Advanced Nanostructures for Environmental Health 0128158824, 9780128158821

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Advanced Nanostructures for Environmental Health

Advanced Nanostructures for Environmental Health Edited by

Lucian Baia Zsolt Pap Klara Hernadi Monica Baia

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-815882-1 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

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Contributors Simion Astilean Nanobiophotonics and Laser Microspectroscopy Center, Interdisciplinary Research Institute in Bio-Nano-Sciences; Department of Biomolecular Physics, Faculty of Physics, Babeş-Bolyai University, Cluj-Napoca, Romania Monica Baia Faculty of Physics; Advanced Materials and Applied Technologies Laboratory, Institute of Research-Development-Innovation in Applied Natural Sciences, Babeș-Bolyai University, Cluj-Napoca, Romania Lucian Baia Faculty of Physics; Nanostructured Materials and Bio-Nano-Interfaces Center, Interdisciplinary Research Institute on Bio-Nano-Sciences; Advanced Materials and Applied Technologies Laboratory, Institute of Research-Development-Innovation in Applied Natural Sciences, Babeș-Bolyai University, Cluj-Napoca, Romania Mihaela Baibarac National Institute of Materials Physics, Laboratory of Optical Processes in Nanostructured Materials, Bucharest, Romania Marianna Bellardita “Schiavello-Grillone” Photocatalysis Group, Dipartimento di Ingegneria (DI), University of Palermo, Palermo, Italy Annika Blohm Leibniz Institute of Photonic Technology, Jena, Germany Cristina Bogatu R&D Centre, Renewable Energy Systems and Recycling, Transilvania University of Brasov, Brașov, Romania Ioan Botiz Nanobiophotonics and Laser Microspectroscopy Center, Interdisciplinary Research Institute in Bio-Nano-Sciences, Babeş-Bolyai University, Cluj-Napoca, Romania Lavinia Florina Ca˘linoiu Faculty of Food Science and Technology; Institute of Life Sciences, University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca, Cluj-Napoca, Romania Andreea Campu Nanobiophotonics and Laser Microspectroscopy Center, Interdisciplinary Research Institute in Bio-Nano-Sciences, Babeş-Bolyai University, Cluj-Napoca, Romania Dana Cialla-May Leibniz Institute of Photonic Technology (IPHT); Friedrich Schiller University Jena, Institute of Physical Chemistry and Abbe Center of Photonics; Research Campus InfectoGnostics, Jena, Germany

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Maria Covei R&D Centre, Renewable Energy Systems and Recycling, Transilvania University of Brasov, Brașov, Romania Agatino Di Paola “Schiavello-Grillone” Photocatalysis Group, Dipartimento di Ingegneria (DI), University of Palermo, Palermo, Italy Anca Duta R&D Centre, Renewable Energy Systems and Recycling, Transilvania University of Brasov, Brașov, Romania Monica Focsan Nanobiophotonics and Laser Microspectroscopy Center, Interdisciplinary Research Institute in Bio-Nano-Sciences, Babeş-Bolyai University, Cluj-Napoca, Romania Carmen Ioana Fort Faculty of Chemistry and Chemical Engineering; Institute of Research-Development-Innovation in Applied Natural Sciences, “Babeş-Bolyai” University, Cluj-Napoca, Romania Torsten Frosch Leibniz Institute of Photonic Technology (IPHT); Friedrich Schiller University Jena, Institute of Physical Chemistry and Abbe Center of Photonics, Jena, Germany Florin Gherendi National Institute for Laser, Plasma and Radiation Physics, Bucharest, Romania € mru € kc¸ u € og˘lu Department of Chemistry, Faculty of Science, Karadeniz Abidin Gu Technical University, Trabzon Klara Hernadi Department of Applied and Environmental Chemistry, University of Szeged, Szeged, Hungary Dana Maniu Nanobiophotonics and Laser Microspectroscopy Center, Interdisciplinary Research Institute in Bio-Nano-Sciences; Department of Biomolecular Physics, Faculty of Physics, Babeş-Bolyai University, Cluj-Napoca, Romania Laura Mitrea Faculty of Food Science and Technology; Institute of Life Sciences, University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca, Cluj-Napoca, Romania € € han Ocak Department of Chemistry, Faculty of Science, Karadeniz Technical Umm u University, Trabzon Zafer Ocak Department of Mathematics and Science Education, Faculty of Education, Kafkas University, Kars, Turkey Mirac¸ Ocak Department of Chemistry, Faculty of Science, Karadeniz Technical University, Trabzon Leonardo Palmisano “Schiavello-Grillone” Photocatalysis Group, Dipartimento di Ingegneria (DI), University of Palermo, Palermo, Italy

Contributors

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Zsolt Pap Nanostructured Materials and Bio-Nano-Interfaces Center, Interdisciplinary Research Institute on Bio-Nano-Sciences, Babeș-Bolyai University, Cluj-Napoca, Romania; Institute of Environmental Science and Technology, University of Szeged, Szeged, Hungary Francesco Parrino “Schiavello-Grillone” Photocatalysis Group, Dipartimento di Ingegneria (DI), University of Palermo, Palermo, Italy Dana Perniu R&D Centre, Renewable Energy Systems and Recycling, Transilvania University of Brasov, Brașov, Romania Lucian Cristian Pop Faculty of Chemistry and Chemical Engineering; Institute of Research-Development-Innovation in Applied Natural Sciences; Interdisciplinary Research Institute on Bio-Nano-Sciences, “Babeş-Bolyai” University, Cluj-Napoca, Romania € rgen Popp Leibniz Institute of Photonic Technology (IPHT); Friedrich Schiller Ju University Jena, Institute of Physical Chemistry and Abbe Center of Photonics; Research Campus Infectognostics, Jena, Germany Monica Potara Nanobiophotonics and Laser Microspectroscopy Center, Interdisciplinary Research Institute in Bio-Nano-Sciences, Babeş-Bolyai University, Cluj-Napoca, Romania Ga´bor Ra´khely Department of Biotechnology, Faculty of Science and Informatics; Institute of Environmental Science and Technology, University of Szeged, Szeged, Hungary Anne Sieburg Leibniz Institute of Photonic Technology, Jena, Germany Bianca Eugenia Ştefa˘nescu Institute of Life Sciences, University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca; Department of Pharmaceutical Botany, Iuliu Hațieganu University of Medicine and Pharmacy, Cluj-Napoca, Romania Katalin Szabo Institute of Life Sciences, University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca, Cluj-Napoca, Romania Ioana Tismanar R&D Centre, Renewable Energy Systems and Recycling, Transilvania University of Brasov, Brașov, Romania N’ghaya Toulbe National Institute of Materials Physics, Laboratory of Optical Processes in Nanostructured Materials, Bucharest, Romania b Institute of Process Engineering, Faculty of Engineering, University of Ga´bor Vere Szeged, Szeged, Hungary Dan Cristian Vodnar Faculty of Food Science and Technology; Institute of Life Sciences, University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca, Cluj-Napoca, Romania

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Contributors

Karina Weber Leibniz Institute of Photonic Technology (IPHT); Friedrich Schiller University Jena, Institute of Physical Chemistry and Abbe Center of Photonics; Research Campus InfectoGnostics, Jena, Germany

1 When the nanostructures meet the environmental health key issues Monica Baiaa,b, Zsolt Papc,d, Klara Hernadie, Lucian Baiaa,b,c FAC UL TY OF PH YS ICS, BA BE Ș-BOL YAI UNIVERSITY, CLUJ-NAPOCA, ROMANIA b ADVANCED MATERIAL S AND AP PLIED T ECHNOLOGIES LA B ORAT ORY, I NSTI TUT E OF RE SEARC HDEV ELO PME NT- INNO VATI ON I N A PPL IE D NAT UR AL SCI ENCES , BABEȘ- BO LYAI UNI VE RSI TY , CLUJ-NAPOCA, ROMANIA c NANOSTRUCTURED MATERIALS AND BIO- NANO-INTERFACES CENTER, INTERDISC IPLINARY RESEARCH INSTITUTE ON B IO-NANO-SCIENCES, BABEȘ -B OLY AI UNIVERS IT Y, CLUJ-NAP OC A, ROMANI A d INSTITUTE O F E NV IRONMENTAL SCIENCE AND TEC HN O L O GY , UNI VE R S I T Y OF SZEGED, SZEGED, HUNGARY e DEPARTMENT OF APPLIED AND ENVIRONMENTAL CHEMI STRY, UNIVERSITY OF SZEGED, SZEGED, HUNGARY a

1.1 About environmental health and nanostructures and their relation with the real life The first impression if someone hears the word “health” is indisputably the human condition. However, a disambiguation page of Wikipedia [1] gives the following definition: “Health is the level of functional and/or metabolic efficiency of an organism.” In turn, for “Health care, the prevention, treatment, and management of illness.” Both approaches are strongly correlated to any form of life. Health, as defined by the World Health Organization (WHO), is “a state of complete physical, mental, and social wellbeing and not merely the absence of disease or infirmity” [2]. Environmental health [3] was first defined in a 1999 document by the WHO as those aspects of human health and disease that are determined by factors in the environment. It also refers to the theory and practice of assessing and controlling factors in the environment that can potentially affect health. A few years later in 2016, the WHO website on environmental health stated, “Environmental health addresses all the physical, chemical, and biological factors external to a person, and all the related factors impacting behaviors. It encompasses the assessment and control of those environmental factors that can potentially affect health. It is targeted toward preventing disease and creating health-supportive environments. This definition excludes behavior not related to the environment as well as behavior related to the social and cultural environment as well as genetics” [3]. Premonitory signs regarding the recrudescence of environmental health have been dramatically augmented in the last few decades. The impact of human activity on the environment is unquestionable; however, its extent is still controversial. Anthropogenic activity such as pollution, overconsumption, clearcutting, overpopulation, etc., inevitably Advanced Nanostructures for Environmental Health. https://doi.org/10.1016/B978-0-12-815882-1.00001-X © 2020 Elsevier Inc. All rights reserved.

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affect the Earth’s balance related to its health. Man-made chemicals (plastics, pharmaceuticals, industrial/agricultural additives, etc.) that are unknown in natural circles can also be emerging hazards for the environment. It is known that even new-born babies are affected by pollution, according to a study released in 2009 and conducted by the Environmental Working Group in the United States. That study showed that 232 toxic chemicals were found in the umbilical cord blood of 10 babies. One can easily imagine that the data has not changed in a positive direction. A few years ago, the Nature journal stated that a study of 3 million infants suggested a connection between inhaled particles and birth weight [4]. Moreover, air pollution also affects children’s neurodevelopment and cognitive ability and can trigger asthma and childhood cancer. Children who have been exposed to high levels of air pollution may be at a greater risk for chronic diseases such as cardiovascular disease later in life. More recent data state that daily, approximately 93% of the world’s children under the age of 15 (1.8 billion children) breathe polluted air that can seriously affect their health and development. Tragically, many of them die. WHO estimates that in 2016, 600,000 children died from acute lower respiratory infections caused by polluted air [5]. Contrasts between human health and environmental health seems to be antagonistic. Nevertheless, a responsible and ecoconscious turn of mind might stop and shunt mischievous trends. It is well known that material properties change drastically on the nanoscale, which may differ from those of single crystals or conventional materials. Humankind has been utilizing nanomaterials since ancient times (e.g., the famous Lycurgus cup, Damascus alloys). Since the last decades of the 20th century, nanoscience and nanotechnology have been rapidly emerging disciplines dealing with the synthesis, characterization, investigation, and manipulation of nanostructured materials. Because of the possible hazards, as with all artificial products, nanomaterials must be handled with widespread particular care; nevertheless, nanotechnology definitely has the potential to achieve fundamental breakthroughs in many fields.

1.2 Approaches regarding the advanced nanostructures used in environmental health issues This book is supposed to present possibilities and advantages via the application of advanced nanostructures for environmental health. Selected topics confirm that these new types of materials are able to open innovative perspectives in environmental protection. Using the nanotechnology “toolkit,” solutions can be provided for a great diversity of environmental problems such as sensitive detection for organic pollutants, metals and metalloids, gas molecules, or microbial contaminants as well as photocatalytic water and air cleaning. At the beginning of the book, from the large amount of organic pollutants, Chapter 2 titled “Sensitive detection of organic pollutants by advanced nanostructures” describes the detection of hydrocarbons, that is, fuels and polycyclic aromatic hydrocarbons (PAHs),

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organic solvents as well as volatile organic compounds (VOCs), pesticides and persistent organohalides and finally, compounds from industrial production and water treatment such as phenols, dye molecules, and disinfection agents by using advanced nanostructures. By taking advantage of advanced nanostructures, the most-used sensing strategies are based on electrochemical, colorimetric, and optical sensing schemes [6]. The reason behind choosing such approaches is simple if we think that nanomaterials and nanostructures exhibit excellent optical, electrical, and catalytic properties along with mechanical stability [6]. Thus, the chapter describes the application of carbon nanostructures, metallic nanoparticles (NPs), silicon nanomaterials, and quantum dots for sensitive detection by using field-effect transistors, electrochemical approaches, fluorescence, surfaceenhanced Raman spectroscopy (SERS), and colorimetry in environmental monitoring. Precisely, in the case of PAH detection, SERS is frequently applied not only in its conventional form but also with increased sensitivity by using either alkyl thiol layers or host molecules to capture PAHs and so to enrich them on the sensor surface [7, 8]. Moreover, for the sensitive detection of organic solvents and VOCs, electrical detection schemes employing electrodes prepared with advanced nanostructures have proven to be favorable [9]. Portable systems are still requested to permit the fast tracking of organic solvents in environmental samples as well as VOCs in indoor settings. In view of the variety of chemical classes of pesticides, various detection schemes are required. Thus, for the specific and sensitive detection of organohalide pesticides in environmental samples, aptamer- and immunoassay-based detection schemes combined with electrochemical and optical techniques are introduced [10]. For the detection of organophosphorous pesticides, the inhibition of the acetylcholinesterase (AChE) enzyme is employed in both electrochemical and optical detection schemes [11–14]. Lastly, for dithiocarbamate pesticide detection, SERS-active advanced nanostructures were used by taking advantage of the strong binding affinity of these molecules toward the metallic sensor platform [15–17]. The category of halogenated biphenyls and bisphenols is monitored by fluorescence- and SERS-based techniques and also by electrochemical methods with nanostructured sensor platforms [18]. With the aim of increasing the specificity to singular target molecules, aptamers, antibodies, molecular-imprinted polymers, or cyclodextrin units are used to specifically transform the nanostructured sensor surface [18, 19]. Besides, the SERS method is also the right choice for the detection of low concentrations of dyes in industrial wastewater, as the resonance Raman enhancement supplementary contributes to the overall SERS spectra [20]. The detection of the disinfection agent sodium hypochlorite is accomplished by electrochemical, colorimetric, and spectroscopic approaches [21–23]. This chapter highlights the fact that it is not feasible to draw an overall conclusion about the architecture of advanced nanostructures and the related detection limit of target analytes because many sensing platforms with similar performances exist. It is obvious that in SERS sensing strategies, the presence of hot spots is related to lower detection limits. In all sensing schemes, the specific enrichment of the target analyte on the sensor surface by specific sensor layers or capture molecules is extremely important. On the other

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hand, because molecules from different chemical classes have different chemical properties, it is evident that different detection schemes are necessary. Therefore, no universal approach is available for monitoring all organic pollutants with the same nanosensor platform. To overcome this drawback, the research might be directed toward finding a suitable combination of different sensing schemes such as SERS with electrochemical methods. The next chapter, Chapter 3 titled “Nanostructure-based detection of pharmaceuticals and other contaminants of emerging concern,” reports on the performances of sensing platforms based on nanostructures/nanocomposites in the detection of several pharmaceutical compounds and also of hormones, toxins, and neurotoxins. Throughout the chapter, the detection of five active pharmaceutical compounds, namely acetaminophen (AC), folic acid (FA), α-lipoic acid (ALA), melatonin (MEL), and azathioprine (AZA), is described. The motivation behind the selection of these pharmaceutical compounds is related to the therapeutic treatment’s schema of the late onset diseases of the brain that contain drugs with these compounds as active substances [24–30]. By taking into account the excellent and specific properties of nanostructures such as enhanced electron transfer and increased interfacial adsorption, a high surface-to-volume ratio, a large specific surface area, outstanding optical properties, and good biocompatibility, the most-used sensing strategies for these pharmaceutical components are based on electrochemical and optical sensing methods [31]. The chapter describes the available sensing platforms prepared from nanostructures/nanocomposites based on carbon nanotubes, graphenes, metallic nanoparticles, and oxides in unfunctionalized and functionalized states with various macromolecular species for the electrochemical and/or optical detection of the selected pharmaceutical compounds, hormones, toxins, and neurotoxins. Thus, for the detection of acetaminophen (AC) and folic acid (FA), considerable effort was made to develop sensitive platforms based on carbon-based nanostructures or oxides used in electrochemical schemes [32–36], whereas only a few such electrodes were developed for the electrochemical detection of α-lipoic acid (ALA), melatonin (MEL), and azathioprine (AZA) [37–39]. As compared to the sensitive platforms containing carbon-based nanostructures or oxides employed for electrochemical detection, the ones based on metallic nanostructures were proved to be more versatile, as they were used both in the electrochemical and optical detection schemes of the drugs [40–42]. The chapter concludes that in the last years, the detection of different compounds by electrochemical methods has proved to be more sensitive and cheaper, but very difficult to be used for real applications. On the other hand, optical detection remains one of the most employed methods for environmental monitoring in real applications in spite of its drawbacks, namely the high cost and the need for expertise in this field. In the case of hormone detection, several sensing platforms based on nanomaterials for electrochemical and optical detection were described. It was found that optical detection by SERS spectroscopy is the more efficient one [43–45]. Among the detection strategies of toxins/neurotoxins reported in recent years, it was concluded that the most adequate ones are those based on optical detection by means of SERS and fluorescence methods [46, 47].

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The improvement of sensing platforms for drug detection is expected to occur as a consequence of in depth knowledge of the electrochemical and optical properties of the employed nanomaterials and pharmaceutical compounds. Nowadays, heavy metal and metalloid residues imply a major environmental concern because of their potential risk to the environment, thus making them a concern to both human and animal health. Consequently, the proper detection of heavy metals and metalloids is a rapidly emerging research field these days. The overwhelming part of the elements belongs to metals and metalloids (approximately 80%) including light metals, alkaline, or earth alkaline, and heavy metals (transition metals, lanthanides, and actinides). However, there are no strict categories to classify heavy metals. Among these, only some metals might be essential for mammal health, too. In spite of their possible biological role, heavy metals are often considered as highly toxic or damaging to the environment in general. Irrespective of the level of toxicity, their proper detection is a highly necessary issue. Chapter 4 is titled “Sensitive detection of metals and metalloids by using nanostructures and fluorimetric methods” and provides an overview of the recently reported results on the determination of metals that are toxic or essential for the human body (e.g., copper, iron, zinc, mercury, cadmium, chromium, lead) and metalloids by fluorimetric methods with emphasis on different detection strategies as well as their advantages and limitations. The origin of fluorescence phenomena by using nanostructures can be explained by different mechanisms, depending on the used analytical strategies. In some cases, a standard fluorescent dye is attached to a nanostructure and its fluorescence response is pursued in the system. Such systems are known as labeled nanostructures and the use of a fluorescent nanomaterial is not necessary at all. In other systems, an intrinsically fluorescent nanostructure is used as a fluorophore moiety without any labeling. However, many systems based on using nanostructures need a selective or specific binding moiety to a metal ion, which can be either a synthetic molecular ligand or an aptamer. Thus, throughout the chapter, the authors first describe the nanostructures that can be used in fluorimetric determinations of metal and metalloids, namely noble metal nanoparticles (e.g., Ag, Au), metal chalcogenide quantum dots (e.g., CdTe, CdSe, ZnS, ZnSe, MoS2), metal oxide nanoparticles (e.g., SiO2 Fe3O4 magnetite, etc.), upconversion nanomaterials (e.g., NaYF4, NaGdF4, LiYF4, YF3, Gd2O3), and carbon-based nanomaterials (e.g., carbon dots). Then, the detection of different metals and metalloids by fluorimetric methods and the use of such nanostructures are presented. Specifically, for silver detection, inorganic-based nanomaterials (e.g., MoS2 nanosheets, Fe3O4 nanoparticles, Ag and Au nanoclusters, etc.) and carbon-based nanomaterials (carbon nanotubes, boron and nitrogen doped-carbon nanosheets, etc.) have been used [48–51], whereas only a small number of organic nanomaterials have been found to be employed in detection schemes. In the case of aluminum ion detection, inorganic nanomaterials such as Cu nanoclusters and Au nanoclusters, carbon-based nanomaterials (e.g., carbon nanodots and graphene oxide nanoparticles), organic nanoaggregates and nanofibers, and organic-inorganic hybrid nanostructures were mainly

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used [52–55]. Regarding cobalt detection, both inorganic (e.g., CePO4:Tb3+ nanocrystals, noble metal nanoclusters, etc.) and organic nanostructures [56–59] were reported to be used with limits of detection in the microM range. Due to the severe dangerous effects of heavy metal pollution, various strategies for their detection with the help of fluorimetric methods and the use of nanostructures, most of them inorganic nanostructures such as AuNPs, Cu nanoparticles, magnetite (Fe3O4) nanoparticles, and carbon dots, are available and were introduced in the chapter [60–63]. The detection limits of mercury in some detection schemes were in the picoM range [60, 64]. In the case of cadmium detection, the mainly used nanostructures are among the metal nanoclusters the Au ones, from the metal chalcogenides class especially, cadmium chalcogenides such as CdS, CdTe and CdSe are the most commonly used chalcogenide quantum dots, a few organic NPs are also proposed, and also functionalized metallic-organic frameworks (MOF) and upconversion nanoparticles are among the nanostructures used in Cd2+ determination [65–67] with detection limits mainly in the nanoM range. For zinc detection, several nanosensors were introduced based on metal nanomaterials (e.g., Au nanoclusters, Si nanoparticles, etc.), magnetite nanomaterials, metal chalcogenide quantum dots (e.g., CdSe/ZnS core/shell QDs modified with azamacrocycles, CuInS2 QDs, etc.), carbon-based nanomaterials (e.g., carbon nanotubes, CdTeQDs-CDs), and other nanomaterials proved to detect concentrations in the microM range [68–73]. The chapter further presents some nanostructures-based fluorimetric detection schemes of lead. Thus, some of them proved to be very sensitive and selective for lead detection using metal nanoparticles and metal chalcogenides, usually based on using aptamers and DNAzymes [74, 75]. Magnetite nanoparticles were also used to detect Pb2+ [76]. Moreover, carbon-based nanomaterials such as carbon nanotubes and carbon dots are also employed to detect lead [77, 78]. Upconversion nanoparticles are other nanomaterial types proposed for lead detection by fluorimetric methods with extremely low detection limits (10
13 M) [79]. Also, the fluorimetric detection methods of iron, which is one of the most investigated metals, by means of nanostructures are described. Thus, from the category of inorganic nanomaterials, the most common nanostructures are the Au, Ag, Cu, and Pt nanoclusters able to detect iron even at the nanoM concentration [80]. Then, many iron detection methods are based on carbon nanomaterials such as carbon dots, graphene oxide, and graphene quantum dots [80–83] and also on other nanostructures such as metal-organic framework nanosheets [84], mesoporous silica nanoparticles [85], organic nanoparticles [86, 87], and upconversion nanomaterials [88]. Various nanostructures were also used for copper determination by fluorimetric methods. The most recommended metal nanomaterials for copper determination are Au nanoparticles and Au nanoclusters functionalized or not with different groups [89, 90] besides others metal chalcogenide quantum dots or magnetite-based nanoparticles [91, 92]. Carbon nanodots were also proposed as fluorescent materials for copper detection [93], together with other carbon-based nanostructures such as fluorescent group labeled-carbon nanotubes [94], graphitic carbon nitride nanosheets [95], carbon nitride

