Quantum Dots and Polymer Nanocomposites: Synthesis, Chemistry, and Applications 9781032210148

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Quantum Dots and Polymer Nanocomposites Quantum Dots and Polymer Nanocomposites: Synthesis, Chemistry, and Applications reviews the properties, fabrication, and current and potential users of quantum dots-based polymer composites. It offers a much-needed update on the essential components of polymer nanocomposites by exploring the synthesis, processing, classification, characterization, and applications of quantum dots. Topics include modern fabrication technologies, processing, nanostructure formation, and the mechanisms of reinforcement. This book also covers biocompatibility, suitability, and toxic effects of quantum dots-based polymer nanocomposites. Applications such as biomedical, pollution mitigation, sensors, and catalysis are explored, as are opportunities and future research directions. This edited book acts as a one-stop reference book for researchers, academics, advanced students, and scientists studying epoxy blends. It will be of interest to materials scientists, polymer technologists, nanotechnologists, chemical engineers, physicists (optics, plasmonics), chemists, and mechanical engineers, among others.

Quantum Dots and Polymer Nanocomposites Synthesis, Chemistry, and Applications

Edited by

Sayan Ganguly, Poushali Das, and Jyotishkumar Parameswaranpillai

First edition published 2023 by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742 and by CRC Press 4 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN CRC Press is an imprint of Taylor & Francis Group, LLC © 2023 selection and editorial matter, Sayan Ganguly, Poushali Das, Jyotishkumar Parameswaranpillai; individual chapters, the contributors Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www. copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. For works that are not available on CCC please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. ISBN: 978-1-032-21014-8 (hbk) ISBN: 978-1-032-21050-6 (pbk) ISBN: 978-1-003-26651-8 (ebk) DOI: 10.1201/9781003266518 Typeset in Times by MPS Limited, Dehradun

Contents Editor Biographies....................................................................................................ix List of Contributors ..................................................................................................xi

Chapter 1

Introduction to Quantum Dots and Their Polymer Composites........1 Jyotishkumar Parameswaranpillai, Poushali Das, and Sayan Ganguly

Chapter 2

What Are Quantum Dots?................................................................. 21 Deepak Kumar Jarwal, Chandan Kumar, Kamalaksha Baral, and Anuradha Bera

Chapter 3

Synthesis of Quantum Dots .............................................................. 45 Sourav Paul and Uttam Kumar Ghorai

Chapter 4

Optical Properties of Quantum Dots ................................................69 Poushali Das, Syed Rahin Ahmed, Seshasai Srinivasan, and Amin Reza Rajabzadeh

Chapter 5

Surface Properties of Quantum Dots................................................87 Poslet Shumbula, Bambesiwe May, and Mokae Bambo

Chapter 6

Impact of Doping on Efficiency of Quantum Dots ....................... 105 Shibam Das, Rohit Bhatia, and Bhupinder Kumar

Chapter 7

Fabrication Methods of Quantum Dot–Polymer Composites........125 Mokae Bambo, Bambesiwe May, and Poslet Shumbula

Chapter 8

Reinforcement Mechanisms of Quantum Dots–Polymer Composites ......................................................................................151 Sayan Ganguly

Chapter 9

Quantum Dots Modified Thermoplastic and Thermosetting Plastic Composites.................................................. 171 Niranjan Patra and Malvika Shukla

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Contents

Chapter 10 Quantum Dots–Rubber Composites................................................189 Sayan Ganguly Chapter 11 Biomedical Applications of Quantum Dot–Polymer Composites ......................................................................................207 Fouad Damiri, Yahya Bachra, and Mohammed Berrada Chapter 12 Quantum Dot–Polymer Composites as Sensors .............................227 Pradip Kumar Sukul, Monalisa Mukherjee, and Chirantan Kar Chapter 13 Quantum Dot–Polymer Composites in Light-Emitting Diode Applications.......................................................................... 259 Madhusudan B. Kulkarni, Kishore Upadhyaya, N.H. Ayachit, and Nalini Iyer Chapter 14 Quantum Dot–Polymer Composites in Catalytic Applications ..................................................................................... 281 Tushar Kanti Das Chapter 15 Synthesis and Applications of Polymer–Quantum Dots Gels .........................................................................................299 Satyabrata Si, Smrutirekha Mishra, and Priti Sundar Mohanty Chapter 16 Biocompatibility of Polymer–Quantum Dot Composite................335 Sazzadur Rahman, Gitanjali Majumdar, and Devasish Chowdhury Chapter 17 Photoluminescence Property of Quantum Dots in Polymer Matrices ............................................................................359 Ankita Saha and Rama Ranjan Bhattacharjee Chapter 18 Environmental Impact of Quantum Dots and Their Polymer Composites........................................................................ 377 Snehlata Katheria, Daniel Amoako Darko, Aseel A. Kadhem, Prachi Parmar Nimje, Bhawana Jain, and Reena Rawat

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Chapter 19 Quantum Dots and Their Polymer Composites for Supercapacitor Applications ............................................................395 Himadri Tanaya Das, Swapnamoy Dutta, Payaswini Das, and Nigamananda Das Chapter 20 Polymer Composites: Processing, Safety, and Disposal ................413 Tanima Bhattacharya, Zhaoquan Ai, Hitesh Chopra, and H.C. Ananda Murthy Index ......................................................................................................................441

Editor Biographies Dr. Jyotishkumar Parameswaranpillai is currently an associate professor at Alliance University, Bangalore. He has research experience in various international laboratories such as Leibniz Institute of Polymer Research Dresden (IPF), Germany; Catholic University of Leuven, Belgium; University of Potsdam, Germany; and King Mongkut’s University of Technology North Bangkok (KMUTNB), Thailand. He has published around 130 papers in high-quality international peer-reviewed journals, 70 book chapters, and has edited 25 books. Recently, he was named in the world’s top 2% of the most-cited scientists in Single Year Citation Impact 2020, by Stanford University. Dr. Poushali Das is a postdoctoral researcher in the School of Biomedical Engineering, Faculty of Engineering, McMaster University, Hamilton, Ontario, Canada. Previously, she worked as a senior postdoctoral research fellow at the Department of Chemistry, Bar-Ilan University, Ramat Gan, Israel. She received a PhD degree in 2019 from the Indian Institute of Technology Kharagpur, India. She has more than 50 research publications in reputed international journals. She has been serving as a topic editorial board member and reviewer of reputed journals and an international consultant. She has presented papers in many international conferences. Her research interests include multifunctional luminescent nanoparticles and their application in sensors, antioxidant property and biomedical field, polymer/quantum dot nanocomposites, sonochemical synthesis of graphenebased nanocomposites, and MXene composites. Dr. Sayan Ganguly is a senior postdoctoral researcher at Bar-Ilan University, Israel. He received his PhD from the Indian Institute of Technology, Kharagpur. He obtained his BSc degree in chemistry (honours) from Ramakrishna Mission Vidyamandira, Belur Math, University of Calcutta; and then his BTech from University of Calcutta. After that he then went on to obtain his MTech from the University of Calcutta in polymer science and technology. His primary research interests include superabsorbent hydrogels, fluorescence in hydrogels, nanomaterials in hydrogels, and hydrogels in the biomedical fields. He has published more than 80 papers and chapters in international journals and books.

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Contributors Syed Rahin Ahmed School of Engineering Practice and Technology McMaster University Hamilton, Ontario, Canada Zhaoquan Ai Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials College of Chemistry & Chemical Engineering Hubei University Wuhan, China N.H. Ayachit School of Advanced Sciences KLE Technological University Hubballi, Karnataka, India and Visvesvaraya, Technological University (VTU) Belagavi, Karnataka, India Yahya Bachra University Hassan II of Casablanca Faculty of Sciences Ben M’sick Department of Chemistry Laboratory of Biomolecules and Organic Synthesis (BIOSYNTHO) Casablanca, Morocco Mokae Bambo DSI/Mintek Nanotechnology Innovation Centre Advanced Materials Division Mintek Randburg, South Africa Kamalaksha Baral VR Siddhartha College of Engineering Vijayawada, Andhra Pradesh, India

Anuradha Bera Defence Laboratory, DRDO Jodhpur, Rajasthan, India Mohammed Berrada University Hassan II of Casablanca Faculty of Sciences Ben M’sick Department of Chemistry Laboratory of Biomolecules and Organic Synthesis (BIOSYNTHO) Casablanca, Morocco Rohit Bhatia Department of Pharmaceutical Chemistry ISF College of Pharmacy Moga, Punjab, India Rama Ranjan Bhattacharjee Amity University Newtown Kolkata, West Bengal, India Tanima Bhattacharya Innovation, Incubation, Industry(i-cube) Laboratory, Techno India NJR Institute of Technology Udaipur, Rajasthan, India and Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials, College of Chemistry & Chemical Engineering Hubei University Wuhan, China Hitesh Chopra Chitkara College of Pharmacy Chitkara University Rajpura, Punjab, India

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Devasish Chowdhury Material Nanochemistry Laboratory Physical Sciences Division Institute of Advanced Study in Science and Technology Guwahati, Assam, India Fouad Damiri University Hassan II of Casablanca Faculty of Sciences Ben M’sick Department of Chemistry Laboratory of Biomolecules and Organic Synthesis (BIOSYNTHO) Casablanca, Morocco Daniel Amoako Darko Institute for Environment and Sanitation Studies University of Ghana Legon, Accra, Ghana Himadri Tanaya Das Centre of Excellence for Advanced Materials and Applications Utkal University Bhubaneswar, Odisha, India Nigamananda Das Department of Chemistry Utkal University Bhubaneswar, Odisha, India Payaswini Das CSIR-Institute of Minerals and Materials Technology Bhubaneswar, Odisha, India Poushali Das School of Biomedical Engineering McMaster University Hamilton, Ontario, Canada Shibam Das Department of Pharmaceutical Analysis ISF College of Pharmacy Moga, Punjab, India

Contributors

Tushar Kanti Das Institute of Physics – Center for Science and Education Silesian University of Technology Krasińskiego, Katowice, Poland Swapnamoy Dutta Department of Chemistry Utkal University Bhubaneswar, Odisha, India Sayan Ganguly Bar-Ilan Institute for Nanotechnology and Advanced Materials Department of Chemistry Bar-Ilan University Ramat-Gan, Israel Uttam Kumar Ghorai Department of Industrial and Applied Chemistry Swami Vivekananda Research Centre Ramakrishna Mission Vidyamandira Kolkata, West Bengal, India Nalini Iyer School of Electronics and Communication Engineering KLE Technological University Vidyanagar, Karnataka, India Bhawana Jain Department of Chemistry Govt. V.Y.T. PG. Autonomous College Durg, Chhattisgarh, India Deepak Kumar Jarwal Defence Laboratory, DRDO Jodhpur, Rajasthan, India Aseel A. Kadhem Iraqi Ministry of Education/Wasit Education Directorate Wasit, Kut City, Alley, Iraq

Contributors

xiii

Chirantan Kar Amity Institute of Applied Sciences Amity University Kolkata, India

Priti Sundar Mohanty School of Chemical Technology KIIT Deemed to be University Bhubaneswar, India

Snehlata Katheria Department of Chemistry University of Lucknow Lucknow, Uttar Pradesh, India

Monalisa Mukherjee Amity Institute of Click Chemistry Research and Studies Amity University Noida, Uttar Pradesh, India

Madhusudan B. Kulkarni School of Electronics and Communication Engineering KLE Technological University Vidyanagar, Karnataka, India Bhupinder Kumar Department of Pharmaceutical Chemistry ISF College of Pharmacy Moga, Punjab, India Chandan Kumar IIT Bombay Mumbai, India Gitanjali Majumdar Department of Chemistry Assam Engineering College Jalukbari, Guwahati, Assam, India Bambesiwe May Analytical Chemistry Division Mintek Randburg, South Africa and Institute for Nanotechnology and Water Sustainability (iNanoWS) College of Science, Engineering and Technology University of South Africa Roodeport, Johannesburg, South Africa Smrutirekha Mishra School of Chemical Technology KIIT Deemed to be University Bhubaneswar, India

H.C. Ananda Murthy Department of Applied Chemistry School of Applied Natural Science Adama Science and Technology University Adama, Ethiopia Prachi Parmar Nimje Department of Chemistry Shrishankaracharya Professional University Bhilai Junwani, Bhilai, Chhattisgarh, India Jyotishkumar Parameswaranpillai Department of Science Faculty of Science & Technology Alliance University Bengaluru, Karnataka, India Niranjan Patra Department of Engineering Chemistry College of Engineering Koneru Lakshmaiah Education Foundation Vaddeswaram, Andhra Pradesh, India and Department of Biotechnology and Bioengineering Institute of Advanced Research Koba Institutional area Gandhinagar, Gujarat, India

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Sourav Paul Department of Industrial and Applied Chemistry Swami Vivekananda Research Centre Ramakrishna, Mission Vidyamandira Kolkata, West Bengal, India Sazzadur Rahman Material Nanochemistry Laboratory Physical Sciences Division Institute of Advanced Study in Science and Technology Guwahati, Assam, India Amin Reza Rajabzadeh School of Biomedical Engineering McMaster University Hamilton, Ontario, Canada and School of Engineering Practice and Technology McMaster University Hamilton, Ontario, Canada Reena Rawat Department of Chemistry Echelon Institute of Technology Faridabad, Haryana, India Ankita Saha Amity University Newtown Kolkata, West Bengal, India Malvika Shukla Department of Biotechnology and Bioengineering Institute of Advanced Research Koba Institutional area Gandhinagar, Gujarat, India

Contributors

Poslet Shumbula Department of Chemistry University of Limpopo Sovenga, South Africa Satyabrata Si School of Chemical Technology KIIT Deemed to be University Bhubaneswar, India Seshasai Srinivasan School of Biomedical Engineering McMaster University Hamilton, Ontario, Canada and School of Engineering Practice and Technology McMaster University Hamilton, Ontario, Canada Pradip Kumar Sukul Amity Institute of Applied Sciences Amity University Kolkata, India Kishore Upadhyaya School of Advanced Sciences KLE Technological University Hubballi, Karnataka, India

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Introduction to Quantum Dots and Their Polymer Composites Jyotishkumar Parameswaranpillai Department of Science, Faculty of Science & Technology, Alliance University, Bengaluru, Karnataka, India

Poushali Das School of Biomedical Engineering, McMaster University, Hamilton, Ontario, Canada

Sayan Ganguly Bar-Ilan Institute for Nanotechnology and Advanced Materials, Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel

CONTENTS 1.1 1.2 1.3 1.4 1.5 1.6

Introduction.......................................................................................................1 Synthesis Overview of Quantum Dots ............................................................2 Optical Properties of QDs................................................................................5 Upconversion Optical Characteristics..............................................................6 Polymer-Based Bioactivation of QDs .............................................................7 Applications of QD–Polymer Composites ...................................................... 8 1.6.1 Biomedical Area...................................................................................8 1.7 Environmental Pollution Remediation Area..................................................11 1.8 Summary......................................................................................................... 15 References................................................................................................................ 16

1.1 INTRODUCTION Optical and electronic properties can be changed because semiconductor nano­ crystals are so small (1–20 nm). Because of this, they have different properties than bulk semiconductor materials and atoms or molecules alone. Over the last two decades, substantial scientific study has been conducted on quantum dots (QDs), and tremendous progress has been made in both their synthesis and our knowledge of their optical and electrical characteristics, as well as in their applications. Indeed, quantum dots have reached maturity, and a plethora of QD-based DOI: 10.1201/9781003266518-1

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Quantum Dots and Polymer Nanocomposites

applications in biodiagnostics, bioimaging, photonics, optoelectronics, and sensors have arisen. Numerous publications and review papers on the synthesis, physical, and optical characteristics of QDs, as well as their applications, have been pub­ lished to document the rapid advancement of this field of study [1]. Across many cases, either the surface of the QDs must be chemically modified or the QDs must be implanted in a solid matrix in order for them to function properly. Being that most synthetic polymeric materials are transparent in the visible spectrum, they are frequently used as a matrix for nanocomposite materials in optical applications due to their transparency in this region [2]. Polymers, in addition to serving as the matrix for the nanocomposite material, also contribute to the mechanical and chemical stability of the material. Additionally, polymers have the potential to inhibit nanocrystal agglomeration and provide processability into technologically important structures such as thin films, micro- and nano-spheres, and other microand nano-structures [3–5]. Despite the numerous benefits that a combination of QDs and polymeric materials has to offer, progress in this field of study has been modest, particularly in recent years. The most significant issues encountered were poor compatibility of the QDs with the polymers, as well as impairment of the electrical or optical characteristics of the QDs when they were mixed with the polymers [6–8]. As a result, research efforts have shifted to the chemical engineering of QD surfaces using polymers, as well as the development of tech­ niques for encapsulating QDs in polymer matrices, in order to circumvent these shortcomings [9–11]. Due to the fact that the chapter unintentionally deals with colloidal QDs, the production of these nanomaterials as well as their primary features is briefly dis­ cussed. The next sections demonstrate how to create the required structures of QD/polymer assemblies using various techniques. It then talks about how polymer chains can be attached to nanoparticle surfaces directly. These include hydrophobic interactions with QD surface ligands, multivalent passivation of the nanoparticle surface, and the “grafting to” and “grafting from” approaches.

1.2 SYNTHESIS OVERVIEW OF QUANTUM DOTS Recent studies have touched on the production of the most studied group of col­ loidal semiconductor nanocrystals (groups II–VI in the periodic table of elements), which is the most studied group of colloidal semiconductor nanocrystals. Due to the fact that the emission wavelength is dependent on the size of the nanocrystals, it is critical to acquire nanocrystals with a limited size deviation. This has proven to be a difficult endeavor, and substantial research into the synthesis of II–VI semi­ conductor nanocrystals has been carried out over the previous two decades in order to meet this challenge. For the production of high-quality nanocrystals, colloidal routes based on controlled precipitation and syntheses in constrained geometries have been extensively researched and investigated. In addition to luminescence, CDs and GQDs possess significant oxygen content due to the presence of carboxyl, carbonyl, or epoxy groups. Since the initial paper documenting the acetic oxidation of carbon soot generated by arc-discharge, by collecting soot from burning candles, or laser ablation, synthesis techniques for these

Introduction to Quantum Dots

3

luminous nanomaterials have grown explosively. In practice, synthetic methods to CDs may be divided into two groups: top-down approaches and bottom-up ap­ proaches. Top-down techniques are the more common. The top-down technique is implemented by the cleaving or breaking down of carbonaceous materials, which can be accomplished through a chemical, physical, or electrochemical manner. The latter involves the carbonization of tiny molecules, pyrolysis, or the chemical fusing of small organic molecules in a stepwise fashion, among other methods. The physico­ chemical properties of nanoparticles are determined by their size, oxygen or nitrogen content, optical characteristics, cystallinity, surface functionalities, compatibility with different solvents, and colloidal stability. The synthesis method and precursors are important factors in this process. The most prevalent techniques for synthesizing QDs produce materials that are insoluble in polar solvents, such as water, as a result of their synthesis. In part, this poor solubility might be attributed to the selection of surface stabilizing ligands, which frequently comprise alkyl chains in their structural composition. Aqueous solubility is required for many applications, particularly in biology, where quantum dots must be soluble in aqueous solutions. Alternative approaches for the manufacture of water-soluble, functionalized QDs have been investigated in order to eliminate the use of superfluous ligand exchange reactions. Every technique proposed to synthesize QDs, both from the top-down and from the bottom-up, can be classified as physical (without the need for chemical alteration of matter), chemical (through the use of a chemical reaction for the formation of a substance), or biological (through the use of living organisms). Because the diameter of QDs varies depending on the pro­ duction procedure, the diameter can range from a little nanometer to a little micro­ meter, with particle size distribution being regulated to within 2% by using accurate growing methods. According to their kinds, the preparation procedures used by QDs range from standard routes to new approaches (Cd-based QDs or Cd-free QDs). Organometallic synthesis was one of the earliest procedures used to synthesize QDs, and it continues to be utilized today. A “bottom-up” way to fabricating QDs with good optical characteristics, the organometallic process was invented around 20 years ago and is still in use today. This process consists of three steps: nu­ cleation, growth, and termination, all of which are carried out in organic solvents at a high temperature in an inert atmosphere, as described above. QDs may be tuned to have certain sizes and emission wavelengths by controlling the chemical conditions under which they are formed. As an illustration, a color shift from red to blue in the light output may be observed in conjunction with a decrease in the size of the crystal. High monodispersity, high quantum yields, and a narrow emission peak are all characteristics of the QDs created from this technique, making them excellent candidates for bioimaging applications. QDs have been produced by organometallic techniques such as CdTe/CdS and PbSQDs, to name a few examples [12,13]. Furthermore, the QDs produced using this method are often hydrophobes, and therefore need be subjected to additional surface modification. Due to the fact that the QDs (orQDs) generated from the organic-metallic method are capped with hydrophobic molecules, additional solubilization in aqueous solutions is required prior to their use in practical applications [14]. A variety of approaches have been used to achieve this goal, including swapping hydrophobe ligands with hydrophilic

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Quantum Dots and Polymer Nanocomposites

ones (e.g., thiol-containing compounds), encapsulating QDs in a layer of amphi­ philic copolymers (diblock, tri-block), and adding extra ornamentation with a water-soluble silica shell. It was discovered that other techniques, which did not rely on the creation of polymer micelles but rather on the direct coating of QDs with amphiphilic polymers, could be used. Low-molecular-weight polyacrylic acids that are amphiphilic and alkyl-modified (with octylamine or isopropylamine) have been demonstrated to coat TOPO-protected nanocrystals and solubilize quantum dots (QDs) in water [15]. As previously indicated, aqueous solution-based synthesis is another method for fabricating QDs that is commonly employed. When compared to the organometallic route, this approach is more environmentally friendly and has higher reproducibility because of the use of mild synthetic conditions, the elim­ ination of the solubilization step, and the use of simple and low-cost reaction in­ struments, which allows the preparation of a wide range of nanomaterials without the use of expensive instruments. The potential applications and the properties are depicted in a graphical illustration, as shown in Figure 1.1. Many kinds of QDs,

FIGURE 1.1 QDs in size-dependent fluorescence and their applications in brief.

Introduction to Quantum Dots

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including CdS QDs [16], AgInS2–ZnS QDs [17], and ZnSe QDs, have been pro­ duced using an aqueous solution technique to date. QDs manufactured in water (aqQDs) have been shown to be naturally waterdispersed, owing to the huge number of hydrophilic ligand molecules present on their surfaces, as previously reported [18]. As a result, as compared to orQDs, aqQDs have a significantly smaller hydrodynamic diameter (about 5.0 nm). A number of reagents are commonly used as stabilizers in aqueous synthetic pro­ cedures, including 3-mercaptopropionic acid (MPA), 2-mercaptoethylamine acid (MA), thioglycolic acid (TGA), and l-cysteine. 3-mercaptopropionic acid (MPA), 2-mercaptoethylamine acid (MA), thioglycolic acid (TGA), and l-cystein [19]. The size and emission wavelength of aqQDs can be regulated in the same way that the organometallic pathway can be controlled by adjusting the reaction conditions. Although there are certain limits, such as for wet chemical treatments when hard templates or catalysts are unavailable, there are some advantages as well. Furthermore, one of the disadvantages of aqQDs has been identified as having poor optical characteristics. In addition to the two methods already mentioned, the biomolecule-templated technique is also used to synthesis QDs [20]. As a new approach for assembling inorganic nanomaterials, the area of biotemplating has evolved and progressed significantly. Biomolecules like nucleic acids, peptides, and polysaccharides have been applied as templates to synthesize QDs, and this has been shown. In this revolutionary technique, QDs assembled from a DNA template, protein cages created from heat shock proteins, and inherently engineered viruses are examples of what may be achieved [21–23]. The order and assembly of biomolecules (e.g., nucleic acids) have been observed to be important variables in the regulation of the size, shape, and optical properties of QDs from a practical point of view [24].

1.3 OPTICAL PROPERTIES OF QDS The incomparable optical features of QDs have made them one of the most attractive choices for imaging applications in both the in vivo and in vitro en­ vironments. Inorganic cores, which are responsible for the essential semi­ conducting qualities and optical properties of QDs, are at the root of the optical properties of these nanostructures. Before capping a QD with ligands, it is common practice to passivate the active core surface with another inorganic shell first. This enhances the optical properties of QDs because the band gap of the shell is larger than the band gap of the core material, preventing electrons and holes from entering the shell and into the core material [25]. Electron hole pairs (excitons) are trapped inside nanocrystal grain boundaries, allowing QDs to exhibit unique optical features. With their unique photophysical properties like broad absorption spectra, size-tunable narrow emission spectra, size-andcomposition-tunable light emission, high fluorescent quantum yields and en­ ormous absorption extinction coefficients as well as photochemical robustness, resistance to photo bleach effect, large stokes shift, and reduced fluorescence intermittency, QDs have become ideal candidates for a wide variety of biome­ dical applications [26].

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Quantum Dots and Polymer Nanocomposites

The presence of surface imperfections in CQDs is another source of fluores­ cence emission. Surface defects are referred to be sites that are not ideal sp2 domains, and they operate as energy traps on the surface of the semiconductor. The presence of sp2 and sp3 hybridized carbon atoms, as well as functionalized surface defects, in CQDs can contribute to the fluorescence emission of the material. Excitation-capture spots can be formed on the surface of CQDs as a result of surface oxidation, which introduces flaws into the crystal structure. Increasing the degree of oxidation results in the formation of additional flaws on the CQD surface, which results in the narrowing of the energy levels [27]. The recombination of electron-hole pairs in sp2 domains results in the emission of fluorescence. Particle localization (PL) is caused by electron-hole pair combi­ nations in the localized sp2 domains, which are located between the band gaps of the s and s* states in the sp3 matrix. The combination of electron-hole pairs at these levels results in the formation of PL. As a result of surface passivation and functionalization, more permanent surface defects are created, allowing for quicker recombination of the electrons and holes that are trapped on the surface, and therefore, more intense emission is observed [28].

1.4 UPCONVERSION OPTICAL CHARACTERISTICS In a process known as “photon upconversion,” many low-energy photons are sequentially absorbed, resulting in the production of high-energy luminescence. Nicolaas Bloembergen described this phenomenon as a theoretically feasible event in 1959, and it has since gained widespread acceptance [29]. Upconversion is advantageous in biological applications because it allows for in-vivo mon­ itoring and because longer wavelength photons (e.g. NIR) penetrate deeper into tissues than shorter wavelength photons (e.g. visible). This phenomena is characterized by low photon toxicity, low photon interference, and great spatial resolution, among other properties [30]. QDs are capable of undergoing upcon­ version. The following examples show how CQDs with a diameter of less than 5 nm, which had been surface passivated using propionyl ethylenimine/coethylenimine, were subjected to laser treatment with an argon ion laser as well as with a femtosecond pulsed titanium: sapphire laser. Due to the quadratic relationship between the strength of the excitation laser and the luminescence intensity, it was determined that the visible luminescence seen was caused by the excitation with two near-infrared photons. Despite the fact that the majority of academics feel that the PL attribute of CQDs should be attributable to the mul­ tiphoton process, there is some disagreement [31]. Shen et al. suggested that the multi-photon mechanism would be unable to explain the upconversion of gra­ phene quantum dots (GQDs) [32]. The energy gap between the emission and excitation is continuous, and it corresponds to the energy difference between the σ and π orbitals in the atom. In reality, a low-energy photon excites a π-electron (which is at an intermediate energy level) to the LUMO state. The relaxation into the HOMO σ orbital results in the emission of a photon with a shorter wavelength.

Introduction to Quantum Dots

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1.5 POLYMER-BASED BIOACTIVATION OF QDS They are a vital component of biomaterials, and they are used in nanomedicine, drug delivery systems and biosensors to a wide range of applications. In medicine, bioactive compounds are being used for a variety of purposes, including the ad­ vancement and present state of medicine. The name graphene was first used in 1986, and it was coined by combining the word graphite with the suffix (n), which stood for polycyclic aromatic hydrocarbons at the time of its introduction. Graphene has progressed from being a mysterious chemical to being a shining star in a variety of disciplines of science and technological research. As a result of graphene’s outstanding features, which include high current density, ballistic transport, che­ mical inertness, high thermal conductivity, optical permeability, and superior hy­ drophobicity at the nanoscale scale, it is becoming increasingly popular. Liu et al. published the first study on the usage of functionalized graphene oxide polyethylene glycol as a nanocarrier for the delivery of anticancer medications [33]. Graphene nanomaterials and their toxicity were studied by Sanchez et al. based on their ability to interact with biomolecules, cells, and tissues, as well as the number of layers and the dimensions of chemical functionalization that were applied [34]. QDs’ photo­ luminescence is a key characteristic that helps explain the wide range of biological applications for which they are useful. Complexity and unknown surface functions of QDs have hindered our understanding of QDs’ photoluminescence processes. Since biological applications are affected by optical characteristics, it is difficult to get the best possible results. So as of yet, no mechanism has been identified that completely explains the photoluminescence phenomena of QDs, placing a restriction on the ability to control their optical characteristics. It is obviously a reference to secondary metabolites generated by microbes as “bioactive compounds.” These metabolites are often not required for the growth and survival of organisms and, unlike primary metabolites and vital macromolecules, are not the foundation of the organism’s fundamental steps and do not play a vital role. Primary and secondary metabolites in bacteria are created during the idiophase period of the organism’s existence, and they are formed as a consequence of certain environmental circumstances, such as a lack of food. The majority of these che­ micals are beneficial to the organism when it is exposed to particular environmental circumstances (such as competition in the ecosystem). Zobell and Rosenfeld pub­ lished the first study on the creation of biologically active compounds from marine bacteria in the context of antibiotic manufacturing, which was the first of its kind. In the intervening period, a number of reports have been received on this. When it comes to biological chemicals, 16,000 have been extracted from marine micro­ organisms, with antibacterial, antiviral, and anticancer substances among those found in the sea [35]. As a result of the better qualities of graphene as compared to polymers, graphene-based polymer composites are used in a variety of applications. When compared to pure polymer, graphene-based polymer composites exhibit improved gas barrier, electrical, mechanical, flame retardant, and thermal capabilities. Graphene-based polymer composites also exhibit superior mechanical, flame retardant, and thermal qualities [36]. Graphene nanofibers are utilized as a

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Quantum Dots and Polymer Nanocomposites

two-dimensional model to organize polymers, hence enhancing the solubility of the polymers in the water they are placed in [37]. Despite the fact that carbon nanotubes (CNTs) have mechanical qualities that are equivalent to graphene, graphene is a superior nanofiller in several areas, such as thermal and electrical conductivity, than carbon nanotubes [38]. The physicochemical characteristics of nanocomposites are influenced by the interfacial bonding between graphene layers and the polymer matrix, as well as the distribution of graphene layers within the polymer matrix. Pure graphene does not mix well with organic poly­ mers and does not combine well with other materials to generate homogenous composites. Since the usage of graphene oxide (GO) sheets as nanofillers in polymer nanocomposites has received a great deal of attention, it may be asserted that, in contrast to graphene, GO is preferred to organic polymers [39]. For so­ phisticated hybrid nanomaterials and applications, polymer dots have been created and employed, especially essential and unique quantum dots. Therefore, polymer dots can be generated by utilizing conjugated and non-conjugated polymers [40].

1.6 APPLICATIONS OF QD–POLYMER COMPOSITES 1.6.1 BIOMEDICAL AREA Most medications can be loaded into QD based polymer composites via π-π and electrostatic interactions because they are bioactive. Bioactive QD-based polymer composites, on the other hand, have strong membrane permeability and biocompatibility, which can raise the drug efficiency of the loaded drug and increase efficacy in the face of drug-resistant cells [41]. For in vivo drug ad­ ministration, bioactive QD-based polymer composites have been extensively researched. The average size of bioactive QDs reduces absorption by the re­ ticuloendothelial system and renal clearance. For in vivo drug administration, bioactive QD-based polymer composites have been extensively researched. The average size of bioactive QDs reduces absorption by the reticuloendothelial system and renal clearance. Another factor contributing to the efficacy of bioactive QDs is their fast delivery rate, which increases the amount of time that blood circulates in the body [42]. A unique core-shell MOFs/CDs@OCMC nanoparticle was designed by Lin et al., and it was produced and validated for use in the detection and treatment of cancer in vitro [43]. Nanoscale metal organic frameworks loaded with an anticancer medication for combined cancertargeted drug delivery and optical imaging were described in another study [44]. As seen in Figure 1.2, this platform is comprised of three phases that work together. First, super paramagnetic iron oxide nanoparticles were coated with an O-carboxymethyl chitosan (Fe3O4@OCMC) to create a super paramagnetic iron oxide nanoparticle. Carbonyl chitosan is employed for the pH-responsive release of drug molecules into acidic tumor endosomes, which trigger death. The Fe3O4 core may be used as a T2-weighted magnetic resonance imaging (MRI) contrast agent and for magnetically guided drug administration in this application.

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FIGURE 1.2 Graphical representation of QD-based polymeric formulation for magnetic field guided drug delivery and contrast agent for MRI. Reproduced with the permission from ref. [©, 44,2016 American Chemical Society.

It was also discovered that the nanoengineered materials might be used as a wound dressing material to treat both acute and chronic wounds. Several studies have been conducted on a bandage material composed of fibrin nanoparticles embedded in chitosan hydrogel that has demonstrated great cell survival, im­ proved blood clotting, and wound closure rates of 98% when tested on male Sprague–Dawley rats [45]. Some of the publications also discuss wound-healing materials that are capable of delivering smart drugs. Ninan et al. developed a wound-dressing material composed of carboxylated agarose composite hydrogel scaffolds and capable of delivering drugs in a pH-responsive manner [46]. Using a pH-responsive hydrogel layer to carry drug-loaded chitosan nanoparticles, Kiaee et al. developed another electronic wound-dressing material that demon­ strated transdermal drug administration in a controlled environment [47]. It was attempted to address and construct a smart, stimuli-responsive system that could be used as a hybrid cotton patch medication delivery system by employing carbon-based QDs in the process [48]. As a result, taking into consideration that there is a reduction in pH after any cut or wound owing to the growth of harmful bacteria, the work was constructed in such a manner that the drug releases in vitro more at a lower pH (pH 5), rather than a higher pH (pH 7) As part of this re­ search, a variety of hybrid cotton patches were created by integrating several CDs derived from various sources such as tea, ascorbic acid, aloe vera, and jute (Figure 1.3). In addition, a herbal formulation was used in which neem leaf ex­ tract was used as a model drug to investigate the drug delivery method, which was successful. This work also describes for the first time the synthesis of lu­ minous CDs from a green source, jute, which is described in detail elsewhere. On the basis of these findings, it was discovered that, of all the various hybrid cotton patches constructed using various CDs and graphene oxide, only the one

10

Quantum Dots and Polymer Nanocomposites

FIGURE 1.3 (a) Using different CDs, the fabricating procedure used to prepare the na­ nocomposite hydrogel bound cotton patch. (b) Drug release pattern of hydrid cotton patches at two pHs (5 and 7). Reproduced with the permission from ref. [©,48,2020 American Chemical Society.

containing the jute CDs exhibits pH-dependent drug release. There is a greater discharge at pH 5 than there is at pH 7. For this reason, in contrast to other pH-dependent releases previously discovered, the system created in our labora­ tory is a nanocomposite hydrogel in which the nanomaterial is carbon dot. It is worth noting that the CDs utilized in this study are generated from natural pro­ ducts. Aside from being significantly easier, the manufacturing process does not require the use of any other solvents than water. An article on C-dots made of κ-carrageenan, biopolymer with sulfur, and folic acid has already been published [49]. Naturally present κ-carrageenan can con­ tribute to the formation of carbogenic core, and folic acid, which served as the surface ligand, was chosen because of its compatibility with carbogenic core, which is composed of a high concentration of π-electron cloud and a folate recognized terminal, and its compatibility with carbogenic core. With the C-dots, researchers discovered a bright blue luminous material with a high quantum yield, excellent photostability and biocompatibility, and minimal cytotoxicity. The cell lines used in this investigation were HeLa cells that were overexpressing the folate receptor (FR)

Introduction to Quantum Dots

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and fibroblast cells that were negative for the folate receptor (FR). By utilizing the advantages of C-dots, an effective approach for distinguishing FR-positive cancer cells from normal cells was created and validated. FR expressing cancer cells were specifically targeted by the C-dots, which demonstrated selective straining for these cancer cells on the cell’s surface. It was discovered that these C-dots played a key role in drug binding and distribution, indicating that they had remarkable nanovehicle-like capability. Furthermore, these C-dots were used in applications such as invisible marker and finger print recovery, among other things. Because of this, it was believed that this nanoprobe would be potentially relevant in the biological and biomedical fields. In another study, the microwave-irradiated thermal coupling approach was used to create multifunctional compact discs (CDs) [50]. Alginate, a polysaccharide that is plentiful in nature, has been chosen as the core forming for spherical CD particles, with urea serving as the N-doping ligand for the surface decoration of the particles. Because the CDs are composed of polar groups, they have a proclivity to bind themselves to polar or hydrophilic medicines. As a model drug, DOX (a widely used chemotherapeutic agent) has been selected in this study (Figure 1.4). Furthermore, the surface moieties of CDs were responsible for the pHresponsive drug release propensity, which is why they may be utilized as a drug carrier on purpose.

1.7 ENVIRONMENTAL POLLUTION REMEDIATION AREA Quantum dots (QDs) exhibit distinctive luminescent and electronic features, in­ cluding broad and continuous absorption spectra, limited emission spectra, and great light stability, among others. They absorb white light and then re-emit certain colors in a matter of nanoseconds, which are dependent on the band gaps of the materials being used in the experiment. According to one study, quantum dots (QDs) can be used as visible-light photocatalysts for water splitting [51]. According to their findings, because of their huge surface area, quantum dots can capture the whole visible light spectrum, resulting in a high output of catalytic activity. Photocatalyst materials have a serious difficulty with electron and hole recombination that decreases the photocatalytic efficiency. A high-efficiency photocatalyst may be produced by using CQDs because of their ability to capture and transport electrons [52]. It is possible for the photocatalyst to be activated by photons with a sufficient amount of energy in order to form electron-hole pairs, which are subsequently transported to the photocatalyst’s surface. Because of the photo-induced electrons in the conduction band, reactive oxidative species are formed in the environment. Meanwhile, as a result of the photo-induced holes, hydroxyl radicals with a high oxidation capability are formed in the environment. Figure 1.5 depicted the fundamental mechanism of photocatalytic degradation of organic contaminants in the presence of a photocatalyst. There have only been a few research conducted on the direct use of CQDs as photocatalysts. The majority of research, on the other hand, has reported the use of CQDs in combination with other semiconductors [53]. The application of CQDs for photocatalytic de­ gradation of organic pollutants has been described in a recent research [54]. In order to create a photocatalyst, composite CQDs were linked to a metal-organic

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Quantum Dots and Polymer Nanocomposites

FIGURE 1.4 Synthesis of drug-carbon dots conjugate by covalent surface chemistry ap­ proach. Reproduced with the permission from ref. [ 50] © 2020 American Chemical Society.

framework by using a chemical bonding method. Based on that research, CQDs have been synthesized using ascorbic acid as a starting material, with the pro­ duction process of microwave irradiation being employed. An aqueous photocatalyst composite containing various concentrations of CQDs and ligands were homogeneously distributed in distilled water and heated at 90 degrees Celsius for 3 h. Another research effort has also reported on a study that is comparable to this one [55]. After 90 minutes of visible light irradiation, the percentage of

Introduction to Quantum Dots

13

FIGURE 1.5 Photocatalytic degradation of organic contaminants.

rhodamine B (RhB) degradation was 99% when CQDs/La2Ti2O7 composite was used as a photocatalyst. The improved photocatalytic ability of CQDs is hy­ pothesized to be responsible for the observed conversion of visible light to shorter wavelengths based on this finding. The La2Ti2O7 nanosheets may be excited to create electrons and holes using shorter wavelengths of light. CQDs, on the other hand, will take electrons from La2Ti2O7’s conduction band, resulting in the separation of photo-generated electron-hole pairs. Direct oxidation of RhB mo­ lecules might be achieved by hydroxyl radicals generated by the reaction between aqueous oxygen and La2Ti2O7’s valance band. Additionally, as previously de­ scribed, CQDs can be employed for the photocatalytic breakdown of methylene blue (MB) [56]. In that specific work, the researchers created a photo-catalyst composite by integrating CQDs into the surface of zirconia, which was then tested (ZrO2). Zirconia is a non-toxic nanoparticle and high-performance en­ gineering material that is used in a variety of applications. The photocatalyst was added to an MB solution and sonicated for 30 seconds to activate the enzyme. Researchers have proposed photocatalytic destruction of p-nitrophenol and acid violet 43 as a method of reducing their levels [57]. Their research has effectively synthesized CQDs/P25 photocatalyst by chemical adsorption of CQDs onto the surface of pyrogenic nanoparticles, and they have demonstrated that this method is effective (P25). A combination of the photocatalyst and the samples was placed under a mercury lamp for a period of time to see what happened. Following that, the mixture was centrifuged to remove the photocatalyst, and the results were analyzed using an ultraviolet-visible spectrophotometer. According to the find­ ings, acid violet 43 degraded at a rate of 93%, whereas p-nitrophenol degraded at

14

Quantum Dots and Polymer Nanocomposites

a rate of 96%. The deterioration may be explained by the presence of CQDs as an electron pool, which decreased the band gap and allowed the electron-hole se­ paration to be activated with lower energy, resulting in a smaller band gap. A nanocomposite material incorporated onto the surfaces of CQDs has the po­ tential to significantly enhance their photocatalytic activity. This integration can be accomplished by electrostatic interactions between atoms and hydrogenbonding interactions between molecules [58]. Several studies have revealed that potassium hexatitanate (K2Ti6O13) nanotubes possess an unusual tunnel crystal structure with excellent optical characteristics [59]. Moreover, because of the high recombination rate of photo-generated electron-hole pairs, K2Ti6O13 can only be activated by UV light and so cannot achieve the desired photo­ catalytic degradation. The integration of K2Ti6O13 onto the surfaces of CQDs can therefore result in the formation of an a binary composite system that can greatly reduce the recombination rate of charge carriers that have been stimulated by photons [60]. The rapid recombination of charges and the restricted light utili­ sation characteristics of photocatalyst materials are the two most difficult diffi­ culties to overcome. Due to the ease with which QDs may be spread onto the surfaces of other nanocomposites, the vast majority of studies have chosen QDs as the combination material for the production of QDs-based nanocomposites, particularly for photocatalysis applications [61]. Furthermore, CQDs have good electron-transfer characteristics, which can aid in the improvement of charge separation. CQDs have the potential to extend the light-utilization range of wide band gap semiconductor photocatalysts from the ultraviolet to the visible light spectrum. According to yet another study, CQDs/TiO2 composite nanofibers were shown to be effective in the photocatalytic degradation of methylene blue (MB) dyes [61]. In that study, TiO2 nanofibers were found to degrade around 71% of MB in 95 minutes, whereas CQDs/TiO2 composite nanofibers were shown to breakdown practically all MB dyes in the same amount of time. Using this finding, it has been demonstrated that CQDs/TiO2 composite nanofibers exhibit superior photocatalytic properties when compared to TiO2. CQDs absorb light and re-emit it at a lower wavelength during the photocatalytic degradation activity induced by ultra­ violet light irradiation. The TiO2 nanofibers will be excited by the shorter wave­ length, resulting in the generation of electron-hole pairs. As a result, CQDs serve as both acceptors and transporters for electrons created by photons [62]. The electronhole pairs that generated as a result of the electron excitation were trapped by hydroxyl groups on the catalyst surfaces, resulting in the formation of OH• radicals. The hydroxyl radical is required for the breakdown of dyes. The oxygen molecules in solution have been reacting with the excited electrons, resulting in the formation of superoxide radical anions (O2-). A by-product of the protonation process is the formation of hydroperoxyl radicals (HO2•) and hydroxyl radicals (OH•). These radical molecules will operate as powerful oxidizing agents, causing all of the dye molecules to break down into non-toxic gases such as carbon dioxide and water molecules. Figure 1.6 depicts an illustration of the recycling process of photocatalytic activity.

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FIGURE 1.6 The photocatalytic destruction of organic contaminants utilizing QDs as photocatalyst is depicted in this schematic picture.

1.8 SUMMARY QDs are discussed in this review paper, which focuses on their chemical and physical characteristics as well as the raw materials utilized in their manufacture as well as the applications domains in which they may be applied. Citric acid, ascorbic acid, graphite, plant sources, fruits, sugars, gelatin, cholesterol, glucose, bio-waste lignin, and ammonium citrate are examples of raw materials that have been em­ ployed in the fabrication of QDs. However, because of its cost-effectiveness, wide availability, low toxicity, and environmental friendliness, the manufacturing of QDs from sustainable resources (plant-based sources and carbon wastes) should be en­ couraged. Aside from that, it has the potential to minimize the use of chemicals and the creation of trash. The immobilization of such nanodots inside polymer matrix has a substantial impact on the optical characteristics of the resulting nanostructures. A large number of polymer matrices have been selected for their flexibility, surface passivation properties, synergistic optical properties, low corrosion, ability to sense a wide range of molecules (inorganic ions or organic molecules), and simplicity of production procedures. QDs have a high proclivity to tolerate

16

Quantum Dots and Polymer Nanocomposites

polymer or macromolecular chains on their surfaces, which indicates that surface passivation or ligand enrichment have a high likelihood of achieving synergistic improvements in optical characteristics. The surface ligands of polymer chains and nanodots are interacting with one another, which strengthens the overall nanocomposite while maintaining its flexibility. These characteristics of flex­ ibility include toughness, stretchability, necking-like behavior, and minimal fa­ tigue. These are often considered to be fundamental characteristics of elastomers. As a result, in nature, carbon dot-based polymeric composites are frequently referred to as ’elastomer mimics’. The most important characteristic of carbondot-polymer nanocomposite hybrids is the surface adsorption of polymer chains onto the carbon dots, which is the most crucial property. Carbon dots have a significantly greater surface energy than other standard nanofillers, which means they might be the best choice for materials scientists when building composites without compromising their basic characteristics. The thermal stability, on the other hand, is something to be concerned about, and it may be enhanced with the inclusion of carbon dots in small quantities. Quantum dots in polymer matrices can be used in a variety of ways, as the description above shows. A tiny domain does not limit the scope of application.

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What Are Quantum Dots? Deepak Kumar Jarwal Defence Laboratory, DRDO, Jodhpur, Rajasthan, India

Chandan Kumar IIT Bombay, Mumbai, India

Kamalaksha Baral VR Siddhartha College of Engineering, Vijayawada, Andhra Pradesh, India

Anuradha Bera Defence Laboratory, DRDO, Jodhpur, Rajasthan, India

CONTENTS 2.1 2.2 2.3 2.4

Introduction.....................................................................................................22 General Properties of Quantum Dots ............................................................22 Characteristics of Quantum Dots...................................................................25 Types of Quantum Dots.................................................................................27 2.4.1 Core-Type QDs .................................................................................. 28 2.4.2 Core-Shell QDs .................................................................................. 29 2.4.3 Alloyed QDs.......................................................................................29 2.4.4 Doped Quantum Dots......................................................................... 31 2.5 Methodology for Developing Quantum Dots................................................31 2.5.1 Stranski-Krastanow Growth ............................................................... 31 2.5.2 Nanoscale Patterning.......................................................................... 31 2.5.3 Colloidal Nanosynthesis.....................................................................31 2.6 Optoelectronic Properties of Quantum Dots .................................................32 2.7 Application of Quantum Dots........................................................................ 36 2.7.1 Optoelectronic Devices ......................................................................36 2.7.2 Quantum Computing .......................................................................... 38 2.7.3 Biological and Chemical Applications ..............................................38 2.7.4 QDs in Memory Applications............................................................38 2.8 Summary......................................................................................................... 39 Acknowledgments....................................................................................................40 References................................................................................................................ 40

DOI: 10.1201/9781003266518-2

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Quantum Dots and Polymer Nanocomposites

2.1 INTRODUCTION The electronic band gap of the semiconductor material is an essential key factor for optoelectronic and other device applications [1]. It is found that the semiconductor materials having a band gap withen the span of ~1.1 eV to ~3.2 eV are suitable for photovoltaics, photodetectors, and LED application due to their wide absorption range varying from 1,200 nm to 400 nm [2]. Various low-dimensional materials such as nanorods, nanowires, 2D semiconductor material, and the quantum dots have shown feasibility for band gap tunability and attracted a huge amount of interest among re­ searchers. Moreover, the large shifting in the band gap of the semiconducting mate­ rials can be achieved if the dimentions of the semiconducting material is maintained below the exciton Bohr radius (mostly below 10 nm) [3]. The generated excitons in such nanocrystals can modify their energy spectrum according to the observed boundaries of the nanoparticles. This size-dependent electronic and optical phenom­ enon is known as the quantum size or confinement effect [4]. This quantum size or confinement effect comes into existence when the dimentions of nanoparticles is comparable to or less than the ordinary separation length of the e-h (electron-hole) pairs. The nanoparticles with this characteristic are called quantum dots (QDs). The QDs are used extensively to fabricate electronic and photonic devices due to their excellent opto-electronic properties and capability to store energy. They are small in dimention in the span of 2 to 10 nm depending on the Bohr radius of the atom [5]. These nanoparticles consist of a few atoms ranging from ~10 to ~100, and therefore, the properties of QDs varries between the molecules and bulk materials. The quantum confinement effect is dominated at low dimensions, and properties differ from larger particles. The QDs were first discovered in solids by physicist Alexei Ekimov in 1980 [6]. Later, American chemist Louis E. Brus discovered them in a colloidal solution (i.e., minute particles of one element are scattered all over in another element) while working in the Bell laboratory [7]. The color of light emitted or absorbed by QD changed over a period. Larger QDs emit red-shifted color, whereas smaller QDs emit blue-shifted color. Moreover, the specific colors may vary depending on the exact dimention and construction of the QDs. Hence, the confinement of electrons is re­ sponsible for the tailorable particle quantum properties at the nanoscale level [8]. The size-dependent variations in optical and electronic characteristics of semi­ conductor nanocrystals have motivated scientists to explore the potential for lowdimensional semiconductor materials. Changing the size and quantum state or quantum confinement of QDs make it possible to change the optical band gap, emission energies, and fluorescent properties. In this context, the present chapter discusses the various aspect of QDs: characteristics of QDs, their classification (types of QD), electrical and optical properties, synthesis methods, and finally, their optoelectronic applications.

2.2 GENERAL PROPERTIES OF QUANTUM DOTS The nomenclature quantum dot is due to the quantum confinement in the smallest object (because of zero-dimensional). The quantum dots are confined in all three dimensions and therefore called 0D nanomaterials. The 0D nanomaterials have

What Are Quantum Dots

23

some unique properties compared to the bulk material. The physical, chemical, optical, electrical and biological properties of 0D materials differ widely from the properties of individual atoms and molecules or bulk matter. Different types 0D nanostructure materials are nanoparticles, nanograins, nanoshells, nanocapsules, nanorings, fullerenes, collidal particles, activated carbon, nanoporous silicon, and quasi crystals. When the size of the particle becomes comparable with the de Broglie wavelength of the electrons, the particle nature of electrons changes to wave nature due to carrier confinement. Due to the wave nature of electrons, quantum mechanical tunneling occurs between two adjacent particle. Thus, semiconductors consisting of QDs carry electricity (electrons and holes) in highly confined and well-defined energy levels. Their optoelectronic properties varies with both size and shape. The quantum dots (QDs) are widely used for manufacturing of low-cost, large area, and flexible thin film optoelectronic devices because of increase in surface to volume ratio of the QDs. QDs are crystals having width of few nanometers, and typically cointains few dozen atoms across, which vary from a few to thousands of atoms. They’re crystals, with behaviors of individual atoms and are therefore nicknamed artificial atoms. The discrete energy levels of the confined nanocrystal (0D) are closer to the atom than bulk (3D) material. The electrical and optical properties of the bulk (3D-three dimensional) semiconductor material can be easily changed by changing the dimensionality (2Dtwo dimensional, 1D-one dimensional, 0D-zero dimensional) of the material. The advent of synthetic methods to make different-sized quantum dots has offered a new opportunity to study their electrical and optical properties as their shape/size functions. QDs have discreate energy levels and are defined by a non-classical model called a particle in a box. Discretized energy levels are primarly depend on the sizes of QDs, and so their band gap [9]. The optical properties of QDs with dimentions lower than the de Broglie wavelength are significantly affected by their electrical properties. Furthermore, it is defined that the lifetime of fluorescence is determined by the shape and size of the QDs. The QDs have excellent luminescent char­ acteristics in a wider range of excitation with fine and symmetric emission peaks after excitation, as illustrated in Figure 2.1. The bigger size QDs are closer so energy levels are spaced so that the electron–hole pair can be trapped. Therefore, electron–hole pairs in bigger dots have a longer lifetime. To enhance the fluores­ cence quantum yield, quantum dots can be fabricated with shells; semiconductors around them have a larger band gap. In some cases, improvement is due to the reduced access of electrons and holes to non-radiative surface recombination pathways, but it may be due to reduced Auger recombination in others. The emission is mostly more slender than ordinary fluorophores or organic dye mole­ cules. Afterward, the excited electron is pumped straight to the valence band from the conduction band and ejects a photon [10]. The optical properties of quantum dots are known to vary and can be predicted by certain factors. The material that the quantum dot is constructed from plays a role in determining the intrinsic energy of the particle, but the most important factor that affects the optical properties is the size of the dots. Different-sized quantum dots change the color emitted or absorbed by the crystal, due to the energy levels within the crystal.

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Quantum Dots and Polymer Nanocomposites

FIGURE 2.1 Dimention-dependent fluorescence spectra of quantum dots. (a) Normalized fluorescence and (b) size of quantum dot. Reprint with permission from [ 11]. Copyright 2017: Royal Society of Chemistry.

• Energy Levels on Fluorescence Spectrum In the fluorescence spectrum, the color of the light differs according to the energy emitted by the crystal. Red light is associated with lower energy and blue light with higher energy. The band gap energy of a quantum dot is the difference in energy level between the dot’s excited energy state and its resting state. The quantum dot can absorb fluorescent light at the frequency of its band gap to become excited, or emit the same frequency of light to return to its resting state. Effect of size: The size of a quantum dot is inversely proportional to the band gap energy level, and therefore alters the frequency of light emitted and has an effect on the color. Smaller dots emit higher energy light that is bluer in color, whereas larger dots emit lower energy red light. It is also possible for larger quantum dots to posses several energy levels that are more closely aligned. This allows for the absorption of photons with different frequency levels, such as those on the red end of the light spectrum. Effect of shape: Recent studies has also suggested that the shape of quantum dots may play a role in the band level energy of the dots and, as a result, affect the fre­ quency of fluorescent light emitted or absorbed. However, there is insufficient evi­ dence to support this hypothesis and the currently available information does not aid the construction of quantum dots to optimize their shape for specific optical properties. Effect of structure: The crystal lattice of the quantum dot semiconductor has an effect of the electronic wave function. As a result, a quantum dot has a specific energy spectrum equal to the band gap and a specific density of electronic state on the outside of the crystal. Quantum dots can also be synthesized with a protective shell to lengthen its life span and increase the frequency of fluorescent emission.

What Are Quantum Dots

25

For example a quantum dot composed of cadmium selenide may have a thicker protective shell made of cadmium sulfide. Optimizing optical properties for imaging: The most important aspect of the quantum dot that affects the optical properties it displays is its size. The size of the dot can be manipulated in manufacturing processes to create a quantum dot suitable for the purposes of optical imaging. The shape and structure of the quantum dot should also be considered, as well as the material used in the construction process. However, as the size has a direct effect on the optical properties and the frequency of fluorescent light emitted or absorbed by the crystal, it therefore should be given appropriate consideration.

2.3 CHARACTERISTICS OF QUANTUM DOTS It is obvious from the basic physics hypothesis that if external energy is provided to an atom, it “excites” and pumps an electron inside to an energy level higher than the previous when the same electron returns to the previous level its emits light (photon) of the same energy that it initially absorbed. The excitation due to incident photon and transition of an electron during absorption and emission is illustrated in Figure 2.2. The color (wavelength and frequency) of light emitted depends on the incident photon energy. These variations in wavelength and frequency are possible due to the different energy levels in atoms (i.e., quantized) [12]. QDs possess si­ milar characteristics, i.e. quantized energy levels in the same material so give different colors of light depending on sizes. The explanation for this is very clear. A small QD has a bigger band gap, and therefore takes more energy to excite it. A larger QD has energy levels in close proximity, and thus emit lower frequencies (longer wavelengths). As mentioned earlier, the semiconductor crystallite size is smaller than twice the size of its exciton Bohr radius; the excitons are squeezed, leading to quantum confinement. The energy levels can then be predicted using the particle in a box model in which the energies of states depend on the length of the box. Comparing the quantum dot’s size to the Bohr radius of the electron and hole wave functions, three regimes can be defined. A ’strong confinement regime’ is defined as the quantum dots radius being smaller than both electron and hole Bohr radius, ’weak confinement’ is given when the quantum dot is larger than both. For semi­ conductors in which the electron and hole radii are markedly different, an ’in­ termediate confinement regime’ exists, where the quantum dot’s radius is larger than the Bohr radius of one charge carrier (typically the hole), but not the other charge carrier. Band gap energy: The band gap can become smaller in the strong confinement regime as the energy levels split up. The exciton Bohr radius can be expressed as (2.1) B

=

r

m

B

(2.1)

FIGURE 2.2 (a) Schematic representation of change in the state under excitation in the classical Bohr model. Absorption of quantum (excitation) forces an electron (in the ground state) to jump to a higher energy level called an excited state. The electron return to a lower energy level for the emission. (b) Movement of an electron in different energy levels due to excitation and emission using Jabłoński diagram. Reprint from [ 13]. Published 2012 by MDPI as open access.

26 Quantum Dots and Polymer Nanocomposites

What Are Quantum Dots

27

where aB = 0.053 nm is the Bohr radius, m is the mass, μ is the reduced mass, and εr is the size-dependent dielectric constant. This results in the increase in the total emission energy and the emission at various wavelengths. If the size distribution of QDs is not enough peaked, the convolution of multiple emission wavelengths is observed as a continuous spectra. Confinement energy: The exciton entity can be modeled using the particle in the box. The electron and the hole can be seen as hydrogen in the Bohr model with the hydrogen nucleus replaced by the hole of positive charge and negative electron mass. Then the energy levels of the exciton can be represented as the solution to the particle in a box at the ground level (n = 1) with the mass replaced by the reduced mass. Thus, by varying the size of the quantum dot, the confinement energy of the exciton can be controlled. Bound exciton energy: There is a Coulomb attraction between the negatively charged electron and the positively charged hole. The negative energy involved in the attraction is proportional to Rydberg’s energy and inversely proportional to the square of the size-dependent dielectric constant of the semiconductor. When the size of the semiconductor crystal is smaller than the exciton Bohr radius, the Coulomb interaction must be modified to fit the situation. Therefore, the sum of these energies can be represented as (2.2–2.4) Econfinement =

2 2

2 2 1 1 + = me mh 2 a2

2a2

Eexciton =

1 2 r

me

Ry =

(2.2)

Ry

E = Ebandgap + Econfinement + Eexciton = Ebandgap

(2.3) 2 2

2 a2

Ry

(2.4)

where μ is the reduced mass, a is the radius of the quantum dot, me is the free electron mass, mh is the hole mass, and εr is the size-dependent dielectric constant.

2.4 TYPES OF QUANTUM DOTS As mentioned earlier, the electronic and optical properties of quantum dots are highly dependent upon the semiconductor material composition, doping, alloy, and heterostructure [14,15]. The quantum dots can be categorized as organic material quantum dots, hybrid semiconductor material quantum dots, and inorganic semi­ conductor quantum dots based on the composite material. These nanocrystal QDs bridge the gap between individual molecules and bulk materials. More specifically, the compositional structure of the QDs defines the physical properties of the QDs. Therefore, depending on their charge conduction and electronic behaviors, quantum dots are classified into four types: core type quantum dots, core-shell quantum dots, alloyed quantum dots, and doped quantum dots.

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Quantum Dots and Polymer Nanocomposites

2.4.1 CORE-TYPE QDS Core-type quantum dots consist of an inorganic core passivated with organic li­ gand material to prevent the dangling bonds present over the surface. This in­ organic core can be single-component materials with uniform internal compositions, such as chalcogenides (tellurides, phosphide, selenides, arsenides, or sulfides) of metals like cadmium, indium, zinc, or lead; for example, CdS, CdSe, CdTe, PbSe, InP, or PbS. The surface passivation offers electronic and chemical stability for the prevention of agglomeration of QDs due to uncontrolled growth. The schematic of core-type QDs with surface passivation is shown in Figure 2.3(a). We observe a significant increase in the surface-to-volume ratio of a particular volume of material when it is converted into nanoparticles in the form of QDs. In such nanostructured materials, a significant number of atoms of the QDs lie on the surface (e.g., ~ 15% of the total atoms lie on the surface for a ~5 nm thin CdS QD layer) [12]. The optical properties such as photo- and electro­ luminescence of core-type nanocrystals can be fine-tuned by simply changing the crystallite size [5,16,17].

FIGURE 2.3 Different types of quantum dots: (a) Core-type quantum dots, (b) core-shell quantum dots, (c) alloyed quantum dots, and (d) doped quantum dots.

What Are Quantum Dots

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2.4.2 CORE-SHELL QDS Core-shell QDs are layered particles with one core and over the core a shell. Core-shell QDs exhibit enhanced optical characteristics over simple (core-only) QDs with improved stability and photoluminescence because of the shell sur­ rounding the core of QD. Covering of QDs with suitable results in core-shell QDs and exhibit improved quantum yields [2,3,8]. Also, the growth of shell encloses the excitation to the core and therefore protect the core against oxidation and chemical degradation [18]. The structure of the core-shell QDs is illustrated in Figure 2.3(b). ZnS is a suitable shell material for overcoating, and the process for growth of shell can happen in organic or aqueous synthetic methods [19]. The overcoated QDs displayed an enhanced quantum yield of 50% at normal room temperature [20,21]. Core-shell QDs are sub-divided into three groups according to band gap and energy levels of the components: type I, reverse type I, and type II. Type I. In this variant of QD, either the CB (conduction band) or the VB (valance bands) of the core align within the band gap of the shell, so electrons and holes are localized. The band gap of the CdSe core is 1.74 eV, and the band gap of the CdS shell is 2.42 eV in type-I coreshell CdSe/CdS QDs. Hence, both the holes and electrons are trapped to the CdSe core. CdSe/ZnS and InAs/CdSe are other type-I coreshell QDs. Inverse type I. The band gap of the core is larger than the band gap of the shell, and both the CB and VB of the shell are therefore limited within the band gap of the core in inverse type-I coreshell QDs. Subsequently, the holes and electrons are localized in the shell. CdS/HgS, CdS/CdSe, and ZnSe/CdSe are the example of inverse type-I core-shell QDs. Type II. Both edges of the VB and CB of the core are either lower or higher than the shell, so hole and the electron are localized in the core. Examples of type-II core-shell QDs are CdSe/ZnSe, CdTe/CdSe, and CdS/ZnSe. Type-I and Type-II core-shell QDs are illustrated in Figure 2.4 [22].

2.4.3 ALLOYED QDS The optical and electronic properties of QDs are commonly tuned by changing the crystallite size. However, property tuning is difficult in many applications by only changing the crystallite size due to the limitation of size. Alternatively, multi­ component QDs provide further tuning of optical and electronic properties without altering crystallite size. Alloyed semiconductor QD is a multicomponent QD with both homogeneous and gradient internal structures, as illustrated in Figure 2.3(c). The optical and electronic properties are typically tunned by merely

30

Quantum Dots and Polymer Nanocomposites

FIGURE 2.4 (a) Band distribution in the quantum dot, (b) energy distribution in type-I core-shell quantum dot, and (c) energy distribution in type-II core-shell quantum dots. Adapted with permission from [ 22]. Copyright 2014: Royal Society of Chemistry.

composition and internal structure engineering without altering the crystallite size. An example of alloyed QD is CdSxSe1-x/ZnS of 6 nm diameter, which emits light of various wavelengths by just changing the makeup or the composition. Alloyed semiconductor QD made of two semiconductors with different band gap energies has exciting properties different from bulk and individual semi­ conductors. So, alloyed QD possesses distinct and additional composition-tunable optical and electronic properties [23]. Alloyed semiconductor QD (cadmium selenium telluride) having both homogeneously similar and inclined inner structures are made to realize fine-tuning of the opto-electronic properties without altering the particle dimention. Results have demonstrated that the composition and inner structure are two important parameters that can be applied to fine-tune the opto-electronic behavior of multicomponent, alloyed quantum dots. An in­ teresting observation shows nonlinear relationship between the composition and the absorption/emission energies, giving rise to novel properties not found in the parent binary systems. With light shifted in the red region, we observe emissions up to 850 nm and quantum yields up to 60%; this novel class of alloyed quantum dots opens state-of-the-art pospect in band gap engineering and in developing near-infrared fluorescent probes for in vivo molecular imaging and biomarker detection [24,25].

What Are Quantum Dots

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2.4.4 DOPED QUANTUM DOTS QDs passivated with organic molecule ligands provide back to back-barrier between the QDs, which reduces the charge transfer rate between the self-assembled QDs. Doped QDs are used to improve the electron transfer rate of the QDs by modifying the optical characteristics of the QDs. The schematic diagram is shown in Figure 2.3(d). The doping in QDs improves the conductivity of the nanostructured material and makes them suitable for photovoltaic and photodetection applications [26–28].

2.5 METHODOLOGY FOR DEVELOPING QUANTUM DOTS Monodisperse and uniform-sized QDs are essential for high performance. After preliminary work on QDs by the molecular beam epitaxy (MBE) method, the research on methodology for developing QD-based devices has increased exponentially [29]. Some standard methodology for developing QDs has been discussed in this section.

2.5.1 STRANSKI-KRASTANOW GROWTH It is one of the popular methods for growing the thin layers of QDs of a material epitaxially at the surface of the target material. The epitaxially deposited QDs on the substrate experience strain due to lattice mismatch. The continuous growth of the epitaxial layer will lead to the island-like formation after a certain critical thickness. This island-like formation is due to the reorganization of the energy to achieve a stable structure [30]. The small island growth structure leads to the for­ mation of a QDs array in an ordered structure called self-assembly of the QDs. This process requires highly sophisticated and expensive fabrication techniques to achieve a high order of growth control (at the molecular level), such as MBE or metal-organic chemical vapor deposition (MOCVD).

2.5.2 NANOSCALE PATTERNING This type of patterning technique used for the preparation of QDs is also called the topdown approach. It uses a lithography-based technology mostly by the combined usage of electron beam lithography and subsequent etching shown. The use of lithography techniques to achieve QDs can result in high precision control of QD positioning. The film of QDs prepared using a lithography technique faces several drawbacks such as defect formation, contamination, poor interface quality, and size non-uniformity [31].

