sample before and after a total of 270 min of Ar cleaning: The overview spectra indicate O, C and Zn to be present at the sample surface. The inset graph shows the Zn LMM Auger region before and after Ar cleaning at a higher resolution. A small increase of the intensity related to Sn at first glance can be seen after the Ar cleaning b) High-resolution scan of the Zn LMM signal with references of the Sn 3d according to [2].

3.3 XRD Characterization The crystal structure of Sn-doped ZnO-Zn2 SnO4 nanorod arrays grown by the hydrothermal method on glass substrate was investigated by X-ray diffraction (XRD) in the range of 20–80° 2θ values with a scanning step of 0.1°. XRD peaks have been attributed according to the card (PDF 00-036-1451) for ZnO peaks and (PDF 00-024-1470) for Zn2 SnO4 peaks. Two Zn2 SnO4 peaks have been clearly detected at 28° (220) and 72.4° (622) 2θ values, indicating the formation of Zn2 SnO4 due to the interaction of ZnO and Sn. No SnO2 peaks according to PDF 00-033-1374 were detected. From the analysis, high crystallinity of the investigated nanostructures was observed with good signal-to-noise ratios. The highest peak for the ZnO peaks was observed at 34.3° for the (0002) ZnO diffraction peak, which means that the nanorods are preferentially oriented along the c-axis [21] (Fig. 5). 3.4 Raman Characterization Fig. 6 represents a typical configuration for all contributions in this region for 2× higher Sn-doped ZnO-Zn2 SnO4 nanorod arrays on glass substrate. The general characteristics of the spectra are as follows. Peaks B1g (563 cm−1 ) and B2g (660 cm−1 ) correspond to tin oxide and E 2 (99, 315, 434, cm−1 ) to ZnO, here the shifting was observed. MicroRaman studies confirm the crystalline quality of obtained nanostructures, as observed in SEM and XRD results. The Raman band at 660 cm−1 matches the vibrational modes reported for the crystalline spinel structure of Zn2 SnO4 , similar results were reported by previous study [11].

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Fig. 5. XRD pattern of 2× higher Sn-doped ZnO-Zn2 SnO4 nanorod arrays grown by the hydrothermal method.

Fig. 6. a) Raman spectrum of Sn-doped ZnO-Zn2 SnO4 nanorod arrays and nanostructures. b) Raman mapping of the Sn-doped ZnO-Zn2 SnO4 nanorod arrays calculated from the ZnO peak at 434 cm−1 .

3.5 Gas Sensing Properties of the Sn-Doped ZnO-Zn2 SnO4 Nanorod Arrays and Nanostructures In this paragraph, the sensing properties of Sn-doped ZnO-Zn2 SnO4 nanorod arrays on glass substrate were investigated. The Keithley 2400 source meter was used as a measuring device, it possessed a very high precision, which was necessary to measure currents of the nano order. Connecting the source meter to the computer made it possible to collect data using the LabView multifunctional software (from National Instruments). Sensor sensitivity, response and recovery times were measured, the target gas being 100 ppm ethanol. The sensitivity (S) of the sensor was determined in percentage according to the following formula, where Ggas is the conductance during gas exposure, Gair is the

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conductance in the air: S=

Ggas − Gair ∗100% Gair

(1)

The measurements were carried out at different operating temperatures from 200 °C up to 350 °C, with the highest response value S = 130% obtained at 250 °C. For the response and recovery time, the best result was also obtained at the temperature of 250 °C, where the response time was ~1 s and the recovery time was about 39 s. Response and recovery time has been determined, as reported before in [20]. The response time was ~1 s and recovery time about 39 s. These results show that Sn-doped ZnO sensors have an improved response and recovery times compared to some results reported by other authors [7]. Gas testing was performed at bias voltage of 0.98 V, and the initial current was approximately 146 nA. The nanostructures also generated a response to CO2 S = 15% at a temperature of 350 °C, having a fairly good response and recovery time (for details see Supporting information). These results make possible the application of nanostructures as sensors for environmental monitoring, chemical safety control and many industrial applications. The Sn-doped ZnO-Zn2 SnO4 nanorods were also exposed to several gases, but the response was generated only for ethanol and carbon dioxide, which demonstrates a high selectivity of the nanostructures as sensors. CO2 being a gas of interest for monitoring in all fields (Figs. 7 and 8).

Fig. 7. Dynamic response to 100 ppm ethanol at an operating temperature of: (a) 200 °C, (b) 250 °C, (c) 300 °C and (d) 350 °C for Sn-doped ZnO-Zn2 SnO4 nanorod arrays on glass substrate.

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Fig. 8. Dynamic response to 100 ppm CO2 at an operating temperature of 350 °C for Sn-doped ZnO-Zn2 SnO4 nanorod arrays.

4 Conclusions In this paper, the morphological and sensing properties of Sn-doped ZnO-Zn2 SnO4 nanorod arrays obtained by the hydrothermal method are presented. A rapid and inexpensive hydrothermal approach was developed to growth large area of nanorod arrays on SiO2 /Si and glass substrates without seeds. SEM study shows that nanostructures are deposited uniformly on the surface of the substrate, and nanorods are with a welldefined hexagonal form. X-ray diffraction XRD analysis showed high crystallinity of the Sn-doped ZnO-Zn2 SnO4 nanorod arrays nanostructures with the highest peak at 34.3° for the (0002) ZnO peak, and formation of Zn2 SnO4 . The EDX and XPS analysis show the presence of Sn dopant with a low doping concentration of 0.27 at% and that Sn is uniformly distributed. The XPS analysis shows that the presence of Sn on the sample surface can be indicated by a high-resolution scan of the Zn LMM and Sn-3d region after 270 min of Argon cleaning. However, the overall intensity of the Sn-related signal is small and the Sn doping concentration could be too small to detect using our XPS set-up. Sensing results shows a fast response time of ~1 s and high gas response to 100 ppm ethanol ~130% at 250 °C. The presented results can be used for further improvement of the Sn-doped ZnO-Zn2 SnO4 nanorod arrays sensors for fast and reliable detection of dangerous gases, by optimizing the synthesis parameters. Following research on optimization of the growth regimes such as heating rate and time to control the aspect ratio of the nanorods is underway. Acknowledgments. The authors are grateful to the Kiel University, especially Prof. Adelung Rainer and Prof. Faupel Franz, and Technical University of Moldova, the Center for Nanotechnology and Nanosensors for support. This work was partially supported by the Technical University of Moldova and ANCD-NARD Grant No. 20.80009.5007.09 at UTM.

Conflict of Interest. The authors declare that they have no conflict of interest.

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References 1. Barsan, N., Schweizer-Berberich, M., Göpel, W.: Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report. Fresenius. J. Anal. Chem. 365, 287– 304 (1999). https://doi.org/10.1007/s002160051490 2. Shu, S., Wang, M., Yang, W., Liu, S.: Synthesis of surface layered hierarchical octahedral-like structured Zn2 SnO4 /SnO2 with excellent sensing properties toward HCHO. Sens. Actuators B: Chem. 243, 1171–1180 (2017). https://doi.org/10.1016/j.snb.2016.12.110 3. Choi, S.-H., et al.: Amorphous zinc stannate (Zn2 SnO4 ) nanofibers networks as photoelectrodes for organic dye-sensitized solar cells. Adv. Funct. Mater. 23, 3146–3155 (2013). https:// doi.org/10.1002/adfm.201203278 4. Mirzaei, A., Leonardi, S.G., Neri, G.: Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: a review. Ceram. Int. 42, 15119– 15141 (2016). https://doi.org/10.1016/j.ceramint.2016.06.145 5. Zhou, T., Liu, X., Zhang, R., Wang, Y., Zhang, T.: Shape control and selective decoration of Zn2 SnO4 nanostructures on 1D nanowires: boosting chemical-sensing performances. Sens. Actuators B: Chem. 290, 210–216 (2019). https://doi.org/10.1016/j.snb.2019.03.048 6. Sinha, S.K.: Synthesis of 1D Sn-doped ZnO hierarchical nanorods with enhanced gas sensing characteristics. Ceram. Int. 41, 13676–13684 (2015). https://doi.org/10.1016/j.ceramint.2015. 07.166 7. Lupan, O., et al.: Properties of a single SnO2 :Zn2 SnO4 – functionalized nanowire based nanosensor. Ceram. Int. 44, 4859–4867 (2018). https://doi.org/10.1016/j.ceramint.2017. 12.075 8. Wang, B., Zheng, Z.Q., Zhu, L.F., Yang, Y.H., Wu, H.Y.: Self-assembled and Pd decorated Zn2 SnO4 /ZnO wire-sheet shape nano-heterostructures networks hydrogen gas sensors. Sens. Actuators B: Chem. 195, 549–561 (2014). https://doi.org/10.1016/j.snb.2014.01.073 9. Wang, R.-C., Hung, J.-S.: ZnO–Sn:ZnO core-shell nanowires and ZnO–Zn2 SnO4 comb-like nanocomposites. J. Nanosci. Nanotechnol. 10, 5634–5640 (2010). https://doi.org/10.1166/ jnn.2010.2471 10. Pang, C., et al.: Synthesis, characterization and opto-electrical properties of ternary Zn2 SnO4 nanowires. Nanotechnology 21, 465706 (2010). https://doi.org/10.1088/0957-4484/21/46/ 465706 11. Postica, V., et al.: Multifunctional materials: a case study of the effects of metal doping on ZnO tetrapods with bismuth and tin oxides. Adv. Funct. Mater. 27, 1604676 (2017). https:// doi.org/10.1002/adfm.201604676 12. Yang, H.M., et al.: Synthesis of Zn2 SnO4 hollow spheres by a template route for highperformance acetone gas sensor. Sens. Actuators B: Chem. 245, 493–506 (2017). https://doi. org/10.1016/j.snb.2017.01.205 13. Yang, X., et al.: Highly efficient ethanol gas sensor based on hierarchical SnO2 /Zn2 SnO4 porous spheres. Sens. Actuators B: Chem. 282, 339–346 (2019). https://doi.org/10.1016/j. snb.2018.11.070 14. Zhou, C., et al.: High sensitivity and low detection limit of acetone sensor based on NiO/Zn2 SnO4 p-n heterojunction octahedrons. Sens. Actuators B: Chem. 339, 129912 (2021). https://doi.org/10.1016/j.snb.2021.129912 15. Lupan, O., et al.: Development of 2-in-1 sensors for the safety assessment of lithium-ion batteries via early detection of vapors produced by electrolyte solvents. ACS Appl. Mater. Interfaces (2023). https://doi.org/10.1021/acsami.3c03564 16. Brinza, M., et al.: Two-in-one sensor based on PV4D4-coated TiO2 films for food spoilage detection and as a breath marker for several diseases. Biosensors 13, 538 (2023). https://doi. org/10.3390/bios13050538

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Characterization of Films Prepared by Aerosol Spray Deposition in the (MgO)x (In2 O3 )(1−x) System Vadim Morari1(B)

, Daniela Rusu3 , Emil V. Rusu1 and Ion M. Tiginyanu2

, Veaceslav V. Ursaki2

,

1 Technical University of Moldova, D. Ghitu Institute of Electronic Engineering and

Nanotechnologies, Chisinau, Republic of Moldova [email protected] 2 National Center for Materials Study and Testing, Technical University of Moldova, Chisinau, Republic of Moldova 3 Petru Poni Institute of Macromolecular Chemistry, Romanian Academy, Iasi, Romania

Abstract. In this paper nanostructured thin films with thickness of 150 nm have been prepared by aerosol deposition method on p-Si in the system (MgO)x (In2 O3 )(1−x) with the composition range x = 0.2, 0.4 and 0.6, using indium chloride and magnesium chloride as precursors. The produced films were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) to determine the morphology and roughness, energy dispersive X-ray (EDX) analysis for the chemical composition estimation, and X-ray diffraction (XRD) for establishing the structural and crystallographic phases. It was found that the nano-crystallites sizes grow with increasing the Mg content, therefore influencing the roughness of the films. The film surface roughness calculated from topographic AFM images is in the RMS range from 5.7 to 7.5 nm with increasing Mg concentration, but the value of the Coefficient of Kurtosis parameter is from 0.18 to 0.64. The evolution of the crystalline phases content with increasing the x value from 0.2 to 0.6 was established. The electrical and photoelectrical properties were studied by I-V characterization under the illumination with the light with the wavelength of 365 nm. It was shown that the films are sensitive to this radiation with the ratio of the photocurrent to the dark current from 5 to 7 at the excitation density of 2.4 mW/cm−2 . Keywords: Aerosol spray deposition · In2 O3 cubic bixbyte · MgIn2 O4 cubic spinel · SEM · X-ray diffraction · I-V characteristics

1 Introduction Various systems of oxide semiconductors are used for a wide range of applications. Among them, the ZnO-MgO system was widely explored in view of its application, especially in the field of ultraviolet (UV) photodetectors [1], since the band gap of ZnMgO solid solutions covers a wide ultraviolet (UV) spectral region ranging from © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 52–59, 2024. https://doi.org/10.1007/978-3-031-42775-6_6

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the direct band gap of 3.36 eV in ZnO to that of 7.8 eV in MgO at room temperature, and due to their preparation by a variety of methods, such as magnetron sputtering [2], plasma-assisted and molecular beam epitaxy (RF-MBE) [3], spray pyrolysis [4], metalorganic chemical vapor deposition (MOCVD) [5], chemical bath deposition (CBD) [6], sol-gel spin coating [4, 7, 8], and other technologies. The (MgO)x (In2 O3 )(1−x) system is less investigated, in spite of the fact that indium oxide (In2 O3 ) have attracted a great deal of attention in the last decade for its potential use in various high-performance optoelectronic applications due to its high transparency, n-type conductivity, relatively low fabrication temperature [9], wide band gap (3.6 eV) [10] and low-cost process. The crystallographic structure of indium oxide is cubic bixbyite-type (Ia-3 space group). A cubic spinel MgIn2 O4 phase can be obtained if a Mg content is added to the oxide film. Ezhil Raj et al. obtained a single-phase ternary oxide by spray pyrolysis method, and revealed the octahedral and tetrahedral sites occupation by In3+ and Mg2+ ions in an inverse spinel phase [9]. The precursor solutions were prepared in the ratio Mg/In = 0.5 and sprayed on glass substrates at temperatures up to 450 °C with a stoichiometry of polycrystalline films. MgIn2 O4 thin films have been previously obtained by various techniques, such as co-sputtering deposition [11], RF-magnetron sputtering [12], pulse laser deposition [13, 14], combustion of aqueous solutions of metal nitrates [15], plasmaassisted molecular beam epitaxy [16] and MOCVD [17]. In this work we present results on the study of morphology, chemical composition, roughness, crystallographic structure and I-V characteristics of films prepared on silicon substrates by aerosol deposition method in the (MgO)x In2 O3(1−x) system with the composition range of x = 0.2, 0.4 and 0.6 Mg.

2 (MgO)x (In2 O3 )(1−x) Films Preparation (MgO)x (In2 O3 )(1−x) films were prepared by aerosol deposition method onto p-Si (100) substrates. The chemical solutions of 0.35 M indium chloride (InCl3 ) and 0.35 M magnesium chloride (MgCl2 ) dissolved in ethanol (C2 H5 OH) were mixed in an ultrasonic bath at a temperature of 50–60 °C for 30 min. Before starting the experiments, Si wafers were degreased at a temperature of 80 °C for 5 min in a solutions consisting of distilled water, hydrochloric acid (37%) and hydrogen peroxide in volume fractions of H2 O:HCl:H2 O2 = 8:1:1. Then they were washed in distilled water and dried at 100 °C in air atmosphere. Throughout the deposition process, the substrate temperature was maintained at 450– 480 °C. A solution injection rate of 1.25 mL/min was used and the deposition process last 10 min for each prepared sample. The solution was sprayed using a sprayer with an oxygen flow rate obtained at 0.1 Bar above the normal atmospheric pressure.

3 Study of Morphological and Chemical Composition The morphology and chemical composition of the prepared solid solutions films were studied by Verios G4 UC Scanning Electron Microscope (Thermo Fisher Scientific) equipped with an energy dispersive spectrometer (EDS, EDAX Octane Elite). Figure 1 shows the morphology of (MgO)x (In2 O3 )(1−x) films with the range of x compositions from 0.2 to 0.6. The thickness of the obtained films measured in the cross section of

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the sample was approximately 150 nm. From the SEM images one can conclude that a higher concentration of Mg leads to an increase in nanocrystallites size, contributing to the reduction of their number on the surface of the substrate.

Fig. 1. SEM top view images of (MgO)x (In2 O3 )(1−x) films with the composition range x a) 0.2; b) 0.4; c) 0.6 obtained by aerosol deposition method.

The chemical composition determined from EDX analysis of (MgO)x (In2 O3 )(1−x) films (Fig. 2) showed the presence of all the elements (In, Mg, O). As the concentration of Mg in the precursor solution increases, there is an increase in the Mg content in the film, while the In content decreases. INTEGRA Spectra Atomic Force Microscope (NT-MDT, Russia) with rectangular silicon cantilevers NSG10 was used for the determination of the roughness of the oxide films obtained. The measurements were realized at room temperature (300 K) in air, in tapping mode. For data processing a Nova v.1.98 software was utilized. Due to the fact that the morphology of the films changes considerably with the increase of Mg content in In2 O3 from the AFM images it can be observed how the crystallites change, at the same time their roughness also changes, which starts even at a Mg content of 0.2, where some increased crystallites are seen (Fig. 3). Different scan sizes were analyzed 5, 10 and 20 μm2 , but for our study, the size of 10 μm2 was chosen, their images are shown below. The AFM data are corroborated with SEM measurement results, which indicate an efficient incorporation of Mg atoms into the cubic lattice of indium oxide films. The film surface roughness calculated from topographic AFM images is in the RMS range from 5.7 to 7.5 nm with increasing Mg concentration, as shown in Table 1.

Fig. 2. Chemical composition determined from EDX analysis of (MgO)x (In2 O3 )(1−x) films with the composition range x a) 0.2; b) 0.4; c) 0.6 obtained by aerosol deposition method.

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Fig. 3. AFM images in 3D (10 × 10 μm) of (MgO)x (In2 O3 )(1−x) films with the composition range x a) 0.2; b) 0.4; c) 0.6 obtained by aerosol deposition method.

The surface skewness parameters also change from 0.16 to 0.32, but the coefficient of kurtosis changes the most from 0.18 to 0.64. These values signify the increase of the surface roughness of the films with the increase of the Mg concentration. Table 1. Roughness parameters of the obtained films. Composition range of Mg

Root Mean Square (RMS)

Surface skewness (Ssk)

Coefficient of kurtosis (Ska)

0.2

5.77006 nm

0.164393

0.181408

0.4

6.72214 nm

0.195087

0.234746

0.6

7.50108 nm

0.326644

0.647469

4 Study of X-ray Diffraction Pattern The analysis of structural properties was performed with a Rigaku Miniflex 600 Benchtop Diffractometer (Japan) with CuKα radiation (λ = 1.5406 Å), in the range of 20–80° with a step of 0.01° and a recording speed of 2 rpm, using “Zero background” sample holder. At a low content of Mg, the XRD pattern of films is dominated by the (222), (400), (431), (440) and (622) reflexes from the In2 O3 cubic bixbyte phase with the Ia-3 space group, according to the JCPDS#06-0416 card [18]. The (311), (511) and (442) reflexes from the MgIn2 O4 spinel cubic phase with the space group Fd-3m start to emerge in the XRD pattern at the x value of 0.2, according to the JCPDS#73-2414 card [9, 19]. MgIn2 O4 is an inverse spinel, according to the lattice substitution of Mg and In cations in the tetrahedral and octahedral voids in the anion sublattice. The general formula for an AB2 O4 spinel is (A1−y By )[Ay B2−y ]O4 , where cations in the square brackets occupy the octahedral sites and cations in the parentheses occupy the tetrahedral sites. A normal spine corresponds to the y value of 0, while y value of 1 corresponds to the inverse spinel structure [9, 20]. With increasing the Mg content in the prepared films, the intensity of reflexes from the MgIn2 O4 phase increases, while that from the In2 O3 phase decreases. So, that at the x value of 0.6 the XRD pattern is dominated by the (311) reflex from the MgIn2 O4 phase, while the reflexes from the In2 O3 phase are still present in the pattern. Apart

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from that, (111) and (200) reflexes from the MgO phase are observed in the pattern for this x-value, indicating the presence of MgO crystallites in the film. The fact that other reflexes from the MgIn2 O4 phase, except for the (311) reflex, are absent in the pattern, is an indicative of the oriented growth of the MgIn2 O4 crystallites (Fig. 4).

Fig. 4. XRD diffraction of (MgO)x (In2 O3 )(1−x) films with the composition range x a) 0.2; b) 0.4; c) 0.6 obtained by aerosol deposition method.

5 Study of Photosensitivity of Films The photosensitivity of the prepared films has been investigated with Ag coplanar contacts deposited on the surface. The volt-ampere (I-V) characteristic measured at applied voltage of −3 to +3 V in the dark and under irradiation with UV light from a LED with 365 nm wavelength are shown in Fig. 5. The optical power of 2.4 mW was focused on a spot with an area of 1.0 cm2 , resulting in an irradiation power density of approximately 2.4 mW/cm−2 . The I-V characteristics suggest that the contacts are nearly ohmic. Nevertheless, the ratio of the direct current to the reverse current at the bias of 2 V under illumination is around 1.5 for the x value of 0.2, and it is around 1.0 for the x value of 0.6. The current through the films decreases by an order of magnitude with increasing the x value from 0.2 to 0.6, therefore indicating the increase of their resistivity with increasing the Mg content in films. At the same time the ratio of the photocurrent to the dark current (Iphoto /Idark ) does not change significantly, it being equal to 7 for the x value of 0.2 and to 5 for the x value of 0.6 at the applied bias of 2 V.

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Fig. 5. I-V characteristics of (MgO)x (In2 O3 )(1−x) films with the composition range x a) 0.2; b) 0.4; c) 0.6 obtained by aerosol deposition method.

6 Conclusions The results of this study demonstrate that nanocrystalline composite films are produced by the aerosol deposition method in the (MgO)x (In2 O3 )(1−x) system. At low content of Mg (x = 0.2) the films are mainly formed from In2 O3 cubic bixbyte phase crystallites with a small content of MgIn2 O4 spinel cubic phase crystallites, while at high content of Mg (x = 0.6) the films are by the contrary mainly formed from with a small content of MgIn2 O4 spinel cubic phase crystallites with a small content of In2 O3 cubic bixbyte phase crystallites and some of rock salt MgO phase. The SEM and AFM analysis suggest that the nano-crystallites sizes grow with increasing the Mg content, therefore influencing the roughness of the films. The electrical and photoelectrical characterization demonstrate that the produced films are suitable for UV sensor applications. Particularly, under the UV radiation with the wavelength of 365 nm and excitation power density of 2.4 mW/cm−2 the ratio of the photocurrent to the dark current varies from 5 to 7 for films with x value from 0.6 to 0.2. Acknowledgments. This work was supported financially by the ANCD through grant no. #20.80009.5007.02.

Conflict of Interest. The authors declare that they have no conflict of interest.

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References 1. Jia-Lin, Y., Ke-Wei, L., De-Zhen, S.: Recent progress of ZnMgO ultraviolet photodetector. Chin. Phys. B 26(4), 47308 (2017). https://doi.org/10.1088/1674-1056/26/4/047308 2. Jr-Shiang, S., Sanjaya, B., Chuan-Pu, L., Jow-Lay, H.: Ultraviolet photodetectors based on MgZnO thin film grown by RF magnetron sputtering. Thin Solid Films 620, 170–174 (2016). https://doi.org/10.1016/j.tsf.2016.09.037 3. Hierro A., et al.: ZnMgO-based UV photodiodes: a comparison of films grown by spray pyrolysis and MBE. In: Oxide-Based Materials and Devices VII, vol. 9749, pp. 70–75. SPIE, Bellingham, WA, USA (2016). https://doi.org/10.1117/12.2213697 4. Morari, V., Ursaki, V.V., Rusu, E.V., Zalamai, V.V., Colpo, P., Tiginyanu, I.M.: Spin-coating and aerosol spray pyrolysis processed Zn1−x Mgx O films for UV detector applications. Nanomaterials 12(18), 3209 (2022). https://doi.org/10.3390/nano12183209 5. Wang, L.K., et al.: Single-crystalline cubic MgZnO films and their application in deepultraviolet optoelectronic devices. Appl. Phys. Lett. 95, 131113 (2009). https://doi.org/10. 1063/1.3238571 6. Chawla, S., Jayanthi, K., Chander, H.: Enhancement of luminescence in ZnMgO thin-film nanophosphors and application for white light generation. Phys. Stat. Sol. A 205(2), 271–274 (2008). https://doi.org/10.1002/pssa.200723149 7. Tsay C.Y., Chen, S.T., Fan, M.T.: Solution-processed Mg-substituted ZnO thin films for metal-semiconductor-metal visible-blind photodetectors. Coatings 9(4), 277 (2019). https:// doi.org/10.3390/coatings9040277 8. Morari, V., et al.: Band tail state related photoluminescence and photoresponse of ZnMgO solid solution nanostructured films. Beilstein J. Nanotechnol. 12(11), 899–910 (2020). https:// doi.org/10.3762/bjnano.11.75 9. Moses Ezhil, R.A., Selvan, G., Ravidhas, C., Jayachandran, M., Sanjeeviraja, C.: Magnesium indium oxide (MgIn2 O4 ) spinel thin films: chemical spray pyrolysis (CSP) growth and materials characterizations. J. Colloid Interface Sci. 328(2), 396–401 (2008). https://doi.org/10. 1016/j.jcis.2008.08.052 10. Morari, V., Zalamai, V., Rusu, E.V., Ursaki, V.V., Colpo, P., Tiginyanu, I.M.: Study of (In1−x Gax )2 O3 thin films produced by aerosol deposition method. Advanced Topics in Optoelectronics, Microelectronics, and Nanotechnologies XI. In: Proceedings of SPIE, vol. 12493, pp. 324–330 (2023). https://doi.org/10.1117/12.2642127 11. Chih-Chiang, Y., Kuan-Yu, C., Wei-Sheng, Y., Yan-Kuin, S., Zi-Hao, W.: Ultraviolet photodetection application in magnesium indium oxide thin film transistors via co-sputtering deposition. Appl. Sci. 10(15), 5128 (2020). https://doi.org/10.3390/app10155128 12. Xiaobin, B., Jianke, Y., Shengdong, Z.: Magnesium-doped indium oxide thin film transistors for ultraviolet detection. In: IEEE International Conference on Electron Devices and Solid State Circuits, 14986075 (2014). https://doi.org/10.1109/EDSSC.2014.7061093 13. Florian, S., Daniel, S., Stefan, M., von Holger, W., Marius, G.: Electronic defects in In2 O3 and In2 O3 : Mg thin films on r-plane sapphire. Phys. Stat. Sol. B 252(10), 2304–2308 (2015). https://doi.org/10.1002/pssb.201552328 14. Tarsa, E.J., English, J.H., Speck, J.S.: Pulsed laser deposition of oriented ln2 O3 on (001) InAs, MgO, and yttria-stabilized zirconia. Appl. Phys. Lett. 62(2332) (1993). https://doi.org/ 10.1063/1.109408 15. Suzy, S., Dominique, G., Mickaël, D., Gianguido, B., Stéphane, U., David, S.: Rapidly synthesis of nanocrystalline MgIn2 O4 spinel using combustion and solidvstate chemistry. Solid State Sci. 13, 42–48 (2011). https://doi.org/10.1016/j.solidstatesciences.2010.10.004 16. Oliver, B., James, S.S.: Mg acceptor doping of In2 O3 and overcompensation by oxygen vacancies. Appl. Phys. Lett. 101, 102107 (2012). https://doi.org/10.1063/1.4751854

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17. Jun, N., Lian, W., Yu, Y., He, Y., Shu, J., Tobin, J.M.: Charge transport and optical properties of MOCVD-derived highly transparent and conductive Mg and Sn doped In2 O3 thin films. Inorg. Chem. 44(17), 6071–6076 (2005). https://doi.org/10.1021/ic0501364 18. Prasad, K.H., et al.: Structural, magnetic and gas sensing activity of pure and Cr doped In2 O3 thin films grown by pulsed laser deposition. Coatings 11(5), 558 (2021). https://doi.org/10. 3390/coatings11050588 19. Moses, E.R.A., Subramanian, B., Senthilkumar, V., Jayachandran, M., Sanjeeviraja, C.: Synthesis and characterization of spray pyrolysed MgIn2 O4 spinel thin films for novel applications. Physica E 40(3), 467–473 (2008). https://doi.org/10.1016/j.physe.2007.07.004 20. Wei, S.H., Zhang, S.B.: First-principles study of cation distribution in eighteen closed-shell AII BIII 2 O4 and AI VBI I2 O4 spinel oxides. Phys. Rev. B 63, 045112 (2001). https://doi.org/ 10.1103/PhysRevB.63.045112

Nanocomposite Films Based on Photosensitive Azopolymer with Gold Nanoparticles: Synthesis, Film Deposition, Diffractive Optical Elements Recording and Characterization Elena Achimova1 , Vladimir Abashkin1(B) , Alexei Meshalkin1 , Constantin Losmanschii1 , Vladislav Botnari1 , and Giancarlo Pedrini2 1 Institute of Applied Physics, Moldova State University, Chisin˘au, Republic of Moldova

[email protected] 2 Institut Für Technische Optik, Universität Stuttgart, Stuttgart, Germany

Abstract. In the present study, the photosensitive nanocomposite was fabricated based on azopolymer and Au nanoparticles (Azo-Au NPs). For the first time, the synthesized polymeric poly-N-(2,3-epoxypropyl) carbazole with azo dye SY 3 (PEPC-co-SY 3) was the basis of the nanocomposite. As a medium for polarization holographic recording, thin films of a nanocomposite a number of concentrations deposited on a glass slide by the rod coating method were studied. Diffraction gratings were recorded on films by direct and single-stage polarization holography. For recording, the polarization states of the beams were P-P, S-S, and ± 45 º and left-right circular. In nanocomposites, the optical path of the beam is defined by the summary changes in surface topography and refractive index. The periodically modulated polarization/amplitude interference patterns produced by the gratings were investigated by in situ measurements of the diffraction efficiency (DE) kinetics in the first diffraction order when the DE saturation value was reached. A maximum DE value of 35% was obtained for the nanocomposite PEPC-co-SY3 with 0.0006mg/ml of Au NPs. The gratings were studied using a polarization-sensitive digital holographic microscope to reveal their optical phase features using the full-field method. The surface relief was measured by AFM. A comparison of the behavior of azopolymer films during the recording of surface relief gratings with and without Au NPs was carried out. The results of diffraction gratings recording by the polarization holography method are presented, confirming the possibility of recording not only amplitude and phase of light, as in scalar holography, but also polarization states.

1 Introduction Inorganic/organic nanocomposite materials have been extensively studied in the past few years because they extend the range of possible applications [1]. Different types of materials can be combined to create a new material that not only inherits the properties of each of the constituent materials, but can also exhibit new properties depending on the chemical or physical interaction between them. Nanocomposites can be generated not only to improve their optically induced properties, but also to produce new functionalities. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 60–69, 2024. https://doi.org/10.1007/978-3-031-42775-6_7

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Azopolymers are organic polymers with a photosensitive response, and under an appropriate illumination they produce macroscopic and microscopic changes, in addition, they meet the polarization sensitive requirements [2]. The responsiveness of azopolymers to light is now known to be due to photon-triggered isomerization between the two isomers of azodye molecules. Photo-isomerization is a highly efficient and chemically completely reversible process between the two states: the thermally stable trans form and the metastable cis form. The metastability of the cis isomer refers to its ability to trans form back to the trans state thermally. The steady-state composition is unique to each system, and is dependent on the quantum yields for the two isomerization processes and the thermal relaxation, and the radiating light can enhance both quantum yields, leading to rapid cycling between the isomerization states. Cycling plays a significant role in photo-induced mass transport. Azopolymer films develop surface reliefs under irradiation with ultraviolet violet/visible light. The phenomenon is described in terms of a light-driven macroscopic mass transport of the polymer chains, triggered by the microscopic photo-isomerization dynamics of the azodye molecules embedded into the polymeric matrix. The mass migration also takes place only in illuminated areas of the surface of the azopolymer, and it is highly directional, with a very peculiar sensitivity to the polarization and intensity distributions of the irradiating light field. Normally, mass migrates from brightly illuminated areas to dark ones with a velocity that is proportional to the gradient of intensity. This phenomenon is typically studied by illuminating the material with an interference pattern to induce a surface relief grating (SRG) and simultaneously measuring the diffraction efficiency with another laser at a wavelength that doesn’t produce interactions with the azopolymers [3]. In order for the azobenzene to perform the fast movements that occur during photoisomerization and the reorientation of the transition dipole moment, a certain free volume in the polymer matrix should be available. Merkel showed that nanoparticles (NPs) with diameters smaller than 50nm can act as nanospacers to prevent the polymer chain from packing close [4], increasing the number and size of the free volume elements, and consequently allowing higher and faster transport through the polymer. Different functional composite materials were fabricated in the past few years by combining azo derivatives with inorganics [5], including noble metal (Ag or Au) composites, and these composites showed novel properties. NPs make a significant applied contribution to the general optical features of azopolymer films. Promising devices can be structures in the form of diffractive optical elements fabricated on nanocomposites films. The main purpose of this work consists of the study of new photosensitive NPsdoped azopolymers for one step relief formation of gratings by using the polarization holographic recording method. The current study aims to prepare and characterize thin films of a nanocomposite based on azopolymer PEPC-co- Solvent Yellow 3 (SY 3) and Au NPs and compare the optical properties of the nanocomposite with those of pure azopolymer. The interference light patterns with periodically modulated polarization/amplitude induce surface relief formation, as investigated by in-situ DE measurement. We studied the photosensitive properties of the PEPC-co-DO with and without Au NPs in dependence on the polarization states of the recording beams. The study of

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relief gratings obtained with a digital polarization holographic microscope and an atom force microscope was carried out.

2 Materials and Methods 2.1 Azopolymer Synthesis and Thin Film of Nanocomposites Based on Azopolymer with Au NPs Preparation In the present study, a nanocomposite based on an azopolymer and gold nanoparticles was fabricated. The photosensitive azopolymer was synthesized by the polymerization of poly-N-(2,3-epoxypropyl) carbazole (PEPC) with the azodye SY 3 chromophore. A SY 3 (4’-amino-2,3’-dimethylazobenzene) with a dye content of 90% is manufactured by Sigma-Aldrich Company, without further purification. Azopolymers were obtained by dissolving 1.0 g of PEPC and 0.3 g of SY 3 in 8 ml of toluene and refluxing for 8 h at 220 °C. The reaction of the azodye attachment to the PEPC polymer matrix was carried out through the epoxy group, which was opened by the primary amine of the azodye. After the end of the reaction, the azopolymer solution was cooled down to room temperature and passed through cellulose filter paper (0.015% ash nominal). This final operation was carried out to separate the possible and unwanted unpolymerized crystalline particles of azodye. A polymerization reaction was utilized for the chemical bonding of SY 3 chromophore to PEPC polymer. A chemical structure of PEPC-co-SY3 is published in [6]. A basic solution of gold nanoparticles (spheres with a diameter of around 25 nm from Sigma-Aldrich®) was obtained by dispersing 0.1 mg Au NPs in 2 ml toluene. Three solutions of Au NPs were prepared by dissolving 0.25 ml of the basic solution in 1, 2, and 3 ml of toluene. Each prepared solution consisted of dissolving 0.25 ml in 2 ml of PEPC-co-SY 3 azopolymer. After this, three azopolymer solutions with different concentrations of Au NPs were obtained. Fabrication of the nanocomposites thin films from solutions was done via rod-coating (using a programmable rod-coater made by ourselves in the lab). The rod is fixed, and the glass substrate on the platform moves relative to the rod. A smoothly rotating stepper motor mechanically connected to the platform and smoothly moving it at a constant speed, which is set according to the viscosity of the solution. This method provides films that are uniform in thickness, which is determined by a fixed distance between the rod and the glass substrate. The micrometer screw ensures the deposition of thin films with a thickness, controlled to tenths of a micrometer. Drying for 12 h at room temperature for all sample thicknesses was done. The film substrates were chosen as microscope standard slides from soda lime glass (refractive index nD = 1.52). 2.2 Transmittance Transmittance spectra of polymer films were recorded on the Specord UV/VIS twobeam spectrophotometer at room temperature in the spectral wavelength range from 350 to 800 nm. In the reference beam of the spectrophotometer, the glass substrate was

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mounted to compensate for substrate influence. The comparative spectra of Azo-Au NPs films with a number of concentrations of gold nanoparticles do not exhibit peaks at these wavelengths. The transmittance spectrum for 30 wt% concentrations of azodye has a minimum absorption of about 347 nm, which is attributed to the electron transition of transisomers. The minimum of absorption at 359 nm may be referred to as the cis-transition of azopolymers. Note that the azodye concentration does not strongly affect the absorption spectrum in the wavelength range of 500–650 nm [6] (Fig. 1). Sample thicknesses prepared for our experiments were: PEPC-co-SY 3 (30 wt%): 760 nm, PEPC-co-SY 3 (0.001 mg/ml Au NPs): 740 nm, PEPC-co-SY 3 (0.0006 mg/ml Au NPs): 850 nm, PEPC-co-SY 3 (0.0004 mg/ml Au NPs): 560 nm.

Fig. 1. Transmittance spectra of nanocomposite based on Au NPs and azopolymer.

2.3 Grating Recording Process with Polarization Holography The thin film of obtained nanocomposites, Azo-Au NPs, was used to perform the investigation of direct diffraction grating recordings without wet development. The experimental geometries for holographic recording and controls are reported in Figs. 2a and 2b. A DPSS single traverse mode laser as a pump beam at wavelength 473 nm (power 75 mW) was used for polarizing holographic recording in photosensitive azopolymer thin films. The grating period in all grating recordings was installed at about  = 7 µm. Some polarization states of beams were used for direct relief recording, such as P-P, S-S, and ± 45 º and left-right circular. The transmittance spectra of nanocomposite based on Au NPs and azopolymer have shown that for probe beams, a red laser diode may be applied. A continuous probe laser diode beam (wavelength 640 nm and power 1 mW) was passed through the sample grating and fell onto two photodiodes. They were mounted in 0 and 1st diffraction orders,

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and the time dependences of the photocurrent were registered on a computer. The probe beam of the laser diode was purified in the polarization sense by a Glan-Thompson polarizer with a high extinction rate: 100 000:1. The Soleil-Babinet compensator is a continuously variable, zero-order retarder. In our setup, it was applied as a wave plate to create a horizontal azimuth state of polarization and minimum ellipticity. The pump and probe beams intersect at the spot of recording. Optical set-ups are shown separately for clarity. The polarimeter was always used for pre-adjusting the polarization states of two interfering beams because the adjustment is critical for accurate holographic recording experiments. The software’s “Alignment Reading Indicator” option on the PAX1000VIS/M polarimeter helps greatly with this important adjustment step to eliminate artifacts. As recording diffraction efficiency was done without switching off the pump laser, it was very important to eliminate its influence on photodiodes. A dichroic mirror for the highwavelength region of the transmission spectrum was used to prevent the diodes readings from being affected by the direct and scattered light of the pump laser beam. This component (cold mirror FM04, Thorlabs, not shown on optical set-ups) was set at an angle of incidence of 45° with a diode laser beam in front of the diodes. Optical transmission of the wavelength 640 nm by the mirror is more than 85%, and reflection is more than 90% for the wavelength 473 nm. The optical phase properties of recorded surface gratings on thin films were studied on a microscope in transmission mode. Microscope resolution with a dry objective of 40x and a numerical aperture of NA = 40 is approximately 1 µm. In this work, we used the polarization digital holographic microscope based on the modified Mach-Zehnder interferometer with a liquid crystal variable retarder for producing all-optical 4 phaseshifts in the reference beam. A 3 mW red He-Ne linearly polarized output beam laser was applied. The microscope operates in phase-shifting mode with the LabVIEW™ software, which allows acquiring four hologram sequences from the camera with respect to four phase shifts as well as controlling the liquid crystal retarder. From the four recorded holograms, using the hybrid reconstruction algorithm and least squares unwrapping of MATLAB code, the optical phase distribution on the gratings was extracted.

3 Results and Discussion Surface relief diffraction gratings were recorded directly by polarization holography on the obtained nanocomposite Azo-Au NPs thin films. The experimental data refer to gratings recorded with a density value of laser energy of 200 mW/cm2 and an exposure time of 25 min. For all gratings, the diffraction efficiency (DE) in the 1st diffraction order was recorded when saturation was reached, the time of which was 25 min. The obtained kinetics for four concentrations of Au NPs allow comparison of the diffraction properties of gratings in dependence on polarization states (see Fig. 3). As a rule, about ±6 diffraction orders were observed for all our samples and compositions. The scattered light from the polymer film was blocked by iris diaphragms in front of photodiodes. From Fig. 3(a, b, c, and d), one can see that for all linear polarizations of recording beams (P-P, S-S, ±45º) the DE of azopolymer Azo-Au NPs exceeds the DE of pure azopolymer. The maximum DE of 35% was obtained on a nanocomposite with

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Fig. 2a. Optical set-up for diffraction elements recordings: DPSS single traverse mode laser (wavelength of 474 nm, power 100 mW); HWPhalf waveplate; PBS-polarizing beam splitter; M-mirrors; QWP-quarter wave plate; Sample-nanocompozite polymer film deposited on microscope slide with recording grating.

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Fig. 2b. Optical set-up for diffraction efficiency control during recording of elements. Optical set-ups are shown separately for clarity. Laser diode (wavelength of 640 nm, power 10 mW, single mode), Glan-Thompson polarizer, Soleil-Babinet compensator SBC-VIS and two photodiodes.

0.0006 mg/ml Au NPs at P-P polarizations. But for left-right circular polarizations, the DE of pure azopolymer is greater than the DE of nanocomposite. This fact may be explained by the formation of diffraction gratings with different polarization distributions for different states of polarization of recording beams. The linear polarizations form the linear polarization in the grating, and the formed polarization has a constant direction varying only in value. With left-right circular polarizations, a complex polarization structure is created in the recorded grating, so the interaction between the electric fields of the grating and nanoparticles is different in different places on the sample surface. The reported results show that the polymeric film undergoes a light-guided structural modification after illumination with a proper polarized light pattern, including linear, orthogonal, and circular polarizations. The study of relief gratings obtained with a digital polarization holographic microscope (DHM) was carried out. DHM’s results show only optical phase presence on the surface of films, not direct surface relief. The optical phase may consist of the surface relief and photoinduced anisotropy in the volume of the film. In Fig. 4, the profiles of phase grating with 0.0006 mg/ml Au NPs and without nanoparticles are presented. Comparing the presented results (see Fig. 4), it is seen that samples with nanoparticles have a deeper phase profile than pure azopolymer, but the noisiness of the profile is high for

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Fig. 3. Diffraction efficiencies of gratings in the 1st diffraction order vs. recording time for four polarization states ((a) P-P, (b) S-S, (c) ± 45º, (d) left-right circular polarizations) and for four chemical compositions. Chemical compositions presented on the figures are: PEPC-co-SY3 (30 wt%) -, PEPC-co-SY3 (0.001 mg/ml Au NPs)- ˛, PEPC-co-SY3 (0.0006 mg/ml Au NPs)- ▲, PEPC-co-SY3 (0.0004 mg/ml Au NPs)- ●.

nanocomposite. Note that the DHM measures the total phase of a transparent sample in transmission mode, i.e., the bulk and surface phases. The image of the surface sample in white light presented in Fig. 4c proves the complex structure of the surface relief grating. The study of SRG recorded by left-right circular polarizations of interfering beams demonstrates that these gratings have a very smooth profile, but the amplitude of this profile is smaller than for gratings registered at other states of polarization (see Fig. 5). The AFM method allows us to measure only the surface relief of the grating, excluding the volume contribution. Figure 6 shows the AFM measurements of SRG recorded on a thin film of PEPC-co-SY3 with 0.0006 mg/ml Au NPs at P-P polarization states of recording beams. It is clear that Au NPs addition to polymer leads to deep surface profile of the grating (see Fig. 6). The grating recorded on a nanocomposite with Au nanoparticles has a phase relief more than three times deeper than on a pure azopolymer. Diffraction efficiency for nanocomposite based on azopolymer with Au NPs at S-S states of polarization is 30%

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Fig. 4. DHM measurements of surface relief grating for PEPC-co-SY3 with 0.0006 mg/ml Au NPs and without nanoparticles at P-P polarization states of recording beams: (a) the phase profile of PEPC-co-SY3; Au NPs; (b) the phase profile of PEPC-co-SY3; (c) the surface of the sample in white light of an LED; and (d) the 3D optical phase image of the grating.

Fig. 5. DHM measurements of the surface relief grating for PEPC-co-SY3 at left-right circular polarization states of the recording beam: (a) profile perpendicular to fringes; (b) 3D optical phase surface.

and for P-P polarizations is 35%. From AFM measurements it is clear that these grating form on the surface of samples.

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Fig. 6. AFM measurements of the surface relief grating for PEPC-co-SY3 with 0.0006 mg/ml Au NPs at P-P polarization states of recording beams: (a) 3D surface; (b) profile perpendicular to fringes.

4 Conclusions We report the experimental investigation of surface relief gratings recorded on photosensitive nanocomposite Azo-Au NPs thin films by polarization holographic recording. The polarization states of beams P-P, S-S, ±45º and left-right circular were used for direct surface relief recording. The kinetics of diffraction efficiencies of gratings for all polarization states were studied. The reported results show that PEPC-co-SY3 with 0.0006 mg/ml of Au NPs shows maximum diffraction efficiency of 35% for P-P polarizations of the recording beams, and for S-S and ±45º polarizations of the recording beams, nanocomposites exhibit increasing diffraction efficiency in comparison with azopolymer without Au NPs. The exception is gratings recorded by left-right circular polarizations of beams; the diffraction efficiency of pure azopolymers is greater than the diffraction efficiency of nanocomposites. These results were confirmed by grating measurements with digital holographic and atomic force microscopes. Possible explanations lie in the features of the polarized surface relief gratings, which create different conditions for mass transfer in materials with various contents of Au NPs. Summarizing the obtained result, we can conclude that the nanocomposite polymer Azo-Au NPs are promising media for polarization holographic recording, which requires further investigations. Acknowledgments. The work was partly supported by the project of National Agency for Research and Development of Moldova (ANCD 20.80009.5007.03) and International Project of Program ERA.Net Rus Plus (ANCD 21.80013.5007.1M). Conflict of Interest.. The authors declare that they have no conflict of interest.

References 1. Darwish, M.S., Mostafa, M.H., Al-Harbi, L.M.: Polymeric nano-composites for environmental and industrial applications. Int. J. Mol. Sci. 23(3), 1023 (2022). https://doi.org/10.3390/ ijms23031023

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2. Sekkat, Z.: Vectorial motion of matter induced by light fueled molecular machines. OSA Continuum 1(2), 668–681 (2018). https://doi.org/10.1364/OSAC.1.000668 3. Nikolova, L., Ramanaujam, P.S.: Polarization Holography. Cambridge University, Press (2009) 4. Aleksejeva, J., Reinfelde, M., Teteris, J.: Direct surface relief pattering of azo-polymers films via holographic recording. Can. J. Phys. 92(7/8), 842–844 (2014). https://doi.org/10.1139/ cjp-2013-0598 5. Pagliusi, P., Audia, B., Provenzano, C., Piñol, M., Oriol, L., Cipparrone, G.: Tunable surface patterning of azopolymer by vectorial holography: the role of photoanisotropies in the driving force. ACS Appl. Mater. Interfaces 11(37), 34471–34502 (2019). https://doi.org/10.1021/acs ami.9b12624 6. Merkel, T., et al.: Ul-trapermeable, reverse-selective nanocomposite membranes. Science 296(5567), 519–522 (2002). https://doi.org/10.1126/science.1069580 7. Zhou, J., et al.: Effect of silver nanoparticles on photo-induced reorientation of azo groups in polymer films. Thin Solid Films 515(18), 7242–7246 (2007). https://doi.org/18.7242-7246 8. Hammami, I., Alabdallah, N.M., Jomaa, A.A., Kamoun, M.: Gold nanoparticles: synthesis properties and applications. J. King Saud Univ. Sci. 33(7), 101560 (2021). https://doi.org/10. 1016/j.jksus.2021.101560 9. Na, S-K., et al.: Efficient formation of surface relief grating on azo-polymer films by gold nanoparticles. J. Appl. Phys. 104(10), 103117–1–103117–5 (2008). https://doi.org/10.1063/ 1.3031278 10. Losmanschii, C., Achimova, E., Abashkin, V., Botnari, V., Meshalkin, A.: Photoinduced anisotropy in azopolymer studied by spectroscopic and polarimetric parameters. In: Tiginyanu, I., Sontea, V., Railean, S. (eds) 5th International Conference on Nanotechnologies and Biomedical Engineering. ICNBME 2021. IFMBE Proceedings, vol. 87, pp. 314–321. Springer, Cham (2021).https://doi.org/10.1007/978-3-030-92328-0_42

MOF-Coated 3D-Printed ZnO Tetrapods as a Two-in-One Sensor for H2 Sensing and UV Detection Barnika Chakraborty1 , Philipp Schadte1 , Mirjam P. M. Poschmann3 , Cristian Lupan2 , Tudor Zadorojneac2 , Nicolae Magariu2 , Ajay Padunnappattu3 , Fabian Schütt1 , Oleg Lupan1,2 , Leonard Siebert1(B) Norbert Stock3 , and Rainer Adelung1

,

1 Department of Materials Science, Chair for Functional Nanomaterials, Faculty of

Engineering, Kiel University, Kiel, Germany [email protected] 2 Department of Microelectronics and Biomedical Engineering, Center for Nanotechnology and Nanosensors, Technical University of Moldova, Chisinau, Republic of Moldova 3 Institute for Inorganic Chemistry, Kiel University, Kiel, Germany

Abstract. As the world rapidly transitions towards renewable energy sources, the use of hydrogen (H2 ) as a green energy carrier has become increasingly important. The various applications of hydrogen in the energy sector require sensor materials that can efficiently detect small amounts of H2 in gas mixtures. One solution is the use of a Metal-organic Framework (MOF)-functionalized oxide gas sensor, specifically a MOF-functionalized ZnO sensor. The sensor is composed of tetrapodal ZnO microparticles coated with a thin layer of MOF, which results in a core@shell composite structure. Prior to the conversion to MOF, these microparticles are 3D printed to create macroscopic sensor circuitry. The sensor demonstrated selectivity and sensitivity to 100 ppm H2 in air at an operating temperature of 250 °C. The sensor is based on crystalline t-ZnO as a core which is partially converted to ZIF-8 (zinc dimethylimidazolate, Zn(MeIM)2 ). MOF are a class of porous materials composed of metal ions or clusters connected by organic ligands. They have a high surface area and can be tailored to exhibit specific properties, such as selective adsorption of gases. The sensor also reliably detected H2 gas in air and is selective versus methane, acetone, butanol, and propanol. Such a selectivity is important for determining the H2 dilution level in natural gas pipelines. Analysis was performed using X-ray diffraction, SEM, UV radiation, and gas sensing measurements. This innovative two-in-one sensor for UV radiation and H2 gas has significant implications for the energy sector’s transition to renewable energy sources. Keywords: Metal Organic Framework (MOF) · t-ZnO · H2 sensing · Nanomaterials · Zeolithimidazole Framework-8 (ZIF-8)

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 70–79, 2024. https://doi.org/10.1007/978-3-031-42775-6_8

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1 Introduction With the ongoing shift towards renewable, zero-carbon energy sources and the decreasing reliance on fossil fuels, there is a growing interest in green H2 as a replacement for natural gas. To utilize the existing infrastructure, up to 20% H2 may be fed into natural gas pipelines, but safety concerns like leaks must be considered. Therefore, there is a need for tiny and selective solid-state devices to be deployed along pipelines and constantly check the concentration of H2 . Metal oxide materials like ZnO are commonly known as metal oxide semiconductors (MOS) and are promising for gas sensing [1, 2], but they lack selectivity. They are also easily fabricable, stable with increasing heat, chemically resistant and budget friendly [3]. ZnO can be synthesis in different morphologies and so can its morphological versatility have been explored in previous work [4]. Metal-organic Frameworks (MOFs), especially ZIF-8, are important for gas sensing due to their characteristics, allowing them to act as molecular sieves [1, 2]. ZIF-8, which stands for Zeolitic Imidazolate Framework-8, is a type of metal-organic framework (MOF) that has gained significant attention in recent years due to its unique structural properties and potential applications. MOFs are a class of porous materials composed of metal ions or clusters connected by organic linkers, resulting in a highly ordered and porous three-dimensional structure. ZIF-8, in particular, is known for its exceptional stability, high surface area, and tunable pore size. MOF are a versatile class of materials with tremendous potential for a wide range of applications. The ability to tailor their properties at the molecular level makes them ideal for solving complex challenges in fields such as energy, environmental remediation, sensing and healthcare [5, 6]. The structure of ZIF-8 is derived from the zeolite framework, which is a class of crystalline aluminosilicate materials with well-defined microporous structures. However, unlike zeolites, ZIFs employ organic linkers, specifically imidazolate ligands, instead of inorganic oxo-anions. The imidazolate ligands coordinate with metal ions to form the backbone of the framework. In the case of ZIF-8, zinc (Zn) ions are commonly used as the metal centers, although other metal ions such as cobalt, nickel, and copper can also be utilized to form different ZIF structures. The basic building block of ZIF-8 is a tetrahedral zinc ion coordinated with four imidazolate ligands. The imidazolate ligand consists of two nitrogen atoms connected by a carbon-carbon double bond, forming a five-membered ring. The coordination of the imidazolate ligands with the metal ions results in the formation of a three-dimensional network with interconnected pores. The size of the pores can be controlled by varying the length and functional groups of the organic linkers, allowing for the customization of ZIF-8’s properties. The structure of ZIF-8 exhibits a cubic crystal system, specifically known as the sodalite topology. In this topology, the zinc ions and imidazolate linkers form a cubic framework with large cages and interconnected windows. The cubic structure gives rise to high porosity and surface area, enabling ZIF-8 to accommodate guest molecules within its pores. MOF have been shown to exhibit sensing properties for a wide range of gases, including volatile organic compounds, ammonia, carbon dioxide, and hydrogen gas [1, 2]. The sensing mechanism in MOF-based sensors involves the interaction of the gas molecules with the metal nodes and/or organic ligands in the framework. This interaction

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can result in changes in the electronic, magnetic, or optical properties of the MOF, which can be detected by various sensing techniques [1–6]. The performance of MOF-based gas sensors can be further enhanced by functionalizing the MOF with specific groups or nanoparticles to improve selectivity and sensitivity towards the target gas [1–9]. A combination of ZnO and the MOF ZIF-8, with a thickness of 50–1000 nm, may lead to highly selective and sensitive gas sensors, with low response time and the ability to detect H2 gas in various mixtures. Several studies have investigated the use of ZnOMOF composites for gas sensing applications. For example, a study by Cui et al. (2017) [10] reported the development of a hydrogen gas sensor using ZnO@ZIF-8 (core@shell) nanorods. The sensor exhibited high sensitivity to H2 and selectivity even in the presence of other gases such as CH4 , CO, and hydrocarbons. Another study by Wang et al. (2018) [10] investigated the use of ZnO/ZIF-8 composites for the detection of volatile organic compounds (VOCs) and found that the composites exhibited high sensitivity and selectivity to several VOCs. Other studies have investigated different MOF and ZnO morphologies for gas sensing applications. For example, a study by Sui et al. (2018) [10] reported the development of a H2 gas sensor using ZnO nanorods coated with a thin layer of ZIF-8. The sensor exhibited high sensitivity and selectivity to H2 even in the presence of other gases. Another study by Li et al. (2018) [10] investigated the use of the MOF UiO-66-NH2 (UiO = Universitetet i Oslo) coated on ZnO nanowires for the detection of H2 S gas and found that the composites exhibited high sensitivity and selectivity to H2 S gas [7–10]. The performance can be further improved by using single-crystalline low defect ZnO and adjusting the layer thickness [10]. Real devices can be made by 3D-printing a slurry containing the tetrapods and subsequent surface conversion into the MOF. This study presents the conversion of 3D-printed, single crystalline low defect tetrapodal t-ZnO to the MOF ZIF-8, resulting in composites that show high selectivity and sensitivity to H2 . In this work, we tried to elaborate how the presence of UV response operates sensors at relatively low working temperature, preferably room temperature. Gas responses are however most prominent at 250 °C for developed t-ZnO@ZIF-8 3D-printed tetrapodsbased sensors as 2-in-1 device for UV radiation and H2 detections.

2 Materials and Methods 2.1 Synthesis and Characterization All chemicals were used without further purification. Mishra et al. developed the flame transport synthesis technique to synthesize zinc oxide tetrapods [11, 12]. To execute this method, a pure zinc powder with a purity greater than 99% was heated together with a sacrificial polymer, specifically polyvinylbutyral (PVB), at 900 °C in air ambient. The zinc can heat up without oxidation as the polymer burns. Once the PVB is completely converted into carbon dioxide by reaction with oxygen, the high vapor pressure of zinc causes it to evaporate and react with oxygen to form zinc oxide. The flame’s convection creates the ideal conditions for producing zinc oxide tetrapods. Although the precise mechanism of tetrapod formation remains a topic of debate [13–15], this technique is reproducible and can be scaled.

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For 3D printing, tetrapodal t-ZnO powder was again mixed with PVB and ethanol in the weight ratio of 1:1:3. This forms a viscous slurry of t-ZnO with 20% ZnO concentration. The slurry was mixed well in a syringe to form a paste. A suitable sized piston pushed inside ensures the absence of air bubbles. The syringe is then fixed with a holder onto the 3D printer. A structure to be printed consists of meandering lines and is sliced with the open-source software Slic3r. The printing was performed onto a glass slide. After printing, the whole slide is held over the flame of a Bunsen burner to burn out the PVB from the deposition. To create t-ZnO@ZIF-8 samples, a method similar to the one described by Stassen et al. [16] was utilized. In brief, the 3D-printed samples were placed in one half of a 30 mL Teflon container, while a smaller Teflon vessel containing 2-Methylimidazole (HMeIM) was placed in the other half of the larger Teflon insert. The autoclave was sealed and the reaction was conducted at 140 °C for 2 and 4 h reaction time. The resulting product was obtained after cooling to room temperature and removing excess HMeIM through activation for 6 h at 100 °C under reduced pressure (2σ (I)]

0.0726; 0.1657

R1 , wR2

0.1568; 0.1919

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3 Results and Discussions The reaction of trinuclear [Fe3 O(piv)6 (H2 O)3 ](piv)·Hpiv cluster with terbium(III) nitrate, sodium azide and triethanolamine (H3 tea) ligands in MeCN/EtOH (1:1) mixture under ultrasonic irradiation leads to a wheel-shaped [Fe18 Tb6 (piv)12 (Htea)18 (tea)6 (N3 )6 ]·n(solvent) (1) nano-cluster (Fig. 1). The IR spectrum of 1 displays strong band at 1560 cm−1 and 1411 cm−1 that arise form asymmetric and symmetric vibrations of the coordinated pivalate moieties. The C–H asymmetric and symmetric stretching vibrations of –CH2 – and –CH3 groups in the pivalate and triethanolamine moieties are observed between 2971 and 2901 cm−1 , whereas the asymmetric and symmetric bending vibrations for these groups produce strong bands at 1481 cm−1 and 1375 cm−1 , respectively. A very strong peak at 2064 cm−1 corresponds to the N≡N stretching vibrations of azide ligands. The presence of the uncoordinated hydroxyl groups of Htea2− and solvent molecules caused the appearance of broad absorption bands in the region of 3671–3247 cm−1 . Single-crystal X-ray diffraction analysis revealed that compound 1 crystalizes in the space group R-3 of the trigonal system. The wheel-shaped nano-cluster resides around special position of three-fold inversion axis thus having C 3i molecular symmetry. Similar to the reported {Fe18 Ln6 }-rings [13, 14], the core structure of 1 has a ring-like arrangement of 18 FeIII atoms and 6 TbIII bonded via 12 pivalate and 24 triethanolamine ligands. All iron and terbium atoms are in the +3 oxidation state which has been confirmed by BVS calculation (3.19 for Tb and 2.94–2.97 for Fe atoms). In the wheel core, three FeIII and one TbIII ions, generate six repeated {Fe3 Tb} sequential fragments along the ring, with Fe···Fe···Fe angles of 134.85(5)º, Fe···Fe···Tb 121.55(5) and 135.07(5)º, and Fe···Tb···Fe 113.98(5)º. The shortest Fe···Fe distances equal 3.172(2) and 3.213(2) Å and Fe···Tb distances 3.415(1) and 3.530(1) Å. The outer diameter of the wheel is ca. 3.5 nm, the inner diameter is ca. 1.0 nm, and the thickness of this molecular wheel is ca.1.3 nm, which is close to the reported by us earlier for related compounds [13, 14].

Fig. 1. View of wheel-shaped nano-cluster with {Fe18 Tb6 } metals core in (a) ball and stick (hydrogen atoms are omitted for clarity) and (b) spacefill mode.

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All iron atoms are hexacoordinated, displaying NO5 or N2 O4 distorted octahedral surrounding. The environment of Fe1 atom consists of one nitrogen atom and three oxygen atoms from two Htea2− , one oxygen atom from tea3− and one oxygen atom from pivalate bridging ligand. The coordination sphere of Fe2 atom consists of one nitrogen atom and three oxygen atoms from tea3− and two oxygen atoms from two Htea2− . Coordination sites of Fe3 atom are occupied by nitrogen atom and three oxygen atoms of two Htea2− , one oxygen atom of tea3− and one nitrogen atom from azide anion. The terbium atoms are octacoordinated displaying NO7 coordination sphere, which consists of nitrogen atom and five oxygen atoms from three Htea2− and two oxygen atoms from terminal and bridging pivalate ligands. The Fe–O bond distances are in range of 1.917(6)–2.028(5) Å, Fe–N fit the interval of 2.280(7)–2.314(7) Å, Tb–O are in the range of 2.2324(5)–2.2398(6) Å and Tb–N equals to 2.645(7) Å. Six uncoordinated and protonated branches of tridentate Htea2− form six strong O–H···O = 2.64(1) Å intramolecular hydrogen bonds with the coordinated and deprotonated branches of tea3− ligands, and another six coordinated and protonated branches of tetradentate Htea2− form very strong O–H···O = 2.48(1) Å intramolecular H-bonds with mono-coordinated terminal pivalate anions. These twelve intramolecular H-bonds stabilize the mutual arrangement of the ligands in the coordination wheel.

Fig. 2. (a) Ball and stick representation of crystal packing of 1 into a layer with centroid-centroid separations of 3.5 nm. (b) Space-filling model of honeycomb-like layers of wheels, layers stacked in an ABCABC fashion creating hourglass channels. (c) Side-view of the stacked layers.

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In the crystal structure of 1, the packing of bulk wheel-like clusters is supported by twelve O–H···O = 2.62(2) Å intermolecular hydrogen bonds between the uncoordinated branches of Htea2− ligands. The packing of wheels can be viewed as alternation along the crystallographic c axis in ABCABCABC … fashion the honeycomb-like layers of wheels which are parallel to the (ab) crystallographic plane (Fig. 2a). Such packing creates infinite hourglass channels (Fig. 2b). In the crystal structure of 1, the packing of bulk wheel-like clusters creates infinite channels parallel to the crystallographic c axis and filled by solvent water/acetonitrile molecules. Upon removal of solvent molecules this structure reveals a huge total potential solvent accessible volume (SAV) of ca. 26% per unit cell volume (Fig. 2), as calculated by PLATON, the similar channels were founded in previous reported trigonal crystals of {Fe18 Ln6 }-rings [13, 14]. It is interesting to note that a very similar crystal packing has been reported for an essentially different {Cu6 } cyclic coordination cluster [21] and the well-known pure inorganic 3R-MoS2 polytype structure [22].

4 Conclusions A simple and environmentally friendly synthetic procedure using ultrasonic irradiation and the well-known [Fe3 O(piv)6 (H2 O)3 ](piv)·Hpiv complex as a starting material results in a new nanosized heteronuclear 3d/4f cluster with wheel-like shape, [Fe18 Tb6 (piv)12 (tea)6 (Htea)18 (N3 )6 ]·n(solvent). The crystal structure of this material has been studied by a single crystal X-ray method. It has been proven that such cluster structure represents a reliable and steady architectural skeleton for the generation of a family of wheel-like heteronuclear clusters with various lanthanide atoms and different carboxylic acids. The variety of components involved can lead to different physicochemical properties. Moreover, the observed crystal packing seems to be very common for laminated honeycomb structural compositions and can be expected for many different types of cyclic wheel-like molecules. The wheel-shaped clusters reported and related to them, considering the porosity of their structures, can be attractive candidates for surface deposition for potentially practical applications in catalysis, as molecular sensing, and others. Acknowledgements. Authors thank the support of State Programs of the National Agency for Research and Development of R. Moldova ANCD 20.80009.5007.15.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Taft, K.L., Delfs, C., Papaefthymiou, G.C., Foner, S., Gatteschi, D., Lippard, S.J.: [Fe(OMe)2(O2CCH2Cl)]10, a molecular ferric wheel. J. Am. Chem. Soc. 116(3), 823–832 (1994). https://doi.org/10.1021/ja00082a001 2. Tasiopoulos, A.J., Vinslava, A., Wernsdorfer, W., Abboud, K.A., Christou, G.: Giant singlemolecule magnets: a Mn84 torus and its supramolecular nanotubes. Angew. Chem. Int. Ed. 43(16), 2117–2121 (2004). https://doi.org/10.1002/anie.200353352

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3. Zhang, Z.M., Li, Y.G., Yao, S., Wang, E.B., Wang, Y.H., Clerac, R.: Enantiomerically pure chiral {Fe28} wheels. Angew. Chem. Int. Ed. 48, 1581–1584 (2009). https://doi.org/10.1002/ anie.200805827 4. Liu, C.-M., Zhang, D.-Q., Hao, X., Zhu, D.-B.: Nestlike C4-symmetric [Co24] metallamacrocycle sustained by p-tert-butylsulfonylcalix[4]arene and 1,2,4-triazole. Chem. Eur. J. 17(44), 12285–12288 (2011). https://doi.org/10.1002/chem.201101607 5. Whitehead, G.F.S., Moro, F., Timco, G.A., Wernsdorfer, W., Teat, S.J., Winpenny, R.E.P.: A ring of rings and other multicomponent assemblies of cages. Angew. Chem. Int. Ed. 52(38), 9932–9935 (2013). https://doi.org/10.1002/anie.201304817 6. Jiang, H., et al.: A gigantic molecular wheel of {Gd140}: a new member of the molecular wheel family. J. Am. Chem. Soc. 139(50), 18178–18181 (2017). https://doi.org/10.1021/jacs. 7b11112 7. Li, M., et al.: A family of 3d–4f octa-nuclear [MnIII4LnIII4] wheels (Ln = Sm, Gd, Tb, Dy, Ho, Er, and Y): synthesis, structure, and magnetism. Inorg. Chem. 49(24), 11587–11594 (2010). https://doi.org/10.1021/ic101754g 8. Mondal, A., Raizada, M., Sahu, P.K., Konar, S.: A new family of Fe4Ln4 (Ln = DyIII, GdIII, YIII) wheel type complexes with ferromagnetic interaction, magnetocaloric effect and zerofield SMM behavior. Inorg. Chem. Front. 8(21), 4625–4633 (2021). https://doi.org/10.1039/ D1QI00781E 9. Zou, L.-F., et al.: A dodecanuclear heterometallic dysprosium–cobalt wheel exhibiting singlemolecule magnet behaviour. Chem. Commun. 47(30), 8659–8661 (2011). https://doi.org/10. 1039/C1CC12405F 10. Baniodeh, A., et al.: Heterometallic 20-membered Fe16Ln4 (Ln = Sm, Eu, Gd, Tb, Dy, Ho) metallo-ring aggregates. Dalton Trans. 40(16), 4080–4086 (2011). https://doi.org/10.1039/ C0DT01742F 11. Zhang, Z.-M., et al.: Wheel-shaped nanoscale 3d–4f CoII16LnIII24 clusters (Ln = Dy and Gd). Chem. Commun. 49(73), 8081–8083 (2013). https://doi.org/10.1039/C3CC45075A 12. Leng, J.-D., Liu, J.-L., Tong, M.-L.: Unique nanoscale CuII36LnIII24 (Ln = Dy and Gd) metallo-rings. Chem. Commun. 48(43), 5286–5288 (2012). https://doi.org/10.1039/C2CC30 521F 13. Podgornii, D., et al.: Heterometallic Fe18M6 (M = Y, Gd, Dy) pivalate wheels display solventinduced polymorphism. Cryst. Growth Des. 22(9), 5526–5534 (2022). https://doi.org/10. 1021/acs.cgd.2c00620 14. Botezat, O., van Leusen, J., Kravtsov, V.C., Kögerler, P., Baca, S.G.: Ultralarge 3d/4f coordination wheels: from carboxylate/amino alcohol-supported Fe4Ln2 to Fe18Ln6 rings. Inorg. Chem. 56(4), 1814–1822 (2017). https://doi.org/10.1021/acs.inorgchem.6b02100 15. Gerbeleu, N.V., Batsanov, A.S., Timko, G.A., Struchkov, Y.T., Indrichan, K.M., Popovich, G.A.: Synthesis and structure of tri- and hexanuclear μ3-oxopivalates of iron(III). Dokl. Akad. Nauk SSSR 293, 364–367 (1987) 16. Sheldrick, G.M.: A short history of SHELX. Acta Cryst. A64(1), 112–122 (2008). https:// doi.org/10.1107/S0108767307043930 17. Sheldrick, G.M.: Crystal structure refinement with SHELXL. Acta Cryst. C71(1), 3–8 (2015). https://doi.org/10.1107/S2053229614024218 18. Spek, A.L.: PLATON SQUEEZE: a tool for the calculation of the disordered solvent contribution to the calculated structure factors. Acta Crystallogr. C71, 9–18 (2015). https://doi.org/ 10.1107/S2053229614024929 19. Brese, N.E., O’Keeffe, M.: Bond-valence parameters for solids. Acta Crystallogr. B47, 192– 197 (1991). https://doi.org/10.1107/S0108768190011041 20. Spek, A.L.: Structure validation in chemical crystallography. Acta. Crystallogr. 65(2), 148– 155 (2009). https://doi.org/10.1107/S090744490804362X

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Organic Nanostructured Crystals for Thermoelectric Cooling in Medical Applications Ionel Sanduleac(B)

, Silvia Andronic , and Ion Balmus

Faculty of Electronics and Telecommunications, Technical University of Moldova, Chisinau, Republic of Moldova [email protected]

Abstract. In this study we performed theoretical calculations and numerical modeling of a thermoelectric p-n pair composed of organic nanostructured crystals. Specifically, we focus on two highly promising materials: TTT2 I3 and TTT(TNCQ)2 crystals, which exhibit promising thermoelectric properties attributed to their unique molecular arrangements and electron-phonon interaction mechanisms. Our theoretical investigations demonstrate that tuning the concentration of charge carriers can significantly enhance the thermopower and electrical conductivity of these materials. However, such manipulations can also introduce impurities and lattice dislocations that affect the thermoelectric properties. Through detailed numerical calculations, we explored the thermoelectric characteristics of these crystals within specific temperature ranges, charge carrier concentrations, and impurity scattering parameters. Numerical calculations reveal that, within a certain range of temperature, charge carrier concentration, and impurity scattering parameters, these crystals exhibit highly promising thermoelectric characteristics. Building on these findings, we investigate the cooling properties of a thermoelectric device composed of these materials, with potential applications as local cooling systems for medical use or accurate temperature controllers for biomedical laboratories. Our results demonstrate the potential of these organic nanostructured crystals as small-scale, efficient, reliable, and environmentally friendly cooling devices. Moreover, their non-toxic nature makes them particularly suitable for diverse medical and biomedical applications, such as localized cooling systems and precise temperature controllers. Keywords: Organic crystal · TTT2 I3 · TTT(TCNQ)2 · Thermoelectric coefficient of performance · Thermoelectric cooler · Medical applications · Temperature control · Local cooling systems

1 Introduction A thermoelectric module (TEM) is a solid-state device that allows for the direct conversion of heat energy into electrical energy, or vice versa, through the Seebeck or Peltier effect. The TEM is composed of two different types of semiconducting materials, which © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 88–95, 2024. https://doi.org/10.1007/978-3-031-42775-6_10

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are connected electrically, but thermally insulated from each other. When a temperature gradient is applied across the TEM, a voltage is generated that can be used to power electrical devices or charge batteries. Conversely, applying a voltage to the TEM can create a temperature difference, causing one side to cool and the other to heat up, thus allowing for temperature regulation or cooling. TEMs have a wide range of applications, from power generation to thermal management in electronic devices, and are being explored for fields including aerospace, automotive, and biomedicine [1]. Typical applications of thermoelectric coolers (TECs) is refrigeration, electronics cooling, air conditioning, thermal comfort and thermal convenience. However, a wide area of applications is biomedical domain, such as cooling of biological samples, surgical tools, and therapeutic devices. In biomedicine, TECs have been used for applications such as cryopreservation of tissues and cells, laser surgery, and pain management. Additionally, TECs can be integrated with other biomedical devices to enable advanced functionality and enhanced performance. Overall, the use of TECs in biomedicine has shown great promise in improving patient outcomes and advancing medical research [2]. TECs are able to achieve stable and uniform cooling of samples, making them ideal for applications such as the cryopreservation of cells, tissues, and organs. TECs also offer precise temperature control, which is crucial in many biological applications. For example, in biomedical research, the success of many experiments and procedures is highly dependent on maintaining a consistent and precise temperature. TECs can provide the necessary temperature stability for experiments such as PCR, DNA sequencing, and protein crystallization, where even slight temperature variations can result in inaccurate or unreliable results [3]. Organic thermoelectric materials are a promising class of materials for thermoelectric applications in biomedicine. These materials are composed of organic polymers or small molecules that exhibit high thermoelectric performance due to their high electrical conductivity and low thermal conductivity. Additionally, they are often solution-processable, allowing for low-cost and scalable production. Organic thermoelectric materials have the potential to revolutionize biomedical applications such as implantable medical devices, where they can be used for localized heating or cooling to enhance device performance and reduce the risk of infection. They can also be used for thermoelectric power generation to power devices without the need for batteries or external power sources [4]. Significant achievements were reported in several conducting polymers such as poly(3,4-ethylenedioxythiophene) [5, 6]. The development of novel thermoelectric materials is primarily focused on achieving the Phonon-Glass/Electron-Crystal principle, which involves reducing thermal conductivity while maintaining high electrical conductivity. One approach to achieving this goal is using nanostructuring techniques, such as injecting nanodots and nanotubes into polymeric structures. However, it is important to note that thermoelectric performance is affected by various factors, and ongoing research continues to explore new strategies for improving these materials [7–9]. In this paper, we investigate the thermoelectric cooling properties of a p – n junction of two different organic nanostructured crystals, namely tetrathiotetracene-iodide (TTT2 I3 )

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[10] and tetrathiotetracene-tetracyanoquinodimethan (TTT(TCNQ)2 ) [11]. These crystals have been shown to exhibit tunable thermoelectric properties through manipulations of the stoichiometry of charge carriers and impurity concentrations [12]. Previous studies have also demonstrated that the concentration of charge carriers or the filling level of the conduction band can significantly impact the charge transport [13]. In addition, p–n modules made of these crystals have been previously investigated in the regime of power generation for biomedical applications such as infrared sensors and wearable devices [14].

2 Crystals of TTT2 I3 and TTT(TCNQ)2 Crystals of tetrathiotetracene-iodide (TTT2 I3 ) are a type of organic semiconductor material that have been found to exhibit promising electronic properties. These crystals have a layered structure with alternating tetrathiotetracene and iodide layers. The tetrathiotetracene molecules arrange in a herringbone pattern, which allows for efficient charge transport along the crystallographic b-axis. The iodide layers act as insulating spacers between the tetrathiotetracene layers, and they also contribute to the structural stability of the crystal. This allows for efficient charge transport and high electron mobility, making them potentially useful in thermoelectric applications. The electrical conductivity along b direction is of p – type and appears due to a conduction band of −0.64 eV [10]. TTT molecules acts as acceptors and iodine chains, arranged in parallel direction, are donors of electrical charge. The electrical conductivity is high anisotropic. The distance between TTT molecular chains is −9 Å while the nearest TTT molecules along the chain are spaced at −5 Å. Thus, along the chains an electrical conductivity of band-type occurs, while in transversal to chain direction only hopping processes of electrons are possible. The internal structure of tetrathiotetracene-tetracyanoquinodimethan (TTT(TCNQ)2 ) crystals is similar to the structure of TTT2 I3 crystals. But in this case, the electric conductivity is provided by electrons along the TCNQ chains. The electrical conductivity, thermopower and electronic thermal conductivity of the mentioned materials are described in previous our papers [15, 16].

3 Charge Transport in TTT2 I3 and TTT(TCNQ)2 Crystals The electrical charge is primarily carried along the molecular chains. This process can be described using the method of two-particle, retarded Green functions, in a weak electrical field at low temperatures, typically near room temperature. The resulting electrical conductivity, thermopower (also known as the Seebeck coefficient), and thermal conductivity provided by charge carriers along the molecular chains can be expressed by the following equations:    −1 R2 2|w1 | R1 e , κxx = 4 e2 T σ0 (R2 − 1 ) (1) σxx = σ0 R0 , Sxx = ± eT R0 R0 where σ0 =

2 |w |3 z 2e2 Mvs1 1

π 3 abc(k0 T )2 w21



T0 T

2

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is a coefficient with the parameters: e – elementary electric charge, M -mass of TTT or TCNQ molecule, vs1 – sound velocity along molecular chains, w1 – transfer energy of a charge carrier between nearest molecules along TTT chain (or TCNQ in case of TTT(TCNQ)2 crystal), z – the number of molecular chains in a elementary cell, a, b, c – lattice constants in the y, x and z directions, k 0 – Boltzmann constant and w1 ’ – the derivative of transfer energy w1 with respect to the intermolecular distance along the chains. A temperature dependence of T −2 is introduced. For S xx the upper sign is for p type crystal and lower – for n – type. R0 , R1 , R2 are the transport integrals as follows: Rn =

 π 

dxdydz|sin x|3 [1 − cos x + d1 (1 − cos y) + d2 (1 − cos z) 2  −(1 + d1 + d2 )εF ]n nk (1 − nk )/{ 1 ∓ |γ1 |[1 + βxx (T − T0 )]5 cos x  d12  1 + 2 sin2 y ∓ 2|γ2 | cos y + |γ2 |2 + 2 8 sin x   d22 + 1 + 2 sin2 z ∓ 2|γ3 | cos z + |γ3 |2 + D0 + D1 exp(−E0 /k0 T )} 2 8 sin x −π

(2)

where nk is the Fermi function, k = (kx b, ky a, kz c) is the dimensionless quasi momentum, εF = EF /2w1 is the dimensionless Fermi energy, d1 = w2 /w1 and d2 = w3 /w1 , βxx is the coefficient of thermic dilatation of the lattice along the x direction of molecular chains. The upper sign in “∓” is for p – type crystals and the lower for n – type ones. D0 and D1 are free dimensionless parameters introduced to describe the charge scattering on point-like impurities and on thermally activated lattice dislocations; E 0 is the activation energy of a lattice dislocation; γ1 , γ2 , γ3 are parameters describing the ratio of two internal electron-phonon interaction mechanisms and is related to the mean polarizability of molecules as follows [14] γ1 = 2e2 α0 /b5 |w1 |, γ2 = 2e2 α0 /a5 |w2 |, γ3 =

2e2 α0 c5 |w3 |

(3)

In order to perform numerical calculations of the coefficients (1) the following crystal parameters were considered: For TTT2 I3 : b = 4.96 Å, a = 18.35 Å and c = 18.46 Å, M = 6.5•105 me , w1 = 0.16 eV, w1 = 0.26 eVÅ−1 , vs1 = 1.5·103 m/s, z = 4, d1 = d2 = 0.015, D0 = 0.001, D1 = 0.2, E0 = 0.024eV , α 0 = 46 Å−3 , β = 0.000145 K−1 . For TTT(TCNQ)2 : b = 3.75 Å, a = 12.97 Å and c = 19.15 Å, M = 3.72·105 me , w1 = 0.125 eV, w1 = 0.22 eVÅ−1 , vs1 = 2.8 ·103 m/s, z = 2, d1 = 0.01, d2 = 0.015, D0 = 0.001, D1 = 0.2, E0 = 0.024eV , α 0 = 10.2 Å−3 , β = 0.000145 K−1 . Figure 1 shows the electrical conductivity and Seebeck coefficient plotted as a function of dimensionless Fermi energy (left side). Two temperature values of 280 and 320 K were considered. At εF = 0.35 the crystals are stoichiometric with carrier concentration nh = 1.2· 1021 cm−3 (h representing holes). Experimental studies have demonstrated that this value can be altered by sublimation of iodine from the crystalline lattice [17]. As shown in Fig. 1, decreasing the carrier concentration (i.e. εF from 0.35 to 0.2) results in a decrease in electrical conductivity from 12·103 −1 cm−1 to 4.5·103 −1 cm−1 , but

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an increase in Seebeck coefficient from 60 μV/K to 120 μV/K. This implies that each conducting hole carries more energy within the same temperature gradient.

Fig. 1. Electrical conductivity and Seebeck coefficient of TTT2 I3 crystals of p – type (on the left). Electrical conductivity vs electronic thermal conductivity (on the right). The dimensionless parameter D1 describes the scattering of charge carriers on thermal activated lattice defects.

Manipulating the carrier concentration can result in the creation of more lattice dislocations, which are partially reflected in the parameter D1 . The thermal conductivity e ≈ 3.6W /m · K and it due to electrons at stoichiometric concentration is about κxx decreases as the dimensionless Fermi energy εF is reduced.

Fig. 2. Electrical conductivity and Seebeck coefficient of TTT(TCNQ)2 crystals of n – type (on the left). Electrical conductivity vs electronic thermal conductivity (on the right). The dimensionless parameter D1 describes the scattering of carriers on lattice defects.

The calculations were conducted at two different temperatures, 280 K and 300 K and e slightly increase as the temperature decreases, the results showed that σxx , Sxx and κxx which can be attributed to an increase in charge carrier mobility due to reduced phonon scattering. In Fig. 2 the calculations performed for TTT(TCNQ)2 crystals are presented. e to the right of ε is due to the fact The displacement of the maximums of σxx and κxx F that the charge carriers in TTT(TCNQ)2 are electrons. The stoichiometric concentration of electrons in these crystals is ne = 1.1· 1021 cm−3 , corresponding to εF = 0.36. Doping with donor impurities can improve the thermoelectric performance of these

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crystals, as increasing the concentration of conducting electrons by two times (εF from 0.35 to 1.0), leads to an increase in electrical conductivity from 0.5·103 −1 cm−1 to 4·103 −1 cm−1 while the Seebeck coefficient remains almost constant at −100 μV/K. Thus, precise tuning of the charge carrier concentration can optimize the electrical conductivity, Seebeck coefficient, and electronic thermal conductivity, making TTT2 I3 and TTT(TCNQ)2 promising thermoelectric materials. Additionally, Fig. 1 and Fig. 2 demonstrate that the thermoelectric properties of these crystals remain relatively constant within the range of temperatures from 280 to 320 K, which is important for further modeling of a thermoelectric converter using these organic compounds.

4 A Thermoelectric Module made of TTT2 I3 and TTT(TCNQ)2 Crystals Further, let us consider a TEM made of two organic crystals joined together. To assess the suitability of this module for cooling applications, we performed numerical calculations of the electrical conductivity, Seebeck coefficient and electronic thermal conductivity. Using experimentally reported data on lattice thermal conductivity, we estimated the coefficient of performance (COP) of the TEC, which is a measure of its efficiency in removing heat energy Q from the cold side relative to the electrical energy IV consumed to power the device. COP = Q/IV

(4)

A higher COP indicates a more efficient TEC, as it is able to remove more heat energy for each unit of electrical energy consumed. However, the COP of a TEC is limited by its inherent design and materials, and it is difficult to achieve high COP values in practice. The COP of a thermoelectric cooler depends in both the material parameters and the operating conditions and can be estimated by several measurements: the maximum temperature difference Tmax in the absence of a heat absorber (Q = 0), the maximum electrical current I max through the junction that provides this ΔT max and the maximum heat flow Qmax when T = 0.

Tmax = ZTc · Tc /2; Imax = STc · σ ; Qmax = σ S 2 Tc2 /2 = ZTc κ/2

(5)

where ZT = σ S 2 T /κ is the thermoelectric figure of merit, σ - electrical conductivity, S – Seebeck coefficient, κ – thermal conductivity of the TEC device. Thus, COP is a parameter determined by the material parameters as well as the operating conditions (I and T). Finally, for optimal electric current, the COP is defined as: √ 1 + ZTav − Th /Tc Tc · √ (6) COPmax =

T 1 + ZTav + 1 where T h and T c are the temperatures of the hot and cold sides, T av = (T h + T c )/2. It is important to notice that the ratio T c /ΔT is the thermodynamic maximum of the COP of the Carnot cycle. In the following, we assumed a TEM consisting of a TTT2 I3 and a TTT(TCNQ)2 crystal joined in a p-n pair with a length of leg l = 0.005 m. Temperatures

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of the hot and cold sides: T h = 320 K, T c = 280 K. Figure 3 displays the calculated values of ZT ave and COP for a single p – n pair made of organic nanostructured crystals of TTT2 I3 and TTT(TCNQ)2 . The left side plot shows the dependence of ZT ave and COP on the electrical conductivity of p – type crystal. For stoichiometric crystals with p εFn = 0.35, the electrical conductivity σxx = 12 · 103 −1 cm−1 resulting in ZT ave ≈ 0.2 and COP ≈ 0.1. When TTT(TCNQ)2 is doped with donors (εFn = 1.0) and iodine is p sublimated from TTT2 I3 , the electrical conductivity reduces σxx = 4.5 · 103 −1 cm−1 resulting in ZT ave ≈ 0.7 and COP ≈ 0.5.

Fig. 3. Thermoelectric figure of merit (ZT ave ) and Thermoelectric coefficient of performance (COP) of a single p – n pair made from TTT2 I3 and TTT(TCNQ)2 crystals.

On the right side plot, ZT ave and COP are presented as a function of the electrical conductivity of TTT(TCNQ)2 . The results show that changes in the carrier concentration of TTT2 I3 have a more significant effect on achieving high thermoelectric performance of the TEM than the increase in electronic conductivity of TTT(TCNQ)2 .

5 Conclusions The study conducted numerical calculations to determine the thermoelectric figure of merit (ZT ) and coefficient of performance (COP) of a p – n pair composed of TTT2 I3 and TTT(TCNQ)2 crystals at different charge carrier concentrations. The investigations focused on thermoelectric efficiency (ZT ) under a temperature gradient and refrigerator performance (COP). Results showed that the thermoelectric properties of the crystals remains stable within a temperature range of 280–320 K. The device could achieve ZT up to 0.7 and a COP up to 0.5 if the charge carrier concentration is precisely tuned. Based on these results, the study suggests potential experimental realizations of such a thermoelectric cooling device for biomedical applications. Acknowledgments. This research was performed with the support of ANCD national project 45/22.10.19 F.

Conflict of Interest. The authors declare that they have no conflict of interest.

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References 1. Tritt, T.M.: Thermoelectric materials: principles, structure, properties, and applications. 2nd edn. Encyclopedia Mater. Sci. Technol. 1–11. Elsevier, Amsterdam (2002). https://doi.org/ 10.1016/b0-08-043152-6/01822-2 2. Wang, Y., Li, S., et. al.: Heat and cold therapy reduce pain in patients with delayed onset muscle soreness: a systematic review and meta-analysis of 32 randomized controlled trials. Phys. Ther. Sport 48, 177–187 (2021). https://doi.org/10.1016/j.ptsp.2021.01.004 3. Hu, B., Shi, X.: Thermoelectrics for medical applications: progress, challenges, and perspectives. Chem. Eng. J. 437, 135268 (2022). https://doi.org/10.1016/j.cej.2022.135268 4. Zhao, Y., Liu, L., Zhang, F., Di, C., Zhu, D.: Advances in organic thermoelectric materials and devices for smart applications. SmartMat 2(4), 426–445 (2021). https://doi.org/10.1002/ smm2.1034 5. Sun, Z., Shu, M., Li, W., et.al.: Enhanced thermoelectric performance of PEDOT:PSS selfsupporting thick films through a binary treatment with polyethylene glycol and water. Polymer 192, 122328 (2020). https://doi.org/10.1016/j.polymer.2020.122328 6. Wang, H.L., Wang, M.X.: Spin thermoelectric effects in organic single-molecule devices. Phys. Lett. 381(20), 1738–1744 (2017). https://doi.org/10.1016/j.physleta.2017.03.024 7. Jin, W., Liu, L., Yang, T., et al.: Exploring peltier effect in organic thermoelectric films. Nat. Commun. 9, 3586 (2018). https://doi.org/10.1038/s41467-018-05999-4 8. Ding, J., Liu, Z., Zhao, W., Jin, W., et.al.: Selenium-substituted diketopyrrolopyrrole polymer for high-performance p-type organic thermoelectric materials. Angewandte Chemie Int. Ed. 58(52), 18994–18999 (2019). https://doi.org/10.1002/anie.201911058 9. Zhou, D., Zhang, H.: Recent adv and prospects of small molecular organic thermoelectric materials. Small 18(23), 2200679 (2022). https://doi.org/10.1002/smll.202200679 10. Isset, L., Perez-Albuerne, E.: Low temperature metalic conductivity in bis (tetratiotetracene) triiodide, a new organic metal. S. S. Comm. 21(5), 433–435 (1977). https://doi.org/10.1016/ 0038-1098(77)91368-0 11. Buravov, L.I., et al.: Structure and electromagnetic properties of a new high-conductive complex (TTT)+(TCNQ)2-. ZhETF Pis. Red. 20, 457 (1974) 12. Sanduleac, I., Pflaum, J., Casian, A.: Thermoelectric properties improvement in quasi-onedimensional organic crystals. J. Appl. Phys. 126(17), 175501 (2019). https://doi.org/10.1063/ 1.5120461 13. Andronic, S., Casian, A.: Metal-insulator transition of peierls type in quasi-one-dimensional crystals of TTT2I3. Adv. Mat. Phys. and Chem. 7, 212–222 (2017). https://doi.org/10.4236/ ampc.2017.75017 14. Sanduleac, I., Andronic, S.: Organic crystals of p - Type TTT2I3 and n - Type TTT(TCNQ)2 as prospective thermoelectric materials for biomedical sensors. In: 5th ICNBME 2021. IFMBE Proceedings, vol. 87, pp. 528–533. Springer, Cham (2022). https://doi.org/10.1007/978-3030-92328-0_70 15. Sanduleac I., Casian A.: High thermoelectric properties in quasi-one-dimensional organic crystals. Thin Film Flex. Thermoelectric Gen. Dev. Sens. 259–280 eBook (2020). https://doi. org/10.1007/978-3-030-45862-1 16. Sanduleac, I., Casian, A.: State of the art and new possibilities to increase the thermoelectric figure of merit of organic materials. J. Thermoelectricity 6, 29–39 (2016) 17. Pudzs, K., Vembris, A., Rutkis, M., Woodward, S.: Thin film organic thermoelectric generator based on tetrathiotetracene. Adv. Electr. Mater. 3(2), 1600429 (2017). https://doi.org/10.1002/ aelm.201600429

General Nature of Serration Effect in Crystals and Other Materials Under Indentation Daria Grabco1(B)

, Constantin Pyrtsac1,2

, and Olga Shikimaka1

1 Institute of Applied Physics, Moldova State University, Chisin˘au MD2028, Republic of

Moldova [email protected] 2 Technical University of Moldova, Chisin˘au MD2004, Republic of Moldova

Abstract. The nature of the manifestation of the “serration effect” (SE) during nano-microindentation of materials of various types was studied in this work: ionic and covalent crystals (LiF, MgO, Si), coated systems (CSs) of the film/substrate type, metals (Cu, austenitic steel AISI 316L) and laser phosphate glasses doped with rare earth elements (SP-R). The serration effect on the nano-microindentation P(h) curves was revealed both at the loading and unloading stages. It has been established that serration effect is a property of all studied materials. General regularities were revealed: SE is most pronounced in single crystals and CSs, less in metals, and the weakest in glasses. With an increase in the load on the indenter and an increase in the loading rate, the amplitude and step of oscillations decrease. The characteristic dependences obtained in the paper correlate with the literature data, which also confirm the wavelike nature of indentation for various materials. It has been suggested that the effect may be associated with the elasticplastic recovery (relaxation) of the material, which takes place throughout the entire indentation process. The fluctuations revealed on the load-displacement curves may indicate the wave nature of the indentation process and the universal character of the oscillatory effect in the process of depth-sensing indentation. Keywords: Serration effect · Model single crystals · Composite systems · Metals · Glasses · Oscillation character of nano-microindentation

1 Introduction Currently, micro- and nanoindentation is one of the most popular and indispensable non-destructive methods for determining the mechanical properties of various avantgarde materials: crystals, amorphous materials, film/substrate CSs, structures doped with nanoparticles with special properties, plastics, biological objects, etc. Today, the depthsensing indentation, instead of the traditional method of quasi-static microindentation, is used intensively in a very wide range of loads: micro-, nano-, and even pico-indentation, for which special high-precision devices, nanotesters, have been created. With the help of depth-sensing indentation, a large number of mechanical parameters, more than twenty, can be estimated. These are micro- and nanohardness (H MI , H NI ), determined by the depth of penetration of the indenter into the test material (h), sclerometric microhardness © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 96–105, 2024. https://doi.org/10.1007/978-3-031-42775-6_11

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(H s ), microbrittleness (γ), microstrength (σ), crack resistance coefficient (klc ), adhesive film properties, etc. One interesting phenomena revealed on the strain curves P(h) of a large number of both bulk materials and film/substrate CSs is the “pop-in” and “pop-out” effects. In most cases, these effects occur skippingly and are accompanied by a rapid immersion of the indenter into the material under the action of a constant load (“pop-in”) or its abrupt displacement at the unloading stage (“pop-out”). Much work has been devoted to elucidating the question of the mechanism for the occurrence of these effects [1–3]. Another common property, characteristic of a large number of materials, was discovered during depth-sensing indentation. The deformation curves P(h) exhibit the character of a weak oscillation both at the stage of loading and at the stage of unloading. The effect is referred to as the “serration effect” (SE) and is better seen at low indenter loads. Thus, the authors of [4] observed oscillations in the P(h) curves during nanoindentation of silico-calcium-sodium glasses, which were called “serrated defects”. Oscillations were clearly manifested in the case of indentation at a low speed, and decreased with a gradual increase in speed. In [5], the authors revealed a characteristic “ladder structure” on the strain curves of thin single-crystal Al films (t = 400 nm). The “toothed effect” during nanomicroindentation of TCO/Si and Cu/LiF composite structures was also revealed in [6, 7]. The oscillation effect correlates with the concept of the undulating nature of plastic deformation developed by Zuev et al. [8], when studying crystals subjected to a uniaxial load. As follows from the information presented above, “jagged effect” is a property inherent in materials of various types, and its magnitude is not constant, but depends on various factors. In this regard, in this work, the task was set to study in more detail the nature of the manifestation of the “toothed effect” during nanomicroindentation of materials of various types: ionic and covalent crystals, composite structures of the film/substrate type, metals and glasses.

2 Materials and Methods Model single crystals of LiF, MgO, Si and composite structures of the film/substrate type based on them, namely Cu/LiF, Cu/MgO, Cu/Si, were chosen for the study. Composite thin metal film/substrate systems are used in micro/nanoelectronics for the production of integrated circuits, magnetic and optical devices, in medicine and the aviation industry, etc. Cu/LiF belongs to soft-on-soft coated systems (CSs), while Cu/MgO and Cu/Si are soft-on-hard CSs. Polycrystalline copper, AISI 316L austenitic steel and phosphate glasses were also taken as test objects. The microhardness of LiF, MgO, and Si single crystals is 1.1, 7.5, and 8.2 GPa, respectively, and that of polycrystalline copper is 0.6 GPa. LiF crystals have an ionic chemical bond, MgO has an ionic-covalent bond, and Si has a covalent bond. For each type of substrate, similar composite structures were fabricated. Cu films were deposited on the (001) face of LiF, MgO and Si single crystals by magnetron sputtering using a Magnetron Sputtering RF device in the P = 200W, T = 500C mode. CSs with different thicknesses of Cu films were obtained: t 1 = 85; t 2 = 470 and t 3 = 1000 nm. The surface roughness of the films (R) was estimated by atomic force microscopy and was R = 15–20 nm. The copper films in the

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CS had a nanocrystalline structure with the texture of Cu crystals according to (111). The mechanical properties were studied by depth-sensing indentation on a NanotesterPMT-NI-02 instrument equipped with a Berkovich indenter. In the work, the indenter indentation procedure was applied in the mode of gradually increasing the load applied to the indenter from 0 to Pmax , being briefly held under the load, and then gradually decreasing to 0. Continuous recording of the penetration depth of the indenter into the material under study makes it possible to control the loading/unloading process. When testing for nano- and microindentation, the following steps were performed for each sample: loading –20 s, exposure at maximum load (Pmax ) –5 s, unloading –20 s. For each load, 5 imprints were made. The results were calculated as the average of 5 tests. Calculations were carried out according to the Oliver-Pharr method [9]. All calculations were performed automatically using the instrument software.

3 Results and Discussions During the study, the main attention was paid to several main issues: the dependence of the “serration effect” on the sample type, the effect of the indentation load on the manifestation of the “serration effect”, the effect of the loading rate on the SE value, and the dependence of the effect on the film thickness in composite structures. Figure 1 shows the strain curves P(h) for Cu, LiF, MgO, and Si at similar loads.

Fig. 1. Loading–unloading curves P(h) obtained on Cu (a), LiF (b), MgO (c), and Si (d) crystals at close peak loads. Pmax , mN: a, d – 5; b – 4; c – 3. On Fig. 1 a-c arrows mark the “pop-in” steps.

As follows from the curves, samples of various types (polycrystalline Cu metal, single crystals of LiF, MgO, and Si, respectively, of the ionic, ionic-covalent, and covalent

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types of chemical bonding) exhibit a pronounced “serration effect.” The frequency and magnitude of oscillations are approximately the same for single crystals of LiF, MgO, and Si, whereas on polycrystalline Cu, oscillations are rarer and noticeably smaller in magnitude. In the following figure, as an example, several samples of Cu/LiF, Cu/MgO and Cu/Si CSs with the same film thickness of 85 μm are selected (Fig. 2). This figure shows curves at low loads (Pmax = 3, 4 and 10 mN) (Fig. 2 a, c, e), and at higher loads (Pmax = 100 mN) (Fig. 2 b, d, f). It can be seen that the serration effect is clearly seen at low loads, and only weak oscillations are visible at high loads. The same pattern was noted for similar CSs with t 2 = 470 and t 3 = 1000 μm. Here, too, the oscillation amplitude is greater for low loads (Fig. 3 a, c), than for high ones (Fig. 3 b, d).

Fig. 2. Loading–unloading curves P(h) obtained on Cu/LiF (a, b), Cu/MgO (c, d) and Cu/Si (e, f) CSs (t 1 = 85 nm) at low loads (Pmax , mN: a, c - 3, 4; e –10), and at higher ones (b, d, f –100).

Attention should be paid to another point in the deformation of coated systems. At low loads, the depth of the indentations is much smaller than the film thickness; therefore, the Cu film makes a large contribution to the indentation formation. At high loads, the indenter reaches depths of the order of the film thickness (hin ≈t f ) and it also comes

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Fig. 3. Loading–unloading curves P(h) obtained on Cu/Si (a, b), (t 2 = 470 nm), and Cu/MgO (c, d) CSs (t 3 = 1000 nm) at low and higher peak loads (Pmax , mN: a – 5; b – 200; c – 3; d – 100).

up against a certain resistance on the side of the substrate. Judging by the shape of the deformation curves, this contribution is more noticeable for a load of 200 mN (Fig. 3 b), at which the elastic recovery of the indentation significantly increased compared to (Fig. 3 a), which is typical for Si (see Fig. 1 a and 1 d). At a load of 100 mN, this contribution is small (Fig. 3d). The effect can manifest itself at much higher loads, when the depth of the indentation significantly exceeds the film thickness. To more vividly demonstrate the effect of a decrease in the amplitude of oscillations on the deformation curves with an increase in the load on the indenter, Fig. 4 shows the P(h) curves for the LiF ionic crystal at different loads Pmax = 1, 10 and 100 mN on the same scale. It is clearly seen that with an increase in the load on the indenter, on the one hand, the oscillation amplitude slightly decreases, on the other hand, the length of the translational step of the indenter increases and, accordingly, the oscillation frequency decreases. The microstructure of the imprint obtained at a load of Pmax = 1mN is shown in Fig. 5 a, and in Fig. 5 b, its profile in section is presented. In Fig. 5 b, one can notice a different distance between the points of the profile, which indicates the inhomogeneous nature of the recovery of the imprint at the stage of unloading. The following figure (Fig. 6) shows the loading-unloading curves on the same scale for a covalent Si crystal, also obtained at three loads Pmax = 4, 20, and 100 mN. Comparing the curves P(h) with each other, you can observe the evolution of the change in the shape of the curves with the load increase. An increase in load is accompanied with a decrease in the amplitude of oscillations and an increase in the length of the translational step of the indenter. The same effect is also clearly visible when testing the Cu/MgO CSs, (Fig. 7).

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Fig. 4. Loading–unloading curves P(h) obtained on LiF single crystal at low and higher peak loads (Pmax , mN: a – 1; b – 10; c – 100).

Fig. 5. AFM. Single crystal LiF. a – surface relief pattern around the imprint in 3D image at atomic force microscope (P = 1 mN), and b – its image in section.

A somewhat less pronounced serration effect was found in AISI 316L steel, (Fig. 8 a, b), which has a typical polycrystalline austenitic structure with a grain size in the range of (2–50) μm and high ductility combined with a fairly high microhardness (H = 1.8 GPa). Here, as well as on the materials discussed above, an increase in load leads to a decrease in the serration effect. The Vickers imprint is shown on a single crystallite of AISI 316L steel (Fig. 8 c). You can see the wrinkled surface of the imprint, as a consequence of the oscillatory process at the relaxation stage when the load is removed. Next, we shall consider the specifics of the deformation process on samples of yet another structure, namely on SP-Li2 0-Na2 O, SP-Gd and SP-Tb, representatives of glassy materials doped with rare earth elements. Laser phosphate glasses form an important

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Fig. 6. Loading–unloading curves P(h) obtained on Si single crystal at low and higher peak loads (Pmax , mN: a – 4; b – 20; c – 100).

Fig. 7. Loading–unloading curves P(h) obtained on Cu/MgO CS at low and higher peak loads (Pmax , mN: a – 2; b – 50).

group of substances used in quantum electronics as the active media for lasers, optical amplifiers, photosensitive elements, sensors, and so on. The microhardness of phosphate glasses varies within H = (4.5–7.5) GPa depending on the doping rare earth element. As can be seen from Fig. 9 a, b phosphate glasses are distinguished by a slight serration effect with a small oscillation amplitude. Increasing the load also leads to a decrease in the serration effect. Figures 9 c, 9 d show indentation curves of SP-Tb phosphate glasses with the same load Pmax = 20 mN, but at different rates, which differed by an order of magnitude: Vin = 0.5 mN/s and Vin = 5 mN/s. The low speed indentation curves (Fig. 9 c) showed a dense wavy appearance, while the high speed indentation curves had a smooth run without hesitation (Fig. 9 d).

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Fig. 8. Austenitic steel AISI 316L: a, b – the loading–unloading curves P(h) obtained at low and higher peak loads (Pmax , mN: a – 10; b – 200), stain rate (Vin , mN/s: a - 0,5; b – 10); c – the microstructure of the Vickers imprint, made on the AISI steel at load P = 200 mN.

It was demonstrated above that the effect of increasing the uniformity of the indentation process with an increasing load is found on all the samples under study. Considering that in our experiments the duration of loading and unloading was the same for all loads (20 s), the rate of the indenter penetration process (Vin ) gradually increases when moving from a minimum load to a higher one. For example, when going from Pmax = 1 mN to Pmax = 100 mN, the indentation rate will increase by two orders of magnitude, from Vin = 0.05 mN/s to Vin = 5 mN/s. Therefore, the effect of increasing the uniformity of the indentation process with the increasing load corresponds to the effect of increasing the uniformity of the process with the increasing speed. The factor responsible for the increase in the uniformity of the deformation process with the increasing Vin may be the limited ability of the material to relax internal stresses that accumulate during indentation. The characteristic dependences obtained in this work correlate with the results of [4–8], in which the wavy nature of indentation was also confirmed on a wide range of materials in a wide range of loads and speeds. The fluctuations revealed on the loaddisplacement curves make it possible to do an assumption about the wave-like nature of the elastic-plastic mass transfer in the region of maximum shear stresses under the application of a concentrated load and, in particular, during nano-microindentation.

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Fig. 9. Loading–unloading curves P(h) obtained on phosphate glasses of the SP-Li2 0-Na2 O (a), SP-Gd (b), SP-Tb (c, d) at low and higher loads (Pmax , mN: a – (10–40); b – (10–900); c, d – 20).

4 Conclusions In this paper, the question of the manifestation of the serration effect during the indentation of materials of various types is studied: model single crystals of LiF, MgO, Si and CSs of the film/substrate type based on them (Cu/LiF, Cu/MgO, Cu/Si), Cu polycrystal, austenitic steel AISI 316L and phosphate glasses doped with rare earth elements. The serration effect on the P(h) curves was revealed. It was found that the oscillation effect depends on the type of sample. The effect manifests itself most clearly on single crystals and CSs based on them. The oscillation effect is somewhat less pronounced in metals, and the effect is weaker in glasses. The most pronounced oscillatory effect is manifested at low loads and low strain rates; as they grow, the effect decreases. It has been suggested that the oscillation effect may be associated with the elastic-plastic recovery (relaxation) of the material that occurs throughout the entire indentation process. This may indicate the wave nature of the indentation process and the universal character of the oscillatory effect in the process of depth-sensing indentation. Acknowledgments. This work is supported financially by the National Agency for Research.and Development of the Republic of Moldova under the grant no.20.80009.5007.18. SP-Tb phosphate glasses were measured at the Polytechnic University of Bucharest, Romania.

Conflict of Interest. The authors declare that they have no conflict of interest.