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nanodots [96], etc. A large variety of polymeric nanomaterials, which includes amidine/ Schiff base dual-modified polyacrylonitrile nanoparticles, crosslinked polythiophenes, metal-organic coordination polymer nanoparticles including Tb3+, luminol, and guanine monophosphatem [97–99], were also used for copper detection alongside other nanostructures. For chromium detection, mainly two groups of nanomaterials are used in fluorimetric detection schemes. Thus, Au nanoparticles are mostly employed for chromium detection from various environments with limits of detection of 10
9 M [100, 101]. Metal chalcogenide quantum dots were also proposed to detect chromium in the same range of concentrations [102]. In addition to the metal nanostructures, the chapter also presents some examples of carbon nanomaterials used for chromium detection such as carbon nanodots, undoped and doped with nitrogen and/or phosphorus; carbon nanoglobules from natural products [103–105]; and also some other nanostructures such as nanogels, core-shell type upconversion nanostructures, LiYF4:Yb3+/Ho3+/Ce3+@LiYF4, etc. [106, 107]. Only a few arsenic detection schemes are mentioned in the chapter. These are based mainly on inorganic nanomaterials (e.g., Au nanoclusters, L-cysteine capped CdTe quantum dots), but there are also examples of other proposed materials (e.g., branchpolyethyleneimine modified multiwalled carbon nanotubes, cysteamine-capped CdTe/ ZnS core-shell quantum dots) [108–110]. There are several fluorimetric approaches for detecting gold, calcium, manganese, and tin by using nanostructures. These are mainly based on metal nanomaterials and, as expected, are specifically designed for each metal and were able to detect concentrations in the microM range [111–113]. Finally, the chapter highlights the fact that fluorimetric methods for metal detection with nanostructures are of real interest because they are fast, simple, precise, and costeffective methods. It also provides an overview of the metals that can be detected by the most recent fluorimetric detection strategies, underlying that there is a need to develop further detection schemes, especially for metalloid detection. Moreover, Chapter 5 titled “Heavy metal and metalloid electrochemical detection by composite nanostructures” delivers an overview of the available composite nanostructures-based materials used as electrode materials for heavy metal and metalloid electrochemical detection. This chapter highlights the current progress in the design and development of nanocomposites-based electrochemical electrodes used as sensors for heavy metal and metalloid detection. Hopefully, this outlook will further inspire the development of innovative nanocomposite materials such as electrode materials for the improved electrochemical detection of heavy metals and metalloids. The main features of electrode performances such as sensitivity, reproducibility, stability, selectivity, fast response, efficiency in the multiplexed detection, and onsite detection ability are summarized in this review. Because of the drawbacks (difficult sample preparation, preconcentration procedures, qualified personnel, and constraint to a single composition detection) of other techniques such as AAS, ICP-MS, IC-UV-Vis, ICP-AES, ICP-OES, CE,

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etc., the user-friendly, low-cost electrochemical techniques with good sensitivity, reproducibility, stability, and selectivity are very attractive in this field. However, there are strict requirements regarding the electrode materials. They must be green and biocompatible; moreover, their applicability (also their miniaturization and automatization) is essential for widespread claims from environmental monitoring to the clinical, safety, aerospace, and defense and security fields. Due to their unique properties, nanocomposites-based electrode materials have proven to be the best candidates to overcome many challenges [114–117]. Because of their extraordinary features, carbon nanocomposites-based electrodes showed the most basic common sensing platforms for metal and metalloid detection [118–123]. Therefore, in this chapter, the advantages of using graphite, mesoporous carbon, carbon nanotubes, and graphene nanocomposites as electrode materials were highlighted in four subchapters. Among the carbon nanocomposite-based electrode materials, the graphene-based composite, with specific architectures and morphologies, revealed excellent performances such as sensitivity, detection limit, selectivity, linear range of detection, stability, repeatability, etc., for heavy metal and metalloid detection [124–129]. The perspectives, trends, and up-to-date developments in nanocomposite material-based electrochemical sensors for the detection of heavy metals and metalloids are given as an erudite summary in Chapter 5. Precise monitoring of greenhouse gases and other pollutants in the atmosphere is a crucial issue that is becoming an ever more important global concern. Because many gases can be harmful to organic life, such as humans or animals, gas detection methods and gas sensors possess accentuated importance. Chapter 6, titled “Detection of gas molecules by means of spectrometric and spectroscopic methods,” presents several spectroscopic and spectrometric techniques applied in environmental gas analysis, with the attention focused on the fundamental mechanisms in the analysis and instrumentation. Moreover, the advantages and limitations of all described techniques are also summarized. Thus, in addition to some mass spectrometric techniques, ion mobility spectroscopy and laser spectroscopic methods (e.g., IR absorption and fluorescence methods, Raman spectroscopy) are introduced to provide a valuable overview of currently available and established techniques for environmental gas analysis. The need for versatile analytical techniques able to measure gaseous compounds in the environment is closely related to the large number of application areas such as urban (e.g., traffic emissions), industrial (e.g., petrochemical industry, combustion sites, waste retrieval, process control), environmental studies (e.g., background concentrations, greenhouse gases, fruit storage), medical diagnostics (e.g., exhaled breath gas monitoring), indoor air quality (e.g., exposure and emission limits at the workplace) [130], and the large variety and quantity of the analyzed compounds. The chapter describes one of the standard techniques for gas analysis, namely gas chromatography coupled to mass spectrometry (GC-MS), which is able to analyze compound mixtures at the ppb to ppt level [131]. The main disadvantages of this method are the sophisticated procedures for sample collection and preconcentration and the high

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instrument costs. From the category of mass spectrometry methods, the chapter describes the following techniques: isotope-ratio mass spectrometry (IR-MS) [132], which was developed mainly for analysis of isotopes of carbon, oxygen, and hydrogen and in many cases is used in combination with gas chromatography; membrane-inlet mass spectrometry (MI-MS) [9, 18, 133–135], which can be used for the selective elimination of water and particulate matter that would obstruct the mass spectrometric analysis of the sample; selected-ion flow-tube mass spectrometry (SIFT-MS), which is a flow-drift tube chemical ionization analytical method and uses H3O+, O+2 , and NO+ ions as precursor ions [133, 136] and is capable of simultaneous analysis of several compounds down to ppb concentrations on the time scale of a few seconds [133, 136, 137]; proton-transfer reaction mass spectrometry (PTR-MS), which is a flow-drift tube chemical ionization analytical method that uses only H3O+ precursor ions [136, 138, 139] and enables high sensitivity, real-time analysis with detection limits in the sub-ppb regime [137–139]; and direct analysis in realtime mass spectrometry (DART-MS), which uses reactant ion generation, helium, or nitrogen that is introduced into a discharge chamber, where an electrical discharge produces ions, electrons, and excited state species [140–143] and is able to detect very low concentration in the lower femtoM range [141, 144, 145]. The chapter summarizes all discussed mass spectrometric methods with their advantages and limitations as well as other important characteristics. Moreover, the authors present ion mobility spectrometry (IMS), which is based on the determination of ion mobility in electric fields where the sample compounds are ionized and enter a drift tube [137, 146]. The detection limit achieved by IMS is usually in the ppb range [146] and strongly depends on the substance measured and the experimental conditions [146]. The IMS technique can detect a broad range of analytes in the gas phase, mostly as protonated molecule ions [146], in real time because one measurement takes as little as a few ms [137, 146, 147]. Therefore, its principal applications are in security and military venues for the detection of chemical warfare agents and explosives [137, 146]. Finally, the authors introduced the laser spectroscopic techniques used for gas sensing. Because optical gas sensing techniques are based on the ability of molecules to absorb or emit electromagnetic radiation of a well defined wavelength, they are extremely selective compared to other gas-analytical measurement methods. From the category of absorption methods, the authors describe the following techniques: infrared absorption spectroscopy (IR), based on the fact that different gaseous molecules with a temporally changeable dipole moment absorb different frequencies of light, which can be used for compound identification [148], the amount of absorbed light depending on the gas concentration; Fourier transformation infrared absorption spectroscopy (FTIR), which takes the advantage of computer technology development and has some decisive advantages over the dispersive spectrometers (e.g., a significantly higher signal-to-noise ratio, shorter measurement times, increased limits of detection by increasing the optical pathlength); nondispersive infrared (NDIR) spectrometers, which, unlike the standard IR spectrometer, have no dispersive components and are commonly applied for environmental gas analysis in specific situations where one or only a few compounds are to be measured;

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Advanced Nanostructures for Environmental Health

gas chromatography coupled with infrared absorption spectroscopy, GC/FTIR, which permits the real-time analysis of the GC effluent using a “light-pipe” measurement chamber [149] with internal gold film coatings for multiple reflections and passes of the beam through the cell and can be used for organic volatile pollutants analysis; tunable (mid-IR) diode laser absorption spectroscopy (TDLAS) in which a tunable laser with a very narrow line width is scanned over the absorption lines of the gases to be detected (e.g., O3, NOx, CO, CO2, H2O2, HCHO, NH3, CH4, C2H2, C2H4, HCl, HF, SiF6, CF2Cl2, H2S, and OCS) with detection limits down to the ppt range in a few minutes; cavity ringdown spectroscopy (CRDS), which is based on a short laser pulse coupled into an optical cavity between two high-reflectivity mirrors, with the high number of reflections of the pulse within the cavity enhancing the absorption pathlength in the cavity by several orders of magnitude and was employed for studies of greenhouse gas emissions (e.g., N2O, CO2, CH4, and NH3) [150, 151], atmospheric trace gas analysis [152], isotope ratio measurements [153], and breath analysis [154]; continuous wave cavity enhanced absorption spectroscopy (CW-CRDS), where the cavity is excited with continuous wave radiation as an alternative to the classical pulsed CRDS and can be used for trace gas detection or to study nonlinear effects with detection limits in the ppb levels [155]; integrated cavity output spectroscopy (ICOS), which makes possible the measurement of the absorption spectra by direct attenuation methods, providing detection sensitivities comparable to classical CRDS of specific molecules including CO2, N2O, NO, CO, CH4, H2S, NH3, and SO2 down to ppt levels; differential optical absorption spectroscopy (DOAS), which is based on a set up that consists of a broadband light source, an optical set up that transfers the light through the atmosphere, and a telescope-spectrograph-detector system to record the absorption spectra [156] of very reactive species such as free radicals (e.g., OH, NO3, ClO, BrO, and IO) as well as a wide range of other trace gases (e.g., SO2, O3, NOx, NH3, etc.); light detection and ranging (LIDAR) methods that can provide altitude profile information of the atmosphere by means of the measurements recorded with high spatial and temporal resolution, offering the possibility of observing the atmosphere at ambient conditions and the potential of covering the height range from the ground to more than 100 km in elevation [157]; photoacoustic spectroscopy (PAS), which provides absorption measurements, but instead of using the Beer-Lambert law, the absorbed radiation is directly measured by a calorimetric method [158] and thus it is a powerful technique to quantify gases in the ppb or ppt concentration range; laser-induced fluorescence (LIF), which relies on the excitation of gas molecules by absorption of laser radiation corresponding to a transition from the molecule’s ground state to an (electronic) excited state and is a very sensitive and specific technique for the detection of atmospheric trace gases at the lower ppt level [159]; chemiluminescence, which is used as the “gold standard” and also as a reference method in European standards [160] for NOx measurements in atmospheric [161] and environmental [162] research, but also for breath analysis [163, 164] because detection limits in the ppb range are possible [165]; Raman spectroscopy, which is based on the inelastic scattering of photons at molecules that occurs when an externally applied electric field induces a dipole in the molecule during the molecular vibration, in contrast to an intrinsic

Chapter 1 • When the nanostructures meet the environmental health key issues

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dipole moment required for IR absorption spectroscopy [166–168] and is able to measure a wide range of gases with detection limits in the range between 10 and 100 ppm; and cavity-enhanced Raman spectroscopy (CERS), fiber-enhanced Raman spectroscopy (FERS), coherent anti-Stokes Raman spectroscopy (CARS), and electronic-resonanceenhanced CARS (ERE-CARS) spectroscopy, which are alternative methods aimed at enhancing the Raman signal of gas samples and pushing forward the limits of detection. In conclusion, the chapter provides a comparison of different instrumental techniques discussed throughout the book, together with the main characteristics while also pointing out the main advantages and drawbacks of the methods employed for gas molecule analysis. Furthermore, Chapter 7, titled “Advanced composite nanostructures as gas sensors,” gives a comprehensive review of the development of gas sensors, revealing the necessity of further improvements in this field. The chapter is aimed at both informing the reader about up to date gas sensing technologies and giving basic guidance to those who want to try novel materials and sensor geometries. This summary deals mainly with the use of composite nanostructures as gas sensors, as the demand for more and more efficient sensors is very frequently fulfilled by composite nanostructured materials. Besides accuracy and gas type selectivity, cost and portability are also very important factors and are therefore widely investigated in recent publications. There are also direct detection methods for gas sensing, which are commonly based on direct measurement of one of more physical properties such as various spectroscopic techniques, mass spectrometry, or gas chromatography [169]. Even though they have excellent accuracy and selectivity, the latter methods are expensive and often require large and heavy equipment. Also, for achieving a good signal, most of these methods need a larger volume of analyte gas. Therefore, the demand for miniaturization and a large surface-to-volume ratio obviously links indirect detection gas sensors to advanced composite nanostructures. The need for cheap and portable sensors is a pressing demand that requires indirect detection mechanisms, where the sensor generates a response of a certain type when exposed to the detected gas. The response can be induced by either a chemical reaction, physical adsorption, or absorption, and can be measured by simple and small electrical or electrooptical set ups, allowing a small detector device [170, 171]. In this chapter, a review of the most significant gas sensor structures and geometries applied nowadays was presented. Specific nanocomposite/nanostructured materials and their applications were also discussed [171–173], with basic suggestions about the technological construction procedures. Because gas sensors generally show imperfect selectivity by themselves, the possibility of gas filtering methods using zeolites, porous carbon nanocomposites, or thin films was described in order to improve selectivity [174–182]. A short summary of possible specific applications together with the main characteristics and performances of nanocomposite materials and nanostructures in gas sensors were given as well. A useful table about the general comparison of gas sensors is shown, providing a quick selecting guide for material classes and fabrication technologies as a function of the desired application, performance, and available budget.

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Further details about performance comparisons of various nanocomposites/nanostructures in gas sensing are also available in a following subchapter. It is prominently highlighted in this chapter that the variety of possible nanocomposite structures is nearly unrestricted, and the large number of references allows the reader to expand his or her relevant knowledge quite easily. The actual trends cover further hybrid organic-inorganic composites, which generally also contain nanostructures such as nanotubes, nanowires, and two-dimensional (2D) materials, either in pristine, reduced, or doped form. Such nanostructures provide the most promising results in terms of stability, linearity, detection limit, and the possibility to work at room temperature. The category of microbial pollutants consisting of harmful bacteria (e.g., Salmonella typhimurium, Escherichia coli, Staphylococcus aureus, Listeria innocua, etc.) and viruses (e.g., Poliovirus, Rotavirus) represents an important class of pollutants typically found in food, wastewater, aquatic ecosystems, and drinking water [183]. Human exposure to these contaminants can provoke some medical issues, from light infections to severe and even fatal diseases [184]. Therefore, to avoid disease outbreaks, humans understand that there is an urgent need to find cost-effective strategies that are fast and portable for the real time detection, identification, and monitorization of microbial contaminants. Efforts are currently under way to develop such methods to detect [185] and remove [186] them from different environments. Over the years, a few standard methods for the selective and sensitive detection of pathogenic microorganisms [187–190] have been used (e.g., plating, culturing, enzymelinked immunosorbent assay (ELISA), polymerase chain reaction (PCR)), but most of them are difficult, time-consuming, expensive, or unavailable for onsite detection. By taking advantage of nanostructure development, Chapter 8, titled “Advanced nanostructures for microbial contaminant detection by means of spectroscopic methods,” provides an overview of some recent and successful progress related to the analysis and monitoring of microbial pollutants by Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), infrared spectroscopy (IR), surface-enhanced infrared absorption (SEIRA), fluorescence spectroscopy, localized surface plasmon resonance, and impedance spectroscopy, highlighting the advances in the fabrication of innovative nanostructured metallic platforms for the ultrasensitive and selective detection of pathogens, and the simultaneous identification and discrimination of multiple contaminants in environmental samples. The chapter first sums up the successful approaches using SERS for the detection of various microbial contaminants, with the emphasis on the plasmonic substrate designs proposed for efficient SERS sensing. Thus, a few detection strategies are described such as the direct SERS detection of microbial contaminants by using different metallic platforms; the in situ preparation of SERS active substrates by their direct attachment, either on the cell wall or inside bacteria for the direct detection of microbial contaminants; the direct SERS detection of membrane fouling; the indirect SERS detection strategy of microbial contaminants where the substrate is modified with Raman reporters and other specific receptors to ensure the capture of the target analytes; and finally the SERS detection of phatogenic contaminants in microfluidic devices. Further, the chapter focuses on

Chapter 1 • When the nanostructures meet the environmental health key issues

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the detection of microbial contaminants by IR spectroscopy with a special focus on the SEIRA technique that has proven to be often used, alone or combined with other techniques, especially with multivariate statistical analysis, as a faster, more sensitive, and highly specific method to detect microbial contaminants [191, 192]. Also, different fluorescent sensors developed to detect several strains of pathogenic bacteria and to improve the limits of detection of such contaminants as compared to conventional detection tools are presented. Up to now, many efforts have been devoted to explore and improve the performance of fluorescent biosensors for the detection of relevant pathogens in liquid or solid food. Then, the chapter summarizes the latest developments of the sensing tools based on localized surface plasmon resonance (LSPR) or surface plasmon resonance (SPR). Finally, it presents sensors based on electrochemical impedance spectroscopy that are part of the powerful class of electrochemical biosensors with their names derived from the type of resulting signal that is measured. Thanks to their simplicity, rapidity, low cost, sensitivity, portability, practicability, and multifunctional character, these are essential devices that find valuable applications in the food industry and health monitoring, in portable medical devices for onsite diagnosis, in the prevention and therapy of diseases caused by various bacterial infections, etc. [193, 194]. In conclusion, the chapter highlights the great effort invested lately to develop robust, stable, and reproducible nanostructured metallic substrates to be used in sensing schemes that demonstrate promising perspectives for the exploitation of the spectroscopic techniques rapid, ultrasensitive and selective detection and discrimination of various pathogenic contaminants, but on the other side, it is emphasized that their implementation in real life applications is still difficult. The earliest mention of photocatalysis dates back to 1911, when German chemist Dr. Alexander Eibner integrated the concept in his research of the illumination of ZnO on the bleaching of the dark blue pigment Prussian blue [195]. However, a real breakthrough occurred in photocatalysis research in 1972, when Japanese scientists Akira Fujishima and Kenichi Honda published their results about the electrochemical photolysis of water taking place between connected TiO2 and platinum electrodes, in which ultraviolet light was absorbed by the former electrode and electrons would flow from the platinum electrode (anode; site of oxidation reaction) to the TiO2 electrode (cathode; site of reduction reaction) with hydrogen formation at the cathode [196]. Heterogeneous photocatalysis can be defined as a discipline that might involve a great variety of different reactions: mild or total oxidations, dehydrogenation, hydrogen transfer, gaseous pollutant removal, etc. The heterogeneous photocatalysts used most frequently are transition metal oxides and semiconductors with unique features. In contrast with metallic catalysts having continuous electronic states, due to their void energy region where no energy levels are available, semiconductors are able to promote the recombination of an electron and hole produced by photoactivation in the solid. The recombination of the exciton is objectionable and can lead to the severe deterioration of the photocatalyst. Therefore, many attempts aim to develop photocatalysts that provide an extended exciton lifetime, improving the electron-hole separation. Among others, various approaches trust different

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Advanced Nanostructures for Environmental Health

structural features such as nanocomposite semiconductors containing phase heterojunctions, which might be responsible for enhanced photocatalytic activity. Chapter 9, titled “Semiconductor mixed oxides as innovative materials for the photocatalytic removal of organic pollutants,” gives a critical survey of the extensive literature concerning the potential applications of nanostructured mixed oxides for environmental remediation. This work focuses on the possibility of applying nanostructured mixed oxide materials as photocatalysts for the elimination of toxic organic contaminants from pollutant containing water and wastewater. The advantages and perspectives of such photocatalytically active nanocomposite semiconductors are also discussed in the chapter. Organic chemicals as pollutants turning up in water and wastewater are strongly advised to be removed or defanged for the sake of environmental health. Heterogeneous photocatalysis using semiconductor materials is a promising process, making it an emerging technology for environmental remediation, especially for the removal of water and wastewater containing organic contaminants. Photocatalysis generally utilizes oxygen from the air as the oxidant, consequently requiring no expensive oxidizing chemicals (e.g., hydrogen peroxide, ozone, etc.) compared to other treatment methods available these days such as adsorption or air stripping that only convert these contaminants into other, even more harmful compounds. Moreover, photo-activated semiconductors can completely degrade many types of toxic pollutants under mild conditions (both temperature and pressure) and the photocatalysts can be reused or recycled quite easily. There have been many attempts in the scientific approach to increase the effectiveness of the photocatalytic materials by changing the electronic band structure of semiconductors by various techniques (metal ion/nonmetal ion doping, surface sensitization by organic dyes or metal complexes, or noble metal deposition) [197–200]. A promising alternative is to combine different semiconductors with suitable band structures, forming heterojunction systems able to afford an efficient spatial charge separation and to increase the lifetimes of the charge carriers. The coupling of an n-type semiconductor with a p-type semiconductor can be very efficient and interesting from an application point of view, as it resembles a photovoltaic solar cell. This chapter gives an outstanding summary of nanostructured mixed semiconductors having significant photocatalytic activity for water treatment. In particular, emphasis was placed on the coupling of various metal oxides in order to enhance not only the activity under UV irradiation, but also the visible light efficiency [201–217]. In most but not all cases, TiO2 was one of the constituents. A brief survey of the most important papers reporting on mixed metal oxides having a size-dependent band gap and crystalline structure investigated for the degradation of organic pollutants is reported in Chapter 9. As mentioned above, this is a very popular research topic for environmental applications as it offers many possibilities for developing depollution processes, even at very low pollutant concentrations. The method proved to be effective in removing organic pollutants (dyes, pharmaceutical products, or personal care products) at very low concentrations but higher than the discharge limit for wastewater treatment technologies.