2.5.3 COLLOIDAL NANOSYNTHESIS Colloidal synthesis of QDs is more reliable and cost-effective for large-area devices. Since the properties of QDs are size-dependent, the achievement of uniformity in the size of the QDs by using a low-cost method for QD preparation is highly suitable. Colloidal nanosynthesis by a hot injection method is the most successful route re­ garding quality, size monodispersity, and preparation cost of QDs [32]. These QDs are prepared by using the principle of La Mer’s growth technique, in which hot

32

Quantum Dots and Polymer Nanocomposites

FIGURE 2.5 Experimental setup for QD preparation and change in particle size with re­ spect to time under constant temperature. Reprint from [ 35]. Published 2021 by Royal Society of Chemistry as open access.

injection of the precursor is performed on hot coordinating ligands, resulting in instant nucleation. The nucleation formation decreases the concentration of precursors and reduces the temperature of the coordinating ligand, which immediately stops the nucleation process. Further, the reaction follows the growth of monomers which can be classified into two parts: first is the rapid growth of monomers to form nuclei followed by the stage of slower growth rate called Ostwald ripening or re­ crystallization [33,34]. The preparation of QDs starts with a quick hot injection fol­ lowed by nucleation instantaneously for a very short time. This short nucleation time and very long growth time allow achieving monodisperse QDs. Further, the prepared QDs are passivated by an organic ligand which slows the rate of growth of monomers and provides a tool to control the size of the QDs. The schematic of colloidal na­ nosynthesis for core-shell type QD is shown in Figure 2.5 [35]. The prepared QDs are in colloid form and can be easily deposited over any substrate using low-cost solutionprocessed techniques such as spin coating, dip coating, and inject-printing [36,37].

2.6 OPTOELECTRONIC PROPERTIES OF QUANTUM DOTS The electronic structure of semiconductor material compound typically depends upon the size of the atom and arrangement of electrons within the atom and further the interaction between the multiple atoms or molecules. Niels Bohr explained the atomic structure of hydrogen atoms, as shown in Figure 2.2. He observed that the electrons have different levels of energy states around the nucleus in the atom [38,39]. When the radii of the semiconductor nanoparticles are less than the exciton

What Are Quantum Dots

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Bohr radius, the electron is confined in the smallest region that is called confined in three dimensions. Hence, there is the quantization of the energy levels in ac­ cordance with Pauli’s exclusion principle. Every atom has bundles or packets of energy that are called photons [40]. In general, in the quantum dots, electrons move from one energy level to another energy level so they can absorb or emit light in the form of photons. When an electron absorbs a photon, it jumps to a elevated energy level. Similarly, the electron emits a photon when it comes to lower energy levels called excitation or relaxation of the electrons within the atom [41,42]. The particle size plays a vital role in the optical properties of semiconductor materials. The particle size of the semiconductor quantum dots depends on the Bohr radius of the semiconductor atoms. The different parameters such as size, shape, synthesis process, and elemental composition of semiconductor compounds affect the quantum dots’ optical properties. Therefore, the optical properties of quantum dots for specific purposes can be altered efficiently by changing the size. In general, semiconducting materials possess an intrinsic band gap. The electron transfer between QDs in the case of a solid thin film is of prominence for the overall efficiency and working of the photodetectors prepared using colloidal QDs. The position of the QDs is fixed and separated by the organic ligand used for surface passivation. The separation and size of QDs define the electron transport properties in a thin film of QDs. The electron transport between QDs can be approximated using the Marcus theory, as given in (2.5) [43,44]: 2

Ket =

V12 2 (4

kT )

1 2 exp (

E / kT )

(2.5)

where Ket is the rate of electron transfer, |V12|2 is the electronic coupling matrix between initial and final states of transition on resonance. Furthermore, E=

(

G0 4

) and λ denotes the total reorganization energy for the QDs.

Reorganization energy consists of two parts: internal (λi) and external (λo). The first approximation is made by assuming λi to be a constant. The value of λo can be approximated by using the relation given in (2.6) [43,44].

0

=

e2 1 1 + 4 0 2r1 2r 2

1 ( s) d

(2.6)

where r1 and r2 are the radius of two QDs separated by a distance d. Since the electric field is applied in one direction, the above equation can be solved by ap­ proximating V12 2 S where S is the one-dimensional tunneling probability. Thus, V12 2 can be expressed in terms of the tunnel barrier height (ϕ) and applied field (E) as (2.7) [43,44]: V12 2

S = exp

4 3

2m 1 2 eE

3 2

(

3

eEd ) 2

(2.7)

34

Quantum Dots and Polymer Nanocomposites

FIGURE 2.6 Electron transfer rate for the different radius of CdSe and CdTe QDs. Reprinted with permission from [ 45]. Copyright 2017: American Chemical Society.

where the tunneling probability depends upon the size and dielectric constant of the QDs. Further, the thickness of organic ligands providing surface passivation also affects the electron transfer rate between two QDs. The electron transfer rate be­ tween the different sizes of the CdSe and CdTe QDs is shown in Figure 2.6 [45]. The tunneling probability will decrease exponentially with the increasing barrier width. The other transport phenomenon with the increasing barrier width is the hopping of charge carriers from one QD to another QD. The hopping transport is a field-dependent phenomenon, and hopping rate R(E) is directly related to the ef­ fective mobility of the carriers by (2.8) R (E ) =

where

eff

=

n free free ntotal ,

eff

(E ) × E d

(2.8)

where d is the hoping distance in the direction of the

electric field, ntotal is the total carrier density, nfree is the free carrier density, μeff is the effective carrier mobility, and hopping rate is the combined effect of R (E ) = Rforward (E ) Rback (E ), where Rforward (E ) and Rback (E ) are forward and backward hopping, respectively. Since some of the carriers can occupy trap states, effective mobility is smaller than the mobility of the free carriers (nfree). The quantum confinement of the nanoparticle semiconductors varies according to the size of the particles. In bulk semiconductors, the valence band (Ev) and conduction band (Ec) are continuous due to interactions with other molecules. As the particles are not confined to the bulk semiconductors, these types of semi­ conductor structures are called three-dimensional (3-D) structures. In the quantum wells, particles are confined only in one dimension, due to which the density of states (DOS) is also modified accordingly. The semiconductor

What Are Quantum Dots

35

FIGURE 2.7 Representation of systems DOS as system dimensionality is reduced. The DOS in different confinement configurations: (a) bulk, (b) quantum well, (c) quantum wire, (d) quantum Dot. Reprinted from [ 50]. Published 2016 by Springer Nature as open access.

structures in which particles are confined in only one direction are called twodimensional (2-D) nanostructures. Similarly, the carrier confinement in quantum wire structures occurs in two directions where the DOS does not remain con­ tinuous anymore. These types of nanostructures are called one-dimensional (1-D) nanostructures. The particles are confined in every possible direction in the case of QDs. The QDs are called zero-dimensional (0-D) nanostructures. The con­ finement of particles in QDs leads to the quantization of the conduction and valence band of the nanostructure. The variation of DOS for different types of nanostructures is shown in Figure 2.7. The optical properties of the QDs can be easily assumed as a surface and size-dependent phenomenon [46,47]. The quantum confinement of the exciton increases with the decrease in the size of QDs. The band gap (Eg) of the material is also a size-dependent property for the QDs and is given by (2.9) [48]: Eg =

2 n, l v 2a2meff

+ Eg,0

(2.9)

where ħ is h/2π (with h as the Plank’s constant (6.63 × 10–34 m2kg/s)), is the nth v zero of the Bessel function, a is the radius of the QDs, meff is the effective mass 1

of the free particle, and Eg,0 is the energy of state at k (wave number) = 0. Eg a2 is deducted from the above equation. Thus, the band gap of the QDs shows a blue shift with the decrease in the QD size. The quantization of energy states and shifting in the band gap of QDs are illustrated in Figure 2.8 [49]. The energy quantization and band gap tuning can be utilized to achieve tuneable spectrum selective photodetectors.

36

Quantum Dots and Polymer Nanocomposites

FIGURE 2.8 Spitting of the continuous band into discrete energy levels with the decrease in size of the quantum dots. Adapted with permission from [ 49]. Copyright 2019: Elsevier.

2.7 APPLICATION OF QUANTUM DOTS The exciting opto-electronic effects of the QDs make them advantageous for multitasking applications. These particles release light in the visible and nearinfrared regions. QDs have shown extensive use in electronic devices such as SETs (single electron transistors) or micro-LED arrays, in energy applications such as solar cells (photovoltaic devices), or LEDs [49]. The improved charge transport and optical properties of ZnO QDs is used for efficient ETL in the organic solar cells [51–53]. ZnO QD seed layers exhibit improved nucleation sites for the ZnO na­ norod ETL for perovskite solar cells [54,55]. QDs are also used in biomedical applications for imaging, detecting, and labeling. The application of QDs in dif­ ferent electronic devices is shown in Figure 2.9. The use of ZnO QDs and CdSe QDs results in the spectrum selective and high-performance photodetectors [33,56–64]. Functionalizing the QDs either by hydrophilic functional groups/ ligands, or with capping agent (organic coating), is used for transforming them to aqueous soluble QDs. Additionally, aptamer, DNA oligonucleotide or antibody are grafted on QDs through thiol, amine, or carboxyl groups, giving cross-linking with molecules for biological applications. Functionalized QDs have additional benefits and are suitable for cell labeling, cell targeting, imaging, and drug delivery. The edge over the use of QDs with respect to organic dye molecules are their higher quantum efficiency, high brightness, and stability [65,66].

2.7.1 OPTOELECTRONIC DEVICES QDs have captivated the interest because of their facinating optical properties. They are used for various purposes where exact control of colored light is essential. A thin filter made of QDs has been manufactured and attached on the top of a fluorescent or LED lamp to change its light from a blueish color to a warmer,

What Are Quantum Dots

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FIGURE 2.9 Device applications of quantum dots in photodetectors, LED, solar cell, phototransistor, laser, and biosensor.

redder, more beautiful shade. QDs can also be used in place of dyes, pigments, and in high-quality reflective paints. QDs take in the incoming light of a particular color and emits light of an entirely different color, when embedded in other materials, which are brighter and relatively more manageable than organic dyes (artificial dyes made from synthetic chemicals). QDs are highly appreciated in the field of advanced optics due to advance technology in the manufacturing of extremely efficient solar cells. Photons (from sunlight) knock out electrons from semiconductors and transfer them into a circuit to make useful electric power. But the efficiency of the solar cell through this process is quite low in a conventional solar cell. QDs generally give rise to more electrons (or holes) for each photon that bombard them, therefore potentially en­ hancing the efficiency. Charge-coupled devices (CCDs) and complementary metaloxide semiconductor (CMOS) sensors, which are the image-detecting devices in digital cameras and webcams, work by transforming incoming light into patterns of electrical signals. QDs have shown efficient performance improvement in CCD and CMOS to counter the limitation of conventional structures. QDs are also being used in computer screens and displays, where they give three important advantages. First, QDs can be used to give the light of any color, so that colors of a QD display will become much more realistic. Second, QD generates light, so it does not require a backlight and make the display unit much more energy efficient (reduce battery life). Third, the size of QDs is much smaller compared to liquid crystals, thus, resulting in a higher-resolution image. QDs are also brighter

38

Quantum Dots and Polymer Nanocomposites

than other competitive technology like organic LEDs (OLEDs). QDs are also used in a QD TV to yield brighter pictures and are much more realistic than other conventional technologies.

2.7.2 QUANTUM COMPUTING Currently, the fast processing of information in computing is limited by the speed of the electron in material under conventional technology. To overcome this, data should be stored and transmitted in the form light instead of electrons. Optical computers could use QDs similar to electronic computers using transistors (elec­ tronic switching devices), as fundamental components for memory and logic gates. The bits (binary digits) are stored by individual atoms/ions (not by transistors) and transmitted as photons (not electrons); thus called quantum bits (or qubits) in a quantum computer. These QD-based “switches” can store necessary data values and, at the same time, work on other problems simultaneously. Practically it is impossible to control individual atoms, but a QD (on a considerably larger scale) system is highly feasible and easy to control.

2.7.3 BIOLOGICAL

AND

CHEMICAL APPLICATIONS

QDs have numerous useful applications in medical sciences, including deadly cancer detection and treatment. QDs are designed to accumulate in specific parts of the body and later transfer anti-cancer drugs bound to QDs to the target body part. The main advantage of QD is that it is targeted at single organs (i.e., liver), so the unwanted side effect of the drug on the other organs can be reduced (generally happen in chemotherapy). QDs can be used as a replacement for organic dyes in various biological research. Contrary to organic dyes, which operate over a limited range of colors and generally degenerate quickly, QDs are very bright, can be used to produce any color of light, and relatively much (assumed to be photostable). QDs are also being tested for the potential use as sensors to detect chemical and bio­ logical agents for warfare (i.e., anthrax). Some basic research in the recent year also supports the use of QDs to provide specific colors to the plant for maximum overall growth. At the same time, the use of specific light color for maximum photo­ synthesis can be used for maximum solar cell efficiency.

2.7.4 QDS

IN

MEMORY APPLICATIONS

Memory devices made from tiny islands of semiconductor QDs have shown extreme potential for maximizing the capacity of today’s computer, allowing fast storage and lifetime. It is found that information can be written in just a few nanoseconds using QD-based memory devices. A computer has fast DRAMn(dynamic access memory), to store information for the short term. But, DRAM does not store data for long, and it must be refreshed more than several times per second to save data. Moreover, flash memory, which is commonly used in memory cards, is smaller and more realistic for long-term storage. Flash memory is capable of storing data for years without the need of refreshing, but writing is ~1,000 times slower than DRAM.

What Are Quantum Dots

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FIGURE 2.10 Schematic illustration for (a) storage, (b) write, and (c) erase processes in the hole storage-based QD flash memory. Reprint with permission from [ 68]. Copyright 2008: AIP Publishing.

New research displays that quantum dots-based memory devices can provide the best of both: long-term storage and write speeds nearly equal to DRAM. A highly dense array of the tiny islands (~15 nanometers each) can store one terabyte of data in one square inch. Dieter Bimberg et al. have shown that it is possible to write information quickly in just six nanoseconds in a QD-based device [67,68]. The storage, write, and erase process in the QD flash memory is illustrated in Figure 2.10 [68]. The initial prototype work on QD-based memory has already demonstrated excellent performance and almost as fast as a DRAM. The quickest write time using a QD-based device is possible by blending two semiconductors, indium arsenide, and gallium arsenide. Similarly, QDs of other materials have also shown very fast writing of about 14 nanoseconds. On the other hand, digital data 1s and 0s are transformed by shunting electrons in/out of an electrically isolated silicon in flash memory. But an “energy barrier” must be overcome in this process, which is a relatively slow process that gradually degrades the quality and lifetime of a memory device. QD works in a somehow similar manner: data storing involves shunting the electrons of a QD into other energy bands. But QD memory is faster and more robust due to an electric field that temporarily lowers the energy barrier. QDs can provide much faster performance. Compared to DRAM or flash, the physical limit for write time in QD-based memory is in the picosecond range. Thus, a better device prototype is 100 times faster than the present DRAM. The use of QDs in the memory applications is very important for the computing industry. Therefore, this device promises to bridge the gap be­ tween DRAM and flash [69,70].

2.8 SUMMARY QDs are extremely tiny particles in the range of a few nanometers that possess distinct optical and electronic properties compared to larger particles due to quantum confinement. These exciting optical and electronic properties make them very important in nanotechnology. Colloidal QDs are easier to synthesize and fabricate QDs-based electronic devices. QDs are extremely useful in optical

40

Quantum Dots and Polymer Nanocomposites

applications due to their bright, pure colors and their ability to emit a rainbow of colors, coupled with their high efficiencies, longer lifetimes, and high extinction coefficient. Applications of QDs include LEDs, other solid-state lighting, displays, photovoltaics, etc. QDs are also used in quantum computers and flash memories. Recent applications of QDs are in biomedical applications and thus have the potential to revolutionize the medical industry.

ACKNOWLEDGMENTS We gratefully acknowledge the director, the divisional head, and officer-in-charge of Defence Laboratory, DRDO, Jodhpur (Rajasthan), India, for their support and encouragement. Dr. Deepak Kumar Jarwal acknowledges DRDO, Ministry of Defence for providing the Research Associateship.

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Synthesis of Quantum Dots Sourav Paul and Uttam Kumar Ghorai Department of Industrial and Applied Chemistry, Swami Vivekananda Research Centre, Ramakrishna Mission Vidyamandira, Kolkata, West Bengal, India

CONTENTS 3.1 3.2 3.3

Introduction.....................................................................................................45 Physical Strategy ............................................................................................ 46 Chemical Strategy ..........................................................................................47 3.3.1 Colloidal Quantum Dot Synthesis via Micellar Synthesis ...............47 3.3.2 High-Temperature Injection Organometallic Synthesis of QDs.......47 3.3.3 Organometallic Synthesis of QDs by Noninjection Method ............ 49 3.3.4 Synthesis of Hydrophilic QDs ...........................................................50 3.4 Preparation of QDs Using Biosynthetic Approach .......................................56 3.5 Scaling-Up Aspect of the QD Synthesis .......................................................56 3.5.1 Microreactor or Microfluidic Synthesis of QDs ...............................57 3.5.2 Synthesis of QDs by Rotating Packed Bed Reactor.........................59 3.5.3 Synthesis of QDs Using Spray-Based Technique.............................60 3.6 Conclusion and Prospect................................................................................60 Acknowledgments....................................................................................................62 Conflict of Interest...................................................................................................62 References................................................................................................................ 62

3.1 INTRODUCTION The synthesis of quantum dots (QDs) has made rapid progress in the last score of years; various techniques have been developed not only to prepare but also engineer the size of the QDs. Conventionally there are two strategies employed to synthesize QDs; firstly the physical strategy (does not involve any chemical changes) and secondly the chemical strategy (necessarily involving chemical changes leading to the formation of a product). The techniques used to synthesize GQDs can be categorized into two sections on the basis of nanotechnology: 1) “top-down perspective” in which QDs originate from exfoliated or fragmented through physical, chemical, and electrochemical methods; 2) “bottom-up perspective” where the formation of QDs can be controlled by a sequential step-by-step chemical reaction using molecular precursors. DOI: 10.1201/9781003266518-3

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Quantum Dots and Polymer Nanocomposites

FIGURE 3.1 The three layered pie chart demonstrating the four broad domains of QDs synthesis strategy, like physical, chemical, pilot scale, and biosynthesis approach.

In terms of engineering scalability, QD synthesis can be classified on the basis of reactor and the continuous and the batch process. Figure 3.1 Herein we explain the synthesis process in a synergistic way so that we can classify the QDs’ synthesis in a holistic manner and thereby express it in accordance with the conventional and nanotechnology as well as in an engineering aspect.

3.2 PHYSICAL STRATEGY The physical strategy involves the seeding (nucleation) and growth of particles in the vapor phase or in the liquid phase epitaxial growth for theoretical prerequisite of QDs with specified size distribution [1,2] The practical applicability of the physical approach of preparing QDs lies in the molecular beam epitaxy [3] In this process, the beams in atomic or molecular regime produced by a specific source are de­ posited on a crystalline substrate with atomically smooth surface in an ultrahigh vacuum condition to minimize the turbulence caused by air molecules. Thereby, QDs are spontaneously formed and get aligned into a periodic way on the crys­ talline substrate that shows the self-organization of materials in thin epitaxial films to facilitate the phenomenon of stress relaxation. The process demonstrates a unique feature to prepare QDs with skewed size distribution and investigate the quantum size effect. The negative aspect of the method lies in the requirement of capitalintensive complex equipment and material of very high purity and also the difficulty to separate the QDs from the substrate in the case of a vapor phase process [4,5].

Synthesis of Quantum Dots

47

3.3 CHEMICAL STRATEGY In the following text, we will discuss various techniques of chemical methods employed to synthesize QDs. The liquid phase can produce highly dispersed QDs with a lower-energy intensive process as it involves preparation of QDs’ colloidal nature involving micellar synthesis, high-temperature colloidal synthesis by hot injection organometallic synthesis, refluxing, non-injection organometallic synthesis, biosynthesis, and aqu­ eous synthesis; all the synthesis techniques are carried out in batch-type reactors. Thus, from the scale-up point of view, continuous reactors are preferred, which includes the spray-based technology [6], microchannels [7], and rotating packed bed reactor [8]. In this section, we will elaborate in a very specific manner about the synthesis of colloidal QDs prepared by a chemical route in a solution phase, focusing on the green synthesis with large-scale preparation of QDs.

3.3.1 COLLOIDAL QUANTUM DOT SYNTHESIS

VIA

MICELLAR SYNTHESIS

QDs prepared by colloidal synthesis gain prominence during early 1990s through the ‘water-in-oil’ reverse micro-emulsion proposition. The process stands on the intermicellar exchange of reactants during the ongoing chemical reaction, resulting in nucleation following the growth of QDs. The process has been used for suc­ cessful synthesis of metal [9], metal oxide [10,11], metal halide [12], and other products [13]. It was assumed for quite a while that after nucleation the growth of the QDs in reverse microemulsions is constricted by the reverse micelle shell, followed by the fact that as a reverse micelle is monodisperse in nature, therefore the QDs synthesized by the reverse micellar technique should be monodisperse. Therefore, abiding by the mentioned argument, the size of the QDs can be con­ trolled due to the fact that the reverse micelle size can be precisely controlled by adjusting the surfactant and water concentration, thereby fine-tuning the hydrophilic and lyophilic balance. Tovstun et al. [14] studied and concluded that a complex mechanism is undergone during the stabilization of the nanoparticles, and other bottlenecks of the process lie on the fact that the yield of synthesized QDs is re­ latively low with a widespread variance recorded in the size distribution of the formed QDs.

3.3.2 HIGH-TEMPERATURE INJECTION ORGANOMETALLIC SYNTHESIS

OF

QDS

A conventional way to prepare QDs is by the route of high-temperature colloidal synthesis that involves heating organic solvents and injecting the semiconductor precursors, which was pioneered by Murray, Norris, and Bawendi [15], where they have prepared CdS, CdSe, and CdTe colloidal QDs with bright tunable lu­ minescence employing a simple single-step reaction by the pyrolysis of organo­ metallic precursors at 300°C. To synthesize CdSe QDs, a room-temperature solution mixture of precursors (elemental Se and dimethylcadmium (Cd(CH3)2)) in tri-n-octylphosphine was injected rapidly into heated tri-n-octylphosphine oxide (at 300°C) in the presence of inert atmosphere. The reaction propagated with

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Quantum Dots and Polymer Nanocomposites

sudden nucleation and the solution temperature dropped to 180°C; no further nu­ cleation was noticed. The temperature of the solution was gradually increased to 240 ± 260°C and maintained in this temperature range for the entire nanocrystallite growth process. The tri-n-octylphosphine oxide (TOPO) was used for dual role, first it acted as a solvent having high boiling temperature and secondly it played the role of stabilizing agent. QDs with varying sizes from 1.5 to 11.5 nm are produced in a couple of hours by administering the temperature of growth [15]. This synthetic strategy yields high-quality QDs; that is, (i) nucleation and growth are separated in time and (ii) annealing of nanoparticles is possible. The high-temperature technique allows one to produce defect-free QDs with a fairly high quantum yield of fluor­ escence and narrower size distribution of particles compared with other liquidphase methods. However, the negative aspect of the process lies in the use of re­ actants like dimethyl cadmium, which is expensive, highly toxic, and explosive in nature and hence shuts the door for large-scale synthesis of QDs. The achievement of the process of high-temperature organometallic synthesis of QDs was demon­ strated by Peng et al., where they showed a greener route to produce CdSe QDs using cost-effective and less toxic CdO replacing the Cd(CH3)2 [16]. It should be heeded that the rudimentary constituents of metal(s) QDs prevalently uses reagents of toxic nature. Although the QDs synthesized using this strategy hold a reputation in research and of its practical application, to cite an example, CdSe QDs showed great potential in terms of luminescence properties; however, owing to the carci­ nogenic effect possessed by cadmium, its practical utility was minimized [17]. The problem of the application CdSe QDs can be partly mitigated by the coating of the former being coated or wrapped precisely using SiO2 or polymer of amphiphilic nature [18–20], this strategy minimizes the lixiviating of Cd2+ ions from the QDs core; however, the problem exists with respect to the practical utility of CdSe QDs. Due to the growing concern of environmental safeguard worldwide research are going for “greener synthetic route” to produce QDs using less toxic reagents, solvents of lower hazards, and increasing the efficiency and yield of the process, and after arduous effort “greener” high-temperature organometallic synthetic technique have been established [21], the use of TOPO, a coordinating solvent has been well substituted with the use of octadecene (OE) having non-coordinating nature [22], yet by the replacement of TOPO did not cause any depreciation of the properties exhibited by the synthesized QDs implying new parameters of synthesis [23]. To further simplify the reaction for a greener approach and to cut down the cost of synthesizing QDs, Deng et al. used oleic acid and paraffin liquid as the medium of reaction instead of TOPO [24]. The judicious approach of selecting the precursor and optimizing the parameters of high-temperature organometallic synthesis different QDS can be prepared [25–27]. The lower toxicity cadmium-free QDs are researched and have been successfully synthesized which includes the InP, GaAs, InAs, etc. The InP showed brilliant properties and a potential substitute of CdSe [28]. The problem of synthesizing InP QDs using the high-temperature organometallic synthesis using the solvents, ligands, precursors were used for CdSe QD synthesis and gave inferior results in terms of properties’ attributes. The seeding followed by the growth of the QDs of a pre-determined size took a longer duration for its preparation [29].

Synthesis of Quantum Dots

49

FIGURE 3.2 (a) Synthesis of Cs2SnI6 nanocrystals (left side) and picture of the asprepared Cs2SnI6 samples under UV light (right side); (b) TEM images of Cs2SnI6 QDs (inset of B provides HRTEM image). Reproduced with permission from ref 34. Copyright © 2016, American chemical society.

Simultaneously, enormous research have been carried out on QDs with colloidal nature, namely perovskite QDs [30–32]. Protesescu et al. [33] synthesized per­ ovskite CsPbX3 (X = Cl, Br, I) inorganic nanocrystals by injecting cesium oleate, a precursor solution into lead halide solution (PbX2 (X = Cl, Br, I)). The lumi­ nescence properties of the synthesized CsPbX3 perovskite QDs seem to be com­ parable to the brilliantly luminescent CdSe QDs [33]. The further movement of research on the road of detoxification production of QDs, Wang et al. [34] produced lead-free yet stable perovskite derivative of Cs2SnI6 nanocrystals using the facile high-temperature injection process with the capability to control the shape of the nanocrystal. This process opened up new horizons to synthesize and precisely control the shape of the new array of perovskite QDs. Figure 3.2

3.3.3 ORGANOMETALLIC SYNTHESIS

OF

QDS

BY

NONINJECTION METHOD

The high-temperature injection organometallic synthesis of QDs involve solution injection of multi-precursor solutions into the heated mixture of organic solvents in a batch process, ensued by rapid uniform mixing of the solution. However, the method requires optimization of the chemical reaction parameters like injecting speed of the precursor solution, temperature control, and rate of stirring for synthesis of QDs of superior quality achieving the desired physico-chemical properties, which is achievable in lab scale but when the scaling up of the process comes in question, the process meets with serious difficulties like the reprodu­ cibility in properties of the synthesized QDs varies from one batch to another. This problem elevates and amplifies when large-scale synthesis of QDs is taken into consideration. Hence, to address the mentioned problem synthesis of QDs by the organometallic synthesis by noninjection strategy are adopted with the ob­ jective of production of QDs in massive quantities. Pradhan et al. [35] reported a one-pot versatile synthetic method to produce tunable metal sulfide nanoparticles using a common precursor at ambient condition. They showed by applying heat to the metal xanthate that acts as a precursor present in a strong lewis-base solvent, the metal sulfide nanoparticles forms at a

50

Quantum Dots and Polymer Nanocomposites

low-reaction temperature of 50°C [35]. They have also demonstrated the control of particle size control by regulating the reaction temperature and concentration that follows the synthesis procedure of the long-established colloidal La Mer and Dinegar behavior [36]. Several decades later, CaO et al. [37] produced superior quality CdS nanocrystals by a one-pot technique relying on the concept of ther­ modynamic and kinetic control parameters effective on the nucleation step. They have used the nucleation initiators like the chemical accelerators used for elastomer vulcanization, and found success in separating the nucleation from the growth stage, with similar properties of CdS QDs with respect to the high-temperature organometallic synthesis by injection method [37]. The versatile one-pot organometallic syntheses by noninjection strategy have been used for the preparation of various QDs like CdSe, PbS, Ag2S [38–40]. Liu et al. [39] prepared PbS QD by the noninjection organometallic synthesis with low-reaction temperature, the as synthesized QD showed narrow bandwidth with a band gap with wavelengths less than 900 nm. The growth of the PbS QDs occurs at ambient temperature, demonstrating resulting emission in the wave­ length range of 700–900 nm by varying the temperature of growth, duration of the growth, concentration of the reactants, acid: metal feed ratio [39]. Silver sulfide (Ag2S) QDs demonstrated NIR emission for the first time, the pioneering work was reported by Du et al. [40] where they have shown that the pyrolysis of silver diethyldithiocarbamate [(C2H5)2NCS2Ag] in a solution of OE, oleic acid, and octadecylamine resulted in the formation of Ag2S QDs. They have used a combination of various capping agents and solvents that played a vital role to prepare Ag2S QDs of monodisperse nature [40]. However, they have also observed that when only oleic acid as a capping agent and solvent was used instead of combining a solvent and capping agents resulted in the production of soluble yet aggregated Ag2S nanomaterial and it was also found that the Ag2S QDs size can be controlled by varying the composition of the solution [40]. Specifically, the synthesized Ag2S monodisperse QDs with a size of around 10 nm and when excited with 785 nm electromagnetic wave showed NIR emission at 1,058 nm, and also has the potential application for in vivo bioimaging due to the biocompatible nature of Ag2S QDs [40,41]. The advantage of the organometallic synthesis via noninjection process can be utilized for easy scale-up of the reaction as it does not require injection of precursor solution and temperature optimization; hence, an industry friendly process to produce QDs in large quantities. Figure 3.3

3.3.4 SYNTHESIS

OF

HYDROPHILIC QDS

The organometallic synthesis of QDs (high-temperature injection or noninjection process) demonstrates QDs preparation with excellent properties like high quantum yield and high crystallinity with narrow size distribution but they are generally stabilized using hydrophobic ligands like long-chain organic molecules thatare highly soluble in nonpolar solvents for the specific applications like biological and medicinal domain, photovoltaic, and electroluminescence systems [42–44]. So for the use of QDs in the above-mentioned fields, the QDs

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FIGURE 3.3 (a) Schematic representation of synthesis of Ag2S QDs from a single pre­ cursor source of silver salt of diethyldithiocarbamate; (b) TEM images of Ag2S QDs (inset of B provides HRTEM image). Reproduced with permission from ref 40. Copyright © 2010, American Chemical Society.

synthesized by organometallic route require the use of phase transfer chemical reagents to be dispersed in aqueous media, and also the waster generation and the safe disposal of organic solvents used in the organometallic synthesis technique possess a serious threat to environment and an established hazard in terms of safety concern, so when large-scale synthesis of QDs need to be addressed the route is not at all a safer way, whereas water is a “green” solvent and the aqueous route is a safer and environmentally friendlier way to synthesize QDs. The focus of this chapter is to address and explain in a short yet simple manner, so we will not elaborate about the tons of research activities, but we will discuss the strategy behind the development. The following features are adapted for synthesis of hydrophilic QDs: a. b. c. d. e.

Synthesis of QDs in water medium Hydrothermal process Microwave-aided synthesis Refluxing Substituting the hydrophobic ligands used for stabilization of the particles with the hydrophilic ligands f. Surface functionalization with hydrophilic ligands which are beneficial for biological application. g. Wrapping the hydrophobic particles with the secondary shell showing hydrophilicity. h. Ultrasonic treatment/sonochemical method

Thus aqueous phase syntheses of QDs are an alternative route for QD preparation other than organometallic synthesis. The striking advantage of the process of hy­ drophilic QDs preparation via the aqueous synthetic route lies in the lower use of toxic reagents of predominantly organic nature and the reaction temperature to be quite low as compared to the other methods, by exploiting the advantages of this method the question of scalability of QD synthesis can be answered.

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Quantum Dots and Polymer Nanocomposites

Rogach et al. [45] first reported on the successful synthesis of QDs in a water medium possessing fluorescence properties similar to that of the organometallic synthesis route. Generally the synthesis of CdTe QDs in a water medium is carried out by the reaction of Cd2+ and NaHTe in reflux, set up for few hours at 96°C, the as synthesized QDs show photoluminescence emission [45]. The pio­ neering work led to the evolution of aqueous synthesis of CdSe [46], PbS [47], HgTe [48], and CdHgTe [49]. QDs are arduously studied and major improve­ ments have been made in aqueous synthesis of QDs, which includes the strategies like hydrothermal method [50], ultrasonic treatment [51], and microwave-assisted synthesis [52]. The hydrothermal method generally involves heating the reactants for few specified time interval at a temperature of 100°C [50]. The hydrothermal technique has been used to synthesize various QDs like CdS [50], CdSe [53], ZnS [54], and carbon QDs [55]. The advantage of the process comes from the fact that it does not require post-synthetic operations and also the synthesized QDs shows high quantum yield; it also allows for precise control of the QDs’ growth by tuning the parameters like reaction temperature and the time duration of the reaction. Emphasis needs to be made for microwave-aided synthesis, as the research for this process has been on a great rise due to the following reasons: the reaction time of synthesizing QDs is much less and synthesized QDs show high purity with narrow size distribution. The microwave synthesis strategy involves homogeneous and quickly heating the entirety of the reaction solution in short duration. Various QDs have been produced using this technique: CdS [56], CdTe [57], ZnSe [58], composite QDs (exhibiting core shell structure) [59,60], and doped QDs [61]. The versatility of the process is revealed by the fact that hydrophobic QDs can be synthesized using this technique, along with QDs preparation in solvent-free form by solid-state reaction [62–65]. By precisely controlling the experimental para­ meters like the concentration of the precursor, solution pH, reaction temperature, time of the microwave reaction then the QDs size can be controlled and vary the luminescence properties. However the deficiency of the process is revealed by the fact that the produced QDs have relatively wide size distribution. Refluxing method to synthesize QDs is yet another effective tool to produce QDs of specific size, precise control of the QDs particle size during the progress of the reaction as well as enhance the quantum yield of luminescence [66,67]. It has been observed that before refluxing was carried out the absorbance spectra of nano­ particles showed a well resolved band depicting the first excitonic transition al­ though the nanoparticles did not show any luminescence, when the nanoparticles were taken and refluxed at 100°C for few minutes it showed luminescence of weak nature as the colloids have enhanced color intensity. When refluxing process is carried out for a long duration, QDs of different sizes can be produced. The quantum yield of luminescence of QDs depends on the precursor concentration and also on solution pH; Li et al. [68] observed for CdTe QDs stabilized by 3-mercaptopropionic acid luminescence efficiency increase upon refluxing; re­ fluxing made an impact by dramatically increasing the quantum yield of CdTe QDs by 40–67%. The luminescence bandwidth increased from 30 nm to 60 nm with the increase in time duration of reflux treatment, and the synthesized QDs were found

Synthesis of Quantum Dots

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to highly stable. In case of composite QDs having core–shell structure refluxing process aided in enhancement of quantum yield [69]. Water-soluble QDs are prepared by involving the ligand exchange reaction where the QDs stabilized by hydrophobic ligands are substituted with the hy­ drophilic ligands. This technique is used not only to prepare water-soluble QDs but also QDs of biocompatible nature [70]. Generally the QDs prepared in or­ ganic solvents are stabilized with the hydrophobic ligands like oleic acid, TOPO, TOP, dodecylamine, etc., these ligands can undergo exchange reactions, where the hydrophobic end groups are attached to the QDs surface atoms and the other end comprised of the hydrophilic group [71]. Commonly to serve this purpose hydrophilic sulfur containing groups predominantly thiols are used; these include the examples like mercaptocarboxylic acid, cysteine, 2-aminoethanethiol, dihy­ drolipoic acid and dithiothreitol [72–74]. A conventional inference was drawn that almost every QD of hydrophilic nature shown distinct feature that were characterized by luminescence of lower intensity as compared to the initial hy­ drophobic QD. Rahman et al. [75] carried out synthesis of composite QD (CdSe–ZnS) stabilized with oleic acid (hydrophobic nature) then hydrophilized by substituting the hydrophobic ligand using different ligand exchangers of thiol molecules like L-cysteine, thioglycolic acid, mercaptopropionic acid, mercapto­ succinic acid and mercaptoundecanoic acid. All the QDs showed a decrease in intensity of luminescence with an exception to the thioglycolic acid use as ligand exchanger, it exhibited the most intense luminescence among all the thiols put into experiment, the reasons of the observation are described thioglycolic acid are the smallest molecule among all the thiol molecules, and hence it may assist in grater passivation of the QD surface by forming a dense shell in the core shell structure of the QD system. Blum et al. [76] have demonstrated the relation of the QD fluorescence as a function of the alkyl chain length for water-soluble com­ posite QD (CdSe–ZnS), the study revealed that the intensity of the fluorescence of hydrophilic QD increased with the increase in the alkyl chain length with thiol as a terminating group. All the hydrophilic QDs prepared by ligand exchange re­ action showed a lower quantum yield of fluorescence with respect to the initial hydrophobic QD. The reason lies on the interplay of the efficient removal of water molecules in the local environment near to the surface of the composite QD. Figure 3.4 Hence, how to mitigate the problem of the decrease in luminescence intensity which occurs when hydrophobic capping agents are replaced by the hydrophilic ones? A strategy is developed to use polythiols to form stronger bonds with the QD surface. Wang et al. [77] demonstrated that the TOPO functionalized CdSe QDs were phase transferred from organic phase to aqueous environment by ligand exchange reaction using dithiocarbamate. It was observed that the luminescence quantum yield for hydrophilized QD was higher than its hydrophobic counterpart, the reason being effective passivation of the QD surface by dithiocarbamate. Generally, the uses of polydentate ligands with terminal groups are not the guaranteed route to obtain successful results for biphasic reactions of hydrophilic QDs. As shown by Bloemen et al. [78] where they have performed ligand

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Quantum Dots and Polymer Nanocomposites

FIGURE 3.4 Schematic representation of capping agent exchange of TOPO and thiol in order to make the QDs water soluble (left side); Right shows absorbance of QDs solutions with different capping agent in water and toluene; top picture in ambient light and bottom picture under UV light. Reproduced with permission from ref 76. Copyright © 2008, American Chemical Society.

exchange reaction and have replaced the hydrophobic ligands like TOPO and nhexadecylamine used for stabilizing CdSe–ZnS composite QDs by hydrophilic ligands of catechol family to cause the aqueous phase transfer of the QDs, the hydrophilic ligands stabilized composite QDs showed luminescence of lower intensity as compared to the hydrophobic ligands stabilized QDs. Hence, the future of this technique seem to have hit a bump because of the following reasons, first the ligand exchange reactions for biphasic transfer makes a direct impact on the physicochemical properties of the surface atoms of the QDs and generally causes a decrease in the quantum yield of luminescence, second ideal surface passivation is not feasible in an organic phase by the use of hydrophilic ligands, and third the family of thiol stabilizers does not produce QDs, which are highly stable in water and finally due to stability issue aggregate formation occurs and thus leads to precipitation of the QDs. Another important strategy for inducing hydrophilicity in QDs involves the architectural formation of secondary shell of hydrophilic nature on the primary hydrophobic shell encircling the QDs. Thus, any sort of defects produced during the synthesis of QDs encircled by the hydrophobic moiety remains constant throughout the treatment process. Only the secondary shell of hydrophilic nature can be designed in various ways like the micellar formation route, wrapped by the organic shell of the QDs then captured into the secondary layer composed of inorganic moiety (like SiO2) or polymeric wrapping which solubilize in aqueous medium. Hydrophilic colloidal QDs produced by this technique show a very good quantum yield of luminescence. Even though the formation of secondary shell by silicate or polymeric phospho–lipid layer significantly increases the size of the QDs but secures about the prospect of the stability of the QDs in an extensive pH range. As an example, the hydrodynamic diameter of CdSe–ZnS QDs encapsulated with amphiphilic block copolymer system increased from 4 ± 8 to 16 ± 32 nm [79]. The process of biphasic transfer of QDs from polar organic solvent to the aqueous media are feasible by the encapsulation of QDs

Synthesis of Quantum Dots

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with amphiphilic chemical system by the aid of several interactions like specific host–guest, hydrophobic, and electrostatic [70]. The hydrophobic moiety present in the amphiphilic system can interact with the primary hydrophobic layer sur­ rounding the QDs and form the secondary shell encapsulating the QDs, hence assisting in the aqueous solubility by the help of exterior hydrophilic layer [80]. Yong et al. [81] have shown that the CdSe–CdS/ZnS QDs encapsulated with primary layer of TOPO and oleic acid, the alloyed QDs were dispersed in water by wrapping the QD system with the secondary layer formation composed of phospholipids/polyethylene glycol); thus, the absorbance spectra as well as the luminescence spectra remained almost unaffected and the luminescence quantum yield showed a great stability in a wide range of temperature (25 ± 70°C) and pH values from 3 to 10, the microscopic analysis confirms that the aggregate for­ mation does not occur and the growth of QDs occur during the propagation of the encapsulation process and forming the phospholipid micelles. Even different class of cyclodextrins was investigated for the process of water dispersion of the hydrophobic QDs, Depalo et al. [82] conducted a pioneering work in which CdS stabilized by oleylamine were successfully dispersed in water by using β cyclodextrins, the hydrophilization process did not have any impact on the size or the size distribution of the QDs. Organosilicates are also used for the hydro­ philization of the QDs owing to its nontoxic and biocompatible nature due to the existence of functional groups like amine and thiols present in the organosilicate system [83]. Ultrasonic treatment or sonochemical strategy are used to synthesized water dispersible composite QDs (CdSe–ZnS) encapsulated with α, β and γ cyclodextrins as capping agent, based on specific host–guest interaction between hydrophobic primary TOPO layer and the cyclodextrins, it was observed that the peaks of the absorbance spectra and the luminescence spectra did not undergo any shift with respect to the initial QDs encapsulated with only primary shell of TOPO. The transmission electron microscopy results confirms that the size of QDs encapsulated with primary shell are similar to the QDs with primary shell wrapped with a sec­ ondary shell, results in formation of uniform and monodispersed QDs in water. The hydrophilic cyclodextrin wrapped QDs showed superior stability in water and the fluorescence intensity were unaffected in the pH range of 6 to 9 [84]. The prospect of practical application and mass scale production of QDs have faced a setback due to the toxic nature of the composition of the QDs made of lead, mercury, cadmium, etc. which are classified as heavy metal and can cause metal poisoning. Hence for sustainability as well as biocompatibility perspective the eco-friendlier “green” route are developed which includes the synthesis of QDs of nonheavy metals like ZnS [85], ZnO [86], and also core shell QDs like CuInS2–ZnS [87] by the simple aqueous synthesis process. The QDs showing fluorescence in the near-infrared region (NIR) have generated a lot of attention because for the biomedical imaging purpose like in vivo animal organ imaging immobilized within tissues or in vivo dynamic tracking of the tagged stem cell in animals [88,89]. Specifically, Ag2S QDs demonstrated its novelty due to ex­ hibition of brilliant NIR emission along with biocompatible nature and very low toxicity of the QD system [89].

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Quantum Dots and Polymer Nanocomposites

3.4 PREPARATION OF QDS USING BIOSYNTHETIC APPROACH QDs of specifically metal sulfide class like Ag2S [90], ZnS [91], PbS [92], and CdS [93] are biosynthesized using the enzymatic action of the microbes, the bio­ synthesis process involves metal and sulfide ions as the reactant species to prepare metal sulfide QDs and it also open a new dimension of green alternative route to produce QDs. Broadly, there are two biosynthesis routes to produce QDs based on the site of the reaction, whether outside the cell (extracellular) or inside the cell (intracellular). The extracellular biosynthesis technique allows the formation of QDs in medium or on the cell membrane by the enzymatic activity [94]. The intracellular biosynthesis method employs the transport of the ions (mag­ nesium) from outside the cell (medium) to the cytoplasm of the cell and by the action of the enzymes produced within the cytoplasm results into the formation of the QDs [95]. Genetic engineering technique has been employed by Li et al. [96] in yeast cells to biosynthesize CdSe QDs of uniform size in intracellular environment by glu­ tathione metabolic pathway. Spangler et al. [97] have produced PbS and composite QDs PbS–CdS by extracellular biosynthesis technique using genetically engineered strain of stenotrophomonas maltophilia which produces a specific enzyme cy­ stathionine γ-lyase that plays the key role for the biomineralization process of metal sulfide QDs in a buffered conditioned aqueous solution of metal salts and L-cysteine. Another bacterial biosynthesis technique to produce CdS QDs have been reported by Gallardo et al. [98] in which antarctic bacteria possessing psy­ chrotolerant, oxidative stress–resistant behavior have been used to biosynthesize CdS QDs at temperature as low as 15°C. In a head-to-head comparison of biosynthesis of QDs with the chemical synth­ esis of QDs, the biosynthesis route has the following advantages; first, QDs pre­ pared at a relatively lower temperature; second, the entire reaction occurs in the aqueous phase; third, the biosynthesis process is an inherently greener synthesis process; fourth, the probable morphology of QDs achieved that are not feasible by the chemical process [99]; fifth, QDs with enhanced functionality can be prepared [99]; and finally the precursor along with microbes availability can significantly bring down the cost open the channel of scalability.

3.5 SCALING-UP ASPECT OF THE QD SYNTHESIS The engineering aspect for scalable production of QDs are discussed from here onwards, from the industrial production viewpoint continuous reactor become the obvious choice to produce QDs, the critical chemical engineering parameters like rate of stirring, reaction temperature, solution mixing phenomena, and precursor insertion point needs to be precisely optimized to obtain good quality QDs. However, using batch reactors, progress has been made to produce QDs in the order of gram scale. Kim et al. [100] have been successful in producing QDs of subkilogram order where they have synthesized 200 g of CdSe–ZnSe composite QDs after drying in a

Synthesis of Quantum Dots

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single batch process; they employed the strategy of fast injection of precursor so­ lution in a temperature monitored environment. InP QDs have also been synthesized in the order of gram scale per batch (6 g of QDs in a single batch) and reported by Bang et al. [101], they have sublimed easily available red phosphorus to produce white phosphorus (P4), which played the role of precursor; the great result in terms of scalable production of QDs was feasible due to the kinetically slow reaction among the precursors. The size of the synthesized InP QDs can be controlled by tuning different reaction parameters like reaction temperature, time of reaction and the precursor concentration. The drawback of the QD synthesis using batch reactors lies in the difficulty to control and optimize the reaction condition(s) and control the engineering para­ meters, when working with higher volume of solution a necessary step for largescale QD synthesis then the rate of reaction goes pretty high [102], hence, making it further difficult to achieve the reproducibility of the process. As mentioned earlier preparation of QDs using continuous reactor is the pre­ ferred choice over batch reactor from the industrial productivity purpose, as the name suggests the “continuous” reactor process involves precursor feeding into the reactor in a continuous fashion and the production of QDs is in a continuum stream cycle, hence the reproducibility of the process is much higher as the chemical engineering controls like reaction temperature, pressure, and reaction time can be controlled in a precise and superior way than the batch reactor. The properties of the synthesized QDs are consistent and without much degree of variation in properties like size, size distribution, and quantum yield.

3.5.1 MICROREACTOR

OR

MICROFLUIDIC SYNTHESIS

OF

QDS

A flow-controlled microreactor or microfluidic synthesis of QDs; this is a re­ latively new technique [103]. In a gist the microreactor is a continuous reactor where the process demands the precursor QDs to be passed through micro­ channels using a fluid movement control system to the blending zone and the solution moves through various zones with predefined temperature gradient; the temperature control is done by different heating element integrated to the spe­ cific reaction zones [104], conventionally nucleation occurs in the hightemperature zone while the growth of the QDs occur in the lower-temperature zone, the significance of the process lies in the fact that the nucleation step can be separated from the growth phase of QDs [105]. The reaction process control for optimized synthesis of QDs is much better for the microreactor process compared to the traditional batch reactor technique [104], and also the micro­ fluidic process under optimized condition allows one to produce QDs with a very narrow size distribution. The size of the QDs can also be precisely con­ trolled by monitoring the flow rate or in other words reaction time is the mi­ crofluidic system. Microfluidic synthesis was carried out to prepare QDs like CdS, CdSe, CdSeS, and also surface functionalized CdSe and CdTe QDs [106,107]. The ongoing chemical reaction of QD synthesis in the continuous flow microfluidic approach is similar to the high-temperature organometallic synthesis technique (batch process) with the only difference that for microfluidic

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technique the reaction temperature is only 160°C as compared to the 300°C required for the organometallic synthesis [107]. The core-shell or composite QDs like CdS–ZnS, CdSe–ZnS, CdTe–ZnS can also prepared by utilizing this process [107,108]. The advantage of the system over other conventional methods of synthesizing QDs can be summed in these salient features like small-sized continuous reactor setup, a very minute amount of precursor required to produce QDs, the reaction time is quite less, real-time monitoring of the reaction parameters to control the size and size distribution of the produced QDs. The bottleneck of the process lies in the effective heat transfer and mass transfer during the fluid flow of the precursor solution; for instance, if the precursor so­ lution is viscous (long carbon chain solvents) in nature, then mass transfer is not proper owing to the viscous drag faced during the fluid flow, drag force of the fluids, and the wall of the microchannel, which results in turbulence in the fluid when flowing thorough the microchannels and ultimately leads to production of QDs with less precision and wider size distribution. Marre et al. [109] have reported on the issue of viscous drag and its effect, in their work supercritical continuous micro flow synthesis of QDs with narrow size distribution they have applied high pressure to tame the viscous drag. The current scenario of this process seems to be still on lab scale, although there is high possibility of large-scale synthesis of QDs after addressing a few issues like the experimental setup condition while scaling up, mixing of the precursor solutions, effective heat transfer, and most importantly mass transfer. Figure 3.5

FIGURE 3.5 (a) Schematic diagram of the microfluidic reaction setup for the synthesis of CdSe/ZnS and CdS/ZnS QDs (S1-syringe pump with Se precursor, S2-syringe pump with S precursor, S3-syringe pump with Cd-OA-OLA, S4-syringe pump with Cd-OA-OLATOPO, Y-Y conjunction, M-micromixer, V-stop valve, C-channel). (b) PL spectra and photo of synthesized CdS/ZnS and CdSe/ZnS QDs in ambient light and UV light, re­ spectively. Reproduced with permission from ref 108. Copyright © 2009, American Chemical Society.

Synthesis of Quantum Dots

3.5.2 SYNTHESIS

OF

QDS

59 BY

ROTATING PACKED BED REACTOR

The rotating packed bed reactor ensures proper mass transfer by using the prin­ cipal application of high gravity field; the reactor has a fixed casing containing a packed bed-type rotator which rotates at a high speed of a few thousands rpm. The liquid precursor solution enters through the hopper of the reactor through the fluid inlet tube and then uniform spraying is done by the slots present in the tube distributor on the interior edge of the rotator. The reactant solution moving through the packed bed rotator is made to split into droplets of micro or nano dimension, or into film shape by the action of the fluid flow in the radial direction due to rotational effect of the reactor [110], moving through packing rotator and exterior side of the shell; ultimately collection is made and the product solution leaves the reactor through the exit tube. The gas enters into the system by the inlet and flows in countercurrent direction with respect to the fluid flow and finally exits from the outlet by the action of pressure gradient. This principle ensures proper mixing at the micro or even in nano order and hence optimized mass transfer takes place, which does not necessarily happen in traditional batch process or even in microreactors [111,112]. The process leads to intensification of the mass transfer phenomena; thus, several QDs like ZnS [113] and ZnO–SnO2 [114] have been synthesized using this technique, although the size of the QDs prevailed in the nano dimension but the synthesized QDs did not exhibit any luminescence properties [113,114]. Thus, by using the rotating packed reactor, the mass transfer phenomena can be intensified along with homogeneous micro mixing, thereby ensuring homogeneity and reproducibility in the process of production of QDs. It is to be clearly mentioned that to the best of our knowledge, we have not come across any public scientific reports on producing various other types of QDs using this process; hence, there is a huge prospect for future research related to this specific topic. Figure 3.6

FIGURE 3.6 (a) Schematic representation of experimental setup in rotating packed bed reactor. (b) Picture of actual rotating packed bed reactor. Reproduced with permission from ref 114. Copyright © Elsevier.

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FIGURE 3.7 (a) Schematic representation of pneumatic thermospray apparatus. (b) Demonstration of stream production of unsupported cadmium sulfide nanocrystals when solvent vaporization occurs from the stream droplets. Reproduced with permission from ref 115. Copyright © 2005, American Chemical Society.

3.5.3 SYNTHESIS

OF

QDS USING SPRAY-BASED TECHNIQUE

In general, the synthesis of QDs uses organic ligands for various reasons like dispersion purposes and resist the aggregation of the QDs, or for alloyed QDs. The use of such ligands severely enervates chemical viability of the QDs morphological interaction with electromagnetic waves and thus creates limitation when transfer of energy of QDs is subjected, or electron movement becomes an important issue due to its photoluminescence application. Hence, to overcome the limitations of the traditional liquid based synthesis strategy, a probable solution is based on the spray technique to synthesize QDs, the unique technique has been developed by Amirav et al. [115]. They have success­ fully prepared solely QDs (CdS, MnS) without coating and without substrate support, by strategically using this process where the precursor solutions are sprayed at an elevated temperature using a nebulizer to form spherical droplets of monodisperse nature. With the forward movement of the droplets, the precursor achieves saturation and thereby the solvent starts evaporating and spontaneous condensation of the salt occurs. Thus, when the entire solvent vaporizes the droplet from a single spray, it can produce QDs [115,116]. Apart from QDs like CdS [115] amd MnS [6,116], various other QDs can be prepared by this process like MoS2 [118] and ZnS [117,118]. There is a huge prospect for this spray-based technique when uncoated or unencapsulated QDs need to be prepared, or substrate-free QDs in a packed condition are to be supplied. Figure 3.7

3.6 CONCLUSION AND PROSPECT QDs have garnered significant interest due to their potential multifaceted applica­ tions ranging from the fields of energy, electronics, optoelectronics, and bio­ technology. From the galaxy of published research articles on the synthesis of QDs, a single chapter will surely not be able to explain all the aspects of the problem. Therefore, we have solely focused on the key arena of QD preparation from the

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61

industrial scalable perspective. As of now, the production of QD still remains a capital intensive process with hefty cash burns and thus creates an important barrier for the technology to grow; therefore, QD technology is taking a very long time to mature [119]. The widespread application of QDs will be exploited if and only if the cost of the production of QDs are reduced significantly and large-scale synthesis processes are not developed with size tunable QDs with the specified morphological structure. From a practical utility viewpoint, the preparation followed by the aftertreatment of QDs needs to be done with the focus of increasing the quantum yield, narrowing the size distribution and finally enhancing the stability of QDs. The engineering aspect of the QD synthesis has just initiated and working to­ wards the goal of successfully developing a robust large-scale process to prepare QDs; in order to achieve the goal, there will be communication between the en­ gineers (chemical, mechanical, etc.) and the material scientists (chemists, physicist, etc.) who have already achieved the optimized QD synthesis in the lab scale setup. We have tried to represent from the pilot scale viewpoint, addressing the problem in a balanced way, where laboratory scale and the large-scale viability of a process (continuous process and batch process) are discussed, keeping in mind the less expensive route and also the eco-friendly green route of synthesis of QDs. The prospect of the growth in the field of QDs synthesis is immense, as a great number of synthesis processes have already been reported and newer synthesis routes are con­ tinuously developed. There is ample scope to create a new synthesis strategy of QD preparation with well-defined specified properties usable for tailored-made products. With the galloping progress in the field of QD synthesis, some very important features need to be addressed in order to safeguard the environment: a. In order to make a green process, a green precursor needs to be used instead of using less expensive hazardous chemicals. The heavy metal QD does not allow any use of green precursors entirely, thus promoting toxicity to the environment and enhancing pollution. b. An alternative of heavy metal QDs should be researched upon; if we can develop novel green QDs with similar properties like the heavy metal QDs, then it will be a big step towards a true greener alternative. c. The green process to synthesis of QD with optimized reaction parameters are still in the research and development stage. d. Development of an analytical control by digital or analog devices for realtime monitoring and optimization of the synthesized QDs. e. We should strike a balance of cost effectiveness and eco-friendly process in order to develop an industrial feasible process, ensuring the quality of the synthesized QDs. f. From our perspective, the continuous production process of QDs like microreactors and rotating packed reactors has shown great potential due to their micromixing and process intensification. Worldwide arduous research efforts and the following advances are made to develop QDs for their widespread application of paramount importance. It is our sincere belief that further research will be investigation about the concerned research problem and it

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will boost the overall development of this field of study. We aspire to the fact that this chapter can generate further new ideas to produce QDs and venture on the fostering of large-scale QD preparation for industrial applications.

ACKNOWLEDGMENTS U.K.G. acknowledges SERB, Govt. of India, for providing the Teachers Associateship for Research Excellence (TARE) fellowship and research grant (TAR/2018/000763). U.K.G. acknowledges the central DST-FIST program (SR/ FST/College-287/2015) for financial support. U.K.G. thanks the DBT Star College Scheme (BT/HRD/11/036/2019) for funding.

CONFLICT OF INTEREST There is no conflict of interest.

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90. Suresh, A. K.; Doktycz, M. J.; Wang, W.; et al. 2011. Monodispersed Biocompatible Silver Sulfide Nanoparticles: Facile Extracellular Biosynthesis Using the γ-Proteobacterium, Shewanella Oneidensis. Acta Biomater. 7, 4253−4258. 91. Labrenz, M.; Druschel, G. K.; Thomsen-Ebert, T.; et al. 2000. Formation of Sphalerite (ZnS) Deposits in Natural Biofilms of Sulfate-Reducing Bacteria. Science 290, 1744−1747. 92. Seshadri, S.; Saranya, K.; Kowshik, M. 2011. Green Synthesis of Lead Sulfide Nanoparticles by the Lead Resistant Marine Yeast, Rhodosporidium Diobovatum. Biotechnol. Prog. 27, 1464−1469. 93. Prasad, K.; Jha, A. K. 2010. Biosynthesis of CdS Nanoparticles: An Improved Green and Rapid Procedure. J. Colloid Interface Sci. 342, 68−72. 94. Narayanan, K. B.; Sakthivel, N. 2010. Biological Synthesis of Metal Nanoparticles by Microbes. Adv. Colloid Interface Sci. 156, 1−13. 95. Hosseini, M. R.; Sarvi, M. N. 2015. Recent Achievements in the Microbial Synthesis of Semiconductor Metal Sulfide Nanoparticles. Mater. Sci. Semicond. Process. 40, 293−301. 96. Li, Y.; Cui, R.; Zhang, P.; et al. 2013. Mechanism-Oriented Controllability of Intracellular Quantum Dots Formation: The Role of Glutathione Metabolic Pathway. ACS Nano 7, 2240−2248. 97. Spangler, L. C.; Lu, L.; Kiely, C. J.; 2016. Biomineralization of PbS and PbS-CdS Core-Shell Nanocrystals and Their Application in Quantum Dot Sensitized Solar Cells. J. Mater. Chem. A. 4, 6107−6115. 98. Gallardo, C.; Monrá s, J. P.; Plaza, D. O.; et al. 2014. Low-Temperature Biosynthesis of Fluorescent Semiconductor Nanoparticles (CdS) by Oxidative Stress Resistant Antarctic Bacteria. J. Biotechnol. 187, 108−115. 99. Yang, Z.; Lu, L.; Kiely, C. J.; et al. 2016. Biomineralized CdS Quantum Dot Nanocrystals: Optimizing Synthesis Conditions and Improving Functional Properties by Surface Modification. Ind. Eng. Chem. Res. 55, 11235−11244. 100. Kim, J. I.; Lee, J. K. 2006. Sub-Kilogram-Scale One-Pot Synthesis of Highly Luminescent and Monodisperse Core/Shell Quantum Dots by the Successive Injection of Precursors. Adv. Funct. Mater. 16, 2077−2082. 101. Bang, E.; Choi, Y.; Cho, J.; et al. 2017. Large-Scale Synthesis of Highly Luminescent InP–ZnS Quantum Dots Using Elemental Phosphorus Precursor. Chem. Mater. 29, 4236−4243. 102. Haber, J.; Kashid, M. N.; Renken, A.; et al. 2012. Heat Management in Single and Multi-Injection Microstructured Reactors: Scaling Effects, Stability Analysis, and Role of Mixing. Ind. Eng. Chem. Res. 51, 1474−1489. 103. Mello, A. J.; Habgood, M.; Lancaster, N. L.; et al. 2004. Precise Temperature Control in Microfluidic Devices using Joule Heating of Ionic Liquids. Lab Chip 4, 417. 104. Marre, S.; Jensen, K. F. 2010. Synthesis of Micro and Nanostructures in Microfluidic Systems. Chem. Soc. Rev. 39, 1183−1202. 105. Yang, H.; Luan, W.; Tu, S. T.; et al. 2008. Synthesis of nanocrystals via micro­ reaction with Temperature Gradient: Towards Separation of nucleation and Growth. Lab. Chip. 8, 451. 106. Abdelhady, A. L.; Afzaal, M.; Malik, M. A.; et al. 2011. Flow Reactor Synthesis of CdSe, CdS, CdSe/CdS and CdSeS Nanoparticles from Single Molecular precursor (s). J. Mater. Chem. 21, 18768. 107. Kikkeri, R.; Laurino, P.; Odedra, A.; et al. 2010. Synthesis of CarbohydrateFunctionalized Quantum Dots in Microreactors. Angew. Chem., Int. Ed. 49, 2054−2057. 108. Yang, H.; Luan, W.; Wan, Z.; et al. 2009. Continuous Synthesis of Full-Color Emitting Core/Shell Quantum Dots via Microreaction. Cryst. Growth Des. 9, 4807.

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4

Optical Properties of Quantum Dots Poushali Das School of Biomedical Engineering, McMaster University, Hamilton, Ontario, Canada

Syed Rahin Ahmed School of Engineering Practice and Technology, McMaster University, Hamilton, Ontario, Canada

Seshasai Srinivasan and Amin Reza Rajabzadeh School of Biomedical Engineering, McMaster University, Hamilton, Ontario, Canada School of Engineering Practice and Technology, McMaster University, Hamilton, Ontario, Canada

CONTENTS 4.1 4.2

Introduction.....................................................................................................69 Approaches for Tuning the Optical Characteristics......................................70 4.2.1 Regulating the Intrinsic Characteristics of QDs ...............................70 4.2.2 Modulation of the Surface ................................................................. 72 4.2.3 Doping Methods .................................................................................73 4.3 Optical Properties ...........................................................................................74 4.4 Photostability of Quantum Dots ....................................................................76 4.5 Current Theories for PL Mechanisms ...........................................................77 4.5.1 Recent Developments in Understanding Photoluminescence ...........78 4.6 Conclusion and Future Perspectives.............................................................. 79 References................................................................................................................ 79

4.1 INTRODUCTION The foundation of the nanoscience and nanotechnology was built upon and improved by consistent breakthroughs in the synthesis and fabrication of new materials [1–3]. Without tunable optical features of QDs, the nanoscience and technology and nano­ biotechnology (together referred to as “the Nanoworld”) would not be nearly as in­ triguing as it is today. As the sizes of materials approach the nanoscale, their optical and electrical characteristics become size- and shape-dependent and significantly DOI: 10.1201/9781003266518-4

69

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Quantum Dots and Polymer Nanocomposites

differ from those at the bulk and atomic/molecular levels [4]. QDs are nanometer-sized semiconductor crystals made of elements from groups II to VI or III to V. Particles with sizes smaller than the exciton Bohr radius are called QDs [5]. They are zerodimensional nanostructured materials with limited numbers of electrons that corre­ spond to discrete quantized energy in the density of states. Quantum confinement is considered to be the most important property of QDs [6–8]. The quantum confinement effects occur when the size of the nanoparticles decreases to a point below a critical size, at which point the distance between adjacent energy levels becomes greater than kT (where k is Boltzmann’s constant and T is temperature) [9]. When particle size falls below a certain size, then a blue shift or an elevated band gap energy can be detected. The phenomenon is termed quantum confinement effect. This effect allows to modify the band gap by varying the size of the QDs. The composition of the QDs is another factor that influences the band gap [10]. The synthesis of QDs can be categorized into top-down and bottom-up methods [11]. QDs possess broad absorption bands, narrow emission spectra, and good light stability [12]. Besides semiconductor QDs, there are many other types of QDs including silicon quantum dots [13,14], carbon quantum dots [15–17], graphene quantum dots [18–20], graphitic carbon nitride quantum dots [21,22], magnetic QDs [23–25], etc. QDs have been employed for different applica­ tions in sensing [26–28], cell labeling, photocatalysis, light-emitting diodes [29,30], solar cell [31–34], and batteries and supercapacitors [35–38] etc. These QDs have been attracting a great deal of attention due to robust properties including multicolored fluorescence, excellent biocompatibility, nontoxicity, good water solubility, and costeffectiveness in comparison to semiconductor QDs and commercial dyes [39–43]. Because of their unique chemical compositions, these QDs have an abundance of surface groups and possess strong photoluminescence [9]. Nevertheless, the pho­ toluminescence mechanism is the topic that has generated the greatest debate. The dominating photoluminescence (PL) center of QDs involves the quantum confinement effect of conjugated π-domains and surface/edge state, along with the synergistic effect of these two variables [44]. This chapter discusses the optical characteristics of QDs, with a focus on adjusting photoluminescence, photostability, existing theories, and new advancements to understand photoluminescence process.

4.2 APPROACHES FOR TUNING THE OPTICAL CHARACTERISTICS 4.2.1 REGULATING

THE INTRINSIC

CHARACTERISTICS

OF

QDS

In general, QDs can be prepared using either a top-down or a bottom-up strategy [45]. The top-down technique is breaking down from the bulk state to the nanoscale level, and the bottom-up is building up from atomic scale to nanoscale. Since the preparation processes are so versatile, there are significant differences in the in­ herent properties of the resulting QDs, such as size, shape, or surface arrangements, which ultimately result in a diversity of the optical properties. Ajayan et al. revealed that the size of the QDs has a major impact on the band gap, which can actually result in various PL emissions depending on the size of the QDs [46]. By con­ trolling the temperature, they fabricated three different types of QDs with distinct particle size distributions (1–4 nm, 4–8 nm, and 7–11 nm). As the energy gap

Optical Properties of Quantum Dots

71

decreased from 3.90 to 2.80 eV, the resulting QDs displayed a range of PL emission colors, from blue and green to yellow. Figure 4.1a displayed the UV visible ab­ sorption spectra of GQDs synthesized at different temperatures (80, 100, and 120°C), respectively. As the temperature increases, a clear blue shift was observed from 330 to 270 nm. The finding indicates that the reaction temperature can have an effect on the absorption characteristics of the as-synthesized GQDs. The digital images of three different GQDs irradiated under UV light are dis­ played in the inset of Figure 4.1a. The GQDs fabricated at different temperatures showed different emission color (blue, green, and yellow). Figure 4.1b indicates that the temperature can influence the emission wavelength distribution of fabri­ cated GQDs. The relationship between the energy gap and the size of GQDs is shown in Figure 4.1c. It is obvious that the energy gap falls from 3.90 to 2.80 eV as the size of the GQDs increases. Figure 4.1d shows the luminescence decay characteristics of the blue GQDs. The lifetime signals of the blue and green GQDs were extremely well-fit to a three-exponential function. The nanosecond lifetime of the QDs indicated that they are promising candidates for optoelectronic and biological applications. Zhang et al. used gel electrophoresis to obtain three sets of GQDs with narrow size distributions, particularly 5.5 nm, 12.5 nm, and 16 nm, denoted as GQDs-1, GQDs-2, and GQDs-3, respectively [47]. The authors demonstrated that the photoluminescence emission of these QDs originated mostly from the peripheral carboxylic functionalities

FIGURE 4.1 (a) UV visible spectra of GQDs prepared at different temperatures A (120°C), B (100°C), and C (80°C), respectively. Inset shows the corresponding GQDs under 365 nm UV light. (b) Photoluminescent emission spectra of GQDs with different emission color excited at 318, 331, and 429 nm, respectively. (c) Relationship between the energy gap and the size of GQDs. (d) TRPL decay profile of blue GQDs. Reproduced with permission from ref. [ 46].