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Trends in Evolution of the Energy Band Structure of Chalcopyrite CuBIII XVI 2 Compounds with Variation of the B and X Compositions Alisa Ma¸snic1

, Victor Zalamai2(B)

, and Veaceslav Ursaki2,3

1 Technical University of Moldova, Chisinau, Republic of Moldova 2 National Center for Materials Study and Testing, Technical University of Moldova, Chisinau,

Republic of Moldova [email protected] 3 Academy of Sciences of Moldova, Chisinau, Republic of Moldova

Abstract. Bulk and nanostructured AI BIII XVI 2 chalcopyrite materials, including quantum dots on their basis, are widely used in the development of optical filters, solar cells, optoelectronic devices and photocatalysis. Physical properties of both bulk and nanostructured chalcopyrite compounds are determined by their energy band structure. The optical spectroscopy is one of the powerful and nondestructive method for determination of physical properties. This paper presents results of investigation of optical reflectance spectra of CuBIII XVI 2 compounds with B = Al, Ga, and In, and X = S and Se, performed in a wide spectral range from 1.7 eV to 7.5 eV. The measured spectral position of peaks in the reflectance spectra are assigned to electronic transitions in different points of the Brillouin zone, on the basis of the electronic band structures of these materials deduced from theoretical calculation performed in previous works. Trends in the evolution of the energy band structure with changing the composition of materials have been revealed, which are important for practical applications. Apart from that, the observed trends in the evolution of the energy band structure of chalcopyrite CuBIII XVI 2 compounds with variation of their composition are helpful for a right assignment of the observed peaks in the reflectance spectra to respective electronic transitions in various points of the Brillouin zone. Keywords: AI BIII XVI 2 chalcopyrite materials · Solid solutions · Optical reflectance spectra · Energy band structure · Brillouin zone · Electronic transitions

1 Introduction AI BIII XVI 2 chalcopyrite materials and their solid solutions, as anisotropic materials with inherent birefringence properties, present especial interest in elaboration of optical filters [1–5]. Many of these materials are suitable for the production of narrow stopband or pass-band filters, since they have accidental isotropic wavelength, at which the © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 106–114, 2024. https://doi.org/10.1007/978-3-031-42775-6_12

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ordinary and extraordinary refractive indexes become equal to each other, this being a necessary condition for anisotropic uniaxial crystals to be suitable for the development of such filters. Particularly, optical stop-band filters are usable for applications in Raman spectroscopy, making possible to measure Raman scattering at frequencies very closed to the laser excitation line with a non-expensive single monochromator, instead of expensive double or triple ones. Stop-band filters based on uni-axial optically active AgGaSe2 crystals have been proposed for Raman spectroscopy in combination with lasers based on GaAs semiconductor [2–4, 6]. Narrow bandgap CuIn1-x Gax Se2 solid solutions are widely used in solar cells [7– 9], while applications in nonlinear optics have been demonstrated for large bandgap materials, such as AgGaS2 and AgGaSe2 [10–13]. CuAlx Ga1-x Se2 thin films have also been proposed for photovoltaic applications [14]. CuAlSe2 thin films have been prepared for multi-junction or semi-transparent device applications [15]. Apart from bulk materials, nanostructured AI BIII XVI 2 compounds are used in light emitting devices [16, 17] and photocatalysis [18, 19]. Chalcopyrite CuAlS2 and CuAlSe2 nanorods have been synthesized via hydrothermal method [20], while CuAlS2 nanocrystals have been shown to be perspective for application in bio-imaging [21]. It was shown that atomic-scale incorporation of CuAlSe2 inclusions within the Cu2 Se matrix are enabling high thermoelectric performance of Cu2 Se/CuAlSe2 composites, due to the significant improvement in ZT, since the nanostructuring near the CuAlSe2 /Cu2 Se interface, as well as the extensive atomic disorder in the Cu2 Se and CuAlSe2 phases, significantly increases phonon scattering, leading to suppressed lattice thermal conductivity [22]. A series of applications have been demonstrated with CuAlS2 based quantum dots (QDs). Particularly, colloidal copper chalcogenide-based CuAlS2 QDs are emerging building blocks for optoelectronic technologies. Tunable, transparent emitters operating over the entire visible spectrum have been demonstrated on CuAlS2 /CdS QDs with waveguided optical excitation and a clear aperture of 7.5 cm2 [23]. High performance photodetectors have been fabricated using CuAlS2 /ZnSe QDs, which parameters are even further enhanced through functionalization with Au-QD heterostructures [24]. CuAlS2 /ZnS QDs are known to exhibit remarkable photocatalytic properties, particularly by directly converting aqueous solutions of bicarbonate ions to oxygen and organic molecules such as formate with high enough efficiency even under sunlight [25]. Physical properties of both bulk and nanostructured chalcopyrite compounds are determined by their energy band structure. The goal of this paper is to reveal trends in the evolution of the energy band structure of chalcopyrite CuBIII XVI 2 compounds with variation of the B and X composition via a comparative analysis of their optical reflectance spectra in a large spectral range.

2 Methods and Materials Chalcopyrite crystals from three series of solid solutions, such as CuAl1-x Gax Se2 , CuAlS2x Se2(1-x) , and CuIn1-x Gax S2 with x = 0 and x = 1 have been grown by chemical vapor transport with iodine as transport agent [26]. The chemical transport was performed in quartz ampoules with the inner diameter from 15 to 22 mm and the length

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from 170 to 180 mm. The used iodine concentration was from 4 to 6 mg/cm3 . The ampoules consisted of two compartments, namely, one compartment for placing the precursors, and another one in the form of a capillary with iodine, beforehand evacuated and sealed. Later-on the precursors have been loaded and the ampoules were evacuated to a vacuum of 10–3 Pa. After evacuation, the capillary with iodine was opened and the iodine passed to the chamber with precursors. The prepared ampules have been placed in a horizontal furnace with two zones, which temperatures were set independently. For crystals with Al, boron nitride crucibles or graphitized ampoules were used, in order to avoid the reaction of the Al with the quartz ampoule walls. The reflectance spectra were measured at room temperature from mirror-like surfaces containing the optical c-axis of crystals, by means of an installation described elsewhere [27], with halogen or a hydrogen lamp as light sources.

3 Results Figure 1 shows spectral dependences of reflectance spectra measured in Ec polarization for CuAlS2x Se2(1-x) solid solution crystals with x = 1 (CuAlS2 ) and x = 0 (CuAlSe2 ). The observed peaks in the reflectance spectra are related to electron transitions at specific points of the valence and conduction bands. Therefore, data related to the energy band structure of CuAlS2x Se2(1-x) solid solutions can be extracted from the spectral position of features observed in the reflectance spectra.

Fig. 1. Reflectance spectra measured in E||c polarization for CuAlS2 and CuAlSe2 crystals.

The evolution of the reflectance spectra for CuAl1-x Gax Se2 solid solution crystals with changing the composition from x = 0 (CuAlSe2 ) to x = 1 (CuGaSe2 ) was analyzed in a previous paper [27]. It was found that the distance between the positions of the low energy A1 and A6 peaks increases with increasing the x value from 0 to 1, while the distance between the positions of the high energy A8 and A10 peaks decreases with increasing the x value from 0 to 1. At the same time, the distance between the intermediate energy peaks A6 and A7 peaks, as well as between the A7 and A8 peaks is practically constant with variation of the x value from 0 to 1.

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Table 1 summarizes the energy position of the peaks observed in the reflectance spectra of CuAlSe2 , CuGaSe2 , and CuAlS2 crystals. Table 1. The energy position (in eV) of the peaks observed in the reflectance spectra of CuAlSe2 , CuGaSe2 , and CuAlS2 crystals. Composition

A1

A2

A3

A4

A5

A6

A7

A8

A9

A10

CuAlSe2

4.01

4.12

−

4.40

4.68

4.94

5.21

5.36

5.63

6.31

CuGaSe2

3.19

3.29

3.48

3.69

4.02

4.71

5.03

5.27

−

5.75

CuAlS2

4.20

4.49

4.65

4.78

5.29

5.46

5.59

5.95

6.25

7.13

Similar reflectance spectra measured for the CuIn1-x Gax S2 solid solution crystals with x = 0 (CuInS2 ) and x = 1 (CuGaS2 ) are presented in Fig. 2, while the position of the observed peaks is summarized in Table 2.

Fig. 2. Reflectance spectra measured in E||c polarization for CuGaS2 and CuInS2 crystals.

Table 2. The energy position of the peaks observed in the reflectance spectra of CuGaS2 and CuInS2 crystals. Composition

A1

A2

A3

A4

A5

A6

A7

A8

A9

A10

CuGaS2

3.72

3.85

4.15

4.38

4.73

5.16

5.42

5.65

6.10

-

CuInS2

3.33

3.47

3.83

4.15

4.30

4.57

4.75

5.00

5.16

5.35

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4 Discussions The experimentally observed peaks in the reflectance spectra for CuAl1-x Gax Se2 solid solution have been related to the electronic band structure of materials, computed in previous works, and have been interpreted on the basis of a generic band diagram for AI BIII XVI 2 compounds deduced from electronic structure calculated by the method of full-potential linear muffin-tin orbital (FPLMTO) [27]. This model is shown in Fig. 3a for CuAlSe2 crystals in T, , and N points of the Brillouin zone. The band structure calculated in the same critical points for CuGaS2 crystals, which band gap value (2.5 eV) is close to that of CuAlSe2 crystals (2.7 eV), is shown in Fig. 3b [28].

Fig. 3. The calculated band diagram of electronic structure of CuAlSe2 (a), and CuGaS2 (b) crystals in T, , and N points of the Brillouin zone.

Figure 4 compares the band structure diagram in the T, , and N points for CuAlS2 and CuInS2 , which band daps are essentially different (3.4 eV and 1.6 eV, respectively).

Fig. 4. The calculated band diagram of electronic structure of CuAlS2 (a), and CuInS2 (b) crystals in T, , and N points of the Brillouin zone.

The assignment of peaks observed in the reflectance spectra of CuAlSe2 , CuGaSe2 , and CuAlS2 to electronic transitions according to the band diagram in Fig. 3a is presented

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in Table 3, while the assignment of peaks observed in the reflectance spectra of CuGaS2 and CuInS2 to electronic transitions according to the band diagram in Fig. 3b is presented in Table 4. Table 3. The assignment of peaks in the reflectance spectra of CuAlSe2 , CuGaSe2 , and CuAlS2 to electronic transitions in the T, , and N points of the Brillouin zone. A1

A2

A6

A7

A8

A9

A10

(V1 ) – (C2 )

(V3 ) – (C2 )

N(V1 ) – N(C1 )

T(V1 ) – T(C1 )

N(V2 ) – N(C1 )

T(V2 ) – T(C1 )

N(V3 ) – N(C1 )

Table 4. The assignment of peaks in the reflectance spectra of CuGaS2 and CuInS2 to electronic transitions in the T, , and N points of the Brillouin zone. A1

A2

A3

A4

A5

A6

A7

A8

A9

A10

N(V1 ) – N(C1 )

N(V2 ) – N(C1 )

N(V3 ) – N(C1 )

(V1 ) – (C2 )

(V2 ) – (C2 )

T(V1 ) – T(C1 )

T(V2 ) – T(C1 )

T(V1 ) – T(C2 )

T(V2 ) – T(C2 )

T(V3 ) – T(C2 )

As mentioned above, the distance between the positions of the low energy A1 and A6 peaks in CuAl1-x Gax Se2 solid solution increases with increasing the x value from 0 to 1. It means that the rate of the shift of the A1 peak position to lower photon energies with increasing the x value is higher than the rate for the A6 peak. Then, according to the assignment in Table 3, one can conclude that the rate of the decrease of the energy interval N(V1 ) – N(C1 ) is lower than the rate for the (V1 ) – (C2 ) interval. Similarly, the decrease of the distance between the positions of the high energy A8 and A10 peaks, with increasing the x value from 0 to 1, means that rate of the shift of the A8 peak position to lower photon energies is lower than the rate for the A10 peak, i.e. the rate of the decrease of the energy interval N(V2 ) – N(C1 ) is lower than the rate for the N(V3 ) – N(C1 ) interval. From the comparative analysis of the shift of A6 , A7 , and A8 peaks, one can conclude that the rate of the decrease of the energy intervals N(V1 ) – N(C1 ), T(V1 ) – T(C1 ), and N(V2 ) – N(C1 ), with increasing the x value from 0 to 1, is nearly the same. Moreover, from the comparative analysis of data in Tables 1, 2, 3 and 4, and the band diagram of electronic structure in Figs. 3 and 4, one can conclude that the energy interval between the valence band V1 and the conduction band C2 in the  point of the Brillouin zone of CuIn1-x Gax S2 solid solutions is larger than the energy interval between the valence band V1 and the conduction band C1 in the N point, while for CuAl1-x Gax Se2 solid solutions the relations are vice-versa. The observed trends in the evolution of the energy band structure of chalcopyrite CuBIII XVI 2 compounds with variation of their composition are helpful for a right assignment of the observed peaks in the reflectance spectra to respective electronic transitions in various points of the Brillouin zone.

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5 Conclusions The results of this study demonstrate possibilities of revealing the trends in the evolution of the energy band structure of chalcopyrite CuBIII XVI 2 compounds from the optical reflectance spectra with changing their composition, and the helpfulness of the found trends for a right assignment of the observed peaks in the reflectance spectra to respective electronic transitions in various points of the Brillouin zone. The experimental observations point to the following trends: (i) the rate of the decrease of the energy interval N(V1 ) – N(C1 ) in CuAl1-x Gax Se2 solid solution with increasing the x value from 0 to 1is lower than the rate of the decrease for the (V1 ) – (C2 ) interval; (ii) the rate of the decrease of the energy interval N(V2 ) – N(C1 ) is lower than the rate of the decrease for the N(V3 ) – N(C1 ) interval; (iii) the rate of the decrease of the N(V1 ) – N(C1 ), T(V1 ) – T(C1 ), and N(V2 ) – N(C1 ) energy intervals with increasing the x value from 0 to 1 is nearly the same; (iv) the energy interval between the valence band V1 and the conduction band C2 in the  point of the Brillouin zone of CuIn1-x Gax S2 solid solutions is larger than the energy interval between the valence band V1 and the conduction band C1 in the N point, while for CuAl1-x Gax Se2 solid solutions the relations are vice-versa. The deduced trends are important for practical applications. Acknowledgments. This research was funded by National Agency for Research and Development of Moldova under the Grant #22.80009.5007.20.

Conflict of Interest. The authors declare no conflicts of interest. The funding sponsors had no role in the following actions: the design of the study; the collection, analyses, or interpretation of data; the writing of the manuscript, and the decision to publish the results.

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Optical and Photoelectric Properties of Cadmium Diarsenide and Surface-Barrier Structures Based on It Ivan Stamov(B) and Dmitry Tkachenko Pridnestrovian State University, Tiraspol, Republic of Moldova [email protected]

Abstract. The optical properties of CdAs2 single crystals are studied in polarized light. The features of the fundamental absorption edge and the splitting of bands in the field of the crystal lattice are studied. Birefringence indices in the wavelength range from 3 μm to the absorption edge were determined by the method of interference of polarized beams. The necessity of taking into account the complex nature of the dielectric functions of a crystal when calculating birefringence in the near-edge region of intrinsic absorption is shown. A technology has been developed for obtaining ohmic and rectifying contacts on CdAs2 crystals. The electric and photoelectric properties of surface-barrier structures based on CdAs2 have been studied. The determining influence of deep levels and the ratio of deep to shallow levels on the properties and parameters of the contact barrier is found. The optimal properties of the semiconductor and metal are determined to obtain the most pronounced photoelectric effect. It has been established that the photocurrent spectrum has features of the band structure of the crystal, an indirect-gap extreme fundamental development with a band gap of ~ 1 eV, and optical transitions in the deep zone. Impurity absorption bands are found, one of which is adjacent directly to the absorption edge and has a significant effect on the photoelectric effect boundary. The dependence of the height of the potential barrier of metal contact with cadmium diarsenide on the work function of the metal has been established. In addition, a significant decrease in the barrier in the near-contact region was found, which has a significant or decisive effect on the spectral distribution of the photocurrent, depending on the electrical properties of the semiconductor. For example, the Fowler electron emission photocurrent is detected only when the barrier is reverse-biased, while in the short-wavelength region of fundamental absorption, the photocurrent is suppressed or reverses its sign. The presented studies show the potential possibilities of using CdAs2 to create active structures and the need to improve the technology for obtaining crystals with desired properties. Keywords: Optics · Crystals · Birefraction · Chromatic polarization · Photoelectric effect · Surface-barrier structures

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 115–122, 2024. https://doi.org/10.1007/978-3-031-42775-6_13

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1 Introduction Cadmium diarsenide (CdAs2 ) is a promising semiconductor for creating optical elements and optoelectronic devices, the principle of operation of which is based on the features of the crystal and energy structure, anisotropy of properties [1]. The optical properties of CdAs2 were examined in [2–4], from which it follows that the fundamental absorption edge is formed by forbidden and allowed indirect transitions to exciton states and the participation of phonons. In addition, the edge splitting is insignificant ~ 5 meV, while the birefringence is the largest of known birefringent crystals [3]. It was also determined in [2, 5] that CdAs2 is a gyrotropic crystal with a significant gyration coefficient. It is also known that CdAs2 crystals have a significant anisotropy of electrical properties in electric and magnetic fields, and the nature of the anisotropy is interpreted in a ambiguous way in a number of works. The photoconductivity of cadmium diarsenide (CdAs2 ) was studied in [2, 4, 6]. The light absorption coefficient was found for its values less than 20 cm−1 from the photoconductivity spectra. However, the photoconductivity spectra were measured in a narrow wavelength range and do not contain information on absorption in the impurity wavelength region and in the fundamental absorption depth. The elucidation of the reasons for the anisotropy of the optical and electrical properties of these crystals is an important task for the creation of optoelectronic devices based on them and for the technology of obtaining starting materials and crystals. In this paper, we present the results of studying the transmission and interference spectra of polarized rays for CdAs2 crystals, as well as the electrical and photoelectric characteristics of surface-barrier structures based on them.

2 General Results Cadmium diarsenide crystals were grown from the gas phase in a temperature gradient of 30 °C at a synthesized material temperature of 550 °C. Undoped crystals with electronic type of conductivity and resistivity of 50 and 500 ·m were studied. Surfacebarrier structures were created on natural growth faces (010) and machined plates of the required orientation. We assumed that, by analogy with cadmium and zinc phosphides of the electronic type of conductivity, a potential barrier, the Schottky barrier, whose photoelectric properties are much more informative than the photoconductivity spectra [7], will form at the interface of the metal with cadmium diphosphide [7]. Au and Pt were chosen as the metal for creating the barrier, taking into account the above circumstances. Semitransparent barrier metal films on the verge of natural growth and cut out plates from single crystals were obtained by electrochemical deposition from electrolytes. Optical and photoelectric properties were measured at room temperature on a spectrometer with a resolution of 0.1 nm. In the study of the optical and photoelectric properties of cadmium diarsenide in polarized light, Glan-Thomson polarizers were used. Figure 1 shows the transmission spectra in polarized light of CdAs2 crystals. The absorption edge, as shown in [1–4, 6], is characteristic of indirect optical transitions. The band gap Eg, determined from the dependence of the absorption coefficient, is 0.998 and 1.006 eV for light polarizations Ec and E⊥c, respectively. The obtained

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Fig. 1. Transmission spectra of the crystal in polarized light: 1 - E⊥c, 2 - Ec.

values of Eg are close to the values obtained in the works cited above and the theoretical values from the electronic spectrum of CdAs2 calculated from first principles based on the density functional method [8]. A feature of these spectra is the difference in the edge splitting from the values in other studies, which is ~ 30 meV. The interference spectra of the polarized rays of the crystal in the system polarizer (P) - crystal - analyzer (A), excited in the directions Ec and E⊥c are shown in Fig. 2. The optical axis of the crystal in these experiments is located in the plane of the crystal plate and at an angle of 45° with respect to the light polarization vector. The transmission coefficient for parallel oriented transmission directions of the polarizer and analyzer, without taking into account reflection and losses in the polarizers and the plate under study, is determined by the expression [9]: T = cos2 [π d (ne − no )/λ]

(1)

or T* = 1 − T - at perpendicular transmission directions of the polarizer and analyzer. Here no and ne are the refractive indices of the ordinary and extraordinary rays; d - is the plate thickness; λ - is the wavelength. The theoretical description of the reflection and transmission of light in anisotropic absorbing media is presented in [9, 10]. When calculating the refractive and birefractive indices from the spectra of multipath interference and birefractive interference, the recurrent relations nm = λm /λm-1 ·nm-1 + λm /4·d and nm = λm /λm-1 ·nm-1 + λm /2·d and known, quite accurately, the values of these quantities in the long-wavelength region (transparency region). It was also assumed that their values change monotonically between interference extrema.

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Fig. 2. The interference spectra of polarized beams of a crystal in the system polarizer (P) - crystal - analyzer (A) in the configuration: 1 - P⊥A, 2 - PA.

The birefringence dispersion was determined from these spectra (Fig. 3). The coincidence of the birefringence value with the known data measured by the small-angle prism method at a wavelength of 1.3 μm does not exceed the errors of the methods. It follows from the dispersion curve that the contribution of edge absorption to birefringence is not as significant as in the dispersion of birefringence in the region of the absorption edge during direct transitions.

Fig. 3. The birefringence dispersion of CdAs2 crystals.

Figure 4 shows the photocurrent spectra of the Pt - CdAs2 structure in unpolarized light in the photon energy range 1.1–3.25 eV at positive and negative bias voltages. The edge of the photocurrent corresponds to the absorption of light associated with indirect

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forbidden transitions in accordance with the interpretation of the band structure of the crystal [8].

Fig. 4. The photocurrent spectra of Pt-CdAs2 in polarized rays: 1 - Ec, 2 - Ec (A); cpektpy fototoka Au- CdAs2 in unpolarized rays for different values of the reverse bias (B).

The photocurrent in the long-wavelength region (λ >1.2 μm) corresponds to the emission of electrons from the metal (Fig. 5, A, B). Figure 5c shows the root dependence of the photocurrent on the photon energy. The potential barrier height for the Pt-CdAs2 and Au-CdAs2 structures, obtained from the Fowler dependence, is 0.52 eV and 0.47 eV, respectively. The spectrum of the long-wavelength signal in the Pt, AuCdAs2 structures shows features at photon energies of 0.574, 0.580, and 0.744 eV, the nature of which can be associated with impurity absorption. In the spectral region of the beginning of the fundamental absorption edge, the thermionic current decreases. A similar effect is observed in Schottky barriers on zinc and cadmium diphosphides of electronic conductivity. This effect increases with an increase in the height of the contact barrier and is associated with the generation of holes and recombination processes when the height of the contact barrier is ≥ ½ Eg. Up to the fundamental absorption edge, the photocurrent spectra in the wavelength range 0.82–0.98 eV exhibit a broad band, the intensity of which varies from sample to sample, the nature of which is of impurity origin. In the region of fundamental absorption, the photocurrent contains a number of features a1 –a4 , due to optical transitions to higher extrema of the band structure of the crystal. In [12], according to the calculations of optical functions based on the Kramers-Kronig relations for the reflection spectra, a1 appears as a small maximum in the polarization Ec, a2 , a3 , a4 in the form of an inflection. According to theoretical calculations, the maximum a1 (1.313 eV) in the polarization Ec corresponds to direct transitions, the features a2 (1.61), a3 (2.065) and a4 (2.405) are possibly associated with transitions at the Z, G, P points of the band structure [7], but their values differ significantly from the calculated ones. In the surface-barrier structures under study, the photovoltage spectra contain features of a general nature. The emission photocurrent from the metal at zero bias is absent and appears at a slight reverse bias voltage; in the short-wavelength region, the photocurrent value significantly decreases when a certain wavelength is reached in structures on

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Fig. 5. Photocurrent spectra of Au-CdAs2 in the long-wavelength region (A), root dependence of the photocurrent for structures (B): 1 - Au-CdAs2 , 2 - Pt-CdAs2 .

machined surfaces and changes sign in structures obtained on natural growth faces. This behavior of the photocurrent is associated with the formation of an opposite sign potential in the near-contact region of the semiconductor, the field of which prevents the thermionic transition of electrons from the metal to the semiconductor and is responsible for the decrease or inversion of the sign of the photocurrent during the generation of charge carriers in the short-wavelength region of the spectrum. The electrical characteristics of the contact current-voltage (CV) (A), capacitancevoltage (CV) (B) and impedance frequency characteristics are presented on the Fig. 6A,B,C from which it follows that the barrier is formed by a slight bending of the zones, the concentration of shallow donors is less than deep ones, the depth of which is ~ 0.25 eV [13]. The current in the Au-CdAs2 and Pt-CdAs2 structures is limited by the series resistance, both in the forward and reverse bias voltages. The current-voltage characteristics of the structures under study are typical for the cases of barriers with a slight bending of the bands. The decrease in the barrier at the interface Eb , in addition to the Schottky effect, may be associated with an increase in the concentration of defects in the near-surface region (Fig. 6D). The presented diagram is heuristic due to the lack of complete information on the spectrum of defect levels in the band gap of cadmium diphosphide. The mechanism of the formation of such a barrier profile cannot be associated with the pinning of the Fermi level by the surface states of the contact, but recombination processes involving deep levels in the near-surface region of the semiconductor can take part in its formation. The construction of an energy diagram of the Pt -, Au - CdAs2 surface-barrier structures, which will make it possible to explain the features of the electrical and photoelectric properties, will require additional research.

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Fig. 6. The electrical characteristics of surface-barrier structures Pt – CdAs2 on the crystals with the concentrations of of charge carriers n, cm−3 : 1–1017 , 2–1014 (A, C); the characteristics of complex conductivity of surface-barrier structures Au - CdAs2 for the frequency 1 kHz, n = 1017 cm−3 (B); the contact energy diagram Au -, Pt - CdAs2 (D).

3 Conclusions Thus, from the presented studies of the optical properties of cadmium diarsenide, it follows that it is necessary to take into account the complex nature of the dielectric functions of the crystal in the spectral regions adjacent to the fundamental absorption edge. The absorption edge is formed by indirect transitions, with slightly different values in polarized light. In accordance with the experimental data obtained, a potential barrier is formed at the interface of the semiconductor with the metal, the value of which depends on the work function of the metal and the electronic affinity of the semiconductor and is ~ ½ E g with Au and Pt. Features of the photoelectric effect manifest themselves in the appearance of a decelerating field in the near-contact region and lead to limitation of electron emission from the metal into the semiconductor in the structures under study.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Marenkin, S.F., Trukhan, V.M.: Phosphides, Zinc and Cadmium Arsenides. Publisher Varaskin A.N, Minsk (2010) 2. Morozova, V.A., Semenenya, T.V., Loseva, S.M., Koshelev, O.G., Marenkin, S.F., Raufman, A.M.: Determination of band structure parameters of CdAs2 by optical transmission and photoconductivity methods. Phys. Solid State 29(3), 393–399 (1995)

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3. Marenkin, S.F., Raufman, A.M., Matiyuk, I.N., Lasarev, V.B.: Birefrigence and optical activity of cadmium diarsenide single crystals. Opt. Eng. 33(9), 3034–3037 (1994) 4. Marenkin, S.F., Morozov, V.A.: Synthesis and optical properties of CdAs2 single crystals. Inorg. Mater. 35(10), 1190 (1999) 5. Lazarev, V.B., Malinko, V.N., Marenkin, S.F., Matyuk, I.N., Sokolovsky, K.A.: Gyrotropy of semiconductor single crystals CdAs2. Izvestiya AN SSSR. Inorgan. Materials 21(7), 1082– 1085 (1985) 6. Morozova, V.A., Marenkin, S.F., Semenenya, T.V., Koshelev, O.G., Raukhman, A.M.: Tech. peculiarities of photoelectric properties of cadmium diarsenide. Inorganic materials 33(10), 1183–1189 (1997) 7. Stamov, I.G., Tkachenko, D.V.: Photoelectronic phenomena and charge transfer in surfacebarrier structures based on n-type zinc and cadmium diphosphides IV. Bulletin of PSU 3(35), 66–75 (2009) 8. Basalaev, Y.M., Kopytov, A.V., Poplavnoy, A.S., Polygalov, Y.I.: Ab initio study of the electronic and vibrational structures of tetragonal cadmium diarsenide. Phys. Technol. Semiconductors 51(6), 815–820 (2017) 9. Volkova, I.E.: Polarization Measurements. Publishing house of standards, Moscow (1974) 10. Fedorov, F.I.: Optics of Anisotropic Media. Editorial URSS (2004) 11. Furs, A.N., Petrov, N.S.: Light reflection and transmission by the plane-parallel anisotropic plate. J. Appl. Spectrosc. 83(1), 163–166 (2016) 12. Kozlov, A.I.: Features of the optical spectra of zinc and cadmium di-arsenides in the range 1–12 eV/V. In: Kozlov, A., Croitoru, S.G., Sobolev, V.V. (eds.) Abstracts of the 7th All-Union Conference on Physics of Vacuum-Ultraviolet Radiation. (Riga) Leningrad State University (1986) 13. Mollaev, A.Y.: Electronic transport phenomena in binary and ternary semiconductors in a range of the polymorphous transformation at high pressure (Review). Phys. High Pressures Technol. 14(1), 34–43 (2004)

Preliminary Study on Silver Nanoparticle Synthesis Through Chemical and Biological Methods Ramona Mirela Plesnicute1 , Anamaria Vacariu1 , Iuliana Motrescu2 and Dorina Creanga1(B)

,

1 Physics Faculty, Alexandru Ioan Cuza University, Iasi, Romania

[email protected] 2 Horticulture Faculty, Ion Ionescu de la Brad University of Life Sciences, Iasi, Romania

Abstract. This research was focused on the synthesis and characterization of silver nanoparticles, known for their various utilizations in the people daily life. Aiming the promotion of eco-friendly nanoproducts, we present a comparison between the properties of silver nanoparticles synthesized by chemical reduction method and biological method based on the reduction with antioxidants from plant extract. The formation of colloidal silver nanoparticles was evidenced by the recordings of the characteristic spectral band (due to the phenomenon of localized surface plasmon resonance) in the UV-Vis range for both types of samples. The citrate-AgNP suspension, synthesized by reduction with trisodium citrate reagent, was characterized by relatively broad band with maximum at 425 nm while green tea-AgNP suspension, synthesized with green tea leaf extract, presented the characteristic band with maximum at about 445 nm. Highest intensity of LSPR band was noticed for the AgNPs synthesized by the eco-friendly method denoting remarkably better efficiency of the biological reducers. The good granularity in the nanometric domain was revealed by Scanning Electron Microscopy for both samples while the elemental composition was confirmed by Energy Dispersive Spectroscopy. The optimization of the biological synthesis with green tea extract is designed to efficiently provide silver nanoparticles for biomedical applications that are more friendly with the environment. Keywords: Silver nanoparticles · Colloidal suspensions · Green synthesis

1 Introduction Among the various metallic nanoparticles used in biomedical applications, silver nanoparticles (AgNPs) are ones of the most studied and interesting nanomaterials, with many utilizations in cosmetics (antiseptic lotions and unguents), textiles (silver nanoparticles incorporated into the fabric), sanitation (water purification, sterile dressings), dentistry (implant coatings, periodontal biofilms) and environment remediation by removing contaminants from wastewater [1, 2]. In [3] the authors described the impregnation of textile material by conducting the silver reduction in situ, i.e., by supplying first the solution of silver precursor and then the reducer onto the material fibers and thus, finally, yielding a product with antibacterial features, mainly against B. subtilis. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 123–133, 2024. https://doi.org/10.1007/978-3-031-42775-6_14

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Due to their antibacterial, antifungal, antiviral, anti-inflammatory, and anti-cancer properties the silver nanoparticles can be successfully used in biomedicine [4–6] for drug carriers, and contrast agents in imagistic. In [7] the authors reported the production of silver based drug delivery novel system that includes AgNPs provided by reduction with hyaluronic acid which balance silver nanotoxicity on healthy cells and, in the same time, is able to bind various anti-cancer drug molecules. Nevertheless, the exceptional optical and spectral features made nanosized silver very useful in optoelectronics (surface enhanced Raman spectroscopy (SERS), catalysis (bleaching of some organic dyes), biosensing, and biological labelling [8, 9]. SERS (surface-enhanced Raman spectroscopy) based on silver nanoparticles can be useful tool in the detection and identification of food-born pathogens by means of SERS as described in [10] with focus on E. coli., S. aureus and Salmonella. Various studies were dedicated to the optimization of the synthesis methods with high efficiency, good stability, nanoparticle controllable properties and reduced costs, that are more and more careful regarding the impact on the environment [11–13]. A promising way to reach these objectives is the application of green chemistry or biological procedures to yield ecofriendly silver nanoparticles by using plants and plant extracts. Due to the high content of antioxidants (polyphenols, mainly catechines) the green tea leaves are studied by several research groups for the eco-friendly synthesis of AgNO3 . The authors of [14] reported AgNP synthesis by using relatively large amounts of reagents - 750 mL silver nitrate 10 mM and 25 mL extract of green tea leaves (14.8 g boiled in 100 mL deionized water). The AgNP suspension was centrifuged and rinsed before the analysis. The main UV-Vis band of about 0.7 intensity at 450 nm wavelength was accompanied by a small shoulder at larger wavelengths denoting large size particle sub-population while SEM diameter was found between 25 nm and 75 nm. In [15] the authors synthesized AgNPs using 100 mL AgNO3 1 mM and 12 mL of green tea leaf extract (10 g dried leaves boiled in 100 mL deionized water). The spectral band revealed in the visible domain has about 2 units intensity in the maximum at 436 nm being accompanied also by relatively intense shoulder at 396 nm. The authors of [16] used 75 mL of green tea extract (2 g leaves in 100 mL deionized water) and 75 mL AgNO3 1 mM in the presence of NaOH 1 M. The spectral band in the visible range had 0.75 intensity in the maximum at 410 nm. In [17] it was reported the silver nanoparticle yielding from 1 g green tea leaves boiled in 30 mL of deionized water and diluted at 20% and 10 mL 10 mM AgNO3 solution. The characteristic band of 0.9 intensity. We have adapted the procedure described in literature [14] for silver reduction with green-tea aiming the synthesis optimization.

2 Experimental 2.1 Materials Crystallized powders of silver nitrate (AgNO3 ) and trisodium citrate (TSC, C6 H9 Na3 O9 ) dihydrate were used [18] without any purification for silver nanoparticle chemical synthesis. Green tea dry leaves of commercial provenience and silver nitrate were used for silver nanoparticle biological synthesis.

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2.2 Synthesis Chemical method. Based on Frens method proposed for gold nanoparticles [19] 5 mL of 1% trisodium citrate (1 g trisodium citrate dihydrate to 100 mL distilled water) were added to 50 mL of 0.001 M AgNO3 (0.0084 g AgNO3 in 50 mL of distilled water) and let to about 85 °C on the heating plate under magnetic stirring (700 rpm) until the color turned out to light yellow (Scheme 1). Then the reaction product was left to cool down at room temperature. Biological method. Green tea dry leaves (about 7 g) were crushed in a granite mortar and added to 400 mL distilled water in an adequate Erlenmeyer flask that was put on the heating plate at about 60 ºC and let for 50 min with 10 more minutes for boiling. Separately, 0.0084 g of AgNO3 was added to 50 mL of distilled water (1 mM solution) and mixed with 5 mL of green tea extract at room temperature. The mixture was placed for 5 min on the heating plate with magnetic stirrer at 40 ºC and 700 rotations per minute. After 5 min, the mixture was let to cool down at room temperature while its color changed from yellow to brown which indicated the formation of silver nanoparticles (Fig. 1).

Fig. 1. Chemical and biological synthesis of silver nanoparticles

This procedure was meant to be an optimization of that proposed in literature [14] as presented below in the next chapter. 2.3 Investigation Methods The laboratory devices that we used for the synthesis of silver nanoparticle colloidal suspensions were: (i) heating plate with magnetic stirrer, type Witeg MSH-20D; (ii) semi analytical balance (with an accuracy of 10–4 g) type Adam, having a maximum weighing threshold of 250 g; (iii) Hettich type laboratory centrifuge operating at 15000 rotations/minute; (iv) UV-VIS spectrophotometer type Shimadzu PharmaSpec 1700, with 1 cm quartz cuvettes, provided with specialized software for data acquisition and processing.

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The investigations by Scanning Electron Microscopy (SEM) and EDS (Energy Dispersive X-Ray Spectroscopy) were carried out on the samples directly pipetted on aluminum stubs and let to dry. An ESEM device was used, (type Quanta 450 from FEIThermoScientific, USA, that does not require covering the samples with a conductive layer. The analyses were performed in low vacuum conditions (100 Pa) in water vapor atmosphere with an electron beam accelerated at 10 kV. After the imaging by SEM recordings, the elemental composition of the samples was analyzed by EDS using the detector of EDAX module (type Ametek Inc., USA) mounted inside the ESEM device chamber. The measurements were semi-automatically done on a selected area from each sample, with specific spots of interest. For processing, the background of the spectra was manually selected, and the software TEAMS used to record the measurements, automatically calculating the chemical composition in relative percentage for the main detectable elements in the samples.

3 Results and Discussion 3.1 AgNPs Synthesized by Chemical Method UV-Vis recording revealed (Fig. 2) a large band suggesting size polydispersity, generated mainly by localized surface plasmon resonance (LSPR) phenomenon, characteristic to citrate coated AgNPs [20] with the maximum intensity at 425 nm and about 0.34 intensity.

Fig. 2. The AgNP characteristic spectral band for the chemical synthesis with TSC confirms the successful reduction of silver ions to silver atoms associated in nanoparticles. Relatively small intensity in the band maximum suggests the modest reduction capacity of TSC. The maximum intensity at 425 nm was of about 0.34.

In Fig. 3, the AgNPs visualized by SEM are presented. The smaller AgNPs are marked in circles while larger particles and particle associations are marked in squares. The smallest nanoparticles evidenced in the pictures (in black circles) appears to have tens of nm order (up to 20 nm size) and symmetrical shape while largest particles and particle aggregates have cubic crystallization form and up to hundreds of nanometers.

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Fig. 3. SEM image with silver particles resulted from chemical reduction with TSC shows individual nanoparticles under 20 nm size together with certain nanoparticle aggregates.

EDS analysis, carried out on larger particle groups has revealed the elemental composition of the sample - deposited on aluminum support, that appears with highest peak (Fig. 4).

Fig. 4. Elemental analysis on citrate-AgNPs synthesized by chemical reduction evidenced the silver peak corresponding to L shell emission line.

In Fig. 4, apart from the highest peak corresponding to the aluminum support, the silver presence is emphasized (0.65% from the sample elements weights at the analyzed spot and 0.09% from elements abundance, Table 1). Carbon and oxygen are originating from the organic components (mainly citrate ions either unreacted or attached to AgNP surface and acting as stabilizing capping agent); copper could be an impurity in the installation or in the sample metal support. The silver nanoproduct remained stable in refrigerator for couple of months.

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Weight (%)

Atomic (%)

Error (%)

CK

53.56

60.41

9.06

OK

10.94

10.54

10.32

CuK

0.63

0.16

4.24

AlK

34.31

19.80

4.67

AgL

0.65

0.09

7.80

3.2 AgNPs Resulted from Biological Method In Fig. 5a, the result of the spectral investigation is presented for the freshly prepared green tea-AgNP sample.

Fig 5. a) The characteristic band of green tea-AgNP fresh suspension (30 min after cooling) confirms the good reducing capacity of the used plant extract. Several time higher intensity in the band maximum was revealed compared to previous case. b) LSPR band characteristic to green tea-AgNP sample immediately after centrifugation and resuspension has smaller intensity.

We found that at 30 min after cooling, the main absorption peak is positioned at 447 nm wavelength with 2.35 intensity (Fig. 5b), compared to 455 nm reported in [14]. This difference could be caused by some differences in the coating molecular shell, since the green tea composition (qualitative and quantitative) could differ (and the coating shell contributes to the characteristics of the UV-Vis recorded spectrum, due to the changings in the refractive index and dielectric constant of the proximal vicinity). Further we searched for the optimization of the synthesis procedure by centrifugation for 10 min at 1500 rpm of AgNP pristine suspension and the deposit resuspension in water as well as by allowing the suspension ageing at room temperature. After this process, the resuspended deposit in aqueous suspension was analyzed by UV-Vis, the results being presented in Fig. 6a. A relatively large band of small intensity (about 0.4) having the maximum at about 445 nm was evidenced.

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Fig. 6. a). The increased spectral band of green tea-AgNPs, 3 h after resuspension. The biological reduction of silver has continued, the new AgNPs increasing the band intensity. b). The LSPR band of 72 h aged suspension of green tea-AgNPs resuspended. The band intensity increased during the storage at room temperature denoting the completing of the reduction process.

After 2 h and 30 min (sample kept at room temperature for total time duration of 3 h), the suspension of AgNPs obtained by biological reduction with green tea was analyzed again assuming that the reduction reactions continue to develop in time. Indeed, as presented in Fig. 6b the intensity of the band maximum has increased significantly (to over 3.05), shaping the spectral band as having the maximum positioned at about 448 nm. The sample was further studied for ageing modifications being let in darkness for 72 h in a refrigerator. The new UV-Vis spectrum (Fig. 7) of the aged suspension is rather similar to previous one (maximum position at 445 nm) except the characteristic band was better shaped at shorter wavelengths. We could assume that in the range of UV domain (under 300 nm) the spectral recording evidenced the contribution of certain free antioxidant molecules from the green tea extract which, after ageing, have reacted with free silver ions remained unreacted previously, thus the absorption under 300 nm diminishing together with slight increase (over 3.37 intensity) in the main band intensity (due to new nanoparticle formation). After that time interval no longer changes in the sample spectral band intensity could be noticed, the AgNP suspension remaining stable for two months. In contrast, no changes were observed in the chemical reduced AgNPs. In Fig. 7 the nanoparticles synthesized with green tea extract as complex reducer are presented – having dimensions of tens of nm (under 50 nm) up to hundreds of nm aggregates and apparently cubical shape; part of them is associated in aggregates or clusters of hundreds of nm size. The remained compounds from the tea extract, especially mineral ones could be responsible for the apparent higher density of the green tea-AgNP nanostructures in the suspension- compared to citrate-AgNP sample. In Fig. 8, the results of the EDS investigation are presented; along with the highest peak corresponding to the aluminum support, the silver is present (1.35% from the sample elements weights at the analyzed spot and 0.18% from elements abundance, Table 2) accompanied by carbon and oxygen

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Fig. 7. SEM image of AgNPs synthesized by biological reduction with green tea extract. Along with individual nanoparticles up to 50 mn size) certain particle aggregates were evidenced as short AgNP clusters.

Fig. 8. Elemental analysis on AgNP synthesized by biological reduction with green tea extract. Silver presence is given by the L shell emission line.

from the organic components; these ones contributed partially to the silver ion reducing and also to their capping – that resulted in the colloidal silver nanoparticles. In Table 3 the summary of the spectral results is presented. Copper and magnesium together with other elements (calcium, iron, potassium, phosphorus etc.) are known to cover about 5% of the green tea leaf dry weight [21]. The differences in the spectral behavior of the AgNP band from the visible domain could be better described by discussing the nature of the capping molecular shell. While the citrate ions from citrate-AgNP sample are the only organic interaction partners of silver grains, in the green tea-AgNP colloidal suspension there are at least several groups of organic antioxidants like catechin and its derivatives, flavonoids and phenolic acids [22], generally higher molecular structures than citrate, covering the silver cores with a

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Table 2. The results of the elemental analysis for the green tea-AgNPs. Element

Weight (%)

Atomic (%)

Error (%)

CK

51.71

66.21

10.47

OK

18.54

17.82

11.67

CuL

1.40

0.34

14.74

MgL

0.19

0.12

33.96

AlK

26.91

15.33

4.13

AgL

1.35

0.18

20.98

Table 3. Summary of spectral band quantitative features. Reducer

Light extinction (a.u.)

Maximum SPR band (nm)

Trisodium citrate

0.344

425

Green tea at 30 min

2.354

447

Green tea at 3 h

3.055

448

Green tea at 72 h

3.374

445

more complex dielectric shell able to have higher influence of the suspension interaction with light. The efficiency of the described biological synthesis consists in the higher concentration of nanoparticles in the reaction mixture as could be seen from the relative intensities of the characteristic band intensity (0.34 for citrate-AgNPs and over 2.35 in green tea-AgNPs, Table 3) – in accord to previous reports that assessed the citrate as a modest reducer for silver, thus in the mixture some unreacted silver ions as well as the and citrate ones [23], could remain. The green synthesis proposed inhere lasts considerably less time than that based on blueberry extract where the reaction completion takes up to 14 days [24], or the green synthesis using strawberry fruit extract which involves the total reaction time of 18 days [25]. More, regarding the biological impact of green tea-AgNPs designed for biomedical applications, the benefits of the investigation/therapy methods are supplemented by the benefic role of antioxidant coated nanoparticles.

4 Conclusions Colloidal silver nanoparticles were synthesized by adapted biological synthesis with green tea extract, that resulted in much higher intensity of the characteristic spectral band than in the case of chemical synthesis with trisodium citrate. This was the prove that silver reduction occurred with high efficiency following the utilization of green tea extract, rich in antioxidants. Resuspension and ageing led to higher intensity of LSPR band than after the basic steps proposed by other authors. Thus, controlled ageing for about three days (72 h) appeared as suitable step in the reaction completing, the synthesized suspension of the

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silver nanoparticles remaining stable for two more months. This could represent the main improvement in the proposed synthesis protocol, along with the specific array of concentrations and volumes of the used reagents. Nevertheless, the reaction completion of this eco-friendly procedure of preparing silver nanoparticles requires 5–6-time shorter time duration than in the case of using various berries. Further research will be dedicated to nanotoxicity comparison between the AgNPs produced by the two types of silver reduction.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Ganguly, K., Dutta, S.D., Patel, D.K., Lim, K.T.: Silver nanoparticles for wastewater treatment. In: Aquananotechnology, pp. 385–401 (2021). https://doi.org/10.1016/B978-0-12-821 141-0.00016-1 2. Beyene, H.D., Werkneh, A.A., Bezabh, H.K., Ambaye, T.G.: Synthesis paradigm and applications of silver nanoparticles (AgNPs), a review. Sustain. Mater. Technol. 13, 18–23 (2017). https://doi.org/10.1016/j.susmat.2017.08.001 3. Montes-Hernandez, G., et al.: In situ formation of silver nanoparticles (Ag-NPs) onto textile fibers. ACS Omega 6(2), 1316–1327 (2021). https://doi.org/10.1021/acsomega.0c04814 4. Abou, E.N.M., Eftaiha, A., Al-Warthan, A., Ammar, R.: Synthesis and applications of silver nanoparticles. Arab. J. Chem. 3(3), 135–140 (2010). https://doi.org/10.1016/j.arabjc.2010. 04.008 5. Ma, Y., Shi, L., Liu, F., Zhang, Y., Pang, Y., Shen, X.: Self-assembled thixotropic silver cluster hydrogel for anticancer drug release. Chem. Eng. J. 362, 650–665 (2019). https://doi.org/10. 1088/0957-4484/20/16/165501 6. Gherasim, O., Puiu, R.A., Birca, A.C., Burdusel, A.C., Grumezescu, A.M.: An updated review on silver nanoparticles in biomedicine. Nanomaterials 10(11), 2318 (2020). https://doi.org/ 10.3390/nano10112318 7. Hussein, H.A., Abdullah, M.A.: Novel drug delivery systems based on silver nanoparticles, hyaluronic acid, lipid nanoparticles and liposomes for cancer treatment. Appl. Nanosci. 12, 3071–3096 (2022). https://doi.org/10.1007/s13204-021-02018-9 8. Li, W., Guo, Y., Zhang, P.: SERS-active silver nanoparticles prepared by a simple and green method. J. Phys. Chem. C 114(14), 6413–6417 (2010). https://doi.org/10.1021/jp100526v 9. Serra, A., et al.: Non-functionalized silver nanoparticles for a localized surface plasmon resonance-based glucose sensor. Nanotechnology 20(16), 165501 (2009). https://doi.org/10. 1088/0957-4484/20/16/165501 10. Wei, C., Li, M., Zhao, X.: Surface-enhanced raman scattering (SERS) with silver nano substrates synthesized by microwave for rapid detection of foodborne pathogens. Front. Microbiol. 9, 2857 (2018). https://doi.org/10.3389/fmicb.2018.02857 11. Islam, M.A., Jacob, M.V., Antunes, E.: A critical review on silver nanoparticles: from synthesis and applications to its mitigation through low-cost adsorption by biochar. J. Environ. Manage. 281, 111918 (2021). https://doi.org/10.1016/j.jenvman.2020.111918 12. Yaqoob, A.A., Umar, K., Ibrahim, M.N.M.: Silver nanoparticles: various methods of synthesis, size affecting factors and their potential applications–a review. Appl. Nanosci. 10(5), 1369– 1378 (2020). https://doi.org/10.1007/s13204-020-01318-w

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13. Zahoor, M., et al.: A review on silver nanoparticles: classification, various methods of synthesis, and their potential roles in biomedical applications and water treatment. Water 13(16), 2216 (2021). https://doi.org/10.3390/w13162216 14. Nakhjavani, M., Nikkhah, V., Sarafraz, M.M., Shoja, S., Sarafraz, M.: Green synthesis of silver nanoparticles using green tea leaves: experimental study on the morphological, rheological and antibacterial behaviour. Heat Mass Transf. 53, 3201–3209 (2017). https://doi.org/10.1007/ s00231-017-2065-9 15. Loo, Y.Y., Chieng, B.W., Nishibuchi, M., Radu, S.: Synthesis of silver nanoparticles by using tea leaf extract from camellia sinensis. Int. J. Nanomed. 7, 4263–4267 (2012). https://doi.org/ 10.2147/IJN.S33344 16. Rolim, W.R., et al.: Green tea extract mediated biogenic synthesis of silver nanoparticles: characterization, cytotoxicity evaluation and antibacterial activity. Appl. Surf. Sci. 463, 66–74 (2019). https://doi.org/10.1016/j.apsusc.2018.08.203 17. Widatalla, H.A., et al.: Green synthesis of silver nanoparticles using green tea leaf extract, characterization and evaluation of antimicrobial activity. Nanoscale Advances 4, 911–915 (2022). https://doi.org/10.1039/D1NA00509J 18. http://www.sigmaldrich.com 19. Xia, H., Xiahou, Y., Zhang, P., Ding, W., Wang, D.: Revitalizing the Frens method to synthesize uniform, quasi-spherical gold nanoparticles with deliberately regulated sizes from 2 to 330 nm. Langmuir 32(23), 5870–5880 (2016). https://doi.org/10.1021/acs.langmuir.6b01312 20. Oprica, L., et al.: Citrate-silver nanoparticles and their impact on some environmental beneficial fungi. Saudi J. Biol. Sci. 27(12), 3365–3375 (2020). https://doi.org/10.1016/j.sjbs.2020. 09.004 21. Chacko, S.M., Thambi, P.T., Kuttan, R., Nishigaki, I.: Beneficial effects of green tea: a literature review. Chin. Med. 5(1), 1–9 (2010). https://doi.org/10.1186/1749-8546-5-13 22. Lorenzo, J.M., Munekata, P.E.S.: Phenolic compounds of green tea: health benefits and technological application in food. Asian Pac. J. Trop. Biomed. 6(8), 709–719 (2016). https://doi. org/10.1016/j.apjtb.2016.06.010 23. Yan, S., et al.: Time-resolved small-angle X-ray scattering study on the growth behavior of silver nanoparticles. J. Phys. Chem. C 118(21), 11454–11463 (2014). https://doi.org/10.1021/ jp502482c 24. Kumar, B., Smita, K., Cumbal, L., Debut, A.: Green synthesis of silver nanoparticles using andean blackberry fruit extract. Saudi J. Biol. Sci. 24(1), 45–50 (2017). https://doi.org/10. 1016/j.sjbs.2015.09.006 25. Umoren, S.A., Nzila, A.M., Sankaran, S., Solomon, M.M., Umoren, P.S.: Green synthesis, characterization and antibacterial activities of silver nanoparticles from strawberry fruit extract. Pol. J. Chem. Technol. 19(4), 128–136 (2017). https://doi.org/10.1515/pjct-20170079

Advanced Nanotechnology-Based Approaches to Waste Water Purification from Organic Pollutants Tatiana Datsko1(B)

, Veacheslav Zelentsov1

, and Dmitri Dvornikov2

1 Institute of Applied Physics, Moldavian State University, Chisinau, Moldova

[email protected] 2 IEEN D. Ghitu, Technical University of Moldova, Chisinau, Moldova

Abstract. Advanced nanotechnology-based approaches to waste water purification is attracting more and more attention at the present time. Among several types of advanced oxidation processes (AOPs), heterogeneous photocatalytic decomposition using a solid semiconductor photocatalyst and UV radiation of low concentrated, hihgly toxic, hardly decomposable impurities should be distinguished. One such organic substance that requires wastewater treatment before being discharged into the aquatic system is phenol and its derivatives, which are known to be endocrine disruptors. Heterogeneous photocatalysis using titanium dioxide and ultraviolet radiation has been successfully used in suspended slurry photoreactors. A photocatalyst (NTD) based on nanosized anatase and diatomite as a substrate has been synthesized in an electrolyzer and applied for the photodecomposition of phenol in a slurry-type photoreactor. The parameters affecting the adsorption and the degree of photodecomposition were determined: the initial phenol concentration, pH of the solution, dose of the photocatalyst, and duration of UV irradiation. It is shown that photocatalysis with NTD under UV radiation makes it possible to achieve the degree of purification of the aqueous phenol solution up to the MAC (maximum allowable concentration) level for wastewater (5 mg/l) at an initial phenol concentration of 11 mg/l, a catalyst dose of 2 g/l, pH = 4.5 during 32 min of the process. Keywords: Heterogeneous photocatalysis · Nanosized anatase · Hybrid photocatalyst · Phenol

1 Introduction Activated carbon adsorption, solvent extraction, chemical oxidation and electrochemical processes are the most commonly used methods for removing phenol and phenolic compounds from wastewaters. These approaches are often ineffective because they just transfer the organic pollutants from aqueous phase to solid one without any changes. Nanotechnology offers original approaches to the destruction of organic matter in water up to complete mineralization and has a number of environmental benefits. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 134–146, 2024. https://doi.org/10.1007/978-3-031-42775-6_15

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Heterogeneous photocatalysis with solid catalyst, one of the methods used in today’s advanced oxidation processes (AOP), has been used to remove or mineralize a wide variety of organic and inorganic contaminants [7, 8]. Several semiconductors can act as photocatalysts, but TiO2 stands out as the most effective photocatalyst and has been extensively used in water and wastewater treatment studies because it is cost-effective, thermally stable, non-toxic, chemically and biologically inert, strong photoactive and is capable of oxidation of organic compounds until their complete mineralization to safe compounds, CO2 , H2 O et al. [9]. Heterogeneous photocatalysis using titanium dioxide and ultraviolet radiation has been successfully used in suspended slurry-type photoreactors [10–12]. It is known that TiO2 has the highest activity in the form of nanosized particles (TNPs), which imposes certain restrictions on its use during the process, namely: a strong tendency of nanoparticles to aggregate, which significantly reduces their photoactivity up to its complete disappearance, difficulties in extracting them from the solution after treatment, and low adsorption capacity. To overcome these shortcomings, many researchers have recently focused on the immobilization of nano-TiO2 on supports with a large surface area and high adsorption capacity [13, 14]. This approach can improve the distribution of TNPs in suspension, which increases the adsorption and concentration of target substances, as well as facilitates the unloading of the reactor after catalysis. Diatomite is an important porous non-metallic mineral resource with nontoxic and good chemical stability. Application of diatomite as carrier material may enhance the TiO2 nanosized particles distribution in suspension which enables to adsorb and concentrate the target substances. Basically, the method of obtaining of a composite photocatalyst on the base of diatomite and TiO2 is a heterogeneous chemical deposition of titanium dioxide from titanium alkoxides or titanium tetrachloride as a precursor of nanosized titanium dioxide [15, 16]. A common disadvantage of the mentioned methods is the use of chemical reagents, the multistage and long duration of the process of the composite obtaining. The electrochemical method developed by the authors for the preparation of a hybrid photocatalyst based on nanosized titanium dioxide and diatomite as a substrate is free from these drawbacks. It is as follows: 2.0 g of purified diatomite was dispersed in a solution of TiCl4 of the required concentration (to give about 20% of TiO2 in the composition of the photocatalyst) in the cathode chamber of the electrolyzer and stirred for 30 min. Then a constant current is applied to the terminals of the electrolyzer until a certain pH is reached in the cell. Then the current is turned off, the contents of the cell are filtered, washed with distilled water, dried at room temperature, then in an oven at 100 °C, calcined at 400 °C for the development of the anatase phase. The resulting product NTD- is further stored in a desiccator for further physicochemical and photocatalytic studies.

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2 Experimental 2.1 Materials and Methods Titanium(IV) chloride (TiCl4, 99.9%), as titania precursor, was purchased from Sigma Aldrich. The diatomite used in the study was supplied from the deposits in the Viscauti, Orgeev region, Moldova after an appropriate purification. Phenol (C6H6O), was purchased from Sigma Aldrich (Germany) and used for all the experiments on the adsorption and photodegradation. Hydrochloric acid (HCl, 35 wt.%) and sodium hydroxide (NaOH) of analytical grade (for pH adjusting) were purchased from Aldrich (Germany). Double distilled water was used for the preparation of all solutions. Phenol concentration was determined by UV–vis spectroscopy in the range of 190– 400 nm, using Spectrophotometer T80 (UK), pH of water solutions was measured using a pH-meter ADWA AD 1000 (Hungary). For study of photocatalytic properties of NTD photocatalyst under UV irradiation a mercury lamp with a wavelength of ∼370 nm and a luminous flux power of 3.67 mW/cm2 was used. All the experiments were carried out in a slurry-type glass photoreactor. The adsorption-structure properties of the prepared materials were studied at 77 K using a Quantachrome Autosorb 1MP surface area analyzer. Prior to analysis, samples were degassed in nitrogen at 90 °C for 1 h and then at 200 °C for 4 h. The crystal structure was investigated by XRD method on a DRON-UM1 diffractometer using FeKα radiation (λ(Kα1) = 1.9406 A) and a Mn filter. Analyzed samples were compared with an anatase standard sample and raw diatomite. The elemental composition of surface compounds was analyzed by XPS on a PHI Versa Probe II 5500 spectrometer. 2.2 Characterization of Adsorption-Structural Properties and Crystal Structure of NTD Samples The measurements have been carried out of nitrogen low-temperature adsorption on the samples; the adsorption-structural characteristics provided by BET method: specific surface area, pore volume, effective pore radius have been calculated. In Fig. 1 the isotherm of nitrogen low-temperature adsorption-desorption is presented, the effective pore radius distribution is shown in the insert of the diagram.

Advanced Nanotechnology-Based Approaches 0,008

180

0,006

160

dV/dr

200

0,004 0,002

140

0,000

120

20

3

aN, см /г

137

40

ref, A

100 80 60 40 20 0 0,0

0,2

0,4

0,6

0,8

1,0

p/p0

Fig. 1. Low-temperature nitrogen adsorption –desorption isotherm (77 K) for NTD composite.

The nitrogen low temperature adsorption-desorption isotherm has an S-shape and capillary condensation hysteresis, which indicates their heterogeneous porous structure. According to BDDT classification, the isotherm mainly corresponds to type IV, which is typical for samples whose porous structure contains both micro- and mesopores. Specific surface area, pore volume, and effective pore radius have been determined by BET and BJH analyses. According to XRD data the titanium dioxide in the photocatalyst samples was represented by the crystalline phase of anatase (Fig. 2).

Fig. 2. XRD pattern of synthesized NTD composite.

Figure 2 shows the x-ray diffraction patterns of NTD particles. The experimental spacings were compared with those reported for anatase (101) to identify the particle structure [17]. The XRD results revealed that the synthesized particles are mainly composed of anatase. The anatase crystallite size was determined from the full width at half-maximum of a characteristic peak using the Scherer equation: d=

kλ β · cosθ

(1)

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where d is the mean crystallite size, nm; λ is the wavelength of X-ray radiation (λ(FeKα) = 1.940 A); β is the full width at half-maximum of a peak of interest, rad; θ is the diffraction angle (peak position is given by2); and k = 1. The values of β and θ were taken for crystal plane 101. The results are presented in Table 1. Table 1. Adsorption-structural properties of initial diatomite D and NTD composite according to BET analysis and XRD data Sample

Specific surface area, Sorption pore volume, Pore radius, Crystallite TiO2 size, S, m2 /g V s , cm3 /g Ref , Å nm

D initial 36,5

0,136

17.0

–

NTD

0.230

15.3

8

198.1

Modification of the diatomite surface with nanosized TiO2 leads to a significant growth of surface area: from 36.5 to 198.1 m2 /g, the fact which can be very important for further adsorption and photodegradation of target substance. In Table 2 there are data on concentration of main elements on the surface of initial diatomite and the composite of diatomite and nano TiO2 , NTD, according to XPS analysis. Table 2. Surface concentration of chemical elements in samples of initial diatomite D and NTD composite according to XPS data. Element

Al

Si

K

Ca

Sample

Concentration, at.,%

D initial

3,06

NTD

2,55

Ti

Fe

93,41

0,96

0,52

0.37

0.83

61,54

0,75

0.18

30.26

0,55

From the data in Table 2 it is seen, that the surface of diatomite is enriched with titanium, and silicium and some oxides contents decrease during the modification of the surface of diatomite with titanium compounds. 2.3 Photocatalytic Activity of NTD When studying the photocatalytic degradation of an organic impurity in an aqueous solution, one should always keep in mind that the mineralization process itself can be divided into several stages, the contribution of each of which is important to a different extent in the overall effect [18]. One of the parameters determining the overall success of photo purification is the adsorption capacity of the solid photocatalyst. Therefore, any study on photocatalysis must be preceded by a study of the adsorption of a target substance without exposure to irradiation. Consequently, it was necessary to study the adsorption of phenol by the composite.

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2.4 Phenol Adsorption on NTD All the experiments on the adsorption study were carried out under the following conditions: into 50.0 mL of aqueous phenol solution with an appropriate initial concentration 0.1 g of the sorbent sample was added with constant stirring. After adsorption, the solid phase was separated from liquid by centrifugation. The amount of adsorbed phenol, a, mg/g was determined using the equation: ae =

(C0 − Ce ) · ν , m

(2)

where C 0 and C e , mg/L – initial and equilibrium phenol concentration in the solution, respectively; v, L – the solution volume; m, g – the sorbent weight. All the experiments were conducted in duplicate to verify the results reproducibility. The adsorption study of phenol was performed depending on its initial concentration the adsorption isotherm, pH of the suspension, the sorbent dose, and the process duration - adsorption kinetics. The Dependence of Phenol Adsorption on Its Initial Concentration. The adsorption isotherm of phenol on NTD composite (Fig. 3) shows that the maximum adsorption capacity of phenol by NTD is equal to 22.83 mg/g. Initial concentrations of phenol varied from 10 to 250 mg/L.

Fig. 3. Isotherm of phenol adsorption on NTD composite. pH = 4.75; t = 120 min; T = 20 °C; sorbent dose 2g/L.

The adsorption isotherm was fitted by Langmuir model and the equation parameters were determined: Langmuir constant and maximum adsorption capacity (am = 22.33 mg/g, KL = 0.0368 L/mg, R2 = 0.9970). The Effect of Solution pH on Phenol Adsorption by NTD Composite. Phenol adsorption has been studied within the solution pH range from 4.0 to 7.5. The adsorption value of phenol on NTD as a function of the solution pH showed that the maximum value of the adsorption is at the pH value about 4.5 (Fig. 4). The most favorable pH for the phenol adsorption is in the acid region, which is related to the pH of the zero point of charge of the composite, being equal 5.2. At pH

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Fig. 4. Dependence of the phenol adsorption value on NTD on the solution pH. Cin = 100 mg/L; T = 20 °C. t = 120 min; T = 20 °C; sorbent dose 2 g/L.

below this value, the surface charge of the sorbent is positive, which contributes to the adsorption of the negatively charged substances from the solution, and at a pH above this value, the surface of the adsorbent is negatively charged. Since phenol in this range of pH is in a molecular form, the pH close to this of zero point of charge favors the phenol adsorption. The Effect of the Sorbent Dose upon the Adsorption of Phenol by NTD. The dose of sorbent (the other parameters unchanged) ranged from 0.5 to 15 g/L.

Fig. 5. The effect of NTD dose on the value of the remaining phenol concentration. Cin = 250 mg/L; pH 4.75; T = 20 °C.

As can be seen from the curve in Fig. 5 the optimum sorbent dose was determined by 2 g/L, at this sorbent consumption, the residual concentration of phenol is minimal, a further increase of the dose does not lead to a visible decrease of the remaining concentration and it is not necessary, since it increases the cost of the process. Kinetics of Phenol Adsorption by NTD. The adsorption kinetics of phenol by NTD composite has been studied. The contact time of the sorbent with the phenol solution was 10 to 200 min. Samples with a volume of 1 ml were taken for analysis every 20 min. The equilibrium time was determined as 120 min (Fig. 6);

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Fig. 6. Adsorption kinetics of phenol on NTD composite. Cin = 100 mg/L; pH 4.75; T = 20 °C, sorbent dose 2 g/L.

The phenol adsorption kinetics in dark is well described by Lagergren model of pseudo-first kinetic reaction [19]: a = am ∗ (1 − exp(−kL ∗ t))

(3)

a, mg/g, is phenol adsorption at equilibrium; am , mg/g, is the adsorption capacity of phenol; k L , min−1 is Lagergren constant; t, min, is the reaction time. The results are shown in Table 3. Based on the results obtained, the optimal conditions for phenol adsorption on the NTD nanocomposite were as follows: the solution pH 4.75; the sorbent dose 2g/L; the adsorption time to equilibrium - 120 min. These parameters were taken as the basis for the phenol photodegradation study.

2.5 Photodegradation of Phenol by the NTD Photocatalyst Under UV Irradiation The parameters of the process of phenol photodegradation were obtained under the action of UV irradiation in the photoreactor with NTD as a photocatalyst depending on the initial concentration of phenol, the solution pH, the photocatalyst dose and the UV irradiation time. The Role of the Initial Concentration of the Phenol. The initial pollutant concentration influences the degree of photodestruction of phenol in such a way: the higher the initial concentration, the lower the degree of removal of the phenol from the solution, and the faster the maximum purification of the solution was obtained, (Fig. 7). The kinetics of the reaction of phenol photodegradation was modeled using nonlinear regression and fit the Langmuir–Hinshelwood model [14, 20], The L-H model is described by the equation: Ce = Co ∗ exp(−KLH ∗ t + A)

(4)

where C e , mg/L, is phenol concentration at equilibrium; C o , mg/L, is the initial concentration of phenol; K LH is Langmuir-Hinshelwood’s constant; t, min, - reaction time; A is the coefficient of the equation.

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Fig. 7. The effect of the phenol initial concentration on the degree of its photocatalytic destruction by applying NTD under the UV irradiation. pH 4.75; T = 20 °C, dose = 2 g/L

The Kinetics of Phenol Photodegradation. The kinetics of photodegradation of phenol on NTD photocatalyst is well described by Langmuir-Hinshelwood model (with a high correlation coefficient R2 ) showing that the reaction belongs to the pseudo first order. However, in order to draw a conclusion about the limiting stage - whether the reaction is limited by the kinetic phase or diffusion of agents to the surface, some additional studies are needed, for example, the dependence of the reaction rate on the initial concentration of phenol, the pH of the solution, the rate of mixing of the solution with the catalyst. The calculated parameters according L.-H. model in dependence on initial phenol concentration are presented in Table 3. Table 3. The results of adsorption and UV photocatalysis obtained conform Lagergren and Langmuir-Hinshelwood models depending on initial phenol concentrations. Cin , mg/L

Adsorption in dark kL , (min−1 )

a, mg/g

Photocatalysis % of removal

R2

kL-H , (min−1 )

% of removal

R2

42

0.020

6.94

33.02

0.9929

0.124

80,39

0.9191

100

0.024

17.23

31.21

0.9868

0.070

59,16

0.9964

The dependence of the rate constant on solution pH (not showed) and the constant of L.-H. model on the initial phenol concentration speaks in favor of the fact that the external diffusion is the determining step of the process and a slurry-type reactor providing the best contact of the solid catalyst with the solution will be the best solution for the problem under study. Further studies of the influence of the parameters of the process of phenol photodegradation on its residual content in water solution have been carried out with a solution phenol concentration close to that occasionally occurring before discharge into the sewer - about 10mg/L. The Effect of the Catalyst Dose. The NTD catalyst loading plays a specific role in the process of photocatalytic oxidation of phenol (Fig. 8): initially increasing the dose up to

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2g/L increases the purification degree of the solution, then with increasing dose of the photocatalyst the degree of phenol removal decreases.

Fig. 8. The effect of NTD photocatalyst dose on the degree of phenol degradation under UV irradiation. Initial phenol concentration 10 mg/L; pH 4.75; T = 20 °C

This phenomenon is associated with the initial increase of the number of photocatalytic centers during the increase of the catalyst dose, after that the turbidity factor starts to play a role and the higher the mass of the catalyst, the photocatalytic centers are less available for source radiation. Thus a dose of the photocatalyst NTD 2g/L was taken as an optimal for the studied photodegradation process. Comparison of the data of photolytic degradation of phenol (without photocatalyst) with the results obtained by adsorption on NTD in the dark and photocatalytic degradation in the presence of NTD photocatalyst under UV irradiation showed a significant increase in the degree of phenol removal under the same experimental conditions. The results clearly show that the photolytic degradation of phenol occurs only to a small extent, and the removal of phenol observed in the presence of NTD photocatalyst is due to the activity of the catalyst alone (Fig. 9). At an initial phenol concentration of 10.74 mg/L, MAC (Maximum Allowable Concentration) is achieved by photocatalytic decomposition in 32 min, while during the same time only ~ 30% of the pollutant is removed by simple adsorption, in addition, photocatalysis mineralizes pollutants to harmless compounds, and when adsorbed, they simply pass into another phase without any changes.

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Fig. 9. Comparison of efficiencies of photolysis, adsorption and photo destruction of phenol by NTD photocatalyst under UV irradiation (Cin = 10.74 mg/L; pH = 4.75; T = 20 °C, catalyst loading 2 g/L).

3 Conclusions The heterogeneous photocatalysis, as one of the applications of the AOP (Advanced Oxidation Processes), friendly to the environment, can be successfully applied to decomposition of low concentrated hard-to-remove harmful substances such as phenol. The composite photocatalyst NTD synthetized by electrochemical method on the base of nanosized TiO2 and diatomite as career has proved its high activity in the photocatalytic degradation of phenol from aqueous solution under UV irradiation in a slurry-type photoreactor – 80.0% of phenol removal compared to adsorption – 32.9%. It is worth emphasizing that the photocatalytic oxidation of harmful organics leads to the formation of harmless compounds such as CO2 and H2 O, while during adsorption only the transfer of toxic compounds from one phase to another occurs without any changes. The rate of phenol photo destruction depends on initial phenol concentration, solution pH and photocatalyst dose. It was found that at the NTD doze of 2 g/L, pH = 4.75 and the initial phenol concentration 10,74 mg/L, under UV irradiation phenol degradation to a maximum allowable concentration (MAC) = 5mg/L takes place in 32 min of being in the photoreactor. Acknowledgments. The work was carried out within the framework of the projects ANCD 20.80009.5007.06 and JOP Romania-Republic of Moldova, RO-MD_2SOFT_1.2_139.

Conflict of Interest. The authors declare that they have no conflict of interest.

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Micro-Raman Analysis of Some As-S-S-Te Nanostructured Semiconductors Oxana Iaseniuc(B) and Mihail Iovu Institute of Applied Physics, Moldova State University, Chisin˘au, Republic of Moldova [email protected]

Abstract. In the present work some nanostructured chalcogenides of the As-SSb-Te system have been investigated by non-contact Micro-Raman spectroscopy which is a powerful technique for obtaining information on the local structure of the crystalline as well as disordered materials, especially when the composition and structure is varied. In this paper we report micro-Raman spectra of As1.17 S2.7 Sb0.83 Te0.40 , As1.04 S2.4 Sb0.96 Te0.60 , As0.63 S2.7 Sb1.37 Te0.30 , and As0.56 S2.4 Sb1.44 Te0.60 , of bulk semiconductor compounds and thin films. These semiconductor alloys are interesting and important from the point of view of assessing their physical properties, primarily the structure, as well as for determining the scope of technical application. It was established that the Raman spectra of light scattering of bulk samples differs from the spectra of thin films with a higher As content and a low Sb content, but samples prepared as bulk and powder exhibit the same behavior. All spectra have characteristic intense bands which are assigned to the Te-Te (ν = 119 cm−1 ), As-As (ν = 234 cm−1 ), AsS3/2 (ν = 345 cm−1 ), As4 S4 (ν = 495, 236, 223, 189, 168 cm−1 ), As4 S3 (270–273 cm−1 ), S8 rings (ν = 146, 220 cm−1 ) and SbO (ν = 255 cm−1 ) structural units. It was also found that the sample As0.63 S2.7 Sb1.37 Te0.30 have a more amorphous phase, while As0.56 S2.4 Sb1.44 Te0. 60, As1.17 S2.7 Sb0.83 Te0.40 and As1.04 S2.4 Sb0.96 Te0.60 samples are more polycrystalline. Keywords: Nanostructured quaternary amorphous semiconductors · Micro-raman spectra · Vibration modes

1 Intorduction Nanostructured semiconductors of quaternary As-S-Sb-Te system are of great interest for physical processes in non-crystalline solids, as well as for technical applications, such as IR optical elements, diffraction optics, holography, photonic and gas sensors, etc. [1, 2]. As2 S3 , Sb2 S3 , and Sb2 Te3 chalcogenides and its ternary and quaternary compounds are the most intensively studied chalcogenide glasses because of its ease of formation, its excellent infrared transmission and its resistance to atmospheric conditions and chemical stability. Despite the fact that As and Sb belong to the same group of the periodic table, As2 S3 , Sb2 S3 , and Sb2 Te3 compounds do not show the same tendency to glass formation. The addition of As2 S3 to Sb2 S3 increases the glass-forming ability, and it is possible to obtain glasses in a mixed As-Sb system [3]. It is well known the © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 147–155, 2024. https://doi.org/10.1007/978-3-031-42775-6_16

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fact that three-dimensional As2 S3 glass network is constructed of trigonal bipyramid formation of the Ac3/2 blocks, which are interconnected by As-S-As bridges. The main structural units of vitreous Sb2 S3 are the trigonal pyramidal geometry of SbS3/2 bonded to each other by S atoms [4]. The crystal structure of Sb2 Te3 exhibits the layered atomic arrangement in the rhombohedral lattice which consists of three quintuplet layers (QLs) and each quintuplet layer contain five atoms in the order of Te1 -Sb-Te2 -Sb-Te1 [5]. Study of the Raman spectra of Asx S100-x glasses showed the presence in them of such structural groups as AsS3/2 pyramidal units (strong band ν = 336, 343 cm−1 ), the homopolar bonds As-As belonging to the As4 S4 structural units (ν = 495, 236, 223, 189, 168 cm−1 ), and S8 - rings (ν = 474, 234, 155 cm−1 ) [6]. For optical fibers As2 S3 Sb2 S3 , the intensity of the Raman scattering signal increases in comparison with optical fibers made of pure glass As2 S3 , where Raman spectra are considered from the point of view of the molecular model of the glass structure, and sharp features in the region ν = 100–250 cm−1 can be attributed to molecular particles of As4 S4 , As4 and Sn glass types [7]. For the alloy As-Te-Si, the detected vibration bands at ν = 210 and ν = 240 cm−1 are associated with As clusters. The large pesk at ν = 375 and ν = 175 cm−1 are characteristic to As2 Te3 compounds, and the peak at ν = 120 cm−1 can be due to Te clusters [8]. Some glasses with Sb2 Te3 constant concentrations in Sb2 S3 -As2 S3 -Sb2 Te3 system have been studied by means of 121 Sb Mössbauer spectroscopy. The obtained results have been used in order to investigate the local environment of 121 antimony in this glass system [9]. The optical and physical properties of the As11.2 S48.0 Sb28.8 Te12.0 and As20.8 S48.0 Sb19.2 Te12.0 nanostructured polycrystalline semiconductors were reported in [10, 11]. It is established that the difference in the Raman spectra of bulk and amorphous thin films is due to the different disorder of the structural network. This was confirmed by identical images for different studied areas. The main purpose of this work was to study the micro-Raman spectra of the some As-S-Sb-Te nanostructured chalcogenide semiconductors in the form of bulk and thin film samples to identify phases and vibrational modes of the corresponding structural units. The experimental results presented in this paper are original and represent new information for the physics of non-crystalline materials.

2 Expiremental The nanostructured As1.17 S2.7 Sb0.83 Te0.40 , As1.04 S2.4 Sb0.96 Te0.60 , As0.63 S2.7 Sb0.1.37 Te0.30 and As0.56 S2.4 Sb0.1.44 Te0.60 semiconductors were prepared from the preliminary synthesized compounds (As2 S3 )0.35 (Sb2 S3 )0.65 , (As2 S3 )0.65 (Sb2 S3 )0.35 , and Sb2 Te3 , containing 6N purity elements (As, Sb, S, Te) by conventional melt quenching method. The initial compounds were placed and mixed in quartz ampoules, which then were evacuated up to pressure of P ~ 10–5 Torr and sealed. At last, the mixtures were melted at 850–900 o C in rocking furnace during 10 h for homogenization, and then quenched at the room temperature. The average mean coordination number for all samples is Z = 2.40 as in the case of the binary components. Part of the synthesized ingots have been ground into powders with a grain size of about d = 100 nm and were used for the X-ray and MicroRaman measurements. Another part of them has been used for the preparing thin film layers

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(d = 0.13 – 0.54 μm) onto glass substrates by thermal vacuum evaporation (P = 10–5 Torr, deposition rate was about 50 Å/s). First, samples were characterized by X-ray fluorescence analyzer (XRF), which permits to estimate the presence of different chemical elements in the investigated material. For further research, compositions with the most optimal content of chemical elements close to the initial one were selected. The Raman study of the samples was carried out at room temperature by Confocal Micro - Raman Spectroscopy, using a LabRam HR800 system. All Raman spectra were generated by exposing the specimens for 100 s to λ = 632 nm (D0.6 filter) wavelength red excitation laser and dispersing the sample emitted signal onto the CCD detector using a 600 lines/mm grating. The samples were also analyzed using Secondary electron images (SEM) were acquired at different magnification: x1.5 K, x3K, x5K, x10K, x30K, x50K, x100K, x200K, and x500K.

3 Results and Discussion Some results on XRF, XRD, EDS and SEM analyses of As-S-Sb-Te compounds were reported in [10, 11]. The X-ray diffraction patterns of studied materials show the presence of amorphous phases for As0.63 S2.7 Sb1.37 Te0.30 and nanocrystalline phases for As0.56 S2.4 Sb1.44 Te0. 60, As1.17 S2.7 Sb0.83 Te0.40 and As1.04 S2.4 Sb0.96 Te0.60 with the main structural units AsS3 , Sb2 S3 , and Sb2 Te3 . For example in the XRD spectra for the ´ As1.04 S2.4 Sb0.96 Te0.60 compound the peak with the interlayer distance d = 3.21 Å 0 (2θ ≈35 ) is related to the Sb2 S3 units, and the peaks with the interlayer distance d ´ (2θ ≈490 – 480 ) and d = 3.157 Å ´ (2θ≈34.40 ) are related to the Sb Te = 2.35–2.37 Å 2 3 structural units, respectively. A possible overlap of some peaks for Sb2 S3 and Sb2 Te3 is assumed. The SEM analyses show that the investigated samples don’t present a uniform morphology. This is clearly evidenced in the mapping analysis. The mapping analysis of elements distribution spectra (EDS) indicate that for each investigated area, the intensity corresponded to sulfur (S), arsenic (As), antimony (Sb) and tellurium (Te) remain practically unchanged. The EDS analyses show the presence of all the initial elements (As, S, Sb, Te) with a slight deviation. Depending on the content of the material present, different morphologies are observed. Tellurium showed an acicular morphology while the regions containing more Sb are flatter. By comparison, the As1.04 S2.4 Sb0.96 Te0.60 bulk sample exhibit higher content of S and As, around above 34 at.% having the content of Sb around 16 at.%. This is caused by a significant difference in the composition of the samples. Some structural investigations of the chalcogenide glasses on the Sb2 S3 -As2 S3 -Sb2 Te3 and (As2 S3 )1-x (Sb2 S3 )x system also were reported in [12–14]. It is well know that micro-Raman spectroscopy is an efficient method for obtaining relevant information about the local structure of the amorphous and non-crystalline semiconductors, especially when the composition is varied. The micro-Raman spectroscopy was also successfully used for investigation the photoinduced transformations during light exposure and heat treatment due to the photostructural changes in the bulk, thin films and optical chalcogenide fibers [7, 14–16].

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Figures 1–4 illustrates the micro-Raman spectra for bulk (a) and thin film (b) studied samples for different areas, as well as the SEM images for the one of the selected zone.

Fig. 1. Micro-Raman spectra of the bulk (a) and thin film, L = 0.135 μm (b) of As1.17 S2.7 Sb0.83 Te0.40 samples for different areas and the SEM image.

It can be seen that the behavior of the spectra is identical for both bulk and thin film samples, but the signal intensity for different zones is different due to the morphological specification of the local structure. In the micro-Raman spectra for different areas of the bulk and thin film As1.17 S2.7 Sb0.83 Te0.40 (Fig. 1), As1.04 S2.4 Sb0.96 Te0.60 (Fig. 2) and As0.56 S2.4 Sb1.44 Te0.60 (Fig. 4) samples were determined some intense peaks, which are assigned to the vibrational modes corresponding to the structural units in the range ν = 117–139 cm−1 (Te–Te), at 142–146 cm−1 (S), at 187–189 cm−1 (As4 S3 ), at 220 cm−1 (S), at 231–235 cm−1 (As-As), at 255–257, 295 cm−1 (Sb-O), at 270–273 cm−1 (As4 S3 ) and at 310–362 cm−1 (AsS3/2 ).

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Fig. 2. Micro-Raman spectra of the bulk (a) and thin film, L = 0.54 μm (b) of As1.04 S2.4 Sb0.96 Te0.60 samples for different areas and the SEM image.

Some similar vibration bands were also observed in Asm Sn and (As2 S3 )0.95 (Sb2 S3 )0.05 chalcogenide optical fibers [7]. The vibration modes corresponding to the Sb2 S3 and Sb2 Te3 units is not observed in the micro Raman spectra due to the inversion symmetry in the metastable cubic phase belongs to the F3 m3 space group [17]. Besides that, the micro-Raman spectra for all investigated As-A-Sb–Te powder samples are similar to the spectra for bulk samples.

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Fig. 3. Micro-Raman spectra of the bulk (a) and thin film, L = 0.27 μm (b) As0.63 S2.7 Sb1.37 Te0.30 samples for different areas and the SEM image.

On the all figures are also represent the SEM images of the surface for bulk amorphous specimens, where a layered polycrystalline structure is clearly visible. Only As0.63 S2.7 Sb1.37 Te0.30 compound has a more amorphous phase as well for both bulk and thin film sample. This fact was also confirmed during the study of samples using XRD spectroscopy. The position of the additional peaks was deduced from Lorentzian curves deconvolution of the experimental Raman spectra. An example of the deconvolution of the As1.17 S2.7 Sb0.83 Te0. 40 bulk sample is presented in Fig. 5.

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Fig. 4. Micro-Raman spectra of the bulk (a) and thin film, L = 0.18 μm (b) of As0.56 S2.4 Sb1.44 Te0.60 samples for different areas and the SEM image

According to the [18] the monocrystals Sb2 Te3 , Sb2 Se3 and Sb2 S3 are well-known layered bulk structures with Van der Waals interactions. It was shown, that, in the crystal structure of Sb2 X3 (where X = S, Se, Te) each Sb atom is surrounded by six X atoms and each X atoms encompassed by four Sb atoms.

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Fig. 5. Deconvoluted Raman spectrum of the As1.17 S2.7 Sb0.83 Te0. 40 bulk sample.

4 Conclusions The micro-Raman spectra and SEM images for bulk and thin-film nanostructured layered quaternary semiconductor compounds As1.17 S2.7 Sb0.83 Te0.40 , As1.04 S2.4 Sb0.96 Te0.60 , As0.63 S2.7 Sb1.37 Te0.30 , and As0.56 S2.4 Sb1.44 Te0.60 were investigated and discussed. Spectra have been studied for various regions for both bulk and film samples. Analysis of the experimental data showed a presence of polycrystalline and amorphous phases for different compounds with the main structural units located in the range ν = 117– 139 cm−1 (Te–Te), at 142–146 cm−1 (S), at 187–189 cm−1 (As4 S3 ), at 220 cm−1 (S), at 231–235 cm−1 (As-As), at 255–257, 295 cm−1 (Sb-O), at 270–273 cm−1 (As4 S3 ) and at 310–362 cm−1 (AsS3/2 ). The results of structural studies have shown that specimens prepared in the form of films are more homogeneous compared to bulk samples and powders. Acknowledgements. This work was financially supported by the project ANCD 20.80009.5007.14 and by the Romanian Ministry of Education and Research, under the following ECSEL-H2020 Projects: PIn3S—Contract no. 10/1.1.3H/03.04.2020, POC-SMIS code 135127and BEYOND5—Contract no. 12/1.1.3/31.07.2020, POC-SMI_S code 136877.

Conflict of Interest. The authors declare that they have no conflict of interest.

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3. Kamitsos, E., Kapoutsis, J., Culeac, I., Iovu, M.: Structure and bonding in As-S-Sb chalcogenide glasses by infrared reflectance spectroscopy. J. Phys. Chem. B 101, 11061–11067 (1996). https://doi.org/10.1021/jp972348v 4. Indu, R., Sumesh, R., Rudra, P.J., Archana, L.: Crystal Growth and X-ray Diffraction Characterization of Sb2 Te3 Single Crystal. AIP Proc. 2100, 020070 (2019). https://doi.org/10.1063/ 1.5098624 5. Dally, B., Kouame, N., Houphouer-Boigny, D.: Study of some characteristic parameters of Sb2 S3 -As2 S3 -Sb2 Te3 vitreous compositions calculated from their chemical formula obtained by EDS experiments. Chalcogenide Lett. 18(11), 681–691 (2021) 6. Wagner, T., Kasap, S., Vlcek, M., Sklenar, A., Stronski, A.: The structure of Asx S100-x glasses studied by temperature-modulated differential scanning calorimetry and Raman spectroscopy. J. Non-Cryst. Solids 227–230, 752–756 (1998). https://doi.org/10.1016/S0022-3093(98)001 94-X 7. Iovu, M., Culeac, I., Koudelka, L., Voynaroych, I., Vlcek, M.: Raman spectra in As-based chalcogenide optical fibers. J. Nanoelectronics Optoelectron. 9(2), 1–4 (2014). https://doi. org/10.1166/jno.2014.1577 8. Popescu, M., et al.: Structure and properties of As25 Te35 Si40 glass. J. Non-Cryst. Solids 326–327, 389–393 (2003). https://doi.org/10.1016/S0022-3093(03)00444-7 9. N, dri, K., Coullibaly, V., Sei, J., Houphouet-Boigny, D., Jumas, J.-C.: Investigations of antimony local environment in some Sb2 S3 -As2 S3 -Sb2 Te3 glasses by Mössbauer spectroscopy. Chalcogenide Lett. 10(12), 533–541 (2013) 10. Iaseniuc, O., Iovu, M.: Characterization of some optical and physical properties of As11.2 S48.0 Sb28.8 Te12.0 and As20.8 S48.0 Sb19.2 Te12.0 nanostructured polycrystalline semiconductors. Chalcogenide Lett. 19(2), 117–124 (2022) 11. Iaseniuc, O., et al.: Structural analysis of As-S-Sb-Te polycrystalline nanostructured semiconductors. Chalcogenide Lett. 19(11), 841–846 (2022) 12. N, dri, K., Coullibaly, V., Houphouet-Boigny, D.: XRD and EDS characterization of some Sb2 S3 -As2 S3 -Sb2 Te3 glasses prepared by rapid quenching method. J. Ovonic Res. 9(4), 113–121 (2013) 13. Sava, F.: Structure and properties of chalcogenide glasses in the system (As2 S3 )1–x (Sb2 S3 )x . J. Optoelectron. Adv. Mater. 3(2), 425–432 (2001) 14. Stronski, A., Revulska, L., Shportko, L. et al.: The influence of composition on short-range order of amorphous As2 S3 -Sb2 S3 chalcogenide alloys: a XRD and Raman study. Funct. Mater. 27(2), 315–321 (2020). https://doi.org/10.15407/fm27.02.315 15. Iovu, M., Shutov, S., Andriesh, A., et al.: Spectroscopic sudies of bulk As2 S3 glasses and amorphous films doped with dy, sm and mn. J. of Optoelectronics and Advanced Materials 3(2), 443–454 (2001) 16. Iaseniuc, O., Enachescu, M., Dinescu, D., Iovu, M., Sergheev, S.: Influence of heat treatment and illumination on the vibration modes of (As4 S3 Se3 )1-x Snx thin films. J. Optoelectron. Adv. Mater. 18(1–2), 34–38 (2016) 17. Bragagfia, V., et al.: Far-infrared and Raman spectroscopy investigation of phonon modes in amorphous and crystalline epitaxial GeTe-Sb2 Te3 alloys. Sci. Rep. 6, 28560 (2016). https:// doi.org/10.1038/srep28560 18. Bafekry, A., Mortazavi, B., Faraji, M. et al.: Ab initio prediction of semiconductivity in a novel two-dimensional Sb2 X3 (X=S, Se, Te) monolayers with orthorhombic structure. Sci. Rep. 11(1), 10366 (2021). https://doi.org/10.1038/s41598-021-89944-4

Optical Properties and Photoinduced Anisotropy of PEPC-co-SY3 Nanocomposite Constantin Los, manschii(B) , Elena Achimova , Vladimir Abaskin , Alexei Mesalchin , Alexandr Prisacar , and Vladislav Botnari Institute of Applied Physics, Moldova State University, Chisinau, Republic of Moldova [email protected]

Abstract. In the present work, the optical properties of the PEPC-co-SY3 nanocomposite doped with gold nanoparticles are brought to light. The samples were prepared with 3 different concentrations of Au as well as a control sample of undoped azopolymer. Thin films were studied with UV/Vis and polarimetric spectroscopy. The paper’ results show bandgap shifts depending on the concentration of gold nanoparticles. At the same time, the analysis of the spectral dependence of the transmittance, the reflectance, the absorption coefficient as well as the refractive index, indicates a proportional dependence of these parameters on the nanoparticle concentration. It was found that the band gap narrows with increasing concentration of nanoparticles, it decrease from Eg = 2.38 eV for undoped azopolymer to 2.3 eV for azopolymer with a concentration of nanoparticles C = 0.001 mg/ml.The angular dependences of the azimuth and ellipticity of probe beam are also analyzed, which indicates the appearance of the polymer chirality change. Also, from the study of the spectral properties of azopolymer films and azopolymer nanocomposites, a change in the values of the spectral dependence of the refractive index was revealed, which was calculated taking into account the reflection spectrum, which has more pronounced interference peaks which leaves its mark on the refractive index spectra, and the dependence refractive index on the concentration of nanoparticles. Keywords: Azopolymer · Nanocomposite · Surface plasmon resonance · Photoinduced birefringence · Optical reversibility · Anisotropic optical material

1 Introduction Azobenzenes have been found to possess the remarkable property of photoisomerization, specifically the ability to undergo trans-cis-trans isomerization when exposed to light with wavelengths near their absorption maximum. Typically, this absorption occurs within the UV-blue range of the electromagnetic spectrum [1]. This property was a premise that in 1984 Todorov discovered the property of recording the polarization states of light on materials containing azobenzenes, as a consequence of the trans-cis-trans cyclic photoisomerization, which takes place until the azobenzene molecules will orient perpendicular to the electric field vector of incident light [2]. At the same time, there © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 156–165, 2024. https://doi.org/10.1007/978-3-031-42775-6_17

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is a macroscopic change in the photoinduced dichroism (D) and the photoinduced birefringence n [3]. The introduction of the azobenzene molecule in the polymer structure allowed to increase the temporal stability of the photoinduced effects [4]. At the same time, the interaction of the azopolymer with light can also induce changes in its surface topography, which made it possible to use these materials in practice for direct holographic recording [5–7]. Initially, the orientation of the azobenzene residues in the azopolymer is random, respectively this medium can be considered an isotropic medium. The interaction of the azopolymer with the polarized light induces the initiation of the trans-cis-trans cyclic photoisomerization until the molecule is oriented perpendicular to the electric field vector, and in the end, they stay inert to the light [8]. This transition from an isotropic medium to an anisotropic one also induces a change in the value of the refractive index of the azopolymer [9]. Noble metal nanoparticles are intensively studied in field of light-matter interaction, and they represent a class of promising materials with properties that differ drastically from bulk materials [10]. The doping or dispersing of gold nanoparticles (Au NPs) in azopolymers to improve the diffraction efficiency of the recorded gratings, and also to increase the sensitivity of nanocomposites is already known [11, 12]. The interaction of the diazo group with the incident light induces the appearance of rapid transitions as a consequence of the cyclic photoisomerization and reorientation of dipoles of azobenzene molecules. This fact requires the existence of sufficient space in the volume of the polymer to carry out these movements. Free space in the volume can be created by introducing nanoparticles with diameter less than 50 nm that will act as nanospacers preventing the polymer chain from being packed tightly [13]. By doping with Au NPs the azopolymer, it becomes possible to attain higher photoinduced birefringence value compared to the undoped material. Similarly, the diffraction efficiency of the doped azopolymer will exhibit greater values when compared to undoped azopolymer [14]. Likewise for the photoinactive functional groups, they also reorient in space cooperatively [15]. The purpose of this work is to investigate the poly-n-epoxypropilcarbazole (PEPC) with Solvent Yellow 3(SY3) azopolymer nanocomposite with different content of Au NPs and to evaluate how the concentration of nanoparticles can modify the absorption coefficient, the refractive index, the bandgap as well as the magnitude of the photoinduced anisotropy, i.e. photoinduced changes in azimuth and ellipticity. For this, we performed the study of photoinduced anisotropy depending on the angle between the linear polarizations of the pumping and the probing beams.

2 Material and Methods 2.1 Thin Film Preparation and Thickness Measure The azopolymer that was used is of the side chain type. Its structure is represented in Fig. 1. The azopolymer with a SY3 content of 30 wt% was obtained by the method previously described in [16]. The gold nanoparticle’s (purchased from Sigma Aldrich, with diameter 25 nm) basic solution was obtained by dispersing 0,1 mg gold nanoparticles in 2 ml toluene. After

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was prepared three solutions of gold nanoparticles through dissolving 0,25 ml of basic solution in 1, 2 and 3 ml toluene. From each prepared solution was dissolving 0,25 ml in 2 ml of PEPC-co-SY3 azopolymer. After this, three azopolymer solutions with different concentrations of gold nanoparticles were obtained (0 mg/ml; 0,0004 mg/ml; 0,0006 mg/ml and 0,001 mg/ml). The thin nanocomposite films were obtained by the rod-coating method, at a rod feed rate of 13mm/s and the rod’s height from the substrate’s surface was 270 μm. The film thickness was determined from transmittance spectra, according to the Swanepoel method.

Fig. 1. Chemical structure of azopolymer PEPC-co-SY3

2.2 Spectroscopic and Polarimetric Analysis of Samples Transmittance and reflectance spectra were recorded on a Varian Cary 5000 spectrophotometer in the 330–2500 nm range. The spectral dependence of the absorbance was determined from the transmittance and reflectance spectra, and the calculation was made from the relation (1) [17]. ⎡ ⎤  2 4 (1 − R) (1 − R) 1 + + R2 ⎦, (1) α = log⎣ d 2T 2T 2 where d - film thickness, R - reflectance and T - transmittance. The spectral dependence of the refractive index was calculated from the reflectance spectra, which was determined from the relationship:  4R (1 + R) + − k 2, (2) n= (1 − R) (1 − R)2 where R is the reflectance and k is the extinction coefficient, defined as k = λα/4π. The band gap width of the PEPC-co-SY3: NP nanocomposite was determined according to the Tauc method, from the relationship: αhv = B(hv − Eg)x ,

(3)

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where hν – photon energy of the beam, Eg represents the width of the optical band gap, B – constant. The value of x denotes the type of electronic transition in the material and four types of electronic transitions are distinguished: direct allowed (x = 1/2), direct not allowed (x = 3/2), indirect allowed (x = 2) and indirect not allowed (x = 3). To determine the type of electronic transition, respectively the value of x, the dependence ln(hν) will be constructed as a function of hν [18] and the straight line segment in the absorption region will be chosen, in accordance with the Tauc method [19]. Due to the fact that the possibility of modulating the photoinduced anisotropy by changing the polarization angle between the excitation and probe beams was previously observed for the PEPC-co-SY3 azopolymer samples [16], the dependence of the photoinduced anisotropy on the angle between the linear polarizations of the pumping and the probing beams was studied using the optical scheme in Fig. 2a.

Fig. 2. The optical setup for photoinduced anisotropy measurements.

Two lasers were used in this optical scheme. The single mode DPSS laser λ = 473 nm, P = 100 mW with vertical polarization was applied as the pumping irradiation. In order to realize the gradual change of the state of polarization of this beam, the λ/2 wave retarder mounted in the rotary mount K10CR1 was used. Afterwards, the beam was expanded with a lens to obtain a spot with a diameter larger than the diameter of the red laser spot. The probe laser, semiconductor type, with λ = 635nm, P = 5 mW, was used. The beam was attenuated to 2 mW by means of neutral filters and polarized with a Glan-Thompson calcite polarizer (extinction ratio - 100 000:1). Attenuation of the beam was performed to exclude photoinduced modulation of the azopolymer surface. Setting the linear polarization angle of the probe beam was achieved with the Soleil-Babinet compensator, used as a wave retarder, which the azimuth orientation under the angle of 45° was set as shown in Fig. 2b. Finally, the ellipticity and azimuth of the probe beam were recorded by the Thorlabs PAX1000 polarimeter.

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3 Results and Discussions 3.1 UV/Vis Spectroscopy The optical parameters of the PEPC-co-SY3 film, as well as the PEPC-co-SY3:Au NP nanocomposite as a function of wavelength, were determined from the transmittance and reflectance spectra.

Fig. 3. Transmission spectra of PEPC-co-SY3 film with different concentrations of Au NPs.

To determine the optical parameters, such as the absorption coefficient or the extinction coefficient and the refractive index of the film, it is important to know the thickness of the films. The thicknesses were determined from the transmittance spectra, according to the Swanepoel method [20] and are given in Table 1. The optical Eg calculated by Tauc method (Fig. 5) and the initial refractive index nin photoinduced in the samples at the beginning of irradiation are also presented in Table 1. Table 1. Thin films thickness and Au NPs concentrations Au NPs concentration

Thickness

Eg

nin

0 mg/ml

1830 ± 10 nm

2,38 eV

1.79

0,0004 mg/ml

1015 ± 10 nm

2,36 eV

1.83

0,0006 mg/ml

1041 ± 10 nm

2,32 eV

1.86

0,001 mg/ml

1160 ± 10 nm

2,30 eV

1.97

From transmission spectra (Fig. 3) we determined that the PEPC-co-SY3 film has a strong adsorption in the blue region, but we can distinguish that the transparency of the film decreases with the increase in the concentration of NPs in the nanocomposite. This behavior is attributed to surface plasmon resonance [21]. The dependence of the absorption coefficient on the wavelength is shown in Fig. 4. It can be observed that doping with nanoparticles shifts the absorption edge of the nanocomposite. Note that with the increase in the content of nanoparticles, there is a red

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shifting of the absorption edge, which can be attributed to the Au NPs contribution and this can be seen more clearly from Fig. 5 and Table 1, where the optical band gaps are indicated.

Fig. 4. The dependence of the absorption coefficient on the wavelength for different concentrations of Au NPs.

The Eg values for the thin films obtained were identified by the Tauc method. First of all, it should be noted that the analysis of the presented curves indicates that the type of electronic transitions in the nanocomposite is direct allowed and respectively in formula (3) the value of x = 1/2.

Fig. 5. Tauc plot for direct bandgap transitions of PEPC-co-SY3: Au NP nanocomposites.

Figure 5 shows Tauc plot curves, by which the optical Eg of the nanocomposites were determined. The values are included in Table 1. From these dependencies it is seen that there is a shift of Eg towards lower energies and this shift can be attributed to the influence of surface plasmon resonance [10]. The spectral dependence of the refractive index of samples before irradiations (see Fig. 7) was determined from the reflectance spectra (see Fig. 6) using the formula (2). High values of reflection were observed in the 500–800 nm regions of the spectra, which correlates with the transmittance spectrum and can be attributed not only to surface plasmon resonance, but rise scattering with increase of Au NPs content.

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Fig. 6. Reflection spectra of PEPC-co-SY3 film with different content of Au NPs.

The spectral dependence of the refractive index is represented in Fig. 7. Note that for the spectral range corresponding to surface plasmon resonance, one can observe an increase in the refractive index with an increase in the concentration of nanoparticles.

Fig. 7. Spectral dependence of the refractive index of PEPC-co-SY3 film with different content of Au NPs.

3.2 Polarimetry Initially, before exciting the nanocomposite with the pump laser, the ellipticity of the beam passing through the polymer was set to 0 degrees ± 0.05°, having a linear polarization of the probe beam. However, it is observed that when changing the angle between the pump and probe beams polarization planes, we obtain the ellipticity modulation (Fig. 8b). Changing the ellipticity and at the same time, the azimuth angle (8a) induces the appearance of chiral structures in the polymer volume [22]. The optical scheme described in the previous chapter was used to determine the dependences of azimuth and ellipticity on the angle between the linear polarization of the incident pump beam and the horizontal for all the described concentrations. Each point represents the average of measurements taken over 6 min, during saturation is reached for each polarization angle. Thus, the results of the experiment confirmed that the interaction of the azopolymer with laser radiation is determined not only by the wavelength of the pump beam, but also

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Fig. 8. Angular dependence of probe beam azimuth (a) and ellipticity (b)

changes the polarization of the probe beam passing through the sample, turning it from linear to elliptical, and such optical activity is typical for 3D chiral materials. On the other hand, a decrease in the ellipticity and azimuth with an increase in the concentration of nanoparticles can be explained by the pronounced absorption of nanoparticles.

4 Conclusions In this paper we presented the spectral and polarimetric study of thin layers of azopolymer PEPC-co-SY3 doped with Au NPs nanoparticles. The photoinduced anisotropic behavior in these materials was shown. It was observed that the adsorption of the films increases with the increase in the concentration of nanoparticles. At the same time, increasing the concentration leads to a decrease in Eg, from Eg = 2.38 eV for undoped azopolymer to 2.3 eV for azopolymer with a concentration of nanoparticles C = 0.001 mg/ml. At the same time, the influence of nanoparticles was shown in the reflectance spectra, which drastically change the value of the refractive index, especially in the region where surface plasmon resonance takes place. The polarimetric analysis showed that this azopolymer, as well as the nanocomposite based on it, demonstrate photoinduced threedimensional chirality. Like the azimuth, the ellipticity demonstrated a correlation with the concentration of nanoparticles, having larger magnitudes at lower concentrations of Au NPs.

Conflict of Interest. The Authors Declare that They Have no Conflict of Interest.

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Ground and Excited States of Excitons in GaSe Single Crystals Ecaterina Cristea1,3

, Ivan Stamov2 , and Victor Zalamai3(B)

1 Technical University of Moldova, Chisinau, Republic of Moldova 2 T.G, Shevchenko State University of Pridnestrovie, Tiraspol, Republic of Moldova 3 National Center for Materials Study and Testing, Technical University of Moldova, Chisinau,

Republic of Moldova [email protected]

Abstract. Gallium selenide is a layered crystal with outstanding nonlinear optical properties. Due to layered structure and weak van der Waals bonds it is perspective as two dimension material. GaSe possesses two-phonon absorption and can be uses in THz diapason. Investigation of optical properties is important for further development of optoelectronic devices on its base. In this work photoluminescence, reflection and absorption spectra of GaSe single crystals were studied in a wide temperature range (10 – 300 K). The presence of series of excitonic levels in the region E > Eg was shown. At excitation by 448 nm laser of GaSe crystal electrons were resonantly excited from V1 ( 1 ) band to C1 ( 6 ) and C2 ( 5 ) bands. The luminescence from excitonic levels (nA = 1, 2 … 5) of conduction band C1 ( 6 ) to valence band V1 ( 1 ) was observed. Recombination from excitonic level of C1 ( 6 ) band to V2 and V3 bands (maxima nB = 1–2.1751 eV and nB = 2–2.2222 eV) and to V4 and V5 bands (maxima nC = 1 – 2.311 eV and nC = 2–2.350 eV) was observed. Luminescence maxima nD = 1 (2.399 eV) and nD = 2 (2.434 eV) attributed to transitions between C1 – V6 , V7 bands and E3 maximum caused by recombination C2 – V1 were found out. A model of energy bands responsible for observed transitions was suggested. Keywords: Gallium selenide · Excitonic states · Band structure · Photoluminescence · Wavelength modulation reflection spectra

1 Introduction GaSe compound is a layered crystal with unequal properties. Physical properties of ε-GaSe are important for researchers and designers developing various optoelectronic devices. This crystal possesses outstanding nonlinear optical properties, including twophoton absorption in the THz region. Detectors of optical emission in visible and near infrared spectral ranges were developed based on this material [1–6]. It is predicted using of this material in quantum electronics, to create highly efficient photovoltaic converters, gas sensors and thermoelectric converters, effective sources of terahertz laser radiation, etc. [4–8]. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 166–173, 2024. https://doi.org/10.1007/978-3-031-42775-6_18

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A strong covalent bond in layers and a weak interlayer van der Waals bond with a small ion-covalent contribution leads to a strong anisotropy of GaSe properties. The crystals have a high susceptibility, high optical birefringence (n - 0.3 at 700 nm), high transparency (in the range 0.7–18.0 μm) with a low absorption coefficient (α < 0.3 cm−1 ). Optical characteristics, electroconductivity, thermoconductivity etc. have a significant difference along c-axis (perpendicular to layers) and along layers. GaSe single crystals grown by the Bridgman method usually have ε-polytype structure [9, 10]. In the present work photoluminescence, reflection and absorption spectra of GaSe crystals were investigated in temperature range 300 – 10 K in near edge region. Series of excitonic levels was discovered. The luminescence spectra from the exciton levels of C1 (G6 ) and V1 (G1 ), V2 , V3 , and V4 – V6 bands were studied.

2 Methods and Materials GaSe crystals grown in ampoules by the zone melting method were ingots with size of 1 × 1.2 × 1.5 cm. They were easy cleaved with receiving plates with mirror surfaces and different thicknesses (100 nm–5 mm). The thin layers with thickness of 100 nm were exfoliated by a scotch type. Crystals quality and its correspondence to the certain known space group were controlled by Raman and x-ray spectroscopy methods. The existence of over structural phases in a crystal ingot was checked with help of SEM imaging and Raman spectra measuring in different point of ingot. Optical spectra of transmission and reflection were measured with help of double grating high aperture spectrometer SDL-1 with aperture 1:2 and linear dispersion 0.8 nm/mm. Low temperature spectra of crystals deposed in closed helium cryostat LTS-22 C 330 were recorded at entrance and exit slits of spectrometers ≤70 μm i.e. with resolution ~0.7 Å. Surface of investigated plates is perpendicular to c-axis and has high reflectivity characteristic for metallic aluminum mirrors. Wavelength modulation reflection spectra were measured by spectrometer MDR-2 with aperture 1:2 and linear dispersion 0.7 nm/mm.

3 Results and Discussions A large amount of experimental and theoretical data about excitonic and electronic spectra exist for layered GaSe crystals [1–9]. It was mentioned in [9–12] that properties of excitonic spectra near the absorption edge for layered semiconductors do not differ from the known properties of exciton spectra in three-dimensional crystals. It should be mentioned that a some moments and properties were observed in layered crystals and were not found out in unlayered crystals. Edge absorption spectra of GaSe crystals with thicknesses 0.5 mm - 300 nm measured at temperatures 300 and 10 K are shown in Fig. 1A. Curve γ was measured for nanocrystals with thickness of 100 nm. In the absorption spectra, an edge onset at 300 K begins at energy of 1.9 eV and reaches a maximum at 2.02 eV. At temperature of 10 K the absorption edge shifts to higher energies and the absorption maximum is observed at 2.1 eV. A temperature shift coefficient β = E/T is 2.7 × 10–4 eV/K. A wellpronounced maximum (n = 1) due to excitonic states in the band gap minimum at low

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and room temperatures is observed in GaSe nanocrystals. At 10 K n = 1 maximum (2.1137 eV) possesses a lower half-width at thickness of 100 nm and its excited states n = 2 (2.1299 eV) and n = 3 (2.1332 eV) are clear recognized. The binding energy of this exciton Ry is 20.9 ± 0.5 meV. In abovementioned, experimental data an interesting fact is the fact that the ground state of exciton is observed at room temperature even at such low value of binging energy. Some absorption spectra shown in Fig. 1A were measured on 100 and 300 nm thick crystals. Interference fringes in absorption spectra in the crystals with such thicknesses are not practically observed. This intensive maximum n = 1 at energy of 2.1137 eV can be associated with Wannier-Mott exciton.

Fig. 1. A – Absorption spectra (K) of nanocrystals with different thicknesses (from 0.5 mm to 300 nm) measured at 300 and 10 K (γ corresponds to 100 nm sample). Insert illustrates images of investigated crystals. B – Photoluminescence spectra (PL) from cleavage plane (a – E||a, b – E||b and c – non-polarized light) measured at 10 K and excited by 325 nm laser line.

Photoluminescence (PL) spectra from cleavage plane (a – E||a, b – E||b, c – in nonpolarized light) measured at 10 K and excited by laser line 325 nm are shown in Fig. 1B. The crystal thicknesses (~1 mm) were chosen to avoid interference fringes in measured PL spectra. In PL spectra excited by 325 nm laser and received from cleavage plane (ab plane) a doublet intensive maximum at energies of 2.1134 and 2.1155 eV was found out. In PL spectra measured from the cleavage plane the sharp maximum at 2.1134 eV is more recognized in E||a polarization and the maximum at 2.1155 eV is observed in E||b polarization. It can be suggested that these maxima can be attributed to transversal (ωT ) and longitudinal (ωL ) energies of exciton polaritons. Thus, a longitudinal transversal splitting of exciton polaritonic branches is equal to 2.1 meV. At higher energies in photoluminescence spectra lines at energies of 2.1199 and 2.1332 eV due to excited states n = 2 and n = 3, respectively are observed. Insert of Fig. 1B illustrates another excited states (n = 4 and n = 5). The binding energy of exciton according PL data is 20.5 meV. Wavelength modulation reflection spectra (R/λ) at 300 and 10 K are shown in Fig. 2 A and B, respectively. In R/λ spectra even at room temperature in addition to the maxima nA = 1 (2.0234 eV) and nA = 2 (2.038 eV) wider maxima nB = 1 (2.140 eV), nB = 2 (2.185 eV), nC = 1 (2.295 eV), nC = 2 (2.312 eV), nD = 1 (2.351 eV), nD = 2

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(2.380 eV) i nE = 1 (2.482 eV) are found out, see Fig. 2A. With temperature decreasing down to 10 K maxima in R/λ spectra can be recognized more clearly at higher energies (nA = 1 (2.1115 eV) i nA = 2 (2.038 eV), nA = 3). These excited states (n = 1, n = 2 and n = 3) of excitons are formed by excitonic states nearby V1 – C1 bands. The wider maxima nB = 1 (2.1760 eV), nB = 2 (2.223 eV), nB = 3 (2.2321 eV), nC = 1 (2.320 eV), nC = 2 (2.351 eV), nD = 1 (2.400 eV), nD = 2 (2.431 eV), nD = 3 (2.439 eV) and nE = 1 (2.530 eV) are recognized at higher energies, see Fig. 2B.