Chapter 1 • When the nanostructures meet the environmental health key issues

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Heterogeneous photocatalysis can be successfully applied in atmospheric depollution, too, the processes of which mainly focus on removing nitrogen oxides and other gaseous pollutants issued by automobiles [218, 219]. For extended applications, photocatalytically active surfaces are also applied in self-cleaning surfaces such as antimicrobial layers for indoor applications, as transparent coatings for the glazing of solar energy convertors (photovoltaic modules, solar-thermal collectors), or for opaque surfaces on building facades, terraces, or roofs that are properly oriented [220, 221]. Chapter 10, titled “Composite nanostructures as potential materials for water and air cleaning with enhanced efficiency,” provides an overview of photocatalytic applications focusing on emerging composite thin films, resembling the third generation photovoltaic solar cells that can be used as efficient, Vis-active photocatalytic materials for different environmental applications, including wastewater treatment, air depollution, or selfcleaning surfaces [222–225]. In this chapter, the challenging progress of composite photocatalysts is also discussed, considering the requirements on the structural (suitable band-gap alignment, allowing Vis activation, and fast charge separation) and surface properties (specific surface area, surface charge) as well as the stability (both mechanical and chemical stability under working conditions) and the possibility of photocatalyst recovery after the procedure (thin films versus powders). Sustainable alternatives are presented in terms of materials composition and deposition technologies [226–232]. The authors reviewed the perspectives of successful design for photocatalytic composites, having extended photocatalytic response to Vis. The major issues can be listed as follows: (i) the composite material is supposed to be active under visible light irradiation, namely electron-hole pairs should be generated and can be further involved in producing oxidant species such as hydroxyl radical (HO%), which has the ability to decompose the pollutant molecules; (ii) to avoid the electron-hole recombination, the composite is expected to allow an extended charge carrier flow at the interface, which has an evident significance in the reduction during the photocatalytic reaction steps. Thus, a well designed band gap alignment is targeted, allowing the fast separation of the charges produced during irradiation; (iii) because water is the main intermedium reactant in photocatalytic depollution procedures, the photocatalyst must be stable in aqueous solutions; in other words the composing materials have to be insoluble; (iv) the application of the immobilized photocatalysts as thin films is mostly recommended for both advanced wastewater treatment and air depollution. Compared to powdered photocatalysts, immobilization might decrease their contact surface available for pollutants; however, it allows a simplified technological procedure of better acceptance; and (v) the thin photocatalytic films must be mechanically stable over long periods of continuous flow processes. The promising conclusion from this chapter is that the application of composite thin films as photocatalytic layers provided good removal/mineralization of the organic pollutants and thus can be recommended for future missions. Self-cleaning cement products are available on the market even now to reduce air pollution when deposited on the building’s facades while self-cleaning windows covered with doped TiO2 layers are already widely used in Japan. The photocatalytic composite thin films introduce a practicable

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Advanced Nanostructures for Environmental Health

route for emerging processes that are beyond the laboratory or pilot scale; different potential wastewater recycling procedures are prospective to enter the market, therefore contributing to the reduction of the water crisis. Over time, noble metal nanoparticles (e.g., gold, silver, platinum) were the most-used nanostructures, mainly in the biomedical field, and are still in use as valuable biostructures capable of eliminating microbial threats due to their well known antimicrobial activity. Bacteria, fungal strains, actinomycetes, yeasts, and some plants are only a few of the cost-effective organisms able to produce noble bionanoparticles. Chapter 11, titled “Removal of bacteria, viruses, and other microbial entities by means of nanoparticles,” provides an overview regarding the features of noble metal nanoparticles, oxide nanoparticles, and other metal nanoparticles, which were synthesized either by specific microorganisms that have biomedical applicability and/or for the removal of microorganisms. Moreover, the chapter describes the antimicrobial activity of these nanoparticles together with their mechanisms of action. Generally, the action mechanism of nanoparticles is explained by one of the following models: oxidative stress induction, metal ion release, or nonoxidative mechanisms [233, 234] that can take place either simultaneously or independently. The main processes that underline the antibacterial effects [235] of nanoparticles are bacterial cell membrane disruption, reactive oxygen species (ROS) generation, the penetration of the bacterial cell wall, and the stimulation of intracellular antibacterial effects, including interconnections with proteins and DNA [235, 236]. Among noble metal nanoparticles, the gold ones are extremely efficient as antimicrobial agents, as demonstrated from the beginning of modern medicine. Gold nanoparticles behave differently from other metal nanoparticles. Thus, their antimicrobial activity does not include any ROS-implicated processes, but instead involves a two-phase mechanism [237]. First, the nanoparticles attach to the bacteria membrane, causing membrane potential alteration and simultaneously a decrease of the adenosine triphosphate (ATP) level. In the second phase, the binding of tRNA to the ribosome is inhibited. Moreover, viruses, which are impressive pathogens responsible for major causes of death worldwide, can be identified by means of gold nanoparticles with considerably lower detection time and costs [238] as compared to the usual procedures. On the other hand, the silver nanoparticle is one of the most investigated nanoparticles with biomedical applications [239, 240] that include antimicrobial activity but also efficacious virucidal activity against some of the common viral strains [241–243]. In order to induce cell death, silver nanoparticles act on the outer membrane of bacteria. There are different hypotheses regarding their mechanism of action, including that silver nanoparticles might cause hollows and orifices in the bacterial membrane, producing cell fragmentation [244, 245], or the silver ions interrelate with the enzyme groups disulfide or sulfhydryl, causing metabolic process disturbances that lead to cell death [246, 247]. In addition to gold and silver nanoparticles, the chapter also provides examples of the biomedical applications of platinum complexes, copper and copper oxide nanoparticles, titanium dioxide nanoparticles, selenium nanoparticles, and magnesium oxide nanoparticles. Among the oxide nanoparticles, the attention was focused on TiO2, which was

Chapter 1 • When the nanostructures meet the environmental health key issues

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comprehensively studied. Then, throughout the chapter, several aspects related to the bionanomaterials (nanocomposites) produced by microbes considered as “living devices” are discussed, together with the possibility of pathogen removal from different environments by means of nanotechnology. In conclusion, the chapter underlines that nanoparticles are essential structures that deserve to be particularly studied in future research because they can be effectively engineered for multiple purposes and have considerable perspectives, such as in the removal of microbial entities from different environments. As discussed, environmental health significantly depends on how we maintain our environment. Unfortunately, the seemingly obvious first step, namely prevention, due to the quite frequent lack of an ecoconscious attitude, does not work in general. Neither society nor industry takes the long term effects of the various problems affecting environmental health into consideration, which is why more expressive action should first be implemented. As long as remediation and purification are the preferable feasible options, photocatalysis is one of the most promising methods that can interpose in such situations, as the number of the scientific papers on the field show. Nevertheless, an astonishingly low number of papers report on reactor development and even fewer deal with their standardization approaches. The scientific and engineering efforts mainly focus on the fabrication of various semiconductor materials that can be activated under solar irradiation instead of a more costly UV light source. Very promising results are obtained in the removal of pollutants in the upper micro- and millimolar range; however, the concentration of contaminants with emerging concerns is generally only a few micromolars. Chapter 12, titled “Pilot-plant scaled water treatment technologies, standards for the removal of contaminants of emerging concern based on photocatalytic materials,” elaborates a suppletory topic, namely, photocatalytic reactors and standardization. There are ISO standards readily available [248]; however, they are not linked and not present in the publications that focus on the reactors [249–251]. This fact reveals that there is a linkage hiatus between reactor and catalyst development. This chapter gives a prominent summary about various kinds of reactor designs that were developed for the degradation of organic pollutants even under solar light, the so-called contaminants of emerging concern. This chapter reveals that hydrodynamic features, illumination, and mass-transfer aspects were the actuating parameters that finally determined the type, geometry, and functioning modality of the reactors. Occasionally, mathematical modeling was also used to further improve the features of the reactors. However, a significant number of various concerns (such as too slow removal of dissolved organic carbon, degradation of the semiconductor nanoparticles, scaling-up difficulties, etc.) are still existent while results of great promise are always coming out and standardization procedures are also emerging. Nevertheless, while photocatalysis is considered mainly in photodegradation procedures, new approaches, including standardization, have also been started for photocatalytic syntheses [252–256]. It is well known that developing a good photocatalyst under certain standardization conditions is mandatory. This chapter and also the entire book make it evident that environmental health perception is a collection of several approaching issues that includes many subjects such as the recognition, mindfulness, action, and remediation processes. Under the given

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Advanced Nanostructures for Environmental Health

circumstances, the photocatalytic removal of contaminants is an important constituent in the wide spectrum of remediation. However, the method alone is not sufficient to solve the problem of environmental health [257, 258]. On the other hand, photocatalysis together with other methods has the potential to be an important tool for the safekeeping of the environment and finally for humanity itself. Regarding the excessive complexity of the current book covering diverse topics, the closing chapter is required to give a synoptic overview about possible correlations. Chapter 13, titled “Perspectives of environmental health issues addressed by advanced nanostructures,” provides a comprehensive summary on environmental problems and solutions related to nanostructure materials that emerged in the previous chapters. The chapter gives a brief but very substantial summary about nanostructures and also about the effect of synthesis methods, properties, and varieties [259]. The concern of efficient environmental remediation is minatory, thus requiring early activity; photocatalysis is a promising tool for many applications, consequently, through photocatalytic approaches, new trends, perspectives, and issues arose [260–263]. Vibrational spectroscopy has an important role in the characterization of nanostructures, and its use is essential. However, there are still challenges in this field. The possibility for full qualitative and quantitative understanding is discussed, which helps to make the best use of nanostructures to improve the vibrational sensing of chemical and biological entities from water and air. Finally, the detection of gaseous pollutants is also a crucial issue related to different kinds of nanostructures, and this is briefly summarized in Chapter 13. The careful test of ecotoxicological effects of nanomaterials and nanostructures is another emerging key element that is summarized briefly as well [264]. It is revealed that appropriate control experiments are pivotal for the standardization and maintenance of the consistency of the data. However, although Chapter 13 provides a valuable summary and perspectives about major issues discussed in this book, it is not capable of substituting for reading the foregoing chapters with detailed information.

1.3 Predictions about nanostructure involvement in environmental health applications Maybe the most important aspect related to the involvement of the advanced nanostructures in environmental applications is the impact the nanoentities will have on our lives. The data collected from the scientific literature database (Web of Science) make us confident that things are moving in the right directions because a clear tendency toward an increasing number of papers published related to nanostructures and environmental health issues can be noticed (see Fig. 1.1). A close analysis of the presented data reveals that a progression of the number of such studies has occurred only in the last 10 years, with a significant increase in the last five years and a really important jump in the last two years. Furthermore, the interest in such topics related to obtaining and using nanostructures for environmental health applications has also continuously increased in the last period, as can be easily observed in the representation illustrated in Fig. 1.2, where the number of papers that cited the publications related to nanostructures and environmental health is

Chapter 1 • When the nanostructures meet the environmental health key issues

19

40 33

Count

30

27*

20

18 16 14

10

9 4 1

1

1

1

4

5

5

5

2

2 20 19

20 18

20 17

20 16

20 15

20 14

20 13

20 12

20 11

20 10

20 09

20 08

20 07

20 06

20 05

20 04

20 03

0

Year FIG. 1.1 The increasing trend of the number of publications during time related to nanostructures and environmental health (information acquired using the Web of Science Core Collection, July 29, 2019, search parameters TOPIC: (nanostructures) AND TOPIC: (environmental health) Timespan: All years). * Please note that 2019 is not a whole year.

1500 1203* 1074 980 753

785

20 15

20 16

Count

1000

680 467

500

523

569

614

388 293 204 88

20 19

20 18

20 17

20 14

20 13

20 12

20 11

20 10

20 09

20 08

20 07

20 06

19 20 05

0

Year FIG. 1.2 The increasing trend of the number of publications during time that cited the publications related to nanostructures and environmental health (information acquired using the Web of Science Core Collection, July 29, 2019, search parameters TOPIC: (nanostructures) AND TOPIC: (environmental health) Timespan: * Please note that 2019 is not a whole year.

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Advanced Nanostructures for Environmental Health

presented. It can be seen that the number of publications increases starting with 2005, but the maximum interest for such topics was also recorded in the last two years. Keeping in mind all these data, we can infer that there are strong reasons to be optimistic when thinking about the future of research on environmental health and nanostructures.

Acknowledgment This work was supported by a grant from the Romanian Ministry of Research and Innovation, CCCDI– UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0350/01.03.2018 (Graphene4Life), within PNCDI II.

References [1] https://en.wikipedia.org/wiki/Health_(disambiguation). [2] https://en.wikipedia.org/wiki/Health. [3] https://en.wikipedia.org/wiki/Environmental_health. [4] H. Hoag, Air pollution delivers smaller babies, Nat. News (2013). https://doi.org/10.1038/ nature.2013.12358. [5] https://www.who.int/news-room/detail/29-10-2018-more-than-90-of-the-world%E2%80%99s-chi ldren-breathe-toxic-air-every-day. [6] S. Su, W. Wu, J. Gao, J. Lu, C. Fan, Nanomaterials-based sensors for applications in environmental monitoring, J. Mater. Chem. 22 (35) (2012) 18101–18110. [7] H.-X. Gu, K. Hu, D.-W. Li, Y.-T. Long, SERS detection of polycyclic aromatic hydrocarbons using a bare gold nanoparticles coupled film system, Analyst 141 (14) (2016) 4359–4365. [8] X. Gu, S. Tian, Q. Zhou, J. Adkins, Z. Gu, X. Li, J. Zheng, SERS detection of polycyclic aromatic hydrocarbons on a bowl-shaped silver cavity substrate, RSC Adv. 3 (48) (2013) 25989–25996. [9] T. Wang, Z. Sun, Y. Wang, R. Liu, M. Sun, L. Xu, Enhanced photoelectric gas sensing performance of SnO2 flower-like nanorods modified with polyoxometalate for detection of volatile organic compound at room temperature, Sensors Actuators B Chem. 246 (2017) 769–775. [10] A. Verdian, Apta-nanosensors for detection and quantitative determination of acetamiprid—a pesticide residue in food and environment, Talanta 176 (2018) 456–464. [11] L. Zhou, X. Zhang, L. Ma, J. Gao, Y. Jiang, Acetylcholinesterase/chitosan-transition metal carbides nanocomposites-based biosensor for the organophosphate pesticides detection, Biochem. Eng. J. 128 (2017) 243–249. [12] B. Jiang, P. Dong, J. Zheng, A novel amperometric biosensor based on covalently attached multilayer assemblies of gold nanoparticles, diazo-resins and acetylcholinesterase for the detection of organophosphorus pesticides, Talanta 183 (2018) 114–121. [13] M.L. Satnami, J. Korram, R. Nagwanshi, S.K. Vaishanav, I. Karbhal, H.K. Dewangan, K.K. Ghosh, Gold nanoprobe for inhibition and reactivation of acetylcholinesterase: an application to detection of organophosphorus pesticides, Sensors Actuators B Chem. 267 (2018) 155–164. [14] Q.J. Luo, Z.G. Li, J.H. Lai, F.Q. Li, P. Qiu, X.L. Wang, An on–off–on gold nanocluster-based fluorescent probe for sensitive detection of organophosphorus pesticides, RSC Adv. 7 (87) (2017) 55199–55205. [15] J. Zhu, M.-J. Liu, J.-J. Li, X. Li, J.-W. Zhao, Multi-branched gold nanostars with fractal structure for SERS detection of the pesticide thiram, Spectrochim. Acta A Mol. Biomol. Spectrosc. 189 (2018) 586–593.

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ti, P. Berki, [250] L. Baia, E. Orba´n, S. Fodor, B. Hampel, E.Z. Kedves, K. Saszet, I. Sze A. Vulpoi, K. Magyari, A. Csavda´ri, C. Bolla, V. Coşoveanu, K. Herna´di, M. Baia, A. Dombi, V. Danciu, G. Kova´cs, Z. Pap, Preparation of TiO2/WO3 composite photocatalysts by the adjustment of the semiconductors’ surface charge, Mater. Sci. Semicond. Process. 42 (2016) 66–71. [251] Z. Ka´sa, K. Saszet, A. Dombi, K. Herna´di, L. Baia, K. Magyari, Z. Pap, Thiourea and Triton X-100 as shape manipulating tools or more for Bi2WO6 photocatalysts? Mater. Sci. Semicond. Process. 74 (2018) 21–30. [252] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, J.M. Herrmann, Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania, Appl. Catal. B Environ. 39 (2002) 75–90. [253] N.P. Xu, Z.F. Shi, Y.Q. Fan, J.H. Dong, J. Shi, M.Z.C. Hu, Effects of particle size of TiO2 on photocatalytic degradation of methylene blue in aqueous suspensions, Ind. Eng. Chem. Res. 38 (1999) 373–379. [254] M. Rochkind, S. Pasternak, Y. Paz, M. Rochkind, S. Pasternak, Y. Paz, Using dyes for evaluating photocatalytic properties: a critical review, Molecules 20 (2014) 88–110. [255] A. Mills, J. Wang, M. McGrady, Method of rapid assessment of photocatalytic activities of selfcleaning films, J. Phys. Chem. B 110 (37) (2006) 18324–18331. [256] M.J. Powell, C.W. Dunnill, I.P. Parkin, N-doped TiO2 visible light photocatalyst films via a sol–gel route using TMEDA as the nitrogen source, J. Photochem. Photobiol. A Chem. 281 (2014) 27–34. [257] M. Kobielusz, P. Mikrut, Photocatalytic synthesis of chemicals, Adv. Inorg. Chem. 72 (2018) 93–144. [258] C.“.C.”. Le, M.K. Wismer, Z.-C. Shi, R. Zhang, D.V. Conway, G. Li, P. Vachal, I.W. Davies, D.W. C. MacMillan, A general small-scale reactor to enable standardization and acceleration of photocatalytic reactions, ACS Central Sci. 3 (2017) 647–653. [259] C. Dhand, N. Dwivedi, X.J. Loh, A.N. Jie Ying, N.K. Verma, R.W. Beuerman, R. Lakshminarayanan, S. Ramakrishna, Methods and strategies for the synthesis of diverse nanoparticles and their applications: a comprehensive overview, RSC Adv. 5 (127) (2015) 105003–105037. [260] X. Wang, H. He, Y. Chen, J. Zhao, X. Zhang, Anatase TiO2 hollow microspheres with exposed {001} facets: facile synthesis and enhanced photocatalysis, Appl. Surf. Sci. 258 (15) (2012) 5863–5868.
kely, G. Kova´cs, L. Baia, V. Danciu, Z. Pap, Synthesis of shape-tailored WO3 micro-/nanocrystals [261] I. Sze and the photocatalytic activity of WO3/TiO2 composites, Materials 9 (4) (2016) 258. [262] F. Chen, H. Huang, L. Guo, Y. Zhang, T. Ma, The role of polarization in photocatalysis, Angew. Chem. 58 (30) (2019) 10061–10073. [263] X. Xue, W. Zang, P. Deng, Q. Wang, L. Xing, Y. Zhang, Z.L. Wang, Piezo-potential enhanced photocatalytic degradation of organic dye using ZnO nanowires, Nano Energy 13 (2015) 414–422. [264] E.J. Petersen, T.B. Henry, J. Zhao, R.I. MacCuspie, T.L. Kirschling, M.A. Dobrovolskaia, V. Hackley, B. Xing, J.C. White, Identification and avoidance of potential artifacts and misinterpretations in nanomaterial ecotoxicity measurements, Environ. Sci. Technol. 48 (8) (2014) 4226–4246.

Further reading [265] Y. Huang, S. Tao, An optical fiber sensor probe using a PMMA/CPR coated bent optical fiber as a transducer for monitoring trace ammonia, J. Sensor Technol. 21 (2011) 29–35.

2 Sensitive detection of organic pollutants by advanced nanostructures € rgen Poppa,b,c Dana Cialla-Maya,b,c, Karina Webera,b,c, Ju a LEI BNIZ I NS TIT U TE OF PHOT ONI C T E C H N OL OG Y (I P H T ), JE NA , G E R M A N Y b FR IE DR ICH SCHI LLER UNI VERSI TY JENA, INSTI TUTE O F P HY SICAL CHEMISTRY AND ABBE CENTER OF PHOT ON ICS, JENA, GE RMANY c RESEAR CH CAMPUS INFECTOGNOSTICS, JENA, GE RMANY

2.1 Introduction The pollution of our ecosystem with endocrine-disrupting chemicals, a substance class with biological effect on our health such as pesticides, polychlorinated biphenyl, and polycyclic aromatic hydrocarbons (PAHs), is currently a major concern all over the world [1,2]. The chemical industry is producing substances and compounds for every aspect of our life such as agriculture, health, mobility, fashion and cosmetics, habitation as well as leisure. As a consequence, the inappropriate use or disposal of these products results in the contamination of the ecosystem, that is, soil, air, and water as well as plants and animals living in the contaminated areas. Pollutants are defined as primary pollutants which are harmful when exposed to the environment and secondary pollutants which are created by less harmful precursors [3]. As an example, occurrence data of emerging organic compounds such as pharmaceuticals, personal care products, and industrial compounds in groundwater are summarized [4,5]. Due to long residence times in groundwater, it is expected that organic pollutants are a threat to freshwater source for decades. As primary source of pollutants in water samples with >100 chemicals including pharmaceuticals, waste-indicator compounds, and pesticides, collected in different US regions, the wastewater discharge into aquatic systems was assumed [6]. Based on the development and application of innovative and advanced analytical detection schemes, the monitoring of pollutants in environmental samples is achieved down to the sub-ng/L range [7]. Moreover, protection strategies, regulations of pollutants as well as a powerful risk assessment of the entire ecosystem are of high importance in the future. To initiate fast measures to reduce the ecological burden, powerful on-site pollutantdetection schemes are required. As nanomaterials and nanostructures are showing

Advanced Nanostructures for Environmental Health. https://doi.org/10.1016/B978-0-12-815882-1.00002-1 © 2020 Elsevier Inc. All rights reserved.