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Quantum Dots and Polymer Nanocomposites

FIGURE 4.2 HRTEM images of QDs showing their primary shapes and corresponding populations (p) with the increasing average size of QDs. Reproduced with permission from ref. [ 48].

and the conjugated carbon skeleton. For the QDs having sizes less than 6 nm, the surface states was involved to the photoluminescence characteristics, and the con­ tribution of the carbon skeleton only turns up when they are excited at a longer wa­ velength. For QDs with large sizes (12 and 16 nm), the larger carbon skeleton is a major contributor to their photoluminescence behavior, while peripheral carboxylic groups have a minor impact. Increasing the size of the QDs would cause a red shift in the emission, while the red photoluminescence emission was not very strong as the quantum effect became less important when the sizes became larger. The sizedependent shape and edge-state phenomena that affect the absorption and PL prop­ erties of GQDs were reported by Kim et al. [48]. Below 17 nm, the QDs tend to form circular or elliptical morphology with zigzag and armchair edges. A polygonal shape with an armchair edge was obtained when the sizes became larger than 17 nm (Figure 4.2). The change in the absorption peak is consistent with the quantum con­ finement effect. On the other hand, the photoluminescent spectra of QDs showed nonmonotonic patterns as the size increased to 17 nm. These non-monotonic behaviors can be related to the size-dependent shape and corresponding edge variations of the QDs.

4.2.2 MODULATION

OF THE

SURFACE

Another efficient method for tuning the optical characteristics of QDs is by regulating the surface chemistry. The presence of different reactive sites and functional groups at the surface and edges of the QDs, such as carbonyl (C=O), epoxy (–C–O–C–), and carboxyl (–COOH), enable them to further functionalize

Optical Properties of Quantum Dots

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with other molecules [49,50]. A high degree of surface oxidation often leads to a red shift in the photoluminescence emission of the QDs. This phenomenon is caused by the surface functional groups generating the surface oxidation states with a series of emissive traps [51]. Although the presence of oxygen function­ alities contributes to the green or red emission of the QDs, they are also prone to cause the non-radiative recombination of localized electron-hole pairs, which will hinder the intrinsic state emission [52]. The interaction of alkylamines with the functional groups such as carboxyl or epoxy leads to the formation of -CONHR and -CNHR, as well as a decrease in non-radiative recombination. The procedure described above not only paves the way for tuning the photoluminescence emission of the QDs from yellow to blue but also elucidates two primary surface modulating strategies for tuning the optical performance of QDs, including sur­ face functionalization and surface oxidation/reduction. The structure and optical characteristics of the QDs can be fine-tuned through surface functionalization. Due to its unique protonation process, diamines are the most efficient organic compounds used to functionalize GQDs [53]. During diamine functionalization, certain cyclic structures are developed at the surface of GQDs, allowing the transfer of ammonium moiety of GQDs to the conjugated structure and inducing strong PL emission [54,55]. Besides amines, polyethylene glycol (PEG) and polyethylenimine (PEI) are also effectively applied for the surface passivation of GQDs [56–58]. The surface passivation not only enhances their PL emission but also provides an intermediate for further modification. The concept of fabricating protecting shells on the core of CdSe QDs is widely recognized. This method eliminates surface defects, enhances the photo­ luminescence quantum efficiencies, offers photostability to the core, and minimizes toxicity by restricting the dissolution of cadmium ions. The synthesis of core-shell quantum dots was mainly reported in 1996 and 1997 by Bawendi et al. [59,60] and Hines and Guyot-Sionnest [61]. These publications paved the way for the possibilities of colloidal QDs to be used in technology and biology. ZnS shells on CdSe cores were obtained from diethyl zinc and hexamethyldisilathiane in a TOP–TOPO mixture at temperatures between 140– 230°C. Although direct aqueous phase synthesis of cadmium chalcogenide nanoparticles was developed more than 40 years ago, the fabrication of size-controlled and highly luminescent CdSe QDs was achieved later [62–66]. The water solubility of the QDs is one of the main limitations of their use in biological applications. Generally, chemical modification of the surface and ligand exchange procedures have been employed to achieve QDs synthesized in organic phases into water-soluble QDs. Mattoussi and colleagues comprehensively categorized different procedures for the conversion of organic QDs to aqueous phases [67]. Recently, numerous varieties of bioconjugated and water-soluble CdSe–ZnS QDs are readily available for purchase in the marketplace for their use in different in vitro and in vivo applications [4].

4.2.3 DOPING METHODS Compared to traditional luminescent materials, doping of QDs with heteroatoms has gained popularity as a unique technique to tune the inherent features of QDs for

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novel applications. Doping with heteroatoms will inevitably lead to structural and electronic distortions, making it feasible to effectively tailor QDs’ electronic features, surface chemistry, elemental composition, and band gap, all of which ultimately have a significant impact on the optical properties [68–70]. Different dopants with various valencies and sizes have been explored and shown to be feasible for modifying the optical characteristics of QDs, particularly photo­ luminescence performance, for applications like biosensing or bioimaging. As a dopant, nitrogen is a good candidate due to its comparable atomic size to carbon and the availability of valence electrons. Nitrogen (N) is a good example of a dopant because its atomic size is com­ parable to that of carbon and because it has plenty of valence electrons that may be used to establish robust valence bonds with carbon atoms [71]. Zeng and coworkers reported the fabrication of N-doped GQDs where ammonium hydroxide was used as a dopant [72]. A simple alteration in the concentration of ammonium hydroxide was done to modulate the N/C atomic ratios. This resulted in the for­ mation of either the pyrrolic structure or substitutional N in the graphene lattices. After N doping, the quantum yield (QY) of GQDs attained as high as 34.5%, which is much higher than the undoped GQDs. The high electron-withdrawing capacity of nitrogen atoms changes the electronic structure of the doped GQDs, further restoring sp2 hybridization and donating delocalized electrons to the π* states, re­ sulting in a more effective photoluminescence radiative emission in N-doped GQDs. Some methods have been attempted to obtain tunable photoluminescence by boron (B) doping. Qiu et al. proposed a hydrothermal approach to achieve B-doped GQDs from B-doped graphene [73]. According to several characteriza­ tions, B atoms were effectively doped into the graphene structures with the atomic percentage of 3.45% and emitted blue color at 440 nm excitation. In addition to the common dopants such as N and B, several other dopants like sulphur [74,75], phosphorus [76,77], chlorine [78], fluorine [79], and potassium [80] have been employed to increase the optical performance of QDs.

4.3 OPTICAL PROPERTIES In general, the electron-hole pair are created in semiconductors due to the ab­ sorption of photonic energy. The diameter of a semiconductor plays a vital role in confining electrons and holes that leads to the quantum confinement effect. In particular, this effect appears when the size of the semiconductor nanocrystal is smaller than its exciton Bohr radius and dominant with decreasing the size of na­ nocrystals. The PL properties of quantum dots occurs when excited electrons relax to the ground state and recombines with the hole. The emission wavelength depends on the size of the quantum dots, for example, the larger quantum dot has higher emissive wavelength (lower energy) and the smaller quantum shows shorter emissive wavelength (higher energy) (Figure 4.3). Over the last several years, considerable progress has been created in developing QDs including ZnS, CdSe, and CdTe because of their optical properties and numerous applications. Moreover, the synthesis of core-shell structure of QDs opens an op­ portunity to improve the photoluminescence properties; for example, CdS or ZnTe

Optical Properties of Quantum Dots

75

FIGURE 4.3 Emission properties of QDs with various sizes.

(shell)-CdSe(core) QDs and CdSe shell-CdTe (core) QDs showed significant im­ provement in emission properties. Furthermore, Cd-based QDs are often coated to lower their toxicity; for example, the use of ZnS QDs as a shell on the top of CdTe QDs has been reported as non-toxic with strong photoluminescence properties. Besides optical properties and biocompatibility, the core-shell QDs are well-known for enhanced photostability and quantum yield. The third-order optical nonlinearity can be achieved from core-shell QDs through fine-tuning of their shell thickness and composition, which has an essential function in optical switching devices and optical data storage. The surface ligands have a strong influence on the optical properties of QDs. Abdelhameed et al. have reported the synthesis of silicon QDs (SQDs) with phenanthrene, pyrene, and perylene chromophores. The results revealed that the PL peak position of SQD was red-shifted by 69 and 65 nm, and blue-shifted by 50 nm with perylene, phenanthrene, and pyrene, respectively, compared to SQDs-heptene. Moreover, the quantum efficiency was enhanced from 8% in SQDs-heptene to 18% and 11% in SQDs-perylene and SQDs-pyrene, respectively [81]. Recently, copper (Cu)-based QDs have received lots of research attention because of their tunable optical properties via changing the ratio of cations that compose the QDs. The in­ troduction of zinc (Zn) into Cu QDs causes a blue shift in its absorption and emission properties [82]. Graphene QDs (GQDs) are very attractive for biocompatibility and attractive optical properties in biological fields. The optical properties of GQDs not only depend on the size and shape but also the carboxylic and carbonyl groups on the surface and edges that provides an opportunity to tune the optical properties through conjugation with different polymer, organic and inorganic molecules, and biological substances [83]. Those groups create “surface oxidation states” and result in the tunable of GQDs emission properties. Zhang and co-workers have reported a synthesis method of different-sized GQDs with varying emissive properties because of the π*-n transitions of carboxylic or carbonyl groups of GQDs [47]. Aminofunctionalized GQDs also contribute to tailoring the optical emission properties of GQDs. For example, Tetsuka et al. reported the amino-functionalized GQDs emitting multiple colors with single excitation [54]. In some cases, the PL intensity of GQDs

76

Quantum Dots and Polymer Nanocomposites

shifts to blue emission after functionalization with GQDs, for example, green GQDS with a PL intensity at 500 nm shifted to 405 after functionalization with the amino group [55]. Moreover, red-shifting of GQDs PL intensity was observed when func­ tionalized with the alkylamines group [84].

4.4 PHOTOSTABILITY OF QUANTUM DOTS The photostability of quantum dots is very important in considering their practical applications in different fields. Several factors can influence the stability of quantum dots; for example, temperature and oxygen and water molecules [85,86]. A con­ tinuous exposure of photoexcited QDs in the presence of oxygen molecules induces the creation of surface etching and ultimately initiates fluorescence quenching and broadening of the emission spectrum [87]. A protective shell would be a promising idea to overcome this drawback. The water sensitivity of organic amino salts is another reason for photobleaching for several QDs. Photoexcited QDs generate free radicals in the presence of oxygen or water molecules. Then, amino salts react with free radicals, resulting in QD surface defects [88–90]. The stability of QDs at higher temperatures is important since most of the light-emitting device operate at above the room temperature. Thermal heating may create temporary QD surface defects that can trap emissions and cause PL quenching. However, repeated heating and cooling may cause permanent surface defects even at core-shell QDs [91–93]. The stability of QDs can be enhanced by proper design of the shell, ligand, and overcoating. One of the possible ways to protect the surface of QDs from oxygen and water is to design an overcoating shell or introduce ligand on the surface. The introduction of protecting shells on the surface of QDs has a significant role in maintaining its stability, quantum yields, and avoiding photodegradation. Here, the role of shell is to confine excitons in the core that helps to avoid QD surface defects and protects the core from photodegradation. A summarized result of enhanced QD stability by introducing shells has been presented in Table 4.1. The introduction of ligands on the surface of QDs can enhance the photostability and a summarized result has been presented in Table 4.2.

TABLE 4.1 Stability of Dhell Dtructured QDs Quantum fots

Stability yest

Remained PL

Ref.

CdSe/CdS/ZnS InP/GaP/ZnS

Under 365 nm UV light, for 144 h Under 254 nm UV light for 100 h

100% 90%

[ 94] [ 95]

CdSe@ZnSe

Under 365 nm UV light for 6 h

90%

[ 96]

InP@ZnS/ZnS CdSe/CdS

In 150°C for 72 h Under UV light for 80 h

70% 90%

[ 97] [ 98]

InP@ZnS

Under 352 nm UV light for 24 h

57%

[ 99]

CdSe@ZnS/ZnS

In 120°C for 360 h

65%

[ 100]

Optical Properties of Quantum Dots

77

TABLE 4.2 Stability of Ligand Introduced QDs Quantum dots

Ligand

CdSe/ZnS CuInS2/ZnS/ZnS

Dodecanethiol Dodecanethiol

CsPbI3

2,2′-aminodibenzoic acid

CsPbBr3 CsPbI3

3-(N,N-dimethyloxtadecylammonio)propanesulfonate Trioctylphosphine-PbI2

CsPbBr3

Trioctylohosphine oxide

Remained PL

Ref.

63% 100%

[ 101] [ 102]

90%

[ 103]

100%

[ 104]

85%

[ 105]

95%

[ 106]

4.5 CURRENT THEORIES FOR PL MECHANISMS Shortly after the discovery of CQDs in 2004, researchers started to speculate on the photoluminescence nature of the CDs and the reason behind their excitation-de­ pendent emission characteristics [107]. The three primary concepts evolved were surface state, quantum confinement, and molecular state [9]. It was necessary to investigate the concept of quantum confinement in CQDs, as it is widely established that metal-based QDs exhibit emissions based on this phenomena [108]. Another widespread concept involves the generation of different molecular fragments at­ tached to the surface during the synthesis process of CQDs. This hypothesis is termed “molecular state.” This theory is supported by a large number of studies that provide strong evidence; however, the scope of this argument is restricted to par­ ticular CQDs preparations. This interpretation of the photoluminescence me­ chanism has been utilized in a few CQDs derived from citric acid [109,110]. In addition, there were few reports that used fluorescent precursors to elucidate the photoluminescence mechanism of CQDs using a molecular state. According to these studies, the precursor or precursor component was on the surface of CQDs. Although this idea is supported by substantial data in such CQDs systems, its ap­ plicability is obviously constrained by the precursor, and unable to offer a com­ prehensive explanation for the excitation-dependent emission that CQDs often exhibit. The third and most prevalently employed hypothesis is the surface statecontrolled photoluminescence. This explanation has become widely accepted as a valid explanation for photoluminescence mechanism of CQDs [9,15,111,112]. Using density-functional theory (DFT) and time-dependent DFT (TDDFT) simu­ lations, Chen et al. conducted a systematic investigation to elucidate the mechan­ isms that were responsible for the tunable photoluminescence features of QDs. It was found that the emission of zigzag-edged pure QDs may cover the full visible light spectrum by changing the diameter in the range between 0.89 and 1.80 nm [113]. The QD PL emission was blue-shifted by the armchair edge and pyrrolic N-doping, while the red shift is caused by chemical functionalities and defects. The number of isolated small sp2 domains in heterogeneously hybridized QDs determines the photoluminescence emission of the QDs. Therefore, QDs can be tailored to emit a broad spectrum of wavelengths.

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Quantum Dots and Polymer Nanocomposites

In semiconductors, extrinsic surface states can be caused by defects in the crystal lattice at the surface, adsorbates that attach to the semiconductor, or interfaces between two different types of materials. Characterizing and modeling extrinsic surface states is a significantly more difficult task than doing so with intrinsic surface states [114]. In addition, these surface states are usually unique for a specific system. This is because these surface states are determined by the atoms and structures and the defects present in those systems [115].

4.5.1 RECENT DEVELOPMENTS

IN

UNDERSTANDING PHOTOLUMINESCENCE

The concept of surface state-controlled photoluminescence in CQDs was first pro­ posed by Sun and group in 2006 [116]. These CQDs were prepared by laser ablation and passivated by PEG1500N. They envisioned surface energy traps could be sta­ bilized by passivating with PEG1500N. Since that time, there have been a lot of developments and variations on this concept. Ding et al. synthesized CQDs derived from p-phenylenediamine, and urea, and used column chromatography to extract various fractions [117]. From the FTIR and XPS analysis, they found that the wa­ velength of photoluminescence improved with the increasing degree of oxidation in the sample. They reported that the PL of CQDs was responsible for conjugated sp2 carbons on the surface of CQDs, whose band gap can be lowered by the amount of oxidative surface defects, as can be seen in Figure 4.4. Modeling graphene oxide using DFT confirms that oxidation of sp2 carbons distorts the electrical environment [118]. Luo et al. adopted a hydrothermal technique with hydroquinone and ethyle­ nediamine to synthesize CQDs and separated them using silica-gel chromatography to obtain blue, green, and yellow emissive CQD fractions [119]. It was found that the multicolor emissive photoluminescence property was caused mainly by their surface state. The photoluminescence emissions of these QDs were highly related to the C=N

FIGURE 4.4 Different CQD fractions under UV irradiation and the modeling of their band gap based on surface oxidation. Reproduced with permission from ref [ 117].

Optical Properties of Quantum Dots

79

functionalities present at the surface. With growing C=N content at the surface of the QDs, the band gap decreased, leading to the red shift of the emission peak. The electronic transition involving the connection of the core and surface states may also be used to explain the photoluminescence process [120]. According to the research conducted by Yu et al., both the surface states and the carbon core play an essential role in the regulation of photoluminescence emission [121]. This is because the band gap of the surface states (4.5 eV) is smaller than that of the core (5.0 eV). Additionally, impurity levels in the band gap would be generated due to the presence of oxygen and nitrogen components as well as associated chemical bonds. This results in a shift in the excitation and emission spectra of CQDs.

4.6 CONCLUSION AND FUTURE PERSPECTIVES QDs have a profound impact on the current status of nanotechnology. Since the publications of CQDs have grown significantly in the recent decade, numerous advancements have been made in optical characteristics, such as QY and photo­ luminescence lifetime. Advances in QD fabrication, experimental analysis, and the­ oretical understanding of their optical characteristics expedited their practical applications in different areas, from optical devices to biological detection and ima­ ging. The introduction of different synthesis techniques of QDs at different conditions has improved their quality. Also, studies of electronic and optical characteristics of QDs enhanced their applications in nanobiotechnology. The advantages of the QDs compared to the commercially available fluorescent dyes can be understood by rea­ lizing their optical properties. QDs, especially CQDs, are superior to fluorescent dyes in terms of bright emission, photostability, less toxicity, and biocompatibility. The optical features of CQDs have attracted a lot of attention due to their unusual property of excitation-dependent emission, high fluorescence QY, long photoluminescence lifetimes, and photostability. These characteristics encourage researchers to use CQDs in bio-imaging, sensing, and various biomedical applications.

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5

Surface Properties of Quantum Dots Poslet Shumbula Department of Chemistry, University of Limpopo, Sovenga, South Africa

Bambesiwe May Analytical Chemistry Division, Mintek, Randburg, South Africa Institute for Nanotechnology and Water Sustainability (iNanoWS), College of Science, Engineering and Technology, University of South Africa, Roodeport, Johannesburg, South Africa

Mokae Bambo DSI/Mintek Nanotechnology Innovation Centre, Advanced Materials Division, Mintek, Randburg, South Africa

CONTENTS 5.1 5.2

Introduction.....................................................................................................87 Surface Ligands.............................................................................................. 88 5.2.1 Organic Ligands .................................................................................89 5.2.2 Inorganic Ligands...............................................................................91 5.3 Surface Modification of QDs......................................................................... 92 5.4 Surface Modification Strategies to Improve Solubilization and Stability of the QDs ........................................................95 5.4.1 Solubilization by Ligand Exchange...................................................95 5.4.2 Solubilization by Hydrophobic Interaction .......................................96 5.4.3 Silica Encapsulation ........................................................................... 97 5.5 Characterization of QD Surfaces ...................................................................97 5.6 Conclusions.....................................................................................................99 Acknowledgments.................................................................................................. 100 References.............................................................................................................. 100

5.1 INTRODUCTION Semiconductor quantum dots (QDs) have been a hot issue in materials chemistry in recent years because of their technical and fundamental importance [1–5]. QDs are inorganic semiconductor particles with organic surface ligands that have been used in DOI: 10.1201/9781003266518-5

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FIGURE 5.1 An increase in quantum dot size causes a red-shift in the fluorescence emission peak [Adapted from 14].

various technological applications that span across electronics to bio-imaging. Surface ligands are typically added during the QD preparation process and they are crucial components because they control the growth of nanoparticle and size, it also offers the stability of the colloidal, and passivates the surface traps [3−10]. In contrast to the known fluorophores, QDs consist of broad absorption spectra (which means that multiple colors can be excited by a monochromatic source), narrow, symmetric, and size-tuneable emission spectra, making it relatively easy to distinguish one QD population from another, as shown in Figure 5.1. They have a high resistance to physical and chemical degradation (suited for long-term ima­ ging), high extinction coefficients, and quantum yields (enough for single molecule measurements) [2,5,8,11–13]. QDs are made up of two parts: an inorganic nano-crystalline particle and the organic ligands that passivate the inorganic particle’s surface [15]. In addition, the organic component is required for modifying QD surface characteristics and solubilizing QDs in various solvents. In most cases, ligand engineering on the surface of CDs is done in a solution, using either “in-synthesis” or “post-synthesis” approaches. The impact of ligands on monomer activities is thought to be greater at the nucleation stage of the wetchemical synthesis process, which could affect both monomer activities and nano­ crystal development [16,17]. As a result, not only the size and size distribution of QDs but also their morphology can be influenced by ligand types and concentrations. QDs are frequently produced by an organometallic process, in which they are stabilized by hydrophobic surfactants and hence initially soluble in non-polar solvents [18,19].

5.2 SURFACE LIGANDS Throughout the synthesis process, a ligand-linked ‘capping’ layer saturates dangling bonds, screens the particle from its surroundings, and controls nucleation and

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growth kinetics [20–25]. The hydrocarbon tail of such surface ligands is directed away from the nanocrystal surface, while the anchoring head-group is linked to the surface. The inorganic core’s equilibrium shape reduces the energy of the exposed surface area and facet-specific energy of broken bonds [22,26,27]. Anderson et al. considered the quantity of electrons involved, as well as the identification of the electron donor and acceptor groups, to distinguish the metal-ligand interaction. This involves the known classes of these metal-ligand interactions, which are (i) L-type ligands that have a single electron pair and are neutral two-electron donors that datively coordinate surface metal atoms. These types of ligands are represented by the L-type compounds ligands that are amines (RNH2), phosphines (R3P), and phosphine oxides (R3PO). (ii) Secondly, the X-type ligands are neutral compounds with an odd number of valence-shell electrons that require one electron transfer from the nanocrystal surface site to create a two-electron covalent connection. As a result, X-type ligands can be neutral radicals that bind neutral surface sites (each with an unpaired electron) or monovalent ions that bind oppositely charged sites at the nanocrystal surface, which is the more prevalent case. Thiolates (RS–), car­ boxylates (RCOO–), and phosphonates (RPO (O)) are X-type ligands; these also include inorganic ions (such as Cl–, InCl4−, and AsS33–) and bound ion pairs (for example, NEt4+I–) in nonpolar solvents. Electron-rich (nucleophilic) ligands and Xtype ligands bind to electron-deficient (electrophilic) surface sites with high Lewis acidity at the nanocrystal surface, which are frequently occupied by coordinated metal ions. (iii) Furthermore, the Z-type ligands, such as Pb (OOCR)2 or CdCl2, attach to the metal atom as two-electron acceptors. Because of their electron-rich Lewis basic sites on the surfaces of metal chalcogenides, oxides, and other com­ pound nanocrystals can interact with Z-type ligands [28]. Additionally, protons (H+), positively charged compounds, and electrophilic X-type ligands can attach to the surface of oxide nanocrystals [29,30]. Surface ligands regulate the solibility of QD dispersions in nonpolar and polar liquids through steric and electrostatic processes. For instance, oleophilic ligands which contain the hydrocarbon tail can increase the spectroscopic characteristics of QDs while also increasing their intrinsic nonpolar solvent solubility. According to various additional studies when analyzing the structural and electrical properties of colloids, the type of ligands used and their distribution on nanocrystal surfaces are crucial [31–39].

5.2.1 ORGANIC LIGANDS Organic compounds such as trioctylphosphine/trioctylphosphine oxide (TOP/ TOPO), long-chain alkyl amines, and alkyl thiols are among the recognized ligands [40]. High boiling point of TOPO as a surfactant, solvent, and a capping agent allows reactions to run at 350°C on a regular basis, allowing for high-temperature precursor decomposition and nanoparticle annealing that would be impossible with other aqueous-based routes, and it is compatible with organic solvents which allow for a completely inert reaction environment, making it easier to use air sensitive precursors in the reaction. QDs tend to be liquid because of their long alkyl chains, allowing them to be handled like any other organic reagent. However, the solvents

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employed must have a very high dielectric constant to overcome the van der Waals forces among the colloidal particles [40]. In TOPO-capped CdSe, the lone pairs of electrons on the phosphine oxide moiety create dative interactions with the surface cadmium sites [41,42]. TOPO, on the other hand, has been shown to move from cadmium to selenium sites after lighting, forming TOPO–Se complexes that are intimately linked to the photobrightening process, in which the emission quantum yield of nanoparticles increases for a brief period after synthesis [43]. During the synthesis of TOPO-capped CdSe nanocrystals, TOP is frequently utilized as a surfactant and selenium-delivery solvent in the form of a trioctyl­ phosphine selenide (TOP-Se) solution. TOP, like TOPO, is intended to bind to the surface of the particles via selenium sites (as shown in Scheme 5.1), allowing for a more thorough surface passivation [44–46]. Surface reconstruction, particularly a lattice contraction during growth, has been observed when amines are used on CdSe particles as capping agents, which could explain the enhanced emission [47]. Long-chain amines are better suitable for II-VIbased semiconducting systems as surfactants. Without the use of an inorganic shell, CdSe nanoparticles produced with primary amines of long-chain have been seen to exhibit 60% emission quantum yields. This has been linked to ligand packing on the nanoparticle surface and the etching of surface defects, with amines contributing to the oxygen etching process [48–50]. Furthermore, the impact of amines (primary, secondary, and tertiary) on the emission of quantum yield of CdSe particles was investigated, and it was discovered that primary amines significantly increased emission, although secondary and tertiary amines had little or no effect [51].

SCHEME 5.1 Molecular seeding showing the nucleation and growth of CdSe QDs with TOP surface ligands [Adapted from 45].

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To passivate QD surfaces, Shakiba et al. used several main amines, ranging from NH3 to hexylamine carbon chain lengths [52]. Experiments show that primary amine ligands cause a blue shift in the band gap of ligated QDs, despite the fact that the alkane chain itself shows no changes in the band gap. The binding energy between ligands and QDs increases as chain length grows, but the rate drops due to increased steric hindrance between the ligands. The involvement of van der Waals forces in such behavior is revealed by geometry optimization for each system with and without dispersion correction effects [53]. Other chemical ligands, including carboxylic acids, phosphoric acids, and thiols terminated ligands, were used as surfactants and capping agents in the synthesis QDs by Knauf et al. [54]. In oleic acid, the most commonly used surfactant is the carboxylic acid, the double bond and kink in the alkyl chain are discovered to be important factors for the stability of the colloids [55,56]. Most semiconducting and metal nanoparticles seem to perform well with long chain thiols as a capping agent. Rajh et al. were the first to describe thiol-stabilized CdTe particles, which paved the door for the ligand’s widespread use in QD synthesis [57]. Alkylamine, triphenylphosphine (TPP), TOP, and TOPO have all been found to have an effect on the photoluminescence (PL) efficiency of CdSe QDs [58]. According to Wuister et al., thiol is used in both the organic and aqueous phases as a PL quencher. It is also suggested to be the cause of the hole trap development in CdSe QDs [59].

5.2.2 INORGANIC LIGANDS It has recently been established that adding inorganic-type ligands to QD solu­ tions enhances their electrical conductivity. Inorganic ligands such as metal-free chalcogenides, chalcogenidometallates, halides, pseudohalides, halometallates, and oxoanions/polyoxometallates were developed by Talapin et al. for electrical, optoelectronic, and thermoelectric applications [60]. Electronic solid-state de­ vices based on colloidal nanocrystals have benefited significantly from the functionalization of the inorganic surface, which is employed to remove native organic capping ligands from nanocrystal surfaces [61,62]. To make all-inorganic nanocrystals, organic surface ligands are often exchanged with metal-free anionic species (like S2− and SCN−) or molecular metal chalco­ genide complexes (MCCs). A two-step ligand exchange or direct phase transfer technique can be used for ligand exchange [63]. Murray et al. published a generalized ligand exchange procedure that removes organic ligands from nanocrystal surfaces using nitrosonium tetrafluoroborate (NOBF4), resulting in nearly ligand-free positively charged nanocrystals that can be dispersed in solvents such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), or acetonitrile (ACN) [64]. Accordingly, Protesescu et al., after X-type oleate-to-thiostannate ligand exchange, CdSe nanocrystals preserve their Cd-rich stoichiometry, with a stoichiometric CdSe core and surface Cd adatoms functioning as binding sites for the thiostannates ligands with terminal S atoms, resulting in an all-inorganic CdS. When the thiostannates SnS44− and Sn2S76− attach to nanocrystal surfaces, the tetrahedral SnS4 shape is preserved [65]. Zhang et al. also showed that halide, pseudohalide, and halometallate ligands can help colloidal nanocrystals stay

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together [66]. These types of ligands further enriches the current family of inorganic ligands used in QD synthesis.

5.3 SURFACE MODIFICATION OF QDS During the surface modification process, a series of physical or chemical events bind a range of organic, inorganic, or biological components to the QD surface. To make the QD work with the ligands in a specific way, surface modification is used. Chemosensing and biological applications are two possible uses for the modifica­ tion of the surface QDs. Thereafter, the ability of surface ligands to fluorescence QDs is assessed. Reactions that use ligand exchange as a surface modification technique increase the adaptability of nanocrystal materials by permitting the use of targeted species for application such as ions (inorganic or organic), polymers, and clusters, in place of ligands optimized for synthesis [67–69]. Colvin et al. were able to demonstrate, for the first time, surface modification of lead-based QDs by substituting 11-mercaptoundecanoic acid for oleates on a PbSe QD surface, allowing them to change to hydrophilic phase from their initial hy­ drophobic phase [70]. The particles were stable in water but not in physiological saline buffers, according to the researchers. Various other scholars have since proposed a variety of surface-altering strate­ gies, as seen in Figure 5.2 [71–75]. Pichaandi et al. described four different surface modification strategies such as: (i) coating QD with silica; (ii) polyvinylpyrrolidone ligand exchange; (iii) polyethyleneglycol-oleate (PEG-oleate) intercalation into the oleate ligands on the surface of QDs; and (iv) poly (maleicanhydride-alt-1-octadecene) (PMAO) intercalation into the oleate ligands on the surface of the QDs and further crosslinking of the PMAO. They were able to achieve coating with silica and exchange of the ligand in water and exceptional dispersion stability; however, their pho­ toluminescence was lost [72]. Nanocrystal surface ligand exchange parallels the coordination of complex substitution reactions. The polarity and coordinating abilities of the kinetics and

FIGURE 5.2 Colloidal-stable QDs modified by dual-functional group polymer [Adapted from 75].

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mechanism of ligand exchange activities at the nanocrystal surface are influenced by the solvent. The dissociative process is favored by the steric crowding of mo­ lecules in the capping layer, which needs a bound ligand to desorb from the na­ nocrystal before a new one can enter from the solution and connect to the surface. All engaged species in the exchange reaction in nonpolar liquids should be elec­ trically neutral. As a result, the capabilities of the outer shell are crucial for boosting contact possibilities and selectivity. Post-synthetically produced the secondary molecule layer that is covalent conjugation or non-covalent interaction alongside or onto the first ligand layers can further fine-tune the flexibility of QDs activities [76]. Lui et al. used two methods, by using the multidentate ligand (MDL) surface modification and sulphydrycoupling ligand modification. In one method, the ligands surround the center ion in MDL, and the atoms of the ligands are directly bonded to the center ion. When two or more ligand atoms connect to one center ion si­ multaneously in MDL, to perform surface modification, the MDL surface mod­ ification technology relies on coordinated complexation, with the ligand’s amino or carboxyl binding to the CdSe/CdS QDs center ion. In another method, the sul­ phydryl coupling, sulphydryl which is a functional group of the thiophenol (Ph-SH), mercaptan (R-SH), and thiol carboxylic acid, is employed. Sulphydryl coupling and QDs with their strong interaction between sulphydryl and Zn or Cd coupling on the surface which forms the basis of QD surfaces. To improve the hydrophilicity of QDs, a strong force binds the thiol carboxylic acid to the QD shell. Meanwhile, the carboxyl functional groups on the surface of QDs can be linked to the amino of biological molecules such as polypeptide and protein, allowing QDs to be used in a wide range of applications [77]. The hydrophilic “head” and lipophilic “tail” of amphiphilic compounds are used to modify the surface of these compounds. In the “head,” polar species such as amine and choline salt are typically found. The “tail” usually contains a long chain aliphatic group. The formed QD surfaces in the non-aqueous phase (oil) is coated with hydrophobic molecules (such as TOPO). Using ultrasonic emulsifi­ cation, the “tail” of amphiphilic molecules can be attached directly to the TOPO. The usage of amphiphilic molecules has simplified manufacturing procedures and eliminated the time-consuming process of molecular replacement [78]. The hy­ drophilic “head” can also bind H2O, enhancing QD solubility in water and aqueous phase synthesis efficiency. The surface of QDs is bent by amphiphilic molecules, generating a cap layer [77]. To make QDs water-soluble and improve electrostatic interaction with re­ combinant proteins, Mattoussi and co-workers overcoated them with negatively charged dihydrolipoic acid (DHLA), which is a bidentate thiol [79]. DHLA is unable to retain the luminescence of QDs in cellular applications, which is found to be stable only in basic circumstances (pH 7) and therefore results in not binding specifically to positively charged proteins. To get around this, QDs coated with carboxylated DHLA can be modified to include functional groups of various sites that serve multiple functions. As demonstrated below in Figure 5.3, Susumu et al. prepared a new family of ligands that are water-soluble based on DHLA and polyethylene glycol (PEG). Biomolecules quickly conjugate with the functional groups linked to them (such as hydroxyl, amino, carboxylic acid, and biotin) [80].

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FIGURE 5.3 Modular design based on DHLA-PEG, QD hydrophilic ligands with various terminal functional groups [Adapted from 80].

Similarly, surface silanization is an alternative route for creating a silica layer around the QD by entirely removing the pre-existing hydrophobic ligands. QDs can acquire a silica coating in two ways: direct exchange of the ligand (surface silanization) or indirect ligand encapsulation [81–83]. In a common approach, in alkaline methanol solution, QDs coated with TOPO are combined with mercap­ topropyltrimethoxysilane (MPTS). The mercapto group can connect to the ZnS layer of QDs and so replace the TOPO layer once the solution is heated to enable silanol group cross-linking. To assure covalent binding to biomolecules, the silica shell is further modified using thiols, amines, or carboxyl groups, as shown in Figure 5.4 [84]. QDs are increasingly being used as scaffolding to create a variety of energy transfer pathways. Electronic coupling processes like electron transfer (ET) and fluorescence resonance energy transfer (FRET) are possible due to the nature of the interaction between QDs and surface ligands. In an ET procedure, electrons from QDs’ conduction band travel to a conjugated acceptor with the lowest un­ occupied molecular orbital (LUMO) energy level, dimming fluorescence [85,86].

FIGURE 5.4 Silica-coated QDs with functional groups that can easily cross-link with biological molecules are designed in a modular fashion [Adapted from 84].

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For the dipole-dipole mechanism of FRET, as well as finite overlapping of the donor emission and acceptor absorption spectra, donors, and acceptors must be close together (up to nearly double the Förster distance). This non-radiative energy transfer has the potential to increase surface acceptor emissions while decreasing the loss of QD emissions. The use of a non-fluorescent acceptor can limit QD emissions. When loading donor or acceptor molecules, end-functionalized ligands enable interaction [87].

5.4 SURFACE MODIFICATION STRATEGIES TO IMPROVE SOLUBILIZATION AND STABILITY OF THE QDS QDs must consequently be coated in order to be fully stabilized. QDs that have been passivated with organic or inorganic ligands differ from those that have not been passivated. Their differences can be determined using absorption spectra, emission spectra, and confirmation of particle sizes using transmission electron microscopy (TEM), among other methods. The presence of electron and/or hole traps on a poorly QD capped surface has a major impact on the luminescence and other properties. To achieve the appropriate radiative qualities, it is necessary to carefully manage the surface imperfections as well as the size of a dot. QDs synthesized in the presence of capping ligands that are soluble in organic solvents, such as hex­ adecylamine (HDA), TOPO, and others, are typically monodispersed, and synthesis procedures may be easily controlled. Surface passivation of QDs is commonly used to overcome stability issues. However, the applicability of the nanocrystals tend to be limited, particularly in biological applications. As a result, they need to be modified on the surface to optimize their aqueous dispersion and biocompatibility. QDs that are soluble in inorganic solvents have been converted to water-soluble quantum dots using a variety of ways [88–90].

5.4.1 SOLUBILIZATION

BY

LIGAND EXCHANGE

This approach involves replacing hydrophobic ligands in the outermost layer with hydrophilic moieties that coordinate with surface atoms. On one end of the molecule, the hydrophilic ligands should be reactive toward the surface atoms of the nanoparticles, and on the other end of the molecule, they should be water compatible. Ligand exchange is one popular approach to modification of QDs because it allows for changes in surface chemistry without affecting particle size or shape [91]. Mercaptoacetic acid (MAA), which has an S-H binding group and a carboxylic group, provides water solubility through the repulsive electrostatic interactions of the charged COO- groups is one of the recognized ligands previously employed [92]. In addition to ligand exchange, several other strategies for converting hy­ drophobic nanoparticles to hydrophilic nanoparticles were used. As seen in Scheme 5.2, these strategies are hydrophobic interaction and silica encapsulation. Yu et al., on the other hand, described silica encapsulation as difficult [93]. The latter approach, while capable of producing stable quantum dots, has the drawback of producing large-sized particles.

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SCHEME 5.2 Diagram showing the depiction of methodologies used to make water-soluble nanoparticles [Adapted from 93].

5.4.2 SOLUBILIZATION

BY

HYDROPHOBIC INTERACTION

Surfactants such as 1, 2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy (polyethylene glycol)] or 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine can be used to induce hydrophobic contact. These surfactants cover QDs in the core by producing oil-in-water micelles via hydrophobic interactions between their hydrophobic ends and the QDs surface ligands, and give water solubility via their hydrophilic exterior ends [94]. Long-chain amphiphilic polymers are effective surfactants for creating micelle-like structures in which water-insoluble quantum dots can be converted to water. In some cases, poly (acrylic acid) was partially grafted with octylamine via EDC-coupling (EDC = N-(3-dimethyl aminopropyl)-N’-ethyl carbodiimide hydro­ chloride) to become amphiphilic and then formed micelle-like structures with hy­ drophobic QDs encapsulated in the core and –COOH facing outward [95]. Introduction of carboxylic acid groups (–COOH) and graft them with aminoterminated poly (ethylene glycol) (PEGNH2) to achieve stability and highly watersoluble structures. Depending on the synthesis technique, QDs pegylation usually proceeds through main routes. As previously stated, QDs made in organic solvents require ligand exchange to phase transfer the particles to water. In the presence of PEG-polyethylenimine, CdSe/CdS/ZnS QDs produced in chloroform using cota­ decylamine as a stabilizing ligand may be transported into buffer solutions [96]. In the presence of bis (6-aminohexyl) amine, which helped to increase the stability by cross-linking the polymers, poly (maleic anhydride-alt-1tetradecene) was employed to convert hydrophobic QDs into water [97]. The use of amphiphilic polymers gives this method an advantage over ligand exchange in the sense that (a) the method prevents quantum dots from interacting with hydrophilic surfactants; (b) the polymer’s large number of hydrophobic side chains strengthen the hydrophobic interaction, resulting in more stable watersoluble quantum dots; and (c) some of the polymers used are inexpensive when compared to other molecules such as phospholipids.

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5.4.3 SILICA ENCAPSULATION Coating the QD through a layer of silica, as indicated in Scheme 5.2, is another way of creating water-soluble QDs [81,98]. Organosilicone compounds containing –NH2 or –SH are required, which provide surface functions for biomedical appli­ cations [81,84,99]. This strategy, like the ligand exchange technique, resulted in a drop in quantum efficiency [100]. If this method is to be used, some drawbacks include (a) the procedures being difficult and (b) large quantum production being impossible due to the dilute conditions used [99]. To overcome these drawbacks, Ying and co-workers reported a simple reverse microemulsion method for coating quantum dots with silica [101].

5.5 CHARACTERIZATION OF QD SURFACES Developing an effective and accurate approach for detecting and quantifying li­ gands on QD surfaces has proven difficult despite significant efforts to understand the chemistry of QD surface ligands. To describe and study QD surfaces, several techniques and methodologies for ligand analysis have been explored. Some wellknown approaches are discussed, with an emphasis on their utility in probing for QD surface characteristics. A number of techniques such as imaging, spectroscopic, scattering, and computational methodologies were used to fully give surface characteristics of QD surfaces [102,103]. When used independently or in combi­ nation, these methods can provide additional information about the interface, such as (i) ligand concentration and structure, (ii) as well as chemical interactions of the QD core and surface ligands, and (iii) the overall effective features of the capping ligand, such as the ligand thickness, density, and dielectric constant. The spectroscopic technique of Fourier transform infrared spectroscopy (FTIR) is used to probe the QD surface and provide information on the fingerprint structure of ligand molecules. Significant infrared absorption bands in the 3,000 cm–1 and 1,500 cm–1 wavelength ranges are observed in QDs capped with organic ligands, such as oleic acid, confirming the presence of surface-bound hydrocarbon molecules with C–H stretching and bending modes [104]. The absorption and emission spectra of the as-synthesized oleate capped PbS QDs synthesized using a hot-injection technique can be seen in the FTIR data. A strong excitonic peak was observed in the absorption spectra at 945 cm–1, while a narrow emission peak was found at 1,050 cm–1. FTIR spectra of oleate before and after ligand exchange with mercaptoethanol (ME) were used to confirm this. Following this ligand exchange, the C-H (2,850–3,000 cm−1), C-O (1,700–1,725 cm−1), and symmetric O-C-O (1,360–1,400 cm−1) vibrations of the oleate ligands vanish completely, while the new IR absorbance of the ME’s O-H (3,200–3,600 cm−1) and C-O (1,050–1,150 cm−1) vibrations appear, indicating a higher percentage replacement [105]. The displacement of hydrocarbon ligands can also be studied using FTIR, for example the exchange of organic for inorganic spe­ cies. The ligand exchange may also be confirmed using FTIR and other techniques such as the X-ray photoelectron spectrometer (XPS), which showed that the assynthesized QDs have a cation-rich surface. On the thin films of Br-terminated PbS QDs, XPS analyses were performed and analyzed in detail to obtain information

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about the relative content and bonding of the halide ions. The S2p peak at 163.5 eV represents the binding energy of Pb–S, while the Br 3d peak at 68.6 eV represents the binding energy of Br with Pb. As a result of spin-orbit coupling, both fitted peaks clearly show substantial splitting [106]. Another spectroscopic technique, nuclear magnetic resonance (NMR) spectro­ scopy is a well-known and used for studying structure [103]. In this instance of QD analysis, based on NMR fingerprints of spin-active nuclei of 1H, 13C, 31P, and others, NMR spectroscopy was utilized to characterize the capping layer of nanocrystal. The signal quality of solution-state NMR spectroscopy is determined by the isotropic tumble rates of the target molecules. Despite the fact that ligand compounds are relatively small and tumble quickly enough to generate a crisp peaks, ligand mole­ cules connected to QD tumble much slower, resulting in wide NMR signals that are insufficient for even simple two-dimensional (2D) NMR investigations [107]. Due to dipolar coupling effects, the NMR peaks of surface-tethered ligands are greatly widened, which cancel out for fast-tumbling free ligand molecules in solution. The utility of typical one-dimensional NMR for nanocrystal surface investigation is limited as a result of this broadening. Diffusion-ordered spectroscopy (DOSY) associates a diffusion coefficient, D, with each resonance, allowing signals to be distinguished from surface-bound and free ligand molecules. Some scientists were able to discriminate between unbound ligands and those coordinated on QD surfaces by combining solution phase 31P and 1H NMR with DOSY [23]. To investigate the structure and composition of the organic capping shell dis­ covered on the surfaces of CdSe-ZnS quantum dots produced, Zeng et al. used a combination of 31P, 1H, HSQC, DOSY NMR, and mass spectroscopy (MS) [103]. The composition and stoichiometry of the organic shell were determined to be dependent on the extent of purification given to the QD dispersion. Alkylamine, phosphorus molecules (e.g., TOP and TOPO), and various phosphonic acid (HPA) surfactants make up the surface coating of CdSe-ZnS QDs, according to the find­ ings. Because TOP and TOPO have a low coordination affinity, during purification, they easily detach from the QD surface, leaving just a small fraction of zinc- and sulfur-complexed TOPO and TOP molecules in the form of TOPO-Zn and TOP-S. Surface-bound HPA and to a lesser extent HDA on the other hand are detected in larger amounts on three-time purified QDs. This suggests that HPA ligands have the strongest affinity for the metal-rich surfaces of the nanocrystals, which is in line with previous findings on the organic cap of core-only QDs [103]. They were able to develop a model structure for the ligand shell composition on the surface of the nanocrystals by combining 31P, 1H, 13C, and DOSY NMR data collected from CdSe-ZnS QDs subjected to various rounds of purification with matrix assisted laser desorption ionization (MALDI)-MS data collected from supernatant solutions of displaced ligands. They were also able to show that the DOSY-derived QD hydrodynamic radius corresponded to those obtained from dynamic light scattering (DLS) experiments [103]. Another well-known technique for distinguishing free and bound ligands from 1 H NMR signals is nuclear overhauser spectroscopy (NOESY). It investigates the increased cross-coupling NMR signals caused by dipole–dipole interactions be­ tween protons within the same molecules when immobilized on slowly tumbling

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objects like nanocrystals [108]. Nanocrystals with inorganic surface ligands such as SnS44– have been studied using heavy-nuclear NMR [65]. Raman spectroscopy can also be used to characterize inorganic ligands that have heavy atoms that vibrate at low frequencies. This allows scientists to differentiate between the ligand and the surface of QD in different types of QDs [109]. The identification and content of organic ligands on the QD surface can also be determined using the gas chromatography-mass spectrometry (GC–MS) technique. Similarly, because GC–MS identifies organic compounds in a gaseous state, ligand molecules must be easily converted to this state before being analyzed by GC–MS. QD ligands, on the other hand, are usually strongly bound to the nanoparticle surface, preventing gasification in some cases [110]. TEM or scanning electron microscopy (SEM) techniques which also may in­ clude atomic force microscopy (AFM), DLS, small-angle X-ray scattering (SAXS), and nanoparticle tracking analysis (NTA), all of these methods can be used for determining nanoparticle size, morphology and concentration are well-known and can provide some information about the ligand and particles. Microscopy can be used to examine nanoparticle shape and size and, in certain cases, their surface ligands in real-time. Researchers can explore nanocrystals at the single-particle level with microscopy techniques, revealing features of these systems that would otherwise be concealed by ensemble averages. In the oleate capped PdS QDs by Hine et al., according to a high-resolution TEM (HRTEM) micrograph, the average diameter of as-prepared QDs was found to be 2.9 nm, and the QDs are highly crystalline with a narrow size distribution [37,106]. SAXS and the effective thickness of the nanocrystal capping layer and its contribution to total particle hydrodynamic radius were investigated using DLS [111]. Molecular dynamics (MD) and density functional theory (DFT) provide some alternatives to computer-aided simulation in some circumstances where realistic information is insufficient. This will help to compensate for the difficulty of ex­ perimentally probing nanoparticles and surfaces of the nanoparticle. Among them, DFT is utilized to characterize the electrical interactions between various nano­ particle characteristics and the molecules that attach to them, allowing for the prediction of surface atom configurations, ligand binding modes, and nanocrystal optoelectronic properties. A recent study has combined ab initio electronic structure with MD simulations to rebuild a complete nanocrystal, which includes the in­ organic core as well as the whole ligand shell structure [112]. These and other computer modeling tools aid in both the analysis of experimental data and the identification of new experimental pathways.

5.6 CONCLUSIONS Surfaces of QDs play a crucial role in the properties and applications of QDs in a variety of disciplines. The ability to manipulate the surface chemistry thereby changing the behavior of the QDs is one of the driving forces in QD applications. Attachment of ligand to the bare QDs and also further surface modification by ligand exchange are furthering the use of QDs. Understanding these manipulations and their resultant properties is of utmost importance in this field of research.

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This is mostly due to the interaction of the QDs with the ligand, which gives the QD tailorable features. This has allowed the changing of QDs from being hydrophobic in one environment to hydrophilic by changing the ligand, such as attaching a ligand that is favorable to the favored media. Understanding the underlying factors that control such behaviors requires in-depth knowledge and probing of these surfaces, thereby making surface information available within a reasonable time and certainty. This will require state-of-the-art analytical techniques to be able to handle sensitive structures from QD surfaces. This will enable the establishment of the property-structure relationship of QDs. This allows QDs to continue to be the favorite material in electronics and biological applications.

ACKNOWLEDGMENTS The authors would like to thank the following institutions: Council of Minerals and Technology (Mintek), University of South Africa (Unisa), and the University of Limpopo (UL).

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6

Impact of Doping on Efficiency of Quantum Dots Shibam Das Department of Pharmaceutical Analysis, ISF College of Pharmacy, Moga, Punjab, India

Rohit Bhatia and Bhupinder Kumar Department of Pharmaceutical Chemistry, ISF College of Pharmacy, Moga, Punjab, India

CONTENTS 6.1 6.2

6.3

6.4 6.5

Introduction...................................................................................................106 Methods of Preparation................................................................................ 108 6.2.1 Top-Down Approach...................................................................... 108 6.2.2 Bottom-Up Approach .....................................................................108 Significant Applications of Quantum Dots .................................................109 6.3.1 Plant Bioimaging............................................................................109 6.3.2 Animal Bioimaging ........................................................................ 109 6.3.3 Prokaryote Bioimaging................................................................... 109 6.3.4 Tracking of Particles ...................................................................... 109 6.3.5 In situ Imaging ...............................................................................109 6.3.6 Drug Delivery................................................................................. 110 6.3.7 Detection of Various Cancers........................................................110 6.3.8 Imaging and Sensing of Infectious Diseases.................................111 Doping .......................................................................................................... 111 Significance of Doping into Quantum Dots................................................111 6.5.1 Electrochemical Doping of Quantum Dots ...................................111 6.5.2 n-type Doping by Lithium Ion Intercalation ................................. 113 6.5.3 Elemental Doping of Graphene QDs.............................................113 6.5.4 Doping on InAs/GaAs QD Solar Cells ......................................... 114 6.5.5 Effects of Dopants (N and P) on the Size and Quantum Yield... 114 6.5.6 Effect of Doping on the Structural and Optical Properties .......... 114 6.5.7 Effect of Doping on the Electrons and Holes............................... 115 6.5.8 Silver-Doped PbSe Quantum Dots ................................................115 6.5.9 Effect of Copper Doping on Electronic Structure ........................ 115

DOI: 10.1201/9781003266518-6

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6.5.10 6.5.11 6.5.12

Effect of Heteroatom-Doped Carbon Quantum Dots ...................116 Effect of Mg and Cu Doping on ZnS Quantum Dots ..................116 Effect of Mn Doping on CdS Quantum Dot–Sensitized Solar Cells ................................................................................................ 117 6.5.13 Effect of Si Doping on InAs/GaAs Quantum Dot Solar Cells ....117 6.5.14 Effect of Silicon Delta-Doping...................................................... 117 6.5.15 Diffusion Doping in Quantum Dots ..............................................118 6.5.16 Significance of p-Doping for Quantum Dot Laser ....................... 118 6.5.17 Impact of Modulation p-Doping in InAs Quantum Dot Lasers ... 118 6.5.18 Mn:Cu Co-Doped CdS Nanocrystals............................................. 119 6.6 Conclusion .................................................................................................... 119 Conflict of Interest.................................................................................................119 References.............................................................................................................. 120

6.1 INTRODUCTION Nanomaterials are intriguing because they can overcome the gap between bulk and molecular levels, opening up totally new application possibilities, particularly in optoelectronics, electronics, and biology. A nanostructure is a solid that exhibits a distinct variation of optical and electronic properties with a particle size variation of less than 100 nm and is classified as (1) two-dimensional, such as thin films or quantum wells; (2) one-dimensional, such as quantum wires; or (3) zerodimensional or dots(1,2). Russian physicists Alexei Ekimov and Alexander Efros discovered quantum dots in materials in 1980 while working at the Vavilov State Optical Institute. Quantum dots (QDs) are zero-dimensional, nanometric fluor­ escent, semiconductor crystals made up of elements from groups II to VI or III to V, having a physical size smaller than the exciton Bohr radius(3). Wide and continuous absorption spectra, limited emission spectra, and great light stability are only a few of the remarkable luminescence and electronic properties of QDs(4). They absorb white light and, depending on the band gap of the material, re-emit a certain color a few nanoseconds later(5).The energy difference between the highest valence band and the lowest conduction band grows as the size of the crystal shrinks. The dot requires more energy to excite, and the crystal releases more energy when it returns to its ground state, resulting in a color shift from red to blue in the emitted light (Figure 6.1). As a result of this phenomena, these nanomaterials may emit any color of light just by changing the dot size from the same material. These semiconducting structures can also be adjusted during manufacturing due to the high level of control over the size of the nanocrystals produced(6). Since QDs are tiny particles with radii similar to that of Bohr, quantum confinement effects are easily seen. When the size of a material is close to the magnitude of the de Broglie wavelength of the electron wave function, quantum confinement is observed(7). QDs have a semiconductor core that is overcoated with a shell (e.g., ZnS) to increase optical characteristics, as well as a cap that allows for improved solu­ bility in aqueous buffers(8) (Figure 6.2). The application of QDs as a new bio­ systems technology has primarily been researched in mammalian cells. In plant research, there is a growing trend toward using QDs as markers(9,10). Because of

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FIGURE 6.1 Splitting of energy levels in quantum dots.

FIGURE 6.2 Schematic representation of quantum dot.

their tiny size, brightness, independence of emission from the excitation wave­ length, and stability in very hostile settings, QDs could be used as markers of cells or cell walls for plant bioimaging. They also have great photostability(11) and overcome the challenges of photobleaching. Some physical features of QDs, such as optical and electron transport characteristics, differ significantly from those of bulk materials due to their tiny structures(12). Surface solubilization and functionalization provide a significant challenge when utilizing QDs in live cells. Because QDs are hydrophobic and are manufactured in nonpolar solvents, their surfaces must be coated with ampiphilic coatings to make them hydrophilic for biological purposes. In order to make QDs that are both tiny and biocompatible,

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Fabien et al. has created QDs coated with phytochelatin-related peptides, where carefully chosen amino-acid sequences solubilize QDs in aqueous solution, providing a biological interface required for live cell imaging.

6.2 METHODS OF PREPARATION QDs have been synthesized using a variety of methods. In general, QD synthesis strategies are classified as either top-down or bottom-up approaches.

6.2.1 TOP-DOWN APPROACH To make QDs, a bulk semiconductor is thinned in top-down approaches. To manufacture QDs with a diameter of less than 30 nm, electron beam lithography, reactive-ion etching, and/or wet chemical etching are often utilized. For systematic experiments on the quantum confinement phenomenon, controlled forms and sizes with the necessary packing geometries are possible. Alternatively, focused ion or laser beams have been utilized to make zero-dimension dot arrays. Impurities in the QDs and structural defects caused by patterning are two major downsides of these methods. Etching, which has been used in nanofabrication for over 20 years, is critical. In case of dry etching, a reactive gas species is put into an etching chamber, and a radio frequency voltage is used to form a plasma, which splits down the gas molecules into more reactive fragments. These high-kinetic-energy species collide with the surface and produce a volatile reaction product, which is used to etch a patterned sample. This etching procedure is known as reactive ion etching (RIE) when the energetic species is ions. This method allows for a great deal of versatility in the development of nanostructured structures. This approach can be used to create any shape of Qdots, wires, or rings with precise separation and periodicity. This approach was used to make III-V and II-VI Qdots with particle sizes as tiny as 30 nm with great success(13).

6.2.2 BOTTOM-UP APPROACH The QDs were synthesized using a variety of self-assembly approaches, which can be generally classified into wet-chemical and vapor-phase methods(13): (a) Wet-chemical approaches primarily follow traditional precipitation methods, with careful parameter control for a single solution or mixture of solutions. Both nucleation and restricted development of nanoparticles are inevitably in­ volved in the precipitation process. Homogeneous, heterogeneous, and sec­ ondary nucleation are the three types of nucleation(14). When solute atoms or molecules mix and attain a critical size without the help of a pre-existing solid contact, this is known as homogeneous nucleation. Microemulsion, sol–gel(15), competitive reaction chemistry, hot-solution decomposition(16), sonic waves or microwaves(17), and electrochemistry are examples of wet-chemical pro­ cesses. (b)Vapor-phase approaches for generating QDs begin with atom-byatom layer growth processes. As a result, QDs self-assemble on a substrate without any patterning(18,19).

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6.3 SIGNIFICANT APPLICATIONS OF QUANTUM DOTS 6.3.1 PLANT BIOIMAGING CdSe QDs attach to cellulose and lignin in the cell wall of the Picea omorika branch, according to Djikanovi et al. Interaction with the chains of C = C and C-C alternating bonds, as well as interaction with the OH groups, are used to bind to lignin and cellulose, respectively. Data indicated that QDs can be used to label the entire cell wall uniformly. This is due to the cell wall polymers’ structural ar­ rangement in the entire cell wall network, as well as the QDs’ incredibly small size. These properties make it possible for nanoparticles to penetrate the polymer structures in the cell wall composite(20).

6.3.2 ANIMAL BIOIMAGING For labeling living HeLa cells, Goldman et al. employed biotinylated CTxB in combination with QD-avidin conjugates(21). In another study, Jaiswal et al. used sulfo-NHS-SS biotinylating reagent to label live HeLa cells, which were subsequently treated with avidin-conjugated yellow-emitting QDs(22).

6.3.3 PROKARYOTE BIOIMAGING Yang and Li reported the simultaneous detection of E. coli O157:H7 and Salmonella typhimurium using QDs with different emission wavelengths (525 nm and 705 nm)(23). Smith’s group demonstrated sensitive and selective staining of bacterial mutants using QD labels. This detection approach is based on the Zn(II)dipicolylamine coordination complex’s preferential affinity for phospholipids on the bacterial cell surface of a given strain(24,25). In another study, magnetic beads coated with anti–E.coli O157 antibodies and streptavidin-coated QDs were used by Xiao-Li et al. to measure the bacterial cell concentration(26).

6.3.4 TRACKING

OF

PARTICLES

Howarth et al. demonstrated a method for tracking endogenous cell-surface proteins without cross-linking by purifying monovalent antibody-QD conjugates(27). In numerous situations, QDs were employed to target membrane proteins and explore their mobility and entry-exit kinetics: (1) Transmembrane proteins such as integrins, channels, and aquaporines; (2) GABA, glycine, interferon, and HER receptors; and (3) neurological synapse(28–37).

6.3.5 IN

SITU IMAGING

Since it is physically difficult to see living satellite cells localised inside skeletal muscle in situ, it is unclear how satellite cell migration is involved in muscle re­ generation after damage. Ishido and Kasuga sought to visualize satellite cells in both healthy and wounded rat skeletal muscle in situ using quantum dots linked to

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anti-M-cadherin antibody. They are the first to achieve real-time imaging of satellite cells within skeletal muscle in situ(38). Stylianou and Skourides used near-infrared QDs to image mesoderm migration in-vivo with single-cell resolution for the first time and offer quantitative information on migration rates in-vivo(39).

6.3.6 DRUG DELIVERY Since QDs can target the delivery system and quickly discriminate sick cells from healthy cells through metal affinity-driven self-assembly between artificial poly­ peptides and the semiconductor core shell QDs, they have almost no side effects. Nanoparticles of QDs provide key characteristics for effective targeted adminis­ tration, including adequate blood circulation, cargo protection from degradation, substantial drug loading capacity, regulated drug release profile, and the integration of numerous targeting ligands on their surface(40). Wee beng et al. employed chitosan nanoparticles packed with quantum dots (QDs) to deliver HER2/neu siRNA. The inclusion of fluorescent QDs in the chitosan nanoparticles used to monitor the delivery and transfection of siRNA using such a design(41). Chen et al. used Lipofectamine 2000 to cotransfect QDs and siRNA, and QD fluorescence was used to monitor transfection effectiveness(42). Figure 6.3 demonstrates the application of QDs in a drug delivery system.

6.3.7 DETECTION

OF

VARIOUS CANCERS

The clinical result of a cancer diagnosis is strongly reliant on the stage at which the illness is discovered, hence early screening is critical in any kinds of cancer (43). Cancer biomarkers could be detected using QDs with intense and stable fluorescence properties. Liu et al. developed multicolor QD-antibody conjugates to detect a panel of four protein biomarkers (CD15, CD30, CD45, and Pax5) in human tissue samples simultaneously. This method of multiplexing enables for the quick detection and distinction of uncommon HRS cells from invading

FIGURE 6.3 QDs as drug delivery systems.

Impact of Doping on Efficiency of Quantum Dots

111

immune cells like T and B lymphocytes. They describe the use of multiplexed QDs with wavelength-resolved imaging in Hodgkin’s lymphoma to detect and characterize a type of low-abundant tumor cells(44). GSH-TGA-QDs-ND-1 probes were used to mark colorectal cancer cells CCL187, according to Yu et al. They made QDs with the monoclonal antibody ND-1 for a specific reaction with the antigen LEA(45).

6.3.8 IMAGING

AND

SENSING

OF INFECTIOUS

DISEASES

QDs are a new device with a lot of potential in neuroscience research, and they’re great for investigations where the anatomy of neuronal and glial connections is limited(46). Malaria is a severe global health issue that affects over 300 million people and kills approximately 1 million people each year(47). Tokumasu et al. employed QD-Ab to illustrate the different pattern of protein distribution and to examine erythrocyte membrane deformation during invasion of erythrocytes by Plasmodium falciparum(48). Ku et al. demonstrated the use of a QD-based probe to label P. falciparum-infected RBC in a simple and efficient approach, as well as its utility as an anti-malarial drug screening probe(47).

6.4 DOPING Doping is the process of creating extrinsic semiconductors by adding substances to a pristine semiconductor for the aim of altering its electrical characteristics. Semiconductors are doped to produce either an excess or a scarcity of valence electrons. Dopants are classified into two types: n-type (“n” stands for negative) and p-type (“p” stands for positive). Extra valence electrons with energies near to the conduction band are found in n-type dopants, which operate as electron donors. When n-type dopants are introduced into the atomic lattice of a semi­ conductor, their valence electrons can be easily stimulated to the conduction band. By accepting electrons, p-type dopants aid in conduction. When a p-type dopant is integrated into the atomic lattice of a semiconductor, it can accept electrons from the conduction band, allowing positive holes to develop easily. Figure 6.4 and 6.5 depict doping of a silicon crystal with n-type and p-type doping, respectively.

6.5 SIGNIFICANCE OF DOPING INTO QUANTUM DOTS 6.5.1 ELECTROCHEMICAL DOPING

OF

QUANTUM DOTS

Gudjonsdottir et al. examined various high-melting-point nitriles as electro­ chemical solvents in order to generate stable electrochemically doped semi­ conductor films at room temperature. They discovered that n-doping stability in ZnO QD films is linked to both electrochemical stability windows and solvent melting points. Finally, both the solvent stability and the melting point dictate the solvent’s capability for stable doping by freezing at ambient temperature. Because the electrolyte ions are totally immobilized at normal temperature,

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FIGURE 6.4 Doping a silicon crystal with a n-type doping.

FIGURE 6.5 Doping a silicon crystal with a p-type doping.

cyanoacetamide produced the best results for ZnO QD films. The doping density of ZnO QD films charged with cyanoacetamide is entirely stable over 20 hours at room temperature, according to conductivity measurements. The conductivity gradually decreases over longer periods, and after ten days, roughly 20% of the injected electrons have left the ZnO QD sheet. This drop is most likely due to oxidation of the frozen cyanoacetamide by molecular oxygen. Finally, we mea­ sured the electrochemical properties of two additional semiconductors, PbS QDs and the conductive polymer P3DT, in cyanoacetamide. The n-doped PbS QD layer remained stable for more than five days, however roughly 10% of the

Impact of Doping on Efficiency of Quantum Dots

113

injected electrons had vanished after 40 days. The p-doped P3DT film, on the other hand, remained steady over the entire 76-day experiment and conductivity reduced by only 2%. Finally, we show a pn-junction diode built of a PbS QD sheet in which the p and n regions were electrochemically generated in cya­ noacetamide at extreme temperatures and then stabilised by cooling to room temperature. These findings suggest that electrochemical doping combined with solvent freezing at ambient temperature is a potential approach for producing permanently doped semiconductor thin films, such as colloidal QDs or organic semiconductors(49).

6.5.2

N-TYPE

DOPING

BY

LITHIUM ION INTERCALATION

Excess electrons or holes can be introduced into the delocalized bands of colloidal quantum dots (QDs) to modify their optical characteristics. Appropriate charge compensation is critical for reliable and energy-efficient electrical doping of QDs. Surface modification and substitutional impurities, on the other hand, are in­ sufficiently controlled to allow effective QD doping. Chang et al. described electro­ chemical n-type doping of CdSe QDs, in which injected electrons are compensated by interstitial Li+ to generate LixCdSe, x ≤ 0.3. n-type degenerate doping reduces ab­ sorption into the QD’s lowest-energy excitonic state, initiates intraband optical transitions, and moves the QD’s photoluminescence to a higher-energy state. A de­ crease in the intensity of band-edge excitonic absorption, a hypsochromic shift of the photoluminescence (PL) spectrum due to the MossBurstein effect, and the de­ velopment of intraband electron transitions in the mid-infrared spectrum are all optical repercussions of n-type doping(50).

6.5.3 ELEMENTAL DOPING

OF

GRAPHENE QDS

Graphene quantum dots (GQDs) have received a lot of attention in recent years because of their unique structural and optoelectronic features, as well as their huge potential in applications like bioimaging, medical diagnostics, catalysis, and photovoltaic devices. Because of their unique combination of several key merits, such as tunable photoluminescence (PL), superior photostability, excellent bio­ compatibility in physiological conditions, and ease of bioconjugation, GQDs are expected to surpass current fluorescent reporters such as fluorescent proteins, organic dyes, and semiconductor quantum dots in several applications. However, the absence of controllable synthesize methods and a lack of knowledge of GQDs’ adjustable PL features are currently impeding their wider application. Wang et al. used a simple chemical approach to make GQDs and N/S/P/Bdoped GQDs. By refluxing with strong nitric acid, GQDs were made from thermally exfoliated graphite oxide. Then, using a pyrolysis process, N/S/P/B-doped GQDs were made utilizing melamine, dibenzyl disulfide, triphenylphosphine, and boric acid as N, S, P, and B sources, respectively. GQDs’ PL characteristics are changed via elemental doping. The quantum yield of all N/S/P/B-doped GQDs was found to be higher than that of GQDs(51).