Fig. 2. Wavelength modulation reflection spectra (R/λ) measured at 300 K (A) and 10 K (B).

Intensive emission maxima in wide interval (2.1 – 2.6 eV) are observed in PL spectra from cleavage plane of GaSe crystals at excitation of 448 nm laser, Fig. 3A. The intensive maxima n = 1 and n = 2 due to Wannier-Mott excitons are recognized at energy around 2.1 eV. Maxima nB = 1 and nB = 2 at energies of 2.1751 and 2.2222 eV, respectively have a bit lower intensity. At higher energies maxima nC = 1 (2.311 eV), nC = 2 and nD = 1 (2.399 eV), nD = 2 (2.434 eV) and E3 (2.623 eV) can be recognized.

Fig. 3. A – Photoluminescence spectra excited by 448 nm lasers and measured at 10 K from cleavage surface (curves b1 and b2 measured in different points of crystal surface). B – Absorption spectrum measured at room temperature. C – Photoluminescence spectrum measured at 10 K and excited by emission of 448 nm laser.

It should be mentioned, that in luminescence spectra at room temperature reported in Ref. [12] maxima at energies of 2.177, 2.335 and 2.5516 eV were observed. These maxima taking into account temperature shift have a good agreement with our data.

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Figure 3B illustrates an absorption spectrum of GaSe nanocrystals with thickness of 595 nm at room temperature, where absorption maximum at 2.251 eV is revealed. PL spectrum of sample with the brightest emission measured at 10 K in the range 2.36–2.48 eV and excited by 448 nm laser is shown in Fig. 3C. Features at energies of 2.3151 eV, 2.3522 eV and 2.3611 eV due to recombination processes from excitonic states nC = 1, nC = 2 and nC = 3 were found out in the luminescence spectrum. One can believe that, electrons from band C1 and holes from valence band V3 form this excitonic series. In immediate vicinity of the longest wavelength exciton series, namely, in the energy range 2.1–2.25 eV, the absorption maximum nB = 1 (2.176), nB = 2 (2.223eV) and nB = 3(2.232eV) is found both at room and at low temperatures. Figure 4A illustrates absorption spectra of nanocrystals with thickness 87 nm measured at temperature range 10–50 K. Energies of absorption maxima nB = 1, nB = 2 shift with temperature changing in energy range 2.007–2.185 eV. The maxima shits toward higher energies at temperature decreasing: 2.007 eV at 300 K, 2.103 eV at 50 K, 2.170 eV at 30 K, 2.176 eV at 20 K and 2.185 eV at 10 K. These absorption maxima can be due to exciton and electron transitions between valence bands marked as V2 and conduction band C1 , Fig. 4C.

Fig. 4. A – Absorption spectra of GaSe nanocrystal with thickness of 87 nm measured at temperatures 50, 30, 20 and 10 K. B – Band structure and Brillouin zone. C – Model of electron transitions explains PL spectra.

To date, the band structure for layered GaSe crystals has been calculated by many authors [13–20]. Apparently, the most detailed calculations were carried out by the empirical pseudopotential method for III-VI semiconductor compounds [9, 10]. Recently, data on the band structure of layered IV-VI crystals have appeared [13]. Band structure calculations for GaSe, performed in two-dimensional approaching, well describe the order of states arrangement in valence band and the energy gaps between bands extrema. However, a complete picture of optical transitions, their polarization features, and finally, ideas about the distribution of electron density in a layered crystal were obtained by considering the real three-dimensional crystal structure of gallium selenide. For β-GaSe crystals with space group of symmetry D6h 4 a fragment of band structure in Brillouin zone center are shown in Fig. 4B. The top of valence band in this case is − → situated in center of Brillouin zone (state  1 ). Band symmetry in k = 0 is described in [9, 10]. A conduction band has a minimum also in Brillouin zone center ( 6 ). Energies

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of indirect transitions and minimal direct transitions are very close to each other [1, 6, 17, 19, 21]. Direct and indirect transitions become allowed as a result of  1 and  6 states interband mixing due to spin-orbit interaction. An important circumstance is the three-dimensional character of direct excitons in gallium selenide. Experimental studies have shown that an excitonic series in GaSe is well described by the three-dimensional dependence: En = −Rn−2 , (n = 1,2,3…) [9, −2  (n = 0,1,2,3…). 10], but not by the two-dimensional En = −R n + 21 An anisotropy of electrons and holes effective masses in gallium selenide (m|| and m⊥ - marks || and ⊥ mean along and across layers, respectively) is small [1]. The band structure of gallium selenide is built on the base of band structure considered by authors of Refs. [1, 2, 6, 9, 10, 17]. The band gap magnitude for gallium selenide at 4.2 K Eg dir is equal to 2.132 eV [22]. According our data the band gap at 10 K is equal to 2.1325 eV and the energy of excitonic ground state is 2.1121 eV. Experimental results received by us were examined on the base of recent theoretical calculations of the band structure [2, 8, 9, 23, 24]. According the theoretical calculations in GaSe crystals in conduction band in interval 5 – 6 eV around 6 – 7 levels take place. The fragment of band structure built on the base of theoretical calculations is shown in Fig. 4B [1, 2, 6]. According the our data the energy interval V1 (G1 ) → C2 (G5 ) is equal or very close to the energy of exciting laser (~2.767 eV). Thus at excitation of GaSe crystals by 448 nm laser (2.767 eV) electrons are resonantly excited and transited from the top valence band V1 (G1 ) and simultaneously in C1 (G6 ) and C2 (G5 ) bands. As a result in photoluminescence spectra (Fig. 3) a set of emission lines, due to excitons in the vicinity of C1 (G6 ) and V1 (G1 ) bands, E(ωT ) 2.1134 eV, E(ωL ) - 2.1155 eV, n = 2 (2.1199 eV), n = 3 (2.1332 eV), n = 4 (2.1341 eV) and n = 5 (2.1379 eV) is found out. This group of lines is due to the most long-wavelength excitonic series of Wannier-Mott excitons. Near top valence band, there are three closely spaced zones V1 -V2 , V3 -V4 and V5 -V6 Fig. 4B,C. Discovered in photoluminescence spectra intensive lines series A and B are caused by recombination of electrons from excitonic band C1 to split bands V2 and V3 . Maxima C and D more probably can be associated with recombination of charge carriers from C1 ( 6 ) level to lower valence bands (marked in Fig. 4C as V3 (V4 ) and V5 (V6 )). Luminescence maxima C and D (Fig. 3A) are due to recombination of electrons from C1 (G6 ) band to the group of split from double degenerated valence bands (marked in Fig. 4C as V6 and V7 ). High-energy maxima E3 can be associated with transitions from C2 to V1 –V3 bands. Received excitonic and band parameters have a good agreement with our previously published data [25, 26].

4 Conclusions It was sown the presence of excitonic levels in region of E > Eg (V1 –C1 ) by investigation of absorption, reflection and luminescence spectra of GaSe nanocrystals in wide temperature rage (10–300 K). At excitation of 448 nm laser the electrons in GaSe crystals are resonantly excited from V1 (G1 ) to C1 (G6 ), C2 (G5 ) bands. In luminescence spectra the emission lines due to recombination from excitonic levels C1 (G6 ) to band V1 (G1 ) were found out. Recombination from C1 (G6 ) to V2 , V3 bands (luminescence maxima series B) and from C1 (G6 ) in V4 ,V5 bands (maxima series C) take place at this excitation too.

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High-energy maxima E are more probably associated with transitions from C1 to V1 –V3 bands. Parameters excitons and bands V1( 1 ) and C1 ( 6 ) were determined by investigation of absorption, reflection and photoluminescence spectra. Thus in the region of interband gap minimum in Brillouin zone center there are at least 7 valence bands split by crystal field, spin-orbit interaction, and layered structure of GaSe crystal. Acknowledgments. This research was funded by National Agency for Research and Development of Moldova under the Grant #22.80013.5007.4BL “Nano- and hetero-structures based on zinc oxide and A3B5 compounds for optoelectronics, photonics and biosensorics”.

Conflict of Interest. The authors declare no conflicts of interest. The funding sponsors had no role in the following actions: the design of the study; the collection, analyses, or interpretation of data; the writing of the manuscript, and the decision to publish the results.

References 1. Yagmurcukardes, M., Senger, R.T., Peeters, F.M., Sahin, H.: Mechanical properties of monolayer GaS and GaSe crystal. Phys. Rev. B 94(7), 245407 (2016). https://doi.org/10.1103/Phy sRevB.94.245407 2. Allakhverdiev, K.R., Yetis, M.Ö., Özbek, S., Baykara, T.K., Salaev, E.: Effective nonlinear GaSe crystal. Opt. Prop. Appl. Laser Phys. 19(5), 1092–1097 (2009). https://doi.org/10.1134/ S1054660X09050375 3. Guo, J., et al.: Doped GaSe crystals for laser frequency conversion. Light Sci. Appl. 4, e362– e367 (2015). https://doi.org/10.1038/lsa.2015.135 4. Hsu, Y.-K., Chen, C.-W., Huang, J.Y., Pan, C.-L.: Erbium doped GaSe crystal for mid-IR applications. Opt. Express 14(12), 5484–5491 (2006). https://doi.org/10.1364/OE.14.005484 5. Tonndorf, P., et al.: Single-photon emitters in GaSe. 2D Mater. 4(2) 021010(7) (2017). https:// doi.org/10.1088/2053-1583/aa525b 6. Pham, K.D., Phuc, H.V., Hieu, N.N., Hoi, B.D., Nguyen, C.V.: Electronic properties of GaSe/MoS2 and GaS/MoSe2 heterojunctions from first principles calculation. AIP Adv. 8(5), 075207 (2018). https://doi.org/10.1063/1.5033348 7. Brudnyi, V.N., Sarkisov, S.Y., Kosobutsky, A.V.: Electronic properties of GaSe, InSe, GaS and GaTe layered semiconductors: charge neutrality level and interface barrier heights. Semiconductor Sci. Technol. 30(9), 115019 (2015). http://iopscience.iop.org/0268-1242/30/11/ 115019 8. Agekyan, V.F., Serov, A.Y., Filosofov, N.G.: Optical spectra of GaSe and GaS crystals of different thicknesses. Phys. Solid State 60, 1223–1232 (2018). https://doi.org/10.1134/S10 63783418060021 9. Belen’kii, G.L., Stopachinskii, V.B.: Electronic and vibrational spectra of III-VI layered semiconductors. Physisc-Uspekhi 140, 234–239 (1983). https://doi.org/10.1070/PU1983v02 6n06ABEH004420 10. Belen’kii, G.L., Godzhaev, M.O., Salaev, E., Aliev, E.T.: High-temperature electron-hole liquid in layered InSe. GaSe Gas Cryst. Soviet Phys. JETP 64(5), 1886–1890 (1986) 11. Bernier, G., Gagnon, R., Jandl, S.: Thermoreflectance study of direct and indirect excitons near the fundamental edge of GaSe. Solid State Commun. 63(5), 431–437 (1987). https://doi. org/10.1016/0038-1098(87)91144-6

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12. Kyazym-zade, A.G., et al.: Structure, optical, and luminescent properties of GaSe nanoparticles. Nanotechnol. Russia 10(9–10), 794–803 (2015). https://doi.org/10.1134/S19950780 15050122 13. Asensio, M.C., Lhuillier, E., Eddrief, M., Fal’ko, V.I., Ouerghi, A.: Valence band inversion and spin - orbit effects in the electronic structure of monolayer GaSe. Phys. Rev. B 98(6), 115405 (2018). https://doi.org/10.1103/PhysRevB.98.115405 14. Ertap, H., Mamedov, G.M., Karabulut, M., Bacıoglu, A.: Pool-Frenkel thermoelectric modulation of exciton photoluminescence in GaSe crystals. J. Lumin. 131, 1376–1381 (2011). https://doi.org/10.1016/j.jlumin.2011.03.037 15. Dey, P., et al.: Biexciton formation and exciton coherent coupling in layered GaSe. J. Chem. Phys. 142, 212422–212429 (2015). https://doi.org/10.1063/1.4917169 16. Abdinov, A.S., Babaeva, R.F.: Peculiarities of kinetic coefficients of single crystals of a layered p-GaSe semiconductor. Russ. Phys. J. 61(9), 1667–1673 (2019). https://doi.org/10.1007/s11 182-018-1585-1 17. Jappor, H.R.: Electronic structure of novel GaS/GaSe heterostructures based on GaS and GaSe monolayers. Physica B 524, 109–117 (2017). https://doi.org/10.1016/j.physb.2017.08.054 18. Sarkisov, S.Y., Kosobutsky, A.V., Brudnyi, V.N., Zhuravlev, Y.N.: Ab initio calculations of optical constants of GaSe and InSe layered crystals. Phys. Solid State 57, 1735–1743 (2015). https://doi.org/10.1134/S1063783415090309 19. Ben Aziza, Z., et al.: Valence band inversion and spin-orbit effects in the electronic structure of monolayer GaSe. Phys. Rev. B 98, 115405–115412 (2018). https://doi.org/10.1103/Phy sRevB.98.115405 20. Yetis, M.Ö., Özbek, S., Baykara, T.K.: Effective nonlinear GaSe crystal. Optical Properties and Applications. Laser Phys. 19(5) 1092–1104. (2009). ISSN 1054–660X 21. Robertson, J.: Electronic structure of GaSe, Gas, InSe and GaTe. J. Phys. C: Solid State Phys. 12, 4777–4785 (1979). https://doi.org/10.1088/0022-3719/12/22/019 22. Tang, Y., Xie, W., Mandal, K.C., McGuire, J.A., Lai, C.W.: Exciton spin dynamics in GaSe. J. Appl. Phys. 118, 113103–113112 (2015). https://doi.org/10.1063/1.4930809 23. Pham, K.D., Phuc, H.V., Hieu, N.N., Hoi, B.D., Nguyen, C.V.: Electronic properties of GaSe/MoS2 and GaS/MoSe2 heterojunctions from first principles calculation. AIP Adv. 8(9), 075207 (2018). https://doi.org/10.1063/1.5033348 24. Errandonea, D., et al.: Crystal symmetry and pressure effects on the valence band structure of γ-InSe and ε-GaSe: Transport measurements and electronic structure calculations. Phys. Rev. B 71(8), 125206 (2005). https://doi.org/10.1103/PhysRevB.71.125206 25. Syrbu, N.N., Zalamai, V.V., Stamov, I.G.: Interference of exciton polariton waves in GaSe nanocrystals. Mater. Today Commun. 27(7), 102355 (2021). https://doi.org/10.1016/j.mtc omm.2021.102355 26. Zalamai, V.V., Syrbu, N.N., Stamov, I.G., Beril, S.I.: Wannier-Mott excitons in GaSe single crystals. J. Optics 22(7), 085402 (2020). https://doi.org/10.1088/2040-8986/ab9f17

New Characteristics of Blue Self-pulsating InGaN Lasers Eugeniu Grigoriev , Spiridon Rusu , and Vasile Tronciu(B) Department of Physics, Technical University of Moldova, Chisinau, Moldova [email protected]

Abstract. In this paper, we present results of numerical calculations on the influence of blue light laser parameters on self-pulsation regimes. The adopted Yamada model to InGaN laser is used for numerical calculations. We start our investigations presenting the dependence of output power on the injected current. A threshold current of 90mA is obtained. We use the bifurcation analysis to plot the lines of Hopf - bifurcation in the plane of different parameters. The region of self-pulsation in the plane differential amplification coefficient injected current is obtained. The region of self-pulsation is wide and appears for large values of injected current. We studied also the influence of the thickness of the saturation absorber, the length of the laser, as well as the lifetime of the charge carriers on the self-pulsation region in terms of several parameters. The region of self-pulsations for different values of the reflection coefficient of the back facet of the laser is obtained. The higher reflectivity implies the wide self-pulsation region. Finally, we calculated the lines of the same frequency in the plane of laser length – injected current. We also report the regions of pulsations with higher frequency. Keywords: Self-pulsations · bifurcations · blue lasers · saturable absorber

1 Intorduction In recent years, due to applications in medicine, blue and blue-violet light lasers (450 and 405 nm) seem to represent an interesting approach for several clinical treatments [1]. On the other hand, blue and violet InGaN lasers are widely used in interferometers, laser printing, digital photofinishing, data recording, etc. Data recording is one of the important issue for the development of blue laser diodes. The production of blue and violet lasers is motivated by the relatively short wavelengths [2]. Room-temperature operation of self-pulsating InGaN lasers was reported both theoretically and experimentally at a wavelength of 395 nm. Self-pulsation in the range from 1.6 to 2.9 GHz has been confirmed. Good agreement was obtained between measurements and theoretical simulations [3]. Studies of the dynamics of a multi-quantum well InGaN laser with a saturable absorber have been reported in [4]. In particular, the self-pulsation and excitable operation of the laser were investigated. In this paper, we present theoretical results of the influence of blue light laser parameters on self-pulsations. We studied the influence of the thickness of the saturation © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 174–181, 2024. https://doi.org/10.1007/978-3-031-42775-6_19

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absorber, the length of the laser, as well as the lifetime of the charge carriers on the self-pulsation region in terms of several parameters. The paper is organized as follows: in Sect. 2, we present the laser configuration and equations, in Sect. 3, we summarize the theoretical model, discuss the parameter variation and the influence on the pulse generation, also the numerically simulated results are presented in this section. Conclusions are presented in Sect. 4.

2 Laser Structure and Equations Figure 1 shows a structure of the investigated laser which consists of the InGaN active layer and a saturation absorber. The active layer is composed of 5 quantum wells of the InGaN type. On the other hand, the saturation absorber could be fabricated by single or multi quantum wells. The thickness of the active region and saturation absorber is 18 nm and the wavelength is 405 nm. The length of the active layer is 650 µm.

Fig. 1. Schematic of the InGaN laser.

The theoretical model used to describe the dynamics of laser shown in Fig. 1 is based on the model proposed in [5] and [4]      N a ξ − N dS M i ai ξi Ni i i i gi i = − BS − Gth S + (1) dt Vi Vi   Iji − Iij dNi Ni Ni  Nj ai ξi  (2) + − + Ni − Ngi S − =− dt Vi τsi Tij Tji e j=i

where S is the total number of emitted photons, Ni is the number of charge carriers injected in region i, ai is the differential amplification coefficient, ξi is the field limiting factor. Ngi is the number of charge carriers transferred through region i, τsi is the lifetime

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of the load carriers and Tij is the duration of time that is equivalent to the life time of charge carriers during their diffusion from region j to region i. Iji is the intensity of charge carriers injected from region j into region i. Vi is the volume of the laser sections determined by the expression Vi = Wi di L, where L is the length of the laser. di and Wi are, respectively, the thickness and width of sections. a1 = 1, 85 · 10−12 m3 /s and a2 = 20 · 10−12 m3 /s are the differential amplification coefficients of region one and region two, respectively. ξ1 = 0.045, ξ2 = 0.0033 represents the limiting factors of the field. Ntr is the number of photons at transparency, τs is the lifetime of charge carriers. Ii is the carrier injection in region i. M is the equivalent total number of longitudinal modes [5]. We consider the amplification saturation coefficient B with the following form   a1 ξ12 , B = Bc Ni − Ngi V12

(3)

where Bc =

9π cτin2 |Rcv |2 . 2ε0 n2r λ0

For the threshold level of amplification Gth we use the following expression  1 1 c ln Gth = k+ , nr 2L Rf RB

(4)

(5)

where Rf = 0, 25 and Rb = 0, 95 are the reflectivity coefficients of the front and back facets, respectively. k = 1000 is the loss coefficient. For a detailed description of the remaining parameters of the model we refer to [4].

3 Results and Discussion In what follows we study the dynamics of the InGaN laser shown in Fig. 1 using Eqs. (1)– (5). We start in Fig. 2 showing the dependence of the output power of the emerging light on the injected current, obtained based on numerical simulation. We observe a threshold current of 0.09 A. When the intensity of the injected current increases, immediately after its threshold value, the operation of the laser in continuous wave mode is observed (solid line). After this, the laser begins to produce pulses through the Hopf bifurcation, marked in Fig. 2 through a square. Unstable steady states are drawn by red dotted line. We observe that in our case the pulsations start after threshold and continuous wave operation different from that described in [6], where the self-pulsations start immediately from the threshold value of the current intensity. We mention that, in [6] it was assumed that the lifetime of charge carriers in the saturation absorber region is the same as in the active region. In our case, since the active region and the saturation absorber are separated by an anti-evaporation layer, we could assume that in the active region and that of the saturation absorber layer, the charge carriers have different lifetimes. Finally, we mention that both Hopf points shown in Fig. 2 are supercritical.

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Fig. 2. Dependence of the output power on the intensity of the injected current. Unstable stationary states are represented by dotted line, and stable ones by solid lines.

It is well known, that the bifurcation diagram gives an information on the location of stable and unstable regions in the plane of different parameters. Figure 3 shows one example of bifurcation diagram, where the solid black line (Hopf bifurcation) is calculated with help of AUTO-2000 [7]. The green colored region shows self-pulsation operation in the plane: differential amplification coefficient depending on the injected current in the active region. The red line in Fig. 3 represents the border between the operating regions of the laser in “off” and “on” mode. The yellow region marked with CW corresponds to the operating mode of the laser with continuous waves. One can see that, for a wide region of injected current the laser is operating in self-pulsations regime.

Fig. 3. Bifurcation diagram for a laser length of 650 mm. The region of self-pulsations in the plane: differential amplification coefficient depending on the injected current (green region). The current threshold value is indicated by the red line. The black line indicates the Hopf bifurcation.

Depth of the saturation absorber in the limits from 1 to 5 nm is considered the better one for the manufactured lasers. In what follows, we consider two values of depth 5 nm

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and 3.75 nm. For a thin absorber, the region of self-pulsations becomes narrower (see Fig. 4). For these set of parameters exist a threshold value of saturable absorber depth of 3.5 nm for which the self-pulsations disappears (see Fig. 4b). This can be explained in the following way: as a result of decreasing the thickness of the saturation absorber, the limiting factor of the field decreases. On the other hand, decreasing the thickness of the saturation absorber leads to decreasing the injected current threshold (see dotted lines in Fig. 4). Thus, we conclude that a thinner saturation absorber reduces the injected current threshold, but also narrows the self-pulsation region. On the other hand, a higher increase of depth of saturable absorber leads to high injected currents for laser operation.

Fig. 4. The influence of the thickness of the saturation absorber on the self-pulsation region in the plane of different parameters. Dotted lines show the threshold currents for a laser with a cavity length of 650 µm and different absorber thicknesses. Solid lines – Hopf bifurcation.

Next, we analyze the influence of the reflection coefficient of the back facet of the laser on the self-pulsation region. Usually, in the laser growth process it is necessary that this coefficient to be kept close to unity. However, in some cases, following laser degradation, the reflection coefficient decreases. As seen from Fig. 5 the reduction of the reflection coefficient leads to the reduction of the self-pulsation region and of course of the emission intensity of the laser. To maintain the operating point in the region of selfpulsations in the case of small reflection, it is necessary to increase the injected current and also to increase the length of laser. On the other hand, an increase of laser length result in big losses. We consider that the lasers with length between 400 µm to 700 µm

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are suitable for getting self-pulsation operation for lower injected currents keeping the back facet reflectivity appropriate to unity.

Fig. 5. The region of self-pulsations for different values of the reflection coefficient of the back face of the laser.

In the follows, we analyze the frequency of self-pulsations. To estimate the frequency of self-pulsations, the numerical modeling was performed in the plane of two parameters laser length – injected current. The obtained results are presented in Fig. 6, where the black line corresponds to the Hopf bifurcation and the colored lines are the ones with self-pulsations of the same frequency. It can be observed, how the frequency of selfpulsations changes. For a fixed current intensity, the frequency of pulsations becomes higher for smaller lengths of the resonator and for high values of the injected current. The higher frequency of self-pulsations in this figure is approaching the relaxation frequency.

Fig. 6. Self-pulsation region and its frequency.

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Next, we calculate the frequency of self-pulsations in terms of the following parameters: the saturation absorber and the length of the laser. As mentioned above, the properties of the saturation absorber determine the appearance of self-pulsations and ultimately their frequency. In Fig. 7 the regions of self-pulsations are illustrated by the dependence of the length of the laser resonator on a) the differential amplification coefficient and b) the lifetime of the charge carriers in absorbent. These regions were obtained using Eqs. (1)–(5) for the InGaN laser parameters and the current intensity fixed at 150 mA. We mention that the colored regions are those with self-pulsations, and the white region corresponds to operation with continuous waves or without emission. We observe that in the range of resonator lengths from 400 to 500 µm, the regions of self-pulsations are wider. Small resonator lengths imply high self-pulsation frequencies: however, these regions are narrow and have high absorption levels and high threshold currents. Thus, the range of resonator lengths 400–500 µm are the most favorable for the generation of self-pulsations with frequencies between 2 and 3 GHz.

Fig. 7. Variation of the frequency of self-pulsations in the plane: the length of the resonator depending on a) coefficient of differential amplification in the absorber and b) the lifetime of the charge carriers in absorbent. The current intensity is 150 mA.

4 Conclusions In this paper we investigate the dynamics of blue lasers with self-pulsations. The domains of self-pulsations were obtained in the plane of different parameters of the laser. A laser structure grown in the vertical direction, so-called “sandwich”, was investigated. It was concluded that the thickness of the absorber, as well as the life time of the charge carriers in the absorber play an essential role in the dynamics of the laser, in particular, to the appearance of self-pulsations. Self-pulsations with frequencies in the 0.55–3.00 GHz range were detected by numerical calculations.

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Acknowledgments. This work was supported by the National Agency for Research and Development of Moldova within the project 20.80009.5007.08 "Study of optoelectronic structures and thermoelectric devices with high efficiency".

Conflict of Interest. “The authors declare that they have no conflict of interest”.

References 1. Fornaini, C., Fekrazad, R., Rocca, J.-P., Zhang, S., Merigo, E.: Use of blue and blue-violet lasers in dentistry: A narrative review. J. Lasers Med. Sci. 12(1), e31 (2021). https://doi.org/ 10.34172/jlms.2021.31 2. Nakamura, S., Pearton, S., Fasol, G.: The Blue Laser Diode, 2nd edn. Springer, Berlin, Germany (2000). https://doi.org/10.1007/978-3-662-04156-7 3. Tronciu, V.Z., Yamada, M., Ohno, T., Ito, S., Kawakami, T., Taneya, M.: Self-pulsation in an InGaN laser - theory and experiment. IEEE J. Quant. Electron. 39, 1509–1514 (2003). https:// doi.org/10.1109/JQE.2003.819541 4. Tronciu, V.Z., Yamada, M., Abram, R.A.: Analysis of the dynamics of a blue-violet InxGa1−xN laser with a saturable absorber. Phys. Rev. E 70(2), 026604 (2004). https://doi.org/10.1103/ PhysRevE.70.026604 5. Yamada, M.: A theoretical analysis of self-sustained pulsation phenomena in narrow-stripe semiconductor lasers. IEEE J. Quant. Electron. 29(5), 1330–1336 (1993). https://doi.org/10. 1109/3.236146 6. Mirasso, C.R., et al.: Self-pulsating semiconductor lasers: theory and experiment. IEEE J. Quant. Electron. 35(5), 764–770 (1999). https://doi.org/10.1109/3.760324 7. Doedel E.J., et al.: AUTO 2000: Continuation and bifurcation software for ordinary differential equations (with HomCont). Technical Report. Caltech, February (2001)

Parametric Anomaly of the Phonon Spectrum of a Thin Free-Standing Membrane Sergiu Cojocaru(B) Horia Hulubei National Institute for Physics and Nuclear Engineering, 077125 Magurele, Romania [email protected]

Abstract. We consider modification of the acoustic Rayleigh-Lamb phonon spectrum in a thin homogeneous membrane upon variation of parameters characterizing the medium. It is emphasized that single-valued parametric dependence of the frequency-wavenumber spectrum is related to the performance of acoustic sensing applications and a single layer membrane has been known to fully support this requirement. To capture the behavior in the full three-dimensional parametric space of the considered basic structure we analyze the solutions in terms of variables scaled with material parameters. The main finding is that not all the branches of the spectrum demonstrate the regular monotonic evolution in the parametric space (e.g., an increase of the bulk velocity is associated with an increase of the phonon eigenfrequencies). Thus, the resonance frequencies of a whole range of phonon modes are shown to decrease if the bulk velocity of the propagation medium is increased. It is noteworthy that the respective anomalous branches, S1 and A2, are the ones which show the strongest dispersion anomalies, like zero group velocity and backward wave propagation. However, the anomalous parametric variation of the spectrum is located in the region of shorter wavelengths than the dispersion anomaly of the same branches. So that the “abnormal” parametric behavior is observed for the modes on the S1 and A2 branches which a have a “normal”, i.e., positive group velocity. Keywords: Ultrathin Layer · Vibration Spectra · Rayleigh-Lamb Phonon Modes

1 Intorduction Membranes of a nanometric thickness consisting of layers of different materials are commonly used in nanotechnology and microelectronics, and have a large variety of applications in information technology [1], wireless communication [2], heat transport [3], acoustic microsensors monolithically integrated in smart microsystems [4], etc.. These are just a few examples from the plethora of modern applications where acoustic phonon physics comes to the forefront. Thus, due to a huge difference in propagation velocity an important advantages phonons have over photon-based technologies is a much smaller spatial scale of the devices operating in a similar frequency range. It ensures that microwave-frequency phonons, as opposed to photons, can be a natural coupling © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 182–190, 2024. https://doi.org/10.1007/978-3-031-42775-6_20

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agent in hybrid quantum nano-systems [5]. In electronic radiation nano-detectors and micro-coolers the main functional element is composed of a stiff insulating membrane supporting a soft metallic thin layer, so that electron relaxation mainly involves acoustic phonons which dominate the low energy spectrum [6]. Very similar structures constitute the core of modern acoustic membrane-based sensors of temperature, pressure, humidity, mass and gas [7]. It has been established that performance and sensitivity of such devices actually strongly increases with their miniaturization on account of increasing the resonance frequencies and phonon phase velocities [8]. Dissipation of heat from an excited electronic system towards the phonon thermal bath is also strongly enhanced in sufficiently thin sample upon the temperature crossover from bulk-like to the confined, quaisi-2D, regime of the phonon subsystem. This highly beneficial correlation for the modern electronics is due to the properties of Rayleigh-Lamb modes which are characteristic of the thin layered structures. It is generally true that characteristic frequencies are higher for thinner layering provided the phonon wavelengths are comparable to the width of membrane. This explains the shift from bulk to nano-membrane substrate in modern technologies. However, increasing the frequencies to the GHz range in sensing applications on account of reducing the finger width of interdigital transducers has also its technological limitations at some 100–150 nm so that optimal excited wavelengths are about 10–15 times the total thickness of the membrane [9]. An alternative path is related to selection of materials and stacking structure. This perspective has been explored in some detail in the previous works [5, 10] based on the analytic solutions for the long wavelength part of the RL spectrum in multilayers. Here we explore the variation of the RL spectrum with material parameters for a free-standing single layer in a finite wavelengths region where no analytic solutions are available. It should be mentioned that although the quantum nature of the acoustic displacement field reveals itself in the behavior of the amplitudes transformed into quantum operators, the associated excitation spectra can be obtained from the underlying classical elasticity model [11]. Connection between the rather complex multi-branched spectrum of the Rayleigh-Lamb waves and such characteristics as mass densities, thicknesses of the layers, elastic moduli, bulk sound velocities and other parameters is of both practical and fundamental interest [12]. Thus, for an acoustic sensing application it is important that the frequency shift of the chosen resonance mode is uniformly (preferably linearly) dependent on the change of parameters. The latter are in one-to-one correspondence with such external factors as temperature, pressure etc. within sufficiently large intervals of values. E.g., elastic moduli are decreasing functions of temperature until the melting point [13]. This requirement seems not to pose difficulties in both bulk and confined systems that can be understood, e.g., by a simple estimation of the time-of-flight across an infinite layered medium γ = 1, 2, .... Thenthe effective   velocity  of a transverse   hγ / hγ /sγ with bulk shear wave propagating at normal incidence is seff = γ

γ

velocities sγ and layer thicknesses hγ . Shear and longitudinal velocities of a homoge√ neous√substance are determined by mass density and Lame elastic constants, s = μ/ρ,  = (λ + 2μ)/ρ. It is easy to see that effective velocity is a monotonic function of any of its parameters, so that softening of one of the substances with temperature would decrease the velocity in the respective layer causing a decrease of the effective velocity

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of the total system. If thickness of a layer with lowest velocity is increased relative to the others, then also the seff decreases. A similar uniformity of the parametric dependence is demonstrated by the frequency-wavenumber ω (q) spectrum of the horizontally polarized shear  wave (SH) in a single layered membrane [14].

ωn = s (π n/h)2 + q2 , n = 0, 1, 2.., and can even be found in some models of functionally-graded materials [15]. Here we have an example that growth of the resonance frequencies or phase velocities is achieved by thinning of the structure in addition to selecting a faster material. One can also see that every n-th spectral curve becomes a hypersurface in a larger, 4-dimensional, space when the two independent material parameters, s and h, are considered. The Rayleigh-Lamb (RL) modes propagating in the x-direction of the (x,y) plane of a homogeneous layer have a mixed polarization and are represented by the displacement field U = (ux (z), 0, uz (z)) exp(iqx − iωt). The characteristic equations corresponding to these straight wavefront waves are sufficiently complex to avoid explicit analytic solution except for some limits. Nevertheless, the RL equations have been thoroughly investigated and make the conceptual basis for interpretation of experimental data, e.g., [11–14] (remarkably, the same equations hold for the RL waves with circularly crested shape). There are three independent parameters characterizing materials composition of the layer. These are also commonly used to define the physically meaningful scaled variables Q = hq and Ω = (ωh / s) (e.g., the dimensionless frequency is an integer multiple of π for the cutoff modes corresponding to the q = 0 thickness resonances), √ ⎞± ⎛   2 −Q2 2 2 2 2 2 tan −4Q j − Q − Q ⎠ = ⎝ √ 22 (1) 2 2 2 j −Q2 (2Q − ) tan 2 where j = s2 /2 is an involution of Poisson’s ratio, σ = (1/2 − j)/(1 − j). The +, − signs correspond the symmetric S n and antisymmetric An branches ω = ωn (q) of the spectrum. It may be expected that parametric dependence of the band structure in this extended space will not be qualitatively different from the uniform trend discussed above. Indeed, the known “irregularities” only refer to the important features in frequency-wavenumber dispersion ω(q), Fig. 1: presence of zero-group-velocity (ZGV) points at q = qmin . And the associated phenomenon of backward wave propagation at longer wavelengths where the group and phase velocities have opposite signs [16–19]. These special features are currently explored for the development of new kinds of devices, including resonators, super-resolution flat acoustic lenses, filters, energy harvesters, and acoustic cloaks. Thus, ZGV modes correspond to excitations localized at the excitation region within half-wavelength range because the energy cannot propagate in the sample due to vanishing of the group velocity. Measurement of corresponding resonance values is used for non-destructive evaluation of local characteristics of material, e.g., bulk velocities or Poisson’s ratio. Existence of negative group velocity has allowed to demonstrate the phenomenon of negative refraction and wave focusing in thin membrane structures which have several advantages over the engineered phononic crystals or acoustic metamaterials [20]. In the parametric space these anomalies are observed in some areas on the hypersurfaces identified by the band index “n” for both symmetric and antisymmetric modes,

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Fig. 1. The frequency-wavenumber Rayleigh-Lamb phonon band structure of the symmetric modes in dimensionless variables for the GaN thin film, Wurtzite crystal, < 100 > direction, s = 4130 m/s,  = 7960 m/s. Note that the zero-group-velocity point, ZGV, is at Qmin = qmin h ≈ 2.

e.g., S1, A2, etc. [21]. For instances, the ZGV is only observed if Poisson’s ratio varies from 0 to 0.451. For higher values no frequency minimum exists at finite q and the S1 Lamb mode resonance occurs only at the cutoff frequency ωS1 (q = 0) = π s/h. Still, no anomalies have been reported so far as concerns the variation of eigenfrequencies along the parametric “coordinates”: even if the wavenumbers correspond to anomalous regions {0, qmin }, solutions of the Eq. (1) appear to be monotonously increasing functions of the three parameters. It should also be mentioned that a change of boundary conditions affects the parametric dependence of spectral lines as well. For instance, existence of the so called “diabolical crossings”, where conical band intersection is realized at some points in the two-dimensional parametric space {σ , s}, has been recently proven for the RayleighLamb waves in a layer with fixed bottom (LFB) [22]. From the perspective of the present analysis, it would be important to clarify whether parametric variation of the spectrum is monotonic also for the LFB. From the Figs. 6 and 8 in the paper it appears to be the case for the σ – dependence even in the presence of band degeneracy.

2 A Parametric Anomaly of the Single Layer Spectrum In a traction-free layer the monotonic variation of the S and A solutions of Eq. (1) with layer thickness is a general property of the spectrum, as illustrated in the Fig. 2, with the remark that the A0 branch is the only one which increases with h. In the Q < 1 region it corresponds to the fundamental flexural mode C F = Q s (6 (1- σ ))−1/2 , where C = ω/q and becomes the Rayleigh surface acoustic wave (SAW) at large h where its velocity depends on {σ , s} in a regular, continuously increasing way, as can be seen from the approximation obtained in [23]: CR /s = 0.874 + 0.196σ − 0.043σ 2 − 0.0553σ 3 .

(2)

As explained above, the parametric variation of the phonon spectra has direct implications for acoustic sensing applications where the essential quantitative measures are

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Fig. 2. The first few branches of the frequency (Hz) versus thickness (μm) for the RL band structure (antisymmetric modes, ν = ω/2π) excited in a Sapphire membrane at the fixed wavelength λ = 3μm ( = 11 km/s, s = 6.5 km/s, σ = 0.23, ρ = 3980 kg/m3 ). The GHz frequency-range is typical for the membranes of a few microns used in microelectronics.

defined as sensitivity F to a parameter (e.g., h) or f , the response coefficient, [8, 12, 14] Fh =

dlnωn d ωn , or fh = . dh dh

(3)

We can then see from Fig. 2 and above expressions that their values are highest for thinner membranes and tend to decrease in the bulk limit. Indeed, from the known analytic solutions in these limits [12] we find Fh (A0 , h → 0) = −Fh (An , n ≥ 1, h → 0) = 1, Fh (An , h → ∞) = 0.

(4)

When calculating the sensitivities with respect to external factors, e.g., temperature, humidity or mass response, one should include also variation of the other material parameters [7, 14]. For instance, temperature is known to cause a decrease of the elastic moduli, but also to increase thickness of the sample due to dilatation, particularly in metals [24]. Along with the requirement of maximized sensitivity, the other general characteristic important for applications, is the single valuedness of the measurements. This translates into the requirement of the monotonicity of parametric dependence. In the case of RL the phonon spectral lines ωn (q) unroll into a hypersurfaces in the 5- dimensional space upon variation of parameters {h, s, σ } or {h, s, }. Choosing one or another set for the bulk constants may be a matter of convenience when comparing materials within a specific group. However, in exploring the extended space each choice assumes an independent variation of “coordinates” and therefore gives complementary information on the structure of spectral surfaces. However, irrespective of any given set of material constants, the layer thickness cannot be responsible for a non-monotonic behavior since, as already mentioned, the whole spectrum of the S and A modes changes monotonically with h. It then allows introduce thickness as a scaling parameter when considering the evolution of the band structure in the parametric space since it would not affect the general behavior. Then it should be taken into account that the set commonly used to identify the parameters is {s, σ} because variation of the Poisson’s ratio is relatively restricted while the

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two velocities bulk velocities may differ quite significantly between substances. Indeed, the ratio is confined to the interval −1 ≤ σ ≤ 1/2 by the requirement of mechanical stability or positive definiteness of strain energy [12, 14]. This interval is further reduced to acoustically “normal” (non-auxetic) materials by the applicability of the linear elasticity theory, 0 ≤ σ ≤ ½, or, equivalently √ 0 ≤ s ≤ / 2  0.71. (5) If we now consider s and σ as independent variables then from Eq. (1) it follows that for any value of σ all the solution branches are linearly increasing with bulk shear velocity. This proves that the whole band structure is a continuous single valued function in the two variables {h,s}. The above arguments actually justify the use of dimensionless variables in clarifying dependence of the phonon spectrum on all parameters. The solutions in Fig. 3 represent the typical dependence on the Poisson’s of the two symmetry classes of solutions at different wavenumbers.

Fig. 3. Monotonically increasing dependence of the RL symmetric and antisymmetric spectra with Poisson’s ratio for some representative values of the wavenumbers Q.

Hence, we have demonstrated that the standard choice of parameters {h,s,σ } leads to a completely regular single valued dependence of the RL phonon spectrum of a single free-standing layer. However, let us now note that σ is actually a composite quantity defined by the ratio of the two bulk velocities σ = (2 – 2s2 ) / 2(2 –s2 ) and consider the alternative set {h,s, } of independent parameters. In this case we can no longer separate the contribution of the two parametric variables in the Eq. (1) and cannot use the above scaled variables. We may still use the scaling with respect to thickness h to define a new quantity ω0 = ω h. When considering the independent variation of the two parameters we should also take into account the restriction in (5). It can be seen that considering variation of the longitudinal velocity at a fixed value of transverse velocity will not produce any irregularity since the situation is captured already by the results in Fig. 3, i.e., the resonance frequencies will increase in a monotonic but a non-linear way. We can now explore the evolution of the band structure with s at a fixed value of the longitudinal velocity. We may then take this fixed value as a velocity unit,  = 1.

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The calculations have been carried out for different wavenumbers, covering the region corresponding to the spectral anomalies, ZGV and negative group velocities. The results are presented in Fig. 4 for the symmetric solutions of Eq. (1), similar behavior is found for the antisymmetric ones.

Fig. 4. The scaled frequency ω0 = ω h of the few lowest branches of the symmetric RL spectrum versus bulk shear velocity s for a fixed value of the longitudinal velocity  = 1. The wavenumber Q = q h is taken at a value Q = 3.3 (left), larger than at the spectral anomaly ZGV in Fig. 1, and close to the anomaly, Q = 2 (right). Note the descendent trend of the resonance frequencies for the S1 branch as compared to the other branches.

We can see from Fig. 4 that the S1 branch of the RL spectrum shows a downwards bending of its dependence on the bulk shear velocity provided the longitudinal velocity of the medium is kept constant. A similar behavior is found for the A2 branch of the antisymmetric RL modes. These two branches have been long known for their anomalous dispersion in the region of finite wavenumbers (see, e.g., Fig. 1). The associated parametric anomaly is found to come into effect at wavenumbers larger than the region of dispersion anomaly, as can be seen from Fig. 1 and Fig. 4.

3 Conclusions We have found the previously undetected anomaly of the Rayleigh-Lamb spectrum of a single thin layer with traction-free boundary conditions related to its dependence on the parameters of the propagation medium. It has turned out to be associated to the S1 and A2 branches of the spectrum which are well known for their anomalous dispersion features, such as zero and negative group velocities. The anomalous parametric variation of the thin layer spectrum is located in the region of shorter wavelengths than the dispersion anomaly of the same branches. Namely, for the wavenumbers in this region the “abnormal” decrease of the phase velocity with increase of the bulk shear velocity is observed for the phonon modes which a have a “normal”, i.e., positive group velocity. On the other hand, for the wavenumbers corresponding to the backward propagating waves (i.e., negative group velocity at longer wavelengths) the parametric variation is completely regular (i.e., a decrease of the bulk velocity causes a decrease of the eigenfrequency of the phonon mode).

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Thus, our analysis of the basic single layer model suggests that to reveal the full structure of the dispersion anomalies, which are currently discussed in relation to new classes of membrane-based acoustic devices, it is necessary to consider dependence of the spectrum on the parametric variables also in more specialized models. Acknowledgments. This work was financially supported by ANCS Romania (project no. PN 23 21 01 01/ 2023).

Conflict of Interest. The author declares having no conflict of interest.

References 1. Dumur, É., et al.: Quantum communication with itinerant surface acoustic wave phonons. npj Quant. Inf. 7(1), 173 (2021). https://doi.org/10.1038/s41534-021-00511-1 2. Xie, J., Shen, M., Xu, Y., Fu, W., Yang, L., Tang, H.X.: Sub-terahertz electromechanics. Nat. Electron. 6(4), 301–306 (2023). https://doi.org/10.1038/s41928-023-00942-y 3. Cojocaru, S.: Unusual size dependence of acoustic properties in layered nanostructures. In: Tiginyanu, I., Sontea, V., Railean, S. (eds.) ICNBME 2019. IP, vol. 77, pp. 23–27. Springer, Cham (2020). https://doi.org/10.1007/978-3-030-31866-6_5 4. D. Vasilache, A. et al.: Müller: Development of high frequency SAW devices devoted for pressure sensing. In: 2022 International Semiconductor Conference (CAS), pp. 91–94 (2022). https://doi.org/10.1109/CAS56377.2022.9934404 5. MacCabe, G.S., et al.: Nano-acoustic resonator with ultralong phonon lifetime. Science 370, 840–843 (2020). https://doi.org/10.1126/science.abc7312 6. Cojocaru, S., Anghel, D.V.: Low-temperature electron-phonon heat transfer in metal films. Phys. Rev. B. 93, 115405 (2016). https://doi.org/10.1103/PhysRevB.93.115405 7. Zhang, Y., Tan, Q., Zhang, L., Zhang, W., Xiong, J.: A novel SAW temperature-humiditypressure (THP) sensor based on LiNbO3 for environment monitoring. J. Phys. D Appl. Phys. 53, 375401 (2020). https://doi.org/10.1088/1361-6463/ab9138 8. Müller, A., et al.: GaN membrane supported saw pressure sensors with embedded temperature sensing capability. IEEE Sens. J. 17, 7383–7393 (2017). https://doi.org/10.1109/JSEN.2017. 2757770 9. Takagaki, Y., Hesjedal, T., Brandt, O., Ploog, K.H.: Surface-acoustic-wave transducers for the extremely-high-frequency range using AlN/SiC(0001). Semicond. Sci. Technol. 19, 256–259 (2004) 10. Cojocaru, S.: Surface adapted partial waves for the description of elastic vibrations in bilayered plates. Wave Motion 92, 102430 (2020). https://doi.org/10.1016/j.wavemoti.2019.102430 11. Stroscio, M.A., Dutta, M.: Phonons in Nanostructures. Cambridge University Press (2005) 12. Auld, B.A.: Acoustic Fields and Waves in Solids, 2nd ed. (Krieger, Malabar, FL) (1990) 13. Kausel, E.: Fundamental Solutions in Elastodynamics: A Compendium. Cambridge University Press (2006) 14. Cheeke, J.D.N.: Fundamentals and Applications of Ultrasonic Waves, 2nd edn. CRC Press(2012) 15. Cao, X., Jin, F., Jeon, I.: Characterization of the variation of the material properties in a freestanding inhomogeneous thin film. Phys. Lett. A 375(2), 220–224 (2010). https://doi.org/ 10.1016/j.physleta.2010.11.004

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16. Clorennec, D., Prada, C., Royer, D.: Local and noncontact measurements of bulk acoustic wave velocities in thin isotropic plates and shells using zero group velocity Lamb modes. J. Appl. Phys. 101(3), 034908 (2007). https://doi.org/10.1063/1.2434824 17. Kausel, E.: Number and location of zero-group-velocity modes. J. Acoust. Soc. Am. 131(5), 3601–3610 (2012) 18. Glushkov, E.V., Glushkova, N.V.: Multiple zero-group velocity resonances in elastic layered structures. J. Sound Vib. 500, 116023 (2021). https://doi.org/10.1016/j.jsv.2021.116023 19. Forbriger, T., Gao, L., Malischewsky, P., Ohrnberger, M., Pan, Y.: A single Rayleigh mode may exist with multiple values of phase-velocity at one frequency. Geophys. J. Int. 222(1), 582–594 (2020). https://doi.org/10.1093/gji/ggaa123 20. Legrand, F., et al.: Cloaking, trapping and superlensing of lamb waves with negative refraction. Sci. Rep. 11, 23901 (2021). https://doi.org/10.1038/s41598-021-03146-6 21. Negishi, K.: Existence of negative group velocities in lamb waves. Jpn. J. Appl. Phys. 26, 171 (1987). https://doi.org/10.7567/JJAPS.26S1.171 22. Malischewsky, P.G.: Diabolical points and Rayleigh-wave propagation. In: 2022 Days on Diffraction (DD), pp. 88–94 (2022). https://www.pdmi.ras.ru/~dd/ 23. Malischewsky, P.G.: Comparison of approximated solutions for the phase velocity of Rayleigh waves. Nanotechnology 16, 995 (2005). https://doi.org/10.1088/0957-4484/16/6/N01 24. Müller, A., Nicoloiu, A., Dinescu, A., Stavrinidis, A., Zdru, I., Konstantinidis, G.: The influence of metallization on resonance frequency and temperature sensitivity of GHz operating III-Nitride SAW based sensor structures. In: 2018 IEEE/MTT-S International Microwave Symposium – IMS, pp. 938–941 (2018). https://doi.org/10.1109/MWSYM.2018.8439423

Photoluminescence and Cathodoluminescence of Layered ZnIn2 S4 and Zn2 In2 S5 Compounds Thermally Processed in Sulfur Vapor and Vacuum Efim Arama1 , Valentina Pîntea2 , and Tatiana Shemyakova3(B) 1 ”Nicolae Testemiteanu” University of Medicine and Pharmacy, Chisinau, Moldova , 2 Technical University of Moldova, Chisinau, Moldova 3 Institute of Applied Physics, Moldova State University, Chisinau, Moldova

[email protected]

Abstract. The phenomena of nonradiative recombination have been investigated for various semiconductors of the family of Znx In2 S3+x (x = 1, 2, 3) compounds that exhibit photoluminescence in a wide energy range. These properties are promising for a vast range of applications in various branches of engineering. The luminescent properties of the layered compounds were investigated under the action of accelerated electrons – cathodoluminescence - and X-ray radiation. The experimental results of investigations conducted on the luminescent properties of layered compounds ZnIn2 S4 (three-packet polytype III), Zn2 In2 S6 (III), and Zn3 In2 S6 (I) are presented. Processing of layered compounds of the Znx In2 S3+x (x = 1, 2, 3) family in sulfur vapor leads to the displacement of the photoluminescence emission maximum to the low energy region, and the thermal processing in vacuum shifts it towards the high energy range. The obtained results show that it is possible to change the limits of the spectral range of light radiation from 1.36 to 2.71 eV. The calculation of the excitation rate on the material’s surface and in its volume (Rs and Rv ) under the action of accelerated electrons and X-ray radiation, a high level of excitation energies and excitation current allow one to specify the structure and nature of the energy levels in the forbidden energy band. The basic bands in the luminescent emission spectrum of zinc and indium bisulfides Znx In2 S3+x (x = 1, 2, 3): a red band with the maximum at 1.79 eV, an orange band with the maximum of 2.08 eV, and a yellow-green band with a maximum at 2.34 eV were identified. Keywords: Layered compounds · Photoluminescence · Cathodoluminescence · Bisulfides · Accelerated electrons · X-ray radiation · Nonradiation recombination

1 Introduction The phenomena of nonradiative recombination have been investigated for various semiconductors of the family of Znx In2 S3+x (x = 1, 2, 3) compounds that exhibit photoluminescence within a wide energy range. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 191–196, 2024. https://doi.org/10.1007/978-3-031-42775-6_21

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The layered compounds with stoichiometric vacancies in the crystal lattice of the Znx In2 S3+x (x = 1, 2, 3) family possess major advantages due to their strong properties, such as: intensive luminescence [1], wide spectrum of forbidden energy gap values, low sensitivity to impurities and high stability to ionizing radiations [2]. Moreover, zinc and indium bisulfides exhibit intensive luminescence and high photosensitivity within the visible and ultraviolet spectrum. These properties are promising for a vast range of applications in various branches of engineering. Recently, there has been a constant interest in photocatalytic activity of ZnIn2 S4 with the release of hydrogen [3–5] and for CO2 reduction [6–8]. Defect engineering on the photocatalysts has been reported to increase the carrier separation efficiency, in which the introducing defects could serve as the centers for capturing photoexcited electrons and hence prevent their recombination with the photogenerated holes. Since the technology of synthesis and growth of nanosize crystals of these ternary semiconductors has reached a high level [9], they are of considerable interest not only from the point of view of extremely useful applications, but also of fundamental research. The expansion of the excitation energy spectrum by applying the X-ray radiation to investigate the luminescent properties and conductivity of these layered compounds under the action of accelerated electrons and X-ray radiation is also reasonable. The experimental research of luminescence and conductivity under the action of accelerated electrons and X-ray radiation is expedient due to the fact that the effectiveness in these cases does not exceed 25% for samples with the thicknesses of about two millimeters, since the activation degree is high. The main purpose of this work was to study the experimental results of investigations conducted on the luminescent properties of layered compounds ZnIn2 S4 (three-packet polytype III), Zn2 In2 S6 (III), and Zn3 In2 S6 (I).

2 Experimental Method Focusing on the complex theoretical and experimental study of the cathodoluminescence of the investigated compounds, we aimed to essentially broaden the spectrum of excitation energies. The only research methodology for semiconductor materials is the method using electron microscopy, which employs as a radiation source a beam of highly accelerated electrons in an electric field. Upon the impact of an electron beam and X-rays with the substance under investigation, secondary electrons, X-rays, and photons are emitted, which also provide information about the topography of the analyzed surface. The scheme of the installation applied for experimental research is described in [10, 11]. According to the theoretical study and the experimental investigations carried out, ternary semiconductors ZnIn2 S4 are characterized by the phenomenon of polytypism, among which two-pack polytyps exist. It is known, that zinc thioindate belongs to the group of materials with a high concentration of intrinsic defects. Their concentrations in the investigated materials varied between 1.2·1019 and 1.9·1020 cm−3 . On the basis of experimental research, photoluminescence spectra were determined for crystals grown from a gas phase with a thickness up to 250 μm and surface area up to about 0.3 cm2 . Photoluminescence was excited using a HBO-500 (366 nm) mercury lamp.

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3 Experimental Results In the photoluminescence spectra of the ZnIn2 S4 (II) two-packet polytype, thermally processed in sulfur vapor for 28 h at a temperature of 80 K, the emission band with the approximate energy maximum (1.42 ± 0.02) eV predominates (Fig. 1). The half-width of this emission band confirms the participation of various optical transitions in the recombination process. This experimental result assumes that the photoluminescence spectrum contains several radiation bands due to the emission of recombination centers of different nature. Indeed, as seen from Fig. 1, the main energy maximum of the spectrum is recorded at approximately 1.47 eV, and on the high-energy wing an energy band with the maximum at 1.73 eV and a plateau at approximately 1.54 eV appear. Obviously, the peculiarities of the structure of the radiation spectrum are determined by optical transitions with the participation of various impurities located in the forbidden energy band. Figure 2 shows the cathodoluminescence spectra of layered compounds ZnIn2 S4 (II) in arbitrary units, for monocrystals not thermally processed (Fig. 2, curve 1) and thermally processed in sulfur vapor (Fig. 2, curve 2). Spectra have been recorded at a temperature of 80 K. The luminescent radiation was excited with an electron beam accelerated with the energy of 60 keV and current density of the electron beam j = 10–5 A cm−2 .

Fig. 1. Photoluminescence spectrum of ZnIn2 S4 (II) single crystals after thermal treatment in sulfur vapor within 28 h at a temperature of 80 K.

Analysis of the structure of the spectrum shows that for single crystals that were not thermally processed in sulfur vapor no particularities are revealed, but in the case of zinc thioindate thermally processed in sulfur vapor the absolute maximum is recorded at 1.64 eV. On the low energy wing a relatively weak peak is observed at 1.36 eV. In the spectrum of semiconductors thermally processed in sulfur vapor there is no obvious displacement observed. Thus, after some analyses, however, there is a difference in the

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half-width of the emission band of the processed single crystals (0.40 eV) from the unprocessed ones (0.30 eV). So, we can state that the increase in the half-width of the cathodoluminescence band is due to the displacement of the low and high energy segments, respectively. Here we have to take into account that both zinc and sulfur are volatile elements, and the investigated compounds were thermally processed in vacuum. In order to highlight the changes taking place in the optical and luminescent properties under the action of radiation, that is, to influence more strongly, in the sense of obtaining a more pronounced change in the shape of the spectra and their structure, thermal processing regimes of different time intervals were experimented [12, 13]. Alongside with the above-mentioned experimental investigations, the cathodoluminescence of layered compounds from the Znx In2 S3+x (x = 1, 2, 3) family thermally processed in vacuum for 78 h were also investigated, especially for ternary compounds Zn2 In2 S5 .

Fig. 2. CL spectra of ZnIn2 S4 (II) compounds at a temperature of 80 K; 60 keV; 10–5 A·cm−2 . (1) Thermally unprocessed single crystals; (2) thermally processed in sulfur vapor.

In Fig. 3 cathodoluminescence spectra for layered compounds of Zn2 In2 S5 type at a temperature of 300 K with current density 10–6 A·cm−2 and electron beam with energy 60 eV are shown. In the emission spectrum of thermally processed compounds in vacuum, a band was recorded with the maximum energy placed at 2.08 eV in comparison with 1.88 eV for thermally unprocessed single crystals. In the spectrum of thermally processed in vacuum Zn2 In2 S5 single crystals on the low energy wing a very weak energy peculiarity is observed at 1.97 eV, which determines a complicated character of the emission bands.

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Fig. 3. CL spectra of Zn2 In2 S5 (II) compounds at a temperature of 300 K; 60 keV; 10–6 A·cm−2 . (1) Thermally unprocessed single crystals; (2) thermally processed in vacuum.

4 Conclusions Analyzing the experimental investigations, the results of which are graphically presented above, we can conclude that the processing of zinc and indium bisulfides, especially ZnIn2 S4 (II) single crystals in sulfur vapor, leads to the displacement of the emission band maximum in the low energy range, and the thermal processing in vacuum – in the high energy range. These results allow one to modify the spectral range of light radiation from 1.36 up to 2.7 eV. Applying different methods of excitation – accelerated electrons and X-rays – we detected the basic bands in the luminescent emission spectrum of zinc and indium bisulfides Znx In2 S3+x (x = 1, 2, 3): a red band with the maximum at 1.79 eV, an orange band with the maximum 2.08 eV, and a yellow-green band with a maximum 2.34 eV.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Aram˘a, E.: Luminescence of Mn in ZnIn2 S4 (III). In: Scientific Annals of the State University of Moldova. Series “Physico-Mathematical Sciences”, pp. 159–162. (2000) (in Romanian) 2. Abramova, T., Arama, E., Bazakutsa V., et al.: Recombination effect in γ – quanta irradiated ZnIn2S4. In: Proceedings of the 8th International Conference on Ternary and Multiparty Compounds, Chisinau, vol. 1, pp. 405–408 (1990) 3. Chen, Y., Huang, R., Chen, D., et al.: Exploring the different photocatalytic performance for dye degradations over hexagonal ZnIn2 S4 microspheres and cubic ZnIn2 S4 nanoparticles. ACS Appl. Mater. Interfaces 4, 2273–2279 (2012). https://doi.org/10.1021/am300272f 4. Yang, W., Zhang, L., Xie, J., et al.: Enhanced photoexcited carrier separation in oxygen-doped ZnIn2 S4 nanosheets for hydrogen evolution. Angew. Chem. Int. Ed. 55, 6716–6720 (2016). https://doi.org/10.1002/anie.201602543

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5. Lee, J., Kim, H., Lee, T., et al.: Revisiting polytypism in hexagonal ternary sulfide ZnIn2 S4 for photocatalytic hydrogen production within the Z-scheme. Chem. Mater. 31, 9148–9155 (2019). https://doi.org/10.1021/acs.chemmater.9b03539 6. Jiao, X., Chen, Z., Li, X., et al.: Defect-mediated electron−hole separation in one-unit-cell ZnIn2 S4 layers for boosted solar-driven CO2 reduction. J. Am. Chem. Soc. 139, 7586–7594 (2017). https://doi.org/10.1021/jacs.7b02290 7. Wang, G.M., Ling, Y.C., Li, Y.: Oxygen-deficient metal oxide nanostructures for photoelectrochemical water oxidation and other applications. Nanoscale 4, 6682–6691 (2012). https:// doi.org/10.1039/C2NR32222F 8. Zhang, L., Wang, W.Z., Jiang, D., et al.: Photoreduction of CO2 on BiOCl nanoplates with the assistance of photoinduced oxygen vacancies. Nano Res. 8, 821–831 (2015). https://doi. org/10.1007/s12274-014-0564-2 9. Peng, S., Li, L., Wu, Y., et al.: Size-and shape-controlled synthesis of ZnIn2 S4 nanocrystals with high photocatalytic performance. Cryst. Eng. Comm. 15, 1922–1930 (2013). https://doi. org/10.1039/C2CE26593A 10. Aram˘a, E., Pîntea, V., Jitari, V., et al.: Growth of Znx In2 S3+x (x = 1,2,3) compounds by chemical transport reaction method. Eng. Meridian (TUM) 4, 48–50 (2008). In Romanian 11. Maciuga, A., Zhitari, V., Arama, E.: Catodoluminescence of ZnIn2 S4 . In: Abstracts of International Conference on Materials Science of Chalcogenide and Diamond Structure Semiconductors, Chernivtsi, v. 2, 150 (1994) 12. Zhitar, V., Raylyan, V., Radautsan, S.: The peculiarities of ZnIn2 S4 luminescence. Il Nuovo Cimento D 2, 1919–1922 (1983). https://doi.org/10.1007/BF02457887 13. Maciuga, A., Radu, R., Pîntea, V., Stratan, I., Nistiriuc, I.: Luminescence of ternary compounds under the action of accelerated electrons and X-rays. Eng. Meridian (TUM) 3, 45–47 (2006)

ZnO Microtetrapods Covered by Au Nanodots as a Platform for the Preparation of Complex Micro-nano-structures Eduard V. Monaico1(B) , Armin Reimers2 , Vladimir Ciobanu1 , Victor V. Zalamai1 , Veaceslav V. Ursaki1,3 , Rainer Adelung2 , and Ion M. Tiginyanu1,3 1 National Center for Materials Study and Testing, Technical University of Moldova, Chisinau,

Republic of Moldova [email protected] 2 Christian-Albrechts-Universität Zu Kiel, Kiel, Germany 3 Academy of Sciences of Moldova, Chisinau, Republic of Moldova

Abstract. We propose to use hybrid networks of ZnO microtetrapods produced by flame transport synthesis and Au nanodots deposited by pulsed electroplating, for the preparation of more complex 3D micro-nano-structures via Au catalystassisted vapor-liquid-solid growth of semiconductor nanowires on the surface of ZnO microtetrapod arms. The pulsed electrochemical deposition of Au nanodots with optimized pulse parameters was realized in pressed pellets containing the ZnO tetrapods with the density 1 g cm−3 . The mechanical stability was increased by means of thermal treatment of pressed hybrid networks of ZnO microtetrapods at 950 °C for 1 h. The morphology of the ZnO microtetrapod networks and the density of the deposited Au nanodots were investigated by scanning electron microscopy. The deposition of Au nanodots with various densities and of monolayers of selfassembled nanodots was demonstrated on ZnO microtetrapods possessing different conductivities. The optical quality of the ZnO microtetrapods was investigated by photoluminescence (PL) spectroscopy in the temperature interval from 10 to 300 K. PL bands related to neutral donor bound excitons D0 X and donor–acceptor pairs (DAP) recombination were observed at low temperature. We assume that the presence in the spectrum of PL bands related to excitonic radiation is indicative of a high enough quality of the investigated ZnO microtetrapods for various optoelectronic and photonic applications. Keywords: ZnO microtetrapods · Au nanodots · Pulsed electroplating · Semiconductor nanowires · Scanning electron microscopy · Photoluminescence

1 Introduction Zinc oxide adopts a large variety of nanostructures which are suitable for a wide area of applications [1, 2]. Among these nanostructures, ZnO tetrapods and their networks, Produced by a cost effective and high output flame transport synthesis (FTS) [3] have been © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 197–205, 2024. https://doi.org/10.1007/978-3-031-42775-6_22

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used as sacrificial templates for the preparation of various aeromaterials, such as aeroGaN [4], aero-ZnS [5], aero-Ga2 O3 [6], and aero-TiO2 [7]. Furthermore, a wide variety of functional nanowires have been grown by means of catalyst-assisted or self-catalyzed vapor-liquid-solid (VLS) processes on a variety of semiconductor substrates. This variety includes nanowires based on InP [8], GaAs [9], GaP [10], GaN [11], ZnO [12], and ZnS [13], as well as a series of core/shell nanowires [14]. Among technologies applied for the VLS growth of semiconductor nanowires, one can mention chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), metalorganic vapor phase epitaxy (MOVPE) or metal organic chemical vapor deposition (MOCVD), the gold being the most frequently used catalyst. The produced semiconductor nanowires with bandgaps covering a wide spectral range from near infrared (NIR) to ultraviolet (UV) have been used for wide applications in energy, photocatalysis, sensors, electronics, optoelectronics, photonics, piezoelectric generators, and other emerging applications. However, most of these nanowire structures have been prepared on flat substrates, while deposition of semiconductor nanowire arrays on ZnO microtetrapod substrates is expected to enlarge even further the area of their applications. The goal of this paper is to demonstrate the deposition of Au nanodots on ZnO microtetrapod networks with density controlled both by technological conditions of deposition and by microtetrapod electrical conductivity. The resulting hybrid structures can be further used as catalyst nucleation points for the growth of semiconductor nanowires with needed composition. Considering that the preferential direction of nanowires growth is perpendicular to the used substrate, it is expected that their growth in radial directions of the tetrapod arms will result in the fabrication of more complex micro-nano-structure assemblies with controlled design and morphology, for various applications.

2 Methods and Materials The ZnO tetrapods used in the experiments were obtained from a mixture of Zn metal microparticles (sigma-Aldritch, Germany) and polyvinyl butyral (PVB) powder (Kuraray, Germany) in a weight ratio of 1:2 by means of a simple FTS approach schematically depicted in Fig. 1a. The process is described in detail in [3] and was optimized for controlled growth of different ZnO nano-microstructures for the production of interconnected ZnO networks in a scalable process [15]. For electrochemical deposition, the ZnO powder, containing tetrapods of 1–10 μm length of arms, were pressed in pellets with the density 1 g cm−3 Using the Compression Mold Shown in Fig. 1b. The required mass of tetrapods, expressed in grams, was determined according to the relation: m = (ρp /1000) × V

(1)

where, ρp is the density of the pellets to be obtained, and V is the final volume of the pellets. The porosity (F) of the obtained pellets was determined as follows:  = (1 − ρp /ρb ) × 100%

(2)

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in which, ρb is the density of bulk ZnO (5.61 g cm−3 ). After compression, a thermal treatment of the obtained pellets was carried out in order to increase the mechanical stability. During sintering for 1 h at 950 °C the microtetrapods form an interconnected network.

Fig. 1. (a) Schematic representation of the obtaining procedure of ZnO tetrapods. (b) Image of ZnO pellet pressing mold. (c) Image of powder from ZnO tetrapods and pressed pellet in the inset.

Pulsed electrochemical deposition of Au was realized in a commercially available gold bath containing 5 g L−1 Au (DODUCO GmbH, Germany) at temperature of 25 °C in a common two-electrode plating cell, where the sample served as working electrode, while a platinum wire was used as a counter electrode. The pulsed electroplating with controlled parameters of pulse width (ton ), delay between pulses (toff ), and voltage pulse amplitude (U) were performed according to the experimental setup and methodology described in detail in a recent paper [16]. In the case of electroplating on aero-GaN pellets a half of the sample was immersed in plating electrolyte while the electrical contact was realized on the other side using silver paste. The morphology (top view and cross-sectional view) of samples was investigated using TESCAN Vega TS 5130 MM scanning electron microscope (SEM). The continuous wave (cw) photoluminescence (PL) was excited by the 325 nm line of a He-Cd laser and analyzed with a double spectrometer ensuring a spectral resolution higher than 0.5 meV. The samples were mounted on the cold station of an LTS-22-C-330 optical cryogenic system.

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3 Results and Discussions 3.1 Morphology Investigation of the As-Grown ZnO Tetrapods The morphology of the produced ZnO microtetrapod networks is illustrated shown in Fig. 2. The mean length of tetrapod arms is around 10 μm, however there are some arms with lengths up to 50 μm.

Fig. 2. (a) SEM image of ZnO tetrapods pressed in pellet after thermal annealing at 950 °C. (b) Magnified view of a single ZnO tetrapod.

The thorough analysis of the arm morphology revealed their hexagonal cross section, indicating on the wurtzite structure. The microtetrapods consist of a core and four arms. The mechanism of their growth was previously investigated. It was initially suggested that the octahedral core nucleus is of zincblende-type, which is formed in the high pressure and high temperature growth conditions, and it might be transformed into the wurtzite form in ambient conditions, or when it passes through the region in the flame in a convection flow, and thereafter arms might grow with the wurtzite structure [17]. Later investigations suggested another growth mechanism, since none of the experiments showed that the center core of the tetrapod contains an octahedral seed crystal [18]. It was observed that the center cores of the tetrapod nanorods consist of four grains of hexagonal structure, which are directly related to the four elongated arms. These four tiny grains form a distorted tetrahedral-like structure with a large number of structural defects mediating the misfit stress between grains. As a result of this analysis [18], a freestanding self-assembly nucleation and growth process was proposed, which adopted the characteristics of a phase transition proposed in the initial model [17]. This growth model was confirmed by other investigations [15, 19, 20]. 3.2 Photoluminescence Study of the As-Grown ZnO Tetrapods In order to be suitable for optoelectronic or photonic applications, the final complex ZnO nano-micro-structures should have a high optical quality. To assess their optical properties, the photoluminescence spectra have been investigated in a temperature range

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from 10 to 300 K, as presented in Fig. 3. The spectra measured at low temperatures in the near band edge spectral region consist of a series of five PL bands. The highest energy band at 3.360 eV is related to a neutral donor bound excitons D0 X, previously assigned as I8 and suggested to be associated with the Ga impurity (with the donor binding energy around 55 meV) [21]. The position of a lower intensity band at 3.346 eV is close to the position of the I11 band related to a neutral donor bound exciton with the binding energy higher than 80 meV [21]. However, it could also be related to a neutral donor bound exciton A0 X.

Fig. 3. PL spectra of ZnO microtetrapods measured at different temperatures in the near UV spectral range related to band edge electron transitions (a), and in the visible spectral range related to deep impurity energy levels (b).

The nature of the most intense PL band at around 3.3 eV was previously analyzed, and it was suggested that it is most probably associated with radiative recombination of free carriers via donor–acceptor pairs (DAP) [22, 23]. The other two bands at lower photon energy represent the 1LO and 2LO replicas of the DAP with the LO phonon energy of 72 meV. The near bandgap emission is quenched with increasing temperature, so that at room temperature the spectra are dominated by the visible emission as shown in Fig. 3b. The green PL band with the maximum around 2.4 eV is the most often observed band in ZnO samples. Various recombination channels have been proposed as origin of this PL band, the most probable one being associated with VO oxygen vacancies [24]. The presence in the spectrum of PL bands related to excitonic radiation suggests that the quality of the investigated ZnO microtetrapods is high enough for various optoelectronic or photonic applications, even for lasing effects. 3.3 Estimation of the Electrical Conductivity of ZnO Tetrapods via Electrochemical Deposition Figure 4 illustrates ZnO microtetrapods covered with self-assembled Au nanodots produced by pulsed electroplating at 50 μs pulse length. One can see that the density of the deposited Au nanodots is different for different microtetrapods. The microtetrapods from the first region (Fig. 4a, b) are very rarely covered by Au nanodots, or by a moderate

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density of nanodots, while the microtetrapods from the second region (Fig. 4c, d) are covered by a dense monolayer of self-assembled Au nanodots.

Fig. 4. SEM images of a ZnO microtetrapods covered by Au nanodots taken from different regions of a pressed pellet at low magnification (a, c), and high magnification (b, d). The parameters of electroplating: ton = 50 μs; toff = 1 s; U = −15 V, tdep = 120 s.

These observations indicate that the electrical conductivity of the ZnO microtetrapods used in this work, differs from the usually high resistivity found for ZnO microtetrapods in previous studies [3, 25, 26]. According to a previous study, the Au nanodots deposition on semiconductor substrates is controlled by the applied cathodic voltage, by the width of voltage pulses, and by the number of applied voltage pulses [16]. The mechanism of pulsed electrochemical deposition of metal nanodots was found to be governed by the Schottky barrier height at the interface of the metallic nanodot with the semiconductor template. The size of the metal nanodots after their nucleation is limited by the height of this barrier [27]. After reaching the threshold size, a new nanodot is nucleated at another point on the surface of the substrate, and the deposition continues following this “hopping electrodeposition” process until the entire surface of the semiconductor template is covered by a monolayer of self-assembled Au nanodots, as illustrated in Fig. 4d. It was also found that the

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density of metal nanodots on the surface of semiconductor nanostructures is determined by the conductivity of the nanostructure, and it was proposed to use the pulsed electrochemical deposition of metals as a tool for assessing the conductivity of semiconductor nanostructures [28]. The rare nanodots deposition on the microtetrapod 1 in Fig. 4b indicates its high electrical resistivity, as observed previously [3, 25, 26], while the Au nanodots deposition on the microtetrapod 2 suggests a moderate conductivity. The image in Fig. 4d demonstrates that the conductivity of ZnO microtetrapods can even reach a high enough value for the deposition of a monolayer of Au nanodots as observed for the microtetrapod 3. These observations suggest that additional investigations are necessary to elaborate technological conditions enabling to produce ZnO microtetrapods networks with controlled electrical conductivity, which could be suitable for the production of complex micro-nano-structure assemblies with controlled design by means of VLS growth of semiconductor nanowires with various chemical composition on the surface of ZnO microtetrapods arms.

4 Conclusions The results of this study demonstrate the possibility of self-assembled Au nanodots deposition on ZnO microtetrapod networks grown by flame transport synthesis. The density of nanodots is determined not only by the parameters of pulsed electroplating, such as pulse width, pause between pulses, voltage pulse amplitude and number of pulses, but also by the conductivity of the ZnO microtetrapods. The optical quality of the ZnO microtetrapods is high enough for some optoelectronic or photonic applications, as demonstrated by the presence of PL bands related to excitonic radiation in the emission spectrum. However, technological conditions enabling to produce ZnO microtetrapods networks with controlled electrical conductivity have to be elaborated for the production of complex micro-nano-structure assemblies with controlled design by means of VLS growth of semiconductor nanowires with various chemical composition on the surface of ZnO microtetrapods arms. Acknowledgments. This research was funded by National Agency for Research and Development of Moldova under the Grant #22.80013.5007.4BL “Nano- and hetero-structures based on zinc oxide and A3B5 compounds for optoelectronics, photonics and biosensorics”. E.V.M. acknowledge financial support for his postdoctoral grant #21.00208.5007.15/PD “Micro- and nanoengineering of semiconductor compounds based on electrochemical technologies for electronic and photonic applications”.

Conflict of Interest. The authors declare no conflicts of interest. The funding sponsors had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, and in the decision to publish the results.

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Synthesis Technology for CdSe/CdTe Heterojunctions and Characterization of Their Photoelectric Properties Ludmila Gagara1

, Ion Lungu1(B)

, Lidia Ghimpu2

, and Tamara Potlog1

1 Laboratory of Organic/Inorganic Materials for Optoelectronics, Moldova State University,

Chisinau, Moldova [email protected] 2 Ghitu Institute of Electronic Engineering and Nanotechnologies, Technical University of Moldova, Chisinau, Moldova

Abstract. This paper presents the results of studying the photoelectric properties of CdSe/CdTe heterojunctions synthesized by the hot-wall epitaxy method. The CdSe/CdTe heterojunctions were manufactured by consecutive growth of CdSe and CdTe layers on a conductive ITO/glass substrate purchased from Solaronix Swiss. As ohmic contact for CdTe, Ni was deposited by thermal evaporation. The CdSe layer thickness (1–3 μm) was controlled according to the time of deposition of the layer. The temperature of the substrate and the source for CdTe growing were 400 °C and 520 °C, respectively and reached the thickness 15 μm. The synthesis process for heterojunctions with CdTe layers includes the treatment of the entire structure in a CdCl2 solution, followed by annealing in air at a temperature of 450 °C for 30 min. Upon the deposition of CdTe layer, due to the diffusion of Se into the growing CdTe film, a transition layer of the CdSex Te1–x solid solution is formed at the interface, evidenced by the spectral dependence of the photocurrent. The investigation of the current-voltage characteristics at different intensity of illuminations shown that nonideality factor n has a value of 1.7–2.0, which indicate a generation-recombination mechanism of current in the CdSe/CdTe heterojunctions. The best photovoltaic parameters for CdSe/CdTe heterojunctions were achieved for structures with thicker CdSe layer and are as follows: J SC = 24.6 mA/cm2 , U OC = 730 mV, FF = 0.5, η = 7.6%. Keywords: heterojunction · solar cell · photovoltaic parameters

1 Introduction The use of environmentally friendly energy sources, in particular, solar cells (SCs), is a widespread problem in world practice. It is well known that A2 B6 compounds are materials exhibiting fairly high photosensitivity for the entire visible spectrum of solar radiation; therefore, most of these compounds are commonly used for synthesizing thin-film SCs. A distinctive feature of these compounds is their pronounced monopolarity; therefore, these materials are used in the form of heterojunctions [1]. The most widely known © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 206–213, 2024. https://doi.org/10.1007/978-3-031-42775-6_23

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thin-film heterojunction SCs are CdS/CdTe. However, the possibility of synthesizing other pairs of heterojunction systems based on A2 B6 and SCs, in particular, CdSe/CdTe, is of scientific interest. Recently, interest in these heterojunctions has increased, because it has been proven that a small percentage of Se leads to an increase in the efficiency of these SCs [2–6]. In recent years, the efficiency of CdTe/CdS SCs has increased to ~ 22% [11] from a record value of 16.5%, primarily due to the incorporation of Se into CdTe. Cadmium selenide (CdSe) is a group II–VI semiconductor; it has a bandgap of 1.7 eV; however, the solubility of CdSe in CdTe is significantly higher than that of CdS; this feature provides a higher interfacial diffusion during the deposition of CdTe and a heat treatment after the deposition of the CdTe layer [4, 6]. The diffusion of Se from the CdSe layer leads to the formation of a CdTe1–x Sex alloy with a smaller band gap than that of CdTe [4]. This decrease in the band gap in the region of the interface between the CdTe and CdSe layers leads to an increase in the short-circuit current. In addition, due to the higher diffusion of Se, the CdSe layer will effectively decrease; this process will lead to a decrease in losses in the short wavelength region of the spectrum. However, a study exclusively of the properties of the CdSe–CdTe heterojunction has not been conducted directly. Since the semiconductor layers that constitute the heterojunction should not necessarily exhibit crystalline perfection, cheap synthesis technologies can be used; therefore, the cost of the photoconverters can be reduced [7–10].

2 Experimental A CdSe layer was deposited on a glass substrate coated with a thin highly conductive ITO layer by the hot-wall epitaxy method. The reactor, which provided the formation of CdSe and CdTe layers in a single technological cycle without open vacuum in the reactor, made it possible to monitor the temperatures of the evaporators and the substrate and control the thickness of the layer. The thickness of the CdSe layer was varied in accordance with the deposition time; it was no more than 1–2 μm. During the formation of CdSe layers, the temperature of the substrate and the evaporator was 330 and 521– 523 °C, respectively. A CdTe layer was deposited on the surface of the CdSe layer in accordance with a specially developed procedure [13]. The temperature of the substrate and the evaporator was 370–400 and 510–520 °C, respectively; the layer thickness was 8– 10 μm. A specific feature of the synthesis process for heterojunctions with CdTe layers is the treatment of the entire resulting sample in a CdCl2 solution and subsequent annealing of it in air at a temperature of 450 °C for 30 min [13]. After that, the heterojunctions were thoroughly washed and dried; an ohmic contact layer of Ni was deposited on the CdTe surface.

3 Experimental Results For the synthesized heterojunctions, the current–voltage characteristics (I-U) were measured at room temperature in the solar cell (SC) operation mode (see Fig. 1). For all the synthesized heterojunctions, at forward biases in a region of 0–0.5 V, an exponential dependence of the current on the applied voltage was observed. Forwardbias currents vary in a range of 10–4 –10–2 A. Reverse-bias currents vary in a range of

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10–7 –10–5 A. The ln I = f (U) characteristic without illumination shows the presence of three linear regions; this fact indicates the different current flow mechanisms in a voltage range of 0–0.76 V. The illumination causes a photocurrent independent of the applied voltage, i.e., I 0 (U) = I f (0). The (I-U) characteristics for three CdSe/CdTe heterojunction structures differing in the thicknesses of the layers components of the heterojunctions are shown in Fig. 1. Their main photovoltaic parameters are given in Table 1. The measurement of the (I-U) characteristics at different illumination intensities (Fig. 2) showed that an increase in the intensity of illumination of the heterojunction leads to a shift of the curves toward higher voltages; in addition, all the curves successively intersect in the same point. This change is due with the high photosensitivity of the junction components and, as a consequence, a decrease in the series resistance of the heterojunction, with an increase in the illumination intensity, which is a characteristic feature of heterojunctions based on A2 B6 compounds. Thus, the (I-U) characteristic takes the following form, where Rs is the series resistance:     e(V − IRS ) − 1 − If I = IS exp (1) nKT where I S – -saturation current, Rs – series resistance of heterojunction, n-non ideality factor, T – measured temperature, I f – photocurrent.

Fig. 1. Current–voltage characteristics of CdSe/CdTe heterojunctions with the different thicknesses of the CdSe layer at 100 mW/cm2 illumination intensity: (1) 3 μm, (2) 2 μm, and (3) 3.5 μm. Insert: Dark I-U characteristic in the ln I = f(U) coordinates.

Table 1. Photovoltaic parameters of the CdSe/CdTe SCs Sample

J SC , A/cm2

U OC , V

J max , A/cm2

U max , V

FF, %

η, %

d CdSe μm

d CdTe μm

5.11

0.019

0.74

0.013

0.43

46.1

5.61

3

15

5.12

0.021

0.76

0.012

0.47

49.1

5.64

2

12

5.29

0.013

0.78

0.009

0.41

34.1

5.52

3.5

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The replotting of the (I-U) characteristics in the ln I = f (U) coordinates (Fig. 3 a) make possible to calculate the main physical characteristics of this heterojunction in the SC mode. The saturation current I S is 0.8–1.3 μA, as determined from the above dependences. In the dark, the nonideality factor n is 5–7. This value of n suggests that mixed current transfer mechanisms take place in this structure under forward bias; however, under illumination of the heterojunction, the nonideality factor n has a value of 1.7–2 (Fig. 3 b, a), which provides evidence of an advantage of the generation–recombination mechanism of current generation in CdSe/CdTe heterojunctions in the SC operation mode.

Fig. 2. Current–voltage characteristics of the CdSe/CdTe heterojunction at different intensity of the illumination.

Fig. 3. (a) Dependencies ln I = f(U) of the CdSe/CdTe heterojunction at different illumination intensities and (b) dependencies of the nonideality factor with illumination.

An important characteristic of an SC is open-circuit voltage U OC ; it is related to short-circuit current I SC by the following relationship (Fig. 4): UOC =

nKT   eln I0ISC +1

(2)

provided that I f = I SC and assuming that I 0 « I SC , we obtain I SC = BE, where B-const., E-intensity of illumination. Then UOC =

nKT elnBE

(3)

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Thus, we can compare the nonideality factor values determined from the (I-U) characteristics. A fairly large set of CdSe/CdTe heterojunctions, in which the thickness of the contacting CdSe and CdTe layers was varied, was synthesized. The obtained samples were tested in the SC operation mode and the photovoltaic parameters are presented in Table 2. Table 2. The photovoltaic parameters of CdSe/CdTe SCs for different thicknesses of the CdSe and CdTe thin films. Sample number

Surface area S, cm2

J SC. , mA/cm2

U OC , V

P, mW

FF

η, %

d CdSe , μm

d CdTe , μm

5–2

0.5

16.8

0.600

2.52

0.250

2.52

4.0

18

5–10

0.5

19.0

0.669

5.98

0.470

5.98

3.0

15

5–11

1.0

24.6

0.703

6.42

0.492

7.61

3.0

15

5–12

1.0

18.8

0.692

6.14

0.512

6.14

2.0

12

5–14

0.4

17.7

0.560

3.41

0.344

3.41

2.5

16

5–25

0.45

16.5

0.650

5.71

0.544

5.83

1.5

16

The highest efficiency (η), 7.71%, has been achieved for CdSe/CdTe SC with thickness of CdSe 3 μm. The photoelectric characteristics of the CdSe/CdTe heterojunctions were measured using integrated and monochromatic radiations. The spectral dependences of the short-circuit current were studied (Fig. 5); it is evident that the region of the spectral sensitivity of the CdSe/CdTe heterojunction lies in a range of 0.55–0.86 μm. In addition, the constant photosensitivity region is broad, and the two contacting materials-both CdSe and CdTe-contribute to the photoresponse (a plateau is observed in the region of 0.65–0.84 μm). The short-wavelength decay of the photocurrent is controlled by the technological conditions for synthesizing the heterojunction and the thickness of the contacting materials. In the Fig. 6, the dependences of the photovoltaic parameters of the heterojunction on the incident power of the monochromatic radiation were studied; irradiation was conducted both using integrated radiation in a wide irradiation intensity region and using a laser with a wavelength of 532 nm (Fig. 6). One of the main parameters of the SC based on p-n junctions is quantum efficiency (U): γ =

If Ne

(5)

I f- - numbers of the electrical carriers formed in the result of the incident photons with energy 2.33 eV, N e - numbers of the incident photons on the surface of the heterojunction.

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Fig. 4. Dependences of U OC (left) and I SC (right) at different intensity of illumination.

Fig. 5. Spectral dependencies of photocurrent for CdSe–CdTe heterojunctions.

Fig. 6. Photocurrent of CdSe/CdTe solar cell at different monochromatic intensity of irradiation at λ = 532 nm.

The (U) for the CdSe/CdTe heterojunction was calculated. These calculations were conducted for the entire spectrum of irradiation intensities; it was observed that, with an increase in the illumination intensity, the quantum efficiency coefficient slightly decreases (Table 3).

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Table 3. Dependence of the quantum efficiency of CdSe/CdTe SC on the intensity of illumination E, mW/cm2

U

35

0.510

28

0.526

21

0.596

14

0.635

7

0.682

4 Conclusions Using the hot-wall epitaxy method, CdSe/CdTe heterojunctions with different thicknesses of the CdSe and CdTe components have been synthesized. The studies of the (I-U) in a wide range of temperatures and illuminations using both integrated and monochromatic radiation at a wavelength of 532 nm have been described. In the SC operation mode, the photovoltaic parameters of the synthesized heterojunctions have been calculated; it has been shown that the short-circuit currents achieve a value of 24 mA/cm2 and the open-circuit voltage is U OC ≈ 0.73 V. The efficiency of heterojunctions depends on the technological conditions of their synthesis, namely, temperature and thickness of the CdSe layer. It has been proven that CdSe/CdTe heterojunctions have fairly high photoelectric parameters and can be used as both visible-light photodetectors and SCs. Acknowledgments. The authors thank the Ministry of Education and Research of the Republic of Moldova for funding research grants 20.80009.5007.16.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Simaschevici, A., Gorceac, L., Serban, D.: Conversia fotoltaic a energiei solare. Cent. Ed. USM, Kixinev (2002) 2. Munshi, A.H., et al.: Polycrystalline CdSeTe/CdTe absorber cells with 28 mA/cm2 shortcircuit current. IEEE J. Photovolt 8(1), 310–314 (2018). https://doi.org/10.1109/JPHOTOV. 2017.2775139 3. Swanson, D.E., Sites, J.R., Sampath, W.S.: Co-sublimation of CdSexTe1−x layers for CdTe solar cells. Sol. Energy Mater. Sol. Cells 159, 389–394 (2017). https://doi.org/10.1016/j.sol mat.2016.09.025 4. Paudel, N.R., Yan, Y.: Enhancing the photo-currents of CdTe thin-film solar cells in both short and long wavelength regions. Appl. Phys. Lett. 105(18), 183510 (2014). https://doi.org/10. 1063/1.4901532 5. Fiducia, T., et al.: Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells. Nat. Energy 4(6), 504–511 (2019). https:// doi.org/10.1038/s41560-019-0389-z

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6. Yang, X., et al.: Preparation and characterization of pulsed laser deposited CdS/CdSe bi-layer films for CdTe solar cell application. Mater. Sci. Semicond. Process. 48, 27–32 (2016). https:// doi.org/10.1016/j.mssp.2016.03.009 7. Poplawsky, J.D., et al.: Structural and compositional dependence of the CdTe x Se 1 x alloy layer photoactivity in CdTe-based solar cells. Nat. Commun. 7, 12537 (2016). https://doi.org/ 10.1038/ncomms12537 8. Baines, T., et al.: Incorporation of CdSe layers into CdTe thin film solar cells. Sol. Energy Mater. Sol. Cells 180, 196–204 (2018). https://doi.org/10.1016/j.solmat.2018.03.010 9. Fang, X., et al.: Investigation of recombination mechanisms of CdTe solar cells with different buffer layers. Sol. Energy Mater. Sol. Cells 188, 93–98 (2018). https://doi.org/10.1016/j.sol mat.2018.08.015 10. Grice, C.R., Archer, A., Basnet, S., Paudel, N.R., Yan, Y.: Characterization of CdS/CdSe window layers in CdTe thin film solar cells. In: 43rd Photovoltaic Specialists Conference (PVSC) (2016). https://doi.org/10.1109/PVSC.2016.7749859 11. Green, M., Dunlop, E., Hohl-Ebinger, J., Yoshita, M., Kopidakis, N., Hao, X.: Solar cell efficiency tables (version 57). Progress in Photovoltaics: Research and Applications 1–13 (2020). https://doi.org/10.1002/pip.3371 12. Potlog, T.: The production of new types of CdTe photovoltaic devices with high efficiency. Pub. By Research Signpost, 87–121 (2014). ISBN: 978-81-308-0533-7 13. Potlog, T., Ghimpu, L., Gashin, P., Pudov, A., Sites, J.: Influence of annealing in different chlorides on the photovoltaic parameters of CdS/CdTe solar cells. Sol. Energy Mater. Sol. Cells 80(3), 327–334 (2003). https://doi.org/10.1016/j.solmat.2003.08.007

Illumination-Dependent Photovoltaic Parameters of CdS/ZnTe Solar Cells Ion Lungu1(B)

, Lidia Ghimpu2 , Victor Suman2 and Tamara Potlog1

, Dumitru Untila1

,

1 Laboratory of Organic/Inorganic Materials for Optoelectronics, Moldova State University,

Chisinau, Moldova [email protected] 2 Ghitu Institute of Electronic Engineering and Nanotechnologies, Technical University, Chisinau, Moldova

Abstract. This paper focuses on the influence of the illumination of CdS/ZnTe solar cells with different ZnTe thin film thicknesses. The devices were analyzed through current-voltage measurements. The values of the open circuit voltage (V OC ) and the short circuit current density (J SC ) depend on the substrate and source temperatures. The J SC is observed to decrease from 224 μA/cm2 to 95 μA/cm2 with increasing the source temperature from 560 °C to 600 °C, while the V OC increases from 0.41 V to 0.54 V, respectively. The value of V OC increasing from 0.68 V to 0.76 V, but J SC decreasing from 760 μA/cm2 to 500 μA/cm2 , when ZnTe thin film thickness increasing. Besides, the impact of the light intensity on the photovoltaic parameters of the CdS/ZnTe solar cells with different ZnTe thin film thicknesses was analyzed. The increasing in the light intensity from 20 mW/cm2 to 100 mW/cm2 rise the V OC from 0.67 V to 0.76 V tending further to saturation. Regardless of ZnTe thin film thickness, η increases logarithmically with the light intensity, but for the Jsc is observed linear dependence. The Rs increases with the increasing ZnTe thin film thickness, but decreases with the increasing of light intensity. Also, the Rsh , changes under changing the ZnTe thickness and the light intensity. Keywords: CdS/ZnTe solar cells · Close space sublimation method · Photovoltaic parameters

1 Introduction Due to the practical implementation of the intermediate band solar cells (IBSCs), zinc telluride (ZnTe) seems to be the appropriated solution. The IBSCs offer a concept of absorption based on multi-photon absorption with assistance of an IB situated in the bandgap of an active semiconductor [3, 4]. The IB must be partially filled to allow absorption of photons with energy below the energy bandgap. ZnTe has a direct wide bandgap 2.26 eV (300 K) [1, 2]. Furthermore, ZnTe is a p-type semiconductor which can form with n-type CdS thin films, a p-n junction more useful for photovoltaic solar energy conversion because it undertakes a high-energy part of the solar spectrum in © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 214–222, 2024. https://doi.org/10.1007/978-3-031-42775-6_24

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the conversion process. For ZnTe is possible to generate an IB by doping with oxygen forming ZnTe:O alloy that allowed increasing the limiting conversion efficiency of single gap semiconductor based solar cells from 40.7% to 63.2% for IBSCs [3, 5, 6]. In this paper, we replaced the CdTe material usually used as an absorber in CdS/CdTe solar cells by ZnTe layer doped with oxygen [7] and describe the correlation of device parameters with illumination energy to explore the performance of a CdS/ZnTe solar cell.

2 Experimental Details For CdS/ZnTe solar cells preparation, as transparent electrode, ZnO thin films on glass substrates by DC magnetron sputtering from ZnO:Ga:Cl target were deposited [8]. The ZnO:Ga:Cl thin films had the resistivity value of 2.5·10–4 ·cm, a thickness of 400 nm, an average transparency of 90% and a mobility of 25 cm2 /V·s. A technology for synthesizing an epitaxial CdS layers by the closed space sublimation method was developed earlier for CdS/CdTe junctions. For CdS and ZnTe thin films deposition, CdS and ZnTe powders 99.999% purity were purchased from Alfa Aesar, Germany. The effect of the source and substrate temperatures on the structure and electrophysical parameters of CdS and ZnTe thin films were studied to determine the optimum source (Ts) and substrate (T sub ) temperatures: T S = 590 °C and T sub = 380 °C for CdS and T S = 590 °C and T sub = 320 °C for ZnTe. The deposition temperatures for the ZnTe thin films were chosen so that to hinder the reevaporation of the CdS thin films. The CdS layer obtained in the optimal technological growth conditions had the thickness of 0.45 μm and the electron concentration of 1.6 x 1018 cm–3 . The ZnTe thin film had the thickness of 4–8 μm and the hole concentrations of 4.2 x 1015 cm–3 . Figure 1 (a) presents the SEM cross section image CdS/ZnTe deposited of ZnO glass substrates. From the SEM image, relatively abrupt interfaces between the different layers is observed. The ZnTe grains are formed by columnar structures. Figure 1 (b) illustrates the morphology of the ZnTe thin film obtained under optimal technological condition.

Fig. 1. Cross-section (a) and SEM image (b) for the CdS/ZnTe heterostructure.

The current-voltage (I–V ) characteristics of the CdS/ZnTe structures were performed at room temperature using Keithley 2400 source, under illumination and in the dark were

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given at the same graph in order to find the electrical and photovoltaic parameters. The measurements of these characteristics and of solar energy conversion efficiency have been carried out under standard conditions (AM1.5, 100 mW/cm2 , 25 °C) with the solar simulator ST 1000. The current data obtained from I–V measurements have been converted to the current value per unit area.

3 Results and Discussion 3.1 The Correlation Between Technological Regime and Photovoltaic Parameters of the Solar Cells The photoelectric properties have been investigated at the room temperature under different illumination through the ZnO/CdS interface. Two sets of samples were fabricated, where for the first, the substrate temperature and for the second, the source temperature was varied. For the first set, the substrate temperature was modified from 320 °C to 360 °C and the source temperature was kept constant at 580 °C. The current-voltage characteristics of the CdS/ZnTe solar cells fabricated at different substrates are shown in Fig. 2(a). The best value of the open circuit voltage of 0.48 V for T sub = 340 °C was reached, while the best value of the short circuit current density of 1,15 A/cm2 exhibits the structure deposited at T sub = 360 °C. For the second set, where the source temperature was changed from 560 °C to 600 °C, and the substrate temperature was kept constant at 340 °C, the best value for the short circuit current density is estimated at TS = 580 °C, while the best value of the open circuit voltage for TS = 600 °C. The data can be fitted to the usual expression which includes the effect of series and shunt resistances:       U − JRS U − JRS . (1) −1 + J = J0 expq AkT RSh where J 0 is the reverse saturation current density, A is the diode factor, RS the series resistance and Rsh the shunt resistance. The photovoltaic parameters: the fill factor (FF), short circuit current density (J SC ), open circuit voltage (V OC ) and the efficiency of the conversion (η), estimated from these dependencies are presented in Table 1. The Rs and Rsh resistances in this paper were calculated from the line slope of the J-V curves at the open-circuit voltage and short-circuit current points, respectively. The improvement observed for J SC is revealed for T sub = 320 °C with the highest reverse saturation current density. For this T sub , V OC decreases. A lower V OC in the cell having a higher defect density by considering the approximate analytical formula which ignores the shunt and series resistances: VOC = (nkT /q)ln[(JSC /J0 ) + 1]

(2)

indicates that as the dark reverse saturation current density J 0 increases, V OC decreases. The dark J-U characteristics are not presented here. We present only in Table 1 some electrical parameters for the dark condition of the above-named solar cells. In the Fig. 2 (a, b) are presented J-U characteristics of the CdS/ZnTe solar cells fabricated by different substrate and source temperatures, under illumination. (Cred asa e mai bine)!

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Fig. 2. Current-voltage characteristics of the CdS/ ZnTe solar cells fabricated by different substrate (a) and source (b) temperatures under illumination 100 mW/cm2 .

Table 1. The photovoltaic parameters of the ZnTe/CdS solar cells fabricated by different substrate and source temperatures Tsub , °C VOC , V JSC *10–4 , FF A/cm2

RS , 

RSh , 

nlight ndark J0 *10–7 , A/cm2

320

0,44

1,15

0,32 3066,6 52850.4 2.13

1.97

12.2

340

0,48

0,98

0,29 4706,4 83930.0 2.29

2.76

5.1

360

0,40

1,01

0,24 2239,3 51630.8 1.14

1.19

3.6

TS , °C

VOC , V

JSC *10–4 A/cm2

FF

nlight

ndark

J0 *10–7 , A/cm2

560

0,41

2,24

0,29 1759,2 3495,5

2.08

2.44

0.10

580

0,47

1,06

0,12 5148,6 7888,4

0.54

3.84

8.10

600

0,54

0,95

0,19 5994,0 8566,6

1.51

5.05

0.14

RS , 

RSh , 

The improved V OC observed for T sub = 340 °C has relate with the diminishing bandgap defect density states, after increasing the substrate temperature with 20 °C, which causes a reduced electron–hole recombination rate. The fill factor is small because the value of the current at the maximum power point (I mp ) depends on RS . The fill factor according to [9] can be represented as: (FF)RS =

2 Imp

VOC ISC



RS + nVth IL + I0 ± Imp

 (3)

where Vth = kT /q, k-Boltzmann constant, T-Kelvin temperature, q-elemental charge carrier. As one can see from formula (3), the fill factor depends also on reverse saturation current density J o . The value of the Rs increases with the increasing J o . The reverse saturation current depends on the T sub , with other words depends on the crystalline quality of the ZnTe. From Table 1 results that the best technological parameters are T sub = 340 °C and T S = 600 °C.

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3.2 The Influence of the Back Contact on the Short Current Density and Open Circuit Voltage The CdS/ZnTe solar cell efficiency and fill factor degrades as a result of the shunting effects. Therefore, to obtain a stable CdS/ZnTe solar cell is highly important the back contact. A set of CdS/ZnTe solar cells with different metals at the back contact of ZnTe were fabricated. The current-voltage characteristics of the CdS/ZnTe solar cells under illumination 100 mW/cm2 with different metals at back contact are presented in Fig. 1 (left) and the schematic image is shown (right). The back contact has great influence the efficiency of solar cells [10,11].

Fig. 3. Current-voltage characteristics (left) and the schematic image of the structure (right) of the CdS/ZnTe solar cells with different metals at back contact, under illumination 100 mW/cm2 .

From Table 2 it can be seen that the most suitable metal for the CdS/ZnTe structure is Ag deposited from the paste composed of a conductive phase, a matrix resin binder phase, a solvent, and an auxiliary agent. Table 2. The photovoltaic parameters of the CdS/ZnTe solar cells with different metals at back contact. Back Contact

VOC , V

JSC *10–4 , A/cm2

Work function, eV

RS , 

RSh , 

Cu

0.43

0.28

4.53

10952.4

23172.7

Pd/Ag

0.36

0.23

5.30

14823.5

21419.7

Ni

0.30

0.12

5.04

28911.8

25961.8

Ag (evaporated)

0.43

1.15

4.50

3320.9

5775.9

Ag (paste)

0.52

1.29

4.70

2973.3

5477.8

3.3 Influence of the ZnTe Deposition Time on the Photovoltaic Parameter Figure 4 shows the current-voltage characteristics of CdS/ZnTe solar cells made by different deposition time of ZnTe (6 min, 12 min and 18 min) at the optimal technological

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219

parameters T sub = 340 °C and T S = 600 °C at illumination with intensity of 100 mW/cm2 . The cells fabricated with the deposition time of 12 min and 18 min show the highest of 0.76 V of the open circuit voltage and the short circuit current density of 5·10–4 A/cm2 . The increasing of the deposition time of ZnTe thin film until 18 min reduces slightly the series resistance (RS ) of the structure and respectively increases the Rsh and the FF, see Table 3.

Fig. 4. Current-voltage characteristics of the CdS/ZnTe solar cells fabricated by different deposition time of ZnTe under illumination 100 mW/cm2 .

Thus, the values of V OC and J SC increase when the thickness increases (increasing the deposition time). This will subsequently lead to an increase in the absorption coefficient of the ZnTe thin films and to an increase in the FF, and the efficiency (η) of the solar cell. Table 3. The photovoltaic parameters of CdS/ZnTe solar cells fabricated by different deposition time of ZnTe thin films t, min

VOC , V

JSC *10–4 , A/cm2

6

0.68

3.76

31.51

0.08

4300

27821.9

1.9

12

0.76

5.00

28.53

0.11

6200

22899.1

1.7

18

0.76

5.00

33.14

0.12

5900

26798.6

0.9

FF, %

η, %

RS , 

Rsh , 

n

The fill factor depends on both Rs and Rsh in a complex way. As one can see from the table, both Rs and Rsh changed with the deposition time. Comparative studies of photovoltaic parameters show the modest values of the efficiency for all CdS/ZnTe solar cells fabricated by different deposition time of ZnTe thin films. When a solar cell absorbs light, an electron-hole pair is formed, which must then be separated, so the charges can be put to use in a circuit. The problem is that the electron and hole can recombine before they are used. On the other hand, if the cell is very thin it will not be able to absorbs as much of the incident light as a thick one, because the light absorption of a sample is a function of the ZnTe depth. An effective solar cell must balance these competing effects to achieve maximum efficiency.

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3.4 The Influence of the Illumination on the Photovoltaic Parameters The influence of the light intensity on the short circuit current density voltage, is given by the equation: ISC = KE · E

(4)

Fig. 5. The dependences of V OC = f(E) (left) and J SC = f(E) (right) for CdS/ZnTe solar cells fabricated by different deposition time of ZnTe thin films.

The short circuit current is directly proportional to the light intensity, while the open circuit voltage tends to saturation with illumination:   E nkT ln VOC = Vocn + (5) q En As one can see from the Fig. 5 and Fig. 6, the open circuit voltage really tends to saturation and the current density (J SC ) increases as the light intensity increases. The light generated current density (J L ≈ J SC ) is proportional to energy flux as long as the minority carrier lifetime in the absorber is constant. At higher energy fluxes however, increased carrier traffic begins to saturate the recombination centers, increasing the lifetime and thus producing an increase in the quantum efficiency. According to the Fig. 6 the fill factor does not change almost with increasing the intensity of the light. The lower values of FF usually are a result of higher parasitic resistances and higher recombination. The series resistance is parasitic in solar cell. It works to reduce the maximum output power which is defined as the point near the inflection of the (J-V) curve, which corresponds to the rectangular area under the characteristics curve represented by the output characteristics of the curve J mp ·V mp and thus the fill factor of the cell decreases. As shown in the Fig. 7 (a, b), both the Rs and Rsh decreases with the increasing of the light intensity. The low shunt resistance causes power losses in the solar cell by providing an alternate current path for the light-generated current. Such a diversion reduces the amount of current flowing through the solar cell junction and reduces the voltage. By varying the intensity of the light, the cell efficiency increases (see Fig. 8). According to the theory, the open circuit voltage increases with the natural logarithm, and consequently the fill factor increases slightly, too. In summary the efficiency is

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221

Fig. 6. Dependence FF = f(E) for CdS/ZnTe solar cells fabricated by different deposition time of ZnTe thin films.

Fig. 7. Dependencies of RS = f(E) (a) and Rsh = f(E) (b) for CdS ZnTe solar cells fabricated by different deposition time of ZnTe thin films.

higher. If there is significant series resistance the fill factor may decrease at higher levels of the illumination. Theoretically, the conversion efficiency should increase with the light intensities.

Fig. 8. Dependence η = f(E) for CdS/ZnTe solar cells fabricated by different of deposition time of ZnTe thin films.

So, the results reveal that all parameters of the solar cells, especially, the open circuit voltage, short circuit current, fill factor, and conversion efficiency, are affected by the change of the intensity of illumination. Series and shunt resistances have a substantial influence on solar cell efficiency. The fill factor and open circuit voltage are influenced

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by the series resistance. The increasing of the illumination intensity affects the short circuit current density, while V OC is less sensitive to the illumination. The decrease in shunt resistance decreases fill factor as the intensity of the light increases.