35

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Advanced Nanostructures for Environmental Health

excellent optical, electrical, and catalytic properties accompanied by mechanical stability, a wide selection of nanoparticle (NP)-based sensors or devices for monitoring pollutants in environmental samples with electrical or optical detection schemes are available [8]. The authors have introduced different detection schemes, such as field-effect transistor, electrochemical approaches, fluorescence, surface-enhanced Raman spectroscopy (SERS) as well as colorimetry and illustrated the application of carbon nanostructures, metallic NPs, silicon nanomaterials as well as quantum dots to allow for a sensitive detection using these sensing techniques for environmental monitoring. In the case of graphene, the surface area-to-volume ratio is maximal, accompanied with low noise in the aspect of electrons [9,10]. Moreover, the resistivity is significant when compared to traditional materials. Finally, the exceptional electronic structure of graphene results in different detection schemes such as measuring the increase in the conductivity due to the interaction with target molecules and the creation of resistive-type sensors. In the case of carbon nanotubes, the wide application of these carbon nanostructures in sensing devices are assigned to their conductive properties as well as resonance-free transmission capacities [10]. NP-based electrochemical biosensors using the inhibition of enzymes to analyze pollutants such as pesticides and phenols are promising in environmental sensing and it is expected that further improvements in this sensing concept will result in real sample applications [11]. The SERS as detection scheme for environmental monitoring is promising due to the molecular specificity of this method and the high sensitivity achieved by the application of powerful plasmonic materials [12,13]. As a consequence, SERS is applied for the detection of organic pollutants in environmental samples with high sensitivity [14]. The sensitivity is further improved by employing precision target structures to guide the analyte molecule toward the metallic surface, for example, hydrophobic coating or tailored hosts [15]. The interested reader is referred to the aforementioned citations and references cited therein with regard to the applicable electrochemical and optical detection schemes. This chapter summarizes the sensitive detection of organic pollutants by the application of advanced nanostructures. First, the detection of hydrocarbons, that is, fuels and PAHs are introduced. Second, the detection of organic solvents as well as volatile organic compounds (VOCs) is briefly summarized. Third, the analysis of pesticides and persistent organohalides is discussed. Finally, the detection of compounds such as phenols, dye molecules, and disinfection agents from industrial production and water treatment will be illustrated. The detection of pharmacologically active substances, such as drugs and pharmaceuticals, hormones as well as (neuro)toxins are discussed in Chapter 3.

2.2 Detection of hydrocarbons: Fuels and PAHs The contamination of the environment such as water sources or soil by PAHs is associated with the incomplete combustion of fuels and coal [16,17]. The concentration of highly carcinogenic and persistent chemicals could reach up to 300 g/kg in coal gasification areas as

Chapter 2 • Sensitive detection of organic pollutants

37

well as mg/L concentration levels in water sources. As gold standard, liquid or gas chromatographic techniques are applied in combination with mass spectrometry.

2.2.1 Fuels As an example for the rapid detection of fuels in environmental samples, a carbon nanotube-based piezosensor was presented to estimate kerosene in soil [18]. Large ecological issues are associated with kerosene leaks, especially on military airfields. Heavier fractions of kerosene remain on the soil surface while lighter fractions penetrate water sources and finally groundwater. Thus, a detection scheme allowing for the assessment of the degree of contamination of soil is envisaged without sampling or sample preparation steps. The authors have demonstrated a detector with an open input to be placed directly on the soil and the kerosene vapor interacts with a quartz crystal resonator with modified electrodes. It is concluded that the measurement error is a function of the kerosene concentration as well as the temperature and the moisture of the soil sample. The temperature range of the sensor is between 15°C and 35°C and the investigated kerosene concentration range is between 5 and 600 mg/kg. Thus, a concept in monitoring fuels is presented to pave the way toward on-site application schemes identifying contaminants in the environment.

2.2.2 Polycyclic aromatic hydrocarbons SERS is mostly reported as powerful method for the detection of PAHs in environmental samples, and in this section different SERS sensing schemes are introduced targeting and quantifying PAHs in water samples. To do so, AuNPs were incubated with PAH-containing river water samples and dropped on the surface [19]. Due to the drying process and coffee ring effect, the AuNPs and the analyte molecules are densely packed and SERS spectra are recorded for six different PAHs allowing for quantification down to the μM range. Specific Raman modes of the investigated molecules are identified in a water sample spiked with a PAH mixture. Another approach is based on using a smooth gold film equipped with a hydrophobic layer to enrich PAHs from a methanolic solution and in a second step, bare AuNPs are dropped on the surface [20]. Thus, plasmonic coupling effects occur due to the generation of hot spots allowing for the sensitive detection of PAHs down to 10
8 M range. A similar approach has been outlined for using silver as plasmonic material [21]. Here, bowl-shaped silver cavities are modified with alkyl dithiol to enrich the PAHs close to the metallic surface and to bind AgNPs for increased SERS sensitivity (see Fig. 2.1). The estimated detection limit of anthracene was 8 nM and pyrene was detected down to 40 nM in ethanol solutions. Thus, the authors of both studies have illustrated that nonpolar pollutants such as PAHs are enriched on sensor surfaces by alkyl modifications. This sensor concept was transferred on magnetic SERS-active beads [16]. Here, magnetite beads were modified with AgNPs and functionalized with pentanethiol to allow an enrichment of PAHs on the metallic surface and a subsequent separation from the PAHcontaining solution due to magnetic forces. A portable Raman spectrometer was applied

38

Advanced Nanostructures for Environmental Health

Functionalization

–SH Ag cavity PAHs

1,10-Decanedithiol

PAHs

Adsorption

Ag cavity array

Ag nanoparticle

Adsorption Ag FIG. 2.1 Schematic illustration of capturing PAHs in alkyl layers with subsequent immobilization of AgNPs allowing for the recording of high-quality SERS spectra [21]. Reproduced with permission from The Royal Society of Chemistry, X. Gu, S. Tian, Q. Zhou, J. Adkins, Z. Gu, X. Li, J. Zheng, SERS detection of polycyclic aromatic hydrocarbons on a bowl-shaped silver cavity substrate, RSC Adv. 3(48) (2013) 25989–25996; permission conveyed through Copyright Clearance Center, Inc.

for the SERS measurements and for the applied PAH substances, detection limits between 10
5 and 10
7 M were achieved. Furthermore, magnetite beads were modified with AuNPs in order to create efficient hot spots on the bead surface as well as due to aggregation [17]. Thus, the specific and sensitive detection of 16 priority PAHs was shown for spiked river water samples. The estimated LODs (limits of detection) for all PAHs were between 5 and 100 nM. Carbon nanotubes were applied in combination with AgNPs to enhance the Raman scattering of the PAH pyrene [22]. A charge transfer occurs from the AgNPs to the nanotubes. The analyte molecule and carbon nanotubes interact via π-π stacking and the detection down to 10
9 M was observed. To allow for on-site monitoring, graphene was decorated with hexanethiol-coated AgNPs to concentrate PAHs on the sensor surface and to detect subsequent high sensitivity and specificity [23]. The authors have illustrated that the nanocomposite could be used as solid-phase extraction (SPE) adsorbent. A portable Raman spectrometer was applied for recording the SERS spectra employing seawater samples. Six PAHs were investigated and the LODs were in the range 10
8–10
10 M. Finally, the results were compared with GC-MS (gas chromatography coupled with mass spectrometry) illustrating the potential of identifying and quantifying PAHs from complex water samples employing SERS. Fig. 2.2 summarizes the on-site detection capability of the proposed sensor as well as the SERS-based simultaneous detection of PAHs and the validation with GC-MS. The concept of combining SPE with SERS was further demonstrated by employing Ag-Cu fibers coated with propanethiol-modified AgNPs [24]. To extract the PAHs, the SERS-active fiber was immersed in water and the marker mode of propanethiol at 1030 cm
1 was used as an internal standard. Thus, SERS spectra with excellent reproducibility are recorded and the detection limit of fluoranthene was 8  10
10 M. The SPE fiber was further applied in combination with GC-MS, the gold standard in the detection of PAHs in water matrices. The same group has recently

Chapter 2 • Sensitive detection of organic pollutants

(A)

(B)

39

PER PYR ANT FLU NAP TOL

d

c

b a

40

Concentration (mM)

m/z : 202 PYR m/z : 252 PER

m/z : 92 TOL

Spiked concentration 20 of sample 2

Spiked concentration of sample 1

60

m/z : 202 FLU

m/z : 125 NAP

20

(D)

m/z : 178 ANT

15 45 SPE-SERS

SPE-SERS

GC-MS

GC-MS

10

30

5

15

0

Concentration (mM)

Relative intensity

(C)

0

0 5

10

15

Time (min)

20

25

TOL

NAP

Analytes

ANT

FLU

PYR

PER

Analytes

FIG. 2.2 On-site SERS detection of PAHs. Application of the SERS-active SPE device in combination with a portable Raman spectrometer (A). SERS spectra of seawater samples before (a, b) and after spiking with PAHs (c, d), the marker modes of the identified PAHs are assigned (B). GC-MS spectrogram of all investigated PAHs (C). Comparison of the SERS-based results with GC-MS (D). (PER, perylene; PYR, pyrene; ANT, anthracene; FLU, fluoranthene; NAP, naphthalene; TOL, toluene) [23]. Reproduced with permission from The Royal Society of Chemistry, S. Jia, D. Li, E.K. Fodjo, H. Xu, W. Deng, Y. Wu, Y. Wang, Simultaneous preconcentration and ultrasensitive on-site SERS detection of polycyclic aromatic hydrocarbons in seawater using hexanethiol-modified silver decorated graphene nanomaterials, Anal. Methods 8(42) (2016) 7587–7596; permission conveyed through Copyright Clearance Center, Inc.

published a study to illustrate the feasibility of propanethiol-modified SERS-active surfaces for swabbing extraction of PAHs on toys for children and the subsequent on-site detection by a portable Raman system [25]. An hierarchical SERS-active structure was equipped with octadecanethiol in order to allow the enrichment of PAHs on the surface of the applied superhydrophobic substrates [26]. Pyrene was detected down to 10
8 M plotting the SERS intensity of marker modes as a function of the concentration. In order to illustrate the usability of SERS in routine analysis of PAHs in water samples, a commercially available Au-based SERS substrate was modified with 4-dodecyl benzenediazonium-tetrafluoroborate to allow for an enrichment of the nonpolar target molecules benzo(a)pyrene, fluoranthene, and naphthalene and for a subsequent specific SERS-based detection employing a portable system [27]. The authors have showed for all

40

Advanced Nanostructures for Environmental Health

investigated analyte molecules the concentration dependency of the SERS signal and estimated the LOD, that is, 0.026 mg/L for benzo(a)pyrene, 0.064 mg/L for fluoranthene, and 3.94 mg/L for naphthalene. The plot of the LOD as a function of the octanol-water partition coefficient (KOW ) showed the trend that the more nonpolar the analyte is the more efficient is the SERS-based detection using the hydrophobic functionalization on SERS substrates. As an alternative, an alginate network was equipped with AuNPs to create hot spots allowing for high SERS signals as well as the enrichment of PAHs from water samples [28]. The quantitative analysis of the SERS spectra of benzo(a)pyrene showed a detection limit of 0.365 nM employing a portable Raman spectrometer. The same research group also published TiO2 nanotube arrays decorated with AuNPs as efficient SERS substrate for the detection of benzo(a)pyrene in various water matrices, that is, river, tap, and spring water [29]. The application of viologen-based host molecules to enrich PAHs on SERS-active sensor surface was investigated using lucigenin, diquat, and paraquat as host molecules [30]. Here, it was demonstrated that the best and most stable SERS intensities were achieved by using lucigenin to capture pyrene via π-π stacking. In further studies, the viologen host lucigenin was applied to modify the SERS-active surface [31,32]. The characteristic fingerprint peaks of the investigated PAHs were identified and plotted as a function of the concentration. The authors found that the host molecule has a higher affinity toward PAH molecules with four fused benzene rings [31]. Moreover, lucigenin-modified Au nanoplates were successfully used for the detection of pyrene down to 5  10
10 M [33]. After rinsing with methanol, the pyrene molecules were washed from the surface and the recycled SERS substrate was used again for the detection of pyrene (four cycles). As a further strategy, SERS-active structures were modified with β-cyclodextrin illustrating the enhanced capturing efficiency for perylene via the increased SERS intensity compared with unmodified SERS substrates [34]. The feasibility for environmental sensing was demonstrated by investigating four different PAHs employing the same SERS substrate. Finally, calixarene were used to modify the SERS-active surface to capture pyrene from water samples [35]. Using calixarene-modified silver colloids, hot spots for an increased SERS activity are associated with capturing PAHs from both sides by the host molecules [36]. The authors have illustrated that in the presence of the guest, that is, PAHs, the intramolecular cavity shows a closer configuration and a more perpendicular orientation of the involved benzene rings toward the metallic surface. To conclude the SERS-based detection schemes, a high number of SERS studies for detecting PAHs from water samples exists employing powerful nanostructures as sensor surface (see Table 2.1). As illustrated in Table 2.1, the SERS intensity of the nonpolar target analytes and therewith the limit of detection are increased via modification of the SERSactive nanostructures with alkyl thiol layers or host molecules as well as utilizing the gap mode in designed nanogap arrangements. Moreover, the enrichment of the target by using SPE techniques in combination with SERS is beneficial. The specific SERS marker modes associated with the alkyl modifications are used as an internal standard for the

Chapter 2 • Sensitive detection of organic pollutants

Table 2.1

41

SERS sensing platforms for the detection of PAHs

SERS-active nanostructure

λexc [nm]

PAH mix

AuNPs (dried in coffee ring)

785

+

AuNPs@Au film (gap mode)//LSL Bowl-shaped silver cavities decorated with AgNPs (gap mode)//LSL Magnetic Fe3O4@Ag particles//LSL Magnetic Fe3O4@AuNPs satellite-core particles

785

+

River water (PAH stock solutions in acetone and ethanol) Methanol

514, 633

+

785

LOD range for investigated PAHs

References

0.25–5 μM

[19]

[20]

Ethanol/water

1.2  10
8 M– 6.3  10
8 M 8–40 nM

+

Ethanol

0.25–20 μM

[16]

785

+

5–100 nM

[17]

CNT bundles @ AgNPs AgNP decorated graphene// LSL, SPME AgNPs@metallic fibers//LSL, SPME

514.5 785


+

10
9 M 0.03–6 nM

[22] [23]

785

+

8  10
10 M

[24]

Flexible Ag nanowire membrane//LSL AuNPs on nickel foam//LSL Commercial Au nanorod array//LSL AuNPs@alginate gel network

785

+

35–45 ng/cm2

[25]

633 691


+

River water (PAH stock solutions in acetone and ethanol) Aceton Seawater (PAH stock solutions in ethanol) Ultra-pure water (PAH stock solutions in ethanol) Swabbing extraction on toys Water Methanol/water

[26] [27]

785

+

10
8 M 0.1–31 μM (0.026–3.94 mg/L) 0.365–0.485 nM

AuNPs@TiO2 nanotube arrays

785




12.6 nM

[29]

AgNPs//HM




10
9 M

[30]

AgNPs//HM AgNPs//HM Au nanoplates//HM AgNPs@silica beads//HM

514, 785, 1064 785, 1064 785 633 532


+
+

10
7 M 10
6–10
8 M 5  10
10 M 10
8 M

[32] [31] [33] [34]

AgNPs//HM

785

+

10
7–10
8 M

[36]

Matrix

River, spring, and tap water samples (PAH stock solutions in acetone) River, spring, and tap water samples (PAH stock solutions in toluene) Water (PAH stock solutions in acetone) Acetone/water Acetone/water Water Ethanol/ dichloromethane Water (PAH stock solutions in acetone)

[21]

[28]

λexc, Excitation wavelength; PAH mix, identification of single components in PAH mixtures; LOD, limit of detection; LSL, lipophilic sensor layer, i.e., alkyl chain modification via SH group to enrich PAHs on sensor surface; SPME, solid-phase microextraction for preconcentration; HM, SERS active surface is modified with specific host molecules.

42

Advanced Nanostructures for Environmental Health

normalization of the SERS intensity of PAHs. The differences in the SERS spectra of PAHs from various studies might be attributed to a change in orientation of the analyte molecules toward the metallic surface. As in many studies the high potential for on-site testing and the application of portable and handheld Raman systems were demonstrated, future applications in routine analysis to monitor PAHs pollutants are feasible. Therefore, a portable or handheld Raman system should be combined with a stable SERS substrate prepared via a cost-efficient and simple fabrication protocol (equipped with hydrophobic or host modifications). In future, studies should be conducted employing real samples and the validation should be performed by employing the gold standard, that is, GC-MS. In order to allow the parallel detection of PAHs and to avoid spectral overlapping of marker modes, SERS might be combined with separation techniques in the future work. Besides the application of SERS-based techniques for PAH monitoring, a fluorescent quantum dot-graphene oxide nanocomposite was developed and characterized thoroughly by imaging and spectroscopic methods [37]. Here, the fluorescence emission of the quantum dots is enhanced due to the interaction of PAHs, acting as π-electron donors, with the graphene oxide sheets and detection limits in the range of 0.2–0.3 μg/L are achieved for four different PAH substances. Another approach was reported based on the fluorescence resonance energy transfer (FRET) mechanism [38]. Therefore, CdTe quantum dots were fabricated on TiO2 nanotubes and are excited at 270 nm. As the emission around 370 nm overlaps with the absorption of benzo(a)pyrene, FREToccurs with the target molecule as receptor. The normalized fluorescence intensity at 410 nm is plotted as a function of the concentration within the range of 40 pM and 400 nM and the LOD is estimated to 15 pM. As the specificity to single PAHs components was not demonstrated in both studies using quantum dots, these detection schemes might be suitable to detect the overall PAH contamination as fast on-site testing. Finally, an electrochemiluminescence detection scheme was introduced employing AgNP-modified TiO2 nanotube films as working electrode and S2 O8 2
as coreactant [39]. For the model analyte benzo(a)pyrene, a detection limit of 10
12 M was found and the linear range was between 3  10
12 and 10
9 M. The authors have illustrated that PAHs with more than four rings are detectable by this method. Thus, this detection scheme is not available for all PAH substances but shows excellent sensitivity for PAHs with more than four rings.

2.3 Organic solvents and VOCs Organic solvents show, due to its broad use in chemical, pharmaceutical, gas and oil industries, a high potential of polluting the environment. Moreover, the final appearance in everyday life products increases the tendency of pollution with organic solvents. Owing to the high vapor pressure of organic solvents and small organic molecules, they are often found in the gas phase as VOCs. For example, formaldehyde has a low boiling point (around
20°C), thus the molecule is released at room temperature from materials that contain the chemical such as furniture [40]. Formaldehyde is considered as a priority

Chapter 2 • Sensitive detection of organic pollutants

43

indoor pollutant and carcinogenic for humans. Thus, a specific and selective detection of organic solvents in environmental samples as well as VOCs in air is of huge interest.

2.3.1 Detection of organic solvents in aqueous environment and contaminated water To detect ethanol in environmental samples, CuO nanosheets were applied to decorate glassy carbon electrodes and the measured current was plotted as a function of the concentration [41]. The authors found a detection limit of 0.143 mM in a buffer solution. The solvent 2-butoxyethanol is often used as surface coating materials in paint as well as ink formulations and was found in high concentrations in wastewater from hydraulic fracking [42]. Thus, it is considered an indicator of organic pollution. In this study, the authors have illustrated a combination of colloidal crystal templating and molecular imprinting to create a sensitive detection platform to monitor this solvent in aqueous solutions. The reflectance spectra of the final polymer sensor film showed characteristic Bragg peaks. As a function of the concentration of 2-butoxyethanol (1 ppb to 1 ppm) the spectral position of the Bragg peaks also changes. When used in wastewater samples, the authors found that the order of magnitude of the concentration can be estimated showing the potential for semiquantitative analysis at the hydraulic fracturing site. A further study following a similar detection scheme is presented for the organic solvents such as chloroform, chlorobenzene, tetrahydrofuran, dichloromethane, and dimethoxyethane [43]. A hybrid material is generated from densely packed polystyrene NPs, which are silver-coated NPs and embedded within a PDMS (polydimethylsiloxane) matrix. The reflectance peak of this iridescent sensor material is around 550 nm and upon exposure to the target solvents, a shift of the peak (recorded under Wulff-Bragg conditions) is observed. The authors have concluded that this shift is associated with the PDMS swelling and thus, the change in lattice parameters of the detection sensor platform. It is concluded that the proposed sensor shows a great potential for the detection hazardous solvents in contaminated water. Finally, a chemiluminescence-based method for the detection of formaldehyde is introduced employing graphene quantum dots [44]. Due to the presence of formaldehyde, the chemiluminescence of the rhodamine B-H2O2 system is quenched (see Fig. 2.3) and the target analyte is detected in an aqueous environment with an LOD of 6 ng/L. The selectivity of the detection scheme is demonstrated by using other aldehydes and the authors have concluded that the system has an acceptable selectivity for formaldehyde. To conclude, by applying NPs, the detection of organic solvents in aqueous environment and contaminated water sources is demonstrated by employing electrical detection as well as reflectance spectroscopy of a photonic crystal lattice sensor. To increase the applicability in environmental study, the selectivity for different organic solvents needs to be demonstrated in the future work. One approach might be the application of a chemiluminescence system based on the oxidation of rhodamine B showing selectivity for formaldehyde.

44

Advanced Nanostructures for Environmental Health

FIG. 2.3 Schematic illustration of a chemiluminescence-based method for the detection of formaldehyde (HCHO) [44]. Reproduced with permission from A. Khataee, J. Hassanzadeh, R. Lotfi, S.W. Joo, Graphene quantum dots/bisulfite assisted chemiluminescence of rhodamine B-H2O2 system for sensitive recognition of HCHO, Sens. Actuators, B 254 (2018) 402-410, Copyright 2017 Elsevier.