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Quantum Dots and Polymer Nanocomposites

6.5.4 DOPING

ON INAS/GAAS

QD SOLAR CELLS

For several years, self-assembled InAs/GaAs quantum dot solar cells (QDSC) have been researched for the practical implementation of the intermediate band (IB) idea in the GaAs material system. Despite substantial technological advancements and experimental verification of core IB operating principles by QDSCs, many reported devices have failed to meet expectations in terms of improved conversion effi­ ciency. A mild rise in the short-circuit current (Jsc) paired and a considerable fall in the open circuit voltage (Voc) with respect to bulk cells are the two causes. Cappelluti et al. analyzed the effect of doping on quantum dot (QD) solar cells in terms of photovoltaic characteristic, external quantum efficiency, and photo­ luminescence (PL) at room temperature to study the effect of doping on quantum dot (QD) solar cells. To acquire a full device-level assessment of the impact of doping profile and density on solar cell behavior, the study looks at the two most used approaches for QD selective doping, namely modulation and direct doping. In a semi-classical drift-diffusion-Poisson model, devices are simulated using a physics-based model that precisely characterizes QD carrier dynamics. They ex­ plained that the detrimental effect of p-type doping on attainable Voc in high-quality crystal samples is due to the asynchronous and quicker dynamics of holes in comparison to electrons. Different crystal quality scenarios are considered: sub­ stantial open circuit voltage recovery is projected in high-quality materials towards the radiative limit, due to the suppression of radiative recombination through QD ground state. Due to the inhibition of both nonradiative and QD ground state ra­ diative recombination, considerable photovoltage recovery is also achieved in the faulty material. At high doping density, PL emission from extended wetting layer states becomes dominant in both situations(52).

6.5.5 EFFECTS

OF

DOPANTS (N

AND

P)

ON THE

SIZE

AND

QUANTUM YIELD

Shelby Hall explored the effect of nitrogen and phosphorus dopants on the fluor­ escence wavelength of CQDs in a study. N-doped dots fluorescence emission was generally in the 450 nm region, compared to undoped dots with fluorescence emission somewhat over 500 nm. The decrease in the size of the CQDs is re­ sponsible for the shift in emission wavelengths. N-doping inhibited CQD growth by causing the dots to glow at a shorter wavelength, indicating a larger band gap. In addition, the fluorescence intensity of the nitrogen-doped dots was higher than that of the undoped CQDs. P-doped dots emitted light with a wavelength in the 495 nm range. The decrease in the size of the CQDs is responsible for the wavelength shift. P-doping inhibited the growth of CQDs in the same way as N-doping did. This was indicated by the fluorescence at lower wavelengths indicated this when compared to undoped CQDs(53).

6.5.6 EFFECT

OF

DOPING

ON THE

STRUCTURAL

AND

OPTICAL PROPERTIES

In a study by Bailon-Ruiz et al., pure and Cu-doped crystalline QDs were synthesized in the aqueous phase under microwave irradiation. The formation of a

Impact of Doping on Efficiency of Quantum Dots

115

core-shell ZnSe@ZnS structure was proposed by XRD measurements. The coreshell QDs’ luminescence properties were tweaked by doping with the right amount of Cu species. The internal inclusion of copper in the lattice of the host chalco­ genide was attributed to the intense green emission detected in the doped nano­ crystals(54).

6.5.7 EFFECT

OF

DOPING

ON THE

ELECTRONS

AND

HOLES

The exploration of carrier heating in quantum dot optical amplifier (QD SOAs) using carrier temperature modeling is an intriguing concept, as the carrier tem­ perature is a more convenient method of characterizing semiconductor device heating than the nonlinear gain coefficient. Theoretical studies of reservoir carrier temperature in the conduction and valence bands have revealed that the reservoir of electron temperature in the conduction band is higher than that in the valence band. In addition, as the doping concentration rises, the carrier temperature rises as well. The majority of carrier due to doping adds to the increasing of carrier temperature, where doping with n-type leads to a considerable variance in electron temperature, but doping with p-type leads to the opposite(55).

6.5.8 SILVER-DOPED PBSE QUANTUM DOTS Kroupa et al. investigate electronic impurity doping of PbSe QDs with Ag+ cations (Ag:PbSe QDs) and discuss the changes in QD optical and electronic properties as a result of Ag+ incorporation in this study. They show an effective approach of integrating Ag+ cations into PbSe QDs utilizing a postsynthetic cation exchange process that does not appreciably disrupt the initial PbSe QD crystalline matrix in this study. They discovered that Ag+ incorporation is followed by a Z-type ligand exchange at the QD surface, in which Pb(oleate)2 is displaced by AgNO3. They also noticed spectroscopic indications of electronic impurity doping in QDs, such as a bleaching of the first exciton absorbance characteristic and the development of a quantum confined infrared intra-band absorbance. With increasing Ag+ incorpora­ tion, they also observed a quenching of band edge photoluminescence and an ac­ celeration of a fast exciton decay channel, implying the introduction of additional, nonradiative relaxation pathways. They unambiguously identified Ag+ as a p-type dopant for PbSe QDs using photoelectron spectroscopy, which is consistent with earlier Ag doping of bulk PbSe(56).

6.5.9 EFFECT

OF

COPPER DOPING

ON

ELECTRONIC STRUCTURE

The oxidation state of Cu impurity can alter the photophysical characteristics of Cu-doped CdSe quantum dots (QDs), yet there is still controversy about the Cu oxidation state (+1 or +2) in these QDs, which has been discussed and poorly understood for many years. Zhao et al. used the Heyd-Scuseria-Ernzerhof (HSE) screened Coulomb hybrid functional in a systematic density functional theory (DFT) model to investigate how the location and concentration of Cu dopants affect the structural, electronic, energetic, and optical properties of a magic-sized

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Quantum Dots and Polymer Nanocomposites

Cd33Se33 QD. Cu dopants incorporated into the surface of the magic sized Cd33Se33 QD result in non-magnetic Cu 3d orbitals distribution and Cu+1 oxi­ dation state, whereas Cu atoms doped in the core region of QDs result in both Cu+1 and Cu+2 oxidation states, depending on the local environment of Cu atoms in the QDs. Cu dopants with a +1 oxidation state are energetically more stable than those with a +2 oxidation state in Cd33Se33, according to binding energy calculations. The optical absorption coefficient of Cu-doped Cd33Se33 in the in­ frared zone was also discovered to be closely related to Cu oxidation state rather than Cu concentration. In the visible range of 380–574 nm, however, doping only one Cu atom introduces sharper absorption peaks than doping two Cu atoms, suggesting that the absorption coefficient is sensitive to Cu concentration. This research shows that the dopant concentration and location can have a big impact on the electronic structure of Cu 3d states. To improve the efficiency and per­ formance of CdSe QD-based optoelectronic devices, adequate control of dopant concentration and location is essential(57).

6.5.10 EFFECT

OF

HETEROATOM-DOPED CARBON QUANTUM DOTS

Eryiğit Ş et al. investigated the absorption and fluorescence properties of QD-Pc structures in solution. When the QDs were doped with heteroatoms B and N, the quantum yields of the QDs reduced marginally from 30% to 25%. For BCNQD/ZnPc, the Förster resonance energy transfer efficiency was determined to be 33%. There was essentially little energy transfer from QDs to Pc cores for the other conjugates. Because the energy transfer between QDs and Pc cores differed significantly from that between QDs and photosynthetic pigments, we inferred that heteroatom doping in the QD structure and the presence of zinc metal in the phthalocyanine structure are both required for optimal energy transfer(58).

6.5.11 EFFECT

OF

MG

AND

CU DOPING

ON

ZNS QUANTUM DOTS

Heiba et al. published a study in which they found that a simple chemical approach was used to make undoped and 10% Mg or Cu-doped ZnS quantum dots (Zn0.9Mg0.1S and Zn0.9Cu0.1S) at a low temperature (300°C). The Rietveld profile method and highresolution transmission electron microscopy techniques were used to evaluate the effect of doping on structural features. Differential scanning calorimetry (DSC) was used to investigate the effect of doping on the thermal kinetic mechanism and elec­ trical behavior of nano-ZnS. According to diffuse UV reflectance, the optical energy gap increased when ZnS was doped with Mg, but decreased when ZnS was doped with Cu. The band gap energy for Zn0.9Mg0.1S increased somewhat from 3.3 to 3.39 eV due to a shift in Mg–s and p states to higher energy when compared to Zn and S states, but it decreased from 3.3 to 3.26 eV due to the presence of Cu-d state in the upper section of the valence band. According to the composition and excitation wavelength utilized in the study, the PL spectra displayed violet, blue, and green colors. When comparing doped samples to pure ZnS, the intensity of PL has in­ creased. The ability to tune the energy gap of ZnS through doping enables uses in the visible region for light harvesting and photocatalytic degradation(59).

Impact of Doping on Efficiency of Quantum Dots

6.5.12 EFFECT OF MN DOPING SOLAR CELLS

ON

117

CDS QUANTUM DOT–SENSITIZED

Due to the properties of quantum dots (QDs), such as high absorption coefficient, variable band gap, and multiple exciton generation (MEG) effect, quantum dot–sensitized solar cells (QDSSCs) have recently attracted a lot of attention. However, when compared to the theoretical value, its photoelectric conversion efficiency is still low. The carrier’s recombination with redox couple on the semiconductor interface, a slower rate of hole transport, and the electrode char­ acteristics are the key issues limiting the efficiency of QDSSCs. Many recent in­ itiatives to improve quantum dots, electrolytes, and electrodes have achieved significant progress. Li et al. used the impurity element Mn2+ in the precursor so­ lution of cadmium sulphide to examine the influence of Mn doping on the properties of CdS quantum dot–sensitized solar cells (CdS). In addition, Mn-doped-CdS QDs have been deposited in situ on TiO2 mesoporous substrates via ionic layer ad­ sorption and reaction (SILAR). In this investigation, the doped ratio was set to 1: 10 as the most optimal Mn-doped concentration. The photoelectric conversion efficiency of Mndoped-CdS QDSSCs reaches its highest value (1.51%) under air mass 1.5 condition (100 mW/cm2) after adjustment of experimental parameters (the doped ratio of Mn: CdS kept at 1: 10 and SILAR six cycles)(60).

6.5.13 EFFECT

OF

SI DOPING

ON INAS/GAAS

QUANTUM DOT SOLAR CELLS

Naito et al. investigated the influence of Si-doping density, Si-doping methods (both modulation/δ-doping and direct doping), and direct doping on the performance of three types of InAs/GaAs quantum dot solar cells (QDSCs) (0, 2, and 8 Si/QD). In QDSCs with Si doping, there is a clear recovery of open circuit voltage (VOC). In both δ-doped and direct-doped QDSCs, the VOC gradually recovers with in­ creasing Si doping and rises by ∼40 and 80 mV for Si-doped QDSCs with 2 and 8 Si/QD, respectively, as comparing to nondoped QDSC (0 Si/QD)(61).

6.5.14 EFFECT

OF

SILICON DELTA-DOPING

Kim et al. investigated the optical characteristics of type-II InAs/GaAsSb QDs doped with Si. The increased electron population in the QDs caused by δ-doping slows down the PL blueshift rate due to enhanced carrier-carrier scattering, and tends to increase the PL intensity ratio of the ground state (GS) emission to the first excitation state emission, indicating faster radiative recombination at the GS subbands, at a low temperature of 10 K. Furthermore, when the doping density is increased to 5 × 1011 cm−2, the redshift rate of the GS emissions with temperature (>130 K) is accelerated, and when the doping density is increased to 2 × 1012 cm−2, the red shift rate of the GS emissions is reduced. The overall radiative lifespan is extended when the -doping level of 5 × 1010 cm−2 is employed, according to time-resolved photoluminescence (TRPL) measurements. When the δ-doping density is increased to 2 × 1012 cm−2, however, the total radiative lifetime reduces due to the increased radiative re­ combination caused by the fast carrier relaxation. The overall radiative lifetime of

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Quantum Dots and Polymer Nanocomposites

QDs is heavily impacted by the electronic subband occupation of the QDs, which can be controlled by calibrating the δ-doping levels(62).

6.5.15 DIFFUSION DOPING

IN

QUANTUM DOTS

Semiconducting materials that are evenly doped with optical or magnetic impurities have been found to be beneficial in a variety of applications. Aggregation or phase separation during synthesis, on the other hand, has made this job difficult. Saha et al. recently published a study in which they described a universal technique for producing high-quality luminescent, ferromagnetic, semiconducting multifunctional QDs with strong controllable fluorescent emission in the visible and near-infrared regions and room temperature ferromagnetic properties. They did it by diffusing distinct transition metal sulphide and oxide cores inside a semiconducting shell matrix while adhering to the thermodynamic parameters of the transition metals. Inside the semiconductor matrix the bond dissociation energy of the core as well as the diffusivity of the transition metal ions are the two major factors that affect diffusion doping of transition metal ions throughout thermal annealing, according to their findings, while the ease of cation exchange is not. Controlling these parameters reveals various unique characteristics, including exceptional composition tunability, and can enable uniform doping in QDs with desired size and dopant concentration. In comparison to previous research that reveals the direct association between in­ ternal doping and magnetic property, all of these QDs exhibit broad and strong optical emission due to doping and a comparably superior magnetic response(63).

6.5.16 SIGNIFICANCE

OF P-DOPING FOR

QUANTUM DOT LASER

Quantum dot lasers with p-type modulation doping of the active region have been shown to increase high temperature and dynamic performance. The necessity of p-modulation doping (pMD) in reaching the full potential of QD lasers, particularly for epitaxial integration on silicon, was discussed by Norman et al. At optimal doping levels, the extra holes considerably boost gain and differential gain, low­ ering the linewidth enhancement factor (LEF), enhancing high-temperature lasing properties, and increasing the dependability of QD lasers on Si. Performance begins to decline when doping levels are too high, most likely due to nonradiative re­ combination. A spectrally resolved assessment of the effect of pMD on QD laser gain finds anomalous net gains, mostly on the blue side of the gain spectrum, that vanish at high temperatures and contribute to the high typical temperatures in pMD QD lasers(64).

6.5.17 IMPACT

OF

MODULATION P-DOPING

IN INAS

QUANTUM DOT LASERS

Semiconductor lasers’ epitaxial growth on silicon has been hailed as a significant step forward in the integration of light sources in silicon photonics. Threading dislocations (TDs) are naturally introduced to the laser expitaxy due to the sub­ stantial lattice mismatch among silicon (Si) and the grown III-V material. Quantum dot lasers have been shown to be significantly less vulnerable to the destructive

Impact of Doping on Efficiency of Quantum Dots

119

effects of TDs than quantum well (QW) lasers due to the three-dimensional con­ finement of carriers. In a work by Zhang et al. direct estimation of material gain and transparency current in QD laser active regions with differing levels of modulation p-doping was demonstrated. The introduction of excess holes by p doping caused enhancement of population inversion and thus enables the laser to attain higher gain and lower transparency current. At greater doping densities, however, the statefilling effect and increased carrier scattering rate limit the amount of gain that can be gained for the same quantity of extra dopant. Increased Shockley-Red-Hall recombination is also caused by p doping, which saturates the gain-current re­ lationship over a specific doping concentration. Thus, limited threshold and en­ hanced differential gain can be achieved by insertion of suitable p doping density in the QD active region(65).

6.5.18 MN:CU CO-DOPED CDS NANOCRYSTALS Gbashi et al. investigated thin films of Mn- and Mn:Cu co-doped CdS (QDs) which was deposited on a micro-glass substrate using a pulsed laser deposition method. According to the findings, Mn doping increased the energy gap i.e., 2.97 eV compared to CdS bulk materials i.e., 2.42 eV, and adding Cu ions as co doping resulted in a significant reduction in the energy gap (2.97–2.74 eV) as particle size increased. Furthermore, due to the quantum confnement effect, doped and co-doped CdS exhibited a blue shift from bulk CdS (512 nm). In a Cu0.03:Mn0.97 co-doped CdS sample, the photoluminescence intensity fell, then increased. It suggests that the Cu0.03:Mn0.97 ratio is the best for increased PL emission. According to the findings, Cu:Mn as a co-doped material with CdS might be used to improve the structural, luminescent, optical, and morphological properties of nano-semiconductor materials by forming traps and discrete energy states in the energy gap for excited electrons(66).

6.6 CONCLUSION Quantum dots are described as colloidal fluorescent semiconductors of nanosize. This book chapter includes the structure of QDs along with the approaches of QDs synthesis. The reported applications of QDs in various fields such as bio-imaging, biological labeling as well as its clinical applications have been discussed. The addition of an impurity in an intrinsic semiconductor, which is also called doping, is done in QDs to modulate its properties and thus enhance its performance. Dopants are of two types; the n-type dopants include extra valence electrons with energies near the conduction band and act as electron donor, whereas in the case of p-type doping, the dopants aid conduction via accepting electrons. In this chapter, we also included the reported effect of doping in QDs.

CONFLICT OF INTEREST Declared None

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Quantum Dots and Polymer Nanocomposites

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7

Fabrication Methods of Quantum Dots–Polymer Composites Mokae Bambo DSI/Mintek Nanotechnology Innovation Centre, Advanced Materials Division, Mintek, Randburg, South Africa

Bambesiwe May Analytical Chemistry Division, Mintek, Randburg, South Africa Institute for Nanotechnology and Water Sustainability (iNanoWS), College of Science, Engineering and Technology, University of South Africa, Roodeport, Johannesburg, South Africa

Poslet Shumbula Department of Chemistry, University of Limpopo, Sovenga, South Africa

CONTENTS 7.1 7.2 7.3 7.4

Introduction...................................................................................................126 Quantum Dots............................................................................................... 127 QD Polymer Nanocomposites...................................................................... 129 Fabrication Techniques for QD Polymer Nanocomposites ........................ 129 7.4.1 Blending Methods ............................................................................129 7.4.1.1 Melt Blending Method ...................................................... 130 7.4.1.2 Solution Blending Method ................................................131 7.4.2 Chemical Grafting Method .............................................................. 136 7.4.3 In situ Polymerization Method ........................................................138 7.4.4 Layer-by-Layer Method ...................................................................138 7.4.5 Microwave Methods......................................................................... 141 7.5 Challenges in QD–Polymer Nanocomposite Formation .............................143 7.6 Conclusions................................................................................................... 143 Acknowledgments.................................................................................................. 144 References.............................................................................................................. 144

DOI: 10.1201/9781003266518-7

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7.1 INTRODUCTION Polymer nanocomposites are inorganic-organic hybrid materials that are a rapidly growing field of study. They exist as either basic inorganic-organic material or specialized material with unique and promising physical properties for a particular application. Their remarkable mechanical qualities, including as high elastic stiff­ ness and strength with low nanoadditives concentrations, have made them popular in academics and industry [1–5]. These materials also exhibit outstanding properties such as wear and flame retardancy, barrier resistance, as well as magnetic, optical, and electrical properties [1,4,6–10]. Polymer nanocomposites consist of a polymer that serves as the matrix material, and nanofillers or nanoadditives that serve as reinforcement materials. Because polymers may be easily tuned to a variety of physical properties, they are thought to be suitable hosting matrices for composite materials [4,7,8], and are often stable over time and easy to use [3,5,7]. A variety of polymers have been used to make polymer nanocomposites such as thermoplastics, thermosets, and elastomers [2,11,12]. Inorganic nanoparticles have unique electrical, optical, catalytic, and magnetic capabilities that set them apart from their bulk counterparts [13–15]. Polymer nanocomposites are nanometer-scale materials that combine the best features of each component, which are often recognized in the constituent materials, with high expectations in terms of their new applications [6–11,16]. The advantages of polymer nanocomposites can be stated in many parts: (i) nanocomposites are much lighter in weight than conventional composites due to the addition of a small percentage of nanofiller materials, (ii) nanomaterials with size-dependent properties enhance the properties of the matrix material in nano­ composites, compared to conventional composites that require a high concentration of microparticle to improve properties, and (iii) nanomaterials with size-dependent properties enhances the properties of the matrix material in the nanocomposites [11,17]. Therefore, the types of reinforcing materials and matrix materials utilized in nanocomposites are categorized. Some essential ideas in polymer nanocomposites must be considered since they have a direct impact on the properties of the resultant materials. The shape, size, volume fraction, and state of dispersion of the nanofillers are all aspects that affect the characteristics of the nanocomposites [6,11,18]. Understanding the relationship between the physicochemical characteristics of the nanofillers and the final prop­ erties host matrices and the design of new materials with specific functionalities is critical. In comparison to typical composites, nanocomposites have a higher interfacial area because of the nanoscale inorganic fillers. Because of the increased interfacial area, a substantial volume of interfacial polymer is produced, resulting in a lower nanofiller content and characteristics that are distinct from the bulk polymer [11,19,20]. The interactions between the organic and inorganic components at the interface are partly attributed for the creation of these novel features. A material’s property can vary substantially at the nanoscale, according to research. Materials can exhibit new qualities such as increased strength, electrical conductivity, in­ sulating behavior, elasticity, different color, and stronger reactivity with merely a

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reduction in size and no change in the substance itself [11,21]. Furthermore, as dimensions approach the nanoscale level, interactions at phase interfaces improve dramatically, which is critical for improving material properties. The surface area/ volume ratio of reinforcement materials used in the preparation of nanocomposites is critical for understanding their structure-property correlations in this context. Based on the nature and strength of these interactions, polymer nanocomposites materials can be categorized into different classes; Class I, which are hybrids with only weak interactions (e.g., van der Waals or hydrogen bonds) between the organic and inorganic components, whereas Class II hybrids have strong chemical bonds at the interfaces [22,23]. The size, shape, size distribution, and dispersion state of the nanoparticle fillers are all linked to these interactions. The uniform dispersion of nanofillers in the polymer matrix is one of the most difficult aspects of nano­ composites synthesis. Nanomaterials, on the other hand, have a tendency to clump together in order to reduce their high surface energy [23–27]. The agglomeration of nanoparticles in nanocomposites reduces the interfacial area and interactions with polymers, negating the potential benefits of adding nanoscale fillers. Agglomeration can cause material qualities to deteriorate and behave as a defect in the system in some situations. When the physical size of a material is lowered to nanoscale scales, quantization and surface area play a key role in causing dramatic changes in nanomaterial properties. Polymer nanocomposites have been made with a variety of nano reinforcements of various forms. These include three-dimensional (3D) nanospheres/particles, two-dimensional (2D) nanosheets/thin films, one-dimensional (1D) nanowires, and zero-dimensional (0D) quantum dots (QDs) and are some of the geometries and morphologies of nanomaterials that are used [26,28]. Different types of semi­ conductor QDs have been investigated for photovoltaic devices, field-effect tran­ sistors, lasers, light-emitting diodes, and biological applications among nanosized materials [29–32]. The utilization of QDs as nanofillers in the creation of polymer nanocomposites will be the discussion of this chapter.

7.2 QUANTUM DOTS QDs, also known as semiconductor nanocrystals, are nanoscale inorganic materials that are typically 1–10 nm in size [26,29,30]. QDs have unique electrical properties that are midway between those of bulk semiconductors and discrete molecules, which is due in part to their huge surface area [29,33–35]. QDs increase the effi­ ciency of solar photoconversion by generating multiple excitons (photo-induced electron-hole pairs) from the absorption of a single photon [34,35]. Fluorescence is one of the most intriguing features of QDs, in which the na­ nocrystals can create unique colors based on their size (as seen in Figure 7.1) [29,32–35]. When the electrons in QDs return to stable ground states after reaching their excited states, they can emit photons corresponding to their respective energy band gaps. The QDs band gaps can be modified by altering the type and/or size of the QDs, which can be adjusted by altering reaction parameters such as reaction time and temperature [29,36–38].

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FIGURE 7.1 The top side showing the colors of emission, from small (blue) to large (red) CdSe QDs excited by a near-ultraviolet lamp and bottom part showing the photo­ luminescence spectra of certain CdSe QDs [Adapted from 29].

Because of quantum effects (’quantum confinement’) the chemical, physical, and electrical properties of these nanomaterials vary substantially. As a result of the direct influence of the ultra-small length scale on the energy band distribution in the material, the quantum phenomenon, the most prominent word in the nanoworld, is vital because of the modifications in the material [38–40]. The extraordinary electrical, mechanical, optical, and magnetic capabilities of nanoscale materials are due to quantum phenomena, which has piqued the interest of nanotechnology researchers, particularly those in the fields of electronics, medicine, biology, chemistry, and computer science. The most prevalent QDs are binary semiconductor compounds made up of II–VI elements like cadmium sulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe), zinc selenide (ZnSe), lead sulfide (PbS), and mercury sulfide (HgS), among others. Indium phosphide (InP), gallium nitride (GaN), indium arsenide (InAs), and other group III–V elements are also of great interest. There have also been reports of QDs made up of a single element (such as silicon) or ternary elements with two of them in either the cation or anion side (such as CdTeS, CdZnS, CdSSe, InP, and so on). All of these QDs are semiconductors, with a huge band gap energy of less than 4 eV in some cases [38,39,41]. Since the semiconductor nanocrystals were first identified in solids in 1980 by Russian physicist Alexei Ekimov [42], remarkable progress has been achieved in

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the use of QDs to reinforce polymer matrices. Exploration of QD behavior has grown in popularity, extending from basic science to various prospective economic fields aimed toward future high-profit industries. The use of these QDs in polymer composites they do not just allow for the creation of one-of-a-kind structures, but also allows for the development of one-ofa-kind characteristics and functions not attainable with traditional reinforced composites. Over the last few years, several detailed evaluations of QDs reinforced polymer composites have been published [30,43–45]. These are intended to em­ phasize the importance of these materials in a variety of applications. Carbon-based QDs (polymer dots, graphene QDs, and carbon dots) have recently emerged as a new type of semiconductor nanocrystals with better properties such as water solubility, chemical inertness, biocompatibility, and passivation and mod­ ification potentialities [46–49]. In this chapter, we will focus more on inorganic QDs and their application in preparing polymer nanocomposites.

7.3 QD POLYMER NANOCOMPOSITES In QD-polymer nanocomposites, the polymer typically serves as the continuous phase, while the QDs serve as nano-fillers or property enhancers. Usually QDs can be used to enhance or add to the qualities or functionalities of materials that are currently in use for certain applications. The control of elemental composition and stoichiometry in the nanophase, however, makes nanocomposite fabrication processes a bit difficult. Despite the fact that several polymeric materials tagged with QDs have been prepared previously [30,43–45], there is still a search for new synthetic procedures ongoing in order to obtain; a good compatibility between the QDs and the polymer matrix, (ii) better distribution of the QDs in the polymer, (iii) specific characteristics and/or chemical functionalities in the polymer phase, and (iv) biocompatibility through chemical shielding. Several synthetic approaches to creating these QD-polymer nanocomposites have been proposed in the literature [26,30,43,45,50,51]. They will be examined herein. Physical blending, chemical grafting, in-situ growth, layer-by-layer, and microwave methods are some of the current methods used for constructing QDs-polymer nanocomposites.

7.4 FABRICATION TECHNIQUES FOR QD POLYMER NANOCOMPOSITES 7.4.1 BLENDING METHODS Direct mixing of nanoparticles with the polymer matrix is known as the simplest way for generating inorganic/polymer nanocomposites [11,26]. Mechanical blending or physical techniques such as coprecipitation, and grafting can all be used to create a blend of two or more molecular species with no chemical bonds between them. Because nanoparticles have a strong tendency to form agglomerates, the fundamental challenge in the mixing process is establishing an effective dispersion of the nanoparticles in the polymer matrix. However, achieving good dispersion of

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FIGURE 7.2 Overview of melt processing and film casting approaches [Adapted from 26].

the nanofiller in the polymer matrix can be more difficult compared to other ap­ proaches [51–53]. The key challenge in the mixing process is nanofiller agglom­ eration, which can be solved by altering the surface of inorganic particles and other methods. As indicated in Figure 7.2, the mixing can be done via melt blending or solution blending [26]. Melt blending involves integrating particles into a melt polymer, whereas solution blending involves solubilizing the polymer matrix in a solvent and then adding particles to the solution. 7.4.1.1 Melt Blending Method In comparison to other blending methods, melt blending is the most extensively utilized and easy approach for preparing thermoplastic nanocomposites because it is straightforward, industrially advantageous, low cost, and ecologically viable [54,55]. It is also well understood that the processing method has a significant impact on nanofiller dispersion and distribution, interfacial interactions, and the physical and structural properties of polymer nanocomposites. Extrusion, injection molding, high shear mixing, and thermal spraying are some of the methods used to integrate melts [27,54–56]. The fundamental benefit of these techniques, especially extrusion, is the vast quantity of nanocomposites that may be produced. Typically, the polymer matrix is melted in the mixer at the appropriate regulated temperature, then the fillers are added, and the particles are dispersed using spinning rotors. An inert gas (such as argon, nitrogen, or neon), is used to combine the melts. Internal mixers are often only used to compound master batches that are then extruded. For thermoplastics, extrusion is the most common com­ pounding procedure. Co-rotating twin-screw extruders are the best for a complex thermoplastic matrix with inorganic fillers out of all the numerous types of ex­ truders. Two screws allow for greater mixing and good particle dispersion.

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Particles can be introduced directly with the polymer in the main hopper at the start of the barrel, or separately in a side hopper lower down. The final dispersion and characteristics of compounded nanocomposites are influenced significantly by processing parameters such as melt temperature, screw speed, and profile. Due to the lack of organic solvents, melt blending is ecologically friendly, and nanofiller or treated nanofiller can be combined directly in the molten polymer phase [57]. The main benefit of this method is that it is free of toxins; however, melt blending has some disadvantages, including poor particle dispersion in the matrix, especially at larger filler loadings, and also degradation of polymer or surface modifiers due to the high temperatures required for processing. This is related to the higher viscosity of the composites materials [11,58]. Alternatively, the polymer and nanofiller can be dry combined before being heated in a mixer and sheared to produce the desired polymer nanocomposites [58]. Thermal spray, despite its rarity, has shown to be an effective surface coating method [59]. In this method, particles and polymer matrix are mixed together and subsequently heated, and then placed onto a surface. Chu et al., for example, de­ veloped water-insoluble bovine serum albumin (BSA) microspheres coated with CdTe nanoparticles using a spray-drying and thermal denaturizing method [60]. In a short period of time, the microspheres are formed and the CdTe nanoparticles are incorporated into the microspheres synchronously. 7.4.1.2 Solution Blending Method Another simple blend approach for polymer nanocomposites is solution blending, also referred to as solution mixing. To achieve good nanofiller dispersion in elastomeric polymer matrices, solution mixing is commonly performed. It commonly entails dissolving a polymer in a solvent and nanofiller suspension in similar or compatible solvent [57,61]. In using energetic agitation, controlled solvent evaporation, and composite film casting, the nanofiller can be dispersed in a polymer solution [57]. The most important need for using this technology is to select a solvent that combines the polymer matrix’s high solubility with the dispersibility of nanoparticles. Usually, particle surface modification is required to ensure that they are properly disseminated in the solvent. Toluene, tetrahydrofuran (THF), chloroform, dimethylformamide (DMF), acetone, and cyclohexane are among the solvents utilized in the method [58]. The agitation stage can also significantly improve the dispersion of the particles. Magnetic stirring, shear mixing, reflux, and, most often, sonication can all be used for agitation. The solvent is then evaporated to obtain a thin film of polymer na­ nocomposites after adequate dispersion [58]. Combining solution blending with techniques like printing, film casting, dip coating, and spin coating makes it simple to make films. All of these processes entail depositing the solution of particles/ polymer into a mould or onto a substrate, then evaporating the solvent to produce a nanocomposite. The blended solution is splattered across the surface. The polymer/ nanoparticle solution spreads over the substrate during a high-speed, controlled rotation, and a homogeneous coating is recovered following quick evaporation of the solvent [21]. Typical thin films with thicknesses of 1–100 nm can be produced using spin coating. The cost of the solvent and the possibility of toxicity from evaporation are disadvantages of solution mixing [58].

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FIGURE 7.3 The fabrication of PMMA/ZnO nanocomposite films with pure ZnO QDs via solution mixing is depicted in this diagram [Adapted from 63].

Polymethyl methacrylate (PMMA) based polymeric nanocomposites are one of the most commonly synthesized nanocomposites through solution mixing. For in­ stance, Sun et al., detailed the preparation of PMMA/ZnO QDs nanocomposite at different concentrations of ZnO QD (i.e. 0.2, 0.4 and 0.8%). First, the ZnO QDs were synthesized through hydrolyzation of zinc acetate with KOH/methanol mix­ ture, purification, and redispersion in methanol, followed by solution mixing with PMMA solution (see Figure 7.3) [62]. TEM images revealed a gradual increase in aggregation when QD concentration is increased, which was also confirmed by redshifted absorption peaks in UV-Visible spectra. The QD polymerization at 0.2 and 0.4% resulted in the dispersion of QDs indicative from the transparent polymeric nanocomposite solution; however, at 0.8% QD concentration transparency was reduced, likely due to the increased degree of aggregation at higher concentra­ tions [63]. In another study, Yingming et al. described the fabrication of PMMA/CdSe/ZnS core/shell QD nanocomposites by doping the PMMA polymer with QD con­ centrations ranging from 0.03 to 0.1 mg/mL [64]. The CdSe/ZnS core/shell QDs were synthesized using a liquid-solid method that involved dissolving the pre­ cursors in water under magnetic stirring at 180°C. QDs and PMMA were mixed in chloroform solutions, then cast on a quartz substrate, and dried for 72 hours. Uniform distribution of QDs was established within the polymeric films at lower QD concentrations while higher concentrations produced closely packed or ag­ gregated QDs [64].

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In many cases, when nanoparticles are incorporated into organic polymers through solution mixing they tend to form non-uniform dispersion of QDs which can result in aggregation and phase separation within the polymer, especially when they are not coated with organic coupling agents and the solvent evaporation is slow [65]. Several authors have made attempts to produce well-dispersed QDs within polymeric nanocomposites by (i) pre-polymerization with monomers followed by post-polymerization [66], (ii) improving compatibility between QDs and the polymer by selecting suitable solvents and surfactants/ionic liquids for surface modification of QDs [67,68]. Pang et al., for example, used a two-step (before and post) radical poly­ merization technique to fabricate tri-n-octylphophine oxide (TOPO) capped CdSe/ ZnS core/shell QD/PMMA [66]. The pre-polymerization process involves adding CdSe/ZnS core/shell QDs in toluene to MMA solution in the presence of a radical initiator, azobisisobutyronitrile (AIBN) at 0.1% wt., and then heating for 20 minutes at 90°C in a water bath. Then, to polymerize the MMA to PMMA, the postpolymerization procedure was carried out in an oven at 60 C for around 20 hours. The pre-polymerization of the QDs with MMA monomer prevented aggregation and separation within the polymer matrix, which can lead to ununiformed disper­ sion of the QD in the final solid phase of the polymer nanocomposite. As a result, non-uniform distribution of QDs with clusters was observed when the prepolymerization step was not applied. In another development, Song et al., reported the preparation of TOPO capped CdSe/ZnS core/shell QD/PPMA nanocomposite by dispersing the CdSe/ZnS QDs in MMA monomer in the presence of AIBN at 0.5%w/w to obtain a clear QD/MMA solution [69]. Then a solution of 28% w/w/ PMMA was added to the clear QD/MMA solution, and stirred to obtain a homo­ genous polymeric nanocomposite. Zeng et al., constructed a transparent ZnO/PMMA-co-poly-styrene (PS) nano­ composite with well-dispersed ZnO nanoparticles within this polymer, PMMA-PS [70]. Here, the as-synthesized ZnO nanoparticle surface was modified with a monoethanolamine (MEA) coupling agent to make them compatible with the PMMA-PS polymer (as shown in Figure 7.4). A solution of ZnO nanoparticles dispersed in toluene was mixed with MEA at a 5/1 MEA/ZnO ratio at 80 C. Finally, the nanoparticle solution and PMMA-PS polymer solution were sonicated for 10 min to obtain a homogenous polymeric nanocomposite [70]. Highly luminescent polymeric nanocomposites were fabricated by incorporating water-soluble thiocholine bromide (TCB)-capped CdTe QDs into acrylate func­ tionalized polymerizable ionic liquid [71]. A colored solution of water-soluble TCB capped CdTe QDs as mixed with a clear solution of 1-(3-acrylolyloxypropyl)3-methylimidazolium bis(trifluoromethanesulfonyl)-imide (apmium TFSI) ionic liquid monomer under stirring. Then, the TCB-capped QDs were immediately extracted into the apmiumTFSI, indicative of the color change of the aqueous TCBcapped QD phase from color to clear and clear to color for the ionic liquid layer. The fabrication of CdTe QD/polymer nanocomposite was employed by mixing the TCB-capped CdTe QD-apmiumTFSI colored mixture with diethyleneglycol di­ methacrylate cross-linker at 1:4 mol ratio respectively, in the presence of 1 mol% of

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FIGURE 7.4 Fabrication of transparent ZnO/PMMA-PS nanocomposite [Adapted from 70].

AIBN radical initiator. The mixture was degassed and polymerized at 60°C for 2 hours in a glass tube [71]. A similar approach was described where water-soluble 2-dimethylaminoethanethiol (DAET)-capped CdSe/ZnS QD were extracted into 1-hexyl-3-methyl imidazolium bis (trifluoromethane sulfonyl) imide (HMITFSI) via cation exchange. In the presence of AIBN radical initiator and ethylene glycol dimethacrylate (EGDMA) as cross-linker, the CdSe/ZnS-HMITFSI mixture generated a compatible medium for PMMA polymerization (see Figure 7.5) [67]. Some studies have demonstrated the preparation of perovskite QDs/polymer nanocomposites [72,73]. For example, Raja et al., described the incorporation of perovskite CsPbBr3 QDs into high molecular weight hydrophobic polymers by means of a QD-ligand-hydrophobic matching scheme [72]. To generate homo­ geneous dispersion of QDs inside the polymer, toluene solutions of oleylamine functionalized-CsPbBr3 QDs and poly (styrene-ethylene-butylene-styrene (SEBS) containing alkyl chain ligands of very comparable composition to the native alkyl chain ligands were combined and sonicated [72]. In another development, Qaid et al., demonstrated the fabrication of CsPbBr3 QDs/poly (9,9-di-n-octylfluorenyl2,7-dial) (PFO)/nanocomposite thin films by combining toluene solutions of per­ ovskite QDs and PFO polymer and spin-coating on glass substrates [73].

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FIGURE 7.5 Preparation of PMMA/HMITFSI/CdSe/ZnS nanocomposite [Adapted from 67].

Furthermore, highly fluorescent (92% QY) ternary CuInS2 (core) ZnS/ZnS (double shell) QDs were used to prepare CuInS2/ZnS/ZnS QD-PMMA nano­ composites for applications in QD-based white light emitted diodes (WLED). The oleic acid-capped CuInS2/ZnS/ZnS QDs were dispersed in chloroform and com­ bined with a PPMA solution in chloroform for 30 minutes at room temperature. However, prolonged LED operations can lead to instability and quench fluorescent properties thus a silica over layer (sol-gel) was grown to improve its operational stability [74]. James et al., used spin coating to make thin films in which cerium oxide na­ noparticles were integrated into the polystyrene phase using three distinct ways to raise the refractive index of the resulting nanocomposite thin film [75]. The first step for making the polymer nanocomposite was to deposit cerium oxide (ceria) nanoparticles onto the PS thin film; then followed by incorporating the ceria na­ noparticle onto the cross-linkable PS thin film, and then the third step was to bulk mix functionalized ceria nanoparticles into the cross-linkable PS and spin coat the composite solution to make the thin film. PS was dissolved in tetrahydrofuran with the help of surface-modified ceria nanoparticles, which Parlak and Demir exploited to create transparent nanocomposites. The solution was spin-coated into thin films with a thickness of 2.5 µm [76]. Mallakpour et al., created two types of transparent

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nanocomposites from polyvinyl alcohol (PVA) in ethanol solutions blended with modified TiO2 using a combination of solution blending and solution casting methods [77]. Chae and Kim generated a polymer nanocomposite by dispersing unmodified ZnO particles in PS/N, N-dimethylacetamide solutions using a simple solution mixing method [78].

7.4.2 CHEMICAL GRAFTING METHOD Because of nanoparticle aggregation, distributing nanoparticles uniformly into a polymer or organic solvent is problematic. The mechanical characteristics of the polymer matrix, as well as the qualities of the interfacial areas between the surface of nanoparticles and the matrix polymer, are thought to influence the properties of nanocomposite with nanoparticles [23,79]. As a result, several re­ searchers have examined the chemical and physical changes on the surface of QDs nanoparticles extensively [80–82]. Modification of the surface by the chemical process is permanent, whereas physical modification is just temporary. Surface grafting of polymers, i.e. chemical binding of polymers, onto nano­ particle surfaces has been shown to greatly increase the dispersibility of QD nanoparticles [82–85]. The grafting of polymer onto nanoparticle surfaces is being studied for the purpose of developing functionalized organic-inorganic hybrid materials with the best of both nanoparticles and grafted polymers [83,85,86]. Chemical modifications to establish covalent connections with poly­ mers, such as esterification, etherification, epoxidation, ethacrylation/acrylation, and acylation, are made possible by the abundance of multifunctional groups on the QD surface. Chemical grafting of QDs to polymer matrix outperforms phy­ sical mixing in terms of enhancing mechanical strength and retaining character­ istics over time [83,85]. The production of nanocomposites with more uniform characteristics is another advantage of chemical grafting [11,83,86]. For surface grafting of polymers onto nanoparticles, several methods have been devised [83,86]. These are highlighted as follows (see Figure 7.6);

FIGURE 7.6 Schematic diagram showing the “grafting to” and “grafting from” methods [Adapted from 87].

Fabrication Methods of Quantum Dots–Polymer Composites

137

i. ‘Grafting onto’ method; in the presence of nanoparticles, traditional initiators are used to graft polymerize various vinyl monomers. The ter­ mination of developing polymer radicals, cations, and anions produced during polymerization is used to guide the grafting of polymers onto the surface. ii. ‘Grafting from’ method: graft polymerization of diverse monomers is initiated from radical, cationic, and anionic initiating groups which are previously introduced onto the nanoparticle surface, and iii. Polymer reaction method: the reactivity of functional groups on nano­ particles with polymers containing functional groups including hydroxyl, carboxyl, and amino groups results in grafting onto the nanoparticle surface. Grafting can also be accomplished by using functional groups on the nanoparticle surface to deactivate the ends of live polymer chains. iv. Stepwise growth method: Dendrimer synthesis methodology is used to grow grafted polymer chains from surface functional groups on nanoparticles. In other study, atom transfer radical polymerization (ATRP) was used to graft polymers from nanoparticles initiated by surface functional groups and transition metal complex systems [88–90]. It has been observed that silica nanoparticles can undergo in-situ radical transfer addition polymerization and emulsion polymeriza­ tion. In situ bulk radical polymerization was employed by Bawendi et al. to create a highly luminous nanocomposite of CdSe–poly (lauryl methacrylate) which can be used to display colors [91]. Esteves et al., used activators generated by electron transfer (AGET) ATRP in a miniemulsion to create QD–polymer nanocomposites with regulated polymer chains grafted directly from the QD surface. The QDs were first modified with a trialkylphosphine oxide with a chlorine-based ATRP used as an initiator and then polymerized from the functionalized surface. This polymerization technique uses the AGET catalytic system, which eliminates the usage of traditional radical in­ itiators, which can degrade QDs and can begin free polymer chains [92]. Reversible addition-fragmentation chain transfer (RAFT) and nitroxide-mediated polymerization (NMP) have also been used to create QD–polymer nanocomposites with better structural control [93,94]. The presence of typical free-radical ATRP initiators can diminish the optical characteristics of semiconductor nanoparticles, therefore ATRP has received less attention in this context. [95,96]. Furthermore, utilizing a “grafting from” method, QDs/polymer nanocomposites were synthesized in miniemulsions by RAFT polymerization. Initially, a chain transfer agent, a trisalkylphosphine oxide with 4-cyano-4-(thiobenzoylsulfanyl) pentanoic acid moieties, was used to functionalize the surfaces of CdS and CdSe QDs. A PS block was grafted from the surface of the QDs using a free radical initiator (AIBN) to initiate the RAFT process. The nanocomposite formed was found to have quantum confinement effects, indicating that the QDs were not da­ maged during the polymerization process. The presence of free PS chains in the final nanocomposite suggests that RAFT polymerization from the surface of the QDs was complemented by traditional free

138

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radical polymerization. A second poly(n-butyl acrylate) block was tentatively gen­ erated from the previous PS block after isolating the nanocomposite particles [93]. Due to its capacity to compartmentalize the components in nanoreactors (monomer droplets) and create well-dispersed nanocomposite materials, mini­ emulsion polymerization has recently been described as particularly favourable for the polymer encapsulation of inorganic materials [97]. In fact, investigations in this method have demonstrated that under standard free radical polymerization condi­ tions, organically capped QDs may be efficiently encapsulated using the in situ miniemulsion technique [92]. During the polymerization process, QDs were trapped within the polymer particles and remained as clustered nanocrystals in the final nanocomposite beads [91]. The integrity of the nanocrystals and their photo­ luminescent characteristics were preserved in these materials. Individual CdSe–ZnS–trioctylphosphine oxide (TOPO) nanocrystals were suc­ cessfully encapsulated in polystyrene beads by Esteves et al. using the same miniemulsion method. However, there was some aggregation and segregation of the QDs toward the surface of the polymer particles [98]. Since complicated reaction processes such as filtration, centrifugation, and solvent extraction are required for the synthesis of polymer-grafted nanoparticles, and large amounts of waste solvent are obtained, scaled-up synthesis of polymer-grafted nanoparticles is difficult to achieve [86].

7.4.3 IN

SITU

POLYMERIZATION METHOD

The process of in situ polymerization also referred to as chemical oxidative poly­ merization, is widely utilized in the preparation of composites since it increases the dispersion and interaction of QDs in the polymer matrix [99]. The use of in situ polymerization in the synthesis of QD-polymer nanocomposites has been shown to improve electrical characteristics significantly when compared to other preparation methods (e.g., ex-situ) [100]. Due to its advantages, such as the capacity to synthesize nanocomposites with insoluble and thermally unsteady matrices that cannot be gen­ erated by other methods, such as solution/melting procedures, it has previously been used in the synthesis of QDs-polymer composites by a number of scientists. Filler, such as inorganic nanoparticles, are first distributed in a liquid monomer in this process. Heat/radiation, diffusion of the proper initiator, organic initiator/catalyst put on the surface of nanoparticles under the right temperature, pressure, and stirring conditions commence the polymerization reaction (see Figure 7.7). Fabrication of polymer nanocomposites is actually a hybridization process that involves an organic/inorganic polymer matrix and an inorganic/organic nanofiller to create a single material with integrated matrix and filler properties [101]. The in­ teraction between the matrix and the nanocrystalline fillers gives the nanocomposite mechanical [102], optical [103], and electrical properties [30].

7.4.4 LAYER-BY-LAYER METHOD In the creation of polymeric shells, various approaches based on polymerization straight from the QD surface have been disclosed. The thicknesses of the shells,

Fabrication Methods of Quantum Dots–Polymer Composites

139

FIGURE 7.7 In situ polymerization method of preparation of conductive and insulating polymers [Adapted from 100].

on the other hand, were a considerable issue because they were found to be difficult to manage. In order to remedy the situation, layer-by-layer (LbL) as­ sembly technique, introduced by Decher and Hong [104–108], was employed and found to solve most of these shortcomings. This method has been utilized to create molecular-level structures and multilayer organic-inorganic films via noncovalent interactions. It has become an enabling technology in bottom-up nanofabrication due to the ease with which it can produce smooth, homogenous, defect-free, and high-quality thin films with thicknesses that can be regulated at the nanoscale. The substrate form is rarely a limitation when employing aqueous solutions for construction. Despite the fact that the technique was originally developed for the assembly of polyionic polymers, other materials such as metal and semiconductor nanoparticles, organic dyes, polymer nanospheres, electro­ chemically active materials, and bio-macromolecules have been incorporated into the nanostructured films to obtain assemblies with desired optical, electro-optical, mechanical, electrical, magnetic, and electromagnetic properties. However, as compared to other fabrication methods, the LbL method has one major disadvantage in that it is a relatively slow process. On the other hand, the capacity to control polymer film formations on the nanoscale is so important that this disadvantage is typically overlooked. This LbL assembly method has also been presented as a way to include QDs into layered double hydroxide (LDH) without compromising their photoluminescence efficiency, resulting in composites that are extremely luminous and photostable [109]. Before being encased in polymer, the QDs were produced in an organic solvent (maleic acid-alt-octadecene). To make QD-polymer-LDH composites, negatively charged polymer-encapsulated QDs with negative zeta potentials were electrostatically linked with positively charged LDH nanosheets, as shown in Figure 7.8. The photoluminescence properties of the hybrid films were revealed to be similar to those of organic QD solutions, and the QD-polymer-LDH composites provide better photostability by sheltering the QD surface from the environment using polymers and LDH nanosheets. The fluorescence spectra of the composite did not change when isolated from the colloidal state, whereas QDs and polymerencapsulated QDs lacking LDH composite formations were red-shifted. In contrast,

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Quantum Dots and Polymer Nanocomposites

FIGURE 7.8 Schematic of the assembly procedure for QD-polymer-LDH composites [Adapted from 109].

a photoluminescence quantum yield decline of QD-polymer film was discovered due to a non-uniform distribution of fluorophores (see Figure 7.9). In addition to electrostatic interactions, other supramolecular interactions like hydrogen bonding can be employed to generate LbL structures [110,111]. Poly (vinyl pyridine) (PVP)/CdSe multilayer films have been created using the interac­ tions between CdSe coated with a carboxyl group and the pyridine moiety of PVP, for instance. X-ray diffraction studies revealed that the hydrogen bond method created homogeneous thin films. Polyacrylic acid (PAA) was used as an interaction medium between QDs and an N-vinyl carbazole/4-vinyl pyridine copolymer by

FIGURE 7.9 Photoluminescence spectra of films of QD-polymer solution (I) and QDpolymer-LDH composite solution (II) with an inset photograph of the QD-polymer film (left inset) and the QD-polymer LDH composite film (right inset) under UV light irradiation for color recognition [Adapted from 109].

Fabrication Methods of Quantum Dots–Polymer Composites

141

FIGURE 7.10 Schematic graph for the buildup of an alternating film of CdTe/Co-VCz4VP. [Adapted from 112].

Zhang et al. in a similar manner [112]. By varying the number of pyridine units in the copolymers and the quantity of PAA in the solutions (see Figure 7.10), Zhang et al., were able to adjust the amount of deposited QDs during each adsorption step [112]. PAA has been used a ligand on the QD surface. Before the PAA-coated QDs are deposited, a layer of polyelectrolyte that is positively charged is put to the substrate. Polymer deposition is used to create the next layer (N-vinyl carbazole-co-4-vinyl pyridine) between the carboxyl groups in polyacrylic acid and the pyridine units in the copolymer, then hydrogen bonds develop.

7.4.5 MICROWAVE METHODS Microwave treatment methods are efficient and rapid strategies of synthesizing QDs with improved quality and yield. Microwave heating offers a considerable ad­ vantage in time over the thermal form of heating. Some microwave-assisted reactors offer multi-functions such rapid heating and pressure control features that makes the method even more desirable. Microwave-assisted approaches have been widely utilized in fabrication of QD/polymer nanocomposites, either as (i) one-pot synthesis or for (ii) the prior synthesis of the native QDs followed by conventional heating/mixing with the polymer to form the nanocomposite [113]. A one-pot microwave-assisted approach was used to fabricate carbon QD/ polymer nanocomposites using carbon sources (either citric acid or ascorbic acid) and polyethylene glycol (PEG), PVP, and BSA as polymers. Aqueous solutions of citric acid or ascorbic acid and the polymers were prepared by dissolving in water and stirring for 30 min at room temperature. After stirring, the carbon source/ polymer mixture is heated between 60 and 140°C at 300 W for 20 minutes in a microwave reactor (Figure 7.11). Generally, enhanced fluorescence properties were observed for carbon QDs synthesized using citric acid than ascorbic acid and BSA for carbon QD/BSA polymeric nanocomposites [114]. A multi-step microwaveassisted approach for the synthesis of chitosan- ZnO/CdS QD polymer nano­ composites was described by Midya et al. In the first step involved the microwaveassisted synthesis of cross-linked chitosan, where chitosan was dissolved in acetic

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Quantum Dots and Polymer Nanocomposites

FIGURE 7.11 Fabrication of G-QD/PEG, G-QD/PVP, and G-QD/BSA nanocomposites [Adapted from 114].

acid under stirring for 4 hours at 60°C then, the solution was heated in a microwave reactor at 75°C at 24 W power at inert atmosphere. This was followed by the addition of potassium persulphate, methacrylic acid (MAc), and diethylene glycol dimethacrylate (DEGDMA) cross-linker, and the reaction was allowed to run for 4 minutes. The second step is the microwave-assisted synthesized CdS QDs, where CdO and sulphur precursors in ODE are mixed and heated in a microwave reactor at 240°C for 10 minutes. The third step is a microwave-assisted synthesis of ZnO QDs, where zinc nitrate precursors are dissolved in ammonia solution and then heated at 70°C in a microwave reactor for 5 minutes. Lastly, a cross-linked chitosan aqueous solution was mixed with the ZnO QD solution and then stirred in a mi­ crowave reactor at 70°C for 7 min. A solution of CdS QD in acetone was also added and the reaction was continued for further 3 minutes [115]. In another study, a microwave-assisted/hydrothermal method was implemented for the fabrication of graphene QD/polyaniline (PANI) nanocomposites (Figure 7.12). First, graphene QDs were synthesized by irradiating a solution of D-glucose in a

FIGURE 7.12 Fabrication of PANI/GQD nanocomposites [Adapted from 116].

Fabrication Methods of Quantum Dots–Polymer Composites

143

microwave oven at 180 C for 5 minutes at 300 W and at varied pressures between 165 to 350 psi. This was followed by mixing the aqueous GQD solution with aniline salt solution at 80 C for 30 minutes, leading to PANI/GQD composite formation through COO-, NH3+ charge-charge interaction between GQD and aniline functio­ nalization, respectively. Lastly, purified ammonium sulphate crystals are added to the mixture, and polymerization is carried out at 4 C for 30 minutes [116]. Similar ap­ proaches were also followed to produce N-doped carbon QDs/PANI nanocomposites [117], and carbon QD/PAFP [118].

7.5 CHALLENGES IN QD–POLYMER NANOCOMPOSITE FORMATION The usage of nanofillers in polymer nanocomposites has a substantial impact on the overall qualities of the nanocomposite. Some of the difficulties that the approaches for synthesizing QD–polymer nanocomposites face have already been mentioned. The key challenge is creating a perfectly homogeneous material by evenly dis­ persing the nanofiller in the polymer matrix. This can be attributed to a number of causes, including (i) incomplete nanofiller mixing in the polymer and also (ii) na­ nofiller agglomeration. Because QDs are small particles, the quantum effect is highly experienced, as is the compatibility of the nanofiller with the polymer. In the melt blending method, the main drawback is the occasionally persistent poor dispersions of particles in the matrix specifically at higher filler loadings and also degradation of polymer matrix or surface modifiers due to high temperatures needed for melt processing. Solution mixing, which has been suggested as a method of choice, tends to involve the use of toxic solvents, and in some circumstances large amounts of these solvents are required to make composite materials. Chemical grafting, for example, is difficult to use on a big industrial scale. This makes it difficult for to be used widely.

7.6 CONCLUSIONS Polymer nanocomposites provide a tremendous opportunity to investigate new functions that are not available in traditional materials. The methods for fabri­ cating QD–polymer nanocomposites were discussed in this chapter. The ad­ vantages and disadvantages of the approaches for producing QD–polymer nanocomposites are discussed. Because of their ease of use and ability to produce homogeneous materials with better physicochemical properties, some of the processes are still in use. Even with industry-leading methods like melt blending, attaining 100% homogeneous dispersion of the nanofiller remains a challenge. The development of QD–polymer nanocomposites is progressing thanks to the utilization of methods such as LbL and microwave techniques. As a result, nano­ composites are finding various niche applications. Some of the disadvantages of these methods are also noted, but progress is being made to enhance them. Continuous advancement in the pursuit of homogeneous dispersion of QDs in the polymer matrix is still a hot topic in academia and industry.

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ACKNOWLEDGMENTS The authors would like to thank the following institutions: Mintek, University of South Africa, and the University of Limpopo.

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8

Reinforcement Mechanisms of Quantum Dot–Polymer Composites Sayan Ganguly Bar-Ilan Institute for Nanotechnology and Advanced Materials, Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel

CONTENTS 8.1 8.2 8.3 8.4

Introduction...................................................................................................151 Benefits and Complexities of Polymer-Based Nanocomposites.................153 Dispersions and Agglomeration of Nanofillers in Polymer Matrices ........154 Various Nanofillers for Polymer Matrices .................................................. 155 8.4.1 Shape Dependency Reinforcement .................................................. 155 8.4.2 Nanofiller Chemistry ........................................................................ 156 8.4.3 Nanofiller Size and Shape................................................................ 157 8.5 Carbon Dots: Features and Surface Properties ........................................... 158 8.6 Quantum Dots............................................................................................... 159 8.7 Polymer Dots and Their Hybrids.................................................................160 8.8 Reinforcement Behaviors of Fillers into Polymer Matrices ....................... 161 8.9 Summary and Outlook ................................................................................. 164 References.............................................................................................................. 165

8.1 INTRODUCTION Nanotechnology is “the study and control of matter at dimensions of around 1–100 nanometers,” according to the National Nanotechnology Initiative Strategic Plan, and it has become one of the most popular topics of development and research in many technological disciplines in the previous 20 years. Manipulation of nanometer-scale structures, in fact, enables the use of physical and chemical characteristics of matter that are only revealed at this size [1]. The utilization of nanoparticles in particular has resulted in the creation of improved polymeric matrix nanocomposites for the production of high-performance goods. These nanoparticles have a large interfacial area for their size, allowing for improved molecular inter­ actions with the polymeric matrix, resulting in unexpected and improved DOI: 10.1201/9781003266518-8

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characteristics above typical composites. A composite is a material that consists of one or more discontinuous phases encased in a continuous phase (the matrix). The word “nanoreinforcement” is frequently used in nanocomposites to denote nano­ fillers that improve a matrix’s mechanical characteristics [2–5]. Nanofillers can also be employed to improve the heat resistance or barrier qualities of a nonconductive polymer (nanoclays), or to improve the toughness or wear resistance of a non­ conductive polymer (carbon nanotubes) [6–9]. From vehicle bumpers to sophisticated optoelectronic devices, polymer na­ nocomposites have been suggested or are being employed for a variety of purposes. Understanding the effects of nanofillers on the mechanical char­ acteristics of composites is crucial to the success of all of these applications. As a result, several research groups are working to build a generic framework for predicting, or at the very least understanding, how the chemistry and mor­ phology of the polymer matrix interact with the surface chemistry, size, and shape of a nanoscale filler to determine mechanical characteristics [10]. The fundamental mechanisms are found at the junction of chemistry, physics, ma­ terials science, and continuum mechanics in this comprehensive framework [11]. As a result, the researchers working in this crucial field of science come from a wide range of experiences and perspectives [12–14]. The driving topic in this branch of study is: what is the “nano” influence on macroscopic mechanical properties? Although this issue appears to be straightforward and focused, it has a significant deal of depth and intricacy. To begin, what exactly is “nano”? The term “nano” is commonly used in the field of polymer nanocomposites to refer to the shortest length scale connected with the filler. Nanocomposite fillers typically have length scales ranging from 0.1 to 100 nanometers [15]. On these length scales, the filler items approach the length scale of a single polymer coil and form unique interactions that allow for optimal mechanical property control, such as failure qualities. The properties of the interphase itself determine the majority of the final properties of nanocomposites: because the surface-to-volume ratio of nano­ particles is so high, the resulting interphase fills a large portion of the compo­ site’s volume and can become the most important factor in determining the properties of the nanocomposite. Herein, it would be highly interesting in characterizing mechanisms that arise on nanoscale length scales, influence the microscale, and impact the macroscale in order to better comprehend the “nano” effect on mechanical characteristics. The presence of nanofillers can lead to the creation of novel microstructures, improved stress transmission between mate­ rials with complimentary qualities, or an increase in the density of shear de­ formation events. The combined influence of the material components in the nanocomposite determines the dominance of these or other processes. In today’s world, polymer composite materials are frequently employed. Fiber-reinforced polymer matrices with Young’s moduli of up to 150 GPa [16], such as carbon- and glass-fiber-epoxy composites, are common examples. These glassand carbon-fiber-polymer composites are often used in structural parts for aerospace [17], boating [18], sports [19], and the automotive industry [16]. The hardness, stiffness, and fracture toughness of non-continuous polymerparticulate

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composites are also improved [20]. Polyvinyl alcohol (PVA) and polylactic acid (PLA), which are extensively used in biomedical applications, are examples of biodegradable polymers [21].

8.2 BENEFITS AND COMPLEXITIES OF POLYMER-BASED NANOCOMPOSITES PNCs (polymer-based nanocomposites) are a type of polymer composite mate­ rial. PNCs have a long history of design. Previous research [22] on the re­ inforcing effects of fibers [23] and particulates [8] on polymer matrices emphasized the need of a strong interface and the presence of an interphase. The border area of carbon-fibers embedded in epoxy, for example, was examined to better understand and improve these polymer-filler interactions [24]. The characteristics of the polymer in the region of the inserted fibers clearly differ from the properties of the real polymer matrix, generating a third phase in the two-phase polymer-filler system, according to atomic force microscopy (AFM) investigations [25]. The interphase phase may be found in both micro- and na­ nocomposites, as well as particle and fiber-based composites (Figure 8.1). A robust contact between the particle and the interphase is required to provide improved load and heat transmission from the polymer matrix to the filler. A nanocomposite, in contrast to a microcomposite, is a multiphase material in which one of the phases has a diameter of less than 100 nanometers (nm) or the composite phases are separated by nanoscale distances [26]. We shall solely investigate polymer matrix composites with nanoparticles from now on (fillers). Nanofillers have a large surface area due to their tiny size, which increases the amount of polymer in contact with the filler. When the nanofiller volume content is high enough, the interphase takes over as the dominant phase in the composite. Controlling the interphase characteristics provides a lot of possibilities for modifying the properties of the nanocomposite and can result in a whole new set

FIGURE 8.1 Composite specific surface area versus particle diameter.

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of qualities that aren’t found in the separate components. It should be noted that the interphase has a significant influence on overall composite characteristics, and depending on the interaction between the nanofiller and the polymer and electrical to optical interactions, can result in dramatic increases or decreases [27,28]. Furthermore, multifunctional composites can be created by mixing various fillers with the necessary qualities or by employing fillers with numerous properties [29,30]. Nonetheless, a number of challenges must be addressed be­ fore nanofillers may be fully utilized in PNCs. One of the difficulties is estab­ lishing uniform dispersion and reducing nanofiller aggregation. Agglomeration is influenced by dispersion qualities such as viscosity and chemical structure. Particle characteristics, on the other hand, primarily influence dispersion and agglomeration, with particle size, surface charge, and chemical structure being major factors [31]. The size of agglomerates is determined by surface charge and chemical structure, which has a direct impact on the percolation threshold [7]. All of these elements interact, making the creation of nanocomposites a difficult task that also has a lot of promise.

8.3 DISPERSIONS AND AGGLOMERATION OF NANOFILLERS IN POLYMER MATRICES The diverse states of dispersion and agglomeration may be categorized into four categories: (a) uniform dispersion (Figure 8.2a); (b) random dispersion (Figure 8.2b); (c) clustered random dispersion (Figure 8.2c); and (d) agglomerated dispersion (Figure 8.2d). Uniform and random dispersions are preferred because they allow for

FIGURE 8.2 Various states of filler dispersions (a) uniform, (b) random, (c) cluster for­ mation, and (d) agglomerated particles.

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the use of nanofillers’ vast surface area. Clustered and agglomerated dispersions are undesirable because agglomerates and clusters have qualities that differ sig­ nificantly from filler properties and are dependent on interparticle interactions [32]. Agglomerates can behave as voids at a crucial agglomeration size, preventing the polymer matrix from infiltrating. Although this feature can be useful in some cases, agglomeration of nanofillers is not necessary in most applications [33]. As a result, the agglomeration undermines the unique benefits of nanoparticles in composites. Many studies have shown that the degree of nanoparticle dispersion in polymer matrices has a significant impact on physical qualities. The impact of dispersion degree on mechanical and rheological characteristics is well understood. Because a strong interfacial bonding may efficiently transfer the load from the matrix to the reinforcement, the lack of chemical interaction between the polymer and the particle, which has a link with dispersion, might impact the mechanical characteristics of composites. Nanoparticles tend to clump together and form tightly bound aggregates, which can then grow into bigger formations called agglomerates. The repulsion be­ tween particles in a solvent cast system may be adjusted by adjusting the pH or electrolyte content.

8.4 VARIOUS NANOFILLERS FOR POLYMER MATRICES The nanofillers may be divided into three categories: 0-, 1-, and 2-dimensional fillers. Zero-dimensional fillers are fillers with only one nanometer in dimension. As a result, one- and two-dimensional fillers have diameters more than 100 nm in both one and two dimensions.

8.4.1 SHAPE DEPENDENCY REINFORCEMENT The volume and form of filler nanoparticles are another factor that might influence mechanical characteristics. There is a maximum volume fraction of particles that can be integrated into the polymer matrix for each filled polymer system. Otherwise, stress concentration sites form, resulting in a brittle material with decreased ultimate tensile strength. Liu et al. investigated the mechanical reinforcing efficiency of two types of nanoparticles, nanotube and nanoplatelet, from a micromechanics standpoint [34]. Their findings show that nanotubes have a greater mechanical reinforcing ef­ ficacy than nanoplatelets of the same aspect ratio for longitudinal characteristics of aligned composites, whereas nanoplatelets’ high in-plane isotropic modulus allows for superior reinforcing in random orientations in most circumstances. Multi-walled carbon nanotubes (MWCNTs) enhanced composite elastic behavior, according to Breton et al. [35]. Compared to nanoplatelets (at the same volume fraction and degree of dispersion), nanotubes form a substantially higher number of interfaces, which can overcome the fundamental nanoparticle impact and result in higher stiffness for all nanotube composite configurations. These findings might explain why graphite na­ noparticles (expanded graphite) haven’t improved their characteristics to the same extent as carbon nanotube (CNT) reinforced materials [36].