4 Conclusions Two sets of CdS/ZnTe solar cells were fabricated, where for the first, the substrate temperature and for the second, the source temperature was varied. The best photovoltaic parameters were obtained at T sub = 340 °C and T S = 600 °C. The effect of the intensity of illumination on the performance of the solar cell was studied, where the results showed that the short circuit current is the most affected parameter by changing the intensity of the illumination, while the open circuit voltage V OC and the efficiency increase logarithmically. Acknowledgments. This work was supported by the Ministry of Education, Culture, and Research of Republic of Moldova, research grant 20.80009.5007.16.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Mahmood, W., Shah, N.A., Akram, S., Mehboob, U.: Investigation of substrate temperature effects on physical properties of ZnTe thin films by close spaced sublimation technique. Chalcogenide Lett. 10, 273–281 (2013) 2. Wang, W., Xia, G., Zheng, J., Feng, L., Hao, R.: Study of polycrystalline ZnTe (ZnTe:Cu) thin films for photovoltaic cells. J. Mater. Sci. Mater. Electron. 18, 427–431 (2007) 3. Luque, A., Martí, A.: Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels. Phys. Rev. Lett. 78(26), 5014–5017 (1997) 4. Dhomkar, S., Manna, S., Noyan, U., Tamargo, I., Kuskovsky, M.: Vertical correlation and miniband formation in submonolayer Zn(Cd)Te/ZnCdSe type-II quantum dots for intermediate band solar cell application. Appl. Phys. Lett. 103(18), 181905 (2013) 5. Araújo, G., Martí, A.: Absolute limiting efficiencies for photovoltaic energy conversion. Sol. Energy Mater. Sol. Cells 33(2), 213–240 (1994) 6. Luque, A., Martı, A.: Numerical study of the influence of ZnTe thickness on CdS/ZnTe solar cell performance. Prog. Photovolt. 9, 173 (2001) 7. Lungu, I., Zalamai, V.V., Monaico, E.I., et al.: Effect of deposition temperature on structural, morphological and optical properties of ZnTe thin films. J. Mater Sci. 58, 4384–4398 (2023). https://doi.org/10.1007/s10853-023-08285-x 8. Colibaba, G., Rusnac, D., Fedorov, V., Monaico, E.: Effect of chlorine on the conductivity of ZnO: Ga thin films. J. Mater. Sci.: Mater. Electron. 32(13), 18291–18303 (2021) 9. Dadu, M., Kapoor, A., Tripathi, K.: Effect of operating current dependent series resistance on the fill factor of a solar cell. Sol. Energy Mater. Sol. Cells 71(2), 213–218 (2002)

Fine Dispersion and Intensification of Heat Transfer at Boiling in Electric Field on the Modified Surfaces Ion Chernica(B)

and Mircea Bologa

Institute of Applied Physics, Chisinau, Moldova [email protected]

Abstract. The research tasks were formulated on the basis of scientific and applied aspects of engineering thermophysics and bioengineering. In the first part of the work the results of experimental study of heat transfer at boiling of a dielectric liquid in electric field on the modified surfaces are presented The surfaces are received using the electric spark alloying. The experimental conditions and the results of investigation of the influence of field strength, interelectrode distance and the specifics of heat supply are described. The maximum influence of the field is observed for an underdeveloped boiling regime, where the relative heat transfer coefficient increases with increasing of field strength and decreases with increasing of heat flux density. The optimal interelectrode distance is determined, at which the effects of heat transfer intensification under the influence of the field are most pronounced. The heat exchange has been intensified up to 6 times, compared with boiling in the absence of a field. In the area of developed bubble boiling, the field effect weakens and, depending on the experimental conditions, may even become negative. The influence of the electric field on the hydrodynamics of the vaporization process is discussed. In the second part of the work, on the basis of visual observations and high-speed filming, the features of generating of steam bubbles and the mechanism of microdispersion of a dielectric liquid under the influence of an electric field are analyzed. The regime parameters have been established at which the splitting of steam jets, the formation of a cloud of finely dispersed charged bubbles, and the behavior of micro– and nanofilms on the heat exchange surface are observed. The importance of determining of the number of vaporization centers, tear-off diameters and the frequency of bubble separation, the possibility of using microbial bio-coatings for the degree of cooling and thermostating under controlled exposure to an electric field is emphasized. The obtained results can be used in calculations of the intensity of electroconvective heat exchange during boiling of weakly conducting heat carriers. Keywords: Heat transfer · Boiling · Thin dispersion · Micro- and nanofilms · Heat flux · Temperature head · Heat transfer coefficient · Intensification · Electric field

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 223–230, 2024. https://doi.org/10.1007/978-3-031-42775-6_25

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1 Introduction One of the main problems of the modern engineering thermophysics is the improvement of the process and intensification of heat transfer at boiling the solution of which is particularly important when developing and creating new types of evaporative cooling systems – closed electrohydrodynamic systems with active regulation [1–4] that ion its turn suggests expansion and deepening of research of hydrodynamics and heat exchange at phase transformations in the electric field. The investigation of the regularities of field interaction with two-phase vapor-liquid systems with the aim to optimize cooling, maintain temperature modes of the heat-emitting elements and the nodes of computing modules and electronic devices is of significant interest [5]. The analysis of the methods to intensify the heat transfer at boiling by means of finning the heat transfer surface, changing its roughness, applying porous coatings, action of external fields (acoustic, electric, electromagnetic, magnetic), modeling of the process on the modified surfaces, using of combined methods of macro- mesomicro- and nanomodified surfaces [5, 6] shows that the efficiency of one or the other method is mostly determined by the conditions under which the evaporation process takes place and is associated with certain technical requirements. When boiling is used in the restricted space under high electric potential the electric method is preferable from the engineering point of view [1]. The action of electric fields on the formation of a vapor phase can lead to the change in the parameters of phase equilibrium and the boundaries of thermodynamic stability, to creation of additional forces influencing the hydrodynamics of the phase transformation process [1, 7, 8]. In its turn, the hydrodynamic effects can have a significant impact on thermal processes at boiling of a dielectric liquid. Thus, the nonuniform field provides an additional buoyant force acting on a vapor bubble that causes an increase in the frequency of the formation and separation of a vapor bubble and the relief of their ascent. The electric field significantly deforms the vapor bubbles that causes a substantial change in the interphase surface, and thus the physical state of the vapor-liquid system on the heat-emitting surface. On the other hand, at the boundary with solid surface the field greatly influences the interfacial tension that also influences the intensity of the heat removal to the boiling liquid. The interest to the usage of highly potential electric fields for technical purposes also grew due to the development of the model of the dielectric liquid dispersion from the metal capillary [9]. It turned out that if a high electric potential is applied to the capillary than at its certain value the value is sprayed at a high degree of dispersion. These investigations occurred specially efficient at the analysis of the behavior of the metastable liquid nano- microfilm (10–9 –10–6 m thick) formed under the vapor nucleation site growing on the heating surface and of the development of crisis phenomena at the maximum heat fluxes. The knowledge of the peculiarities of interaction between electric field and relatively thin layer of liquid under the vapor bubble is also important from the point of view if this is a bubble boiling or film boiling mode. At the same time, exceptionally high values of the heat transfer coefficient during boiling led to the use of microbial bio-coatings for cooling systems. Studies of Ahmad Motezakker et al. using a thermoacidophilic Sulfolobus Solfataricus coatings confirmed the possibility of increasing of the relative heat transfer coefficient during the phase transition [10]. The proposed method is biocompatible and environmentally friendly

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and can open up new opportunities for the application of biotechnologies in energy systems. This work presents the research results for the action of an electric field on dispersion and heat transfer at boiling of dielectric liquid on the modified surfaces received using the electric spark method. The study is focused on the action of the field intensity and the interelectrode distance on the main characteristics of boiling, as well as the development of the process in the electric field, including the formation of a cloud of finely dispersed charged vapor bubble.

2 Methodical Support of the Experiment The experimental setup consisted of a closed sealed chamber, a working section, and a measuring equipment. The chamber is made in the form of a metal cylinder case 300 mm in internal diameter and 200 high. A high voltage electrode in the form of a round brass grate 60 mm in diameter was arranged in the upper part, it could be moved in the vertical plane with respect to the working section. Certain size notches were applied to the both sides of the grates in order to create an additional flow of liquid in the liquid layer near the solid surface. An experimental section with zero electric potential being a stainless tune 4 mm in diameter 80 mm long heated due to the direct passage of an electric current was arranged horizontally under the high voltage electrode. A layer of stainless steel 0.05–0.075 mm thick with a developed roughness was applied to the its outer surface using the electric spark method. Hexane with a boiling point of T s = 68.7 °C was used as a dielectric liquid. The interelectrode distance, i.e., the smallest distance between the high voltage electrode and the heat-emitting (the second electrode) was varied in the range 2–8 mm. The electrical circuit with a negative high voltage potential was used which was changed stepwise up to 25 kV. The experimental setup and the procedure was described in detail in work [2]. The experiments were carried out at atmospheric pressure within the parameters of specific heat flows of q = 5·102 –1.5·105 W/m2 and wall overheating values relative to the saturation temperature T = T w – T s = 0.2–30 K. The measurement errors for the temperature head T, the heat flux density q and heat transfer coefficient α do not exceed 2, 8, and 15%, respectively. Along with the study of heat transfer, visual observations and high-speed filming of the boiling process were carried out.

3 Results and Discussion The influence of the field intensity and the physical parameters of the liquid determining the development of electric convection is the most significant factor in the intensification of heat transfer at the boiling of a dielectric liquid under the action of electric forces. Figure 1 presents the heat flux densities q on the temperature head T and the field intensity E at the interelectrode distance δ = 3 mm. For clarity some data from Fig. 1 are presented in Fig. 2 in the coordinates α e /α – E, where α e is the heat transfer coefficient at boiling in the electric field, and a is the heat transfer coefficient at boiling without any field. At low and moderate temperature heads, in the region of underdeveloped bubble boiling, even comparatively low field intensities cause a considerable intensification of

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heat transfer, for example, at T = 1.30 K and E = 33.33 kV/cm the heat transfer coefficient is almost 6 times more than in its absence. A strong influence of field on the intensity of bubble boiling under conditions of large volume is apparently explained by the capacitive effect of the field which destabilize the vapor cavities on the heating surface.

1 2 3 4 5

Fig. 1. Heat flux density q versus temperature head T and field intensity E at interelectrode distance δ = 3 mm: 1 – E = 0; 2 – E = 8.33 kV/cm; 3 – E = 16.67 kV/cm; 4 – E = 25.00 kV/cm; 5 – E = 33.33 kV/cm.

As for the region of a developed bubble boiling the field effect is negligible, and it can be even negative. This is especially noticeable at large heat fluxes and slight field intensities where suppression of heat transfer is observed. It is assumed that the attenuation of the effects of heat transfer intensification by the electric field in this mode is due to the decrease in the dominant influence of electric convection. Similar regularities of heat transfer at boiling are also received at other distances between the high voltage electrode and the heat-emitting surface. Thus, it can be deduced that the electric field substantially intensifies the heat transfer in the most modes of bubble boiling. The character of the dependences presented in Fig. 2 is determined by the relation between the heat transfer coefficients at boiling in electric field and without it. The relative heat transfer coefficient at boiling α e /α is a complex function of the field intensity E and the heat flux density q or the temperature head T. It is supposed that the degree of intensification is diminished with the growth in q due to sharper increase in α with q according to α = Kq0.7 where K is the constant depending on the physical properties of the liquid and the heat-emitting surface.

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1 2 3 4 5

Fig. 2. Relative heat transfer coefficient α e /α versus field intensity E: 1 – q = 4100 W/m2 ; 2 – q = 12605 W/m2 ; 3 – q = 26400 W/m2 ; 4 – q = 49425 W/m2 ; 5 – q = 97915 W/m2 .

The highest level of heat transfer intensification is at an interelectrode distance of δ = 3 mm (Fig. 3) that almost coincide with previously set result at boiling in electric field on the surfaces with capillary-porous structure [3]. The influence of the interelectrode distance on the heat transfer intensity manifests itself through the interaction of the pulsating motion of the liquid due to the evaporation and disturbances of the liquid caused by the electrical convection. At the distances less than optimum the electroconvection develops mainly within the limits of the wall overheated layer where the liquid temperature is close to the temperature of the solid surface. In this case the evacuation of vapor bubbles from the heat-emitting surface takes mainly place due to the difference in the dielectric constants of the liquid and vapor phases, and the heat transfer intensity is lower than at the optimal interelectrode distance. The interelectrode gap growing at a certain field potential there is a hydrodynamic adjustment of the boiling liquid fluxes, as a result no sufficient portions of cold liquid can come to the heat-emitting surface. In some places near the upper tube generatrix there is possible the formation of dry spots, and the removal of heat directly to vapor is less intensive that leads to the deterioration of heat transfer when the interelectrode distances are more than optimal. However, under certain conditions the electric field leads to the drastic change in the hydrodynamic situation which is determined by the dispersion of dielectric liquid in the interelectrode space. In the region of the underdeveloped bubble boiling when the heat-emitting surface generates a comparatively small number of vapor bubbles, at the influence of field E > 12.5 kV/cm on the surface of film there appear waves with single cones on their crests. Small liquid drops and jets break from the tops of the cones. The character of hydrodynamic processes in the aria of the field action changes with the growth in its intensity and the heat flux density. Increase in the heat flux density leads to the growth in the film thickness due to the increase in the number of the existing nucleation sites and their frequency, and with the growth in the field intensity it leads to

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1 2 3 4 5

Fig. 3. Heat transfer coefficient α e versus interelectrode distance δ and field potential  at q = 30000 W/m2 : 1 –  = 0; 2 –  = 2.5 kV; 3 –  = 5.0 kV; 4 –  = 7.5 kV; 5 –  = 10.0 kV.

more significant Coulomb forces and adjustment of the hydrodynamic structure. There is a considerable increase in the number of formed jets and drops of the dispersed liquid with the decrease in their sizes. In the lower part of chamber under the heat-emitting element there is formed a cloud of finely dispersed charged vapor bubbles which, the field intensity growing, is carried out from the action area of the electric forces. Dispersed vapor bubbles perform a complex chaotic movement. Under the action of the Coulomb forces they first move down and, reaching a certain depth due to the field intensity, float. Rising they merge together and form vapor cavities floating to the free surface. The heat flux increasing the density of nucleation sites and the growth rate of vapor bubbles rise that leads to an intensive merging of them with forming large vapor cavities. A cloud of finely dispersed bubbles forming the degree of heat transfer intensification is not so significant than at moderate heat fluxes. Thus, foe example, at q = 33500 W/m2 and δ = 6 mm the intensifying action of field is 160%. The cloud of finely dispersed vapor bubbles was observed at the heat flux densities q = 1.5·104 –4·104 W/m2 and the field intensities E > 12.5 kV/cm. Visual observations and filming of the process allowed to display the approximation scheme of the vapor phase removal from the heat-emitting surface. At the underdeveloped boiling with no field the bubbles are formed almost uniformly on the whole heat-emitting surface and uniformly break from it (Fig. 4). Under action of an external electric field the evaporation on the upper generatrix of the tube disappear, and on the rest of the surface it concentrates in the roughness pits where vapor is thrown out in the form of individual jets (Fig. 5). The diameter of the bubbles separated from the lower generatrix of the tube is greater than that of the bubbles separated from the lateral generatrix. The heat flux density growing the number of nucleation sites increases.

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Fig. 4. Boiling of liquid without electric Fig. 5. Dispersion of liquid in electric field: q field: q = 17850 W/m2 ; α = 1685 W/(m2 ·K); = 17850 W/m2 ; α e = 2380 W/(m2 ·K); T = T = 10.60 K; E = 0. 7.50 K; E = 25 kV/cm.

4 Conclusions The possibility to intensify heat transfer at boiling in electric field on the modified surfaces received by alloying using the electric spark method. The heat exchange is intensified by 6 times in comparison with boiling without any field. The heat exchange at boiling on the modified surfaces in the electric field has a number of significant peculiarities, namely: a weak influence of field at large heat fluxes and moderate field intensities; the presence of an optimal interelectrode gap at which the heat transfer intensity is maximal, the change in the hydrodynamic situation determined by the dispersion of dielectric liquid and formation of a cloud of finely dispersed vapor bubbles. Important internal characteristics of the boiling process depend on the conditions of the field action such as tear-off diameter, bubble break-away frequency, number of acting nucleation sites which must be determined for the engineering calculation of heat exchange. The problem becomes more complicated because of the absence of reliable data concerning dispersion of a dielectric liquid in the electric field. Acknowledgments. This work was supported by the Government of Moldova within the framework of the project ANCD 20.80009.5007.06.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Bologa, M., Smirnov, G., Didkovskii, A., Klimov, S.: Heat Transfer at Boiling and Condensation in Electric Field. Shtiintsa, Chisinau (1987) 2. Chernica, I., Bologa, M., Mardarskii, O., Kozhevnikov, I.: Action of electrohydrodynamic flow on heat transfer at boiling. J. Electrostatics 109 (2021). https://doi.org/10.1016/j.elstat. 2020.103524

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3. Chernica, I., Bologa, M., Kozhevnikov, I., Mardarskii, O.: Heat transfer at boiling at the porous surface under the electric field action. In: Proc. Jubilee Conf. Nation. RAS Committee on Heat and Mass Transfer and XXI School-workshop of Young Scientists under Acad. RAS A. Leontiev (May 22–26, 2017, St. Petersburg), vol. 1, pp. 119–122. Moscow (2017) 4. Chernica, I., Bologa, M., Kozhevnikov, I., Motorin, O.: Intensification of Heat Transfer at Boiling in Electrohydrodynamic Flow. In: Int. Conf. on heat and mass transfer (May 16–19, 2022, Minsk), pp. 449–452. Minsk (2021). https://www.itmo.by/conferences/abstracts/mif16/mif16.pdf 5. Volodin, O., Pecherkin, N., Pavlenko, A.: Intensification of Heat Transfer at Boiling and Evaporation of Liquids on the Modified Surfaces. High Temp. 59(2), 280–312 (2021) 6. Volodin, O., Pecherkin, N., Pavlenko, A., Kataev, A., Mironova, I.: Metods of intensification of heat transfer at boiling and evaporation of draining films on the packets of horizontal tubes. In: Int. Conf. on heat and mass transfer (May 16–19, 2022, Minsk), pp. 303–306. Minsk (2021). https://www.itmo.by/conferences/abstracts/mif-16/mif16.pdf 7. Ahmad, S.: Combined effect of electric field and surface modification on pool boiling of R-123. Brunel University (USA) (2012) 8. Eronin, A.: Peculiarities of Heat Processes at Boiling of Dielectric Liquids in Nonuniform Electric Field. United Inst. High Temperatures, Moscow (2012) 9. Verdiev, M.: Production of thin films of liquids by dispersion in electric field. Surf. Eng. Appl. Electrochem. 4, 36–41 (1991) 10. Motezakker, A., Sadaghiani, A., Akkoc, Y., Parapari, S., Gözüaçik, D., Ko¸sar, A.: Surface modifications for phase change cooling applications via crenarchaeon Sulfolobus solfataricus P2 bio–coatings. Scientific Reports 7(1), (2017). https://doi.org/10.1038/s41598-017-181 92-2

Photodetector Based on β-Ga2 O3 Nanowires on GaSx Se1-X Solid Solution Substrate Veaceslav Sprincean1 , Mihail Caraman1 , Haoyi Qiu2 , Tim Tjardts3 Alexandr Sereacov5 , Cenk Aktas3,4 , Rainer Adelung2 , and Oleg Lupan2,3,5(B)

,

1 Faculty of Physics and Engineering, Moldova State University, 60 Alexei Mateevici Street,

MD-2009 Chisinau, Republic of Moldova 2 Department of Materials Science, Faculty of Engineering, Chair for Functional

Nanomaterials, Kiel University, Kaiserstraße 2, 24143 Kiel, Germany [email protected] 3 Chair for Multicomponent Materials, Department of Materials Science, Faculty of Engineering, Kiel University, Kaiserstraße 2, 24143 Kiel, Germany 4 Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara, Turkey 5 Center for Nanotechnology and Nanosensors, Department of Microelectronics and Biomedical Engineering, Technical University of Moldova, 168, Stefan Cel Mare Av., MD-2004 Chisinau, Republic of Moldova

Abstract. The detection of radiation in the ultraviolet C (UVC) region (100– 280 nm) is of great importance for numerous technical applications, such as fire detection in security devices, tracking astronomical missile trajectories, chemicalbiological analyses, and medicinal applications. Wide bandgap semiconductors have emerged as ideal materials for electronic devices operating in this spectral range. Among the various promising materials, β-Ga2 O3 , a gallium oxide with a monoclinic crystal lattice, has garnered significant attention along with Alx Ga1-x N, AlN, and BN. While thin layers of Alx Ga1-x N suffer from structural instability, AlN exhibits photosensitivity to radiation with wavelengths shorter than 215 nm. On the other hand, cubic BN possesses an absorption band fundamental in the UV-vacuum region (λ ≤ 195 nm). Notably, β-Ga2 O3 , with its direct n-type energy bands and a bandgap of 4.5–4.9 eV, demonstrates high photosensitivity in the UVC range, making it an excellent material for photoreceptors in the 220–280 nm range. In this study, we investigate the elemental chemical composition, absorption band edge, vibrational and photoresponsive properties of β-Ga2 O3 nano-wire/nano-ribbon assemblies on a substrate of monocrystalline lamellae from GaSx Se1-x solid solutions (x = 0.17). The nano-wire assemblies were grown using thermal oxidation of gallium compound semiconductors at temperatures ranging from 750 to 950 °C in an oxygen or water vapor-enriched atmosphere. Our findings provide valuable insights into the potential of β-Ga2 O3 nanostructures as efficient photoreceptors for UVC radiation detection. Keywords: Ultraviolet C radiation · Wide bandgap semiconductors · β-Ga2 O3 · Alx Ga1-x N · Nano-wires · Photoreceptors · Thermal oxidation · GaSx Se1-x · Photodetection © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 231–242, 2024. https://doi.org/10.1007/978-3-031-42775-6_26

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1 Introduction The detection of radiation in the ultraviolet C (UVC) region (100–280 nm) holds significant importance for a wide range of technical applications, including security devices for fire detection, tracking astronomical missile trajectories, chemical-biological analyses, and medicinal applications [1–3]. Wide bandgap semiconductors have emerged as promising materials for electronic devices operating within this spectral range. Notably, β-Ga2 O3 , a gallium oxide with a monoclinic crystal lattice, has attracted considerable attention alongside Alx Ga1-x N, AlN, and BN. However, challenges such as structural instability in thin layers of Alx Ga1-x N and AlN’s limited photosensitivity to wavelengths shorter than 215 nm have spurred interest in exploring alternative materials [4–7]. Cubic BN exhibits an absorption band fundamental in the UV-vacuum region (λ ≤ 195 nm), while β-Ga2 O3 , with its direct n-type energy bands and a bandgap of 4.5–4.9 eV, showcases high photosensitivity in the UVC range, making it an exceptional candidate for photoreceptors operating within the 220–280 nm range [8–10]. The fabrication of nano-wire assemblies involves the thermal oxidation of gallium compound semiconductors (GaN, GaAs, GaSe, GaS) [11–13] in an oxygen or water vapor-enriched atmosphere at temperatures ranging from 750 to 950 °C [14, 15]. In this study, we present a comprehensive investigation of the elemental chemical composition, absorption band edge, and photoresponsive properties of β-Ga2 O3 nanowire/nano-ribbon assemblies grown on monocrystalline GaSx Se1-x solid solution (x = 0.17) substrates. Through our systematic characterization, we provide valuable insights into the potential of β-Ga2 O3 nanostructures as efficient photoreceptors for detecting UVC radiation.

2 Experiment Methodology and Sample Preparation GaS0.17 Se0.83 single crystals were obtained by the Bridgman method in a furnace with three thermal sectors. Ga(5N), Se(5N) and spectrally pure S taken in stoichiometric amounts were used as primary material. The synthesis of the material and the growth of single crystals was carried out in quartz ampoules with an internal diameter of 14 mm coated with graphite. The graphitization of the internal walls of the ampoule took place by pyrolysis of acetone vapors at a temperature of ~500 °C. The synthesis of the compound with a mass of ~20 g took place in the first two thermal sections of the furnace fixed at an angle of 30° to the horizon. Initially, the central sector of the furnace where the primary material is placed was slowly heated at the rate of ~200 °C/hour up to 1250 °C. The end of the ampoule is found outside the furnace where sulfur and selenium accumulate. As the amount of S and unsynthesized Se decreases, the ampoule is inserted into the furnace and the temperature in the outer sector of the furnace is gradually raised to ~1280 °C throughout the heating of the furnace, the ampoule rotates around the axis of the ampoule at the speed of ~15 rpm. This regime lasts ~8 h, after which the oven is fixed vertically. Sector three of the furnace is heated for ~2 h to ~700 °C after which the ampoule of synthesized substance is passed through the temperature gradient between the furnace sectors two and three at the rate of ~2 mm/hour. After the melt solidifies, the temperature in the furnace decreases at the rate of ~100 °C per hour to room temperature.

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From massive single crystals, plane-parallel plates with a thickness of 50–200 μm are obtained by splitting, which serve as the primary material for the manufacture of β-Ga2 O3 layers. Layers consisting of nano-formations (nano-wires, nano-ribbons) of β-Ga2 O3 with and without remainings of GaS0.17 Se0.83 primary material were obtained by thermal treatment of GaS0.17 Se0.83 plates in a tube furnace at a temperature of 900 °C and in a water vapor enriched atmosphere for 6 h. Single crystals of GaS0.17 Se0.83 and also GaS or GaSe are formed by planar packings with four atomic layers of the type Se/S-Ga-Ga-Se/S with iono-covalent bonds of the atoms inside the packing and the bond weak polarizations (type Van-der-Waals) between packings. The Se/S atoms in the honeycomb packings are arranged in such a way that cracks are formed between them, through which O2 and H2 O molecules easily diffuse inside the plates. Thus, in a long-term process, homogeneous β-Ga2 O3 plates can be obtained both on the surface and in the volume. The crystalline structure, chemical and elemental composition of the synthesized material were studied with the help of X-ray diffraction (XRD) diagrams recorded with monochromatized X-rays with the wavelength λCuKα of 1.54060 Å at 40 kV and 40 mA, which was carried out using a Seifert 3000 TT instrument and a Micro-Raman spectrometer (WITec Alpha 300 Ra, Ulm, Germany) with arc laser radiation excitation at a wavelength of 532 nm. The surface morphology and the type of nano-formations were determined using a scanning electron microscope (SEM) (Carl Zeiss AG, Oberkochen, Germany) at a voltage of 7 kV and a current of 8 μA. With the help of this equipment aided with accessories for determining the X-ray energy emitted by the sample under the influence of the electron beam, the EDX spectra were measured. The diffuse reflection spectra of the radiation from the nano-structured surface of the sample were measured at the spectrometric installation based on the MDR-2 type monochromator completed with a UVC radiation source (DVS-25 lamp) and a sphere for diffuse reflection measurements under the angle 90°. A microstructured layer of BaSO4 was used as a diffuse reflection standard. As a source of UVC radiation (220–290) nm, the 1000 W Xe electric arc lamp was used. The required spectral region was selected from the spectrum of the Xe lamp by means of a monochromator with mirrors and quartz prism of the type ZMR-3. The photocurrent through the sample was measured with a V-30 type electrometer. Thin layers of Al deposited on the surface of the β-Ga2 O3 layer at a distance of ~2 mm from each other were used as electrodes.

3 Experimental Results and Discussion 3.1 Crystal Structure Figure 1 shows the XRD pattern of the material obtained by thermal treatment in the atmosphere enriched with water vapor at the temperature of 900 °C for 6 h. The XRD diffractions in the range of 2θ angles from 30° to 90° are well identified, according to PDF card № 43–1012, as reflections from crystallographic plane assemblies of the β-Ga2 O3 monoclinic lattice with unit cell parameters: a = 12.23 Å, b = 3.04 Å, c = 5.80 Å and β = 103.7°. Similar XRD pattern was recorded from β-Ga2 O3 nanowire layers obtained by CVD method on sapphire substrate [16]. As demonstrated in previous research [17–19], at nanometric crystallite sizes, the width of the contour of the

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Fig. 1. XRD pattern of the material obtained by thermal treatment in the atmosphere enriched with water vapor at the temperature of 900 °C for 6 h of the GaS0.17Se0.83 single crystal plate, the contour of the XRD line with 2θ = 54.40° (Inset).

diffraction lines increases with the decrease in the respective crystallite sizes. Figure 1 (Inset) shows the contour of the X-ray diffraction line with 2θ = 54.40°. The average size of β-Ga2 O3 crystals can be determined using the Debye-Scherrer formula [20]: d=

kλ βcosθhkl

(1)

where k is the Scherrer constant, equal to 0.94, λ is the X-ray wavelength, θhkl is the Bragg diffraction angle, β is the width of the X-ray diffraction reflection contour at half maximum intensity. For λ = 1.5406 Å, θhkl = 27.20°, β = 0.55° we obtain d = 17 nm. 3.2 Microscopic Studies Figure 2 shows SEM images of the surface of the β-Ga2 O3 layers formed on the surface of the GaS0.17 Se0.83 monocrystalline plate by thermal treatment in a water vapor enriched atmosphere at a temperature of 900 °C for 6 h. Figure 2 (a) shows the general view of the surface, where it can be seen that microplates covered with sub-micrometric formations are formed on the surface of the singlecrystalline GaS0.17 Se0.83 plate. The area of the plates varies from units of μm2 to hundreds of μm2 . Formations in the form of threads and ribbons have a thickness from several tens of nm to ~10–12 μm. Nano formations are chaotically oriented with respect to the surface of β-Ga2 O3 microplates. At the same time, β-Ga2 O3 microplates are present in the assembly of nanowires/nano-ribbons. The presence of micro-plates with surface areas of 2–4 μm2 of β-Ga2 O3 in nanowire assemblies was observed in β-Ga2 O3 layers obtained by the CVD method [21], by thermal treatment at temperatures in the range of 900–950 °C of GaAs plates [22], as well as of Sn, In metal oxide layers [23]. Figure 2 (c) shows the SEM image of the cleft in the direction parallel to the C6 axis of a β-Ga2 O3 /GaS0.17 Se0.83 structure with a thin β-Ga2 O3 layer on the (0 0 0 1) surface. The layer of β-Ga2 O3 nanoformations was grown for 30 min at a temperature of 900 °C.

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Fig. 2. SEM image of the surface of the β-Ga2 O3 layer formed on the surface of the GaS0.17 Se0.83 plate: a) the island structure of the β-Ga2 O3 formations; b) assemblies of β-Ga2 O3 nano-wires on mono-crystalline substrate; c) loose nano-wires with lengths of 6–8 μm; d) the cross-section of the β-Ga2 O3 /GaS0.17 Se0.83 structure.

3.3 Elemental Composition The compositional analysis of β-Ga2 O3 nano-formations was performed using X-ray photoelectron spectroscopy (XPS) in the range of binding energies from 0 to1200 eV. The wide scan spanning accross the whole probed energy range indexed with corresponding characteristic binding energies for the elements present in the β-Ga2 O3 surface layer on the GaS0.17 Se0.83 substrate is shown in Fig. 3 (a). Characteristic energies of Auger electrons are indexed as well. The binding energies obtained from this XPS spectrum were corrected to the C-1s reference line of energy 284.6 eV. The maximum thickness of the β-Ga2 O3 layer that was analyzed (the thickness of the layer from which electrons with the maximum energy of 1.2 keV can be emitted) was determined using the Kanaya-Okayama formula [24]: R=

0.0276 · A · E0n Z 0.89·δ

(2)

where R – penetration depth, A – atomic weight (g·mol), n – constant equal to 1.35 for electron energies E 0 < 5 keV, E 0 – electron energy (kV), Z – atomic number, ρ – density (g/cm3 ). In the approximation that electrons lose energy by interacting with gallium atoms (AGa = 69.7, Zga = 31); for E 0 = 1.2 kV, ρ = 5.86 g/cm3 from (2) we obtain R = 8.8 nm. As can be seen from (a) the XPS spectrum shows the emission lines of the electrons from the Ga3d, Ga2d shells, the Auger peak Ga-LMM, O1s, O-KLL and C1s. Peaks with maximum intensity centered at 1150 eV and 1120 eV (Fig. 3(c1)) correspond to the Ga2p1/2 and Ga2p3/2 states characteristic of the Ga-O bond [25]. As can be seen from Fig. 3 (c1) the XPS peaks of the Ga2p1/2 and Ga2p3/2 layers in the β-Ga2 O3 layer on the GaS substrate are shifted to high energies by ~4.3 eV compared to those on the GaS0.17 Se0.83 substrate. Such a shift of the peak energies has been reported to be related to the presence of impurity atoms on the surface, in particular oxygen atoms [26]. The peak with energy 531.5 eV (Fig. 3 (d)) corresponds to the O-1s state in the βGa2 O3 lattice [27]. As can be seen from Fig. 3 (c3) and (c4) the sulfur and selenium in the

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Fig. 3. XPS (a-c) and EDX (d) spectrum of β-Ga2 O3 layer on mono-crystalline GaS0.17 Se0.83 substrate obtained by thermal treatment in air with water vapor at temperature 900 °C for 6 h.

layer on the β-Ga2 O3 surface is below the background level of the analysis methodology. Quantitatively, the distribution of oxygen and gallium in the β-Ga2 O3 layer was analyzed from the EDXS spectrum (Fig. 3(d)), which shows that Ga and O are found in a ratio of 2/3. 3.4 Raman Spectroscopy Micro-Raman spectra at room temperature of the nanostructured layer (nanofibers, nano lamellae) of β-Ga2 O3 with monoclinic crystal lattice space group C2/m [28] excited with laser radiation with a wavelength of 532 nm, power 5–10 mW in the range of wave numbers 100–900 cm−1 is presented in Fig. 4. The elementary cell consists of two units/formulae – one the GaO6 octahedron and the other GaO4 – tetrahedron with 27 active vibrational modes (15 Raman modes, and 12 IR modes). As can be seen, the vibrational modes are energetically selected into three groups: the low-frequency vibrational modes 145.5 cm−1 , 173 cm−1 and 200 cm−1 correspond to bonds and translations of atom chains, the mid-frequency range 322 cm−1 , 349 cm−1 , 415 cm−1 and 473 cm−1 are attributed to formational modes in Ga2 O6 octahedra, and the high frequency bands (600–763) cm−1 correspond to bond stretching and comparison in GaO4 tetrahedra. Raman frequencies (cm−1 ) obtained in this work on β-Ga2 O3 nanowire/nanoribbon assemblies and previous experimental results are included in Table 1 for comparison [14, 18, 30]. In this table the symmetry of the respective Raman vibration modes is

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Fig. 4. Micro-Raman spectrum of β-Ga2 O3 nanoformations obtained by thermal treatment in the air with water vapor at 900 °C for 6 h.

shown [29]. It is easy to see that the frequencies of the Raman vibration modes of the nano-structured β-Ga2 O3 compound [18] Ga2 O3 nanowires [14] and in single crystals [29] are in good correlation. Frequency variations of 1–2 cm−1 are determined by the type of nano-formations and the manufacturing technology. So, from the analysis of the XRD diagrams, the XPS and EDXS spectrograms and the Raman spectra, we can conclude that by thermal treatment of mono-crystalline plates from GaS0.17 Se0.83 solid solutions at a temperature of 900 °C for 6 h in an atmosphere enriched with water vapor, layers of β-Ga2 O3 nanowires and nanoribbons are obtained. Table 1. Comparison of the Raman vibration frequencies with the values in the β-Ga2 O3 nanostructured layers obtained by thermal treatment of single crystals of GaS0.17Se0.83 at a temperature of 900 °C for 6 h in air. #

Raman shift, cm−1

Intensity, u. a

Nanowire Ga2 O3 [14]

Crystal [29]

Symmetry of models [18]

1.

146.0

233.9

144.0

147.0

Bg

2.

173.0

234.1

171.0

170.0

Ag

3.

200.1

611.7

201.0

199.0

Ag

4.

322.0

141.6

321.0

318.0/321

Ag

5.

349.0

247.7

347.0

346.0

Ag

6.

415.0

248.1

417.0

415.0

Ag

7.

473.0

140.3

475.0

475.0

Ag

8.

629.5

140.6

631.0

628.0

Ag

9.

655.4

210.3

654.0

651.0

Ag

10.

762.5

276.8

767.0

763.0

Ag

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3.5 Photoresponsive Properties The fundamental absorption band edge of the nano-structured β-Ga2 O3 layer formed by thermal treatment of single-crystalline GaS0.17 Se0.83 solid-solution plates in air with water vapor at 900 °C for 6 h was analyzed from the spectrum of diffuse reflection in the 220–400 nm wavelength region (Fig. 5 (a)).

Fig. 5. Diffuse reflection spectrum (a) and the direct band gap of the nanostructured layer of β-Ga2 O3 on GaS0.17 Se0.83 substrate (b).

The diffuse light reflection coefficient Rdif and the linear absorption coefficient α are related by the Kubelka-Muunk formula [31]. 

F Rdif



2  1 − Rdif α = = 2Rdif s

(3)

where s is the light scattering factor, is a wavelength-independent parameter for microgranular media with dimensions larger than the wavelength of the incident radiation. The absorption coefficient α and the band gap E g in semiconductors with direct electronic bands are defined by the equality [32]:  1 αhν = hν − Eg 2

(4)

From the formula (3) and (4) the expression results: 

F Rdif



1  hν − Eg 2 · hν = s

(5)

In the Fig. 5 (a) shows the diffuse reflection spectrum of the β-Ga  2 O3 nano-formation layer. In the range of wavelengths from 290 nm to 247 nm the αs absorbance increases ~5.0 times. By extrapolating the straight line segment of the photon energy dependence

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(αhν)2 to zero, the width of the direct band gap was determined equal to 4.86 eV (Fig. 5 (b)). The band gap of β-Ga2 O3 compound in bulk crystals and in the form of thin layers and nano-formations (nano-wires/nano-blades) depends on the type and technology of the formations and varies in the range (4.4–5.2) eV [18, 33–35]. The photoresistor was made of single crystal GaS0.17 Se0.83 plate with dimensions 5x0.2x8 mm3 subjected to thermal treatment in air with water vapor at a temperature of 900 °C for 6 h. As a result, a 0.28 mm thick β-Ga2 O3 plate was obtained. On the surface of this plate, as electrodes, two strips of Al were deposited by thermal vacuum evaporation at a distance of 2 mm from each other. The illuminated surface area is 10 mm2 . The Hg vapor lamp with the power of 500 W was used as a radiation source. The radiation band with wavelengths 240–270 nm was selected from the spectrum of the lamp with a 1200 mm−1 diffraction grating monochromator. As a source of radiation with a continuous spectrum in the range of wavelengths 230–400 nm, an electric arc lamp in Xc with a power of 1000W served. Typical I-V characteristics in the β-Ga2 O3 layer in the dark and under radiation beam illumination with wavelengths of 240–270 nm and flux density of 6 μW/cm2 are presented in Fig. 6 (a). The dark current through the sample at 20 V applied voltage is 17 pA. When illuminating the sample with UVC radiation of 18 μW/cm2 , the current increases to 122 nA. As seen from Fig. 6 (a) the I-V characteristics in the voltage range -30 to + 30 V are linear. The photoresponsivity determined as the ratio of the photocurrent to the power of the incident radiation beam on the photoresistor (Rp = If/p) at 30 V voltage is ~3.0 A/W.

Fig. 6. a) I-V characteristic of the photoresistor based on the nano-structured β-Ga2 O3 layer formed by thermal treatment in water vapor enriched air at 900 °C for 6 h in the dark (1) and upon excitation with radiation in the range of 240–270 nm with the density of 6μW/cm2 (2); b) Spectral distribution of the photocurrent.

We note that the photoresponsiveness of β-Ga2 O3 receptors varies in a wide range of values. In metal-semiconductor-metal structures based on β-Ga2 O3 thin layers obtained by the MBE method it was 3.7·10–2 A/W [36], while in thin layers of β-Ga2 O3 grown on a sapphire substrate (c-plane) reached 259 A/W [33]. Upon illumination of the sample with a pulse of radiation, the photocurrent intensity reaches the steady state in ~22 s. The spectral distribution of the photocurrent recorded on points in the stationary photocurrent

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regime is presented in Fig. 6 (b). In the wavelength range from 290 nm to the wavelength corresponding to the maximum photocurrent value of 246 nm (5.04 eV) the photocurrent increases ~10 times. At wavelengths smaller than 246 nm the photocurrent is decreasing and at wavelengths of 220 nm it reaches ½ of the maximum value. The direct band gap determined from the analysis of the fundamental absorption band edge contour is equal to 4.86 eV. So the maximum of the photoconductivity spectral band is shifted towards high energies by ~0.2 eV. Such displacement is characteristic of semiconductors in various forms (crystalline, thin layers, micro- and nano-formations) [37–39].

4 Conclusions Dense layers of β-Ga2 O3 nano-wires/nano-blades were obtained by thermal treatment in water-vapor enriched air at 900 °C for 6 h of mono-crystalline slabs from GaS0.17 Se0.83 solid solutions. The elemental composition, vibrational and structural properties of the synthesized material were determined from XRD, XPS, EDXS and Raman spectra, which demonstrated the structural homogeneity of the manufactured samples. From the analysis of the edge of the fundamental absorption band, the width of the direct band gap of the nano-structured layers of β-Ga2 O3 on mono-crystalline GaS0.17 S0.83 substrate was determined to be equal to 4.86 eV. Based on the homogenous layers of β-Ga2 O3 , photosensitive photoresistors were fabricated in the wavelength range 220–300 nm with a photoresponse of ~3 A/W. Acknowledgments. It was financially supported by Moldova State University through the Grant Nos. 20.80009.7007.05, of National Agency for Research and Development (Republic of Moldova). We would also like to acknowledge the Technical University of Moldova for support.

Conflict of Interest. The authors declare that they have no conflict of interest.

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Aero-Materials Based on Wide-Band-Gap Semiconductor Compounds for Multifunctional Applications: A Review Ion Tiginyanu1,2(B)

and Tudor Braniste1

1 Technical University of Moldova, Chisinau, Republic of Moldova

[email protected] 2 Academy of Sciences of Moldova, Chisinau, Republic of Moldova

Abstract. Over the last decades, controlling 3D micro-nano-architectures of semiconductor materials has been used to bring to light new characteristics and even new phenomena. This approach is especially promising when applied to the design of hybrid micro-nano-architectures. The aim of this paper is to review the research efforts undertaken last years to develop novel hybrid threedimensional micro-nano-architectures based on wide-band-gap binary compounds for multifunctional applications. Special attention will be paid to 3D micro-nanoarchitectures based on GaN, but results of investigation of architectures based on Ga2 O3 , ZnS, ZnO will be presented as well. Self-interaction of aero-tetrapods of GaN on water surface leads to the formation of elastic membranes that exhibit high degree of porosity with impressive cargo capabilities. Wrapping liquid droplets into aero-GaN we demonstrate the formation of liquid marbles, that show unique characteristics like self-propulsion on water surface at record velocities, pulsed rotations and pendulum-like oscillations of liquid marbles. Higher photocatalytic response was achieved by functionalizing aero-nanomaterials with noble metal nanoparticles. Besides microfluidic applications, aero-GaN proves to be highly efficient in shielding electromagnetic fields in the GHz and THz region, while aero-Ga2 O3 is completely transparent in the same spectral region. Keywords: Aerogalnite · Gallium nitride · Liquid marbles · Self-organization

1 Introduction The properties of semiconductor materials can be modified in a controlled fashion or even new characteristics may be brought to light by building hybrid 3D nanoarchitectures. The goal of this paper is to review the research efforts undertaken over the last years to develop novel bio-inspired hybrid 3D nanoarchitectures based on binary compounds such as GaN, ZnS, ZnO, Ga2 O3 etc. One of the most promising 3D nanoarchitectures proves to be the so-called aero-GaN or Aerogalnite, which represents the first artificial material exhibiting dual hydrophobic-hydrophilic behavior, its characteristics being close to those inherent to a biological cell membrane. The Aerogalnite consists of gallium nitride (GaN) hollow micro-tetrapodal structures with nanoscopic thin walls, the inner surface © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 243–248, 2024. https://doi.org/10.1007/978-3-031-42775-6_27

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being covered by an ultrathin film of zinc oxide (ZnO). The technological route of aero-GaN fabrication includes epitaxial growth of a thin film of GaN on sacrificial ZnO tetrapods with simultaneous decomposition of ZnO in the harsh environment of the Hydride Vapour Phase Epitaxy system [1]. The lateral faces of GaN tetrapods were found to show hydrophobic properties, while the free end of the arms – hydrophilic ones. This new result was achieved in close collaboration with other research groups (see [1] and https://physicsworld.com/a/hydrophobic-or-hydrophilic-aero-gallium-nitride-is-both/). Once placed on the water surface, the free-floating individual hollow GaN tetrapods interact with each other that results in the formation of waterproof rafts showing impressive stretching and cargo performances. The interaction between tetrapods resembles the interaction of fire ants forming live rafts on the water surface which enable the insects to survive during floods [2, 3]. The elasticity and stretching performances of self-assembled aero-GaN membranes were studied using communicating vessels, as shown in Fig. 1. It was found that the aerotetrapods of GaN interact with each other on the water surface until a consolidated membrane forms. The membrane is elastic and can be used as a separation barrier between liquids, avoiding direct contact and mixing, but keeping the gas exchange due to a very high degree of porosity. We found that the membranes can withstand liquid droplets hundreds of times heavier than the membrane [1].

Fig. 1. Self-assembled membrane based on Aerogalnite hollow microtetrapods on one end of two communicating vessels: (a) The picture of the communicating vessels; (b) the freestanding membrane in contact with water; (c) a droplet of colored water on the membrane; (d) the stretched membrane using the pressure of water from the second arm of the communicating vessels

It was found that aero-GaN pellets with the thickness of 1–2 mm exhibit impressive shielding capabilities against electromagnetic radiation in a wide range of frequencies including Gigahertz and Terahertz ones [4, 5]. The shielding effectiveness in the frequency range from 0.25 to 1.37 THz exceeds 40 dB, which places Aerogalnite among the known best Terahertz shields.

2 Liquid Marbles Liquid marbles (LM) represent the droplets of liquid embedded in a layer of hydrophobic coating, which usually is in the form of powder [6]. The hydrophobic coating allows the marbles to maintain their shape, avoiding collision or disintegration. As a result the

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liquid marbles are freely floating on other liquids without mixing the phases and are highly mobile and can roll on solid surfaces. A special interest in the exploitation of Liquid marbles is the possibility to use them in the applications that require controlling of chemical reactions in confined space. These properties make LM useful in many applications as chemical micro-bio-reactors, drug delivery containers etc. 2.1 Aero-GaN Self-Assembled Liquid Marbles Aero-GaN liquid marbles are formed by encapsulating water droplets in the hollow nano-microtetrapods based on GaN. The water droplets are first distributed on a bed of freestanding microtetrapods and by shaking the substrate, the nano-micro-structures of aero-GaN will start to attach to the surface of the water droplet. After several minutes of shaking a homogeneous porous mantle of aero-GaN is covering the water droplet. The self-interaction of GaN hollow micro-tetrapods on water surface can also be stimulated by exposure to ultrasound treatment for 1–2 min, which leads to a stronger interpenetration of floating microtetrapods.

Fig. 2. Aero-GaN Liquid Marble: (a) formation of aero-GaN liquid marble; (a) digital image of an aero-GaN LM on glass; (c) Uniaxial deformation of the aero-GaN Liquid Marble between two glass plates

Similar to the membranes, the aero-GaN liquid marbles exhibit remarkable elasticity withstanding many cycles of uniaxial compression (see Fig. 2c). 2.2 Aero-GaN Self-Propelled Liquid Marbles Taking into account the practical importance of technologies to manipulate fluids at very small scales, in the past years many approaches to control Liquid Marbles have been developed. By controlling the chemical composition, reactants, or surface properties one can design self-propelled liquid marbles with specific propulsion mechanisms. The self-propulsion mechanisms of LMs are typically achieved through a combination of chemical reaction at the interface, which generate gas within the liquid droplet or generate the modification of the surface tension in the near proximity of the LM inducing the Marangoni flow [7].

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In order to explore the aero-GaN liquid marble properties, different deviations from the spherical symmetry were induced during the fabrication process. It was found that aero-GaN based liquid marbles exhibit energy-efficient long-term translational movement for several hours and fast velocity of rotation up to 750 rpm [1]. The rotation speed and the time decay of spinning liquid marbles are highly dependent on their weight. The lighter liquid marbles show higher rotation speed, while the heavier ones are characterized by a much higher inertia keeping the spinning for a longer time [8]. The rotation of liquid marbles is highly dependent on the specific architecture of the enveloping shell consisting of GaN hollow microtetrapods with dual hydrophilic – hydrophobic properties and the deviations from the spherical shape leads to behavioral changes of the marbles. It was found that elongated liquid marbles exhibit pulsed rotation, attaining the same maximum speed of rotation at each pulse, after which the speed of rotation drops down sharply. This phenomenon was described by using a simple analytical model which takes into account the uplift of the marble and formation of water columns underneath during the spinning process. When the rotation speed increases the marble tends to detach from the water surface, which leads to interruption of the propulsion mechanism and consequently the marble drops on the water surface and continues the rotation at much lower speed [8]. It was found that spherical aero-GaN liquid marbles can freely oscillate at the interface of water with glass. The marbles rotate along the glass, periodically changing the direction of the translational motion. The speed of rotation and the amplitude of the pendulum-like translational motion are constant with the time. Graphical representation of the oscillation behavior is presented in Fig. 3. The blue dots show the turnover points of the liquid marble.

Fig. 3. Aero-GaN liquid marble oscillating along the glass border in the V-shape of water-glass interface

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3 Wide-Bandgap Semiconductor Aero-Nanomaterials Aero-GaN serves as template for the fabrication of aero-Ga2 O3 , a highly porous aeromaterial with the same architecture as aero-GaN, but with completely different properties. In particular, the aero-Ga2 O3 is completely transparent to the electromagnetic spectrum both in the GHz and THz region [9], and shows promising photocatalytic properties when functionalized with noble metal nanoparticles [10]. The technological conditions of aeroGaN transformation into aero-Ga2 O3 , taking into account the remnant layer of ZnO on the inner surface of the aero-tetrapods, leads to the formation of the ZnGa2 O4 ternary compound, which shows characteristics promising for energy storage applications [11]. A more complex technological route is used for the fabrication of the aero-ZnS material. The process starts with epitaxial growth of an ultrathin layer of CdS on the sacrificial template of ZnO microtetrapods. During the second technological step, the ZnO sacrificial template is being decomposed at high temperature in a gaseous hydrogen atmosphere. During this process Zn atoms replace the Cd atoms in the CdS compound, resulting in its transformation into aero-ZnS [12]. ZnS is an important semiconductor compound with remarkable optical properties widely used in practical applications including optoelectronics [13], photocatalysis [14], or biomedicine [15]. The three-dimensional architecture of aero-ZnS represents a material with very low density of about 5 mg/cm3 , which has dual hydrophobic/hydrophilic properties and exhibits a remarkable photo response [12].

4 Conclusions Novel aero-materials based on GaN, Ga2 O3 , ZnS have been successfully developed over the last years. Aero-GaN exhibits unique wetting properties being the first artificial material with dual hydrophobic/hydrophilic behavior. The self-organization of micronanostructures of aero-GaN in elastic membranes led to the development of mechanically resistant liquid marbles that showed record speeds of self-propulsion. Along with microfluidics applications of aero-GaN and aero-ZnS, it was demonstrated that aero-materials can be used for applications requiring shielding (aero-GaN) or complete transparency (aero-Ga2 O3 ) to electromagnetic spectrum in GHz or THz region. Acknowledgments. This research was funded by National Agency for Research and Development of Moldova under the Grant #20.80009.5007.20 “Nanoarhitecturi în baz˘a de GaN s, i matrici tridimensionale din materiale biologice pentru aplicat, ii în microfluidic˘a s, i inginerie tisular˘a” and by the European Commission under the Grant #810652 “NanoMedTwin”.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Tiginyanu, I., et al.: Adelung, R: Self-organized and self-propelled aero-GaN with dual hydrophilic-hydrophobic behav-iour. Nano Energy 56, 759–769 (2019). https://doi.org/10. 1016/j.nanoen.2018.11.049

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2. Hu, D.L., Chan, B., Bush, J.W.M.: The hydrodynamics of water strider locomotion. Nature 424, 663–666 (2003). https://doi.org/10.1038/nature01793 3. Mlot, N.J., Tovey, C.A., Hu, D.L.: Fire ants self-assemble into waterproof rafts to survive floods. Proc. Natl. Acad. Sci. U. S. A. 108, 7669–73 (2011). https://doi.org/10.1073/ pnas.1016658108, Hu, D.L., Chan, B., Bush, J.W.M.: The Hydrodynamics of Water Strider Locomotion. Nature 424, 663–666 (2003). https://doi.org/10.1038/nature01793 4. Braniste, T., et al.: Terahertz shielding properties of aero-GaN. Semicond. Sci. Technol. 34(12), 12LT02 (2019). https://doi.org/10.1088/1361-6641/ab4e58 5. Dragoman, M., et al.: Electromagnetic interference shielding in X-band with aero-GaN. Nanotechnology 30(34), 34LT01 (2019). https://doi.org/10.1088/1361-6528/ab2023 6. Aussillous, P., Quéré, D.: Liquid Marbles. Nature 411, 924–927 (2001). https://doi.org/10. 1038/35082026 7. Bormashenko, E., Bormashenko, Y., Grynyov, R., Aharoni, H., Whyman, G., Binks, B.P.: Self-propulsion of liquid marbles: leidenfrost-like levitation driven by marangoni flow. J. Phys. Chem. C 119, 9910–9915 (2015). https://doi.org/10.1021/acs.jpcc.5b01307 8. Braniste, T., et al.: Tiginyanu, I: Self-Propelled Aero-GaN Based Liquid Marbles Exhibiting Pulsed Rotation on the Water Surface. Materials 14(17), 5086 (2021). https://doi.org/10.3390/ ma14175086 9. Braniste, T., et al.: Aero-Ga2O3 nanomaterial electromagnetically transparent from microwaves to terahertz for internet of things applications. Nanomaterials 10, 1 (2020). https:// doi.org/10.3390/nano10061047 10. Plesco, I., et al.: Highly porous and ultra - lightweight aero - Ga2 O3 : enhancement of photocatalytic activity by noble metals. Materials 14, 1985 (2021). https://doi.org/10.3390/ma1 4081985 11. Wolff, N., et al.: Synthesis and nanostructure investigation of hybrid β-Ga2O3/ZnGa2O4 nanocomposite networks with narrow-band green luminescence and high initial electrochemical capacity. Small 19, 2207492 (2023). https://doi.org/10.1002/smll.202207492 12. Plesco, I., et al.: Aero-ZnS architectures with dual hydrophilic-hydrophobic properties for microfluidic applications. APL Mater. 8, 061105 (2020). https://doi.org/10.1063/5.0010222 13. Raza, A., Noor, H., Riaz, S., Naseem, S.: Modifying the optical properties of ZnS for optoelectronic applications. Engineering Proceedings 32(1), 18 (2023). https://doi.org/10.3390/ engproc2023032018 14. Cao, J., et al.: Highly enhanced photocatalytic properties of ZnS nanowires-graphene nanocomposites. RSC Adv. 4, 30798–30806 (2014). https://doi.org/10.1039/C4RA04164J 15. Labiadh, H., Lahbib, K., Hidouri, S., Touil, S., Chaabane, T.B.E.N.: Insight of ZnS nanoparticles contribution in different biological uses. Asian Pac. J. Trop. Med. 9, 757–762 (2016). https://doi.org/10.1016/j.apjtm.2016.06.008

Technological Features of Creating Hole Structures on the Base of MoS2 and the Electrochemical Behavior of MXene/Holey MoS2 Hybrids in Oxygen Reduction Reactions Havva Nur Gurbuz1,2 , Hasan H. Ipekci1,2 , Vladimir Goremichin3 , Nikita Siminel3(B) , Leonid Kulyuk3 , and Aytekin Uzunoglu1,2 1 Metallurgical and Materials Engineering, Faculty of Engineering, Necmettin Erbakan

University, Konya, Turkey 2 BITAM, Necmettin Erbakan University, Konya, Turkey 3 Institute of Applied Physics, Moldova State University, Chisin˘au, Republic of Moldova ,

[email protected]

Abstract. High-performance noble metal-free two-dimensional (2D) electrochemical catalysts have gained great importance to replace the Pt-based catalysts in oxygen reduction reactions (ORR) to reduce not only the cost of the fuel cells but also enhance the energy efficiency. Herein, we designed a novel ORR catalyst by forming MXene/holey MoS2 hybrids. The holes were created on the basal plane of MoS2 both to create electroactive defective regions and enhance the diffusion of the reactants in the catalyst layer. Holey 2D MoS2 layers were characterized using transmission electron microscopy (TEM), UV-ViS spectroscopy, scanning electron microscope (SEM), and Raman spectroscopy. The TEM images indicated the formation of nano-holes on the basal plane of MoS2 . The increased defect concentration was revealed from the Raman spectra of the samples. The successful synthesis of the V2 C MXene layers was confirmed using SEM and EDS results. The holes created on the basal plane of 2D MoS2 boosted the electrochemical ORR performance compared to the pristine 2D counterparts, which is attributed to the defect-rich active sites on the edge of the holes and enhanced diffusion of the reactants. In conclusion, our designed MXene/holey MoS2 hybrid catalyst exhibits superior electrochemical performance in ORR, offering a promising approach for the development of cost-effective and efficient catalysts for fuel cell applications. Keywords: MoS2 · MXene · Holey Structure · Catalyst · Oxygen Reduction Reaction

1 Introduction It is of great interest to develop highly active and durable electrocatalysts for oxygen reduction reaction (ORR) since the sluggish kinetics of the cathode electrode in metal-air and fuel cells limit the energy conversion efficiency and performance. Precious platinum © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 249–256, 2024. https://doi.org/10.1007/978-3-031-42775-6_28

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(Pt) and its alloys continue to be used in high-performance electrocatalysts developed and widely used for ORR [1]. However, high cost, limited reserves, and various instability issues are emerging as major problems hindering the large-scale production and thus commercialization of Pt [2, 3]. Therefore, in recent years, non-precious metal catalysts and metal-free catalysts have been investigated in ORR applications. Graphene and graphene-like 2D materials have gained a great deal of interest in catalysis applications due to showing large surface area, unique electron transfer properties, high chemical stability, and tailorable surface properties [4–6]. Among various 2D materials, 2D transition metal dichalcogenides (TMDs) have been widely exploited in various electrochemical applications ranging from energy storage and conversion to sensors and biosensors [7]. MoS2 , which is one of the typical TMDs used in electrochemistry poses various advantages over graphene such as a direct band gap of 1.9 eV in the monolayer, which is dependent on the structural properties [8]. The use of 2D MoS2 for oxygen reduction reactions as a low-cost electrocatalyst has been reported by various researchers [9]. The results indicated that while MoS2 displays various advantages including low cost and high abundance compared to noble metal-based catalysts, the electrochemical activity of pristine MoS2 should be improved. It is known that the electrocatalytic performance of 2D materials can be improved by the creation of defectrich regions in the basal plane. Therefore, the electrochemical performance of defective MoS2 layers was also investigated and the results indicated a prominent improvement in the activity. In addition, the formation of hybrid materials for electrochemical application results in a significant enhancement in the performance of the catalysts due to the synergetic effects between dissimilar 2D materials. MXene, which is also graphene-like 2D material, has been used in electrochemical applications due to its high electronic conductivity, tailorable surface property, and large physical surface area [10]. Herein, we designed novel ORR electrocatalysts based on MXene/MoS2 hybrids. To enhance the electrochemical performance of the MoS2 component, holes were created on the basal plane using an H2 O2 treatment. The change in the physical and chemical properties of the holey MoS2 layers (h-MoS2 ) was characterized using TEM, SEM, UV-vis, and Raman spectroscopy methods. The electrochemical ORR activity of the h-MoS2 -containing catalysts was evaluated. The electrochemical results indicated an improvement in the ORR activity which is attributed to the increase in the defective sites on the basal plane and enhancement in the diffusion of the reactants through the catalyst layer.

2 Materials and Methods 2.1 Chemicals Bovine serum albumin (96%) (BSA), molybdenum (IV) sulfide (99%, < 2 um), and hydrogen peroxide (H2 O2 , 30%) were purchased from Sigma-Aldrich. MAX phase, V2 AlC, was obtained from Nanografi Inc. TURKEY. The chemicals were used without further modification.

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2.2 Synthesis of Layered MoS2 and Holey MoS2 The synthesis of layered MoS2 was carried out by several methods to obtain a higher concentration of exfoliated flakes as well as to determine better compatibility with the downstream technology to produce perforated MoS2 . Firstly, liquid-phase exfoliation (LPE) mediated by carbon nanotubes was used [11]. A dispersion solution of carbon nanotube (CNT) powder was prepared by mixing 1 g of carbon nanotube powder with 250 g of ethanol. The resulting mixture was then sonicated for 40 min to produce a homogeneous dispersion. Then 2 g of the previously obtained bulk MoS2 powder was added to the dispersion. The resulting mixture was sonicated for 60 min using an ultrasonic bath. To further remove the CNT, the mixture was heated to 55 °C for 30 min, during which time the CNTs precipitated out. The mixture was then centrifuged to collect the supernatant. The resulting supernatant containing MoS2 nanosheets and CNT residues was evaporated at 70 °C for 30 min to give a solid powder. Secondly, the more traditional LPE method using Nmethyl-pyrrolidone (NMP) was used [12, 13]. Exfoliated MoS2 flakes were obtained by sonication of a 100 mg/ml solution of bulk CVT MoS2 powders in NMP (Sigma-Aldrich, CAS 872–50-4) for 10 h using a horn-tip in on/off cycles of 5 s. The flat-bottomed beaker with the solution was connected to a cooling system to avoid overheating the processor. This procedure was then repeated 3 times to increase the final concentration of MoS2 in the solution. The samples were then centrifuged at 1500 rpm for 60 min. To remove NMP, the dispersions were diluted 40 times in isopropanol (IPA) and treated in a bath for 30 min. Thirdly, the MoS2 nanosheets and holey-MoS2 were prepared by direct exfoliation of bulk MoS2 powder in a BSA-containing solution [14]. 500 mg of bulk MoS2 was dispersed in a 100 ml BSA solution with a concentration of 1 mg/ml BSA and sonicated for 48 h in a cooled ultrasonic bath. The obtained solution was centrifuged at 1500 rpm for 45 min to collect precipitates. The resulting supernatant was named layered (l-MoS2 ). The first and third methods showed more acceptable results for further etching of holes in MoS2 layers. The formation of the holes on the basal plane of l-MoS2 was carried out via chemical dissolution in the presence of H2 O2 . In this method, 50 mM H2 O2 was introduced into the supernatant containing l-MoS2 solution and which was followed by an ultrasonic homogenization process. The solution was then centrifuged at 6000 rpm for 45 min to remove excessive BSA. The collected precipitate was redispersed into 100 mL of pH 4.0 solution via sonication followed by further centrifugation at 1500 rpm for 45 min to obtain a brownish supernatant of holey MoS2 nanosheets (h-MoS2 ). 2.3 Synthesis of V-MXene The synthesis of V-MXene was conducted based on a paper reported by Guan et. al. [15]. 1 g of lithium fluoride was dissolved in 12 M hydrochloric acid (36 ~ 38 wt%, 10 mL) in a polytetrafluoroethylene vessel with magnetic stirring for 30 min. Then, 0.5 g of V2 AlC powders were gradually added to the solution. The reaction vessel was sealed and transferred to a stainless-steel autoclave. The autoclave was heated to 90 °C for 144 h. Subsequently, the solution was cooled down to room temperature. The mixture was taken out from the vessel and washed several times with ethanol and deionized water to remove impurities and dried in a vacuum oven at 60 °C for 24 h. The obtained

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V-MXene powder was dispersed in 25 mL DI water by ultrasonic treatment for 2h. And then the solution was centrifuged at 2000 rpm for 60 min. After this process, the supernatant is collected by removing the sediment and freeze-dried. 2.4 Characterization of the Catalysts The phase analysis of the samples was conducted using a PANalytical EMPREAN Xray diffractometer equipped with Cu Kα (λ = 0.15406 nm) radiation. The SEM images were taken using a Zeiss Gemini SEM 500 SEM unit equipped with an EDS analyzer. A Jeol JEM-2100 PLUS transmission electron microscope operating at 200kV was implemented to record the TEM images. The Raman results were obtained using a Renishaw inVia Reflex Raman spectrometer. The electrochemical performance analysis of the catalysts was performed using a Gamry References 1010E electrochemical workstation connected with a rotating disk electrode (RDE, BASi RDE-2). The catalyst slurries were prepared by dispersing 5 mg catalyst in 1 ml MoS2 supernatant. The solution was tip sonicated for 30 min. After the homogenization of the slurry, 6 μl of the slurry was dropped on a polished glassy carbon electrode (GC, 3 mm in diameter) and dried under an ambient atmosphere. The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) experiments were performed in O2 and N2 -saturated 0.1M KOH. Ag/AgCl (3M NaCl sat.) and Pt wire were used as reference and counter electrodes, respectively.

3 Results and Discussions The 2D layered structure and the holes formed on the basal plane of MoS2 were investigated by TEM analysis. The recorded images are displayed in Fig. 1 a-b. The TEM images indicated the successful synthesis of layered h-MoS2 structures. The XRD reflection of h-MoS2 was given in Fig. 1c. The reflection obtained at the 2θ of 14.45 is attributed to the (002) planes of 2H-MoS2 (JCPDS 37–1492). Therefore, the TEM and XRD results confirmed the successful synthesis of h-MoS2 . The UV-ViS result of the h-MoS2 is given in Fig. 2a. The recorded spectrum is compatible with typical MoS2 spectra given in the literature. This method was also implemented to determine the concentration of the supernatant. The calibration curve obtained from the UV-ViS absorbance values is given in Fig. 2b. The Table in Fig. 2c indicated the measured results used to create the calibration curve. The concentration of the supernatant was found to be 0.37 mM of MoS2 . This concentration value was further used to form MXene/h-MoS2 hybrids with a constant weight ratio (Table 1). Raman spectrum was performed to confirm the thickness and defect structure of h-MoS2 nanosheets. The thickness is related to wavenumber differences between inplane vibrational (E12g) and out-of-plane (A1g) modes. The reflections located at 382.5 and 406.9 cm−1 are attributed to the E12g and A1g modes for MoS2 , respectively. In particular, the broad and low intensity of E12g is associated with in-plane disorders or high defect density. The k values of the peak distances are 24.11 and 26.29 cm−1 for the b-MoS2 and h-MoS2 , respectively, as shown in Fig. 3 a-b. The SEM images of V-MXene (V2 C) are given in Fig. 4a. As observed, nanoflowerlike MXene shapes were obtained after the synthesis process. The EDS results indicated

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Fig. 1. A-b) TEM images and c) XRD results of h-MoS2 .

Fig. 2. UV-vis results of holey MoS2 : a) UV-vis spectrum, b) calibration curve

a decrease in the Al content, indicating the formation of the MXene phase. On the other hand, it should be mentioned that the Al layer could not be removed completely from the MXene phase. The electrochemical performance of the catalysts is given in Fig. 5. As observed in the CV results displayed in Fig. 5 a-c for V-MXene, V-MXene/l-MoS2 , and VMXene/h-MoS2 , respectively, the highest reduction current was obtained from the hMoS2 -containing sample, indicating the importance of the holes created on the basal plane of the MoS2 layer. The ORR activity of the samples at different rotating speeds is also given in Fig. 5 d-f, which shows that the highest kinetic activity was observed from the holey MoS2 -containing sample. Therefore, it may be alleged that the creation

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Concentration (mM)

Slope

Concentration of the supernatant

0.315

0.34

Average: 0.37 mM

0.625

0.60

Standard deviation 0.007

1.25

1.24

3.125

3.11

Fig. 3. Raman Spectra of bulk (a) and holey (b) MoS2 .

Fig. 4. a) SEM and b) EDS results of V-MXene.

of holes in the basal plane of MoS2 shows a promising result for the development of ORR catalysts.

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Fig. 5. CV curves of (a) V-MXene (b) V-MXene/l-MoS2 and (c) V-MXene/hMoS2 in N2 and O2 -saturated 0.1 M KOH solution. (d) LSV curves of V-MXene, (e) V-MXene/l-MoS2 (f) VMXene/hMoS2 in O2 -saturated 0.1 M KOH solution at a rotation of at different rotations from 400 to 2500 rpm.

4 Conclusions Herein, we designed MXene/holey MoS2 -based electrocatalysts for oxygen reduction reactions. The electrochemical activity of MoS2 was enhanced by the creation of holes in the basal plane. The TEM and XRD results indicated the successful synthesis of holey MoS2 layers. In addition, flower-like V2 C MXene nanostructures were also obtained. The electrochemical performance of the MXene/MoS2 hybrids was evaluated using CV and RDE experiments. The results showed that the creation of the holes in the basal plane enhanced the electrochemical activity of the hybrids against ORR, which is attributed to the increased defect concentration and enhanced diffusion of the reactants.

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˙ Acknowledgments. The authors thank the TÜBITAK - NARD Bilateral Cooperation Project “Development of Fully Inkjet Printed FET Biosensors Using 2D Transition Metal Dichalcogenides for E.coli Bacteria Detection”, TUBITAK 122N920, NARD 23.80013.5007.2TR.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Gan, J., et al.: Mechanistic understanding of size-dependent oxygen reduction activity and selectivity over Pt/CNT nanocatalysts. Eur. J. Inorg. Chem. 27, 3210–3217 (2019). https:// doi.org/10.1002/ejic.201801521 2. Dong, F., et al.: Heteroatom (B, N and P) doped porous graphene foams for efficient oxygen reduction reaction electrocatalysis. Int. J. Hydrogen Energy 43(28), 12661–12670 (2018). https://doi.org/10.1016/j.ijhydene.2018.04.118 3. Dombrovskis, J.K., Palmqvist, A.: Recent progress in synthesis, characterization and evaluation of non-precious metal catalysts for the oxygen reduction reaction. Fuel Cells 16(1), 4–22 (2016). https://doi.org/10.1002/fuce.201500123 4. Jahandideh, H., et al.: Fabrication of graphene-based porous materials: traditional and emerging approaches. Chem. Sci. 13(31), 8924–8941 (2021). https://doi.org/10.1039/D2SC01 786E 5. Sinha, A., et al.: MXene: An emerging material for sensing and biosensing. TrAC, Trends Anal. Chem. 105, 424–435 (2018). https://doi.org/10.1016/j.trac.2018.05.021 6. Cherusseri, J., et al.: Recent trends in transition metal dichalcogenide based supercapacitor electrodes. Nanoscale Horizons 4(4), 840–858 (2019). https://doi.org/10.1039/C9NH00152B 7. Lam, C.Y.K., et al.: Recent advances in two-dimensional transition metal dichalcogenide nanocomposites biosensors for virus detection before and during COVID-19 outbreak. J. Compo. Sci. 5(7), 190 (2021). https://doi.org/10.3390/jcs5070190 8. Park, H., et al.: Ultrasensitive and selective field-effect transistor-based biosensor created by rings of MoS2 nanopores. ACS Nano 16(2), 1826–1835 (2021). https://doi.org/10.1021/acs nano.1c08255 9. Xu, H., et al.: Two-dimensional MoS2 : structural properties, synthesis methods, and regulation strategies toward oxygen reduction. Micromachines 12(3), 240 (2021). https://doi.org/10. 3390/mi12030240 10. Zhang, X., Zhang, Z., Zhou, Z.: MXene-based materials for electrochemical energy storage. J. Energy Chem. 27(1), 73–85 (2018). https://doi.org/10.1016/j.jechem.2017.08.004 11. Han, W., et al.: Exfoliation of large-flake, few-layer MoS 2 nanosheets mediated by carbon nanotubes. Chem. Commun. 57(36), 4400–4403 (2021). https://doi.org/10.1039/D1C C00673H 12. O’Neill, A., Khan, U., Coleman, J.: Preparation of high concentration dispersions of exfoliated MoS2 with increased flake size. Chem. Mater. 24(12), 2414–2421 (2012). https://doi.org/10. 1021/cm301515z 13. Jawaid, A., et al.: Mechanism for liquid phase exfoliation of MoS2 . Chem. Mater. 28(1), 337–348 (2016). https://doi.org/10.1021/acs.chemmater.5b04224 14. Guan, G., et al.: Surface-mediated chemical dissolution of two-dimensional nanomaterials toward hole creation. Chem. Mater. 30(15), 5108–5115 (2018). https://doi.org/10.1021/acs. chemmater.8b01540 15. Guan, Y., et al.: A hydrofluoric acid-free synthesis of 2D vanadium carbide (V2C) MXene for supercapacitor electrodes. 2D Materials 7(2), 025010 (2020). https://doi.org/10.1088/20531583/ab6706

The Water-Soluble Zinc Phthalocyanine Substituted with Sulfur-Containing Groups Iacob Gutu1

, Victor Suman2

, Alic Barba3

, and Tamara Potlog4(B)

1 The Faculty of Chemistry and Chemical Technology, Moldova State University, Chisinau,

Moldova 2 Ghitu Institute of Electronic Engineering and Nanotechnologies, Technical University,

Chisinau, Moldova 3 Institute of Chemistry, Moldova State University, Chisinau, Moldova 4 Laboratory of Organic/Inorganic Materials for Optoelectronics, Moldova State University,

Chisinau, Moldova [email protected]

Abstract. In this paper we describe [phthalocyaninato]zinc octakis (methylene isothiuronium) chloride and [phthalocyannato]zinc octamethanethiol having as the starting substance octakis(cloromethyl) phthalocyanine zinc obtained by chloromethylation reaction of zinc phthalocyanine. The structures of the synthesized compounds were characterized by elemental analysis, FTIR and 1 H-NMR spectroscopies. The UV–Vis spectra of mentioned compounds depend on its concentration and generally present, a wavelength region with – the B band situated at approx. 300–400 nm and the Q band at approx. 600–800 nm. The UV–Vis spectra of [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride presented a broader Q-band in water solution with a shoulder on the red side. It is noticeable, the disappearance of Q peak splitting, with a slight hypsochromic shift at 639 nm, characteristic of the α-form of aggregation. Laser flash photolysis has been used to characterize the triplet state of [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride compound in dilute DMSO: H2 O, NVP: H2 O and H2 O solutions. The fluorescence decay curves for [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride at the interval of excitation wavelengths (λexc = 341…703 nm) show a biexponential behavior with lifetime values being yielded 2.31 μs and 1.23 μs in DMSO: H2 O, 1.22 μs and 9.22 μs in NVP: 9H2 O solvents. The decay curve of phosphorescence of [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride in H2 O are multi-exponential and are represented by the relatively long triplet lifetimes of 1.09 μs, 4.96 μs and 15.23 μs. The triplet lifetime and triplet quantum yield values of [phthalocyannato]zinc octamethanethiol in DMSO: H2 O are lower than of [phthalocyaninato]zinc octakis (methylene isothiuronium) chloride compound. Keywords: ZnPc derivatives · Isothiouronium group · Thiol group · Absorbance · Fluorescence

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 257–267, 2024. https://doi.org/10.1007/978-3-031-42775-6_29

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1 Introduction Phthalocyanines (PCs) are macrocyclic compounds consisting of four isoindole units (Fig. 1). Phthalocyanines play a major role in modern photochemistry and the nature of metal ions influences their photophysical properties [1–3]. These applications require compounds with various solubilities, photoconductive and optical characteristic. Numerous derivatives can be prepared from phthalocyanines because of the stability of the core macrocycle. Their properties are determined by the nature of the substituents bound to the macrocycle unit and the coordinated metal ion in the macrocycle. Most phthalocyanine derivatives are colored substances. It has been found that some of these dyes qualify as effective phototoxic drugs for photodynamic therapy (PTD). An important requirement in the design of PCs for PTD is the solubility in aqueous solution [4–6]. The intrinsic solubility in water to PCs is conferred by the incorporation of ionic (anionic, cationic) or hydrophilic substituents (Fig. 1). Such anionic substituents as sulfonate, carboxylate and phosphate groups are used for the solubilization of PCs in water. Most phthalocyanine with cationic substituents are generated by the quaternization of amines or pyridine nitrogen. Such types of non-ionic fragments as polyethylene glycol or carbohydrates allow the solubilization of PCs in aqueous solutions.

Fig. 1. Chemical structures of metal phthalocyanines and of substituent.

The introduction of functional groups containing sulfur into the molecules of ZnPc gives them properties that are important for their application in photodynamic therapy (PDT). The previously research results showed ZnPc improved triplet state parameters and high triplet quantum yields. In addition, the presence of S and N atoms on the Pc complex facilitates formation of Ag-N or Ag-S bonding. According to Zafar Iqbal et. al., replacement of the oxygen with sulfur in the non-peripherally tetrasubstituted complexes resulted in red shifting of the Q-band to higher wavelengths due to stronger electron donating ability of the sulfur atoms. Authors of [7] explored NanoAzoPcS nanoparticles ability to regulate the self-assembly behavior of 4-sulfonatophenoxysubstituted ZnPcS. The authors of [8] reported the synthesis of (24-(4- (benzo[d]thiazol-2-yl) phenoxy)2,10,17-tris (4-(2 carboxyethyl) phenoxy) phthalocyanine-29-yl) zinc (II) linked to AuNPs through S-Au and Au-N self-assembly. The ZnPc and the conjugated (Au, Ag)

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Ps nanoparticles have been reported to enhance the triplet and singlet oxygen quantum yields of the phthalocyanines due to the heavy atom effect which encourages intersystem crossing to the triplet state. The improved quantum yields of the triplet state guide the generation of higher singlet oxygen, as it is generated when the triplet state of Pc interacts with ground state molecular oxygen. In this paper, a series of zinc phthalocyanines containing fragments with sulfur containing groups have been synthesized and some of their properties have been studied, Herein, we describe the synthesis and photophysical properties of octakis(chloromethyl)zinc phthalocyanine as a starting material for finding a water-soluble derivative of zinc phthalocyanine with sulfur-containing group for future binding to noble nanoparticles.

2 Experimental Details 2.1 Synthesis of Substituted Zinc Phthalocyanines The synthetic pathway for all compounds in this paper is shown in Fig. 2.

Fig. 2. Synthesis of substituted zinc phthalocyanines.

Octakis (cloromethyl) phthalocyanine zinc 2 used as the starting substance was obtained following a chloromethylation reaction of phthalocyanine zinc. This reaction was originally described in patent [9], in which the chloromethylation process is performed with bis(chloromethyl) ether. Due to the fact that exposure to bis(chloromethyl) ether is associated with an increased risk of developing lung cancer, processes have

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been developed to obtain chloromethyl phthalocyanines using reagents that generate bis(chloromethyl) ether in situ. It is obtained by the interaction of paraform with thionyl chloride in the presence of AlCl3 and triethylamine [10]. Following the known procedure, a product with a chlorine content of 28% corresponding to a compound containing eight chloromethyl groups was obtained. In the FTIR spectrum of the product obtained along the band’s characteristic of the phthalocyanine cycle, the characteristic bands corresponding to the oscillations of the C-Cl bonds (700 cm−1 ) and the oscillations of the Ar-CH2 group at 1222 cm−1 are observed. The chloromethyl derivative reacts easy with thiourea to reflux in ethanol. The reaction product obtained it is slightly soluble in water. In the FTIR spectrum of the product there is a wide absorption band in the 3200 cm−1 regions attributed to the amino formamide group, and the weak band characteristic of the C-S bond, the CH2 group, as well as those of the phthalocyanine ring. Under the conditions of a basic hydrolysis of the thiouronium salt, the respective thiol derivative results on the acidification of the reaction medium. The product obtained is poorly soluble in water, better in alkaline medium and polar solvents. The IR spectrum showed the presence of the peak attributed to the thiol group at 1333 cm−1 . 2.2 Materials and Methods All purchased chemical reagents (Merck) were of the highest commercially available purity and were used without previous purification. A Bruker FTIR spectrometer was used to provide information about chemical composition. The 1 H-NMR spectra were recorded in deuterated DMSO solution on a Nuclear Magnetic Resonance spectrometer (NMR) Bruker Advance III 400 MHz model. Chemical shifts are reported in δ ppm. The UV-Vis spectra of the solutions were measured using a UV-Vis spectrophotometer (Lambda 25, Perkin Elmer Inc., Shelton, CT, USA) from 200 nm to 1200 nm in 10 mm quartz cuvettes. The steady-state fluorescence spectroscopy was performed using a spectrometer (LS55 Perkin Elmer, Inc., Shelton, CT, USA) equipped with double grating excitation and emission monochromators. The concentrations of the solutions were adjusted to obtain an absorbance of ∼0.1 at 345 nm. Time-Correlated Single Photon Counting (TCSPC) was used to determine the fluorescence lifetime. The time-resolved fluorescence spectra were recorded on a spectrometer (Edinburgh FLS980, Livingston EH54 7DQ, Oxford, UK). All the measurements were made at room temperature (295 ± 1 K). The most general and reliable method used for the analysis of the collected data is that of non-linear least-squares analysis, which involves deconvolution of the instrument response function, X(t), with a chosen function and non-linear least-squares fitting. This entails recording an excitation pulse profile, convolving this instrument response function with a theoretical exponential decay, and then optimizing the parameters of the decay to match the measured intensity decay. The fitting procedure is carried out using the Solver function in Microsoft Excel, and yields values for the amplitude (A) and lifetime (τ) of the fluorescence decay. To analyze the decay of the phosphorescence lifetime of zinc phthalocyanine derivatives, a multiexponential model was used [11]:   M t (1) Ai exp − I (t) = i=1 ti

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where Ai represents the fluorescence intensity at time t = 0 for the nth component, τ i the lifetime for the nth component and I(t) the total intensity of fluorescence at time t. The average decay time is given by:  τe  = Ai τi (2) The quality of fits obtained for the lifetime was judged according to reduced chi squared “χ2 ”. If only random errors contribute to “χ2 ”, the value is expected to be near the unity. Usually, if 0.9< χ2 < 1.2 the fit is considered to be good.

3 Results and Discussions 3.1 UV-Vis and Fluorescence Analysis The UV–Vis spectra of 2–4 compounds recorded in DMSO/H2O are shown in Fig. 3 (left). The absorption spectra show characteristic Soret bands and Q bands as previously reported for other ZnPc [24–26]. In the case of [octakis(chloromethyl)phthalocyaninato] zinc, the Q band of the absorption spectrum is shifted by 18 nm to longer wavelengths compared to [phthalocyaninato]zinc octakis (methylene isothiuronium) chloride and [phthalocyaninato]zinc octamethanethiol, while the Q band of phthalocyaninato] zinc octamethanethiol shows a slight hypsochromic shift. In the case of octakis(chloromethyl)phthalocyaninato] zinc and [phthalocyaninato]zinc octamethanethiol derivatives, a strong broadening of the Q band is observed, which indicates the aggregation of phthalocyanine molecules. This broadening is characteristic of H-type aggregates [12] with low photodynamic efficiency. The two main Q and B bands are both considered to be π-π* transitions. Since phthalocyaninato] zinc octamethanethiol is less soluble in water, we will continue to analyze in more detail the compound [phthalocyaninato]zinc oktakis chloride (methylene isothiuronium) in different aqueous solutions. The most significant experiment is the absorption spectra of [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride in 1NVP:9H2 O solution shown in Fig. 3 (right) because of its good water solubility. The aggregation behavior of the [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride compound investigated at 1,83·10−4 g mL−1 (red) and 0,61·10−4 g mL−1 (blue) concentrations in 1 mL NVP: 9 mL H2 O solution show Q-band splitting of this compound into two distinct peaks in the region (550–800) nm, typical for the monomeric form of ZnPc or for insignificant aggregation of ZnPc molecules in solution. So, a clear split of Q band even at low concentrations is observed. The Q band splits into two peaks of the highest isomer symmetry, the splitting Q band is decreased with a decreasing symmetry [13]. Also, the UV–Vis spectra of [phthalocyaninato] zinc oktakis (methylene isothiuronium) chloride in H2 O are recorded and presented in Fig. 4 (left). The Q-band in water solution is seen to be broader with a shoulder on the red side. It is noticeable, the disappearance of Q peak splitting, with a slight hypsochromic shift at 639 nm that is likely characteristic of the α-form of aggregation [14]. The concentration-dependent aggregation showed that the Q and B bands are not affected, and the Lambert-Beer law is also obeyed for 1.46·10−4 g mL−1 and 0.81·10−4 g mL−1 concentrations in the H2 O.

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So, because the [phthalocyaninato]zinc oktakis(methylene isothiuronium) chloride compound possesses good solubility in solvents mixed with water and even in water, we also studied its fluorescence properties. The fluorescence emission spectra of [phthalocyaninato]zinc oktakis(methyleneisothiu-ronium) chloride in DMSO/H2 O at different excitation wavelengths are shown in Fig. 4 (right).

Fig. 3. Absorbance spectra of 2–4 compounds in DMSO/H2 O solvent (left) and absorbance of [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride with 1,83·10−4 g mL−1 (red) and 0,61·10−4 g mL−1 (blue) concentrations recorded in stock solution: 0.5 mg substance in 1 mL NVP: 9 mL H2 O (right).

Fig. 4. The UV–Vis spectra of [phthalocyaninato] zinc oktakis (methyleneisothiuroni um) chloride in H2 O (left) and the fluorescence emission spectra in DMSO: H2 O (right).

The fluorescence emission spectra of this compound in DMSO/H2O excited at 347 nm showed three emission bands situated at 385 nm, 694 nm and 764 nm. At the excitation with 638 nm the positions of the bands situated at 694 nm and 764 nm are kept. The fluorescence spectra of [phthalocyaninato] zinc oktakis (methylene isothiuronium) chloride in 1 mL NVP: 9 mL H2 O solution presented in Fig. 5 (left) and excited at 347 nm showed also three bands as in in the case of DMSO/H2 O solution without changing the position of the bands in 650–800 nm region. The same situation, in the case at excitation with 682 nm is observed, the 764 nm band does not change its position. In Fig. 5 (right) is shown the fluorescence spectra of [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride compound in H2 O. At excitation wavelength with 347 nm, three emission bands are also revealed at 382 nm, 694 nm and 764 nm. Regardless of the solvent used, the position of the peaks in the Q band does not changing.

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The B band of the [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride in 1 mL NVP: 9 mL H2 O has become wider and slightly shifted to longer wavelengths in comparison with bands position of compound solubilized in DMSO: H2 O and 1NVP:9H2 O solutions. Also, an increase in intensity at 850 nm (close to the measurement limit of the device) when excited with both 347 nm and 638 nm (Fig. 5, right) is observed. The limits of the measurement equipment did not allow us to record fluorescence above 850 nm. The fluorescence quantum yields of the isothiouronium substituted ZnPc are significantly lower than values obtained for unsubstituted compounds. This is indicative that substitution of the electron-donating NH2 + groups goes some way in promoting intersystem crossing from the excited state to triplet state. The fluorescence emission spectra of [phthalocyaninato]zinc octamethanethiol are presented in Fig. 6 (left). The fluorescence spectra of [phthalocyaninato]zinc octamethanethiol presented at excitation wavelengths of 347 nm exhibit only band in shorter wavelengths region situated at 385 nm. Also, an increase in intensity at 850 nm as in the case of [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride derivative solubilized in H2 O is displayed. At excitation with 639 nm, a very slightly emission at 783 nm and an increase in intensity at 850 nm are observed. The [octakis(chloromethyl) phthalocyaninato] zinc does not show fluorescence.

Fig. 5. The fluorescence emission spectra of [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride in 1 mL NVP: 9 mL H2 O (left) and H2 O solutions (right).

Fig. 6. The fluorescence emission spectra (left) and the fluorescence decay curve (right) of [phthalocyaninato]zinc octamethanethiol in DMSO:H2 O solution.

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3.2 Triplet Lifetimes and Quantum Yields The phosphorescence could not be recorded in the compounds analyzed above because there are many deactivation processes that have faster rate constants, probably, such as nonradiative decay and quenching [15–17]. Figure 6 (right) displays the triplet decay curve of [phthalocyaninato]zinc octamethanethiol at excitation with 343…387 nm. The triplet lifetime values of 1.04 μs and 8.6 μs in DMSO: H2O solution are lower than for unsub-stituted ZnPc (350 μs) in DMSO [19], suggesting that the substituents quench the triplet state. The decay of fluorescence emissions of [phthalocyaninato]zinc oktakis (meth-ylene isothiuronium) chloride in DMSO:H2O (left) and 1NVP:9H2O (right) solutions are illustrated in Fig. 7. The fluorescence decay curves for [phthalocyaninato]zinc ok-takis (methylene isothiuronium) chloride at the interval of excitation wavelengths (λexc = 341…703 nm) show a biexponential behaviour with lifetime values being yielded 2.31 μs and 1.23 μs in DMSO: H2O, 1.22 μs and 9.22 μs in NVP: 9H2O solvents.

Fig. 7. Bi-exponential triplet decay profile for [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride in DMSO:H2 O (left) and 1NVP:9H2 O (right) solutions.

The decay curve of phosphorescence of [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride in H2 O are presented in Fig. 8. The decay curves are multiexponential and are represented by three lifetimes τ1 , τ2 , and τ3 . The measured photophysical properties of synthesized compounds (3) are summarized in Table 1. No significant changes were observed for molar extinction coefficient values (ε). The highest values of ε were obtained for [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride (C48 H56 Cl8 N24 S8 Zn) dissolved in 1NVP:9H2 O and water solvents. The small red shift (C48 H56 Cl8 N24 S8 Zn, Table 1) in 1 NVP: 9 H2 O and H2 O solvents compared to DMSO/H2 O indicate that the linked sulfur group are not π-conjugated with the Pc aromatic ring. The values of the triplet excited-state lifetimes are comparable to those of other developed ZnPcs [17], and are the best for C48 H56 Cl8 N24 S8 Zn dissolved in water solvent. So, we have developed a water-soluble compound with the relatively longer lifetimes 1.09 μs 4.96 μs and 15.23 μs, respectively.

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Fig. 8. The decay curve of phosphorescence for [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride in H2 O. Table 1. Comparison of the photophysical properties of ZnPc compounds containing sulfur group. Samples (solvent)

λabs (nm) Q-Band, ε (M−1 cm−1 ))

λem (nm)

C40 H24 Cl8 N8 Zn (DMSO/H2 O)

638 (7.1 × 103 ) 685 (1.2 × 104 )

-

C48 H56 Cl8 N24 S8 Zn (DMSO/H2 O)

638((5.7 × 103 ), 703(7.1 × 103 )

694, 764

C48 H56 Cl8 N24 S8 Zn (1 NVP: 9 H2 O)

639 (1.3 × 104 ) 682(1.4 × 104 )

C48 H56 Cl8 N24 S8 Zn (H2 O) C40 H32 N8 S8 Zn (DMSO/H2O)

X2

τT1 , μs

τT2 , μs

τT3 , μs

-

-

-

1.012

2.31

1.23

-

694,764

0.996

1.22

9.22

-

639 (1.8 × 104 ), 676(1.1 × 104 )

694,764

1.085

1.09

4.96

15.2

638 (7.1 × 103 ), 685 (6.4 × 103 )

385, 783, 850

0.988

1.04

8.6

-

The most suitable parameter values for the given values of triplet lifetimes (τT ) are found at “X2 ” close to unity, with “amplitudes” distributed close to the zero; ε- molar extinction coefficient.

4 Conclusions Herein, we report the development of a [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride and [phthalocyaninato]zinc octamethanethiol compounds based on octakis(cloromethyl) phthalocyanine zinc obtained by the chloromethylation reaction of ZnPc. The presence of isothiuronium group at the end of peripheral substituents of the macrocycle improve, on the one hand, solubility in aqueous media and, on the other hand, could improve their linkage to noble metal nanoparticles. The UV–Vis spectra of [phthalocyaninato]zinc oktakis(methylene isothiuronium) chloride show Q band split in DMSO: H2 O and 1NVP: 9H2 O solvents. The fluorescence spectra of [phthalocyaninato]zinc oktakis (methylene isothiuronium) chloride in all types of solvents showed the same position of the peaks in the 650–800 nm region, while the position of the fluorescence emission band from short wavelengths region changes in 1 mL NVP: 9 mL

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H2 O solution. The photophysical studies reveal that the water-soluble oktakis (methylene isothiuronium-substituted ZnPc derivative exhibit low ground-state absorption but strong triplet excited-state absorption from 400 to 700 nm with a relatively longer triplet excited-state lifetimes compared to compound soluble in DMSO: H2 O and 1NVP: 9H2 O solvents. Acknowledgments. This work was supported by the Ministry of Education, Culture, and Research of Republic of Moldova, research grant 20.80009.5007.16.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Allen, M., Sharman, W., Van Lier, J.: Current status of phthalocyanines in the photodynamic therapy of cancer. J. Porphyrins Phthalocyanines 5, 161–169 (2001) 2. Miretti, M., Prucca, C., Tempesti, T., Baumgartner, M.: Current phthalocyanines delivery systems in photodynamic therapy: an updated review. Curr. Med. Chem. 28, 5339–5367 (2021) 3. Karges, J., Basu, U., Blacque, O., Chao, H., Gasser, G.: Polymeric encapsulation of novel homoleptic bis(dipyrrinato) zinc(II) complexes with long lifetimes for applications as photodynamic therapy photosensitisers. Angew. Chem. Int. Ed. Engl. 58, 14334–14340 (2019) 4. Furuyama, T., et al.: Design, synthesis, and properties of phthalocyanine complexes with main-group elements showing main absorption and fluorescence beyond 1000 nm. J. Am. Chem. Soc. 136, 765–776 (2013) 5. Nemykin, V.N., Lukyanets, E.A.: Synthesis of substituted phthalocyanines. Arkivoc 2010(1), 136–208 (2010) 6. Apostol, P., Bentaleb, A., Rajaoarivelo, M., Clérac, R., Bock, H.: Regiospecific synthesis of tetrasubstituted phthalocyanines and their liquid crystalline order. Dalton Trans. 44, 5569– 5576 (2015) 7. Lukyanets, E., Nemykin, V.: The key role of peripheral substituents in the chemistry of phthalocyanines and their analogs. J. Porphyr. Phthalocyanines 14, 1–40 (2010) 8. Kobayashi, N., Ogata, H., Nonaka, N., Lukyanets, E.A.: Effect of peripheral substitution on the electronic absorption and fluorescence spectra of metal-free and zinc phthalocyanines. Chem. A Eur. J. 9, 5123–5134 (2003) 9. Patent № US2761868A, USA; MPK C09B 47/16. Sulfonated and unsulfonated imidomethyl, carboxyamidomethyl and aminomethyl phthalocyanines: № US354897A: 13.05.1953: published 04.09.1956 / Lacey Harold T. by Wyeth Holdings LLC. 9 p. 10. Włodarczyk, J., Kierdaszuk, B.: Interpretation of fluorescence decays using a power-like model. Biophys. J. 85(1), 589–598 (2003) 11. García, M., et al.: Novel thiol-derivatized Zinc(II) phthalocyanines. Tetrahedron Lett. 50, 2467–2469 (2009) 12. Dilbert, G., et al.: Non-aggregated zwitterionic Zinc(II) phthalocyanine complexes in water with high singlet oxygen quantum yield. Dyes Pigm. 160, 267–284 (2019) 13. Kobayashi, N., Furuyama, T., Satoh, K.: Rationally designed phthalocyanines having their main absorption band beyond 1000 nm. J. Am. Chem. Soc. 133, 19642–19645 (2011)

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14. Idowu, M., et al.: Photophysicochemical and fluorescence quenching studies of tetra- and octa-carboxy substituted silicon and germanium phthalocyanines. J. Photochem. Photobiol., A 204, 63–68 (2009) 15. Farag, A.: Optical absorption studies of copper phthalocyanine thin films. Opt. Laser Technol. 39(4), 728–732 (2007) 16. Palewska, K., et al.: Molecular aggregation in soluble phthalocyanines - chemical interactions vs. π-stacking. Opt. Mater. 34, 1717–1724 (2012) 17. Potlog, T., Popusoi, A., et al.: Photophysics of tetracarboxy-zinc phthalocyanine photosensitizers. RSC Adv. 12, 31778–31785 (2022) 18. Dolotova, O., Kaliya, O.: Synthesis and some physicochemical properties of the aqua complexes of covalent conjugates of platinum (II) with octacarboxy-substituted cobalt phthalocyanine. Russ. J. Coord. Chem. 33, 111–115 (2007)

Effect of Particle Size and Roughness on Contact Angle of ZnTe Thin Films Ion Lungu1

, Simon Busuioc2

, Elena I. Monaico2(B)

, and Tamara Potlog1

1 Laboratory of Organic/Inorganic Materials for Optoelectronics, Moldova State University,

Chisinau, Republic of Moldova 2 National Center for Materials Study and Testing, Technical University of Moldova, Chisinau,

Republic of Moldova [email protected]

Abstract. Zinc telluride (ZnTe) thin films were prepared by close space sublimation method, and their detailed structural properties and wettability characterization were performed. The XRD analysis of all ZnTe thin films revealed the cubic structure, F-43m space group, regardless of the substrate and source temperatures. The vary of the substrate temperature with 10 °C in the interval (320–360) o C, lead to insignificant changes in the value of the crystallite size, from 32 nm to 27 nm. The same trend is also kept by changing the source temperature. The SEM analysis showed changes in the particle size that is directly related to the substrate/source temperatures. The tendency of the contact angle to increase with the increasing in the substrate and source temperatures of the ZnTe was also observed. The same behavior was revealed for the roughness deducted from the AFM measurements and shown that increasing RMS roughness enlarges the surface area, potentially enhancing the hydrophobicity of ZnTe thin films. The contact angle method shows that hydrophobicity of ZnTe thin films is well tailored by changing the substrate and the source temperatures. The increasing of the hydrophobic properties may lead to the increase of the self-cleaning properties of the solar cells elaborated on the basis of ZnTe thin films. Keywords: ZnTe thin film · Grain size · Particle size · Roughness surface · Contact angle · Wettability

1 Introduction A2 B6 materials such as zinc telluride (ZnTe), cadmium telluride (CdTe), cadmium sulfide (CdS) and others have received significant attention due to their low cost and high absorption coefficients in their applications to a variety of solid-state devices. The CdTe absorber has a bandgap of 1.54 eV at 300 K and by adding the Se, the value can be reduced to < 1.4 eV, due to the bandgap bowing effect, which is closer to the optimum bandgap value for single-junction solar cells per the Shockley-Queasier limit [1, 2]. ZnTe thin films are broadly used in manufacturing different solid-state optoelectronic devices e.g., photo detectors, solar cells, light emitting diodes, laser diodes, microwave devices, etc. due to its specific optical and electrical properties (high transparency in visible and infrared regions and low electrical resistivity) [3–6]. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 268–277, 2024. https://doi.org/10.1007/978-3-031-42775-6_30

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Among fabrication techniques, close space sublimation is a versatile technique for fabrication of A2 B6 thin films, which exhibit excellent chemical stability, thermal stability, and optical properties. This method is attractive because it offers high deposition rates, is capable of producing thin films with larger crystallite size than those deposited by other techniques, and can be easily scaled up for manufacturing purposes. Recently, we presented the physicochemical features and structural properties in function of substrate/source temperatures in the wide bandgap ZnTe polycrystalline thin films [7]. Recently, the technological approaches aiming controlling the wetting properties of GaAs crystals via anodization and metal deposition were reported [8, 9]. Reliable values of the contact angle for a chemically homogeneous ZnTe surface are not reported in the literature. The ZnTe thin films with controllable wettability, in particular superhydrophobic ones, can be developed with the knowledge of the contact angle method. For this purpose, in this paper the relationship between morphological and surface properties of ZnTe thin films and wetting is investigated.

2 Experimental Details The polycrystalline ZnTe thin films were obtained by the closed space sublimation (CSS) method on glass substrate, reported in details in our recent paper [7]. Briefly, the ZnTe powder of 99.99% purity acquired from Merck Company was used as the source material. In order to determine the optimal technological conditions for the growth of ZnTe thin films, two sets of samples were prepared. First set was prepared in function of the substrate temperature (T sub ) from 320 °C up to 360 °C, maintaining source temperature (T S ) constant at 580 °C and for the second set, the T S was varied from 560 °C to 600 °C, while T sub = 340 °C was maintain constant. The deposition time was the same (6 min) for both sets. The structure of deposited films was investigated using a Rigaku X-ray diffractometer with CuKα radiation (λ = 1.54056 Å), Ni filter. The X-ray diffraction (XRD) analysis was performed using Rigaku software PDXL. The surface topology was investigated by means of Atomic Force Microscope (AFM) NanoStation II from Surface Image System (SIS) operated in non-contact mode with Si tip. The AFM images were scanned of the film’s surfaces with sizes of 51 μm × 51 μm using Ultra Objective SIScanPanel and analyzed in Gwyddion software. The interaction behavior between a 3 μL volume of distilled water drops and deposited ZnTe film surface was evaluated using the KRUSS DSA25 (drop shape analyzer) equipped with a camera. The sample was illuminated from behind so that the shadow contour of the substrate-drop contact was visible and well highlighted to allow the software to determine the contact angle. The measurements were performed at 23 °C. The morphology from the top view and cross-section view of deposited ZnTe films was investigated using TESCAN Vega TS 5130 MM scanning electron microscope (SEM), with a thin gold sputtered film on the surface and contacting the sample with the silver paste to the ground, to prevent the charge accumulation due to the glass substrate.

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3 Results and Discussions 3.1 X-Ray Diffraction Analysis The X-ray diffraction analysis for ZnTe thin films in function of T sub (left) and T S (right) exhibit a polycrystalline structure with preferential orientation along the (111) plane. All diffraction peaks fit well with JCPDS data cards # 00–015-0746 [10]. The microstructural parameters (crystallite size (D), lattice strain (ε), interplanar distance (d) were estimated from XRD diffractograms reported in our recent work [7]. The crystallite size and lattice stress were calculated by Scherrer’s equation and Williamson-Hall method [11–13]. The Williamson-Hall relation assumes a uniform deformation model according to which the strain is uniform in all crystallographic directions. Williamson-Hall analysis, derived from Bragg’s equation (D = kλ/βhkl cosθ, where k = 0.94, θ-diffraction angle, λ = 1.54056 Å, β represent full width at half-maximum (FWHM) of (hkl) XRD peak corrected by instrumental broadening), is a simplified integral breadth method that can distinguish between size and strain related peak broadening [14]. This is due to the crystallite size depending on 1/cos θ, while the strain varies as tan θ. The change in the temperature of the substrate does not change grain size being about 27 nm. The intensities of ZnTe peaks were enhanced with increasing source temperature from 560 °C to 600 °C, this may be due to higher surface kinetic energies produced by higher deposition temperature. Changing the value of T S also leads to a slight increase in d as in the case of T sub , but this change is more pronounced, with values increasing from 3.5226 Å (T S = 560 °C) to 3.5250 Å (T S = 600 °C). The same trend as in the substrate temperature, the temperature of the source weakly changes grain size. The change of the T sub with 10 °C slightly changes the value of the crystallite size, being in the interval from 32 nm to 27 nm. Figure 1 presents microstructure parameters of crystallite size (D) and microstrain (ε) of the ZnTe films in function of different T sub and T S deposited on glass substrates.

Fig. 1. Microstructural parameters D and ε of ZnTe thin films in function of T sub (left) and T S (right).

It is observed that the crystallite size and microstrain have the same trends with the increase of the substrate temperatures (Fig. 1, left). On the other hand, the microstrain exhibited an opposite behavior with the increasing of the source temperature, i.e., it decreases with increasing T S . Such a decrease in microstrain may be due to the decrease in lattice defects among the grain boundaries with the grain size increasing. It can be

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concluded that XRD studies showed that the ZnTe films are polycrystalline in nature with the ZnTe crystallites having the zinc blende structure. 3.2 Particle Size and Contact Angle Particle size measured by means of SEM demonstrated a conglomeration of small crystallites into a single large particle and occasionally it is difficult to resolve individual crystallites in SEM. As can be seen from the SEM images presented in Fig. 2, the deposited ZnTe films consists from uniformly distributed nanoparticles.

Fig. 2. Top view SEM images of deposited ZnTe thin film surfaces obtained at various T sub and TS.

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It is obvious that the particle size is directly related to the substrate/source temperature. According to Fig. 2 and Table 1, the SEM analysis showed changes in the particle size with the variation of Tsub of the ZnTe thin films. With the increase of T S from 560 °C to 600 °C, the regularity of the increase in the thickness of the films from 0.738 μm to 1.08 μm is observed. The XRD calculates crystallite size, while SEM shows physical particles, so we can see the mismatch in measured particle size. The coalescence effect is more prominent in the ZnTe thin film deposited at 600 °C, and a larger particle size can be observed compared to the ZnTe thin film grown at 560 °C. It can be seen from Fig. 2 that variation in T sub does not lead to essential changes in the particle size. In the case of T S varying from 560 to 600 °C the particle sizes increase from 0.738 μm to 1.083 μm. Also, the measured contact angles for different samples are given in Table 1. It was observed that the contact angle increases with an increase in particle size and for a larger size of about 1.083 μm the contact angle value reaches 137.6°, accordingly to the images for deposited water droplets on the ZnTe surface, shown in Fig. 3. The surface energy may be one of the reasons for the variation between the contact angle and the particle size. The tendency of the contact angle measurements to increase was observed also with the increase in the substrate temperature of the ZnTe film at Ts = 580 °C. The same situation is observed when the source temperature of the ZnTe film increased and at Tsub = 340 °C is kept constant. As a result of these investigations, the wettability studies in function of the substrate and source temperatures indicates to the high hydrophobicity of the deposited ZnTe thin films. Table 1. The systematized structural and morphological parameters, and contact angle measurements in function of the deposition temperatures of ZnTe thin films. TSub , °C

TS , °C

SEM Diameter, nm

Contact angle, °

AFM Sq, nm

Sa, nm

Ssk

320

580

845

125.6

91.94

77.60

−0.1765

330

580

961

127.4

83.84

68.19

0.0061

340

580

968

134.0

96.89

79.61

0.0096

350

580

845

138.2

82.30

67.00

−268.80

360

580

926

140.0

84.8

0.0206

340

560

738

134.5

91.60

75.22

−0.0908

340

570

837

138.1

95.90

78.06

−229.70

340

580

920

138.5

85.9

0.0126

340

590

997

136.6

68.39

−0.0370

340

600

1083

137.6

83.4

−0.0875

104.0

107.0 85.36 104.2

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3.3 The Relation Between Roughness and Contact Angle Figures 3 and 4 shows the AFM topography of Zne thin film surfaces deposited at different T sub and T S of each parameter value on 51 μm × 51 μm surface scans, while their surface parameters are shown in Table 1. It can be easily observed that surface roughness (S a ) tends to decrease as the film thickness increases. The same situation is observed for the mean square roughness (S q ). In particular, an overall positive skewness (S sk ) with values close to zero indicates the presence of a very limited number of above-average height values in the case of T sub variation from 320 °C to 360 °C. In case of T S variation, a positive Ssk value is deduced only for 580 °C. For the other TS , skewness values indicating more valleys than peaks on the ZnTe surface, as a result, it can be concluded that the surface morphology of the deposited films is influenced by both, the substrate and source temperatures. The systematization of dependence of contact angle for ZnTe thin film surfaces obtained at various T sub and T S are shown in Fig. 5. It is observed an increase in the contact angle with the increasing the substrate temperature. At the same time, the roughness (S a ) deducted from the AFM measurements, also exhibited an increase in the value, with some fluctuations at the increase of the substrate’s temperature. The same behavior can be observed also in the case of the increase of the source temperature, the valley spacing between the ZnTe particles became narrower with a more uniform distribution. There are two possible reasons to explain the increase in the contact angle as roughness effect by the Wenzel theory [15] and/or air space effect by the Cassie– Baxter theory [16]. According to the Wenzel equation, if the surface roughness scale is smaller than the size of the droplet, the contact angle of the droplet on the surface is cos θ = Sa cos θ0 where θ is the contact angle, θ0 is the contact angle on a flat surface (without roughness), and Ra is the roughness factor: Ra = ASL /AF, where ASL is the solid–liquid interface area and AF is the projection of ASL on the flat surface [15]. A higher surface roughness (S a ) corresponds to a higher contact angle. According to the Cassie–Baxter theory, when the droplet does not entirely penetrate the valley of the rough hydrophobic surface, an air space can be formed between the liquid and solid surfaces, which increases the contact angle [17]. The results of the investigations, as well as the Cassie–Baxter theory, allow us to assume that in the case of deposition of ZnTe thin films at different parameters of the substrate temperature and evaporation, the hydrophobic behavior is related to the air spaces between the nanoparticles. Figures 3 and 4 show that the ZnTe films exhibited relatively high contact angle. A higher surface roughness corresponds to a higher contact angle, means that Wenzel effect is a dominant factor for the increase in the contact angle, owing to the hydrophobicity of the ZnTe surface. As a result, a rougher surface would contain more air, which would prevent the expansion of a water droplet at the surface more effectively, and hence reinforce the hydrophobicity of film surface. Consequently, the AFM results revealed that increased RMS roughness enlarges the surface area, potentially enhancing the hydrophobicity. From the values of the contact angle and the SEM images, we can observe that the value of contact angle is directly correlated with the surface structure of the deposited ZnTe film. It was recently reported that the control the hydrophobic properties will give

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Fig. 3. AFM topography and contact angle measurements of deposited ZnTe thin films obtained at various T sub .

the possibilities to increase the self-cleaning capacity of the solar cells, providing high transmittance for energy conversion [18–20].

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Fig. 4. AFM topography and contact angle measurements of deposited ZnTe thin films obtained at various T S .

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Fig. 5. The contact angle measurements for ZnTe thin films obtained at various T sub (left) and T S (right).