2.3.2 Monitoring of VOCs by gas sensors As VOCs are considered major organic pollutants, a high number of studies are presented in the literature to detect these compounds with a high sensitivity. In the case of solid-state gas sensors it is expected that these sensors will have a key role in environmental science, chemical processing, personal safety control, etc., to monitor VOCs in the near future [45,46]. However, the selective detection of single components in a gas mixture of benzene, toluene, and xylene is challenging due to the similar chemical structures and properties [46]. The detection of VOCs by means of gas sensors is briefly summarized in the following section. For more information on gas sensing, the interested reader is referred to Chapter 6. The photoelectrical sensing of the most relevant indoor pollutant formaldehyde is performed by using flower-like SnO2 nanorods doped with polyoxometalate for an improved sensitivity at room temperature [47] The same device is also used for methylbenzene. Moreover, by using hierarchical SnO2 nanostructures prepared by implementation of the topological transformation approach into the fabrication procedure, a fast response and sensitive detection down to 1 ppm is achieved illustrating the potential for indoor detection of formaldehyde [48]. The sensing mechanism is depicted in Fig. 2.4. Another modification strategy of SnO2 nanostructures is described with the addition of Zn2SnO4 creating a nanocomposite-based sensing platform for the detection of formaldehyde based on resistance measurements [49]. Employing the concept of quartz microbalance measurements, metallic nanostructures equipped with molecular imprinted polymer layers were coated onto the crystal and formaldehyde mass detection is achieved down to 1 pM [50]. In comparison to the quartz microbalance concept, film bulk acoustic resonators are associated with a single molecule mass sensitivity due to the application of piezoelectric films [51]. By coating the sensor device with polyethylene imine nanofibers,

Chapter 2 • Sensitive detection of organic pollutants

45

FIG. 2.4 Sensing mechanism of formaldehyde employing hierarchical SnO2 nanostructures [48]. Reproduced with permission from K. Xu, D. Zeng, S. Tian, S. Zhang, C. Xie, Hierarchical porous SnO2 micro-rods topologically transferred from tin oxalate for fast response sensors to trace formaldehyde, Sens. Actuators, B 190 (2014) 585-592, Copyright 2013 Elsevier.

the LOD for mass-sensitive detection of formaldehyde is found to be 37 ppb. Thus, both electrical and mass-sensitive detection schemes are used to monitor gaseous formaldehyde and the sensitivity is improved by the application of innovative nanostructures. In order to estimate benzene concentration in the atmosphere with high selectivity against other indoor pollutants, a Pd-loaded SnO2 yolk-shell platform is equipped with catalytic-active Co3O4 layer [52]. A resistance ratio of 88 is achieved for 5 ppm of benzene and the response to other relevant molecules such as toluene, xylene, and formaldehyde is negligible. The same group published an NiO/NiMoO4 nanocomposite structure for the selective detection of 5 ppm of p-xylene with a maximum resistance ratio of 101.5 [53]. The achieved selectivity is associated with the catalytic promotion of the relevant xylene sensing reaction. In the case of nitrobenzene and o-toluidine, polystyrene nanobeads were functionalized with fluorophores [54]. Upon the interaction with nitroaromatics or amines, the fluorescence of the applied dye molecules is selectively quenched resulting in different fluorescence patterns under UV (ultraviolet) light excitation. The sensors were subjected to eight cycles of testing. To increase the selectivity to trifluorotoluene, nanostructured ZnO fibers were modified with receptor cavities and their sensing capabilities were illustrated by employing XPS and bulk Raman measurements for characterization [55]. A mass-sensitive approach for the detection of monoaromatic compounds (e.g., benzene, toluene, and xylenes (BTX)) is illustrated by using an organic polymer modified

46

Advanced Nanostructures for Environmental Health

quartz crystal microbalance approach [56]. The sensor surface could also be applied in wet environments (up to 40% relative humidity) and at room temperature. As a further relevant indoor organic pollutant, acetone was studied as target analyte. A photochromism approach is introduced based on the application of tungsten-doped TiO2 nanofibers [57]. Here, the photocatalytic oxidation of acetone causes a color change which is observable with the naked eye. An interdigitated electrode was modified with polymeric nanostructures to capture acetone from the atmosphere [58]. The authors have found that the best results in detecting acetone were achieved by employing poly (4-vinylphenol) and the response was further improved after the addition of silicon NPs. The measurement was performed at room temperate in 1 min. As a further sensor layer, poly(porphyrin) is applied to detect acetone vapor and the poor conductivity is overcome by the application of carbon nanotubes as transducer [59]. As a detection limit, 9 ppm was found and the dynamic range for detection is between 50 and 230,000 ppm. A bimetallic plasmonic nanogap structure was used to detect acetone vapor using SERS as readout technique [60]. The marker mode of acetone at 791 cm
1 can be detected down to the limit of 9.5 pg, illustrating the potential of the molecular specific and sensitive detection scheme of SERS for VOC analyses. Finally, an approach based on flexible electronics for the detection of acetone and other VOC substances in wearable applications was introduced [61]. Here, a nanocomposite prepared from graphene and ethyl cellulose was used. To detect ethanol, a three-dimensional (3D) spinel cobalt oxide nano-arrangement was developed showing excellent sensing properties as well as selectivity to the target analyte [62]. Here, the resistance of the device was measured in the presence of the VOCs as well as air. A similar detection scheme is used applying anisotropic SnO2 nanostructures as sensing platform [63]. Here, the influence of the crystal size on the response was investigated by using ethanol and acetone as target analytes with a concentration of 100 ppm. As a further sensing platform, ZnO nanosheets are employed and the resistance is estimated in the presence of gaseous ethanol as well as isopropyl alcohol [64]. To increase the sensitivity and open the way toward trace gas sensing, ZnO nanosheets are decorated with silver NPs and a detection of ethanol down to 1 ppb is achieved [65]. In order to estimate the general pollution with volatile organic molecules, a total VOC amount was determined [66,67]. Porous single-crystalline ZnO nanoplates were decorated with AuNPs and the resistance change was monitored after exposure to the VOC vapor. For the VOC substance classes, the contribution ratios to the overall signal were determined to characterize the correlation of target components in different mixtures and the signal response. As an alternative to electrical detection schemes, an optical hollow metal-organic framework (MOF) nanoshell-based etalon sensor was introduced for the detection of VOCs [68]. After exposure to VOCs, a color change was observed due to changed reflectance of the optical sensor platform, whereas different VOC substances cause different peak shifts which might be the starting point to distinguish VOC substance classes in the future work. To conclude, the detection of VOCs is performed with good sensitivity by electrochemical detection schemes employing NP-decorated sensor platforms. In Table 2.2, the resistance-based sensing platforms are summarized as this detection strategy is mostly

Chapter 2 • Sensitive detection of organic pollutants

47

Table 2.2 Overview about resistance-based measurements of VOCs using advanced nanostructures

Sensing platform

TO [°C]

Response sensitivity

LOD for investigated VOCs

Hierarchical porous SnO2 microrods on gold electrodes decorated with SnO2 NPs (40–70 nm) Nanostructured SnO2-Zn2SnO4 composites Pd-loaded SnO2 yolk-shell platform decorated with a thin Co3O4 overlayer NiO/NiMoO4 nanocomposite hierarchical spheres SWCNT equipped with poly(porphyrin) sensor layer Flexible graphene/ethyl cellulose nanocomposite platform Hierarchical Co3O4 needleshaped microspheres Anisotropically shaped SnO2 nanocrystals Sn-doped ZnO nanocomposites

330

Ra/Rg ¼ 3.86 (@1 ppm formaldehyde)

550 ppb formaldehyde

[48]

162

Ra/Rg ¼ 8.1 (@100 ppm formaldehyde) Ra/Rg
1 ¼ 12.3 (@0.25 ppm benzene)

(100 ppm formaldehyde) 12.7 ppb benzene

[49]

Rg/Ra ¼ 1.2 (@0.02 ppm p-xylene) ΔR/R0 ¼ 5.9 (@50 ppm acetone)

0.02 ppm p-xylene

[53]

9 ppm acetone

[59]

ΔR/R0 ¼ 0.15–0.28 (@25 ppm VOCs and 20 wt%) Rg/Ra ¼ 13.4 (@50 ppm ethanol)

25–100 ppm for various VOCs 10 ppm ethanol

[61]

3.1 ppb ethanol

[63]

NA

[64]

360

Ra/Rg > 100,000 (@100 ppm ethanol) (Ra/Rg
1)  100% ¼ 91% (@4 vol% ethanol) Ra/Rg ¼ 5–25 (@50 ppm VOCs)

[66]

400

Ra/Rg ¼ 7–59 (100 ppm VOCs)

(2–20 ppm for various VOCs) NA

AuNP decorated porous singlecrystalline ZnO nanoplates AuNP decorated porous singlecrystalline ZnO nanoplates

375

375 Room temp. Room temp. 170 250 250

References

[52]

[62]

[67]

TO, operating temperature; LOD, limit of detection; Ra, electrical resistance in dry air; Rg, electrical resistance in the tested gas; R0, baseline resistance before analyte exposure; ΔR ¼ R
R0, R, resistance of the sensor platform exposed to the target; NA, value is not available.

used in VOC sensing. An increased specificity to single components or VOC substance classes is achieved by, for example, specific polymer sensor layers to capture target analytes from the environment, by recording molecular specific SERS spectra or by using etalon sensors monitoring reflectance change after exposure. The application scenarios of VOC sensing require detection schemes to be performed in different (indoor) settings, that is, portable systems are requested, as well as by employing wearable sensors.

2.4 Persistent organic pollutants: Detection of pesticides and halogenated biphenyls and bisphenols The discovery of the effective insecticidal behavior of the organochlorine dichlorodiphenyltrichloroethane (DDT) and its application in agriculture is accompanied by the pollution of soil and water sources [3]. The application of DDT was banned in the United States

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Advanced Nanostructures for Environmental Health

in 1973 and in United Kingdom in 1984. Due to the long persistence of this chemical, it remains in the environment for decades. After the World War II the application of organochlorine as pesticides was preferred, currently, available as pesticides organophosphates and carbamates. Pesticide residues are defined as pesticides which are found in or on food and accumulated in the food chain [69]. The knowledge that only 0.1% of the applied pesticides reach the target pest [69] as well as 1% of all insect species are classified as significant pests [3] shows that alternatives for agriculture need to be found to avoid the pollution of our ecosystem. Polychlorinated biphenyls (PCBs) are classified as persistent organic pollutants with a high environmental toxicity found all over the world in the atmosphere, water sources, and soil samples. Its sensitive and specific detection is of extreme interest as the compounds are known to cause serious health issues such as cancer [70]. To detect pesticides or halogenated persistent organic pollutants in environmental samples, NP-based sensor systems are developed, which will be introduced in this chapter.

2.4.1 Pesticides Organohalides. The pesticide acetamiprid was detected employing an aptamer-based colorimetric approach [71]. In this study, it was demonstrated that acetamiprid is captured specifically by an aptamer, and the formed complex is bound to the surface of AuNPs accompanied by an increased catalytic activity to oxidize a reporter substance. Thus, the color change of the solution is assigned to the formation of a green oxidation product. The readout is performed with a UV-VIS (visible) absorption spectrometer and the detection limit was found to be around 1 μg/L. Moreover, two specific aptamer sequences were applied to modify a sensor surface equipped with PtNPs allowing for the specific detection of acetamiprid and atrazine [72]. The successful capturing of the target is monitored by a shift of the impedance, that is, the charge transport in the NP film is hindered. The detection limits for both analytes were estimated: 0.6  10
11 M for acetamiprid and 0.4  10
10 M for atrazine. In general, to detect organohalide pesticides such as acetamiprid in environmental samples, aptamers in combination with advanced nanostructures have gained attention in the last years [73]. As read-out techniques, optical (e.g., fluorescence and colorimetry) and electrochemical (e.g., electrochemical impedance spectroscopy, differential pulse voltammetry, and photoelectrochemical (PEC) biosensing) are available. As the potential of aptamer-based nanosensors for the detection of the organohalide pesticide acetamiprid was illustrated the author of this review article concluded that in the future, the development of commercial sensor devices should be focused. To monitor atrazine in water samples, a nanoporous sensor platform utilizing the selectivity of an antibody toward atrazine was presented [74]. The change in impedance is plotted as a function of the concentration demonstrating a dynamic range between 10 pg/L and 1 μm/L in spiked river and drinking water samples. The SERS-based detection of atrazine was illustrated employing commercially available SERS substrates [75]. Atrazine was detected down to 0.1 ppb spiked in tap water. Furthermore, amino silanemodified quantum dots were used as optical sensing platform [76]. Here, target molecules

Chapter 2 • Sensitive detection of organic pollutants

49

with good leaving groups such as –Cl bind to the amino groups of the modified quantum dots. Since each investigated pesticide, that is, atrazine, aldrin, tetradifon, and glyphosate, shows a different binding affinity toward the quantum dots, the differentiation between the pesticides became possible via change in fluorescence decay or resistance. To investigate thiamethoxam [77] as well as pyridaben [78] Ag-based SERS substrates were presented allowing for recording molecular specific fingerprint spectra of organohalide pesticides. In the case of dicofol, it was found that glutathione-modified quantum dots are aggregated in the presence of this target molecule which is associated with an increase in fluorescence intensity [79]. The detection limit was estimated to 55  11 ppb. A copper nanowire-based sensor was demonstrated for the sensitive detection of trifluralin in spiked urine and soil samples [80] To increase the specificity, immunoassays are performed in electrochemical detection schemes. In the case of 2,4-dichlorophenoxyacetic acid [81], diuron [82], and picloram [83], competitive immunoassays were developed using advanced nanostructures as electrochemical sensing platform allowing for an indirect detection with high specificity. In summary, to allow for a specific and sensitive detection of organohalide pesticides in environmental samples, aptamer- and immunoassay-based detection schemes combined with electrochemical and optical techniques with high potential for on-site applications could be achieved by miniaturization and system integration in the future work. A tabular overview of different optical and electrochemical sensing strategies alongside with the nanostructure sensing platform and LOD of acetamiprid are reported in the literature [73] illustrating the potential for aptamer-based sensors. In order to avoid specific biomolecules, that is, aptamers and antibodies, in the analysis, organic-modified quantum dots associated with changed properties upon interaction with target analytes as well as utilizing powerful SERS substrates for recording fingerprint spectra are available. Organophosphates. The organophosphorus pesticide (OP) malathion was detected via a colorimetric detection scheme employing Au nanoprobes and their aggregation in the presence of a peptide structure [84]. When no target analyte is present, the specific aptamer interacts with a peptide and the added AuNPs remain free associated with no color change, that is, the AuNPs remain red in color. In the presence of malathion, a complex with an aptamer is formed and thus, the peptide remains free to interact with AuNPs causing subsequent aggregation and a color change from red to blue. The LOD is estimated to 1.94 pM. To detect OPs with high sensitivity, the property to inhibit the activity of the enzyme acetylcholinesterase (AChE) is used [85–87]. A transition metal carbide nanocomposite is equipped with AChE and chitosan and the biosensor is characterized by cyclic voltammetry and electrochemical impedance spectroscopy [85]. The detection of malathion is achieved down to 0.3  10
14 M. A similar approach was demonstrated by employing Pd-Cu nanowires to modify the electrode surface [86]. After immobilization of AChE and chitosan, the malathion down to 4.5 pM is detected. The estimation of the target analyte in fruit and vegetable samples shows the potential also for the analysis of pesticide residues. Finally, an AChE equipped electrode modified with AuNPs was used for the detection of both malathion and methyl parathion with the LODs of 5.12 and

50

Advanced Nanostructures for Environmental Health

5.85  10
13 g/L, respectively [87]. Thus, powerful and sensitive detection schemes with a high affinity toward OPs are presented paving the way toward fast and on-site detection in environmental samples. An electronic tongue utilizing graphene hybrid nanocomposites equipped with AuNPs was presented for the trace level detection of the OPs malathion and cadusafos [88]. As illustrated in Fig. 2.5, the electrical resistance data were analyzed by means of principal component analysis which shows a discrimination of the investigated OPs at the nanomolar concentration level which is important for the investigation of OP mixtures. Further well-studied OPs for a sensitive detection in environmental samples are parathion and methyl parathion. A colorimetric approach was developed using the enzyme activity of AChE to hydrolyze acetylthiocholine to thiocholine, which reduces Au3+ resulting in a slight growth of the AuNPs, that is, the color of the solution changes to dark red [89]. In the case of parathion present in the solution, the enzyme activity in inhibited, thus, the reducing agent thiocholine is not produced. As a consequence, the AuNPs in the solution are dissolved by Au3+ and by the assistance of CTAB, a colorless Au+-CTAB complex is formed. Parathion is detected in different aqueous solutions such as apple washing solution, tap, and seawater in the ppm range and recovery values between 90% and 130% are achieved. As an alternative colorimetric approach, AuNPs are aggregated by thiocholine, thus the inhibition of the enzyme activity of AChE in the presence of OPs prevents the aggregation [90]. The sensor scheme was successfully applied for the detection of parathion, paraoxon, fenitrothion, and diazinon. A PEC assay was developed utilizing triphenylamine dye-modified TiO2 NPs on an electrode surface and the authors have found that

FIG. 2.5 Principal component (PC) analysis of the electrical resistance measurements for the investigated malathion and cadusafos solutions in PBS buffer and tap water [88]. Reproduced with permission from M.H.M. Facure, L.A. Mercante, L.H.C. Mattoso, D.S. Correa, Detection of trace levels of organophosphate pesticides using an electronic tongue based on graphene hybrid nanocomposites, Talanta 167 (2017) 59–66, Copyright 2017 Elsevier.

Chapter 2 • Sensitive detection of organic pollutants

51

this sensor shows a high photocatalytic activity toward thiocholine, the hydrolysis product of AChE [91]. As the activity of AChE is inhibited by OPs, this platform was applied to detect parathion in a linear range of roughly 10
6–10
12 g/mL. Organophilic nanohybrid kaolinite is applied to modify glassy carbon electrode surfaces for the detection of methyl parathion in natural water sources [92]. As an alternative to enzyme inhibition biosensors, an electrode surface was modified with CuO nanostructures and pralidoxime chloride was immobilized [93]. When OPs (such as methyl parathion, chlorpyrifos, and fenthion) are present, the electrooxidation is inhibited and the pesticides are detected down to 10
9 M range. A fluorescence-based sensing scheme was presented by employing rhodamine B-modified metallic NPs [94]. Due to the interaction of the dye with the metallic surface, the fluorescence is quenched; however, after incubation with OPs, the dye is displaced from the surface, regaining the fluorescence signal. The LOD of methyl parathion in fruit and water samples is 0.0018 ng/mL. The same group published a fluorescence-based sensor employing the AChE inhibition by methyl parathion [95]. In the presence of methyl parathion, the fluorescent intensity increases and the detection is achieved within the range of 0.33–6.67 ng/mL. Furthermore, quantum dots are used to detect 4-nitrophenol, a hydrolysis product of methyl parathion under alkaline conditions, due to photoluminescence quenching [96]. The platform was used for the analysis of methyl parathion in rice and tap water in the range of 0.5–10 μg/mL. Finally, a regularly ordered SERS substrate was demonstrated to detect methyl parathion with a high molecular specificity down to the nM range [97]. The geometry of multiple hot spots allows for recording the SERS spectra of methyl parathion with high quality and plotting the Raman intensity of the marker mode as a function of the concentration and illustrated the linear range between 10 nM and 1 μM. Several studies were published to detect OPs and to illustrate the potential of the applied techniques in environmental monitoring, such as paraoxon [98,99], chlorpyrifos [100–102], dichlorvos [103], omethoate [104], glyphosate [105], phorate [106], phosmet [107,108], acephate [109], and fenitrothion [110,111] by using NP-based techniques which are employed for the detection of malathion and parathion as already explained above as well as its variants. To conclude, in many biosensor applications in the field of OPs, the inhibition of AChE is utilized to detect pesticides with a high sensitivity. A summary of AChE-based sensing strategies is illustrated in Table 2.3. As this inhibition process is not specific for an individual OP component, these sensor systems are capable of monitoring the overall contamination with OPs. As detection schemes, both electrochemical and optical techniques are widely used and the specificity to individual OP components is achieved via the specific detection of hydrolysis products of OPs or via SERS as readout method. Carbamates. Dithiocarbamate pesticides such as thiram or ferbam are used to protect fruits and vegetables from fungal diseases [112]. For the detection of thiram, multibranched Au nanostars were developed and the marker mode at 1376 cm
1 was chosen for plotting the SERS intensity as a function of the concentration [113]. A detection limit of 10
10 M was estimated which is associated with 0.24 ng/cm2 on apple peels. The

52

Advanced Nanostructures for Environmental Health

Table 2.3 Overview about AChE-based detection schemes of organophosphates (pesticides) Sensing method DPV

Advanced nanostructured platform

DPV

Chitosan-modified Ti3C2Tx nanosheets Pd-Cu nanowires (NWs)

CV

Colloidal AuNPs

Colorimetry

AuNPs

LSPR

AuNPs

PEC

TiO2 NPs modified with a di-branched di-anchoring triphenylamine dye T(TA)2 Au nanoclusters (NCs)

Fluorescence DPV

DPV

Composite of graphene oxide (GO) network and multiwalled carbon nanotubes (MWCNTs) TiO2-graphene nanocomposites

Biosensor

LOD

References
14

AChE/CS-Ti3C2Tx/GCE

0.3  10

AChE-CS/Pd-Cu NWs/ GCE AuNPs/DAR/AChE films on GCE

4.5 pM malathion

[86]

0.0016 pM malathion; 0.0022 pM methyl parathion 35 ppb parathion

[87]

0.13 ng/mL paraoxon; 0.37 ng/mL parathion; 0.42 ng/mL fenitrothion; 0.20 ng/mL diazinon 5.6  10
13 g/mL parathion

[90]

0.14 ng/mL parathionmethyl 0.015 ng/mL carbofuran; 0.025 ng/mL paraoxon

[95]

AuNPs in Au3+-CTAB solution Citrate stabilized AuNPs

T(TA)2–TiO2/FTO

BSA-protected AuNCs AChE-GO-MWCNTs/ GCE

AChE-immobilized CS@TiO2-CS/rGO/GCE

M malathion

29 nM dichlorvos

[85]

[89]

[91]

[99]

[103]

LOD, Limit of detection; DPV, differential pulse voltammetry; CV, cyclic voltammetry; LSPR, localized surface plasmon resonance technique; PEC, photoelectrochemical technique; CS, chitosan; GCE, glass carbon electrode; DAR, diazo resins; CTAB, cetyltrimethylammonium bromide; FTO, fluorine doped tin oxide electrode; rGO, reduced graphene oxide.

authors have shown that OPs do not interfere with the SERS signal of thiram which might be attributed to the strong affinity of thiram toward the metallic surface in comparison to other pesticides. Inspired by the nanoscale tentacles of geckos, a flexible SERS substrate was introduced which is pressed to vegetable and fruit surfaces and peeled of, accumulating thiram as target analyte [114]. Thiram is detected down to 1.6 ng/cm2 when tested on apple, grape, and cucumber surfaces. A sponge-like SERS sensor was fabricated from reduced graphene oxide and Ag nanocubes to allow for a selective enrichment of aromatic pesticides on the graphene-based surface and of dithiocarbamate pesticides on the metallic nanostructures [112]. Thus, the interference of different pesticides is avoided resulting in SERS spectra dominated by the contributions of thiram and ferbam, respectively, enriched from mixture samples. For both target analytes, detection down to the nM range is achieved. Moreover, a melamine-based sponge structure is modified with AgNPs and the detection of thiram down to the nM range was illustrated employing a portable Raman

Chapter 2 • Sensitive detection of organic pollutants

53

spectrometer for readout [115]. In order to enrich thiram from aqueous solutions accompanied by a subsequent forming of hot spots for an efficient SERS investigation, magnetic nanospindles equipped with a silver layer are fabricated [116]. In this study, the detection down to 10
7 M is observed. A comparable study illustrated the fabrication of cube-like magnetic Au–Ag nanocomposite structures for the detection of 5  10
11 M of thiram [117]. A macroscale lattice of Au nanorods and magnetic beads were fabricated on a hydrophobic surface, detached and incubated in an aqueous solution containing thiram [118]. By applying magnetic forces, the macroscale lattice is separated from the matrix solution after the incubation time and SERS spectra of thiram are recorded down to a concentration of 10
8 M. Furthermore, magnetite beads are modified with Ag seeds and as a consequence, highly branched Ag flower composites are created [119]. A detailed investigation illustrated the excellent SERS characteristics associated with the detection of thiram down to 10
11 M. The magnetic cores allow for creating hot spots after applying magnetic forces. The SERS-based detection scheme is illustrated in Fig. 2.6. After washing with hydrochloric acid, ammonia, and ethanol, no SERS signal of the target is detectable and the recycled SERS-active composites are used again for the analysis. In conclusion, different SERS-based approaches have been illustrated in this section showing, for example, the selective enrichment of dithiocarbamate pesticides on metallic surfaces and the application of flexible SERS substrates or magnetic nanocomposites creating excellent hot spots (see Table 2.4). As dithiocarbamate pesticides are applied on crops to prevent fungal diseases, detection schemes on the peels are developed. For the sensitive detection of thiram, the SERS-based detection is predominant the literature, which might be attributed to the chemical structure containing sulfur and its excellent binding to metallic surfaces, which is essential for recording SERS spectra with high signal-to-noise ratio.