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8.4.2 NANOFILLER CHEMISTRY Two key features of the polymer/nanofiller interaction are influenced by the chemistry of the nanoscale filler. To begin, the nanofiller’s chemistry adds to the enthalpic interaction with the polymer chain. The effectiveness of stress trans­ mission across the nanofiller/polymer interface is heavily influenced by enthalpic interactions [37]. The van der Waals interactions between the nanoparticle and polymer chains can define these enthalpic interactions, or they can be linked to particular interactions like covalent bonds. The shape of a polymer nancompo­ site is heavily influenced by the intensity of these interactions [38]. As pre­ viously stated, the polymer blend theory essentially describes the equilibrium morphology of a polymer nanocomposite. The interaction parameter between particle and polymer can modify the miscibility boundaries for given filler and polymer size ratios, much as it can in the polymer blend theory [39]. The filler chemistry, which is commonly defined in terms of the material’s Hamaker constant, has a significant impact on the van der Waals contact between sur­ rounding filler particles. These interparticle interactions are significant in the strength of the filler aggregates and at large volume percentages of the filler in the nanocomposite [40]. Nanofillers can phase segregate and generate domains that are rich in the nanofiller but deficient in the polymer in this last effect. If the inter-particle interactions are very favorable in these domains, the aggregate will behave more like a big filler particle than individual nanoscale fillers [41]. If the interparticle attractions are weak, the aggregate’s deformation processes may play a significant role in the storage and dissipation of applied mechanical en­ ergy. The development of mechanical properties in the polymer nanocomposite will be greatly influenced by these deformation processes [41]. The field of polymer-clay nanocomposites is one example of these inter-filler interactions and their influence on mechanical characteristics. Many research groups have shown the influence of on mechanical character­ istics in this area [42]. The filler particles are well-dispersed and behave in­ dependently in context with the polymer matrix in the exfoliated morphology. The exfoliated filler has a tiny length scale, but the geometric stiffness of the filler particle is similarly minimal. The clay fillers are well-dispersed in intercalated morphologies; however, the clay filler is made up of numerous layers of clay sheets that are firmly attracted to one other [43]. The intercalated clay fillers react similarly to bigger filler particles with lower surface-to-volume ratios due to these strong attractions. The mechanical characteristics of intercalated clay na­ nocomposites are changed when more molecules are introduced to reduce intersheet contacts. The chemistry of the filler also determines its mechanical quali­ ties. The local constraint is defined by the mechanical characteristics of the filler in relation to the mechanical properties of the polymer matrix, much like in traditional composites [44]. Several research groups have investigated the influence of the filler’s mechanical characteristics on the overall mechanical properties of the nanocomposite, both experimentally and conceptually. A recent contribution by Liu and Brinson covers much of this work from the perspective of continuum mechanics.

Reinforcement Mechanisms

8.4.3 NANOFILLER SIZE

AND

157

SHAPE

The “nano” impact on mechanical characteristics of nanocomposites is driven by the filler size. The form of the nanofiller is closely related to its size. The filler size and shape, in general, may be thought of as influencing two major contributions to the total polymer nanocomposite properties: a. the filler’s surface-to-volume ratio b. excluded volume interactions The fundamental impetus for the creation of polymer nanocomposites is the filler’s surface-to-volume ratio. This ratio represents the amount of interfacial area in the composite relative to bulk material. The interfacial area governs new structural configurations on the molecular scale and is responsible for the effective passage of stress across composite components, among other things. As a result, increasing the amount of interfacial area increases the possibility of defining new material attributes. The surface to volume ratio of the filler for spherical particles with radius r is simply:

3 As 4 r2 = = 3 r Vs 4/3 r This connection shows that when the spherical radius lowers, the available surface area (As) per volume (Vs) of filler rises. The entire amount of interfacial area inside a nanocomposite determines the level of property change or control by the interfacial region. In a nanocomposite with filler volume fraction φ, the total interfacial area-tovolume ratio scales directly with φ, such that: Ai,total 3 = Vtotal r

This connection has two significant ramifications. If φ remains constant, lowering the filler radius increases the surface area accessible for interfacial interactions. If stress is delivered through adsorbed chain segments at the filler interface in a linear polymer nanocomposite, then each chain/filler interaction will occupy a certain interfacial area. As a result, increasing the interfacial area available increases the number of chain/filler interactions. This action improves the stress transmission efficiency between the fill and the polymer matrix. This connection also shows that if φ is regulated separately, two composites with similar interfacial areas may be manufactured with different filler sizes. This method can help decouple size effects from other mechanisms connected to the “nano” effect. 2 r 2 + 2 rL 2 2 Ac = = + r 2L r L Vc

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When identical volume fractions of spherical and cylindrical fillers are compared, the surface-to-volume (SV) ratio for the sphere vs the cylinder scales as: SVs 3 = SVc 2(1 + r / L )

This equation shows that the surface to volume ratio for cylindrical fillers is larger than the ratio for a spherical particle for all plates (r > L) and short rods (L < 2.0r). However, as compared to long fibers (L > 2.0r), spherical fillers have a higher surface-to-volume ratio, although the increase is only 50%. If the surface-to-volume ratio is the most important design factor, plate geometries, like clay sheets, clearly have a major advantage at equivalent volume fraction loadings. Other design ele­ ments are unfortunately important for the design of mechanical characteristics in filled composites, and form must be addressed equally for these aspects. Rigid cylindrical fillers, for example, are difficult to scatter in an isotropic way at large volume fractions. As a result, non-isotropically distributed fillers’ mechanical characteristics will most likely be anisotropic. For certain applications, this design constraint may be useful, but not for all. In the subject of colloidal dispersions, the difficulty of dispersing non-spherical fillers isotropically has been investigated and is mostly attributable to the excluded volume effect of inflexible fillers and the ease of packing non-spherical objects. The excluded volume contribution arises from the fact that two hard filler objects cannot occupy the same space at the same time, resulting in a repulsive force owing to the filler objects’ entropy [45]. Textbooks provide straightforward analyses of the consequences of omitted volume contributions. Excluded volume contributions reveal that when the aspect ratio, L/r, grows, high-aspect ratio objects, such as long cylinders, become more difficult to scatter isotropically. In other words, for low filler volume percentages, ordered phases comparable to liquid crystalline transi­ tions will form. The spatial distribution of nanofillers in polymer matrices has been demonstrated to be influenced by the excluded volume of nanoscale fillers com­ bined with the polymer chain’s configurational entropy [46]. Non-mechanical ex­ periments, such as the control of nanoparticles inside ordered block copolymer domains, have established this control. The excluded volume and configurational entropy of the polymer chains can lead nanoscale fillers to defects in surface coatings, such as cracks, as established by Lee et al. using molecular-based simu­ lations and Gupta et al. using experimental evidence [47,48]. Crosby et al. found that nanoscale processes influence the deformation and failure mechanisms of linear glassy polymers in a way that is similar to these effects [49].

8.5 CARBON DOTS: FEATURES AND SURFACE PROPERTIES Carbon dots (CDs), a carbonaceous nanomaterial with a diameter of less than 10 nm, have gotten a lot of press recently because of its many intriguing properties, such as low photobleaching, high water solubility, strong biocompatibility, and chemical stability, to name a few. The oversupply of surface functional groups on

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CDs’ surface demonstrated such flexibility that it dynamized CDs’ emergence as a nanofiller in the polymer matrix. From improving the physico-mechanical char­ acteristics of CD-reinforced polymers to adding a zing like fluorescent, anti­ microbial, or photocatalytic activity, it all stems from the CDs’ intrinsic and extrinsic nature. Despite the fact that CDs, DNs, and GQDs are all quantumconfined fluorescent carbon materials that have been widely used as biosensors and bioimaging agents [50], the varied spatial arrangements of carbon atoms provide diverse physical and chemical characteristics [51]. A sp3-hybridized core and a graphitic carbon shell make up the majority of DNs. CDs and GQDs, on the other hand, are mostly made up of sp2 carbon, oxygen, and nitrogen components, as well as other doped heteroatoms [52]. CDs, unlike GQDs, do not have excellent crystal structures [53]. Furthermore, luminous CDs have a lateral dimension of less than 10 nm, but luminescent GQDs have a lateral dimension of up to 100 nm [54]. CDs are widely recognized as a promising contender in biosensing, bio-imaging, and other physiologically related applications due to their low cost, high quantum yield, plentiful supply, minimal cytotoxicity, and great chemical and light stability.

8.6 QUANTUM DOTS Quantum dots (QDs) are nanoparticles with exceptional semiconducting, optical, and electrical characteristics [55]. Inorganic quantum dots and carbon quantum dots are the two types of quantum dots. Inorganic carbon dots, on the other hand, have outstanding photoluminescence. Quantum dot photoluminescence is mostly determined by their size. Zinc sulfide (ZnS), cadmium sulfide (CdS), cadmium selenide (CdSe), zinc selenide (ZnSe), and other inorganic nanoparticles are ex­ amples of inorganic quantum dots [56]. Carbon-based dots are an important type of quantum dot. Polymer dot (PD), graphene quantum dot (GQD), and carbon dot are the several types of carbon-based dots. Carbon dots are the building blocks of semiconductor nanocrystals. Photoluminescence in carbon dots is also extremely adjustable. The photoluminescence characteristics of ultra-small quantum dots (carbon dots/inorganic dots) between 1.5–10 nm are superb. The luminescent, electronic, and optoelectronic characteristics of QDs are affected by their size and composition. If the particle size is too tiny to be similar to the electron wa­ velength, quantum confinement effects are seen. The energy levels, valence bands, conduction bands, and electron energy band gaps are all defined by the quantum confinement effect. QDs can emit a variety of hues, including red, orange, blue, green, violet, and others. In quantum dots, the light emission phenomenon (pho­ toluminescence) is related to the existence of electrons and holes, as well as the production of discrete and quantized energy levels. The color is determined by the varying energy levels of each atom. As a result, electron excitation from the valence band to other energy levels occurs, making the quantum dot electron conducting and leaving a hole behind. Due to its incredibly tiny size, the electron-hole pair (exciton) is quantum bound. Quantum dots are created using a variety of bottom-up and topdown processes [57]. QD have good water solubility and miscibility, allowing them to interact with a variety of biomolecules and ions. Improved optoelectronic and fluorescence qualities may result from functionalizing the quantum dot surface.

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Quantum dots have been employed in a variety of industries, including solar cells, light emitting diodes, biomedical imaging, and so on.

8.7 POLYMER DOTS AND THEIR HYBRIDS A polymer dot (PD) is a significant quantum dot and a remarkable carbon dot compound that is used in many applications. Particle deposition (PD) is a com­ bination of polymer properties and quantum dot fluorescence features [58]. Polymer dots are 0D polymer-based nanoparticles that are used in a variety of applications. Polymer dots can be chemically changed and doped in order to achieve certain functional characteristics. Polymer dot offers a number of fa­ vorable properties, like water solubility, solvent dispersibility, ease of processing, and the capacity to perform sophisticated semiconducting circuits at high tem­ peratures. The optical, electrical, luminescent, and physical characteristics of a nanocomposite are improved when PD is used [59]. Polythiophene, polypyrrole, polyaniline, and their analogues have all been utilized to produce PD, and they are among the conjugated polymers that have been employed. Additionally, nonconjugated polymers have been employed to create PD nanomaterials [60]. Polymer dots have been created using polyethylene glycol (PEG), poly­ saccharides, polyacrylamide, polyvinyl alcohol, and other materials. In addition to showing high quantum yield and stability, the PD also demonstrated excellent characteristics. When it comes to polymer nanoparticles, the direct polymeriza­ tion method was first utilized in the 1980s to create them. Afterwards, coupling processes catalyzed by transition metals were employed to achieve the desired results. Up to this point, the most frequent method of forming polymer dots is by the use of various precursor polymers and nanoprecipitation, miniemulsion, and the hydrothermal method [61–63]. Nanoprecipitation is the process of forming polymer nanoparticles as a result of the displacement of polymer chains from a semi-polar solvent that is miscible in water at the interface of two liquid surfaces. Nanoprecipitation is a simple method for encapsulating hydrophobic or hydrophilic substances. The process of creating nanoparticles from two immiscible liquid phases is known as miniemulsion. It is common practice to use shear mixing and surfactants in this procedure. A technique known as hydrothermal synthesis is used to create nanoparticles under extreme pressure and temperature conditions. Applications involving bioimaging or biosensing of biological molecules, metal ions, temperature, pH, and other parameters are inevitable in PD. PD may also generate luminous assemblies in conjugated polymers, which can be used in a variety of applications such as sensors, capacitors, and light-emitting diodes, among others. The characteristics of QD and PD are compared in Figure 8.3. Many different types of technological applications have been identified for polymeric nanoparticle and quantum dot-based hybrid materials in recent years. As a result, research has concentrated on the creation and application of hybrid na­ nostructured materials, which are composed of two or more distinct nanoparticles. Polymer dots and composite nanoparticles have been employed in a wide range of technologies, including optoelectronics, multi-imaging, and nanomedicine, among

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FIGURE 8.3 Comparative properties study between QDs and PDs.

others. It has proven possible to create hybrids by combining fluorescent inorganic quantum dots with other organic and inorganic nanoparticles.

8.8 REINFORCEMENT BEHAVIORS OF FILLERS INTO POLYMER MATRICES The reinforcing effectiveness of particulate fillers is influenced by the physical size, the structure of the filler, and the surface area of the filler [64]. The particle size of the granular fillers is one of the factors that are used to categorize the materials. Reinforcing fillers with very tiny particle sizes (in the range of 10–100 nm) have a high specific surface area, which means they have more active contact locations with the polymer matrices than larger particles [65]. It is well known that the surface morphology of the filler has a direct relationship with the physical characteristics of loaded polymers. In order to provide a strong connection be­ tween the filler and the polymer matrices, the kind or form of functionalities on the filler surface should be determined. This interaction should be related to the filler’s affinity for and capacity to react with the polymer. The “structure” of fillers, which is generated during the manufacturing process by the aggregation or agglomeration of the parent particles, comprises the shape, density, and size of the fillers. The higher the structure, the greater would be the efficacy of the strengthening [66]. The polymer network, hydrodynamic effects, filler-polymer interactions, and filler-filler interactions are the four basic aspects of the re­ inforcement idea that have often been used to describe the reinforcing in filled polymer compounds. In the case of particle fillers, the research conducted by Payne on the relationship between three-dimensional filler aggregates and the dependency of the storage modulus on strain resulted in a better understanding of the reinforced polymers’ reinforcing process [67]. When a modest amount of strain is applied to an aggregate network or a inter-filler network, it has been proved that the network may readily rupture. Following that, the Payne idea has

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FIGURE 8.4 Reinforcement effect and the contributing factors for strengthening fillerpolymer composites.

been widely utilized to represent the filler-filler interaction as well as the fillerpolymer interaction, as seen in Figure 8.4. Using the Einstein-Smallwood concept as a starting point, Huber and Vilgis addressed the process of hydrodynamic reinforcement in elastic composites and the main challenges involved [68]. In accordance with their theoretical findings, the relative increase in the elastic modulus as a function of the filler volume fraction of different elastic composite systems was determined and analyzed. The result for an elastic matrix filled with spherical core-shell particles with a hard core and a soft shell is depicted in Figure 8.5 for a hard core and a soft shell, respectively.

FIGURE 8.5 Filler particles with hard core: relative increase of the elastic modulus as a function of filler volume fraction for different values of the ratio Eshell/Em.

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FIGURE 8.6 Various filler-polymer matrix systems and their plausible attachment models.

It is also crucial to note the input to modulus enhancement made by the fillerpolymer interactions that result in the formation of bound polymer. For carbonaceous-filled compounds, many models have been presented to char­ acterize the filler-polymer interactions, as seen in Figure 8.6. The models are divided into three categories. The presence of immobilized chains connected with the filler surface may be found inside the composites. Fillers with active prop­ erties can self-assemble in the polymer matrix to form inter-filler networks. Such inter-filler network can trap a portion of the polymer in the void created by their structure. These trapped chains within the voids of the filler aggregates are protected from distortion by the filler aggregates. Polymer that has become im­ mobilized is referred to as confined or occluded polymer. Another model stated that polymer molecules are capable of attaching to the filler surface by physical or chemical interactions as well as chemical reactions, and the concept of shell polymer with an immobilized or glassy state character was offered as a result of this model. The glassy polymer shell concept has also been published by O’Brien et al., who describe it as follows: They do not contribute to the elastic behavior of the polymer matrix, but rather operate as hard filler particles that prevent the polymer matrix from deforming [69]. As a result, the in-polymer structure en­ hances the effective filler loading and, as a result, the contribution to the modulus that is independent of strain. Essentially, bound rubber is that portion of a rubber molecule that is strongly interacted with a filler surface and hence cannot be removed by a suitable polymer-solvent combination. As a result of the fillerrubber interactions, a three-dimensional structure is formed in uncured filled rubber compounds, which is commonly used as a measure of the filler surface activity [70]. It is the entangled phase in which molecules are closely bonded to the surface of the filler and their mobility is rigidly confined, whereas the loosely bound phase contains molecules that are somewhat less constrained than those found in empty gum rubber. Advanced methods, such as nuclear magnetic re­ sonance (NMR) spectroscopy, have been used to investigate the two bonded layers with differing amounts of molecular mobility [71]. The closely bonded layer has a thickness of around 0.41.3 nm, according to current estimates [72]. It had been discovered experimentally by Sternstein and Zhu that the mechanism for reinforcing in nanocomposites is a result of interactions between the filler and the matrix, rather than filler agglomeration or percolation [73]. Depending on the

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FIGURE 8.7 Glassy junction/bridges between filler particles.

quantity or severity of interactions between the filler particles and the rubber chains, rubber chains that are directly adherent to the filler particles and/or in close proximity to the filler rubber interface exhibit varying mobility as compared to the surrounding matrix. Numerous papers have been published on the local segmental dynamics of polymer chains in the proximity of filler surfaces, and gradients of chain mobility that progress from polymer in direct contact with filler surfaces to bulk behavior, as well as “bridges” of glassy polymer between filler aggregates where extreme confinement of polymeric macrochains is achieved. As illustrated in Figure 8.7, the existence of glassy layers around the fillers is also a proposal for a microscopic model, as explored by Merabia and colleagues [74]. According to this concept, substantial reinforcement is achieved when the glassy layers between the fillers are overlapping. Because of the little overlap of the glassy layers and/or because of the high temperature of the process, only the actual filler volume fraction is raised at intermediate volume fractions and/or at high temperature. However, when the filler volume fractions are increased and/or the temperature is lowered, the glassy layers overlay and the system approach the re­ gime of highly reinforced materials. The applicability of interfacial effects in terms of change in dynamics, and in particular the rise of glass transition temperature generated by a solid substrate with strong contacts with the polymer, should be kept in mind when considering this model.

8.9 SUMMARY AND OUTLOOK It is quite popular to employ nanofillers to reinforce polymers since they have the ability to provide not only improved mechanical qualities but also certain functional capabilities to the polymer when combined with other additives. Unlike homo­ geneous systems, the interfaces, filler-filler interactions, and filler-polymer chain dynamics at or on top of the carbon nanodot filler surface all play a role in re­ inforcing properties of carbon nanodot polymer nanocomposites. The reinforcing efficacy of the most widely used particulate fillers is influenced primarily by particle

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size and specific area, structure, filler-polymer, and filler-filler interactions, as well as the amount of dispersion in the polymer. Several models, including bonded polymer/macrochains and immobilized or restricted polymer chains, have been developed to characterize the reinforcing process in combination with this fillerpolymer interaction. If the layers are entirely exfoliated and the load can be properly transmitted between the polymer and the filler, a considerable improvement in re­ inforcement efficiency can be attained. The amount of polymer matrix limited in the vicinity of the particles, which is dependent on filler-polymer interactions, will also have an impact on the reinforcing efficiency. A number of cross-link junctions and, consequently, the modulus of the materials are produced as a result of the physical entanglements formed by the rubber chains as they adsorb on the wide interfacial area provided by nanofillers. Regardless of their differences in shape, the injection of nanofillers into the rubber matrix has certain similar effects, namely, changes in the characteristics of the rubber matrix near to or in the neighborhood of the filler surfaces, which are discussed further below. Solidification of the filler network occurs above the critical concentration or percolation threshold, with the existence of restricted polymer chains or glassy layers, which all contribute to an increase in nano­ composites’ stiffness. Filler networks play a crucial role in reinforcing at tiny deformations when linear viscoelastic behavior is exhibited. As the strain in­ creases, the modulus of the filler network breaks down and re-establishes itself; i.e., polymer chains are released from the filler surface. While strain amplification predominantly affects reinforcement and desorption/absorption of polymer chains at the filler surface when materials exhibit nonlinear response under substantial deformation, this method has also been used to explain nonlinear behaviors under high strain. In spite of the fact that much information has been gathered and progress has been made, no model has yet been able to satisfactorily describe the reinforcing mechanism. If novel nanofillers and polymer-nanofiller composites are developed and studied in detail in both experimental and theoretical settings, we can get a deeper understanding of the reinforcing process. The specific process of nanoscale reinforcement in nanocomposites will hopefully be discovered with the assistance of new upgraded technologies of characterization instruments and theoretical predictions.

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9

Quantum Dots Modified Thermoplastic and Thermosetting Plastic Composites Niranjan Patra Department of Engineering Chemistry, College of Engineering, Koneru Lakshmaiah Education Foundation, Vaddeswaram, Andhra Pradesh, India Department of Biotechnology and Bioengineering, Institute of Advanced Research, Koba Institutional area, Gandhinagar, Gujarat, India

Malvika Shukla Department of Biotechnology and Bioengineering, Institute of Advanced Research, Koba Institutional area, Gandhinagar, Gujarat, India

CONTENTS 9.1 9.2 9.3 9.4 9.5

9.6

9.7

Introduction...................................................................................................172 Polymer Nanocomposites.............................................................................173 Typical Polymers in QDs/Polymer Composites.......................................... 174 Quantum Dots............................................................................................... 175 Synthesis Methods of Quantum Dots..........................................................176 9.5.1 Top-Down Approach........................................................................ 176 9.5.1.1 Chemical/Electrochemical Oxidation................................ 176 9.5.1.2 Arc Discharge ....................................................................176 9.5.1.3 Laser Ablation ...................................................................177 9.5.2 Bottom-Up Approach (Self-Assembly) ........................................... 177 9.5.2.1 Wet Chemical Methods..................................................... 177 9.5.3 Vapor Phase Methods ...................................................................... 178 Preparation of QDs/Polymer Composites.................................................... 179 9.6.1 Physical Mixing................................................................................ 179 9.6.2 Chemical Grafting ............................................................................180 9.6.3 In situ Polymerization Method ........................................................181 Dispersion of QDs in Polymer Matrix ........................................................181

DOI: 10.1201/9781003266518-9

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9.8 Applications of QD/Polymer Composites ................................................... 182 9.9 Conclusion and Future Perspectives............................................................ 183 References.............................................................................................................. 184

9.1 INTRODUCTION Outstanding properties of nonmaterial such as quantum dots limit the applications until and unless it has been incorporated in some matrix such as a polymer to realize potential applications in real life. Improving the functional and structural properties by combining the nonmaterial with polymeric matrix has been wellproven [1–7]. Since classical time composites have been made in nature by imitating natural mechanism for everyday applications. Composite materials are based on filler (reinforcement) and a matrix (polymer) which binds it together. The filler could be of macro-, micro-, or nanoscale. Recently, nanoscale fillers such as quantum dots have been immensely exploited to prepare polymeric composites which surpassed the performance of conventional composites mate­ rials using macro- or microscale particles in the past decades by developing new functional and structural properties. Incorporation of nanoscale materials into polymers display advanced unparallel multifunctional properties led to diverse applications in a range of fields. In the fast-pace growth of composites materials, polymer composites using nanoscale fillers are indispensable for high-end applications. A nanocomposite comes in the family of composite material where one of the constituents has a dimension in the range of 1 to 100 nm. The nanoscale con­ stituent improves the end product properties resulting from the molecular inter­ actions which influence the materials properties as a whole. The nanoscale constituents composed of few atoms, where the properties of particles or the nanoaggregates are reflective of few atoms rather than the bulk materials. Tuning the size can play a pivotal role in influencing their physical and chemical attri­ butes. Recent studies show that nanocrystals can be incorporated into molecules and solids, just like atoms and these solid materials comprise tens of thousands of atoms that are more generally known as quantum dots, nanocrystalines, nano­ phases, or clusters of aggregates. Quantum dots (QDs) are the most promising nanomaterial that attracted the attention of the scientific community recently. QDs can be classified into inorganic (e.g., CdS, CdTe, ZnO, Si QDs, CuInS2) and organic-based (e.g., carbon QDs, graphene, graphene oxide or carbonized polymer quantum dots) materials [8,9]. They exhibited properties over their conventional counterpart such as high specific surface area, excellent photostability, unique up-conversion photoluminescence, tunable fluorescence property which is due to their quantum confinement effect, surface defect, band gap transition of conjugated π-domains, and can be modified using post-synthetic or in situ synthesis method [8–10]. This enables them to ex­ plore new potential applications in the fields of sensors, optoelectronics, biome­ dical, energy storage, and photocatalysis. Preparation of the QD-based polymer composites by integrating QDs into compatible polymer matrices through intermolecular interactions or covalent

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bonding is a feasible and suitable method. Furthermore, polymers are ideal solid matrices for many applications. A suitably selected polymer matrix can play a pi­ votal role in uniform dispersion and bonding QDs to effectively avoid and reduce the aggregation. Furthermore, this approach can control and enhance the properties of the QDs/polymer composites in many ways like rapid electron transfer or thermal conductivity as well as enhanced chemical and thermal stability for the QDs. The unique features of QDs can provide polymers with other potential capabilities e.g., mechanical strength, excellent fluorescence performance, ionic conductivity, bioactivity, sensing, self-healing, and stimuli-responsive characteristic [11,12]. These multifunctional QD/polymer composite materials possess broad application prospects in diverse fields, ranging from electronics, biomedical, fuel cells, solar cells, and drug delivery. In this context of QD/polymer composites, this chapter aims to summarize a systematic picture from the synthesis of QDs and preparation of QD/polymer composites to the applications of these composites. We highlight the synthesis methods of QDs and preparation of QDs/polymer composites are discussed and listed. Furthermore, the recent advances in some cutting-edge ap­ plications of the QD/polymer composites in different fields are discussed. Finally, the outlook on the current challenges and future prospects in this topic are provided to open up new horizons for expanding their application scope.

9.2 POLYMER NANOCOMPOSITES In the late 1980s, polymer nanocomposites were first realized. In 1984, Roy and Komameni coined the word “nanocomposites,” which essentially composed two phases where one of the constituent is in the size of nanometer range. The term nanocomposite has been widely accepted since then. The incorporation of non­ material (particularly, silicates and carbon nanotubes make for a far attractive choice over spherical nanoparticles) in polymer matrix greatly enhances viscous behavior and shear modulus owing to hydrodynamic and elasticity theories and improves existing attributes such as being stable, biodegradable, permeable, and flammable as well as produces new attributes owing to the surface modification and intrinsic physical and chemical complexities. Silicates are highly compatible with other polymers, facilitating easy tuning of their characteristics with simple alteration of silicate concentration. It was further reported that these nanocompo­ sites could be synthesized without adding any organic solvent, which makes it a feasibly attractive point of research interest thus, leading to high-scale production with lesser evolution of volatile compounds. Nanocomposites can also be clay based by using epoxy and various polymers. The shape, size, and structure of na­ nocomposites can be easily altered and are directly correlated to their exceptional properties, which in turn improves the performance of polymer nanocomposites and makes it a far better choice than its conventional composite and filler counterparts [13]. Filler concentration, particle size, shape, and filler interaction with a matrix play a key role in altering polymeric properties. Over the years, mineral, metallic, and fibrous fillers have been amalgamated with thermoplastic or thermoset to produce variety of composites. The surface interactions occurring between non­ material and polymer matrix, along with a nanoscale, mark the point of difference

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between a polymer nanocomposite and its conventional composite and filler counterparts. The development of polymer nanocomposites has proven to be useful in an array of wide-ranging applications including catalysis, sensing, storage of energy, coating of surfaces, biocompatible platforms, etc.

9.3 TYPICAL POLYMERS IN QDS/POLYMER COMPOSITES Polyaniline (PANI) is one of the most important conducting polymeric materials for the rapidly growing technology development, especially in the optoelectronic field. However, the major limitations of conducting PANI are inability to process easily by conventional methods due to its rigid backbone as well as having poor mechanical properties. These limitations can be overcome by preparing con­ ducting PANI-based composites using some quantum dots, which not only confers mechanical strength but also increases electrical properties of the composites. The shortfalls of PANI alternatively have led to the development of the PANI-based composites and blends. Luk et al. [14] studied the optical properties of graphene quantum dot–polyaniline (PANI–GQD) composite films synthesized by a chemical oxidation polymerization method. The mole con­ centration of PANI and the size of the GQDs were varied. A sandwich device (Au/PANI–GQDs/ITO) was realized to investigate the transport properties of the composites. A stable hysteresis loop was observed in the composites in response to the applied voltage. The electrical conductance behavior of the device can be tuned by varying the size of the GQD and the PANI content within the hys­ teresis loop (Figure 9.1) Cellulose acetate (CA) is a semi-synthetic, brittle, and biocompatible deriva­ tive product of cellulose produced through acetylation of the hydroxyl groups on cellulose [15]. Acetylation gives rise to the emergence of thermoplastic proper­ ties. Cellulose acetate remains a difficult material to melt process due to the narrow and small temperature window between the glass transition (Tg) and degradation temperatures [16]. The physical properties of CA, such as solubility and biodegradability, are highly affected by the degree of substitution [17]. Cellulose acetate (CA) is a brittle material, which is why it needs processing aids like plasticizer and other additives to make it tougher [18]. Joshi et al. [19] synthesized well-dispersed graphene quantum dots (GQDs) in a cellulose acetate

FIGURE 9.1 Schematic representation of the preparation of PANI-GQD composites. Source: Adapted from Ref [14] with permission. (Copyright 2014 Royal Society of Chemistry).

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(CA) polymer matrix. The carboxylic and hydroxyl functional groups present in GQD interacted with the main hydroxyl group of cellulose acetate that led to stabilization of the composite. Concentration of GQDs play an important role in the amorphous and semicrystalline domain disparity that eventually influences the properties of the composites. Uniform distribution of GQDs was observed in the matrix, which gives rise to good photoluminescence. GQD loading improves the ductility of the CA matrix. Epoxy resin, one of the most important high-performance thermosetting poly­ mers, has applications in high-performance composites, structural adhesives, coating, and microelectronics due to their excellent mechanical properties such as high stiffness, strength, good thermal chemical resistance, dimensional stability, and excellent electrical insulation [20]. Due to low weight and excellent adhesive and mechanical strength, epoxy resins became a prominent material for highperformance applications in structural composites materials. However, the shortfall in epoxy resin is its poor toughness and high brittleness in its cured state comes from their cross-linked structure [21]. In order to overcome this problem, several approaches have been used, including blending with an elastomeric polymer as well as the incorporation of nanofillers. Nanoparticles have paved the way for tailoring the properties of epoxy matrix [22]. A number of studies have incorporated na­ noparticles fillers in an epoxy matrix such as nanoclay, silica, MWCNTs, and graphene oxide [7,9,13,14,19,23–26]. In one recent work, Gobi et al. [5] show that infusing graphene QDs in an epoxy matrix results in a 2.6-fold increase in tough­ ness and 2.25-fold increase in tensile strength of epoxy resin compared to the pristine epoxy resins without compromising the optical properties of the GQD/ epoxy composite.

9.4 QUANTUM DOTS Quantum dots (QDs) are fundamentally semiconductor particles, made up of elements falling in groups II to VI or III to V with sizes smaller than Bohr radius exciton [27]. QDs exhibit tunable optical properties due to a change in band gap energy caused by quantum confinement effects [28]. On absorption of light, electrons are promoted from the valence band to the conduction band, producing an electron hole called as an “exciton”. When the electron and hole recombine, energy is released in the form of a photon called a radiative recombination. Upon reducing the particle size in the range of Bohr radius, the energy required to create an exciton increases. This effect is popularly called as “quantum con­ finement”, which is generally observed in ultra small crystalline and semi­ conducting particles. The smaller the size of the QDs, the larger the band gap energy, leading to emission of photons of higher energy (blue shift) and vice versa [27]. QDs reaped a boundless deal of desirability due to their utility in electronics and allied fields. When the size of QDs increased, absorption and emission experience a red-shift. Similarly, for colloidal agglomerated systems, when the accumulate size increases, QDs again experience a red-shift. It is ob­ served that adjacent QDs experience electronic energy transfer via coupling at segregation of up to 10 nm but once the QDs come into direct contact with each

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other and achieve a certain accumulate size, they exhibit bulk attributes. QD powder without any ligands and agglomerated QDs free of ligands in polymer matrix also display similar characteristics. Thus, regulation of QD distribution in composites is a highly critical mandate to generate the desired characteristics. Same constituents can be used to regulate the absorption and emission, with the size of QDs as the most important control point. The tuning of QDs size can produce a shift up to 1 eV. As the surface of QDs is organophobic, it is a great challenge to distribute them in polymer matrix. For materials to possess improved transparency and luminescence efficacy, improved dispersion of the QDs in the polymer matrix is required, which can be taken care of by using various surface functionalization methods. As reported by Fu et al., clear ZnO-SiO2/epoxy na­ nocomposites were designed by integrating calcinated ZnO-SiO2 nanoparticles in a clear epoxy matrix based on a filler-matrix refractive index matching principle. The produced nanocomposites exhibited enhanced luminescence along with a wide emission spectrum.

9.5 SYNTHESIS METHODS OF QUANTUM DOTS Various methods have been developed to fabricate QDs, which are categorized into top-down and bottom-up approaches.

9.5.1 TOP-DOWN APPROACH The top-down method essentially denotes the breakdown of bulk matter into smaller QDs, which includes techniques such as chemical/electrochemical oxi­ dation, arc discharge, ultrasonication, molecular beam epitaxy (MBE), ion im­ plantation, electron beam, and X-ray lithography. The shortcomings of this method comprise the requisition of costly resources, severe reaction environment, and longer processing time. Some of the commonly used techniques follow. 9.5.1.1 Chemical/Electrochemical Oxidation It is the most commonly used size-controlled process as it offers incredibly high yield and purity in exchange for minimal cost. The very first synthesis of QDs from CNT using this method was reported by Zhou et al. [29]. Later on, Ray et al. fabricated QDs using carbon soot, which could be used for mg scale fabrication [29]. 9.5.1.2 Arc Discharge Arc discharge is a process for rearrangement of atoms decomposed by bulk carbon at the anode electrode using gaseous plasma in a sealed reactor with a temperature rising up to 4000 K under the influence of electric current to give rise to plasma containing high energy while at the cathode, QDs are formed by selfassembly of carbon vapor [30,31]. The QDs produced by this technique are highly water soluble, but due to their large particle size range, the specific surface area available is greatly reduced, which inhibits the number of active sites available during the reaction [32].

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9.5.1.3 Laser Ablation Laser ablation makes use of a high-energy pulse for surface irradiation to a ther­ modynamic state wherein high temperature and pressure are produced along with heat, followed by evaporation to a plasma state and finally crystallization occurs to produce nanoparticles from vapor [33]. The photoluminescence attributes can be easily altered by choosing a suitable solvent during the laser ablation which fa­ cilitates surface modification of QDs [34]. Yu et al. [35] reported that the size of QDs could also be controlled using laser furnace. Moreover, Nguyen et al. [36] concluded that not only size but also photoluminescence attributes could also be altered by varying the size of spot and time period taken for irradiation and laser fluence. For instance, small QDs can be developed by prolonging the time period of irradiation. Thus, it is an efficient process to fabricate QDs with small size, high water solubility, and fluorescence attributes, but shortcomings such as complex procedure and large cost, impede their growth.

9.5.2 BOTTOM-UP APPROACH (SELF-ASSEMBLY) The bottom-up approach essentially denotes the buildup or self-assembling of smaller particles arrangements into required size of colloidal QDs, which can be realized by the following. 9.5.2.1 Wet Chemical Methods They usually follow typically controlled precipitation processes, which comprise of controlled nucleation and growth for an individual solution or a solution mixture. Nucleation can be classified into homogenous, heterogeneous, and secondary. Homogenous nucleation is facilitated without the aid of any preformed solid in­ terface. Commonly used techniques include hydrothermal/solvothermal, micro­ emulsion, sol-gel, competitive reaction, thermal decomposition, ultrasonication, or microwave-assisted synthesis like pyrolysis and electrochemistry. From the afore­ mentioned techniques, some of them are described below. 9.5.2.1.1 Hydrothermal/Solvothermal Method It is a frequently used method because of its rather simple arrangement which provides consistent sized particles with high yield. Other advantages include cost effectiveness and environmental safety. Using this method, QDs can be easily fabricated by using saccharides, amines, acids, and derivatives. This method in­ cludes the dispersion of small organic molecules or polymers in water or any solvent to give rise to a precursor, which is then shifted to Teflon lined autoclave. The combination of organic molecules and polymers at elevated temperatures generate carbon seed cores which then proliferate into QDs of size less than 10 nm. The controlled doping facilitates the development of an electrocatalyst with variable doping configuration and electronic arrangements [37]. 9.5.2.1.2 Microwave-Assisted Pyrolysis It is a prevalently accepted method owing to faster synthesis and industrialization. Zhu et al. [38] described a method to fabricate QDs by coalescing polyethylene

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glycol (PEG 200) with a saccharide to give rise to a transparent mixture, followed by thermal treatment in a microwave. The excitation-dependent photoluminescence attributes were greatly enhanced for the formed QDs. This is an easy, relatively faster and ecological method, particularly useful for QDs possessing groups con­ sisting of oxygen that can easily become metal coordination sites for C-based electrocatalyst development [38]. 9.5.2.1.3 Ultrasonication The number of reports wherein this method has been used to fabricate QDs, is inadequate. Li et al. [39] first developed fluorescent QDs possessing water solubility in the size range 5-10 nm via acid aided ultrasonication of glucose. Then, during the same year, Li et al. [40] fabricated fluorescent QDs possessing water solubility by using activated carbon via one step hydrogen peroxide aided ultrasonication, which produced QDs with size range of 5-10 nm and surface containing hydroxyl groups.

9.5.3 VAPOR PHASE METHODS These processes focus on layer proliferation via atom-by-atom buildup. Similarly, QDs can be self-assembled on a substrate with no pattern. Various techniques under these methods include- self-assembly using MBE, sputtering, liquid metal sources or gaseous monomer accumulation. QDs belonging to groups 3 to 5 and 2 to 6 are predominantly self-assembled using MBE by large lattice mismatching. Figure 9.2 shows a general schematic representation of synthesis of QDs (Top down (topdown and Bottom up)bottom-up)

FIGURE 9.2 General schematic representation of synthesis of QDs (top-down and bottom-up). Source: Adapted from reference [ 54] (Unrestricted use with proper reference).

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9.6 PREPARATION OF QDS/POLYMER COMPOSITES Composites materials are majorly based on two different phases where one phase is continuous which is called as the matrix and other is the discontinuous phase which is known as the filler/reinforcement. In case of nanocomposite, the re­ inforcement material has at least one dimension in the nanometer scale of less than 100 nm. The filler in the nanoscale size determines the functional role of improving the properties of the final material for specific applications. For QD/ polymer nanocomposites, the matrix could be of a linear or branched thermo­ plastic or thermosets (crosslinked polymer). The quantum dot filler can either covalently grafted or non-covalently mixed with the polymer matrix depending on the method of preparation. The polymer material is chosen based on the ap­ plication in specific area. In polymer composite the main function of the matrix is to bind the QD filler and to provide the required strength depending on the chemical structure of the thermoplastic or thermoset polymer. The chemical structure of the polymer also determines the possibility of attaching the QD which will influence the dispersion of QD in the matrix. There are several approaches to prepare QD/polymer composite based on physical, chemical, and in situ growth approach.

9.6.1 PHYSICAL MIXING Physical mixing of the two components (QD and the polymer) is the simplest ap­ proach to prepare a QD/polymer composite that involves adding a small amount of QDs in the polymer in a solution or by melt blending using a rotating screw followed by casting. In this case, the nanocomposite formed is due to the noncovalent interactions such as electrostatic interaction, hydrogen bonding or the π-π-interaction between the QDs and the polymer [2–4,6,41–43]. Electrostatic in­ teraction which is considered as the weaker link as compared to ionic or covalent bonding but stronger than van der waal interaction between the non polar mole­ cules. Hydrogen bonding mainly takes place via electrostatic interaction. Hydrogen bonding takes place between a partially positively charged hydrogen atom in one molecule and a negatively charged F, N, or O atom in a nearby molecule. The electrostatic interactions can either be related with the attractive or repulsive forces between charged molecules. In the physical mixing method of QD/polymer com­ posites, the attractive interactions are the dominant driving forces for uniform mixing leading to improved properties. Quantum dots exhibit negative charge due to presence of functional group like carboxylic group interact with the positive charge of polymers. Normally quantum dots exhibit negative surface charges due to the presence of carboxylic functional group, thus tends to form interactions with polymers with a positive charge. Xu et al., [44] utilized physical mixing method to prepare a carbon QD/polymer nanocomposite. In this study, incorporation of coffee ground–derived highly oxygenated GO-QDs in polylactide acid (PLA) bionano­ composites films allowed the creation of favorable interfacial interactions, bene­ fitting the exfoliation and uniform dispersion of nGO-QDs in the matrix (Figure 9.3) [45].

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FIGURE 9.3 QD/polymer composite materials prepared via physical mixing method. (a) Schematic representation showing the preparation method of coffee ground-derived nGO-QDs and the multifunctional homogenous nGO-QD/PLA composite film. Source: Adapted from Ref [ 45] with permission. Copyright 2017 American Chemical Society.

FIGURE 9.4 Schematic representation of the synthesis of starch-derived nGO-QDs and the process for electro­ spinning nGO-QD/starch nanofibrous scaffold. Source: Adapted from Ref [ 46] with permission. Copyright 2018 American Chemical Society).

Wu et al. [46] prepared electrospun nanofibrous scaffolds by using the starchderived nGO-QDs as the property modifiers (Figure 9.4) [46]. The addition of nGOQDs tremendously improved the electrospinnability of starch and improves the thermal stability of the spunned fibers. QDs, especially the graphene-based QDs, are rich in conjugated sp2 domains. Graphene-based QDs readily form π-π interactions with polymers possessing the aromatic groups such as polypyrole and polystyrene or with polymers functiona­ lized with p-orbital rich groups e.g., pyrene, phenyl [43]. The physical mixing approach has the advantage of simple operation, low cost, and also possible to scale it up in industries especially for melt processing approach. However, the agglomeration creates a problem that is connected to the self-π-π stacking inter­ actions between QDs and polymer may impede ununiform dispersion of QDs in the polymer, leading to poor mechanical behavior and unstable optical properties.

9.6.2 CHEMICAL GRAFTING A quantum dot surface has abundant functional groups that expand the possibility for chemical modifications to form a covalent network with the polymer for

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reactions like esterification [47], methacrylation/acrylation, acylation, etherifica­ tion, and epoxidation [48]. The chemical grafting method is superior compared to the physical mixing technique, which gives the advantage of uniform mixing, mechanical strength, and life cycle due to the formation of covalent bonds be­ tween the QDs and the poymer chain. Gustavsson et al. [49] prepared a ther­ moplastic “all-cellulose” oxidized carbonized cellulose/cellulose composite film with the help of covalent functionalization of cellulose acetate by cellulose-based OCC prepared by microwave-assisted HTC of paper tissues and cellulose.

9.6.3 IN

SITU

POLYMERIZATION METHOD

Though the physical mixing approach is the facile approach to prepare QD/ polymer composites, it lacks a strong intermolecular interaction between the QDs and the polymer matrix. The in situ chemical grafting method helps to forms a strong adherence of QD with the polymer matrix by the formation of strong covalent bonding. However, in situ growth is a time- and energyconsuming process due to multiple steps involvement, complex reaction pro­ cesses and use of toxic chemical reagents and solvents. Contrary to this in situ synthesis of QDs within the matrix is preferable as the bonding between the QDs and the polymer involves chemical and physical interactions leading to better attachment. In situ synthesis approach can be achieved using a one-pot thermal treatment, which includes pyrolysis or low temperature heating of the mixture containing QDs precursors and the monomer or by hydrothermal process [23,50]. The formation of QDs, along with the polymer precursor matrix, takes place simultaneously in one pot. Immobilization of QDs into the matrix has certain demerits such as low compatibility of the two components, leading to anisotropic properties. However, the QDs will nucleate and grow on the avail­ able polymer matrix active sites directly generating highly stable and homo­ genous system [51]. After synthesis, the QDs/polymer composites solution may be cast as a film/sheet. The size and uniform distribution of QDs in the polymer matrix can be tuned by tailoring the structure of the polymer. Dispersion of QDs in the matrix can also be effectively achieved with the aid of either sonication or by using a suitable surfactant.

9.7 DISPERSION OF QDS IN POLYMER MATRIX Nanocomposites attributes are largely dependent on the extent of nanoparticles effective dispersion in a polymer matrix. While the degree of QD accumulation rises, the surface area to volume ratio declines, which in turn impedes its utility as QD accumulation acts as an unfavorable factor, owing to diminished boundary area which leads to lower conversion of power that ultimately has a negative impact on the effectiveness of photovoltaic devices. Moreover, the diminished boundary area caused by QD accumulation has a detrimental impact on the physical attributes of polymer matrix as well. Furthermore, the highly accumulated particles residing in the matrix scatter light rather significantly this inhibits the utility of nanocomposites to a restricted number of optical applications.

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Colloidal QDs are stabilized by surface functionalization with the help of covalently attached ligands or adsorption of large molecules. The mixing of solution is another method for filler particles to attain improved distribution in the matrix with the only condition being that the nanoparticles should be stable in a dispersion solvent which is mixable with the polymer. Various studies show that partition of phases between filler particles and matrix takes place following solvent extraction. Latest investigations indicate that adding nanoplatelets to the polymer matrix ac­ companied by QDs aids in efficient dispersal as platelets, particularly silicates in­ duce efficient dispersal. For instance, in a study, the existence of zirconium phosphate nanoplatelets directly influences the matrix or association between QDs by locally polarizing the medium to facilitate constant accumulation and dispersal, the rate being directly correlated to the QD concentration and exfoliated nanoplatelets. To attain efficient dispersal for metal oxide nanoparticles such as zinc oxide in matrix, zirconium nanoplatelets were utilized. With simple alteration of volume fraction of zirconium phosphate nanoplatelets, the average size of zinc oxide QDs accumulate were scaled down from microns to discrete dots. The association of zirconium phosphate na­ noplatelets with the matrix and solvent like acetone varies the thermodynamic characteristic of the polymer, enabling zinc oxide nanoplatelets’ dispersal by ex­ panding their favorable association with the neighboring medium. Moreover, dispersal alteration was attained at reduced dispersant volume fractions owing to the nanoplatelets’ geometry.

9.8 APPLICATIONS OF QD/POLYMER COMPOSITES QDs incorporated with polymer composites have shown promising applications in a variety of fields and continuously emerging due to their peculiar tunable prop­ erties of QDs, easy preparation, and structure compositions. All these potential characteristics with abundant polar functionalities integrated QDs with tremendous potential for further applications in QD/polymer composites, which can take re­ volutionary multifunctional materials. Recently, QDs/polymer composites have shown applications in various fields in energy storage, biomedical, environmental, and optoelectronics. This part comprehensively summarizes and highlighted the progress made so far in the aforementioned field. For instance, the development of QD/polymer nanocomposites as optical clear compounds have found immense use in the optoelectronic field as well as in the fabrication of displays, lenses, and dental equipment [52]. The ongoing expeditious progress of the aforementioned technology has led to the improvement of various attributes such as robustness, thermal expansion and conductivity, UV tolerance and rigidity, all of which are immensely accredited to the incorporation of nanoparticles in clear polymeric materials. Apart from these attributes, high transmittance is also a critical mandate for the development of optical field. Another example includes the development of LEDs by using QDs/epoxy nanocomposites, as seen in a UV-WLED lamp, which was fabricated by en­ closing it with a zinc oxide/T-epoxy nanocomposite. The zinc oxide nanoparticle content incorporated in the clear epoxy resin was negligible (0.07%), which

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enabled easy preservation of the attributes. The QD/epoxy nanocomposites also possessed luminescence [5]. Moreover, these nanocomposites have also paved the way into the aerospace industry and can be widely found in various aircraft bodies as the polymer attributes can be further enhanced by dispersing nanofillers in the polymer matrix. Another interesting application was explored by exploiting the antibacterial as­ pect of QDs incorporated in the polymer matrix using photodynamic therapy. Kovacova et al. [53] was the first one to report the fabrication of QDs incorporated in antibacterial nanocomposites, which exhibited antibacterial activity against bacteria such as E. coli and S. aureus. Further investigations with biocompatibility established that the aforementioned nanocomposites comprising of polyurethane did not exhibit cell toxicity and no hemolytic effect was observed in the red blood cells. QDs can also be incorporated in polyacrylamide for application as a hydrogel owing to their various attributes such as fluorescence. Similarly, polyvinyl alcohol/QD nanocomposites can be explored for their wide-ranging applications in biomedical devices, biosensors/sensors, controlled drug release, anti-counterfeiters, and ac­ tuating systems. Furthermore, polyacrylonitrile/QDs that can be developed by electrospinning can find utility in biomedical imaging, photochemical reactions, and smart clothing [53].

9.9 CONCLUSION AND FUTURE PERSPECTIVES QDs show immense potential as the building blocks of the new generation of multifunctional composites. This chapter has represented a comprehensive knowledge from synthesis of QDs to the preparation of QDs/polymer composites. Recently, physical mixing, chemical grafting, and in situ growth are considered as the main strategies for the preparation of QDs/polymer composites, among which the in situ polymerization method stands out as combining the merits of the other two methods, including facile preparation, better compatibility of each component, and strong bonding between QDs and matrix. Moreover, QD/polymer composites with cutting-edge applications in a wide range of fields such as in biomedical, energy storage, and environment have been highlighted. Despite the outstanding properties and remarkable progress in exploring the synthesis methodologies and QD/polymer composites developed in the past few years, there are still some challenges for the future. For instant, physical mixing of QDs into polymers is the easiest way to prepare QD/polymer composites but there is still advantage of inhomogeneous dispersion of QDs in polymer matrix. Though the in situ polymerization can overcome many disadvantages, but they are still scope out futher improvements. Lack of adequate methods characterize QDs when they grow in in situ polymerization. Therefore, there are still scopes of advanced development for the QD/polymer composite preparation and characterization. Environmentally friendly and greener resources and fabrication processes are also needed. Though QDs find application in a number of fields, there is a need to explore QD applications on the agricultural side as in the past few studies high­ lighted that QDs could find application in plant growth in agriculture. Looking at the pollution that plastic creates, there is also immense scope to study QDs/polymer

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composites for the battle against waste. Finally, the compatibility of QDs with a matrix is also an important factor in enhancing the electronic properties, which is yet to be investigated.

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Quantum Dots–Rubber Composites Sayan Ganguly Bar-Ilan Institute for Nanotechnology and Advanced Materials, Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel

CONTENTS 10.1 Introduction................................................................................................. 189 10.2 Background and Challenges.......................................................................190 10.3 Surface Modification of QDs by Polymer Phases ....................................191 10.4 QDs in Elastomer Matrices........................................................................ 195 10.5 Summary..................................................................................................... 202 References.............................................................................................................. 202

10.1 INTRODUCTION In part due to the unique optical and electrical properties of quantum dots (QDs), the incorporation of QDs into polymer nanocomposites results in significantly improved function for QD-containing nanocomposites that are able to take ad­ vantage of these distinctive traits [1]. Maintaining the polymeric matrices’ varied bulk mechanical capabilities, but reducing their dimensional stability [2]. Precision control over the QD structure has been achieved through the development of refined synthetic processes, yielding improved electrical and optical features. Proper monomer selection, on the other hand, allows for total control over the underlying polymer matrix and the ability to adjust mechanical characteristics to specific re­ quirements [3]. As a result of the exact engineering of the QD structure, the quantum yield (QY) may be improved, photoluminescence lifetimes (PL) can be prolonged, and impacts from nonradiative decay processes can be minimized [4,5]. However, while encapsulation of QDs in a polymer matrix with uniform dispersion and inhibited aggregation has been proved to not only give additional benefits from the bulk characteristics of the polymer but also to improve QD optical qualities, it is still a difficult task [6–8]. Aspects of these nanocomposites that have potential uses include advanced photonic parity-time (PT) symmetric systems, such as Bragg reflectors and linked waveguides, which have the ability to exhibit phenomena such as unidirectional invisibility [9]. Semiconductor nanocrystals (likewise identified as quantum dots, QDs) display changed optical and electrical characteristics as a result of their tiny size (1–20 nm) [10,11]. These properties may be classified as being somewhere among those of DOI: 10.1201/9781003266518-10

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bulk semiconductor constituents and those of sequestered molecules or atoms. Over the past two decades, substantial scientific study has been conducted on quantum dots (QDs), and great developments have been made in both their production and our knowledge of their optical and electrical characteristics, as well as in their applications [12]. However, quantum dots have reached maturity, and a wide range of applications for QDs in biodiagnostics, bioimaging, photonics, optoelectronics, and sensors have arisen in recent years [13]. Numerous publications and review papers on the synthesis, physical and optical characteristics of QDs, as well as their uses, have been published to document the rapid advancement of this field of study [14]. Across many purposes, either the exterior of the QDs must be chemi­ cally modified or the QDs must be implanted in a compact medium in order for them to function properly. Being that utmost artificial polymeric constituents are see-through in the visible spectrum, they are frequently used as matrix for nano­ composite resources in optical applications due to their transparency in this region. Polymers, in addition to serving as the matrix for the nanocomposite material, also contribute to the mechanical and chemical stability of the material. Furthermore, polymers have the potential to inhibit nanocrystal clumping and provide workability into operationally important structures such as thin films, micro- and nano-spheres, and other micro- and nano-structures [15]. Despite the numerous benefits that a combination of QDs and polymeric materials has to offer, progress in this field of study has been modest, particularly in recent years [16]. The most significant issues encountered were meagre compatibility of the QDs with the polymers, as well as impairment of the electrical or optical characteristics of the QDs when they were mixed with the polymers [17]. As a result, research efforts have shifted to the chemical engineering of QD surfaces using polymers, as well as the development of techniques for encapsulating QDs in polymer matrices, in order to circumvent these shortcomings [18]. The current chapter discusses latest investigation in the creation of rubber QD composites, as well as their creation and production processes, as well as the optical characteristics and uses that have resulted as a result of this work. The next portions demonstrate how to create the required structures of QD/polymer composites using various techniques. There follows an argument on the undeviating alteration of nanoparticle surfaces with macromolecules using a variety of strategies such as hydrophobic interactions with QD surface ligands, multivalent passivation of the nanoparticle surface, and the “grafting to” and “grafting from” approaches for the connection of polymer chains straightforwardly to the QD surface, among others. At the conclusion, the applications of hybrid nanocomposite materials in the fields of natural science, photonics, and optoelectronics are discussed in detail.

10.2 BACKGROUND AND CHALLENGES In electrical actuators, adhesives, and soft tissue substitutes, viscoelastic elastomers with high yield strain and low Young’s modulus have been employed because of their great toughness and long-term endurance [19]. Polymer nanocomposites (PNCs) have lately attracted a lot of attention because of their new features obtained from the effective combining of the characteristics of parent components into a

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single material. In addition to their numerous advantages such as cost-effective processability, low weight, and adjustable mechanical, magnetic and electrical characteristics, PNCs are also proving to be promising materials in a variety of applications. The effectiveness of QDs as nanofillers in polymers for the fabrication of composite subwavelength optical waveguides and as efficient photoconductive devices at infrared wavelengths has been established [20]. Elastomer fibers and nanofibers have an elastic character, which may hold great promise in the field of muscle restoration and replacement [21]. When used in conjunction with the fluorescent tagging of QDs embedded in elastomer fibers, the cell integration and dynamic mobility may be tracked more readily and accurately, allowing for more accurate cell tracking. Moreover, till now, only a small amount of research has been published on the manufacturing of fluorescent nanocomposite nanofibers, which may improve the effectiveness and responsiveness of cell separation and medical diagnostics by increasing their reactivity.

10.3 SURFACE MODIFICATION OF QDS BY POLYMER PHASES The polymers on the QD surface may also serve as a link between the QDs and the surrounding matrix, according to certain theories. For particular, electron transfer mechanisms between quantum dots and their adjacent matrix are required in a variety of optoelectronic devices, like solar cells, to function properly. The possi­ bility of functionalizing the QD surface with electroactive polymers is being in­ vestigated in order to facilitate charge relocation through the QD/polymer contact. To create polymer-coated QDs, a variety of different techniques have been devised. Using hydrophobic interactions among the nanocrystals’ superficial ligands and polymers, for example, it is possible to deposit a thin polymeric covering of quantum dots. Due to the fact that this method does not include ligand exchange processes, it is not expected to have any negative impact on the optical char­ acteristics of the QDs. Additionally, QD-functionalization can be accomplished by the direct bonding or addition of macromolecules to the QD external via multiple or single bonds, or through the direct depolymerization of polymeric chains from the QD surface. All of the later approaches necessitate the use of an intermediary ligand exchange phase and, as a result, frequently result in changes in the photophysical characteristics of the quantum dots. The most common approaches applied to manufacture QDs outcome in con­ stituents that are insoluble in polar solvents. In part, this poor solubility might be attributed to the selection of surface stabilizing ligands, which frequently comprise alkyl chains in their structural composition. Aqueous solubility is required for so many applications, particularly in biology, where quantum dots must be dissolvable in water. Different solutions for the manufacture of water-soluble, functionalized QDs have been investigated in order to eliminate the use of superfluous ligand exchange reactions. In spite of this, the most significant advantage of using the hydrophobic interaction approach is that it eliminates the need for performing li­ gand exchange processes. The literature has just a few accounts on such tactics, despite the fact that they have such an appealing characteristic to them. It should be noted that a similar strategy has already been used with considerable success, but

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using tiny organic compounds rather than polymers as the building blocks. Even though it is possible to generate semiconductor nanocrystals that have been coated with tiny organic molecules such as amino acids, phosphines, or thiols, the stability of such a system in complicated settings is restricted. Since the ligands on the QD surface maintain a balance with the surrounding medium’s free species, this is possible. In the absence of the ligand on the QD surface, the nanocrystal ag­ gregation is likely to occur. In order to overcome this obstacle, it would be ne­ cessary to coat the nanocrystals with molecules that give several anchoring sites on their surfaces. This would increase the stability of the QDs and expand their ap­ plication range dramatically. As a result of the cooperative binding of multidentate ligand molecules to nanocrystals, desorption of the whole molecule from the na­ nocrystals surface occurs at a significantly slower rate. The key problem in this multivalent passivation strategy is the management of the photophysical char­ acteristics of the QDs (in most cases, the ability to keep the luminescence quantum yields) while also providing the essential colloidal stability and providing chemical functionality to the QDs. It has been investigated as multidentate coatings whether they are made of synthetic linear or hyperbranched polymers or if they are made of biomacromolecules or bioengineered macromolecules that include large densities of functional groups. In most cases, ligand exchange processes are used to passivate the surface of nanocrystals; however, this may also be done by directly combining them with functionalized polymers. Figure 10.1 shows the plausible approaches for functionalization of QDs. Carboxylic acid and thiol functionalities can be added to the polymer chain or to its side groups to enhance its properties [22,23]. Various academic researchers have employed ligand exchange to alter the sur­ face of colloidal QDs, and it has shown to be a successful method [24–26]. It is necessary to replace the as-grown ligand (for example, phosphine oxide) that was

FIGURE 10.1 Different types of surface engineering adopted by various researchers for QDs’ surface functionalization.

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introduced during the QDs production with fresh biocompatible polymers during the ligand exchange operation. This class of biocompatible polymers often contains functional anchor groups such as thiol, amine, and carboxyl, which have the ability to passivate QDs more strongly than the original ligand [27]. Because of the strong affinity of thiols for metallic surfaces, biocompatible polymers containing thiols have been shown to be beneficial in ligand exchange. Examples are Uyeda et al. [28] and Yildiz et al. [29], who used thiol ended poly(ethylene glycol) (PEG), that is a recognized biocompatible polymer with several uses ranging from engineering production to medication, in order manufacture biocompatible QD fluorophores. Chinese researchers recently developed thiol-PEG-peptide hybrid polymers that were employed in the manufacture of pH-responsive quantum dots (QDs). In water, the resulting polymer/QDs exhibit excellent dispersibility and biocompatibility [30]. Despite this, thiol-stabilized QDs are discovered to be unbalanced due to photooxidation of the thiols and diminished photoluminescence of the QDs while they are being generated. Different research teams have proven the usefulness of amine-linked polymers as functionalities for the ligand interchange mechanism in their experiments. Poly(acrylic acid) (PAA), a non-toxic and water-soluble polymer, has been treated with amine (–NH2) to produce PAA/QDs hybrids that have excellent colloidal stability over long periods of time [31,32]. In the “grafting to” approach, functional polymers that have been synthesized independently of one another are covalently bonded to the surface of QDs by anchor groups that are located at the end of or along the polymer chain. A variety of linear and hyperbranched polymers have been “grafted” onto QDs in order to produce biocompatible polymer/QD hybrids that are biocompatible. As an example, linear PEG has been linked to CdSe QDs using the 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS) process [33,34]. A group led by Peng and colleagues has grafted hyperbranched PEI on CdSe QDs to produce biocompatible PEI/QDs that have high water-soluble character [35]. The thermal stability of the resulting PEI/QDs has been proved across a wide temperature range, which was required [36]. Additionally, “grafting to” is an effective approach for attaching biopolymers (such as protein, peptide, and DNA) to QDs. Biopolymers include a large number of amine groups along their molecular chains, which allows them to easily react with carboxylic acid functionalized QDs through EDC/NHS chemistry. Because of their biopolymer/QDs conjugation, these materials have excellent biocompatibility. To detect antibodies in biological samples, CdSe/ZnS core/shell QDs have been covalently “grafted” to CdSe/ZnS core/shell QDs that are extremely bright [37]. Figure 10.1 illustrates another effective way for producing biocompatible polymer/QDs: the “grafting from” chemistry, in which the macrochain is matured superficially from the QDs’ external surface by the use of surface-initiated poly­ merization chemistry. The QDs are initially coated with tiny molecules, which serve as a source of energy for the polymerization process. In addition to ring-opening polymerization (ROP) [38], reversible addition-fragmentation chain transfer (RAFT) polymerization [39], nitroxide-mediated radical polymerization (NMRP) [40], and atom transfer radical polymerization (ATRP) [3], there are other poly­ merization techniques to choose from. and the polymerization of oxyanionic vinyl

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compounds (OAVP) [41]. Poly(caprolactone) (PCL) was polymerized onto QDs by Carrot et al., who used ROP to polymerize the biocompatible and biodegradable polymer onto the QDs [38]. Because of the ease with which PCL may be degraded by hydrolysis of its ester bonds under biological circumstances (such as those seen in the human body), PCL/QDs may garner considerable interest for usage as an implantable biomaterial as well as a drug release and delivery mechanism. Additionally, hyperbranched PEG was “grafted from” QDs by ROP to produce a biocompatible PEG/QDs material in addition to PCL [42]. In addition to capping polymers onto QD surfaces through physical interaction, such as hydrophobic or electronic interactions, between polymers and the original ligands on QDs, another widely used strategy for fabricating polymer/QD hybrids is to cap polymers onto QD surfaces through electrostatic interaction. In order to change the surface of QDs using the capping approach, a number of amphiphilic copolymers and polyelectrolytes were produced. Hydrophobic portions of amphi­ philic copolymers link the initial ligands of QDs to hydrophobic segments of the copolymer, whereas hydrophilic segments are exposed to water to facilitate QD dispersion in aqueous conditions [43]. As an example, significant research has established that amphiphilic PEG polymers may effectively enclose QDs as the hydrophobic core and stable QDs in water when used in this manner [44]. The biocompatible PEG/QDs that were produced have been proven to have the cap­ ability of detecting and imaging biomolecules. Additional to this, alkyl-modified amphiphilic PAA has been used to coat QDs using the capping approach for biodetective applications [45]. Poly(maleic anhydride) copolymers, which are easily hydrolyzed into PAA, have also been frequently employed to produce QDs with excellent biocompatibility by the capping approach, owing to their ease of hydro­ lysis (Figure 10.2) [46]. Polyelectrolytes, on the other hand, have the ability to “cap onto” the surface of QDs by electrostatic contact. Using a multifunctional poly (acrylamide) (PAM) as an example, it has been possible to change CdSe/ZnS core/ shell QDs over electrostatic attractions among the positively charged lateral chains of PAM and the negatively charged moieties of QDs [47]. Biopolymers, which are a specific type of polyelectrolytes, may also couple with QDs via the capping ap­ proach, which is based on electrostatic contact, in order to produce biocompatible conjugates. As example, positively charged DNA fragments have been coupled

FIGURE 10.2 Graphic illustration presentation the steps involved to prepare water-soluble QDs using poly(styrene-co-maleic anhydride). Reproduced with the permission from ref. [©, 46,2009 American Chemical Society.

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FIGURE 10.3 (a) Transmission electron micrographs of individual Au-core/polyelectrolyte shell nanoparticles at various phases of the dissolution of the Au core. After the breakdown of the gold core and staining with uranyl acetate, an impression transmission electron mi­ crograph of “empty nanospheres” is shown in (b). Reproduced with the permission from ref. [ 49] © 2004 American Chemical Society.

with negatively charged CdSe/ZnS QDs for use in photoluminescence and bioassay claims, respectively [48]. The capping approach, as opposed to “grafting from” and “grafting to”, produces extra-dense polymer/QDs amalgams with smaller hydrodynamic diameters in water as a result of the many interactions between QDs and a single polymer chain, which is advantageous in water. Scientists have used unlike polyelectrolytes with con­ trasting charges to consecutively coat QDs by layer-by-layer association in order to produce diverse types of polymer/QDs assemblies with the similar QDs core. Single gold QDs have been sequentially coated with up to 20 layers of polyanion poly­ styrene sulfonate (PSS) and polycation polyaluminum phosphate (PAL), according to a typical example (Figure 10.3) [49].