4 Conclusions Zinc telluride thin films with various thicknesses were deposited by close space sublimation method on glass substrates. The structure was found to be cubic with preferential orientation along a (111) diffraction plane and crystallite size dependent of the substrate/source temperatures. Characteristic morphological and surface parameter values varied with changing substrate/source temperatures, and the surface texture evolved with the thickness increase. The values of the contact angle and the SEM images indicate that the value of contact angle is directly correlated with the surface structure of the deposited ZnTe film. A higher surface roughness corresponds to a higher contact angle. An increase in the contact angle from 125.6 º to 140 º with the increasing the substrate temperature from 320 ºC to 360 ºC was observed. At the same time, the roughness (S a ) deducted from the AFM measurements, also exhibited an increase in the value from 77.60 nm to 84.80 nm with the increasing in the contact angle from 125.6 º to 140 º. So, a higher surface roughness corresponds to a higher contact angle, owing to the hydrophobicity of the ZnTe surface. Acknowledgments. This research was funded by National Agency for Research and Development of Moldova under the Grant 20.80009.5007.16.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Fiducia, T.A.M., et al.: Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells. Nat. Energy 4, 504–511 (2019). https://doi.org/10.1038/s41560-019-0389-z 2. Wolf, H., et al.: Doping of ZnSe, ZnTe, and CdTe with group V elements. Mater. Sci. Forum 196–201, 309–314 (1995). https://doi.org/10.4028/www.scientific.net/MSF.196-201.309 3. Baron, T., Saminadayar, K., Magnea, N.: Nitrogen doping of Te-based II-VI compounds during growth by molecular beam epitaxy. J. Appl. Phys. 83, 1354–1370 (1998). https://doi. org/10.1063/1.366838

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4. Soundararajan, D., Mangalaraj, D., Nataraj, D., Dorosinskii, L., Kim, K.H.: Magnetic properties of Cr doped ZnTe alloy powder. Mater. Lett. 87, 113–116 (2012). https://doi.org/10. 1016/j.matlet.2012.07.042 5. Späth, B., Fritsche, J., Klein, A., Jaegermann, W.: P-ZnTe for back contacts to CdTe thin film solar cells. MRS Online Proc. Libr. 865, 83 (2005). https://doi.org/10.1557/PROC-865-F8.3 6. Potlog, T., et al.: The performance of thin film solar cells employing photovoltaic ZnSe/CdTe, CdS/CdTe and ZnTe/CdTe heterojunctions. In: Proceedings of the 2011 37th IEEE Photovoltaic Specialists Conference, pp. 001365–001370. 19–24 June 2011 https://doi.org/10.1109/ PVSC.2011.6186211 7. Lungu, I., Zalamai, V.V., Monaico, E.I., Ghimpu, L., Potlog, T.: Effect of deposition temperature on structural, morphological and optical properties of ZnTe thin films. J. Mater. Sci. 58, 4384–4398 (2023). https://doi.org/10.1007/s10853-023-08285-x 8. Monaico, E.V., Busuioc, S., Tiginyanu, I.M.: Controlling the degree of hydrophilicity/hydrophobicity of semiconductor surfaces via porosification and metal deposition. In: Proceedings of the 5th International Conference on Nanotechnologies and Biomedical Engineering. Tiginyanu, I., Sontea, V., Railean, S. (eds.) pp. 62–69. Springer International Publishing, Cham (2022). https://doi.org/10.1007/978-3-030-92328-0_9 9. Monaico, E.V., Monaico, E.I., Ursaki, V.V., Tiginyanu, I.M.: Porous semiconductor compounds with engineered morphology as a platform for various applications. Phys. Status Solidi (RRL) 2300039 (2023). https://doi.org/10.1002/pssr.202300039 10. Venkatachalam, S., Mangalaraj, D., Narayandass, Sa.K., Kim, K., Yi, J.: Structure optical and electrical properties of ZnSe thin films. Phys. B: Condens. Matter 358, 27–35 (2005). https:// doi.org/10.1016/j.physb.2004.12.022 11. Wagner, C.D., Riggs, W.M., Davis, L.E., Moulder, J.F., Muilenberg, G.E.: Handbook of XPS. Perkin Elmer Corporation, Eden Prairie, MN, USA (1979) 12. Langer, D.W., Vesely, C.J.: Electronic core levels of zinc chalcogenides. Phys. Rev. B 2, 4885–4892 (1970). https://doi.org/10.1103/PhysRevB.2.4885 13. Wang, W., Xia, G., Zheng, J., Feng, L., Hao, R.: Study of polycrystalline ZnTe (ZnTe:Cu) thin films for photovoltaic cells. J. Mater. Sci.: Mater. Electron 18, 427–431 (2007). https:// doi.org/10.1007/s10854-006-9044-0 14. Bozzini, B., Lenardi, C., Lovergine, N.: Electrodeposition of stoichiometric polycrystalline ZnTe on N+-GaAs and Ni–P. Mater. Chem. Phys. 66, 219–228 (2000). https://doi.org/10. 1016/S0254-0584(00)00339-4 15. Wenzel, R.N.: Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 28, 988–994 (1936). https://doi.org/10.1021/ie50320a024 16. Jung, Y.C., Bhushan, B.: Contact angle, adhesion and friction properties of micro-and nanopatterned polymers for superhydrophobicity. Nanotechnology 17, 4970 (2006). https://doi.org/ 10.1088/0957-4484/17/19/033 17. Ebert, D., Bhushan, B.: Wear-resistant rose petal-effect surfaces with superhydrophobicity and high droplet adhesion using hydrophobic and hydrophilic nanoparticles. J. Colloid Interface Sci. 384, 182–188 (2012). https://doi.org/10.1016/j.jcis.2012.06.070 18. Baquedano, E., Torné, L., Caño, P., Postigo, P.A.: Increased efficiency of solar cells protected by hydrophobic and hydrophilic anti-reflecting nanostructured glasses. Nanomaterials 7, 437 (2017). https://doi.org/10.3390/nano7120437 19. Ferrari, M., Piccardo, P., Vernet, J., Cirisano, F.: High transmittance superhydrophobic coatings with durable self-cleaning properties. Coatings 11, 493 (2021). https://doi.org/10.3390/ coatings11050493 20. Ferrari, M., Cirisano, F.: High transmittance and highly amphiphobic coatings for environmental protection of solar panels. Adv. Coll. Interface Sci. 286, 102309 (2020). https://doi. org/10.1016/j.cis.2020.102309

Patterning Nanoelectronic Devices Using Field Emission Scanning Electron Microscope Adrian Dinescu1(B) , Mircea Dragoman1 , Andrei Avram1 and Daniela Dragoman2

,

1 IMT Bucharest, Bucharest, Romania

[email protected] 2 Faculty of Physics, University of Bucharest, Bucharest, Romania

Abstract. Many advances in fabrication processes at micro and nanoscale in the past two decades were possible due to scanning electron microscopy, which is now an indispensable tool for analyzing and fabricating new nanostructures and nanomaterials. The development of very efficient in-lens detectors for SEM and the capability to use low energy electron probes are the gateway to the revelation of new features and new properties of nanomaterials that have been hidden by the use of high accelerating voltages and large interaction of volume, in the high-resolution SEM. Electron beams have been used for lithography for decades and pattern generators can be fitted to all modern SEMs, converting them in very powerful nanolithographic tools, without degrading or limiting their imaging capabilities. The SEM became a very versatile tool for micro and nanofabrication, the same equipment used for fabrication being used to view the resulting nanostructures. Electron Beam Lithography (EBL) is one of the highest resolution lithographic technologies and a key technique for fabrication of nanoelectronic devices, allowing direct patterning of structures with critical dimensions down to 10 nm [1]. Apart of resolution, a very important point of EBL is that it can be easily implemented in a research laboratory by converting a scanning electron microscope (SEM) for lithography with an external pattern generator. To illustrate the patterning capabilities of electron microscopy, in this review we describe the EBL based fabrication processes of some nano-devices like field effect transistors on graphene and other 2D materials. Keywords: Nanoelectronic devices · Patterning · Electron microscope

1 Introduction There are four main techniques used in the micro and nano-fabrication of electronic devices: thin film deposition, lithography, etching and doping (ion implantation or diffusion). Very high-resolution lithography (like EBL or extreme UV) is possibly the most important techniques in the nano-fabrication of a large variety of structures. Focused electron beams have been used for more than fifty years for patterning of microelectronic devices [2]. From the very beginning the scanning electron microscope was used as the source for the electron beam, and the developments in the SEM technology (field © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 278–283, 2024. https://doi.org/10.1007/978-3-031-42775-6_31

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emission guns, capability to work at low accelerating voltages and digital acquisition of images to name just a few) pushed the e-beam lithography performances further, in terms of resolution, throughput and versatility. There are many dedicated electron beam based writing systems with very high performances but when research applications are the primary use the converted SEM has some advantages over them: the initial cost, easier maintenance and imaging capabilities. All patterning processes described in this work were done with an e-beam system based on Zeiss Gemini electron column and featuring a field emission electron gun with accelerating voltages from 100 V to 30 kV. The beam current can be variated in the range from 10 pA to 4.5 nA, in six steps, using six apertures: 7.5, 10, 20, 30, 60 and 120 µm diameter. There are two electron detectors: an in chamber secondary electron detector, useful for topographical images and slightly insulating specimens and an in-lens detector, more sensitive to backscattered electrons and able to provide an excellent compositional contrast for samples of high interest in EBL, like silicon wafers covered with patterned electron resist. The pattern generator imposes a writing speed up to 20 MHz, and the sample stage is controlled by a laser interferometer, allowing 40 nm stitching and overlay accuracy. The basic EBL process involves: electron resist spinning on the substrate, electron beam irradiation (exposure) and development. Then the pattern can be transferred on the substrate by etching - reactive ion etching (RIE), ion milling, etc., or by thin film deposition and lift-off (Fig. 1). Depending on the application, a positive electron resist, like PMMA or negative resist like HSQ is used.

Fig. 1. Basic EBL process technology

In order to facilitate a reliable lift-off, process the thin films should be deposited by evaporation in a highly directional e-beam evaporation system.

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2 Nano-fabrication of Nanotransistors on 2D Materials In the last 15 years electronic devices based on 2D materials have been widely investigated. Among them, field effect transistors (FETs) induced a special interest in research labs and industry. However, after years of focused research on 2D based transistors, important issues preventing their integration in solid-state microelectronic devices are still to be solved [4]. In the next few examples, our own methods developed to fabricate FETs with 2D materials are presented. Among the first FETs on graphene fabricated in our laboratory was a ballistic transistor with oblique top gate [5]. It is presented in Fig. 2, where the source, drain, gate and channel are clearly denoted. The transistor was fabricated on CVD graphene, transferred on a 4-inch silicon wafer covered with silicon oxide (285 nm thickness). Register marks were fabricated in the first step on the whole wafer in order to assure the overlay accuracy for the following processes. The marks were patterned by e-beam lithography, then a metal layer (Ti/Au – 5/50 nm) was evaporated. In the second step the graphene channel was shaped by EBL patterning and RIE etching in oxygen plasma. The electrical contacts for the source and drain were created using the same processes used for the alignment marks, but the short length of the channel (200–400 nm) imposed two patterning steps, one for the inner part of the S-D contacts and one for the pads in order to reduce the exposure/structure time to a reasonable value.

Fig. 2. SEM micrograph of the FET based on a graphene monolayer, with an oblique gate.

PMMA 950k A4 was the electron resist of choice for all steps. It was developed in a mixture of MIBK/IPA (1/3) and before metal deposition the samples were subjected to a few seconds oxygen plasma descum in order to remove all resist residues. Hydrogen silsesquioxane (HSQ) as the insulating layer for the gate. It was spin coated at 3000 rpm, backed at 250 °C for two minutes on a hotplate and then exposed and developed in a mixture of tetramethylammonium hydroxide and DI water (2.3%). The resulting thickness of cross-linked HSQ layer was 50 nm. Then a very high overlay accuracy was

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required in order to fabricate the gate electrode (40 nm width) on top of the gate, by e-beam patterning, metal deposition (Ti/Au – 5/40 nm) and liftoff. An overview of the entire fabrication process is presented in Fig. 3

Fig. 3. Optical microscope images of various nano-fabrication steps

The shape and the inner geometry of the graphene channel proved to be the key factor for getting a bandgap in graphene. We reported field effect transistors with perforated channel showing very high on-off ratio is due to bandgap opening by nanoperforations [6]. Such a channel is depicted in Fig. 4a, the whole transistor being presented in Fig. 4b. The holes have a diameter of 20 nm and a period of 100 nm, the lengths of the channel being of 2 µm. Using the same nano-fabrication methods an electrostatic superlattice consisting of an array of tilted metallic electrodes deposited on a graphene monolayer was produced in order to show the formation of minibands at room temperature, negative differential resistance and the evidence of Bloch oscillations [7]. Such a multiple gate FET on graphene is presented in Fig. 5.

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Fig. 4. SEM micrographs of a graphene FET with nano-perforated channel: (a) detail of the channel, (b) the whole transistor structure

Fig. 5. Image of the superlattice FET. The inset shows the grating area with 10 electrodes.

Apart of graphene, other two-dimensional materials like molybdenum disulfide (MoS2 ) and tin sulfide (SnS) have been attracting a great research interests. We fabricated self-switching diodes (SSD) on a two dimensional few-layer MoS2 thin film grown on Al2 O3 layer deposited on silicon [8]. All the fabrication steps were similar with those used for graphene based devices, excepting the RIE of molybdenum disulfide layer that was done with argon milling. On 10 nm thin tin sulfide we fabricated for the first time and with high reproducibility a field-effect-transistor with subthreshold slope below 60 mV/decade [9].

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3 Conclusions The e-beam lithography based on converted scanning electron microscope has been used for successful fabrication of a series of nano devices like field effect transistors on graphene, molibdenum disulfide and tin sulfide. EBL is a complex technique with many process parameters that can affect the functionality of fabricated devices and the reliability of the process. However, it proved to be a versatile technique that can be adapted for the fabrication of almost any nanoelectronic devices. There are no limitations for the substrates as long as they are reasonably flat. Combined with RIE or ion milling, it can shape any 2D material at the nanoscale and combined with metal deposition and lift-off it can deposit an unlimited variety of electrical contact. The scanning electron microscope is a very versatile research tool, both for imaging and patterning at the nanoscale, showing an extreme importance in the field of micro and nano-fabrication. Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Rai-Choudhury, P. (Ed.): Handbook of Microlithography, Micromachining, and Microfabrication: Microlithography, Vol. 001. SPIE Press, June 1997. https://doi.org/10.1117/3.226 5070 2. Broers, A.: Combined electron and ion beam processes for microelectronics. Elec. Reliab. 4, 103. Pergamon Press (1965). https://doi.org/10.1016/0026-2714(65)90267-2 3. Dragoman, M., Dinescu, A., Dragoman, D.: 2D Materials nanoelectronics: new concepts. Fabrication, characterization from microwaves up to optical spectrum. Phys. Stat. Sol. (A), 216(8) (2019). https://doi.org/10.1002/pssa.201800724 4. Jing, X., et al.: Engineering field effect transistors with 2D semiconducting channels: status and prospects. Adv. Funct. Mater. 30(18) (2020). https://doi.org/10.1002/adfm.201901971 5. Dragoman, M., Dinescu, A., Dragoman, D.: Negative differential resistance in graphene based ballistic field-effect transistor with oblique top gate. Nanotechnology 25 (2014). https://doi. org/10.1088/0957-4484/25/41/415201 6. Dragoman, M., Dinescu, A., Dragoman, D.: Solving the graphene electronics conundrum: high mobility and high on-off ratio in graphene nanopatterned transistors. Physica E Lowdimensional Syst. Nanostruct. 97 (2018). https://doi.org/10.1016/j.physe.2017.12.011 7. Dragoman, M., Dinescu, A., Dragoman, D., Comanescu, F.: Bloch oscillations at room temperature in graphene/h-BN electrostatic superlattices, Nanotechnology 32(8) (2021). https:// doi.org/10.1088/1361-6528/ac02e6 8. Dragoman, M., et al.: Multifunctionalities of 2D MoS2 self-switching diode as memristor and photodetector. Physica E Low-dimensional Syst. Nanostruct. 126, February 2021. https://doi. org/10.1016/j.physe.2020.114451 9. Dragoman, M., et al.: Ultrathin tin sulfide field-effect transistors with subthreshold slope below 60 mV/ decade. Nanotechnology 33 (2022). https://doi.org/10.1088/1361-6528/ac7cf8

Controlling Hydrophobic/Hydrophilic Properties of ZnO Microtetrapods Structures by Means of Thermal Treatment Vladimir Ciobanu1 , Veaceslav V. Ursaki1,2 , Armin Reimers3 , Geanina Mihai4 , Victor V. Zalamai1 , Eduard V. Monaico1(B) , Rainer Adelung3 , Marius Enachescu4,5 , and Ion M. Tiginyanu1,2 1 National Center for Materials Study and Testing, Technical University of Moldova, Chisinau,

Republic of Moldova [email protected] 2 Academy of Sciences of Moldova, Chisinau, Republic of Moldova 3 Christian-Albrechts-Universität zu Kiel, Kiel, Germany 4 Center for Surface Science and Nanotechnology, University Polytechnica of Bucharest, Bucharest, Romania 5 Academy of Romanian Scientists, 050094 Bucharest, Romania

Abstract. We report on possibilities to convert, by means of thermal treatment, the wettability properties of networks consisting of ZnO microtetrapods from hydrophobic to super-hydrophilic. The ZnO microtetrapods were produced by flame transport synthesis. The ZnO powder containing the ZnO tetrapods were pressed in pellets with the density of 1 g cm−3 using a compression mold. The comparative study was performed on two sets of samples, and namely: the asgrown ZnO tetrapods pressed in pellets and the annealed pellets. The wettability conversion proved to be an irreversible process for a long period. As a result, the thermal treatment process not only increase the mechanical stability of the ZnO pellets but also essentially increase the hydrophilic behavior of ZnO tetrapods, which is a very important issue for further chemical or electrochemical functionalization. Apart from wettability characteristics investigated by Water Contact Angle (WCA) measurements, the structural and optical properties were investigated by X-ray diffraction (XRD) and photoluminescence (PL) techniques, respectively. The XRD patterns revealed the hexagonal wurtzite structure and a high structural quality of both as-grown samples and annealed networks of microtetrapods at 950 °C. Their high quality was also confirmed by the presence of PL bands related to exciton recombination in the emission spectrum. The possible nature of other PL bands, especially green emission band attributed to specific recombination channels and their evolution with thermal treatment are discussed. Keywords: ZnO microtetrapods · Flame transport synthesis · Thermal treatment · Scanning electron microscopy · Photoluminescence · Hydrophobic and hydrophilic properties

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 284–292, 2024. https://doi.org/10.1007/978-3-031-42775-6_32

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1 Introduction The control of hydrophobic and hydrophilic properties of materials is important for biological and microfluidic applications. A nanomaterial formed from self-assembled GaN tetrapods exhibiting both hydrophilic wetting and hydrophobic dewetting properties, named aerogalnite (aero-GaN), has been demonstrated for applications in microfluidic devices and microrobotics [1]. Waterproof rafts carrying liquid droplets much heavier than the raft itself have been demonstrated with such a material. ZnS hollow micro-tetrapod structures with nanoscale thin walls, called aero-ZnS, proved also to be hydrophilic under tension and hydrophobic when compressed against water, which opened perspectives for micro-fluidic applications [2]. These materials have been prepared on the basis of ZnO micro-tetrapodal networks used as a sacrificial material. Networks of ZnO tetrapods produced by a cost effective and high throughput method of flame transport synthesis (FTS) [3], have been also used as sacrificial templates for the preparation of other aero-materials, such as aero-Ga2 O3 [4], aero-graphite [5], aero-BN [6] and aero-TiO2 [7], which have been applied in photocatalysis or optical diffusers. Recently it was reported that the wetting properties of bulk and porous semiconductor materials can be controlled via electrochemical treatment, and namely electrochemical etching and pulsed electrochemical deposition [8, 9]. It was demonstrated that wetting properties is strongly influenced by the morphology and the degree of porosity of semiconductor material. The decoration of the porous GaAs surface with gold nanodots, resulted in an increase of the contact angle from 33.6° up to 117.6° [9]. The possibilities to control the wetting properties, especially for highly porous materials is a very important issue that will give the opportunities for further chemical or electrochemical functionalization of the prepared aero-materials. The goal of this paper is to demonstrate that the wettability properties of ZnO microtetrapod networks used for the preparation of the mentioned variety of aero-materials can also be tailored by a simple procedure of annealing in air.

2 Methods and Materials The ZnO tetrapods used in the experiments were obtained by means of the mentioned above FTS approach [3]. The ZnO powder containing tetrapods of 10–50 μm length of arms were pressed in pellets with the density of 1 g cm−3 using a specially elaborated compression mold, for further processing. The annealing of the samples was conducted at 950 °C for 1 h in air conditions. The morphology analysis was performed with a VEGA TESCAN 5130 SEM instrument equipped with an EDX detector from Oxford Instruments. The structural properties were investigated by X-ray diffraction (XRD) measurements performed using a Rigaku SmartLab X-Ray Diffractometer with Kα1 = 1.540598 Å, in a standard 2θ Bragg–Brentano configuration, operating at 45 kV beam voltage and 200 mA beam current. The photoluminescence (PL) spectra were measured using a MDR2 monochromator with aperture 1:2 and linear dispersion 0.7 nm/mm under excitation by the 325 nm line of He-Cd laser. The emission was analyzed in a quasi-backscattering geometry. The signal

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from a FEU-100 photomultiplier with an SbKNaCs photocathode was amplified by a lock-in amplifier and introduced into a PC via a GPIB interface for further processing. For low temperature measurement, a helium-closed optical cryogenic system LTS-22 C 330 was used. The working volume of the cryostat was evacuated using a T-STATION 85 oil-free vacuum station. The Water Contact Angle (WCA) measurements were performed with a 3 μL volume of distilled water drops at room temperature (T = 22 °C) using a Kruss DSA25 Drop Shape Analyzer equipped with a digital camera. The software used for analysis was Kruss ADVANCE 1.9, where the Young-Laplace fitting method was applied for CA determination.

3 Results and Discussions 3.1 Morphology Study of the ZnO Microtetrapods The SEM images of the ZnO microtetrapods are shown in Fig. 1. The length of the tetrapods arms vary in the range of 10 to 50 μm, while their diameter is in the range of 2 to 6 μm. The microtetrapods consist of a core, from which the arms are growing. Their growth mechanism has been previously discussed in a series of papers [10–14]. One can observe that the arms have a hexagonal cross section, indicating on the wurtzite structure, which is further confirmed by the XRD analysis.

Fig. 1. SEM images of ZnO microtetrapods obtained by FTS approach.

3.2 XRD Measurements The wurtzite structure of the ZnO tetrapods is confirmed by the XRD pattern presented in Fig. 2. All the observed lines are indexed to the ZnO wurtzite crystalline structure with the space group P63mc , according to the PDF Card No. 2300112. The high crystalline quality of both as grown and annealed structures is indicated by very narrow lines, and there are no significant differences between these two patterns.

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Fig. 2. XRD pattern of as grown ZnO tetrapods (a), and of tetrapods annealed at 950 °C in air during 1 h (b).

3.3 Photoluminescence Study The PL spectra of as grown and annealed ZnO microtetrapod structures are presented in Fig. 3. The emission consists of two bands related to near-bandgap recombination of the carriers at around 3.2 eV and a green emission band centered around 2.5 eV.

Fig. 3. PL spectra of ZnO microtetrapods measured at room temperature from as grown structures (curve 1), and from structures annealed at 950 °C in air (curve 2).

One can see that annealing leads to the decrease of the luminescence efficiency, especially of the near-bandgap emission band. One may suggest that this luminescence quenching is due to the diffusion to the bulk of ZnO tetrapods of some impurities, present at the surface of the as prepared tetrapods, during the annealing at high enough

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temperature of 950 °C for 1 h, which leads to the emergence of some non-radiative recombination channels. Various recombination channels have been proposed as origin of the green PL band, the most probable one being associated with VO oxygen vacancies or Cu impurity [15]. However, VZn Zinc vacancies [16], or surface defects [17, 18] have been proposed as an origin of the green luminescence in ZnO. The origin of the near-bandgap emission is better to be discussed in relation with spectra measured at low temperature. The analysis of PL spectra measured in air and in vacuum (Fig. 4) suggests that the origin of the green luminescence band is associated with effects on the surface of the prepared samples, both as-grown and annealed. However, these effects are more significant in as-grown samples, since the decrease of the intensity of the PL band at 2.5 eV is stronger in the as-grown samples. It was previously found that the prepared ZnO nanoparticles may have residual intermediate compounds on the surface in the form of various chemical groups, which acts as defect centers for the emission of green luminescence [18]. These compounds could be evaporated from the nanoparticles surfaces during evacuation, while their nature could be changed upon annealing.

Fig. 4. PL spectra of as grown (a) and annealed (b) ZnO microtetrapods measured in air (curve 1) and in vacuum (curve 2).

As concerns the near bandgap emission measured at low temperature, one can see that it consists of a series of PL bands (Fig. 5), and the spectra are nearly identical for the as grown and annealed sample. The spectra are dominated by a PL band situated at around 3.31 eV with its LO-phonon replicas with phonon energy around 72 meV. The nature of this PL band at around 3.3 eV was previously analyzed, and it was suggested that it is most probably associated with radiative recombination of free carriers via donor– acceptor pairs (DAP) [19, 20]. The two higher energy PL bands at around 3.37 eV and around 3.36 eV are related to a neutral donor bound excitons D0 X, previously assigned as I1 and I8 , respectively, the I8 line being associated with the Ga impurity [21]. The presence in the spectrum of PL bands related to excitonic radiation suggests that the quality is high enough for both the as-grown and annealed samples.

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Fig. 5. PL spectra of as-grown (a) and annealed (b) ZnO microtetrapods measured at low temperature (10 K).

3.4 Contact Angle Measurements In order to investigate the wettability property of the ZnO microtetrapods, pellets with the material density of 1 g cm−3 were prepared. As it can be seen in Fig. 6a, the sample with the non-annealed tetrapods demonstrates hydrophobic behavior and shows a contact angle of about 145°. The high hydrophobicity of the sample is also demonstrated by the nearly spherical shape of the water droplet on the surface of the sample shown in Fig. 6b. The sample with tetrapods annealed at 950 °C becomes super-hydrophilic, as shown in Fig. 6c, which demonstrates that the ZnO as-grown microtetrapods are floating on the water surface, while the tetrapods are flooding after the thermal treatment. Note that the measurement of the contact angle becomes impossible after annealing, since the water droplet is immediately infiltrated into the ZnO microtetrapods pellet. The reasons for wettability change might be related to chemical composition change on the surface of tetrapods, since morphology changes of the microstructures were not observed by SEM investigations, even after annealing samples for 5 h at 1150 °C. The annealing effect on the surface states might be related to the transformation between the oxygen-vacant-state and the oxygen-reach-state [22]. Another possible explanation could be the presence of carbon contamination, formation of COOH groups on the tetrapod’s surface which make them hydrophobic. These species evaporate during the annealing process, or their nature is changed. Many researchers describe the changes of wettability properties introduced by annealing to be reversible, however we have not observed recovery even after 30 days.

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Fig. 6. (a) WCA measurements of the pressed as-grown ZnO tetrapods (before thermal annealing) and the photo image shape of a water drop on their surface (b). (c) Comparative study of ZnO tetrapods on water demonstrating strong hydrophobic behavior of the as-grown (left) and strong hydrophilic behavior of the same ZnO tetrapods after thermal annealing at 950 °C for 1 h.

4 Conclusions The results of this study demonstrate possibilities to control the hydrophobic/hydrophilic properties of networks of ZnO microtetrapods by means of thermal treatment. The ZnO microtetrapod networks produced by flame transport synthesis proved to be hydrophobic when as-prepared, but they became super-hydrophilic upon annealing at 950 °C. The high crystalline and optical properties of the prepared ZnO microtetrapod structures have been demonstrated by XRD analysis and PL spectroscopy investigations. The changes introduced by thermal treatment to PL and wettability properties were explained in terms of the annealing effects on the surface states or on different chemical groups present on the surface of microtetrapods.

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Acknowledgments. This research was funded by the National Agency for Research and Development of Moldova under the Grant #22.80013.5007.4BL “Nano- and hetero-structures based on zinc oxide and A3B5 compounds for optoelectronics, photonics and biosensorics”. This research was funded by ECSEL JU under the following grant agreements: No. 876124 (BEYOND5) and No. 875999 (IT2). The JU receives support from the European Union’s Horizon 2020 research and innovation program and Germany, Belgium, Sweden, Austria, Romania, France, Poland, Israel, Switzerland, Netherlands, Hungary, United Kingdom. This work is financially supported by the Romanian Ministry of Research, Innovation and Digitalization, under the following ECSELH2020 Projects: BEYOND5-Contract no. 12/1.1.3/31.07.2020, POC-SMIS code 136877 and IT2-Contract. no. 11/1.1.3H/06.07.2020, POC-SMIS code 136697. Conflict of Interest. The authors declare no conflicts of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References 1. Tiginyanu, I., et al.: Self-organized and self-propelled aero-GaN with dual hydrophilic/hydrophobic behavior. Nano Energy 56, 759–769 (2019). https://doi.org/ 10.1016/j.nanoen.2018.11.049 2. Plesco, I., et al.: Aero-ZnS architectures with dual hydrophilic-hydrophobic properties for microfluidic applications. APL Mater. 8, 061105 (2020). https://doi.org/10.1063/5.0010222 3. Mishra, Y.K., et al.: Fabrication of macroscopically flexible and highly porous 3D semiconductor networks from interpenetrating nanostructures by a simple flame transport approach. Part. Part. Syst. Charact. 30, 775–783 (2013). https://doi.org/10.1002/ppsc.201300197 4. Plesco, I., et al.: Highly porous and ultra-lightweight aero-Ga2 O3 : enhancement of photocatalytic activity by noble metals. Materials 14, 1985 (2021). https://doi.org/10.3390/ma1408 1985 5. Mecklenburg, M., et al.: Aerographite: ultra lightweight, flexible nanowall, carbon microtube material with outstanding mechanical performance. Adv. Mater. 24, 3486 (2012). https://doi. org/10.1002/adma.201200491 6. Schütt, F., et al.: Conversionless efficient and broadband laser light diffusers for high brightness illumination applications. Nat. Commun. 11, 1437 (2020). https://doi.org/10.1038/s41 467-020-14875-z 7. Ciobanu, V., et al.: Aero-TiO2 prepared on the basis of networks of ZnO tetrapods. Crystals 12, 1753 (2022). https://doi.org/10.3390/cryst12121753 8. Monaico, E.V., Busuioc, S., Tiginyanu, I.M.: Controlling the degree of hydrophilicity/hydrophobicity of semiconductor surfaces via porosification and metal deposition. In: Tiginyanu, I., Sontea, V., Railean, S. (eds.) 5th International Conference on Nanotechnologies and Biomedical Engineering. ICNBME 2021. IFMBE Proceedings, vol. 87. Springer, Cham (2022). https://doi.org/10.1007/978-3-030-92328-0_9 9. Monaico, E.V., Monaico, E.I., Ursaki, V.V., Tiginyanu, I.M.: Porous semiconductor compounds with engineered morphology as a platform for various applications. Physica Status Solidi (RRL) – Rapid Research Letters, 2300039 (2023). https://doi.org/10.1002/pssr.202 300039 10. Mishra, Y.K., et al.: Versatile fabrication of complex shaped metal oxide nano-microstructures and their interconnected networks for multifunctional applications. KONA Powder Particle J. 31, 92–110 (2014). https://doi.org/10.14356/kona.2014015

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11. Nishio, K., Isshiki, T., Kitano, M., Shiojiri, M.: Structure and growth of tetrapod-like ZnO particles. Philos. Mag. A 76, 889–904 (1997). https://doi.org/10.1080/01418619708214216 12. Ronning, C., Shang, N.G., Gerhards, I., Hofsäss, H.: Nucleation mechanism of the seed of tetrapod ZnO nanostructures. J. Appl. Phys. 98, 034307 (2005). https://doi.org/10.1063/1.199 7290 13. Wang, B.B., Xie, J.J., Yuan, Q., Zhao, Y.P.: Growth mechanism and joint structure of ZnO tetrapods. J. Phys. D: Appl. Phys. 41, 102005 (2008). https://doi.org/10.1088/0022-3727/41/ 10/102005 14. Yang, D., Gopal, R.A., Lkhagvaa, T., Choi, D.: Oxidizing agent impacting on growth of ZnO tetrapod nanostructures and its characterization. Environ. Res. 197, 111032 (2021). https:// doi.org/10.1016/j.envres.2021.111032 15. Özgür, Ü., Alivov, Ya.I., Liu, C.: A comprehensive review of ZnO materials and devices. J. Appl. Phys. 98, 041301 (2005). https://doi.org/10.1063/1.1992666 16. Janotti, A., Van de Walle, C.G.: Fundamentals of zinc oxide as a semiconductor. Rep. Prog. Phys. 72, 126501 (2009). https://doi.org/10.1088/0034-4885/72/12/126501 17. Wang, Z.G., Zu, X.T., Zhu, S., Wang, L.M.: Green luminescence originates from surface defects in ZnO nanoparticles. Physica E 35, 199–202 (2006). https://doi.org/10.1016/j.physe. 2006.07.022 18. Kumar, S., Sahare, P.D.: Effect of surface defects on green luminescence from ZnO nanoparticles. In: AIP Conference Proceedings, Chandigarh, India, vol. 1393, pp. 159–160 (2011). https://doi.org/10.1063/1.3653658 19. Ursaki, V.V., et al.: Photoluminescence of ZnO layers grown on opals by chemical deposition from zinc nitrate solution Semicond. Sci. Technol. 19, 851–854 (2004). https://doi.org/10. 1088/0268-1242/19/7/012 20. Ursaki, V.V., et al.: Photoluminescence and resonant Raman scattering from ZnO-opal structures. J. Appl Phys. 96, 1001–1006 (2004). https://doi.org/10.1063/1.1762997 21. Meyer, B.K., et al.: Bound exciton and donor–acceptor pair recombinations in ZnO. Phys. stat. sol. (b) 241, 231–260 (2004). https://doi.org/10.1002/pssb.200301962 22. Mozumder, M.S., Mourad, A.H.I., Pervez, H., Surkatti, R.: Recent developments in multifunctional coatings for solar panel applications: a review. Sol. Energy Mater. Sol. Cells 189, 75–102 (2019). https://doi.org/10.1016/j.solmat.2018.09.015

Quantum Oscillations in Topological Insulator Bi2 Te2 Se Microwires Contacted with Superconducting In2 Bi Leads Leonid Konopko1(B)

, Albina Nikolaeva1

, and Tito Huber2

1 Ghitu Institute of Electronic Engineering and Nanotechnologies,

Technical University of Moldova, Chisinau, Moldova [email protected] 2 Howard University, Washington, USA

Abstract. We studied the magnetoresistance (MR) of polycrystal Bi2 Te2 Se topological insulator (TI) microwires contacted with superconducting In2 Bi leads. Bi2 Te2 Se has a simple band structure with a single Dirac cone on the surface and a large non-trivial bulk gap of 300 meV. To study the TI/SC interface, the Bi2 Te2 Se glass-coated microwire with a diameter of d = 17 μm was connected to copper leads on one side using superconducting alloy In2 Bi (T c = 5.6 K), and on the other side using gallium. The topologically nontrivial 3D superconductor (SC) In2 Bi has proximity-induced superconductivity of topological surface states. To eliminate conventional contribution to superconductivity from the bulk, the resulting edge states of the TI/SC contact area were studied in magnetic fields above H c2 in In2 Bi. The h/2e oscillations of magnetoresistance (MR) in longitudinal and transverse magnetic fields (up to 1 T) at the TI/SC interface were observed at various temperatures (4.2 k–1.5 K). To explain the observed oscillations, we used magnetic flux quantization, which requires a multiply connected geometry where flux can penetrate into normal regions surrounded by a superconductor. The effective width of the closed superconducting area of the TI/SC interface is determined to be 15 nm from an analysis of FFT spectra and the beats of the MR oscillations for two different directions of magnetic field. Keywords: topological insulator · superconductivity · thin microwire · proximity effect · magnetoresistance · h/2e oscillations of magnetoresistance

1 Introduction Topological insulators (TI) are a type of material that exhibit unique electronic properties [1, 2]. They are classified as insulators in their bulk, but they possess conducting states on their surfaces or edges. The key feature of topological insulators is the existence of a “topological order” or a unique arrangement of electronic states. This order is robust against certain types of perturbations and imperfections, making the surface or edge states highly stable. It means that even in the presence of impurities or defects, the conducting states on the surface or edge of a topological insulator can remain unaffected. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 293–302, 2024. https://doi.org/10.1007/978-3-031-42775-6_33

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The robust surface states of topological insulators are highly desirable for creating lowpower and high-speed electronic devices and enabling the manipulation of electron spin for information storage and processing [3–5]. Bismuth-based binary compounds, such as Bi2 Te3 and Bi2 Se3 , have been long known as excellent thermoelectric materials due to their unique near-gap electronic structure [6, 7]. Recently, it has been shown that these materials host novel topological surface states inside the bulk energy gaps that are protected by the time reversal symmetry. These surface states are characterized by a helical Dirac cone structure, which arises due to the spin-orbit coupling and the band inversion in the material [8, 9]. These unique properties make it a promising material for various applications in spintronics, electronics, and quantum computing. Recently, Aharonov-Bohm (AB) oscillations have been observed in single-crystal Bi2 Te3 nanoribbons with a rectangular cross section of 120 * 55 nm, and rather strongly suppressed Altshuler-Aronov-Spivak (AAS) oscillations have also been observed [10]. The area of contact between a topological insulator and a superconductor (SC) is an intriguing interface that has been the subject of extensive research. This interface is of particular interest because it can give rise to novel phenomena, such as the creation and manipulation of Majorana fermions [11]. These quasiparticles are their own antiparticles and exhibit non-Abelian statistics, making them potential building blocks for quantum computing. Recent efforts to detect and control Majorana fermions in solid state devices have used topological insulator nanowires proximity coupled to superconducting wires [12–14]. In this study, the quantum oscillations of magnetoresistance (MR) in longitudinal and transverse magnetic fields (up to 1 T) at the TI/SC interface, obtained by contacting microwires of a Bi2 Te2 Se topological insulator with In2 Bi superconducting leads, are explored.

2 Samples and Experimental Details To prepare Bi2Te2Se TI microwires, we used the Taylor technique. The following materials with a high degree of purity were used as initial components: Bi (99.999%), Te (99.999%), and Se (99.999%). Synthesis was performed at a temperature of 700–720 °C in a cylindrical furnace. To prepare a Bi2 Te2 Se microwire based on semiconductor materials with many components and volatile impurities at high temperatures, the Taylor-Ulitovsky method with thermal furnace heating has been developed (see schematic diagram in Fig. 1a) [15]. The main element of the novel method was the use of a furnace with resistive heating and stable temperature regime, which was ensured by a temperature controller with accuracy of ± 0.5 °C, as a heater in the Taylor–Ulitovsky installation. The smallest and largest diameter obtained was d = 5 and 100 μm, respectively. A thermal treatment of the prepared Bi2 Te2 Se glass-coated microwires at different temperatures (430–525 K) and time intervals (24–72 h) was performed to improve the microwire characteristics. Isothermal annealing of the microwires leads to an increase in the thermopower and a decrease in the resistivity. The larger the temperature and annealing time, the higher the physical parameters obtained. X-ray studies showed that the microwire core was in general a

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polycrystal consisting of big disoriented single-crystal blocks with an approximate size of 10–15 μm. At 300 K for the samples with diameter d in the range 15–20 μm, the thermopower is S = –(100–140) μV/K and the resistivity is ρ = 1 × 10–3 to 7 × 10–3  cm. A scanning electron microscope image of the cross section of the 17-μm glass-coated Bi2 Te2 Se microwire is shown in Fig. 1b.

Fig. 1. (a) The Taylor method for synthesizing long small-diameter wires in a glass coating. (b) Scanning electron microscope image of the cross section of the 17-μm Bi2 Te2 Se wire in a glass coating.

The samples for measurements were cut from long microwires; the length of the samples was 2 mm. They were then mounted on special foil-clad fiberglass plastic holders. Electrical contacts between the microwire and the copper foil were made on one side with a Ga solder and on the other side using an In2 Bi [16–19] superconducting solder with T c = 5.7 K and the melting point of T m = 80 °C. Sketches of the location of glasscoated microwire on a substrate made of foil-coated fiberglass are shown in the insets to Figs. 3b and Fig. 4b. Gallium has superconductivity at temperatures below 1 K, so it was a normal metal in our measurements. In2 Bi is a topological non-trivial 3D superconductor; it has proximity-induced superconducting topological surface states that arose due to a non-trivial band structure and strong spin-orbit coupling [19]. The prepared wire samples were held in special holders and inserted in a cryostat for low temperature measurements. We carried out magnetic field-dependent resistance R(B) measurements in a range of 0 to 1 T at the International Laboratory of High Magnetic Fields and Low Temperatures (Wroclaw, Poland) and employed a device that tilts the sample axis with respect to the magnetic field and also rotates the sample around its axis. We used the magnetic field modulation technique to measure magnetoresistance oscillations (Shubnikov–de Haas oscillations, Aharonov-Bohm oscillations). The amplitude of the oscillatory field is 45 Oe. This very sensitive technique allowed us to register the oscillation amplitude directly at the lock-in amplifier output.

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Fig. 2. (a) Magnetic field dependences of transverse MR of thick In2 Bi tape measured in the temperature range 1.5–5.85 K. (b) Temperature dependence of the critical magnetic field Bc in thick In2 Bi tape.

3 Results and Discussions One of the main ways to detect and manipulate majorana fermions in solid-state devices is to use TI nanowire proximity-coupled to superconducting leads. This combination holds some promises for the fundamental physics and applications. The In2 Bi compound with T c = 5.7 K was used as a superconductor. In2 Bi is a topological non-trivial 3D superconductor; it has proximity-induced superconducting topological surface states that arose due to a non-trivial band structure and strong spin-orbit coupling. The characteristic superconductivity parameters of In2 Bi were obtained from DC magnetization curves: the lower and upper critical fields H c1 ≈ 490 Oe and H c2 ≈ 950 Oe, respectively, the coherence length  ≈ 60 nm, the magnetic field penetration depth  ≈ 65 nm, and the Ginzburg–Landau (GL) parameter K = 1 [19]. Topological surface states of In2 Bi have a distinct signature in surface magnetization and can be detected above the bulk critical field for superconductivity, H c2 . The critical field for surface superconductivity, H c3 = 2 H c2 at low temperatures. According to our R(B) measurements at various temperatures of a thin (10 × 2 × 0.2 mm3 ) polycrystal In2 Bi tape in a transverse magnetic field Bc ~ 0.22 T (see Fig. 2); it was actually a measurement of surface superconductivity. Since this compound has a low melting point (T m = 80 °C), it can be used to make contacts by soldering the microwire to copper leads, without damaging or modifying the studied TI microwires. To eliminate ambiguity in measurements, contacts to TI microwires made of In2 Bi were used only on one side of the microwires. On the other side of the samples, the contacts were made using Ga solder (T c = 1 K), which in the range of studied temperatures of 300–1.5 K was in a normal state. It should be noted that it is difficult to obtain contacts with a microwire when using In2 Bi solder. As a consequence, the superconductor is probably not in contact with the microwire over the entire cross-sectional area of the microwire. in our experiment we used 17-μm ntype Bi2 Te2 Se microwires with an outer diameter of 29 μm. The R(T ) Dependence for microwire sample shown in Fig. 3b, exhibits a “metallic” behavior with a pronounced superconducting transition at 5.2 K. At helium temperatures in a longitudinal magnetic

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field, the resistance of this sample increases and exhibits a nonmonotonic behavior (see Fig. 4b). We observed the non-monotonic changes of longitudinal and transverse MR that are equidistant in a magnetic field (preliminary results are published in [20]), and the onset of periodic oscillations occurs after reaching a certain magnetic field, comparable to the value of H c2 in In2 Bi. The amplitude of the MR oscillations decreases with increasing temperature. The oscillations almost disappear when the measurement temperature reaches the superconducting transition temperature in In2 Bi. The amplitude of the MR oscillations also decreases with increasing magnetic field. Oscillations were observed in the region of magnetic fields 0.1–0.6 T, that is, within the region of critical fields H c2 - H c3 . The oscillation part of magnetic field derivative of the transverse MR at T =

Fig. 3. (a) Magnetic field dependence of the derivative of transverse MR, T = 1.5 K (monotonic part is subtracted) for the Bi2 Te2 Se microwire in a glass coating, D = 29 μm, d = 17 μm. The derivative of transverse MR curve exhibits a clear beating pattern. The vertical lines indicate the position of the beat nodes. (b) Temperature dependence of resistance of the same sample; Inset: Sketch of the location of the glass-coated microwire on a substrate made of foil-coated fiberglass. Contacts to the microwire are made using In2 Bi and Ga solders. (c) FFT of the oscillating part of the derivative of transverse MR. The black vertical lines show the location of two frequencies determined from the analysis of beats in the oscillations. Insert: sketch of a superconducting TI-SC contact area. The outside edge determines the area of the quantization trajectory of h/2e oscillations (0.124 μm2 ) corresponding to the frequency F 2 FFT, the inside edge determines the area of the quantization trajectory (0.101 μm2 ) corresponding to F 1 FFT. r = 16 μm is the width of the superconducting area at the TI-SC interface.

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1.5 K is shown in Fig. 3a. This curve exhibits a clear beating pattern. The vertical lines indicate the position of the beat nodes. The FFT spectrum of this oscillation is shown in Fig. 3c. The black vertical lines show the location of two frequencies determined from the analysis of beats in the oscillations. Frequencies F 1 = 51.1 T−1 and F 2 = 59.75 T−1 correspond to periods B1 = 0.0196 T and B2 = 0.0167 T, respectively. To explain the observed oscillations, we will use fluxoid quantization, which requires a multiply connected geometry where flux can penetrate normal regions encompassed by a superconductor. This is quite reasonable, because oscillations are observed in the region of magnetic fields greater than H c2 , which is, the contact area of the TI-SC is in the normal non-superconducting state, and only at the edges of this area is the contact of the topological surface superconductivity of In2 Bi with the topological insulator Bi2 Te2 Se. According to this assumption, the observed oscillation periods implies characteristic geometrical areas – inside area A1 and outside area A2 (see inset in Fig. 3c; A1 = 0 (B1 )−1 = (h/2e) (B1 )−1 = 0.105 μm2 , A2 = (h/2e)(B2 )−1 = 0.124 μm2 , where h is Planck’s constant, e the electron charge. If we imagine √ the contact area as a circle, then the width of the annular area is r = (A2 - A1 ) / 2π (A2 + A1 ) = 16 nm. Above the bulk critical field H c2 , the superconductivity in In2 Bi is retained within a thin surface sheath of thickness ≈ 2ξ = 120 nm and exists up to the critical field for surface superconductivity, H c3 . The edge states of the TI-SC interface proximity-induced by surface superconductivity have a width of 16 nm. The oscillation part of magnetic field derivative of the longitudinal MR at T = 1.5 K is shown in Fig. 4a. This curve exhibits a clear beating pattern. The vertical lines indicate the position of the beat nodes. The FFT spectrum of this oscillation is shown in Fig. 4c. The black vertical lines show the location of two frequencies determined from the analysis of beats in the oscillations. Frequencies F 1 = 18.5 T−1 and F 2 = 23.7 T−1 correspond to periods B1 = 0.054 T and B2 = 0.042 T, respectively. Inside area A1 = 0.038 μm2 and outside area A2 = 0.049 μm2 , r = 15 nm (see inset in Fig. 4c). The TI-SC contact area forms a complex surface, the edge states of which are superconductive. These states form a closed superconducting area inside the TI-SC contact area, along which the h/2e quantization of the magnetic flux occurs, and the difference in the quantization areas along the outer and inner edges of the closed superconducting area determines the h/2e oscillation frequency shift in the FFT spectrum. A sketch of the experimental setup is shown in Fig. 5, a glass-coated microwire of TI Bi2 Te2 Se in proximity coupled to a bulk In2 Bi superconductor and subjected to an external magnetic fields B⊥ and B// . The occurrence of different quantization areas S ⊥ and S // for transverse and longitudinal magnetic fields is shown. The reliability of the interpretation of our experiment is proven by the comparability of the obtained data on the width r of the superconducting area at the TI-SC interface in magnetic fields above H c2 .

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Fig. 4. (a) Magnetic field dependence of the derivative of longitudinal MR, T = 1.5 K (monotonic part is subtracted) for the Bi2 Te2 Se microwire in a glass coating, D = 29 μm, d = 17 μm. The derivative of longitudinal MR curve exhibits a clear beating pattern. The vertical lines indicate the position of the beat nodes. (b) Magnetic field dependence of resistance of the same sample, T = 1.5 K; Inset: Sketch of the location of the glass-coated microwire on a substrate made of foil-coated fiberglass. Contacts to the microwire are made using In2 Bi and Ga solders. (c) FFT of the oscillating part of the derivative of longitudinal MR. The black vertical lines show the location of two frequencies determined from the analysis of beats in the oscillations. Insert: sketch of a superconducting TI-SC contact area. The outside edge determines the area of the quantization trajectory of h/2e oscillations (0.049 μm2 ) corresponding to the frequency F 2 FFT, the inside edge determines the area of the quantization trajectory (0.038 μm2 ) corresponding to F 1 FFT. r = 15 μm is the width of the superconducting area at the TI-SC interface.

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Fig. 5. (a) Illustration of the real setup. Glass-coated microwire of TI Bi2 Te2 Se in proximity coupled to a bulk In2 Bi superconductor. At magnetic fields greater than H c2 in In2 Bi, the TISC contact area forms a complex surface, the edge states of which are superconductive. These states form a closed superconducting area inside the TI-SC contact area along which the h/2e quantization of the magnetic flux occurs, and, the difference in the quantization areas along the outer and inner contours of the closed superconducting area determines the frequency shift of the h/2e oscillations in the FFT spectrum. (b) The TI-SP contact area, for example, is presented in the form of a circle. The occurrence of different quantization areas S ⊥ and S // for transverse and longitudinal magnetic fields is shown.

4 Conclusions We have investigated the transport properties of 17-μm glass-coated Bi2 Te2 Se microwires contacted with In2 Bi topological superconductor (T c = 5.7 K) leads. The effects of topological superconductivity were isolated from the obscuring conventional contribution from the bulk by using magnetic fields above H c2 . The h/2e oscillations of MR in longitudinal and transverse magnetic fields (up to 1 T) at the TI/SC interface were observed at various temperatures (4.2 k–1.5 K). To explain the observed oscillations, we used fluxoid quantization, which requires a multiply connected geometry where flux can penetrate normal regions encompassed by a superconductor. This is quite reasonable, because oscillations are observed in the region of magnetic fields greater than H c2 , which is, the contact area of the TI-SC is in the normal non-superconducting state, and only at the edges of this area is the contact of the topological surface superconductivity of In2 Bi with the topological insulator Bi2 Te2 Se. The effective width of the closed superconducting area of the TI/SC interface is determined to be 15 nm from an analysis of FFT spectrum and the beats of the MR oscillations for two different directions of magnetic field. Acknowledgments. This work was supported by Moldova State Project #20.80009.50007.02, NSF through STC CIQM 1231319, the Boeing Company and the Keck Foundation. Conflict of Interest. The authors declare that they have no conflict of interest.

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Biomaterials for Medical Applications

Design and Simulation of a Biocompatible Prosthesis Ti-15Mo-XTa Alloy: An Analysis of Mechanical Integrity Using Finite Element Modeling A. Najah Saud1,2(B) , Hasan Sh. Majdi1 , Erkan Koç2 and Mohammed Al Maamori1

,

1 Biomedical Engineering, Al-Mustaqbal University College, Babylon, Iraq

[email protected] 2 Biomedical Engineering, Karabuk University, Karabuk, Turkey

Abstract. The main focus of this work is the development and simulation of a prosthesis using a high entropy alloy known as Ti-15Mo-XTa. The selection of this alloy is based on its compatibility with the human body, which is a crucial factor when choosing materials for medical implants. Traditional metal implants can cause several problems for patients, including toxic reactions from the release of metal ions, wear and tear of joint replacements from movement, and structural failure from repetitive loading. To address these concerns, the present study creates a three-dimensional finite element model of the prosthesis using COMSOL software. The model includes both isotropic and anisotropic materials and is subjected to various mechanical loads based on experimental studies. The finite element method is used to analyze the distribution of stress and strain across adjacent elements of the prosthesis. By simulating the behavior of the prosthesis under different loading conditions, valuable insights into its performance and durability can be gained. To assess the static design, the prosthesis is tested using COMSOL simulation software and subjected to loading conditions of 70, 90 and 110 kg. The objective of this assessment is to determine the robustness and ability of the design to withstand real-world mechanical demands. By conducting these simulations and tests, the researchers hope to contribute to the development of improved prostheses that can offer better functionality, longevity and overall patient satisfaction. Keywords: Ti-alloy · Femur · Finite element analysis · COMSOL software

1 Introduction In recent decades, several authors have researched the biocompatibility of materials [1– 4]. For many researchers, titanium is one of the most advisable materials, as the patient does not need to perform any revision surgery for 15 years on average. Optimizing the shape of the implant to reduce the effect of stress shielding has also been the subject of extensive scientific discussion through finite element analysis [5, 6]. Within the scope of © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 305–312, 2024. https://doi.org/10.1007/978-3-031-42775-6_34

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implant design, experimental techniques have been applied, complemented by numerical and analytical analysis, taking into account the shape and material to be used, as investigated by [7, 8]. The increasing aging of the population means that the most frequent orthopedic surgery in adults is the application of a total hip prosthesis. This replaces the hip joint components to improve the patient’s mobility and quality of life. When an implant is introduced into the human body, it has to be biocompatible. For this, materials such as silicone, carbon fiber, titanium, chromium-cobalt, and stainless steel, among others, are used, which in addition to having characteristics accepted by the body, are also considerably light, resistant, and energy storage [9]. The hip is affected by a disease characterized by progressive destruction of the joint that leads to decreased functions, accompanied by more or less intense pain, depending on the wear it suffers. In some cases, surgical intervention is necessary to remove the damaged bone or cartilage. The process involves realigning or altering the joint surfaces carrying the load or remodeling the joint by replacing it with a synthetic material. Modern orthopedic implants are usually more rigid than the bone close to the prosthesis. Because implants are so stiff, bone resorption occurs around them because the applied stress is not distributed uniformly across the contact area with the adjacent bone. This causes the bone to break and the implant to become loose. The high corrosion resistance, biocompatibility, osseointegration, strength-to-weight ratio, fracture toughness, and fatigue strength make titanium and its alloys appealing for biomedical applications. Titanium alloys have significantly lower Young’s modulus and density values than metallic biomaterials [10]. The femur bone is a key structural component that helps humans maintain an upright posture and steady movement when seated, standing, and moving about. The femur is the longest and one of the most robust bones in the human body. The femur bone is both intricately shaped and uniquely made. The thigh bone, or femur, is composed of a head, a body, and a thigh. The body’s form is long and nearly cylindrical [11, 12]. Due to individual differences in bone geometry and mechanical properties, replicating experimental results with femur bone is challenging. For this study, a 3D scan of a human femur bone served as the basis for the CAD geometry, which was then modified using AutoCAD 2017 [13]. Finite element analysis (FEA) is a popular method for complex stress analysis in various disciplines due to its speed and accuracy. Analysis of stress distribution and stress shielding in THR have been successfully measured using finite element analysis [14]. Furthermore, FEA techniques are used to aid in [15, 16] and optimize [17, 18] the design of hip prostheses, contributing to an understanding of biomechanics under different types of loading, which is crucial to the durability and longevity of prostheses. This study aims to develop a simulation model utilizing FEM COMSOL to determine the stress distribution and vulnerable region in the femur due to applying a load to its head.

2 Experimental Starting with a femur specimen from a 40-year-old male patient scanned using a 3D Laser Scanner, E. Isaza and his colleagues developed a process to get the solid geometry model [13]. Because of a lack of local patient-specific scan model equipment and resources, we

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used this scanned model in this study. The scanned model (Fig. 1) is a 3D model in.stl format. The 3D finite element model necessary for analysis is constructed by discretizing the geometric model. The discretization was carried out in the ANSYS environment. The implant’s 3D geometry is modeled separately. This file is opened in the same environment as the implant model. Finally, the model is ready for analysis. Tria elements are utilized to make 86188 elements. Several boundary conditions are supplied and analyzed using ANSYS 14.5. Figure 2 depicts the FEA model of the implant, whereas Fig. 3 shows the boundary conditions used in the analysis model.

Fig. 1. 3D scanning for the femur.

Mesh information for the femur design Mesh vertices 2630 Tetrahedra 9409 Triangles 3790 Edge elements 1613 Vertex elements 488 Number of elements 9409 Minimum elements 0.1865 quality Average elements 0.6227 quality Elements volume ratio 0.00978 Mesh volume 5.32E-4 m3

Fig. 2. Finite element model for femur with mesh information.

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After importing the geometric model, we set the materials’ attributes, the components’ kinds and sizes, and the boundary conditions. Ti-15Mo-XTa alloy prepared as described in [19] with parameters shown in table 1 with varied loading forces F = 70, 90, and 110 kg. In order to mimic compression, the constraints were defined as symmetric boundary conditions, and the model was loaded by applying forces to nodes on the end face of the sample. Consequently, von Misses stresses and displacements were the computation outcomes for each simulated sample.

Fig. 3. Boundary Conditions.

Table 1. Mechanical properties of the Ti-15Mo-XTa biomaterial. Materials

Property Density

Elastic modulus

Passion ratio

14.78 ± 0.73

101.16

0.3052

Ti-15Mo-10Ta

5,03 ± 0.25

11.32 ± 0.56

89.24

0.288

Ti-15Mo-15Ta

5,32 ± 0.26

10.06 ± 0.50

80.74

0.283

Ti-15Mo

4.87 ± 0.24

Porosity

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3 Result and Discussion Using the material characteristics of Ti-15Mo-XTa, a finite element analysis was performed on a static model of an artificial hip implant, yielding von Mises stresses and static displacement. Three different loads (70 kg, 90 kg, and 110 kg) were chosen as the applied load. The safety requirement is met if the von Mises stresses generated are less than the tensile strength of the chosen material. Selected materials were verified to be safe after comparing the produced von Mises stresses to the allowed strength of Ti-15Mo-XTa. The distribution of strains in the different locations of the femur implant and the static displacement of given loads of 70 kg, 90 kg, and 110 kg are illustrated in Table 2 and Figs. 4, 5 and 6, respectively. A static load of 70 kg, 90 kg, or 110 kg resulted in von Mises stresses of 2.5 MPa, 3–3.1 MPa, and 3.5–3.7 MPa, respectively. The equivalent stress created on the femur body’s shaft is very high, at 3.7 MPa. The area of the femur is where the equivalent elastic strain has accumulated the most. As a result, our study has pinpointed the region of the human femur geometry where bending loads are too great. Table 2. Von Mises stresses and total displacement for prepared alloy. Materials

Load

Von Mises stresses (MPa)

Total displacement (m)*10–3

Ti-15Mo

70

2.5

26

90

3

35

110

3.5

41

70

2.5

30

90

3.05

37

Ti-15Mo-10Ta

Ti-15Mo-15Ta

110

3.6

46

70

2.5

25

90

3

32

110

3.5

37

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Fig. 4. Von Mises stresses and total displacement for Ti-15Mo alloy.

Fig. 5. Von Mises stresses and total displacement for Ti-15Mo-10Ta alloy.

Fig. 6. Von Mises stresses and total displacement for Ti-15Mo-15Ta alloy.

4 Conclusions This study aimed to develop and simulate orthopedic implants using functionally grade materials (FGM), particularly Ti15Mo-XTa high entropy alloy, to determine whether or not they were feasible. To do this, a COMSOL model of a femur implant was developed and simulated under loads of 70, 90, and 110 kg to account for the various physical

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qualities of functionally graded materials. The findings of the static load analysis provide a distribution of stress, strain, and displacement that is acceptable according to the Von Misses theory. The following inferences may be drawn from the findings and comments. For a 70-kg patient, the highest stress on titanium alloys was 2.5 MPa, and the maximum displacement was 30*10–3 . Two, the Von Misses stress was raised by adding Ta at a weight percentage of 15%. The resulting stresses are well within the yield limit. Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Elen, L., Turen, Y., Cicek, B., Bozer, B.M., Saud, A.N., Koc, E.: The cytotoxic and genotoxic assays of Mg-Ag Alloy doped with Zn, Ca, and Nd elements. J. Mater. Eng. Perform. 1–11 (2022) 2. Jain, S., Parashar, V.: Analytical review on the biocompatibility of surface-treated Ti-alloys for joint replacement applications. Expert Rev. Med. Devices 19, 699–719 (2022) 3. Pesode, P., Barve, S.: Additive manufacturing of metallic biomaterials and its biocompatibility. Mater. Today Proc. (2022) 4. Soni, R., Pande, S., Salunkhe, S., Natu, H., Abouel Nasr, E., Shanmugam R,., et al.: In Vitro and electrochemical characterization of laser-cladded Ti-Nb-Ta alloy for biomedical applications. Crystals 12, 954 (2022) 5. Naidubabu, Y., Mohana Rao, G., Rajasekhar, K., Ratna, S.B.: Design and simulation of polymethyl methacrylate-titanium composite bone fixing plates using finite element analysis: optimizing the composition to minimize the stress shielding effect. Proc. Inst. Mech. Eng. C J. Mech. Eng. Sci. 231, 4402–4412 (2017) 6. Nizam Ahmad, M., Shuib, S., Hassan, A., Shokri, A., Ridzwan, M., Ibrahim, M.: Application of multi criteria optimization method in implant design to reduce stress shielding. J. Appl. Sci. 7, 349–355 (2007) 7. Simões, J., Monteiro, J., Vaz, M.: Numerical–experimental method for the validation of a controlled stiffness femoral prosthesis. J. Biomech. Eng. 123, 234–8 (2001) 8. Simoes, J., Marques, A., Jeronimidis, G.: Design of a controlled-stiffness composite proximal femoral prosthesis. Compos. Sci. Technol. 60, 559–567 (2000) 9. Merola, M., Affatato, S.: Materials for hip prostheses: a review of wear and loading considerations. Materials. 12, 495 (2019) 10. Gao, X., Fraulob, M., Haïat, G.: Biomechanical behaviours of the bone–implant interface: a review. J. R. Soc. Interface 16, 20190259 (2019) 11. Mughal, U., Khawaja, H.A., Moatamedi, M.: Finite element analysis of human femur bone. Int. J. Multiphys. 9 (2015) 12. Sylvester, A.D., Merkl, B.C., Mahfouz, M.R.: Assessing AL 288-1 femur length using computer-aided three-dimensional reconstruction. J. Hum. Evol. 55, 665–671 (2008) 13. Isaza, E., García, L., Salazar, E.: Determination of mechanic resistance of osseous element through finite element modeling. In: Proceedings of the 2013 COMSOL Conference in Boston (2013) 14. Pal, B., Gupta, S.: New AM.: Influence of the change in stem length on the load transfer and bone remodelling for a cemented resurfaced femur. J. Biomech. 43, 2908–2914 (2010) 15. Kluess, D., Martin, H., Mittelmeier, W., Schmitz, K.-P., Bader, R.: Influence of femoral head size on impingement, dislocation and stress distribution in total hip replacement. Med. Eng. Phys. 29, 465–471 (2007)

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16. El’Sheikh, H., MacDonald, B., Hashmi, M.: Finite element simulation of the hip joint during stumbling: a comparison between static and dynamic loading. J. Mater. Process. Technol. 143, 249–255 (2003) 17. Li, Y., Yang, C., Zhao, H., Qu, S., Li, X., Li, Y.: New developments of Ti-based alloys for biomedical applications. Materials 7, 1709–1800 (2014) 18. Bennett, D., Goswami, T.: Finite element analysis of hip stem designs. Mater. Des. 29, 45–60 (2008) 19. Majdi, H.S., Saud, A.N., koç, E., Al Juboori, A.M.: Investigation of the effect of adding tantalum on the microstructure and mechanical properties of biomedical Ti-15Mo Alloy. In: Tiginyanu, I., Sontea, V., Railean, S. (eds.) 5th International Conference on Nanotechnologies and Biomedical Engineering. ICNBME 2021. IFMBE Proceedings, vol. 87. Springer, Cham (2022). https://doi.org/10.1007/978-3-030-92328-0_81

Modification of Acrylic Paint by Acetamide to Be Antibacterial Used for Medical Applications Mohammed Al Maamori1 , Hasan Sh. Majdi1 and A. Najah Saud1,3(B)

, Ali Kareem2

,

1 Biomedical Engineering, Al-Mustaqbal University College, Babylon, Iraq

[email protected]

2 Polymer and Petrochemical Industrials, University of Babylon, Babylon, Iraq 3 Biomedical Engineering, Karabuk University, Karabuk, Turkey

Abstract. This study aimed to develop an innovative approach to produce an organic antibacterial composite material by combining acrylic paint and acetamide through a simple mixing method. Acetamide, known for its potent antibacterial properties, underwent a thorough evaluation to assess its effectiveness in the composite. The antibacterial properties were evaluated using established methods such as the minimum inhibitory concentration (MIC) and the agar well diffusion test. These tests provided quantitative and qualitative measures of inhibitory activity against two common bacterial strains, namely S. aureus and S. epidermidis. The results showed a clear correlation between the concentration of acetamide in the composite and its antibacterial activity. Higher concentrations of acetamide led to a significant increase in the effectiveness of the composite material against the targeted bacterial strains. In addition to the antibacterial properties, the mechanical and physical properties of the composite material were also analyzed comprehensively. Parameters such as wettability, swelling ratio and chemical structure were thoroughly investigated using Fourier Transform Infrared (FTIR) analysis. This comprehensive characterization enabled a detailed understanding of the behavior and performance of the composite material. The results of this study are auspicious in the context of operating rooms. The proposed composite antibacterial polymer coatings, utilizing organic or inorganic agents at low concentrations, represent an effective solution to eliminate bacteria and maintain a sterile environment. These coatings can be applied to operating room walls and offer improved infection control and reduced bacterial contamination risk. Keywords: Antibacterial polymer · Acrylic paint/acetamide · MIC and Agar well diffusion

1 Introduction Controlling microbial infections, mainly bacterial and fungal infections, is critical in hospitals, especially the operating theatre. In general, there are two ways to stop the growth of pathogenic microorganisms. The first is the use of disinfectants, and these are useless © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 313–323, 2024. https://doi.org/10.1007/978-3-031-42775-6_35

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because bacteria build strains resistant to these disinfectants. The second is the use of antibacterial surfaces, these are designed by coating them with materials containing biocides in their internal structure. These coatings are antibacterial polymers, a new type of disinfectant that can be used as an alternative to disinfectants. Interestingly these polymers can be linked with nanoparticles, atoms, or groups with antibacterial properties [1]. Coatings are closely related to human health. Hospital infections occur within 48 h, three days, or 30 days after surgery. These infections are the result of microbes, especially bacteria. Staphylococcus aureus, for example, is the most common type; although it does not cause infection, it can be a cause of death for people with weak immune systems, followed by other species such as Pseudomonas, E.coli, and B.subtills. Hospital infections are transmitted through ventilation, walls, floors, equipment, etc. [2]. The concept of antibacterial surfaces are surfaces containing agents that have the potential to inhibit growth or reduce bacterial attack. These surfaces have become more widespread over a wide range of applications in clinics, factories and even homes and be used in different environments. This study aims to modify acrylic paint whose properties have been improved against bacteria by adding acetamide. These coatings are non-toxic, low-cost, and lethal to microorganisms. The results obtained show that antimicrobial activity increases with increased acetamide concentration [3].

2 Materials and Method 2.1 Materials The acrylic paint used in this study is a commercial paint purchased from the market and manufactured in Jordan by TAPCO Company with a low cost of 17 thousand per five litre, as shown in Fig. 1. It has properties as follows: 1. Provides high coverage, gloss, high resistance to washing, weather resistance and high resistance to must and moisture. 2. It is used for internal and external surfaces on wooden and concrete surfaces. 3. Apply it with a brush, roll or spray on dry, clean surfaces free from grease and dirt. It was diluent with water by 25–50%. 4. It becomes insoluble in water permanently and flexibly when it dries. 5. Acrylic paint dries by evaporation of its water component. Thin paint films will dry in 10–20 min, while thick paint films may take from an hour up to several days [4]. Acetamide (systemic name: ethane amide) is an organic compound with CH3CONH2. It is the simplest form of Amide, an acetic acid derivative [5]. Acetamide is used as a solvent, plasticizer, humidifier, and penetration agent. Physical Properties of acetamide: acetamide has hexagonal crystals, it has odour at a concentration of 140 to 160 mg per cubic meter, molecular weight is 59.07 g/mol, and steam pressure for acetamide is 1 mm Hg at 65 °C. Acetamide contains the NH2 group in the chemical composition, Fig. 2 [6]. The mechanical of this kind is summarized as follows. An interaction between the acetamide molecules carrying the positive charge and the membrane cells of the bacteria carrying a negative charge by electrostatic attraction results in the

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Fig. 1. Acrylic paint.

leakage of protein and cellular contents within the bacteria. Acetamide acts as a surfactant that contains a positive charge in an NH2 group and by an electrostatic reaction with the negative charge in the cell wall and thus ruptures the cell membrane and loses the cell containing.

Fig. 2. Chemical structure of Chitosan and Acetamide.

2.2 Preparation of Bacterial Suspension The inoculum was prepared by adding 5 isolated colonies of the previously identified bacterial isolate to 5 ml of sterile brain heart infusion (BHI) broth and incubated at 37 ºC for 18–24 h to produce a standard bacterial suspension of moderate turbidity equal to McFarland standard tube. 2.3 Preparation of Acetamide for Minimum Inhibitory Concentration (MIC) 2.4 g of acetamide was dissolved in distilled water 3.6 ml under mixing for 5 min. Then 100 μg/ml of this solution was taken and diluted in a microplate plate in a twofold method for five residues at 30, 15, 7.5, 3.7, and 1.8.

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2.4 Preparation of Acrylic/Acetamide Solutions for Agar Well Diffusion Method The acrylic paint/acetamide is prepared in a simple mixing method. Acetamide is added within a range of concentrations (20, 10, 5, 2.5, 1.25 mg/ml) to distilled water, and the solution is mixed for 10 min. Then add acrylic paint to the solution during mixing and leave for 30 min to homogenize at room temperature. 2.5 Determination the Antimicrobial Activity of Acetamide by MIC Method Antimicrobial activities of acetamide were calculated against bacteria of E.coli, S.aureus, Pseudomonas, and Candida fungus by serial dilution during minimum inhibitory concentration (MIC) in the culture broth. In this method, they used a series of twofold diluents where the concentration was diluted in half. 1 mL of media was taken into the test tube, 1 ml of acetamide solution was added, and then 0.1 mL of all bacterial strains used were prepared in 0.9% NaCl to the test tube containing media and acetamide solution. It was diluted five times and gave dilutions of 30-15-7.5-3.7-1.8. Nutrient broth, where samples test and controlled for 24 h, was incubated at 37 °C. Control samples contain only the media and bacteria. Microplate was placed inside the Eliza device (BioTek, El × 800) and exposed to light at 600 nm to calculate the bacterial effectiveness ratio for acetamide by MIC. Then antibacterial activity values are determined by the following equation [7]; Percentage antibacterial activity =

OD of control × 100 OD of control − OD of test

(1)

2.6 Determination the Antimicrobial Activity of Acrylic/Acetamide by Well Diffusion The antibacterial activity of the sample solutions was determined following the agarwell diffusion method described by Irobi et al. (1994) [8]. The studied bacterial isolates for acrylic/acetamide are only on gram-positive bacteria (Staphylococcus aureus; Staphylococcus epidermidis). 0.1 mL of bacteria suspensions 0.5 McFarland tube (1.5 X 108 CFU/ml) standards were distributed into the nutrient agar (NA) medium. A sterile swab obtained an inoculum from the bacterial suspension and spreader on a MullerHinton agar plate. Using a sterile plastic pipette, five holes were punched in each culture plate with a diameter of (6) mm. One of the holes was punched in the center of the plate, where 50 μl of PVA was added as positive control; 50 μl of the hybrid solution was added as a negative control in the other hole at the Muller-Hinton agar plate. One-hour pre-diffusion time was allowed, after which the plates were incubated at 37 °C for 18 h [9]. The zones of inhibition were then measured in millimetres.

3 Result and Discussion 3.1 Minimum Inhibitory Concentration (MIC) of Acetamide This study studied the antibacterial activity for Acetamide against E. coli, S.aureus, Pseudomonas and Candida albicans in a nutrient broth by calculating MIC values. These values were taken at the least concentration necessary for the growth of bacteria in the

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test tube after incubation. This method depends on the solution’s turbidity change, as shown in Fig. 3, where No. 1: E.coli No. 2: Pseudomonas No. 3: S.aureus No. 4: Candida. Five dilutions of acetamide were taken as follows (30-15.7.5-3.7-1.8). Results showed that all microbes were fully inhibited at 3.7. Antibacterial activity was not noticeable at 1.8. The growth rate of bacteria and fungi was calculated by calculating the optical density by Eliza at 600 nm. The equation determines the rate of inhibition of bacterial growth. No. (1). The growth of these bacteria was measured at 600 nm after 24 h. The optical density measurements of the reference samples were measured without the addition of acetamide. The results showed that the optical density of all the bacteria used was noticed with concentrations (20, 10, 5, 2.5, 1.25% mg/ml) of E. coli, S.aureus, Pseudomonas and Candida. The growth inhibition ratio indicated that the optical density values were reduced by decreasing turbidity to resistant bacteria and increased inhibition with concentration. Inhibition of growth increases with increased concentration. Figure 4 below shows the rate of inhibition of growth. The results showed that all bacteria and fungi had the highest inhibitory growth for acetamide (91.84%) for Pseudomonas and 61.38% for E. coli and 44.36% for Candida, and 31.7% for S.aureus at the highest concentration of 20 wt% mg/ml. (13.38%) for Pseudomonas and 11.8% for E. coli and 10.8% for Candida, and 9.42% for S. aureus at the lowest concentration of 1.25 w% mg/ml. Some optical densities were found to be similar and stable at different concentrations. 3.2 Agar Well Diffusion Method for Acrylic/Acetamide Composites The activity of composite (acrylic/acetamide) against bacteria was calculated by calculating the diameter of the inhibition zone in millimetres for both S.aureus and S.epidermides bacteria. Compared with the acrylic sample only, we note that the activity against the bacteria increased with increasing acetamide concentration, as shown in Fig. 5.

Fig. 3. The change of the turbidity with the change of concentration

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Fig. 4. Growth inhibition ratio of acetamide.

Acrylic was taken as a reference sample against S. aureus and S.epidermidis bacteria. The results showed that antibacterial activity for composite material depended on increased acetamide concentration. Figure 6 shows that acrylic/acetamide has the highest kill rate at 20% mg/ml (16% mm) for S.aureus bacteria. This ratio is constant for all acetamide concentrations of this type of bacteria. These results are identical to those of the MIC test.

Fig. 5. The rate of inhibition of the composite material (acrylic/acetamide).

3.3 Fourier Transforms Infrared (FTIR) Spectroscopy The changes in the chemical structure were studied using infrared spectroscopy. As shown in Fig. 7, we do not notice the emergence of new curve peaks, meaning there is

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Fig. 6. Antibacterial activity of (acrylic/acedamide)

no chemical reaction between acrylic and acetamide. The acrylonitrile butadiene rubber has an acrylic acid, which shows that it is an acrylic coating as shown in Fig. 8, where OH appears at 3610 cm−1 and the C-HX at 2830 cm−1 and C = O at 1720 cm−1 and CO at 1372 cm−1 and 815 cm−1 . These apparent bonds are similar in their location and composition to acrylic acid, indicating that the material used is acrylic acid with a slight creep. As a result of the increased length of the finger at 3610 cm−1 and 1720 cm−1 and decrease in the finger’s length at the rest of the bonds.

Fig. 7. FTIR curves of the composite (acrylic/acetamide)

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Fig. 8. FTIR spectra for Acrylic

3.4 Swelling Studies Swelling behavior is an essential property for specific applications. The study of swelling behavior was performed on acrylic and acrylic/acetamide in the form of 10 × 10 cm2 films, the absorption of films in water with the change of acetamide concentration over time. “The results showed that the swelling rate of pure acrylic was little (9%) because that acrylic paint does not adsorb to water, and this little percentage was due to the porosity of the sample, but when added acetamide, note shrinkage of samples with increasing concentration as shown in Table 1, these results were calculated by Eq. (2) below, because acetamide dissolves in water, the sample had more significant shrinkage at a high concentration of acetamide (20%) wt mg/ml. swelling = Ws − Wd /Wd × 100%

(2)

Table 1. The swelling results Concentration of Acetamide

Weight before immersed in Weight after immersed in water (g) water (g)

% Swelling

Pure acrylic

0.11

0.12

9

20%

0.38

0.11

−71

10%

0.12

0.08

−35

5%

0.16

0.13

−17.5

2.5%

0.15

0.135

−10

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3.5 Wettability In this test, the mechanism was reversed, where the composite material solution was placed as the liquid in the device instead of the distilled water. The purpose is to know the wettability of the coating on the surface, and here the used surface is concrete. Wettability was calculated through the contact angle of the coating, as shown in Fig. 9. The results showed that adding antibacterial agents reduced the contact angle compared to the pure sample, indicating increased hydrophilicity of the coating. As observed from the images below, the greater the concentration decreases θ, thus the more wettability.

Fig. 9. Wettability measurements of composite material Acrylic / Acetamide.

3.6 Tensile Test and Young Modulus. Figures 10 and 11 below shows the effect change of acetamide concentration on tensile strength and young modulus. Acrylic film has a lower tensile strength than composite (Acrylic/Acetamide) at concentration Acetamide (10, 5 wt% mg/ml), which means that within this range of Acetamide content the tensile properties of the coating are improved, but at 20 wt% mg/ml, the coating loses its mechanical properties when it is dry 9and becomes unstable gelatin material, because than naturally Acetamide was working as plasticizer of polymer. The results showed that the mechanical properties decreased by increasing acetamide because acetamide works on the spacing of the chains and does not form any bonds with the main or secondary chain in the acrylic. This is evident in FTIR spectrum.

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Fig. 10. Tensile strength of composite material films with the change of concentration of acetamide.

Fig. 11. Young modulus of composite material films with a change in the concentration of acetamide.

4 Conclusions The possibility of preparing antibacterial composite polymeric coatings by adding organic or inorganic agents kills bacteria at low concentrations. The results showed that the composite material coating/acetamide possesses properties against bacteria and tensile properties at the concentration of% p. Also, the properties of water and adhesion were achieved on the surface of the concrete. Conflict of Interest. The Authors Declare that They Have no Conflict of Interest.

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References 1. Siedenbiedel, F., Tiller, J.C.: Antimicrobial polymers in solution and on surfaces: overview and functional principles. J. Polymers. 4, 46–71 (2012) 2. Ali, Z.A., Yahya, R., Puteh, R.: Antibacterial polymer based transparent coating for elimination of Staphylococcus aureus. J. Procedia-Soc. Behav. Sci. 195, 2218–2220 (2015) 3. Hasan, J., Crawford, R.J., Ivanova, E.P.: Antibacterial surfaces: the quest for a new generation of biomaterials. Trends Biotechnol. 31, 295–304 (2013) 4. Liquitex: The Acrylic Book: A Comprehensive Resource for Artists: Liquitex Artist Materials (2007). 116 p. 5. Speight, J.G.: Handbook of Industrial Hydrocarbon Processes. Gulf Professional Publishing (2019) 6. Frost, R., Kristof, J., Paroz, G., Kloprogge, J.: Intercalation of kaolinite with acetamide. Phys. Chem. Minerals 26, 257–63 (1999) 7. Banjara, R., Jadhav, S., Bhoite, S.: MIC for determination of antibacterial activity of di-2ethylaniline phosphate. J. Chem. Pharm. Res. 4, 648–652 (2012) 8. Irobi, O., Moo-Young, M., Anderson, W., Daramola, S.: Antimicrobial activity of bark extracts of Bridelia ferruginea (Euphorbiaceae). J. Ethnopharmacol. 43, 185–190 (1994) 9. Majdi, H.S., Saud, A., Al-Mamoori, M.: A novel nanocomposite (SR/HA/-nZnO) material for medical application. In: Tiginyanu, I., Sontea, V., Raileanof, S. (eds.) ICNBME 2019. Springer, Cham (2020). https://doi.org/10.1007/978-3-030-31866-6_63

Interaction Between Thin Layers of Polysaccharides Studied by Quartz Crystal Microbalance with Dissipation (QCM-D) Sergiu Coseri(B)

, Gabriela Biliuta , and Andreea Laura Chibac-Scutaru

“Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, Iasi, Romania [email protected]

Abstract. In this paper, the adsorption behavior of both unoxidized pullulan and its C6 oxidized correspondent, onto cellulose films derived from trimethylsilyl cellulose (TMSC), was monitored using one of the most sensitive techniques namely Quartz Crystal Microbalance with Dissipation (QCM-D). Pullulan (Pu) was converted into its oxidized counterpart using a selective oxidation protocol that targets only the C6 atom of the anhydroglycosidic unit and involves the presence of a trio of reagents: sodium hypochlorite, sodium bromide (co-oxidants), and a stable radical, i.e., 2,2,6,6-tetramethylpiperidin-1-yl (TEMPO), in the role of mediator in water. The oxidation reaction was carried out at room temperature, and the resulted product, oxidized pullulan (OxP) analyzed using FTIR and 13 C-NMR. Buffer solutions of Pu and OxP were prepared at various pH values, and added into contact with thin cellulose layers, the interaction being in situ monitored by QCM-D. The cellulosic matrix deposited on the QCM-D crystals has been prepared by using trimethylsilyl cellulose (TMSC) as a precursor, following spin coating procedure and subsequent hydrolysis under acidic environment. Under the experimental conditions, the QCM-D studies demonstrate that at pH 5 and higher electrolyte concentrations, the highest adsorption occurs. Pullulan that hasn’t been oxidized, adsorbs more effectively than its 6-carboxy derivative, which may be explained by the former’s low water solubility and the potential for weak repulsive forces to form between OxP’s anionic charged groups and the surface of the cellulose. Keywords: Pullulan · Cellulose · QCM-D · SPR · Oxidation

1 Introduction Pullulan is a biodegradable water-soluble extracellularly polysaccharide, produced from starch by the yeast-like fungus Aureobasidium pullulans, consisting of maltotriose units connected through α-(1 → 4) glycosidic bonds, whereas consecutive maltotriose units are linked to each other by α-(1 → 6) glycosidic bonds. Pullulan is known for its nontoxicity and biocompatibility [1, 2]. It has several commercial applications, primarily in the food and pharmaceutical industries. Pullulan is also being investigated for its biomedical applications in several areas, for example targeted drug and gene delivery, © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 324–331, 2024. https://doi.org/10.1007/978-3-031-42775-6_36

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tissue engineering, and wound healing. Due to greater reactivity and lower steric hindrance than other two hydroxyl groups in pullulan, the primary OH group, which exists on two out of the three glucose units, has attracted considerable attention for chemical modifications [1–3]. Thereby, the incorporation of carboxylic acid groups in the pullulan backbone will provide anionic compounds, which allows obtaining polymer solutions with various properties and viscosity. Today, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is widely used for surface carboxylation of different polysaccharides, employing sodium hypochlorite, sodium bromide in homogeneous conditions, using water as solvent, at pH ~ 10, the primary hydroxyl groups being selectively converted to sodium carboxylates [4]. Notably, other non persistent nitroxyl radicals such as phthalimide-N-oxy (PINO) were tested as efficient mediators in the case of different polysaccharides’ oxidation [5– 9]. The degree of oxidation can be easily controlled by adjusting the sodium hypochlorite stoichiometry. The oxidized product, in the case of pullulan, OxP, is even more watersoluble than pullulan itself, and possesses exceptional features for drug delivery and other applications. [10, 11]. The purpose of this study was to investigate the attachment efficiency of pullulan (Pu) and oxidized pullulan (OxP) onto thin cellulose films derived from trimethylsilyl cellulose (TMSC), deposited on quartz crystals by using quart crystal microbalance with dissipation (QCM-D) and surface plasmon resonance (SPR) techniques.

2 Experimental Section 2.1 Materials Pullulan (Mw = 150 kDa) purchased from TCI Europe was dried under vacuum at 120 °C overnight prior to use. 2,2,6,6-tetramethylpiperidin-1-yl (TEMPO) 99% Sigma-Aldrich, sodium hypochlorite (NaOCl, 9% chlorine, Chemical Company Romania) and sodium bromide (99% Alfa Aesar) were used as received. Trimethylsilyl cellulose (TMSC, degree of substitution 2.8, Mw = 185,000 g mol−1 ) from TITK, Rudolstadt, Germany. The water used for sample solution preparation and rinsing was of Milli-Q ultrapure grade with a resistivity of 18.2 M−1 cm−1 . 2.2 Pullulan Oxidation Oxidation of pullulan was carried out as follows: 1 g of pullulan was dispersed in 30 mL distilled water, then 0.1 mmol TEMPO and 1 mmol sodium bromide were added to the pullulan dispersion. After the pH was adjusted to 10 with a few drops of 0.5 M hydrochloric acid, a volume of sodium hypochlorite corresponding to 10 mmol/g pullulan was added. The pH was carefully checked with a pH meter instrument and maintained at 10 by adding 0.4 M sodium hydroxide solution. After 4 h, the reaction was stopped with 5 mL of methanol and then acidified to a pH of 6.8. A large volume of ethanol has been used to precipitate the reaction products and the formed precipitate was collected by centrifugation. The oxidized pullulan was re-dissolved in water, dialyzed and finally freeze-dried.

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2.3 Sensors Cleaning, Preparation of Cellulose Films and Adsorption Studies Prior to spin coating, both QCM-D and SPR gold sensors were subject to an intensive cleaning treatment, in a 5:1:1 mixture of Milli-Q water, H2 O2 (30%) and NH4 OH (25%) for 5 min at 70 °C, then extensively rinsed with MilliQ water and blow dried with nitrogen gas. All QCM-D experiments were performed at 30 °C. 50 μl toluene solution of TMSC were spin coated at 2,000 rpm at an acceleration of 2,500 rpm s−1 for 120 s onto a 14-mm quartz crystal. The spin-coated TMSC films were subsequently exposed for 120 s in a 2 M HCl vapor chamber, which caused the full conversion of the TMSC into cellulose. The cellulose films thus prepared were used for anchoring the pullulan (Pu) and the oxidized pullulan (OxP). Pu and OxP solutions (0.1%) were prepared by dissolving 0.1 g Pu or OxP in 99.9 mL of 1 mM or 100 mM NaCl solution. 2.4 NMR Determinations The NMR spectra were obtained on a Bruker Avance DRX 400 MHz spectrometer, equipped with a 5 mm QNP direct detection probe and z-gradients. 2.5 QCM-D Measurements The QCM-D experiments were performed with a quartz crystal microbalance with a dissipation unit (QCM-D) type Q-Sense (Goteborg, Sweden). The analyses were performed on cellulose surfaces, prepared using spin coating technique. The quartz crystals (supplied by Q-Sense AB) were AT-cut quartz with gold plate electrodes and with gold on the active surface. The fundamental frequency of quartz crystals is f0 ≈ 4.95 MHz and sensitivity constant C = 0.177 mg/m2 Hz.

3 Results and Discussion TEMPO-Mediated Oxidation of Pullulan. Pullulan has been oxidized by using sodium hypochlorite and sodium bromide, at room temperature, and TEMPO serving as mediator for the oxidation reaction. The possibility for complete oxidation of pullulan to its carboxylate, and the high regioselectivity of this reaction for C6-OH were the motivation to employ this methodology to obtain pullulan derivatives containing carboxyl groups. The oxidized sample is fully oxidized at C6-OH (all OH groups are converted to carboxyl groups), confirmed by means of 13 C NMR spectroscopy (Fig. 1), since the CH2 OH peak is absent. Moreover, there is no resonance in the 195–205 ppm region in the spectra, indicating that no ketone group is present after the oxidation. The spectrum of unmodified pullulan, showing the signal of C6 atom (6g1 → 4) at 69.26 ppm and C6 atom (4g1 → 4 and 4g1 → 6) at 63.19 ppm. Also the signals of the C1 atom around 100.68–102.99 ppm, C4 atom at 80.56 ppm and C2,3,5 atoms between 72.26 and 76.21 ppm can be observed (Arnosti & Repeta, 1995).

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Fig. 1. 13 C NMR spectra of unoxidized (native) and C6 fully oxidized (OxP) pullulan samples.

The FTIR spectra of oxidized pullulan and pullulan were recorded to evaluate possible structural changes after oxidation (Fig. 2). The FTIR spectra of oxidized pullulan in acidic form shows a carboxylic acid band at 1701 cm−1 , showing that there is evident transformation from hydroxyl to carboxylic acid group upon oxidation of pullulan.The main absorption bands of pullulan can be found at: 3500 cm−1 region due to OH stretching vibration (νOH), 2926 cm−1 corresponding to the CH stretching vibration (νCH), 1653 cm−1 adsorption band of bounded water, 1458 cm−1 assigned to symmetric CH2 bending vibration and 930 cm−1 assigned to COC stretching at β-(1 → 4)-glycosidic linkages. The adsorption band at 1610 cm−1 can be also attributed to the CO stretching of free carboxylate groups, since the adsorption band at 1417 cm−1 represents the CO symmetric stretching of dissociated carboxyl groups (Saito et al., 2009). The other bands appear nearly unchanged and therefore a successful oxidation could be concluded. The cellulose films, were prepared starting from trimethylsilyl cellulose (TMSC), by spin coating, followed by the cellulose regeneration by exposing the films to the 10% (by weight) vapors of aqueous hydrochloric acid, at room temperature. The cleavage of trimethylsilyl side groups by acid vapor leads to well-defined regenerated cellulose films (Fig. 3). Characterization of TMSC and Regenerated Cellulose Films. TMSC is a quite suitable precursor to obtaining cellulose films. The solubility of TMSC depends of degree of substitution (DSSi ), thus the TMSC with DSSi smaller than 1.5 is soluble in polar

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Fig. 2. FTIR spectra of the pullulan (Pu) and oxidized pullulan (OxP) samples.

Fig. 3. Hydrolysis of TMSC (DSSi = 2.5) to cellulose by 10% HCl solution treatment.

solvents, like ethanol or DMSO while TMSC with DSSi bigger than 2.5 is soluble in unpolar solvents, like toluene and chloroform (our case). The solubility of TMSC is an important parameter when studying the interaction of regenerated cellulose with other polymers, the morphology and thickness of the layers depending on the TMSC solubility. For the preparation of the cellulose films, TMSC was deposited on the QCM crystals by spin coating, followed by cleavage of the silyl groups under a vapors precinct of HCl. Adsorbtion of Pullulan and Oxidized Pullulan on Cellulose Film Monitored by QCM-D and SPR. The QCM-D technique provides valuable information in real time on the adsorption of various components onto a quartz crystal. The method principle is based on the change in resonance frequency of a quartz crystal, due to an increase of mass as a consequence of deposition of different materials. Regeneration of cellulose from TMSC provides free –OH groups, which serves for further anchoring of Pu and OxP as proposed in Fig. 4. To ensure the anchoring of Pu and OxP to cellulose films, all the reactions were performed in situ, in the QCM-D chamber, by introducing a continuous flow of a

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Fig. 4. Proposed film stacking and subsequent reaction with Pu and OxP.

0.1 mL min−1 of 0.1% solution of Pu or OxP either in 1 mM or 100 mM NaCl solutions respectively, in order to emphasize the role of the added salt concentration. Pullulan adsorb better than OxP on cellulose films in both 1 and 100 mM NaCl solutions. Another parameter that can influence decisively the adsorption processes is pH value. Therefore, we considered useful to carry out the study of the adsorption of the Pu and OxP at three different pH values, ranging from 2 to 8. The pH value has a great influence on the adsorption behavior. The optimum value of the maximum adsorption of both Pu and OxP is found to be 5, which is close to isoelectric point of the pullulan. Figure 5 shows the changes in the frequency and dissipation after in situ deposition of Pu in 100 mM NaCl at pH = 5 on cellulose films. It can be seen that the Pu in the 100 mM salt solution at pH = 5, exhibits the highest adhesion with a change in the vibration frequency of −12.5 Hz and the dissipation shows an increase of Pu immobilized on the cellulose film and a change of 1.40 at the time of washing with the 100 mM salt solution.

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Fig. 5. Changes in frequency (f 3 ) and dissipation (D3 ) in time after in situ deposition of pullulan (a) and (b) (in 100 mM NaCl solution) at pH = 2, and oxidized pullulan (c) and (d) at pH = 5 on cellulose film.

4 Conclusions One of the most challenging polysaccharides with applications in the healthcare and pharmaceutical industries, pullulan has seen significant growth in recent years in the adsorption of active principles. In this paper, the pH and electrolyte type influence on pullulan and 6-carboxy pullulan adsorption on the cellulose thin films is studied using one of the most sensitive, mass-dependent techniques, i.e. QCM-D. Pullulan was oxidized using a selective system, which includes the stable radical 2,2,6,6-tetramethylpiperidin1-yl (TEMPO) as mediator, sodium hypochlorite and sodium bromide, as actual oxidants, at pH = 10, at ambient temperature. The QCM-D experiments show that the maximum adsorption occur at pH 5 using larger concentration of electrolyte. Unmodified pullulan adsorbs better in comparison with its 6-carboxy derivative, which could be explain by the poor water solubility of the first and the weak repulsive forces might appear between the anionic charged groups in OxP and cellulose surface. Acknowledgments. This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI – UEFISCDI, project number PN-III-P4-ID-PCE-2020–0476, within PNCDI III.

Conflict of Interest. The Authors Declare that They Have no Conflict of Interest.

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References 1. Spatareanu, A., Bercea, M., Budtova, T., Harabagiu, V., Sacarescu, L., Coseri, S.: Synthesis, characterization and solution behaviour of oxidized pullulan. Carbohyd. Polym. 111, 63–71 (2014). https://doi.org/10.1016/j.carbpol.2014.04.060 2. Coseri, S., et al.: Green synthesis of the silver nanoparticles mediated by pullulan and 6carboxypullulan. Carbohyd. Polym. 116, 9–17 (2015). https://doi.org/10.1016/j.carbpol.2014. 06.008 3. Coseri, S., Spatareanu, A., Sacarescu, L., Socoliuc, V., Stratulat, I.S., Harabagiu, V.: Pullulan: a versatile coating agent for superparamagnetic iron oxide nanoparticles. J. Appl. Polym. Sci. 133(5), 42926 (2016). https://doi.org/10.1002/app.42926 4. Denooy, A.E.J., Besemer, A.C., Vanbekkum, H.: Highly selective nitroxyl radical-mediated oxidation of primary alcohol groups in water-soluble glucans. Carbohyd. Res. 269(1), 89–98 (1995). https://doi.org/10.1016/0008-6215(94)00343-e 5. Coseri, S.: A new and efficient heterogeneous system for the Phthalimide-N-oxyl radical (PINO) generation. Eur. J. Org. Chem. 11, 1725–1729 (2007). https://doi.org/10.1002/ejoc. 200601072 6. Coseri, S.: N-Hydroxyphthalimide (NHPI)/lead tetraacetate, a peculiar system for the Phthalimide-N-Oxyl (PINO) radical generation. Mini-Rev. Org. Chem. 5(3), 222–227 (2008). https://doi.org/10.2174/157019308785161675 7. Coseri, S.: N-Hydroxyphthalimide (NHPI)/lead tetraacetate reactions with cyclic and acyclic alkenes. J. Phys. Org. Chem. 22(5), 397–402 (2009). https://doi.org/10.1002/poc.1466 8. Coseri, S.: Phthalimide-N-oxyl (PINO) radical, a powerful catalytic agent generation and versatility towards various organic substrates. Catal. Rev. Sci. Technol. 51(2), 218–292 (2009). https://doi.org/10.1080/01614940902743841 9. Coseri, S., Biliuta, G., Simionescu, B.C.: Selective oxidation of cellulose, mediated by Nhydroxyphthalimide, under a metal-free environment. Polym. Chem. 9(8), 961–967 (2018). https://doi.org/10.1039/c7py01710c 10. Dulong, V., Le Cerf, D., Picton, L., Muller, G.: Carboxymethylpullulan hydrogels with a ionic and/or amphiphilic behavior: swelling properties and entrapment of cationic and/or hydrophobic molecules. Colloids Surfaces A-Physicochem. Eng. Aspects 274(1–3), 163–169 (2006). https://doi.org/10.1016/j.colsurfa.2005.08.045 11. Posey-Dowty, J.D., Watterson, T.L., Wilson, A.K., Edgar, K.J., Shelton, M.C., Lingerfelt, L.R.: Zero-order release formulations using a novel cellulose ester. Cellulose 14(1), 73–83 (2007). https://doi.org/10.1007/s10570-006-9079-7

The Critical Size Bone Defects - In-Vivo Experimental Method of the Treatment with the Decellularized Vascularized Bone Allografts Elena Pavlovschi1(B)

, Alina Stoian1

, Grigore Verega2

, and Viorel Nacu2

1 Laboratory of Tissue Engineering and Cellular Culture, Nicolae Testemitanu State University ,

of Medicine and Pharmacy Chisinau, Chisinau, Republic of Moldova [email protected] 2 Department of Orthopedics and Traumatology, Nicolae Testemitanu State University of , Medicine and Pharmacy Chisinau, Chisinau, Republic of Moldova

Abstract. The critical sized defects occur due to various factors (trauma, tumor resection, congenital anomalies, infections). Contemporary reconstructive orthopedic surgery cannot offer standardized treatment for all CSD, so now it’s a therapeutic dilemma. Modern methods that users have a high level of morbidity and complications. Transplantation of live allogeneic vascularized bone can be potentially the “perfect” solution, only if significant and unjustified risks of long-term immunosuppression will be avoidable. For these reasons, the scientific community has focused its activity on studying simple or combined vascularized bone grafting (with local and systemic factors that grow bone bioactivity). Our work aims to study the local and paraclinical postoperative manifestations after the plasty of the critical bone defect with vascularized bone allotransplantation in the rabbit model METHODS: The 12 rabbits (New Zealand White Rabbits) were divided into three groups, weighing 2.6–4.6 kg. Lot 1 - plasty of critical bone defects with vascularized bone autograft. Lot 2 - plasty of critical bone defects with native vascularized bone allograft. Lot 3 - plasty of critical bone defects with decellularized vascularized bone allograft. We have studied the local (on the 1st, 5th, 10th, and 15th postoperative days) and the paraclinical postoperative manifestations (at 14 and 30 days postoperatively) after the lateral intermuscular and the medial approach of the thigh. Keywords: critical size bone defects · decellularized composite grafts · in-vivo experiment

1 Introduction The use of bone transplants has been a successful step in the treatment of a large number of diseases of the osteoarticular system. Bone grafting is a surgical procedure that replaces missing or sick bone and becomes the second most transplanted tissue, next to blood transfusion [1–3]. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 332–347, 2024. https://doi.org/10.1007/978-3-031-42775-6_37

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The history of bone grafting becomes relevant with the initial studies on bone anatomy, vascularization, and microscopic structures, dating from the 17th century, made by Antoni van Leeuwenhoek and Havers [1]. The first well-described autograft was performed by the German Philips von Walter in 1820 [4]. And the first reported use of allograft bone was in 1880, made by MacEwen [5]. The concept of vascularized bone graft was first reported by Huntigton in 1905 when he used the fibula on the pedicle for bone defect restructuring [6]. The vascularization of bone was investigated together with the process of graft implementation in orthopedics treatment. The blood supply in long bones is provided by multiple sources (the nutrient arteries, periosteal arterial supply, and arterio-venous sinuses in the medulla). Recent studies demonstrated that the principal blood supply of healthy long bones was received from the primary nutrient arteries [7]. Blood vessels transport oxygen, nutrients, and regulators factors to tissues, and waste. That way, vascularization is vital for bone defects to heal. The trauma or other major bone injury damages the blood supply, which may be maintained by the subsequent inflammation. The resulting hypoxia and the acidosis have an exacerbated negative influence on matrix mineralization and stimulate mature osteoclasts to resorb bone tissue. The osteoblast’s functions are inhibited in hypoxia, so osteoblasts begin a latent state. On the other hand, hypoxia stimulates the reproduction of osteoclasts from mononuclear precursor cells, resulting in increased bone resorption [7, 8]. Large bone defects of bones occur due to various factors (trauma, tumor resection, congenital disabilities, infections). Critical-sized bone defects (CSD) are defined as too large and will not heal through only natural processes within a patient’s lifetime. Such defects are generally accepted to be ≥ 1.5 - 2 times the diameter of the long bone diaphysis, or in adult patients, those have circumferential loss ≥50% or a length of ≥2 cm2 . [9] Their critical size exceeds the intrinsic capacity of self-regeneration, and consequently, bone healing is disrupted and insufficient [3, 10, 11]. Skeletal reconstruction of large bone defects is a significant challenge for the reconstructive surgeon. Contemporary methods that are used in these types of bone defects have a high level of morbidity and complications. Modern reconstructive orthopedic surgery cannot offer standardized treatment for all CSD, so now it’s a therapeutic dilemma. The treatment is based on multiple clinical factors, including defect size, patient comorbidities, soft tissue, and vascularization condition [12, 13]. The current gold standard approach for CSD is an autologous free vascularized bone flap because of the patient’s cells, growth factors, and a vascularization bed. However, impediments to this option include lack of supply, donor site morbidity, infection, and the need to sculpt the anatomy of the residual bone defect. Other alternatives implemented in the treatment of CSD, allogeneic and xenogeneic bone grafts, have those impediments such as immunogenicity, lack of vascularization, disease transmission, and in some cases, donor shortage [3, 11]. Specialized literature indicates the ideal method for treating a CSD as being functionally and biologically identical. The graft should provide immediate mechanical strength, heal promptly, and remain biologically active. In other words, this one must possess

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characteristics similar to those of autografts without their limitations. But, vascularization has been identified as a central component of influencing bone healing and callus regeneration. In circumstances of limited blood supply, the choice of a vascularized bone graft seems imminent. Transplantation of live allogeneic vascularized bone can be a potential “perfect” solution, only if significant and unjustified risks of long-term immunosuppression can be avoided [14–16]. Decellularization involves the removal of cells and associated antigens from the scaffold by chemical and enzymatic agents. At the same time was keep the integrity of the extracellular matrix (ECM), including the architecture of the vascular wall. Another important advantage of these techniques lies in preserving the integrity of the vascular channel which makes it suitable for cell repopulation (recellularization) and for ensuring cell migration and proliferation [20–22]. In this context, we set out to investigate whether vascularized decellularized bone allotransplantation re-included in the host’s vascular circuit produces bone consolidation more efficiently than vascularized bone autotransplantation (H0 ). Our work aims to study the local and paraclinical postoperative manifestations after the plasty of the critical bone defect with vascularized bone allotransplantation in the rabbit model.

2 Methods 2.1 General Aspects This study is part of the first author’s Ph.D. research. The project was approved by the Nicolae Testemit, anu State University of Medicine and Pharmacy Ethics’ Research Committee on 21.05.2018. The first phase of our study was performed on a New Zealand White Rabbit animal model in the Laboratory of Tissue Engineering and Cellular Culture, in Chisinau. The study was supported by Nicolae Testemit, anu State University of Medicine and Pharmacy, and the research 20.80009.5007.20 was offered by the National Agency for Research and Development of the Government of the Republic of Moldova. During the period 01. 06.–02.12.2022, the twelve laboratory animals (New Zealand White Rabbit - NZWR) were deployed in the vivarium of the Nicolae Testemit, anu State University of Medicine and Pharmacy. The animals weighing 2.6–4.6 kg, were divided into three lots (the group distribution), four animals each. Lot 1 - plasty CSD of bone defects with vascularized bone autograft (control lot) Lot 2 - plasty CSD of bone with native vascularized bone allograft Lot 3 - plasty CSD of bone defects with decellularized vascularized bone allograft. We have studied the local and paraclinical postoperative manifestations after the lateral intermuscular and the medial approach of the thigh to determine the stages and mechanisms of acute transplant rejection [26]. A. Local postoperative manifestations: – the appearance of the wound: tumor, heat, redness, elimination, scarring – functionality of the operated limb: support on the limb, walking, jumping - on the 1st , 5th , 10th , and 15th postoperative day. B. Postoperative paraclinical manifestations:

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– General blood analysis with leukocyte formula – Blood biochemical analysis with bone indices: Ca, P, Alkaline Phosphatase – X-ray of the operated hind limb at 14 and 30 days postoperatively. 2.2 Operative Procedure The research carried out allowed the protocol of the lateral intermuscular surgical access of the lateral region on the thigh, for the orthotopic grafting of the femur segment for artificially performed CSD plasty at NZWR. The protocol was divided into preoperative and four intraoperative stages. The preoperative stage was performed according to the Guidelines on Anesthesia and Analgesia in Rabbits (Michigan University) and the Rabbit-Specific Anesthesia (the University of Texas at Austin), adapted to our study intraoperative needs [23, 24]. The animals were kepted in quarantine for minimum of 72 h, adapted to handling, and evaluated for obvious clinical signs of disease. NZWR were weighed prior to surgery to ensure the correct drugs dministration. A pre-anesthetic fast of 1–4 h was recommended. The rabbit’s head was slightly elevated to reduce pressure on the diaphragm [24]. The sedation started with the inhalation of Isoflurane solution 1000 mg/250 ml 3–5% in the induction chamber. The induction continued with intramuscular gluteal administration of Ketamine 44 mg/kg combined with Xylazine solution 3 mg/kg, and the maintenance was achieved with 2–3%. Isoflurane 1000 mg/250 ml until the end of the surgery. Ophthalmic ointment should be applied on the eyes to prevent corneal drying during anesthesia or sedation. Providing fluid support during anesthesia is important particularly if a procedure lasts one-half an hour or more. Intravenous saline was administered through the lateral auricular vein, 12 ml/kg/h. Preoperative antibiotic prophylaxis was performed through intramuscular Ceftriaxone solution 24 h before surgery, 100 mg/kg. The Ps, Pa O2, TA monitoring was performed by Advanced® PO-100B Pulse Oximeter. Intrarectal temperature monitoring and body temperature maintenance during the intervention were possible using the Harvard Homeothermic Blanket Control Unit115 V, 60 Hz 55 VA (Fig. 1).

Fig. 1. The preoperative care and support during anesthesia in the NZWR model

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The surgery protocol was divided into the lateral and the medial approach to the thigh. A.The lateral intermuscular approach of the femur in the rabbit model was divided into four intraoperative stages: 1. Preparation of the operative field. 2. Incision of the skin tissue, superficial fascia lata, preparation of the access between the vastus lateralis muscle and the short head of the biceps femoris muscle, with access to the lateral part of the diaphysis of the femoral bone located between the crureus muscle and the vast intermedius muscle 3. Performing a segmental osteotomy (2.5 cm) of the femur with the preparation of the vascular pedicle up to the internal iliac artery, with the formation of a massive bone defect. In our previous part of the study, we determined the optimal segment for vascularized allografting (the rabbit model): the upper third of the femur with the up to the level of the internal iliac artery [25] (Fig. 2). 4. Layered suturing of the operative wound. B. The medial approach for the end-to-side anastomosis of the decellularized bone allograft pedicle with the femoral artery in the host circuit was divided into three stages. 1. The first stage was the preparation of the operative field and the medial macroscopic approach of the pedicle. The neurovascular bundle of the thigh (the artery, vein, and femoral nerve) was located on the obturator and pectineus muscles. 2. Microsurgical preparation and end-to-side anastomosis in the femoral triangle was the second stage. From the host artery. 3. At the end of the stage, anatomical muscle-to-skin closure followed by 5/0 monofilament nylon suture (Fig. 3). According to the established anesthesia and operator protocol (the lateral intermuscular approach - for osteotomy and osteosynthesis and the medial approach for the lateral-terminal anastomosis of the pedicle with the femoral artery) in the 3rd stage of the research, the comparative research was carried out on three groups of 4 laboratory animals NZWR: Lot 1 - plasty of vast bone defects with vascularized bone autograft Lot 2 - plasty of vast bone defects with native vascularized bone allograft Lot 3 - plasty of vast bone defects with decellularized vascularized bone allograft Postoperative research in each subject of the study of the content: A. Local postoperative manifestations: • the appearance of the wound: tumor, heat, redness, elimination, scarring - on the 1st, 5th, 10th, 15th postoperative day • functionality of the operated limb: support on the limb, walking, jumping - on the 1st, 5th, 10th, and 15th postoperative day B. Postoperative paraclinical manifestations. • General blood analysis with leukocyte formula • Blood biochemical analysis with bone indices: Ca, P, Alkaline Phosphatase

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• X-ray of the operated hind limb at 14 and 30 days postoperatively.

Fig. 2. The intraoperative scheme and stages of the lateral intermuscular approach of the thigh region on the rabbit model

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Fig. 3. The intraoperative scheme and stages of the medial approach for the end-to-side anastomosis of the decellularized bone allograft pedicle with the femoral host artery.