2.4.2 Halogenated biphenyls and bisphenols By employing aptamers as recognition unit, the compound PCB77 was detected with a high specificity and the readout is performed by fluorescence [70]. Here, Au NPs were decorated with DNA strands being specific for the aptamer sequence. As the aptamer is labeled with a fluorescent dye, the fluorescence signal is quenched due to the vicinity to the metallic surface. In the presence of PCB77, the fluorescent-labeled aptamer is removed from the Au NP surface to bind to the target molecule. Thus, the fluorescence is increased and the linear detection range is between 0.1 and 100 ng/mL. Moreover, SERS was used for the specific and sensitive detection of PCB77 [120,121]. The first approach is based on the decoration of Fe3O4 microspheres with Au NP to enrich the analyte molecule on the metallic surface [120]. By applying magnetic forces, hot spots are generated. SERS spectra are recorded in the concentration range between 10
3 and 10
7 M. A second approach is based on the modification of the SERS substrate with β-cyclodextrin to capture the target PCB77 from the sample [121]. Here, a specific SERS signal of PCB77 is recorded in the μM range. It is worthmentioning, that in both cases, the SERS spectra are different, that is, different marker modes dominate the SERS spectrum,

54

Advanced Nanostructures for Environmental Health

FIG. 2.6 SERS-based detection scheme to monitor thiram employing magnetic SERS-active beads (A), concentrationdependent SERS spectra (B), and the Raman intensity of 1385 cm
1 marker mode as a function of the concentration (C) [119]. Reproduced with permission from The Royal Society of Chemistry, C. Wang, J. Wang, P. Li, Z. Rong, X. Jia, Q. Ma, R. Xiao, S. Wang, Sonochemical synthesis of highly branched flower-like Fe3O4@SiO2@Ag microcomposites and their application as versatile SERS substrates, Nanoscale 8(47) (2016) 19816–19828; permission conveyed through Copyright Clearance Center, Inc.

which might be attributed to a changed orientation of the analyte molecule toward the metallic surface as different SERS substrates are employed. To detect the compound PCB101 in environmental samples, TiO2 nanorods were decorated with molecular imprinting sites to achieve a high selectivity over chemical similarity of molecules and the readout was performed via PEC sensing [122]. Moreover, due to the structure of the TiO2 nanorods, the separation of photogenerated electrons and holes are more efficient, leading to an improved sensitivity of the sensor platform. The photocurrent response was plotted as a function of the logarithmic concentration and a linear profile was found between 0.08 pM and 30 nM. Finally, three contaminated water samples from Shanghai were investigated and by standard addition, the PCB101 concentration was

Chapter 2 • Sensitive detection of organic pollutants

Table 2.4

55

SERS sensing platforms of dithiocarbamates (pesticides)

SERS-active nanostructure Ag nanocube modified rGO (sponge-like SERS substrate) Multibranched Au nanostars AgNPs on 3D flexible nanotentacle array Ag film-grafted melamine sponge Fe3O4@SiO2@Ag magnetic nanospindles Cube-like Fe3O4@SiO2@Au@Ag magnetic nanoparticles Au nanorod/Fe3O4 NP assemblies Fe3O4@SiO2@Ag microflowers

λexc [nm]

Mix

Matrix

LOD

References

785, 633, 532 633

+

Acetone

10 ppb thiram; 16 ppb ferbam

[112]




Ethanol

[113]

633




785 633, 785 785





Pressed on surface and peeled off Water Ethanol

10–10 M and 0.24 ng/cm2 thiram 1.6 ng/cm2 thiram




633 785





[114] [115] [116]

Ethanol

1.88 nM thiram 10–7 M (0.024 ppm) thiram 5  10–11 M thiram

Water Ethanol

10
9 M thiram 10
11 M thiram

[118] [119]

[117]

λexc, excitation wavelength; Mix, recording SERS spectra in pesticide mixtures; LOD, limit of detection; rGO, reduced graphene oxide.

estimated to be in the range 63.7–120.2 pM. The detection limit of PCB101 is improved to 50 fM by utilizing molecular imprinted TiO2 nanorods decorated with Pd quantum dots [123] PCB101 was successfully detected in spiked lake water (0.5–15 μg/L) and soil (0.5–1.5 μg/g). A further strategy to increase the selectivity is illustrated by the modification of impedimetric sensor surfaces with mercapto-β-cyclodextrin to detect persistent toxic substances such as PCBs [124]. Finally, an electrochemical immunosensor was developed equipped with Ag NPs modified glassy carbon electrodes and immobilized polyclonal anti-PCB-antibodies employing square wave voltammetry [125]. For the target analyte PCB28 (spiked into water and juice), a linear electrochemical response was achieved between 0.2 and 1.2 ng/mL and the estimated LOD as well as limit of quantification values were 0.063 and 0.209 ng/mL, respectively. However, the authors found a poor specificity against benzyl chloride and PCB180, which was associated with the chemical similarity of the molecular structures. As a consequence, this sensor platform might be suited for the estimation of the total PCB content in environmental or food samples. The bromine analogue 4,40 -dibrominated diphenyl (PBB15) was detected via a competitive FRET immunoassay [126]. Here, carbon dot-modified PBB15 antigens and AuNPs immobilized with anti-PBB15 antibodies are used. In the absence of PBB15, the carbon dot-modified antigen interacts with antibody-modified AuNPs. Thus, the fluorescence is quenched due to the vicinity of the carbon dots to the metallic surface. In the case of PBB15 present, the target is competing with the carbon-dot-modified antigen for free binding sites on the AuNP surface and the fluorescence intensity is recovered. The fluorescence intensity was found to be proportional to the concentration within the range of 0.05–4 μg/mL and the LOD was 0.039 μg/mL. Moreover, the proposed method showed an excellent selectivity over chemically similar molecules such as PCBs.

56

Advanced Nanostructures for Environmental Health

To detect tetrabromobisphenol A, a brominated flame retardant, voltammetry was used [127]. In order to allow for an increased sensitivity, the carbon paste electrodes are modified with FeOOH nanorods. As a result, oxidation peak from differential pulse voltammetry measurements is increased by a factor of 8 in comparison to unmodified electrodes. The detection limit is estimated to 0.6 nM. To summarize, the detection of halogenated biphenyls and bisphenols is achieved by fluorescence- and SERS-based as well as electrochemical detection platforms. As strategy to increase the specificity to individual target molecules, aptamers, antibodies, molecular imprinted polymers, and cyclodextrin units are applied to modify the nanostructured sensor surface.

2.5 Detection schemes for molecules relevant in industrial production and water treatment Phenol and its derivates such as aminophenol or nitrophenol are important chemicals in industrial production and traces of these toxic compounds are widely distributed in our ecosystem, particular in water sources [128]. The gold standard methods such as GC-MS or high-performance liquid chromatography (HPLC)-based methods are assigned as expensive, laborious, and time-consuming techniques, which illustrate the requirement for new approaches for on-site applications. In this section, innovation detection schemes utilizing the exceptional properties of advanced nanostructures are summarized to detect phenols and dye molecules relevant in industrial production as well as disinfection agents used in water treatment.

2.5.1 Phenols To quantify phenol in spiked wastewater and untreated lake water samples, graphene quantum dots are used [128]. As a result of a quantitative chemical reaction between phenol and the graphene quantum dots in the presence of H2O2 and horseradish peroxidase, the light scattering is enhanced allowing for the sensitive and specific detection of phenol down to 2.2  10
8 M. Alkylphenols, which are associated with toxic effects on aquatic organisms as well as decrease in reproductive health, are detected by utilizing a nanocomposite gel consisting of carbon nanotubes and an ionic liquid [129]. The sensor platform is equipped with horseradish peroxidase to oxidize the target molecules using H2O2 as oxidation agent. The linear range for the analysis of 4-t-octylphenol and 4-n-nonylphenol is μM and detection limits around 1 μM. The environmental pollution with the toxic compound 4-aminophenol is among others due to its application for the production of dyes [130] or for the synthesis of pharmaceuticals [131]. Employing MoS2 nanostructure-modified glassy carbon electrodes for the sensitive detection of 4-aminophenol, excellent electrocatalytic activity was demonstrated [131]. The LOD was around 30 nM and the target analyte was successfully investigated in spiked water sources with recovery values between 96% and 99%. Later, the

Chapter 2 • Sensitive detection of organic pollutants

57

detection limit was further improved to 2 nM with a comparable sensor concept [130]. A further electrochemical sensing platform is based on graphene oxide-modified SnO2 hollow spheres allowing for the individual and simultaneous monitoring of 4-aminophenol and 4-chlorophenol achieving detection limits of 2.2 and 3.1 nM, respectively [132]. Water samples from different sources were spiked with both analytes (3–5 μM) and the concentration values were compared with HPLC results illustrating high potential for real application scenarios. Moreover, graphene is decorated with PdAg nanoalloy structures allowing for the simultaneous and sensitive detection of phenol isomers (the LOD is given in brackets), that is, 4-aminophenol (6.7 nM), 2-aminophenol (13.7 nM), hydroquinone (0.05 μM), and catechol (0.06 μM) [133]. In Fig. 2.7, the results by using differential pulse voltammetry for readout are illustrated for all four target molecules. The authors have demonstrated additionally the reduction of 4-nitrophenol to

–0.27

H2Q

0.8

0.4

2.4

CC

I/mA

CC 0.4

–0.45

1.2

1.1

0.0

1.2

1.3

1.4

1.5

1.6

Log [CC]

–1.2

0.0

–2.4 0.2

0.0

0.6

0.4

(B)

0.8

0.00

E/V vs Ag/AgCI 3M KCI 2-AP

–0.6

1.2

–0.9

–1.5 0.0

4-AP

0.3

0.6 0.9 1.2 Log [2-AP]

1.5

I/mA

–1.2

0.42

0.56

0.00

4-AP

–0.3 Log Ip

0.8

0.28

–0.38 Log Ip

1.0

0.14

E/V vs Ag/AgCI 3M KCI

1.5

0.0

I/mA

0.1

Log [H2Q]

0.2

0.6

0.2

0.0

1.44 1.60 1.76 1.92 2.08

(A)

H2Q

–0.54 –0.63

0.3

Log lp

0.6

I/mA

Log lp

–0.36

0.9

–0.76 –1.14 –1.52 0.0

0.6

0.3

0.6

0.9

1.2

1.5

Log [4-AP]

2-AP

0.4 0.3 0.2 0.0 –0.2

(C)

0.0

0.2

0.4

0.6

E/V vs Ag/AgCI 3M KCI

–0.2

0.8

(D)

0.0

0.2

0.4

0.6

0.8

E/V vs Ag/AgCI 3M KCI

FIG. 2.7 Differential pulse voltammetry investigations using graphene decorated with PdAg nanoalloy nanostructures as sensor platform: hydroquinone from 6 to 56 μM with a constant concentration of catechol of 30 μM (A), catechol from 7 to 53 μM with a constant concentration of hydroquinone of 30 μM (B), 2-aminophenol from 1 to 22 μM with a constant concentration of 4-aminophenol of 2 μM (C), and 4-aminophenol from 1 to 22 μM with a constant concentration of 4-aminophenol of 4 μM (D) [133]. Reproduced with permission from S.A. Bhat, N. Rashid, M.A. Rather, S.A. Pandit, G.M. Rather, P.P. Ingole, M.A. Bhat, PdAg Bimetallic nanoalloy-decorated graphene: a nanohybrid with unprecedented electrocatalytic, catalytic, and sensing activities, ACS Appl. Mater. Interfaces 10(19) (2018) 16376–16389. Copyright 2018 American Chemical Society.

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Advanced Nanostructures for Environmental Health

4-aminophenol, which is important for the synthesis of pharmaceuticals. Thus, a sensor system assigned to high yield rates, low costs, and simple handling is presented with high potential for future on-site environmental monitoring. Finally, an SERS-based detection scheme, utilizing silica-coated Ag nanocomposite structures, was demonstrated for the estimation of 4-aminothiophenol down to 54 ppb [134]. As a relevant water pollutant, 4-nitrophenol was estimated using green fabricated flower-shaped Zn-based microstructures to modify glassy carbon electrodes [135]. Here, the binder-free and nonenzymatic amperometric detection is achieved down to 0.013 mM accompanied with high sensor stability. For the electro-catalysis of nitrophenol, a spinel ZnMn2O4 nanocomposite material was developed allowing for an LOD of 20 μM as well as high stability and repeatability accompanied by short response times [136]. Carbon paste electrodes were modified with Mg(Ni)FeO nanostructures for an improved estimation of 4-nitrophenol down to 0.2 μM [137]. The indirect oxidation peak current of the analyte molecule is proportional to the concentration within the range of 2 and 200 μM. Finally, a ratiometric fluorescence approach is presented employing photoluminescent carbon dots [138]. The nanocomposite was equipped with a molecular-imprinted polymer being specific for capturing 4-aminophenol and upon enrichment of the analyte, the fluorescence of the carbon dots is quenched while the fluorescence of the encapsulated CdSe quantum dots remains constant. Thus, the estimation of 4-aminophenol is achieved down to 0.026 μg/mL. In order to detect 2,4,6-trinitrophenol (picric acid) Fe-doped ZnO nanoellipsoids were fabricated and the authors have characterized the luminescence quenching upon interaction with the analyte molecule [139]. As detection limit, the value of 2.9 μM is determined and a linear response is achieved between 5 and 60 μM. Mesoporous structured NPs were molecularly imprinted and a ratiometric fluorescence detection scheme is applied [140]. As reference, CdTe quantum dots are embedded and due to the interaction of picric acid with a target sensitive dye, the fluorescence is quenched. A detection limit down to 43 nM is demonstrated with 3 min of analysis time. Picric acid was spiked in different water sources with a concentration of 100 nM and recovery values between 95% and 102% were found. Finally, a phosphorescent chemosensor for the detection of picric acid is available based on CaTiO3 nanostructures doped with Pr3+ [141]. The phosphorescence is quenched in the presence of picric acid and as detection limit, 20 nM is determined illustrating the high sensitivity. Moreover, the authors have demonstrated the selectivity for picric acid over structural similar analytes. To monitor 4-chlorophenol, an electrode surface is decorated with nanostructured Au/ BiOCl composite material for an improved PEC detection approach [142]. The authors found a linear response between 0.16 and 20 mg/L with an LOD of 0.05 mg/L. In the case of 3,5-dichlorophenol, a glassy carbon electrode was equipped with reduced graphene oxide V2O5 nanosheets to monitor this hazardous chemical in an aqueous environment over a wide concentration range down to 5 nM [143]. The high potential for real applications was demonstrated by detecting 3,5-dichlorophenol electrochemically in industrial wastewater, river water, and soil samples.

Chapter 2 • Sensitive detection of organic pollutants

59

Hydroquinone (1,4-dihydroxybenzene) as potential carcinogen substance and xenobiotic micro-pollutant were estimated by modifying an electrode with cerium oxide nanocrystals [144]. Employing cyclic voltammetry as read-out technique, a detection limit for hydroquinone of 0.111 mM was estimated. The sensitivity is increased by utilizing reduced graphene oxide Cu2O nanocomposite to modify electrode surfaces [145]. The peak current showed a linear dependency between 5 μM and 1 mM. Finally, a new nanosensor concept was demonstrated with the development of laminated samarium borate modified with the enzyme laccase [146]. Under optimized cyclic voltammetry sensing conditions, a linear response within the range from 1 to 50 μM was achieved and the LOD was found with 0.3 μM. To monitor catechol (1,2-dihydroxybenzene), a CuO reduced graphene oxide nanocomposite was presented [147]. To improve the interaction of the target analyte with the sensor platform, hollow carbon nanospheres were equipped with AuNPs and further functionalized with cyclodextrin [148]. Thus, excellent electrochemical sensing capabilities of carbon-based nanostructures are combined with the electrocatalytic activity of AuNPs as well as with an improved recognition of the target analytes, that is, 1,2-dihydroxybenzene (catechol) and 1,4-dihydroxybenzene (hydroquinone), by the host cyclodextrin. A simultaneous detection is demonstrated as the oxidation peaks of both substances are well separated and as detection limits 0.01 and 0.02 μM are calculated for 1,2-dihydroxybenzene and 1,4-dihydroxybenzene, respectively. The feasibility of the sensor platform in environmental sensing was shown by the investigation of spiked water samples (1–4.5 μM for both components) and the validation with HPLC as gold standard. Finally, the detection of bisphenol A, a chemical widely used in plastic industries, is achieved selectively by reduced graphene oxide/hydroxyapatite nanocompositedecorated glassy carbon electrodes [149]. In this study, the excellent sensing properties were demonstrated as the linear dynamic range is between 0.2 and 2.0 mM as well as the detection limit is calculated with 60 pM. To conclude, for the sensitive detection of phenols, electrochemical sensing strategies are preferred. The electrode surfaces are equipped with advanced nanostructures and composite materials allowing for electrocatalytic reactions. For most sensor platforms, the authors have stressed the high stability under the measuring conditions, high sensitivity for environmental monitoring applications, low-cost fabrication, and simple handling. The potential for real application scenarios was demonstrated by the investigation of spiked water samples. Simultaneous detection of phenolic compounds, achieved due to the separation of the redox peaks, is promising for real applications in the future.

2.5.2 Dye molecules Dyes associated with environmental pollution or health problems due incomplete purification of wastewater are used in industrial production as dying agent. As an example, the dye Congo red is an azo dye and once released by the textile industry, toxic and carcinogenic intermediates are formed during degradation [150]. Based on the application of

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plasmonic-active materials, SERS is used for the detection of Congo red [150,151]. Nanotubes are decorated with SERS-active gold NPs and due to the large surface area Congo red is removed from the aqueous solution [150]. The successful enrichment of the organic dye in the nanotube network is confirmed by recording fingerprint-specific SERS spectra of Congo red. Moreover, a nanoporous TiO2 aerogel matrix decorated with Ag NPs was used for SERS-based detection of Congo red [151]. Here, specific Raman marker modes are detected down to 5  10
6 M to illustrate the potential for the detection of organic pollutants in water samples. As further analytes, dye molecules from the Rhodamine family are investigated and detected using metallic nanostructures as sensors. Silver NPs were coated with molecularly imprinted polymers (MIP) to allow for an efficient enrichment of the analyte Rhodamine B on the metallic surface due to the high affinity resulting in a high SERS intensity of the recorded fingerprint information [152]. The authors have demonstrated that the LOD is in the picomolar range illustrating high potential of MIPdecorated SERS-active nanostructures for the detection of organic dyes in water samples. In order to take advantage of the properties of graphene, a hybrid structure with magnetite NPs were generated allowing for an adsorption of the organic pollutant Rhodamine 6G from aqueous solutions combined with a sensitive detection [153]. Here, the organic pollutant is removed from water samples after magnetic separation and the authors have found a detection limit of 5  10
7 M in aqueous solutions. Finally, the degradation of Rhodamine 6G on the surface of the hybrid structure was monitored by recording time-dependent SERS spectra. Thus, an innovative approach for wastewater treatment and monitoring is presented which might find application in the textile industry. As an alternative, TiO2 substrates with a three-dimensional (3D) structure are decorated and used for the SERS-based detection of Rhodamine 6G [154]. Again, the catalytic activity to decompose the dye molecules is demonstrated. As last example, the SERS-based detection of malachite green as significant organic pollutant in the environment is demonstrated by employing a silver NP-loaded nanofiber matrix [155]. A good linearity of the SERS signal was shown between 0.5 and 100 μM. To conclude, as demonstrated by the presented studies, SERS is a perfect candidate to detect dye molecules in low concentrations in aqueous environments such as wastewater. However, since additionally the resonance Raman enhancement contributes to the overall SERS spectra (creating surface-enhanced resonance Raman spectroscopy—SERRS [156]), the successful detection of dyes shows not the potential of the nanostructured SERS substrate to detect nonresonant molecules, i.e., other organic pollutants, in trace concentrations. The synthetic and most commonly used dye Rhodamine 6G was detected by the application of pyrene NPs as a novel probe for FRET [157] The pyrene NPs exhibit a negative zeta potential and thus, the cationic analyte molecules are efficiently adsorbed toward the NP’s surface. As a consequence, an energy transfer occurs from pyrene NPs to Rhodamine 6G molecules and the fluorescence of the pyrene NPs is quenched. The authors have finally demonstrated the potential of the proposed detection scheme by determining Rhodamine 6G in the matrix of industrial textile effluents with a recovery value of almost 100% and a relative standard deviation of H2O, not C1-C4 hydrocarbons

Quantification is easy if certain parameters are kept constant, real-time analysis, low water cluster formation, good sensitivity, no sample preparation