10.4 QDS IN ELASTOMER MATRICES Since Goodyear discovered that sulfur could vulcanize natural rubber in 1844, rubbers have recognized to be useful in a wide range of technical claims due to their entropic elasticity, high straining, and low rigidity, among other character­ istics. As a result of vulcanization, linear polymer chains are interconnected to form a three-dimensional (3D) linkage that improves mechanical quality and shape constancy, allowing for long-term permanency and dependability under a variety of

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environmental circumstances. Various additives, such as activator and accelerator, are required for the improvement of the vulcanization efficiency and mechanical qualities, just as they are for many other classical chemical processes. These ad­ ditives, on the other hand, typically produce harmful volatile organic compounds (VOCs), which result in a disagreeable odor during vulcanization and practical uses. Some vulcanizers and additives, on the other hand, are cytotoxic and ecologically hazardous. For example, ethylene thiourea (ETU) is a potent carcinogen, and zinc oxide (ZnO) is extremely harmful to aquatic species, despite the fact that both of these chemicals are commonly employed as vulcanizers or additives in a variety of products. As a result, the rubber sector is in desperate need of easy-to-use and ecologically safe cross-linking agents, yet doing so is extremely difficult. One option is to discover new non-toxic vulcanizers and accelerators, which has prompted some researchers to turn their focus to natural substances as a possible answer. Conventional fillers, such as carbon black, can need vast quantities of material, which is both ecologically unfriendly and energy-intensive. As a result, developing nanofllers that can perform the twin functions of chemical cross-linker and reinforcing fller is a potential technique. Based on its chemical inertness of traditional fillers such as carbon black and silica, as well as advanced nanofillers such as carbon nanotube and graphene, neither traditional nor advanced nanofillers can cross-link rubbers. Surface modification with functional groups is usually re­ quired to solve this issue, and some classic processes such as the epoxy/carboxy reaction, Si–H/vinyl hydrosilylation, and TESPT/sulfur reaction are all examples of such treatments. These modifications commonly necessitate the use of large amounts of organic solvent and hazardous chemicals, and the resultant nano­ particles typically exhibit poor cross-linking efficiency. The inherent oxygenic groups in styrene-butadiene rubber (SBR) were shown to be capable of homolytic cleavage upon heating, which allowed them to produce radicals that were highly effective in cross-linking the rubber. This was in contrast to the intended surface modification [50]. Synthetic rubber chloroprene (CR) is frequently used because of its mechan­ ical qualities and chemical resistance. Chloroprene rubber’s vulcanization process differs significantly from that of other diene rubbers [51]. ‘Cl’ atoms have an electrical action that prevents sulfur from vulcanizing carbon rubber. As a result, chloroprene chains are vulcanized with ETU and ZnO by interacting with the chloroprene chains’ 1,2-units (allyl chloride). As a result, despite their negative effects on the environment, ZnO and ETU are essential in the vulcanization of CR because of their high mechanical performance. We are compelled to design new vulcanizers for CR in this aspect [52]. Using nitrogen-doped carbon nanodots (N-CDs) as both a cross-linker and a reinforcing nanofiller, Kong et al. developed a green approach for fabricating a kind of mechanically robust chloroprene rubber (CR) that is both environmentally friendly and mechanically robust. The N-CDs are generated on a large scale using a green bulk high-temperature carbonization process that is environmentally friendly, and they have the ability to cross-link CR by nucleophilic substitution [53]. Curemeter is used to investigate the vul­ canization kinetics. The covalent cross-links between N-CDs and CR also result in a strong interfacial contact between the two materials, which allows for

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remarkable strengthening of the rubber matrix. Therefore, NCDs cross-linked CR exhibits good mechanical characteristics, with tensile stress up to 30 MPa and an elongation at break of up to 1353% being achieved. In this study, a novel technique for producing superior green rubber composites without polluting the environment is introduced, with no carcinogenic or ecologically harmful compounds added. According to Li et al., a simple and effective technique is developed in which CQDs with functional groups are well disseminated into the silicone rubber (SR) matrix by a hydrosilylation procedure that prevents the CQDs from fluorescence quenching as a result of agglomeration is developed. We began by synthesizing the blue-emitting CQDs ethanol solution utilizing an ultrasonic technique, in which starch soluble was used as the carbon source for the reaction [54]. After being modified by KH570, the CQDs as synthesized were seen to successfully mix with SR through a hydrosilylation process, resulting in the formation of the CQDs/SR composite. It was found that the KH570-modified CQDs were well disseminated in the SR matrix, which may effectively avoid the fluorescence quenching of the CQDs as a result of agglomeration. Additionally, when exposed to UV light, the CQDs/SR composite displays a strong blue fluorescence and exhibits excitationdependent PL emission behavior. The CQDs/SR composite has remarkable features, including high transparency, excellent mechanical qualities, and excellent thermal stability, making it suitable for use in a broad range of disciplines, including optoelectronic devices and LEDs. This type of rubber is produced by hydrogenating a nitrile rubber, which results in a rubber with stronger heat resistance and improved age resistance while still retaining the oil resistance of the nitrile rubber as a result of the hydrogenation process. The solution hydrogenation and latex hydrogenation are the two most important procedures for preparation. Due to the complexity of the solution hy­ drogenation process, which necessitated severe reaction conditions and the use of precious metal catalysts, HNBR has a high manufacturing cost. A high hydro­ genation degree has been reported for the diimide generated by the reaction of hydrazine hydrate with NBR and the quick addition of carbon-carbon double bonds throughout the reaction [55]. GQDs solution was utilized to create fluorescent HXNBR latex using an in situ technique, as demonstrated by Xie et al. The GQDs can be grafted onto the HXNBR molecular chain in a consistent manner. Preparation of the GQDs and HXNBR self-cross-linking films was accomplished by the casting process. By examining the mechanical characteristics and optical properties of the composites, it was discovered that the interpenetrating network model of HXNBR films led to the improvement of both mechanical and optical qualities [56]. It is possible that the inclusion of GQDs may result in the formation of an interpenetrating network structure, which will aid in the dispersion of GQDs in the rubber matrix. The tensile strength of the interpenetrating network film is up to 11.9 MPa, according to the mechanical characteristics of the network film. It has been demonstrated that, when 0.1 percent of GQDs are spread throughout the system, fluorescence intensity of the interpenetrating network system is 1.66 times more than that of the blend system. Anti-counterfeiting and flexible display applications are possible with this material.

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Graphene quantum dots (GQDs) have attracted considerable interest as an enticing structure that poses a barrier to the development of electrical, biomo­ lecular, and photonic gadget applications, among other things. As graphene has a zero band gap, the possibility of the material releasing light is virtually im­ possible. This disadvantage can be overcome by cutting graphene into nanometerscale pieces and creating a hole inside it. This is the most promising method of doing so. These nanoscale components were referred to as graphene nanoribbons (GNRs) and graphene quantum dots (GQDs). This method is dependent on the quantum confnement, which is lawful for any restricted size of graphene, starting with its unlimited exciton Bohr span, which is the starting point for this me­ chanism. Chemical doping can be used to modify the synthetic characteristics of GQDs in a practical manner. Chemical doping is a widely utilized approach for the creation of GQDs from a variety of carbonaceous nanomaterials, and the results are remarkable for their unique marvels and unexpected capabilities. Because of the extremely high location level, the likelihood of identifying a single molecule is quite low. Because graphene-based chemical sensors can achieve extremely low noise levels, they are becoming increasingly popular. Edayadiyil et al. produced and investigated the characteristics of natural rubber/ graphene quantum dots (GQDs) nanocomposites, which were then used to various applications [57]. Addition of GQDs into the natural rubber matrix has been shown to have a significant impact on the mechanical and thermal characteristics of the rubber. By adding 1phr of GQD to natural rubber nanocomposites, the tensile strength increases by 30%, the elongation at break increases by 13%, and the Young’s modulus increases by 23% when compared to pure natural rubber nanocomposites, according to the researchers. The thermal stability of the NR/ GQD nanocomposites containing 1.5 phr of GQD is significantly improved. The enhanced thermal stability was attributed to the homogenous distribution of GQD in the NR matrix, as well as the interaction of GQD with other elements in the matrix, according to the researchers. The GQD will operate as a protective barrier, which will prevent thermal degradation from occurring by absorbing the heat released by the matrix throughout the process. Increased loading of GQD in the NR matrix results in agglomeration, which reduces the characteristics of the NR matrix. Another set of researchers achieved high thermal conductivity properties in polyamideimide/boron nitride composite films by doping boron nitride quantum dots into the polyamideimide matrix [58]. In this study, the authors used an evaporation-induced self-assembly approach to produce thermoplastic polyamideimide (PAI) composite films in order to introduce boron nitride quantum dots (BNQDs) into the PAI matrix based on the addition of boron nitride (BNNS). This will be accomplished by filling up the gaps between the filler and matrix of the PAI/BNNS composite film, so increasing the number of thermal conductive routes even more. The synergistic impact between the BNNS and BNQDs is critical in the construction of a continuous thermal conductive route. The thermal conductivity value obtained is greater than the majority of those obtained for polymeric composites with minimal filler content in the literature. It is note­ worthy that the photoluminescence of the BNQDs, which is distinctive to the

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material, may be used to observe thermal conductive routes in real time. This study gives useful assistance for the development of thermal management ma­ terials that are capable of meeting the efficient heat dissipation needs of electronic devices. According to Wang et al., the production of aminopropylmethylpolysiloxane (AMS, or aminopropylmethyl silicone oil) functionalized luminous CDs (AMSCDs) and their application in WLEDs could be possible [59]. Given their high stability, AMS might be an appropriate candidate, and the carboxyl (from the carbon source) and aminopropyl (from AMS) groups on the surface of the AMS-CDs produced would react with one another to form a totally cross-linking network, as seen in Figure 10.4. As a result, AMS-CDs may be used to generate CDs cross-linked silicone rubbers (SRs) by bulk self- or co-cross-linking with AMS at temperatures ranging from 50 to 80 degrees Celsius. CDs served as crosslinking sites. The concentration of AMS-CDs doped in silicone rubber was regulated between 10 and 100 weight % (100 wt % silicone rubber was referred to the self-cross-linking AMS-CDs without AMS). More importantly, when compared to commercial WLEDs with a two-component system, these CDs-based SR coatings with high thermal stability could serve as both a CCL and an en­ capsulation layer at the same time, allowing for more effective optimization of the compatibility between luminescent materials and the encapsulation matrix, as previously demonstrated. The visualization of the samples and their photo­ luminescence behaviors have been depicted in Figure 10.5. Quantum dots are also playing a crucial role as antioxidants in elastomer ma­ trices. Heat, particularly at high temperatures, an excess of oxygen, UV light, and chemicals are all known to cause the degradation of organic polymer materials over time. The resulting alterations have a negative impact on the performance and dependability of certain materials, and they have a significant impact on their

FIGURE 10.4 Synthesis of silicon based quantum dots and their cross-linking mechanism. Reproduced with the permission from ref. [ 59] © 2016 Americal Chemical Society.

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FIGURE 10.5 (a) Optical photographs of CDs cross-linked SRs with different ratios from 10 to 100 wt %, upon visible light (top) and 365 nm UV illumination (below); (b) bended 100 wt % SR; (c) absorption and PL emission spectra of the luminescent SRs at different AMS-CDs concentrations excited at (d) 360 nm and (e) 460 nm. Reproduced with the permission from ref. [ 59] © 2016 Americal Chemical Society.

applications. One particularly noticeable example is the deterioration of rubber tires over time. When subjected to heat, dienic elastomers become extremely susceptible to thermo-oxidation. This is due to the fact that the unsaturated skeleton containing allylic hydrogens may be easily activated, resulting in the generation of reactive radicals, which function as initiators in the process of aging. Several antioxidants, including those for radical inhibition and scavenging, have been used to preserve elastomeric materials from aging and to increase the length of time that they may be used. Regardless of the fact that traditional antioxidants have shown a delay in the aging process of elastomers, these petrol-derived antioxidants have not been found to be satisfactory due to their short working period, toxicity, and low effectiveness when used directly in the elastomer formulation. For sufficient de­ fense over long periods of time, such as months or years, effective encapsulation of sustainable antioxidants in nanocontainers doped into elastomers for sustained agent release throughout those periods is required, as demonstrated by these find­ ings. Nanomaterials have been developed for the encapsulation and sustained re­ lease of additives in a variety of applications. Wu et al. developed an imaginative architectural concept utilizing tubule halloysite clay as hollow nanocontainers for the loading and sustained release of antioxidative carbon nanodots, which was made possible by the use of tubule halloysite clay [60]. If added to a rubber matrix (Figure 10.6), the halloysite loaded with CDs exhibits prolonged anti­ oxidative action, overpowering the oxidative progression of the organic background and conserving its physical and chemical characters when being subjected to rigorous thermo-aging for 20 days at 100°C. This implies several months of shield at poorer facility temperatures. A facile surface adaptation with silane bis

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FIGURE 10.6 Graphical illustration of quantum dots protective activity against nabber oxidation. Reproduced with the permission from ref. [ 60] © 2017 American Chemical Society.

(triethoxysilylpropyl)-tetrasulfide) results in the formation of an extra shell on the nanotubes, which slows the release rate of the loaded carbon nanodots and improves the aging-resistance characteristics of the rubber composites. A consequence of the cross-linking caused by the self-catalytic transfer and recombination of reactive radicals that occurs as a result of the aging process is that the elastomer’s high stretchability will be severely deteriorated. As a result, the withholding of physicmechanical belongings of SBR compounds could provide the most lineal valuation of the degradation procedure: entire rubber composites showed a decrease in elongation at break, EAB, and an increase in tensile strength and cross-link density after aging. In addition, all rubber composites showed an increase in cross-link density after aging (Figure 10.7). For instance, the stretchability of SBR/HNTs was virtually completely eliminated during the long-term thermo-aging test. The direct integration of 1.4-phr CDs was able to greatly retain the elongation at break (EB) of SBR/HNTs and CDs in the initial aging, but the composite’s oxidation resistance drastically degraded as the aging period progressed. Significantly, during 20 days of thermo-aging at 100°C, the retention of EB for SBR/Loaded HNTs is approxi­ mately 50% greater than the retention of EB for SBR/HNTs and CDs combined. The continuous discharge of CDs from halloysite lumen suggests that the com­ pound has long-acting antioxidant effect, which may be beneficial in delaying the aging process. Furthermore, following admixing with an identical quantity of modified loaded halloysite, the EB increased by 64%, which is about 1.5 times greater than the EB of the rubber with HNTs and CDs added separately (in the nonformulated form). In this study, we demonstrate that the addition of a silane shell to a loaded halloysite results in a boost in the effectiveness of CD delivery while also providing long-acting radical-scavenging action. During long-term thermoaging, this formulation effectively removes reactive radicals from the SBR matrix while also inhibiting the oxidation of the matrix itself.

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FIGURE 10.7 Withholding of (a) elongation at break and (b) tensile strength for SBR composites with pristine and loaded halloysite vs. aging time (c) Cross-link density for all SBR composites vs aging time. Reproduced with the permission from ref. [ 60] © 2017 American Chemical Society.

10.5 SUMMARY It can be summarized that quantum dot–loaded rubber matrices have several ver­ satile applications from reinforcement to radical scavenging. Initially, this chapter summarized how quantum dots could be compatiblized for polymer matrices by adopting surface chemistry pathways. Next, the composites’ fabrication and ap­ plicability were nurtured as per the various scientists’ discoveries. It is seen that quantum dots are difficult to disperse in polymer matrices because of their high surface energy. But when it is accomplished, the rise of the synergistic properties cannot be ruled out. In the near future, elastomeric-quantum dot composites could be excellent alternatives of materials research problems.

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11

Biomedical Applications of Quantum Dot–Polymer Composites Fouad Damiri, Yahya Bachra, and Mohammed Berrada University Hassan II of Casablanca, Faculty of Sciences Ben M’sick, Department of Chemistry, Laboratory of Biomolecules and Organic Synthesis (BIOSYNTHO), Casablanca, Morocco

CONTENTS 11.1 11.2 11.3

Introduction.................................................................................................207 Chemical Structure of CQDs .....................................................................208 Preparation Methods of CQDs...................................................................209 11.3.1 Top-Down Route .......................................................................... 209 11.3.2 Bottom-Up Route ......................................................................... 211 11.4 Strategies to Change Biodistribution and Toxicity ...................................212 11.4.1 Biodistribution ..............................................................................212 11.4.2 Toxicity .........................................................................................212 11.5 Applications of Carbon-Based Quantum Dots (CQDs) ............................213 11.5.1 CQDs in Diagnosis....................................................................... 214 11.5.2 CQDs with Dual Functions (Phototherapy and Radiotherapy)... 215 11.5.3 Role of CQDs in the Drug Delivery Field.................................. 215 11.5.4 Gene Therapy ...............................................................................217 11.5.5 Biosensing and Immunosensors ................................................... 217 11.5.6 Bone Tissue Enginnering ............................................................. 221 11.5.7 Use in the Environment ............................................................... 221 11.6 Conclusions and Prospects for the Future.................................................222 References.............................................................................................................. 222

11.1 INTRODUCTION Quantum dots (QDs) are semiconductor nanocrystals materials with unique sizedependent optical and electrical characteristics that have emerged in recent years as a result of the quantum size effect [1,2]. Carbon-based quantum dots (QDs) are primarily classified into two subcategories, known as carbon quantum dots (CQDs) DOI: 10.1201/9781003266518-11

207

208

Quantum Dots and Polymer Nanocomposites

and graphene quantum dots (GQDs), have received considerable attention in sci­ ence and engineering fields. In 2004, CQDs were discovered through the pur­ ification process of SWCNTs (single-walled carbon nanotubes). When compared to typical nanomaterials, they have better features such as low toxicity, wide surface area, high photo stability, high resistance to photo bleaching, and ease of mod­ ification, making them intriguing materials for biomedical applications [2,3]. However, two main synthesis routes can obtain CDs: the top-down and the bottom-up route [4,5]. The first technique depends on the decomposition of en­ ormous graphitic compounds are broken down into different carbon-based com­ pounds. Top-down techniques, on the other hand, typically need severe reaction conditions, costly materials/equipment, and lengthy production periods. Bottomup approaches, on the other side, are perhaps the most extensively utilized synthesis methods. They typically include the pyrolysis of smaller organic mo­ lecules in powder form (calcination) or in a solution using hydrothermal or microwave-based methods. Bottom-up techniques have the benefit of being massproducible, ecologically benign, and highly adaptable [6]. Recent research in this subject has focused on generating less toxic QDs, building compact and bio­ compatible QDs using novel capping ligands, developing innovative imaging and sensing methods based on QDs, and creating multifunctional QD nanocomposites [7]. Despite the fact that QDs have beneficial qualities that make them excellent nanomaterials for biological applications, scientists are actively researching po­ tential approaches to increase their properties. In this chapter, we first explain the chemical structure of QDs, method of preparation of QDs, strategies to modify biodistribution, and toxicity. We then discuss their application in biomedical. The aim of this chapter is to offer a review of the most recent advances of QDs in biomedical research.

11.2 CHEMICAL STRUCTURE OF CQDS Carbon is a dark substance that has long been thought to have poor fluorescence and limited water solubility. Carbon-based quantum dots have garnered a lot of interest because of their intense luminosity and excellent solubility; therefore, they’re called carbon nanolights [8]. Carbon-based quantum dots’ architectures control their varied characteristics. Numerous carboxyl groups on the CQD surface contribute to excellent biocompat­ ibility and water solubility [1]. CQDs may also be used for surface passivation and chemical modification using a variety of polymeric, biological, organic, or inorganic materials. Surface passivation improves the physical and fluorescent characteristics of CQDs. CQDs based on carbon have strong photochemical stability, a benign chemical composition, and conductivity. The chemical structures of CQDs vary depending on the production process. CQDs have a spherical form and are classified as carbon nanoparticles with or without a crystal structure. The distance between the layers of CQDs is approximately 0.34 nm, which corresponds to the (002) spacing of crystalline graphite. On the surface of CQDs, there is a network of linked or changed chemical functional groups,

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such as oxygen-based and amino-based groups. The matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) method may be used to determine the functional group of CQDs based on their physical and chemical structures [9]. CQDs are produced from polycyclic aromatic compounds using bottom-up procedures such as dehydration and carbonization. The hydrothermal, carbonization in a micro-reactor, microwave-hydrothermal [10], and plasma-hydrothermal tech­ nologies are among those utilized for dehydration and carbonization processes [11]. Nonetheless, these approaches provide good control over the end product’s qualities [12].

11.3 PREPARATION METHODS OF CQDS CQD preparation may be divided into two categories: “top-down” and “bottom-up” (Table 11.1) (Figure 11.1). The former entails the breakdown of carbonaceous materials via chemical, electrochemical, or physical methods. The latter, on the other hand, is achieved by the carbonization of tiny organic molecules followed by chemical fusion. A full chapter on the preparation of CQDs can be found in various review publications, and this section will just provide a quick summary of current advances in the development of CQDs.

11.3.1 TOP-DOWN ROUTE The first reported CQDs separated by accident from arc‐discharged soot were created using the top‐down technique (Table 11.1), in which carbonaceous soot was cleaved by nitric acid oxidation, followed by gel electrophoresis separation to recover CQDs. Following that, laser ablation of graphite using Ar as a carrier gas in the presence of water vapor was shown to be effective for the top-down preparation of CQDs. CQDs have already been created as a product of dimension reduction from graphite, gra­ phene, carbon nanotubes (CNTs), and other materials having sp2 carbon structure using different physicochemical methods such as arc discharge, laser ablation, elec­ trochemical exfoliation, and chemical oxidation. To break down the chemically inert carbon structure of sp2 covalence bonds, however, the top-down method is generally involved with extreme conditions, some of which may be ecologically hazardous, and hence unsuitable for large-scale manufacture. Furthermore, in such top-down procedures, effective control over the particle size of CQD products is frequently difficult to achieve. Francesca Limosani et al. created N-doped carbon quantum dots (N-CQDs) (Figure 11.2) by employing a top-down strategy, i.e., hydroxyl radical opening of fullerene with hydrogen peroxide in a basic environment with ammonia for two distinct reaction durations. The subsequent characterization using dynamic light scattering, SEM, and IR spectroscopy demonstrated a size control that was time dependent, as well as a more pronounced -NH2 functionalization. The N-CQDs were tested for metal ion detection in aqueous solutions and during bio-imaging, and they showed a change in Cr3+ and Cu2+ selectivity with increasing -NH2 functionalization, as well as HEK-293 cell nuclei labeling [25].

Top-down Approach

Bottom-Up Approach

Type

Mitochondria targeting, long time cell imaging Targeted cancer drug delivery

Pyrolysis of konjac flour

Human prostate cancer cell lines Cell line

Phototherapy

Phototherapy

Arc discharge of graphite rod produced nanopowder, which was then refluxed in nitric acid before dialysis Candle soot oxidation by nitric acid

Patient blood samples

Cancer diagnosis

Cl-CQD: CQD synthesized from MWCNT were functionalized with –COCl and conjugated with anti-desmin

CQD-organic dye conjugates human cell lines In vitro

In-vitro

Biosensing and diagnosis (H2S) SNP polymorphism detection (disease diagnosis)

Mitochondria targeting and imaging

Chitosan, ethylenediamine, and mercaptosuccinic acid hydrothermal treatment

In-vitro cell model

Bionanoplatform (Fe3O @ mSiO 4 −TPP/CDs) Nanogel (copolymerized with zwitterionic amino acid ornithine methacrylamide)

Bio-nanoplatform (CQD-HASiO4-DOX)

Model

Chemical treatment is used to create CQD from ethanolamine CQD-ssDNA: Chemical oxidation of candle soot

Theranostic nanoparticle for targeted drug delivery to cancer cells

mPEG-OAL-DOX/CQD: pyrolysis of citric acid, cross-linked with PEGylated oxidized alginate (mPEG-OAL)

Microwave synthesis using acrylic acid and ethylene diamine, followed by glycidyl methacrylate functionalization

Targeted drug delivery to cancer cell

Application

Citric acid, hyaluronic acid, and ethylenediamine hydrothermal therapy

Synthesis Method and Precursors

TABLE 11.1 Methods of Synthesizing CQDs [ 14]

Highly cytotoxic to cancer cells

Cytotoxic to cancer cell

[ 24]

[ 23]

[ 22]

[ 21]

– –

[ 20]

[ 19]

[ 18]

[ 17]

[ 16]

[ 15]

Ref.

–

Less cytotoxic

Low cytotoxicity

Low cytotoxicity

Low cytotoxicity

Low cytotoxicity

Cytotoxicity

210 Quantum Dots and Polymer Nanocomposites

Biomedical Applications

211

FIGURE 11.1 Schematic illustration methods for the synthesis of carbon quantum dots (CQDs) [ 13].

FIGURE 11.2 Preparation of fluorescent N-CQDs through hydroxyl radical-induced de­ composition of fullerene C60 [ 25].

11.3.2 BOTTOM-UP ROUTE Bottom-up approaches, on the other hand (Table 11.1), provide chances to regulate the formation of CQDs by employing organic molecules as carbon precursors. Carbonization may be used on a variety of compounds, followed by aggregation in solution to form CQDs. CQDs have already been prepared via pulsed laser irradiation of toluene, hydrothermal treatment of citric acid, electrochemical carbonization of

212

Quantum Dots and Polymer Nanocomposites

low-molecular-weight alcohols, and microwave-assisted pyrolysis of citric acid for­ mamide solution [26]. Lately, biomass molecules such as sucrose, glucose, cellulose, and amino acids have piqued the interest of researchers as potential precursors for the dehydration and subsequent carbonization of CQDs. Furthermore, as a strategic potential for large-scale production, raw biomass is a viable precursor for the synthesis of CQDs. Hong Hee Kim et al. revealed an unique bottom-up synthetic approach for producing highly crystalline CQDs suited for high-brightness blue light-emitting diodes [27]. The two-step solution procedure begins with time-controlled heat carbonization of citric acid, followed by ligand exchange of the CQDs in solution with oleylamine (OA). Carbonization promotes the formation and development of crystalline CQDs, whereas OA treatment disperses the CQDs and stabilizes the solution, resulting in CQDs with minimal structural flaws and homogeneous sizes. The photoluminescence (PL) investigation revealed the origin of the light emission of OA-treated CQDs, yielding a high quantum efficiency of 30%. When used in an inverted light-emitting diode, the photoluminescence-optimized OA-treated CQDs demonstrate outstanding blue EL performance with a low turn-on voltage of 4 V and high brightness of 308 cd m−2; and a minor voltage-dependent color shift [27].

11.4 STRATEGIES TO CHANGE BIODISTRIBUTION AND TOXICITY Significant progress has been achieved in improving the surface characteristics of QDs, as well as their solubility and stability in biological settings, as mentioned in the preceding section. Nevertheless, many tasks remain to be addressed to realize the clinical potential of QDs. The major boundaries are the capability to control the biodistribution of QDs only to the target organs, to decrease long-term buildup in the body, and long-term toxicity. In this section, we address potential solutions and current progress on these persistent difficulties.

11.4.1 BIODISTRIBUTION The study of biodistribution of biological substances, biomarkers, and associated systems is critical to our understanding of their behavior. For the most part, QDs are delivered directly into the bloodstream through systemic intravenous (i.v.) injection in in vivo applications. As QDs are nanocolloids, their performance in this biological environment is much more challenging than that of conventional organic dyes [28–31]. Particles are subjected to several layers of defense throughout their passage through the circulatory system, and they must overcome a number of significant biological obstacles, ranging from the organ to the cellular level. These impediments hinder accumulation and reduce the efficacy of focused interactions [28].

11.4.2 TOXICITY Long-term toxicity of QDs is another key concern in biological applications [32], and it is one of the most difficult hurdles to overcome in clinical use. As described previously, QD particles have been demonstrated to accumulate and reside in the

Biomedical Applications

213

liver and spleen of living mice. Toxicity comes mainly from four areas [33]: i) heavy metal materials that are widely employed in particle cores; (ii) chemicals that may release toxic constituents from the surface (i.e., especially for coatings con­ taining Cd, Se, and Hg); iii) any free radicals or reactive species created by sti­ mulation; and iv) the interaction between tissue and nano-colloid in biological settings (e.g., small particle size, positive surface charge, chemical composition). Liang Hu et al. evaluated the bioaccumulation and toxicity of three CdSe/ZnS QDs in-depth (COOH CdSe/ZnS525, NH2 CdSe/ZnS 525, and NH2CdSe/ZnS625) confocal laser scanning microscopy, reactive oxygen species (ROS) measurements, and cell viability experiments in Phanerochaete chrysosporium (P. chrysosporium) [34]. The analytical results of confocal laser scanning microscopy showed that all CdSe/ZnS QDs could accumulate largely in the hyphae and induce the generation of ROS, indicating a direct toxicity to P. chrysosporium, and that the bioaccumulation and toxicity of CdSe/ZnS QDs presented dose-dependent and time-dependent effects on P. chrysosporium. Furthermore, the cytotoxicity of CdSe/ZnS QDs was related to their physicochemical properties, including particle size and surface charges: NH2CdSe/ZnS 525 with small size was more cytotoxic than NH2CdSe/ ZnS 625 with large size, and the smaller negative-charged NH2CdSe/ZnS 525 re­ sulted in greater cytotoxicity than the larger negative-charged COOH CdSe/ZnS 525 The acquired results are useful for investigating and comprehending the toxi­ city mechanism of QDs in live cells [34]. Ting Chen et al. investigated the cytotoxicity of InP/ZnS QDs with various surface groups (NH2, COOH, OH) on two lung-derived cell lines (Figure 11.3). The diameter and spectra of InP/ZnS QDs were studied, as well as the hydrodynamic size of QDs in an aqueous solution [35]. The labeling of QDs for human lung cancer cell HCC-15 and alveolar type II epithelial cell RLE-6TN was seen using confocal laser scanning microscopy. Flow cytometry was utilized to establish the qualitative absorption efficiency of QDs, cell death, and ROS production. The results indicated that InP/ZnS-OH QDs were simpler to aggregate in deionized water and had a substantially larger hydrodynamic size than the other InP/ZnS QDs. At low con­ centrations, all of these InP/ZnS QDs were able to penetrate the cells, with greater absorption efficiency for InP/ZnS-COOH and InP/ZnS-NH2. Cell viability was reduced by high dosages of InP/ZnS QDs, and InP/ZnS-COOH QDs and InP/ZnSNH2 QDs appeared to be more hazardous than InP/ZnSOH QDs [35].

11.5 APPLICATIONS OF CARBON-BASED QUANTUM DOTS (CQDS) CQDs have unique physicochemical and catalytic features that make them potential candidates for biological applications [36]. CQDs’ tiny size and biocompatibility, as well as their ability to be tracked throughout the body due to their PL properties, may provide them a significant potential to be exploited as drug delivery vehicles. CQDs with low toxicity, high hydrophilicity, and water solubility, excitation wavelength-dependent PL emission and chemical stability offer a wide range of potential biological uses. We will focus on the primary bio-applications of CQDs and highlight recent achievements in each field in the sections that follow.

214

Quantum Dots and Polymer Nanocomposites

FIGURE 11.3 HCC-15 and RLE-6TN cell viability after treatment with I InP/ZnS-COOH, (ii) InP/ZnS-NH2, and (iii) InP/ZnS-OH QDs, respectively. The cells are treated with various QD doses for 24 (A,C) and 48 h (B,D) [ 35].

11.5.1 CQDS

IN

DIAGNOSIS

Diagnostics is another key use of CQDs in biomedicine. Semi-conductive quantum dots have been employed in illness diagnostics in-vivo [14]. When compared to other current technologies, quantum dot–based nanoprobes are quick and inexpensive. Because CQDs are less hazardous than typical semi-conductive quantum dots, they are preferred for in-vivo labeling. CQDs made from MWCNTs (multiwalled carbon nanotubes) were functionalized with –COCl (through acid and SOCl2 treatment), resulting in Cl-CQDs. To detect the protein desmin, Cl-CQDs were subsequently conjugated with anti-desmin. Desmin is discovered in significant amounts in the serum of people with colorectal cancer. This CQD-based nanoprobe might be utilized to detect desmin in patient serum samples with good specificity and sensitivity. This discovery should pave the way for the creation of a low-cost, sensitive, and specific diagnostic platform based on CQDs [37].

Biomedical Applications

215

CQDs have recently been shown to distinguish between malignant and normal cells based on their redox state. Silicon and N-doped CQD produced by solvothermal treatment of glycerol and N-(3-(trimethoxysilyl)propyl) ethylenediamine demon­ strated Fe3+ ion sensitivity with a detection limit of 16 nM and an on-off fluorescence mechanism [38]. These CQDs with Fe3+ ions (CQDs/Fe3+) may detect cancer cells using an on-off-on mechanism, since the reductive environment of malignant cells lowered Fe3+ ions, resulting in the reactivation of CQD fluorescence. Furthermore, FA-CQDs and carbon quantum dots-apatamer conjugates were able to identify cancer cells preferentially [39]. As a result, these scientists demonstrated CQDs’ immense promise for the creation of low-cost, effective, and sensitive diagnostic nanoprobes and point-of-care (POC) devices. CQDs were recently utilized in the construction of microfluidic paper analytics devices for biological sample diagnostics. POC di­ agnostic devices will not only aid to cut diagnostic costs, but will also help to reach out to the impoverished and inaccessible rural populations in developing nations.

11.5.2 CQDS

WITH

DUAL FUNCTIONS (PHOTOTHERAPY

AND

RADIOTHERAPY)

CQDs have also been widely employed in phototherapy and therapeutic treatment of superficial cancers like skin cancer [40]. This procedure includes concentrating photosensitizers in tumor tissue and then irradiating it to cause the creation of ROS, which promotes apoptosis (programmed cell death) [41,42]. In cell lines, CQDs functionalized with positively charged molecules can promote the generation of ROS. As a result, CQDs can be used in phototherapy. Another essential treatment for cancers is radiotherapy. In addition, CQDs with PEI functionalization (CQDPEI) were employed as photosensitizers in DU145 and PC3 cells. These CQDs exhibited strong photodynamic effects, generating reactive oxygen species (type I mechanism) and singlet oxygen (type II mechanism). CQDs have also been used in radiotherapy. In-vitro, silver-functionalized PEGcoated CQDs (CQD-PEG-Ag) were employed as a radio sensitizer in Du145 and PC3 cells. These CQDs specifically penetrated cancer cells while avoiding the cytotoxicity seen in traditional radiation. When exposed to X-rays, the CQD created reactive species and destroyed cancer cells, resulting in cellular death. In another study, a CQD–chlorine e6–hyaluronate (CQD-Ce6-HA) compound was shown to be effective for photodynamic treatment of melanoma in mice. CQDs were created by thermally decomposing glycerin and then coupled with Ce6, a nontoxic pho­ tosensitizer with a high singlet oxygen production. For targeted distribution into cancer cells, the CQD-Ce6 combination was further conjugated with hyaluronic acid. Therefore, the produced CQD-Ce6-HA combination caused apoptosis in a mouse model of melanoma skin cancer.

11.5.3 ROLE

OF

CQDS

IN THE

DRUG DELIVERY FIELD

In the pharmaceutical area, the use of QDs as a drug delivery and targeting system is a highly promising notion [43]. Their application in the creation of drug delivery has sparked considerable interest, with QDs at the forefront of this focus. They have been the focus of extensive investigation due to their distinct features [44].

216

Quantum Dots and Polymer Nanocomposites

Regarding this, this chapter incorporates their crucial importance as anticancer medication delivery and targeting agents. In this paper, an overview of recent ef­ forts to use QDs as a drug targeting and delivery system has been primarily focused on cancer treatment. While several other approaches have recently been suggested for drug delivery in cancer therapy, many studies have been performed on the application of QDs in this field. Whereas chemotherapy has traditionally been used to treat cancer and other localized disorders, this strategy is often ineffective, causing toxicity and multidrug resistance issues. As a result, approaches based on tailored drug administration have been investigated as potential solutions for improving treatment efficacy while lowering adverse effects. However, in such alternate ways, targeting agents are also harmed by drug leakage before reaching the target location. As a result, there is a tremendous demand for the creation of effective targeting agents, such as CQDs, that provide several distinct benefits. The creation of multifunctional nanosystems has been a thriving field of study in recent years. CQDs have been used to create dual nanocarrier systems containing bioimaging and medicinal drugs. They have emerged as a viable alternative to semiconductor QDs, which have a variety of drawbacks such as poor solubility, high toxicity, and limited drug loading capacity. In this regard, CQDs are exciting nanoparticles that can do both bio-imaging and drug delivery with little cytotoxicity problems. This was not achievable in the early phases of research since most other nanoparticles were cytotoxic. Javanbakht and Namazi presented a novel carboxymethyl cellulose hydrogel nanocomposite film incorporating graphene QDs and doxorubicin (DOX) (2018). Based on the findings, it was concluded that hydrogel nanocomposite films may be employed as an anticancer drug delivery system, and that this technology could accomplish sustained DOX release. L. Tan et al. (2013) showed the development of S-nitrosothiols (SNO) based on chitosan (CS) and encapsulated silver sulfide quantum dots (Ag2S QDs). CS-SNO compounds with NO-storing functional groups were created via amino-modifying chitosan. With the help of ethylenediaminetetraacetic acid, water-soluble Ag2S QDs were produced and coupled with the CS-SNO compounds (EDTA). Under NIR irradiation, the biocompatible Ag2S-CS-SNO nanospheres with dimensions of ∼117 nm showed strong NIR fluorescence and acceptable photostability. The Ag2SCS-SNO nanospheres could release NO when exposed to UV or visible light at physiological pH and temperature, but not when exposed to NIR light. Cell imaging was successful, indicating that the Ag2S-CS-SNO nanospheres could generate visible NIR fluorescence and release NO in live cells. The NIR fluorescence ima­ ging of the Ag2S-CS-SNO nanospheres did not interfere with their light-triggered NO release, opening up new avenues for the use of multifunctional nanostructured materials in diagnostics and imaging [45]. Guoping Li et al. constructed novel nanosponges encapsulated with doxorubicin (DOX) onto hydrazine-functionalized crosslinked carbon quantum dots (CQDs)/ polyethylene glycol bisacrylate (PEGBA) (C-CQDPEGBA-Hy) nanosponges via an acid-labile imine bond for tumor intracellular real-time imaging and pH-triggered DOX release. The proposed prodrug nanotheranostics with a DOX content of 14.57 percent and a mean hydrodynamic diameter of around 116 nm demonstrated

Biomedical Applications

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excellent pH-triggered drug release performance, with a cumulative release of 52.71% in the simulated tumor intracellular microenvironment in 4 days and a premature drug leakage of 7.09% in the simulated normal physiological medium. Furthermore, the fluorescence could be restored following drug release, indicating a possible use for tumor intracellular real-time imaging [46].

11.5.4 GENE THERAPY Gene therapy, which has received a lot of interest in the technological and medical areas, is focused on fixing the root cause of illnesses by providing and expressing exogenous DNA coding for the missing or faulty gene product. As a result, one of the most important aspects of gene therapy is the use of proper gene vectors. For gene delivery, many kinds of nanoparticles and QDs have been employed. Because of their biocompatibility, low toxicity, strong fluorescence emission, broad excitation spectra, and stable PL, CQDs might possibly be em­ ployed as a platform for gene transport. It is demonstrated that CQDs have a superior ability to condense plasmid DNA with high transfection efficiency. Both caveolae- and clathrin-mediated endocytosis routes can be used to transport CQDs/ pDNA complexes into cells. Positively charged CQDs might create a compound with negatively charged siRNAs. The folate-conjugated reducible polyethyleneimine passivated CQDs have been reported to generate a siRNA carrier that releases siRNA in a reducing environment. CQDs have been employed as a gene vector for fibroblast chondrogenesis. The plasmid SOX9 might be condensed to generate nanoparticles in the 10–30 nm range using CQDs. The nanoparticles that were created exhibit out­ standing qualities such as high solubility, minimal cytotoxicity, and fluorescence emission.

11.5.5 BIOSENSING

AND IMMUNOSENSORS

CQDs were also employed in biosensing based on the employment of antibodies and their gene-recombinant fragments. CQDs are primarily used as fluorescent labels in immunoassays in this system. This was demonstrated in an example by PosthumaTrumpie et al., who focused on the application of CQDs in lateral flow and microarray immunoassays. CQDs are less expensive, more stable, and more sensitive than other regularly used fluorescent markers; hence, they were chosen for this investigation. CQDs were proven to be more sensitive as labels in lateral flow assays (LFAs) than gold or latex nanoparticles. It was reported that CQDs had sensitivity in the picomolar range. In the case of nucleic acid LFA (NALFA) (Figure 11.4), their respective an­ tibodies recognize the discriminating tags on the amplicons, and fluorescent signals are given by the attached CQDs. Because of their enormous surface area, QDs may be conjugated to therapeutic agents (e.g., anticancer) and numerous diagnostic agents (e.g., optical, magnetic, radioisotopic). CdTe quantum dot has been shown in several studies to have anticancer properties. QDs can also be used as (bio)sensors to assess the efficacy of anticancer medicines. Muthusankar et al. (2020) described an electrochemical sensor based on the N-CQD@Co3O4/MWCNTs hybrid nanocomposite modified

218

Quantum Dots and Polymer Nanocomposites

FIGURE 11.4 NALFIA is depicted schematically. Printed with permission from [ 47].

glassy carbon electrode (GCE) for the simultaneous measurement of the antibiotic nitrofurantoin (NF) and the anticancer agent flutamide (FLU). The suggested sensor performs well in the simultaneous measurement of NF and FLU in the linear ranges of 0.05–1,220 μM for NF and 0.05–590 μM for FLU, with detection limits of 0.0169 μM and 0.044 μM, respectively [48]. Immunosensors are affinity solid-state based biosensors created for the detection of disease biomarkers that rely on the creation of a stable immunocomplex as a result of the affinity between an antigen and matching antibody. Immunosensors are a valuable method for clinical diagnostics because they provide a selective and sensitive response. Huang et al. created an amperometric label-free immunosensor for measuring afetoprotein (AFP). This immunosensor was created by dumping a 6 mL slurry of chitosane TiO2- graphene (Chit-TiO2-Gr) over a glassy carbon electrode (GCE). Chit-TiO2-Gr/GCE was submerged in Au NPs solution for 10 hours after drying the electrode surface to yield Au NPs/Chit-TiO2-Gr/GCE. Because of the negatively charged Au NPs, the AFP antibody was immobilized via an adsorption method. For AFP, a broad detection range (0.1–300 ng mL−1) was found. At a S/N ratio of 3, the limit of detection (LOD) was 1.3 ng mL−1. Carcinoembryonic antigen (CEA), an oncofetal glycoprotein, is a malignant tumor biomarker used to diagnose malignancies such as pancreatic, lung, colorectal, liver, gastric, and notably breast cancer. The molecular weight of CEA is 180–200 kDa.

Biomedical Applications

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High levels of CEA in human blood (>5 ng mL−1) are symptomatic of cancer cell development. Some researchers have reported determining CEA using various types of immunosensors. Ganganboina et al., for example, used a label-free impedimetric immunosensor based on nitrogen- and thiol-doped GQDs (N,S-GQDs) and goldembedded polyaniline (Au-PANI) nanowires to detect CEA in human serum samples. N,S-GQDs/AuPANI nanowires have high electroconductivity, which speeds up electron transport, and anti-CEA is immobilized on N,S-GQDs as a bifunctional probe that works as an amplifier of electrochemical activity. After the coupling of antibody-antigen, the generated label-free immunoassay platform’s impedance changes due to an increase in charge transfer resistance, resulting in the measurement signal. In a linear range of 0.5 to 1,000 ng mL−1, this impedimetric immunosensor has a detection limit of 0.01 ng mL−1. Yang et al. developed a carcinoembryonic antigen detection ultrasensitive electrochemical immunosensor based on nitrogen-doped GQDs (N-GQDs) sup­ ported by PtPd bimetallic nanoparticles (CEA). The immunosensor has a high electrocatalytic activity for hydrogen peroxide (H2O2) reduction and a broad linear range for CEA detection ranging from 5 fg mL−1 to 50 ng mL−1. 2 fg mL−1 is the detection limit (S/N = 3). Nie and colleagues presented another another GQDs-based immunosensor for the detection of CEA. Poly(5-formylindole)/electrochemically reduced graphene oxide nanocomposite (P5FIn/erGO) was employed as an effective substrate to encapsulate the main antibody (Ab1), and Au NP adorned graphene quantum dots (GQDs@Au NP) was used for signal amplification. A P5FIn/erGO nanocomposite has a greater surface area, which allows for increased Ab1 loading and facilitates ion transport during redox processes. GQDs@Au NP combined with secondary antibody (Ab1) as a label, on the other hand, boosts electron transfer efficacy to obtain sustained ECL intensity. Based on a multiple signal amplification method, such a sandwich-type ECL immunosensor has a wide dynamic linear range of 0.1 pg mL−1 to 10 ng mL−1 and a low detection limit of 3.78 fg mL−1. The new immunosensor, which has a high selectivity as well as outstanding stability and repeatability, enables the detection of CEA in human serum with acceptable re­ covery findings. Tables 11.2 and 11.3 illustrate several uses of QDs as biosensors and immunosensors in this section. Xiwen Feng et al. created innovative xylan-based carbon quantum dots (CQDs) and utilized them as a green in situ reducing agent to create CQDs capped gold nanoparticles (Au@CQDs). MXene with high electrical conductivity was employed as the immobilized matrix in the fabrication of Au@CQDs-MXene nanocomposites with high electrical conductivity and electrocatalysis. Loading the Au@CQDsMXene on a glassy carbon electrode resulted in an electrochemical sensor for nitrite monitoring. Due to the strong catalytic activity of AuNPs and CQDs, the huge specific surface area of MXene, and the outstanding electrical conductivity of AuNPs and MXene, the sensor has high sensitivity, good stability, a wide linear range, and great selectivity. The sensor’s linear detection range ranged from 1 M to 3,200 M under ideal conditions, with a detection limit of 0.078 M (S/N = 3), which was superior to most reported sensors employing the differential pulse voltammetry (DPV) approach [54].

Cholesterol

H2O 2

Graphene quantum dots

CoFe2O4@CdSeQD Juice and milk

52 μM (DPV) -70.4 μM (SWV)

0.38 μM

35 nmol.L−1

0.1 mM

Chlordiazepoxide HeLa cells extracts (1%) samples Human serum samples

84 μM (DPV) -46 μM (SWV) 54 μM (DPV) -56 μM (SWV)

Glutathione

54 μM (DPV) -56 μM (SWV)

7.55×10 M 56 μM (DPV) -73 μM (SWV)

−8

Detection Limit

Diazepam

Serum Serum

Matrix

Clonazepam Oxazepam

Clopidogrel Alprazolam

Analyte

CdSe@ZnS

CdSeQDs CS-Ag/N-GQD-Au electrode

QD material

Amperometric response

Fluorescence lateral flow Chemiluminescence

AdSDPV DPV, SWV

Measuring Signal

TABLE 11.2 Analytical Investigations with Core-Shell Quantum Dots and Their Applications of Various Types of QDs in Biosensing Platforms Were Chosen and Primarily Employed [ 43]

[ 53]

[ 52]

[ 51]

[ 49] [ 50]

Ref

220 Quantum Dots and Polymer Nanocomposites

Biomedical Applications

221

11.5.6 BONE TISSUE ENGINNERING Another critical area of biomedicine is bone tissue engineering, which is used to repair bone abnormalities. The primary components of bone tissues are hydroxyapatite and collagen. A scaffold is required for structural integrity in order to regenerate bone tissue, which can offer cell adhesion, proliferation, and differentiation. After a proper length of time for healing has elapsed, the scaffold should resorb into harmless components. There are a variety of polymers that may be used for this purpose, but they often lack bioactivity, which aids in the formation of new bone tissue. CDs were shown to aid in the growth of osteoblastic cells on a waterborne biodegradable hy­ perbranched polyurethane scaffold [55]. The addition of CDs in situ during thermoset polymerization improved mechanical performance in terms of strength, elongation at break, and toughness, which could also be improved by increasing the CDs’ con­ centration from 0.5 to 1.0 and 1.5 wt %. Carbon nanofillers have indeed been proven to be bioactive and to aid in the de­ velopment of hydroxyapatite crystals, for example, on polymer substrates. CDs are promising for this purpose because of their nanosize and high oxygen capabilities, which are thought to attract calcium ions and promote the formation of hydroxyapatite on the surface. It was discovered that adding CDs to a PCL matrix generated bioactivity for biomineralization [56]. CDs were reduced in an efficient microwave process with caffeic acid as a reducing agent to improve the mechanical performance of the PCL matrix [57]. At the same time, the PCL surface’s potential to induce biomineralization was preserved. The cell viability of MG63 osteoblast-like cells on the decreased CD/ PCL scaffold was also comparable to that of clean PCL scaffold, according to the study. Similarly, with the help of covalently linked CDs, biopolymers such as starch were used as scaffold materials for biomineralization [58]. CDs have also been proven to increase starch fiber electrospinnability [59], when the electrospun starch/CD fibers were de­ graded concurrently. Their combination acted as an artificial interwoven fiber network, mimicking the structure and function of extracellular matrix. Their combination acted as an artificial interwoven fiber network, mimicking the structure and function of ex­ tracellular matrix [60]. The strength and stiffness of the fibers were enhanced compared to the similar neat fibers even at modest CD loadings (0.5–2.5 percent). Electrospinning, according to Mina Ghorghi et al., is a viable and accessible approach for producing scaffolds with desirable physical, chemical, and biological characteristics for tissue engineering. Captopril (CP)-loaded polycaprolactone (PCL)/carbon quantum dots (CQDs) nanocomposite scaffolds for bone tissue re­ generation were created in this work. Scanning electron microscopy and wettability tests were used to evaluate the microstructure and hydrophilicity/hydrophobicity ratio of scaffolds. The inclusion of CQDs and CP in the scaffolds lowered the fiber diameter (1,180 281.5–345 110 nm) while increasing the surface hydrophilicity (137°–0°) of the scaffolds. Attenuated total reflectance-Fourier transform infrared spectroscopy was used to assess the functional groups of the scaffolds [61].

11.5.7 USE

IN THE

ENVIRONMENT

Rapid population increase and worldwide industrialisation have resulted in sig­ nificant water contamination, which has a negative impact on the ecosystem and

222

Quantum Dots and Polymer Nanocomposites

affects human health. CDs have demonstrated considerable potential in the detec­ tion and remediation of contaminants because of their distinctive fluorescence features, low toxicity, strong biocompatibility, and abundance of functional groups. Due to the unique benefits of composite materials, such as greater stability, mo­ bility, recyclability, and wide functionality, there has been an increasing amount of research in this field focused on the usage of CD/polymer nanocomposites rather than CDs alone. The environmental applications of CD/polymer nanocomposites will be divided into two categories: fluorescent probes for contamination detection and adsorbents for contaminant removal.

11.6 CONCLUSIONS AND PROSPECTS FOR THE FUTURE QDs are luminous semiconductor nanocrystals with particle sizes ranging from 2–100 nm. Because of their distinct optical properties, they have gained substantial attention in a variety of scientific domains in recent years. QDs offer several unique features and benefits over typical organic dyes. In this chapter, we described current developments in the synthesis and tailoring of GQD characteristics utilizing various techniques, with an emphasis on their biological applications. We addressed topdown and bottom-up methodologies for GQD synthesis and concluded that each strategy had advantages and limitations. We anticipate the appearance of cheaper, simpler, and more inventive synthesis methods, as well as exciting new applications in the future to better utilize the promise of these increasingly significant carbon materials. In general, CQDs, rather than semiconductor QDs, are potential prospects for biological applications.

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12

Quantum Dot–Polymer Composites as Sensors Pradip Kumar Sukul Amity Institute of Applied Sciences, Amity University, Kolkata, India

Monalisa Mukherjee Amity Institute of Click Chemistry Research and Studies, Amity University, Noida, Uttar Pradesh, India

Chirantan Kar Amity Institute of Applied Sciences, Amity University, Kolkata, India

CONTENTS 12.1

12.2

12.3

Carbon Dot/Polymer Composite-Based Sensors .......................................228 12.1.1 Optical Properties of Carbon Dots/Polymer Composites ........... 229 12.1.2 Sensing Application of Carbon Dots/Polymer Composites........233 12.1.3 Chemical Sensors ......................................................................... 234 12.1.4 Biological Sensors........................................................................ 235 12.1.5 Physical Sensors ........................................................................... 237 Graphene Quantum Dot/Polymer Composite-Based Sensors ...................237 12.2.1 Heavy Metal Ion Sensing Using Graphene Quantum Dot/Polymer Composite-Based Sensors ......................................238 12.2.2 Sensing Disease Biomarkers Using Graphene Quantum Dot/Polymer Composite-Based Sensors ......................................239 12.2.3 Sensing Drugs and Contaminants Using Graphene Quantum Dot/Polymer Composite-Based Sensors ......................................240 Perovskite Quantum Dot/Polymer Composite-Based Sensors..................241 12.3.1 Sensing of Organic Dye Using Perovskite Quantum Dot/Polymer Composite-Based Sensors ......................................243 12.3.2 Sensing of Organophosphorous Pesticide Using Perovskite Quantum Dot/Polymer Composite-Based Sensors...................... 243 12.3.3 Detection of UV Radiation Using Perovskite Quantum Dot/Polymer Composites ............................................................. 245 12.3.4 Sensing of Chloride/Iodide Ion Using Perovskite Quantum Dot/Polymer Composite-Based Sensors ......................................246 12.3.5 Biomolecule Sensing Using Perovskite Quantum Dot/Polymer Composite-Based Sensors ......................................248

DOI: 10.1201/9781003266518-12

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228

Quantum Dots and Polymer Nanocomposites

12.3.6 Development of pH Sensor Using Perovskite Quantum Dot/Polymer Composites ............................................................. 248 12.4 Summary and Future Perspectives of Quantum Dot/Polymer Composites as Sensors .........................................................250 Acknowledgments.................................................................................................. 251 Declaration.............................................................................................................251 References.............................................................................................................. 252

12.1 CARBON DOT/POLYMER COMPOSITE-BASED SENSORS Xu et al.. first discovered fluorescent carbon quantum dots in 2004 by the purification of single-walled carbon nanotubes [1]. Later, in 2006, Sun and co-workers also obtained similar fluorescent particles, both as a dispersion in solvent and in solid form and named them carbon dots [2]. After that, many studies were performed to understand the properties of these particles and many new names are proposed for these new nanosized materials, e.g. carbon quantum dots (CQDs) [3], graphene quantum dots (GQDs) [4], carbon dots (CDs), carbon nanodots (CNDs) [5], and polymer dots (PDs) [6]. Recently a new kind of material has been developed which is a hybrid of carbon dots with polymer/ carbon hybrid structure, also known as carbonized polymer dots [7]. Carbon dots are interesting because of their zero-dimensional state which is the reason behind their remarkable chemical, physical, and optical properties [8]. They have a carbon skeleton of less than 20 nm size with surface functionalities (groups like carboxylic acids, hy­ droxyls, epoxy, and carbonyls), the covalent carbon skeleton of the core gives them the exceptional stability and the surface functionalities provide them the optimum water solubility and an enhanced fluorescence property [7,9]. Due to high biocompatibility, carbon dots are less harmful compared to semiconductor quantum dots [10]. Because of easy and cheap synthetic procedures, less toxicity, and outstanding physical and optical properties, carbon dots are used in many chemical sensors [11], biological sensors [12], optoelectronic devices [13], bioimaging agents [14], and light-emitting devices [15]. Recently, a new material has been developed by the combination of carbon dots with different functional materials; this combination has further reduced the limitations of traditional carbon dots and increased their diversity and this new material is known as composite material. Carbon dots can be combined with various other material: inorganic nano-structures [16,17], biomaterials [18], as well as polymers [19]. Although polymers can be combined with other nanomaterials like nanotubes, nanofibers, and graphene, a composite between carbon dots and polymer is preferable because of its tremendous advantages with very few shortcomings. Carbon dot polymer composites have been made by combining carbon dots with polymeric gel, polymer matrices, and molecularly imprinted polymers [20]. The small size and the high number of functionalities on the carbon dots play a vital role in increasing the interaction with the polymer material, the new composite material formed due to this interaction is found to have excellent characteristics like better optical quality in terms of absorptivity and photobleaching resistance, less toxicity, and enhanced biocompatibility [21]. Due to the abovementioned advantages, a carbon dot/polymer composite is applied in numerous fields such as UV shields development [22], designing of optoelectronic devices [13], and solar cells [23], in supercapacitors [24], solid-state optical films [25], and sensors [26].

Quantum Dots–Polymer Composites as Sensor

12.1.1 OPTICAL PROPERTIES

OF

229

CARBON DOTS/POLYMER COMPOSITES

As already mentioned, carbon dots have an interesting optical property, although most of them doesn’t have any significant absorption in the visible region but displays a broad range of fluorescence emission. They emit light of various wavelengths and show multicolor emission when excited with an appropriate frequency. Their emission range starts from blue light in the visual region and extends up to the near-infrared region [27]. It has been shown by Hu et al.. that fluorescence properties of carbon dots can be kept intact after the composite formation with the polymer [28]. As shown in Figure 12.1a, the carbon dot/poly vinyl acetate has a quench-resistant fluorescence behavior. It has an optimum

FIGURE 12.1 (a) Images of different carbon dots and polyvinyl acetate composite under white light and UV light. (b) Application of the composite material as fluorescence ink. (c) Images of carbon dots and polyvinyl acetate composite films under white light. (d) Images of carbon dots and polyvinyl acetate composite films under UV light. Reprinted with per­ mission from [ 28]. Copyright (2019) The Royal Society of Chemistry.

230

Quantum Dots and Polymer Nanocomposites

viscosity and can produce bright solid state fluorescence at ambient temperature (Figure 12.1b), therefore can be applied in making fluorescence ink of different pattern. This carbon dots/polymer composite can be converted into films (Figure 12.1c) and have significantly high quantum yield and emits blue, green, yellow and orange emission when excited under UV light (Figure 12.1d). It was also shown in the same report that the excitation and emission spectra of the composite is the same as that of the carbon dots; this indicates that com­ posite formation with polymers does not hinder the optical properties of the carbon dots. In another work by Wang et al., it has been shown that carbon dots/polymer composites can also increase the stability of the fluorescence emission as the carbon dots remain protected under the polymer shell. According to the study, their magnetic covalent organic frameworks/polymer /carbon dot composite, showed excellent stability when checked for several days under intense light [29]. The composite was utilized for detecting picric acid and shows significant fluorescence enhancement in the presence of the analyte (Figure 12.2a). There are a few carbon dots/polymer composites that show exciting UV–vis absorption behavior. For example, Zhao et al. [30] synthesized a carbon dot/poly­ vinyl acetate film that exhibited a broad absorption band over the visible region (Figure 12.2b). In similarity to the previous work, the polymer/carbon dot com­ posite reported by Bai et al. showed broad absorbance bands in the UV region; the absorption characteristics of this composite are identical to the carbon dots from which it is synthesized [31]. The carbon dots are implanted on the composite without changing their intrinsic optical properties and the composite is providing the protection to fluorescence properties of the carbon dots. Peak intensities of the composite increases linearly with the increasing concentration of the carbon dots. In Table 12.1, the optical properties of various carbon dots/polymer composites are presented. Although the quantum yields of carbon dots/polymer composites are

FIGURE 12.2 (a) Increase in fluorescence intensity of carbon dot/polymer composite in the presence of picric acid. (b) The broad absorption band of carbon dot/polyvinyl acetate composite. Reprinted with permission from [ 29, 30]. Copyright (2019) Elsevier and Copyright (2020) Springer Nature.

Quantum Dots–Polymer Composites as Sensor

231

TABLE 12.1 Optical Properties of Various Carbon Dots/Polymer Composites [ 32] Composites

λex (nm)

λem (nm)

Quantum Yield (%)

Absorbance (nm)

Fluorescence Color

CD/PVA film PVA/CD film

532 420

585 530

N/A N/A

489 ~220, ~290

N/A Green

CDs@MIPs

370

470

51.8

370

Blue

CS/PVA/CDs PVA/CDs

360 360

436 ~470

N/A 47

360 294/340

Cyan Blue

TPU/CDs

400

470

68

335, 399

Blue

CD-polymer CDs/Fe3O4@MIPs

455 370

550 470

14.86 N/A

200–500 N/A

Yellow Blue Green

CDS@Cu/Alg

400

513

N/A

N/A

CQDs@MIPs C-dots/PVB film

360 400

453 550

N/A N/A

~260 353, 410, 500

N/A Green-blue, orange-red

MIPs-GSCDs

340

410

18.6

~275

CD-polymer CDs@SiO2@MIPs

N/A 380

470 >450

50 N/A

250, 300 288

N/A

WCDs@PS

380

~590

10.7, 15.2

N/A

CDs@MIPs C-dot/PEI gel

360 470

450 ~565

N/A 1.9–4

250–300 290, 340, 380

CDs@PVA

365

420–440

N/A

~350

Blue

C-dots/PVA HMIP@CDs

360 390

459 503

8.64 N/A

282, 341 ~300

Green N/A

Green N/A Orange and blue Blue Cyan

BMIP@CDs

425

520

N/A

N/A

N/A

CD-MIPGlcA Poly(VPBAAAm)-CDs

445 900

~500 515

0.97 N/A

~350 241

Blue Blue

N/A

314, 316, 318

44

286, 355

PAN/CQD nanofibers PVA-N@C-dots

350, 477, 530 560, 598, 660 390

~460

Red, green, blue Cyan

rarely reported inliterature, generally the values are quite significant. As the optical properties of the composite is completely dependent on the optical property of the carbon dots, therefore it is based on the fluorescence emission wavelength of the constituent carbon dots; the composite material exhibits vivid colors like green, blue, cyan, red, yellow, and orange. Additionally, the optical properties of the carbon dots/polymer composite is also influenced by the physiochemical properties of the carbon dots. Technically, the size of a carbon dot can be anything from 1 to 100 nm but generally the average size of these carbon dots are 10 nm, with a few example of larger par­ ticles of 60 nm size [33]. Better-quality quantum dots are created when the

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quantum dot size is lesser than their Bohr excitation radius, this phenomenon is also known as quantum confinement effect [34,35]. Due to the quantum con­ finement effect, the band gap of this particles depends on their size, resulting in several size-dependent optical properties [34]. Apart from the size of carbon dots, various functionalities present in it may also modify the state of emission. For instance, functionalities like C=C, C=O, C-O, and O-C=O bonds significantly contribute to the UV-absorption characteristics with a prominent role in defining the peak positions of the material [36]. On the other hand, the fluorescence property is dependent on the functional group and the rigidity of the carbon core structure. It has been observed that carbon dots with a carboxyl or amide group display green fluorescence; carbon dots with hydroxyl groups show blue fluorescence and replacement of hydroxide with amine groups induce a redshift in fluorescence [37]. As the fluorescence properties of carbon dots/polymer composite are similar to the constituent carbon dot, therefore the fluorescence characteristics of the composites are also affected by the different size of carbon dots and the emissive traps on carbon dot surface. It has often been observed that after the separation, carbon dots with identical functionality show different quantum yields, this is because of various degree of surface functionalization. If the degree of functionalization is less, the quantum yield tends to be lesser [38,39], due to the same reason if the degree of functionalization is increased quantum yield will be higher [27]. Carbon dots also show concentration-dependent fluorescence (intensity and emission maxima); at a high concentration, the polar functional group interact with each other, leading to aggregation. In such cases, fluorescence emission depends on the degree of aggregation [40]. The production yield and the quantum yield of the composite material depend on the composition of the mixture as well as the ratio of carbon dots and polymer molecules in the mixture. Fernandes and coworkers have reported that when ethanolamine carbon dots are prepared with polyethylene glycol, the yield of the PEG/carbon dots composite was 28%, whereas in a similar condition when polyethylene is taken as the polymer, the yield of the composite is 20% [41]. Interestingly, without the addition of polymers, the yield of carbon dots from ethanolamine is 9% under similar conditions. The reason behind this is the decreased evaporation rate of the volatile ethanolamine when dispersed in a polymeric matrix. The decrease in volatilization also improves the quantum yield of the carbon dots. Wang et al. has performed an experiment to find the effect of mass ratio on fluorescence properties of composites. They have varied the feed ratio of polystyrene nanospere/carbon dots from 1:1 to 2:1 and found a decrease in overall fluorescence intensity of the composite. The surplus polystyrene nanosphere present is a cloudy haze and led to the quenching of the fluorescence from the carbon dots [42]. On the other hand, if the amount of polystyrene nanosphere is decreased, the overall yield of the composite material will decrease, keeping the fluorescence intensity constant. Hence, from the abovementioned facts it can be said that the fluorescence quantum yield and the pro­ duction yield depends on various factors. A schematic representation of the structural nature of the polymer/carbon dot composite along with the factor that can affect various properties of the composite is shown in Figure 12.3.

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FIGURE 12.3 A schematic representation of the structural nature of the polymer/carbon dot composite along with the factor that can affect various properties of the composite. Reprinted with permission from [ 32]. Copyright (2021) MDPI.

Noteworthy, the hydrogen-bonding interaction between the functional groups of the carbon dots and the pendant polar groups present in the polymer molecules (shown in Figure 12.3) helps to regulate the homogenous concentration of the carbon dots in the polymer matrix. From the above details, you can understand that the application of a carbon dot/ polymer composite is a robust area of study and discussing all the aspects of this material is beyond the scope of this chapter; here, we will only focus on specific sensing applications of this composite material. We have divided the sensing ap­ plication part into three categories, depending on the nature of the sensing viz chemical, biological, and physical sensors with a brief discussion about some major challenges and future perspectives for carbon dots/polymer composites.

12.1.2 SENSING APPLICATION

OF

CARBON DOTS/POLYMER COMPOSITES

Carbon dots/polymer composites are applied for sensing studies in various im­ portant fields. They are used as chemosensing probes for explosive nitro compounds like picric acid, para nitro phenol, and 2, 4 dinitrotoluene; metal ions like Cu (II), Ag(I), and Fe(III). They are also used as biosensors for detecting epidermal growth factor receptor, thiabendazole, alkaline phosphatase, tetracycline, caffeic acid, and glucose. Apart from chemical entities, sensors for physical factors like temperature and humidity sensors are also developed using carbon dot/polymer composites.

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Citric acid from various sources like mango, cedrus, and rosemary have been used as a precursor for synthesizing most of these carbon dots; along with that, other precursors like glucose, cetylpyridinium chloride, o-phenylenediamine, and am­ monium citrate have also been used. Well-established procedures for carbon dot formation like hydrothermal, solvothermal, microwave, and ultrasound techniques are used to convert the precursors to desired carbon dots. For constructing the composite with carbon dots, various types of polymers are chosen, e.g., chitosan, methyl acrylate, alginate, styrene, dopamine, polystyrene, polyethylene glycol, polythene, etc. Construction techniques of the polymer carbon dot composites vary with the precursors and some of the most popular techniques are bulk poly­ merization, free radical dispersion polymerization, reverse microemulsion poly­ merization, drop casting, sol–gel, in situ polymerization, cross-linking, and polymer-assisted self-assembly. Ultimately, these composite materials are used as sensing probes for targeting various chemical, biological, and physical entities.

12.1.3 CHEMICAL SENSORS Among various chemical entities, compounds that can directly cause harm to the environment or to human health are now of major concern. Scientists are trying to develop sensors that can detect this pollutant at the lowest level possible, so that they can be traced even before showing their detrimental effects. 2,4,6-trinitrophenol or picric acid is such a kind of environmental pollutant; it is mostly used as an explosive, a chemical intermediate in industries, and as a fun­ gicide in agriculture. The problem with picric acid is that it is highly soluble in water, it has low biodegradability, and high toxicity [43]. Even when present in small quantities it can be the reason of a severe health hazard by causing allergies, eye and skin irritation, and dizziness [44]. There are many methods to detect picric acid that have been developed recently, based on liquid chromatography, electro­ chemistry, mass spectrometry, fluorescence, etc. Among these different approaches, the fluorescence method is the most advanced and accessible one, as it can detect the pollutant with better sensitivity while consuming less sample processing time. For complete exploration of this method, several fluorescence carbon dot sensors are also developed that can detect picric acid from a water sample but the major problem is the interference similar molecules such as 2, 4-dinitrophenol. Additionally, for detection of environmentally hazardous species, the sensor’s limit of detection (LOD) must be high but in the case of carbon dot-based sensors, LODs are not very satisfying. Hence, Wang and co-workers have developed a carbon dot/ polymer composite, where they have used carbon quantum dots and iron oxide nanoparticles as a co-nucleus and grafted molecularly imprinted polymer (with picric acid as the template) over it [45]. They have found that the composite ma­ terial not only detects picric acid efficiently but can also be used for the removal of the substance from an aqueous solution. The limit of detection for picric acid is found to be 0.5 nM. The magnetic (due to the presence of iron oxide) composite material is successfully utilized for the removal of picric acid from natural river water and tap water samples (spiked). The equilibrium for the adsorption process is reached within 1 min, indicating a rapid response time.