2.3 Statistical Methods The One Way ANOVA procedure that compares the means of two or more independent groups in order to determine whether there is statistical evidence that the associated population means are significantly different. This statistical test was used to compare groups for local and general postoperative manifestations, to estimate the reject aspects. A requirement for the ANOVA test is that the variances of each comparison group are equal. We have tested this using the Levene statistic that was greater than .05. You don’t want a significant result, since a significant result would suggest a real difference between variances. This is not a significant result, which means the requirement of homogeneity of variance has been met, and the ANOVA test can be considered to be robust. All statistical tests were compared with P values of 0.05) (Table 3). Table 3. The values of Means leucocyte formula analysis on the 14 and 28-day postoperative period. NAME

Day 14

Day 28

Lot I

Lot II

Lot III

Mean

Mean

Mean

WBC

11.6750

14.8325

9.6675

Gran

6.1275

7.6100

6.8400

Lym

20.8575

7.0775

4.0975

Mon

.8150

.7950

.6825

WBC

7.31

18.93

9.05

Gran

4.23

10.37

6.67 (continued)

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Table 3. (continued) NAME Lot I

Lot II

Lot III

Mean

Mean

Mean

Lymf

4.55

8.86

4.41

Mon

.70

.81

.50

- Blood biochemical analysis with bone indices: Ca, P, Alkaline Phosphatase for the examination of the degree of osteolysis (Table 4) [28]. Table. 4. The values of Means and the One-way ANOVA description of the postoperative examination of the blood biochemical analysis with bone indices: Ca, P, Alkaline Phosphatase(ALP)indices of the bone destruction (Ca, P, ALP) on the 14 and 28-day postoperative period. NAME

Day 14

Day 28

Lot I

Lot II

Lot III

Mean

Mean

Mean

Ca

3.3175

4.7400

3.0150

P

1.7300

3.9475

1.5475

ALP

144.40

284.05

178.185

Ca

3.98

6.79

3.27

P

2.98

4.61

1.96

ALP

133.23

402.98

199.78

There was a statistically significant difference between groups as demonstrated by one-way ANOVA (F(2,9) = 9.96–60.97). A Tukey post hoc test showed that the postoperative examination of the blood biochemical analysis with bone indices was able to throw the frisbee statistically significantly further than the type of lots (p < 0.05). - X-ray of the operated hind limb at 14 and 30 days postoperatively by Scoring System for Evaluation of the Fracture Healing of Bone Defects on Radiography [27]. According to the score, the radiographic examinations were performed and analyzed on the study groups (Table 5) (Fig. 7).

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Table 5. System for Evaluation of the Fracture Healing of Bone Defects on Radiography the criteria for comparing lots Criteria

Score

Bone formation

- no evidence of bone formation - bone formation occupying 25% of defect - bone formation occupying 50% of defect - bone formation occupying 75% of defect

1 2 3 4

Total radiographic union

- nonunion - possible union - union

1 2 3

Graft-host bone connection

- no connection - cortex to trabecula - cortex to cortex (one side) - cortex to cortex (two side)

1 2 3 4

Fig. 7. The radiographic investigation of the posterior limb of the NZWR, in the postoperative period on the 1st and 28th day.

There was a statistically significant difference between groups as demonstrated by one-way ANOVA (F(2,9) = 1.929–39.00). A Tukey post hoc test showed that the postoperative examination of the the radiographic investigation of the posterior limb of the NZWR, in the postoperative period was able to throw the frisbee statistically significantly further than the type of lots (p < 0.05). There was no statistically significant difference between the Graft-host junction - day 14th like indicators of the postoperative paraclinical manifestations of the graft reject (p = 0.569) (Fig. 8).

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Fig. 8. The postoperative paraclinical manifestations of the graft reject by the radiographic investigation of the posterior limb of the NZWR.

4 Discussions We have described a step-by-step surgical procedure for the decellularized vascularized femoral bone allotransplant in the rabbit model. Maintaining the osteoplastic properties of the vascularized autograft, and combining them with the orthotopic characteristics of an allogenic bone, can be an optimal option the reconstructive surgery of the skeletal system as long as the immunosuppression is solved. Histologically, rabbits’ long bones have a very different microstructure compared with humans. In contrast with other species, such as primates and some rodents, the rabbit has faster skeletal change and bone turnover (significant intracortical, Haversian remodeling). This can make it difficult to extrapolate results from studies performed in rabbits onto the likely human clinical response. However, rabbits are commonly used as an animal model for experimental bone in vivo research [29]. In our previous part of the study, we determined the optimal segment for vascularized allografting (the rabbit model): the upper third of the femur with the up to the level of the internal iliac artery [25]. Our work aims to study the local and paraclinical postoperative manifestations after the plasty of the critical bone defect with vascularized bone allotransplantation in the rabbit model, and we have selected the most accessible methods for determining the immune response through the body’s inflammatory process to an immunogenic agent with paraclinical and Rgx examination for osteodistruction monitoring.

5 Conclusions The intraoperative protocol has been established for two surgical approaches: the lateral intermuscular approach for osteosynthesis of the graft and the medial approach for the microsurgical anastomosis of the decellularized pedicle of the host circuit. The vascularized decellularized bone allotransplantation re-included in the host’s vascular circuit produces bone consolidation with a statistically significant difference

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between the vascularized bone autograft and the native vascularized bone allograft in the critical size defects. Acknowledgments. We would like to express our sincere gratitude to all the individuals and organizations that have contributed to the publication of this research paper. Contributed to the publication of this research paper. We would like to thank our supervisors, Professor Verega Grigore and Professor Nacu Viorel for their invaluable guidance and support throughout the research process. Their expertise and insights were salient in shaping the direction and focus of our research. We are also grateful to the Laboratory of Tissue Engineering and Cellular Culture at „Nicolae Testemit, anu” State University of Medicine and Pharmacy for providing us with the resources and support we needed to complete this project by the research 20.80009.5007.20 offered by the National Agency for Research and Development of the Government of the Republic of Moldova. We also would like to thank our colleague Gherman Cernei, Veterinarian, Veterinary clinic “NagsVet” SRL, Chisinau, the Republic of Moldova, for his valuable insights and suggestions.

Conflict of Interest. The Authors Declare that They Have no Conflict of Interest.

References 1. Willems, W.F., et al.: Bone Graft: Revascularization Strategies. Universiteit van Amsterdam [Host] (2014) 2. Hung, N.N.: Basic knowledge of bone grafting. In: Zorzi A. Bone Grafting, Chapters, pp. 11– 38. InTech, Croatia (2012) 3. Mishra, R., Bishop, T., Valerio, I.L., Fisher, J.P., Dean, D.: The potential impact of bone tissue engineering in the clinic. Regen. Med. 11, 571–587 (2016) 4. Ollier, L.: Traité expérimental et clinique de la régénération des os et de la production artificielle du tissu osseux. V. Masson et Fils (1867) 5. Caldwell, P.E., 3rd., Shelton, W.R.: Indications for allografts. Orthop. Clin. North Am. 36, 459–467 (2005) 6. Shin, A.Y., Dekutoski, M.B.: The role of vascularized bone grafts in spine surgery. Orthop. Clin. North Am. 38, 61–72, vi (2007) 7. Marenzana, M., Arnett, T.R.: The key role of the blood supply to bone. Bone Res. 1, 203–215 (2013) 8. Malhotra, A., Habibovic, P.: Calcium phosphates and angiogenesis: implications and advances for bone regeneration. Trends Biotechnol. 34, 983–992 (2016) 9. Mauffrey, C., Barlow, B.T., Smith, W.: Management of segmental bone defects. J. Am. Acad. Orthop. Surg. 23, 143–153 (2015) 10. Roddy, E., DeBaun, M.R., Daoud-Gray, A., Yang, Y.P., Gardner, M.J.: Treatment of criticalsized bone defects: clinical and tissue engineering perspectives. Eur. J. Orthop. Surg. Traumatol. 28, 351–362 (2018) 11. Vidal, L., Kampleitner, C., Brennan, M.Á., Hoornaert, A., Layrolle, P.: Reconstruction of large skeletal defects: current clinical therapeutic strategies and future directions using 3D printing. Front Bioeng Biotechnol. 8, 61 (2020) 12. Houdek, M.T., Wagner, E.R., Wyles, C.C., Nanos, G.P., 3rd., Moran, S.L.: New options for vascularized bone reconstruction in the upper extremity. Semin. Plast. Surg. 29, 20–29 (2015) 13. Locker, P.H., Arthur, J., Edmiston, T., Puri, R., Levine, B.R.: Management of bone defects in Orthopedic trauma. Bull. Hosp. Jt. Dis. 76, 278–284 (2018)

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14. Houben, R.H., et al.: Neo-angiogenesis, transplant viability, and molecular analyses of vascularized bone allotransplantation surgery in a large animal model. J. Orthop. Res. 38, 288–296 (2020) 15. Oryan, A., Alidadi, S., Moshiri, A., Maffulli, N.: Bone regenerative medicine: classic options, novel strategies, and future directions. J. Orthop. Surg. Res. 9, 18 (2014) 16. Korompilias, A.V., Soucacos, P.N.: Vascularized bone grafts in trauma and reconstructive microsurgery, part 2. Microsurgery 31, 169–170 (2011) 17. Hutton, B., et al.: The PRISMA extension statement for reporting of systematic reviews incorporating network meta-analyses of health care interventions: checklist and explanations. Ann. Intern. Med. 162, 777–784 (2015) 18. Coleman, J.: Subject Guides: Systematic Reviews - Research Guide: Using PICO or PICo (2019) 19. Higgins, J.P.T., et al.: Cochrane bias methods group, Cochrane statistical methods group: the Cochrane collaboration’s tool for assessing risk of bias in randomised trials. BMJ 343, d5928 (2011) 20. Chandra, P., Atala, A.: Engineering blood vessels and vascularized tissues: technology trends and potential clinical applications. Clin. Sci. 133, 1115–1135 (2019) 21. Badylak, S.F., Freytes, D.O., Gilbert, T.W.: Extracellular matrix as a biological scaffold material: Structure and function. Acta Biomater. 5, 1–13 (2009) 22. Guidelines on Anesthesia and Analgesia in Rabbits Unit for Laboratory Animal Medicine, Universyty of Michigan Nov 1, 2018. https://az.research.umich.edu/animalcare/guidelines/ guidelines-anesthesia-and-analgesia-rabbits 23. Rabbit-Specific Anesthesia. https://research.utexas.edu/wp-content/uploads/sites/7/2020/02/ Rabbit_Anesthesia_guidance_ARC.pdf 24. Pavlovschi, E., Stoian, A., Verega, G., Nacu, V.: In vivo experimental study of the arterial supply of the rabbit posterior limb. Moldovan Med. J. 64, 26–32 (2021) 25. Ponticelli, C.: The mechanisms of acute transplant rejection revisited. J. Nephrol. 25(150– 158), 25 (2012) 26. Oryan, A., Monazzah, S., Bigham-Sadegh, A.: Bone injury and fracture healing biology. Biomed. Environ. Sci. 28, 57–71 (2015) 27. Lowe, D., Sanvictores, T., Zubair, M., John, S.: Alkaline phosphatase. In: StatPearls. StatPearls Publishing, Treasure Island (FL) (2022) 28. Mapara, M., Thomas, B.S., Bhat, K.M.: Rabbit as an animal model for experimental research. Dent. Res. J. 9, 111–118 (2012)

Antigenic and Biodegradable Characteristics of the Extracellular Matrices from the Pig Dermis Olga Macagonova1(B) , Adrian Cociug1 , Tatiana T, aralunga1 Vladimir Ciobanu2 , and Viorel Nacu1

,

1 Laboratory of Tissue Engineering and Cellular Culture, Nicolae Testemitanu State University

of Medicine and Pharmacy of the Republic of Moldova, Chis, in˘au, Moldova [email protected] 2 National Center for Materials Study and Testing, Technical University of Moldova, Chisin˘au, , Moldova

Abstract. The present work demonstrates the possibility for fabrication of extracellular matrices from the pig dermis. The obtained matrices were characterized from the point of view of antigenic, biodegradability and the ability to absorb the fluid from the environment, making them prospective for fabrication of intelligent dressings. Five parallel groups of extracellular matrices were established and the mean value was calculated. The size of the grafts was 10 × 10 × 2 mm and the weight of 87.9 ± 3 mg for all the study groups. Histological examination revealed the presence of fewer number of cells. As a result, we were able to remove around 80.5% of the genetic material from the porcine dermal structures, demonstrated by spectrophotometric DNA quantification. In the in vitro graft degradation study in 0.01 M of phosphate buffer solution with the pH 7.4 combined with collagenase, we determined a significant (p < 0.05) loss of graft mass by 91.3% during 35 h. In the absorption test, we obtained a variable depending on the exposure time, respectively the soaked samples ended up exceeding four times the initial mass of 87.9 ± 3 mg at the 4th hour of immersion in the liquid. Acellular grafts from the porcine dermis can play a key role in the wound care and facilitating tissue engineering strategies by the acting as an acellular and immunologically inert scaffold, as a source of the bioactive molecules with the hydrophilic and biodegradable properties. Keywords: Porcine dermis · decellularization · scaffolds · biodegradability · immunogenicity

1 Introduction Tissue engineering is an effective method for the developing skin substitutes and improving the healing process [1–5]. Collagen structures are widely used for the skin tissue engineering and coating materials and is known as one of the most promising biomaterials for the various applications [6]. Modern and intelligent dressings are becoming © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 348–356, 2024. https://doi.org/10.1007/978-3-031-42775-6_38

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more and more in demand [7]. Their values consist not only in a protective barrier, but also in a beneficial environment for the healing, biocompatibility, low toxicity, structural, physical, chemical, immunological properties, good ability to absorb the fluids from the environment [8–10], the possibility of making therapeutic agents and minimal human involvement, which actively support the wound regeneration process. Extracellular matrix (ECM) dressings derived from porcine dermis are an example of such innovative products that facilitate wound healing by providing a moist environment in which cells can thrive while the wound can still breathe and self-drain exudate [4]. The successful application of the scaffold in tissue engineering depends on many characteristics such as the biocompatibility, biodegradability or strength, mechanical and chemical properties, scaffold architecture and manufacturing technologies [11]. For the decellularization of the porcine dermis, Dar-Jen H. et al. used the superficial carbon dioxide decellularization protocol at 30–50 °C under 200–350 bar pressure for 40–90 min, followed by a neutralization step using sodium hydroxide (0.1–1 N) to produce decellularized ECM scaffolds [12]. We evaluated the literature, which included in vitro and in vivo animal models, as well as case reports on the method of measuring the biodegradability of the scaffolds used in the regenerative medicine that was performed by two of the following methods: either by measuring the mass loss in vitro studies, or by histological evaluation at the certain intervals in vivo study models. In in vitro testing, the solutions used were Phosphate Buffer Solution (PBS) or the simulated body fluids [13–16]. The degradation rate of the synthetic membranes was relatively slow (12–24 months) [17]. Naturally engineered membranes without the cross-linking presented a fast degradation rate of about 7–10 days. The cross-linked scaffolds had a slow rate of the autoresorbtion. Controlled degradation was observed in Mg-based bioceramic scaffolds doped with Zn or Cu ions. The samples doped with Cu and hydroxyapatite showed a faster rate of the autoresorbtion compared to the Zn combined samples. Another example of the controlled degradation of the natural scaffolds was described by Park et et al. [15], who concluded that aqueous silk fibroin scaffolds showed a 95% mass loss. However, scaffolds prepared with hexa-flour-isopropanol showed a mass loss of only 7%. In vitro biodegradation of the collagen/hyaluronic acid/gelatin sponge scaffold showed that both 10,000 and 30,000 U/ml lysozyme degraded the scaffold gradually within one week. Scaffold lost about 61.9 ± 2.6% and 63.6 ± 5.1% from the initial weight after 7-day treatments, respectively. After degradation of the acellular dermis with hyaluronidase of 30 U/ml, the scaffold retained approximately 10% of the tissue after a 5-day examination. With 50 U/ml hyaluronidase, the scaffold was degraded after 7 days. The scaffold immersed in 20 U/ml collagenase I for 3 h was almost degraded, and compared to 10 U/ml, the tissue remained about 45% of the original weight. The non-crosslinked collagen/hyaluronic acid/gelatin scaffolds dissolved in the water within 5 min and could not support the solid constructs. Cross-linked structures with 1-ethyl-3 [3-(dimethyl-amino)-propyl] carbodiimide showed a swelling ratio of over 20 g water/g dry scaffold. Without hyaluronic acid (HA), the swelling ratio value decreased by 15% [5]. Gilbert T. et al. described in vivo degradation of the porcine small intestinal submucosa ECM scaffolds integrated with 14C-proline by approximately 50% after 28 days from implantation and was completely degraded by 75 days when used for musculotendinous reconstruction [18]. SEM of the collagen/HA/gelatin scaffolds revealed highly interconnected porous structures, and the

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pore wall surface appeared smooth and homogeneous. The electron microscopy analysis of the sponge-shaped scaffold indicated that it had open macro-porous structures with the pore sizes of 132.5 ± 8.4 μm [19]. The aim of our study was to evaluate the antigenic and biodegradable properties of the extracellular matrices obtained from the porcine dermis.

2 Materials and Methods 2.1 Skin Preparation To achieve the purpose of the study, we examined 30 decellularized porcine dermal grafts. The samples were obtained from the piglets weighing up to 10 kg euthanized by blunt trauma, following the recommendations of the university ethics committee (decision no. 4 of 24.02.2023). As a result, we obtained 30 samples with an area of 10 × 10 × 2 mm and a weight of about 87.9 ± 3 mg. 2.2 Separation Method By treating the tissues with 0.3% trypsin solution at 37 °C, we obtained the dermoepidermal separation of the grafts, according to the protocol [20]. 2.3 Decellularization Method Tissue decellularization was performed by the treating of the porcine dermis with 1% Triton X-100, 4% sodium deoxycholate and washing thoroughly in PBS [21–23]. 2.4 Morphological Assessment Examination of the decellularized samples was performed by the histological examination with hematoxylin-eosin. Samples were fixed in buffered formaldehyde solution 4% [24, 25]. 2.5 Spectrophotometric Method The spectrophotometric method was applied for DNA quantification. Decellularized and native tissues were quantified using a kit (DNA Extraction Kit, Cygnus Technologies, USA). Extracted DNA was quantified in a spectrophotometric microplate reader (NANODROP 2000C).

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2.6 In Vitro Degradation of Dermal Grafts Degradation was performed in vitro by following the weight loss of the grafts in 0.01 M PBS in acidic, neutral and basic media. The freeze-dried scaffold was weighed (m0 ) and immersed in a centrifuge tube containing PBS. The pH of the buffer was 7.4, 4.0 and 10.0, the exposure time being 1, 7, 14, 21 and 28 days under the incubator conditions at 37 °C. Comparatively, graft degradation was monitored in 0.01 M PBS solution with pH 7.4 combined with collagenase from Clostridium histolyticum (≥250 CDU/mg of solid, Sigma-Aldrich, UK) 10 U/ml, the follow-up period being of 1, 5, 8, 24, and 35 h at 37 . The remaining mass fraction (D, %) was calculated using the following formula: D = mx /m0 × 100%,

(1)

where, mx is the final mass of the tissue [26]. Four parallel groups were established and the mean value was calculated. 2.7 Water Absorption Test The absorption of the water revealed the diffusion of the medium into the tissues being necessary for the resorption of the exudate from the wound and the cells cultured on the ECM obtaining essential nutrients. 0.01 M PBS pH 7.4 was used in the fluid absorption test. The time required to follow the dynamics of the weight of the samples was 1, 2, 4, 8, 12 and 24 h at 25 . The soaked samples were then removed from the solution and weighed, and the excess water on the surface of the samples was gently blotted with a filter paper to obtain the WWET . The percentage of water absorption for the samples at different times was calculated as follows: Water absorbtion(%) = 100 ×

WWET − WDRY WDRY

(2)

where, WDRY and WWET are the weights of the dry scaffolds and wet scaffolds at the required times, respectively [27, 28]. Four samples were tested for each scaffold and the mean values were recorded. 2.8 Scanning Electron Microscopy (SEM) The interior of the graft is crucial for appreciating the “microenvironment”. Pore size and connectivity can affect cell adhesion, nutrient exchange, metabolic waste removal and skin regeneration [29]. The morphological characteristics of the acellular dermis scaffolds were investigated using a VEGA Tescan TS 5130 scanning electron microscopy. After washing with phosphate buffer, the decellularized dermis samples were dehydrated and dried under vacuum. The dried samples were then cut and the cross-section was coated with 10 nm of Au. For SEM we used a representative tissue sample from each study group and a non-degraded control sample to monitor ECM disorganization leading to the tissue weight loss.

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3 Results and Discussions 3.1 Histological and Antigenic Analysis Through the histological examination, we highlighted the presence of a fewer number of cells in the decellularized dermis (Fig. 1). As a result, we managed to remove about 80.5% of the genetic material from porcine dermal structures according to spectrophotometric DNA quantification. Respectively, 2.43 ± 0.5 ng/μl was determined in porcine dermis and compared to 17.43 ± 3.4 ng/μl in the intact sample.

Fig. 1. Two segments of decellularized skin, (a) papillary dermis shows collagen fibers without cells, (b) reticular dermis shows collagen fibers without cells, H-E × 140

3.2 In Vitro Degradation of the Dermal Grafts The rate of the dermal grafts degradation in the skin wound should ideally match the rate of the wound regeneration. Thus, if the acellular scaffold degrades rapidly during the early stage of wound regeneration, it will not provide a good barrier for the regeneration itself, and this will eventually lead to the soft tissue expansion into the skin defect, which is not welcome for the organizational regeneration of the soft tissues. As shown in Fig. 2, the degradation rate of the acellular dermal scaffold in 0.01M PBS pH 7.4 combined with collagenase from Clostridium histolyticum was the fastest, reaching a degradation rate of 91.3% during 35 h, while the degradation volume of the acellular scaffolds in 0.01M PBS solution with pH 4.0 and 10 without enzymes, represented 79.8% and 74% of the total sample from the 21st to the 28th day, and then the degradation tended to be slow (Fig. 2b). In 0.01M PBS solution with pH 7.4, the degradation reached 90.3% at the 28th day. The macroscopic Fig. 2c shows the decrease in volume and significant rupture of the grafts starting from the 1st day to the 28th day. 3.3 Water Absorption Test In the absorption test we obtained a variable depending on the exposure time, respectively the soaked samples reached 350 mg at the 4th hour of the immersion in the liquid, the initial mass being 87.9 ± 3 mg. The absorption rate is shown in the chart from Fig. 3.

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Fig. 2. (a) Degradation rate of the acellular dermal scaffold. Degraded acellular dermal scaffolds represent statistically significant values (p < 0.05) compared to the non-degraded part; (b) degradation behavior of acellular dermis in the presence of collagenase (10 U/ml, PBS pH 7.4, 35 h); (c) distribution of the acellular scaffolds depending on the duration of exposure and the pH of PBS chosen for the study.

350 300 250 200

portion of the initial graft weight (%) water absorption rate (%)

150 100 50 0

1 hour

2 hours 4 hours 8 hours 12 hours 24 hours

Fig. 3. Water absorption rate in the dermal graft.

As shown in Fig. 3, the rate of the water absorption in the acellular dermal sample was significant (p < 0.05) even at the first hour of the exposure, it exceeded the initial mass of the dry scaffold by 222.7%. While the absorption rate reached 307.1% of the total sample after 8 h and 305.7% after 12 h and then the absorption rate tended to be stable. 3.4 Scanning Electron Microscopy During the degradation of the grafts, we recorded macroscopically the denaturation of the samples and the decrease of the mechanical resistance until the subtotal degradation of the tissues on the 28th day. This fact required us to follow the disorganization of tissue ultrastructure at SEM. Degradation of the tissues in PBS 0.001 M pH 7.4 with collagenase led the rupture of the pore walls starting from the first hour (Fig. 4b), resulting in autoresorbtion of the grafts until the 35th hour (Fig. 4e). The samples are disorganizing, resulting in a significant reduction in weight (Fig. 4b-f). Triton/SDS decellularization and degradation removed cells and resorbed tissue resulting in a honeycomb appearance of ECM (Fig. 4b-f) and preservation of the dermal structures (Fig. 4a), which is a landmark for the tissue decellularization and degradation.

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Fig. 4. SEM images of the porcine dermis following the degradation of acellular scaffolds depending on the duration of exposure in days. The pores of the non-degraded porcine dermis (a), the pores of the degraded porcine dermis at one hour (35.9 ± 12.2 μm) (b), at 5 h (100.4 ± 18.0 μm) (c), at 8 h (63.9 ± 22.5 μm) (d), at 24 h (42.8 ± 9.1 μm) (e), at 35 h (52.6 ± 10.9 μm) (f).

4 Conclusions Effective dermal scaffold must consider several important factors: management of the tissue deficit, prevention of the infection, control of the inflammation and improvement of the skin wound margins. Obtaining intelligent biological grafts for the skin wound regeneration is one of the research directions of the modern tissue engineering and represents an attractive segment of the regenerative medicine. A xenogeneic material is easier to obtain in the commercial quantities, which would cover the needs for clinical application. Scaffolds obtained from decellularized porcine dermis have great potential as a source of bioactive molecules, due to the appropriate pore size, immunological inertness, hydrophilic properties, and biodegradability. It is necessary to carry out further studies regarding cross-linking, recellularization and cytocompatibility of the grafts. Acknowledgments. This work was supported by the State Program “GaN-based nanoarchitectures and three-dimensional arrays of biological materials for applications in microfluidics and tissue engineering” with the number 20.80009.5007.20 and the young researchers project “The development of the biological dressings for the regeneration of skin wounds through the tissue engineering”.

Conflict of Interest. The authors declare that they have no conflict of interest.

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16. Gomes, M.E., Azevedo, H.S., Moreira, A.R., Ellä, V., Kellomäki, M., Reis, R.L.: Starch– poly(ε-caprolactone) and starch–poly(lactic acid) fibre-mesh scaffolds for bone tissue engineering applications: structure, mechanical properties and degradation behaviour. J. Tissue Eng. Regen. Med. 2(5), 243–252 (2008). https://doi.org/10.1002/term.89 17. Kawase, T., et al.: The heat-compression technique for the conversion of platelet-rich fibrin preparation to a barrier membrane with a reduced rate of biodegradation: heat-compressed PRF as an effective barrier membrane. J. Biomed. Mat. Res. Part B Appl. Biomater. 103(4), 825–831 (2015). https://doi.org/10.1002/jbm.b.33262 18. Gilbert, T.W., Stewart-Akers, A.M., Badylak, S.F.: A quantitative method for evaluating the degradation of biologic scaffold materials. Biomaterials 28(2), 147–150 (2007). https://doi. org/10.1016/j.biomaterials.2006.08.022 19. Kryscia, L., et al.: Nanoskin® to treat full thickness skin wounds: nanoskin® for full thickness skin wounds. J. Biomed. Mater. Res. Part B Appl. Biomater. 107(3), 724–732 (2019). https:// doi.org/10.1002/jbm.b.34166 20. David, I., John, T., Walsh. B.: Effect of various methods of epidermal-dermal separation on the distribution of 14 c-acetate-labeled polyunsaturated fatiy acids in skin compartments. J. Invest. Dermatol. 62(5), 517–521 (1974). https://doi.org/10.1111/1523-1747.ep1268106 21. Oliveira, A.C., et al.: Evaluation of small intestine grafts decellularization methods for corneal tissue engineering. PLoS ONE 8(6), e66538 (2013). https://doi.org/10.1371/journal.pone.006 6538 22. Gubareva, A., Sotnichenko, S., Gilevich, V., et al.: Optimal decellularization of rat hearts and diaphragms and morphological evaluation. Cell Transplant. Tissue Eng. 4, 20–27 (2012). https://doi.org/10.1371/journal.pone.0066538 23. Macagonova, O., Risnic, D., Cociug, A., Nacu, V.: Comparative analysis of the skin decellularization methods. Moldovan Med. J. 64(2), 79–86 (2021). https://doi.org/10.52418/mol dovan-med-j.64-2.21.14 24. Horobin, R.W.: Biological staining: mechanisms and theory. Biotech. Histochem 77, 3–13 (2002). https://doi.org/10.1080/bih.77.1.3.13 25. Macagonova, O., Cociug, A., Braniste, T., Nacu, V.: Structural and physical characteristics of the dermal decellularized structures evaluation. Moldovan Med. J. 65(2), 36–40 (2022). https://doi.org/10.52418/moldovan-med-j.65-2.22.05 26. Dingkun, L., et al.: Synthesis and characterization of a hydroxyapatite- sodium alginatechitosan scaffold for bone regeneration. Front. Mater. Biomater. 8 (2021). https://doi.org/10. 3389/fmats.2021.648980 27. Choudhury, M., Mohanty, S., Nayak, S.: Effect of different solvents in solvent casting of porous PLA scaffolds – in biomedical and tissue engineering applications. J Tissue Sci Eng. 6(1), 1–7 (2014). https://doi.org/10.1166/jbt.2015.1243 28. Tohamy, M., Mabrouk, M., Soliman, E., et al.: Novel alginate/hydroxyethyl cellulose/hydroxyapatite composite scaffold for bone regeneration: In vitro cell viability and proliferation of human mesenchymal stem cells. Int J Biol Macromol. 112, 448–460 (2018). https://doi.org/10.1016/j.ijbiomac.2018.01.181 29. Nazir, R., Bruyneel, A., Carr, C., Czernuszka, J.: Mechanical and degradation properties of hybrid scaffolds for tissue engineered heart valve (TEHV). J. Funct. Biomater. 12(1), 20 (2021). https://doi.org/10.3390/jfb12010020

Effectiveness of Tissue Engineering in Obtaining the Extracellular Composite Vascularized Bone Matrix Alina Stoian1,2(B)

, Elena Pavlovschi1 , Nicolae Capro¸s2 , Grigore Verega2 and Viorel Nacu1

,

1 Laboratory of Tissue Engineering an Cell Cultures, SUMPh, “Nicolae Testemitanu”, Chisin˘au, ,

Moldova [email protected] 2 Department of Orthopedics and Traumatology, SUMPh, “Nicolae Testemitanu”, Chisin˘au, , Moldova

Abstract. Massive bone defects are considered to be one of the basic causes of functional disability. The gold standard, which nowadays is autologous grafting, is a perfect combination of mineralized extracellular matrix, bone marrow, and osteogenic cells. However, the available amount of such biological material is limited and the bone large defects remain a challenge. The lack of oxygen and nutrient transport actually remains the basic technical challenge in tissue engineering that limits the achievement of an effective bone allograft in the treatment of massive bone defects. The purpose of the paper is to present the results collected from the experimental study in obtaining the biocompatible extracellular composite vascularized bone matrix (vECCBM). We present a universal approach to a decellularization protocol based on the consecutive use of an isotonic solution, a chelating agent, anionic and ionic detergent as well as an enzyme solution. The effectiveness of decellularization was tested by histological examination (H&E and DAPI staining) and DNA quantification. The biocompatibility test was performed using the cultivation of the STEM cells from the bone marrow. Results: we were able to obtain a protocol for decellularization of the composite grafts, bone + vessel (soft and hard tissue) with the preservation of the vascular pedicle integrity and its connection with the bone compartment having in this way the possibility of applying anastomoses between the decellularized matrix and host. Keywords: Bone defects · Extracellular composite bone matrix · Bone allograft · Tissue engineering

1 Introduction Tissue engineering is a promising medical branch with amazing evolution in recent years aimed at tissue regeneration. Through the use of nanotechnologies, 3D bioprinting technologies, multiple cell types, and, of course, extracellular matrices (ECMs), tissue engineering has the goal of creating autologous tissue grafts for the effective use of © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 357–365, 2024. https://doi.org/10.1007/978-3-031-42775-6_39

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replacement therapy of non-functional organs or segments through replacement or cell regeneration [1–5]. Together with regenerative medicine, tissue engineering puts up a new interdisciplinary scientific branch that is based on an inevitable connection with other disciplines, such as biochemistry, chemistry, biology, physics, biophysics, and applied engineering [6]. The treatment of large soft and hard tissue defects such as nerves, vessels, skin, bones, and muscles, has a long history [7, 8]. Overall, the lack or numerical limitation of organs and segments that can be transplanted is caused by a large number of requirements, versus the number of available organs and segments, and the need for immunosuppression further constrains the indications for their use, with the accent being on vital organs like the heart, kidneys, lungs, and liver [1, 9]. Although there are multiple advances in obtaining artificial organs (in vitro), at the moment, their inadequate functionality makes them unacceptable for transplantation [3, 10]. In the last 20 years, various tissues and organs of donors and laboratory animals, such as skin, bone, vessels, nerves, tendons, kidneys, aortic valves, heart, lungs, or even entire upper limbs, have been used to obtain extracellular matrix for further use in biomedical engineering [1, 11–13]. The recent technical advances in decellularization offer a promising alternative by creating the extracellular matrix (ECM) of organs with the preservation of the anatomical and vascular architecture that could later allow the good exchange of nutrients and gasses at the time of transplantation, thus preserving its functionality and giving a weak immune response from the host [10, 14]. The objective of our experimental study on a large animal model is actually based on a noble intention to start a study to solve the problem of massive bone defects, which is one of the basic problems of reconstructive surgery. Massive bone defects, named in some sources as critical bone defects, represent one of the basic causes of functional disability [1]. The gold standard, which nowadays is autologous grafting, is a perfect combination of mineralized extracellular matrix, bone marrow, and osteogenic cells. However, the available amount of such biological material is limited, and the large bone defects remain a challenge [15–19]. Several methods are used in the treatment of large bone defects, such as the Induced membrane method, the Ilizarov method, or other microsurgical or pedicled bone flap transplantation methods. The main disadvantage of these methods is the long time of treatment and the limited use of these methods for large bone defects because the amount of autologous bone that can be actually collected is limited and insufficient for the reconstruction of these types of defects [20–23]. We are going to present the following a new approach to the existing methods of decellularization, used in obtaining the bone extracellular matrices in order to decellularize composite vascularized large bone grafts.

2 Material and Methods The experimental study on laboratory animals, initiated based on the favorable opinion of the research ethics committee no. 76 of May 21, 2018. Composite vascularized bone grafts of porcine origin were obtained from the Animal Facility Department of the Leibniz Research Laboratory for Biotechnology and Artificial Organs (LEBAO) at the Hannover Medical School (MHH) in Germany. Composite vascularized bone grafts from a pig model were collected according to the operation protocol described by the authors’

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group [24] (Fig. 1). Thus, a bone graft (fragment of the tibial bone) with approximately 4.5 cm length was collected, with preservation of the periosteum and vascular pedicle (popliteal artery). The popliteal artery, with a length of approximately 2.5 cm, provides a nutricia artery branch, which has direct connections with the periosteum and penetrates the bone at the level of the border between proximal and distal 1/3, having a length of approximately 2.7 (± 0.6 mm) and a diameter of approximately 1.2 (± 0.1 mm).

Fig. 1. Anatomical dissection of the vascularized composite bone graft. Big animal model. (A) Pig animal model. (B) The femoral bone with vascular pedicle (Popliteal artery). (C) Osteotomy of the tibial bone with graft making. (D) The macroscopic appearance, anterolateral view of the composite graft. (E) The macroscopic appearance, posterior view of the composite graft.

The used protocol for decellularization represented the optimization version of the protocol described for small bone grafts in 2016 by the group of authors Ilyas Jamaica Plain at all. [25]. Our adapted final protocol, included 9 steps, lasting 7 days. The method of infusion of the solutions was carried out with the help of the peristaltic pump through the popliteal artery for some samples and through the diaphysis for other samples (Fig. 2).

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Fig. 2. Methods for perfusing solutions for decellularizing vascularized composite bone grafts with a vascular pedicle. (A) Peristaltic pump. (B) Perfusion of solutions for graft decellularization through the popliteal artery. (C) Perfusion of solutions for graft decellularization through the femoral bone diaphysis.

As the effectiveness of the protocol was previously verified and demonstrated by histological analysis (H&E and DAPI staining), DNA quantification, and the biocompatibility test separately, for simple bone grafts (cortical and cancellous bone samples), periosteum segments, as well as blood vessels of various diameters (large, medium, and small). The composite grafts were tested, after processing, by DNA quantification and the biocompatibility test. DNA quantification was performed for cortical and cancellous bone tissue. Native, unprocessed cortical and cancellous bone segments of porcine origin, taken from the same graft and the same animal, were used as positive controls. Both native and processed/decellularized bone pieces were frozen using liquid nitrogen and subsequently powdered for further lyophilization. The biocompatibility test of the extracellular bone matrix was done using allogenic porcine bone marrow-derived mesenchymal stem cells (BM-MSCs).

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3 Results As the collected graft consists of cortical and cancellous bone tissue, segments from both types of bone tissue were collected for DNA quantification (as shown in Fig. 3). After processing the grafts, a 75% decrease in DNA content was observed in the perfused graft through the popliteal artery in the cancellous bone, and a 4% decrease in DNA content was observed in the cortical bone (Fig. 4A). For the graft perfused through the femoral bone diaphysis, a 70% decrease in DNA content was recorded in the cancellous bone, and a 30% decrease was observed in the cortical bone (Fig. 4B).

Fig. 3. The macroscopic aspect of the decellularized graft. The cortical and spongy components collected for DNA quantification.

Fig. 4. Results of DNA quantification. (A) results from graft perfused through the popliteal artery. (B) results from graft perfused through the femoral bone diaphysis.

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For the biocompatibility test, we used porcine bone marrow-derived mesenchymal stem cells, which were implanted into the bone and vascular pedicle (Fig. 5) after isolation, expansion, and labeling (using the PKH26 cell labeling kit) (Fig. 6). The obtained bone matrix with seeded stem cells was maintained in a cell culture medium and examined over a period of 5 days. Consequently, a decrease in the number of cells was observed, with the visualization of individual cells within the bone matrix and vascular pedicle on the 5th day (Figs. 7 and 8).

Fig. 5. Macroscopic view of the composite bone matrix along with the vascular pedicle on which approximately 17 million mesenchymal stem cells were seeded

Fig. 6. Mesenchymal stem cells labeled with PKH26, stereo microscope images

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Fig. 7. Fluorescent microscopy images, mesenchymal stem cells on the first day after seeding. It shows the surface of the bone matrix, with the labeled cells indicated by red dots. (A) Macroscopic appearance. (B) Microscopic appearance.

Fig. 8. Fluorescent microscopy images, mesenchymal stem cells on the fourth day after seeding. It shows the surface of the spongy region of the bone matrix, with the labeled cells represented by red dots. (A) Macroscopic appearance. (B) Microscopic appearance.

4 Discussions Speaking as a whole about the thrombogenic properties of decellularized vessels, the inability of qualitative revascularization of large decellularized bone segments, and of course about the immune response, the study seems to be an extremely difficult one. However, analyzing the results obtained by us and comparing these results with others from the latest studies worldwide, we believe that we make an important contribution in understanding the properties of the large vascularized decellularized bone segments. We were able to prove the effectiveness of one protocol for decellularization of the soft and hard tissue as well. The biocompatibility test showed the ability of the extracellular bone matrix to have a good environment for cell growth, even the cells were seeded only on the outer surface of the graft. The significant decrease in the amount of DNA in the spongy segment of the graft leads us to the idea that this could decrease the immune response of the host. However, we are convinced that in vivo studies are necessary for the definitive understanding of the integration of these vascularized composite bone allografts obtained by tissue engineering.

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5 Conclusions Based on the previous results and the findings presented above, we can confidently assert that the utilization of a universal protocol for decellularizing composite grafts is indeed possible. In the case of allogeneic bone grafts, preserving the vascular pedicle allows for potential reintegration into the host vascular system. This can address the issues of oxygenation and nutrient exchange by utilizing the newly formed vascular networks. We believe that these network formations can start with the first penetration of blood flow into the interior of the matrix. Acknowledgements. We gratefully acknowledge the contributions and support from colleagues, research team members from Laboratory of Tissue Engineering and Cell Cultures SUMPh „Nicolae Testemitanu”, Republic of Moldova and team members from LEBAO Hannover Medical School, Germany. UTM team, who made possible this collaboration via NanoMedTwin project and state project: 20.80009.5007.20. Their valuable contributions were essential to the successful completion of this research.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Pereira, H.F., Cengiz, I.F., Silva, F.S., Reis, R.L., Oliveira, J.M.: Scaffolds and coatings for bone regeneration. J. Mater. Sci. Mater. Med. 31, 27 (2020) 2. Gerli, M.F.M., Guyette, J.P., Evangelista-Leite, D., Ghoshhajra, B.B., Ott, H.C.: Perfusion decellularization of a human limb: a novel platform for composite tissue engineering and reconstructive surgery. PLoS ONE 13(1), e0191497 (2018). https://doi.org/10.1371/journal. pone.0191497 3. Atala, A.: Methods and compositions for organ decellularization (2002). https://patentima ges.storage.googleapis.com/0b/ab/49/aea2b448149315/US6376244B1.pdf 4. Peneda Pacheco, D., Suárez Vargas, N., Visentin, S., Petrini, P.: From tissue engineering to engineering tissues: the role and application of in vitro models. Biomater Sci. 9, 70–83 (2021) 5. Nacu, V., Cos, ciug, S., Cobzac, V., Tîmb˘alari, T.: Medicina regenerativ˘a în restabilirea t, esuturilor scheletice. Arta Medica 63, 30–33 (2017) 6. Sarvazyan, N.: Tissue Engineering: Principles, Protocols, and Practical Exercises. Springer Nature (2020) 7. Diaz-Siso, J.R., Bueno, E.M., Sisk, G.C., Marty, F.M., Pomahac, B., Tullius, S.G.: Vascularized composite tissue allotransplantation–state of the art. Clin. Transplant. 27, 330–337 (2013) 8. Zor, F.: Facial vascularized composite tissue allotransplantation (2013). https://doi.org/10. 5455/gulhane.39844 9. Messner, F., Guo, Y., Etra, J.W., Brandacher, G.: Emerging technologies in organ preservation, tissue engineering and regenerative medicine: a blessing or curse for transplantation? Transpl. Int. 32, 673–685 (2019) 10. Arenas-Herrera, J.E., Ko, I.K., Atala, A., Yoo, J.J.: Decellularization for whole organ bioengineering. Biomed. Mater 8, 014106 (2013) 11. Baino, F.: Scaffolds in Tissue EngineeringMaterials, Technologies and Clinical Applications. BoD – Books on Demand (2017)

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12. Banfi, A., Holnthoner, W., Martino, M.M., Ylä-Herttuala, S.: Editorial: vascularization for regenerative medicine. Front. Bioeng. Biotechnol. 6, 175 (2018) 13. Basu, J., Ludlow, J.W.: Overview of tissue engineering/regenerative medicine (2012). https:// doi.org/10.1533/9781908818119.1 14. Song, J.J., Ott, H.C.: Organ engineering based on decellularized matrix scaffolds. Trends Mol. Med. 17, 424–432 (2011) 15. Smith, C.A., Richardson, S.M., Eagle, M.J., Rooney, P., Board, T., Hoyland, J.A.: The use of a novel bone allograft wash process to generate a biocompatible, mechanically stable and osteoinductive biological scaffold for use in bone tissue engineering. J. Tissue Eng. Regen. Med. 9, 595–604 (2015) 16. Chiara, G., et al.: Nanostructured biomaterials for tissue engineered bone tissue reconstruction. Int. J. Mol. Sci. 13, 737–757 (2012) 17. Yang, P., et al.: Individual tissue-engineered bone in repairing bone defects: a 10-year followup study. Tissue Eng. Part A 26, 896–904 (2020) 18. Xing, F., Xiang, Z., Rommens, P.M., Ritz, U.: 3D bioprinting for vascularized tissueengineered bone fabrication (2020). https://doi.org/10.3390/ma13102278 19. Petite, H., et al.: Tissue-engineered bone regeneration. Nat. Biotechnol. 18, 959–963 (2000) 20. Borzunov, D.Y., Kolchin, S.N., Malkova, T.A.: Role of the Ilizarov non-free bone plasty in the management of long bone defects and nonunion: problems solved and unsolved. World J. Orthop. 11, 304–318 (2020) 21. Han, W., Shen, J., Wu, H., Yu, S., Fu, J., Xie, Z.: Induced membrane technique: advances in the management of bone defects. Int. J. Surg. 42, 110–116 (2017) 22. Iord˘achescu, R., Stoian, A., Gornea, T., Ivanov, V., Verega, G.: Corticoperiosteal-skin flap in the treatment of septic pseudarthrosis of the calf. Clinical case. (2020) 23. Verega, G.: Lambourile insulare ale membrului pelvin, (2008) 24. Houben, R.H., Kotsougiani, D., Friedrich, P.F.: Outcomes of vascularized bone allotransplantation with surgically induced autogenous angiogenesis in a large animal model: bone healing, remodeling, and material. J Reconstr Microsurg. 36(2), 82–92 (2020) 25. PROCESS FOR BONE TISSUE DECELLULARIZATION - European Patent Office - EP 3095469 A1. https://dx.doi.org/EP-3095469-A1-20161123

Effect of Gold Nanoparticles Functionalized by Arthrospira Platensis on Rats Liliana Cepoi1(B)

, Ludmila Rudi1 , Tatiana Chiriac1 , Inga Zinicovscaia2 Dmitrii Grozdov2 , and Valeriu Rudic1

,

1 Institute of Microbiology and Biotechnology of Technical, University of Moldova, Chisinau,

Moldova [email protected] 2 Joint Institute for Nuclear Research, Dubna, Russia

Abstract. The influence of unmodified engineered AuNPs versus biofunctionalized by cyanobacterium Arthrospira platensis (spirulina) on rats was studied. Au NPs were administered per os in a quantity of 1 μgAu/day per animal for 28 days, followed by a clearance period of the same duration. The accumulation of nanoparticles in different organs, the change in hematological and biochemical parameters in the experimental animals were assessed at the end of the nanoparticle administration and the clearance periods. The amount of gold accumulated in organs was determined by applying neutron activation analysis at the IBR-2 reactor. Biochemical and hematological analyses of the blood were performed using a semi-automated system StarDust MC15. The biofunctionalized nanoparticles were accumulated in larger amounts, and the amount of metal remaining after the clearance period was also higher in the case of the functionalized nanoparticles. Only biofunctionalized nanoparticles accumulated in the ovaries. Both types of nanoparticles possess high biological activity, inclusive of their induced changes in the leukogram, glucose, urea and liver transaminases levels. More pronounced changes being characteristic for unmodified gold nanoparticles. Tested nanoparticles can cause long-term or delayed effects, which include the increase in glucose and urea levels as well as the increase in ALT activity after the clearance period. Keywords: Arthrospira platensis · (spirulina) · Gold nanoparticles · Au accumulation · Au clearance

1 Introduction Gold nanoparticles (AuNPs), due to their unique properties, are intensively applied in biomedicine especially for imaging, drug delivery, diagnosis and treatment of cancer [1, 2]. AuNPs due to selective targeting can improve the efficiency and minimize the side effects of chemotherapy [3]. It is known that the properties of nanoparticles determine their biodistribution in organs and tissues [4, 5]. Thus, nanoparticles with smaller sizes are more actively accumulated in multiple organs and tissues [4, 6]. Directed coating of nanoparticles © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 366–375, 2024. https://doi.org/10.1007/978-3-031-42775-6_40

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with various artificial and/or biological ligands (functionalization/biofunctionalization) allows their stabilization and changes their properties and toxicological profiles. Through nanoparticles functionalization, the clearance time can be changed, or a specific tropism towards a certain organ can be obtained [7]. Biofunctionalization of AuNPs using different biological extracts it is associated with change of the properties of cellular metabolites in extracts, and the subsequent use of nanoparticles require their purification. Biofunctionalization of nanoparticles by living cells (for example, photosynthetic microorganisms) can prevent the formation of toxic metabolites, and nanoparticles can be used together with bioactive components of microbial biomass. In our previous research, it was demonstrated that silver nanoparticles biofunctionalized with living spirulina cells were accumulated and eliminated from rat organs differently compared to unmodified nanoparticles [8]. The use of biofunctionalized nanoparticles together with the matrix which was used for their biofunctionalized can provide additional benefits. This research aimed to study the accumulation and effects induced by engineered unmodified AuNPs compared to biofunctionalized with spirulina one on albino Wistar rats.

2 Materials and Methods 2.1 Nanoparticles The polyethylene glycol coated gold (PEG-AuNPs) nanoparticles, custom-made and purchased from the company “Nanomaterials & Technologies”, Togliatti, RF were used in this study. AuNPs are mainly spherical and form small agglomerations. The size of the nanoparticles ranges from 3 nm to 15 nm, with an average of 4.2 nm. 2.2 Cyanobacterial Strain The strain of the cyanobacterium Arthrospira platensis-CNMN-CB-02 was obtained from the National Collection of Nonpathogenic Microorganisms, Institute of Microbiology and Biotechnology of Technical University of Moldova. 2.3 Functionalization of AuNPs with Arthrospira platensis Biomass To produce biofunctionalized AuNPs on the third day of the culture cycle, which corresponds to the beginning of the exponential growth phase, AuNPs in concentration of 0.5 μM were added to the culture medium. The nutrient mineral medium SP1 used for the growth of cyanobacteria as well as optimal conditions for biomass growth were described previously in [9]. At the 6th day, the biomass was separated from the cultivation medium by filtration, standardized to a concentration of 10 mg/mL, and administrated to animals.

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2.4 Experimental Design The experimental design was approved by the Institutional Research Ethics Committee of the Institute of Physiology and Sanocreatology, Moldova (IREC) (IREC approval no. IREC/12/ 03.11.2022). Thirty-six white laboratory Wistar rats, were divided into six groups, consisting of 3 males and 3 females, placed in separate cages: Negative control group (C1); Positive control group (C2), Experimental groups: 1 (AuNPs); 2 (AuNPs-Sp); 3 (AuNPsC); 4 (ANPs-SpC). Groups 3 and 4 are clearance groups. All animals were maintained in optimal conditions. Water was given ad libitum, while the diet was regular, with the following particularities: (C1) – regular diet (RD); (C2) – RD admixed with spirulina biomass; (AuNPs and AuNPsC) – RD admixed with unmodified AuNPs; (AuNPs-Sp and AuNPs-SpC) – RD admixed with AuNPs biofunctionalized with spirulina. The amount of AuNPs administrated was 1 μgAu/day per animal. C2 group received the same amount of spirulina biomass as the animals from the (AuNPsSp) and (AuNPs-SpC) groups. AuNPs and spirulina were incorporated into whole grain rye flour breadcrumbs, offered as the first meal. Animals of group C1 also received breadcrumbs without additives. The total duration of experiment was 56 days: 28 days of experiment and 28 days of clearance. The animals of groups (C1), (C2), (AuNPs) and (AuNPs-Sp) were sacrificed after 28 days, and the animals from groups (AuNPsC) and (AuNPs-SpC) were sacrificed at the end of clearance period. The blood and organs were collected for analysis. 2.5 Quantitative Analysis of Gold Content in Animal Tissues The amount of gold accumulated in organs was determined applying neutron activation analysis at the IBR-2 reactor (JINR, Dubna, RF). Prior to irradiation, the samples were freeze-dried, homogenized, and packed in aluminum bags. Then, samples were irradiated with epithermal neutrons for 3 days at a neutron flux of 1.2 × 1011 cm−2 s−1 and measured for 30 h. Gamma spectra of induced activity were measured using three spectrometers based on HPGe detectors with an efficiency of 100% and resolution of 1.8–2.0 keV for the 1332 keV total-absorption peak of the isotope 60 Co. The analysis of the spectra was performed using the Genie2000 software by Canberra, and calculation was carried out using the software “Concentration”. Quality control of the analytical results was provided by comparing the calculated and certified concentrations for the standard reference materials: NIST 2710a (Montana I Soil Highly Elevated Trace Element Concentrations), and liquid Au standard (Merk, Germany). The difference between calculated and certified values did not exceed 5%. 2.6 Blood Hematology and Biochemistry Biochemical and hematological analyses of the blood were performed using a semiautomated system StarDust MC15 (DiaSys Diagnostic Systems, Germany).

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3 Results All animals survived the 28-day experiment and showed normal body weight values at the end of the nanoparticle administration period without differences between groups. The appearance of the animals in all groups was healthy and behavioral changes were not recorded. Thus, visible manifestations of nanoparticle toxicity were not found. 3.1 Accumulation of AuNPs in Animal Organs Following Administration of AuNPs and After a Period of Clearance The content of gold accumulated in the brain, liver, spleen, kidneys and reproductive organs (testes and ovaries) was determined following daily oral administration for 28 days of unmodified and biofunctionalized nanoparticles in an amount of 1 μg/day per animal (after Au), as well as at the end of the clearance period, with an identical duration of 28 days. The results are shown in Table 1. Table 1. The content of gold in rats’ organs after administration with gold nanoparticles. Organ

Au content, ng/g

Unremoved Au, %

AuNPs group

AuNPsC group

Brain

3.4 ± 0.201

n.d

Spleen

5.5 ± 0.314

2.0 ± 0.174

36.36

Liver

1.35 ± 0.176

1.0 ± 0.13

Kidney

22.7 ± 0.885

3.5 ± 0.246

Testicles

9.8 ± 0.441

1.6 ± 0.,341

Ovaries

n.d

n.d

Au content, ng/g

Unremoved Au, %

AuNPs-Sp

AuNPs-SpC

4.23 ± 0.245

n.d

6.2 ± 0.184

2.8 ± 0.196

45.16

74.04

3.5 ± 0.246

2.9 ± 0.2

82.85

15.42

74.3 ± 1275

14.4 ± 0.576

19.38

16.33

6.3 ± 0.435

4.3 ± 0.341

68.25

1340.0 ± 49.58

438.0 ± 17.08

32.68

–

–

–

n. d. - not detected.

The maximum content of gold upon administration of unmodified AuNPs was determined in the kidneys (22.7 ± 0.885 ng/g), while in liver the accumulation of gold was the lowest– 1.35 ± 0.176 ng/g. In the brain gold content was 3.4 ± 0.2 ng/g and ovaries did not accumulate AuNPs. The most efficient clearance was established for the brain tissue which in 28 days removed 100% of the accumulated nanoparticles, followed by kidney and testicles. Even in the liver the rate of gold accumulation was the lowest, the removal of metal was not pronounced, with 74% of the gold remaining in the organ. In the case of biofunctionalized AuNPs-Sp the content of gold accumulated in the brain, spleen, liver and kidneys was 24–230% higher compared to the group that received unmodified nanoparticles. Instead, in the testicles the level of gold was by 35.7% lower compared to the unmodified ones. A large amount of gold was identified in the ovaries 1340 ng/g. After a 28-day clearance period, the entire amount of accumulated biofunctionalized nanoparticles was eliminated only from the brain, while significant amount of gold was still present in the other organs. In the ovarian tissue was determined 438.0 ng/g of gold, which constitutes almost 33% of the accumulated gold.

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3.2 Change of Hematological Parameters in Animals Administrated with AuNPs The results of the hematological tests performed on the groups administrated with gold nanoparticles are presented in Fig. 1. Hemoglobin and RBC levels did not change compared to the controls.

Fig. 1. Hematological indices of rats from groups administrated with AuNPs and AuNPs-Sp, as well as control groups (C1 – negative control; C2 – positive control). HB-hemoglobin; RBC – red blood cells; WBC – white blood cells; PLT – platelet count; PMN – polymorph multinuclear neutrophil granulocytes; LY – lymphocytes; EOS – eosinophils; BAS – basophils; RET – reticulocytes.

PLT in females administrated with AuNPs indicated a more than 20% decrease of the parameter compared to the C1 (p < 0.05), 25% in comparison with in females, which received AuNP-Sp. In males there is a decrease in the number of platelets by 40% compared to the C1. In females, WBC level in all groups changed towards an increase compared to the negative control. The level of PMN in males administered with NPs increased approximately twofold (p < 0.05), and the level of LY in both groups decreased approximately 40% compared to C1. In males administrated with nanoparticles the number of eosinophils was reduced, more evident in case of unmodified nanoparticles. The number of basophils increased considerably in males, administrated with biofunctionalized AuNPs, while in females in case of both types of nanoparticles, this index was significantly reduced, more pronounced (more than twice) in the case of AuNP-Sp. The number of reticulocytes increased considerably in males, which were administered with nanoparticles of both types, by 30% higher than the upper limit of the norm for rats. 3.3 Change of Biochemical Parameters in Animals Administrated with AuNPs The following biochemical parameters: total proteins (TP), alanine aminotransferases (ALT), aspartate aminotransferases (AST), glucose (GL), urea (UA), and creatinine

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(CREA) were monitored. The values of some of these parameters differed in animals administrated with nanoparticles in comparison with control (See Fig. 2).

Fig. 2. Results of the biochemical tests of blood serum obtained from animals a administrated with AuNPs and AuNPs-Sp, as well as control groups (C1 – negative control; C2 – positive control). TP – total proteins; GL – glucose; CREA – creatinine; UA – urea; ALT – alanine aminotransferase; AST – aspartate aminotransferase.

TP content has undergone changes only in females administrated with AuNP-Sp, where a tendency of parameter values decrease was observed. The amount of GL in the blood of experimental males did not change, but a significant increase was observed in females, more pronounced in the case of animals administrated with unmodified nanoparticles. However, the obtained values were within the normal limits for the rats. The amount of CREA increased only in females administrated with spirulina (positive control), but the archived values were within the limits for rats. Also, an increase in the amount of UA in males from the positive control group (by 35%) and the group that received biofunctionalized nanoparticles (by 57%) compared to the negative control was observed. Administration with unmodified AuNPs resulted in a significant increase in ALT activity in both males and females, more pronounced in males, where ALT assay values increased by 97% (vs. 66% in females) compared to the negative control group. Biofunctionalized nanoparticles altered ALT activity only in females, an increase of ALT activity by 84% compared to the negative control was noted. 3.4 The Efficiency of the Clearance Period for Normalization of Hematological and Biochemical Indicators In Table 2 are presented the most significant results of hematological and biochemical tests performed for animals administrated with AuNPs and AuNP-Sp at the beginning and end of the clearance period of 28 days. Platelet count after 28 days of administration decreased in both groups of animals that received administrated with AuNPs. After the clearance period, the number of platelets was restored in the group of animals administrated with AuNP-Sp and considerably increased, but did not reach the negative control values in animals administrated with

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Table 2. Hematological and biochemical tests of animals administrated with AuNPs and AuNPSp at the beginning and end of the clearance period of 28 days. Indices WBC, 109 /L PLT, 109 /L

AuNPs, 28th day

AuNPsC, 56th day

AuNPs-SP, 28th day

AuNPs-SpC, 56th day

17.80 ± 3.44

11.50 ± 0.96

12.32 ± 1.52

11.86 ± 2.26

555.0 ± 163.3

915.8 ± 36.34*

575.2 ± 25.25

730.2 ± 107.95

33.90 ± 3.73

42.70 ± 9.13

20.68 ± 1.22*

38.90 ± 0.20

52.48 ± 9.39

PMN, %

56.40 ± 9.96

LY %

51.01 ± 13.54

50.65 ± 3.57

EOS, %

4.17 ± 1.61

7.05 ± 0.73*

4.68 ± 1.26

9.07 ± 0.62*

BAS, %

0.37 ± 0.08

0.90 ± 0.45

0.82 ± 0.49

0.52 ± 0.09

RET, %

6.52 ± 4.02

3.01 ± 0.52

6.48 ± 1.89

TP, g/L

72.86 ± 6.25

70.85 ± 7.79

66.32 ± 6.26

4.96 ± 1.13

7.46 ± 0.87

4.90 ± 0.65

9.59 ± 2.15*

95.2 ± 4.02

105.0 ± 18.02

GL, mmol/L CREA, μM/L UA, mg/dl ALT, U/L AST, U/L

92.8 ± 9.23 23.77 ± 0.94 66.84 ± 30.45 90.82 ± 9.83

110.0 ± 9.50

3.08 ± 0.42 71.2 ± 9.67

42.50 ± 2.43*

34.77 ± 5.38

27.19 ± 7.47

38.37 ± 6.35*

62.17 ± 14.46

51.27 ± 4.47

89.78 ± 2.87

84.4 ± 9.28

83.78 ± 19.70

unmodified AuNPs. The total amount of white blood cells that increased during nanoparticle administration above the limits characteristic for rats had recovered to normal values in both groups of animals after clearance. Likewise, the leukocyte formula returned to normal values by normalizing the level of LY and PNM. The amount of RET also decreased to norms characteristic for rats. Instead, after the clearance period, the amount of EOS increased in the blood of animals administrated with nanoparticles – by ~60% in the case of unmodified AuNPs and by ~95% in the case of AuNP-Sp. The amount of glucose in the blood of the animals in the experimental groups continued to increase after the clearance period. Thus, the value of this parameter in the group of animals administrated with unmodified AuNPs immediately after the administration period was 4.95 mmol/L, and after 28 days of clearance – 7.46 mmol/L. In the group that received AuNP-Sp immediately after the administration period, it was 4.90 mmol/L, and after 28 days of clearance – 9.59 mmol/L. In both groups the level of ALT returned to normal values for rats. The level of UA in rats administrated with unmodified nanoparticles but increased by almost 80% compared to the pre-clearance level.

4 Discussion No morphometric differences, morphological and histological changes, as well the differences in the weight of the organs between experimental and controls groups were observed. It should be mentioned that other researchers, who applied much higher doses of AuNPs, did not observe visible changes in the morphometric parameters of the animals [10].

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In our experiment, essential differences in the accumulation of AuNPs and AuNP-Sp were identified. In brain, spleen, liver and kidney were accumulated in a higher amount AuNPs-Sp compared to unmodified AuNPs. Brain tissue accumulated small amounts of nanoparticles in both experimental variants. However, accumulation of AuNPs-Sp was by 24% compared to unmodified AuNPs. After 28 days of clearance the brain tissue was completely cleared of accumulated gold in both experimental variants. Although it is considered that nanoparticles should not cross the blood-brain barrier, their accumulation in the brain in small amounts and their rapid elimination have been reported by other researchers [11]. In present study, AuNP-Sp showed higher affinity for brain tissue compared to unmodified AuNPs. This specificity after in-depth study can serve as a basis for the development of targeted drugs. Starting from the fact that gold nanoparticles conjugated with CLPFFD peptide were able to destroy toxic β-amyloid aggregates, similar to those found in the brains of Alzheimer’s patients [12], appear a perspective to application of AuNPs biofunctionalized with spirulina biomass in the treatment of degenerative brain conditions. In our experience we observed a different level of accumulation of nanoparticles in different organs. The data regarding the preferential accumulation of nanoparticles in different organs are very diverse [7]. It is assumed that the content of AuNPs accumulated in organs depends on the route of their administration, the oral administration being associated with the lowest amounts of gold accumulated in tissues [13]. At present there is limited information demonstrating reliable changes in hematological parameters in animals that were administrated with AuNPs. Some authors affirm the total lack of hematological changes [14]. Other studies, including present one, identified several changes in the values of the respective parameters (WBC, PLT, PNM, LY, EOS, BAS, RET), some, such as WBC and LY, even went beyond the normal range for rats. The main part of hematological indicators changed as a result of nanoparticle administration after the clearance period return to values normal for rats. However, a serious exception was detected, namely the increase in the number of eosinophils after the clearance period, which indicates a delayed effect in time. Delayed hematological changes have also been established by other authors, for example, after a recovery period in experiments with injection of AuNPs in rats at doses of 10–100 μg/kg/day for 28 days [15]. Regarding the biochemical markers, the modification of the specific indices of the liver and kidney system, which have been subjected to an intense process of accumulation and elimination of gold nanoparticles is expected. An increase in ALT values was found in rats administered with AuNPs. After the clearance period, ALT values were restored only in rats administrated with unmodified nanoparticles. Similar results were obtained by other research teams. Thus, for example Kozics and co-authors showed significant changes in liver enzymes AST and ALT after injection of a dose of 0.7 mg/kg of 10 nm AuNP (PEG). [7]. AuNPs biofunctionalized by propanoic acid and proanthocyanidin led to increased ALT values in laboratory animals [14]. Another parameter, GL level, was higher not only after the nanoparticle administration period, but continued to increase even more after the clearance period. It can be

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assumed an impact on the pancreas in rats that received unmodified nanoparticles, but especially AuNP-Sp.

5 Conclusions Both types of gold nanoparticles - unmodified and biofunctionalized were accumulated in the brain, spleen, liver, kidney and testes, and only the biofunctionalized nanoparticles were accumulated in the ovaries. Accumulation of biofunctionalized AuNPs in the brain, kidney, spleen, liver and ovaries was higher compared to unmodified AuNPs, while the latter have a higher affinity for the testes. After a 28 days period of clearance from the brain, the entire amount of accumulated gold was eliminated, regardless of the type of nanoparticles that were administered. Both biofunctionalized and unmodified nanoparticles possess high biological activity, inclusive they can develop long-lasting (increase and/or maintenance of high level of GL, UA, ALT) or delayed (increase of EOS level after the clearance period) effects. Acknowledgments. The results were obtained within the project 20.80009.5007.05 funded by the National Agency for Research and Development, Republic of Moldova.

Conflicts of Interest. The authors declare no conflict of interest.

References 1. Bansal, S.A., Kumar, V., Karimi, J., Singh, A.P., Kumar, S.: Role of gold nanoparticles in advanced biomedical applications. Nanoscale Adv. 2, 3764–3787 (2020). https://doi.org/10. 1039/D0NA00472C 2. Zhang, S., et al.: Ribbon of DNA lattice on gold nanoparticles for selective drug delivery to cancer cells. Angewandte Chemie Int. Ed. 59, 14584–14592 (2020). https://doi.org/10.1002/ anie.202005624 3. Goddard, Z.R., Marín, M.J., Russell, D.A., Searcey, M.: Active targeting of gold nanoparticles as cancer therapeutics. Chem. Soc. Rev. 49, 8774–8789 (2020). https://doi.org/10.1039/D0C S01121E 4. Chen, J., et al.: Sex differences in the toxicity of polyethylene glycol-coated gold nanoparticles in mice. Int. J. Nanomedicine 8, 2409–2419 (2013). https://doi.org/10.2147/IJN.S46376 5. Siddique, S., Chow, J.C.L.: Gold nanoparticles for drug delivery and cancer therapy. Appl. Sci. 10(11), 3824 (2020). https://doi.org/10.3390/app10113824 6. Khan, H.A., Abdelhalim, M.A.K., Al-Ayed, M.S., Alhomida, A.S.: Effect of gold nanoparticles on glutathione and malondialdehyde levels in liver, lung and heart of rats. Saudi J. Biol. Sci. 19(4), 461–464 (2012). https://doi.org/10.1016/j.sjbs.2012.06.005 7. Kozics, K., et al.: Pharmacokinetics, biodistribution, and biosafety of PEGylated gold nanoparticles in vivo. Nanomaterials 11(7), 1702 (2021). https://doi.org/10.3390/nano11 071702 8. Rudi, L., et al.: Accumulation and effect of silver nanoparticles functionalized with Spirulina platensis on rats. Nanomaterials 11(11), 2992 (2021). https://doi.org/10.3390/nano11112992 9. Cepoi, L., et al.: Effects of PEG-coated silver and gold nanoparticles on Spirulina platensis biomass during its growth in a closed system. Coatings 10(8), 717 (2020). https://doi.org/10. 3390/coatings10080717

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10. Parveen, A., Malashetty, V.B., Mantripragada, B., Yalagatti, M.S., Abbaraju, V., Deshpande, R.: Bio-functionalized gold nanoparticles: Benign effect in Sprague-Dawley rats by intravenous administration. Saudi J. Biol. Sci. 24(8), 1925–1932 (2017). https://doi.org/10.1016/ j.sjbs.2017.11.041 11. Behroozi, Z., et al.: Distribution of gold nanoparticles into the brain: a systematic review and meta-analysis. Nanotoxicology 15(8), 1059–1072 (2021). https://doi.org/10.1080/17435390. 2021.1966116 12. Hirn, S., et al.: Particle size-dependent and surface charge-dependent biodistribution of gold nanoparticles after intravenous administration. Eur. J. Pharm. Biopharm. 77(3), 407–416 (2011). https://doi.org/10.1016/j.ejpb.2010.12.029 13. Bednarski, M., et al.: The influence of the route of administration of gold nanoparticles on their tissue distribution and basic biochemical parameters: in vivo studies. Pharmacol. Rep. 67(3), 405–409 (2015). https://doi.org/10.1016/j.pharep.2014.10.019 14. Jo, M.R., Bae, S.H., Go, M.R., Kim, H.J., Hwang, Y.G., Choi, S.J.: Toxicity and biokinetics of colloidal gold nanoparticles. Nanomaterials 5(2), 835–850 (2015). https://doi.org/10.3390/ nano5020835 15. Lee, J.H., Gulumian, M., Faustman, E.M., Workman, T., Jeon, K., Yu, I.J.: Blood biochemical and hematological study after subacute intravenous injection of gold and silver nanoparticles and coadministered gold and silver nanoparticles of similar sizes. Biomed. Res. Int. 8460910 (2018). https://doi.org/10.1155/2018/8460910

The Significance of Computed Tomography in Diagnosing Pediatric Tuberculosis Constantin Iavorschi(B) , Stela Kulcitkaia , Igor Ivanes , and Nadejda Pisarenco State University of Medicine and Pharmacy “Nicolae Testemitanu”, Chisinau, Moldova [email protected]

Abstract. Pediatric tuberculosis (TB) presents unique diagnostic challenges due to various factors, including the low sensitivity of sputum examination, difficulties in obtaining samples, and the presence of paucibacillary forms of the disease. This retrospective-descriptive study aimed to evaluate the role of Computed Tomography (CT) in diagnosing pediatric TB. A total of 142 pediatric TB cases were analyzed using CT scans. The most common CT finding was enlarged lymph nodes, observed in 86% of cases. Other notable findings included nodules (38%), parenchymal consolidation (27.5%), bilateral dissemination (2%), destructive changes (5%), pleural effusion (3%), Tree-in-bud appearance (1.5%), ground glass opacities (3.5%), bronchiectasis (2%), atelectasis (2%), calcifications in intrathoracic lymph nodes (6%), and calcifications in lung parenchyma (3%). These CT patterns played a crucial role in the accurate and timely diagnosis of pediatric TB, aiding in differentiating it from other conditions and facilitating the initiation of appropriate antituberculosis treatment. The integration of CT in pediatric TB diagnosis offers several advantages. It provides precise lesion localization, allows differentiation from anatomical landmarks, and enables comprehensive evaluation of the disease, especially in cases involving intrathoracic lymph nodes. By providing valuable anatomical and pathological information, CT enhances clinical decision-making and improves the management of pediatric TB cases. In conclusion, CT examination plays a vital role in the diagnosis of pediatric TB. Its ability to provide detailed imaging findings helps in accurate disease identification, differentiation, and prompt initiation of appropriate treatment. Integrating CT into clinical practice improves diagnostic accuracy and enhances patient care in pediatric TB cases. Keywords: Tuberculosis · Children · Computed tomography · Diagnostics · Enlarged lymph nodes · CT patterns

1 Introduction Tuberculosis is a global pandemic. It is a prevalent infectious disease caused by Mycobacterium tuberculosis, affecting over two billion people worldwide, which accounts for approximately one-third of the global population. TB is characterized by a complex © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 376–385, 2024. https://doi.org/10.1007/978-3-031-42775-6_41

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interplay between the human body and the Mycobacterium tuberculosis bacterium, representing a dynamic equilibrium. [Bates and Stead 1993]. Only 10% of individuals infected with Mycobacterium tuberculosis progress to manifest TB disease over their lifetime. In the remaining cases, the bacterium enters a latent state known as Latent TB Infection (LTBI) [6, 19, 20]. The incidence of pediatric tuberculosis reflects the epidemiological situation in society, as it encompasses cases of TB disease characterized by the development of tuberculosis-specific inflammation and corresponding clinical manifestations in children with immunosuppression [6, 19, 21]. The development of TB disease in children not only poses a medical concern but also represents a significant social problem for the healthcare system and the country as a whole [16, 17, 19, 22]. Pediatric tuberculosis has often been overlooked in surveillance efforts, as national tuberculosis programs have traditionally focused on cases of microbiologically confirmed tuberculosis, which are more prevalent among adolescents and adults. However, diagnosing tuberculosis in children poses significant challenges due to various factors. These include the low vigilance among healthcare providers, reliance on sputum examination with its low sensitivity (ranging from 1% to 14%), the difficulty children face in producing sputum samples, and the presence of paucibacillary forms of the disease in children [5, 6, 18, 21, 22]. Computed tomography is a modern diagnostic method that utilizes X-rays to examine the body. In recent years, CT has gained increasing prominence in the diagnosis of respiratory diseases, including tuberculosis in children. This growing trend can be attributed to several factors. Firstly, CT is highly informative when assessing the respiratory organs. By employing advanced techniques such as high-resolution computed tomography (HRCT), CT surpasses traditional radiography in terms of diagnostic capabilities. It can reveal pathological changes that may not be detected or clearly identified on conventional X-rays. While other radiological methods like linear tomography, bronchography, and angiopulmonography exist, they often involve significant costs, high radiation doses, or time-consuming procedures. CT, on the other hand, provides comprehensive diagnostic information that would otherwise require multiple techniques. Additionally, CT helps reduce the need for invasive examinations, thus minimizing patient discomfort and risks. Not only does CT enhance the diagnostic value, but it also streamlines the overall diagnostic process by saving time [3, 10–12, 15]. Therefore, our aim was to assess the role of chest CT in the examination of children suspected of having tuberculosis.

2 Materials and Methods Subjects: In order to evaluate the role of Computerized Tomography in diagnosing tuberculosis in children, we analyzed 142 cases of tuberculosis in children aged between 0 and 18 years. All the children were admitted to the children’s ward of the IMSP Municipal Clinical Hospital of Pneumophtisiology, Chisinau for diagnosis and treatment between 2018 and 2020. Clinical data: The clinical records were examined to gather medical history information. This included assessing the epidemiological context, identifying any risk factors

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that could contribute to immunosuppression, and evaluating imaging findings indicative of tuberculosis in the children. Imaging: The CT images were carefully examined to identify typical changes associated with pediatric tuberculosis. The presence of adenopathy in the hilar, mediastinal, and paratracheal regions was analyzed. These adenopathies could be single or multiple, with a higher frequency of occurrence on the right side. The presence of pulmonary parenchymal changes, such as opacities, with or without accompanying adenopathies (homogeneous, systematized/unsystematized, multifocal) was recorded. Lung involvement was assessed, including ground glass opacity and cyst formation. The study also evaluated the scoring of pleural effusion, lesions with a miliary appearance, and lesions resulting from complicated adenopathies, such as atelectasis or hyperinflation due to compression of the bronchi or lymph nodes. The study results underwent statistical analysis using computerized processing. Analysis programs were utilized to group the obtained data using various grouping methods, including simple, complex, and repeated grouping. Depending on the type of evidence, typological and variational grouping was performed, presenting quantitative signs in tabular form using numerical figures.

3 Results Out of the 142 children included in the study and diagnosed with TB through CT, the age distribution was as follows: 41 children (29%) were between 0 and 5 years old, 80 children (56%) were between 6 and 14 years old, and 21 children (15%) were over 15 years old. The age group between 6 and 14 years old predominated, indicating that CT scans played a decisive role in the diagnosis of TB in this particular age range. Figure 1 presents the data visually. 56%

60% 40%

29% 15%

20% 0% 0-5 years

6-14 years

15+ years

Fig. 1. Pacient characteristics according to age (N = 142).

The analysis of tuberculosis detection methods in the children included in the study revealed that active detection, which involved examining children at higher risk of the disease, was predominantly employed. Specifically, 118 cases (83%) involved children who had close contact with adult patients diagnosed with pulmonary tuberculosis. On the other hand, passive detection, where children presented themselves with symptoms suggestive of tuberculosis, accounted for 24 cases (17%). Clinical manifestations of

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tuberculosis in children are not specific, but certain patterns were observed. The majority of children, 102 cases (68%), exhibited varying degrees of an intoxication syndrome, characterized by decreased appetite, weight loss, sweating (particularly nocturnal), subfebrility (slightly elevated body temperature), and general asthenia (fatigue or weakness). Bronchopulmonary syndrome, in most cases, was mild, presenting with infrequent and non-productive coughs, and was reported in 97 cases (65%). The lack of specific symptoms and the failure to recognize the clinical significance of the symptoms by parents often contribute to a delay in seeking medical attention for children with tuberculosis. Additionally, the non-specific nature of the clinical manifestations further complicates early detection. These factors are particularly relevant in cases where children come from socially disadvantaged or dysfunctional families, with a higher prevalence of neglect or lack of awareness regarding their health. In some instances, parental alcohol abuse can further hinder the recognition and response to potential health concerns in children. Overall, these circumstances highlight the importance of raising awareness among both parents and healthcare providers regarding the clinical presentation of tuberculosis in children and the need for timely medical evaluation. The distribution of tuberculosis forms in the studied children was as follows: intrathoracic lymph node tuberculosis (69%), primary tuberculous complex (17%), infiltrative pulmonary tuberculosis (10.6%), nodular pulmonary tuberculosis (1.4%), and disseminated tuberculosis (2%) (Fig. 2).

2,0%

1,4%

10,6%

Intrathoracic Lymph Node Tuberculosis (ILNT) Primary Tuberculous Complex

17,0% 69,0%

Infiltrative Pulmonary Tuberculosis Disseminated Tuberculosis Nodular Pulmonary Tuberculosis

Fig. 2. Distribution of children with TB according to the form of tuberculosis. In our analysis of children diagnosed with tuberculosis using Computed Tomography, we identified 135 (95%) cases classified as “New Case” of TB, and 7 (5%) children classified as “Retreatment” cases.

The inflammatory process in relation to the established forms of TB was examined. The predominant phase observed was the evolutionary phase, which consisted of infiltration, destruction, and dissemination, accounting for 138 (97%) cases. Only 4 cases (3%) showed the regressive phase of inflammation, characterized by resorption and induration

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components. A detailed analysis of the evolutionary phase revealed that 101 cases (73%) exhibited the infiltration component, 26 children (19%) had dissemination, and 11 cases (8%) showed destruction. Among the risk factors observed in the children included in the study, the most common was contact with adults diagnosed with pulmonary tuberculosis, which was reported in 16% of cases. BCG vaccination is a well-known method of anti-tuberculosis protection. It triggers a strong specific cellular immune response, involving the secretion of cytokines, especially IFN-γ, and the activation of cytotoxic mechanisms and macrophage activity. The type of immune response induced by BCG vaccination is influenced by the population’s previous exposure to various environmental mycobacteria. The BCG vaccine is primarily administered to newborns soon after birth, as this is considered the most effective timing for maximum protection in a non-sensitized host. The presence of a post-vaccination scar on the left arm was assessed in the examined children to determine their BCG immunization status. Among the children, 123 (86.6%) had received the BCG vaccine, while 19 (13.4%) had not, and did not exhibit a post-vaccination scar. Among the vaccinated children, 80 (56%) had a scar smaller than 5 mm, and 43 (35%) had a scar larger than 6 mm. The characteristics of the post-BCG scar are presented in Table 1. Table 1. Distribution of children with TB based on characteristics of the post-BCG scar (N, %) post-BCG scar < 5 mm > 6 mm Total

N 80

% 65

43

35

123

100

The CT patterns detected in the children included in the study varied and included the presence of nodules (38%), parenchymal consolidations (27.5%), bilateral dissemination (2%), destructions/cavitations (5%), enlarged lymph nodes (86%), pleural effusion (3%), Tree-in-bud (1.5%), ground glass (3.5%), bronchiectasis (2%), atelectasis (2%), calcifications in intrathoracic lymph nodes (6%), and calcifications in parenchyma (3%). These findings highlight the diverse range of imaging changes observed in the chest CT scans of the children, providing valuable insights into the manifestations of tuberculosis in the thoracic region (Fig. 3). The CT images of the children were thoroughly examined for the presence of various parenchymal changes and lymphatic involvement. Parenchymal changes, including consolidation, centrilobular nodules, miliary nodules, bronchiectasis, cavitation, and fibrosis, were carefully evaluated. Additionally, the size, necrosis, and calcification of mediastinal and hilar lymph nodes were assessed. Other findings, such as pleural effusion, were also observed. Among the identified patterns, mediastinal lymphadenopathy was found to be the most common finding in primary tuberculosis, either in isolation or in combination with pulmonary lesions. The results of these evaluations provide valuable insights into the distribution and characteristics of CT patterns observed in children

The Significance of Computed Tomography

Enlarged hilum

24%

2%

381

Consolidations

53%

3%

Pulmonary dissemination Destructions

1%

Pleural effusion

17%

Presence of nodules

Fig. 3. Distribution of children with TB according to CT patterns.

with tuberculosis, highlighting the diverse manifestations of the disease in the thoracic region.

4 Discussion Childhood tuberculosis (TB) remains a public health challenge in the Republic of Moldova, as evidenced by our study involving 142 children diagnosed with TB through computed tomography (CT). The persistent incidence of TB in children suggests the presence of undiagnosed sources of infection, despite a global decline in TB cases [2] (Fig. 4). It is worth noting that the ongoing COVID-19 pandemic has further impacted TB services, resulting in decreased diagnosis and management of TB cases worldwide [8, 14]. According to a World Health Organization report, TB case notifications decreased by 18% from 2019 to 2020, with the number of reported cases declining from 7.1 million to 5.8 million [9]. This unfortunate trend is expected to lead to a projected 20% increase in TB deaths over the next five years due to disruptions in TB services, delayed diagnosis, and challenges in accessing TB care and treatment [8, 9, 14, 20]. Distinguishing TB from other respiratory conditions, such as childhood pneumonia and COVID-19, poses a challenge due to overlapping clinical presentations, including fever, cough, and shortness of breath [8, 9, 14, 20]. However, the differences lie in the onset of symptoms, with COVID-19 exhibiting a sudden onset compared to the slower onset typically associated with TB. The clinical and immunopathological interactions between TB and COVID-19, as well as the factors contributing to the increased mortality associated with the dual infection, remain areas requiring further research [8, 9, 14, 20]. Diagnosing TB in children is a laborious task due to various difficulties, including obtaining suitable specimens for bacteriological examination, the limited specificity of radiological examinations, and the inherent limitations of the tuberculin skin test. The

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25 20 15 10

23.8

22.1 19.7

18.8 17.4

20.4

18.6

17 14

15.5 8.9

5 0

Fig. 4. TB incidence in children in the Republic of Moldova, ‰

paucibacillary nature of the disease, along with its predominantly intrathoracic lymph node localization, further complicates the diagnostic process, making it challenging to detect small forms of intrathoracic lymph node tuberculosis accurately. As a result, both overdiagnosis and underdiagnosis of TB in children persist as issues. In practice, the challenges of diagnosing childhood TB typically arise in two scenarios: (1) children presenting with respiratory symptoms raising suspicion of a tuberculous cause, and (2) children with a history of contact with individuals diagnosed with TB. Establishing a definitive diagnosis of TB in children requires consideration of multiple elements, including the epidemiological context, clinical manifestations, tuberculin skin test (TST), serological tests such as Interferon Gamma Release Assay (IGRA), and imaging examination [1, 2, 5]. Computed tomography (CT) plays a crucial role as a diagnostic method for pediatric TB, offering advantages over standard radiography. CT provides precise localization of lesions, differentiation from anatomical landmarks, identification of bronchial changes, and visualization of parenchymal and pleural alterations. Additionally, CT can aid in distinguishing adenopathies from vascular structures without the need for contrast material. The value of high-resolution CT (HRCT) examination is particularly indisputable in confirming the clinical diagnosis of intrathoracic lymph node tuberculosis, which is frequently observed in children [3, 4, 7, 12, 13]. Our study findings indicate that mediastinal lymphadenopathy was the most common CT finding, affecting 86% of the studied children. Among the mediastinal lymph nodes, the right paratracheal and subcarinal nodes were most frequently involved across all age groups, often with multiple groups affected. Bilateral adenopathy was observed in 31% of cases, with a decrease in prevalence observed with increasing age. Calcifications in intrathoracic lymph nodes were detected in 6% of children, while atelectasis due to

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lymph node enlargement was observed in 2% of cases. Acinar/centrilobular nodules, suggestive of disease activity, were present in 38% of patients, often challenging to visualize on standard radiography. Other observed CT findings included the “sprouted tree” pattern in 1.5% of cases, ground glass opacities in 3.5% of cases, parenchymal consolidation in 27.5% of cases, calcifications in the lung parenchyma in 3% of cases, and destruction/cavitation in 5% of cases. Notably, the presence of consolidation with ipsilateral hilar/paratracheal extension of enlarged lymph nodes strongly suggested TB. CT scans played a crucial role in facilitating the diagnostic process by providing valuable information on the form of TB and evaluating the inflammatory process activity [3, 4, 7, 12, 13]. Overall, our study highlights the ongoing challenges in diagnosing TB in children and the importance of utilizing CT as a diagnostic tool in pediatric TB cases. By improving the accuracy of diagnosis and enabling timely initiation of appropriate treatment, CT contributes to better management and control of childhood TB.

5 Conclusion In this study, we analyzed a cohort of 142 children diagnosed with tuberculosis through CT scans. The findings underscore the critical role of CT examination in achieving timely and accurate diagnosis of pediatric tuberculosis. CT imaging not only aids in the correct identification of the disease but also facilitates differentiation from other conditions, particularly in cases of intrathoracic lymph node tuberculosis, including the “small volume” variant. This distinction is essential for initiating appropriate antituberculosis treatment promptly. The utilization of CT scans provides valuable insights into the clinical form and activity of tuberculosis, enhancing the diagnostic process for children with TB. By precisely localizing lesions, distinguishing them from anatomical landmarks, and visualizing bronchial changes, parenchymal alterations, and pleural involvement, CT enables a comprehensive evaluation of disease progression. Furthermore, CT scans help differentiate between adenopathies and vascular structures without the need for contrast material. The findings of this study emphasize the significance of CT as a diagnostic tool for pediatric tuberculosis, allowing for improved management and control of the disease. By facilitating accurate diagnosis and timely initiation of appropriate treatment, CT scans contribute to better patient outcomes and help mitigate the spread of tuberculosis. However, further research and efforts are still required to address the challenges associated with diagnosing tuberculosis in children. These challenges include the paucibacillary nature of the disease, limitations in obtaining suitable specimens for bacteriological examination, and the need to differentiate TB from other respiratory conditions with similar clinical presentations. Continued efforts in these areas will help advance the field and enhance diagnostic strategies for childhood tuberculosis. In conclusion, our study highlights the indispensable role of CT examination in the diagnosis of pediatric tuberculosis. By providing crucial information on disease localization, distinguishing features, and disease activity, CT scans offer valuable insights that aid in the prompt and accurate management of tuberculosis in children. Future endeavors should focus on addressing the remaining challenges to further improve the diagnosis and management of childhood tuberculosis.

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Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Botnaru, V.: Imagistica toracic˘a în cazuri clinice comentate.Tipogr. “Balacron” Chi¸sin˘au (2012) 2. Protocolul clinic na¸tional “Tuberculoza la copil”. Chi¸sin˘au (2020) 3. Baishideng Publishing Group Co.: Pediatric vs adult pulmonary tuberculosis: a retrospective computed tomography study. World J. Clin. Pediatrics (2013). https://doi.org/10.5409/wjcp. v2.i4.70 4. Stern, E.J., Swensen, S.J., Jeffrey, P., Kanne, T.: High-Resolution CT of the Chest: Comprehensive Atlas, Lippincott Williams & Wilkins, Philadelphia (2012) 5. International Union against Tuberculosis and Lung Disease. Desk guide for the diagnosis and management of tuberculosis in children. Paris (2010) 6. Gomez-Pastrana, D.: Diagnosis of pulmonary tuberculosis in children. J. Infect. Dis. Therapeutics 1, 015 (2013). https://doi.org/10.1586/eri.10.9 7. Marchiori, E., et al.: Pulmonary tuberculosis associated with the reversed halo sign on highresolution CT. Br. J. Radiol. 83, e58–e60 (2010). https://doi.org/10.1259/bjr/22699201 8. McQuaid, C.F., et al.: The impact of COVID-19 on TB: a review of the data. Int. J. Tuberc Lung. Dis. 25(6), 436–446 (2021). https://doi.org/10.5588/ijtld.21.0148 9. Migliori, G.B., Thong, P.M.: Country-specific lockdown measures in response to the COVID19 pandemic and its impact on tuberculosis control: a global study. J. Bras. Pneumol. 48(2), e20220087 (2022). https://doi.org/10.36416/1806-3756/e20220087 10. Baghaie, N., et al.: Diagnostic value of lung CT-scan in childhood tuberculosis. Tanaffos 4(16), 57–62 (2005) 11. Vandna, R.: Use of high-resolution computed tomography (HRCT) in diagnosis of sputum negative pulmonary tuberculosis. Turk. Thorac. J. 17(2), 59–64 (2016). https://doi.org/10. 5578/ttj.17.2.012 12. Webb, R.W., Muller, N.L., Naidich, D.P.: David P Naidich, High-Resolution CT of the Lung. Lippincott Williams & Wilkins, Philadelphia (2014) 13. Webb, R.W.: Thoracic Imaging: Pulmonary and Cardiovascular Radiology, 3rd edn. Lippincott Williams & Wilkins, Philadelphia (2020) 14. Rodrigues, I., et al.: Impact of the COVID-19 pandemic on tuberculosis services. Pulmonology 28(3), 210–219 (2022). https://doi.org/10.1016/j.pulmoe.2022.01.015 15. Bolla, S.: Role of HRCT in predicting disease activity of pulmonary tuberculosis. Gujarat Med. J. 69(2), 91–95 (2014) 16. WHO: Latent tuberculosis infection. Updated and consolidated guidelines for programmatic management. WHO (2018). https://www.who.int/tb/publications/2018/latent-tuberculo sis-infection/en/ 17. World Health Organization: Guidance for National Tuberculosis Programmes on the Management of Tuberculosis in Children. World Health Organization, Geneva (2022) 18. World Health Organization. The End TB Strategy- (2015) 19. Walls, T., Shingadia, D.: Global epidemiology of paediatric tuberculosis. J. Infect. 48, 13 (2004) 20. World Health Organization: Global tuberculosis report 2022. World Health Organization, Geneva (2022). Licence: CC BY-NC-SA 3.0 IGO. https://www.who.int/teams/global-tuberc ulosis-programme/tb-reports/global-tuberculosis-report-2022

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21. WHO: Multidrug-resistant tuberculosis in children and adolescents in the WHO European Region, Expert opinion. WHO Regional Office for Europe, Copenhagen (2019). http://www.euro.who.int/en/publications/abstracts/multidrug-resistant-tuberculosisin-children-and-adolescents-in-the-who-european-region-2019 22. WHO: Roadmap Towards Ending TB in Children and Adolescents, 2nd edn. World Health Organization, Geneva (2018)

Mechanical Characterization of Decellularized Blood Vessels: A Valuable Tool to Provide Comprehensive Information About the Scaffold Tatiana Malcova1,2,3(B)

, Gheorghe Rojnoveanu1 and Viorel Nacu3

, Anatol Ciubotaru4

,

1 Department of Surgery No. 1 “Nicolae Anestiadi”, Nicolae Testemitanu State University of

Medicine and Pharmacy of the Republic of Moldova, Chisinau, Republic of Moldova [email protected] 2 Laboratory of Hepato-Pancreato-Biliary Surgery, Nicolae Testemitanu State University of Medicine and Pharmacy of the Republic of Moldova, Chisinau, Republic of Moldova 3 Laboratory of Tissue Engineering and Cell Cultures, Nicolae Testemitanu State University of Medicine and Pharmacy of the Republic of Moldova, Chisinau, Republic of Moldova 4 Department of Cardiovascular Surgery, Nicolae Testemitanu State University of Medicine and Pharmacy of the Republic of Moldova, Chisinau, Republic of Moldova

Abstract. Cardiovascular diseases (CVDs) remain an important global health problem. Surgical revascularization (or bypass surgery) has been established as the most optimal therapeutic approach for patients with severe injury; however, not in all cases a suitable vascular substitute can be identified. The field of vascular tissue engineering and regenerative medicine aim to produce suitable tissue-engineered vascular grafts (TEVGs) for vascular repair, replacement, or reconstructive aims. Decellularization (DC) is a promising approach because it completely removes the antigenic cellular components. The goal of the proposed study was to examine the mechanical integrity of the decellularized porcine carotid arteries (a prototype of small-diameter vascular grafts). The developed DC procedure included osmotic shock, chemical surfactant treatment, and enzymatic digestion. Agree to other DC protocols reported previously, we were able to demonstrate, on the one hand, complete removal of cells throughout the arterial wall by performing H&E staining and DAPI, on the other hand, good biomechanical properties of decellularized tissue by performing the suture retention strength testing. The average suture retention strength of native porcine vessels was 1.08 ± 0.39 N. The average suture retention strength of decellularized vessels was 1.14 ± 0.38 N (p = 0.0731). In summary, the both control and treated vessels exhibited similar mechanical properties; the used combined method had beneficial effect in this study. Keywords: Decellularization · Small-Diameter Blood Vessels · Suturability

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 386–396, 2024. https://doi.org/10.1007/978-3-031-42775-6_42

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1 Introduction CVDs are a group of disorders of the heart and blood vessels, including coronary heart disease, cerebrovascular disease, peripheral arterial disease (PAD) and other conditions. The prevalence of CVDs increases worldwide, the pathology of the circulatory system remaining the leading cause of death globally (in 2019, 17.9 million people died from CVDs, representing 32% of all global deaths) [1, 2]. PAD, the pathology of blood vessels, is characterized by narrowing/occlusion of the luminal diameter associated with the reduction of the oxygen level carried to the arms and legs. According to the latest studies in the field, there is a higher prevalence of PAD in high-income countries vs low-income and middle-income countries with an estimated total number of 236.2 million people >25 years suffering from the disease in 2015 in the world [3, 4]. Most CVDs can be prevented by avoiding the risk factors (tobacco use, unhealthy diet, use of alcohol) and physical activity; even so, majority of patients require invasive therapeutic approaches. With the developments in endovascular techniques, it has been considered as treatment of choice for revascularization in many vascular centers [5, 6]. For the management of severe pathologies surgical revascularization (bypassing procedures using grafts) is recommended, allowing redirecting blood flow around their blocked arteries [7]. Taking into consideration the military settings are on the rise, vascular injuries, including civilian and military traumatic injuries, represent another area of critical need for replacement vessels using grafts [8]. Obviously, the demand for bypass grafting is very large [7, 9–12]. Current clinical strategies for the bypassing of small diameter blood vessels ( 680 nm), high extinction coefficients (εmax > 105 L·mol−1 ·cm−1 ), and tunable photophysical and photochemical properties via chemical substitutes. To date, one Pc (Aluminium Pc, Photosens®, Russia) [1] has © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 437–446, 2024. https://doi.org/10.1007/978-3-031-42775-6_47

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been approved for clinical use and two Pcs (Silicon Pc, Pc4®, USA [2] and Zinc Pc, Photocyanine®, China) [3] have currently reached clinical testing. In the recent years, a special role in medicine, particularly in the treatment of cancer diseases, is played by zinc phthalocyanine (ZnPc), because it affords complexes with both high intersystem crossing effect and long triplet lifetimes, usually showing strong photochemical and photodynamic activities and unsurprisingly becomes the main candidate for clinical PDT. Cauchon et al. [4] found that functionalizing a zinc phthalocyanine (ZnPc) with three sulfonate groups and one hexynyl hydrophobic substituent greatly enhances cellular uptake, with preferential localization at the mitochondrial membranes, leading to a photodynamic effect toward EMT-6 murine mammary tumor cells. A recent example is zinc disulfo-di(phthalimidomethyl)phthalocyanine (ZnPc-S2 P2 ), which was shown to be effective at killing tumor cells in vitro [5, 6] and causing tumor regression in vivo. Positively charged substituents are often introduced into the photosensitizer not only to increase the polarity and solubility of the hydrophobic phthalocyanine ring, but also to increase cellular uptake and to facilitate selective targeting to tumor cells and subcellular targeting to vulnerable intracellular sites. [7] The positively charged Pcs such as Nmethylpyridyloxyphthalocyanine and poly-L-lysine–chlorin e6 [8, 9] have been found to be capable of efficiently photo-inactivating both Gram-negative and Gram-positive bacteria. The authors of [9, 10] publications studied the problem of water-soluble zinc phthalocyanine. These authors solve the problem of the water solubilization of Zn, Fe, and Mn metal phthalocyanines by forming conjugates with highly water-soluble polymers. For solubilization, the cited authors use highly water-soluble synthetic polymers, such as polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG). Above mentioned metal phthalocyanines are dissolved in an extremely small amount of sulfuric acid and dimethylformamide (DMF) and then mixed with dilute PVP or PEG solutions (1–2 wt %) under stirring. The authors of [11] describe an ingenious method, namely, grafting of N-vinylpyrrolidone–acryloyl chloride copolymers with ZnPc. The water-soluble ZnPc:ClAC:N-VP developed photosensitizer presents a narrow absorption band at 970 nm, fluorescence at λem = 825 nm and the decay fluorescence profile with relatively longer lifetimes of 1.2 μs, 4.6 μs, and 37 μs. The phthalocyanine copolymers synthesized by this method are soluble only in organic solvents, such as dimethylsulfoxide, DMF, and others. In this paper we synthetized a highly water-soluble phthalocyanine- polymer compounds for use in PDT. We describe the synthesis of zinc aminomethylphthalocyanine (ZnAMPc) grafted to natural dextran macromolecules. Dextran is a water-soluble polysaccharide that consists mainly α-1,6 linked Dglucopyranose residues with a low percentage of α-1,2, α-1,3 and α-1,4 linked side chains. Usually dextran’s are considered as linear polysaccharides or oligosaccharides. Generally, they are obtained by a species of L. mesenteroides, NRRL B-512F, which produces water-soluble dextran containing 95% linear α-(1 → 6) and 5% α-(1 → 3) linkages. Dextran’s and their derivatives are of considerable biomedical and industrial importance and have a broad spectrum of applications ranging from drilling fluid additives and chromatographic support media, to drug carriers and blood plasma extenders. The best activity against Staphylococcus aureus with minimum inhibitory concentration 60 μg/mL was provided by polymers obtained from dextran with lower molecular mass

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(Mn = 4500), C12 H25 or C18 H37 end groups, and N, N-dimethyl-N-benzylammonium pendent groups. To produce highly water-soluble phthalocyanine polymers for use in the medicinal practice, we describe here the synthesis of zinc aminomethylphthalocyanine (ZnAMPc) grafted to dextran macromolecules. The antimicrobial activity of dextran depends on chemical composition, molar mass, length of end alkyl group and chemical structure of ammonium groups.

2 Experimental Details 2.1 Synthesis of Substituted Zinc Phthalocyanines The mono-ZnAMPc synthesis began with the hydrolysis of mono-(carboxybenzamidomethyl) ZnPc [13] according to the following scheme (Fig. 1):

Fig. 1. Synthesis of mono-zinc aminomethylphthalocyanine compound.

The synthesis of ZnAMPc:dextran was performed in equipped ampoule with a stirrer and a thermometer by charging with a mixture containing zinc phthalocyanine, phthalimidomethyl group, water, a 20% aqueous solution of sodium hydroxide. The mixture was reacted at 170 °C for 2 h with stirring. Then 20% hydrochloric acid was added, and the melt reaction was further maintained at 170 °C for 2 h with stirring and simultaneously applying pressure of 3 to 10 kg/cm2 , and further maintaining the reactants at the attained temperature and pressure for 3 to 10 h. After the reaction, a 20% aqueous solution of sodium hydroxide was added to make the reaction mixture weakly alkaline. The reaction mixture was then filtered and washed with water to give a wet cake of aminomethyl zinc phthalocyanine having one aminomethyl group on an average. The yield of the reaction was 68%. The grafting of ZnAMPc to dextran was performed using ethyl chloroformate in accordance with the following scheme [Fig. 2 and Fig. 3]: Preliminary experiments have shown that the reaction of dextran with ethyl chloroformate either takes place to a small extent or does not occur at all when it is carried out without any catalyst/acceptor system. As catalyst triethylamine was used. Initially, equimolar triethylamine is placed into a beaker containing a 0.5% polymer solution in DMF. The reaction took place at temperatures of 2–5 °C, in one step. The reaction

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Fig. 2. Scheme B for the functionalization of dextran with cyclic carbonate group

between dextran and ethyl chloroformate in the presence of triethylamine as catalyst yields modified polymer with cyclic and acyclic carbonate groups as evidenced by its FTIR. The FTIR spectra of the functionalized polymer show absorptions at 1805 cm−1 and 1745 cm−1 that indicated that both cyclic and acyclic ethyl carbonate groups were present.

Fig. 3. Scheme C for synthesis of zinc aminomethylphthalocyanine (ZnAMPc) – dextran copolymer

Next, in the cold of the functionalized polymer, a ZnAMPc dissolved in DMF is added dropwise. The reaction mixture is warm to room temperature and stirred for 15–20 min, and then held for 1.5–2.0 h. The product obtained is purified by sedimentation in hexane, then in diethyl ether. In order to establish the composition of the developed copolymers, the UV-Vis spectra of solutions of dextran mixture with different concentrations of zinc aminomethylphthalocyanine (ZnAMPc) were initially investigated. At concentrations of 10–20 wt %, the composition of the grafted copolymers corresponds completely to the concentration of ZnAMPc taken for the grafting reaction. In the case of higher concentrations (30–50) wt%, the composition of the copolymers slightly deviates from the theoretical composition, especially for copolymer with (50: 50 wt %) ratio. 2.2 Materials and Methods All purchased chemical reagents (Merck) were of the highest commercially available purity and were used without previous purification. A Bruker FTIR spectrometer was

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used to provide information about chemical composition. The UV-Vis spectra of the solutions were measured using a UV-Vis spectrophotometer (Lambda 25, Perkin Elmer Inc., Shelton, CT, USA) from 200 nm to 800 nm in 10 mm quartz cuvettes.

3 Results and Discussion 3.1 FTIR Analysis of ZnAMPc:Dextran Copolymers In the stage one of the synthesis of ZnAMPc:dextran copolymers, mono(carboxybenzamidomethyl) ZnPc was obtained from ZnPc, which after thorough purification in stage two was transformed into aminomethyl ZnPc. Hydrolysis was carried out with 10% hydrochloric acid solution at T ~ 170 °C. The individual structure of the synthesized compounds was confirmed by FTIR spectroscopy. The composition of the mono-zinc aminomethylphthalocyanine (mono-ZnANPc) was studied by Fourier transform infrared transmission spectroscopy method (FTIR). The mono-ZnANPc based peaks are found at 1510 cm−1 , 1330 cm−1 , 1280 cm−1 , 1175 cm−1 , 1135 cm−1 , 1100 cm−1 and 740 cm−l . The presence of the new amidomethyl-ZnPc compound is confirmed by the position and presence of the carbonyl band. The FTIR spectra of mono-(carboxybenzamidomethyl) ZnPc and mono-aminomethyl ZnPc compounds are shown in Fig. 4.

Fig. 4. FTIR spectra for ZnPc (1), mono-(carboxybenzamidomethyl) ZnPc (CBAMZnPc) (2) and mono-aminomethyl ZnPc (ZnAMPc) (3).

The spectra of unsubstituted ZnPc are also given for comparison. The peaks of C = N at 1655 cm−1 and N-H at 3023 cm−1 are the characteristic signals of zinc phthalocyanine. All these signals illustrate zinc phthalocyanine has been successful attached to the dextran molecule. The spectra of mono-(carboxybenzamidomethyl) ZnPc and monoaminomethyl ZnPc compounds show the appearance of new vibrations at ν = 3300– 3450 cm–1 and ν = 1550–1650 cm–1 , which are characteristic of the –CO–NH2 amide

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group. This fact confirms the formation of the mono-(carboxybenzamidomethyl)ZnPc compound. The FTIR spectrum of a modified dextran with ethyl carbonate groups using triethylamine as catalyst is presented in Fig. 5. From the inspection of this Fig. 5 results those vibrations situated at 1805 cm−1 and 1745 cm−1 indicated both cyclic and acyclic ethyl carbonate groups. The formation of crosslinked structures and cyclic carbonates, when triethylamine is used as catalyst, can be attributed to the higher nucleophilic character of the alcohol-triethyl amine complex. It may facilitate the interchange reactions of the grafted n-alkyl carbonate groups with the neighboring alcoholic groups along the same chain (cyclic carbonate) or from different chains (crosslinking).

Fig. 5. FTIR spectra of dextran and dextran modified with ethyl carbonate group using triethylamine as catalyst.

Figure 6 presents FTIR spectra of the mono-aminomethyl ZnPc:dextran with 10% (1), 20% (2) and 30% (3) of mono-aminomethyl ZnPc derivative. The peaks of C = N at 1655 cm−1 and N-H at 3023 cm−1 are the characteristic signals of zinc phthalocyanine. The peak at 1630 cm−1 refers to carbonyl stretch, which is assigned to the amide bond. With the increasing of the concentration of mono-aminomethyl ZnPc in copolymer this peak disappeared. The peaks at 1323 cm−1 refer to amino and amino-methyl stretching groups in copolymers with 10%, 20% and 30% of mono-aminomethyl ZnPc derivative shifted slightly to higher value with increasing concentration. The appearance of vibrations at ν = 3295 cm–1 and 1651 cm–1 and an increase in their intensity with an increase in the ZnAMPc concentration in the copolymers confirm the presence of covalent bonds between the dextran and ZnAMPc. Also, there are distinct peak variations observed in the ZnAMPc:dextran spectral “fingerprint” region between 1500 cm−1 and 800 cm−1 . All signals depicted in the FTIR spectra of ZnAMPc:dextran copolymers illustrate that zinc phthalocyanine has been successful attached to the dextran molecule.

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Fig. 6. FTIR spectra of mono-aminomethyl ZnPc:dextran with different concentration of monoaminomethyl ZnPc derivative.

3.2 Optical Absorption of Mono-Aminomethyl ZnPc:Dextran Copolymers Usually, MPcs give rise to electronic spectra with two strong absorption bands, one around 300 nm, called the “B” or Soret band, due to electronic transitions from the deeper π-HOMO to n*- LUMO energy levels, while the other at 600–650 nm, called the “Q” band, due to electronic transitions from the π-HOMO to π*- LUMO energy levels [12]. Optical behaviors of our ZnAMPc: dextran copolymers were studied in DMSO/H2 O and water solutions. The UV-vis absorption spectra of mono-aminomethyl ZnPc: dextran copolymers dissolved in DMSO: H2 O mixture are presented in Fig. 7. The mono-zinc aminomethyl ZnPc: dextran copolymers, in DMSO: H2 O solution, a typical absorption of an intense and sharp Q-band in the near-infrared area at 670 nm indicate. With increasing the concentration of zinc aminomethyl ZnPc in copolymer, the band becomes broader and outside of the band located at 670 nm, a shoulder at 745 nm appear. The UV-Vis absorption spectra of mono-zinc aminomethyl phthalocyanine: dextran copolymers dissolved in water are shown in Fig. 8. The absorption bands of ZnAMPc: dextran copolymers in water are much broader than in DMSO: H2 O solution. Grafting of 10–30 wt% zinc aminophthalocyanine with polymer makes it soluble in water. The increase of the concentration of zinc aminophthalocyanine until 30% in copolymer increase the intensity of the absorbance of both B and Q bands. Further increasing of the concentration of zinc aminophthalocyanine decrease the intensity of the absorbance. Regretfully, that the measurement limit of the device does not allow us to record the absorbance further than 800 nm.

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Fig. 7. The UV-Vis absorption spectra of mono-zinc aminomethyl phthalocyanine: dextran copolymers dissolved in DMSO: H2 O solvent.

Fig. 8. The UV-Vis absorption spectra of mono-zinc aminomethyl phthalocyanine:dextran copolymers dissolved in water.

So, hydrolysis of mono-(carboxybenzamidomethyl) ZnPc yielded a phthalocyanine bearing a carboxylic acid group that was covalently attached to polymer by amide bond. This copolymer obtained a positive charge in biological medium due to the availability of the free aliphatic basic amine groups. The formation of cationic groups can increase the interaction of the copolymer with the cellular envelope of the microorganisms favoring cell inactivation. Absorbance and FTIR studies showed the formation of the ZnAMPc: dextran copolymers.

4 Conclusions Herein, we report the development of mono-zinc aminomethyl phthalocyanine: dextran copolymers. We have demonstrated with FTIR and UV-Vis spectroscopy that developed copolymers are significantly different from those of the monomeric phthalocyanine and monomeric polymer. Our main objective, to synthesize new phthalocyanine- 9 polymer

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systems in high yields, with improved solubility in aqueous solutions and improved properties was reached, performing the following steps: a) by hydrolysis of mono-(carboxybenzamidomethyl)ZnPc in an acidic medium using a closed reaction system; b) by modifying dextran with cyclic and acyclic carbonate groups using triethylamine as catalyst; c) by grafting dextran to a mono-ZnAMPc of 10, 20, and 30 wt % concentrations. Aqueous solutions of the synthesized copolymers exhibit an absorption Q band shifted at λ > 700 nm, which is favorable for medicinal applications. Acknowledgments. This work was supported by the Ministry of Education, Culture, and Research of Republic of Moldova, research grant 20.80009.5007.16.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Zhang, Y., Lovell, J.F.: Recent applications of phthalocyanines and naphthalocyanines for imaging and therapy. Nanomed. Nanobiotechnol. 9(1), e1420 (2016). https://doi.org/10.1002/ wnan.1420 2. Liu, Q., et al.: Potent peptide-conjugated silicon phthalocyanines for tumor photodynamic therapy. J. Cancer 9(2), 310–320 (2018). https://doi.org/10.7150/jca.22362 3. Machacek, M., et al.: Far-Red-Absorbing Cationic Phthalocyanine Photosensitizers: Synthesis and Evaluation of the Photodynamic Anticancer Activity and the Mode of Cell Death Induction. J. Med. Chem. 58(4), 1736–1749 (2015). https://doi.org/10.1021/jm5014852 4. Cauchon, N., Ali, H., Hasséssian, H.M., van Lier, J.E.: Structure–activity relationships of mono-substituted trisulfonated porphyrazines for the photodynamic therapy (PDT) of cancer. Photochem. Photobiol. Sci. 9(3), 331 (2010). https://doi.org/10.1039/b9pp00109c 5. Feuser, P.E., et al.: Synthesis of ZnPc loaded poly(methyl methacrylate) nanoparticles via miniemulsion polymerization for photodynamic therapy in leukemic cells. Mater. Sci. Eng., C 60, 458–466 (2016). https://doi.org/10.1016/j.msec.2015.11.063 6. Obata, M., Masuda, S., Takahashi, M., Yazaki, K., Hirohara, S.: Effect of the hydrophobic segment of an amphiphilic block copolymer on micelle formation, zinc phthalocyanine loading, and photodynamic activity. Eur. Polymer J. 147, 110325 (2021). https://doi.org/10.1016/ j.eurpolymj.2021.110325 7. Motloung, M., Babu, B., Prinsloo, B., Nyokong, T.: The photophysicochemical properties and photodynamic therapy activity of In and Zn phthalocyanines when incorporated into individual or mixed Pluronic® micelles. Polyhedron 188, 114683 (2020). https://doi.org/10. 1016/j.poly.2020.114683 8. Lamch, Ł, et al.: Preparation and characterization of new zinc(II) phthalocyanine — Containing poly(l-lactide)-b-poly(ethylene glycol) copolymer micelles for photodynamic therapy. J. Photochem. Photobiol. B 160, 185–197 (2016). https://doi.org/10.1016/j.jphotobiol.2016. 04.018 9. Minnock, A., Vernon, D.I., Schofield, J., Griffiths, J., Parish, J.H., Brown, S.B.: Mechanism of uptake of a cationic water-soluble pyridinium zinc phthalocyanine across the outer membrane of Escherichia coli. Antimicrob. Agents Chemother. 44(3), 522–527 (2000). https://doi.org/ 10.1128/aac.44.3.522-527.2000

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10. Dolotova, O., et al.: Water-soluble manganese phthalocyanines. J. Porphyrins Phthalocyan. 17(08n09), 881–888 (2013). https://doi.org/10.1142/S1088424613500818 11. Potlog, T., Lungu, I., Tiuleanu, P., Robu, S.: Photophysical Properties of Linked Zinc Phthalocyanine to Acryloyl Chloride:N-vinylpyrrolidone Copolymer. Polymers 13, 4428 (2021). https://doi.org/10.3390/polym13244428 12. Jorge Y.-F., Mirna G.H. et.al.: Factorial design to optimize dextran production by the native strain. Leuconostoc mesenteroides SF3 CS Omega 2021 6(46), 31203–31210 (2021). https:// doi.org/10.1021/acsomega 13. Harold, T., Westfield, N.: Sulfonated and Unsulfonated imidomethyl, carboxyamidomethyl and aminomethyl phthalocyanines. Patent Nr. US2761868A (1956)

Effects of Nickel, Molybdenum, and Cobalt Nanoparticles on Photosynthetic Pigments Content in Cyanobacterium Arthrospira Platensis Ludmila Rudi(B)

, Tatiana Chiriac , Liliana Cepoi , and Vera Miscu

Institute of Microbiology and Biotechnology of Technical, University of Moldova, Chisinau, Moldova [email protected]

Abstract. Nanoparticles are utilized in the cultivation media of cyanobacteria and microalgae to enhance productivity and the accumulation of biologically active compounds. This study focused on investigating the impact of Ni, Mo, and Co nanoparticles stabilized with polyethylene glycol, which was added to the cultivation medium of the cyanobacterium Athrospira platensis (spirulina), at concentrations ranging from 0.25 to 2.5 mg/L, on photosynthetic pigments. A. platensis was cultured in a laboratory setting using a mineral medium supplemented with nanoparticles for 6 days. The results revealed that Ni nanoparticles, within the concentration range of 0.25 to 1.5 mg/L, did not alter the levels of chlorophyll and phycobiliproteins but led to an increase in the content of β-carotene in the biomass. On the other hand, the decrease in photosynthetic pigment content caused by Mo and Co nanoparticles was compensated by an augmentation in phycobiliprotein levels. These nanoparticles inhibitory or stimulatory effects correlated with their concentrations in the cyanobacterium’s cultivation medium. The study concluded that the type of nanoparticles plays a crucial role in shaping the response of the spirulina culture by redirecting biosynthetic activity to maintain photosynthetic processes. Mo and Co nanoparticles, particularly at concentrations that stimulated phycobiliprotein synthesis, can be employed as stimulators in the cultivation technologies of the cyanobacterium Arthrospira platensis. Keywords: Arthrospira platensis · Nickel · Molybdenum · Cobalt · Nanoparticles · Chlorophyll · β-carotene · Phycobiliproteins

1 Introduction Nanoparticles of different types, sizes, and origins have become indispensable in numerous fields. They are used in medicine for the precise delivery of drugs, to enhance the properties of packaging in the food industry, in the electronics, information technology, and energy sectors to improve device performance, and in the textile and construction industry to confer new properties to materials. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 447–456, 2024. https://doi.org/10.1007/978-3-031-42775-6_48

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In the biotechnology of cyanobacteria and microalgae, the exploration of nanoparticles started from the factors underlying their toxicity, which over years, expanded to study the possibility of applying nanoparticles as stimulators for promoting the biosynthetic activity of microalgae culture [1]. In nutrient media, nanoparticles compete for the uptake of nutrients, stimulate their rapid consumption, and reorganization of biosynthetic processes in cells [2]. All the stimulatory effects for microalgae cultures are associated with low concentrations of nanoparticles [3]. Thus, nanoparticles can be used in microalgae and cyanobacteria cultivation technologies to increase productivity and accumulation of nutraceutical compounds with beneficial effects on health, as well as to increase the production of important lipids for biofuel production [4]. Cyanobacterium Arthrospira platensis (spirulina) is a well known sourse of biologically active substances, including photosynthetic pigments such as chlorophyll, βcarotene, and phycobiliproteins-light harvesting proteins. Chlorophyll, β-carotene, and phycobiliproteins (especially C-phycocyanin), derived from spirulina, can be used as natural dyes and antioxidants in food industry and cosmetology, as well as valuable nutraceuticals in the composition of pharmaceutical supplements. Phycobiliproteins are applied in diagnostic imaging as they are highly fluorescent compounds [5–7]. Cobalt, molybdenum, and nickel are vital for growth, metabolism, physiological, and protective functions of microalgae and cyanobacteria [8, 9]. Nanoparticles of these metal can also be utilized for targeted synthesis and accumulation of compounds of interest in phycological objects, including the cyanobacterium Arthrospira platensis. This research aimed to study the effects of Ni, Mo, and Co nanoparticles stabilized in polyethylene glycol on the chlorophyll, β-carotene, and phycobiliproteins content in Arthrospira platensis.

2 Materials and Methods 2.1 Nanoparticles The polyethylene glycol stabilized Ni (NiNPs), Co (CoNPs), and Mo (MoNPs) nanoparticles with a size of 10.0 ± 0.6 nm (custom-made and purchased from the company “Nanomaterials & Technologies”, Togliatti, RF) were used in this study. 2.2 Cyanobacterial Strain The research was carried out using the strain Arthrospira platensis CNMN-CB-02 (spirulina), obtained from the National Collection of Nonpathogenic Microorganisms, Institute of Microbiology and Biotechnology of Technical University of Moldova. 2.3 Experimental Procedures Cultivation of Arthrospira platensis CNMN-CB-02 was performed on mineral nutrient medium with the following composition (in g/L): NaNO3 -2.25; NaHCO3 -8.0; NaCl1.0; K2 SO4 -0.3; Na2 HPO4 -0.2; MgSO4 ·7H2 O-0.2; CaCl2 -0.024; FeSO4 -0.01; EDTA0.08; H3 BO3 -0.00286; MnCl2 ·4H2 O-0.00181; ZnSO4 ·7H2 O-0.00022; CuSO4 ·5H2 O0.00008; MoO3 -0.000015. Ni, Mo, and Co nanoparticles were supplemented in different

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concentrations to mineral nutrient medium, after which the spirulina culture was inoculated in the amount of 0.35–0.40 g/L. Cultivation was carried out in Erlenmeyer flasks of 250 mL capacity and experimental volume of 100 mL, temperature of 28–30 °C, optimal pH of the medium 8.0–10.0, continuous illumination with the intensity of 37–55 μM photons/m/2 /s. The cultivation cycle lasted 6 days. In all experimental variants, biomass was separated from the cultivation medium and standardized according to a biomass concentration of 10 mg/mL. 2.4 Analytical Methods Chlorophyll and β-carotene in spirulina biomass were determined spectrophotometrically. This included the following steps. 1 mL of biomass with a 10 mg/mL concentration was centrifuged at 11000 rpm and the supernatant was removed. 1.0 mL of 96% ethyl alcohol was added to biomass sediment. The mixture was subjected to extraction by shaking for 120 min at room temperature, after which it was centrifuged at 11000 rpm for 5 min. The supernatant was separated and the absorbance was recorded at wavelengths of 450 nm (for β-carotene) and 665 nm (for chlorophyll). The content of chlorophyll in cyanobacterial biomass was calculated by using the absorbance coefficient [10]. The content of β-carotene was calculated based on the extinction coefficient specific of β carotene in ethanol [11] and expressed as % biomass. For the determination of phycobiliprotein content, spirulina biomass samples with a concentration of 10 mg/mL were subjected to repeated freezing/thawing procedures (8 times). Then, biomass was centrifuged at 11000 rpm for 5 min. The supernatant was separated and its absorbance was recorded at wavelengths of 620 nm (for C-phycocyanin) and 655 nm (for allophycocyanin). Phycobiliprotein content was calculated on the basis of equations proposed by [12] and expressed in % biomass.

3 Results 3.1 Chlorophyl Content in Spirulina Biomass Grown in the Presence of Ni, Mo, and Co Nanoparticles The addition of nickel nanoparticles to the nutrient medium of spirulina in concentrations between 0.25–1.5 mg/L did not modify the chlorophyll content in the biomass collected at the end of the cultivation cycle (See Fig. 1). Concentration of 2.0 mg/L of these nanoparticles reduced the chlorophyll content by 18.2% compared to control. Nanoparticle concentration of 2.5 mg/L caused a decrease in the content of chlorophyll in cyanobacterial biomass by 40.8%. Molybdenum and cobalt nanoparticles in applied concentrations significantly reduced the chlorophyll content in spirulina biomass. Mo and Co nanoparticles introduced into culture medium at a concentration of 0.25 mg/L reduced the chlorophyll content by 18.8 and 17.4%, respectively. Similarly, chlorophyll decreased in biomass in the case of concentration range from 0.5 to 2.5 mg/L. Both types of nanoparticles reduced this index by 41.5–67.0%. Using a nanoparticle concentration of 1.5 mg/L, the chlorophyll content decreased even more drastically by 64.5–67.0%.

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NiNPs, mg/L

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Fig. 1. The content of chlorophyll in spirulina biomass grown in the presence of Ni, Mo, and Co nanoparticles.

3.2 Change of β-Carotene Content in Spirulina Biomass Under Spirulina Cultivation in the Presence of Ni, Mo, and Co Nanoparticles

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Figure 2 shows the content of β - carotene in biomass under cultivation conditions of spirulina in the presence of Ni, Mo, and Co nanoparticles.

CoNPs, mg/L

Fig. 2. The content of β-carotene in spirulina biomass grown in the presence of Ni, Mo, and Co nanoparticles.

Nickel nanoparticles exerted a stimulatory effect on β-carotene synthesis by spirulina, and stimulatory concentrations were in the range of 0.25–2.0 mg/L. A minimum increase of 24.6% was determined for nanoparticle concentration of 0.5 mg/L. A higher increase of 50.3% in β-carotene content in spirulina biomass was recorded when applying Ni nanoparticle concentration of 2.0 mg/L. Mo and Co nanoparticles reduced the synthesis and, thereby, the accumulation of β-carotene in spirulina. In the presence of concentrations in the range of 0.25–1.0 mg/L of MoNPs and CoNPs, the content of β-carotene in biomass decreased by 12.5–34.5%. In biomass obtained under cultivation conditions of spirulina in the presence of concentrations of 2.0 and 2.5 mg/L MoNPs and CoNPs, a decrease in β-carotene by 43.7–51.9% was found compared to control.

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3.3 Change of Phycobiliprotein Content in Biomass Under Spirulina Cultivation in the Presence of Ni, Mo, and Co Nanoparticles Under conditions of spirulina cultivation in the presence of Ni, Mo, and Co nanoparticles, not only the amount of phycobiliproteins in cyanobacterial biomass has altered, but also changes in the composition of these photosynthetic light-harvesting proteins occurred, namely, in the ratio of C-phycocyanin and allophycocyanin. (See Fig. 3).

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Fig. 3. The content of phycobiliproteins in spirulina biomass grown in the presence of Ni, Mo, and Co nanoparticles.

Nickel nanoparticles introduced into culture medium in concentrations of 2.0 and 2.5 mg/L reduced the content of phycobiliproteins by 23.3–29.2%. The content of Cphycocyanin drastically decreased by 52.4–54.1% compared to control, while the content of allophycocyanin increased by 13.7–26.8%. Concentrations of 0.25 and 0.5 mg/L NiNPs, which did not change the overall content of phycobiliproteins, instead altered the allophycocyanin content, which increased by 12.5–15%. An increase in phycobiliproteins by 13.7–14% was recorded in the experimental variants with the application of NiNPs in concentrations of 1.0 and 1.5 mg/L, where a reduction by 23.4–24.9% in the content of C-phycocyanin in spirulina biomass was also found. Molybdenum (Mo) nanoparticles in concentration range from 0.25 to 1.5 mg/L stimulated the synthesis of phycobiliproteins, and the proportion of their accumulation in spirulina biomass was 15–34.9%. For all concentrations of MoNPs added to nutrient medium of spirulina, an increase in the content of allophycocyanin by 24.6–56.3% was determined. The maximum increase of this index in spirulina biomass was found at a nanoparticle concentration of 1.0 mg/L, for which an increase in C-phycocyanin content by 22.3% was also established. In the case of cobalt nanoparticles, no significant changes in the content of phycobiliproteins in spirulina biomass were revealed, except for nanoparticle concentration of 2.5 mg/L, for which these cyanobacterial constituents were 13.3% lower than in the control. CoNPs maintained their stimulatory effect on allophycocyanin synthesis, which accumulated in biomass in a ratio of 13.3–44.2%. The maximum value was recorded for a concentration of 0.5 mg/L CoNPs. The content of C-phycocyanin in spirulina biomass decreased by 18%.

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4 Discussion This study examined the effects of nickel, molybdenum, and cobalt nanoparticles, stabilized with polyethylene glycol, on photosynthetic pigments (chlorophyll, β-carotene, and phycobiliproteins in cyanobacterium Athrospira platensis (spirulina). The metal nanoparticles stabilized with polyethylene glycol, to a certain extent, prevent their aggregation with the formation of conglomerates and the increase of their toxicity [13, 14]. The analysis of results showed that low concentrations of cobalt and molybdenum nanoparticles did not alter the content of chlorophyll and β-carotene, whereas high concentrations drastically reduced (by more than 50%) the amount of pigments in cyanobacterial biomass. The correlation between the content of chlorophyll in biomass and the concentration of nanoparticles in culture medium was negative and very strong. The Pearson correlation coefficients calculated for the relationship between these two variables were r = −0.871 for CoNPs and r = −0.908 for MoNPs. A similar response to the presence of CoNPs in culture medium was determined for Co2+ , applied to nutrient media of microalgae Monoraphidium minutum and Nitzschia perminuta. When using low concentrations, the content of chlorophyll in biomass increased, and at high concentrations of 2–3 ppm Co2+ , the chlorophyll content decreased by 36–46%. The same changes were registered for β-carotene content. Low concentrations of Co2+ stimulated β-carotene synthesis, while high concentrations reduced its content in microalgae biomass [15]. The response of spirulina culture to cobalt nanoparticles in the nutrient medium was no different from the reaction to Co2+ . Cobalt affected the photosynthetic system of cyanobacteria. Thus, in the case of microalgae Skeletonema costatum, the chlorophyll content decreased dramatically at the presence of 50 mg/L CoNPs in medium during 4 days of contact of mature culture with nanoparticles. The reduction in β-carotene content was less significant [3]. Concentrations of 1, 2, and 3 ppm molybdenum stimulated chlorophyll synthesis in Oscillatoria agardhii, whereas the concentration of 5 ppm determined the minimum content of chlorophyll in biomass. The same changes have been established for β-carotene with maximum value identified for the concentration of 3 ppm molybdenum [16]. Inhibition of carotenoid and chlorophyll synthesis was revealed for various types of nanoparticles [17–19]. Such a response occurred due to the toxicity of nanoparticles that, as a result of excessive accumulation of free radicals in chloroplasts, affected the photosynthetic system [20]. The application of Au and Ag nanoparticles stabilized in polyethylene glycol to spirulina culture medium led to a moderate inhibition upon the synthesis of photosynthetic pigments for nanoparticle concentrations ranging between the values of 1 μM-10 μM [21]. Low concentrations of 0.025–0.5 μM of Au and Ag nanoparticles stabilized in polyethylene glycol slightly enhanced the chlorophyll content without altering the values of β-carotene content in spirulina biomass [22]. Molybdenum and cobalt nanoparticles reduced the synthesis and accumulation of β-carotene in spirulina biomass. The inhibitory effect was the same for both types of nanoparticles. In the presence of MoNPs and CoNPs in concentrations of 0.25–1.0 mg/L, a decrease of β-carotene content in spirulina biomass by 12.5–34.5% occurred. In the collected biomass under cultivation conditions in the presence of 2.0 and 2.5 mg/L MoNPs and CoNPs, β-carotene was found to be 43.7–51.9% lower than in the control sample.

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The inhibitory effect of Co and Mo nanoparticles was confirmed by the correlation coefficients between the concentrations of MoNPs and CoNPs and β-carotene content: r = −0.929 and r = −0.980, respectively, suggesting a very strong negative correlation. In this study, Ni nanoparticles in the applied concentrations had a less dramatic impact on the photosynthetic system of A. platensis culture. Concentrations between 0.25–1.5 mg/L did not affect chlorophyll synthesis and stimulated the synthesis of βcarotene. Inhibition of chlorophyll production in cyanobacteria Anabaena doliolum has been reported in the presence of Ni in concentrations of 10 and 100 μM. [23]. In the case of NiNPs stabilized in polyethylene glycol, their inhibitory effect on chlorophyll synthesis was determined at a concentration of 2.5 mg/L. An inhibitory effect on the synthesis of photosynthetic pigments was found for Ni2+ added to culture medium of green microalgae Ankistrodesmus falcatus at low concentrations of 1–17 μg/L [24]. On the other hand, it is known that low concentrations of Ni2+ significantly induce the growth of photosynthetic pigments (chlorophyll, βcarotene, phycobilinov) in Spirulina platensis, high concentrations strongly suppress the synthesis of these compounds [25]. The inhibitory effect on the synthesis of photosynthetic pigments was caused as a result of oxidative stress induced by the presence of nanoparticles in the culture medium. However, studies have been reported showing an increase in carotenoids involved in the antioxidant response against reactive oxygen species [26]. This effect was observed in present study, but concentrations of NiNPs stimulating β-carotene synthesis did not alter the chlorophyll content in cyanobacterial biomass, and no damage to the photosynthetic system can occur, so further studies are needed to explain this type of response. The response of the adaptation of spirulina culture to the consequences of oxidative stress that affected the photosynthetic system was confirmed by changes in the content of phycobiliproteins in biomass. Phycobiliproteins act as antennae of the photosynthetic apparatus that efficiently capture luminous energy and transfer it to chlorophyll during photosynthesis. Cyanobacterium Arthrospira platensis contains two main phycobiliproteins: C-phycocyanin, which is the primary pigment, and allophycocyanin, both play the role of light-harvesting pigments and antioxidant cell protectors. It is possible that the reduction of chlorophyll in spirulina biomass has been mitigated by an increase in the content of phytobiliproteins, resulting in efficient capture and transfer of light energy. Mo nanoparticles proved to be the most effective in stimulating the synthesis of phycobiliproteins. The result was influenced by molybdenum, which is the main constituent of several enzymes involved in protein synthesis [27]. Elevated levels of phycobiliproteins have also been determined in experimental variants with the application of CoNPs. It can be assumed that the decrease in the content of chlorophyll and β-carotene in cyanobacterial cell disrupted the activity of photosynthetic system and, in order to maintain cell viability, the biosynthetic activity was reoriented towards the synthesis of phycobiliproteins as auxiliary pigments. This assumption is valid if we admit that nanoparticles can function as trace elements, while Mo and Co can be part of specific

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enzymes. Among phycobiliproteins determined in biomass using Co and Mo nanoparticles, allophycocyanin turned out to be the component with the highest values. Allophycocyanin increased the range of the spectrum available for light capture and ensured photosynthesis under conditions of insufficient chlorophyll production. Under the action of NiNPs, the content of phycobiliproteins in biomass had a decreasing tendency with an increase in the concentration of nanoparticles in culture medium. In these variants, no changes in the content of chlorophyll were found and, as a result, the functioning of the photosynthetic apparatus was not disturbed. In this case, we can assume that the biosynthetic activity of culture was oriented towards the synthesis of β-carotene and its use as an antioxidant. Nickel largely influenced the content of phycobiliproteins in spirulina biomass. In the case of spirulina growth in polymetallic systems with a predominance of Ni, the content of phycobiliproteins decreased, while the proportion of allophycocyanin pigment increased [9]. Therefore, it can be assumed that the increase in allophycocyanin in the structure of phycobilins of spirulina biomass grown in the presence of NiNPs was due to a stimulatory effect exerted on allophycocyanin synthesis and an adaptive response to stress. A tendency towards a decrease in the content of phycobiliproteins in biomass was observed under conditions of increasing nanoparticle concentration in culture medium. It should be noted that Person’s correlation coefficient showed negative values for all types of nanoparticles used in the present study, and only for Ni nanoparticles it was r = −0.984, which suggests a very strong negative correlation.

5 Conclusions The type of nanoparticles determines how spirulina culture copes with them and supports the biosynthetic activity that manifested in elevated levels of antioxidants and auxiliary photosynthetic pigments. Mo and Co nanoparticles can be used in the cultivation technologies of cyanobacterium Arthrospira platensis to stimulate the phycobiliproteins synthesis and their accumulation in the biomass. The stimulatory effect of these nanoparticles depends on their concentration in the culture medium. Mo nanoparticles in the concentration of 1.0 mg/L increased the content of phycobiliproteins in the biomass of Arthrospira platensis by 34.9% and the content of allophycocyanin by 56.3%. CoNPs in the concentration of 0.5 mg/L increased by 44.2% the synthesis of allophycocyanin. Ni nanoparticles, applied in a concentration of 2.0 mg/L, stimulated the synthesis of βcarotene, the content of which increased by 50.3% Acknowledgments. The results were obtained within the project 20.80009.5007.05 funded by the National Agency for Research and Development, Republic of Moldova.

Conflicts of Interest. The authors declare no conflict of interest.

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Preservation of Microorganisms of Biotechnological Interest Involving Fe2 O3 , Fe2 ZnO4 , and ZnO Nanoparticles Tamara Sirbu(B)

, Cristina Moldovan , and Olga Turcan

Technical University of Moldova, Chisinau, Republic of Moldova [email protected]

Abstract. Microorganisms are inexhaustible and advantageous sources of bioactive substances, which is why they are widely used in biotechnology. The in-depth study of the interactions between microorganisms and nanomaterials opens new ways to improve the biosynthetic properties of microorganisms of biotechnological interest for their application in various technological fields. The use of nanoparticles in the process of cultivating microorganisms that have a beneficial effect on their biosynthetic properties facilitates obtaining valuable bioactive substances, as well as contributing to maintaining stable biosynthetic properties in the process of conservation and long-term storage. The effect of nanoparticles on the biosynthetic activity of microorganisms varies depending on the chemical composition, size, morphology and concentration of the particles, as well as on the physiological-biochemical particularities of the culture. In the process of lyophilization of microorganisms, numerous factors cause the appearance of various problems related to the safety of the initial properties of strains of microorganisms of biotechnological interest. The results of the research presented in this material, carried out on the basis of micromycetes of the genus Trichoderma and the genus Penicillium, demonstrate that the involvement of Fe2 O3 , Fe2 ZnO4 , ZnO nanoparticles in the preservation process by the lyophilization method can contribute to the stimulation of the strains’ sensitivity to some phytopathogens. Thus, the application of nanomaterials in the process of long-term conservation of microorganisms of biotechnological interest, with the subsequent involvement of crops in obtaining biopreparations for agricultural use, contributes to increasing their efficiency in combating phytopathogens and obtaining ecological products. Keywords: Micromycetes · Nanoparticles · Lyophilization · Lyoprotective Medium · Viability

1 Introduction A new field of modern science is nanotechnology. Nanoparticles (NPs) due to their small size, equivalent to ultrafine particles and their physical, chemical, magnetic, optical, and biological properties, have found application in various fields: biotechnology, medicine, food industry, agriculture, etc. The publications of recent years demonstrate the effect © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 457–466, 2024. https://doi.org/10.1007/978-3-031-42775-6_49

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of NPs on the viability, development, and biosynthetic processes of microorganisms. It was demonstrated that with the help of NPs introduced into the culture medium, the morphological characteristics of microorganisms can be modified and the biosynthetic processes stimulated, thus obtaining the expected microbial product of a higher quantity and quality [10, 16, 25, 28, 29, 32, 35]. According to the results of various scientific publications, the effect of NPs on the biosynthetic activity of microorganisms varies depending on the chemical composition, size, and concentration of the particles, as well as the physiological-biochemical characteristics of the strains studied [9, 10, 12, 20, 27, 30]. Numerous researches demonstrate the beneficial action of NPs on living cells, but like any new technology, the one with NPs presents a particular risk regarding possible adverse effects such as DNA modification, acceleration of aging processes and cell death, etc. [1, 15, 17, 18]. Data are presented that demonstrate the ability of NPs to stimulate the growth and development of microorganisms [5, 13, 32]. It was demonstrated that NPs of Fe3 O3 , Fe2 ZnO4 , in a concentration of 5 mg/L, supplemented in the lyoprotective medium increase the viability of micromycetes of the genus Trichoderma and Aspergillus after lyophilization and storage in a lyophilized state [24]. Also, the use of Fe2 ZnO4 or Fe2 CuO4 NPs, supplemented in the cultivation medium of Trichoderma and Penicillium strains, stimulates their sensitivity to some phytopathogens by 12–30% [30]. It was found that Fe3 O4 NPs, used in the nutrient medium for the cultivation of the yeast Rhodotorula gracilis CNMNY-30, in concentrations of 0.5–30.0 mg/L, do not significantly change the production of cellular biomass, but change the biochemical composition of the yeast, reduce the content of carbohydrates and proteins, and in high concentrations it causes significant disturbances in catalase activity [27]. The study on the relationship between the indices of protein quantity and those of catalase activity in the biomass of Saccharomyces cerevisiae CNMN-Y-20, in contact with ZnO NPs, revealed a medium and weak dependence on the size of the NPs [28]. The strong antimicrobial effect of Fe2 O3 and ZnO NPs has been reported in various studies, so they could be used in medicine in combination with antibiotics for bacterial inhibition [20–22]. At the same time, ZnO and Fe2 O3 NPs can significantly affect the soil microbial community, their associated functions and crop yield. ZnO NPs are more toxic [19]. The use of ZnO NPs in various fields is in many cases due to the demonstrated bactericidal and bacteriostatic effects. The cytotoxic mechanism of ZnO NPs, according to the reports of various researchers, consists in the generation of reactive oxygen species, damage to the cell wall and membrane permeability, internalization of NPs, as a result of which mitochondrial dysfunction and disruption of glucose metabolism occur, intracellular flux and release in gene expression of oxidative stress that leads to DNA damage, causes inhibition of cell growth and can cause cell death [3, 7, 11, 26]. Currently, nanomaterials and nanotechnologies are successfully used in almost all fields of agriculture: plant production, animal breeding, poultry breeding, fish farming, veterinary medicine, processing industry, production of agricultural machinery etc. [2, 4, 6, 23, 31, 33, 34]. At the same time, it was demonstrated that Fe2 O3 and ZnO NPs can significantly affect the soil microbial community, their associated functions and crop yield, the former being relatively more toxic.

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The conservation and preservation of microorganisms of biotechnological interest is the main task of microorganism collections. The beneficial effects of nanoparticles used in the culture medium on the biosynthetic processes of microorganisms were a benchmark to investigate their action in the conservation process.

2 Material and Methods 2.1 Study Objects Eleven strains of micromycetes from the National Collection of Non-pathogenic Microorganisms, representatives of the genera Trichoderma (5) and Penicillium (6); and NPs of Fe2 O3 , Fe2 ZnO4 , and ZnO were used as objects for the study. Experiments were set up to study the impact of the lyoprotective medium skimmed milk + 7% glucose (SM + 7% G) supplemented with NPs of Fe2 O3 , Fe2 ZnO4 , and ZnO, on the antifungal activity of the mentioned strains after lyophilization. The concentration of Fe2 O3 , Fe2 ZnO4 NPs supplemented in the lyoprotective medium was 5 mg/L, and that of ZnO NPs – 0.1 mg/L. These NPs concentrations, supplemented in the lyoprotective medium, were selected as a result of previous research in which the highest viability of the strains after lyophilization was established. The SM + 7% G variant, without the application of NPs, was considered as a control. The NPs used had different shapes and sizes. NPs of Fe2 O3 have a regular cubic shape, inhomogeneous dimensions 2–10 nm, with particles of 2–4 nm in size predominating. NPs of Fe2 ZnO4 have irregular shapes with a tendency towards the cubic shape, more homogeneous sizes, between 8–15 nm. NPs of Fe2 O3 and Fe2 ZnO4 were synthesized at the Institute of Chemistry of Moldova [8]. The sizes of NPs ZnO ranged from 20–30 nm and were synthesized by researchers from South-West State University in Kursk, Russia [14]. We thank our colleagues for making these NPs available to us. 2.2 Lyophilization The lyophilization process consists of freezing the biological material and sublimation it in vacuum. Freezing was carried out in the Ult Freezer DW86L626/386/286 refrigerator. The “LABCONCO 6 plus” sublimation system was used in the lyophilization process. For lyophilization, the cultures taken in the study were grown in tubes on the maltagar medium, for 10–14 days, then the spores were suspended in the protective lyoprotectant medium SM + 7% G and the variants in which the lyoprotectant medium was supplemented with 5 mg/L NPs. One ml of spore suspension was introduced into 5 ml vials, which were later frozen at the initial temperature of −50 °C. 2.3 Rehydration of Lyophilized Cultures In each vial with the lyophilized culture, 1 ml of distilled, sterile water was introduced. Rehydration was performed at 28 °C for 2 h. After 2 h of rehydration, the suspension was inoculated into Petri dishes on Czapek medium. After 2 passages of cultivation, investigations were carried out to evaluate the antimicrobial activity.

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2.4 Determination of Antimicrobial Activity Antimicrobial properties were studied according to the diffusometric method, using agar blocks [20]. The method is based on the diffusion capacity of the metabolites produced by the studied microorganisms in the depth of the agar and the action of the active substance in the diffusion zone on phytopathogenic cultures. Phytopathogens were used as test strains: Aspergillus niger; Alternaria alternata, Botrytis cinerea, Fusarium solani, and Fusarium oxysporum. According to the size of the inhibition zones, the researched strains are divided into sensitive, moderately sensitive, and resistant: • • • •

ø inhibition zone up to 10 mm - low sensitivity; ø inhibition zone of 11–15 mm - medium sensitivity; ø inhibition zone of 15–25 mm - sensitive; ø inhibition zone greater than 25 mm - increased sensitivity. Statistical data processing was performed using MS Office Excel 2010.

3 Results and Discussion The experiments set up with the aim of evaluating the impact of the lyoprotective medium supplemented with Fe2 O3 , Fe2 ZnO4 , and ZnO NPs on the antifungal activity of the strains belonging to the genus Trichoderma and Penicillium after lyophiliza-tion demonstrated that the antifungal activity of the strains of the Trichoderma ge-nus after lyophilization in the control variant is at the level of up to upon lyophiliza-tion or slightly diminished. In the variants with NPs, the zones of inhibition of phyto-pathogens exceed the control variant, or are at the same level, with few exceptions (Fig. 1). According to the data presented in Fig. 2 the antifungal activity of strains of the Trichoderma genus, which were lyophilized in the lyoprotective medium SM + 7% G + 5 mg/L NPs of Fe2 O3 , is higher in comparison with control, with few exceptions. This proves to us that NPs of Fe2 O3 supplemented in the lyoprotective medium, used in the lyophilization process of cultures, act differently on the biosynthetic properties of micromycetes. The stimulation of the antifungal activity of the studied cultures varied between 104.3–124.0%. Insignificant decreases (2.5%) in comparison with control variant were recorded in T. virens CNMN FD 13, the zones of inhibition of the phytopathogen F. oxysporum constituted 97.5% in comparison with control variant. Also, the action of the exometabolites of the Tr. Harzianum FD 16 strain is weaker in comparison with Alt. Alternata, the zone of inhibition of the phytopathogen was 94.3% in comparison with control. The zones of inhibition under the action of exometabolites of Trichoderma strains, obtained in variants with Fe2 ZnO4 NPs, also exceed the control variant (Fig. 3). The most obvious is observed in the T. lignorum CNMN FD 16 strain on phytopathogens Alt. alternata, B. cinerea, and F. oxysporum, the zones of inhibition exceed the control by 22%, 18%, and 24%, respectively. At the same time, this strain shows a decrease in activity in comparison with F. solani (4%). A significant stimulation of the antifungal activity, in comparison with control, showed the T. harzianum CNMN FD 16 strain against the phytopathogens A. niger, B. cinerea, and F. oxysporum. The inhibition zones in this case exceeded the control variant by 24%, 30%, and 30%, respectively. At the same

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Fig. 1. Zones of inhibition of phytopathogens under the action of exometabolites of T. virens CNMN FD 13 strain until (I) and after lyophilization (M) in the presence of Fe2 O3 (Fe) and Fe2 ZnO4 (Zn) NPs

Fig. 2. Antifungal activity of Trichoderma strains after lyophilization in protective media with Fe2 O3 NPs

time, the metabolites of this strain showed a decrease in comparison with phytopathogen Alternaria alternata, where the inhibition zones constituted only 97%, in comparison with control (Fig. 3).

Fig. 3. Antifungal activity of Trichoderma strains after lyophilization in protective media with Fe2 ZnO4 NPs

The lyophilized strains in the lyoprotective medium supplemented with ZnO NPs showed antifungal activity against the tested phytopathogens at the control level or stimulated this activity, with the exception of the Tr. virens FD 13 strain, in which the

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diameter of the phytopathogen inhibition zones was 96.4–111.4%, in comparison with the control (Fig. 4). The diameter of the phytopathogen inhibition zones as a result of the action of the T. lignorum FD 14 strain varied from 104% to 134.4%, in comparison with control. The strain acted more actively, in comparison with control, after lyophilization in the presence of NPs ZnO, on the phytopathogens B. cinerea (127.3%) and F. oxysporum (134.3%). The antifungal activity of the Tr. koninghii FD 15 strain increased due to ZnO NPs supplemented in the lyoprotective medium. The diameter of phytopathogen inhibition zones exceeded the control by 2–29.6%. The strain acted most actively on phytopathogens: A. niger (118%), B. cinerea (129.6%), and F. oxysporum (121%). The strain Tr. harzianum FD 16 showed a higher sensitivity to these phytopathogens. The diameter of the zones of inhibition against these phytopathogens was 121%, 119.4%, and 117.8%, respectively, in comparison with control. NPs of ZnO had a weaker effect on the antifungal activity of the Tr. viride FD 17 strain, the zones of inhibition of the tested phytopathogens exceeding the control by 2.5–6.4%.

Fig. 4. Antifungal activity of Trichoderma strains after lyophilization in protective media with ZnO NPs

Antifungal activity of Penicillium strains after lyophilization in variants with NPs Fe2 O3 does not vary significantly in comparison with control variant (Fig. 5). Antifungal activity in most cases varies within ± 4–5% of the control. P. funiculosum FP 01 in comparison with A. niger (118.5%), P. funiculosum FD 11 in comparison with A. niger (111.5%), and F. oxysporum (118, 2%). P. corylophilum FD 20 in comparison with Alt. alternata (118.2%), and B. cinerea (116.25%). P. corylophilum FP 04 in comparison with Alt. alternata (119%), P. verucosum FP 02 versus A. niger (112.8%), Alt. alternata (124%), and F. oxysporum (125%), P. piceum FD 21 versus A. niger (116%). More active against phytopathogens were the exometabolites of Penicillium strains, after they were lyophilized in the presence of Fe2 ZnO4 NPs (Fig. 6). The zones of inhibition of the phytopathogen A. niger with the exometabolites of these cultures varied within ± 2–3% in comparison with control. A stimulation of the activity, against this phytopathogen, showed only the P. piceum FD 21 strain, the diameter of the inhibition zone constituting 110%, in comparison with control. The zones of inhibition of the phytopathogen Alt. alternata under the influence of exometabolites, from the variant with Fe2 ZnO4 NPs, in comparison with control, varied within the limits of 103.6– 122.4%. The most active against this phytopathogen are the strains: P. corylophilum FP 04 (122.4%), P. verrucosum FP 02 (122%), and P. piceum FD 21 (120%). The antifungal

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Fig. 5. Antifungal activity of Penicillium strains after lyophilization in protective media with Fe2 O3 NPs

activity of the strains from the variant with Fe2 ZnO4 NPs, against the phytopathogen B. cinerea, exceeded the control by 6.1–21.5%. The most active strains are P. funiculosum FP 01 and P. corylophilum FD 20, with inhibition zones of 118.6% and 121.3%, respectively, in comparison with control. In comparison with phytopathogen F. solani, the exometabolites of the studied strains obtained in the variant with NPs Fe2 ZnO4 showed an insignificant stimulation of the antifungal activity against the tested phytopathogens, which exceed the control by 2.0–8.5%. A more significant stimulation, in comparison with phytopathogen, was shown only by the P. corylophilum strain, the zone of inhibition being 114% in comparison with control. Against the phytopathogen F. oxysporum, stimulations of the antifungal activity of Penicillium strains were also obtained. The diameter of the zones of inhibition of this phytopathogen in 5 of the 6 strains studied varied between 102.3 and 125.0%. The most active against the phytopathogen F. oxysporum were the strains P. funiculosum FD 11 (118.2%), P. corylophilum FP 04 (118.8%), and P. verrucosum FP 02 (125%).

Fig. 6. Antifungal activity of Penicillium strains after lyophilization in the presence of NPs Fe2ZnO4

The antifungal activity of Penicillium strains from the variant with ZnO NPs after lyophilization did not change significantly in comparison with tested phytopathogens (Fig. 7). The zones of inhibition of phytopathogens being at the level of the control or exceeding it, insignificantly, by 2–10%. The most significant results showed the strain P. funiculosum FD 11 in comparison with B. cinerea, the diameter of the inhibition zone is

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111.5%, in comparison with control and the P. corylophilum FD 20 strain in comparison with Alt. alternata (114.8% comparison with control).

Fig. 7. Antifungal activity of Penicillium strains after lyophilization in the presence of ZnO NPs

Thus, we can state that strains of the genus Penicillium after lyophilization in the presence of NPs ZnO show in most cases a low sensitivity to the tested phytopathogens. According to the obtained results, we can assume that the penetration of NPs Fe2 O3 , Fe2 ZnO4 , ZnO into the cell acted directly on the metabolic processes in the cell, changing the sensitivity to some phytopathogens.

4 Conclusions As a result of biomolecular interactions with nanoparticles, significant changes can occur in the biosynthetic processes of microorganisms, as a result of which bioactive substances can be obtained in predicted quantities, as well as they could reduce the negative reactions produced in the lyophilization process. The results obtained in this study demonstrated that the action of NPs of Fe2 O3 , Fe2 ZnO4 , ZnO supplemented in the protective medium during lyophilization of micromycetes from the genera Trichoderma and Penicillium act differently on the biosynthetic properties of micromycetes. Thus, NPs can decrease or stimulate the antifungal activity against some phytopathogens. In the case of the mentioned strains after lyophilization, in most cases, the tested nanoparticles contributed to the stimulation of the antifungal activity of the micromycetes against the tested phytopathogens by 5–30%. The most significant results were recorded in the variants with NPs of Fe2 ZnO4 , while the micromycetes from the variants with NPs Fe2 O3 and ZnO recorded a less pronounced antifungal activity. The antifungal activity potential of the micromycetes studied after lyophilization, with the involvement of nanoparticles, decreases in the following order: Fe2 ZnO4 > Fe2 O3 > ZnO. Acknowledgments. The research was funded out within the project 20.80009.7007.09 (ANCD).

Conflict of Interest. The authors declare that they have no conflict of interest.

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Clinical and Cost Effectiveness of Telerehabilitation System in Balance Disorder Patients Karla Mothejlova1(B) , Gleb Donin1

, and Romana Svobodova2

1 Department of Biomedical Technology,

Czech Technical University in Prague, Kladno, Czech Republic [email protected] 2 Department of Information and Communication Technologies in Medicine, Czech Technical University in Prague, Kladno, Czech Republic

Abstract. Balance disorders are a very common consequence of brain damage. Most of these conditions have a chronic nature and require long-term rehabilitation care. Telerehabilitation using the Homebalance system is a suitable alternative or complement to standard rehabilitation. The aim of this study is to evaluate the clinical and cost effectiveness of the Homebalance system in telerehabilitation. The study involved 33 patients who were randomly divided into two groups. The intervention group underwent a 4-week telerehabilitation therapy using the Homebalance system. The comparison group received standard in-person rehabilitation care of the same length. Clinical effectiveness was assessed using the standardized Berg Balance scale test. Quality of life was measured using the EQ-5D-5L questionnaire. The cost part of the study was evaluated from a healthcare payer perspective. Clinical effectiveness of the telerehabilitation was demonstrated by difference in the pre-post BBS scores (p < 0,001), which was comparable to the effectiveness of standard therapy (p = 0,52). No significant changes were observed in the patient’s quality of life during the therapy. The costs of the experimental intervention were estimated at CZK 7,152, while the costs of the comparator were estimated at CZK 9,424. Telerehabilitation brings many benefits for patients allowing to undergo therapy from the comfort of their homes. The results of this study have shown that telerehabilitation using the Homebalance system is clinically effective, and also cost-effective. Keywords: Telerehabilitation · Balance disorder · Homebalance · Cost-effectiveness analysis

1 Introduction Balance disorders are a very common consequence of damage to the central nervous system, most often resulting from neurodegenerative, demyelinating, or oncological diseases, strokes, or traumatic events. Most of these conditions are chronic in nature, requiring long-term rehabilitative care. Regular transportation to rehabilitation facility, © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 467–475, 2024. https://doi.org/10.1007/978-3-031-42775-6_50

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rising fuel prices, long waiting times, etc. could ultimately make it difficult or even impossible for the patient to participate in therapy sessions. Telerehabilitation offers a solution to these problems, as it may allow patients to receive necessary care in the home environment with the possibility of feedback from their therapist [1]. Telerehabilitation typically refers to both telephone consultations and the use of technologies and approaches using interactive therapeutic systems, motion sensors etc. [2]. The Homebalance is an interactive telerehabilitation system for home-based therapy of balance disorders. The system consists of a stabilometric platform, a Microsoft Surface Go 2 tablet and control unit. The platform has four strain gauge sensors in the corners, which detect the patient’s movements. The patient’s movements are displayed on the tablet screen in real time and the patient tries to coordinate his body position to achieve the desired goal. The exercise information is then sent to a secure telemedicine server. This allows remote control of the results by the treating staff. The role of the therapist is to check adherence to the exercise plan or to provide appropriate motivation to meet the therapy goals, or to tele-consult and adapt the difficulty of the therapy to the patient’s current condition. System is certified as a Class I medical device. A number of studies have investigated the technical aspects and clinical outcomes of this system in different indications [3–6], but no economic evaluation relating to this topic was performed yet. This study aims to analyze the clinical and cost effectiveness of the Homebalance system in telerehabilitation of balance disorders in selected neurological diagnoses in comparison with in-person physiotherapy.

2 Methods The study was approved by the local ethics committee and all the participants provided written informed consent. The study was designed as a quantitative comparative analysis with prospective data collection. The clinical and cost effectiveness of the Homebalance telerehabilitation set was compared with standard in-person physiotherapy of balance disorders in patients after stroke and with multiple sclerosis. The cost part of the study was evaluated from a Czech healthcare payer perspective. 2.1 Study Design Participants included in the study were individuals after stroke or diagnosed with multiple sclerosis with acquired brain injury and subjective perceived balance impairment meeting the indication criteria listed below. Participants were recruited for the study at two university hospitals and one rehabilitation health care facility. Inclusion criteria were: stability impairment diagnosed by the standardized Berg Balance Scale test; age at least 18 years; ability to stand independently without support; ability to understand all instructions. Exclusion criteria were: significant spasticity in the lower limbs; severe cognitive deficit (inability to understand the instructions and perform the exercises); a score on the Montreal Cognitive Assessment of 20 points or less; Barthel Index score of 40 points or less; significant stability impairment (inability to stand without support); inability to

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stand upright independently; severe sensory impairment precluding use of the Homebalance system; severe visual impairment making it impossible to view therapy scenes on the tablet; decompensated epilepsy; severe psychiatric disorder (e.g. severe organic psychosyndrome); non-cooperation of the patient or his/her family members. Patients who fulfilled the entry criteria for the study were randomly divided into two groups. The experimental group underwent a four-week intervention in their home environment using the Homebalance system with regular follow-up by a therapist twice a week. The control group attended conventional outpatient physiotherapy for balance and gait disorders twice a week for 4 weeks. Each participant underwent an admission examination using non-invasive standardized tests prior to therapy and then a final examination after the four-week intervention. 2.2 Clinical Evaluation The primary clinical outcome was the Berg Balance Scale (BBS). The BBS is a frequently used test to assess stability and risk of falls in patients after stroke [7]. It is used to objectively determine a patient’s ability or inability to safely maintain balance during 14 specified tasks. These tasks are designed to assess the patient’s ability to maintain balance in both static positions and dynamic movements. Each task is rated on a scale of 0 to 4. 0 represents the lowest level of function, i.e., inability to perform the task. 4 represents the highest level of function, i.e., flawless performance of the task, independently. BBS scores are divided into three categories: 0–20 points represent high risk of falling, 21–40 points represent moderate risk of falling, and 41–56 points represent low risk of falling. Another test used in the study was EQ-5D-5L, the generic health-related quality of life questionnaire [8]. The EQ-5D descriptive system includes five dimensions: mobility, self-care, usual activities, pain/discomfort, and anxiety/depression. In this study, the value set for the UK was used. 2.3 Cost Effectiveness Analysis The total telerehabilitation costs consist of initial and final medical examination costs, telerehabilitation medical procedure costs, and Homebalance system costs. Initial and final examination costs were calculated based on a valid payment catalogue of general health insurance company. Telerehabilitation medical procedure costs were estimated as the sum of relevant personal costs and overhead costs according to the Ministry of health recommendations. The Homebalance system costs were calculated based on its purchase price, lifetime, and maintenance costs. The comparator costs were analyzed on the basis of primary data collection at the university hospital. The costs were analyzed for individual patients who met the study inclusion criteria (main ICD-10 diagnosis: I63, I64, or G35) and therapy was focused on balance disorders. The obtained data were averaged and the costs per patient undergoing 4-week outpatient rehabilitation focused on balance disorders were calculated. Cost minimization analysis was chosen to evaluate the cost-effectiveness of telerehabilitation. This type of analysis can be used to determine which of the treatment

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alternatives provides the least costly way to achieve a particular health outcome. It is used when the effects of the two interventions are comparable, and the decision is made solely on the basis of cost comparisons. 2.4 Statistical Analysis Data are presented as mean (standard deviation; SD). The significance threshold was set at 0.05, with no adjustment for multiple comparisons. Statistical significance was accepted at p < 0.05. Two-sample t-test and paired t-test were used. Statistical processing of data and hypothesis testing were performed in R software. One-way deterministic sensitivity analysis was performed in this study. The number of outpatient visits in controlled group; the purchase price of the Homebalance system, the range of procedures reported under standard therapy for the control group, and telerehabilitation procedure costs were gradually changed.

3 Results 3.1 Clinical Effectiveness A total of 33 individuals participated in the study. Basic participants’ characteristics are presented in Table 1. In the experimental group seventy-eight percent (14/18) were women. The control group included forty-seven percent (7/15) of women. The mean age in the experimental group was 50 years (SD = 16) with a median age of 51 years; in contrast, the mean age in the control group was 54 years (SD = 14) with a median age of 52 years. The diagnosis of stroke was more prevalent in both groups. In eighty-three percent (15/18) in the experimental group and eighty-seven percent (13/15) in the control group. Table 2 shows the mean values for the outcome measures at admission, and after 4 weeks for both groups. There were no significant differences between the groups in terms of the outcome measures at admission (p > 0.05). The mean BBS score measured at the admission examination was 47 for the intervention group and 48 for the control group. The mean score at the final examination was 50 for both groups. There was a statistically significant improvement in the mean BBS score and EQ-5D VAS score after 4 weeks in both groups (p < 0.05). Within the mobility dimension, a statistically significant result was observed only in the experimental group (p = 0.004). In terms of overall utility, no statistically significant improvement was demonstrated in either group. Table 3 shows the mean incremental effects after 4 week treatment for both groups. There was no statistically significant difference between the experimental and control group for all clinical outcomes (p > 0.05). 3.2 Cost Analysis The cost of the 4-week experimental therapy was estimated as follows: relevant related medical procedures - 1 004 CZK, the cost of renting the device for 4 weeks – 6 149 CZK. The total cost of the 4-week telerehabilitation intervention was 7 153 CZK.

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Table 1. Participants’ characteristics. Characteristics

Experimental group, N = 18

Control group, N = 15

p-value 0.064a

Sex, n (%) Male

4 (22%)

8 (53%)

Female

14 (78%)

7 (47%) 0.5b

Age Median (IQR)

51 (41, 64)

52 (44, 66)

Mean (SD)

50 (16)

54 (14)

Diagnosisc , n (%)

>0.9a

Stroke

15 (83%)

13 (87%)

MS

3 (17%)

2 (13%)

Note: IQR = Interquartile range; MS = multiple sclerosis; SD = Standard deviation; a Pearson’s Chi-squared test; Fisher’s exact test, b Wilcoxon rank sum test; c Main diagnosis for inclusion in the study.

Table 2. Clinical outcomes for the experimental and control groups. Admission, Mean (SD)

4th week, Mean (SD)

p-value

Experimental

47 (11.0)

50 (9.9)

A, dbSNP rs4244285). The genotype frequencies obtained were 77.9% (95% IC 71.3, 83.6) (G/G), 19.2% (95% IC13.8, 25.6) (G/A) and 2.9% (95% IC 1.1, 6.3) (A/A). Patients were categorized by CYP2C19 metabolizer status based on *2 genotypes using the common consensus star allele nomenclature as normal metabolizer (G/G), intermediate metabolizer (G/A), and poor metabolizer (A/A), respectively (Table 2).

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Total - 172 patients

Count

%

95% Lower CL

95% Upper CL

Male

140

81.4

75.1

86.7

Female

32

18.6

13.3

24.9

Obesity

63

36.6

29.7

44.0

Abdominal obesity

72

41.9

34.7

49.3

Smoking

44

25.6

19.5

32.5

139

80.9

66.1

97.9

Hypertension Diabetes mellitus

56

32.6

25.9

39.8

Chronic kidney disease

21

12.2

8.0

17.7

152

88.3

79.8

94.1

7

4.0

1.4

11.2

13

7.5

3.5

14.6

Acute myocardial infarction Unstable angina Stable angina

Age mean (range) (years) 61 (31–85) Count: absolute frequency; %: relative frequency, CL: confidence level Table 2. CYP2C19*2 frequency Allele

Phenotype

Frequency n (%)

95% Lower CL

95% Upper CL

GG

Normal

134 (77.9)

71.3

83.6

GA

Intermediate

33 (19.2)

13.8

25.6

AA

Poor

5 (2.9)

1.1

6.3

n: sample size; %: percentage, absolute frequency; %: relative frequency, CL: confidence level

Comparison of Clinical Backgrounds Between the CYP2C19*2 Carriers and NonCarriers Groups There were 38 patients with the CYP2C19*2 A allele in total. The clinical backgrounds include sex, age, body mass index (BMI), smoking, creatinine, blood lipids level clinical presentation and comorbidities. There were no significant differences in any parameter at baseline between the CYP2C19*2 carriers and non-carriers groups (p > 0.05). The details are displayed in Table 3. Effects of CYP2C19*2 Polymorphism on Clinical Outcomes The total clinical outcomes are presented in Table 4. Throughout the follow-up period, the patients experienced various adverse cardiovascular events, including 20 cardiac deaths, 17 cases of myocardial infarction, 4 instances of defined stent thrombosis, and 59 cases of unstable angina and stroke - in 4 cases. We observed a strong trend toward higher frequency of unstable angina, death, myocardial infarction, defined stent thrombosis and MACE in CYP2C19*2 carriers compared to non-carriers.

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Table 3. Baseline demographics and characteristics of patients Characteristics

Total (n = 172) CYP2C19*2 non-carriers p-value carriers (n = 38) (n = 134)

Male sex, n (%)

140

29 (76.3)

111 (82.8)

0.456

Age (years, mean ± SD)

61 ± 10

61 ± 10

61 ± 9.5

0.580

Smoking, n (%)

44 (25.6)

8 (21.1)

36 (20.9)

0.914

Obesity, n (%)

63 (36.6)

14 (36.8)

49 (36.5)

0.321

Abdominal obesity, n (%)

72 (41.9)

18 (47.4)

54 (40.2)

0.721

Hypertension, n (%)

139 (80.9)

33 (86.8)

106 (79.1)

0.658

Diabetes mellitus

56 (32.6)

14 (36.8)

42 (31.3)

0.412

Previous MI

31 (18.0)

9 (23.7)

22 (16.41)

0.487

Chronic kidney disease 21 (12.2) GFR < 60 mL/min/min/1.73 m2

5 (13.2)

16 (11.9)

0.893

TC (mmol/l)

5.26 ± 1.4

5.04 ± 1.40

5.49 ± 1.41 0.191

LDL-C (mmol/l)

3.05 ± 1.13

2.95 ± 1.09

3.24 ± 1.18 0.311

TG (mmol/l)

1.93 ± 1.12

1.96 ± 1.20

1.9 ± 1.05

0.717

AMI

152 (88.3)

33 (86.8)

119 (88.8)

0.771

n: sample size; %: percentage, TG - triglycerides, LDL-C-low density lipoprotein cholesterol, HDL-C-high density lipoprotein cholesterol, TC - total cholesterol, MI - myocardial infarction, GFR - Glomerular Filtration Rate, AMI - Acute myocardial infarction

Table 4. Clinical Outcomes Total = 172 patients

CYP2C19*2 carriers (n = 38)

CYP2C19*2 non-carriers (n = 134)

OR (Odds ratio (95%CI))

Death

10 (26.3%)

10 (7.5%)

4.4 0.003 (95% IC 1.6, 11.6)

Myocardial infarction

11 (28.9%)

6 (4.5%)

8.6 0.001 (95% IC 2.9, 25.5)

Stroke

1 (2.6%)

3 (2.2%)

1.18 0.635 (95% IC 0.1, 11.6)

Stent thrombosis

3 (7.9%)

1 (0.7%)

11.4 (95% IC 1.1, 112.9)

0.034

Unstable angina

20 (11.6%)

11 (8.2%)

3.4 (95% IC 1.3, 9.1)

0.013

n: sample size; %: percentage

p-value

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Over a follow-up period of 6–12 months, CYP2C19*2 carriers had odds ratios of 4.42 (95% CI 1.68, 11.65) for cardiac death, 8.69 (95% CI 2.95, 25.53) for myocardial infarction, 11.4 (95% CI 1.15, 112.98) for stent thrombosis, and 3.47 (95% CI 1.31, 9.14) for unstable angina compared to non-carriers (p < 0.001).

4 Disscusion The patients enrolled in this study had the following risk factors: 36.6% (95% CI 29.7, 44.0) had obesity, 41.9% (95% CI 34.7, 49.3) had abdominal obesity, 80.9% (95% CI 66.1, 97.9) had hypertension, 32.6% (95% CI 25.9, 39.8) had diabetes, and 12.2% (95% CI 8.0, 17.7) had chronic kidney disease. There were no significant differences in any parameter at baseline between the groups of CYP2C19*2 carriers and non-carriers (p > 0.05). Dual antiplatelet therapy a P2Y12 and aspirin is the standard-of-care following PCI to prevent MACE, including cardiovascular death, myocardial infarction, and stent thrombosis. Clopidogrel is the most prescribed P2Y12 inhibitor. Numerous studies have demonstrated significant interpatient variability in platelet inhibition among those treated with clopidogrel. This variability is influenced by several factors, including variation in CYP2C19-mediated formation of the active metabolite of clopidogrel [8]. It is well-known that 85–90% of an oral dose undergoes first pass metabolism by carboxylesterase 1 in the liver, leading to the formation of an inactive carboxylic acid metabolite. Additionally, approximately 2% of clopidogrel is oxidized to 2oxoclopidogrel. The most common alleles associated with CYP2C19 activity are CYP2C19*2 (681G > A) and CYP2C19*3 (636G > A), both of which encode enzymes with decreased activity [9, 10]. The frequency of the CYP2C192 allele is significantly higher in East Asian populations (14–39%) compared to Caucasians (8–16%) and Africans (18–25%) [11]. In this study, the presence of the CYP2C19*2 (681G > A) allele was described in a group of coronary patients, with a genotype frequency of 74.5% for G/G, 21.6% for G/A, and 3.9% for A/A. It is well-established that patients carrying CYP2C19 (LOF) alleles have a reduced capacity for clopidogrel bioactivation, impaired platelet inhibition, and a significantly higher risk of MACE when treated with clopidogrel compared with patients without a LOF allele [12, 13]. In a retrospective study conducted by Yu L. et al., the effect of genetic polymorphisms in CYP2C19*2, 3, and 17, along with clinical and demographic factors, on variation in response to clinical outcomes was investigated in 351 Uygur patients with ACS who were treated with clopidogrel and aspirin for at least 12 months. The authors reported that CYP2C19*2 carriers had an odds ratio of 2.51 (95% CI 1.53, 4.09) for experiencing MACE compared to non-carriers (p < 0.001). They concluded that the CYP2C192 gene polymorphism contributes to the risk of MACE in DAPT [14]. This finding was also demonstrated in the present study. Over a follow-up of 6–12 months, poor and intermediate metabolizers had an odds of cardiac death of 4.42 (95% CI 1.68, 11.65), of 8.69 (95% CI 2.95, 25.53) and of unstable angina by 3.47 (95% CI 1.31, 9.14) compared with normal, rapid and ultra-rapid metabolizers (p < .001).

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Our study has the following limitations: First, the research included only 172 patients who received clopidogrel treatment after PCI. Therefore, our findings regarding the association between CYP2C19 polymorphism and the clinical outcomes of clopidogrel treatment need to be validated in larger clinical trials. Second, due to limitations in the detection technology available, we were unable to establish the value of CYP2C19 genotyping combined with platelet reactivity during treatment (platelet function test). Third, some information about the patients’ MACE was obtained over the phone, which may have led to inaccuracies in data documentation.

5 Conclusions The CYP2C19*2 gene polymorphism is an essential risk factor associated with MACE in patients treated with clopidogrel after PCI. These data provide valuable insights into the genetics polymorphisms affecting clopidogrel metabolism among these patients. The clinical testing of the CYP2C19 genotype has been suggested as a way to personalize antiplatelet therapy selection and reduce the risk of MACE in patients undergoing PCI. By identifying patients with the CYP2C19*2 polymorphism, healthcare providers can consider alternative antiplatelet therapies to optimize treatment outcomes. Acknowledgments. This work was supported by the project 20.80009.8007.26 “Piloting the application of the principles of personalised medicine in the conduct of patients with chronic non-communicable diseases” within the State Program (2020–2023), project leader: Curocichin Ghenadie, contracting authority: National Agency for Research and Development, Republic of Moldova.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Valgimigli, M., Bueno, H.: 2017 ESC focused update on dual antiplatelet therapy in coronary artery disease developed in collaboration with EACTS: the task force for dual antiplatelet therapy in coronary artery disease of the European Society of Cardiology (ESC) and of the European Association for Cardio-Thoracic Surgery (EACTS). Eur Heart J. 39(3), 213–260 (2018). https://doi.org/10.1093/eurheartj/ehx419;6 2. Knuuti, J., Wijns, W., Saraste, A., Capodanno, D.: 2019 ESC Guidelines for the diagnosis and management of chronic coronary syndromes. Eur. Heart J. 41, 407477 (2020). https:// doi.org/10.1093/eurheartj/ehz425 3. Dayoub, E.J., Seigerman, M., Tuteja, S.: Trends in platelet adenosine diphosphate P2Y12 receptor inhibitor use and adherence among antiplatelet-naive patients after percutaneous coronary intervention, 2008–2016. JAMA Intern. Med. 178, 943–950 (2018). https://doi.org/ 10.1001/jamainternmed.2018.0783 4. Simon, T., Verstuyft, C.: Genetic determinants of responseto clopidogrel and cardiovascular events. N. Engl. J. Med. 360(4), 363–375 (2009). https://doi.org/10.1056/NEJMoa0808227

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5. Scott, S.A., Sangkuhl, K.: Clinical pharmacogenetics implementation consortium: clinical pharmacogenetics implementation consortium guidelines for CYP2C19 genotype and clopidogrel therapy: 2013 update. Clin. Pharmacol. Ther. 94, 317–323 (2013). https://doi.org/10. 1038/clpt.2013.105 6. Mao, L., et al.: Cytochrome CYP2C19 polymorphism and risk of adverse clinical events in clopidogrel-treated patients: A meta-analysis based on 23,035 subjects. Arch. Cardiovasc. Dis. 106(10), 517–527 (2013). https://doi.org/10.1016/j.acvd.2013.06.055 7. Neumann, F.-J., Sousa-Uva, M.: 2018 ESC/EACTS Guidelines on myocardial revascularization. Eur. Heart J. 40, 87–165 (2019). https://doi.org/10.1093/eurheartj/ehy394 8. Akkaif, M.: The role of genetic polymorphism and other factors on clopidogrel resistance (CR) in an Asian population with coronary heart disease (CHD). Molecules 2021, 26 (1987). https://doi.org/10.3390/molecules26071987 9. Jiang, X.-L., Samant, S.: Clinical pharmacokinetics and pharmacodynamics of clopidogrel. Clin. Pharmacokinet. 54, 147–166 (2015) 10. Pereira, N.L., Geske, J.B.: Pharmacogenetics of clopidogrel: an unresolved issue. Circ. Cardiovasc. Genet. 9, 185–188 (2016). https://doi.org/10.1161/CIRCGENETICS.115.001318 11. Cedillo-Salazar, F.R.: Association of CYP2C19*2 polymorphism with clopidogrel resistance among patients with high cardiovascular risk in Northeastern Mexico. Arch. Cardiol. Mex. 89(4), 324–329 (2019). https://doi.org/10.24875/ACM.19000033 12. Aradi, D., Kirtane, A.: Bleeding and stent thrombosis on P2Y12-inhibitors: collaborative analysis on the role of platelet reactivity for risk stratification after percutaneous coronary intervention. Eur. Heart J. 36(27), 1762–1771 (2015). https://doi.org/10.1093/eurhea rtj/ehv104 13. Winter, M.P., Schneeweiss, T.: Platelet reactivity patterns in patients treated with dual antiplatelet therapy. Eur. J. Clin. Investig. 49(6), e13102 (2019). https://doi.org/10.1111/eci. 13102 14. Yu, L., Wanget, T.: Association between cytochrome P450 2C19 polymorphism and clinical outcomes in clopidogrel-treated Uygur population with acute coronary syndrome: a retrospective study. BMC Cardiovasc. Disord. 21, 391 (2021). https://doi.org/10.1186/s12872021-02201-4

Antibacterial Activity of “Green” Silver Nanoparticles (AgNPs) in Combination with Benzylpenicillin and Kanamycin Seda Ohanyan , Lilit Rshtuni , and Ashkhen Hovhannisyan(B) Department of Medical Biochemistry and Biotechnology, Russian-Armenian University, H. Emin 123, 0051 Yerevan, Armenia [email protected]

Abstract. Due to the lack of progress in the development of antibiotics, there is a pressing need for innovative approaches to treat bacterial infections. Nanotechnology and repurposing existing drugs are innovative approaches that can potentially replace traditional antimicrobials. Silver nanoparticles (AgNPs) have the potential to not only exhibit antibacterial and antibiofilm properties but also serve as carriers for antibiotics and natural antimicrobial compounds. Our study involved testing the antibacterial activity of biogenic AgNPs and their complexes with the antibiotics benzylpenicillin (BP) and kanamycin (KM) against the growth of the gramnegative bacteria Escherichia coli K-12. The antibacterial activity of preparations studied with the disk diffusion method, determined the colony-forming activity by serial dilutions, and calculated the minimum inhibitory concentrations (MIC) of the preparations. Additionally, we determined the growth phases of Escherichia coli K-12. Our findings indicate that the AgNPs at the studied concentrations do not possess cyto- and genotoxicity. The results showed that the action of AgNPs in combination with BP against the growth of the bacterium Escherichia coli K-12 has a synergistic effect at concentration 0.5 mg/ml and higher, which can reduce the antibiotic dose up to 8 times, while in the complex with KM has additive activity, in this case, AgNPs reduce the active dose of KM by 30 times. The tested complexes have potential antivirulence effects that inhibiting the development of biofilm formation at concentrations below the MIC. These findings suggest that the complexes could be used as safe alternatives to antibiotics. Keywords: Green silver nanoparticles · Escherichia coli K-12 · Benzylpenicillin · Kanamycin · Potential antibacterial biocompatible complexes

1 Introduction The emergence of drug-resistant pathogenic bacteria is one of the major challenges in the field of human healthcare. One of the new strategies to combat bacteria is nanotechnology, particularly the use of nanoparticles made of various metals [1]. The use of nanoparticles demonstrates a unique approach to penetrating infectious biofilms and targeting bacterial communication – the quorum sensing (QS) system, thus addressing the major problem associated with biofilm formation [2]. The use of silver as an © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 537–545, 2024. https://doi.org/10.1007/978-3-031-42775-6_57

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antimicrobial agent dates back to ancient times [3]. Based on current research, silver nanoparticles (AgNPs) are one of the most promising inorganic NPs for the treatment of bacterial infections [4]. Silver nanoparticles have a wide range of antibacterial, antifungal, and antiviral properties [5]. Delivery systems utilizing AgNPs as a therapeutic agent and a probe for visualization in new diagnostic methods are being developed [6]. The effectiveness of AgNPs is due not only to their nanosize but also to a large surface area-to-volume ratio [4]. The mechanisms of action of silver nanoparticles are diverse. In particular, they can increase the permeability of cell membranes [7], produce reactive oxygen species, thereby initiating oxidative stress, act on biopolymers, lead to denaturation of proteins and nucleic acids, as well as interrupt replication and translation [8]. AgNPs also have the ability to penetrate through the walls of bacterial cells and alter the structure of the cell membrane due to their nanosize [9]. Combining natural-based nanoparticles with antimicrobial agents to suppress the activity of bacterial efflux pumps, inhibit biofilm formation, interfere with quorum sensing, and potentially prevent plasmid invasion, are just a few of the strategies to combat bacteria and multiple drug resistance. In addition, silver nanoparticles can participate in bacterial signal transmission [10]. The transmission of bacterial signals is influenced by the phosphorylation of protein substrates, and nanoparticles can dephosphorylate tyrosine residues on peptide substrates, which, in turn, can lead to cell apoptosis and cessation of their replication [6]. Gram-negative bacteria are more sensitive to silver nanoparticles [5]. Thus, AgNPs represent one of the best options for use in combination with antibiotics to enhance their effectiveness against resistant bacteria [11]. AgNPs demonstrate multiple and simultaneous mechanisms of action, and when combined with antibacterial agents such as organic compounds or antibiotics, they have shown a synergistic effect against pathogenic bacteria, such as Escherichia coli [9]. The characteristics of silver nanoparticles make them suitable for use in medicine. Plant extracts have become an alternative approach for the biological synthesis of AgNPs, given their several advantages over chemical and physical methods of synthesis [12]. Plants of all types contain carbohydrates, fats, proteins, nucleic acids, pigments, and secondary metabolites that can reduce metal salts to produce nanoparticles without any toxic by-products [13]. The use of different plants for “green” synthesis can yield nanoparticles of varying sizes and shapes [14–17]. In particular, phenolic compounds play a significant role in nanoparticle formation. Beta-lactam and aminoglycoside antibiotics, such as benzylenicillin (BP) and kanamycin (KM), are commonly utilizing as antibacterial agents. BP (and all other β-lactams) has a bactericidal effect. The target of its action is penicillin-binding proteins (PBP) of bacteria, which act as enzymes at the final stage of peptidoglycan synthesis. BP binds to and inactivates PBPs located within the bacterial cell wall, which interferes with cross-linking of the peptidoglycan chains required for the strength and rigidity of the bacterial cell wall. This leads to a weakening and therefore interruption of the synthesis of the cell wall, which subsequently causes the lysis of bacterial cells. It is effective against most gram-positive and some gram-negative bacteria, in particular E. coli [18]. Kanamycin is an aminoglycoside antibiotic of the first generation. Aminoglycosides work by binding to the bacterial 30S ribosomal subunit, causing tRNA misreading,

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resulting in the bacterium being unable to synthesize proteins vital for its growth. Aminoglycosides are mainly useful in infections caused by aerobic Gram-negative bacteria such as Pseudomonas, Acinetobacter and Enterobacter. The mechanism of inhibitory action includes interruption of peptide chain elongation by blocking the aminoacyl site of the ribosome, misreading the genetic code, or preventing the attachment of oligosaccharide side chains to glycoproteins [19]. The purpose of this study was to investigate the antibacterial properties of biogenic silver nanoparticles and their complexes with antibiotics of different classes, in response to the critical need for new and effective antibacterial agents.

2 Methods and Materials 2.1 Biogenic Synthesis and Characterization of AgNPs Green silver NPs were obtained by adding a solution of Ag+ (Sigma Aldrich, Germany) to O. araratum extract in 1:6 ratios. The synthesis of NPs was carried out at room temperature and was finished in a few minutes. The size and shape of NPs were investigated via UV/Vis spectroscopy (JENWAY 6405, Switzerland), nanosize analyser (BeNano 90 Zeta, China) and scanning electron microscopy (LEO-1430 VP, Carl Zeiss, Germany) [20, 21]. 2.2 Bacterial Strain, Cultivation Conditions and Determination of Growth The bacteria Escherichia coli K-12, a natural lysogenic strain was cultivated in peptone medium in thermostat at 37 °C for 18–20 h. The antibacterial activity of nanoparticles was studied using disc-diffusion method on agar [22]. The monitoring of the optical density of the suspension was carried out using a spectrophotometer. The optical density of the bacterial suspension with a concentration of 1.5 × 108 under visual control complies with a turbidity standard of 0.5 based on the McFarland criteria. Bacterial suspension of 100 μl was disseminated on plates; disks with deposited NPs were placed on agar and incubated in 37 °C for 24 h. Different concentrations of antibiotics BP, KM and their complexes with AgNPs (0.015625 – 1 mg/ml) and only AgNPs (0.125 – 1 mg/ml), were tested to evaluate the possible synergistic effect. To each hole, 100 μl of NPs (0.1– 2 μg/ml) or a solution of NPs with antibiotics were added. Inhibition zones (pixel2 ) with/without core diameter were calculated using “Image Repair” software package [23]. 2.3 Determination of Colony-Forming Activity by Serial Dilution Serial dilution method was used for the determination of the colony-forming unit (CFU), which allows to quantify the sensitivity of the selected microbe to antibacterial drugs and determine the minimum inhibitory concentration (MIC) of the drug. After inoculation, the plates were left at room temperature for drying, then turned over and incubated at 37 °C for 24 h, then the grown colonies were counted [23].

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2.4 Generating Growth Curves for Bacteria The wild-type strain Escherichia coli K-12 was incubated in liquid peptone medium at 37 °C for 24 h to determine its growth phase. The bacterial suspension was adjusted to an optical density of 0.5, which corresponds to the McFarland standard of turbidity. The optical density was measured using a spectrophotometer (SPECTRO UV-18 MRC, Israel) at a wavelength of 625 nm every 30 min, and every 20 min when the logarithmic phase appeared. Growth curves were plotted based on the obtained data. 2.5 Statistical Analysis of Results The statistical analysis of the material was carried out using the integrated standard statistical methods of the Microsoft Excel program: the calculation of the mean values, standard deviations, standard mean errors. The experiments have been repeated 4–6 times with 2–3 series of experiments. Data are expressed as means (standard deviation of at least triplicate determinations). Changes in variables were analyzed by a one-way ANOVA for multiple comparisons. Differences were considered significant at p < 0.05.

3 Results 3.1 The Effect of AgNPs and Complexes on the Growth of Escherichia coli K-12 by Disk Diffusion Method Experiments investigating the effect of AgNPs (at concentrations ranging from 0.015625 to 1 mg/mL) on the growth of E. coli K-12 by disk diffusion method showed that AgNPs dispersed in peptone suppressed the growth of bacteria Escherichia coli K-12 starting from a concentration of 0.25 mg/mL (Fig. 1d). It was found that the O. araratum extract does not exhibit antibacterial properties on its own (Fig. 1a). BP, KM, and AgNPs had independent inhibitory effects on bacterial growth. In our previous studies, spectral analysis demonstrated that AgNPs form complexes with the antibiotics examined [21]. These complexes notably increased the zone of inhibition and exhibited antibacterial activity at very low concentrations (0.03125 mg/mL), as illustrated in Fig. 1 and Fig. 2. Figure 2 displays the areas of lysis zones calculated for AgNPs, antibiotics, and their complexes.

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541

Fig. 1. Lysis area zones of the E.coli K-12 bacteria after adding: extract of O.araratum (a); BP(b); KM (c); AgNP (d); AgNP in extract (e); BP + Ag NPs (f); KM + Ag NPs (g, h) (I - 1 mg/ml; II - 0.5 mg/ml; III - 0.25 mg/ml; IV - 0.125 mg/ml; V - 0.0625 mg/ml; VI - 0.03125mg/ml; VII 0.015625mg/ml).

350000

AgNPs AgNPs + O.araratum

300000

BP

Pix²

250000

AgNPs + BP KM

200000

AgNPs + KM 150000 100000 50000 0 1

0.5

0.25

0.125

0.0625

0.03125

Concentration mg/ml

Fig. 2. The areas of lysis zones of E. coli K-12 bacteria under the action of different concentrations (0.03125–1.0 mg/mL) of AgNPs, BP, KM, and their complexes, p < 0.5.

3.2 The Effect of Antibiotics, AgNPs and Their Complexes on the Growth of E. coli Colonies At a concentration of 0.25 mg/ml, the inhibitory activity of AgNPs against the growth of E. coli microcolonies is 7.5 times greater than that of NPs stabilized in extract.

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Regardless of concentration, benzylpenicillin does not inhibit the growth of E. coli K12 microcolonies, but the combined action of BP with AgNPs at high concentrations (1 mg/ml - 0.5 mg/ml) completely suppresses colony growth. Table 1. Number of CFU and MIC (mg/ml) of samples on growth E.coli K-12. Preparations

CFU

MIC (mg/ml)

Control

188 (±10)

-

BP

50 (± 8)

2

KM

45 (±5)

0.125

AgNPs

15 (±7)

0.125

AgNPs + BP

65 (±9)

0.03125

AgNPs + KM

80 (±11)

0.0625

Effect of silver nanoparticles and antibiotics on bacterial grow at a concentration of 0.25 mg/ml, BP combined with AgNPs begins to suppress microcolony growth, and the activity of the complex is 5 times higher compared to the activity of BP alone. Therefore, the AgNPs + BP complex is highly active against bacterial growth, while BP alone is not effective. Kanamycin inhibits the growth of E.coli K-12 colonies at concentrations of 1 mg/ml - 0.5 mg/ml, and its combined effect with AgNPs is evident up to a concentration of 0.03125 mg/ml. Thus, AgNPs reduce the active dose of the antibiotic by 32 times (by two orders of magnitude). Based on the obtained results, CFU of bacteria were determined MIC for E.coli (Table 1). 3.3 Effect of Silver Nanoparticles and Antibiotics on Bacterial Growth Kinetics The antibacterial activity of an active substance is determined by the duration of the log-phase of growth kinetics, as it is commonly known [24]. This study investigated the impact of silver nanoparticles and antibiotics on the growth kinetics of bacteria. The antibacterial activity of the active substance was determined by the duration of the logphase of growth kinetics. Growth curves were obtained for E.coli K-12, with the addition of different concentrations of silver nanoparticles, antibiotics, and their complexes, and were compared to a control (Fig. 3). The log-phase for the control sample was 2 h, while for BP it was 3 h, AgNPs was 4 h, and KM was 5 h. The results indicated that KM was the most active, followed by AgNPs, and then BP (KM > AgNPs > BP), based on the duration of the log-phase. Furthermore, NPs complexes with antibiotics led to the total inhibition of bacterial growth (93% for AgNPs + BP; 94.7% for AgNPs + KM), as seen from the results after 8–10 h of culturing E. coli bacteria, compared to the control. The AgNPs complexes with BP and KM showed high growth inhibitory activity, as demonstrated by the elongation of the lag phase (up to 10 h) (Fig. 3). Our study examined the impact of silver nanoparticles (AgNPs) stabilized in O.araratum extract on the growth of E.coli bacteria. The lysis zones observed in Fig. 1 (d,

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Fig. 3. The growth kinetics and percentage inhibition of E. coli K-12 bacteria were studied in response to the presence of silver nanoparticles, antibiotics, and their complexes with BP and KM.

e) demonstrate that the action of silver nanoparticles alone caused the observed effects. This result can be attributed to the presence of antioxidants and membrane protective components in the O.araratum extract, which can neutralize the harmful effects of ROS, which are one of the mechanisms of action of nanoparticles [23].

4 Conclusion In recent years, nanotechnology has emerged as a promising tool for preventing and controlling biofilms. There is a growing body of experimental evidence indicating that nanoparticles (NPs) can destroy bacterial membranes and prevent the formation of biofilms, thereby reducing the survival of microorganisms [10, 11]. NPs can be surfacefunctionalized with β-cyclodextrin (β-CD) or N-acylated homoserine lactonase (AiiA) proteins, which are capable of interfering with signaling molecules and preventing them from reaching their cognate receptors, thus inhibiting the ligand/receptor interaction. This process can “turn off” quorum sensing (QS) and prevent bacterial communication [24]. The use of NPs represents an innovative approach to penetrating infectious biofilms and targeting bacterial communication, which is a significant health concern associated with biofilm formation [25]. Our experiments have demonstrated that combining AgNPs with antibiotics can enhance their antibacterial effects either additively or synergistically, depending on the dose. Specifically, we observed synergistic effects in the combination of AgNPs with BP and KM against E. coli K-12 bacteria, which are not susceptible to these antibiotics even at very low NP concentrations. Our results suggest that the combination of AgNPs

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with BP has a synergistic effect against the growth of E. coli K-12 at a concentration of 0.5 mg/ml or higher, reducing the antibiotic dose by up to 8 times. With KM, the effect is only additive, but it reduces the active dose of KM by 30 times. Importantly, the concentrations used in our study were found to be non-cytotoxic for human erythrocytes as confirmed by the RBC test, and the synthesized AgNPs were also found to be nongenotoxic, indicating their potential for use as therapeutic agents [20]. Our results support the synergistic antibacterial effect of NPs and the reduction in MIC of antibiotics. Our findings suggest that the complexes of AgNPs with kanamycin and benzylpenicilline enhance the antibacterial effects of both nanoparticles and antibiotics. Moreover, at concentrations below the MIC, the complexes exhibit potential ant virulent properties by reducing the formation and maturation of biofilms, indicating their potential as safe alternatives to traditional antibiotics. Funding. This research was funded by MESCS RA SC, grant number 10–2/23-I/RAU-BIOL, 21T-1F243, 21APP-1F010. Conflicts of Interest. The authors declare no conflict of interest.

References 1. Lee, S., Jun, B.: Silver Nanoparticles: synthesis and application for nanomedicine. Int J Mol Sci. 20(4), 865 (2019). https://doi.org/10.3390/ijms20040865 2. Reygaert, W.: An overview of the antimicrobial resistance mechanisms of bacteria. AIMS Microbiol. 4(3), 482–501 (2018). https://doi.org/10.3934/microbiol.2018.3.482 3. Meikle, T., Dyett, B., et al.: Preparation, characterization, and antimicrobial activity of cubosome encapsulated metal nanocrystals. ACS Appl Mater. Interfaces 12(6), 6944–6954 (2020). https://doi.org/10.1021/acsami.9b21783 4. Gharpure, S., et al.: A review on antimicrobial properties of metal nanoparticles. J. Nanosci. Nanotechnol. 20(6), 3303–3339 (2020). https://doi.org/10.1166/jnn.2020.17677 5. Khezerlou, A., Alizadeh-Sani, M., et al.: Nanoparticles and their antimicrobial properties against pathogens including bacteria, fungi, parasites and viruses. Microbial Pathogenesis 123, 505–526 (2018). https://doi.org/10.1016/j.micpath.2018.08.008 6. Reshma, V., Syama, S., et al.: Engineered nanoparticles with antimicrobial property. Curr. Drug Metab. 18(11),1040–1054 (2017). https://doi.org/10.2174/138920021866617092512 2201 7. Liao, C., Li, Y., Tjong, S.: Bactericidal and cytotoxic properties of silver nanoparticles. Int. J. Mol. Sci. 20(2), 449 (2019). https://doi.org/10.3390/ijms20020449 8. Noronha, V.T., Paula, A.J., et al.: Silver nanoparticles in dentistry. Dental Mater. 33(10), 1110–1126 (2017). https://doi.org/10.1016/j.dental.2017.07.002 9. Burdus, el, A., Gherasim, O., et al.: Biomedical applications of silver nanoparticles: an up-todate overview. Nanomaterials 8(9), 681 (2018). https://doi.org/10.3390/nano8090681 10. Rudramurthy, G., Swamy, M.: Potential applications of engineered nanoparticles in medicine and biology: an update. JBIC J. Biol. Inorganic Chem. 23(8), 1185–1204 (2018). https://doi. org/10.1007/s00775-018-1600-6 11. Isiaku, A., Sabri, M., et al.: Biofilm is associated with chronic treptococcal meningoencephalitis in fish. Microb. Pathog. 102, 59–68 (2017). https://doi.org/10.1016/j.micpath.2016. 10.029

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12. Hernández-Díaz, J., Garza-García, J., et al.: Plant-mediated synthesis of nanoparticles and their antimicrobial activity against phytopathogens. Sci Food Agric. 101(4), 1270–1287 (2021). https://doi.org/10.1002/jsfa.10767 13. Nisar, P., et al.: Antimicrobial activities of biologically synthesized metal nanoparticles: an insight into the mechanism of action. JBIC J. Biol. Inorganic Chem. 24(7), 929–941 (2019). https://doi.org/10.1007/s00775-019-01717-7 14. Jain, A.S., Pawar, P.S., et al.: Bionanofactories for green synthesis of silver nanoparticles: toward antimicrobial applications. Int J Mol Sci. 22(21), 11993 (2021). https://doi.org/10. 3390/ijms222111993 15. Lee, S., Jun B.H.: Silver nanoparticles: synthesis and application for nanomedicine. Int. J. Mol. Sci. 20(4), 865 (2019). https://doi.org/10.3390/ijms20040865 16. Maˇtátková, O., et al.: Antimicrobial properties and applications of metal nanoparticles biosynthesized by green methods. Biotechnol. Adv. 58, 107905 (2022). https://doi.org/10.1016/j.bio techadv.2022.107905 17. Amini, S.M.: Preparation of antimicrobial metallic nanoparticles with bioactive compounds. Mater. Sci. Eng. C Mater. Biol. Appl. 103, 109809 (2019). https://doi.org/10.1016/j.msec. 2019.109809 18. Anush, K., Shushanik, K., Susanna, T. et al.: Antibacterial effect of silver and iron oxide nanoparticles in combination with antibiotics on E. coli K12. BioNanoSci. 9, 587–596 (2019). https://doi.org/10.1007/s12668-019-00640-0 19. Torsten, J., Trayder, T., et al.: How kanamycin a interacts with bacterial and mammalian mimetic membranes. BBA – Biomembranes 1859(11), 2242–2252 (2017). https://doi.org/10. 1016/j.bbamem.2017.08.016 20. Petrosyan, M., et al.: Testing green silver nano-particles for genotoxicity, antioxidant and anticancer. ifmbe proceedings. 77, 567–571 (2020). https://doi.org/10.1007/978-3-030-318666_10 21. Kazaryan, S., Hovhannisyan, A., et al.: Oxidative stress and histopathological changes in several organs of mice injected with biogenic silver nanoparticles. Artif Cells Nanomed Biotechnol. 50(1), 331–342 (2022). https://doi.org/10.1080/21691401.2022.2149931 22. Bauer, A.: Antibiotic susceptibility testing by a standardized single disk method. Am. J. Clin. Pathol. 45(1), 493–496 (1966). https://doi.org/10.1093/ajcp/45.4_ts.493 23. Ohanyan, S., Hovhannisyan, A., et al.: Improvement of the antibacterial activity of benzylpenicillin in combination with green silver nanoparticles against Staphylococcus aureus. In: 4th International Conference on Nanotechnologies and Biomedical Engineering, IFMBE Proc. 77, 349–353 (2020). https://doi.org/10.1007/978-3-030-31866-6_65 24. Blair, JM., et al.: Multidrug efflux pumps in Gram-negative bacteria and their role in antibiotic resistance. Future Microbiol. 9(10), 1165–77 (2014). https://doi.org/10.2217/fmb.14.66 25. Sharifipour, E., Shams, S., et al.: Evaluation of bacterial co-infections of the respiratory tract in COVID-19 patients admitted to ICU. BMC Infect. Diseases 20(1), 646 (2020). https://doi. org/10.1186/s12879-020-05374-z

The Impact of Biogenic Silver Nanoparticles on the Enzymatic Antioxidant System of Wistar Rats’ Kidney Juleta Tumoyan , Shushanik Kazaryan , and Ashkhen Hovhannisyan(B) Department of Medical Biochemistry and Biotechnology, Russian-Armenian University, Yerevan, Armenia [email protected]

Abstract. Nanotechnology is an advanced and promising field that focuses on creating unique nanoparticles with various properties. Among them, silver-based nanoparticles have gained significant attention and are extensively studied. The liver and kidney are particularly vulnerable to AgNPs because they play a crucial role in excreting exogenous substances. The objective of this research was to comparatively assess the effects of biogenic AgNPs, stabilized in a 50% extract of O. araratum, on the antioxidant system (AOS) of Wistar rats’ kidney, considering different exposure durations. The activity of superoxide dismutase (SOD), peroxidase (PO), and the concentration of malondialdehyde (MDA) in the kidney homogenate of experimental animals were measured using colorimetric methods. The study revealed that regardless of the duration of exposure, there was an increase in SOD activity. However, PO activity was inhibited, leading to elevated levels of hydrogen peroxide, as indicated by the higher concentration of MDA, after 7 days of exposure to stabilized biogenic AgNPs. On the other hand, exposure for 14 days resulted in the normalization of MDA content. Prolonged exposure to AgNPs reduced the destructive effects of rosmarinic acid (RA) and the extract. These outcomes shed light on the diverse properties of biogenic AgNPs responsible for inducing oxidative stress. However, despite this critical mechanism, protective mechanisms are also observed in vivo during long-term exposure. Keywords: Biogenic AgNPs · Biocompatibility · Antioxidant System · SOD · PO · MDA

1 Introduction Silver nanoparticles (AgNPs) belong to a significant class of nanomaterials, characterized by sizes ranging from 1 to 100 nm. Their small size provides a high surface area to volume ratio, which contributes to their unique chemical, physical, and biological properties [1]. AgNPs have gained wide commercial and biomedical applications. They are utilized as catalysts, optical receptors in cosmetics and electronics, and antibacterial agents. AgNPs are also used in wound dressings, surgical instruments, and disinfectants [2]. The increased use of AgNPs in various products has raised concerns about their potential interactions with the environment and their effects on human health [3]. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 546–553, 2024. https://doi.org/10.1007/978-3-031-42775-6_58

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Due to their small size, AgNPs can enter the human body through ingestion, inhalation, and skin contact. When AgNPs enter the body through various routes, they can accumulate and, at certain doses, exhibit toxicity in different organs and tissues [4]. Literature reports indicate that AgNPs can accumulate dose-dependently in organs such as the brain, lungs, liver, and dermis [5, 6]. The liver and kidney are particularly vulnerable to AgNPs because they play a crucial role in excreting exogenous substances. The main mechanism of AgNPs’ cytotoxicity involves the induction of oxidative stress, which is caused by the generation of reactive oxygen species (ROS), depletion of glutathione, increased lipid peroxidation, damage to cellular structures and biopolymers, and reduced activity of antioxidant enzymes [7]. ROS comprise oxidative species such as superoxide anion (O2 -), hydroxyl radical (OH•), hydrogen peroxide (H2 O2 ), singlet oxygen (1O2 ), and hypochlorous acid (HOCl). Under normal physiological conditions, ROS are produced in small amounts in response to various stimuli. Neutrophils and macrophages generate an oxidative burst as a defense mechanism against environmental pollutants, tumor cells, and microbes. Silver NPs, as exogenous factors, induce ROS production as a key mechanism of cytotoxicity [8]. Rohde et al. identified a novel mechanism through which AgNPs kill mammalian cells, independent of the contribution of Ag+ ions released into the extracellular environment [9]. Among various methods for synthesizing AgNPs, the biological approach is the simplest, fastest, non-toxic, and environmentally friendly. This method allows for controlling the size and morphology of the synthesized nanoparticles. The “green” or biogenic synthesis method involves utilizing various products of plant metabolism as reducing and stabilizing agents. This approach eliminates the need for toxic chemicals, ensures environmental safety, and exhibits high biocompatibility, making the resulting nanoparticles suitable for medical applications [10, 11]. The use of biological or “green” synthesis methods avoids many drawbacks associated with physical and chemical methods, thus reducing the toxicity of biogenic AgNPs. For instance, employing plant extracts with positive biological properties instead of toxic chemicals to stabilize the NPs structure can be considered an advantage of the biological approach [12–15]. However, despite the aforementioned advantages of biogenic AgNPs, concerns regarding their cytotoxicity and effects on the organism’s antioxidant system remain unanswered. Therefore, the aim of this study was to comparatively assess the effects of biogenic silver nanoparticles (AgNPs), stabilized in a 50% extract of O. araratum, on the antioxidant system (AOS) of the white outbred Wistar rats’ kidney, depending on the duration of action.

2 Methods and Materials Plant extracts preparation, antiradical activity, total flavonoids content and HPLC analysis were prepared according to [7].

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2.1 Biogenic NPs’ Synthesis and Determination of Their Physicochemical Parameters Green silver NPs were obtained by adding a solution of Ag+ (Carl Roth, Germany) to O. araratum extract in 1:6 ratio. The synthesis of NPs was carried out under the room temperature and was finished in a few minutes [16, 17]. The size and shape of NPs were investigated via UV/Vis spectroscopy (JENWAY 6405, Switzerland), scanning electron microscopy (SEM) (SEMLEO-1430 VP, Carl Zeiss, Germany) [7, 18]. 2.2 Experimental Procedures on Animals A comparative evaluation of the effect of biogenic NPs stabilized in a 50% extract of O.araratum was carried out on mature male white outbred Wistar rats (180–220 g). For the determination of a nonlethal dose of AgNPs 0.9–1.2 µg/kg concentrations were chosen. After the first injection of AgNPs at the concentration of 1.2 µg/kg, only 50% of mice survived, after the second injection – 25% and only 12.5% of mice survived until the end of the experiment, which indicates the high toxicity of this dosage. A completely different effect was registered when AgNPs were injected at the concentration of 0.9 µg/kg. 100% of the animals survived after a month of treatment, no behavioural deviations were observed. Taken together, AgNPs at a concentration of 0.9 µg/kg body weight were used in the follow-up experiment [7]. During the study, animals were randomly divided into 4 experimental groups with 5 animals in each with intraperitoneal injection of: group 1 - control (phosphate buffer pH 7.2, 10 mM), group 2 - rosmarinic acid (5.43 µg/ml), group 3 - 50% ethanol extract of O.araratum, standardized by rosmarinic acid, group 4 - biogenic AgNPs (0.9 µg/kg) stabilized in 50% O. araratum ethanol extract. The study was carried out in two series of experiments, and the influencing agents were administered to the animals at each second day for 7 (1st series) and 14 days (2nd series). All interventions were performed in accordance with the principles of laboratory animal care of the Ethics Committee of Yerevan State Medical University (Yerevan, Armenia) and in accordance with the decision of September 22, 2010 of the Council of European Communities [2010/63/EU] and with “ARRIVE” guidelines (Animals in Research Reporting in vivo Experiments). Euthanasia procedures were consistent with the recommendations of the American Veterinary Medical Association (AVMA) Guidelines on Euthanasia, using intraperitoneal injection of 70% ethanol. 2.3 Determination of Superoxide Dismutase (SOD) Activity The method is based on the ability of SOD to inhibit the autoxidation of epinephrine at pH 10,2. For measuring the activity of SOD, 0,4 ml of 2,25 µM epinephrine solution was added to the mixture of tissue supernatant (0,5 ml), 0,15 M sodium-carbonate buffer (1 ml, pH 10,2) and 5 mM phosphate buffer (0,7 ml, pH 7,8). Epinephrine autoxidation to adrenochrome was measures under the wavelength of 480 nm during 3 min [7].

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2.4 Malonic Dialdehyde (MDA) Content Determination MDA accumulation was studied for evaluating lipid peroxidation. In acidic medium, MDA reacts with 2-thiobarbituric acid (TBA) leading to the formation of colored trimethine complexes. Tissue samples were homogenized on ice using Potter-Elvehjem hand operated homogenizer with the addition of 25 µM TRIS-HCl buffer solution (pH 7,4). Protein sedimentation was carried out by adding 1 ml 17% TCA solution (Sigma Aldrich, Germany) to 2,5 ml of homogenate. After the centrifugation under 400 rpm for 10 min, 2 ml of the supernatant was added to 1 ml of 0,8% TBA (Carl Roth, Germany) and incubated in a boiling water bath for 10 min. After the incubation, samples were cooled down to room temperature and their OD was measured at the 532 nm wavelength (Jenway, UK) [18]. 2.5 Statistical Analysis Statistical analysis of the data obtained was done in Microsoft Excel based on complex application of standard statistical methods, such as calculation of means, standard deviations and standard errors. Experiment was conducted on 4–6 biological replicates with 2–3 series of repeats each. Tables, graphs and diagrams show the arithmetic means and their standard errors (n = 8–12). The differences between the series of experiments were validated by Student criteria (P): the difference was valid if P < 0.05.

3 Results 3.1 Total Flavonoid Content and Antiradical Activity in O. araratum Extract The evaluation of total flavonoid content (TFC) and antioxidant radical activity (ARA) demonstrated that all the extracts exhibited dose-dependent characteristics. Notably, the 50% (0.26 ± 0.04 mg/ml) and 25% (0.26 ± 0.06 mg/ml) ethanol extracts of O. araratum displayed the highest values of TFC and ARA among all the extracts. Therefore, further experiments were conducted using these particular extracts. It is noteworthy that the ARA series, built in descending order of the activity of O. araratum extracts, has the following form: 50% ethanol ≥ 25% ethanol >aqueous > 70% ethanol > 96% ethanol. 3.2 HPLC Analysis of O. araratum Extract HPLC analysis of the 50% ethanol extract of O. araratum revealed the presence of a major component, rosmarinic acid (RA), at a concentration of 5.43 µg/ml [18]. 3.3 Green Synthesis of Silver Nanoparticles with Ocimum arartum Extract Considering the requirement for extracts with a pronounced high redox potential for biogenic synthesis of nanoparticles, the 50% ethanol extract of O. araratum was used for synthesizing silver nanoparticles (AgNPs). In this case, spherical-shaped biogenic AgNPs with diameters ranging from 2 to 17 nm were formed, with the majority of nanoparticles having an average diameter of 11 ± 2 nm [18].

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3.4 Effect of Biogenic NPs on the Antioxidant Enzymatic System of Kidney Tissue The examination of superoxide dismutase (SOD) activity in the kidney homogenate of the experimental animals revealed that, in all the groups and series of the experiment, the inhibitory activity of SOD increased by 1.5 to 1.6 times compared to the control (Fig. 1).

Inhibition of adrenaline autoxidation ,%

100 90 80 70 60 50 40 30 20 10 0

1st series 2st series

Control

O.araratum

Rosmarinic acid

AgNPs

Fig. 1. Percentage of inhibition of autoxidation of adrenaline into adrenochrome under the action of SOD of kidney homogenates of experimental animals.

Peroxidase activity,pkat/mg protein

The findings from the investigation of peroxidase (PO) activity indicate a significant inhibition of the enzyme, regardless of the duration of exposure, in all the groups under study (Fig. 2).

10000 1st series 2 st series

5000

0

Control

O.araratum

Rosmarinic acid

Fig. 2. PO activity of experimental animals’ kidney homogenate.

AgNPs

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3.5 Study of Lipid Peroxidation Assessment Under the Action of Biogenic AgNPs

MDA concentration (μg/ml.)

As can be seen from the results shown in Fig. 3, in the first series of experiments, there is a substantial elevation in the concentration of malondialdehyde (MDA) (1.75 ± 0.08 µg/ml) in response to both O. araratum extract and rosmarinic acid (RA), compared to the control (0.06 ± 0.03 µg/ml), contrasting with the effect of biogenic AgNPs (0.36 ± 0.05 µg/ml). However, with prolonged exposure to the test substances, there is an overall declining trend in MDA content across all study groups. Notably, the reduction in MDA content is more pronounced in the presence of biogenic AgNPs.

1st series 1.5 2st series 1

0.5

0 Control

O.araratum

Rosmarinic acid

AgNPs

The content of MDA in the kidney homogenate, μg/ml. Fig. 3. MDA concentration (µg/ml) in the kidney homogenate of experimental animals.

Our results demonstrate that in the first series of exposure to biogenic AgNPs, there is an increase in MDA content, an elevation in the inhibitory activity of SOD, and a suppression of PO activity. Following extended exposure to nanoparticles, there are slight alterations in MDA content, approaching values similar to the control group, while SOD activity exhibits no significant changes and the increased activity persists, and PO activity becomes increasingly suppressed depending on the duration of exposure (Fig. 1– 3). The heightened SOD activity likely indicates the activation of the antioxidant defense system in the body, leading to the dismutation of reactive superoxide anion radicals into more stable hydrogen peroxide. This effect is possibly attributed to the presence of bioactive components in the extract itself, particularly RA, which aligns with the results obtained in the second and third groups of experiments in both series, as well as existing literature [19].

4 Conclusion Thus, the results of the study show that the simultaneous increase in SOD activity and inhibition of one of the antioxidant defense system enzymes, specifically PO, can result in the excessive accumulation of hydrogen peroxide as ROS, potentially leading to cellular damage during lipid peroxidation. This is supported by the findings of the MDA concentration study.

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Prolonged exposure to AgNPs results in a reduction in the detrimental effect of RA and the extract. These outcomes shed light on the diverse properties of biogenic AgNPs responsible for inducing oxidative stress. However, despite this critical mechanism, protective mechanisms are also observed in vivo during long-term exposure. Acknowledgments. This research was funded by MESCS RA SC, grant number 10–2/23-I/RAUBIOL, 21T-1F243, 21APP-1F010.

Conflicts of Interest. The Authors Declare that They Have no Conflict of Interest.

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13. Zhang, J., et al.: Carbon science in 2016: Status, challenges and perspectives. Carbon 98(70), 708–732 (2016) 14. Maryami, M., Nasrollahzadeh, M., Mehdipour, E., Sajadi, S.M.: Preparation of the Ag/RGO nanocomposite by use of Abutilon hirtum leaf extract: a recoverable catalyst for the reduction of organic dyes in aqueous medium at room temperature. Int. J. Hydrogen Energy 41(46), 21236–21245 (2016). https://doi.org/10.1016/j.ijhydene.2016.09.130 15. Wei, Y., McGrath, P.J., Hayden, J., Kutcher, S.: Mental health literacy measures evaluating knowledge, attitudes and help-seeking: a scoping review. BMC Psychiatry 15(1), 1–20 (2015). https://doi.org/10.1186/s12888-015-0681-9 16. Tippayawat, P., Phromviyo, N., Boueroy, P., Chompoosor, A.: Green synthesis of silver nanoparticles in aloe vera plant extract prepared by a hydrothermal method and their synergistic antibacterial activity. 4, 2589 (2016). https://doi.org/10.7717/peerj.2589 17. Petrosyan, M., Gevorgyan, T., Kirakosyan, G., Ghulikyan, L., Hovhannisyan, A., Ayvazyan, N.: Testing green silver nano-particles for genotoxicity, antioxidant and anticancer. Springer, Cham IFMBE Proceedings. 77, 567–571 (2020). https://doi.org/10.1007/978-3-030-318666_10 18. Kazaryan, S., Petrosyan, M., Rshtuni, L., Dabaghyan, V., Hovhannisyan, A.: Effects of green silver nanoparticles on CCl4 injured Albino rats’ liver. Springer, Cham IFMBE Proceedings. 77, 567–579 (2020). https://doi.org/10.1007/978-3-030-31866-6_27 19. Heidari, F., Komeili-Movahhed, T., Hamidizad, Z., Moslehi, A.: The protective effects of rosmarinic acid on ethanol-induced gastritis in male rats: antioxidant defense enhancement. Res Pharm Sci. 16(3), 305–314 (2021). https://doi.org/10.4103/1735-5362

The Sentinel Surveillance System of Severe Acute Respiratory Infections Associated with Influenza in Children from Republic of Moldova Ala Donos(B)

and Albina-Mihaela Iliev

“Nicolae Testemitanu” State University of Medicine and Pharmacy, Chisinau, Republic of Moldova [email protected]

Abstract. Acute respiratory pathology has the highest incidence in children, the most vulnerable are still those aged up to 5 years. Extremely difficult are the cases with severe acute respiratory infections (SARI), manifested by pneumonia and bronchopneumonia associated with influenza. Essential method used is the sentinel epidemiological surveillance; molecular biology techniques in real time (rRT-PCR) to detect viruses in biological material (nasopharyngeal exudates); isolation of influenza viruses in cell cultures MDCK and MDCK-SIAT1 after WHO methodology; identification by the hemagglutination inhibition test with reference antisera for influenza A(H1N1) pdm09, A(H3N2) and B, provided by the WHO Collaborating Centre, National Institute of Health Researches (London, UK). Thus, it was found that SARI associated with the flu threatens the health and life of children, with a major risk in children aged between 0–4 years. In conclusion, the obtained results show, that the identification and evaluation of phenotypic, genotypic and antigenic properties of the influenza viruses, have a major importance, in the context of fairness policy, for the use of influenza vaccine in compulsory seasonal immunization of the children, optimizing the management of treatment and prophylaxis of influenza, including in combination with SARI, foreseeing epidemic process and reducing the negatively impact on the health system. Keywords: severe acute respiratory infections in children · influenza virus · surveillance · epidemic season

1 Introduction Acute respiratory diseases are the most common diseases in children. Influenza, acute respiratory infections are the leading causes of death of 2.5 million children annually (WHO 2014, 2021). Acute respiratory diseases are the leading causes of morbidity and mortality in children in the early years. The most frequent and severe complication is pneumonia. Influenza and acute viral respiratory infections are the most widespread infectious diseases around the globe, which often triggers pneumonia. Also, globally, © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 554–563, 2024. https://doi.org/10.1007/978-3-031-42775-6_59

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influenza is the second most common pathogen identified in children with SARI (pneumonia, bronchopneumonia) [1]. The share of these infections in the structure of infectious diseases in some countries is up to 95.0%. Annual, flu and SARI suggests significant economic losses by the high rate of illness, a big number of hospitalizations with severe complications and deaths [2]. The epidemic process of influenza is manifested by annual epidemics, which are explained by the minor antigenic variations and pandemics at 10– 40 years, after major antigenic variation and the emergence of new variation of influenza viruses, which has not previously circulated in the human population. During influenza epidemics are affected up to 5.0 to 15.0%, and during pandemics - to 50.0% of a country’s population [3]. Considering permanently change in the genetic apparatus of the flu virus, the risk of new variations of influenza virus with pandemic potential, development of resistance to antivirals, requires a constant monitoring of the epidemiological situation and the circulation of influenza viruses, to detect in time the emergence of new variations of flu virus, including modified antigenic and genetic structure, to select correctly the strains of influenza virus, the vaccine cocktail components for next epidemic season and to manage the treatment of flu cases. To this end, the World Health Organization (WHO) recommended, to all National Centers of Influenza in the world, to oversee the surveillance of influenza, viral respiratory acute infections following standards: geographical spread, intensity and tendency of epidemic process, epidemic threshold, strains of dominant/codominant influenza viruses, antiviral resistance, the impact on the health system for predicting the epidemiological situation and elaboration of the preventive measures depending on the arises situation by creating specific seasonal vaccines [4, 5]. In this article are presented the results of epidemiological and virological analysis of SARI associated with influenza in the sentinel surveillance system, assessment antigenic, genotypic and phenotypic particularities of strains of influenza virus A(H1N1) pdm09, A(H3N2) and B, isolated and identified from samples taken from children in epidemic seasons: 2012–2013, 2013–2014, 2014–2015 in Republic of Moldova.

2 Material and Methods Epidemiological surveillance was realized according to the order of Ministry of Health of the Republic of Moldova 9 points sentinel, during cold seasons 2012–2013, 2013–2014 and 2014–2015 [6]. Detection of influenza viruses in biological material (nasopharyngeal exudate), taken from children with SARI, from 0–4 and 5–17 years of age group, was achieved by molecular biology techniques (rRT-PCR) using equipment CFX96 Real time System (Bio-Rad) amplification kits, developed by CDC (Atlanta, USA) and recommended by the reference laboratories in the world [7]. Isolation of influenza viruses in MDCK and MDCK-SIAT1 cell cultures was performed according to the methodology recommended by WHO [8], identification of isolates strains was performed by hemagglutinin inhibition test (RIHA) with reference antisera, against influenza A(H1N1)pdm09, A(H3N2) and B provided by Collaborating Centre for influenza of WHO, National Institute for Research in Medicine (London,

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UK) with further realization of genetic analysis used for sequencing genes HA and NA techniques of concerned influenza viruses [9]. Determining sensibility of isolates strains to flu remedies (Oseltamivir, Zanamivir) was conducted by neuraminidase inhibition test according to the method recommended by WHO, in collaboration with the National Institute for Medical Researches in London, UK [8]. Estimating the incidence of SARI associated with influenza in children per 100,000 population was performed by method described in the book about estimating the burden of influenza, WHO [10, 12, 13].

3 Results The results of epidemiological analysis of SARI associated with influenza during cold seasons 2012–2013, 2013–2014 and 2014–2015 reveals the fact that the most affected were children of 0–4 and 5–17 age groups. Thus, in 2012–2013 epidemic season incidence of SARI associated with influenza in children 0–4 years aged was 330.0 per 100,000 population, while in the 5–17 age group, incidence was 0 (zero) (Table 1). A similar situation was observed in the 2013–2014 season, where the incidence of SARI associated with influenza in children of 0–4 age group was 459.0 per 100 000 population, and in the 5–17 age group, it was also 0 (zero) (Table 1). This indicates an increase of 1.4 times of the incidence of SARI associated with influenza in 0–4 years children, compared with the 2012–2013 season, which allowed to appreciate the severity of epidemic evolution of the 2013–2014 season, to the given contingent of children. Table 1. Estimating incidence of SARI associated with influenza in children. Indicators

Season 2012–2013

Season 2013–2014

Season 2014–2015

0–4 years

5–17 years

0–4 years

5–17 years

0–4 years

5–17 years

Number of SARI cases, total Number of SARI

3190

56

3168

16

3645

72

cases positive to influenza viruses

7

1

7

0

7

3

Population coverage, estimated

89584 151353 81583 150226 81595 138841

Incidence (SARI associated with influenza 330 at 100,000 people)

0

459

0

344,0 7,0

Note: 1 - severe acute respiratory infections

The incidence of SARI, associated with influenza in children in the age group 0– 4 years, in 2014–2015 season, was 344.0 per 100,000 population, attesting a reduction of 0.7 times. Regarding the age group 5–17 years, the incidence was 7.0 per 100,000 population (Table 1), which can be argued by increasing overall morbidity, related to the increased incidence of biological and socio-economic risk factors [13, 14].

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In the sentinel surveillance system to the presence of influenza viruses in biological material (nasopharyngeal exudate) taken from children in the age group 0–4 years with SARI (pneumonia, bronchopneumonia) were investigated 32 specimens in 2012–2013 season, 66 specimens - in 2013–2014 and 82 specimens - in 2014–2015 season. While in the 5–17 age group were tested 5, 12 and 13 specimens in mentioned above seasons. Detection of influenza viruses by the molecular- biological method in real time (rRTPCR) allowed to conduct further analysis of antigenic, genotypic and phenotypic strains of influenza virus A(H1N1)pdm09, A(H3N2) and B, isolates in cell cultures of native samples. During winter 2012–2013, at the children, from named groups, were identified influenza A(H1N1)pdm09 and A(H3N2). Evaluation of the results, regarding the antigenic particularities of the influenza viruses, circulating in Republic of Moldova, was performed with use of various panels of sera standard specific to each type/subtype of influenza virus, produced by Collaborating Centre for Influenza of WHO, National Institute for Researches in Medicine, London UK. So, antigenic particularities of influenza viruses A(H1N1) pdm09, isolated from samples, taken from children, were evaluated using the panel of reference sera A/California/7/2009, A/Bayern/69/2009, A/Lviv/N6/2009, A/ Christchurch/16/2010, A/Hong Kong/3934/2011, A/Astrakhan/1/2011, A/St. Peters-burg/27/2011, A/St. Petersburg/100/2011, A/Hong Kong/ 5659/2012. Generally, all influenza strains that was tested, reacted with presented panel. But, with the serum reference A/Bayern/69/2009, a reduced reactivity has the A/Moldova /258/2013 strain. This, probably, was due to belonging tested viruses to genetic group 6C, but the A/ Bayern/69/2009 strain is not a part of this genetic group, although, is antigenic similar to the A/California/7/2009 vaccine strain [14–19]. Genically, using the panel of standart sera A/Perth/16/2009, A/Victoria/208/2009, /Alabama/5/2010, A/Stockholm/18/ 2011 A/ Iowa/19/2010, A/Victoria/361/2011, A/Berlin/93/ 2011, A/Victoria/361/2011, A/Athens/112/2012, A/Tex-as/50/ 2012, A/ Hawaii/22/2012 - showing a slightly reduced reactivity. At the same time, it has been noticed, that a moderate reactivity have the virus strains that were tested against the A/Victoria/361/2011 vaccine strain and other similar anti-genic strains belonging to the same genetic group 3C. Because in the 2013–2014 epidemic season, in Moldova were circulating strains of influenza virus A(H3N2), including in the children, they were characterized antigenically with the panel of reference sera A/Perth/16/2009, A/Stockholm/18/2011, A/ Iowa/19/2010, A/Victoria/361/2011, A/Athens/112/2012, A/Texas/50/2012, A/Samara/73/2013 A/Serbia/NS-210/2013, A/ Hong Kong/146/2013, NIB-85 (A/Almaty/2958/2013). Antigenic analysis results have confirmed that strains of influenza A(H3N2), isolated and identified in children with SARI (0–4 and 5–17 age groups) showed a moderate reactivity with almost all reference sera, excluding the A/Perth/16/2009 vaccine strain in the 2010–2011 season, which proves that the strains of flu viruses evolve over time by „antigenic drift” minor antigenic variation, characteristic to almost all over the types of influenza viruses, this is due to point mutation in the viral genome. However, it may notice the antigen similarity with the A/Texas/50/2012 strain vaccines and other strains

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of the panel of standard sera, the A/Moldova/696/2013 strain, isolated and identified in December 2013 as the more representative. This can be explained by membership strains of influenza A (H3N2) in this study at 3C.3 genetic group [12–19]. In the 2014–2015 season, antigenic specificities of influenza viruses A (H1N1) pdm09 isolated and identified in children, were evaluated using the panel of reference sera A/California/7/2009, A/Bayern/69/2009, A/Lviv/N6/2009, A/ Christchurch/16/2010, A/Astrakhan/1/2011, A/St. Petersburg/27/2011, A/St. Petersburg/100/2011, A/Hong Kong/ 5659/2012 and A/South Africa/3626/13. Generally, all influenza strains tested reacted with given panel, showing antigenic similarity with the vaccine strain A/California/7/2009. However, it has been proven an antigenic similarity with A/South Africa/3626/13, due to belonging tested viruses to the genetic group 6B, from which belongs this reference strain [18]. Along with the identification of influenza virus strains A (H1N1) pdm09 in children, was identified an influenza type B. Antigenic particularities of type B influenza viruses, isolated and identified in Republic of Moldova in 2014–2015 epidemic season, were studied at reaction RHAI using the panel of reference sera B/Florida/4/2006, B/Brisbane/3/2007, B/Wisconsin/1/2010, B/Stockholm/12/2011 B/Estonia/55669/2011, B/ Massachusetts/02/2012, B/Phuket/3073/2013, B/Hong Kong/ 3417/2014, showing a relatively low reactivity to reference strain B/Estonia/55669/2011. Meanwhile, tested influenza strains, showed a moderate reactivity to B/Massachusetts/02/2012 strain, that was recommended by WHO to be introduced in the composition of influenza trivalent vaccine for the respectively season and the B/Phuket/3073/2013 reference strain, WHO recommended trivalent vaccine composition for Northern Hemisphere in 2015–2016 season [26]. The results of antigenic characterization, demonstrated that tested influenza strains, were antigenically similar to reference strains belonging line B/Yamagata/16/1988 (B/Yamagata) and taking part of the genetic group 3, frequent met in influenza viruses of type B line, B/Yamagata, assets in 2014–2015 season in other European countries [18]. The results of genetic analysis have confirmed that strains of influenza virus A(H1N1) pdm09, isolated and identified from samples taken from children in 2012–2013 season, were a part of the genetic group 6C, while being antigenically similar to the vaccine strain A/California/7/2009, respectively, with other reference strains that are part of the genetic June. Interestingly, the majority of strains of influenza virus A (H1N1) pdm09 assets in different countries, during the reporting period, were positioned in genetic groups 6 and 7 [15–20]. However, strains of influenza virus A(H1N1) pdm09, isolated and identified from samples taken from children in 2014–2015 season, analyzed at the genetic level in phylogenetic trees (HA and NA genes) confirms antigenic similarity of strains of influenza virus A(H1N1) pdm09 with the same vaccine reference virus A/California/7/2009, but belonging to genetic group 6B, which is typical for circulating strains of this influenza virus in other countries, in the respectively season (Fig. 1) [18]. Analysis of the genes sequence, HA and NA of influenza viruses A(H3N2), isolated and identified in children with SARI in 2013–2014 season, indicated the belonging of both genes at the genetic group 3C.3 - containing HA, that encode T128 amino acid substitution (this results in loss of the glycosylation site) and A43S and M18K

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substitutions in HA1 gene, and the gene NA - S335G and E381K substitutions (Fig. 2) [15]. The results of phylogenetic analysis of strains of influenza B virus, isolated from children with SARI in 2014–2015 season, revealed that studied strains were a part of the genetic group 3, similar to the strains of influenza virus B, world-wide circulating (Fig. 3) [18]. It is necessary to note, that isolates strains of influenza viruses in children with SARI, in the neuraminidase inhibition test, were susceptible to anti-flu remedies (Oseltamivir and Zanamivir) and are similar to strains of flu viruses, included in the formula of influenza vaccine, recommended by WHO for the seasons nominated in this article [12–14, 16–18]. The obtained results, shows that the identification and evaluation of phenotypic, genotypic and antigenic properties of influenza viruses are of a major importance, in the context of fairness policy, for the use of influenza vaccine, in compulsory season-al immunization of children, optimizing the management of treatment and prophylaxis of influenza, including in association with SARI, foreseeing epidemic and reducing the burden on the health system.

Fig. 1. Fragment of phylogenetic comparison of strains of influenza virus A(H1N1) pdm09, HA gene, isolated from children with SARI in 2014–2015 season.

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Fig. 2. Fragment of phylogenetic comparison of strains of influenza virus A(H3N2), NA gene, isolated from children with SARI in 2014–2015 season.

Fig. 3. Fragment of phylogenetic comparison of strains of influenza virus B(B/Yagamata), HA gene, isolated from children with SARI in 2014–2015 season.

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4 Discussions Children are one of the most vulnerable categories of population and the most exposed to different challenges - influenza being one of them. SARI illnesses are part of cold season and usually it is associated with various forms of influenza. The highest respiratory morbidity as the mortality is encountered in young children, in association with various comorbidities [17]. Sentinel epidemiological surveillance in 2012–2013, 2013–2014 and 2014–2015 seasons, confirms the usefulness and significance of this supervision at a local, regional and global levels. During the study period was certified circulation of three types of viruses - A(H1N1)pdm09, A(H3N2) and influenza B virus, isolated from children with SARI. In the 0–4 age group, otherness, the contingent of children 5–17 aged, is a significant presence of SARI illness as well as in association with influenza. In all examined seasons, the total number of SARI among the children aged 0–4 years is up from 3190 to 3645 cases compared with the 5–17 age group, overall SARI cases are fewer (i.e. from 56–72 cases). The incidence of SARI associated with influenza to 100,000 children in the 0–4 age group is at high levels throughout the study (i.e. 330, 459 and 344), while in the 5–17 age group it is fixed - zero. This is largely due to wider contact in communities of all children, parallel is important to consider the morpho-functional particularities of maturation system that is in growth, and that leave their mark on morbidity.

5 Conclusions 1. The incidence of SARI associated with influenza in children, was the highest in 2013– 2014 and 2012–2013 seasons, while in 2014–2015 was lower, especially in children of 0–4 years. 2. In children with SARI were identified influenza A(H1N1) pdm09, A(H3N2) and B, which antigenic and genetic particularities, revealed their similarity to the influenza virus vaccine strains, recommended by WHO to be included in the composition of influenza vaccine for those seasons, objective argument in favor of the obligatory vaccination of the population, especially for children, and taking into consideration their vulnerability. 3. Strains of flu viruses, isolated from children with SARI, were susceptible to flu remedies (Oseltamivir and Zanamivir), which allows us to recommend it in the treatment of children with SARI associated with influenza. 4. The obtained data formed the base of the development and implementation of control measures and response to named infections, in order to reduce the risk of infection and spread, and the morbidity by SARI in Republic of Moldova. Acknowledgments. Carrying out the study “The sentinel surveillance system of severe acute respiratory infections associated with influenza in children from Republic of Moldova” was possible with financial support of World Health Organization, being part of the Regional Project of WHO regarding implementation of the seasonal flu vaccine. This study is still in progress. Conflict of Interest The Authors Declare that They Have no Conflict of Interest.

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References 1. Nair H., et al.: Global burden of respiratory infections due to seasonal influenza in young children: a systematic review and metaanalysis. Lancet 378(9807), 1917–30 (2011) http:// www.ncbi.nlm.nih.gov/pubmed/22078723 2. Benezit, Fr., et al.: Non-influenza viral respiratory infections, vol. 48(4), pp. 489– 495. (2020). https://doi.org/10.1007/s15010-019-01388-1PMCID: PMC7095392 PMID: 32056143 3. WHO Regional Office for Europe guidance for sentinel influenza surveillance in humans, WHO. (2011). http://www.euro.who.int/__data/assets/pdf_file/0020/90443/E92738.pdf 4. WHO Global Influenza Surveillance Network (web site), Geneva, World Health Organization http://www.who.int/csr/disease/influenza/surveillance/en/index.html 5. CDC Real-Time Protocol for detection and characterization of swine influenza (version 2009).http://cidbimena.desastres.hn 6. Virus isolation in cell culture. Book for the laboratory diagnosdis and virological surveillance of influenza. WHO Global Influenza Surveillance Network, pp. 35–38, (2011). https://apps. who.int 7. Identification of the heamagglutining subtype of viral isolates by haemagglutination inhibition teating. Book for the laboratory diagnosis and virological surveillance of influenza. WHO Global Influenza Surveillance Network, pp. 43–57 (2011), https://apps.who.int 8. Use of neuraminidase inhibition assays to determine the sucepti- bility of influenza viruses to antiviral drug. Book for the laboratory diagnosis and virological survellance of influenza. WHO Global Influenza Surveillance Network, pp. 103–116 (2011). https://apps.who.int 9. Book for Estimating Disease Burden Associated With Seasonal Influenza. WHO (2015), https://apps.who.int 10. Cojocaru, R., Spinu, C., Eder, V., et al.: Strengthening of the surveillance system for influenza, ARI and SARI in the Republic of Moldova. Options for the Control of Influenza, Cape Town, South Africa, Abstract LBA-P2–017, p. 643 (2013) 11. Spînu, C., Grama, O., Eder, V., et al.: Studying and evaluating of influenza, ARI and SARI morbidity evolution with control and response measures achieving, 2012–2013 epidemic season, in the Republic of Moldova. Archives Balkan Med. Union 48(suppl. 3), 487–491 (2013) 12. WHO. Weekly epidemiological record. No. 10, 87, 81–96 (2012) http://www.who.int/wer/ 2012/wer8710.pdf?ua=1 13. Joint WHO Regional Office for Europe/ECDC meeting on Influenza Surveillance. Report. 29–31 May, Istanbul, Turkey (2013). http://www.euro.who.int/__data/assets/pdf_file/0007/ 155509/e96072 14. WHO Influenza Center. Report prepared for the WHO annual consultation on the composition of the influenza vaccine for the Southern Hemisphere 2014. September 2013. MRC National Institute for Medical Research (Sep2013). http://www.nimr.mrc.ac.uk/documents/ about/NIMR-report 15. ECDC. Surveillance report. Influenza virus characterisation. Summary Europe (May 2014). http://www.ecdc.europa.eu/en/publications/Publications/influenza-characterisation-report 16. Spinu, C., Eder, V., Scofertsa, P., Donos, A., et al.: Phenotypic and genotypic significance of influenza viruses identified in the Republic of Moldova. In: Poster. 4-th International Influenza Meeting, Muenster, Germany, 21–23 September, P98, p. 145 (2014). http://zoonosen.net 17. European Influenza Surveillance Network (EISN). European Centre for Disease Prevention and Control (ECDC) http://www.ecdc.europa.eu/en/activites/surveillance/EISN/Pages/hom e.asp

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18. WHO Worldwide Influenza Centre. Report prepared for the WHO annual consultation on the composition of the influenza vaccine for the Southern Hemisphere 2016. September 2015. The Francis Crick Institute, Mill Hill Laboratory, London, UK. https://www.crick.ac.uk/media/ 273950/crick_sep2015_vcm_report_to_post.pdf 19. Donos, A.: Community-acquired pneumonia and recurrent respiratory diseases, Kishinev, p. 288 (2015). ISBN 978–9975–58–054–0

Neural Circuits-Adjusted Diagnostic Approach to Predict Recurrence of Atrial Fibrillation Ludmila Sidorenko1(B) , Irina Sidorenko2 , Roman Chornopyshchuk3 , Igor Cemortan1 , Svetlana Capcelea1 , Fliur Macaev4 , Ludmila Rotaru1 , Liliana Badan1 , and Niels Wessel5,6,6 1 Department of Molecular Biology and Human Genetics, Nicolae Testemitanu State University

of Medicine and Pharmacy, Chisinau, Moldova [email protected] 2 Medical Center “Gesundheit”, Chisinau, Moldova 3 Department of General Surgery, National Pirogov Memorial Medical University, Vinnytsya, Ukraine 4 Institute of Chemistry, Moldova State University, Chisinau, Moldova 5 Department of Non-Linear Dynamics, Cardiovascular Physics, Humboldt Universitaet zu Berlin, Berlin, Germany 6 MSB Medical School Berlin GmbH, Berlin, Germany

Abstract. Recently the high informational input on individuals of modern society is a real challenge for the capacity of the central nervous system. It has to overcome not just the big data amount, but also a state of permanent hyperactivity due to informationally-induced neuronal circuits, including artificially-induced neural circuits, originating from advertising and directed informational streams. Pathologically hyperactivated interconnectivity of the neural circuits leads to a permanently increased central component of heart rhythm modulation leading to favorable conditions for atrial fibrillation recurrence in patients with paroxysmal atrial fibrillation. Two new parameters of cardiorhythmogram analysis – lowfrequency (LF) drops and high-frequency (HF) counter-regulation are dynamic indicators for the intensity of affection of the heart rhythm regulation by the pathological hyperactivity of the central nervous system. Here we show in the case-series study of 350 cardiorhythmograms of patients with paroxysmal atrial fibrillation, that the LF drops and HF counter-regulation are sensitive biomarkers to predict the onset of recurrence of atrial fibrillation. The hyperactivity of the central nervous system leads to atrial fibrillation onset. The increased centrallydriven heart rhythm modulation can be visualized on cardiorhythmograms by the feature LF drops. The capacity of the vegetative nervous system the compensate for this state in order to maintain normal sinus heart rhythm can be assessed by the HF counter-regulation. The features HF counter-regulation and LF drops reflect the answer of the heart regulation to the neuronal circuits-induced central hyperactivation and can be evaluated in the cardiorhythmograms for the prediction of atrial fibrillation recurrence in patients with paroxysmal atrial fibrillation. Keywords: Atrial Fibrillation · Recurrence · Prediction · Cardiorhythmogram · Neuronal Circuits · Heart Regulation

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 564–573, 2024. https://doi.org/10.1007/978-3-031-42775-6_60

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1 Introduction Recently the high informational input affecting the humanity of the 21st century is a real challenge for the central nervous system. It has to overcome not just the big data amount, but also a state of permanent hyperactivity due to informationally-induced neuronal circuits, including artificially-induced neural circuits, originating from advertising and directed informational streams [1, 2]. This state is a modern risk precondition leading to a diversity of heart diseases, heart arrhythmias, especially atrial fibrillation [3–6]. From a functional point of view, neuronal circuits represent a functional network in the central nervous system which are permanently created for reaching certain goals of inner demands or external goals. They are comparable to team-works, which has the goal to come functionally together to realize a task. Such a neural circuit, which works like a teamwork of neurons, is functionally active until the task is not realized. The threatening aspect of the neural circuits is that a person can be manipulated through the neural circuits system. The reason is that neural circuits are formed in the central nervous system not just for reaching the inner demands of the organism or the goals of the individuum, but also to reach some induced external goals [7, 8]. It can happen, for instance, due to advertising. The advertising uses some psychological manipulations for getting to the mind and to the emotions of the individuals [2, 9]. Important to mention, is that an advertisement is repeated several times so that a certain amount of repeated impulsatory activity is reached. The above-mentioned criteria make the informational stream of advertising into a neural circuit in the brain of individuals. It means that the person, after having such an advertising-induced circuit, recognizes suddenly an enormous desire to buy a certain product produced by a certain firm. This example is the less harmful one. The big problem is that recently advertising is used not just to force individuals to buy certain things, but also to perform certain changes in their life – to change something in their family life, to change something in the body, to change some moral insights and values of the most fundamental and critical aspects of the human life [10]. This is very dangerous from a neurophysiological point of view. Such artificially induced neuronal circuits get in conflict with the existing fundamental neuronal circuits of the person [11]. The central nervous system maintains all the circuits, but realized are the ones with the higher impulsatory activity. For that reason, the circuits are dependent on the emotional input of the contextual goal of the circuit [12]. So, it is maintained by the emotional impulsatory stream towards the nervous system. For that reason, recently the information in news and advertising is served in a way of provoking intensive emotions. In the central nervous system of a modern individuum, there is a tremendous imoulsatory conflict between the own circuits and the artificial ones. It is a highly energy-demanding process [2]. The maintenance of neural circuits by the central nervous system costs a lot of energy. Every circuit has to be maintained until it is not realized in reality by the person or until the contra–circuit will not suppress the impulsatory activity of the weaker one. During the whole time, before they circuits are realized, they are maintained by the central nervous system [1, 12]. Recently a lot of external induced artificial circuits in the human brain were induced, via the prior-mentioned methods of manipulation. They all have to be maintained by the central nervous system. So, this is one of the important reasons why individuals are so exhausted nowadays [10]. Not just the state of

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exhaustion is so difficult, but also the diseases which are provoked by the state of multicircuit impulsatory conflict between the own fundamental circuits and own interests of a person the artificially induced ones [4, 11]. In this paper, the threatening aspect of such a pathophysiologic state is going to be shown on atrial fibrillation. 1.1 Atrial Fibrillation and Neuronal Circuits Atrial fibrillation is the most common arrhythmia in the world [6]. A lot of reasons underline this arrhythmia, such as age, gender, structural heart diseases, valvulopathies, arterial hypertension, thyrotoxicosis, etc. [13]. Important to mention is that atrial fibrillation can also occur in individuals with a structurally healthy heart, without any underlying reasons. This type of atrial fibrillation is called lone atrial fibrillation [13, 14]. So, here, obviously, pathophysiological mechanisms provoke atrial fibrillation. In this case important is to analyze the regulation of the heart rhythm. The regulation of the heart rhythm ensures an adequate reaction of the heart to different internal and external stimuli, reacting by bradycardia or tachycardia [14, 15]. Regarding the above-mentioned neuronal circuits, their role in the affection of heart rhythm regulation should be investigated. The regulation of the heart rhythm occurs via the vegetative nervous system and some intrinsic mechanisms. Vegetative regulation is divided into segmental and suprasegmental regulation. The segmental component of regulation is the automatic one, which regulates autonomously the heart rhythm, the person cannot influence it directly. The suprasegmental regulation belongs to the central nervous system [15]. So, thoughts and emotions affect the heart rhythm via the central component of heart rhythm regulation. The person can influence the central component of heart regulation directly, consciously, or subconsciously [5]. The hypothesis of this study is that a pathophysiologically increased central regulation affects the heart rhythm. The other way around, if the heart’s regulation is permanently ensured predominantly by the central component of the heart’s regulation, it leads to a dysregulation of the heart rhythm and as a result to atrial fibrillation. A dysregulation means that the permanently increased afferent impulsatory activity towards the heart from the brain is ensured via the sympathetic part of the vegetative nervous system. As a consequence, the action potential is changed, leading to a hyperexcitotory state of the conductive system of the heart and of the cardiomyocytes. Such a state is the pathophysiological favorable condition that the permanently firing impulses from the place around the pulmonary veins can trigger atrial fibrillation [13, 16]. The neural circuits have the ability to increase the central component of the heart’s regulation. As more neuronal circuits are at the same time active in the central nervous system, as higher is the impulsatory activity of the central component of heart regulation [15, 17]. Especially dangerous is the state when many circuits that are in contextual conflict with each other, as described prior, are active. In this case, the central component permanently dominates the heart’s regulation, leading to sympathetic overtone even at rest [16]. The hypothesis stated by the given study is going to be checked by a new approach to cardiorhythmogram analysis, published elsewhere by Sidorenko L. et al. [18].

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1.2 Aim of the Study The aim of this study is to evaluate whether the pathologically increased central heart rhythm regulation, visualized on the cardiorhythmogram, can predict recurrence of atrial fibrillation.

2 Prediction of Atrial Fibrillation Recurrences by Pathologically Increased Central Heart Regulation, Visualized on Cardiorhythmograms 2.1 Material and Methods The object of the study. The study includes the analysis of 350 cardiorhythmograms of patients with paroxysmal atrial fibrillation. Inclusion Criteria. In the study cardiorhythmograms obtained from patients with paroxysmal atrial fibrillation, who were at the moment of biosignal recording in sinus rhythm, were included. Exclusion Criteria. The cardiograms obtained from patients with persistent atrial fibrillation, or with paroxysmal atrial fibrillation, but at the moment of recording a recurrence took place, or without respecting stationarity during biosignal recording were excluded. Also, cardiorhythmograms recorded on pregnant women and on patients with thyrotoxicosis or any structural heart diseases were excluded from further analysis in the frame of the study. Study Design. According to standards, the study is designed according to the STARD international criteria for diagnostic study. The study is designed, completed, data were evaluated, analyzed and presented according to the STARD criteria. This is a case-series study.

2.2 Methods The method for cardiorhythmogram analysis is based on the principles of heart rate variability analysis. The new aspect of the analysis is related to the recognition of additional features in the cardiorhythmogram which represent the increased central component of the heart rhythm regulation. This is an absolute new possibility of the cardiorhythmogram. The evaluation of these features reveals predictive power. The extra features analysis of the cardiorhythmogram are tested in this study of being able to predict the recurrence of atrial fibrillation. Cardiorhythmogram Analysis. The cardiorhythmogram analysis is based on the fundamental principle of heart rate variability. For that reason, the biosignal is an ECG recording [19]. In order to perform a qualitative cardiorhythmogram analysis, several strict standard demands for the ECG recording are stated [20]. The main requirement refers to obtaining a steady-state signal. Otherwise, the regulation of the heart rhythm cannot be evaluated reliably. So, it is important to exclude any external factors which can

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lead to events of nonstationarity in the cardiorhythmogram [20]. It is important because the new approach to cardiorhythmogram analysis is based on the analysis of events of non-stationarity occurring in a steady-state signal. The basic principle of cardiorhythmogram analysis is the ECG R-R peak mathematic analysis which is often applied in heart rate variability analysis. Due to the R-R linear methods of analysis, a cardiorhythmogram is obtained, see Fig. 1. The characterization of the waves’ structure of the cardiorhythmogram delivers information about the predominant regulation of the heart rhythm, whether the segmental – sympathetic or parasympathetic, or suprasegmental components modulate the heart rhythm. It delivers also information on whether events of non-stationarity are present. In the case of this cardiorhythmogram the events of non-stationarity are absent. So, it represents a graphical curve of the R-R peak variations taken from the ECG signal [21]. Complementary to graphical represatation of the biosignal in form of a cardiorhythmogram, a well-known spectral analysis can be done, see Fig. 2. Three groups of frequencies are visible – in blue is the HF – the high-frequency spectrum, which represents the parasympathetic vegetative nervous system, in red - the LF, is the low-frequency spectrum, which evaluates the sympathetic vegetative nervous system. Green represents the VLF component, which represents the central component of heart regulation. In this study under special attention is the new method of nonstationary feature analysis in the cardiorhythmograms. On the cardiorhythmogram several pathophysiological nonstationary features can be observed, see Fig. 3. These very informative features can be analyzed via a visual evaluation by a physiologist or some trained physician or via feature analysis methods. These features represent a special predictive interest. In comparison, by the standard heart variability analysis, these features are eliminated from the analysis [20, 21]. The standard analysis of HRV, as already mentioned above, includes a further Fourier transformation of the cardiorhythmogram into a spectrogram. On a spectrogram is analyzed the contribution of the sympathetic and parasympathetic components of the vegetative nervous system, regulating the heart rhythm [19, 20]. So, if some arousals will appear in a cardiorhythmogram, a standard spectrogram will become invalid, because it will take into analysis, not the real peaks of the cardiorhythmogram, but also the artificial ones [21]. The methodology of analysis of the nonstationarity features in the cardiorhythmograms in frame of a predictive analysis is in detail described elsewhere by Sidorenko et al. [18]. Standard Operating Procedure for Biosignal Recording. The main principle to obtain the appropriate ECG signal consists in excluding all factors which can induce externally provoked nonstationarity into the biosignal. As prior explained, important is to obtain a stationary ECG signal. The recording itself which is going to be applied for further analysis, lasts 5 min. Very important before starting the recording itself to monitor the signal and only after having reached a stationary signal to start the recording. This latent period last individually long and can take from 10 to 30 min. During this time the patient lies in a supine position with connected electrodes. The second standard deviation for ECG recording is applied. During the latent period, the room should be just the patient and the trained medical staff. All possible external stimuli which can provoke arousal, have to be excluded. Such stimuli belong to the phone ringing, the door opening, knocking on the door, any noise provoked by electrical devices, etc. The same conditions have to

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Fig. 1. Cardiorhythmogram. On the x-axis are represented the numbers of the R-R intervals. On the y-axis is presented the distance between the R-R intervals in seconds. Here events of non-stationarity are absent. It is a classical steady-state cardiorhythmogram.

Fig. 2. Spectrogram, obtained from a cardiorhythmogram. On the x-axis are represented the frequencies in Hertz. On the y-axis is presented the distance between the R-R intervals in seconds. All these three spectra make up the total power spectrum in amplitudes, which is represented on the y-axis and is measured in ms power two /Hertz *1000.

be respected not just during the latent period, but also during the recording itself. [19, 20] So, in the study were just the cardiorhythmograms included when the signal was recorded in correspondence with the standards.

2.3 Results and Discussion A total amount of 350 cardiorhythmograms were analyzed. 280 were with paroxysms of atrial fibrillation, observed during 18 months in the frame of follow-up. The other

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70 cardiorhythmograms belonged to patients without paroxysms. Therefore, the predictive potency of the cardiorhythmogram features characterizing the increased central heart modulation component was evaluated on cardiorhythmograms without and with paroxysms. It indicates that both biophysical and pathophysiologic circumstances were reflected, including those that suggest the sinus rhythm prognosis and those that predict the appearance of atrial fibrillation paroxysms. This work does not cover standard HRV evaluation techniques because those methods are applicable for otherwise stetted goals [19, 21]. It provides a new neurophysiological approach to cardiorhythmogram analysis, see Fig. 3. Several biophysical parameters and cardiophysiological biomarkers were used for cardiorhythmogram analysis, however just the most useful, relevant, and convenient variables for analyzing the data are discussed in this paper: In the rest state, low frequency (LF) drops, high frequency (HF) counter-regulation and enhanced central activity are seen. These variables capture the non-stationarity in a steady-state cardiorhythmogram in a suitable manner [18]. On Fig. 3, a representative cardiorhythmogram of a patient who had an AF recurrence. This cardiorhythmogram is characterized by the presence of events of non-stationarity. On the cardiorhythmogram LF drops are present, encircled red, followed by pathological counter-regulation, in blue, which is modulated predominantly by the LF waves. Also remarkable is the feature, visible as a drop-down during the counter-reaction. It is characteristic of a pathologically increased central component of heart regulation, occurring in case of inter-circuital impulsatory conflict. The pathophysiological basis for these variables is explained completely elsewhere [16]. If there is no non-stationarity in steady-state cardiorhythmograms, they may be analyzed using routine HRV analysis. In this scenario, the conventional prediction characterizes a predictive factor to maintain a sinus rhythm - the high HRV, whereas a low HRV indicates the risk for the reappearance of an atrial fibrillation recurrence [14, 19]. The difficulty is that in individuals having atrial fibrillation, the activity of the heart’s regulation systems is working pathologically [15], hence cardiorhythmograms lacking non-stationarity were hardly examined. Standard HRV studies are not achievable in these instances. Only 27 of 350 cardiorhythmograms showed stationarity. Non-stationary feature occurrences were observed in 323 cases. Here the advanced method was applied, applying the features of HF counter-regulation and LF drops. Out of 280 cardiorhythmograms of patients having the paroxysms of atrial fibrillation, 263 cardiorhythmograms showed the presence of LF drops in association with low HF counter-regulation. High counter-regulation following the LF drops, had 43 cases out of 70 paroxysm-free cases of cardiorhythmograms analysis. These results of data analysis show that the cardiorhythmogram features HF counterregulation and LF drops that can predict reliable recurrence of atrial fibrillation. More than that, these can predict significantly whether the patient will remain in sinus rhythm. The association of low pathophysiological HF counter-regulation with LF drops predicts significantly the atrial fibrillation recurrence (p < 0.0001). The combination of LF drops with a high HF counter-regulation predicts significantly the maintenance of sinus rhythm (p < 0.001). The association of high, physiological HF counter-regulation with LF drops predicts significantly that the patient will be recurrence-free (p < 0.001), and will stay

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Fig. 3. Cardiorhythmogram with events of non-stationarity. On the cardiorhythmogram LF drops are present, encircled red, followed by pathological counter-regulation, in blue.

in sinus rhythm. The results of the study show the important component of pathologically increased central heart regulation in triggering recurrences of atrial fibrillation. Explicative aspects were described in some recent studies [3, 15, 23–25] a permanently increased central component is observed in the case of hyperactivity of several neural non-realized circuits [22–24]. This state was shown to be intensified in the case of impulsatory conflict between own and artificial neural circuits [9, 23]. As a consequence, this pathophysiological state leads to an intensive afferent sympathetic downstream to the heart, modulating the heart rhythm. Thus, leading to an increased sympathetic overtone of the heart rhythm even at rest, creating favorable for atrial fibrillation [15, 16].

3 Conclusions Pathologically hyperactivated interconnectivity of the neural circuits leads to a permanently increased central component of heart rhythm modulation leading to favorable conditions for atrial fibrillation recurrence in patients with paroxysmal atrial fibrillation. The increased central heart rhythm modulation can be visualized on cardiorhythmograms by the feature LF drops. The level of physiological compensation to the increased central modulation can be evaluated by the cardiorhythmogram feature HF counter-regulation. The features HF counter-regulation and LF drops can be evaluated in the cardiorhythmograms for the prediction of atrial fibrillation recurrence in patients with paroxysmal atrial fibrillation.

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Acknowledgments. The authors are grateful for the support of this study by the project “Synthesis of steroids containing the azole fragment in the cycle D and/or in the side chain as the basis for creating medicines for the treatment of prostate cancer”. № 22.80013.8007.1BL.

Conflict of Interest. The Authors Declare that They Have no Conflict of Interest.

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13. Calkins, H., et al.: 2012 HRS/EHRA/ECAS Expert Consensus Statement on Catheter and Surgical Ablation of Atrial Fibrillation: recommendations for patient selection, procedural techniques, patient management and follow-up, definitions, endpoints, and research trial design. Europace 14(4), 528–606 (2012). https://doi.org/10.1093/europace/eus027 14. Geurts, S., et al.:Heart rate variability and atrial fibrillation in the general population: a longitudinal and Mendelian randomization study. Clin. Res. Cardiol. 112(6), 747–758 (2023). https://doi.org/10.1007/s00392-022-02072-5. Epub 2022 Aug 13. PMID: 35962833; PMCID: PMC10241681 15. Agarwal, S.K., et al.: Cardiac Autonomic Dysfunction and Incidence of Atrial Fibrillation: Results From 20 Years Follow-Up. J Am Coll Cardiol. 69(3), 291–299 (2017). https://doi. org/10.1016/j.jacc.2016.10.059.PMID:28104071;PMCID:PMC5260487 16. Sidorenko, L., Diaz-Ramirez, I., Vovc, V., Baumann, G.: Fundamental aspects of cardiovascular regulation in predisposition to atrial fibrillation. Moldovan Med. J. 61(4), 42–45 (2018). https://doi.org/10.5281/zenodo.2222313 17. Sidorenko, L., Sidorenko, I., Gapelyuk, A., Wessel, N.: Pathological heart rate regulation in apparently healthy individuals. Entropy 25(7), 1023 (2023). https://doi.org/10.3390/e25 071023 18. Sidorenko, L., Diaz-Ramirez, I., Vovc, V., Baumann, G.: New approach to heart rate variability analysis based on cardiac physiological biomarkers. Moldovan Med. J. 61(3), 39–46 (2018). https://doi.org/10.5281/zenodo.14659264 19. Billman, G.E., Huikuri, H.V., Sacha, J., Trimmel, K.: An introduction to heart rate variability: methodological considerations and clinical applications. Front Physiol. 6, 55 (2015). https:// doi.org/10.3389/fphys.2015.00055.PMID:25762937;PMCID:PMC4340167 20. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Circulation 93(5), 1043–65 (1996). PMID: 8598068 21. Sassi, R., et al.: Advances in heart rate variability signal analysis: joint position statement by the e-Cardiology ESC working group and the european heart rhythm association co-endorsed by the asia pacific heart rhythm society. Europace 17(9), 1341–1353 (2015). https://doi.org/ 10.1093/europace/euv015 22. Farahani, F.V., Karwowski, W., Lighthall, N.R.: Application of graph theory for identifying connectivity patterns in human brain networks: a systematic review. Front. Neurosci. 13, 585 (2019). https://doi.org/10.3389/fnins.2019.00585.PMID:31249501;PMCID:PMC6582769 23. Krupnik, R., Yovel, Y., Assaf, Y.: inner hemispheric and interhemispheric connectivity balance in the human brain. J. Neurosci. 41(40), 8351–8361 (2021). https://doi.org/10.1523/JNEURO SCI.1074-21.2021. Epub 2021 Aug 31. PMID: 34465598; PMCID: PMC8496194 24. Alenina, N., et al.: Growth retardation and altered autonomic control in mice lacking brain serotonin. Proc. Natl. Acad. Sci. USA 106(25), 10332–10337 (2009). doi: https://doi.org/10. 1073/pnas.0810793106. Epub 2009 Jun 11. PMID: 19520831; PMCID: PMC2700938 25. Da Costa-Goncalves, A.C., et al.: Role of the multidomain protein spinophilin in blood pressure and cardiac function regulation. Hypertension 52(4), 702–707 (2008). https://doi.org/10. 1161/HYPERTENSIONAHA.108.114355. Epub 2008 Aug 18 PMID: 18711009

Combination Thermostated Vacuum Gauge Igori Belotercovschii(B) , Anatolie Sidorenko , Elena Condrea , and Vladimir Smyslov Ghitu Institute of Electronic Engineering and Nanotechnologies, Technical University of Moldova, Chisinau, Republic of Moldova [email protected]

Abstract. To expand the measured pressure range, a prototype of a CVG-3 combination vacuum gauge using different physical principles of pressure measurement has been developed and constructed; it includes an electronic controller and a specially designed TTD-2 deformation–thermoelectric combination transducer. The transducer includes thermoelectric and deformation sensors. The sensitive element of the thermoelectric sensor consists of an electrically insulating film with heating and measuring circuits formed on the film surface by vacuum deposition; the measuring circuit is an array of thermocouples. The use of a thin insulating film in constructing the sensitive element has made it possible to significantly expand the measured pressure range toward high vacuum. The sensitive element of the deformation sensor is a silicon chip with a thin membrane in the middle; a tensoresistive bridge is formed on the membrane surface. The decrease in the membrane thickness has made it possible to increase the sensitivity of the sensor at pressures below 1 Torr. To decrease the pressure measurement error depending on changes in ambient temperature, the sensors are thermostated. Each sensor has an individual thermostat. The use of the thermocouple and tensometric principles of pressure measurement has made it possible to expand the measured pressure range from deep vacuum to atmospheric pressure, while maintaining high measurement accuracy. The large overlap of the measurement ranges of the sensors has made it possible to exclude jumps in the vacuum gauge readings upon switching from one physical principle of pressure measurement to another. Keyword: Vacuum gauge · Pressure measurement · Thermocouple · Tensometric

1 Introduction The ever-increasing demand for vacuum-based technologies in modern science, engineering, and production ranging from food production to nanotechnology and space exploration makes the development and application of low-pressure measurement instruments one of the principal and promising directions of modern engineering and technology.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 574–581, 2024. https://doi.org/10.1007/978-3-031-42775-6_61

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The key tendencies in the design of low-pressure measurement instruments, in addition to the conventional increase in measurement accuracy and reliability, are the providing of the embeddability of these instruments in computer-driven control systems for production processes and an acceptable price/quality ratio. The main fleet of instruments used to measure pressure in a range of 5 × 10–3 Torr to atmospheric pressure is composed of thermoelectric and deformation low-pressure measurement gauges. The use of thermoelectric low-pressure measurement instruments meets the most important requirements imposed on modern industrial production equipment, namely, simplicity of design, reliability, and low cost. The principle of operation of thermoelectric vacuum gauges is based on the dependence of the thermal conductivity of air on pressure; the instruments cover a pressure range of 5 × 10–3 Torr to atmospheric pressure. The advantages of the pressure transducers of thermoelectric vacuum gauges include, in addition to the simplicity of design, stability to “breakthrough” of the atmosphere and almost unlimited service life. The disadvantages of thermoelectric vacuum gauges are the low accuracy of pressure measurement, the significant dependence of the pressure measurement accuracy on ambient temperature, and the limited use of these vacuum gauges in technological processes that occur at pressures below 5 × 10–3 Torr (in the latter case, an additional pressure measurement instrument, such as an ionization vacuum gauge, should be used to provide pressure control). Deformation vacuum gauges, unlike thermoelectric gauges, are instruments that measure pressure directly, rather than indirectly; their operation is based on the principle of measuring the magnitude of the pressure-dependant deformation of the elastic member. The main advantage of deformation gauges is the independence of pressure readings on the type of gas. This feature is particularly important in view of the increasing applications of ion-plasma technological processes that occur at pressures of a few Torr. Substantial disadvantages of the existing tensoresistive vacuum gauges are the significant dependence of the pressure measurement accuracy on ambient temperature and the low sensitivity; these features limit the use of these vacuum gauges in processes that occur at pressures below 1 Torr. In addition, the existing combination deformation–thermoelectric vacuum gauges are characterized by jumps in the measured pressure readings, which are associated with switching from one physical principle of pressure measurement to another. The results of the recent studies of semiconductor materials have shown that these materials can be used for designing both integral tensoresistive membrane structures and thermocouples from thin films of highly efficient thermoelectric materials.

2 Design Features and Specification of the Vacuum Gauge To eliminate the disadvantages of the existing thermoelectrical and deformation vacuum gauges, a CVG-3 combination vacuum gauge prototype has been developed and constructed (see Fig. 1). A combination vacuum gauge is an instrument that combines different physical principles of pressure measurement to expand the range and simplify the installation, tuning, and integration.

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Fig. 1. Appearance of the CVG-3 vacuum gauge.

The vacuum gauge includes an electronic controller and a specially designed TTD-2 deformation–thermocouple combination transducer. The transducer includes thermoelectric and deformation sensors. The sensitive element of the thermoelectric sensor consists of an electrically insulating film and a heat-conducting base with heating and measuring circuits formed on the film surface by vacuum deposition; the measuring circuit is an array of thermocouples (see Fig. 2). The sensitive element is mounted between two polished heat-conducting screens, which decrease heat losses from the heating circuit due to radiation at low pressures and losses due to gas convection at high pressures, taking into account the ratio between the

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mean free path of gas molecules and the heat transfer distance from the sensing element (see Fig. 4). This design solution has made it possible to expand the measured pressure range toward high pressures up to 10 Torr and provided an overlap with the deformation sensor. The holes in the screens serve for the interaction of the sensitive element with the gas surrounding the transducer [1]. The use of a thin insulating polyamide film (~12 µm) and planar electrical circuits with a thickness of no more than 70 nm has made it possible to significantly decrease heat losses over the surface compared with conventional wire transducers. This design has provided a decrease in the heating temperature of the heat-exchange surface of the sensitive element from 200 °C in the existing counterparts to 50 °C and, therefore, a decrease in radiation losses. The significant decrease in the total heat loss and the increase in sensitivity due to the use of an array of 28 highly efficient thermocouples based on a bismuth–antimony couple have expanded the measured pressure range to 1 × 10–5 Torr and decreased the power consumption of the transducer by an order of magnitude. The thermoelectric sensor operates as follows. The current flowing through the heating circuit of the transducer leads to the heating of the sensitive element; at any timeinvariant pressure, a constant temperature gradient between the center and edge of the sensitive element is established; it generates thermoEMF on the array of thermocouples of the measuring circuit of the transducer. A change in the gas pressure leads to a change in the concentration of gas molecules and, accordingly, in the amount of heat carried away by the gas molecules from the sensitive element; as a consequence, the temperature gradient along the thermocouples and the thermoEMF change.

Fig. 2. Appearance and output characteristics of the TTD-2 thermocouple sensitive element and the PMT-2 industrial transducer.

Owing to the electrical insulation of the heating and measuring circuits, the transducers can be used under strong interference conditions in various technological processes.

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The planar technology used for the construction of the sensitive element provided a significant increase in the mechanical strength and repeatability of the calibration characteristics and an improvement in the ease of manufacture. The sensitive element of the deformation sensor is a silicon chip with a thin membrane in the middle; a tensoresistive bridge is formed on the membrane surface (see Fig. 3). To decrease the dependence on ambient temperature, a circuit consisting of a transistor and resistors is formed on the chip; the circuit provides the supply of a temperature-dependent voltage to the bridge circuit to compensate for a drift [2].

Fig. 3. Design and appearance of the TTD-2 deformation sensitive element.

The method used to form the elastic members of the sensor, which is based on efficient techniques of the anisotropic etching of crystals, has made it possible to decrease the membrane thickness and thereby increase the sensitivity of the transducer and, accordingly, expand the measured pressure range to 5 × 10–2 Torr with the preservation of the linear output characteristic (see Fig. 4). The base of the design of the transducer is a vacuum-tight connector with thermoelectric and deformation sensors mounted on it through a heat-insulation layer (Fig. 5). Each of the sensors is a housing made of a heat-conducting metal and a sensitive element placed inside. Ohmic heaters are wound on the surface of the housings; platinum thermistors are mounted in the immediate vicinity of the sensitive elements. Owing to this design solution, each sensitive element of the sensors can be thermostated individually and the pressure measurement error can be decreased to 10%. The electrical insulation of the sensor circuits is provided by the electrochemical anodizing of the surface of the housings. To store the calibration characteristics of the transducer and operating modes, a nonvolatile memory (EEPROM) is mounted in the transducer housing. To provide connection to the vacuum system, a stainless steel flange (KF25, KF40, CF25, CF40) is used; it is welded to the connector by laser welding. To maintain the operating modes of the transducer, display pressure values on the indicator, and exchange information with the PC, an electronic controller has been developed; the block diagram of the controller is shown in Fig. 6.

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Fig. 4. Output characteristics of the TTD-2 deformation sensor.

Fig. 5. Schematic diagram and appearance of the transducer.

Owing to the use of a high-performance 32-bit microcontroller (PIC32MX170F256D) and precision analog-to-digital and digital-to-analog converters, the operating modes of the converter are maintained with high accuracy. The sensors

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Fig. 6. Block diagram of the CVG-3 vacuum gauge.

are thermostated using the software implementation of the PI controller. The use of a 24-bit analog-to-digital converter for temperature measurement and a 16-bit pulse-width modulation for controlling the thermostat heaters makes it possible to maintain the temperature of the sensitive elements of the sensors with an accuracy of 0.004 °C and thereby almost completely eliminate the temperature drift of the measured pressure values. The use of a precision multichannel analog-to-digital converter (AD7190) for measuring output signals and the wide overlap of the measurement ranges of the sensors exclude jumps in vacuum gauge readings upon switching from one physical principle of pressure measurement to another. To compensate for the temperature drift of the analog circuit elements, a digital temperature sensor (LM75) is mounted on the controller PCB. Information exchange with a computer is implemented via a common RS-485 interface in accordance with the standard Modbus ASCII industrial automation protocol; therefore, the instrument can be integrated with a variety of industrial and scientific equipment. The developed CVG-3 vacuum gauge has the following parameters: – – – – – – –

Pressure measurement range is 1 × 10–5 to 1000 Torr Relative error of pressure measurement is (1 × 10–5 to 30 Torr) ± 10% Error of pressure measurement is (30–1000 Torr) ± 3 Torr Information exchange with PC, RS-485 interface via MODBUS protocol Power consumption is no more than 3 VA Overall dimensions of the vacuum gauge are 70 × 40 × 160 mm Vacuum connection KF25, CF25, KF40, CF40

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3 Conclusions The conducted tests and experience of practical application of the vacuum gauge have shown the following advantages over the existing counterparts: – – – – – – – –

Wide measured pressure range Independence of the readings on the type of gas in a range of 1–1000 Torr Arbitrary positions of the transducer connection High accuracy and reproducibility Thermostat control of the sensors Resistance to sudden pressure changes over the entire range Low energy consumption Wide range of supply voltages of 7–30 V

Acknowledgements. The study was supported by the Moldova State Program Project «Functional nanostructures and nanomaterials for industry and agriculture» no. 20.80009.5007.11 (sample construction and characterization).

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. Belotercovschii I., Sidorenko A., Condrea E., Morari R.: Patent «Vacuummetru termoelectric» MD 1587 Z 2022.07.31 2. Smyslov V., Yakunin V., Belotsercovskii I., Scutelnic E.: Electronic transducers for measurement of pressure and temperature in automated Sistems. In: Abstract Book Humbold Kolleg Cooperation with Germany-experience, Kishinev, Moldova, p. 45 (2009)

Predicting Pain Scores Using Personality Trait Facets and Personality Trait Domains Assessed by Personality Inventory for DSM-5 Ina Timotin1 , Svetlana Lozovanu1 , Andrei Ganenco1(B) , Ion Moldovanu2 Oleg Arnaut1 , Ion Grabovschi1 , Eugeniu Coretchi1 , Tudor Besleaga1 , and Victor Ojog1

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1 Nicolae Testemitanu State University of Medicine and Pharmacy, Chisinau, ,

Republic of Moldova [email protected] 2 Institute of Neurology and Neurosurgery, Chisinau, Republic of Moldova

Abstract. Personality disorder is different from mental illness because it is more persistent during adulthood, while mental illness results from a morbid process of a certain type and has a more recognizable onset and evolution. Pain is a subjective experience that is influenced by genetic, gender, social, cultural and personal parameters. The link between personality disorders and acute pain has been shown to be increasingly significant because the link is bidirectional and, in both cases, they act as risk factors for each other. The psychometric assessment was performed using the PID-5, the pain test was performed using a submaximal effort tourniquet technique. The relationships of potential predictors and pain perception in the first, second and third minute were estimated using regression analysis, bootstrap being applied for model stability estimation. The analysis of the prediction models for pain perception in 5 domains of personality states that no domain was found as a predictor in first minute model, Negative Affect and Detachment which are included in the internalizing category were found as predictors in the 2nd minute model, same as the facets which are included in this category. Both facets included in Psychoticism domain and the domain itself were found as predictors in the 3rd minute model, with the inverse relationship. Our results allow us to consider that personality changes can change the perception of pain, which requires corrections in the diagnosis and therapeutic approach of disorders associated with pain. Keywords: Pain · Personality Trait Facets · Personality Trait Domains · PID-5

1 Introduction Each person has certain personality traits. Some of these may be so dysfunctional that it is difficult to justify a resolute diagnosis of personality disorder. Personality disorder is considered to be different from mental illness because it is more persistent during © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 582–596, 2024. https://doi.org/10.1007/978-3-031-42775-6_62

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adulthood, while mental illness results from a morbid process of a certain type and has a more recognizable onset and evolution. Personality inventory for Diagnostic and Statistical Manual of Mental Disorders -5th edition (PID-5) is a self-assessment, designed to evaluate 25 facets of personality traits. These facets were designed to be used in combination with other diagnostic criteria to diagnose personality disorder (PD) and are presented in Section III of the DSM-5; (American Psychiatric Association, 2013), describing a new hybrid approach to PD diagnosis, both dimensional and categorical [1, 2]. As such, PID-5 characterizes the personality according to five superior domains of personality trait, focusing on maladaptive traits in these areas: Negative Affect, Detachment, Antagonism, Disinhibition, and Psychoticism. Indeed, a large number of scientific papers sustains the validity of the pathological personality traits represented in PID-5 [3, 4]. Personality disorders (PDs) are common and severe mental disorders associated with impaired quality of life [5]. The adult population prevalence approximates 10% in high income countries [6], while mental health services a prevalence of 40% is reported [7], half of these cases are borderline personality disorder (BPD), which in general population is estimated to be 1.6%. BPD is a common psychiatric disorder whose core features are affective dysregulation, identity disturbances and problems in social interaction, with an intense fear of loss, abandonment or rejection by social partners [8]. The definition of pain was revised in 2018 and accepted as “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage”, as recommended by International Association for the Study of Pain (IASP) [9]. Pain is a subjective experience that is influenced by genetic, gender, social, cultural and personal parameters. The link between personality disorders and acute pain has been shown to be increasingly significant because the link is bidirectional and, in both cases, they act as risk factors for each other. Patients with borderline personality disorder (BPD) frequently present to primary care physicians and pain specialists. They are more likely to experience pain and rate pain as more severe than patients with other personality disorders [10], they report significant pain that interferes with their lives. Several studies confirm that pain processing in patients with borderline personality disorder (BPD) is abnormal primarily in terms of pain thresholds that are typically elevated or perception of phasic nociceptive stimuli that is reduced. BPD exhibits altered pain processing that can be attributed to an altered processing of nociceptive stimuli in prefrontal and limbic brain areas, which may help mechanistically explain clinical behavior [11, 12]. Another study concludes that in contrast to healthy volunteers, BPD patients do not report significant affective ratings and specifically display a reduced sensory component for sharpness [13]. People with borderline personality disorder (BPD), compared to controls, report a relative absence of acute pain. In contrast, BPD is overrepresented among chronic pain patients, suggesting that they experience a relative excess of chronic pain. To date, this ‘paradox of pain’ has only been partially explored; no study has examined both acute and chronic pain in the same sample [12]. Thus, studying the perception of pain in such patients can help to understand the pathophysiology of BPD, but also the interaction between the affective and physical dimensions of pain. Promising therapeutic strategies

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should also target the neuroanatomical and neurobiological dysfunctions that lead to altered pain perception in BPD patients [14]. Based on these considerations, research on the influence of all personality traits, especially those included in BPD, on the psychophysiological reactivity of these subjects in the examination of the degree of pain perception is of interest.

2 Material and Methods The study was performed on 140 subjects selected out of 187 persons visiting Department of Headache and Autonomic Disorders of the Institute of Neurology and Neurosurgery (Chisinau, Republic of Moldova) between March 2018–February 2022. They signed an informed agreement to be included in this study, which continued at Department of Human Physiology and Biophysics of State University of Medicine and Pharmacy (SUMPh) “Nicolae Testemitanu”. The subjects with acute or chronic cardiac or respiratory disease were excluded. Before beginning the study, a set of inventories and psychometric tests was prepared. Verbal Rating Scale (VRS) is used to assess the severity of a patient’s pain. Verbal rating scale consists of a series of descriptive words (e.g., “no pain” to “severe pain”) or grades which show the severity of pain perceived by the patient. The psychometric assessment was performed using the PID-5, which was translated and validated by a working group made up of staff of the Department of Human Physiology and Biophysics of SMPhU “N. Testemitanu” and the Department of Headache and Autonomic Disorders within the Institute of Neurology and Neurosurgery, with the consent of the authors. PID-5 is a questionnaire that contains 220 items, used to obtain score on the 4-point scale for each one of 25 facets, each facet includes from 4 to 14 items. These facets correspond to the disadaptive personality traits described in Section 3 of DSM-5 and are included in the five higher order domains also described in Section 3: Negative Affectivity, Detachment, Antagonism, Disinhibition, and Psychoticism. The pain test was performed using a technique, called the submaximal effort tourniquet technique. Pain was produced by a tourniquet which had been inflated around his upper arm till 200 mm Hg [15]. The assessed parameter was pain sensation in 1st, 2nd and 3rd minute after applying a tourniquet as chosen by subject on the VRS. Statistical analysis was performed using software Statistical Package for Social Sciences version 22 (IBM SPSS 22) [16]. Descriptive statistics for numerical variables was presented by minimum, maximum, mean, standard deviation, median, 25th percentile, 75th percentile. Descriptive statistics for discrete variables was presented by count, relative frequencies,95.0% CI for relative frequencies. Correlation analysis was performed using Spearmen test, completed by bootstrap estimation of 95% CI. The form of relationships of potential predictors on pain perception in the first, second and third minute was estimated using regression analysis, bootstrap being applied for model stability estimation.

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Table 1. Data distribution according to the characteristics of the people included in the research Minimum Maximum Mean Standard 75th Median 25th Deviation percentile percentile Age

17.0

73.0

37.6

18.0

56.5

31.5

21.0

Pain 1

0.0

10.0

4.9

2.3

6.5

5.0

3.0

Pain 2

0.0

10.0

5.5

2.6

7.0

6.0

4.0

Pain 3

0.0

10.0

1. Anhedonia

0.00

2.38

5.8

2.7

8.0

6.0

3.8

0.85

0.48

1.13

0.75

0.50

2. Anxiousness

0.00

3.00

1.33

0.70

1.83

1.22

0.83

3. Attention Seeking

0.00

2.88

1.17

0.72

1.75

1.13

0.63

4. Callousness

0.00

2.57

0.50

0.51

0.71

0.29

0.14

5. Deceitfulness

0.00

2.80

0.89

0.62

1.30

0.70

0.40

6. Depressivity

0.00

2.29

0.50

0.44

0.64

0.40

0.21

7. Distractibility

0.00

2.78

0.98

0.63

1.33

0.89

0.44

8. Eccentricity

0.00

2.77

0.79

0.72

1.04

0.62

0.23

9. Emotional Lability

0.00

4.00

1.32

1.03

1.86

1.14

0.71

10. Grandiosity

0.00

2.50

0.87

0.67

1.33

0.83

0.33

11. Hostility

0.00

2.80

1.12

0.69

1.60

1.00

0.50

12. Impulsivity

0.00

2.67

1.09

0.63

1.50

1.00

0.67

13. Intimacy Avoidance

0.00

2.67

0.86

0.66

1.33

0.67

0.33

14. Irresponsibility

0.00

2.43

0.62

0.49

0.86

0.43

0.29

15.

0.00

3.00

0.90

0.67

1.40

0.80

0.40

16. Perceptual Dysregulation

0.00

2.17

0.60

0.53

0.92

0.50

0.17

17. Perseveration

0.00

2.56

1.00

0.57

1.44

0.89

0.56

18. Restricted Affectivity

0.00

2.57

1.03

0.59

1.43

0.86

0.57

19. Rigid Perfectionism

0.00

2.70

1.28

0.65

1.70

1.30

0.80

20. Risk Taking

0.29

2.64

1.23

0.49

1.54

1.14

0.86

21. Separation Insecurity

0.00

3.00

1.06

0.70

1.57

0.86

0.57

22. Submissiveness

0.00

3.50

0.87

0.76

1.25

0.75

0.25

23. Suspiciousness

0.43

2.43

1.42

0.48

1.79

1.29

Manipulativeness

1.14

(continued)

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I. Timotin et al. Table 1. (continued) Minimum Maximum Mean Standard 75th Median 25th Deviation percentile percentile

24. Unusual Beliefs & Experiences

0.00

2.38

0.61

0.57

1.00

0.50

0.13

25. Withdrawal

0.00

2.80

0.85

0.64

1.25

0.75

0.30

3 Results To achieve the aim of the study, the prediction models in the evolution of pain using 25 personality traits facets were created initially, followed by the models using 5 personality trait domains. Those 5 domains can be also combined into 3 clinical categories, according to the tendency that the individual shows when one domain or another prevails over the conscious and the unconscious. These 3 categories are Internalizing behavior which includes Negative Affect and Detachment, Externalizing behavior which includes Antagonism and Disinhibition, and Psychoticism. Table 1 illustrates the descriptive statistics of the respondents’ data included in the study. One of the characteristics was the age of the patients, which varied between a minimum of 17 years and a maximum of 73 years, with a mean of 37.6 and a standard deviation of 18 years. Afterward, the pain level during the first 3 min was related by patient in grades from 0 to 10. Pain 1 was pain score in the first minute after applying a painful stimulus. Thus, it had a minimum of 0.00 and a maximum of 10.00 points on VRS, and the mean was 4.9 with a standard deviation of 2.3 points. Pain 2 was measured after minute 2 of the application of the painful stimulus. Therefore, its intensity varied between a minimum of 0.00 and a maximum of 10.00, with a mean of 5.5 points and a standard deviation of 2.6. Pain 3 was recorded 3 min after applying the painful stimulus and ranged between 0.00 and 10.00, with a mean of 5.8 and a standard deviation of 2.7. This was followed by the recording and evaluation some personality traits of the people included in the study. Table 2. Data distribution by gender and living environment of the people included in the research Subjects environment Sex

%

95.0% Lower CL 95.0% Upper CL

Rural

47

30.1%

23.3%

37.6%

Urban

109

69.9%

62.4%

76.7%

Women

87

55.8%

47.9%

63.4%

Men

69

44.2%

36.6%

52.1%

Individuals examined to determine associations between personality traits and pain sensitivity at minutes 1, 2, and 3 of painful stimulation were from rural areas in 30.1% (95% CI 23.3, 37.6) and 69.9% (95% CI 62.4, 76.7) of rural regions urban, the majority of respondents were women, about 55.8% (CI95% 47.9, 63.4) (Table 2).

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After examining the correlations between the researched factors in the study patients, it was decided to model the prediction of pain intensity in 1st, 2nd and 3rd minute of continuous pain stimulation to highlight their relationships with personality traits. The characteristics of the models are presented further. 3.1 Model PAIN Facets 1 min Pain intensity at the first minute of continuous painful stimulation was the first variable for which the predictive model was created. The model was developed after all the variables described above were initially included in the calculation. Subsequently, the statistical analysis program formed 19 models, successively removing parameters without predictive power of the dependent variable. The final version of the model with optimal traits is shown in Table 3. Table 3. Model Summary, PAIN facets 1 min Model

R

R Square

Adjusted R Square

19

0.453 s

0.205

0.150

s. Predictors: (Constant), Withdrawal, Separation. Insecurity, Grandiosity, Distractibility, Intimacy Avoidance, Rigid Perfectionism, Impulsivity, Attention. Seeking, Perseveration, Deceitfulness t. Dependent Variable: Pain 1

The variance analysis of the developed model showed a regression sum of squares equal to 171,738 out of 836,686 possible. The correlation coefficient between the dependent variable and the other variables included in the model was equal to 0.453, the determination coefficient, in this case, being 0.205. After adjusting it, the model was able to account for 15% of the variance in pain in the first minute (Adjusted R Square = 0.150, p = 0.159), which is relatively low. The null hypothesis (none of the parameters included in the model can predict pain intensity) was rejected (F = 3.745 p < 0.001), with the Fisher test being statistically significant. The resulting model after eliminating insignificant variables is illustrated in Table 4. It included as variables the constant (B = 4.155, p < 0.001) and the standardized values of Attention Seeking (B = −0.857, p = 0.025), Deceitfulness (B = −0.906, p = 0.027), Distractibility. (B = 0.687, p = 0.089), Grandiosity (B = 0.830, p = 0.035), Impulsivity. (B = 0.719, p = 0.073), Intimacy Avoidance (B = 0.772, p = 0.016), Perseveration (B = −1.214, p = 0.03), Rigid perfectionism (B = 0.658, p = 0.094), Separation Insecurity (B = 0.586, p = 0.043), Withdrawal (B = −0.661, p = 0.068). Thus, it was determined that the pain intensity in the first minute after the painful stimulus could be predicted using the mathematical expression: PAIN facets 1 min = −0.857 * Attention Seeking - 0.906 * Deceitfulness + 0.687 * Distractibility + 0.830 * Grandiosity + 0.719 * Impulsivity + 0.772 * Intimacy Avoidance - 1.214 * Perseveration + 0.658 * Rigid perfectionism + 0.586 * Separation Insecurity - 0.661 * Withdrawal.

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Model

Unstandardized Std. Coeff. B Error

Standardized t Coeff. Beta

Sig.

95.0% Confidence Interval for B Lower Bound

(Constant)

4.155

0.482

8.622 0.000

3.202

Upper Bound 5.107

Attention Seeking

−0.857

0.378 −0.265

−2.265 0.025 −1.605 −0.109

Deceitfulness

−0.906

0.407 −0.240

−2.228 0.027 −1.710 −0.102

Distractibility

0.687

0.401

0.187

1.713 0.089 −0.105

1.479

Grandiosity

0.830

0.389

0.238

2.132 0.035

0.061

1.600

Impulsivity

0.719

0.398

0.194

1.804 0.073 −0.069

1.506

Intimacy Avoidance

0.772

0.317

0.220

2.434 0.016

1.399

0.553 −0.300

0.145

Perseveration

−1.214

Rigid Perfectionism

0.658

0.390

0.183

1.688 0.094 −0.112

1.428

Separation. Insecurity

0.586

0.287

0.177

2.042 0.043

0.019

1.154

Withdrawal

−0.661

−1.840 0.068 −1.371

0.049

0.359 −0.182

−2.197 0.030 −2.307 −0.122

The linear regression conditions for the residuals were met in this case. The distribution was normal, and their scattering was random, without any legality collinearity. 3.2 Model PAIN Facets 2 min The model for predicting the intensity of pain felt in the second minute of continuous painful stimulation according to the technique described in the materials and methods section was created, starting from summarizing the effects of all measured variables on the one of interest. The assessment of association relations between the predictive factors and the dependent variable resulted in 24 models formed by the statistical calculation program. The general characteristics of the model with optimal features are presented in Table 5. The adjusted coefficient of determination (Adjusted R Square) of the variance of pain intensity 2 min after the start of the stimulus application was 0.757. Therefore, the created model explains approximately 76% of the dispersion of the variable of interest (the pain in the second minute after the stimulus application was influenced by personality type). The sum of squares was 780.50 out of a possible 1017,840. The null hypothesis (none of the parameters included in the model can predict pain intensity) was not rejected (F = 81.666 p < 0.001), with the Fisher test being statistically significant.

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Table 5. Model Summary, PAIN facets 2 min Model

R

R Square

Adjusted R Square

24

.876

0.767

0.757

x. Predictors: (Constant), Pain 1, Anhedonia, Suspiciousness, Age, Emotional Lability, Attention. Seeking y. Dependent Variable: Pain 2

To develop the current model, the Backward method was used. The final version of the model included constant variables (B = 0.139, p = 0.75) and the standardized values of Age (B = 0.019, p = 0.002), Anhedonia (B = −0.447, p = 0.06), Attention Seeking (B = −0.353, p = 0.032), Emotional Lability (B = 0.240, p = 0.030), Suspiciousness (B = 0.649, p = 0.014), Pain 1 (B = 0.867, p < 0.001) (Table 6). Thus, it was determined that the pain intensity in 2nd minute could be predicted using the mathematical expression: PAIN facets 2 min = Age * 0.019 - Anhedonia * 0.447 - Attention seeking * 0.353 + Emotional Lability * 0.240 + Suspiciousness * 0.649 + Pain 1 * 0.867. Table 6. Coefficients for PAIN facets 2 min Model

Unstandardized Std. Coeff. B Error

Standardized t Coeff. Beta

Sig.

95.0% Confidence Interval for B Lower Bound

(Constant)

0.139

0.435

Upper Bound

0.319

0.751 −0.721 0.998 0.002 0.007

Age

0.019

0.006 0.134

3.178

Anhedonia

−0.447

0.236 −0.083

−1.892 0.060 −0.913 0.020

Attention Seeking

−0.353

0.163 −0.099

−2.166 0.032 −0.675 −0.031

Emotional Lability

0.240

0.109 0.097

2.193

0.030 0.024

0.456

Suspiciousness 0.649

0.261 0.121

2.484

0.014 0.133

1.166

Pain 1

0.046 0.786

18.682

0.000 0.775

0.959

0.867

0.031

The linear regression conditions for the residuals were respected in this case as well. The distribution was normal, and their scattering was random, without any collinearity. 3.3 Model PAIN Facets 3 min The last model was made based on the initial parameters and the pain intensity values in 1st and 2nd minutes. Correlations between the previously described predictive factors and

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the dependent variable were evaluated by the statistical analysis program, which formed a model with optimal predictive features. The general presentation of the respective model is presented in Table 7. Table 7. Model Summary, PAIN facets 3 min Model

R

R Square

Adjusted R Square

26

0.889

0.790

0.783

z. Predictors: (Constant), Pain 2, Unusual Beliefs & Experiences, Distractibility, Perseveration., Pain 1 a. Dependent Variable: Pain 3

Model PAIN Facets 3 min, illustrated in Table 7, correlated with the variables taken into account with a coefficient of 0.889. The adjusted coefficient of determination (Adjusted R Squared), was 0.783. This means that the elaborated model explains approximately 78% of the variance of the variable of interest (pain in 3rd minute after stimulus application was influenced by personality type). The analysis of the variance of the pain values felt by the study subjects in 3rd minute of painful stimulation showed that the elaborated model had a sum of the squares of the residuals which made up about 0.21 of the sums of the squares of the model, the ratio of the regression squares being different from those residuals (F = 112.603, p < 0.001). The final version of the model proposed the approximation of pain intensity in the 3rd minute through using the formula (Table 8): PAIN facets3 min = −0.361 * Distractibility + 0.594 * Perseveration - 0.580 * Unusual Beliefs & Experiences - 0.351 * Pain 1 + 1.175* Pain 2. Table 8. Coefficients for PAIN facets 3 min Model

Unstandardized Std. Coeff. B Error

Standardized t Coeff. Beta

Sig.

95.0% Confidence Interval for B Lower Bound

(Constant)

1.086

0.308

Distractibility −0.361

0.216 −0.085

Perseveration

0.277 0.128

0.594

3.523 0.001

Upper Bound

0.477

1.695

−1.674 0.096 −0.787

0.065

2.141 0.034

0.046

1.142

Unusual Beliefs & Experiences

−0.580

0.218 −0.124

−2.661 0.009 −1.011 −0.149

Pain 1

−0.351

0.083 −0.304

−4.198 0.000 −0.515 −0.186

Pain 2

1.175

0.075 1.126

15.574 0.000

1.026

1.324

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The hypothesis of the residuals for the regression was respected, they being homoscedastic. The results obtained in the current study indicate that 10 of the 25 facets evaluated using the PID-5 can predict the degree of pain in the first minute, 3 of them are in the inverse relationship, namely Withdrawal which is assigned to the clinical domain Detachment, and Attention Seeking and Deceitfulness predict pain inversely proportional, the other 7 traits are directly related, i.e., emphasizing them will increase the degree of pain perception. The results indicate that Grandiosity attributed to the Antagonism domain, Impulsivity and Distractibility attributed to the Disinhibition domain can predict pain in the 2nd minute. Both of these domains are included in the Externalizing category. These results suggest that the traits assigned to the Internalizing category predict the decrease in the level of pain as the score of the respective trait increases, and the traits that are included in the domains that form the Externalizing category increase the perception of pain. The pain score in 2nd minute can be predicted by only 4 traits out of the 25, two of them, namely Anhedonia attributed to Detachment domain and Attention Seeking, show an inverse relationship, and Emotional Lability and Suspiciousness not assigned to domains show a direct relationship with the score of pain perception. Finally, for the prediction of pain in the 3rd minute, only 3 out of 25 traits remain valuable, namely the Distractibility attributed to the Disinhibition domain predicts the decrease in the pain score, and the Perseveration not attributed to the domain increases the degree of pain. It should be noted that an increase the Unusual Beliefs & Experiences trait score attributed to the Psychoticism domain by one unit can predict a decrease in the pain score by 0.580. Descriptive statistics of some clinical domains of personality disorders in subjects included in the study are illustrated on Table 9. Table 9. Descriptive analysis of clinical domains of personality disorders Minimum

Maximum

Mean

St Dev

25th percentile

Median

75th percentile

Negative Affect

0.04

4.26

1.23

0.66

0.77

1.11

1.67

Detachment

0.00

2.28

0.85

0.49

0.50

0.73

1.17

Antagonism

0.03

2.62

0.89

0.57

0.47

0.81

1.27

Disinhibition

0.00

2.25

0.89

0.49

0.55

0.86

1.23

Psychoticism

0.00

2.35

0.67

0.55

0.22

0.54

1.00

Thus, Negative Affect scored between 0.04 and 4.26, with a mean of 1.23 and a standard deviation of 0.66. Detachment ranged from 0.00 to 2.28, with a mean of 0.85 and a standard deviation of 0.49. Antagonism ranged from 0.03 to 2.62, with a mean of 0.89 and a standard deviation of 0.57. Disinhibition scored between 0.00 and 2.25, with

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a mean of 0.89 and a standard deviation of 0.57, and Psychoticism ranged from 0.00 to 2.35, with a mean of 0.67 and a standard deviation of 0.55. 3.4 Model PAIN Domains 1 min In order to identify what proportion of the pain occurring in the first minute after the application of the stimulus is predicted by the gender of the study participants and their age, a model was created in which the coefficient of determination (Adjusted R Squared) was calculated, which was 0.102, relatively small (Table 10). The sum of squares was 95,310 out of a possible 836,68. This would mean that the elaborated model explains only about 10% of the dispersion of the variables of interest (pain intensity in the first minute after applying the painful stimulus). Table 10. Model Summary, PAIN domains 1 min Model

R

R Square

Adjusted R Square

7

0.338

0.114

0.102

g. Predictors: (Constant), Sex, Age h. Dependent Variable: Pain 1

The null hypothesis (none of the parameters included in the model can predict pain intensity) was not rejected (F = 9.83 p < 0.001), with the Fisher test being statistically significant. When developing the current model, the Backward method was used. The resulting model is illustrated in Table 11. It included as variables the constant (B = 6.173, p < 0.001), the standardized values of Age (B = 0.034, p = 0.001) and Sex (B = −0.004, p = 0.015). Thus, it was determined that the pain intensity in the first minute after the painful stimulus could be predicted using the mathematical expression: PAIN domains 1 min = Age * 0.034–Sex * 0.004. Table 11. Coefficients for PAIN domains 1 min Model

Unstandardized Coeff. B

Std. Error

Standardized Coeff. Beta

t

Sig.

95.0% Confidence Interval for B Lower Bound

(Constant)

6.173

1.152

Age

0.034

0.010

Sex

−0.004

0.002

a. Dependent Variable: Pain 1

Upper Bound

5.357

0.000

3.897

8.449

0.265

3.477

0.001

0.015

0.054

−0.188

−2.466

0.015

−0.007

−0.001

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The linear regression conditions for the residuals were met. The distribution was normal, and their scattering was random. 3.5 Model PAIN Domains 2 min The proportion of pain occurring in the second minute after the application of the stimulus is predicted by the constants indicated in Table 12 (Pain 1, Detachment, Age, Negative Affect); a model was created in which the calculated coefficient of determination (Adjusted R Squared) was 0.75. That means, approximately 75% of the dispersion of the variable of interest (pain score in the 2nd minute after the application of the painful stimulus) is explained by the created model. The sum of squares was 771,487 out of a possible 1017,840. Table 12. Model Summary, PAIN domains 2 min Model

R

R Square

Adjusted R Square

6

,871 f

0.758

0.752

f. Predictors: (Constant), Pain 1, Detachment, Age, Negative Affect g. Dependent Variable: Pain 2

The null hypothesis (none of the parameters included in the model can predict pain score) was not rejected (F = 118.21, p < 0.001), with the Fisher test being statistically significant. When developing the current model, the Backward method was used. The resulting model is illustrated in Table 13. It included as variables the constant (B = 0.167, p = 0.63) and the standardized values of Negative Affect (B = 0.506, p = 0.003), Detachment (B = −0.510, p = 0.026), Age (B = 0.022, p = 0.000), Pain 1 (B = 0.889, p = 0.000). Table 13. Coefficients for PAIN domains 2 min Model

Unstandardized Coeff. B

Std. Error

Standardized Coefficients Beta

95.0% Confidence Interval for B Lower Bound

Upper Bound

0.639

−0.535

0.870

3,030

0.003

0.176

0.836

−2.245

0.026

−0.959

0.157

3,692

0.000

0.010

0.034

0.806

19,217

0.000

0.797

0.980

0.167

0.356

Negative Affect

0.506

0.167

0.131

−0.510

0.227

−0.097

Age

0.022

0.006

Pain 1

0.889

0.046

a. Dependent Variable: Pain 2

Sig.

0.471

(Constant)

Detachment

t

−0.061

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Thus, it was determined that the pain score in the second minute after the painful stimulus could be predicted using the mathematical expression: PAIN domains 2 min = Negative Affect * 0.506 - Detachment * 0.510 + Age * 0.022 + Pain 1 * 0.889. The linear regression conditions for the residuals were respected in this case. The distribution was normal, and their scattering was random, without any collinearity. 3.6 Model PAIN Domains 3 min In Table 14, the coefficient of determination (Adjusted R Squared) was calculated to predict the pain score in the 3rd minute after the application of the stimulus. The sum of squares was 867,198 out of a possible 1109,724. Thus, a result of 0.777 was obtained; therefore, the model created explains approximately 78% of the dispersion of the variable of interest. Table 14. Model Summary, PAIN domains 3 min Model

R

R Square

Adjusted R Square

8

0.884 h

0.781

0.777

h. Predictors: (Constant), Pain 2, Psychoticism, Pain 1 i. Dependent Variable: Pain 3

The null hypothesis (none of the parameters included in the model can predict pain score) was not rejected (F = 181.16, p < 0.001), the Fisher test being statistically significant. To create the given model, the Backward method was used. The model is illustrated in Table 15. It included as variables the Constant (B = 1.259, p = 0.000) and the standardized values of Psychoticism (B = −0.323, p = 0.087), Pain 1 (B = −0.382, p = 0.000), Pain 2 (B = 1.190, p = 0.000). Thus, it was determined that the pain intensity in the third minute after the painful stimulus could be predicted using the mathematical expression: PAIN domains 3 min = - Psychoticism * 0.323 – Pain 1 * 0.382 + Pain 2 * 1.190. The linear regression conditions for the residuals were met. The distribution was normal, and their scattering was random. The analysis of the models regarding the predictors using 5 personality trait domains states that no predictors have been found for the pain score in the first minute; in the second minute, Negative Affect and Detachment, which are included in the Internalizing behavior, were found as predictors; this confirms the results presented previously about the predictors within 25 personality trait facets. Also, the Psychoticism category was found as a predictor of pain score in the third minute, with an inverse relationship, just like the predictors for this pain model found within 25 personality trait facets.

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Table 15. Coefficients for PAIN domains 3 min Model

Unstandardized Coeff. B

Std. Error

(Constant)

1,259

0.285

Psychoticism

−0.323

0.187

Pain 1

−0.382

0.083

Pain 2

1,190

0.076

Standardized Coeff Beta

t

Sig.

95.0% Confidence Interval for B Lower Bound

Upper Bound

4,418

0.000

0.696

1,822

−0.066

−1.725

0.087

−0.693

0.047

−0.332

−4.578

0.000

−0.547

−0.217

1,140

15,678

0.000

1,040

1,340

a. Dependent Variable: Pain 3

4 Conclusions After a complex analysis of all pain models which use personality trait facets and all pain models which use personality trait domains, we can say that the Internalizing behavior can predict the decrease of pain in the first and second minutes, and the Externalizing behavior, inversely, predicts the increase of pain in this time interval; Psychoticism serves as a predictor of pain score only in the 3rd minute. Although a multitude of sources report an altered pain perception in persons with BPD, we did not find as predictors of the pain score any personality trait included in BPD, excepting Impulsivity which is attributed to the Disinhibition domain, part of Externalizing behavior. The results of the study allow us to conclude that the modification of the personality profile can change the perception of pain; that would require an adjustment of the diagnosis algorithm and therapeutic approach in the disorders associated with pain. Acknowledgments. The research was carried out within the framework of the State program “The principles of 4p medicine (preventive, predictive, personalized and participatory) in the analysis of risk factors for debut, perpetuation and progression of chronic pain (4p4pain)”, Project No 20.80009.8007.01.

Conflict of Interest. The authors declare that they have no conflict of interest.

References 1. American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders (DSM–5). https://www.psychiatry.org/psychiatrists/practice/dsm 2. Anderson, J., Snider, S., Sellbom, M., Krueger, R., Hopwood, C.: A comparison of the DSM5 Section II and Section III personality disorder structures. Psychiatry Res. 216, 363–372 (2014). https://doi.org/10.1016/j.psychres.2014.01.007 3. Al-Dajani, N., Gralnick, T.M., Bagby, R.M.: A Psychometric review of the personality inventory for DSM-5 (PID-5): current status and future directions. J. Pers. Assess. 98(1), 62–81 (2016). https://doi.org/10.1080/00223891.2015.1107572

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4. Combaluzier, S., Gouvernet, B., Menant, F., Rezrazi, A.: Validation of a French translation of Krueger’s personality inventory for DSM-5 in its brief form (PID-5 BF). Encephale 44(1), 9–13 (2018). https://doi.org/10.1016/j.encep.2016.07.006 5. Soeteman, D.I., Timman, R., Trijsburg, R.W., Verheul, R., Busschbach, J.J.: Assessment of the burden of disease among inpatients in specialized units that provide psychotherapy. Psychiatr. Serv. 56, 1153–1155 (2005). https://doi.org/10.1176/appi.ps.56.9.1153 6. Winsper, C., et al.: The prevalence of personality disorders in the community: a global systematic review and meta-analysis. Br. J. Psychiatry. 216, 69–78 (2020). https://doi.org/10. 1192/bjp.2019.166 7. Newton-Howes, G., Tyrer, P., Anagnostakis, K., Cooper, S., Bowden-Jones, O., Weaver, T.: The prevalence of personality disorder, its comorbidity with mental state disorders, and its clinical significance in community mental health teams. Soc. Psychiatry Psychiatr. Epidemiol. 45, 453–460 (2010). https://doi.org/10.1007/s00127-009-0084-7 8. Ducasse, D., Courtet, P., Olié, E.: Physical and social pains in borderline disorder and neuroanatomical correlates: a systematic review. Curr. Psychiatry Rep. 16(5), 443 (2014). https:// doi.org/10.1007/s11920-014-0443-2 9. Srinivasa, N.R., et al.: The revised international association for the study of pain definition of pain: concepts, challenges, and compromises. Pain 161(9), 1976–1982 (2020). https://doi. org/10.1097/j.pain.0000000000001939 10. Biskin, R.S., Frankenburg, F.R., Fitzmaurice, G.M., Zanarini, M.C.: Pain in patients with borderline personality disorder. Personal Ment. Health 8(3), 218–227 (2014). https://doi.org/ 10.1002/pmh.1265 11. Schmahl, C., Baumgärtner, U.: Pain in borderline personality disorder. Mod. Trends Pharmacopsychiatry 30, 166–175 (2015). https://doi.org/10.1159/000435940 12. Carpenter, R.W., Trull, T.J.: The pain paradox: borderline personality disorder features, selfharm history, and the experience of pain. Personal Disord. 6(2), 141–151. (2015). https://doi. org/10.1037/per0000112 13. Schloss, N., et al.: Differential perception of sharp pain in patients with borderline personality disorder. Eur. J. Pain. 23(8), 1448–1463 (2019). https://doi.org/10.1002/ejp.1411 14. Pavony, M.T., Lenzenweger, M.F.: Somatosensory processing and borderline personality disorder: pain perception and a signal detection analysis of proprioception and exteroceptive sensitivity. Personal Disord. 5, 164–171 (2014). https://doi.org/10.1037/per0000017 15. Smith, G.M., Egbert, L.D., Markowitz, R.A., Mosteller, F., Beecher, H.K.: An experimental pain method sensitive to morphine in man: the submaximtum effort tourniquet technique. J. Pharmacol. Exp. Ther. 154(2), 324–332 (1966) 16. https://www.ibm.com/support/pages/downloading-ibm-spss-statistics-22

Towards Improved Assistive Inertial Positioning Solutions by Using Finely Tuned Wavelet Functions Ioana-Raluca Adochiei1,2,4(B) , Teodor Lucian Grigorie2 , Felix-Constantin Adochiei2 , Petre Negrea3 , Vidan Cristian3 , and Nicolae Jula4 1 Academy of Romanian Scientists, Bucharest, Romania

[email protected]

2 University Politehnica Bucharest, Bucharest, Romania 3 University of Craiova, Craiova, Romania 4 Military Technical Academy, Bucharest, Romania

Abstract. In this article, we discuss improving navigation accuracy by refining signals achieved from the inertial navigation system’s (INS) detection unit. The accuracy of navigation depends on inertial sensors to a large extent. However, their errors can cause interference with signals. Researchers have developed different calibration procedures to address this issue to integrate INS navigators with other navigators. Two types of errors exist - deterministic and stochastic. Sensor noise greatly affects navigation solution quality. Traditional noise reduction methods cannot directly filter noise in the navigation signal due to its frequency spectrum. An alternative option is to use wavelets to denoise signals from inertial sensors. Our methodology uses fine-tuned wavelet functions and the Directed Transfer Function approach to eliminate noise interference with the sensors’ signals. Reference signals obtained from Global Positioning Satellites (GPS) are utilized during the tuning process. We tested our technique by installing an INS navigator with a micro-electro-mechanical (M.E.M.S.) inertial measurement unit and a GPS navigator in a portable assistive device. We optimized the wavelet filters’ decomposition levels for each inertial sensor in the measurement unit by analyzing experimentally acquired data. This method can significantly impact various industries, including human assistive technologies, transport, and logistics. It can also be extended for indoor monitoring purposes. Keywords: inertial navigation system · tuned wavelet functions · assistive positioning

1 Introduction Navigation systems have become essential today, offering numerous advantages across various age groups and situations. They particularly benefit individuals with health issues and the elderly, compensating for declining functionality and preventing loss. These systems also assist family members and medical professionals monitor and respond to emergencies [1]. The original version of this chapter was revised: An acknowledgement has been revised. The correction to this chapter is available at https://doi.org/10.1007/978-3-031-42775-6_64 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024, corrected publication 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 597–606, 2024. https://doi.org/10.1007/978-3-031-42775-6_63

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For individuals who are advanced in age, personal security devices with integrated navigation systems provide security and guidance, boosting confidence, comfort, and mobility [1, 2]. While most personal navigation systems rely on GPS, it is widely acknowledged that GPS accuracy and integrity degrade in challenging conditions such as radio frequency interferences, jamming, and high dynamics, resulting in signal blockage and loss [3]. To overcome these challenges, INS/ GPS solutions are commonly used. However, the accuracy of INS systems is limited when satellite signals are absent due to inherent errors in inertial sensors [4, 5]. Furthermore, the miniaturization of inertial sensors using technologies like M.E.M.S. and N.E.M.S. (nano-electro-mechanical system) has compromised performance, revealing inadequacy for civil navigation applications [3]. Researchers are focused on developing low-cost, compact, and high-precision INS/ GPS navigators to address GPS limitations. These integrated navigators aim to overcome dependencies on models, prior knowledge, and linearization [4, 5, 7]. This work focuses on the testing of a new three-dimensional inertial navigator. It incorporates an algorithm that effectively reduces noise in miniaturized inertial sensors using a tuning method based on GPS-assisted wavelet transform. The experimental validation will assess the standalone INS positioning and monitoring performance using reference information obtained from an INS/ GPS integrated system.

2 Methodology We previously [3] conducted wavelet function tuning using data from a M.E.M.S. inertial measurement unit (IMU) and a GPS/ INS integrated navigator placed on a human subject in a portable assistive device. To improve the decomposition levels for the wavelet filters associated with each inertial sensor in the IMU, we utilized a software model for a strapdown inertial navigator that provided position data. The optimization subroutine, which employed a Direct Transfer Function (DTF) based algorithm, used this information and GPS data. As a result, we established a 3D strap-down inertial navigator mathematical model and designed software to implement it. Several steps were taken to address navigation issues with the inertial navigator [2, 6, 9, 10]. These included estimating attitude using the IMU gyro data, translating the IMU accelerometer data into the navigation frame, accounting for gravity effects on the translated accelerometer data to calculate acceleration components in the navigation frame, integrating these components to determine the speed in the navigation frame, and integrating differential equations related to positioning to obtain the actual global position in terms of latitude, longitude, and altitude [2, 6, 8–10]. As indicated in [2, 6, 9, 10], various reference frames were used to develop the navigation algorithm. These frames included the Inertial frame (0i X i Y i Z i ), which is centered on the Earth and does not rotate; the Earth-Centered-Earth-Fixed (ECEF) frame (0e X e Z e Y e ), which is fixed to the Earth; the Navigation frame - North-East-Down (NED) frame (0n X n Y n Z n ); and the Body (B) frame (0b X b Y b Z b ) [2, 6, 9, 10]. The position and speed of the subject being monitored can be obtained through numerical integration of the general equation of inertial navigation relative to the navigation frame. For assistive positioning, the North-East-Down (NED-0x l yl zl ) local horizontal frame can be used as the navigation frame due to its time-space characteristics.

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The initial linear speeds in the S-N (S towards North) NED trihedral are = −10 [m/s], S-East = −13.89 [m/s], and S-Down = −1.36 [m/s]. The initial position and speed were obtained directly from the GPS receiver, while the initial attitude angles were read from the GPS/INS integrated navigator: Ra (Roll angle) = −0.95°, Pa (Pitch angle) = + 4.51°, Y a (Yaw angle) = −119.57°. The data from the inertial sensors was collected at 50 samples per second. The proposed method’s analysis covered frequencies between 0 and 25 Hz, and the sensors underwent a basic bias correction process. The initial values of the North and East coordinates in the NED system were considered null. The data acquired from the GPS receiver of the integrated GPS/ INS navigator were used as reference data in evaluating the performance of the proposed method. A sample of the subject’s trajectory registered by an accelerometer on the x-axis is shown in Fig. 1, a., while its time evolution recorded using a gyro, also mounted on the IMU’s x-axis, is illustrated in Fig. 2, b.

Fig. 1. Sample visualizations of signals registered by an accelerometer and a gyro mounted on the “x” axis of the tridimensional inertial navigation system.

In contrast to a previously bi-dimensional navigation case of testing [..], where the wavelet de-noising level of decomposition was adjusted in two stages (firstly on the attitude channel and secondly on the position channel), the research team has decided to tune all six detection channels in this case simultaneously. The tuning process aimed to assess the correlations on the “North”, “East”, and “Down” position channels for different combinations of decomposition level values for the wavelet filters on the six detection channels (3 accelerometers and 3 gyros). Due to many potential test combinations, it was determined that the optimization software subroutine would run for values between the 3rd and 7th decomposition levels for all six detection channels. As a result, the algorithm included six “for” cycles, leading to a total of 56 = 15625 evaluation cases. To build the mathematical model of the inertial navigator, the Matlab/Simulink platform we used. The projected model receives data from the 3 accelerometers and 3 gyroscopes of the IMU and generates various outputs, including accelerations, attitude angles (roll, pitch, and yaw), the speed’s components in the navigation frame (N-d north, E_d east, and D-d down directions), position in the north, east, and down directions (Rx_d, Ry_d, Rz_d), and the global coordinate’s (La_d latitude, Lo-d longitude, and h-d altitude). Figure 2 displays the resulting Matlab/ Simulink model of the inertial navigator.

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Fig. 2. The inertial navigator MATLAB/ Simulink model.

3 Wavelet Fine-Tuning Method Basic Principle The wavelet method has been extensively studied in INS/ GPS systems navigation research, with various combinations like genetic algorithms and artificial neural networks. Recently, we validated a two-dimensional assistive positioning system that used inertial techniques and wavelet denoising [3]. We have developed an optimization tuning method called the coupling level (CL) to determine the optimal decomposition level (WoptLvl) for the Wavelet-based on the current decomposition level (WactualLvl) [3]. The logic is as follows: if the previous coupling level (CL) value is lower than the current one, we set the wavelet decomposition level to the previous CL value and add 1. However, if the previous value equals or exceeds the current one, the optimal decomposition level remains the same as the previous CL value [18]. To fine-tune wavelet filters, it is necessary to calculate the CL values for each positioning channel offline. Each sensor has a unique noise signature due to its internal structure. Once we have the CL parameter values for each positioning channel, we can establish the optimized decomposition level for each detection channel to fine-tune the wavelet filters. This approach is beneficial when adjusting wavelet filters for different Inertial Measurement Units (IMUs), especially when using a more accurate positioning system like DGPS or military GPS as the reference navigator. The offline tuning process helps determine the optimal decomposition levels, improving the positioning systems’ precision. We are expanding this methodology to tune data from six detection channels, including accelerometers and gyroscopes.

4 Experimental Testing of Navigator-Based Wavelet Denoising Improvement and Discussions Our study aimed to assess the effectiveness of the tuning method by analyzing data obtained from M.E.M.S. sensors. Specifically, we examined data related to the position and speed of a human subject along the NED axes. We explored various combinations of decomposition levels for all sensor detection channels to determine the best method. We used input data from a M.E.M.S./ IMU inertial detection unit and a GPS/ INS integrated navigator, both worn as mobile systems. These were experimentally obtained

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and applied to the inertial navigator MATLAB/ Simulink model, as shown in Fig. 3. We selected a representative evaluation sample from a more extended test sequence, illustrated in Fig. 3, and the initial parameter values were provided by the GPS/INS navigator, including the latitude, longitude, and altitude measurements of 44.345973390, 23.87373770, and 221.56 m, respectively. We ran an optimization software subroutine for values corresponding from the 3rd level of decomposition to the 7th level of decomposition. This led to using six combined “for” cycles, which provided, as previously mentioned, 56 = 15625 evaluation cases. We use “CL_N” to refer to the degree of coupling between the reference location in the North direction and the resulting position in the North direction from the filtered inertial input. The analysis explored the evaluation outcomes for different scenarios on the NED axis. We identified that each channel has an optimal combination of filters for the three sensors in the inertial detection unit by examining the three maximum CL values. Our numerical evaluations in various situations showed that even a slight coupling variation in the East direction could significantly impact errors. However, the coupling level on the vertical channel has minimal influence on position errors. It is of utmost importance to prioritize the synchronization of coupling levels in both the North and East directions, particularly in areas where their values are at their highest.

Fig. 3. Representative coupling level evaluation sample for the position channels

Figure 4 shows details for index 6422, while Fig. 5 is for index 7047. Based on the numerical evaluation’s observations regarding the impact of CL_E value, it was anticipated that index 6422 would produce better-filtering results than index 7047. The wavelet filtering decomposition levels for 6422 were levRX = 5, levRY = 3, levRZ = 4, levAX = 4, levAY = 7, and levAZ = 4, while for index 7047 were levRX = 5, levRY = 4, levRZ = 4, levAX = 4, levAY = 7, and levAZ = 4.

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Fig. 4. CL representation for the area around the combination index 6422.

Fig. 5. CL representation for the area around the combination index 7047.

The results of inertial sensor data filtering for the combinations corresponding to indexes 6422 and 7047 are presented in Fig. 6. Below are the absolute maximum values of the deviation between the inertial navigator solutions and the references of position and speed for combinations 6422 and 7047, as shown in Table 1. Based on the data, it was found that the combination with index 6422 is the best option compared to the combination with index 7047. Filtered versus unfiltered signals evolution for the 6422 index is represented in Fig. 7. Filtered navigation solution errors for both options are similar. However, the combination with index 6422 shows a decrease of approximately 1.65 times in the maximum deviation from the reference values for positioning in the North direction, 1.32 times for the East direction, 1.15 times for the

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Fig. 6. Coupling level comparison for the indexes 6422 and 7047.

Table 1. Absolute values of deviations for 6422 index Deviations

INS unfiltered

Index 6422 INS filtered

Ratio

88.3842

53.3588

1.6564

East [m]

112.6123

84.9501

1.3256

Down [m]

33.8312

29.3532

1.1525

Speed N [m/s]

2.5075

1.8117

1.3840

Speed E [m/s]

1.7297

1.3576

1.2740

Speed D [m/s]

1.0037

0.9586

1.0469

Latitude [deg]

7.9540·10–4

4.8019·10–4

1.6564

Longitude [deg]

14.1226·10–4

10.6535·10–4

1.3256

Altitude [m]

33.8312

29.3532

1.1525

North [m]

vertical channel, 1.38 times for the speed in the North direction, 1.27 times for the speed in the East direction, and 1.04 times for the speed in the vertical channel. The graphical results describing the final coupling level between the reference navigation solution and the inertial navigator solution after the wavelet filtering of the six channels input data (with the decomposition levels afferent to the combination with the index 6422: levRX = 5, levRY = 3, levRZ = 4, levAX = 4, levAY = 7, levAZ = 4) are shown in Fig. 8 to Fig. 10. Figure 8 exposes the results for the North, East, and Down positioning channels, the equivalent coupling levels having the following values: CL_N = 0.6985, CL_E = 0.6792, and CL_D = 0.5206. The results for the speed components on the NED frame are shown in Fig. 9, the equivalent coupling levels being evaluated as follows: CL_VN = 0.1439, CL_VE = 0.1192, CL_VD = 0.0643. The low values of

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Fig. 7. Filtered versus unfiltered signals evolution for the 6422 index.

the coupling levels between the speed components provided by the INS and the reference ones can be explained by the fact that the inertial method provides speed due to the numerical integration of the acceleration. At the same time, the GPS realizes a numerical derivation of the position to determine the speed. These procedures lead to obtaining different spectral components in the speed signals of the two systems. Figure 10 presents the evaluation results of the coupling levels in latitude, longitude, and altitude channels; the medium values are CL_La = 0.7061, CL_Lo = 0.7071, and CL_h = 0.5206.

Fig. 8. GPS-filtered INS coupling levels for the North, East, and Down positioning channels.

Fig. 9. GPS-filtered INS coupling levels for the North, East, and Down speed channels.

From the data (Table 1) and Fig. 11, it is easy to observe the performance of the proposed filtering solution. The deviations between the curves that characterize the

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Fig. 10. GPS-filtered INS coupling levels for the latitude, longitude, and altitude channels.

navigation solution’s reference values and the curves that result from the navigation solution of the inertial navigator were substantially reduced.

Fig. 11. Global positioning solution for reference navigator, unfiltered and filtered INSs.

5 Conclusions Our research focused on evaluating a tuning algorithm that utilizes the Directed Transfer Function to enhance the wavelet filters used for de-noising inertial sensors in 3D navigation applications. We employed the Directed Transfer Function to estimate the optimal decomposition levels of filters on six detection channels. We proposed a calculus to adapt the DTF method by determining common information between GPS and INS results. The algorithm tunes the filter offline using experimental data, and once optimal levels are established, they can be used for real-time navigation. We demonstrated the efficacy of this mechanism by using a GPS/ INS integrated navigator as a reference. Still, it can also be used with various IMUs and more precise positioning systems. Our proposed methodology utilizes advanced signal processing techniques and offers a unique approach to optimize inertial navigation systems, ultimately improving their accuracy and reliability. Acknowledgments. This work was supported by Romanian Space Agency (ROSA), project STAR, No. 27/19.11.2012, “High-precision micro and nano smart sensors for space inertial navigation applications”, code 168/2012.

Conflict of Interest.

The Authors Declare that They Have no Conflict of Interest.

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References 1. MIT AgeLab: New transport technology for older people, An OECD – MIT International Symposium, Cambridge, Massachusetts, 23–24 September (2003) 2. Krieg-Bruckner, B., et al.: Navigation aid for mobility assistants. In: Proceedings of the Joint CEWIT-TZIacatech Workshop “ICT meets Medicine and Health” ICTMH (2013) 3. Grigorie, T., Negrea, P., Edu, I., Adochiei, F.: Assistive positioning system based inertial techniques and wavelet denoising. In: Proceedings of the 8th ACM International Conference on Pervasive Technologies Related to Assistive Environments (2015). https://doi.org/10.1145/ 2769493.2769504 4. Mohinder, S., Lawrence, R., Angus, P.: Global Positioning Systems, Inertial Navigation, and Integration. Wiley (2001). ISBN 9780471463863 5. Grigorie, T.L., Botez, R.M.: Modelling and simulation-based Matlab/Simulink of a strapdown inertial navigation system’ errors due to the inertial sensors. In: Matlab Applications for the Practical Engineer, pp. 305–338. InTech (2014). https://doi.org/10.5772/57583 6. Grigorie, T.L., Edu, I.R.: Inertial navigation applications with miniaturized sensors. SITECH, Craiova, Romania (2013) 7. Vanicek, P., Omerbasic, M.: Does a navigation algorithm have to use Kalman Filter. Can. Aeronaut. Space J. 45, 1–9 (1999). ISSN 0008-2821 8. Hasan, A.M., et al.: Comparative study on wavelet filter and thresholding selection for GPS/INS data fusion. Int. J. Wavelets Multiresolut. Inf. Process. 08(03), (2010). https://doi. org/10.1142/S0219691310003572 9. Hasan, A.M., Samsudin, K., Ramli, A.R.: Intelligently tuned wavelet parameters for GPS/INS error estimation. Int. J. Autom. Comput. 8(4), 411–420 (2011). https://doi.org/10.1007/s11 633-011-0598-9 10. Noureldin, A., Osman, A., El-Sheimy, N.: A neuro-wavelet method for multi-sensor system integration for vehicular navigation. Meas. Sci. Technol. 15, 404–412 (2004). https://doi.org/ 10.1088/0957-0233/15/2/013

Correction to: Towards Improved Assistive Inertial Positioning Solutions by Using Finely Tuned Wavelet Functions Ioana-Raluca Adochiei , Teodor Lucian Grigorie , Felix-Constantin Adochiei , Petre Negrea , Vidan Cristian and Nicolae Jula

,

Correction to: Chapter “Towards Improved Assistive Inertial Positioning Solutions by Using Finely Tuned Wavelet Functions” in: V. Sontea et al. (Eds.): 6th International Conference on Nanotechnologies and Biomedical Engineering, IFMBE Proceedings 91, https://doi.org/10.1007/978-3-031-42775-6_63 In the original version of the book following belated correction has been incorporated: An acknowledgement has been revised in Chapter “Towards Improved Assistive Inertial Positioning Solutions by Using Finely Tuned Wavelet Functions”. The correction chapter and the book has been updated with the changes.

The updated original version of this chapter can be found at https://doi.org/10.1007/978-3-031-42775-6_63 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, p. C1, 2024. https://doi.org/10.1007/978-3-031-42775-6_64

Author Index

A Abashkin, Vladimir 60 Abaskin, Vladimir 156 Achimova, Elena 60, 156 Adelung, Rainer 70, 197, 231, 284 Adochiei, Felix-Constantin 597 Adochiei, Ioana-Raluca 597 Aktas, Cenk 41, 231 Al Maamori, Mohammed 305, 313 Andronic, Silvia 88 Arama, Efim 191 Arnaut, Oleg 582 Avram, Andrei 278 Ayvazyan, Gagik 3, 12

Capro¸s, Nicolae 357 Caraman, Mihail 231 Cemortan, Igor 564 Cepoi, Liliana 366, 397, 447 Chakraborty, Barnika 70 Chernica, Ion 223 Chibac-Scutaru, Andreea Laura 324 Chiriac, Tatiana 366, 397, 447 Chornopyshchuk, Roman 564 Ciobanu, Vladimir 197, 284, 348 Ciubotaru, Anatol 386 Cociug, Adrian 348 Cojocaru, Florina-Daniela 407, 417, 427 Cojocaru, Sergiu 182 Condrea, Elena 574 Coretchi, Eugeniu 582 Coseri, Sergiu 324 Creanga, Dorina 123 Cristea, Ecaterina 166 Cristian, Vidan 597 Curocichin, Ghenadie 528

B Baca, Svetlana G. 80 Badan, Liliana 564 Balan, Vera 407, 417 Balmus, Ion 88 Barba, Alic 257 Belotercovschii, Igori 574 Besleaga, Tudor 582 Biliuta, Gabriela 324 Bîrnaz, Adrian 22 Bologa, Mircea 223, 514 Botnari, Vladislav 60, 156 Braniste, Tudor 243 Braun, Barbu Cristian 479 Bulimestru, Ion 437 Bulmaga, Petru 437 Busuioc, Simon 268 Butnaru, Maria 427 Buza, Anastasia 528 Buzdugan, Artur 22, 496

D Dashtoyan, Harutyun 3 Datsko, Tatiana 134 Dinescu, Adrian 278 Dit, u, Iustina-Petronela 427 Dodi, Gianina 417 Dogot, Marta 528 Donin, Gleb 467 Donos, Ala 554 Dragoman, Daniela 278 Dragoman, Mircea 278 Drug˘a, Corneliu-Nicolae 479 Dvornikov, Dmitri 134

C Calistru, Anca Elena 417 Capcelea, Svetlana 564 Capros, Natalia 528

E Enachescu, Marius 284 Eremciuc, Rodica 487 Evseeva, Maria 31

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 V. Sontea et al. (Eds.): ICNBME 2023, IFMBE Proceedings 91, pp. 607–609, 2024. https://doi.org/10.1007/978-3-031-42775-6

608

Author Index

G Gagara, Ludmila 206 Gaidarji, Olga 487 Galea-Abdusa, Daniela 528 Ganenco, Andrei 582 Gardikiotis, Ioannis 417 Ghimpu, Lidia 206, 214 Ghinda, Serghei 397 Goremichin, Vladimir 249 Grabco, Daria 96 Grabovschi, Ion 582 Grib, Andrei 528 Grigorie, Teodor Lucian 597 Grigoriev, Eugeniu 174 Grozdov, Dmitrii 366 Gurbuz, Havva Nur 249 Gutu, Iacob 257 H Hovhannisyan, Ashkhen Huber, Tito 293

537, 546

I Iaseniuc, Oxana 147 Iavorschi, Constantin 376 Iliev, Albina-Mihaela 554 Iovu, Mihail 147 Ipekci, Hasan H. 249 Ivanes, Igor 376 J Jula, Nicolae

597

K Kareem, Ali 313 Kazaryan, Shushanik 546 Khachatryan, Mane 12 Khudaverdyan, Ashot 12 Khudaverdyan, Surik 3, 12 Koç, Erkan 305 Konopko, Leonid 293 Kravtsov, Victor Ch. 80 Kulcitkaia, Stela 376 Kulyuk, Leonid 249 L Lasl˘au, Simina-Andreea Lesnic, Evelina 397 Litra, Dinu 41

427

Los, manschii, Constantin 60, 156 Lozovanu, Svetlana 582 Luca, Andreea 407 Lungu, Ion 206, 214, 268, 437 Lupan, Cristian 22, 41, 70 Lupan, Oleg 22, 41, 70, 231 M Macaev, Fliur 564 Macagonova, Olga 348 Magariu, Nicolae 22, 70 Majdi, Hasan Sh. 305, 313 Malai, Dominic 41 Malcova, Tatiana 386 Martyniuk, Volodymyr 31 Ma¸snic, Alisa 106 Matevosyan, Lenrik 3 Mesalchin, Alexei 156 Meshalkin, Alexei 60 Mihai, Geanina 284 Miscu, Vera 447 Moldovan, Cristina 457 Moldovanu, Ion 582 Monaico, Eduard V. 197, 284 Monaico, Elena I. 268 Morari, Vadim 52 Mothejlova, Karla 467 Motrescu, Iuliana 123 N Nacu, Isabella 427 Nacu, Viorel 332, 348, 357, 386 Negrea, Petre 597 Nikitina, Irina 487 Nikolaeva, Albina 293 O Ohanyan, Seda 537 Ojog, Victor 582 Osadchuk, Oleksandr

31

P Padunnappattu, Ajay 70 Paladii, Irina 514 Pavlovschi, Elena 332, 357 Pedrini, Giancarlo 60 Pîntea, Valentina 191 Pisarenco, Nadejda 376 Plesnicute, Ramona Mirela 123

Author Index

609

Podgornii, Daniel 80 Policarpov, Albert 514 Popusoi, Ana 437 Poschmann, Mirjam P. M. 70 Potlog, Tamara 206, 214, 257, 268, 437 Prisacar, Alexandr 156 Pyrtsac, Constantin 96 Q Qiu, Haoyi

41, 231

R Reimers, Armin 197, 284 Revenco, Ninel 487 Robu, Stefan 437 Rojnoveanu, Gheorghe 386 Rotaru, Aurelian 417 Rotaru, Ludmila 564 Rshtuni, Lilit 537 Rudi, Ludmila 366, 397, 447 Rudic, Valeriu 366 Rusu, Daniela 52 Rusu, Emil V. 52 Rusu, Spiridon 174 S Sanduleac, Ionel 88 Saud, A. Najah 305, 313 Schadte, Philipp 70 Schütt, Fabian 70 Semenov, Andriy 31 Semenova, Olena 31 S, erban, Ionel 479 Sereacov, Alexandr 41, 231 Shemyakova, Tatiana 191 Shikimaka, Olga 96 Shova, Sergiu 80 Sibechi, Maria-Gabriela 427 Sidorenko, Anatolie 574 Sidorenko, Irina 564 Sidorenko, Ludmila 564 Siebert, Leonard 41, 70 Siminel, Nikita 249 Siminiuc, Rodica 505 Siminiuc, Sergiu 505 Simion, Andreea 407 Sirbu, Tamara 457 Smyslov, Vladimir 574

Sprincean, Catalina 514 Sprincean, Veaceslav 231 Stamov, Ivan 115, 166 Stepurina, Tatiana 514 Stock, Norbert 70 Stoian, Alina 332, 357 Suman, Victor 214, 257 Svobodova, Romana 467 T T, aralunga, Tatiana 348 Tiginyanu, Ion M. 52, 197, 243, 284 Timotin, Ina 582 T, islinscaia, Natalia 514 Tjardts, Tim 41, 231 Tkachenko, Dmitry 115 Toader, Claudia Valentina 417 Tronciu, Vasile 174 Tulic˘a, Alexandru-Constantin 479 Tumoyan, Juleta 546 Turcan, Olga 457 T, urcanu, Dinu 505 U Untila, Dumitru 214 Ursaki, Veaceslav V. 52, 106, 197, 284 Uzunoglu, Aytekin 249 V Vacariu, Anamaria 123 Vaseashta, Ashok 3, 12 Vataman, Eleonora 528 Verega, Grigore 332, 357 Verestiuc, Liliana 407, 417, 427 Vrabie, Elvira 514 Vrabie, Valeria 514 W Wessel, Niels

564

Y Yushchenko, Tetyana 31 Z Zadorojneac, Tudor 41, 70 Zalamai, Victor V. 106, 166, 197, 284 Zelentsov, Veacheslav 134 Zinicovscaia, Inga 366