DARTMS

Neutral metastable species formed by electrical discharge in a gas (mostly helium), these react with water and air molecules to produce the reactive ionic species, these react with analytes via Penning ionization, protonation, deprotonation, adduct ion formation and charge transfer Electron impact ionization

fmol range

M < 1000 Da, polar and nonpolar organic compounds

1% of respective gas in atmosphere

Atmospheric gases, noble gases, dissolved gases

No sample pretreatment, negative and positive mode, can directly ionize gases, quantitative analysis possible, rugged construction, generation of singly charged analyte species, no memory effect or sample carryover, fragmentation generally not observed, but can be induced Quasi-continuous and onsite measurements possible, robust, no gas purification/ sample preparation, ambient air can be used as standard for calibration, dissolved gases can be measured

MI-MS

Upper LOD, some cluster ions are present, mostly only molecule ion ! hard to differentiate between isobaric ions, high humidity can cause problems, light hydrocarbons (C1-C4) not detectable, calibration necessary for quantification Many parameters that have to be optimized, not one clearly defined ionization mechanism

Drift calibration frequently necessary (1/h), membrane can deter analytes

Chapter 6 • Spectroscopic detection of gas molecules

PTR-MS

267

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Advanced Nanostructures for Environmental Health

6.3 Ion-mobility spectrometry IMS is based on the determination of ion mobilities in electric fields, where the sample compounds in the gas phase are ionized and then enter a drift tube [14, 26]. The detection limit which can be reached in IMS strongly depends on the substance measured and the experimental conditions [26], but is generally in the ppb range [26] and can even reach ppt levels [14]. IMS can detect a broad range of analytes in the gas phase, mostly as protonated molecule ions [26], in real time because one measurement takes as little as a few milliseconds [14, 26, 27]. The ions are generated via CI [14, 26], where a radioactive source (63Ni) is used normally to generate the ions, as it provides a stable and reliable operation. Recently, other ionization sources like corona and partial discharge sources, photodischarge lamps, lasers, or electron spray ion sources become more common, as radioactive components are always connected with permit and licensing problems. The choice of the ion source affects the ionization chemistry in the sample and thus the resulting spectra [26]. Photodischarge ionization is especially well suited for the analysis of aromatic and unsaturated compounds and results in relatively simple spectra, as it is a primary ionization process without reactant ions and the resulting corresponding spectrum peaks. Corona discharge ionization, on the other hand, results in complex spectra similar to EI, as high energies are used. The chemistry of an ion source can be altered in a controlled way by adding controlled, low levels of a particular substance, also called dopant or reagent gas [26]. This procedure can enhance the sensitivity and selectivity of the instrument response. After ionization of the sample, the sample ions are injected as a swarm into the drift region via an electronic ion shutter [26]. The drift velocity is proportional to the strength of the electric field and the ion mobility is the constant of proportionality. The basis for separation of ions in the drift region of the IMS is the mass and/or structure of the ions, as ions have different drift speeds, depending on these parameters [14, 26]. It is important for the ions to have long enough lifetimes to transit the drift tube and reach the detector, which is a Faraday plate. A schematic illustration of the processes in IMS is shown in Fig. 6.7. During the ionization process, mostly water clusters are formed after several steps, which react with the analytes, resulting in [M + H]+ ions and water clusters with the analyte molecules [14, 26]. IMS can be coupled with GC and MS, respectively, to allow the separation of complex mixtures into the individual compounds and the identification of the ions [26]. Identification of unknown substances is not possible using IMS alone, but in combination with MS, this becomes possible. This combination of analytical techniques can be very helpful, as the mobility of an ion is complimentary to its mass and that way, details about the shape and conformation of a substance can be obtained [14, 26, 27]. The IMS device can be operated in positive and negative mode [26], the positive mode being better for alkanes, alkenes, and similar molecules and the negative mode

Chapter 6 • Spectroscopic detection of gas molecules

269

FIG. 6.7 Schematic illustration of an ion-mobility spectrometer (modified from Borsdorf and Eiceman [26]).

being better for aromatic hydrocarbons, carboxylic acids, nitro-alkanes, nitro-aromatic compounds, and halocarbons. IMS can be used for direct atmospheric sampling and quantitative analysis of volatile and semi-volatile compounds, on-site or infield monitoring is possible [5, 14, 26]. In contrast to many other techniques, IMS is suitable for the analysis of corrosive gases [26]. This analytical method has its principal application in security and military venues for the detection of chemical warfare agents and explosives because of its fast response, reliable performance, rugged construction, and low weight and size of the instruments [14, 26], but is also used in petrochemical and environmental analysis, medical diagnostics, air quality monitoring (one popular example is the monitoring of the cabin atmosphere on the International Space Station), etc. In environmental gas analysis, IMS has been applied to the determination of BTEX in air and water, the analysis of HF as well as the analysis of volatile halogenated compounds with GC-MS with a time resolution of 1 min [26]. There are several difficulties that must be considered for IMS. The detector can be saturated, when the analyte concentrations are high enough to completely deplete the reactant ions [26]. Another problem can occur with complex samples, where several analytes are mixed, or the analyte is in a complex matrix [5, 26]. Without prefractionation of the sample, multiple and competitive ionization reactions occur, leading to complex detector responses [26]. That way, the creation of some ion species can be suppressed, resulting in decreased detected concentration or the inability to detect a certain compound [26]. The detection of low-weight hydrocarbons (C1-C4) is not possible at all using IMS [14]. In general, calibration is required for all analytes and quantification can be difficult [14]. The characteristics of ion-mobility spectroscopy are summarized in Table 6.2.

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Advanced Nanostructures for Environmental Health

Table 6.2 Summary of ion mobility spectroscopy including ionization method, limit of detection (LOD), possible analytes, advantages and limitations.

Method IMS

Ionization method/ source Chemical ionization

LOD

Analytes

Advantages

Limitations

pptV

Polar and nonpolar substances, corrosive gases, explosives

Real-time analysis. Reliable performance, portable, rugged, can be coupled to GC and MS (!complimentary method)

Calibration necessary for quantification, C1-C4 not detectable, complex samples can cause problems (suppression of signals), identification of unknown substances impossible

6.4 Laser spectroscopic techniques Optical gas sensing techniques exploit the ability of molecules to absorb or emit electromagnetic radiation of a defined wavelength λ. The spectral position of this absorption or emission characterizes the respective type of gas and is extremely selective compared to other gas-analytical measurement methods. The interaction of electromagnetic radiation with gaseous molecules is effected by absorption, emission, and scattering [7]. According to the laws of quantum mechanics, the energy transfer that is exchanged in these interactions can only take place in precisely defined energy packages. Hence, molecular energy is quantized [28] and is given by Planck’s law: E ¼hν¼

hc λ

(6.9)

The photon energy E thus results from the frequency ν or the wavelength λ of the radiation and the speed of light c (usually c  3.0  108 ms1). The constant h is the Planck constant (h ¼ 6.626  1034 J s). The spectral position can be expressed in different forms [28]. In physical approaches, the indication as wavelength λ has been proven. In chemistry and spectroscopy, the wave  number ν is quite common as the photon energy is proportional to ν. The wavenumber is 1 the reciprocal of the wavelength and therefore has the unit cm : ν 1 ν¼ ¼ c λ



(6.10)

However, in the visible or ultraviolet (UV), nanometer units are the norm.

6.4.1 Absorption methods Gas molecules can absorb radiation by different mechanisms, which are determined by the photon energy and the molecular structure. There are different types of energy absorption in a molecule [29] depending on the wavelength of the incident light.

Chapter 6 • Spectroscopic detection of gas molecules

271

6.4.1.1 Infrared absorption spectroscopy

The most important spectral range for gas-analytical purposes is between 3 and 12 μm. In this area, almost all gases possess strong, characteristic fundamental absorption lines, which are characterized by rotational vibration transitions [30]. Different gaseous molecules absorb different frequencies of light, which can be used for compound identification. The amount of absorbed light depends on the concentration of the respective gas, with higher absorption at higher concentrations. For each wavelength rage, typical excitations of molecular transitions take place, which can be classified as follows: • • •

The lowest energy level (Erot) in the far-infrared (IR) (400–10 cm1 or 25–1000 μm) is needed for rotational excitation of the molecules. A middle energy level (Evib) in the mid-IR (4000–400 cm1 or 2.5–25 μm) is required for the excitation of vibrations. The highest level (Eelec) in the near-IR (14,000–4000 cm1 or 0.8–2.5 μm) allows an increase of the electron in the next higher electron level.

A prerequisite for the absorption of infrared light is the presence of a temporally changeable dipole moment which fulfills equation 6.11 [30]. dμα dq 6¼ 0

(6.11)

R0

Here, dμα is the change in dipole moment and dq is the change in vibrational amplitude. This equation is fulfilled by heteronuclear molecules, but not for noble gases like helium and homonuclear gases like N2 and H2 which are not detectable using IR spectroscopy, as they do not possess a temporally changing dipole moment [30]. Fig. 6.8 and Table 6.3 show the spectral position of typical gases in the IR. To generate the infrared spectrum of a sample, a beam of infrared light with the intensity I0 is passed through the sample. If a bond in the examined molecules has the same vibrational frequency as is contained in the infrared light beam, absorption occurs. The quotient of I0 and the transmitted light is the transmittance, which relates to the amount of energy absorbed at each frequency (or wavelength) [33]. The IR spectrum is displayed by plotting the quantity T¼

I I0

(6.12)

as a function of wavenumbers, where T is the transmittance, I0 the intensity of the IR beam before, and I after passing through the sample. In most cases the absorbance A is used A ¼  log 10 ðT Þ

(6.13)

since the absorbance at a given wavelength is directly proportional to the concentration of a sample according to Beer’s law. That is why IR spectra can also be used to make quantitative determinations of the number of individual components in a sample mixture [34].

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Advanced Nanostructures for Environmental Health

FIG. 6.8 Absorption lines of typical compounds in environmental studies in the mid-infrared between 800 and 3200 cm1 created using experimental data from the HITRAN database [31].

The measurement of transmittance or absorbance can be achieved by scanning the wavelength range using a monochromator [35]. Infrared spectrometers usually combine a radiation source, a sample arrangement, a device for spectral dispersion or selection of radiation, and a radiation detector, connected to appropriate recording and evaluation facilities. For the selective analysis of analytes, lasers are usually used as radiation source, as they are spectrally narrow and thus enable the recording of high-resolution absorption spectra. Tunability in wavelength allows the recording of spectra of several compounds. The advantage of IR spectroscopic methods is that a considerable number of atmospheric constituents absorb in the mid-IR spectral region (in comparison, significantly fewer atmospheric constituents exhibit strong absorptions in the visible and UV spectral regions). Furthermore, they provide low detection limits. This makes IR absorption techniques very useful for multicomponent analysis; especially if a broadband IR source is used. But the overlap of absorption bands can cause difficulties, particularly the interference from water vapor is a well-known problem. Thus, the selective measurement of

Chapter 6 • Spectroscopic detection of gas molecules

Table 6.3

Important trace gases and their spectral position of selected absorptions [32].

Wavenumber (cm21)

Wavelength (μm)

H2O

7323.945 1684.835

1.365 5.935

HF

7855.643 7515.803 3260.427 3040.220

1.273 1.331 3.067 3.289

3067.300 6057.086 2944.914 5723.301 2781.035 2649.092 7458.082 277.519 2270.294 4989.972 6240.105 2236.224 5117.472 2169.198 6330.167

3.260 1.651 3.396 1.747 3.596 3.775 1.341 4.391 4.405 2.004 1.603 4.472 1.954 4.610 1.580

Gas

C2H2 CH3Cl CH4 HCl H2CO HBr HI CO2

N2O CO

273

Vibration

Gas

Wavenumber (cm21)

Wavelength (μm)

ν1 + ν2 + ν3 OdH stretch

OCS

2052.716

4.872

NO

1875.813

5.331

νCH νas

HNO3 NO2

νas

HO2

1722.372 1600.413 12,500.000 1411.182

5.806 6.248 0.800 7.086

SO2

1371.695

7.290

H2S H2O2

1364.601 1284.205

7.328 7.787

HCOOH O3

1113.068 1052.848

8.984 9.498

O2

7880.637 13,142.570 930.757 6478.000

1.269 0.761 10.744 1.544

νas

νCO

νCO

NH3

Vibration

νas, NO2 νas OdH stretch νas

OdH stretch

some compounds can require high-resolution instruments to measure one absorption band only. Not all compounds contained in the atmosphere can be detected by IR spectroscopy. Homonuclear molecules (e.g., N2, O2, and H2) do not have any IR-active vibrations, as they do not have a dipole moment. Fourier-transformation infrared spectrometers With the development of computer technology, it was possible to automate the Fourier transformation infrared spectrometers (FT-IR) and execute it quickly. This gave rise to the possibility of making FT-IR spectrometers widely available. Here, the entire wavelength range is measured using interferometric modulation of radiation and then a transmittance or absorbance spectrum is generated. Typically, the radiation emerging from the polychromatic IR source (e.g., a globar) is passed through an interferometer to the sample before reaching a detector. The most common used interferometer is a Michelson interferometer [36], where the light is collimated and guided to a beam splitter. The moving mirror produces an optical path difference between the two arms of the interferometer. Reflected by the mirrors, the beams are combined again and focused on a detector (Fig. 6.9). For path differences of (n + 1/2)λ, the two beams interfere destructively in the case of the transmitted beam and constructively in the case of the reflected beam.

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Advanced Nanostructures for Environmental Health

FIG. 6.9 Operating principle of a typical FT-IR spectrometer. Modified from P.R. Griffiths, J.A. De Haseth, Fourier Transform Infrared Spectrometry, John Wiley & Sons, 2007.

This results in interference pattern that are summarized in an interferogram where all the spectral information of the radiation are collected [34]. Upon amplification of the signal, in which high-frequency contributions have been eliminated by a filter, the data are converted to a digital form by an analog-to-digital converter and transferred to the computer for Fourier transformation. To get the spectrum of a gas or vapor, the sample should be placed in a gas cell. Typical bench-top FT-IR spectrometers can use gas cells with optical pathlengths between 1 and 10 cm. As the optical pathlength is proportional to the sensitivity that can be reached, a larger pathlength should be chosen for analytes present at lower partial pressure or which have a low absorptivity (e.g., H2S). In this case, a multipass gas cell can be used (so-called White and Herriott cells) [37]. Most multipass cells are designed to monitor static samples and do not have good flow characteristics. Multipass cells are not made to monitor concentrations in a flow-through manner but are rather designed for the analysis of static samples. For nonstatic systems, the gas cell should be designed in a way that the sample can be flushed out easily, while having a high pathlength/volume ratio to keep up a good sensitivity. For this purpose, an internally gold-coated light-pipe of the type used to monitor the effluent from a capillary gas chromatograph by FT-IR spectrometry (see also the chapter about GC/FT-IR) can be used. Such a gas cell was used by Grutter to analyze the composition of ambient air in Mexico city [38]. He measured low-molecular-weight components like CO, CO2, CH4, and N2O with a time resolution of 5 min over a period of 2 weeks. Exact gas profiles could be created on a daily basis, thus uncovering possible origins and anthropogenic contributions.

Chapter 6 • Spectroscopic detection of gas molecules

275

FT-IR spectrometers offer some decisive advantages over the previously routine used dispersive spectrometers. For example, the FT-IR spectrometers have a significantly higher signal-to-noise ratio (SNR) and require significantly shorter measurement times. To reach a limit of detection in the lower ppm range, measurement times of 1 s are enough. As the measurement sensitivity is dependent on the optical pathlength, the LOD can be improved by an increase in the optical pathlength. Compared to other techniques it has several advantages, including simultaneous analysis of a wide range of gaseous species, in situ and nondestructive measurements requiring no chemical transformation of the sample. Nondispersive infrared spectrometer Unlike in a standard IR spectrometer, all IR radiation emitted by the light source is used at the same time in a nondispersive infrared (NDIR) spectrometer. No dispersion of the light according to wavelength by a prism or a grating takes place. Such instruments are specific for one compound only, as some of the compound under investigation is used as part of the detector. NDIR sensors usually have two chambers; one is a reference and the other is a measurement chamber [39]. The spectrometer has two paths for the light coupled into the spectrometer, one as a reference with a reference cell filled with IR-inactive gases like air or nitrogen, and one for the sample containing the analyte. The detector itself consists of two sealed chambers filled with the same (pure) gas which is to be determined in the air sample. Between both chambers, a flexible diaphragm out of metal and a differential capacitor are placed (Fig. 6.10). As less radiation reaches the detector at the measurement cell entrance, the pressure increase is less than in the other chamber, resulting in a bent of the diaphragm toward to the plate of the capacitor on the sample side. The radiation from both halves of the measurement chamber is cut off by a chopper, resulting in a decrease of the pressure difference. A series of pulses is the output created, where the height of the pulses is proportional to the analyte concentration. High analyte concentrations result in pulses of high intensity. Another NDIR gas sensing principle is the use of band-pass filter, where frequencies within a certain range can pass through while frequencies outside that range were rejected or attenuated [40]. The light from the IR source then passes through two filters; one covering the whole absorption band of the target gas, and the other covering a neighboring nonabsorbed region (Fig. 6.11). Commercially, a variety of these filters are obtainable which show typically transmission in a range between 3000 and 750 cm1 (3.3–13 μm).

FIG. 6.10 Sketch of a nondispersive IR gas analyzer. S ¼ IR source; Ch ¼ chopper; CS, CR ¼ detector cells for sample and reference beam; D ¼ thin metal diaphragm; A ¼ amplifier. Modified from S.S. Srivastava, K. Maharaj Kumari, Formaldehyde, environmental analysis of, Encyclopedia of Analytical Chemistry, John Wiley & Sons, Ltd. 2006.

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FIG. 6.11 Schematic diagram of a linear nondispersive gas sensor with two band-pass filters. Modified from J. Hodgkinson, R. Smith, W.O. Ho, J.R. Saffell, R.P. Tatam, Non-dispersive infra-red (NDIR) measurement of carbon dioxide at 4.2 μm in a compact and optically efficient sensor, Sensor Actuators B: Chem. 186 (2013) 580–588.

Each one is designed to analyze a specific compound and can be easily replaced. For example, to measure CO2, the filter removes all light excepting a particular wavelength around 4.25 μm band. Ideally, the specific wavelength can only absorb CO2 molecules perfectly and the detector signal is not affected by other gases. Since NDIR gas sensing relies on the strength of optical absorption in the mid-IR (which is quite stronger than, e.g., near IR) [41], even with short pathlengths (>10 cm), simple IR sources (microbulbs), and uncooled detectors (e.g., thermophile), respectable LOD can be achieved (1 ppm) [40]. Additionally, NDIR devices are highly sensitive because the heating of the detector gas is due only to the small portion of the spectrum absorbed by the corresponding analyte in the sample. That is why NDIR sensors are very versatile devices, which can be adapted to analyze any infrared-absorbing gas. NDIR systems are applied widely for environmental gas analysis in cases where one or only a few compounds are to be measured. Besides being easy to handle and robust to changing working conditions, the compact design allows portability. Additionally, the simple construction of the spectrometer enables high optical throughput and thus high sensitivity measurements. A major disadvantage is the time resolution, which is relatively low. Because of that, it is not possible to resolve fast changes in a gas composition or concentration. GC/FT-IR Like GC-MS, this is a separation technique followed by a molecular identification; in this case using infrared spectroscopy. Before the advent of (fast) FT-IR instruments, GC/IR measurements were made by depositing effluent on to a salt window and running in the IR instrument. FT-IR instruments allow a real-time analysis of the GC effluent using a “light-pipe” measurement chamber [34]. Commonly, such light-pipes has internal goldfilm coatings, where the IR beam is multiply reflected and passes through the cell. Additionally, the light pipe is heated to avoid condensation of the gas phase. The obtained chromatogram based on changes in total infrared intensity. A big advantage of this technique is that FT-IR is a nondestructive and the output from the light-pipe can be passed into another detector for further characterization. An off-line coupling of gas chromatography with FT-IR can be realized by depositing the effluent on the surface of a rotating disk by freezing in an inert argon matrix (matrix isolation GC/FT-IR) [42]. Thus, each trapped component is surrounded by argon atoms

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and therefore isolated from other molecules of the analyte. As a result of this matrix isolation, the width of bands in the spectra of light molecules is reduced significantly. The combination of these both methods is typically used for organic volatile pollutants, especially the differentiation of similar structures and isomers in families of structures like polyaromatic hydrocarbons (PAHs), dioxins, or polychlorinated biphenyls (PCBs). Childers et al., for example, used matrix-isolation GC/FT-IR for the characterization of different bromated and chlorinated dibenzodioxins and dibenzofurans (PXDD and PXDF) [43]. Furthermore, Grainger et al. perform an almost complete assignment of all congeners of tetra- up to octachlorinated dibenzodioxins using online GC/FT-IR [44].

6.4.1.2 Tunable diode laser absorption spectroscopy Tunable (mid-IR) diode laser absorption spectroscopy (TDLAS) is a special technique in IR spectroscopy, where sensitivity, selectivity, and measurement speed have been optimized for most molecules of interest, such as O3, the NOx family (N2O, NO, NO2, HONO, and HNO3), CO, CO2, H2O2, HCHO, NH3, CH4, C2H2, C2H4, C2H6, HCl, HF, SiF6, CF2Cl2, H2S, and OCS with detection limits down to ppt-range. The concept (e.g., [32]) is quite simple. A tunable laser with a very narrow line width is scanned over the respective absorption lines of the gases to be detected. This process additionally reduces background fluctuations during the measurements. Detection levels to below 100 ppt are reported with temporal resolution of the order of minutes. The main difference between conventional IR spectroscopy and TDLAS is that IR spectroscopy uses a thermal radiation source combined with a spectroscopic filter (such as an interference filter, a monochromator, or a Fourier transform spectrometer), whereas the diode laser radiation source itself is tunable in TDLAS. This makes laser spectroscopy to the method of choice for in situ trace gas analysis, because of its very high sensitivity and specificity due to its high spectral resolution. In TDLAS, a narrow-band tunable laser source is used to scan the area around a suitable gas absorption line. Different methods exist for modulating the frequency of such laser, for example, by varying the diode current [45, 46]. If the laser current has a sawtooth-like waveform, a linear scan across a spectral interval is performed, where a small area around and including the absorption line of interest is scanned. That way, no further wavelength selective elements are needed. The advantages of diode lasers are their compact design and the low power consumption, but stable temperatures ensured by liquid nitrogen or closed cycle helium coolers are needed, making the setups not practical for portable devices. In recent years, tunable diode lasers operating at room temperature in the NIR region have been developed, which allow the implementation of very compact and lightweight instruments [47, 48]. Since the overtone bands in the NIR are typically relatively weak, devices operating in the mid-IR are needed. Recently available quantum cascade lasers (QCLs) allow the generation of narrow band emission even in the mid-IR [49, 50]. The development of laser diodes is still in demand. While impressive developments in the field of CD players or fiber optical communications were obtained, optimized diodes for spectroscopic purposes in the visible or UV are still difficult to obtain.