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Apart from compounds, metal ions like Pb2+, Hg2+, Cr3+, Cr6+, and Cu2+ can also be quite hazardous to the environment. Particularly Cu2+ is linked with many neurological diseases like depression, anxiety, and even Alzheimer’s disease. The sources for copper could be industrial waste, mining waste, and petrochemical waste. Copper (II) salts are highly water soluble and can contaminate ground water and river water sources easily. Therefore, lots of chemosensing probes have been developed for the detection of copper (II) ions in aqueous sources, including expensive and sophisticated techniques like transistor-based devices, mass spec­ trometry, and atomic absorption spectroscopy. Fluorescence methods are also ex­ tensively used to detect Cu(II) metal ions because of the swiftness, cost effectivity, and safety of the detection process. Several quantum dot-based fluorescence sensors are also reported in recent literature, but the major disadvantages of these quantum dots are water solubility and stability. Lack of water solubility is a major drawback and a point of concern especially for water-soluble analytes. Moreover, majority of the quantum dots are toxic and therefore difficult to handle by an untrained person. Formation of a carbon dot/polymer composite can overcome the above-mentioned problems. Yu et al. recently developed a pair of yellow-emitting carbon dots/ polymer film composites that can enable naked eye detection of Cu2+ ions [46]. They have infused the carbon dots in two different polymer matrices impregnated with (a) carboxy methyl cellulose-CMC/poly-vinyl-alcohol and (b) chitosan. The CMC polymer increases the hydrophilicity; hence, it helps the sensor to reach the Cu2+ ions easily and enhance the response. It has been found that the chitosan/ carbon dot composite is more sensitive because of the chelation mechanism of chitosan with the copper (II) ions. The detection limit for the chitosan/carbon dot composite was found to be 10 nM, far below the U.S. EPA limit for Cu2+ ions in drinking water.

12.1.4 BIOLOGICAL SENSORS Biological sensors are generally made for detecting biologically significant spe­ cies present inside a living system. Caffeic acid is a phenolic acid and a precursor for the biosynthesis of lignin in plants, due to the presence of two hydroxyl groups and it also behaves as an antioxidant. It has many pharmacological functions like anti-inflammatory, anti-tumor, and immunomodulatory. Caffeic acid is also helpful in curing diseases like respiratory tract infection and obesity problems. Therefore, caffeic acid is now clinically used in healthcare facilities. It has also been used for the treatment of leukopenia, thrombocytopenia, and he­ mostasis. However, some recent research shows that overuse of caffeic acid has some carcinogenic effects. Due to its broad biological and medicinal significance, several analytical techniques like GC-MS, high-performance liquid chromato­ graphy, electrochemical techniques, etc. have been developed to detect caffeic acid. But, most of these methods are complex and expensive. Recently, Xu et al. have reported a sensing probe by combining molecularly imprinted polymers with photoluminescence carbon dots. The carbon dots they used for the purpose were silane-functionalized and were synthesized from citric acid and aminosilane (as coordinating solvent). The composite of these carbon dots with the polymer

236

Quantum Dots and Polymer Nanocomposites

was prepared via a one-pot sol-gel process using caffeic acid as templates. The so-formed composite material is highly selective towards caffeic acid, which shows a fluorescence quenching of the carbon dots upon caffeic acid binding. The limit of detection of the sensor towards caffeic acid is 0.11 μM. This method was successfully applied to detect caffeic acid in blood plasma and interestingly it doesn’t show any interference in the presence of metal ions present in the medium [47]. Due to the high mortality rate of cancer, early cancer diagnosis is a major area of research now. One of the major strategies for cancer diagnosis is the fluor­ escence imaging of membrane receptors overexpressed in cancer tissues. Epidermal growth factor is one such membrane receptor that is overexpressed in many epithelial tumors. To date, several nanoparticles are used for targeting epidermal growth factor but designing a probe for exclusively targeting epidermal growth factor is difficult and costly. A much more economical method is the use of molecularly imprinted polymers that are more specific towards the target protein. Designing a molecularly imprinted polymer by using proteins as the template remains a challenge. Fluorescent molecularly imprinted polymers are nowadays developed for bio-imaging purposes. In fluorescent molecularly em­ bedded polymers, fluorescent quantum dots are imprinted in the polymeric matrix and in the designing of such composites, carbon dots are preferred as they have low toxicity and wide emission bandwidth. Zhang and co-workers have reported a red emitting carbon dot/molecularly imprinted polymer composite that is uti­ lized for in-vivo imaging of epitopes of epidermal growth factors. The longwavelength emission makes the fluorescent imaging better and helps to distin­ guish between cells that are expressing different levels of epidermal growth factors. The sensing procedure is dependent on fluorescence quenching. Hence, the fluorescence intensity decreased with an increased concentration of epidermal growth factors. The limit of detection for the fluorescence sensor is found to be 0.73 μg and it is considered a significant sensor for in-vivo detection (Figure 12.4) of epidermal growth factors [48]. Using the same templating technique, Jalili and

FIGURE 12.4 In-vivo fluorescence imaging of red emitting carbon dot/molecularly im­ printed polymer composite taken at a different time after injection. Reprinted with permission from [ 48]. Copyright (2020) Springer nature.

Quantum Dots–Polymer Composites as Sensor

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co-workers developed another carbon dot/molecularly imprinted polymer com­ posite for detecting 3-nitrotyrosine. 3-nitrotyrosine is a biomarker of a wide range of diseases, such as osteoarthritis, rheumatoid arthritis, atherosclerosis, cardio­ vascular diseases, Alzheimer’s, diabetes, neurological diseases, etc. The se­ lectivity of the material towards other competing species like glucose, sucrose, lactose, ascorbic acid, uric acid, glycine, creatinine, L-cysteine, and different metal ions were examined, and it was found that the fluorescence quenching of the material is observed only in the presence of the 3-nitrotyrosine. This com­ posite material was also successfully tested for detecting 3-nitrotyrosine in human serum [49].

12.1.5 PHYSICAL SENSORS Thermochromism is a property of matter where the chromogenic matter shows temperature-dependent change in color. Thermochromic devices are used in many temperature-sensitive equipment e.g., baby bottles, reaction vessels, heat ex­ changers, and reactors. Recently, Wang et al. [42] combined blue- and orangeemitting carbon dots assisted by a polystyrene nanosphere (through self-assembly technique) and reported the first solid white light-emitting phosphor. Polystyrene nanosphere are well known for their chemical sturdiness and transparency, which are important characteristics for any thermochromic device. Apart from this, the polystyrene nanospheres also act as blockers for the Forster resonance energy transfer process and the aggregation induced quenching of the multicolor carbon dots. The strong solid-state fluorescence of the composite is maintained by controlling the size of the polystyrene nanospheres and the feed ratio of the polystyrene nanosphere/carbon dots. The temperature-dependent fluorescence response of the material has a remarkably wide working range from 20°C to 80°C. The hue of color can be maintained from white to blue by regulating the environmental temperature within the above range. Along with good linear cor­ relation with the temperature, the repeatability of the material is also checked by repeating the process multiple times. According to the scientists, the material behaves as a temperature-sensitive chameleon that shows different colors at different temperatures.

12.2 GRAPHENE QUANTUM DOT/POLYMER COMPOSITE-BASED SENSORS Graphene was first discovered in 2004 by Novoselov et al. [50], and it has extra­ ordinary thermal, mechanical, electrical, and optical properties [51]. Similar to carbon quantum dots, graphene can also be prepared using versatile synthetic methods, such as graphene oxide reduction [52], chemical vapor deposition (CVD) [53], micromechanical stripping [50], and SiC epitaxial growth. Due to the planar conjugate structure of graphene, the π electrons are highly delocalized, which significantly decrease the band gap, making it a perfect candidate for application in the field of optoelectronic devices and semiconductors. Advanced structural studies show that a graphene quantum dot can have oxygen-containing functional groups

238

Quantum Dots and Polymer Nanocomposites

such as carbonyl, hydroxyl, and epoxy groups. Nowadays the research work on graphene quantum dots is mainly focused on surface modification by the in­ troduction of hetero atoms (hetero atom doping), changing the degree of oxidation and functionalization. These modifications enhance the properties of the material and make them more suitable for applications. Graphene quantum dots have many advantageous properties compared to other traditional semiconducting quantum dots, such as lower toxicity with better water solubility and stable fluorescence properties. The fluorescence property is dependent on the size of the graphene quantum dot, surface functionality, and on the doped hetero atom. Although there is no well-established photoluminescence mechanism for graphene, it has been as­ sumed that the photoluminescence mechanism of graphene quantum dots involves the quantum confinement effect of conjugated π-domains, the surface/edge state in graphene quantum dots, and the combined effect of these two factors. They gen­ erally show fluorescence emission of different colors like blue, green, yellow, and red. Fluorescence emission of graphene quantum dots can even be in the ultraviolet region [54]. To further utilize the properties of graphene quantum dots, one promising route is to form a composite with suitable materials such as polymers. Graphene quantum dots/polymer composites generally have excellent specific strength and specific modulus. Furthermore, a combination of graphene quantum dots in a polymeric matrix enhances their applicability in photonic and optoelectronic devices [55]. As already mentioned, in the case of carbon dot/polymer composite, the introduction of the polymer matrix also slow down the agglomeration process of quantum dots that helps to maintain the emission properties. Hence, due to their very small size and exceptional mechanical properties, graphene quantum dots can be incorporated in polymeric matrix for the development of a wide range of sensing devices [45,174,187,191,202,203] including sensors for heavy metal ions, disease biomarkers, drugs, and contaminants. Some of the examples are mentioned below [56].

12.2.1 HEAVY METAL ION SENSING USING GRAPHENE QUANTUM DOT/POLYMER COMPOSITE-BASED SENSORS Heavy metal ions of mercury, lead, cadmium, etc. have many detrimental health effect. Their salts are water soluble and can easily contaminate the groundwater source. During the mid-20th century, a lot of mercury poisoning cases were re­ ported in Minamata, a coastal town in Japan. This devastating incident has at­ tracted the attention of the scientific community and the United Nations Environment Programme (UNEP) adopted a regulation to reduce mercy con­ tamination, named the Minamata convention. Among all the heavy metallic pollutants, Hg(II) is the deadliest one and its source could be natural as well as anthropogenic (chemical manufacturing, fossil fuel combustion). Due to high water solubility, Hg(II) can be readily spread from the environment to the ground water source and finally into food chain. Accumulation of Hg (II) in the human body can severely damage the nervous system and brain, endocrine system, as well as kidneys and other organs. Recently Tang et. al. have reported a novel

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FIGURE 12.5 (a) Electrochemiluminescence profiles of a mercury sensor in the presence of the different concentration of Hg (II): the concentrations were as follows: (a) 100; (b) 50; (c) 10; (d) 1; (e) 0.1; (f) 0.05; (g) 0.01 nM. Inset: the linear relationship between ECL emission and the logarithm of Hg (II) concentration. (b) Decrease in fluorescence intensity of the Cr (VI) sensor in the presence of increasing amount of Cr (VI) (from top to bottom): 0, 57, 111, 163, 227, 294, 357, 416, and 500 μM. Inset: fluorescence plate with identical GQD-nylon membranes in solutions with different Cr (VI) concentrations under natural and UV light (from left to right): 57, 111, 163, 227, 294, 357, 416, and 500 μM. Reprinted with permission from [ 57, 58]. Copyright (2018, 2016) Elsevier.

electrochemiluminescence sensor based on poly(5-formylindole)/reduced gra­ phene oxide nanocomposite for trace Hg(II) detection (Figure 12.5A). The na­ nocomposite works as the electrochemiluminescence substrate and graphene quantum dot-DNA-gold nanoparticle works as the signaling probe. The sensor is used for detecting the presence of Hg (II) ions in a real water sample and human blood serum. These results showed that the sensor can be satisfactorily utilized for actual sample detection both in-vivo and in-vitro [57]. Similar to Hg(II), Cr(IV) is also another notorious metal ion that may easily get mixed with our water and food sources and can cause stomach ulcer and cancers. Recently, Carrasco and co-workers have developed a graphene quantum dot con­ taining reusable polymeric membrane that can act as a fluorescence-sensing plat­ form for selective and sensitive detection of Cr (VI) in an aqueous medium (Figure 12.5B). They have also shown that the membrane-immobilized graphene quantum dots show a similar fluorescence response as water-dispersed graphene quantum dots. The material also exhibits good repeatability and the analytical performance remains stable after several detection cycles. The regeneration of the membrane after detection is also easy and doesn’t require reducing or chelating agents [58].

12.2.2 SENSING DISEASE BIOMARKERS USING GRAPHENE QUANTUM DOT/POLYMER COMPOSITE-BASED SENSORS Diagnosis of many human diseases is possible by detecting the specific biomarkers for the disease; acetone gas is such a biomarker that can be present in the breath of a

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Quantum Dots and Polymer Nanocomposites

patient suffering from diabetes and halitosis. Hence, detection of acetone gas may lead to early detection of these diseases. In a normal human being, the amount of acetone gas is within a range of 0.2–0.5 ppm in exhaled breath, but in the case of a diabetic patient, the amount will be more than 1.8 ppm. Furthermore, other bio­ markers, such as formaldehyde, trimethylamine, and hydrogen sulphides in exhaled breath, can be monitored for early detection of lung cancer, trimethylaminuria, and halitosis respectively. Zhang et al. has recently synthesized metal organic framework-derived ZnO nanopolyhedra/S, N: Graphene quantum dot/polyaniline nanohybrid by an easy one-step in situ polymerization technique can detect se­ lectively acetone gas vapor. The high-performance acetone gas sensing device was made on a flexible polyethylene terephthalate substrate with interdigital electrodes. The experimental data shows that the material (film) has excellent response value with a short response and recovery time. It is also highly selective with long-term stability and good repeatability [59].

12.2.3 SENSING DRUGS AND CONTAMINANTS USING GRAPHENE QUANTUM DOT/POLYMER COMPOSITE-BASED SENSORS Methamphetamine (also known as METH) and its derivatives are recreational drugs and frequently overused due to their addictive nature. It has a neurotoxic effect and hence not prescribed as a medicine. There are several methods, including chro­ matography (GC), liquid chromatography mass spectrometry (LCMS), ion mobility spectrometry (IMS), and GC-mass spectrometry (GC-MS), that are developed to detect methamphetamine. These methods are generally expensive and need many steps before the procedure. To counter these drawbacks, Majid and co-workers have developed a new fluorescence composite material, based on molecularly imprinted polymers and graphene quantum dots, which can detect and determine the presence of methamphetamine. The graphene quantum dots embedded on the molecularly imprinted polymer matrix is easy to synthesize (Figure 12.6) and shows higher selectivity towards methamphetamine compared to the composite with a nonimprinted polymer. It is a fluorescence turn-off sensor, which means the fluores­ cence intensity of the graphene quantum dots are quenched in the presence of the target analyte. The efficiency of turn-off sensors is generally measured in terms of the degree of emission quenching in the presence of analyte, and it has been found that the fluorescence quenching is fully selective in the presence of methamphe­ tamine only and is not affected by other similar analytes like amphetamine, ibu­ profen, codeine, and morphine. It also has a significant detection limit of 1.7 μg/L. Overall, it is an effective and low-cost method to recognize methamphetamine in solution [60]. Ammonia is a toxic compound and can be found in several industrial wastes. It is a water-soluble gaseous compound that can cause damage especially to the skin and eyes. The threshold limit of ammonia for human respiratory system is 25 ppm in air. Many expensive techniques like conducting polymer-based sensors and electrolytic devices are developed to detect the gaseous substance with a limited detection limit and accuracy. In an attempt to develop a cheaper and easy-to-use gas sensor for selective sensing (Figure 12.7) of ammonia. Gavgani et. al. have developed a

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241

FIGURE 12.6 Simple synthetic pathway of the graphene quantum dot/molecularly im­ printed polymer composite for detecting methamphetamine. Reprinted with permission from [ 60]. Copyright (2020) Elsevier.

sulphur and nitrogen co-doped graphene quantum dots/polyaniline nanocomposite. It was synthesized by a simple in situ chemical oxidative polymerization technique. The conductivities of flexible pure polyaniline and the composite material are measured by a four-point probe technique separately and, at 10 nA applied current, they are found to be 32.8 S.cm-1 and 95.8 S.cm-1, respectively. It points towards a significant increase of charge carrier concentration due to the incorporation of the S and N co-doped graphene quantum dots to the polymer matrix. According to the scientists, this increase in sensing ability is due to the synergistic effect between the quantum dots and the polymers. Detailed analysis of the result shows that acid–base doping/de-doping process, carriers mobility, and swelling process are the most probable sensing mechanism of the S and N co-doped graphene quantum dot/ polyaniline composite gas sensor [61].

12.3 PEROVSKITE QUANTUM DOT/POLYMER COMPOSITE-BASED SENSORS In compared to the traditional classes of quantum dots (QDs), perovskite quantum dots (P-QDs) having chemical formula of ABX3 (A = CH3NH3, CH5N2; B = Sn, Pb; X = Cl, Br, I), have become a new outstanding class of QD materials due to

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FIGURE 12.7 Selectivity of ammonia gas by the graphene quantum dot-embedded poly­ aniline in comparison to other analyte and polyaniline alone. Reprinted with permission from [ 61]. Copyright (2016) Elsevier.

their defect-tolerant structure, high synthesis feasibility, narrower full width at half maximum (FWHW), and high quantum yield [62–64]. P-QDs have been applied in various electronic and optoelectronic applications such as photovoltaic [65], pho­ toemission [66,67], photodetectors [68], photocatalysts [69], and memristors [70] as it possesses high light absorption efficiency, tuneable band gap, and low carrier recombination rate. The P-QDs exhibiting maximum PL quantum yield up to 100% is reported by Dai et al.., exhibits promising application for photo-emission [71]. Although P-QDs have advantages of excellent optical properties, they have the serious disadvantages of poor thermal, chemical, and photostability. They are un­ stable in an atmosphere containing oxygen and water due to photooxidation [72]. When photoexcited, the P-QDs release electrons that react with the oxygen mole­ cules to generate free radicals. The produced free radicals react with the amine salt present in the P-QDs to decompose the structure [73]. P-QDs are also extremely sensitive to many other environmental factors like UV light and high temperatures [74]. Therefore, the improvement of the environmental stability is highly required to apply P-QDs in the real field. Various strategies, including ligand design, overcoating, and shell design, have been explored to improve the stability. One such important development is the pre­ paration of composite materials to form a protection layer or to passivate the surface. In recent years, various materials including metallic ions, polymers, oxides, and many other hybrids have been used. The outstanding improvement in stability was achieved

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243

by diverse structures such as forming P-QDs/QDs nanocomposites, encapsulating the P-QDs into a different matrix, ion doping in the lattice of P-QDs, and loading P-QDs onto the surface. These environmentally stable P-QDs exhibit enhanced performance in sensors, electronics, photonics, and other fields [75]. P-QDs are highly unstable in water and polar solvents where it immediately decomposes and loses photo-luminescence properties due to replacement of the weakly bound ligands in perovskite crystals [76]. This largely limits their applic­ ability in fluorescence detection, which is commonly carried out in an aqueous medium [77]. The encapsulation of P-QDs within a polymer matrix is a successful strategy for enhancing their stability in water and polar solvents. The agglomeration of the P-QDs was suppressed by the polymer matrix. Polymer matrices may also provide mechanical and chemical stability of the QD system. [78] Herein, we systematically discuss the recent development of P-QDs-polymer composite for sensing applications.

12.3.1 SENSING OF ORGANIC DYE USING PEROVSKITE QUANTUM DOT/POLYMER COMPOSITE-BASED SENSORS Wang et. al developed a monolithic superhydrophobic polystyrene fiber membrane encapsulating CsPbBr3 perovskite quantum dots (CPBQDs/PS FM) via a one-step electrospinning method [79]. This (CPBQDs/PS FM) composite exhibits high quantum yields ( ∼91%), narrow half-peak width ( ∼16 nm), and nearly 100% fluorescence retention after being exposed to water for 10 days. The composite exhibits 79.80% fluorescence retention after 365 nm UV light (1 mW/cm2) illu­ mination for 60 h. The outstanding optical property of the composite have been utilized to detect Rhodamine 6G (R6G) with an ultralow detection limit of 0.01 ppm. The surface wettability of the composite was found to be superhydrophobic, due to the lower surface energy of the PS host compared to that of water. Since the emission spectrum of CPBQDs/PS FM overlaps greatly with the absorption spec­ trum of R6G in water, R6G in an aqueous solution serves as the analytical target that meets the prerequisite for the FRET process Figure 12.8.

12.3.2 SENSING OF ORGANOPHOSPHOROUS PESTICIDE USING PEROVSKITE QUANTUM DOT/POLYMER COMPOSITE-BASED SENSORS The specific binding sites for the target analytes could be achievable by functio­ nalizing the P-QDs-polymer composite. The specific cavities that are com­ plementary to the target molecules can be introduced in polymeric materials by molecularly imprinted polymers (MIPs), also known as artificial antibodies [80]. The excellent MIP/P-QDs-based sensors can be created by the integration of MIPs with superior perovskite P-QDs. This MIPs technology is utilized by Tan et. al to produce a highly sensitive and selective method for the detection of phoxim by fabricating novel MIP/CsPbBr3 QD composites [81]. They have synthesized oleylamine (OAm)- and oleic acid (OA)-capped high-brightness CsPbBr3 P-QDs by the hot injection method. The polymer template was designed based on the chemical structure of phoxim. The sol-gel process was followed to encapsulate the

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FIGURE 12.8 (a) Transmission electron microscopic image of the ultramicrotomed CPBQDs/PS FM composite (inset) and its enlarged image with CPBQDs encapsulated in­ side. (b) The fluorescence spectra of CPBQDs/PS FM in an aqueous solution with various concentrations of R6G added stepwise 1 ppm from 1 to 10 ppm. The inset pattern showed the relationship between normalized PL intensity and R6G concentration, respectively, mon­ itored at 513 nm (green) and 560 nm (pink). Reprinted with permission from [ 79]. Copyright (2016) ACS.

CsPbBr3 P-QDs in the polymer matrix. This MIP/P-QDs composite was superior as it exhibited high specificity over the previous studies regarding the detection of organophosphorus pesticides. The fluorescence quenching of the MIP/P-QDs composite was the monitoring factor that had a good linear correlation for phoxim in the concentration range of 5–100 ng/mL, and with a limit of detection of 1.45 ng/mL. They have not only detected the phoxim in laboratory but also achieved in tracing phoxim in potato and soil samples, achieving recoveries of 86.8–98.2% Figure 12.9.

FIGURE 12.9 (a) Schematic illustration of the synthesis of MIP/CsPbBr3 P-QDs compo­ sites. TEOSPI: 3-(triethoxysilyl)propyl isocyanate, BUPTEOS: N-(benzyl)-N’-(3(triethoxysilyl)propyl)urea, TEOS: tetraethoxysilane. (b) Effect of phoxim concentration (0-140 ng/mL) on the fluorescence spectra of the composite. Reprinted with permission from [ 81]. Copyright (2019) Elsevier.

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The same MIP technology was also followed by Huang et al. to produce the (3-aminopropyl)triethoxysilane)-capped CsPbBr3 P-QDs (APTES-capped CsPbBr3 P-QDs) by the one-pot approach [82]. The excellent photoluminescence property of APTES-capped CsPbBr3 P-QDs was the key factor to use it as a fluorescent carrier. The specific binding site was created by using the template, omethoate (OMT, organophosphorous pesticide). The composite was created by sol-gel process where cross-linker was tetramethylorthosilicate (TMOS) and APTES was func­ tional monomer. The final MIP@CsPbBr3 P-QDs composites exhibited selective recognition ability toward the OMT when the template was removed via solvent extraction. The composite sensor was applied successfully in the recognition of OMT for vegetables and soil samples by monitoring the fluorescence quenching degree of MIP@CsPbBr3 P-QDs materials. The high specificity toward the OMT was obtained due to the presence of imprinting cavities in MIP@CsPbBr3 P-QDs composites resulting in the higher quenching efficiency. The detection limit of the composites for OMT was 18.8 ng/mL. The composites were selective toward OMT with respect to size, shape, functional groups over several similar organo­ phosphorus (OP) insecticides, such as phoxim, dimethoate, dichlorvos, chlorpyr­ ifos, and acetamiprid (organochlorine pesticide). Interestingly, the practical ability of the MIP@CsPbBr3 P-QDs composites were tested to detect the level of OMT residue in cabbages and soils Figure 12.10.

12.3.3 DETECTION OF UV RADIATION USING PEROVSKITE QUANTUM DOT/POLYMER COMPOSITES Another important area of the application of the P-QD polymer composites developed was the detection of UV radiation. The ultraviolet (UV) radiation is very important and widely employed in human daily life. The decrease of the ozonosphere is the main reason for the harmfulness of overexposure to solar UV radiation, which is now getting the people’s attention. It is required to develop a simple, low cost, fast,

FIGURE 12.10 (a) Schematic representation of the preparation of MIP@CsPbBr3 P-QDs composites. APTES: 3-aminopropyl) triethoxysilane, TMOS: tetramethylorthosilicate. (b) Effect of OMT (0-400 ng/mL) on the fluorescence spectra of the composite sensor. Reprinted with permission from [ 82]. Copyright (2018) ACS.

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FIGURE 12.11 Schematic representation of the solar UV sensor with day time where the UV ray changes and color of the sensor changes. Reprinted with permission from [ 85]. Copyright (2019) ACS.

and portable sensor such as fluorescence [83] and photochromic [84] sensing stra­ tegies for the detection of solar UV radiation. A paraffin-red fluorescence quantum dot composite (P-r-PQDs) was synthesized by Wu et al. by encapsulating red fluorescent QDs in solid paraffin [85]. The obtained P-r-PQDs were very stable in water and emit strong red fluorescence under solar UV lamination due to the well protection of r-PQDs by hydrophobic paraffin and thus is suitable for long-term and sensitive sensing of solar UV radiation. The fabricated P-r-PQD sensing film can emit bright and pure red fluorescence under UV light due to the high fluorescence quantum yield. The sensing film shows wide color variations from green to red when exposed to different solar UV radiation, which can easily be observed by the naked eye or recorded by a camera Figure 12.11.

12.3.4 SENSING OF CHLORIDE/IODIDE ION USING PEROVSKITE QUANTUM DOT/POLYMER COMPOSITE-BASED SENSORS Park et al. developed a cellulose-CsPbBR3 P-QDs composite-based portable di­ agnosis sensor to detect the presence of disinfection ions such as chloride and iodide from local sewage and drinking water [86]. The cellulose matrix gives the stability of the QDs. A hot injection method was followed to synthesize the composite which enables the deposition of CsPbBr3 P-QDs inside the porous cellulose fibers. The anionic exchange between the halide ions which controls the band gap of the QDs was responsible for the colorimetric change. Because of the high portability and rapid sensitivity, the composite was able to detect iodine and chlorine in drinking water. Stability under elevated humidity conditions plays a vital role in the field of practical optoelectronic application. The composite was

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also highly stable and retains its fluorescence quantum yield under high humidity conditions, which makes it a potential for use in different fields. The detection of I- and Cl- were performed by spiking the water sample with the corresponding ions and by dipping the composite film. As the ion exchange occurred, photo­ luminescence property change was observed and the visual color was changed to orange within 5 seconds. The change in color was noticeable through the naked eye and under UV light. The CsPbBr3 P-QDs/cellulose composite showed ex­ cellent sensitivity toward the detection of I- ions in the range from 0.1 mM to 1 M with a limit of detection (LOD) at 2.56 mM, whereas it exhibits the LOD for Clions of 4.11 mM Figures 12.12 and 12.13.

FIGURE 12.12 (a) Schematic illustration of the nucleation and growth of CsPbBr3 P-QDs on cellulose nanofibers and stepwise procedure for the generation of CsPbBr3 P-QDs/ cellulose composites under (b) visible light and (c) UV field. Scale bar: 1 cm. Reprinted with permission from [ 86]. Copyright (2020) ACS.

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FIGURE 12.13 Colorimetric changes showed by CsPbBr3 P-QDs/cellulose composites in the presence of (a) iodide and (b) chloride ions. Reprinted with permission from [ 86]. Copyright (2020) ACS.

12.3.5 BIOMOLECULE SENSING USING PEROVSKITE QUANTUM DOT/POLYMER COMPOSITE-BASED SENSORS Chen et al. developed the composite of polyvinyl difluoride (PVDF), PVDFCH3NH3PbI3@CNDs by in situ growth of CH3NH3PbI3 in a polymer matrix con­ taining CNDs. The composite exhibited a much higher stability than the pristine CH3NH3PbI3 QDs. The PEC sensor with the formed PVDF-CH3NH3PbI3@CNDs as the photoactive layer was constructed to detect cholesterol (CHO) [87]. The lipid cholesterol (CHO) helps in maintaining the structural integrity and fluidity of cell membranes. But excessive CHO in the serum may disturb blood circulation, re­ sponsible for various cardiovascular diseases [88]. So, the detection of CHO is very important in medical labs. The composite was synthesized by thermal polymerization process to imprint CHO. The synthesis was done using CHO as the template, me­ thacrylic acid, and styrene as the functional monomers, ethylene glycol dimethacylate as the cross-linker and azobisisobutyronitrile as the initiator. The hydrogen bonding and van der Waals interactions in the molecularly imprinted polymers film was the driving force to selectively entrap the CHO. The decrease photocurrent was detected due to the steric hindrances towards the transfer of electrons after entrapment of CHO in the sensor film. This change in the photocurrent was the monitoring factor for the detection of CHO [89]. The concentration of CHO in human blood serum was also determined the by the P-QDs composite-based sensor Figure 12.14.

12.3.6 DEVELOPMENT OF PH SENSOR USING PEROVSKITE QUANTUM DOT/POLYMER COMPOSITES The water-soluble fluorescent P-QDs polymer composite microsphere was prepared by An et. al. to monitor pH, urea, and urease. The composite was synthesized by embedding CsPbBr3 P-QDs in the poly(styrene/acrylamide) (PS-PAA) micro­ spheres using a swelling-shrinking method [90]. The pH detection system was constructed by the combination of P-QDs composites and dopamine (DA). The system exhibited strong fluorescence when exposed to an acidic environment, whereas the fluorescence was quenched at a high pH as DA was oxidized to qui­ none. This oxidation was utilized to build the fluorescent sensor to detect urea and urease. Urea gets hydrolyzed to ammonia in the presence of urease and increases

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FIGURE 12.14 Schematic display of the MIPs@PVDF-CH3NH3PbI3@CNDs PEC sensor fabrication process and the detection mechanism of cholesterol (CHO). Reprinted with permission from [ 87]. Copyright (2021) Elsevier.

the pH of the medium, which leads to the fluorescence quenching of the composite. The limit of detection of urea for this sensor system was 1.67 μM. They have also demonstrated the selectivity of the system for urea detection in the presence of other potentially interference (Gly, Ala, Thr, Phe, Leu, His, Na+, K+ and Mg2+) under the same environment Figure 12.15.

FIGURE 12.15 Schematic representation of the preparation of the composite fluorescence microsphere: (a) P-QDs encapsulated in polymer matrix, (b) pH sensing principle, (c) the urea and urease detection mechanism, (d) fluorescence images of the composites under UV light at 365 nm in aqueous suspension in the presence of different interfering substances. Reprinted with permission from [ 90]. Copyright (2021) Springer.

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12.4 SUMMARY AND FUTURE PERSPECTIVES OF QUANTUM DOT/POLYMER COMPOSITES AS SENSORS Above, in a few examples, we have seen various different kinds of composite materials made by combining several different kinds of polymers and quantum dots. In the case of carbon dot/polymer composites, we have seen carbon dots that have been synthesized from multiple cheaply available natural sources like acerola fruit, cassava peels, cedrus, chitosan, mango peels, starch, citric acid, rosemary leaf, glucose, and oleic acid. Different simple processes like micro­ wave, pyrolysis, hydrothermal, solvothermal, and ultrasonic treatments are ap­ plied for the synthesis of these carbon dots. The synthetic procedures for the composite combining the carbon dots and the polymers are also of a wide range, starting from simple stirring to electrospinning, from interfacial polymerization to bulk polymerization, from conventional solution casting to drop casting. One of the major advantages of carbon dot/polymer composites is the availability of numerous synthetic techniques that make the procedure highly flexible. We have also discussed the fluorescence and UV/vis absorption properties of these com­ posite materials with their applications as physical, chemical, and biological sensors to detect various analytes/targets. The improvement in sensitivity and selectivity will further enhance the potential of carbon dots/polymers composites as sensors. Another major drawback is the sensing mechanism; most of these composite materials are principally turn-off sensors that decrease the selectivity due to a possible interference from other potential quenchers that may be present with the target analytes. Challenges that should be met (or modification that could be made) before potential real-time sensing applications of carbon dots/polymer composites are: 1. Developing one step synthetic methodologies for carbon dots/polymer nano composites. 2. Designing newer precursor for carbon dots containing various hetero atoms like N, P, S, etc. 3. Involving naturally occurring polymers as a component of the composite which will be considered as a greener approach for synthesis. 4. New target analytes with different sensing methods should be explored. 5. Physical sensors based on carbon dots/polymer composites are very rarely known; hence, new material that can detect physical entities like tem­ perature, humidity, or pressure is required. In the case of quantum dots/polymer composites, we have seen that diffusing a small quantity of graphene in a polymer matrix may significantly change their mechanical (tensile strength and elastic modulus), electrical and spectral properties. This sub­ stantial improvement in structural as well as functional properties is utilized for designing sensing devices based on graphene quantum dots/polymer composites. There are a few challenges that should be met (or modifications that could be made) before potential real-time sensing applications of graphene quantum dots/polymer composites are:

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1. Although the toxicity of the composite is lower than graphene quantum dots, it still cannot be considered a non-toxic material. Further research to decrease the toxicity of the material will increase its potential use for in-vivo studies. 2. Graphene quantum dots have a bright fluorescence and the composite material also retains its fluorescence property; hence, most of the fluor­ escence sensors based on graphene quantum dots/polymer are turn-off in nature, which reduces the practical applicability of these devices. 3. Exploration for newer target materials will also expand the versatility of this composite material. In the case of perovskite quantum dots/polymer composites, we have shown that the enhancement of the environmental and photochemical stability was achieved by constructing the polymer composite of the quantum dots. The unique optical properties of the perovskite quantum dots could be preserved during the composite preparation by the protection materials such as organic polymers, oxides, MOFs, glass, and semiconductors. The specificity of the sensor system is achievable by a template-based molecularly imprinted polymer technique or by incorporating a specific ligand in the polymer matrix. Greater water resistant, chemical, and photostability can be achieved by encapsulating perovskite quantum dots within the polymer matrix; these protect the matrix layer on the quantum dot composite and lowers its photoelectronic/optical properties. Hence, the exploration of varieties of polymer matrix with greater transparency towards the preparation of high quantum yield composite materials would be an inter­ esting area of research in the sensing field. The use of a porous matrix including MOFs, transparent polymer, and porous silica matrix with appropriate binding sites for analytes may solve the issue. As the research field related to sensing studies using quantum dot/polymer composite material is still an emerging one, and the science related to these com­ posite materials is not yet completely explored, we hope that many more quantum dot/polymer composite-based sensors in terms of precursors, synthetic methods, optical properties, and applications will be developed in the near future and a new sensing strategy with advanced sensing ability will hopefully come to existence in the upcoming days.

ACKNOWLEDGMENTS PKS and CK acknowledge Amity University Kolkata for the infrastructural support. MM acknowledges Amity University Uttar Pradesh (AUUP, Noida) for providing research infrastructure and library facility.

DECLARATION The authors declare no conflict of interest.

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13

Quantum Dot–Polymer Composites in LightEmitting Diode Applications Madhusudan B. Kulkarni School of Electronics and Communication Engineering, KLE Technological University, Vidyanagar, Karnataka, India

Kishore Upadhyaya School of Advanced Sciences, KLE Technological University, Hubballi, Karnataka, India

N.H. Ayachit School of Advanced Sciences, KLE Technological University, Hubballi, Karnataka, India Visvesvaraya Technological University (VTU), Belagavi, Karnataka, India

Nalini Iyer School of Electronics and Communication Engineering, KLE Technological University, Vidyanagar, Karnataka, India

CONTENTS 13.1 13.2 13.3 13.4 13.5 13.6

Introduction................................................................................................. 260 Evolution of Quantum Dot-Based Light-Emitting Diodes ....................... 262 Role of Quantum Dots in LEDs................................................................ 262 Perovskite Quantum Dots .......................................................................... 264 PbS Quantum Dots..................................................................................... 264 Challenges and Limitations in QD–Polymer Composites in LED Applications................................................................................................265 13.6.1 Challenges................................................................................... 265 13.6.2 Compatibility of QDs with Polymers........................................ 265 13.6.3 Reliability and Lifetime of QD-LEDs.......................................265 13.6.4 Combination of QDs with LEDs ...............................................265 13.6.5 Limitations.................................................................................. 265

DOI: 10.1201/9781003266518-13

259

260

Quantum Dots and Polymer Nanocomposites

13.7

Recent Progress in QD-LEDs....................................................................266 13.7.1 Compatibility of QDs and Polymer Matrix............................... 266 13.7.2 Modification of the QDs Surface Chemistry.............................266 13.7.3 Incorporation of QDs into Polymer Nanomaterials ..................267 13.7.4 Embedding QDs into Polymer Microspheres............................267 13.7.5 Optimization of QD-LED Spectra .............................................267 13.7.6 Color Matching Functions and Chromaticity Diagrams ........... 267 13.7.7 Color Gamut ...............................................................................268 13.7.8 CRI and Color Quality Scale (CQS) ......................................... 268 13.7.9 Luminous Efficacy of Optical Radiation (LER) ....................... 268 13.7.10 Increasing the Consistency and Lifetime of QD-LEDs............ 268 13.7.11 Applications of Quantum Dots .................................................. 269 13.8 Display Devices.......................................................................................... 269 13.8.1 Liquid Crystal Display (LCD) Backlighting .............................269 13.8.2 Phosphors.................................................................................... 270 13.8.3 Solar Cell-Based Light Source .................................................. 270 13.8.4 Photodetectors............................................................................. 271 13.8.5 Biomedical Imaging ...................................................................271 13.8.6 Light Emitting Diodes (LEDs) .................................................. 272 13.8.7 Future Perceptive........................................................................ 273 13.9 Conclusion ..................................................................................................273 References.............................................................................................................. 274

13.1 INTRODUCTION With the rapid development of nanomaterial science, low-dimensional materials like nanowires, nanobars, nanocrystals, nanoflakes, nanocones, and nanodots are being ad­ vanced and applied in several fields of applications using technological strategies because of their unique physical, mechanical, electrical, thermal, and optical properties [1–5]. Modern era science and technology have empowered nanomaterial composition of discrete types with several matrices such as polymers and thin-film substrates in which materials are embedded. Nanomaterials are introduced into polymer hosts to create functionalized polymer-based nanocomposites, a novel material with unique features like size-dominated photoluminescence (PL)-based semiconductor QDs [6]. Further, novel metal nanomaterials have a confined surface plasmon resonance characteristic. Usually, polymers are translucent over a varied spectral range and may be effec­ tively preserved as a versatile platform to serve QDs and metallic nanomaterials-based devices [7]. A polymer is a substance or material made up of many repeating subunits that make up very large molecules or macromolecules. There are three different types of polymers such as thermosets, thermoplastics, and elastomers. Further, the use of block copolymer matrices in a range of semiconductor and metal-based functional nanocomposites has been intensively researched. Nanoparticles (NPs) are small par­ ticles that range in the size of 1 to 100 nanometers (nm) [8]. One of the most exciting strategies for creating stable NP-polymer composites is known as ligand exchange. Pre-synthesized polymer macromolecules bearing functional groups replace low molecular weight ligands at the surface of NPs [9–12]. The stability of

Quantum Dots–Polymer Composites

261

nanocomposites is assured in this condition by a specific macromolecule attachment to the nanoparticle surface via electrostatic and hydrophobic interactions of functional groups. The advantage of this technique is that it allows pre-synthesized macro­ molecules with the proper functional groups to interact with the NP surface. In li­ thographic sequences, layer-by-layer assembly, and even solutions, plasmon NPs have been demonstrated to improve the PL of QDs. [13]. The inter-particle separation, NPs, and semiconductor QD compositions, with photoluminescence and absorption maxima, can influence the degree of photoluminescence enhancement [14]. Chan et al. [15] determined the photoluminescence augmentation in layer-by-layer assemblies of CdSe quantum dots and plasmon nanoparticles with the separating polymer layers of diverse thickness. A twofold improvement of photoluminescence adjacent to Au na­ noparticle was perceived at 11 nm distance between quantum dot and nanoparticle, whereas the maximal improvement of photoluminescence was attained at 8 nm [16]. Differing from the data mentioned above, Kulakovich et al. [17] identified when silica dioxide was employed for layer separation; the maximum fivefold photoluminescence boosting CdSe/ZnS quantum dots at 11 nm distance from Au particles. A quantum dot is a kind of nanomaterial used to fabricate a variety of display devices. This is because of its distinct physical, thermal, and optical properties. Further, the quantum confinement effect will be caused by their variable energy band gap acquired by modifying the particle size [18,19]. QDs usually can be modified by a passivation procedure to upsurge the device luminance efficiency due to their charge immobility, photoluminescence instability, and indissolubility. In order to attain the required objective, high charge mobility and stability-based polymers are often em­ ployed to adapt QDs for a variety of uses, such as displays and photonics. QDs have high surface functionalization, which could increase photoluminescence yield in an organic or inorganic nanoparticles caption. Moreover, inorganic nanoparticles pro­ duce the poor formation of the film due to their powder form, which causes nano­ particles to stop interacting with different parameters [20]. As a result, nanoparticles need to be uniformly disseminated in a conductive reaction mixture. The polymers are capable of connecting inorganic nanoparticles while allowing for significant chargecarrier mobility [21]. Figure 13.1 shows the device that generates visible light by transferring energy from quantum well layers to crystals above the layers.

FIGURE 13.1 Device generating the visible light through energy transfer from quantum wells to crystals above the layers.

262

Quantum Dots and Polymer Nanocomposites

Semiconductor QDs have grabbed the curiosity of scientists and industries who are working in parallel for the betterment of the QD manufacturing process, as it is a potential material for absorbing and delivering light energy. QDs often contain elements from groups II–VI, CdSe [22], groups III–V such as InP [23], and groups I–III–VI, such as CuInS2 [24]. QDs have proven to be well suited for various ap­ plications due to their remarkable optical properties, including electrically and optically driven lasers, vivid displays, biosensors, and LEDs. Two kinds of QDLEDs differ based on photo-excited QDs and electro-excited (electroluminescence) QDs. Here, electro-excited QD-LED efficiency has remained lower than that of PL QD-LEDs due to the difficulty in charge injection. Phosphor-converted LEDs (pcLEDs) are commonly used in conjunction with blue LEDs that have yellow phosphors. Because of the dearth of a red element in the emission spectrum, the color-rendering index (CRI) of pc-LEDs is minimal. Furthermore, because phos­ phors have a large, full width at a half-maximum wavelength (FWHM) in the range from 50–100 nm, tuning the spectrum distribution of pc-LEDs is difficult as greenred and blue-green regions are getting extensively overlaid.

13.2 EVOLUTION OF QUANTUM DOT-BASED LIGHT-EMITTING DIODES Researchers have developed a quantum dot-based light-emitting diode (QD-LED) with better efficiency and flexibility due to the numerous advantages of using QDs and their applications in optoelectronic instruments such as LEDs. After OLED screens, QD-LEDs represent the next phase of display technology. QD-LEDs are light-emitting technology used in smartphones, touch screens, TVs, and digital cameras to create large displays. Table 13.1 summarize the evolution of quantum dot-based light-emitting diodes. Further, the construction of a QD-LED is similar to that of an OLED, except that the light-emitting material is QDs, such as cadmium selenide (CdSe) nanocrystals. A traditional QD-LED has three layers: an emissive core layer of QDs, one outer layer that transports electrons, and one outer layer that transport holes. Colloidal QD is a viable technique to make QD-LEDs because of the numerous advantages in most fields. By incorporating an emissive layer into a single layer of QDs, the holes and electrons may be transferred directly from the surfaces of the electron-transport layer and the hole-transport layer, resulting in a significant increase in recombination ef­ ficiency. It is critical to find and study the fundamental causes of inefficiency and suggest prospective solutions for the systematic progress of QD chemistries and ac­ tive layer designs and innovative device architectures for high-performance QD-LEDs. An important requirement for achieving high-performance QD-LEDs is to investigate the efficiency of the light generation process in the QD-LEDs.

13.3 ROLE OF QUANTUM DOTS IN LEDS Semiconducting materials like quantum dots (QDs) are nanometer size, usually between 2–10 nm. QDs exhibit quantum confinement effects due to their small size, and optical and electrical features primarily determine their size. Cadmium-based

Quantum Dots–Polymer Composites

263

TABLE 13.1 Evolution of Quantum Dot-Based Light-Emitting Diodes Year

Generation

Description of the Work

Ref

1907

1st Gen

The notion of the light-emitting diode was born when British researcher HJ Round described the light emission from a crystal detector. OV Losev, a Russian researcher, reported the first light-emitting diode.

[ 25]

1955

R Braunstein, who worked at Radio Corporation of America, invented standard diodes emit infrared light when applied to a current source in a closed loop.

[ 27]

1962

At GE Company in New York, N Holonyak Jr., known as “the father of LEDs,” invented the first visible-spectrum GaAsP LED. In an early mainframe computer, IBM used LEDs on circuit boards for the first time.

[ 28]

1927

1964 1976

[ 29]

TP Pearsall discovered the first high efficiency and brightness lightemitting diodes which were used in telecommunication for fiber optic application

[ 30]

1979

At Nichia Corporation laboratory, Shuji Nakamura created the initial high-brightness blue LEDs.

[ 31]

1992

Akasaki used GaN to create the first blue LED, which had a 1% efficiency. Later, Shuji Nakamura developed a high brightness white LED.

[ 32]

2006 2014

2nd Gen

[ 26]

3rd Gen

H Amano, I Akasaki, and S Nakamura invented the high efficiency and economical blue LED.

[ 33] [ 34]

QDs were discovered in the 1980s. Various cadmium and non-cadmium-based QDs have been developed and examined during this time [35]. By adjusting the shape and size of QDs, significant enhancements in their optical and electrical properties can be made. These have emerged as a substantial material class with numerous uses in photodetectors, photosensors, light-emitting diodes (LEDs), photovoltaics, field-effect transistors, and lasers. High-performance polymeric nanomaterials for optoelectronic-based applications including, QD-polymer composite light-emitting diodes (PLEDs), have recently received much attention [36]. Quantum dots are semiconductor nanostructures that release a lot of energy. In the case of optoelectronic light-emitting diodes, encapsulating quantum dots in polymers can improve photoluminescence and device stability [37]. The photo­ polymerizable resin composite incorporates QDs. Further, QDs have the potential to enable internal healing. They are biocompatible mediums since their fluorescence qualities are alike to those of natural human teeth. QDs can be used as a substitute for fillers in sticky resins. Trimethylene glycol dimethacrylate (TEG DMA) and glycerol dimethacrylate include zinc oxide quantum dots (G DMA) [38]. Quantum dots are fluorescent semiconductor nanocrystals made up of 200–10,000 atoms. Quantum dots are made up of two primary components: a shell and a core. The band gap in these small crystals is quite significant. Fluorescence can now occur at wavelengths that are shorter than those utilized for excitation [39]. This situation

264

Quantum Dots and Polymer Nanocomposites

FIGURE 13.2 Classification of nanocomposite material used for QD-LEDs.

permits red light to generate the emission from the blue light, which uses photo­ initiation during the process. QDs with a gradually stepping from violet to deep are developed on a kilogram scale [40]. Among several nanocomposite materials for the preparation of the quantum dots, perovskite and lead sulfide (PbS) were widely used due to their excellent properties, good wavelength, and advantages in the QD-LEDs. Figure 13.2 shows the classi­ fication of nanocomposite material used for QD-LEDs.

13.4 PEROVSKITE QUANTUM DOTS Perovskite QDs have recently garnered considerable attention. Because of their high PL quantum efficiency of up to 90% and emission with narrow bandwidth are ideal with the wavelength ranging from 20–30 nm. Perovskite QDs are the most pro­ mising CdSe-based QD alternatives [41]. Depending on the polymer composition and particle size of the halide, the wavelength emission for CsPbX3 QDs can be changed to shield the complete visible spectral range between 400 to 650 nm [42]. When Br partially replaces cl, the emission wavelength of the CsPbCl3 QDs moves to the green regime of the visible spectrum, yielding diverse halide perovskite CsPb (Cl/Br)3. QDs emit green light with a CsPbBr3 composition, which transitions from yellow to red with CsPb(Br/I)3 and CsPbI3, respectively [43]. The CsPbX3 (X = Cl, Br) perovskite QDs with emissions ranging from 450-510 nm are the most stable of the numerous perovskite QDs. These QDs are promising for optoelectronic applications because of their improved optical characteristics and chemical resilience. LEDs, photodetectors, and LCD backlighting benefit these cadmium-free QDs with reduced lead concentration [44].

13.5 PBS QUANTUM DOTS Relying on their particle size, the wavelength emission of lead sulphide quantum dots can be adjusted in the range from 900 to 1,600 nm, which is in the infrared (IR) spectrum of the electromagnetic spectrum varying from 2.5 nm–8 nm [45]. The absorption spectra of PbS QDs are typically broad, with narrow fluorescence bands. PbS QDs are suited for using it as light absorbers or infrared (IR) emitters in solar cells, photodiodes, infrared LEDs, and photodetectors. PbS QDs are of special importance in solar photovoltaic cells due to the avail­ able broad spectrum of absorption spanning from near-infrared (NIR) to infrared (IR) with maximum peak-to-valley having the ratio of greater than 4, and narrow band emission with 100 nm wavelength. Because of these characteristics, PbS QDs can be utilized in pair and multiple junction solar cells to improve solar panel efficacy [46].

Quantum Dots–Polymer Composites

265

13.6 CHALLENGES AND LIMITATIONS IN QD–POLYMER COMPOSITES IN LED APPLICATIONS 13.6.1 CHALLENGES There are a few challenges in the development of QD–polymer composite-based LEDs packaging and applications.

13.6.2 COMPATIBILITY

OF

QDS

WITH

POLYMERS

Because of the reaction-based synthesis technique, where QDs are no longer available in various organic solvents, QDs are unsuitable for direct manufacturing and incorporation of LEDs [47]. Consolidation of precise QDs–polymer composite hybrid nanomaterials are required afore utilizing in these applications. The hy­ drophobic surface of characteristic QDs, on the other hand, is frequently in­ compatible with the traditional LED packaging technique, which involves physically mixing the QDs with epoxy resin or silicone. The hydrophobic organic ligands QDs can obstruct resin epitome polymerization, resulting in the substance poisoning effect. Furthermore, the inconsistency of the superficial of QDs with the composite polymer matrix may cause agglomeration, lowering the photo­ luminescence effectiveness of QDs [48].

13.6.3 RELIABILITY

AND

LIFETIME

OF

QD-LEDS

Moisture and oxygen must be kept out of QD-LEDs to keep them reliable and longlasting. Furthermore, the heat generated by QDs must be carefully regulated since excessive temperatures cause thermal quenching of QDs, which severely reduces the performance of QD-LEDs, as these are sensitive to temperature variations [49–51].

13.6.4 COMBINATION

OF

QDS

WITH

LEDS

A mixture of QDs with composite polymer and LED module is the main element that transforms a portion of blue light into WLs of light. QD-LEDs are preferably made up of discrete types of QDs. The distribution of spectra must tune and modify the WLs with a good fraternization ratio of QDs elements. This necessitates both experimental and theoretical spectrum optimization [52]. Furthermore, when as­ sessing the ultimate optical characteristics of QD-LEDs, the packing assembly has a major impact. Low efficiency or poor color fidelity can cause improper QDs particle size distribution and QDs polymer morphology [53].

13.6.5 LIMITATIONS Quantum dots made of CdSe are highly poisonous and require a solid polymer shell. The optical properties of the surfaces can be altered, and particle size is difficult to manage. Quantum dots can degrade inside a living organism as well. The marginal

266

Quantum Dots and Polymer Nanocomposites

leakage in composite restorations allows germs to penetrate, triggering an in­ flammatory reaction, resulting in uncured monomer seepage into the pulp and ne­ crosis. Overall conversion efficiency is reduced, necessitating operation at a lower temperature. QDs are planned to be employed in the QD-LEDs, a novel LED (lightemitting diode) variant. The production of blue-emitting QDs, on the other hand, is a challenging task. It necessitates smaller sizes than the other color emitting dots and a more magnified emission than the different colors [54]. Further, an organic LED curing device is readily damaged by water. Organic LEDs are highly susceptible to moisture, have a high production cost, and have a short life period. They are also particularly sensitive to ultraviolet rays; hence a protective layer or filter is employed to prevent UV rays. In the emissive zone, the concentration of non-radiative recombination sites and luminescence quenchers promotes deterioration. The four processes of chemical breakdown in semi­ conductors include recombination of charge carriers via UV light absorption, subsequent radical addition events that generate radicals, homolytic dissociation, and disproportionation between two radicals resulting in hydrogen-atom transfer reactions [55]. Furthermore, the restored cavities with light preserved polymer composites and self-etching adhesives, a cavity disinfectant containing 2% chlorhexidine increased micro leakage. A color rendering index (CRI) is used to measure the ability of light to reveal the objects real color as compared to the ideal source of the light, which is natural source light. Here, for low ratings, a high CRI is a prerequisite feature.

13.7 RECENT PROGRESS IN QD-LEDS 13.7.1 COMPATIBILITY

OF

QDS

AND

POLYMER MATRIX

Issues with QDs’ poor compatibility with polymeric environments must be rectified before white LED (WLED) applications can be implemented. Although integrating QDs in thin films have improved, maintaining these QDs in the majority of com­ posite polymer matrices relics is a challenging platform. These techniques are aimed mainly at creating scattered QDs inside the composite polymers without affecting their PL. Moreover, encapsulated within LEDs, QD–polymer composites must be translucent [56]. As a result, new methods for producing QD-based composite nanomaterials with outstanding clarity and regularity are required.

13.7.2 MODIFICATION

OF THE

QDS SURFACE CHEMISTRY

The primary method for optimizing the QD and polymer interoperability and miscibility is to cover suitable ligands on the superficial of the QDs. CdS QDs in pyridine was amended by coating them with functionalized ligands by phenyl groups. In contrast, CdS QDs can get soluble in the substrates such as polymethyl methacrylate (PMMA) and polystyrene (PS) were improved by covering them with oleic acid ligands at the surface. An octylamine on the QDs surface suppresses band-edge emission significantly [57].

Quantum Dots–Polymer Composites

267

Following polymerization, QDs–polystyrene composites were effectively made by using covalent connections like 4-thiomethylstyrene as both a capping ligand and a co-monomer to replace the trioctylphosphine oxide (TOPO) on CdSe [58]. Ligand modification can be accomplished by modifying the peripheral directly with surfactants that can copolymerize with the developing polymer composite chains. Zhang et al. [15] described a way for transporting aqueous CdTe QDs into styrene or methyl methacrylate monomer concentrations utilizing octadecyl-p-vinylbenzyldimethylammonium chloride (OVDAC) as a polymeric surfactant, resulting in a transparent CdTe–PS composite.

13.7.3 INCORPORATION

OF

QDS

INTO

POLYMER NANOMATERIALS

The QDs can be bonded in an optically translucent barrier material to reduce in­ compatibility. In terms of powder-type QD–silica composites, the microemulsion approach, for example, produced QD-silica composites with a silica shell coating individual QDs, diverse QD–silica monolith, and QD-embedding silica matrix. As QDs have a silica coating on their surface, catalyst poisoning does not affect the silicone curing process [59]. Instead of surface ligand modification in nanocrystals, electrostatic interaction between negatively charged QDs and a positively charged building block could be the driving factor for generating the desired hybrid QD-polymer composite. The QDs operate as physical cross-linking hubs, allowing the complexes to be easily molded into various luminous shapes and materials [60].

13.7.4 EMBEDDING QDS

INTO

POLYMER MICROSPHERES

Another effective way to enhance the dispersion and stability of QDs inside polymers is to embed them in microspheres. The most accessible approach for making QD–polymer microspheres is to swell them. The swelling polymers capture the QDs inside the par­ ticle, and then the residue is washed away, causing the polymer to diminish, deceiving the QDs inside the microspheres [61]. Chen et al. [62], to make light microbeans, used a swelling method that mixed mesoporous silica particles with CuInS2 (CIS) QDs (LMBs). To prevent QD aggregation, the mesoporous structures act as a lattice.

13.7.5 OPTIMIZATION

OF

QD-LED SPECTRA

The spectrum distribution of QD-LEDs must be optimized in order to generate high-quality white light appropriate for lighting and display applications.

13.7.6 COLOR MATCHING FUNCTIONS

AND

CHROMATICITY DIAGRAMS

For instance, as light excites green, red, and blue cones at different rates, each person’s perception of color and luminous flux differs significantly. Color per­ ception is also a subjective feature that cannot be measured objectively. As a result, the CIE has standardized color measurement using color matching functions and the chromaticity diagram [63].

268

Quantum Dots and Polymer Nanocomposites

13.7.7 COLOR GAMUT There are three types of LED display: green, red, and blue LEDs are the most prevalent. The three colors are mixed in such a way that the viewer sees a combination of them. White LEDs also emit two or three complementing colors [64].

13.7.8 CRI

AND

COLOR QUALITY SCALE (CQS)

The color rendering index (CRI) measures an illuminants ability to render all of the hues of an object illuminated by a light source [65]. The color rendering capabilities of a test light source are assessed by comparing it to a reference light source. For determining the CRI, a planckian black-body radiator with the same color temperature or CCT as the test light source is utilized as the reference background light. With a CRI of 100, the reference light source color rendering skills are flawless. In addition, Davis and Ohno [66] established the CQS to evaluate the colorrendering capabilities of illuminants. The reference light sources are the same as in the CRI. However, the test sample colors are different. The CQS applies a sa­ turation factor and utilizes 15 reflective munsell samples. Furthermore, the CQS spans from 0 to 100, which is more comprehensible given that CRI98% after 70 minutes of exposure to sunlight) among the tested materials. Furthermore, the nanocomposites were shown to have strong inhibitory effects on the development of bacterial and fungal cells. 20.2.2.2 Animal Protein and Their Nanocomposites 20.2.2.2.1 Gelatin and Nanocomposites The biodegradability, biocompatibility, non-toxicity, and cheap cost of gelatin make it a good choice for usage in culinary applications (Sahoo, Sahoo and Lochan.Nayak 2013). Gelatin is a helpful biopolymer in tissue engineering because it can be used as a wound dressing and bone scaffolding, but its poor mechanical characteristics (particularly in the wet state) prevent it from being used as a structural biomaterial. As a consequence, researchers have a huge challenge when it

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comes to enhancing the strength of gelatin. In addition to gelatin-based composites, such as hydroxyapatite tricalcium phosphate and carbon fiber, efficient vapor crosslinking and orientation processes are also being employed in the development of these materials. In general, the material’s overall strength was lacking, especially when it was moist or swollen, and this was particularly noticeable. The thermal characteristics of gelatin have been significantly enhanced as a result of the use of an orientation technique. High-performance gelatin or gel-based composites still have a long way to go before they can be used in practical applications, but recent developments in polymer-layered silicate nanocomposites have opened the door to some intriguing new opportunities. As an example, consider the following. The electrospinning process was used to successfully construct a nanofibrous core-sheath nanocomposite dual drug delivery system made of poly (vinyl alcohol) (PVA)/chitosan/lidocaine hydrochloride loaded with gelatin nanoparticles in 2015. The results were published in 2015 by Fathollahipour et al. (Fathollahipour et al. 2015). It has been shown that gelatin nanoparticles with an average particle size of 175 nm are effective after being synthesized by nanoprecipitation and then loaded with the antibiotic medication erythromycin. With the use of field emission scan­ ning electron microscopy (FE-SEM), researchers observed that the optimum form of gelatin nanoparticles was obtained at 1.25 weight % of gelatin in aqueous phase, with the addition of 20 liters of glutaraldehyde 5% as the crosslinking agent. Other methods, such as dynamic light scattering, zeta potential testing, and Fourier transform infrared spectroscopy, were used to characterize the nanoparticles in addition to the ones mentioned above (FTIR). One study found that the solution weight ratio of 96/4 resulted in the best bead-free form with the least quantity of beads among the PVA/chitosan nanofibrous mats evaluated. Within 72 hours, the in vitro dual release profile of the core-sheath nanofibers was also evaluated, and the release efficiency for lidocaine hydrochloride and erythromycin was found to be 84.69% and 75.13%, respectively, for the two medications tested, according to the findings. According to the release exponent, it has been found that the release of lidocaine hydrochloride from the matrix sheath is produced by an anomalous or non-Fickian diffusion mechanism, while the release of erythromycin is caused by an anomalous or non-Fickian diffusion mechanism. 20.2.2.2.2 Collagen and Nanocomposites Because of its unique properties, such as biocompatibility, biodegradability, low antigenicity, and ease of fabrication of nanoparticles, collagen is being investigated as a drug delivery vehicle due to its potential to be the most cost-effective proteinpolymer currently available. Collagen is the most abundant protein in the body and the primary structural component of connective tissues. The structural elucidation of the collagen molecule has laid the groundwork for the molecule’s particular physicochemical qualities and stability, which have been shown to be essential. According to comprehensive study, there is no indication of harm linked with the delivery of collagen-based nanoparticles at this time. As a result, a few illustrations of the same are provided below. The researchers at Ali et al. (Pourjavadi and Doroudian 2015) developed a semiconductive nanocomposite for electrically controlled medication delivery,

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which they published in 2015. Polycaprolactone was utilized to alter hydrolyzed collagen, which is a polypeptide that occurs naturally in large quantities. As a consequence of this alteration, the mechanical properties of the hydrolyzed collagen were altered. A hydrogel compound was generated by performing radical copolymerization of acrylic acid on the backbone of a biocompatible polymer in the presence of a cross-linker to form a hydrogel compound. It was decided to apply the Taguchi technique to optimize the response parameters that have an impact on the water absorbency of the hydrogel. The inclusion of conductive nanofiber routes throughout the hydrogel matrix was achieved using in situ aniline poly­ merization. The 1H NMR, TGA, AFM, SEM, FTIR, UV–Vis, cyclic voltammetry, and conductivity measurements were used to characterize this system, among other techniques. In addition, the drug release of hydrocortisone, which was used as a model drug, was investigated in vitro using conductive stimulation. The MTT test revealed that neither the conductive nor the nonconductive hydrogels were cyto­ toxic. The results suggest that this nanocomposite may be utilized as a medication delivery system that can be regulated externally and modified to meet physiological processes, according to the researchers. One of the most serious issues nowadays is the regulated administration of various medicines via biomaterials. A unique possibility exists with the nano­ composite method in particular, which combines the scaffold-forming capabilities and biocompatibility of hydrogels with the varied and controllable drug release features of micro- or nano-carriers. In this way, researchers (Mebert et al. 2018) shown that collagen-silica nanocomposites may allow for the extended release of two topical antibiotics, making them potential medicated dressings for the pre­ vention of wound infections. This was accomplished by combining core–shell silica particles containing gentamicin sulphate and sodium rifamycin with concentrated collagen type I hydrogels. SEM showed a thick fibrillar collagen network with a characteristic periodic banding pattern and a homogenous particle distribution, as indicated by scanning electron microscopy. The release of antibiotics from the nanocomposite allowed for a prolonged antibacterial action against Staphylococcus aureus in vitro for a period of 10 days, demonstrating the potential of nano­ composite materials. In the acute cutaneous irritation test performed on albino rabbit skin, no signs of severe inflammation could be detected. It was discovered that the antibacterial efficacy of nanocomposite was improved by two log steps after the usage of loaded systems in an in vivo cutaneous infection model when loaded systems were employed. Histological analysis of the wound bed indicated that M1 inflammatory macrophages had been eliminated as a result of the therapy as well. As a result of their combined efforts, these results reveal the feasibility of using the nanocomposite technique to produce collagen-based biomaterials that administer regulated dual drugs to prevent infection and improve cutaneous wound healing. 20.2.2.2.3 Albumin and Nanocomposites An animal protein, albumin has been researched since the early 20th century and has been used in a variety of medicinal procedures. It has therapeutic uses on the one hand and is also utilized in a variety of formulations to transport pharmaco­ logical and diagnostic substances on the other. Scientists in the field of

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nanomedicine have been interested in this protein because of the multiple ad­ vantages and potentials it offers. It is generally recognized as one of the most promising techniques to treating all forms of cancer in the near future (Kianfar 2021). Abraxane is a medicine formulation based on albumin nanoparticles that sold more than $2 billion in only one year in 2012. A range of disorders are discussed in this section, with a focus on the most recent uses of therapeutic nanoparticles as selective delivery systems. It was discovered that an unique drug delivery system (DDS) combining human serum albumin, poly(lactic-co-glycolic acid (PLGA), magnetite nanoparticles, and a therapeutic agent(s) might be used in the treatment of illnesses such as rheumatoid arthritis (RA) and skin cancer (Misak et al. 2014). Using a modified oil-in-oil emulsion/solvent evaporation (O/OSE) approach, we were able to produce DDS with diameters that ranged from 0.5 to 2 m, with the diameter being principally regulated by altering the lower viscosity albumin in the O/discontinuous OSE’s phase. It was discovered during the drug-release investigation, that the quantity of drug and albumin released is mostly governed by the albumin content of the DDS, which is associated with the drug occlusion-mesopore model. Cytotoxicity studies demonstrated that raising the albumin level in the new DDS boosted cell survival when compared to a lower albumin content in the new DDS, which was due to the better biocompatibility of the new DDS. As a result of these research, it seems that the suggested system may be a feasible choice for treating a range of ailments, including RA, skin cancer, and breast cancer, among others. An investigation carried out by Akbal et al. describes the production and utili­ zation of novel saponin (SAP)-loaded montmorillonite-human serum albumin (Mt-HSA) nanocomposites (NCs) as an anticancer drug delivery agent (Akbal et al. 2018). In a combination of biodegradable and ecologically acceptable HSA and Mt., which has outstanding mucoadhesive qualities, the resultant NCs were able to pass through the gastrointestinal barrier with no difficulty. HPSA NCs containing SAPMt-HSA were produced utilizing a modified desolvation approach that comprised the use of ethanol and glutaraldehyde (GA) as precipitators and cross-linking agents, respectively. A colon cancer cell line (DLD-1) and a healthy cell type (L929 fibroblast cells) were used to evaluate the efficiency of these NCs in an in vitro gastrointestinal model. A study was conducted to determine the cytotoxic impact and cellular absorption of NCs, as well as the release profile of SAP. Researchers discovered that SAP-Mt-HSA NCs may cause cancer cell death in a dose-dependent manner, while producing little or no damage to healthy cells. 20.2.2.2.4 Silk Fibroin and Nanocomposites Achieving effective medication delivery systems via the design and production of novel materials is crucial in medicine and healthcare. A significant deal of interest has been generated in the area of drug delivery by nanocarrier-based drug delivery systems, especially nanoparticles, due to the fact that they provide new potential to overcome the limits of conventional drug delivery techniques. Natural protein polymer silk fibroin (SF) has a number of distinct properties that make it a suitable material for incorporation into a wide range of drug delivery vehicles capable of delivering a variety of therapeutic agents. Silk fibroin (SF) was discovered in nature

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and is a naturally occurring protein polymer with several distinct properties. It has been shown that SF matrices may effectively transport anticancer medicines, small chemicals, and biological molecules. The silk fibroin nanoparticles (SFNPs) are good candidates for medication or other bioactive material administration in vivo, and this is particularly true in the pharmaceutical industry. However, because of SFNPs’ poor colloidal stability, which leads them to congregate in biological conditions, their potential applications are limited in the future. Wang et al. (Wang et al. 2015) developed SFNP composite materials with a core-shell structure (CS-SFNPs) by electrostatically coating SFNPs with four distinct cationic polymers: glycol chitosan, N,N,N-trimethyl chitosan, polyethylenimine, and PEGylated polyethylenimine, in order to solve this problem. It was found that the CS-SFNPs exhibited much greater colloidal stability in bio­ logical medium than the naked SFNPs, as determined by the DLS and NTA ex­ periments. When treated with human cervical carcinoma (HeLa) cells, the CSSFNPs were rapidly internalized and accumulated in the lysosome, and when loaded with an anticancer agent, DOX, the CS-SFNPs exhibited increased cyto­ toxicity against HeLa cells. CS-SFNPs that have been synthesized and have a suitable colloidal stability in biological media might be employed as drug carriers in an anticancer drug delivery system, according to the results. 20.2.2.3 Animal Lipid and Their Nanocomposites It has been discovered and explored that lipid-coated nanoparticles carrying diag­ nostic or therapeutic substances may be used in biomedical applications (Namiki et al. 2011) and this represents a potential nanomedicine technique. In addition to having cationic headgroups on their surfaces that bind anionic nucleic acids, lipid nanoparticles also carry hydrophobic medications at the lipid membrane and hy­ drophilic drugs inside the empty interior region. Furthermore, researchers may design nanoparticles to interact with external stimuli such as magnetic fields, light, and ionizing radiation, which boosts their value in biological applications by in­ creasing their interaction with external stimuli. For this reason, we will examine lipid nanocomposites and their uses in drug delivery applications in the next portion of this chapter. Salma and colleagues (Mohyeldin et al. 2021) conducted a research in which they studied the influence of customized hybrid lipid-polysaccharide nanocompo­ sites on the biological performance of sulfonamide. Copolymers of chitosan, graft, and tocopherol polyethylene glycol 1000 succinate (TPGS) were synthesized and inserted into a SUL-loaded lipid nanocore as a polysaccharide shell, and the results were published in Nature Communications. Nanocore-shell structures were dis­ covered in the optimized nanohybrids, which had a particle size of 110.1 nm, an electrochemical potential of 23.7 mV, an encapsulation efficiency of 85.42%, a pH-dependent release profile, and a mucoadhesive propensity that was tolerable. SUL intestinal penetration and oral bioavailability were increased by 3.3 and 8.7-fold, respectively, with the addition of TPGS to the chitosan backbone. This was due to the reduction in nanohybrid cellular internalization into Caco-2 intestinal cells. It was discovered that nanohybrids outperformed free SUL and a commercial product in the treatment of reserpine-induced depression in rats in an animal model

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of depression. The nanohybrids enhanced the levels of serotonin and dopamine by 1.87 and 1.47 times, respectively, in the blood. Furthermore, it has been shown that nanohybrids may lower brain oxidative stress as well as the irritating impact of SUL on numerous bodily tissues. In general, the newly designed nanohybrids open the path for breakthroughs in the administration of oral drugs to the patient. In a work conducted by Neeraj et al., a unique photo amenable nanoparticlebased drug delivery system for highly efficient targeted on-demand release, fluor­ escence imaging, and treatment was established by combining zinc phthalocyanine (ZnPc) and gold nanoparticles (AuNPs) into liposomes (Thakur et al. 2019). As a result of the hyperthermia generated by AuNPs under LED light irradiation, the liposomal membrane became more liquid, allowing for the rapid release of ZnPc from the carriers, illustrating the notion of on-demand release. Furthermore, in addition to the photodynamic impact, the local hyperthermia induced thermal damage to cancer cells, resulting in a synergetic effect of photodynamic and photothermal treatment. An international team of researchers (Delgado-Rosales et al. 2018) used the emulsification-diffusion technique to incorporate hydrophobic superparamagnetic iron oxide nanoparticles (SPIONs) with an average size of 13 nm, which were prepared by the salt co-precipitation method, into solid lipid nanoparticles (SLN) and oily-core polymeric nanocapsules (NC) to form a novel lipid. In the case of the NC/SPIONs composite, hydrodynamic diameters of 300–400 nm were ob­ served, whereas the SLN/SPIONs composite had hydrodynamic diameters of 450–500 nm. SPION entrapment efficiency (EE) was determined to be 49.0% and 55.7%, respectively, in NC/SPIONs composite batches manufactured with 25% and 12.5% (w/w) SPIONs, according to the results of the study. Batches made with 25% and 12.5% SPIONs (w/w) were determined to have an EE of 40.8% and 59.1%, respectively, based on the EE calculation for SLN/SPIONs. The sa­ turation magnetization of the NC/SPIONs and the SLN/SPIONs was measured to be 6–12 A m2/Kg and 5–9 A m2/Kg, respectively, for the two materials. Furthermore, the remanent magnetization and coercivity were also close to zero, suggesting that the material had superparamagnetic features that might have substantial consequences for drug transport. It was established via the in vitro release profiles that a model medicine could be released slowly from the nano­ composite without being interfered with by SPIONs.

20.2.3 MICROBIAL POLYMERS The usage of polymeric materials has increased dramatically over the past few decades, with increased use in a variety of sectors including the food, textile, and biomedical industries. It is possible to employ biopolymeric materials to en­ capsulate and transport a variety of functional food components and pharmaceu­ ticals, including bioactive lipids, minerals, enzymes, and peptides, in a controlled and efficient manner. Initial drug delivery applications were made possible by the hydrophobic and nondegradable characteristics of hydrophobic polymers such as poly (dimethyl siloxane) (PS), polyurethanes (PUs), and poly (ethylene-co-vinyl acetate) (EVA). PHAs and PHBs, two biopolymers collected by bacteria and used

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as carrier matrices for pharmaceuticals that may be released by bioerosion, were found to be particularly attractive among the synthetic and natural polymers studied in this study. Materials that disintegrate or denature rapidly after processing, on the other hand, constitute a substantial danger to mass manufacturing and industrial application, resulting in a lack of interest in the field outside of academia. In order to produce biopolymers with a wide range of industrial and medicinal uses, it is preferred to include cross-linking agents into the polymerization process. The ad­ dition of functional groups to biopolymers is a common, but elegant, process for generating biopolymers that are long-lasting and useful in industrial applications. A number of advantages of natural polymeric drug delivery systems over other con­ trolled release formulations include good biocompatibility, a flexible drug release profile that can be adjusted through cross-linking strategies, the degradability of the polymer’s by-products, and the possibility of rapid elimination by the excretory system to avoid accumulation in the body. Natural polymeric drug delivery systems are also less expensive than other controlled release formulations. The following is an example of a microbially generated polymer nanocomposite that may be used to define medication delivery. A salt-tolerant strain of the bacteria Agrobacterium sp. ZX09 generates salecan, a unique water-soluble polysaccharide that is not seen in other bacteria. Poly (di­ methylaminoethyl methacrylate) (PDMAEMA) is a pH, temperature, and ionic strength sensitive polymer with antibacterial characteristics. It may be used in a variety of applications including medical devices. Researchers from the University of Shanghai (Wei et al. 2017) have developed a semi-interpenetrating polymer network (semi-IPN) hydrogel made of salecan and PDMAEMA. pH, ionic strength, and temperature are all affected simultaneously by the hydrogel that results: When pH is 1.2 or larger, the swelling ratio is largest, and when pH is greater than 3, it decreases; also, the water content of the hydrogel decreases as the ionic strength grows; and, when temperature is less than or greater than 40 degrees Celsius, the hydrogel swells or deswells.

20.3 SAFETY ISSUES OF POLYMER NANOCOMPOSITES Over the course of many decades, the literature has described several biological uses of polymer-composite materials, and this chapter presents an overview of such applications. It is intended to provide general knowledge regarding the structure and functions of proteins as well as their applications in the biomedical fields of drug delivery, cancer therapy, and neurodisorders. It is discussed in detail the many forms of polymer composites that are now being used or being explored for usage in various biomedical applications. In addition, the advantages of employing polymercomposite biomaterials in certain applications are discussed. A discussion of crucial concerns and technical obstacles that must be solved before polymer composite materials may acquire recognition in the biomedical sector is also included in this chapter. There have been reports that polymer composite devices must be sterilized prior to being implanted in the human body (Ramakrishna et al. 2001). Even more worryingly, it has been suggested that sterilizing techniques may cause degradation of the polymer composites (Visakh and Nazarenko 2015). Although the autoclave

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temperature is kept as low as possible, there is still a substantial risk of deformation and mechanical characteristics being harmed. Because of their biocompatibility and biodegradability, natural and synthetic polymers have been the subject of intense investigation. The inert nature of other polymers, on the other hand, hindered their use in tissue engineering. Because of this, mixing bioactive materials with other bioactive materials to form fully im­ proved composite materials has emerged as one of the most promising new di­ rections in the development of biomaterials for tissue engineering applications. Several research studies investigated the potential uses of carbon nanotubes in biomedical applications. Because of their remarkable electrical characteristics, carbon nanotubes (CNTs) are frequently employed as biosensors, and their spec­ troscopic qualities make them fascinating materials for photothermal treatment and medical imaging applications. Moreover, due to their extremely high aspect ratio, carbon nanotubes (CNTs) are widely used as nanocarriers for the delivery of drugs, genes, and other therapeutic agents to the body by binding them to the sidewall of CNTs through stacking interactions between the graphitic structure and aromatic nucleotide bases and nucleic acid. Recently, carbon nanotubes (CNTs) have been studied for use in tissue engineering applications. A variety of structural and physiochemical reinforcement properties are demonstrated by the incorporation of CNTs into polymer matrix, including increased strength, flexibility, and bio­ compatibility; induction of angiogenesis; reduction of thrombosis; and gene ex­ pression manipulation for tissue repair. The cytotoxicity of carbon nanotubes (CNTs) is strongly influenced by their physiochemical properties, which include their length, diameter, surface area, agglomeration tendency, as well as the presence and nature of catalyst residuals generated during the CNT fabrication process, among other factors (Huang 2020). Polymeric ceramic nanoparticles have been shown to generate oxidative stress, which in turn causes inflammation of the reticuloendothelial system. Cytotoxic activity occurs in the lungs, liver, and heart as a result of inflammation (Govardhane and Shende 2021). The majority of polymeric ceramic nanoparticles have a huge surface area, which results in a significant degree of toxicity for the cell to which they are attached. Due to the release of free radicals produced by ceramic nano­ particles, oxidative stress occurs, which may result in inflammation as well as cell damage and genotoxicity. In addition to its resilience to ambient pH, polymeric ceramic nanocomposite is a perfect drug delivery technology, but there has been little investigation into its safety, raising worries about its toxicity. The cytotoxicity of polymeric ceramic nanoparticles in the human body is shown in Figure 20.3.

20.4 DISPOSAL/DEGRADATION In light of the increasing usage of nanomaterials in biomedical applications, it has become more important to evaluate their environmental and toxicological con­ sequences. Studies on the degradability and toxicity of nanoparticles, in addition to polymers, may give a comprehensive understanding of the uses of advanced na­ nocomposites (Kausar 2019). Nanofillers, both natural and manufactured, may display a wide range of biodegradation behaviors, which vary depending on their

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FIGURE 20.3 When polymeric ceramic nanoparticles exposed to human body. It accu­ mulates in reticulo-endothelial system and reactive oxygen species induced stress alleviated.

origin, structural features, and erosion media (Dziadek, Stodolak-Zych and Cholewa-Kowalska 2017). Natural fillers (such as cellulose fibers) degrade in the presence of enzymes, making them biodegradable (Siqueira et al. 2011). In this particular example, the erosion rate may be influenced by the molecular weight of the cellulose as well as the degradation media. Additionally, natural fillers have been reported to deteriorate in a photochemical manner (Maria Mucha and Sylwia Ksiazek 2015). Some nanomaterials are also quickly destroyed when exposed to high temperatures. Catalytic biodegradation of synthetic nanocarbon nanofillers such as carbon nanotubes (CNTs) and graphene is a potential (Alpatova et al. 2015). It is determined by enzymatic reactions occurring in the media how much nano­ carbon is biodegraded and how quickly (Khatri, Meddeb-Mouelhi and Beauregard 2018). Inorganic nanofillers may be destroyed by enzymes, photochemical reac­ tions, and UV radiation. There is still much more study that needs to be done in order to completely understand the complicated molecular changes that occur in cells and tissues when they are exposed to nanoparticles or nanostructured polymer nanocomposites in diverse biomedical applications.

20.5 FUTURE PERSPECTIVE AND CONCLUDING REMARKS Polymeric nanocomposites have been extensively researched over the last decade as their importance in the healthcare sector has grown. Many researchers have reviewed recent fabrication strategies for such polymer nanocomposites. Due to the complex fabrication steps involved, large-scale production of polymer na­ nocomposites is a critical issue at the commercial level. Aside from fabrication complexity, appropriate fabrication techniques and operating conditions must also be considered. Several parameters, including the dispersion and stability of a nanofiller in a polymer matrix, interactions between the nanofiller and the polymer matrix, surface charge, polymer chain flexibility, surface chemistry, and the nanofiller’s crystallization ability, must be defined. Aside from these factors, it is also difficult to keep particles at the nano-scale because aggregation occurs with the passage of time. Another factor that contributes to the dispersion in­ stability of nanofillers in polymer matrix is gravity, which is difficult to over­ come. Although the addition of some surfactants or other stabilizers can be used to maintain the nanofiller’s dispersion stability, their addition can affect or harm

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the native properties of the nanomaterials. Coagulation and precipitation tech­ niques can also change the properties of nanocomposites, reducing their extra­ ordinary properties. The main issue with polymer nanocomposites for healthcare applications is their toxicity profile. The size of nanoparticles poses a significant challenge in clinical and in vivo applications. Because nanoparticles can easily cross the blood-brain barrier, they can reach any of the human body’s sensitive organs. Many attempts have been made to conduct toxicological investigations (e.g., through quantification) of free nanoparticles released from polymer nano­ composites. A new method for quantifying the amount of free-standing and protruding multiwalled carbon nanotubes (MWCNTs) in the respirable fraction of particles abraded from a MWCNT–epoxy nanocomposite was developed re­ labeling of MWCNTs with lead ions, nanocomposite production, abrasion and collection of the inhalable particle fraction, and quantification of free-standing and protruding MWCNTs by measuring the concentration of released lead ions are all part of the quantification approach. A549 human alveolar epithelial cells and THP-1 monocyte-derived macrophages were used in in vitro toxicity studies for genotoxicity, reactive oxygen species formation, and cell viability. In the worst-case scenario, approximately 4,000 ppm of the MWCNTs were released as exposed MWCNTs (which could contact lung cells upon inhalation) and ap­ proximately 40 ppm as free-standing MWCNTs in the respirable fraction of the abraded particles. The release of exposed MWCNTs in nanocomposites con­ taining agglomerated MWCNTs was lower. Toxicity testing revealed that the abraded particles had no acute cytotoxic effects. Although polymer nano­ composites have significant potential for healthcare applications, more research is required to investigate their toxicity profile before clinical trials can begin. We have summarized the various healthcare applications of polymeric nanocompo­ sites in this chapter. Polymer nanocomposites have opened up new possibilities in a variety of applications, including nanocarriers, biomedical applications, tissue engineering, sensors, and microbial treatments. Polymeric nanocomposites have piqued the interest of researchers due to their novel properties (e.g., large surface area to volume ratio, high elastic modulus, and high flame retardancy) obtained by incorporating small amounts of nanomaterials into the polymer matrix. The interactions of the nanofiller with the polymer matrix play an im­ portant role in determining the properties of the nanocomposite structure, such as chain conformation, degree of chain ordering, and chain mobility. To improve the dispersion and stability of nanofillers in polymer matrices, numerous func­ tionalization techniques based on covalent and non-covalent schemes have been developed. Polymer nanocomposites have enormous potential for disease diag­ nosis and treatment. The administration of various therapeutic agents (i.e., drugs) has been discussed in the context of cancer and diabetes treatments. Because of their high drug loading capacity and enhanced drug release kinetics at the targeted site, polymer nanocomposites can be used as effective nanocarriers for oph­ thalmic drug delivery systems for nanoformulations. The extraordinary properties of polymer nanocomposites containing magnetic nanoparticles and/or semi­ conducting QDs are expected to broaden their range of applications in bioima­ ging. Polymer nanocomposites also have several smart properties for food

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processing applications (such as antimicrobial properties, enzyme immobiliza­ tion, water vapor permeability, and oxygen scavenging). Polymer nanocompo­ sites have also been investigated for the development of robust electrochemical and/or biosensors due to their synergistic effect and hybrid properties. As a result, the significant potential of polymer nanocomposites as a primary diagnostic and therapeutic agent should be further investigated in order to broaden their fields of application in the healthcare sector. Polymeric nanocomposites have been the subject of substantial study over the past decade, as their value in the healthcare industry has expanded in recent years. A large number of academics have evaluated contemporary manufacturing meth­ odologies for polymer nanocomposites of this kind. Large-scale manufacture of polymer nanocomposites is a serious challenge at the commercial level, owing to the many complicated fabrication procedures that must be completed. Aside from the intricacy of the manufacturing process, it is necessary to consider proper fab­ rication processes as well as operational circumstances. In order to determine the dispersion and stability of a nanofiller in a polymer matrix, interactions between the nanofiller and the polymer matrix, surface charge, chain flexibility of the polymer chain, surface chemistry, and the nanofiller’s crystallization ability, several para­ meters must be determined. Furthermore, it is challenging to maintain particles at the nanoscale because aggregation happens over time, which makes it harder to maintain the nanoscale. Gravity, which is difficult to overcome, is another element that contributes to the dispersion instability of nanofillers in polymer matrixes. Although the addition of various surfactants or other stabilizers may be employed to preserve the dispersion stability of the nanofiller, the inclusion of these agents can have an adverse effect on or degrade the native qualities of the nanomaterials themselves. Techniques such as coagulation and precipitation may potentially alter the properties of nanocomposites, hence decreasing their exceptional character­ istics. The toxicity profile of polymer nanocomposites for use in healthcare appli­ cations is the most significant concern. Clinical and in vivo uses of nanoparticles are complicated by their size, which may be difficult to control. Because nanoparticles may readily breach the blood-brain barrier, they have the potential to reach any of the sensitive organs in the human body. Attempts to undertake toxicological ex­ aminations (for example, by quantification) of free nanoparticles released from polymer nanocomposites have been attempted in a number of different ways. We devised a novel technique for assessing the number of free-standing and projecting multiwalled carbon nanotubes (MWCNTs) in the respirable fraction of particles abraded from a MWCNT–epoxy nanocomposite in the respirable fraction of par­ ticles abraded from a MWCNT–epoxy nanocomposite (Schlagenhauf et al. 2015). The quantification approach includes the pre-labeling of MWCNTs with lead ions, the production of nanocomposite materials, the abrasion and collection of the inhalable particle fraction, and the quantification of free-standing and protruding MWCNTs by measuring the concentration of lead ions released from the particles. Human alveolar epithelial cells (A549 cells) and THP-1 monocyte-derived mac­ rophages (THP-1 macrophages) were employed in in vitro toxicity experiments to examine gene toxicity, reactive oxygen species production, and cell viability. As a result, approximately 4,000 parts per million of MWCNTs were released as exposed

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MWCNTs (which could contact lung cells upon inhalation) and approximately 40 parts per million of free-standing MWCNTs were released as free-standing MWCNTs in the respirable fraction of the abraded particles in the worst-case scenario. It was shown that the release of exposed MWCNTs in nanocomposites including agglomerated MWCNTs was decreased in these nanocomposites. Toxicity tests demonstrated that the abraded particles had no immediate cytotoxic effects when exposed to a cell culture medium. Despite the fact that polymer nanocomposites have tremendous promise for use in healthcare applications, ad­ ditional study is needed to determine their toxicity profile before clinical studies can be conducted on them. In this chapter, we have summarized the numerous healthcare uses of polymeric nanocomposites that have been investigated. A wide range of applications, including nanocarriers, biomedical applications, tissue en­ gineering, sensors, and microbial therapies, have been made possible by the de­ velopment of polymer nanocomposites. Because of its new features (e.g., huge surface area to volume ratio, high elastic modulus, and strong flame retardancy), polymeric nanocomposites have aroused the curiosity of researchers. These quali­ ties are produced by introducing tiny quantities of nanomaterials into the polymer matrix. Nanofiller-polymer matrix interactions are critical in influencing the fea­ tures of nanocomposite structures, such as chain conformation, degree of chain ordering, and mobility of the chains within the structure. A variety of functiona­ lization approaches based on covalent and non-covalent schemes have been de­ veloped to increase the dispersion and stability of nanofillers in polymer matrices. When it comes to illness diagnostics and therapy, polymer nanocomposites offer immense promise. Oncology and diabetes therapy have both prompted discussions on the administration of different therapeutic agents (often referred to as medi­ cines). Polymer nanocomposites, which have a high drug loading capacity and improved drug release kinetics at the targeted location, may be employed as effective nanocarriers for ocular drug delivery systems for nanoformulations, ac­ cording to the researchers. With the exceptional capabilities of polymer nano­ composites including magnetic nanoparticles and/or semiconducting QDs, it is envisaged that their range of applications in bioimaging would be expanded sig­ nificantly. Several smart features of polymer nanocomposites make them particu­ larly well suited for food processing applications (such as antimicrobial properties, enzyme immobilization, water vapor permeability, and oxygen scavenging). On the other hand, because of their synergistic impact and hybrid features, polymer nanocomposites have also been researched for the creation of reliable electro­ chemical and/or biosensors. As a consequence, more research into the enormous potential of polymer nanocomposites as a main diagnostic and therapeutic agent should be conducted in order to widen their areas of use in the healthcare industry.

REFERENCES Akbal, Oznur, Tayfun Vural, Soheil Malekghasemi, Betül Bozdoğan, and Emir Baki Denkbaş. 2018. “Saponin Loaded Montmorillonite-Human Serum Albumin Nanocomposites as Drug Delivery System in Colorectal Cancer Therapy.” Applied Clay Science. 10.1016/ j.clay.2018.09.021.

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Index Note: Page numbers in italics denote figures and page numbers in bold denote tables.

AA, see Acrylic acid Abraxane, 426 Acetoacetylethyl methacrylate-Nvinylcaprolactam copolymer-based microgels, 319 Acrylic acid (AA), 312 Activators generated by electron transfer (AGET), 137 AFM investigations, see Atomic force microscopy investigations AGET, see Activators generated by electron transfer Agglomeration, 127 Albumin and nanocomposites, 425–426 Alginate, 11, 349–350 Alloyed QDs, 29–30 Alloyed semiconductor QD, 30 Aminopropylmethylpolysiloxane (AMS), 199 Aminopropylmethyl silicone oil, 199 Ammonia, 240 Amphiphilic polymers, 96 AMS, see Aminopropylmethylpolysiloxane Anaerobic biodegradation of bioplastics, 340 Animal bioimaging, 109 Animal-derived polymer nanocomposite synthesis techniques, 422 Animal lipid and their nanocomposites, 427–428 Animal polysaccharides and their nanocomposites, 421–423 Animal protein and their nanocomposites, 423 albumin, 425–426 collagen, 424–425 gelatin, 423–424 silk fibroin, 426–427 Anionic cadmium sulfide (CdS) quantum dots, 305 Application of QDs, 36, 269, 269 animal bioimaging, 109 biological and chemical applications, 38 cancer detection, 110–111 drug delivery, 110, 110 infectious diseases, imaging and sensing of, 111 in situ imaging, 109–110 memory applications, QDs in, 38–39 optoelectronic devices, 36–38 plant bioimaging, 109

prokaryote bioimaging, 109 quantum computing, 38 tracking of particles, 109 Applications of QD–polymer composites, 182–183 biomedical area, 8–11 AqQDs, 5 Arc discharge, 176 Artificial antibodies, 243 Atomic force microscopy (AFM) investigations, 153 Atom transfer radical polymerization (ATRP), 137 ATRP, see Atom transfer radical polymerization Auger recombination, 23 Bacillus subtilis, 418 Band distribution in QD, 30 Band gap energy, 24, 25 Band structures of QDs, 360 Bioactive compounds, 7 Bioactive QD-based polymer composites, 8 Biocompatible polymer–QD composite materials, 345, 346 alginate, 349–350 bovine serum albumin (BSA) protein, 346–347 cellulose, 347–348 chitosan, 348–349 gelatin, 347 peptides, 347 poly(L-lactide) (PLA), 351 polyethylene glycol (PEG), 350–351 polyvinyl alcohol (PVA), 352–353 silk, 351–352 Bioconjugation, 344 Biodegradability test of polymer materials, 339 anaerobic biodegradation of bioplastics, 340 dip-hanging method, 340 soil burial and compost conditions, 340 Biodistribution of QDs, 212 Biological and chemical applications of QDs, 38 Biomedical applications of QD–polymer composites, 207 biodistribution, 212 carbon-based quantum dots (CQDs) applications of, 213 biosensing and immunosensors, 217–219 bone tissue enginnering, 221

441

442 chemical structure of, 208–209 in diagnosis, 214–215 in drug delivery field, 215–217 with dual functions, 215 environment, use in, 221–222 gene therapy, 217 preparation methods of, 209–212 future prospects, 222 toxicity, 212–213 Biomedical imaging, 271–272 Biomolecule sensing, 248 Bioplastics, anaerobic biodegradation of, 340 Biopolymer-based nanolaminate films, 415 Biopolymer materials, 337 Biopolymer nanocrystals, 414 Biosensing, 217–219 Biosynthetic approach, preparation of QDs using, 56 Blending methods, 129 melt, 130–131 preparation of QDs/polymer composites by, 283–284 solution, 131–136 Bloembergen, Nicolaas, 6 BNNS nanosheet, see Boron nitride nanosheet BNQDs, see Boron nitride quantum dots Bohr radius of the atom, 22 Bone tissue enginnering, 221 Boron nitride (BNNS) nanosheet, 198 Boron nitride quantum dots (BNQDs), 198 Bound exciton energy, 27 Bovine serum albumin (BSA), 131, 141, 346–347 Bragg reflectors, 189 Brus, Louis E., 22 BSA, see Bovine serum albumin CA, see Cellulose acetate CA–CdTe, see Cysteamine-capped CdTe QDs Cadmium quantum dot (CdQD), 282 Cadmium sulfide-polyacrylamide hybrids (CdSPAM), 312 Caffeic acid, 235 Cancer biomarkers, 110–111 Captopril (CP)-loaded PCL/CQDs nanocomposite scaffolds, 221 Carbon-based materials, 398 Carbon-based quantum dots (CQDs), 129, 207–208, 209, 211–212, 211, 217, 219, 282, 288, 346, 353, 360, 361–362 applications of, 213 biosensing and immunosensors, 217–219 bone tissue enginnering, 221 in diagnosis, 214–215 in drug delivery field, 215–217 with dual functions, 215 environment, use in, 221–222

Index gene therapy, 217 -based chitosan hydrogel film, 320, 320 chemical structure of, 208–209 fluorescence properties of, 364–365 methods of synthesizing, 210 origin of photoluminescence in, 362–364 polymers as support for, 366–372 preparation methods of, 209 bottom-up route, 211–212 top-down route, 209 Carbon dot/polymer composite-based sensors, 228, 231 biological sensors, 235–237 chemical sensors, 234–235 optical properties, 229–233 physical sensors, 237 sensing application, 233–234 Carbon dot polymer composites, 228 Carbon dots (CDs), 158–159, 228, 232, 236, 349, 378 Carbon nanodot (CND), 282 Carbon nanofillers, 221 Carbon nanolights, 208 Carbon nanotubes (CNTs), 8, 430 Carcinoembryonic antigen (CEA), 218 Catalytic applications, QD–polymer composites in, 281 catalytic activity, 288–291 future scope and challenges, 291–292 polymer QD composites, 286 thermoplastic polymer composites, QDs and, 286–287 thermosetting polymer composites, QDs and, 287–288 preparation, 283 by blending techniques, 283–284 in situ preparation of polymers, 284–285 one-step fabrication, 285 structures and properties, 285–286 CCDs, see Charge-coupled devices CD15, 110 CD30, 110 CD45, 110 C-dots, 10–11 CdQD, see Cadmium quantum dot CDs, see Carbon dots; Compact discs CdS-PAM, see Cadmium sulfide-polyacrylamide hybrids CdS quantum dot–sensitized solar cells, Mn doping on, 117 CdTe QDs, 52 CEA, see Carcinoembryonic antigen Cellulose, 284, 347–348, 415 Cellulose acetate (CA), 174 Cellulose nanocrystals (CNCs), 347–348, 416, 417

Index Cellulose-QD hybrid hydrogel, 321, 321 Cerium nitrate hexahydrate, 400 Characteristics of QDs, 25–27 Characterization of QD surfaces, 97–99 Charge-coupled devices (CCDs), 37 Chemical/electrochemical oxidation, 176 Chemical grafting method, 136–138, 180–181 Chitosan (CS), 284, 348–349, 421–422 Chitosan–polyethylene oxide (CTS-PEO), 291 Chloride/iodide ion sensing, 246–247 Chloroprene rubber, 196 CHO, see Cholesterol Cholesterol (CHO), 248 Chondroitin sulphate, 421 Citric acid, 368 CMOS sensors, see Complementary metaloxide semiconductor sensors CNCs, see Cellulose nanocrystals CND, see Carbon nanodot CNTs, see Carbon nanotubes Collagen and nanocomposites, 424–425 Colloidal nanosynthesis, 31–32 Colloidal QD synthesis via micellar synthesis, 47 Colloidal-stable QDs, 92 Color gamut, 268 Color matching functions and chromaticity diagrams, 267 Color quality scale (CQS), 268 Color-rendering index (CRI), 262, 268 Compact discs (CDs), 11, 379 Complementary metaloxide semiconductor (CMOS) sensors, 37 Conducting polymers (CPs), 399 Confinement energy, 27 Copper-based QDs, 73 Copper doping on electronic structure, 115–116 Core-shell quantum dots (CQDs), 6, 11–14, 29, 77, 78, 197, 219, 220, 352 Core-type QDs, 28 Coulomb attraction, 27 Coulomb interaction, 27 CPs, see Conducting polymers CQDs, see Carbon-based quantum dots; Coreshell quantum dots CQS, see Color quality scale CR, see Rubber chloroprene CRI, see Color-rendering index CS, see Chitosan Cs2SnI6 nanocrystals, 49, 49 CTS-PEO, see Chitosan–polyethylene oxide Cu doping on ZnS quantum dots, 116 CV cycles, see Cyclic voltametric cycles Cyclic voltametric (CV) cycles, 371 Cysteamine-capped CdTe QDs (CA–CdTe), 317 Cytotocompatable bismuth sulfide, 348 Cytotoxicity of QDs, 338, 387–388

443 DA, see Dopamine DDS, see Drug delivery system de Broglie wavelength, 23 Density-functional theory (DFT), 77, 99, 115 Density of states (DOS), 34 DFT, see Density-functional theory DHLA, see Dihydrolipoic acid Differential pulse voltammetry (DPV) approach, 219 Differential scanning calorimetry (DSC), 116 Diffusion doping in QDs, 118 Diffusion-ordered spectroscopy (DOSY), 98 Dihydrolipoic acid (DHLA), 93 Dimethylaminoethyl methacrylate, 312 2,4-Dinitrophenol, 290 Dip-hanging method, 340 Direct chemical cutting process, 403 Disease biomarkers sensing, 239–240 Display devices biomedical imaging, 271–272 future perceptive, 273 light emitting diodes (LEDs), 272, 272 liquid crystal display (LCD) backlighting, 269–270, 270 phosphors, 270 photodetectors, 271 solar cell-based light source, 270–271 Disposal/degradation, 430–431 Dopamine (DA), 291 Dopants’ effects on the size and quantum yield, 114 Doped QDs, 31 Doping, 73–74, 111 copper doping on electronic structure, 115–116 diffusion doping in QDs, 118 electrochemical doping of QDs, 111–113 on electrons and holes, 115 graphene QDs, elemental doping of, 113 heteroatom-doped carbon QDs, 116 on InAs/GaAs QD solar cells, 114 InAs quantum dot lasers, modulation p-doping in, 118–119 Mg and Cu doping on ZnS QDs, 116 Mn:Cu Co-doped CdS nanocrystals, 119 Mn doping on CdS quantum dot–sensitized solar cells, 117 n-type doping by lithium ion intercalation, 113 p-doping for QD laser, 118 of QDs for pesticide recognition, 381–382 Si doping on InAs/GaAs quantum dot solar cells, 117 silicon delta-doping, 117–118 silver-doped PbSe quantum dots, 115 size and quantum yield, effects of dopants on, 114

444 on structural and optical properties, 114–115 DOS, see Density of states DOSY, see Diffusion-ordered spectroscopy DOX, see Doxorubicin Doxorubicin (DOX), 11, 12, 216 DPV approach, see Differential pulse voltammetry approach Drug delivery system (DDS), 110, 110, 426 Drugs and contaminants sensing, 240–241 DSC, see Differential scanning calorimetry EDC condensation, see 1-Ethyl 3-(3 dimethylaminopropyl carbodiimide hydrochloride condensation Efros, Alexander, 106 Ekimov, Alexei, 22, 106 Elastomer matrices, QDs in, 195–201 Electrochemical doping of QDs, 111–113 Electrochemical process, 402 Electrode material, nanomaterials and importance of QDs as, 397–400 Electronic structure, copper doping on, 115–116 Electrons and holes, effect of doping on, 115 Electron transfer (ET), 94 Electrospinning process, 424 Empty nanospheres, 195 Encapsulation, 342–343 Energy levels splitting in QDs, 107 Environmental impact of QDs, 377 cytotoxicity of QDs, 387–388 exposure pathways, 386–387 future perspective, 388–399 molecularly imprinted polymers (MIPs), 383–384 pesticide detection polymer/supramolecular surface decorated QDs for, 382–383 silica QD composites for, 382 pesticide detection, QD-based, 381 pesticide recognition, doping of QDs for, 381–382 physicochemical properties, 379–381 QD-embedded thin-film membranes, 385 toxicity of QDs, 385–386 Environmental pollution remediation area, 11–14 Epoxy resin, 173 Escherichia coli, 418 ET, see Electron transfer Ethanol, 426 1-Ethyl 3-(3 dimethylaminopropyl carbodiimide hydrochloride (EDC) condensation, 344 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide/ N-hydroxysuccinimide (EDC/NHS) process, 193 Ethylene thiourea (ETU), 196

Index ETU, see Ethylene thiourea Excitation-capture spots, 6 Exciton, 175 Exposure pathways, 386–387 Fabrication techniques for QD polymer nanocomposites blending methods, 129 melt blending method, 130–131 solution blending method, 131–136 chemical grafting method, 136–138 in situ polymerization method, 138, 139 layer-by-layer method, 138–141 microwave methods, 141–143 QD–polymer nanocomposite formation, challenges in, 143 FESEM, 421 Field emission scanning electron microscopy (FESEM), 424 Flow-controlled microreactor, 57 Fluorescence, 127 Fluorescence methods, 235 Fluorescence resonance energy transfer (FRET), 94 Fluorescence sensor, 383, 383 Folate receptor (FR), 10–11 Förster energy transfer distance, 317–318 Fourier transform infrared (FTIR) spectroscopy, 97, 419 FP, see Frontal polymerization FR, see Folate receptor FRET, see Fluorescence resonance energy transfer Frontal polymerization (FP), 322 FTIR spectroscopy, see Fourier transform infrared spectroscopy Fuel cells, 396 Full width at half maximum (FWHW), 242 FWHW, see Full width at half maximum GA, see Glutaraldehyde Garlic extract (GE), 350 Gas chromatography-mass spectrometry (GC–MS) technique, 99 GCE, see Glassy carbon electrode GC–MS technique, see Gas chromatography-mass spectrometry technique GE, see Garlic extract Gelatin, 347, 423–424 Gene therapy, 217 Genetic engineering technique, 56 Glassy carbon electrode (GCE), 218 Glassy polymer shell concept, 163, 164 Glutaraldehyde (GA), 426 GNRs, see Graphene nanoribbons GOQDs, see Graphene oxide quantum dots GOs, see Graphene oxides

Index GQD, see Graphene quantum dot “Grafting from” approach, 136, 137, 195 “Grafting to” approach, 136, 137, 193, 195 Graphene, 7–8 Graphene-based polymer composites, 7–8 Graphene nanoribbons (GNRs), 198 Graphene oxide quantum dots (GOQDs), 364 Graphene oxides (GOs), 8, 290, 363 Graphene quantum dot (GQD), 6, 45, 71, 71, 75, 113, 159, 174–175, 198, 208, 282, 289, 320–321, 320, 352, 363, 399, 402 Graphene quantum dot–polyaniline (PANI–GQD) composites, 174, 174 Graphene quantum dot/polymer composite-based sensors, 237 heavy metal ion sensing using, 238–239 sensing disease biomarkers using, 239–240 sensing drugs and contaminants using, 240–241 Graphite, 7 Greener synthetic route, 48 Ground state (GS) emission, 117 GS emission, see Ground state emission GT, see Gum tragacanth Gum tragacanth (GT), 419 Hall, Shelby, 114 HC, see Hectorite clay HEA, see Hydroxyethyl acrylate Heavy metal ion sensing, 238–239 Hectorite clay (HC), 325 Heteroatom-doped carbon quantum dots, 116 Heyd-Scuseria-Ernzerhof (HSE) screened Coulomb hybrid functional, 115 High-temperature injection organometallic synthesis of QDs, 47–49 Hodgkin’s lymphoma, 111 Hot injection method, 31 HPCCS, see Hydroxypropyl cellulose cross-linked chitosan HPC-PAA, see Hydroxypropylcellulose-poly (acrylic acid) HSE screened Coulomb hybrid functional, see Heyd-Scuseria-Ernzerhof screened Coulomb hybrid functional Hummers method, 404 Hybrid microgels temperature and pH-dependent swelling behavior of, 305–306 Hydrogels, 302 Hydrophilic QDs, synthesis of, 50–55 Hydrophobic interaction, solubilization by, 96 Hydrothermal method, 400 Hydrothermal/solvothermal method, 177 Hydroxyethyl acrylate (HEA), 420 Hydroxypropyl cellulose cross-linked chitosan (HPCCS), 423

445 Hydroxypropylcellulose-poly(acrylic acid) (HPCPAA), 311–312 Immunosensors, 217–219 InAs/GaAs QD solar cells, doping on, 114, 117 InAs quantum dot lasers, modulation p-doping in, 118–119 Incomparable optical features of QDs, 5 Indium quantum dots (InQDs), 282 Infectious diseases, imaging and sensing of, 111 Inorganic cores, 5 Inorganic ligands, 91–92 In situ imaging, 109–110 In situ polymerization method, 138, 139, 181 In situ preparation of polymers in the presence of QDs, 284–285 In situ synthesis of polymer–QDs gel hybrids, 310–315 Intrinsic characteristics regulation of QDs, 70–72 KH570-modified CQDs, 197 KPS, see Potassium persulfate La Mer’s growth technique, 31 Laser ablation, 177 Lateral flow assays (LFAs), 217 Layer-by-layer (LbL) method, 138–141 Layered double hydroxide (LDH), 139, 140 LbL method, see Layer-by-layer method LCD backlighting, see Liquid crystal display backlighting L-cystein, 5 LDH, see Layered double hydroxide Lead sulfide quantum dots, 264 LEDs, see Light emitting diodes LEF, see Linewidth enhancement factor LER, see Luminous efficacy of optical radiation LFAs, see Lateral flow assays Ligand capping, 343 Ligand exchange, 343–344 between QDs and polymer gels, 318–319 solubilization by, 95 Ligands, 76, 76, 88 inorganic, 91–92 organic, 89–91 Light emitting diodes (LEDs), 272, 272 QD–polymer composites in LED applications, see Quantum dot-based light-emitting diode (QD-LED) Limit of detection (LOD), 234 Linewidth enhancement factor (LEF), 118 Liquid crystal display (LCD) backlighting, 269–270, 270 Lithium ion intercalation, n-type doping by, 113 LOD, see Limit of detection Low-molecular-weight polyacrylic acids, 4

446 LSC, see Luminescence solar concentrators Luminescence solar concentrators (LSC), 322 Luminous efficacy of optical radiation (LER), 268 LUMO state, 6 MA, see 2-Mercaptoethylamine acid MALDI-TOF method, see Matrix-assisted laser desorption ionization time-of-flight method Marcus theory, 33 Matrix-assisted laser desorption ionization timeof-flight (MALDI-TOF) method, 209 MBC, see Minimum bactericidal concentrations MB dyes, see Methylene blue dyes MBE method, see Molecular beam epitaxy method MD, see Molecular dynamics MDL surface modification, see Multidentate ligand surface modification Melt blending method, 130–131 Memory applications, QDs in, 38–39 Mercaptoacetic acid (MAA), 95, 360 2-Mercaptoethylamine acid (MA), 5 3-Mercaptopropionic acid (MPA), 5, 314 Mercaptopropyltrimethoxysilane (MPTS), 94 Metal nanoclusters (MNCs), 341 METH, see Methamphetamine Methamphetamine (METH), 240 Methanol oxidation reaction (MOR), 291 Methodology for developing QDs, 31 colloidal nanosynthesis, 31–32 nanoscale patterning, 31 Stranski-Krastanow growth, 31 Methylene-bis-acrylamide (BIS), 312 Methylene blue (MB) dyes, 13, 14 Mg and Cu doping on ZnS quantum dots, 116 Micellar synthesis, colloidal QD synthesis via, 47 Microbial polymers, 428–429 Microfluidic synthesis of QDs, 57 Microreactor/microfluidic synthesis of QDs, 57–58 MicroRNAs (miRNAs), 351 Microwave-aided synthesis of QDs, 52 Microwave-assisted pyrolysis, 177–178 Microwave methods, 141–143, 401 Minimum bactericidal concentrations (MBC), 421 MIPs, see Molecularly imprinted polymers miRNAs, see MicroRNAs Mn:Cu Co-doped CdS nanocrystals, 119 MNCs, see Metal nanoclusters Mn doping on CdS quantum dot–sensitized solar cells, 117 Modulation p-doping in InAs quantum dot lasers, 118–119 Molecular beam epitaxy (MBE) method, 30 Molecular dynamics (MD), 99

Index Molecularly imprinted polymers (MIPs), 243 surface engineering of QDs by, 383–384 Molecular state, 77 Montmorillonite-human serum albumin (MtHSA) nanocomposites, 426 MOR, see Methanol oxidation reaction MPA, see 3-Mercaptopropionic acid MPTS, see Mercaptopropyltrimethoxysilane Mt-HSA nanocomposites, see Montmorillonitehuman serum albumin nanocomposites MTT test, 425 Multidentate ligand (MDL) surface modification, 93 Multi-walled carbon nanotubes (MWCNTs), 155, 432 MWCNTs, see Multi-walled carbon nanotubes MXene, 219 Nanocore-shell structures, 427 Nanofillers, 153, 430 chemistry, 156 in polymer matrices, 154–155 size and shape, 157–158 Nanoprecipitation, 160 Nanoreinforcement, 152 Nanoscale metal organic frameworks, 8 Nanoscale patterning, 31 Natural fillers, 431 Natural polymer, 421 N-CQDs, see Nitrogen-doped CQDs N-doped dots fluorescence emission, 114 Near-field effect, 367 Near-infrared region (NIR), 55 NFX drug, see Norfloxacin drug N-GQDs, see Nitrogen-doped GQDs NIR, see Near-infrared region Nitrogen, 74 Nitrogen-doped CQDs (N-CQDs), 209, 320 Nitrogen-doped GQDs (N-GQDs), 219 4-Nitrophenol (4-NP), 291 3-Nitrotyrosine, 237 Nitroxide-mediated polymerization (NMP), 137 NMP, see Nitroxide-mediated polymerization NMR spectroscopy, see Nuclear magnetic resonance spectroscopy NOESY, see Nuclear overhauser spectroscopy Non-covalent binding, 343 Noninjection method, organometallic synthesis of QDs by, 49–50 Non-metallic silicon material-based QDs, 282 Norfloxacin (NFX) drug, 323, 325 4-NP, see 4-Nitrophenol N-type dopants, 111 N-type doping by lithium ion intercalation, 113 Nuclear magnetic resonance (NMR) spectroscopy, 98

Index Nuclear overhauser spectroscopy (NOESY), 98 Nucleic acid LFA (NALFA), 217 OA, see Oleylamine O-carboxymethyl chitosan, 8 Octadecene (OE), 48 Octadecyl-4-vinylbenzyl-dimethyl-ammonium chloride (OVDAC), 384 OE, see Octadecene Oil-in-oil emulsion/solvent evaporation (O/OSE) approach, 426 OLEDs, see Organic LEDs Oleylamine (OA), 212 One-step fabrication of QDs and polymer composites, 285 O/OSE approach, see Oil-in-oil emulsion/solvent evaporation approach Optical properties of QDs, 5–6, 23, 69, 74–76 doping methods, 73–74 future perspectives, 79 intrinsic characteristics regulation of QDs, 70–72 modulation of the surface, 72–73 photoluminescence (PL) mechanisms, 77–79 photostability of QDs, 76 Optoelectronic devices, 36–38 Optoelectronic properties of QDs, 32–36 Organic contaminants photocatalytic degradation of, 13 photocatalytic destruction of, 15 Organic dye sensing using perovskite quantum dot/polymer composite-based sensors, 243 Organic LEDs (OLEDs), 38 Organic ligands, 89–91 Organometallic synthesis of QDs, 3, 50 by noninjection method, 49–50 Organophosphorous pesticide sensing, 243–245 Organosilicates, 55 Ostwald ripening, 32 OVDAC, see Octadecyl-4-vinylbenzyl-dimethylammonium chloride PAA, see Polyacrylic acid PAAm hydrogels, see Polyacrylamide hydrogels Packed bed reactor, synthesis of QDs by rotating, 59, 59 PAI composite films, see Polyamideimide composite films PAL, see Polycation polyaluminum phosphate Palladium nanoparticles, 290 PAMAM, see Polyamidoamine dendrimers PANI, see Polyaniline PANI–GQD composites, see Graphene quantum dot–polyaniline composites Partially reduced GOQDs (rGOQDs), 364

447 Particle deposition (PD), 160 Particle in a box model, 23, 25 Particle localization (PL), 6 Pauli’s exclusion principle, 33 Pax5, 110 PCL, see Poly(caprolactone) PCL, see Polycaprolactone PD, see Particle deposition PD, see Polymer dot PDA, see Polydopamine PDMAEMA, see Poly (di-methylaminoethyl methacrylate) P-doping for QD laser, 118 PEG, see Polyethylene glycol PEGDA, see Polyethylene glycol diacrylate PEG-NC hydrogel, see Poly(ethylene glycol)nitrocinnamic acid hydrogel PEG-PCL block copolymer, see Polyethylene glycol poly caprolactum block copolymer PEI-CdS QDs, see Polyethylenimine-capped CdS quantum dots Peptides, 347 Perovskite quantum dot/polymer composite-based sensors, 241 biomolecule sensing using, 248 chloride/iodide ion sensing using, 246–247 organic dye sensing using, 243 organophosphorous pesticide sensing using, 243–245 pH sensor development using, 248–249 UV radiation detection using, 245–246 Perovskite quantum dots (PQD), 241–243, 264, 341, 353 Pesticide detection polymer/supramolecular surface decorated QDs for, 382–383 QD-based, 381 silica QD composites for, 382 Pesticide recognition, doping of QDs for, 381–382 Phanerochaete chrysosporium, 213 pH-dependent photoluminescence properties, 306–308 Phosphors, 270 Photocatalyst materials, 11 Photodetectors, 271, 271 Photoexcited QDs, 76 Photo-induced electrons, 11 Photo-induced holes, 11 Photoluminescence (PL), 70, 77–79, 113 in carbon dots, 159, 235 Photoluminescence lifetimes (PL), 189 Photon upconversion, 6 Photostability of QDs, 76 pH-responsive hydrogel layer, 9 pH sensor development, 248–249

448 Physical mixing, 179–180 Physicochemical properties of QDs, 379–381 PIs, see Printed impressions PL, see Particle localization PL, see Photoluminescence PLA, see Poly (lactic acid) PLA, see Polylactic acid Plant bioimaging, 109 Plant-derived lipids and their composites, 420–421 Plant polysaccharides and their bionanocomposites, 415–419 Plant protein and their composites, 420 Plasmodium falciparum, 111 pMD, see p-modulation doping PMMA, see Poly(methyl methacrylate) p-modulation doping (pMD), 118 PNCs, see Polymer-based nanocomposites PNCs, see Polymer nanocomposites Pneumatic thermospray apparatus, 60 PNIPAM-based microgels, see Poly(Nisopropylacrylamide)-based microgels PNIPAM-co-VP, see Poly-(Nisopropylacrylamide-co-4 vinylpyridine) PNIPAm microgels, see Poly(N-isopropyl polyacrylamide) microgels P-nitrophenol, 13 Poly(2-acrylamido-2-methyl-1-propansulfonic acid) p(AMPS) hydrogel network, 312 Polyacrylamide (PAAm) hydrogels, 313 Polyacrylic acid (PAA), 4, 140–141, 193 Polyamideimide (PAI) composite films, 198 Polyamidoamine dendrimers (PAMAM), 343 Polyaniline (PANI), 160, 174 Polyanion polystyrene sulfonate (PSS), 195 Polycaprolactone (PCL), 194, 284, 425 Polycation polyaluminum phosphate (PAL), 195 Poly(di-methylaminoethyl methacrylate) (PDMAEMA), 429 Polydopamine (PDA), 289 Polyelectrolytes, 194 Polyethylene glycol (PEG), 93, 193, 287, 350–351, 419 Polyethylene glycol diacrylate (PEGDA), 313 Poly(ethylene glycol)-nitrocinnamic acid (PEGNC) hydrogel, 316 Polyethylene glycol poly caprolactum (PEG-PCL) block copolymer, 341 Polyethylenimine-capped CdS quantum dots (PEICdS QDs), 314 Polylactic acid (PLA), 153 Poly (lactic acid) (PLA), 351, 418, 419 Poly(maleic anhydride) copolymers, 194 Polymer-based bioactivation of QDs, 7–8 Polymer-based nanocomposites (PNCs)

Index benefits and complexities of, 153–154 Polymer composites, 413 animal lipid and their nanocomposites, 427–428 animal polysaccharides and their nanocomposites, 421–423 animal protein and their nanocomposites, 423 albumin and nanocomposites, 425–426 collagen and nanocomposites, 424–425 gelatin and nanocomposites, 423–424 silk fibroin and nanocomposites, 426–427 cytotoxicity, 337–338 different tests of biocompatibility of, 337 in-vitro, 337–338 in-vivo, 338 disposal/degradation, 430–431 future perspective, 431–434 microbial polymers, 428–429 plant-derived lipids and their composites, 420–421 plant polysaccharides and their bionanocomposites, 415–419 plant protein and their composites, 420 safety issues of polymer nanocomposites, 429–430 Polymer dot (PD), 159, 160–161, 161, 282 Polymer gels, 301–303 Polymeric ceramic nanoparticles, 430, 431 Polymeric nanocomposites, 433–434 Polymer materials, biodegradability test of, 339 anaerobic biodegradation of bioplastics, 340 dip-hanging method, 340 soil burial and compost conditions, 340 Polymer matrices, 266 dispersion of QDs in, 181–182 nanofillers for, 154, 155 nanofiller chemistry, 156 nanofiller size and shape, 157–158 shape dependency reinforcement, 155 reinforcement behaviors of fillers into, 161–164 Polymer matrices, photoluminescence property of QDs in, 359 carbon quantum dots (CQDs), 361–362 fluorescence properties of, 364–365 origin of photoluminescence in, 362–364 polymers as support for, 366–372 surface defect-derived origins, fluorescence emissions of, 365 surface passivation and quantum yield, 366 Polymer microspheres, embedding QDs into, 267 Polymer nanocomposites (PNCs), 126, 127, 129, 173–174, 190–191 processing techniques, 416 safety issues of, 429–430

Index Polymer nanomaterials, incorporation of QDs into, 267 Polymer–QD composite, biocompatibility of, 335 biocompatible polymer–QD composite materials, 345 alginate, 349–350 bovine serum albumin (BSA) protein, 346–347 cellulose, 347–348 chitosan, 348–349 gelatin, 347 peptides, 347 poly(L-lactide) (PLA), 351 polyethylene glycol (PEG), 350–351 polyvinyl alcohol (PVA), 352–353 silk, 351–352 classification of polymers, 336–337 cytotoxicity, 337 in-vitro, 337–338 in-vivo, 338 methods of coating QDs bioconjugation, 344 encapsulation, 342–343 ligand exchange, 343–344 Polymer quantum dot composites, 286 QDs and thermoplastic polymer composites, 286–287 QDs and thermosetting polymer composites, 287–288 Polymer–quantum dot gel hybrids, 299 applications of, 319–326 properties of, 304 hybrid microgels, temperature and pHdependent swelling behavior of, 305–306 pH-dependent photoluminescence properties, 306–308 poly(N-isopropylacrylamide) (PNIPAM)–QDs hybrids, 304–305 temperature-dependent photoluminescence studies, 308–309 synthesis of, 309 in situ synthesis, 310–315 ligand exchange between QDs and polymer gels, 318–319, 318 by loading of preformed QDs onto polymer gels, 315–318, 316 Polymer reaction method, 137 Polymers, 2, 174–175 classification of, 336–337, 337 Polymer/supramolecular surface decorated QDs for pesticide detection, 382–383 Poly(methyl methacrylate) (PMMA), 132, 284, 287 Poly(N-isopropylacrylamide) (PNIPAM)-based microgels, 304, 314, 319, 326, 326

449 Poly(N-isopropylacrylamide) (PNIPAM)–QDs hybrids, size distribution of, 304–305 Poly(N-isopropylacrylamide)-co-(acrylic acid) (pNIPAm-co-AAc) microgels, 312 Poly-(N-isopropylacrylamide-co-4 vinylpyridine) (PNIPAM-co-VP), 343 Poly(N-isopropyl acrylamide-co-acrylamide-co-2acrylamidomethyl-5fluorophenylboronic acid), 311 Poly(N-isopropylacrylamide-co-acrylic acid) poly-(NIPAM-AAc), 316 Poly(N-isopropyl acrylamide-co-acrylic acid-co2-hydroxyethyl acrylate) hydrogels, 310 Poly(N-isopropylacrylamide-co-methacrylic acid) poly(NIPAM-MAA) microgels, 316 Poly(N-isopropyl polyacrylamide) (PNIPAm) microgels, 305 Polypyrrole, 160 Polysaccharide, 421 Poly (sodium 4-styrene sulfonate) (PSS), 368 Polythiophene, 160 Polyurethane (PU), 290 Polyvinyl alcohol (PVA), 153, 284, 287, 290, 352–353, 367, 418 Poly(vinylidene) difluoride (PVDF) film, 420–421 Polyvinyl pyrrolidone (PVP), 284, 290 Potassium persulfate (KPS), 314 PQD, see Perovskite quantum dots Preparation of QD/polymer composites, 179 chemical grafting, 180–181 in situ polymerization method, 181 physical mixing, 179–180 Printed impressions (PIs), 367 Prokaryote bioimaging, 109 Properties of QD–polymer composites, 285–286 Properties of QDs, 22–25 PSS, see Poly (sodium 4-styrene sulfonate); Polyanion polystyrene sulfonate P-type dopants, 111, 112 PU, see Polyurethane PVA, see Polyvinyl alcohol PVDF film, see Poly(vinylidene) difluoride film PVP, see Polyvinyl pyrrolidone QD-LED, see Quantum dot-based light-emitting diode QD–polymer nanocomposite formation, challenges in, 143 QDSCs, see Quantum dot solar cells QDSi nanoparticles, 382 Quantum computing, 38 Quantum confinement effect, 22, 70, 175 Quantum dot-based light-emitting diode (QDLED), 259 applications of QDs, 269, 269

450 challenges, 265 classification of nanocomposite material used for, 264 color gamut, 268 color matching functions and chromaticity diagrams, 267 color quality scale (CQS), 268 color rendering index (CRI), 268 display devices, 269–273 evolution of, 262, 263 increasing the consistency and lifetime of, 268 lead sulfide QDs, 264 LEDs, combination of QDs with, 265 limitations, 265–266 luminous efficacy of optical radiation (LER), 268 modification of QDs surface chemistry, 266–267 optimization of, 267 perovskite QDs, 264 polymer microspheres, embedding QDs into, 267 polymer nanomaterials, incorporation of QDs into, 267 QDs and polymer matrix, compatibility of, 266 QDs with polymers, compatibility of, 265 reliability and lifetime of, 265 role of QDs in LEDs, 262–264 Quantum dots (QDs), 106, 127–129, 159–160, 175–176, 303–304, 340 Quantum dot–sensitized solar cells (QDSSCs), 117 Quantum dot solar cells (QDSCs), 114, 117 Quantum yield (QY), 189, 361 QY, see Quantum yield Radiative recombination, 175 RAFT polymerization, see Reversible additionfragmentation chain-transfer polymerization Reactive ion etching (RIE), 108 Reactive oxygen intermediates (ROI), 360 Reactive oxygen species (ROS), 387 Recrystallization, 32 Reduced graphene oxide (rGO), 363, 404, 417 Refluxing method to synthesize QDs, 52 Reinforcement behaviors of fillers into polymer matrices, 161–164 Reinforcement mechanisms of QDs–polymer composites, 151 carbon dots (CDs), 158–159 polymer-based nanocomposites (PNCs), 153–154 polymer dots, 160–161 polymer matrices, nanofillers for, 155 nanofiller chemistry, 156

Index nanofiller size and shape, 157–158 shape dependency reinforcement, 155 polymer matrices, nanofillers in, 154–155 polymer matrices, reinforcement behaviors of fillers into, 161–164 Resazurin, 290 Reversible addition-fragmentation chain-transfer (RAFT) polymerization, 137, 284, 313 rGO, see Reduced graphene oxide RhB, see Rhodamine B Rhodamine B (RhB), 13 RIE, see Reactive ion etching Rietveld profile method, 116 Ri-n-octylphosphine, 47 ROI, see Reactive oxygen intermediates Role of CQDs in the drug delivery field, 215–217 ROS, see Reactive oxygen species Rubber chloroprene (CR), 196 Rubber QD composites, 189 background and challenges, 190–191 elastomer matrices, QDs in, 195–201 surface modification of QDs by polymer phases, 191–195 Rydberg’s energy, 27 SBR, see Styrene-butadiene rubber Scaling-up aspect of QD synthesis, 56 microreactor/microfluidic synthesis of QDs, 57–58 packed bed reactor, synthesis of QDs by rotating, 59 spray-based technique, synthesis of QDs using, 60 Scanning electron microscope (SEM), 99, 419 Schematic representation of QD, 107 SCs, see Supercapacitors Self-assembly of QDs, 31 Semiconductor nanocrystals, 127, 189 Semiconductor quantum dots, 87 Sensors, QD–polymer composites as, 227 carbon dot/polymer composite, 228 biological sensors, 235–237 chemical sensors, 234–235 optical properties of, 229–233 physical sensors, 237 sensing application of, 233–234 future perspectives, 250–251 graphene quantum dot/polymer composite, 237 heavy metal ion sensing using, 238–239 sensing disease biomarkers using, 239–240 sensing drugs and contaminants using, 240–241 perovskite quantum dot/polymer composite, 241 biomolecule sensing using, 248

Index detection of UV radiation using, 245–246 development of pH sensor using, 248–249 sensing of chloride/iodide ion using, 246–247 sensing of organic dye using, 243 sensing of organophosphorous pesticide using, 243–245 SF, see Silk fibroin SFNPs, see Silk fibroin nanoparticles sGQD, see Sulfonated grapheme quantum dots Shell-structured QDs, 76, 76 Shockley-Red-Hall recombination, 119 Si doping on InAs/GaAs quantum dot solar cells, 117 Silanized nano-hydroxyapatite (Si-nHA) particles, 420 Silica-coated QDs, 94, 94 Silica encapsulation, 97 Silica QD composites for pesticide detection, 382 Silicates, 173 Silicon based quantum dots, 199, 199 Silicon delta-doping, 117–118 Silicone rubbers (SRs), 197, 199 Silicon QDs (SQDs), 75 Silk, 351–352 Silk fibroin (SF), 351, 426–427 Silk fibroin nanoparticles (SFNPs), 427 Silver-doped PbSe quantum dots, 115 Silver quantum dot, 282 Single-walled carbon nanotubes (SWCNTs), 208 Si-nHA particles, see Silanized nanohydroxyapatite particles Size-dependent fluorescence, QDs in, 4 S-nitrosothiols (SNO), 216 SNO, see S-nitrosothiols Soil burial and compost conditions, 340 Solar cell-based light source, 270–271 Solution blending method, 131–136 Solvothermal/hydrothermal process, 400–401 Sonochemical strategy, 55 Soybean oil, 420 Soy protein isolate/reduced graphene oxide (SPI/ rGO) nanocomposites, 420 SPBs, see Spherical polyelectrolyte brushes Spherical polyelectrolyte brushes (SPBs), 307–308 SPIONs, see Superparamagnetic iron oxide nanoparticles Spray-based technique, synthesis of QDs using, 60 SQDs, see Silicon QDs SRs, see Silicone rubbers Staphylococcus aureus, 425 Starch, 418 Stepwise growth method, 137 Stimulus-responsive polymers, 316 Stranski-Krastanow growth, 31

451 Structures and properties of QD–polymer composites, 285–286 STS, see Surface trap states Styrene-butadiene rubber (SBR), 196, 201 Sulfonated grapheme quantum dots (sGQD), 323 Sulfuric acid, 416 Supercapacitors (SCs), 395, 396, 397 direct chemical cutting process, 403 electrochemical process, 402 Hummers method, 404 microwave synthesis, 401 nanomaterials and importance of QDs as electrode material, 397–400 pros and cons and future scope, 405 QDs and polymer composites in applications of, 404–405 solvothermal/hydrothermal process, 400–401 Superparamagnetic iron oxide nanoparticles (SPIONs), 8, 428 Surface defect-derived origins, fluorescence emissions of, 365 Surface defects, 6 Surface ligands, 88 inorganic ligands, 91–92 organic ligands, 89–91 Surface modification strategies of QDs, 92–95 characterization of QD surfaces, 97–99 hydrophobic interaction, solubilization by, 96 ligand exchange, solubilization by, 95 by polymer phases, 191–195 silica encapsulation, 97 Surface oxidation states, 75 Surface passivation and quantum yield, 366 Surface state-controlled photoluminescence in CQDs, 78 Surface-to-volume (SV) ratio, 158 Surface trap states (STS), 371 SV ratio, see Surface-to-volume ratio SWCNTs, see Single-walled carbon nanotubes Synthesis of QDs, 2–5, 45, 176 biosynthetic approach, preparation of QDs using, 56 bottom-up approach, 108, 177 hydrothermal/solvothermal method, 177 microwave-assisted pyrolysis, 177–178 ultrasonication, 178 wet chemical methods, 177–178 chemical strategy, 47 colloidal quantum dot synthesis via micellar synthesis, 47 high-temperature injection organometallic synthesis of QDs, 47–49 hydrophilic QDs, synthesis of, 50–55 organometallic synthesis of QDs by noninjection method, 49–50 physical strategy, 46

452 scaling-up aspect of QD synthesis, 56 microreactor/microfluidic synthesis of QDs, 57–58 packed bed reactor, synthesis of QDs by rotating, 59 spray-based technique, synthesis of QDs using, 60 top-down approach, 108, 176 arc discharge, 176 chemical/electrochemical oxidation, 176 laser ablation, 177 vapor phase methods, 178 Synthetic nanocarbon nanofillers, 431 Taguchi technique, 425 TCB, see Thiocholine bromide TCH, see Tetracycline hydrochloride TDDFT, see Time-dependent DFT TDs, see Threading dislocations TEM, see Transmission electron microscopy Temperature-dependent photoluminescence studies, 308–309 TEMPO-oxidized CNC (TOCNC), 417, 417 Tetracycline hydrochloride (TCH), 419 TGA, see Thioglycolic acid Thermal gravimetric measurements, 419 Thermochromism, 237 Thermoplastic and thermosetting plastic composites, 171 applications of QD/polymer composites, 182–183 future perspectives, 183–184 polymer matrix, dispersion of QDs in, 181–182 polymer nanocomposites, 173–174 QDs/polymer composites, preparation of, 179 chemical grafting, 180–181 in situ polymerization method, 181 physical mixing, 179–180 QDs/polymer composites, typical polymers in, 174–175 synthesis methods of QDs, 176 bottom-up approach, 177–178 top-down approach, 176–177 vapor phase methods, 178 Thermoplastic polymer composites, QDs and, 286–287 Thermosetting polymer composites, QDs and, 287–288 Thin-film membranes, QD-embedded, 385 Thiocholine bromide (TCB), 133 Thioglycerol/thioglycolic acids, 316 Thioglycolic acid (TGA), 5 Thiol-stabilized QDs, 193

Index Threading dislocations (TDs), 118 Time-dependent DFT (TDDFT), 77 Time-resolved photoluminescence (TRPL) measurements, 117 TOCNC, see TEMPO-oxidized CNC Tocopherol polyethylene glycol 1000 succinate (TPGS), 427 TOP, see Trioctylphosphine Top-down approach, 31 TOPO, see Tri-n-octylphosphine oxide Toxicity of QDs, 212–213, 385–386 TPGS, see Tocopherol polyethylene glycol 1000 succinate Tracking of particles, 109 Transition metal oxides/hydroxides (TMO/OH)based materials, 398 Transmission electron microscopy (TEM), 95, 99 Tri-n-octylphosphine oxide (TOPO), 48, 53, 54, 89–90, 93, 133, 319, 360, 381 Trioctylphosphine (TOP), 381 TRPL measurements, see Time-resolved photoluminescence measurements Types of quantum dots, 27, 28 alloyed QDs, 29–30 core-shell QDs, 29 core-type QDs, 28 doped quantum dots, 31 Ultrasonication, 178 Ultrasonic treatment, 55 Ultraviolet (UV) radiation detection, 245–246 Ulvan nanoparticles (UN), 423 UN, see Ulvan nanoparticles Upconversion optical characteristics, 6 Van der Waals interactions, 156 Vapor phase methods, 178 VOCs, see Volatile organic compounds Volatile organic compounds (VOCs), 196 Water-soluble QDs, 53 Wet chemical methods, 177 hydrothermal/solvothermal method, 177 microwave-assisted pyrolysis, 177–178 ultrasonication, 178 White light emitted diodes (WLED), 135 WLED, see White light emitted diodes Zein, 419 Zero-dimensional (0-D) nanomaterials, 22–23 Zero-dimensional (0-D) nanostructures, 35 Zinc sulfide, 29 Mg and Cu doping on ZnS quantum dots, 116 Zirconia, 13