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6.4.1.3 Cavity-enhanced absorption spectroscopy Cavity ringdown spectroscopy Cavity ringdown spectroscopy (CRDS) (e.g., [51, 52]) for gas analysis has developed rapidly in the last decades and has been implemented for a variety of applications ranging from fundamental spectroscopic studies [53], to trace chemical detection for environmental monitoring [54], isotopic measurements [55], and breath gas analysis [56]. The original CRDS setup is based on a short laser pulse, which is coupled into a high-finesse optical cavity between by two high-reflectivity mirrors. The high number of reflections of the pulse within the cavity enhances the absorption pathlength in the cavity by several orders of magnitude giving an effective pathlength of a few kilometers with initial resonator lengths below 1 m. Nevertheless, the light intensity decreases with time, this time span is called decay time or ringdown time. It is used to determine the optical absorption in the cavity. To measure this ringdown event, a photomultiplier tube (PMT) or a photodiode placed behind the second mirror is used and the observed waveform is fitted to a single exponential function (Fig. 6.12). If laser light is passing through an absorbing sample in the cavity, the ringdown time decreases according to the analyte concentration as fewer bounces through the medium are required before the light is absorbed. This loss and reduction in ringdown time depends on the concentration of the absorbing analyte in the sample cell and the intensity of the transmitted light is, according to Beer’s law, expressed in the following form [57]:   t ICRD ðt Þ ¼ I0 exp  τ

(6.14)

where τ is the ringdown time. If the ringdown time is measured in the absence of a sample gas in the cavity, the reflectivity of the mirrors can be determined. With a known mirror reflectivity, CRDS allows the absolute determination of the absorbance and the absolute density of the analyte for a measured sample. This self-calibration is a great advantage in comparison to other high-sensitivity techniques like laser-induced fluorescence (LIF). The high sensitivity of the CRDS technique is mainly based on the considerable number of passes the light pulse makes through the sample in the cavity.

FIG. 6.12 Scheme for laser-absorption measurements using CRDS. Modified from B.A. Paldus, A.A. Kachanov, An historical overview of cavity-enhanced methods, Can. J. Phys. 8310 (2005) 975–999.

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Recently, CRDS has been employed for studies of greenhouse gas emissions (e.g., N2O, CO2, CH4, and NH3) [58, 59], atmospheric trace gas analysis [60], isotope-ratio measurements [55], and breath analysis [61]. For example, Fleck et al. [59] demonstrate the capability of an analyzer based on CRDS to measure soil fluxes using a static chamber by simultaneously analyzing N2O, CH4. CO2, NH3, and H2O. They were able to measure CH4 rate-of-rises of 0.17 ppb/min with very low uncertainty. Additionally, they were able to follow a simulated rain event, where they could demonstrate quite different trends in the flux rates of the different gases. A drawback of this technique is the sensitivity of the cavity to water vapor, because moisture could condense on the mirrors and lead to drastic sensitivity losses and costly repairs. But moisture traps can be built relatively simply and employed if high humid conditions are required. Continuous wave CRDS As alternative to the classical pulsed CRDS, the cavity can be excited with continuous wave radiation (CW) [62]. Here, the resonator will show attenuation, which can be monitored. Since CW lasers can have very narrow line widths and can be tuned over small spectral increments, very high spectral resolution and excellent wavelength reproducibility can be achieved [63]. The laser light is placed into the cavity only when the laser frequency overlaps the resonance frequency of a cavity mode. When this condition is met, intensity will accumulate in the cavity until the laser beam is switched off. If the switch-off is sufficiently fast, a ringdown of the trapped light intensity will be observed by an external detector. To ensure frequency matching of the laser and a cavity mode, the cavity length can be scanned using a piezoelectric transducer [64]. Higher sensitivities can be achieved by actively locking the cavity and laser in resonance, resulting in higher, and more reproducible, intracavity intensities and therefore signals [65]. CW-CRDS is suitable for ultrahigh-resolution spectroscopy and can therefore be used for trace gas detection or to study nonlinear effects. Detection limits in ppb-levels are possible [51]. Many CW laser sources are compact and have low power consumption (e.g., diode lasers), making them easy to integrate into a portable instrument. But the narrow range of frequency tunability for many CW sources makes them less optimal for detection of species with broad features in their absorption spectra [66]. Integrated cavity output spectroscopy Integrated cavity output spectroscopy (ICOS) enables absorption spectra to be obtained through direct attenuation methods, while providing detection sensitivities comparable to classical CRDS. Here, the transmitted output of the cavity is simply integrated to provide an absorption spectrum as the injection light source is scanned in wavelength [67]. To achieve uniform transmission through the cavity, the cavity modes are systematically disrupted by either moving one of the cavity mirrors or by slightly modulating the angle of injection in the cavity [68]. Advantages of this method compared to time-domain approaches are the system simplicity, cost, and the ability to use broad spectral bandwidth light sources. ICOS is mainly applied to the near-IR spectral region. The characteristic

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narrow absorption bands in the near-IR give excellent specificity to absorbing molecules, including CO2, N2O, NO, CO, CH4, H2S, NH3, and SO2 down to ppt-levels. To overcome the need for modulation, off-axis ICOS was developed. This off-axis configuration spatially separates the multiple reflections within the cavity until the so called “reentrant condition” is fulfilled, referring to the time at which the beam begins to retrace its original path through the cavity [69]. The off-axis design eliminates also optical feedback from the cavity to the light source. Compared to other cavity-enhanced techniques, this approach is characterized by reduced complexity of the spectrometer, high sensitivity, and spectral resolution. Therefore, it is of great importance in the field of spectroscopy [70], and this kind of gas analyzer is already commercially available.

6.4.1.4 Differential optical absorption spectroscopy Differential optical absorption spectroscopy (DOAS) is a technique that identifies and quantifies trace gas abundances with narrow band absorption structures in the near UV and visible wavelength region in the open atmosphere (e.g., [71]). The fundamental setup of a DOAS system consists of a broadband light source, an optical setup that transfers the light through the atmosphere, and a telescope-spectrograph-detector system to record the absorption spectra. DOAS can be used for the determination of very reactive species like free radicals (e.g., OH, NO3, ClO, BrO, and IO) as well as a wide range of other trace gases including SO2, O3, NOx, NH3, etc. To overcome the difficulty to determine, whether the received intensity from the light source is generated by absorption of the target molecule or the atmospheric absorption of, for example, aerosols, clouds, or other absorbers at the same wavelength, the differential absorption is used. The variance of a part of the total absorption of a molecule in dependence of the wavelength is the differential absorption. In contrast to the total light extinction, this value is readily observable. To do so, measurements at several wavelengths are carried out. Each molecule has a characteristic absorption spectrum (its spectral fingerprint) and therefore, simultaneous measurements at different wavelengths enable the separation of the different contributions. The relationship between the number of gaseous molecules in the path and the amount of light absorbed is known as Beer-Lambert’s law: I ðλÞ ¼ I0 ðλÞ  exp ½αðλÞ  c  L

(6.15)

where I0(λ) is the intensity of incident radiation, α denotes the absorption coefficient, while L is the length of the optical path in the absorber with the concentration c. In DOAS, absorption spectra are recorded in the open atmosphere, where optical pathlength is known. The spectra generated are overlapping broad- and narrowband spectral structures, which must be separated. Such broadband or smooth spectral structures are mostly aerosol extinction processes and the effect of turbulence, whereas the narrowband spectral features can be assigned to the different gases to be examined. However, this fact restricts the application of DOAS measurements to gas molecules with narrowband absorption structures. The measuring principle is as follows: the transmitted light intensity is measured over a relatively broad spectral interval (compared to the width of an

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FIG. 6.13 Possible configurations of the DOAS principle. (A) Uses artificial and (B–D) natural light sources. (A) Represents a typical long-path DOAS configuration, (B) shows direct sunlight DOAS, (C) displays multi-axis DOAS (MAX-DOAS), and (D) represents Satellite-borne DOAS using Nadir geometry. Modified from U. Platt, J. Stutz, Differential Optical Absorption Spectroscopy: Principles and Applications, Springer Berlin Heidelberg, 2008.

absorption line). A high-pass filter is used to obtain a differential absorption signal out of the spectra and to eliminate broadband extinction processes like Rayleigh or Mie scattering. The quantitative determination of trace column densities is gained by matching the observed spectral signatures to prerecorded reference spectra using, for example, leastsquare methods. The DOAS technique can be used in both passive and active instrumentation (Fig. 6.13). The active DOAS systems such as longpath (LP) systems [72–74] and cavity-enhanced (CE) DOAS systems [75, 76] have their own light source (e.g., high-pressure discharge lamps or light-emitting diodes), whereas passive ones use the sun as a natural light source, for example, MAX (multi-axial)-DOAS [77–79]. In the active techniques, differential absorption spectra and pathlengths could be controlled which is favorable for local measurements. Very long optical paths in the atmosphere from a few hundreds of meters up to 20 km can be realized, which are essential for the detection of trace gases with mixing ratios in the ppb and ppt range. Passive techniques are however more commonly used for large-scale observations with satellites or ground-based measurements of the stratosphere. Several important atmospheric trace species (especially the radicals OH and NO3) have been discovered and first quantified with the help of active DOAS [80–82]. One of the major advantages of DOAS is the possibility to use mathematical calculations directly to get gas concentrations. This makes calibrations of the instruments in the field obsolete and results in high accuracy. With long light paths, it is additionally possible to reach excellent sensitivities. Passive DOAS, on the other hand, has the advantage of a relatively simple experimental setup. For the performance of scattered light measurements, only small telescopes without any artificial light source is needed. Satellite-borne measurements have become possible over the past decade in passive DOAS. Here, sunlight scattered by the atmosphere, the ground or both is used to measure global concentration fields of trace gases, like O3, NO2, and HCHO can be provided as well as the determination of vertical trace gas profiles.

6.4.1.5 LIDAR methods The power of light detection and ranging (LIDAR) methods is the capability to provide altitude profile information of the atmosphere. High spatial and temporal resolution of the

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measurements, the possibility of observing the atmosphere at ambient conditions, and the potential of covering the height range from the ground to >100 km elevation show the advantages of these methods [83]. Differential absorption lidar (DIAL) is quite similar to DOAS. Laser light is emitted at two wavelengths into the atmosphere: one is tuned to an absorption feature of the compound to be measured, and other is tuned slightly offresonance to this absorption line, where there is little absorption. Both wavelengths are backscattered by Rayleigh scattering at the molecules present in the atmosphere, but the return signal is stronger for the wavelength that is not absorbed. The scattered radiation is collected by a telescope and detected using sensitive photomultiplier. The detected signal is recorded by a function of time. The time delay between the outgoing laser radiation and the return signal is measured, and as a pulsed laser is used and it is well known that the speed of light is 300 km/ms, a range-resolved profile can be calculated for all absorbing gases measured. LIDAR measurements can be performed in the troposphere and the lower stratosphere from ground and airborne LIDAR instruments, the most common gases measured are N2O, H2O vapor, O3, aerosols, and SO2. The major limitation of LIDAR, which prevents the wide usage of this technique is the comparably low sensitivity coupled with the high complexity and cost of the instrumentation and data reduction procedures. Current developments include remote sensing of passing vehicle emissions (especially CO, NO, CH4, and C3H8) with detection limits in the low ppm range [84]. Furthermore, spaceborne LIDAR systems can be used on Earth-orbiting satellites to provide profiles of our atmospheric structure with high accuracy and resolution as well as global coverage. Areas of application range from the determination of global warming and greenhouse effects, to monitoring the transport and accumulation of pollutants in the different atmospheric regions [85–87].

6.4.1.6 Photoacoustic spectroscopy As in classical spectroscopic schemes, photoacoustic spectroscopy (PAS) is also based on absorption measurements. Nevertheless, instead of using the Beer-Lambert’s law, the absorbed radiation is directly measured by a calorimetric method [88]. PAS makes use of the fact that when gases absorb IR light, they undergo a small thermal expansion. As in the photometric gas measurements, the laser is tuned to a suitable absorption line and modulated at a fixed frequency in the audio range. If the gas under investigation is in the PAS cell, the absorbed radiation energy is converted into heat by inelastic collisions of the molecules, which leads to a temperature increase. Since the PAS cell is closed during the measurement, this increase in temperature leads to a periodic increase in pressure. This expansion produces a pressure wave (i.e., a sound wave), which amplitude is directly proportional to the gas concentration and can be detected by a sensitive microphone [89]. Afterwards, the signal from the microphone can be Fourier transformed to generate a spectrum of the sample. A typical PAS instrument consists of an acoustic resonator and an intensity-modulated light source (usually a laser) illuminating the interior of the PA cell. In this cell, periodic

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FIG. 6.14 Scheme of a typical photoacoustic spectroscope. Modified from M.W. Sigrist, Laser photoacoustic spectrometry for trace gas monitoring, Analyst 1194 (1994) 525–531.

pressure changes are caused by the intensity modulation, which are detected by a microphone located in the PA cell. An example of a design for a typical PA spectroscope is shown in Fig. 6.14. The signals created are rather weak, because of that they are sometimes enhanced. This can be done by tuning the modulation frequency of the laser to an acoustic resonance of the absorption cell [88]. A photoacoustic spectrum is recorded by tuning the coupled light source to different wavelengths and measuring the pressure changes in the cell. Via this spectrum, the absorbing compound of the sample can be identified. PAS has become a powerful technique to quantify gases in the ppb or ppt concentration range. To enhance the sensitivity, several modifications can be made. Since the intensity of the generated sound is proportional to the light intensity, intense lasers can be used to illuminate the sample [laser photoacoustic spectroscopy (LPAS)] [90]. Another method of signal enhancement is the amplification of the signals detected by the microphones and the use of lock-in amplifiers as detectors. Additionally, the sample can be placed in a cylindrical chamber, where the modulation frequency can be tuned to the acoustic resonance of this cell to amplify the signal generated. Cantilever-enhanced PAS has the highest sensitivity and allows reliable monitoring at the ppb level. Quartz-enhanced photoacoustic spectroscopy (QEPAS) based on the use of a quartz tuning fork (QTF) as a detector for the acoustic oscillation [91]. The QTF is a piezoelectric element which converts its deformation into separation of electrical charges. For example, QEPAS combined with QCLs has demonstrated sensitivities in the low ppt region with measurement times of 1 s for SF6, while using a compact and robust apparatus [92]. There are two major directions for the application of PAS. The largest part of research is concentrated on the development of high-resolution, high-sensitivity instruments. But some research is also focused on the development of low-cost instruments for specific applications like leakage detection. To reduce the costs of such spectrometers, low-cost microphones and digital signal processors are used. Additionally, analytes diffuse through semipermeable disks to get into the spectrometer and electronically modulated thermal sources are used, which are much cheaper than the components used in “normal” PAS. The ultimate goal for low-cost PAS instruments is fully integrated miniaturized systems.

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6.4.2 Fluorescence methods 6.4.2.1 Laser-induced fluorescence LIF is a very sensitive and specific technique for the detection of atmospheric trace gases at the lower ppt level [93]. It relies on the excitation of gas molecules by absorption of laser radiation, the frequency of which corresponds to a suitable transition from the molecule’s ground state to an (electronic) excited state. Typically, the wavelength at which the species has its largest cross section is selected. The excited species will—after some time (usually in the order of few nanoseconds to microseconds)—de-excite and emit light at a wavelength longer than the excitation wavelength. This fluorescent light is typically recorded with a PMT or filtered photodiodes. The number of fluorescence photons detected is then proportional to the atmospheric trace gas concentration. A central problem with LIF is the separation of Rayleigh scattered exciting radiation of air molecules from the fluorescence signal. To suppress the scattered excitation radiation, several techniques have been developed as follows: (1) using excitation at higher levels above fluorescence, the detected wavelengths are shifted and can be separated from the excitation wavelength using wavelength filters [93] and (2) using short laser pulses for excitation, the fluorescence signal can be recorded after the excitation signal has decayed and thus separated. LIF sensors are capable to make atmospheric measurements of, for example, NO, NO2, and NOx [94] under environmental conditions that might normally be considered unsuitable for a laser technique. These include clouds, rain, and, high atmospheric-aerosol loading conditions [93].

6.4.2.2 Chemiluminescence Chemiluminescence is mainly used as “gold standard” and also as reference method in the European standards [95] for NOx measurements in atmospheric [96] and environmental [97] research, but also for breath analysis [98, 99]. With this technique, detection limits in the ppb range are possible [100]. The principle of chemiluminescence for the measurement of NOx is based on the following reaction between nitrogen monoxide (NO) and ozone (O3): 2NO + 2O3 ! NO2 + NO∗2 + 2O2 NO∗2

! NO2 + hν

(6.16) (6.17)

NO in the sample air reacts with O3, which is generated in the instrument, producing a defined amount of energized NO2*, which returns to its basic energy level by emitting a photon. The emitted radiation (chemiluminescence) passes through an optical filter and is then detected by a PMT. The resulting output signal is determined and corresponds linearly to the NO concentration in the sample. For NO2 measurements, a deoxidation-converter changes the NO2 to NO, which is measured as described above. In other words, the NO2 concentration can be obtained by the difference between (1) the NOx concentration measured when the sample gas is

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directed through a converter and (2) the NO concentration measured when the gas is not run through the converter. Interferences with CO2 in the gas sample by partial quenching of the chemiluminescence is possible, especially in the presence of water vapor [100]. Any necessary corrections can be made either by using correction curves from the manufacturer of the device or by calibrating with gases that have approximately the same CO2 concentration as the sample gas.

6.4.3 Raman spectroscopy In Raman spectroscopy, the signal is derived by inelastic scattering of photons at molecules. This scattering takes place when an externally applied electric field induces a dipole in the molecule during the molecular vibration, in contrast to an intrinsic dipole moment required for IR absorption spectroscopy. A Raman signal can be acquired from the interaction of photons with molecules that change their polarizability during the vibration [101–103]. When light is scattered by molecules, almost all the scattering is elastic (Rayleigh scattering) and no energy transfer is present. Raman spectroscopy relies on inelastic scattering, usually performed with a laser light source. The interaction of the laser light with molecular vibrations results in energy changes of the scattered photons (lower energy for Stokes Raman and higher energy for anti-Stokes Raman). These changes in energy are highly specific for the structure of the gas molecule and allow unambiguous identification. The Stokes Raman signal is more intense than the anti-Stokes Raman signal, since the transition starts from nonexcited energy levels, which are higher populated (Boltzmann distribution). The inelastically scattered light is overall very weak compared to the intense elastically scattered light (Rayleigh scattering). For gases, the low density of analytes is an additional factor decreasing the Raman signal intensity, which is about three orders of magnitude lower than for liquid samples. The overall intensity IStokes of the Raman signal depends on several factors, including the laser intensity I0, the angular frequencies of the laser ωL, and the scattered light ωS as well as the polarizability α and the number of the molecules N [104, 105]: IStokes ∝ N  I0  ðωL  ωS Þ4  jαj2

(6.18)

Consequently, different enhancement techniques were developed to increase the signal intensity: for example, (1) increasing the laser power, (2) increasing the number of molecules which are interacting with the laser light, or (3) increasing the frequency of the interacting light by using UV lasers [106–108]. Typical LODs are in the range between 10 and 100 ppm for a wide range of gases, including homonuclear diatomic molecules like H2, O2, and N2 [109, 110].

6.4.3.1 Enhancement methods A straightforward approach to enhance the Raman signal of gas samples is the use of highpower lasers [111]. Such lasers consume high amounts of energy and mostly require cooling; thus, they are not well suited for portable sensors.

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Another strategy is based on multipass cavities to enhance the optical pathlength and to increase the interaction of the laser light and the molecules in the sample gas. A wide range of different cavity-enhanced techniques [52] is available. One possibility is highfinesse optical cavities, in which two highly reflective mirrors are used to efficiently capture the laser light and operate as “optical capacitator.” This leads to a huge power buildup of the light scattering at the gas molecule while the diode laser source itself only consumes low power [112, 113]. Cavity-enhanced Raman spectroscopy (CERS) is a versatile method for quantification of multiple gases in environmental [110, 114–121] and biomedical [111] research as well as industrial analysis of process and quality control [122–126]. An alternative enhancement technique is fiber-enhanced Raman spectroscopy (FERS), which is based on optical waveguides such as metal-coated capillaries [127] and hollowcore micro-structured fibers [128–133]. The inner hollow core of such fibers serves simultaneously as gas sample container and optical waveguide (light is guided due to total reflection, photonic bandgap, or antireflection [130, 134, 135]), in order to increase the interaction length of the gas molecules and the laser light. This technique also achieves highly efficient collection of the Raman signal, and is perfectly suited for applications requiring high sensitivity and low detection limits (sub-ppm) [136]. Extremely fast measurement times are more difficult to achieve with FERS due to filling, evacuation, and exchange times of the gases. With the development of high-power lasers, nonlinear Raman spectroscopy is available. One technique, which is widely used for the measurement of temperatures and species concentrations in reacting flows [137], is coherent anti-Stokes Raman spectroscopy (CARS). New developments in electronic-resonance-enhanced CARS (ERE-CARS) spectroscopy allow the detection of minor species such NO, C2H2, and other flame radicals [138].

6.5 Summary A large number of spectroscopic and spectrometric analytical techniques is available for the analysis of gas molecules in the environment. Each method has its own advantages and disadvantages and is suitable for different applications. Table 6.4 shows a comparison of different instrumental techniques discussed in this chapter. Up to now, a broad range of techniques have been developed for various analytical problems, each with its own advantages and limitations. For some methods, such as GC-MS, real-time analysis is not possible. Other mass spectrometric methods can be used for real-time analysis, but the substance identification and separation can limit the application and requires complementary methods for a complete analysis of the gas sample. Fast GC methods enable compound separation in few minutes, but the resolution and the sensitivity suffer. Another important consideration is instrument portability. Various highly sensitive techniques are only available as lab-based instruments. Often exists a trade-off between sensitivity and portability. Portability is a focus of recent research, as field measurements

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Table 6.4 Comparison of instrumental techniques for gas analysis discussed in this book chapter. Instrumental technique IR spectroscopy Photoacoustic spectroscopy Chemiluminescent detectors Photometric analyzers (e.g., fluorescence) Raman spectroscopy IMS DART-MS SIFT-MS

PTR-MS IR-MS GC-MS

MI-MS CI-MS

Applicable analytes

Comments

CO, CO2, NOx, N2O, SO2, hydrocarbons, fluorocarbons, etc. SF6, CH4, NH3, NO, CO2, H2O, CO, hydrocarbons, alcohols, fluorocarbons, etc. Ozone, NOx

LOD: