Environmental Nanotechnology

This is the second volume on Environmental Nanotechnology. The first chapter discusses the synthesis of nanomaterial and mainly the green synthesis of inorganic nanomaterials. Furthermore, a comperative discussion about resistive and capacitive measurement of nano-based biosensor is reviewed and the efficient delivery of nutraceutical with the help of nano-vehicles are explained. Moreover, the book also includes reviews on such topics as nanopharmaceuticals, health benefits and the toxic impact of heavy metal nanomaterials and the impact of several nanomaterials on plant abiotic stress and have focussed on the long term impacts of nanomaterials on agroecosystems. The reader will also find presentations on molecularly imprinted polymeric nanocomposites, critical and comparative comments on Nano-biosensors and Nano-aptasensors and on applications of nanotechnology for the remediation and purification of water with a main focus on drinking water. The last chapter presents a comprehensive review on plasmonic nanoparticle based sensors whereby the authors have hypothesized the future applications in the environment which can be plausible in the near future.


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Environmental Chemistry for a Sustainable World

Nandita Dasgupta · Shivendu Ranjan  Eric Lichtfouse Editors

Environmental Nanotechnology Volume 2

Environmental Chemistry for a Sustainable World Volume 21

Series editors Eric Lichtfouse, Aix Marseille Univ, CNRS, IRD, INRA, Coll France, CEREGE, Aix en Provence, France Jan Schwarzbauer, RWTH Aachen University, Aachen, Germany Didier  Robert, CNRS, European Laboratory for Catalysis and Surface Sciences, Saint-Avold, France

Other Publications by the Editors Books Environmental Chemistry http://www.springer.com/978-3-540-22860-8 Organic Contaminants in Riverine and Groundwater Systems http://www.springer.com/978-3-540-31169-0 Sustainable Agriculture Volume 1: http://www.springer.com/978-90-481-2665-1 Volume 2: http://www.springer.com/978-94-007-0393-3 Book series Environmental Chemistry for a Sustainable World http://www.springer.com/series/11480 Sustainable Agriculture Reviews http://www.springer.com/series/8380 Journals Environmental Chemistry Letters http://www.springer.com/10311 Agronomy for Sustainable Development http://www.springer.com/13593 More information about this series at http://www.springer.com/series/11480

Nandita Dasgupta  •  Shivendu Ranjan Eric Lichtfouse Editors

Environmental Nanotechnology Volume 2

Editors Nandita Dasgupta Institute of Engineering and Technology Dr. A.P.J. Abdul Kalam Technical University Uttar Pradesh Lucknow, Uttar Pradesh, India

Shivendu Ranjan Faculty of Engineering & The Built Environment University of Johannesburg Johannesburg, Gauteng, India

Eric Lichtfouse Aix Marseille Univ, CNRS, IRD, INRA, Coll France, CEREGE Aix en Provence, France

ISSN 2213-7114     ISSN 2213-7122 (electronic) Environmental Chemistry for a Sustainable World ISBN 978-3-319-98707-1    ISBN 978-3-319-98708-8 (eBook) https://doi.org/10.1007/978-3-319-98708-8 Library of Congress Control Number: 2018939755 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

We dedicate this book to those who are affected by environmental hazards. We hope that this book may be a small contribution to improving their quality of life Think Environment, Think Nanomaterials

Environmental quality is a major factor ruling the health of living organisms because pollution diseases can come from air, water and food. Pollution issues of air and water can be solved by environmental nanotechnologists, using nanobioremediation, nanonutraceuticals, nanobiosensors, and nano-­degradation. This book is the second of several volumes on Environmental Nanotechnology, which are published in series Environmental Chemistry for a Sustainable World. The first chapter by Sardar et  al. discusses the synthesis of nanomaterial and mainly the green synthesis of inorganic nanomaterials. Then, Bhattacharyya et al. review resistive and capacitive measurement of nano-based biosensor in Chap. 2. In Chap. 3, Jindal et al. explain the efficient delivery of nutraceutical with the help of nano-vehicles. Nanopharmaceuticals and health benefits and the toxic impact of heavy metal nanomaterials are reviewed in Chap. 4 by Yata et  al. Molecularly imprinted polymeric nanocomposites are presented by Ahmad et  al. in Chap. 5. Critical and comparative comments on ­nano-­biosensors and nano-aptasensors are presented by Ebrahimi and co-authors in Chap. 6. Saikia et al. detail in Chap. 7 the applications of nanotechnology for the remediation and purification of water and have mainly focused on drinking water. A brief discussion about polymers-metal v

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Preface

nanoparticles is presented by Theivasanthi et al. in Chap. 8. In Chap. 9, Kudelski and team review plasmonic nanoparticle-based sensors and hypothesize future applications. Thanks for reading. Nandita Dasgupta [email protected]  Shivendu Ranjan [email protected]  Eric Lichtfouse [email protected]

Contents

1 Biomolecules Assisted Synthesis of Metal Nanoparticles ��������������������    1 Meryam Sardar and Jahirul Ahmed Mazumder 2 Resistive and Capacitive Measurement of Nano-Structured Gas Sensors����������������������������������������������������������������������������������������������   25 Partha Bhattacharyya, Debanjan Acharyya, and Koushik Dutta 3 Nanotechnology Based Delivery of Nutraceuticals ������������������������������   63 Dhanashree Hemant Surve, Atish Tulsiram Paul, and Anil B. Jindal 4 Health Benefits and Potential Risks of Nanostructured Materials ��������������������������������������������������������������������������������������������������  109 Sidhartha Singh, Sandeep Kumar, and Vinod Kumar Yata 5 Molecularly Imprinted Polymeric Nanomaterials for Environmental Analysis��������������������������������������������������������������������  143 Rashid Ahmad and Mian Muhammad 6 Nano-biosensors and Nano-aptasensors for Stimulant Detection ��������������������������������������������������������������������������������������������������  169 Saeideh Ebrahimi and Rana Eftekhar Nahli 7 Nanotechnology for Water Remediation������������������������������������������������  195 Jiban Saikia, Abhijit Gogoi, and Sukanya Baruah 8 Polymers-Metal Nanocomposites�����������������������������������������������������������  213 Theivasanthi Thirugnanasambandan 9 Nanosensors for Environmental Analysis Based on Plasmonic Nanoparticles��������������������������������������������������������������������  255 Karol Kołątaj, Jan Krajczewski, and Andrzej Kudelski Index������������������������������������������������������������������������������������������������������������������  289

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Contributors

Debanjan  Acharyya  Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, West Bengal, India Rashid  Ahmad  Department of Chemistry, University of Malakand, Chakdara, Khyber Pakhtunkhwa, Pakistan Sukanya  Baruah  Department of Chemistry, Gauhati University, Guwahati, Assam, India Partha  Bhattacharyya  Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, West Bengal, India Koushik Dutta  Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, West Bengal, India Saeideh  Ebrahimi  Department of Chemistry, Ahar Branch, Islamic Azad University, Ahar, Iran Industrial Nanotechnology Research Center, Tabriz Branch, Ismalic Azad University, Tabriz, Iran Abhijit Gogoi  Department of Chemistry, Handique Girls College, Guwahati, India Surve  Dhanashree  Hemant  Department of Pharmacy, Birla Institute of Technology & Science, Jhunjhunu, Rajasthan, India Anil B. Jindal  Department of Pharmacy, Birla Institute of Technology & Science, Jhunjhunu, Rajasthan, India Karol  Kołątaj  Department of Chemistry, Faculty of Chemistry, University of Warsaw, Warsaw, Poland

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Contributors

Jan Krajczewski  Department of Chemistry, Faculty of Chemistry, University of Warsaw, Warsaw, Poland Andrzej Kudelski  Department of Chemistry, Faculty of Chemistry, University of Warsaw, Warsaw, Poland Sandeep  Kumar  Department of Biotechnology, Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India Jahirul  Ahmed  Mazumder  Department of Biosciences, Jamia Millia Islamia, New Delhi, India Mian Muhammad  Department of Chemistry, University of Malakand, Chakdara, Khyber Pakhtunkhwa, Pakistan Rana  Eftekhar  Nahli  Department of Toxicology, Ahar Branch, Islamic Azad University, Ahar, Iran Jiban Saikia  Nano Institute of Utah, University of Utah, Utah, USA Department of Chemistry, Dibrugarh University, Dibrugarh, Assam, India Meryam  Sardar  Department of Biosciences, Jamia Millia Islamia, New Delhi, India Sidhartha  Singh  Department of Biotechnology, Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India Thirugnanasambandan Theivasanthi  International Research Centre, Kalasalingam Academy of Research and Education (Deemed University), Krishnankoil, Tamilnadu, India Paul  Atish  Tulsiram  Department of Pharmacy, Birla Institute of Technology & Science, Jhunjhunu, Rajasthan, India Vinod Kumar Yata  Department of Biotechnology, Dr. B. R. Ambedkar National Institute of Technology Jalandhar, Jalandhar, Punjab, India

About the Editors

Nandita Dasgupta has completed her BTech and PhD from VIT University, Vellore, India. She has major working experience in Micro/Nanoscience and is currently working as Assistant Professor in the Department of Biotechnology, Institute of Engineering and Technology, Lucknow, India. Earlier at LV Prasad Eye Institute, Bhubaneswar, India, she has worked on mesenchymal stem cell-derived exosomes for the treatment of uveitis. She has exposure of working at university, research institutes, and industries including VIT University, Vellore, Tamil Nadu, India; CSIR-Central Food Technological Research Institute, Mysore, India; Uttar Pradesh Drugs and Pharmaceutical Co. Ltd., Lucknow, India; Indian Institute of Food Processing Technology (IIFPT), Thanjavur; and Ministry of Food Processing Industries, Government of India. At IIFPT, Thanjavur, she was involved in a project funded by a leading pharmaceutical company, Dr. Reddy’s Laboratories, and has successfully engineered microvehicles for model drug molecules. Her areas of interest include Micro/Nanomaterial fabrication and its applications in various fields  – medicine, food, environment, agriculture biomedical. She has published 13 edited books and 1 authored book with Springer, Switzerland, and two with CRC Press, USA. She has finished a contract for three book volumes with Elsevier, one volume with Wiley, two book volumes with CRC Press, and one volume with RSC (UK). She has authored many chapters and also published many scientific articles in international peerreviewed journals. She has received the Certificate for xi

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About the Editors

“Outstanding Contribution” in Reviewing from Elsevier, Netherlands. She has also been nominated for advisory panel for Elsevier Inc., Netherlands. She is the associate editor of Environmental Chemistry Letters – a Springer journal of 3.2 impact factor – and also serving as editorial board member and referee for reputed international peer-reviewed journals. She has received several awards and recognitions from different national and international organizations. Dr. Nandita’s publication list is available at: https://scholar.google.co.in/citations?user= u4p3mNkAAAAJ Dr.  Shivendu  Ranjan has completed his BTech and PhD in Biotechnology from VIT University, Vellore, India, and has expertise in Nano(Bio)Technology. He is currently working as Head, Research and Technology Development at E-Spin Nanotech Pvt. Ltd., SIDBI Center, Indian Institute of Technology, Kanpur, India. After joining E-Spin Nanotech, IIT Kanpur, he has successfully developed prototypes for many products and three patents. He is also serving as a Senior Research Associate at Faculty of Engineering and Built Environment, University of Johannesburg, Johannesburg, South Africa. Dr. Ranjan is also reviewer of Iran National Science Foundation (INSF), Tehran, Iran, and jury at Venture Cup Denmark for the past 3 years. He had founded and drafted the concept for the first edition of the “VIT Bio Summit” in 2012, and the same has been continued till date by the university. He has worked in CSIR-CFTRI, Mysuru, India, as well as UP Drugs and Pharmaceutical Co. Ltd., India, and IIFPT, Thanjavur, MoFPI, Government of India. At IIFPT, Thanjavur, he was involved in a project funded by a leading pharmaceutical company, Dr. Reddy’s Laboratories, and has successfully engineered microvehicles for model drug molecules. His research interests are multidisciplinary and include Micro-/Nanobiotechnology, Nano-toxicology, Environmental Nanotechnology, Nanomedicine, and Nanoemulsions. He is the associate editor of Environmental Chemistry Letters  – a Springer journal of 3.2 impact factor – and an editorial board member in Biotechnology and Biotechnological Equipment (Taylor and Francis, USA). He is research topic editor in Frontiers in Pharmacology (FrontiersIn, USA journal

About the Editors

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of 3.83 impact factor). He is serving as the executive editor of a journal in iMed Press, USA, and also serving as editorial board member and referee for reputed international peer-reviewed journals. He has published six edited books and one authored book with Springer, Switzerland, and two with CRC Press, USA.  He has recently finished his contract of three volumes of book with Elsevier, two volumes with CRC Press, and one with Wiley and RSC (UK), respectively. He has published many scientific articles in international peerreviewed journals and has authored many book chapters as well as review articles. He has bagged several awards and recognitions from various national as well as international organizations. Eric  Lichtfouse,  PhD, born in 1960, is an environmental chemist working at the University of AixMarseille, France. He has invented carbon-13 dating, a method allowing to measure the relative age and turnover of molecular organic compounds occurring in different temporal pools of any complex media. He teaches scientific writing and communication and has published the book Scientific Writing for Impact Factors Journals, which includes a new tool – the Micro-Article – to identify the novelty of research results. He is founder and Chief Editor of scientific journals and series in environmental chemistry and agriculture. He got the Analytical Chemistry Prize by the French Chemical Society, the Grand Prize of the Universities of Nancy and Metz, and a Journal Citation Award by the Essential Indicators. Publications: https://scholar.google.fr/citations?use r=MOKMNegAAAAJ, https://cv.archives-ouvertes.fr/ eric-lichtfouse. Email: [email protected]

Chapter 1

Biomolecules Assisted Synthesis of Metal Nanoparticles Meryam Sardar and Jahirul Ahmed Mazumder

Contents 1.1  I ntroduction 1.2  S  ynthesis of Nanoparticles 1.2.1  Silver and Gold Nanoparticles 1.2.2  Synthesis of Metal Nanoparticles Using Microbes 1.2.3  Synthesis of Metal Nanoparticle Using Plants 1.2.4  Synthesis of Nanoparticles Using Enzymes/Proteins 1.2.5  Synthesis of Nanoparticles Using Carbohydrates 1.3  Conclusions References

 2  3  4  5  8  10  12  15  15

Abstract  The synthesis of metal nanoparticles is an upcoming area of research as these nanoparticles have applications in diverse field. These are beneficial to human beings as they can be used for targeting the diseases like cancer, used as therapeutic agents, as biosensors and in imaging. These are also employed in removal of heavy metals and phenolic pollutants from soil and water and have excellent catalytic properties. Thus, there is a need to develop the protocols or methods which can synthesize these nanoparticles at large scale, also the methods should be environment friendly and economical. Lot of reviews have been published so far on the techniques and methods of synthesis of these nanoparticles citing the advantage and disadvantage of each method. In this chapter biosynthesis of metal nanoparticles have been described, the synthesis of silver and gold nanoparticles is discussed at length. These can be synthesized by physical, chemical and biological methods. Biological methods are considered better over other methods of synthesis as they do not employ any toxic chemicals or reagents; only the biomolecules present in the

M. Sardar (*) · J. A. Mazumder Department of Biosciences, Jamia Millia Islamia, New Delhi, India e-mail: [email protected]

© Springer Nature Switzerland AG 2019 N. Dasgupta et al. (eds.), Environmental Nanotechnology, Environmental Chemistry for a Sustainable World 21, https://doi.org/10.1007/978-3-319-98708-8_1

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M. Sardar and J. A. Mazumder

organisms serves as the reducing and stabilizing agent. In nature we have a great diversity of plants and animals available to us, thus have a wider choice of the reducing agents which can reduce the metal ions. Multicellular and unicellular organisms are known to accumulate metals; this property is mainly exploited in the synthesis process.

1.1  Introduction Bionanotechnology is an emerging technology, which can lead to a new revolution in every aspect of science as it requires the contribution from material scientist, physicists, biologists, molecular biologists on a common platform (Takeda et  al. 2009). The prefix nano is derived from a Greek word meaning “dwarf” refers to things of one billionth (109 m) in size (Rosoff 2001). The preliminary idea of nanotechnology was led down by Richard Feynman in a lecture entitled “There’s plenty of room at the bottom” at the American Institute of Technology in 1959 (Feynman 1960). The term nanotechnology was introduced by Norio Taniguchi, a professor at Tokyo Science University (Sivakumar et al. 2011). The physical and chemical properties of the nanomaterials, when compared with their bulk counterparts, are very different which enhances their applications in the biological field (Stone et  al. 2010). Nanotechnology is considered beneficial for human health care as the nanomaterials have applications in drug development, therapy, imaging and waste water treatment (Shi et  al. 2010; Ranjan et  al. 2014; Dasgupta et al. 2015; Jain et al. 2017). The nanomaterials are formed from the basic elements only by altering the atomic as well as molecular properties of these elements (Garitaonandia et al. 2008). Different types of nanomaterials have been synthesized so far such as fullerenes, graphene nanostructures, nanozymes, nanotubes, nanoparticles(Amin et  al. 2014; Gottschalk et  al. 2009; Kadiyala et  al. 2018; Hosseini et al. 2018; Maddinedi et al. 2017). In this chapter we will focus on metal nanoparticles particularly the synthesis of silver and gold nanoparticles have been discussed at length. The important features of metal nanoparticles are: have very large surface area to volume ratio (Korsvik et al. 2007), large surface energies (Navrotsky 2003), Plasmon excitation, quantum confinement (Alivisatos 2004) and forms a colloidal solution (Ahmadi et al. 1996). Due to these special features they find applications in catalysis, photocatalysis, optics, optoelectronics (Gao et al. 2005) and biology and medicine (Das et al. 2009). The properties and applications of nanoparticles are influenced by their size and morphology (Gan et al. 2005), thus ideally these should be prepared by methods which are reproducible, produce the particles of control size, easy and cheap, employs less toxic chemicals and can be easily scaled up. This chapter gives an overview on various methods for metal nanoparticles synthesis, the emphasis is given on biological methods only.

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1.2  Synthesis of Nanoparticles Several approaches have been developed for the conventional metal nanoparticles synthesis like physical, chemical and biological (Nisbet and Weiss 2010) shown in Fig. 1.1. Over the years many reviews have been published describing the mechanism and details of each method, still the mechanism of biosynthesis is not clearly understood. In chemical and biological methods of synthesis the principle focus is mainly on the choice of reducing agents which reduces the bulk metal into zerovalent metal atom, which colloid in solution with other metal ions or metal atoms to form stable metal nuclei. The size of the nanoparticles is dependent upon the interaction between the metal salt and the reducing agents. The nanostructured colloidal metals are unstable and can easily agglomerate, so to prevent agglomeration a capping agent is added. Chemical methods employ harsh and toxic reducing agents like sodium borohydride, citrate and elemental hydrogen, Although chemical methods are quick and easier for synthesis, the biological methods are considered better and ecofriendly (Clark and Macquarrie 2008). The increasing demand of nanoparticles must be accompanied by “green” synthesis methods, as physical and chemical methods are often associated with limitation such as use of excessive energy and release of hazardous toxic bi-products (Veerakumar et al. 2014). Due to increase in environmental pollutants, the focus has been shifted to the ‘green chemistry’ route (Vigneshwaran et  al. 2006). Use of nontoxic chemicals, solvents and renewable materials are some of the key issues that should be considered while using green synthesis strategy (Raveendran et al. 2003). Green synthesis procedures involve the use of plants, biomolecules, agricultural waste and microorganisms (Borase et  al. 2014). These procedures reduces pollution risk at source level and also utilises wastes for the synthesis procedure (Nelson 2003). Moreover, it has been reported that biosynthesized nanomaterials are biocompatible as compared to chemically

Fig. 1.1  Approaches for the synthesis of nanoparticles

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synthesized, thus these methods are preferred one when the materials are applied in biology and medicine. Synthesis of nanoparticles using plant or plant extract is considered advantageous over microbial synthesis because of high cost and elaborate process of maintaining microbial cell cultures (Kumar et al. 2014b; Balaji et al. 2017; Tammina et al. 2017). The biological synthesis of metal nanoparticles has been discussed in detail in the next section mainly the silver and gold are described in detail.

1.2.1  Silver and Gold Nanoparticles Metal nanoparticles such as Silver and gold draw a keen interest in the field of research, medicine and waste water treatment (Brar et al. 2010; Jain et al. 2008) as shown in Fig. 1.2. They have also been employed as antimicrobial agents, biosensor and in food packaging applications. The properties and function of silver and gold nanoparticle depend upon its composition, size, shape, chemical functionality, surface charge, which are greatly influenced by the preparation methods and reaction conditions (Temperature, pH, time, solvent medium) (Li et al. 2011; Soenen et al. 2015; Dasgupta et al. 2016; Dasgupta et al. 2018). Metal nanoparticles have a characteristic Surface Plasmon Resonance (SPR) absorption in the UV Visible region (Willets and Van Duyne 2007) which is generally used to characterize the nanoparticles. The surface Plasmon band arises from the coherent existence of free electrons in the conduction band due to the small particle size. The surface plasmon resonance of silver nanoparticles lies in the range of 400–450 nm and for gold nanoparticles it lies in the range of 500–550 nm. By making use of surface Plasmon resonance and other useful properties of silver and gold nanoparticles, they can be synthesized and Fig. 1.2  Applications of metal nanoparticles

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functionalized for various biotechnological applications (Fierascu et  al. 2010). To characterize the size and shape of the synthesized nanoparticles, techniques such as UV-Vis Spectroscopy, Dynamic light spectroscopy, X-ray Diffraction, Fourier Transform Infrared Spectroscopy and Transmission Electron Microscopy are used. Silver and Gold Nanoparticles can be chemically synthesized by a variety of techniques such as spray pyrolysis, thermal decomposition, chemical vapour deposition, and laser ablation (Amendola et al. 2010; Teoh et al. 2010; Teoh et al. 2005). But the chemical synthesis involve the use of toxic chemicals as reducing agents, which get adsorbed on the surface of the nanoparticles and thus limits the biomedical applications (Balasundaram et al. 2006; Prabhu and Poulose 2012). The chemicals used for synthesis of silver nanoparticles and gold nanoparticles for their stabilization are also toxic and lead to non-eco-friendly by-products (Sabir et  al. 2014). To avoid the release of toxic by-products, gold and silver nanoparticle are widely synthesized by the use of biomolecules, plant extracts, proteins, micro-­ organisms (Ahmad et al. 2013; Ingale and Chaudhari 2013). Interactions between biological agents and nanostructured materials, develops the technologies connecting the two where bulk materials gets reduced into nanoparticles (Rasmussen et al. 2010). Some report shows that biomolecules and organisms can selectively identify inorganic surfaces or serve as matrices for inorganic growth and nucleation (Naik et  al. 2002). There are several comprehensive reviews that provides the detailed mechanisms of nucleation and growth of inorganic nanoparticles (Wang et al. 2011; Wu et al. 2016). Importantly, surfactants or capping agents are essential for the stabilization and morphological control of the nanoparticles (Zhang et  al. 2009). Capping agent can alter the order of free energies for different crystallographic planes and their relative growth rates through its chemical interaction with the nanoparticulate surface (Xiong and Xia 2007). Long chain hydrocarbons with amine, thiol or carboxylic pendants, chiral ligands, poly-carboxylic acids, polymers and hydroxyl compounds, surfactants are often used as capping agents or ligands for the synthesis of nanoparticles (Datta et al. 2016). The functional groups present in the capping agents interact with the unsaturated surface atoms of the nanoparticles (Badawy et al. 2010). As increasingly electropositive metals are transformed into nanoparticles, the usage of stronger reducing agents is required. Plant extract, biomolecules and microbes serves the purpose of reducing as well as capping agents in case of green chemistry (Kamyshny and Magdassi 2014; Kharissova et al. 2013).

1.2.2  Synthesis of Metal Nanoparticles Using Microbes Haefeli, in 1984, for the first time reported that Pseudomonas stutzeriAG259, and a bacterial strain isolated from silver mine were capable of synthesizing silver nanoparticles(Deepak et al. 2011). It is now well established that bacteria alleviates conversion of metals into nanoparticles of silver and gold (He et al. 2007). The synthesis can be achieved by both extracellular and intracellular mechanism. In addition to bacteria other microbes like fungi, algae, yeast, viruses have also been

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Table 1.1  Synthesis of silver and gold nanoparticles using microbes Time taken 12 h 48 h

Size and type of nanoparticles 19 nm, Ag 10–30 nm, Ag

References Wadhwani et al. (2016) Wang et al. (2016).

10 h 3 h

15.5 nm, Ag 17 nm, Au

Xue et al. (2016) Tidke et al. (2014)

1 h 24 h 4 h

5.42 nm, Au 100 nm. Au 8.4 nm, Au

Chlorella vulgaris green algae

24 h

15 and 47, Au

GordoniaamicalisHS-11 Actinomycete Yeast isolates from termite gut Inonotus obliquus (Chaga mushroom)

24 h

5–25 nm, Au and Ag

Namvar et al. (2015) Honary et al. (2013) González-Ballesteros et al. (2017) Annamalai and Nallamuthu (2016) Sowani et al. (2016)

7 days 4 h

2–22 nm, Ag 23 nm, Au

Eugenio et al. (2016) Lee et al. (2015)

Species Acinetobacter sp. SW30 Bacteria Bacillus methylotrophicus DC3 Bacteria Arthrodermafulvum HT77 Fungi Fusarium acuminatum MTCC-­ 1983 Fungi Sargassummuticum (Algae) Penicillium citrinum Cystoseirabaccata Brown alage.

reported to synthesize metal nanoparticles (Durán et al. 2011; Thakkar et al. 2010). Table 1.1 highlight some of the recent examples of silver and gold nanoparticles synthesized by microbes. As fungi secretes large amounts of enzymes, so the synthesis of nanoparticles using fungi is potentially exciting (Honary et  al. 2012). Marine alga Sargassum wightii can also mediate the synthesis of stable gold nanoparticles by the reduction of aqueous AuCl4. The nanoparticles formed are of high density, very stable and the size ranging from 8 to 12 nm (Singaravelu et al. 2007). Intracellular synthesis of silver and gold nanoparticle has been carried out using a number of fungi and bacteria such as Verticillium sp., Pseudomonas stutzeris, Klebsiella aerogenes, Desulfovibrio desulfuricans, Staphylococcus aureus and Escherichia coli etc. Nair and Pradeep (2002) reported the synthesis of nanocrystals of gold, silver and their alloys by lactic acid bacterial cells. They exhibited that lactic acid bacteria present in the whey of buttermilk on exposure of AgNO3 and HAuCl4 solution, are capable of synthesizing silver and gold nanoparticles respectively (Nair and Pradeep 2002). In addition to silver and gold microbes can reduce other metals also, like TiO2 nanoparticles have been synthesized using bacteria such as Lactobacillus crispatus (Ibrahem et al. 2014) and Bacillus subtilis (Kirthi et al. 2011). Hexagonal Cadmium sulphide(CdS) nanoparticles have been synthesized intracellularly by Schizosaccharomyces pombe (Kowshik et al. 2002). Though several reviews have been published on the synthesis of metal nanoparticles using microbes, still the mechanism of synthesis is not very clear. Different bacteria have different mechanisms for the synthesis of nanoparticles (Kulkarni and Muddapur 2014). Despite major differences in mechanism, synthesis of nanoparticle usually follows the mechanism where metal ions are initially trapped on the

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surface or inside of the microbial cells followed by reduction to nanoparticles via a number of enzymatic or non-enzymatic methods (Bhattacharya and Gupta 2005). The mechanisms for intra and extracellular synthesis process of nanoparticles are different and varied considerable with change in biological agents (Hulkoti and Taranath 2014). Intracellular synthesis method involves a specific ion transportation system in the microbial cell. Cell wall of microorganism plays an important role in biosynthesis of metal nanoparticles. The mechanism associated with this process is that, cell wall of microorganism is negatively charged and the metal ions contain positive charge, as a result of which there will be an electrostatic force of interaction between the two opposite charges (Hulkoti and Taranath 2014). The enzymes present in the cell wall of microorganisms reduce these metal ions to nano form and these nanoparticles get diffused off through the cell wall. On the other hand extracellular synthesis of silver nanoparticles by using microbes is basically dependent on nitrate reductase enzyme. It has been found that most of the fungi contain a nitrate reductase enzyme which helps in the bioreduction process of metal ions for the synthesis of metallic nanoparticles (Hulkoti and Taranath 2014). In microorganisms, extracellular bio-reduction reactions may also occur via microbially produced electron transfer agents (Suzuki et  al. 2010), for example: flavins, a group of compounds secreted by Shewanella sp., helps bacteria to transfer metabolically generated electrons to externally located electron acceptors as well as membrane-associated cytochromes and redox proteins (Kitching et al. 2015). Silver and gold nanoparticle can be synthesized with the help of fungi both by extracellular and intracellular methods (Ahmad et al. 2003; Sastry et al. 2003). In case of extracellular gold nanoparticle formation, the enzymes secreted by the fungi reduces Au3+ ions (Sastry et al. 2003). It is suggested that in case of extracellular synthesis AuCl4− ions are adsorbed on the cell-wall of fungi by electrostatic interaction with positively charged groups (e.g. lysine) (Mukherjee et al. 2001), whereas in the case of intracellular synthesis, first Au3+ ions diffuses through the cell membrane and then further reduced by redox mediators present in the cytosol (Das et al. 2012). There are different mechanisms reported for the biosynthesis of gold nanoparticle by fungus. Candida albicans can synthesize phytochelatins, it is an oligomers of glutathione which is formed by a transpeptidation reaction. Metal ions in presence of glutathione, can trigger phytochelatin synthesis, during which Au3+ ions get reduced to gold nanoparticle, and the capping process is mediated by glutathione (Chauhan et al. 2011). It has been reported that mechanism of microbial reduction of nanoparticles involves reductases and other equivalent reductants (Wing-­ ShanáLin 2014). Newman et al. investigated compounds from Fusarium oxysporum  viz. naphthoquinones and anthraquinones, these compounds bear redox potential which can facilitate both metal ion reduction and could act as electron shuttles (Newman and Kolter 2000). Reduction of AgNO3 to silver nanoparticles can also be mediated by nitrate reductase enzyme utilizing NADPH as cofactor (Kumar et al. 2007). Oxidoreductases and quinones present in the membrane and cytosol of yeast might play an important role in the reduction process. The ­oxidoreductases are pH sensitive and work in an alternative manner, at lower pH

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M. Sardar and J. A. Mazumder

value, oxidase gets activated while at higher pH value reductase gets activated (Jha et al. 2009).

1.2.3  Synthesis of Metal Nanoparticle Using Plants In modern nanotechnology, the interaction between inorganic nanoparticles and biological structures is one of the most exciting field of research (Gardea-Torresdey et al. 2002). Nanoparticles synthesis by plant extract demonstrate, electromagnetic (Chandran et al. 2006), optical (Vilchis-Nestor et al. 2008) and catalytic properties (Kumar and Yadav 2009), these properties are dependent on the shape and size of nanoparticles (Albanese et al. 2012). Several plants/plant extract have been reported to synthesize metal nanomaterials for example silver and gold nanoparticle can be prepared by using plants like alfalfa (Gardea-Torresdey et al. 2002), Myrica leaves (Phanjom et al. 2012), neem leaves (Tripathi et al. 2009) etc. TiO2 nanoparticles can be synthesized using medicinal plant such as Aloe Vera (Venkatesh et  al. 2015). Paskalis Sahaya Murphin et  al. for the first time used flower extract of Hibiscus rosasinesis to synthesize TiO2 nanoparticle (Kumar et al. 2014a). The synthesized TiO2 nanoparticle bears antibacterial activity against different bacterial strains such as Vibrio cholerae, Pseudomonas aeruginosa, Staphylococcus aureus (Gupta et al. 2013). An easily available, inexpensive vegetable Capsicum annuum L. has been used for synthesis of silver nanoparticles (Li et al. 2007). It produces high quantum of biomolecules which can serves as reductants and provides a lane for the production of silver nanoparticles in solution (Mittal et al. 2013). Gold nanotriangles, silver and bimetallic nanoparticles have been synthesized using Aloe vera plant extract (Chandran et al. 2006). Sathishkumar et al. used tuber extract of Curcuma longa to synthesize Palladium nanocrystals (Sathishkumar et al. 2009). Jae Yong Song et all used Diospyros kaki leaf extract to synthesise platinum nanoparticles of 2–12 nm (Song et  al. 2010). P.S.  Schabes-Retchkiman et  al. synthesized Ti/Ni bimetallic nanoparticle by using bio reduction method. The biomass source is provided by using alfalfa extract. The synthesized nanoparticles can be used for different purpose such as catalysis and protective coating uses, as well as in optical materials (Schabes-Retchkiman et al. 2006). Jiao Qu et al. used Physalis alkekengi L. extract to synthesize polydisperse ZnO nanoparticles (Qu et al. 2011). A rapid synthesis of stable silver and gold nanoparticle using leaf extract of geranium brings out the possibility of a faster rate of synthesis using biological agents as compared to chemical agents (Shankar et al. 2003). Few recent examples of silver and gold nanoparticle synthesis using plants are shown in Table 1.2. A number of plant metabolites, including polyphenols, terpenoids, sugars (Shankar et al. 2005), alkaloids, phenolic acids and proteins facilitates the process of bioreduction of metal ions (Mariselvam et al. 2014). It has been reported that the bio-reduction of metal nanoparticle using plants and plant extracts includes three main phases. The activation phase in which the reduction and nucleation of metal ions takes place. The second phase is growth phase, where small adjacent nanopar-

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Table 1.2  Synthesis of silver and gold nanoparticles using various plants or parts of plants Plants Sumac leaf extract

Time taken 1 h

Size and type of nanoparticles 20.83 nm, Au

Oak fruit bark Belosynapsiskewensis leaf extract

48 h 24 h

20–25 nm, Ag 10–28 nm, Ag

Elettariacardamomum seeds Lantana camara berry extract Rifoliumresupinatum seed exudate Hibiscus sabdariffa extract Ecliptaprostrata leaf extract

2 min 12 h 12 h 6 min 30 min

15.2 nm, Au 100 nm, Au 17 nm, Ag 10–60 nm, Au 31 nm, Au

Gnidiaglauca flower extract Allium cepa extract Panax ginseng leaves

20 min 15 min 3 min, Au 45 min, Ag 3 h

10 nm, Au 100 nm, Au 10–20 nm, Au 5–15 nm, Ag

Bamboo (Bambusa chungii) leaf extracts

28 nm, Au

References Shabestarian et al. (2016) Veisi et al. (2015) Bhuvaneswari et al. (2016) Rajan et al. (2017) Kumar et al. (2017) Khatami et al. (2016) Mishra et al. (2016) Rajakumar et al. (2016) Ghosh et al. (2012) Parida et al. (2011) Singh et al. (2016)

Jia et al. (2015)

ticles come together to form particles of a larger size, accompanied by an increase in the thermodynamic stability of nanoparticles and the final termination phase where the shaping of nanoparticle takes place (Makarov et al. 2014). Plant extracts may act as both reducing agents and stabilizing agents in the synthesis of metal nanoparticles (Kasthuri et al. 2009). Zhou et al. (2010) and Huang et al. (2011) suggested that the capping agents may include a number of molecules such as sugars, flavonoids, saccharides and proteins, these molecules are responsible for the bioreduction of metal ions (Huang et al. 2011; Zhou et al. 2010). The mechanism of nanoparticles formation is still an unexplored area and need to be strengthened. The most relevant plant metabolites in capping and bio-reduction of metal NPs include terpenoids, polyphenols, sugars, alkaloids, phenolic acids and proteins (Amin et al. 2012; Vilchis-Nestor et al. 2008) as shown in Fig. 1.3. Santos, Pinto and co-workers have established a strategy to identify the key components involved in the stabilization and reduction of silver and gold nanoparticles using aqueous extract of Eucalyptus globules Labill bark. Their investigations have led to the conclusions that phenolic compounds, particularly galloyl derivatives, are primarily responsible for the reduction of metal-ions, while the stabilization of nanoparticles are mediated by sugars (Santos et al. 2014). Phytochemicals present in plant act as capping agents and helps in synthesizing mono disperse TiO2 nanoparticles, preventing their aggregation and thus enhancing the stability (Kumar et al. 2014a). C. Soundarrajan et al. used leaf extract of Ocimum sanctum to synthesise platinum nanoparticles (Soundarrajan et al. 2012). M. Sathish kumar et al. used Cinnamon zeylanicum bark extract for the biosynthesis of silver nanocrystals. They suggested that with increase

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M. Sardar and J. A. Mazumder

Fig. 1.3  Structure of different component of Plant extract involved in the reduction and stabilization of metal nanoparticles

in the dosage of C. zeylanicum bark extract, the size distribution of nanoparticle varies. This might be attributed due to the variation in the amount of reductive biomolecules. It was also found that size and shape of nanoparticle is pH dependent, at high pH, nanoparticles formed were of small and spherical in size (Satishkumar et al. 2009). Wang et al. used different tea extracts, namely, green tea (GT), oolong tea (OT), and black tea (BT) to synthesize iron nanoparticles (Wang et al. 2014).

1.2.4  Synthesis of Nanoparticles Using Enzymes/Proteins Enzymes are important biomolecules which catalysis almost all the vital reactions necessary for the life of an organism. A very important property of the enzymes is that it can function outside the cell also, which makes the enzymes suitable for biotechnological applications. They are used in food industry, medicine, textile, leather, detergent and in the synthesis of biofuels. One of the recent additions to the application of enzymes is in the synthesis of metal nanoparticles. Size of most of the proteins is just 5 nm, which is comparable with the dimensions of the smallest man made nanoparticles. Enzymes act as both reducing as well as stabilizing agent for the growth of metal nanoparticles. Bovine serum albumin protein is reported to have been employed for the synthesis of silver and gold nanoparticles (Mohanpuria et al. 2008; Singh et al. 2005). Amongst, the various metal nanoparticles, Au, Ag, Pt, Pd and an alloy of Au–Ag have been synthesized by a number of enzyme mediated processes (Balaji et al. 2009). AuCl4 was reportedly reduced to gold nanoparticles

1  Biomolecules Assisted Synthesis of Metal Nanoparticles

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by an enzyme α-amylase, having exposed cysteine group, which is oxidized when reacted with the metal and the metal is reduced (Rangnekar et al. 2007). It is also reported that Lysozyme can also catalysis the synthesis of Ag and Au nanoparticles by the reduction of Ag+ and AuCl− via oxidation of the tyrosine residue present within the enzyme (Ray et al. 2006). Ravindra, 2009, synthesized gold nanoparticles utilizing serrapeptase that serves as both reducing and stabilizing agent; they also reported that lysine is involved in reduction and stabilization of gold nanoparticles (Ravindra 2009). In our lab we reported the synthesis of silver and gold nanoparticles using hydrolytic enzymes like alpha amylase and cellulase, the silver and gold nanoparticles were synthesized at ambient temperature and XRD data reveals they are crystalline in nature (Mishra and Sardar 2015). Another important enzyme, Peroxidase, breaks down H2O2 is also reported to be a suitable candidate for synthesis of silver and gold nanoparticles (Mishra et al. 2015). Ji-Soo Jang et al. used apoferritin, a protein consisting of 24 polypeptide subunits for a facile and versatile synthesis of thin-walled SnO2 nanotubes (Jang et al. 2015). Azizi et al. suggested that proteins and polyphenols from Sargassum muticum can be used for the formation of pure ZnO nanoparticles in the size range of 30–57 nm with hexagonal crystal. He further suggested that ZnO nanoparticles prepared by this process can degrade phenol efficiently (Azizi et al. 2014). It has been speculated that the functional groups of the amino acids present in the enzyme molecules act as reducing agents for the synthesis of metal nanoparticles and rest of the polypeptide chain stabilized the nanoparticles (Rangnekar et  al. 2007). Among the different functional groups, the thiol group (-SH), which is present in the side chain of cysteine is considered the most important one, the chemistry of the –SH group with the silver and gold metals had been known since long. It is hypothesized that the -SH group reacts with Au or Ag metal to form Au-S or Ag-S bonds (Pricker 1996). Gold nanoparticles have also been synthesized using histidine-­ rich peptide. Synthesis using histidine rich peptide requires reducing agents such as citric acid or sodium borohydride (NaBH4) at high temperatures to trigger the nucleation and growth of gold nanoparticles (Djalali et al. 2002). The potential of naturally occurring amino acids in the reduction or binding of Au ions was studied by Tan et al. 2010. According to their studies tryptophan acts as the strongest reducing agents, and histidine act as the strongest binding agents. The interaction of amino acid to the metal ions occurs either by amino or carbonyl group, for example in case of aspartic and glutamic acid, carboxyl groups is used and in case of histidine, nitrogen atom of the imidazole ring is involved. It has also been indicated that the hydroxyl groups of tyrosine is involved in the reduction process of Ag ions. Thiol modified ligands can form gold–sulfur bonds on the surface of gold nanoparticles, this can be used in regulating the size of nanoparticle by varying the gold/thiol and the size of the side chain, more the number of thiol groups, the smaller will be the particle size (Brust et al. 1994; Hostetler et al. 1998). A model for enzymatic peptides that are rich in Arginine and Tryptophan strongly bind the metal ions and due to entropic effects, they are not the good choice for synthesizing the silver and gold nanoparticles (Makarov et  al. 2014; Tan et  al. 2010). It has been observed that

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Table 1.3  Synthesis of silver and gold nanoparticles using Enzymes Enzymes Nitrate reductase α- amylase from A. Oryzae. (free and immobilized on iron oxide nanoparticles)

Sulfite reductase

Time taken 24 h 22 h for Ag (immob) 12 h for Au (immob) 12 h for Ag (free) 6 h for Au (free) 5 h

Size and type of nanoparticles 5–7 nm, Ag 17 nm, Au (immob)

References Talekar et al. (2014) Mazumder et al. (2016)

25 nm, Ag (immob)

Mishra and Sardar (2014)

22–42 nm for Ag (free) 2–20 nm for Au (free) 10 nm, Au

Mishra and Sardar (2012)

Lysozyme Laccase Nitrate reductase Horseradish peroxidase C (HRP)

– 90 min 24 h 24 h

Cellulase from A. niger

12 h

Tyrosinase Keratinase from Bacillus safensis α- amylase from Hog pancreas

24 h 2 h

18 nm, Ag 22 nm, Au 10–20 nm, Ag 5–40 nm, Ag 20–80 nm, Au 5–25 nm, Ag 5–20 nm, Au 50–70 nm, Au 8 nm, Ag

48 h

5–20 nm, Au

Gholami-Shabani et al. (2015) Golubeva et al. (2016) El-Batal et al. (2015) Talekar et al. (2014) Mishra et al. (2015) Mishra and Sardar (2015) Yang et al. (2014) Lateef et al. (2015) Rangnekar et al. (2007)

Peptides rich in amino acids such as glutamic or aspartic acids, which have weak association with tetrachloroauric acid ions, are inefficient in the synthesis of nanoparticles because they easily dissociate from the metal ion complex (Makarov et al. 2014). Karwa et al. 2011, proposed that amide linkages and protein capping enhances the stability of the produced nanoparticles (Karwa et al. 2011). Thus, for synthesizing the nanoparticles one has to identify the suitable protein or enzyme and has to optimize the parameters like concentration of enzyme, pH, temperature, a peptide can also be synthesized and used for large scale synthesis of nanoparticles. If one can control the size of nanoparticles using proteins/peptides, the technique can be used in future for commercial preparation of nanoparticles. Few examples of nanoparticles synthesized using enzymes are given in Table 1.3.

1.2.5  Synthesis of Nanoparticles Using Carbohydrates With the recent advancement in cleaner, sustainable chemistry, natural polymers were exploited for synthesis (Llevot et  al. 2016). Polysaccharides are linear or branched carbohydrates, linked together by glycosidic bonds are the most abundant

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Table 1.4  Synthesis of silver and gold nanoparticles using various Carbohydrates Carbohydrates Alginate Cellulose Hydroxy propyl methyl cellulose (HPMC) Curdlan Gum (Cochlospermumgossypium) Polysaccharides (extracted from four marine macro-algae) Starch Glucomannan Chitosan

Time taken 1 min 5 min –

Size and type of nanoparticles 5–30 nm, Ag 1–5 nm, Ag 17 nm, Ag

References Palza (2015) Mochochoko et al. (2013) Dong et al. (2014)

4 h 60 min 20 min

16 nm, Au 5 nm, Ag 7–20 nm, Ag

Yan et al. (2015) Kora and Sashidhar (2014) El-Rafie et al. (2013)

12 h 3 h 50 min

10–30 nm, Ag 21 nm, Au 30 nm, Ag

Ayala Valencia et al. (2013) Gao et al. (2014) Venkatesham et al. (2014)

macromolecules on earth (dos Santos and Grenha 2015). Kahrilas et al. 2014 synthesized silver nanoparticles using β-d-glucose as a reducing agent and starch as a stabilizing agent (Kahrilas et al. 2014). The biomolecules with functional groups like carboxyl, hydroxyl, and amine have the potential for reduction and capping of metal-ion. Some of the polysaccharides used in the biosynthsis of silver and gold nanoparticles are given in Table 1.4. Irvani et al. reported the reduction of Ag+ ions is also reported to be carried out by saccharides in the presence of ammonia, yielding silver nanoparticles with different shapes and sizes of 50–200 nm (Iravani et al. 2014). Interaction of ammonia with Ag+ leads to the formation of a stable complex Ag(NH3)2+, in this case the principal role in articulating the size of silver nanoparticle is the concentration of ammonia and nature of the reducing agents. Reducing sugars must have an aldehyde in open form to be oxidized to carboxylic acid (Dondi et al. 2012). Various polysaccharides like heparin, alginate, chitosan, and hyaluronic acid have been used to synthesize silver and gold nanoparticles(Valdez and Gómez 2016; Zhao et al. 2015) as shown in Fig. 1.4. The carbohydrate coating on the surface of nanoparticles enhances their biomedical applications (Kang et al. 2015). The large number of OH groups present in starch or polysaccharide facilitate the complexation of metal ions to the molecular matrix and the intra- and inter-­molecular hydrogen bonds provide nanoscopic solution domains for the synthesis of noble metal NPs (Tong et al. 2014). Oxygen-rich natural polysaccharide such as cellulose, consist of anhydroglucose units. The anhydroglucose unit is joined by an oxygen linkage which leads to the formation of linear molecular chain. The high oxygen-­ containing moieties such as ether and hydroxyl groups of the cellulose acts as an effective nanoreactor for synthesis of noble metal NPs (Baruah and Konwar 2015). Dextran is a water soluble polysaccharide composed of number of monomeric glucose units joined together. Amine-modified dextran hydrogels were used as supports for the synthesis of silver nanoparticle (Banerjee and Bandopadhyay 2016). Polyphenolic compound such as tannic acid in present in plants, can be used as a functional support for the stabilization of gold nanoparticle (Watcharaporn et  al. 2014). Santhanam and co-workers have shown that the size of gold nanoparticle can

14

M. Sardar and J. A. Mazumder O OH OH

O O

O

H

O HO

OH

O

OH

OH

O

OH O O

a

O HO

OH

Alginate

OH

OH

OH

O HO

O

OH

O HO

OH

CH2OH O H H OH H O OH H OH

O

NH2

a

O HO

O

NH2

a

Chitosan

CH2OH O H H OH H H

OH

O

O HO

O

Cellulose

H

O

a

Agarose

O

OH OH O

O

OH

CH2OH O H H OH H H

OH

O

O HO

OH

OH

a O HO

OH O b

OH O

Starch Dextran CH2OCH2COONa H

H

OH

OH

H

O H OH H

O H OH

H

H

H

H O CH2OCH2COONa

Carboxy methyl cellulose

O

coo+ O

HO

OH O OH

HO

O O

O AcHN

a

Hyaluronan

Fig. 1.4  Structure of different polysaccharide involved in the reduction and stabilization of metal nanoparticles

be fabricated by controlling the pH of reactants, concentration of reaction mixture and time of reaction (Santhanam 2015). Various types of gum (tragacanth, karaya) and hydrocolloids (carrageenan, alginate, guar) are used as biotemplate for the synthesis of copper oxide nanoparticles (Padil and Černík 2013). Miroslav Černík et al. used gum karaya for the synthesis of copper oxide nanoparticle, they suggested that presence of various sugars, amino acids and fatty acids present in the gum karaya could act as a reducing and capping agent for the formation of metal oxide nanoparticles (Padil and Černík 2013). Mahdavi et al. suggested that the polysaccharides present in the aqueous extract of Sargassum muticum is responsible for the reduction of ferric chloride to ferric oxide (Fe3O4) nanoparticles with cubic morphology (Azizi et al. 2013). There are several parameters that regulate the size and rate of synthesis of nanoparticles, such as the quantity of the bulk material present, the concentration of metal ions and the alkalinity of the environment. Gurunathan et al., proposed that with the increase in alkaline conditions, synthesis of nanoparticle is enhanced and maximum synthesis is observed at pH 10. This might be attributed to the formation of nucleation centres which increases at higher temperature and pH. The increase in nucleation centre increases the reduction of silver ion to silver nanoparticle(Gurunathan et al. 2009a; Gurunathan et al. 2009b).

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1.3  Conclusions With the increase in demand of metal nanoparticles, emphasis is given on the techniques/methods to synthesize these nanomaterials. The methodology should involve those methods which are economical and can be employed at industrial scale. For large scale production the demand is for environment friendly approaches. Biological methods of synthesis fulfil these demands, an array of biological materials are available in nature which can be exploited as reducing agent to reduce the metal ions. Plants, microorganism and pure biomolecules can be used for synthesis. A number of plants have been explored so far for their synthesis potential, still a great variety of plants are available in nature needs to be studied. The emphasis is to be given to search the reducing agents from agricultural wastes for maximum utilization of the available resources. The application of nanoparticles is dependent on the size, shape and the stabilizing agents, thus, if one can control the size and shape of the nanoparticles, which is dictated by the reducing and capping agents, these methods can be used as promising methods in future for large scale synthesis. Acknowledgement  The financial support provided by the Indian Council of Medical Research (ICMR), Government of India, is greatly acknowledged.

References Ahmad A, Mukherjee P, Senapati S, Mandal D, Khan MI, Kumar R, Sastry M (2003) Extracellular biosynthesis of silver nanoparticles using the fungus fusarium oxysporum. Colloids Surf B: Biointerfaces 28:313–318 Ahmad T, Wani IA, Manzoor N, Ahmed J, Asiri AM (2013) Biosynthesis, structural characterization and antimicrobial activity of gold and silver nanoparticles. Colloids Surf B: Biointerfaces 107:227–234 Ahmadi TS, Wang ZL, Green TC, Henglein A, El-Sayed MA (1996) Shape-controlled synthesis of colloidal platinum nanoparticles. Science 272:1924–1925 Albanese A, Tang PS, Chan WC (2012) The effect of nanoparticle size, shape, and surface chemistry on biological systems. Annu Rev Biomed Eng 14:1–16 Alivisatos P (2004) The use of nanocrystals in biological detection. Nat Biotechnol 22:47–52 Amendola V, Riello P, Meneghetti M (2010) Magnetic nanoparticles of iron carbide, iron oxide, [email protected] iron oxide, and metal iron synthesized by laser ablation in organic solvents. J  Phys Chem C 115:5140–5146 Amin M, Anwar F, Janjua MRSA, Iqbal MA, Rashid U (2012) Green synthesis of silver nanoparticles through reduction with Solanum xanthocarpum L. berry extract: characterization, antimicrobial and urease inhibitory activities against Helicobacter pylori. Int J Mol Sci 13:9923–9941 Amin M, Alazba A, Manzoor U (2014) A review of removal of pollutants from water/wastewater using different types of nanomaterials. Adv Mater Sci Eng 2014:1 Annamalai J, Nallamuthu T (2016) Green synthesis of silver nanoparticles: characterization and determination of antibacterial potency. Appl Nanosci 6:259–265 Ayala Valencia G, Cristina de Oliveira Vercik L, Ferrari R, Vercik A (2013) Synthesis and characterization of silver nanoparticles using water-soluble starch and its antibacterial activity on Staphylococcus aureus. Starch-Stärke 65:931–937

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Azizi S, Namvar F, Mahdavi M, Ahmad MB, Mohamad R (2013) Biosynthesis of silver nanoparticles using brown marine macroalga, Sargassum muticum aqueous extract. Materials 6:5942–5950 Azizi S, Ahmad MB, Namvar F, Mohamad R (2014) Green biosynthesis and characterization of zinc oxide nanoparticles using brown marine macroalga Sargassum muticum aqueous extract. Mater Lett 116:275–277 Badawy AME, Luxton TP, Silva RG, Scheckel KG, Suidan MT, Tolaymat TM (2010) Impact of environmental conditions (pH, ionic strength, and electrolyte type) on the surface charge and aggregation of silver nanoparticles suspensions. Environ Sci Technol 44:1260–1266 Balaji D, Basavaraja S, Deshpande R, Mahesh DB, Prabhakar B, Venkataraman A (2009) Extracellular biosynthesis of functionalized silver nanoparticles by strains of Cladosporium cladosporioides fungus. Colloids Surf B: Biointerfaces 68:88–92 Balaji S, Mandal BK, Ranjan S, Dasgupta N, Ramalingam C (2017) Nano-zirconia – evaluation of its antioxidant and anticancer activity. J Photochem Photobiol B Biol 170:125–133. https://doi. org/10.1016/j.jphotobiol.2017.04.004 Balasundaram G, Sato M, Webster TJ (2006) Using hydroxyapatite nanoparticles and decreased crystallinity to promote osteoblast adhesion similar to functionalizing with RGD. Biomaterials 27:2798–2805 Banerjee A, Bandopadhyay R (2016) Use of dextran nanoparticle: a paradigm shift in bacterial exopolysaccharide based biomedical applications. Int J Biol Macromol 87:295–301 Baruah D, Konwar D (2015) Cellulose supported copper nanoparticles as a versatile and efficient catalyst for the protodecarboxylation and oxidative decarboxylation of aromatic acids under microwave heating. Catal Commun 69:68–71 Bhattacharya D, Gupta RK (2005) Nanotechnology and potential of microorganisms. Crit Rev Biotechnol 25:199–204 Bhuvaneswari R, Xavier RJ, Arumugam M (2016) Larvicidal property of green synthesized silver nanoparticles against vector mosquitoes (Anopheles stephensi and Aedes aegypti). J King Saud Univ-Sci 28:318–323 Borase HP, Salunke BK, Salunkhe RB, Patil CD, Hallsworth JE, Kim BS, Patil SV (2014) Plant extract: a promising biomatrix for ecofriendly, controlled synthesis of silver nanoparticles. Appl Biochem Biotechnol 173:1–29 Brar SK, Verma M, Tyagi R, Surampalli R (2010) Engineered nanoparticles in wastewater and wastewater sludge–evidence and impacts. Waste Manag 30:504–520 Brust M, Walker M, Bethell D, Schiffrin DJ, Whyman R (1994) Synthesis of thiol-derivatised gold nanoparticles in a two-phase liquid–liquid system. J Chem Soc Chem Commun 0:801–802 Chandran SP, Chaudhary M, Pasricha R, Ahmad A, Sastry M (2006) Synthesis of gold nanotriangles and silver nanoparticles using Aloevera plant extract. Biotechnol Prog 22:577–583 Chauhan A et al (2011) Fungus-mediated biological synthesis of gold nanoparticles: potential in detection of liver cancer. Int J Nanomedicine 6:2305–2319 Clark JH, Macquarrie DJ (2008) Handbook of green chemistry and technology. Wiley, New York Das M, Saxena N, Dwivedi PD (2009) Emerging trends of nanoparticles application in food technology: safety paradigms. Nanotoxicology 3:10–18 Das SK, Dickinson C, Lafir F, Brougham DF, Marsili E (2012) Synthesis, characterization and catalytic activity of gold nanoparticles biosynthesized with Rhizopus oryzae protein extract. Green Chem 14:1322–1334 Dasgupta N, Ranjan S, Mishra D, Ramalingam C (2018) Thermal co-reduction engineered silver nanoparticles induce oxidative cell damage in human colon cancer cells through inhibition of reduced glutathione and induction of mitochondria-involved apoptosis. Chem Biol Interact Dasgupta N, Ranjan S, Mundekkad D, Ramalingam C, Shanker R, Kumar A (2015) Nanotechnology in agro-food: from field to plate. Food Res Int 69:381–400 Dasgupta N, Ranjan S, Rajendran B, Manickam V, Ramalingam C, Avadhani GS, Kumar A (2016) Thermal co-reduction approach to vary size of silver nanoparticle: its microbial and cellular toxicology. Environ Sci Pollut Res 23(5):4149–4163

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Chapter 2

Resistive and Capacitive Measurement of Nano-Structured Gas Sensors Partha Bhattacharyya, Debanjan Acharyya, and Koushik Dutta

Contents 2.1  I ntroduction 2.2  R  esistive Measurements 2.2.1  Planar Sensor Devices 2.2.2  Vertical Structured Sensor Devices 2.2.3  Field Effect Transistor (FET) Sensor Devices 2.2.4  Hetero/Homo Junction Sensor Devices 2.2.5  Comparison of Different Resistive Sensor Structures for Sensing Different Environmental Gases 2.3  Capacitive Measurements 2.3.1  Effective Parameters 2.3.2  Capacitive Sensor Devices 2.4  Comparison 2.5  Conclusions References

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Abstract  With the advent of industrial renaissance and world population exploration, atmospheric pollution is being elevated beyond the predicted roadmap. Development of effective and inexpensive systems for detection as well as selective quantification of environmentally hazardous species (i.e. NO2, NO, N2O, H2S, CO, NH3, CH4, CO2, volatile organic compounds etc.), for industrial and domestic air quality monitoring, are the timely demand. Presently, the most reliable gas measurement techniques are optical spectroscopy, infra-red spectroscopy and gas chromatography/spectroscopy; which are precise but non-portable, expertize is needed to operate these systems and are expensive ones also. As a cost effective alternative, solid-state gas sensors (with nanostructured material(s) as the sensing layer) have widely been researched for environmental gas detection offering promisingly high sensitivity with easy portability. However, solid-state gas sensors often suffer from the limitations like, high operating temperature, low carrier mobility and poor selec-

P. Bhattacharyya (*) · D. Acharyya · K. Dutta Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, West Bengal, India © Springer Nature Switzerland AG 2019 N. Dasgupta et al. (eds.), Environmental Nanotechnology, Environmental Chemistry for a Sustainable World 21, https://doi.org/10.1007/978-3-319-98708-8_2

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tivity. To mitigate these glitches, various types of gas sensors have been reported by tuning the properties of the sensing materials and/or by employing different transduction/measurement strategies. The major transduction/measurement types include resistive type (includes planar, metal insulator metal, junction, field effect transistor based device structure), capacitive type, surface acoustic wave (SAW) type, quartz crystal microbalance (QCM) type and electrochemical type. Among these techniques, resistive and capacitive type sensors have already been proved to be the potential candidate due to simple electronic interface, ease of use/portability and low maintenance cost. However, for the last three or four decades, most widely investigated/employed transducing technique is the resistive mode/conductometric sensing measurement, unfortunately analysis of which does not provide any information regarding the device parasitic capacitance, and hence fails to correlate the transient response of the device because the equivalent circuit of the device cannot be derived in a quantitative manner (only partial and qualitative explanation is possible). Thus, without proper understanding (quantitative) of underlying sensing mechanism/physics, no efficient sensor device can be fabricated with a predefined functionality. While several review/book chapters have so far been published on theory, synthesis and influence of different nanostructures for gas sensing applications, no work has so far been published by critically discussing the prospects and the constraints of resistive and capacitive type transduction/measurement techniques. In this book chapter, a comprehensive review on the resistive and the capacitive transducing/measurement technique is reported with a focus on the specific advantages of the later over the earlier one. Relatively less explored capacitive measurement technique (depending on the change in dielectric constant of the sensing layer due to gas exposure) allows one to comment quantitatively on the equivalent circuit parameters/elements, in reference ambient (as well as the change in the same due to gas exposure), through the ac impedance (modulus and argument) analysis. As a result, efficient material design (of the nanostructures) can be executed according to the requirement of a specific application through judicious quantitative analysis of the equivalent circuit elements. The capacitive sensing technique is privileged by another dimension of measurement (i.e. the input signal frequency) which in turn paves the path for frequency selective sensing by proper tuning of the resonant frequency. However, the capacitive measurement also faces difficulty in case of test species having lower dipole moments, yielding lower sensitivity and lower selectivity. Therefore, optimization and combination of the two measurement techniques, applied to the sensor array, creates the opportunity for the proper selective detection of a particular vapor species.

2.1  Introduction Owing to monotonically increased emissions from vehicle exhausts, industrial processes, domestic appliances etc., which are the obvious and unavoidable curses of modern industrial era, there is an unbearable outburst of the gases/volatile organic

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compounds (CO, CO2, CH4, NO2, N2O, NO, SO2, H2S, alcohols, ketones, NH3, Benzene, toluene, xylene etc.) in the environment since last two or three decades (Lee and Lee 2001; Fine et al. 2010; Wetchakun et al. 2011; Singh et al. 2011; Roy et al. 2012; Rashid et al. 2013; Thirumalairajan et al. 2014; Song et al. 2016; Wang et al. 2016; Paliwal et al. 2017). These volatile organic compounds/pollutants have adverse effects on human health with a wide contamination spectrum from skin to heart/lungs/nerves (Endo et al. 2007; Dutta et al. 2015a; Xu et al. 2016; Dai et al. 2017). Inhalation of the above-mentioned species (for long as well as short duration), with concentrations above than the respective threshold value, may results in severe health consequences (Zampolli et al. 2005; Dutta et al. 2015a; Acharyya and Bhattacharyya 2016). On the other hand, different environment-threatening species like greenhouse gases and/or toxic gases (like NO2, N2O, NO, SO2, H2S, CO, NH3, CH4, CO2 etc.) jeopardize not only on the human beings but also the nature in a wider sense (Lee and Lee 2001; Fine et al. 2010; Wetchakun et al. 2011). For these reasons, monitoring of the environment, specifically in the industrial belt, has become crucial to set appropriate alarm against the adversity caused by mixing of gases/volatile organic compounds in air. In general, a coating/thin layer of semiconducting metal oxide or conductive polymer (on some insulating substrate) acts as the sensing element which transduces gaseous entity (depending upon type/quantity) into corresponding electrical signal through different techniques (Sahay and Nath 2008; Huang 2013; Dutta et al. 2016a; Menart et al. 2017). Metal oxide based resistive sensors are very popular due to easy and tunable fabrication techniques with cost effectiveness and power effective transduction (Sahay and Nath 2008; Biaggi-Labiosa et al. 2012). However, other principles (apart from resistive ones) like capacitive, surface-acoustic-wave, micro-cantilevers, quartz-crystal-­ microbalances, optical and electro-magnetic are also well in practice (Zhou et al. 2003; Vashist and Vashist 2011; Huang 2013; Shubham et  al. 2013; Kabir et  al. 2015; Menart et al. 2017; Paliwal et al. 2017). Conventionally, the nanostructured metal oxide based gas sensors are characterized in terms of the change in conductivity of the sensing layer upon exposure to the test species (either reducing or oxidizing in nature) with respect to that in the inert ambient like air (Sahay and Nath 2008; Hazra et al. 2015). In such cases, generally, dc bias is applied and the measured parameter is the resistance (so it is called resistive measurements). Nanoarchitecturing of the metal oxide is immensely effective in improving gas sensing performance due to large effective surface coverage and surface free energies of nanostructures (Dutta et al. 2016b). Besides, tunable stoichiometric property of the nanoforms is also a crucial factor as degree of stoichiometry defines the effective adsorption sites on the metal oxide surface (Hazra et al. 2015). Further, different types of device structures (planar, Metal-Insulator-­ Semiconductor, Metal-Insulator-Metal etc.) have several interesting influences on the sensing characteristics which are mainly originated due to modification in effective transport path length of the carriers. However, the selectivity issue is the key problem with the resistive approach as the gas sensing is basically adsorption induced change in the depletion width of the metal oxide surface, which follows the similar pattern for similar species (Hazra and Basu 2006; Sahay and Nath 2008). To

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improve the selectivity, doping with catalytic metal nanoparticles, use of binary/ ternary alloy compounds as electrodes and/or temperature modulation technique has been incorporated with the resistive gas sensor devices (Lee and Reedy 1999; Park et al. 2002; Dutta et al. 2017). To some extent, these approaches became effective especially for the cases where prominent difference in dissociation energy of the test species is evident (Dutta et al. 2017). But in case of the test species, with almost same chemical properties (dissociation energy, molecular size etc.), selectivity features, when measured in conventional resistive mode, are really poor. Moreover, the equivalent circuit, derived through resistive measurement, reflects only the variable resistance between two electrodes. Unfortunately, the effects of grain boundaries as well the parasitic capacitances (associates with metal-­ semiconductor junction, grain boundaries and air-semiconductor junction), which are extremely useful, specifically for understanding the transient characteristics of the device, can’t be obtained from such equivalent circuit derived from resistive (dc) measurement. On the other hand, the capacitive transduction technique, based on ac measurements of the sensor device, depends not only on the temperature, voltage and types of nanostructures, but also on the frequency of the input signal (Dutta et al. 2016c). Judicious choice of the operating frequency lays the foundation stone of the selectivity improvement even for the cases where vapors belonging to the same group (e.g. alcohols or ketones) (Dutta et  al. 2017). In general, the effective dielectric constant varies inversely with the operating frequency (Dutta et al. 2017). However, the sensing layer also has semi-conductive path and hence the effective dielectric constant of the sensor device should be considered as a complex quantity (Barsoukov and Macdonald 2005; Dutta et al. 2016a). The variation of imaginary part of impedance (as a function of frequency) shows a peak at a particular frequency for particular ambient and the said frequency is termed as resonant frequency (Dutta et  al. 2017). The resonant frequency varies inversely as a function of change of effective dielectric constant (square root) as the test ambient changes the effective dielectric constant of the sensor device (through physical mixing in the inter-nanostructure void regions), compared to that with in air ambient (Dutta et al. 2017). The variation of the ambient (due to different test species) shifts the resonant frequency and by observing the shift in the resonant frequency, the selectivity can be achieved. Moreover, the ac analysis paves the path for deriving a more informative equivalent circuit, by analyzing the Cole-Cole plot, where the system capacitance and resistance both are considered (as distributed and as lumped parameters) (Dutta et al. 2016a, c). The plot contains the negative imaginary part of impedance versus real part of impedance of the sensor device at a particular ambient (air ambient is generally considered as the reference). The plot may contain semicircle(s) and/or line with slope of 45° (Barsoukov and Macdonald 2005). Also, non-ideality factors are reflected by depressed semicircle(s) (whose center lies in the fourth quadrant) (Barsoukov and Macdonald 2005). The number of semicircles determines the number of circuital loop, which are interconnected via series combination. On the other hand, the line indicates special type of non-ideal situation called Warburg impedance (Barsoukov and Macdonald 2005). Proper fitting and proper calculations

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enables one to presume quantitatively the different circuit elements through the correlation with the physical aspects of the sensor device (Dutta et al. 2016a, c, 2017). For example, the equivalent circuit of ZnO nanorod based Metal-Insulator-­ Semiconductor (MIM) device was derived with the help of Cole-Cole plot by Dutta et al. (2016a). Here, the detailed analysis of the Cole-Cole plot, in the frequency range 100 Hz to 200 kHz, helps to quantify the parameters of equivalent circuit with minute detailing of the non-ideality factors (deviation from the ideal lumped capacitor). The mechanism of the gas sensing in capacitive mode has been explained with the aid of the obtained equivalent circuit elements by considering the change of the circuit elements when the device was exposed to test ambient (Dutta et al. 2016a). In Fig. 2.1a, b, a typical Cole-Cole plot of the Metal-Insulator-Metal device and the correlation of the same with the corresponding equivalent circuit parameters considering the entire nanostructure assembly are presented. The simplified equivalent circuit of the same, considering the resistance due to contact is shown in Fig. 2.1c. In the capacitive measurement technique, vertical device structures (Metal-­ Insulator-­Metal and/or Metal-Insulator-Semiconductor) are preferred where the nanostructured metal oxide is sandwiched between the two conducting plates (electrodes) which define the basic capacitive structure. However, for assuming the equivalent circuit, the impedance/dielectric constant/admittance measurement (frequency response) is necessary (Dutta et al. 2016a, c, 2017). By the capacitive measurements (applying ac signal), due to gas exposure, the change of effective dielectric constant (ɛreff) of the sensing layer is considered. In this case also, like resistance measurements technique of the nanostructured metal oxide based device, the nanostructures of the metal oxides have great impact on the sensing characteristics (Dutta et al. 2016c). In this chapter, a comparative and comprehensive analysis of these two measurement techniques regarding gas sensing is discussed, considering the prospects and constraints of the individuals. In the present technological scenario, the resistive mode measurement is undoubtedly more popular than that of capacitive counterpart. While resistive measurement offers the easiest way of defining the basic change of circuit elements in steady state (resistance only), the capacitive one opens up a new arena where the transient behavior can be well explained considering the individual contribution from the various resistive and capacitive elements originated due to nano-sensing structures, nano-interfaces, grain boundaries, voids, interconnects and electrodes.

2.2  Resistive Measurements Extensive research endeavours in the field of gas sensor devices, aiming environmental pollution control, for detection of different gases/vapors have spotlighted the various unique features of different metal-oxides based semiconductors (such as SnO2 (Das and Jayaraman 2014), ZnO (Acharyya and Bhattacharyya 2016), TiO2 (Hazra et al. 2014), WO3 (Kim 2009), etc.). This conventional technique for sensor

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(a)

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Decreasing Frequency

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im

(kOhm)

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Measured fitted

0

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CPEVR Rc ∫ Contact resistance CPE ∫ Constant Phase Element (non-ideal Rn ∫ Resistance of capacitance) ZnO nanorod Rs ∫ Resistance of seed CPEVR ∫ Void and nanorod layer depletion assembly CPES ∫ Grains in seed layer

Fig. 2.1 (a) Cole-Cole plot of the Metal-Insulator-Metal device, (b) Co-relation of equivalent circuit parameters with the physical part of the nanostructured device and (c) The simplified equivalent circuit of the same

characterization relies on conductivity change which is due to the surface adsorption/desorption of target gas/vapor, which eventually results in a change in the fractional surface coverage of the acceptor/donor state of the sensing layer and can quantitatively be transformed to fractional change in device resistance defining the response of the sensor. For example, in case of n-type metal oxide based sensors, in the presence of air, oxygen, gets adsorbed as oxygen ions (O−, O2− or O2− depending upon temperature) after capturing electron(s) from the metal oxide surface (Das and Jayaraman 2014; Hazra et al. 2014; Acharyya and Bhattacharyya 2016). These ionized oxygen species can further penetrate into the oxide lattice (after occupying the

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Fig. 2.2  Schematic description of the gas-sensing mechanism and the conduction model based on n-type metal-oxide-semiconductor (MOS) and p-type MOS. (Reproduced from Zhang et al. 2017 with permission from Royal Society of Chemistry)

oxygen vacancy site) and consequently reduce the conductivity of the device (Kim 2009; Hazra et al. 2014; Acharyya and Bhattacharyya 2016). Subsequently, when some reducing vapor (or oxidizing vapor) comes in contact with the oxide surface, test molecules reacts with these adsorbed oxygen ions and consequently increases (or decreases) the electron concentrations in the conduction band of the metal oxide which in turn decreases (or increases) the resistance of the device (Das and Jayaraman 2014). In case of p-type material just the reverse phenomenon takes place. Possible gas sensing mechanism for n-type and p-type sensors (Zhang et al. 2017) are shown in Fig. 2.2.

2.2.1  Planar Sensor Devices Planar device structure is perhaps the most attractive and most frequently/commonly used device structure for gas sensing application, due to the simple design and manufacturing process, with potential cost reduction (Bhattacharyya 2014). The schematic of the planar/resistive structure is shown in Fig. 2.3. Planar sensor device consists of two elements, a gas sensitive semiconducting layer (often in

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Fig. 2.3  Schematic of the planar/resistive gas sensor device structure where two parallel metal contacts are deposited on the sensing layer and electrical connection are taken from these electrodes by using conductive paste

oxide nanoforms) and contact electrodes, lying on the same plane. These electrodes (e.g. palladium (Pd), gold (Au), platinum (Pt), titanium (Ti) or aluminium (Al)) are often inter-digitated which are embedded on the top or at the bottom of the sensing layer. These electrodes may be passive (does not contribute to gas sensing, only act as a collector of carriers generated by gas sensing phenomenon) or active (contributes actively towards improvement of the sensor device performance owing to electrodes’ catalytic capability) in nature. For measurement, dc voltage is applied to these contacts (electrodes) and the current flowing through the device is calibrated in turns of the change in resistance between these electrodes (Kim 2009; Bhattacharyya 2014). However, the reverse phenomenon, i.e. to feed current and measure voltage, is also possible. Yun et  al. (2005) concluded with experimental results that the ratio of the change of surface resistance to the total resistance is inversely proportional to the gap between these electrodes; therefore a reduced current response with increased gap between the electrodes was observed. The two main categories of planar gas sensors are thin and thick film sensors (Shimizu and Egashira 1999). In thick-film sensors, the film thickness is typically lies in the range of 10–400 μm. Thick-film sensors are usually fabricated by screen printing (Williams 1999). On the other hand, in thin-film sensors the film thickness is in the range of 2–500 nm (Shimizu and Egashira 1999). The popular synthesis process for such thin film sensing layer preparation are sputtering, evaporation, chemical vapor deposition, spray pyrolysis, chemical deposition etc. (Williams 1999). Due to highly exposed nanostructure, thin film sensing layer usually offers higher surface to volume ratio as well as high aspect ratio than that of thick film sensors, which eventually improve the sensing performance even at low temperatures. In general sensing materials for such sensors are polymer or compound semiconductor (II-VI usually) (Zhang et al. 2017). Among them, semiconducting oxides have proved device reliability of candidature owing to availability of interstitial cation or anion vacancies (which are the adsorption/desorption sites for target gas). These defects/vacancies of the semiconducting oxide can be tailored easily which in turn enhance the sensitivity of the device. For example, Hazra et al. (2014) tailored the oxygen vacancies inside the TiO2 nanotube which results in a highly sensitive methanol sensor at room temperature. Cantalini et al. (2003) presented resistive gas sensors sensitive to NO2 with a lowest detection limit of 100 ppb. The type of substrate used was a wafer of Si/Si3N4 on which the electrodes (Pt) were deposited and the nanotubes were synthesized by microwave plasma enhanced chemical vapor

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deposition (MPECVD) (Cantalini et al. 2003). Cole et al. (2004) patented micro-­ machined NaBaCO3 based planar device structure (combined with micro heater) for COx, NOx and SOx detection towards indoor and outdoor air quality monitoring. Room temperature NO2 sensor was patented by Li et al. (2017), where single wall carbon nanotube/SnO2 thin film was deposited on flexible substrate (i.e. polyethylene naphthalate, polyisoprene and polyethylene terephthalate). ZnO nanofiber based low ppm level (0.1–4 ppm) NO2 sensor was patented by Park et al. (2010a). In planar structure, surface property (surface morphology and defects which eventually determines the electronic properties like conductivity and mobility) of the sensing material is more important than that of the bulk. Therefore, functionalizing the surface further offers a possibility to improve the sensing ability. However, a common bottleneck for these sensors is poor selectivity (Acharyya et al. 2016). The modulation of energy band due to Pd and Ni modification on ZnO surface is shown in Fig. 2.4 (Acharyya et al. 2016). In this context, by using catalytic noble metals (like Pd, Pt) on the sensing layer a guided reversible interactions between the analytes and the sensing layer is possible (Wang et al. 2005; Acharyya et al. 2016). The co-ordination bonds formed during the detection process can be broken by increasing the temperature or changing the chemical environment of the sensor. Therefore, each receptor (i.e. catalytic noble metals) can be utilized to interact with a specific target vapor, hence increasing the selectivity. Wang et al. (2005) studied the performance of sensor based on collective Pd nanocluster (sputtered) modified multiple ZnO nanorods and found enhanced sensitivity towards hydrogen (at room-

Fig. 2.4  Energy band diagram for (a) Ni–ZnO and (b) Pd–ZnO junctions and effect of gas (reducing) exposure for both of the cases in qualitative manner. (Reproduced from Acharyya et al. 2016 with permission from Royal Society of Chemistry)

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temperature resistance changes approximately a factor of 5 larger than that of unmodified ZnO nanorods upon exposure to hydrogen concentrations in N2 of 10–500 ppm). Mädler et al. (2006) developed Pt doped SnO2 nanoparticles based resistive sensor on alumina substrates. The enhanced sensitivity towards CO at 350 °C for the concentration range of 1 and 50 ppm was observed. Ruiz et al. (2006) used WO3–Cr, SnO2–Pd and TiO2–Cr as sensor materials for detecting NH3. The main advantages of planar sensors are easy fabrication, simple operation, and low production cost, which means that planar sensors can be mass produced at affordable market price (Ramgir et al. 2013). Moreover, planar sensors are compact and durable (Acharyya and Bhattacharyya 2015). Planar sensors also possess advantages of compact size and simple measurement electronics but are often hampered by limitations in signal-to-noise ratio (Ramgir et al. 2013). Moreover, thin film sensors are more expensive than thick film ones, mostly because of the use of advanced and expensive technologies for depositing the sensing layer. Further, post-­ treatments such as calcinations or annealing may be required to improve the stability. To activate reactions of oxygen chemisorption and surface catalysis, high temperature (greater than 200 °C) is required. For these purposes, metal-oxide gas sensors often incorporates in built heaters, which are electrically isolated from the sensing layer, which in turn increase the device fabrication complexity and cost (Bhattacharyya 2014).

2.2.2  Vertical Structured Sensor Devices In planar/resistive sensor device, the electron transport path length between the two electrodes is higher (~few mm) which often results in sluggish response/recovery characteristics (Jiménez-Cadena et  al. 2007; Hazra et  al. 2013; Hazra and Bhattacharyya 2014). In contrast, in case of vertical structure, one electrode is usually on the top of the sensing layer and another is at the bottom of the same i.e. the sensing layer is sandwiched between the two electrodes. As the thickness of the sensing layer is very small (~nm, particularly in case of Metal-Insulator-Metal structure), the electron transport path can be reduced by several orders (tens of nm) which in turn leads to faster device performance (fast response and recovery time). Moreover, due to the shorter electron transport path length, probability of mobility reduction (of electrons) by scattering is also lower in case of Metal-Insulator-Metal devices. This is also another reason for improved response time and recovery time for vertically structured sensor devices (Jiménez-Cadena et  al. 2007; Hazra and Bhattacharyya 2014). Two types of vertical device structures can be developed based on the type of the substrate used. The semiconductor substrates, like Si which has good lattice compatibility with the (specific) metal oxide, is used to develop metal-insulator-­ semiconductor type sensor devices (Jiménez-Cadena et  al. 2007). In Metal-Insulator-Semiconductor device, one electrode (usually Pd/Au/Ti) is taken from top of the semiconducting oxide layer and other one from bottom of the Si

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layer (low work function material like Al is used for bottom Ohmic contact). Metal-­ Insulator-­Metal, on the other hand, is the configuration where semiconducting oxide is grown on the metal itself and the same is used as the bottom electrode (Jiménez-­ Cadena et  al. 2007). In Metal-Insulator-Metal configuration, another electrode is deposited on top of the sensing layer (and bottom metal acts as another electrode). Both the device configurations are shown in Fig. 2.5a, b. Metal-Insulator-Metal configuration was first studied by Fonash et  al. (1974). However, potentiality of the structure for room temperature hydrogen sensor, using ZnO as the sensing layer, was first reported by Basu et al. (2008). Bhattacharyya et al. (2010) reported a Metal-Insulator-Metal methane sensor using sol–gel grown ZnO as the sensing layer sandwiched between Pd–Ag (top) and Zn (bottom) electrodes. (Basu et al. 2008) reported a comparative study of Pd–Ag (26%)/ZnO/Zn and Rh/ZnO/Zn Metal-Insulator-Metal sensors for methane sensing using nanoporous ZnO sensing layer deposited by an electrochemical method. They found that compared to the Rh/ZnO/Zn structure, Pd–Ag/ZnO/Zn showed low response and recovery times for sensing 1% or less concentration of methane. ZnO nanorods based Metal-Insulator-Metal sensor was patented by Park et  al. (2010b) for the detection of different environmental polluting gases like H2, NO2, CH4, CO2 and ethanol. Hazra and Bhattacharyya (2014) compared ethanol sensing performance for TiO2 nanotube array based Metal-Insulator-Metal device (where Pd was used as the top contact and Ti as the bottom one) and planar device (Pd as both the contacts). They observed that fast and low temperature (75 °C) sensing performance in case of Metal-Insulator-Metal structure and high response magnitude for planar structure. Moreover, such phenomenon was explained as; for Metal-Insulator-Metal device configuration, the vertical electron transport (in Z-direction) takes place along the tube length. Therefore, device conductivity, in presence of air and reducing species, is either neck controlled (D-2Ld, where Ld varies) or fully depleted grain controlled. For such neck and grain controlled electron transport, electrons do not have to cross the inter-granular potential barrier (qVb), which eventually reduces the activation

Fig. 2.5  Schematic of vertical (sandwiched) device structure where one electrical contact is taken from the sensing layer and another from the substrate depending on which two configurations can be identified: (a) Metal-Insulator-Metal and (b) Metal-Insulator-Semiconductor device structure

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energy (Ea) and manifest itself as the low-operating temperature for ethanol sensing but limits the higher sensitivity. On the other hand, in case of planar device structure, higher amount of effective gas interaction sites in TiO2 nanotubes between the two electrodes increase the response magnitude (Hazra and Basu 2006; Hazra and Bhattacharyya 2014).

2.2.3  Field Effect Transistor (FET) Sensor Devices Field effect transistor based sensor attracted electronic sensor researchers due to CMOS compatibility and improved stability of such devices (Kanungo et al. 2011). The main advantage of field effect transistor structure is that, this structure can be fabricated using standard CMOS micro-fabrication processes and the conversion of the field effect transistor to an effective chemical sensor (chemical field effect transistor) requires only the replacement of the gate metal/position with a suitable chemically sensitive material (such as Pd) (Bur et al. 2014). The obvious choice of gate material was Pd for last few decades but due to large surface area, and high carrier mobility single wall carbon nanotube and graphene have attracted a great deal of attention in the recent years. Lundström et al. (2007) reported for the first time Pd as a gate material, demonstrating hydrogen-sensitive Pd-Si substrate based transistors. Someya et al. (2003) developed single wall carbon nanotube based field effect transistor for the sensing of alcohol vapors. Literature review revealed that use of catalytically active gate materials like platinum, iridium or palladium can improve gas-sensitivity of field effect transistor based sensor significantly (device schematic and possible sensing mechanism is depicted in Fig. 2.6) (Andersson et al. 2013). For example, on the dense homogenous layer of palladium, hydrogen molecules are adsorbed and then dissociates on the palladium layer. Subsequently, at the metal-substrate (i.e. Pd-SiO2) interface hydrogen atoms forms a polarized layer of hydroxyl groups influencing the density of mobile carriers in the channel of the transistor. Thus, drain current can be used as a potential marker for the presence as well as prediction of the concentration of H2 gas (Andersson et al. 2013). On the other hand, for the detection of non-hydroxyl group containing gases like carbon monoxide (CO), a dense gate layer is not suitable (Kanungo et al. 2011; Andersson et al. 2013). In order to detect these gases, a porous layer (like porous platinum or iridium as a gate material) is necessary for the ease of interaction with the oxide surface. Here, barrier height modulation at the junction of metal and oxide acts as a marker for the detection of the target gas. Kanungo et al. (2011) also compared the methanol sensing performance between Pt and Ir as a gate material. They found that the Pt gate based field effect transistor sensor (operating temperature 200  °C) showed higher sensitivity to methanol compared to the Ir gate based one (operating temperature 250 °C) but the stability of the sensor was found to be better for the Ir based one. Schalwig et al. (2002) suggested that the sensing mechanism of hydrogen and non-hydrogen containing gases can be explained by spill-over effects of adsorbed oxygen. Negatively charged oxygen ions on the sensor surface will

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Fig. 2.6 (a) Schematic diagram of the sensor structure and (b) the mounted SiC-FET sensor (Reproduced from Kanungo et al. 2011 with permission from Elsevier) (c) Possible reaction on sensor surface and the effect of mobile carrier concentration on channel for an enhancement type MISFET device and corresponding changes in (d) the band diagram. (Reproduced from Andersson et al. 2013 with permission from Elsevier)

influence the electric field in the underlying oxide and hence the sensor characteristics (Kanungo et al. 2011). Reducing gases like CO would react with adsorbed oxygen and thereby lower the density of oxygen on the surface (similar to the sensing mechanism of resistive type metal oxide sensors). A field effect transistor based selective volatile organic compounds (i.e. acetonitrile and tetrahydrofura, methanol) sensor was patented by Balandin (2014). Another type of field effect transistor based sensor is back gate field effect transistor which is also relatively less investigated structure (shown in Fig. 2.7). In field effect transistor structure, a semiconductor layer (sensing layer) is deposited on the SiO2/Si substrate. Source and drain electrodes are fabricated using Pt/Ti/Au metal-

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Fig. 2.7 Schematic representation of field effect transistor gas sensor where the two parallel metal-contacts on the sensing layer serves as the drain and the source terminal, and the substrate plays the role of gate (metal-contact) terminal

lization (Feng et al. 2014). By applying a gate bias, VG, with respect to the source electrode, charge carriers can electrostatically be accumulated or depleted in the semiconductor-insulator interface (Han et al. 2015). Zhang et al. also reported similar type of observation in their SnO2 nanowire based field effect transistor sensor (Zhang et al. 2004). In effect, with gas exposure, the local dipole screening effects/ partial charge transfer between the materials or analytes-induced charge trapping causes change in the ID -VG characteristics (Han et al. 2015). Besides silicon, which is commonly used as a substrate material, silicon carbide (SiC) or other wide band gap materials provide the possibility for high temperature application (upto 800 °C for SiC). It is worthy to mention, here the measurement is predominantly governed by the modulation of electrostatic charge in the sensing layer due to presence of target gas. Therefore, although the measurement parameters are ID -VG but principle behind the change of those parameters are due the change in capacitance of the sensing layer. Compared to planar/Metal-Insulator-Metal/p-n junction based sensor devices, field effect transistor sensor offers a higher dynamic operation range at room temperature by changing the substrate bias and gate bias voltage. Nakagomi et  al. (2005) demonstrated that applying application of a substrate bias to chemical field effect transistor structure increases the sensitivity of the sensor towards H2. Further, Bur et al. (2014) established that the gate bias not only influences the threshold voltage of the transistor but also modulates sensitivity of the device. They also studied that tailored (cycled/pulse) gate bias can improve the selectivity as well as stability of the device towards non polar gases such as CO, O2, NO2, NH3 (Bur et al. 2014). The main advantages of the field effect transistor based sensors are; (a) carrier concentration and mobility can be changed with different gases and at different gas concentrations (Bur et al. 2014). The estimation of the change of carrier contraction in a field effect transistor, after the gas exposure, was calculated by Bur et al. (2014) (b) Sensor signal can be amplified in field effect transistor sensor by controlling of the gate bias (c) The field effect transistor structure has higher signal-to-noise ratios than that observed in corresponding metal oxide based planar, Metal-Insulator-

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Metal and p-n junction based sensor counterparts (d) chemical field effect transistors are particularly suitable for miniaturization because the integration of the transduced signal that carries chemical information does not depend on the sensing area. Unlike chemiresistors (planar ones mainly), S/N ratio of chemical field effect transistors do not degrade significantly with miniaturization (Bur et al. 2014). The main disadvantages of the field effect transistor based sensors are; (a) often due to some of gaseous molecule remains adsorbed in the porous gate metal an irreversible memory effect/hysteresis effect is noticed in field effect transistor sensor causing severe threat to the long term reliability (Petit et  al. 2008; Bur et  al. 2014). However, Petit et al. (2008) and Paska and Haick (2012) proposed a promising gate pulse technique to measure the ID-VG hysteresis behaviour for each measurement cycle. (b) In case of back gate field effect transistor, it was reported that the electric field applied on the back gate electrode, significantly influence the sensitivity as the applied field modulates the carrier concentrations. A strong negative field is required to refresh the sensors by an electro adsorption mechanism (Petit et al. 2008; Paska and Haick 2012). A high vacuum environment can also be used expedite desorption of the gas molecules. (c) The field effect transistor based sensor requires more sophisticated system for fabrication and subsequent calibration and measurements (Petit et al. 2008).

2.2.4  Hetero/Homo Junction Sensor Devices Using junctions of two sensing layers of different conductivity/band-gap (i.e. p-n, n-n or p-p), sensor elements can be configured and designated as hetero/homo junction device whose properties can be modulated by the presence of target gas (Bur et al. 2014; Hazra and Bhattacharyya 2014). This hetero/homo junction offers; (i) increases carrier life time and (ii) introduces a built in potential at the junction. As a result, increase the sensitivity of the sensor even at low temperature. It is reported that the presence of built in potential at the junction can be effective in dissociation of the polar molecules (e.g. ethanol), which eventually improves the low temperature sensing performance of the device (Acharyya et al. 2016). A schematic of such junction based sensor device is presented in Fig. 2.8. Hetero/homo junction devices usually made of same/different semiconducting oxides (doped/undoped) or of different conductivity (n-type and p-type). Based on the type of materials, three types of p-n junctions can be formed. Formation of junction by same oxide material (same band gap) having n-type and p-type conductivity (respectively) is known as homojunction (Hazra and Basu 2005), whereas the p-n junction made of two different oxide materials (having dissimilar band gap) is known as heterojunction (Basu and Dutta 1997). However, two different oxide materials having difference in band gap but with same type of conductivity (e.g. n-n or p-p) are also known as homojunction (Hazra and Basu 2005). In all these cases, one electrode is deposited on the p-side and another taken from n-side. However,

40

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Fig. 2.8  Schematic of the hetero junction sensor device where the junction property is used for the measurement and for that reason vertical configuration is incorporated to measure the junction property

very few reports are available on such devices where the such devices have been used as gas sensors (Basu and Dutta 1997; Hazra and Basu 2005). Xue et al. (2008) fabricated 1D nano sized core/shell p-n junction based H2S sensor using n-type SnO2 nanorods coated with p-type CuO nanoparticles. Frank et al. (1998) patented Ga2O3-ZrO2 based p-n junction towards detection of H2 and CO. Xu et al. (2014) and Nie et al. (2014) also fabricated CuO–ZnO nanostructure based p-n junction sensor device for H2S sensing. (Sun et al. 2017) fabricated the PPy/WO3 p-n heterojunction by in-situ chemical oxidation polymerization and loaded on a substrate of flexible PET thin film to sense triethylamine vapor. Sun et al. (2017) reported the better sensitivity of p-n junction based sensor devices than that of bare polypyrrole (PPy) and bare WO3 based ones. According to them, the improvement of sensing performance was mainly attributed to the formation of p-n junction at an interface between inorganic WO3 and organic polypyrrole. The band bending at the junction is shown in Fig. 2.9. A highly sensitive and selective gas sensor for triethylamine (TEA) detection was fabricated by Xu et al. (2016) using SnO2 nanosheets and TiO2 nanoparticles (both having n type conductivity). Due to work function difference between SnO2 (5.58 eV) and TiO2 (4.9 eV) an electron depleted layer is created at the side of SnO2 and bends the energy band which leads to a higher resistance of sensing materials than that of the pure SnO2 sensor. Therefore, change of the resistance in air and in TEA gas is increased significantly which in turn increases the sensitivity. Possible band bending and sensing mechanism are depicted in Fig. 2.10. Gad et al. (2012) fabricated coaxial p-Si/n-ZnO nanowire based heterostructure for oxygen sensing application. A sensor structure based on homojunction of p-ZnO and n-ZnO was fabricated by Hazra and Basu (2006). Hazra and Basu (2006) demonstrated that this homojunction was sensitive to hydrogen at and above 300 °C with appreciable sensitivity but sluggish response kinetics (~minute). Basu and Dutta (1997) compared gas sensing performance of Metal-Insulator-Metal (i.e. Pd/ZnO/Zn) and heterojunction i.e. Pd/ZnO/p-Si. Basu and Dutta (1997) found that although both the devices can detect H2 at room temperature, Metal-Insulator-Metal device offered better sensing performance than that of hetero junction counterpart.

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41

Fig. 2.9 (a) The energy band structure diagram of PPy/WO3 hetero-contact, schematic model for the PPy/WO3 heterojunction based sensor (b) in air and (c) in triethylamine. (Reproduced from Sun et al. 2017 with permission from Elsevier)

2.2.5  C  omparison of Different Resistive Sensor Structures for Sensing Different Environmental Gases With the dramatic advancement of digital control systems since last few decades, the resistive sensor has been coupled with a microprocessor that can process the transduced signal right within the sensor system itself. The compatibility of resistive structure, with complementary metal-oxide semiconductor (CMOS) and micro-­ electro-­mechanical systems technology (MEMS), attracted corporate houses to invest in the development of smart gas sensor system, where complete sensor system consisting of an array of sensors on a single chip, combined with the microprocessor and wireless communication is available at an affordable price. For example, Bosch Sensortec successfully developed MEMS based environmental gas sensor (model no: BME680) for indoor quality monitoring. This sensor offered high linearity and high accuracy towards different volatile organic compounds (e.g. alcohol, aldehydes etc.). Similarly, Uniphos developed smart gas sensor for detection of low ppm level (less than 10 ppm) toxic and flammable gases (viz. CO, H2S, NH3, HCN, HCl, NO2, CH4) as well as volatile organic compounds (viz. formaldehyde, isobutylene). Figaro, one of the pioneer companies in this field, developed different resistive sensors (mostly device only) for detection of the wide range of industrial gases.

42

P. Bhattacharyya et al.

Fig. 2.10 (a and b) The energy band diagram of SnO2 nanosheet in air and in TEA; (c and d) The energy band diagram of n-type TiO2 and n-type SnO2 heterostructure. (e and f) Schematic model for the TiO2/SnO2 n–n heterojunction nanosheets sensor exposed to air and TEA gas, respectively. (Reproduced from Xu et al. 2016 with permission from Elsevier)

They developed metal-oxide-semiconductor based NH3 sensor (model no: TGS2444; for concentration range of 10–300 ppm at 100 °C), NOx sensor (model no: TGS2201; for concentration range of 0.1–10 ppm at room temperature), CO sensor (model no: TGS 3870; for concentration range of 50–1000 ppm at room temperature), CO2 sensor (model no: TGS 4160; for concentration range of 300–5000 ppm at 100 °C). Different environmental gas sensors based on optimized applications were also developed by several companies such as; Alfasense, Appliedsensor, Citytech, Draeger, Microsens, Synkera, Nemoto etc. (Liu et al. 2012). In last few decades,

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43

several collaborations between gas sensor industry and academia have also been initiated. Wales university (USA) and City technology jointly developed room temperature methane sensor. Nanomix developed carbon nanotube based gas sensor array for detection of H2, CH4, CO, and H2S gases with the help of Lawrence Berkeley National Laboratory, USA.  IIT Delhi, India along with Radix Electrosystems Pvt. Ltd. developed highly sensitive H2 sensor. Metal-­oxide semiconductor field effect transistor based environmental gas sensor has been developed by Linkoping University and Volvo for automotive application. Environmentally hazardous gases covers a wide spectrum viz. toxic gases (e.g. H2S, CO and NH3), greenhouse gases (e.g. CH4 and CO2), special gases which are both toxic and greenhouse in nature (e.g. NO2, NO) and volatile organic compound vapours (e.g. ethanol, methanol, 2-propanol, benzene, xylene and toluene). Environmentally hazardous gases can be separated into two groups based on oxidizing and reducing effects (Lee and Lee 2001; Fine et  al. 2010; Wetchakun et  al. 2011). Literature review reveals that planar sensor device structure has been mostly used for detection of these species in the resistive mode (Lee and Lee 2001). Environmental gas sensing performances of different device structures are summarized in Tables 2.1, 2.2, 2.3 and 2.4. A contemporary research review reveals that, certain engineered (structurally and electronically) sensing materials based resistive structure offers more sensitivity and selectivity towards particular species (Wetchakun et al. 2011). For example, Cu-loaded SnO2 are reported to be not only highly sensitive but also highly selective towards H2S (Niranjan et al. 2003; Kumar et al. 2013). On the contrary, Pd modified graphene layer offered room temperature selective sensing performance towards H2 (Johnson et  al. 2010). In this context, LaFeO3 nanocube offered the most promising sensing characteristics towards NO2 (Thirumalairajan et al. 2014). Bare graphene sheets offered very promising room temperature sensing performance towards CO2 (Yoon et al. 2012). The most promising sensing result towards NH3 was found in rGO thin films with very high response magnitude and very good selectivity (Hu et al. 2014). For ethanol, ZnO nanotube based resistive device offered room temperature sensing performance (Acharyya et al. 2016). On the other hand, for CH4 detection, ZnO nano-crystalline (Basu and Dutta 1997) based Metal-Insulator-Metal device were found to be effective. In case of volatile organic compounds sensors, TiO2 nanotube based resistive device offered promising room temperature sensing performance towards methanol with sluggish response time and recovery time (Hazra and Bhattacharyya 2014). On the contrary, TiO2 nanotube based Metal Insulator Metal based device offered fast sensing towards methanol (Hazra and Bhattacharyya 2014) at around 75  °C.  Field effect transistor based sensor offered room temperature sensing towards almost all the gases but the sensor response time and recovery time were very slow (Wetchakun et  al. 2011). In contrast, rGO and metal oxide nanotube based hetero-junction devices were reported to be a room temperature fast responsive gas and vapour sensor (Niranjan et al. 2003; Wetchakun et al. 2011; Kumar et al. 2013).

0.5–10 ppm 0.1–100 ppm

100 °C 220 °C

Pd – multi-layer graphene nanoribbon network Pd-ZnO nanorod

H2

300 °C

250 °C

200 °C

300 °C

CH4 Porous SnO2 nanorods

CH4 Nanocrystalline ZnO thin film

CH4 Nanostructured ZnO thin film

NH3 ZnO-Ga nanowires thin film

0.01–1% CH4 in air 0.01–1% CH4 in N2 5–1000 ppm

125–2500 ppm

200 ppm

100–1000 ppm

RT

220 °C

40–8000 ppm

500–4500 ppm

27 °C

20 °C

20–1000 ppm

22 °C

CH4 Pd-SnO2

H2

H2

Graphene decorated with Pd nanoparticle ZnO nanowire

H2

10–100 ppm 100–1200 ppm

Detection range (ppm) 5–100 ppb

Optimum temp. RT

H2S SnO2 nano wire/rGOnanocomposites 22 °C H2S Cu doped SnO2 thin film 200 °C

Gas Sensing materials H2S Cu2O–functionalized graphene sheet nanocomposite H2S rGO-SnO2 nanostructures H2S Pt doped WO3 thin film

Response magnitude (Rair − Rgas)/Rair = 36% at 100 ppb Rair/Rgas = 78 at 10 ppm (Rair − Rgas)/Rgas = 23 at 1 ppm Rair/Rgas = 33 at 50 ppm Rair/Rgas = 900 at 1000 ppm (Rair − Rgas)/Rair = 33% at 1000 ppm (Igas – Iair)/Iair = 65% at 1000 ppm (Rgas − Rair)/Rair = 55% at 40 ppm (Rgas − Rair)/Rair = 91% at 1000 ppm (Rgas − Rair)/Rair = 97.2% at 200 ppm (Rgas − Rair)/Rair = 87.5% at 2500 ppm (Rgas − Rair)/Rair = 87.3% at 1% in air (Rgas – Rair)/Rair = 97% at 1% CH4 in N2 (Rgas − Rair)/Rair = 36% at 1000 ppm 100 s

47 s

8.3 s

30 s

100 s

18 s

21 s

75 s

1 min

2 s 10 s

7 s 30 s

Response time 2 min.

100 s

75 s

17.8 s

5 min

25 s

130 s

23 s

150 s

24 min

292 s 25 min

135 s 30 s

Recovery time 3 min.

Chang et al. (2010)

Roy et al. (2012)

Haridas and Gupta (2012) Biaggi-Labiosa et al. (2012) Basu and Dutta (1997)

Rashid et al. (2013)

Johnson et al. (2010)

Das et al. (2010)

Chung et al. (2012)

Song et al. (2016) Niranjan et al. (2003)

Yin et al. (2014) Tao and Tsai (2002)

Refs. Zhou et al. (2013)

Table 2.1  Sensing performance of metal oxide based ‘Planar’ devices using resistive mode measurements towards different toxic gases and volatile organic compounds

44 P. Bhattacharyya et al.

RT

300 °C

200 °C 250 °C 210 °C 25 °C

NH3 rGO

NH3 ZnO-in thin film

NO2 NO2 NO2 NO2

10–200 ppm

Hierarchical Pd/SnO2 nanostructures 100 °C

CO

Response magnitude (Rgas−Rair)/Rair = 300% at 1000 ppm (Rgas–Rair)/Rair = 22.5% at 50 ppm (Rgas − Rair)/Rair = 80% at 10 ppm Rgas/Rair = 180 at 5 ppm Rgas/Rair = 50 at 10 ppm Rgas/Rair = 13.5 at 2.1 ppm (Rgas − Rair)/Rair = 160% at 5 ppm Rgas/Rair = 286.8 at 100 ppm (Ggas − Gair)/Gair = 26% at 100 ppm (Rgas − Rair)/Rair = 10% at 750 ppm Rair/Rgas = 22.5 at 100 ppm Rair/Rgas = 7 at 200 ppm 60 s

2 s

4 min

8 s

9 s

43 s 100 s 8 min 11 s

40 s

150 s

Response time 100 s

150 s

160 s

4 min

10 s

25 s

18 s 150 s 9 min 15 s

100 s

200 s

Recovery time 450 s

Wang et al. (2016)

Cuong et al. (2015)

Hafiz et al. (2014)

Yoon et al. (2012)

Sberveglieri et al. (1995) Choi et al. (2014) Han et al. (2013) Lee et al. (2011) Thirumalairajan et al. (2014) Zhang et al. (2009)

Hu et al. (2014)

Refs. Chou et al. (2015)

Rair resistance in air, Rgas resistance in gas/vapor, Iair current in air, Igas current in gas/vapor, Gair conductance in air, Ggas conductance in gas/vapor, RT room temperature

10–100 ppm

1–1500 ppm

300 °C

23 °C

CO2 rGO

10–100 ppm

α-Fe2O3 nanoparticles

22 °C

CO2 Graphene sheet

10–100 ppm

0.5–5 ppm 0.5–10 ppm 0.03–2.1 ppm 1–5 ppm

1–10 ppm

1 ppb–50 ppm

Detection range (ppm) 5–1000 ppm

CO

240 °C

NO2 ZnO hollow spheres

SnO2 nanowire ZnO nanorods ZnO nano fibres LaFeO3 nanocube thin films

Optimum temp. 250 °C

Gas Sensing materials NH3 NiO thin film

2  Resistive and Capacitive Measurement of Nano-Structured Gas Sensors 45

100 ppm

75 °C

RT

RT

SWCT-polypyrrole

Si nanowire-TiO2- rGO

NO2

NH3

Response magnitude (Iair − Igas)/Iair = 0.018 at 200 ppt (Rair − Rgas)/Rair = 42% at 100 ppm (Rgas − Rair)/Rair = 82% at 3000 ppm (Rair − Rgas)/Rair = 43% at 50 ppm N.A.

22 s

30 s

Response time 88 s

N.A.

54 s

60 s

Recovery time N.A.

Rair resistance in air, Rgas resistance in gas/vapor, Iair current in air, Igas current in gas/vapor, N.A. not available, RT room temperature

50 ppm

3000 ppm

Detection range 50–200 ppt

Optimum temp. RT

Gas NH3

Sensing materials Polyaniline nanograin enchased TiO2 fibres Ethanol TiO2 nanotubes

Guo et al. (2017)

Hazra and Bhattacharyya (2014) An et al. (2004)

Refs. Gong et al. (2010)

Table 2.2  Sensing performance of metal oxide based ‘vertical’ devices using resistive mode measurements towards different toxic gases and volatile organic compounds

46 P. Bhattacharyya et al.

300 °C

RT

Pentacene

NH3

RT

CNT on Si/SiO2 substrate

SnO2 thin film

Ethnaol

NH3

Response magnitude Rair/Rgas = 105 at 5 ppm

10–100 ppm

(Iair − Igas)/Iair = 25% at 100 ppm

100–1000 ppm |Vair − Vgas|/Vair = 4.5 at 20 ppm 10–100 ppm (Iair − Igas)/Iair = 14.28% at 100 ppm 0.3–5% |Vair − Vgas|/Vair = 1250 at 1.5% 0.3% (Iair − Igas)/Iair = 70% at 0.3%

Detection Range 0.2–5 ppm

4 min

5 min

N.A.

N.A.

N.A. N.A.

1500 s

N.A.

Recovery time 50 s

188 s

N.A.

Response time 130 s

Kanungo et al. (2011) Someya et al. (2003) Bur et al. (2014)

Yu et al. (2012)

Han et al. (2015)

Refs. Singh et al. (2011)

Rair resistance in air, Rgas resistance in gas/vapor, Iair current in air, Igas current in gas/vapor, Vair voltage in air, Vgas voltage in gas/vapor, N.A. not available, RT room temperature

RT

200 °C

Methanol Pt gate SiC

CO

Sensing materials Au functionlised In2O3 nanowires Pt gate SiC

Gas CO

Optimum Temp. RT

Table 2.3  Sensing performance of metal oxide based ‘Field Effect Transistor’ devices using resistive mode measurements towards different toxic gases and volatile organic compounds

2  Resistive and Capacitive Measurement of Nano-Structured Gas Sensors 47

1–300 ppm 2.5–25 ppm 3000– 10,000 ppm 1–1000 ppb

50 °C

56 °C 150 °C

200 °C

Detection range 20–1200 ppm 10–500 ppm

Optimum temp. 150 °C 85 °C

Rair resistance in air, Rgas resistance in gas/vapor, N.A. not available

NO2 WO3 -SnO2 nanoparticle

Gas Sensing materials H2S CuO–SnO2 thin film H2S CuO–SnO2nano composite H2S CuO-modified SnO2 sensor H2S CeO2 –SnO2 thin film H2 ZnO–SnO2 composite

Response time 15 s 2 min

Rair/Rgas = 4 at 5 ppm (Rair − Rgas)/Rair = 90% at 10000 ppm Rgas/Rair = 370 at 200 ppb

N.A.

40 s 60 s

(Rair – Rgas)/Rgas = 8067 at 1 ppm 15 s

Response magnitude Rair/Rgas = 7400 at 20 ppm Rair/Rgas = 2.15 × 106 at 10 ppm

N.A.

20 s 75 s

225 s

Recovery time 118 s 15 s

Kida et al. (2014)

Fang et al. (2000) Mondal et al. (2014)

Refs. Chowdhuri et al. (2003) Esfandyarpour et al. (2004) Patil and Patil (2006)

Table 2.4  Sensing performance of metal oxide based ‘p-n junction’ devices using resistive mode measurements towards different toxic gases and volatile organic compounds

48 P. Bhattacharyya et al.

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49

2.3  Capacitive Measurements Capacitive measurement technique involves ac input (where the frequency is an additional input parameter) and measures the change of capacitance due to change in effective dielectric constant when the device is exposed to the test ambient (Shubham et al. 2013; Dutta et al. 2016a, c). In contrast to resistive measurements, in capacitive measurements, frequency response is responsible to identify the resonant frequency which depends inversely on the square root of effective dielectric constant (fr∝1/√ɛreff) (Dutta et al. 2017). However, the resonant frequency concept depends on the imaginary counterpart of the measured quantity (impedance or complex dielectric) (Dutta et al. 2017). The resonant frequency may be considered as the selectivity tuning parameter because a particular gas of particular concentration yields the highest response at a particular frequency in comparison to the closest interfering species of the test species (Dutta et al. 2017). The change in effective dielectric constant occurs only by physical mixing of the vapors (and air) in the inter nanostructure void regions and negligible part is influenced by chemisorption of test species on the oxide surface (Dutta et al. 2016a, c). Hence, the measurement in the capacitive mode often results in faster sensing (than the resistive method) (Dutta et al. 2016a, c). Different parameters effectively tune the performance of a sensor in capacitive measurements and the effect of these parameters on sensing performance in capacitive measurement mode are discussed in the next section along with the, different device structures used in capacitive sensor.

2.3.1  Effective Parameters It is well known that, capacitance of any device depends on effective dielectric constant (ɛreff), electrode area (A) and the thickness of the dielectric layer (d) and expressed by Eq. 2.1 (Terzic et al. 2012).



C=

e 0e reff A d



(2.1)

Where, ɛ0 is the dielectric constant of the vacuum (also considered for air). So the device capacitance depends on the ‘Top electrode’ dimension (related to A), metal oxide thickness (related to d) and the nanostructure assembly including voids region (related to ɛreff) for the gas sensing devices. It is interesting that, the device capacitance in the native ambient (generally in air) not only depends on the type of the nanostructure but also on the density of nanostructures and spacing/voids among them. This has already been experimentally reported by Dutta et  al. (2016c). However, due to test gas exposure, physical mixing of the gas in the void regions results in the change of effective dielectric constant (ɛreff). As the relative permittivity of the test species is higher than that in air, the device capacitance is increased

50

P. Bhattacharyya et al.

due to such gas exposure (Dutta et  al. 2016a). The effective dielectric constant changes according to Lichtenecker rule (Eq. 2.2) if two different gas (with εrh and εrl) is being mixed in the volumetric fraction of Vh and Vl (where, Vh + Vl = 1) (Wu et al. 2003; Dutta et al. 2016a).

k e reff = Vhe rhk + Vl e rlk



(2.2)

Where, k = ±1, depending on the combination of gas mixing (Wu et al. 2003; Dutta et al. 2016a). Dutta et al. (2016a) described the different possible combinations for mixing of air and ethanol. On the other hand, due to chemisorption on the metal oxide surface, the dielectric constant of the solid part should be changed which in turn should effect the overall capacitance value of the device (Zhou et  al. 2011; Dutta et al. 2016c). But, the permittivity change due to chemisorption of test gas species is considered to be negligible as the total dimension of the void region is much greater than the portions of solid nanostructure assembly where the chemisorption takes place (which is also demonstrated with the example of Pd/ZnO nanorod/Si device) (Dutta et al. 2016a). Apart from the synthesized sensing layer (nanostructures), the capacitive mode gas sensing also depends on the dipole moment of the test species (Zhou et al. 2011; Shubham et al. 2013; Dutta et al. 2016a, c, 2017). The resultant capacitance value, at a particular frequency, is governed by the probability of the orientation of the dielectric dipoles in response to the applied electric field (Zhou et al. 2011; Dutta et al. 2016a). Hence, the capacitive devices are more sensitive towards the gases having higher dipole moments, whereas low dipole moment of the test species yields relatively lower sensitivity (Dutta et al. 2016a, c). For this reason, the capacitive devices, in most of the cases, are prone to be affected by the presence of humidity (as the water has a dipole moment of 1.85 D) (Ishihara and Matsubara 1998; Shubham et al. 2013). Following the same argument, sensing of ethanol results in higher sensitivity (~7500%) due to higher dipole moment (~1.3 D) of alcohol compared to that of the aromatic hydrocarbons (like Benzene, Toluene and Xylene having dipole moment of 0, 0.3 and 0.64 D, respectively) (Dutta et al. 2016a, c). Like the resistive measurement, the capacitive measurement is also dependent on ambient temperature and input signal magnitude (voltage or current). However, frequency also plays a pivotal role in resultant response magnitude in capacitive cases (Terzic et al. 2012; Shubham et al. 2013; Dutta et al. 2016c). Effects of all the above mentioned parameters, on capacitance of the device were observed experimentally and it was stated that the voltage amplitude has less/negligible effect whereas the temperature and frequency plays a vital role when the device capacitance is measured (Dutta et  al. 2016c). The device capacitance decreases as the frequency increases due to decrement of efficiency in orientation process of the dipoles present in gas phase (Dutta et al. 2016c). On the other hand, with increase in temperature the device capacitance increases in a non-linear fashion as the increased ionic displacement (for increase in temperature) within the solid part of dielectric (metal oxide) helps better electronic polarizations (Dutta et al. 2016c).

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Frequency analysis of the device impedance enables one to find the equivalent circuit of the device and also provides the information about the resonant frequency (Barsoukov and Macdonald 2005; Dutta et al. 2016a; Dutta et al. 2016c). To derive and quantify the equivalent circuit parameters, correlating the nanostructure and void assembly, Cole-Cole plot (imaginary part of impedance vs. real part of impedance) is an essential analysis tool (Dutta et  al. 2016a). Impedance spectroscopy technique directly plots the same but as the technique, generally involves liquid interface (use of electrolyte like NaOH, KOH etc.), which jeopardize the structure and voids of the sensing layer (required for sensing), is not preferred for gas sensor characterization (Babu et al. 2010; Li et al. 2010; Zhao et al. 2016). On the other hand, for gas sensing purpose, equivalent circuit assumption should be made through vapor phase calculation. Hence, by measuring the magnitude (|Z|) and argument (θ) of the impedance with respect to frequency (usually measured by LCR meter), the frequency response (Bode plot) of the device can be achieved (Ponce et al. 2009; Dutta et al. 2016a; Dutta et al. 2016c). The imaginary part of the impedance is calculated as |Z| sinθ, while |Z| cosθ represents the real part of the same. By plotting negative imaginary part versus real part of the impedance, Cole-Cole plot can be obtained. Observing the number of semicircles (and nature of the Cole-Cole plot) the number of loops in the equivalent circuit and the corresponding circuit parameters can be derived (Barsoukov and Macdonald 2005). The elements of equivalent circuit elements can be quantitatively derived by analyzing such ­Cole-­Cole plot with an aim to correlate the physical phenomenon taking place in the nanostructure during gas interactions. Moreover, resonant frequency plays a crucial role in the capacitive measurements for gas sensing. The resonant frequency is defined as the frequency where the minimum imaginary impedance (or the minimum imaginary part of dielectric constant) occurs (Suman et al. 2006; Alaeddin and Poopalan 2010; Dutta et al. 2017). In Fig. 2.11, the resonant frequency (fr) is indicated schematically. As the fr depends inversely on the square root of effective dielectric constant, the resonant frequency is different for different gases (of same concentration and/or different concentration

Z' -Z"

Impedance (a.u.)

Fig. 2.11  An example of variation of impedance (real part: Z′ and negative of imaginary part: -Z″) with respect to frequency where the maxima of the –Z″ determines the corresponding resonant frequency (fr)

fr

Frequency (a.u.)

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for a particular gas). Hence, the resonant frequency tuning may be a potential way for selective detection of test species. In summary, the capacitive mode sensing depends on the parameters like physical mixing of gas, ɛreff, nature and geometrical distribution of nanostructure and void assembly, thickness of the sensing layer, density of nanostructures, dipole orientation capacity of the test species and fr. By tuning any one or the combination of these parameters, the sensing performance of the device can reliably tuned for use in practical fields. Moreover, understandings of the equivalent circuit help one to explain the underlying physics of the gas interaction dynamics in the nanostructure assembly. Further, resonant frequency tuning, preferably incorporating the sensor array, may be a potential solution for addressing the conventional problem (in resistive measurement) of selectivity issue.

2.3.2  Capacitive Sensor Devices The capacitive structure, in general, is made of a dielectric layer sandwiched between two conductive electrodes. In case of gas sensor device, the semiconducting oxide layer is considered as a leaky dielectric layer, which has combination of conductive and insulating path (resistance in parallel with capacitance). Thus, it is expected that, for capacitive measurements, vertical device structure is preferable. Usually, Metal-Insulator-Metal and Metal-Insulator-Semiconductor device is practiced. The device configurations have already been discussed in resistive measurement (Sect.2.2); the same device structures are employed to carry out the capacitive measurements also. However, just a handful of research endeavors have been noticed, particularly since last decade, on the metal oxide based capacitive gas sensor devices (Al-Hardan et al. 2010; Shubham et al. 2013; Dutta et al. 2016a, c, 2017). An extensive study on hydrogen sensing performance of Pd/TiO2/Si based device, incorporating Capacitance-Voltage measurements, in absence and in presence of H2, was reported by Shubham et al. (2013). In this case, room temperature sensing towards 2 ppm hydrogen was reported and the device was capable of sensing 0.2 ppm hydrogen at an optimum temperature of 100  °C.  With similar type of metal-insulator-­ semiconductor device (ZnO nanorods as active insulator), a higher response magnitude (~7500%) towards 700 ppm ethanol, was reported by Dutta et al. (2016a). In continuation, Ti/TiO2 nanotube/Ti based sensor device was also tested to find the response towards benzene (Dutta et al. 2016c). In the above cases, the impedance measurement (by applying an ac signal as the input) allows one to infer about the equivalent circuit precisely and quantitatively (Dutta et al. 2016a, c). It can be envisaged that, the dipole moment of the test species (in vapor phase) is immensely important to ensure higher response magnitude (Dutta et al. 2016a, c). Apart from the vertical device structures, there are reports on the planar configuration also, where the impedance profiles (in presence and in absence of test vapor) was employed for measurement purpose (Al-Hardan et  al. 2010; Dutta et  al. 2017).

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Al-Hardan et al. (2010), reported the hydrogen sensing performance (by measuring the device impedance) of the ZnO thin film, which was sputtered on Pt electrodes. The equivalent circuit in air ambient was quantitatively analyzed for different temperature and the corresponding change of these parameters at different hydrogen concentrations (at 400 °C) was also reported (Al-Hardan et al. 2010). The sensitivity was defined in terms of ratio of the impedance in presence of H2 to that in air and no direct capacitance measurement was presented (Al-Hardan et al. 2010). On the contrary, Dutta et al. (2017), represented the variation of imaginary part of impedance with respect to the frequency (using Pd/TiO2 nanorods/Ti device) which eventually leads to the improved selectivity depending upon the resonant frequency, even among the species belonging to the same family (methanol, ethanol and 2-­propanol) having almost same dissociation energy. However, the frequency based selectivity technique can either be applied to indicate the concentration of a known gas or to identify the vapor when the concentration is constant (Dutta et al. 2017). In most of the cases, interdigitated electrodes were used on which the sensing element (metal oxide) was deposited (Imai et  al. 1983; Meyer and Haeusler 1999; Moon et al. 2010). Meyer and Haeusler (1999) patented highly selective CO2 sensor based on capacitance measurement of the device with interdigitated electrodes. However, in most of these cases, the effect of nanostructures was not described correlating the capacitive detection of test species (Nishino and Yoshida 1980; Imai et al. 1983; Li and Tsai 1999; Meyer and Haeusler 1999; Moon et al. 2010). Very recent research suggests that the capacitive measurement is dominantly dependable on physical properties (spacing, density vertical alignment etc.) of the sensing layer (Dutta et al. 2016a, c, 2017). The recent technological scenario of the capacitive/ac mode sensing of metal oxide based device is duly summarized in Table 2.5. As the change in the effective dielectric constant is the main concern of the capacitive sensing, similar nanostructure should results in similar sensing perforTable 2.5  Sensing performance of metal oxide based ‘vertical’ devices in the capacitive mode measurements towards different toxic gases and volatile organic compounds Gas H2

H2

Sensing materials ZnO nano-­ particle Pd/TiO2/ Si

Optimum Detection temp. range 400 °C 200– 700 ppm 27 °C

275 °C Ethanol Pd/ZnO nanorod/ Si 150 °C Benzene Ti/TiO2 nanotube/ Ti

Response magnitude Za/Zg = 8 at 1000 ppm

0.1–2 ppm (Cg − Ca)/ Ca = 40% at 2 ppm 10– (Cg − Ca)/ 700 ppm Ca = 7500% at 700 ppm 10– (Cg − Ca)/ 200 ppm Ca = 14% at 200 ppm

Response Recovery time time Refs. N.A. N.A. Al-Hardan et al. (2010) ~60 s ~5 min Shubham et al. (2013) 210 s 69 s Dutta et al. (2016a) 69 s

27 s

Dutta et al. (2016c)

Za impedance in air, Zg impedance in gas/vapor, Ca capacitance in air, Zg capacitance in gas/vapor, N.A. not available

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mance for a particular gas; hence irrespective of type of conductivity (p or n) the change should be same if proper nanoarchitecturing is adopted. In case of p-n junction type sensor the variation of the capacitance, due to gas exposure is also subjected to the effect of physical mixing rather than the depletion width variation (considering the negligible effect of chemisorption with respect to physical mixing in voids (Dutta et al. 2016a, c).

2.4  Comparison In the above discussions, a comprehensive understanding of the capacitive and the resistive measurements techniques, employed for gas sensor device characterization, is presented correlating the present technological scenario. In these devices, the conventional resistive sensor measurements offers some advantages like easy fabrication and easy transducing technique (simple circuitry); but the common bottlenecks (viz. selectivity, equivalent circuit, response/recovery time etc.) of this mode can be circumvented by adopting the capacitive mode measurements (Dutta et al. 2015a, b, 2017). In case of metal oxide based gas sensor devices, similar fabrication platform is used for the development of nanostructured device for both the measurement techniques. However, capacitive measurements are restricted within vertical device configurations (Metal-Insulator-Metal and Metal-Insulator-Semiconductor) whereas the change in resistivity (resistive measurement) can be measured using planar, Metal-Insulator-Metal, Metal-Insulator-Semiconductor, p-n junction or field effect transistor structures. Resistive measurements involve dc bias for operation whereas capacitive mode involves the ac bias where apart from the voltage amplitude, frequency is also used as a tuning parameter. Further, physical mixing of the test species takes place before the chemisorption, which enables the capacitive mode sensing faster than resistive counterpart (Dutta et  al. 2016a). Also, for hand-held sensor system, battery (dc input) operated devices are preferred ones where the resistive measurement is more suitable. On the contrary, for capacitive sensor, ac input is mandatory and therefore the sensor device can directly be used employing line (power) supply. The input signal frequency for the capacitive device helps one to tune the gas sensitivity leading towards selective detection at a particular resonant frequency which cannot be achieved in the resistive mode. Also, differentiating the gases belonging to the same group (like different alcohols) is possible as different dielectric constant yields different resonant frequency and the device senses that particular gas with the highest response magnitude at that corresponding resonant frequency. Thus, capacitive mode measurements is beneficial from selectivity point of view; though some sensor array based signal conditioning system (more complex ones compared to its resistive counterparts) is required to detect the unknown gas of unknown concentration.

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On the other hand, difference between reducing gas (CH4, H2, VOCs etc.) and oxidizing gas (NO2, O3 etc.) categorization is possible for resistive measurements as the sensing oxide layer produces opposite kind of resistance change, whereas the capacitance will increase for both (reducing and oxidizing) the cases. Further, in capacitive measurements, gases with low dipole moment (like benzene) are sensed with low response magnitude which has comparable response in the resistive measurements (Dutta et al. 2016c).

2.5  Conclusions In this book chapter, a comparative and comprehensive discussion about the resistive measurement and capacitive measurement technique, concerning the characterization of the nanostructured metal oxide based gas sensor devices, is presented focusing the critical aspects like different device structures employed and effective parameters relating and explaining the possible mechanisms of sensing. Detailed understanding of underlying mechanism of these two transducing/measurement techniques broadens the chance for quantitative analysis of the corresponding equivalent circuit. Conventional resistive sensor devices rely on the chemisorption process whereas capacitive measurements involve physical mixing of the vapors/ gases within intra and inter spaces (voids) of the nanostructures. For transducing the sensor signal, dc bias should be applied in case of resistive one whereas the ac bias is used for capacitive counterpart. Application of ac signal also allows one to quantify the equivalent circuit elements via the impedance analysis (Cole-Cole plot). Further, from the unique behavior of the imaginary impedance with varying frequency, the resonant frequency can be inferred based on which, the selectivity of the sensor device can be improved depending on the difference in dielectric constant of the test species. But the capacitive measurement does not reveal the nature of the test species (reducing or oxidizing) whereas a resistive measurement perfectly indicates this diversity. In addition, capacitive mode sensing can be employed for vertical device structures only (Metal-Insulator-Metal and Metal-Insulator-Semiconductor) where the sensing layer is sandwiched between the electrodes. In contrast, resistive measurement can be extended to planar, Metal-Insulator-Metal, Metal-Insulator-­ Semiconductor, p-n junction and field effect transistor structures. Also, ac input of lower voltage and higher frequency (requirement of frequency multiplier) in case of capacitive measurements increases the cost of the signal conditioning system. But the novelty of the capacitive measurements lies in the selective detection (with employment of resonant frequency) along with the possibility to acquire a more vivid insight regarding the equivalent circuit elements which in turn reflects the various physiochemical phenomenon, useful for explaining the mechanistic framework of gas sensing employing nanostructure.

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Chapter 3

Nanotechnology Based Delivery of Nutraceuticals Dhanashree Hemant Surve, Atish Tulsiram Paul, and Anil B. Jindal

Contents 3.1  I ntroduction 3.2  N  anotechnology- Old Nutraceutical in New Form 3.2.1  Submicron Emulsion 3.2.2  Lipid Nanoparticles 3.2.3  Liposomes 3.2.4  Dendrimer 3.2.5  Polymeric Micelles 3.2.6  Polymeric Nanoparticles 3.2.7  Inorganic Nanoparticles 3.2.8  Nanocomposite 3.3  Conclusion References

 65  66  68  76  81  83  85  88  93  96  98  99

Abstract  With the growing worldwide population, there is increase in prevalence of various diseases like cancer (0.338%), hyperlipidemia (39%), hypertension (44.5%), infectious disease (17 million/ annum), diabetes (18.4%), obesity (25.9%) and osteoporosis (200 million/ annum). Nutraceutical seek their application in prophylaxis and cure of these diseases for instance, coenzyme Q-10, polyphenols, isoflavone, flavones, carotenoid, isocyanidine for treatment of cancer and other life threatening diseases, elements like zinc as prophylactic treatment to maintain metabolism, calcium and magnesium to maintain bone health, Selenium as an anti-­ oxidant, vitamin D for osteoporosis, vitamin E & C as anti-oxidant for prevention of cancer and vitamin B-complex for proper functioning of nervous system. However, nutraceuticals have certain limitations including poor solubility, absorption and bioavailability, chemical instability and higher first pass metabolism which can be overcome by nano-delivery of nutraceutical.

D. H. Surve · A. T. Paul · A. B. Jindal (*) Department of Pharmacy, Birla Institute of Technology & Science, Pilani, Pilani Campus, Jhunjhunu, Rajasthan, India e-mail: [email protected]

© Springer Nature Switzerland AG 2019 N. Dasgupta et al. (eds.), Environmental Nanotechnology, Environmental Chemistry for a Sustainable World 21, https://doi.org/10.1007/978-3-319-98708-8_3

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Various formulation technologies like submicron emulsion, lipid nanoparticles, liposomes, polymeric micelles, polymeric nanoparticles, inorganic nanoparticles and nanocomposites are currently under thorough research in order to eliminate the above said limitations of nutraceuticals. Submicron emulsion enhances solubility, half-life, bioavailability of bioactives along with reduced gastric irritation. Self-­ nanoemulsifying or self-microemulsifying drug delivery system has a property to reduce poor palatability, hydrolysis of bioactive and large amount of water and lymphatic uptake of bioactive causing increased bioavailability. Lipid nanoparticles due to their smaller size and lipophilicity could provide advantages of enhanced permeation of bioactives through blood brain barrier and improved bioavailability. Liposomal delivery system is useful to prevent the sensitive bioactives from oxidation, light and moisture. It can serve as unique system to deliver both hydrophilic and lipophilic bioactive. Highly potent anti-cancer bioactive possess reduced bioavailability and inability to deliver at specific site. Dendrimer and inorganic nanoparticles can be utilized for targeting such bioactive and sustained release. Polymeric micelles are stable compared to submicron emulsion and are useful for triggered release of bioactive and protection of bioactive from enzymatic degradation. Polymeric nanoparticles derived from natural source like zein, β-lactoglobulin, β-casein and nanodiamond can be used to deliver bioactives in controlled manner, protection from harsh acidic environment and reduced toxicity.

Abbreviations ANOVA CMC CO Q-10 EGFR GIT GMS HA HAuCl4 LNCaP LNP NLC NLC-H1299 o/w PDI PSMA PUFA SEDDS SMEDDS SNEDDS

Analysis of variance Critical micelle concentration Coenzyme Q-10 Endothelial growth factor receptor Gastrointestinal tract Glyceryl monostearate Hyaluronic acid Gold hydrogen chloride Human prostate cancer cell lines Lipid nanoparticles Nanostructured lipid carrier Non-small cell lung cancer Oil in water Polydispersity index Prostate specific monoclonal antibody Polyunsaturated fatty acid Self-emulsifying drug delivery system Self-microemulsifying drug delivery system Self-nanoemulsifying drug delivery system

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s-SNEDDS Solid-SNEDDS SiRNA Small interfering RNA TAMRA-TAT 5-(and 6)-carboxytetramethylrhodamine labelled HIV transactivator protein TAT

3.1  Introduction Nutraceutical find its application for treatment and cure of wide arena of disease or disorders. For instance, polyphenols, carotenoids, isoflavones, flavones, anthocyanidins, resveratrol and coenzyme Q-10 used for cancer, hyperlipidemia, hypertension, ageing, infectious disease, inflammation, diabetes, obesity, osteoporosis etc., vitamin E and C as anti-oxidant, elements like zinc for proper metabolism, calcium and magnesium for osteoporosis, selenium as anti-oxidant and chromium for enhancing glucose tolerance, traditional chinese herbs for prevention of thromboembolism and extract of danshen for healing of muscles and tissues (Espín et al. 2007; Acosta 2009; Tehran 2015; Mcclements et al. 2009; Kuppusamy et al. 2015; Devi et  al. 2010; Meghani et  al. 2018; Walia et  al. 2017; Dasgupta et  al. 2017; Dasgupta et al. 2018). However, these nutraceuticals possess limitations like poor absorption, low solubility, low bioavailability, chemical instability and first pass metabolism (Acosta 2009; Espín et al. 2007; Mozafari et al. 2006). Nano-intervention could be a vital strategy to combat these limitations. Nutraceutical market, due to its unique characteristics like safety, efficacy, cost-­ efficiency and patient compliance is blooming. The global nutraceutical market was valued at around United States Dollars (USD) 205.39 billion in 2016 and is expected to reach around USD 294.79 billion by 2022, at a compound annual growth rate (CAGR) of 6.3% from 2017 to 2022 (Mordointelligence.com 2017). Some examples of nutraceutical which possess great market value are Arthriblend SR (marketed by Sabina corporation) which is a blend of glucosamine sulphate, boswelin, curcumin C3 Complex®, bioperine® for healthy joint and connective tissues in the body and Res-Q by Ashoka Life Science Limited as fast dissolving tablet for respiratory distress (Devi et al. 2010). DuPont, ADM, BASF, DSM and Cargill are the leading companies in nutraceutical (Mordointelligence.com 2017). The lucrative market statistics has engrossed research scientists into development of novel nutraceutical formulations overcoming their present disadvantages (Das et  al. 2012). Applying ‘nanotechnology’ based principle for formulation of nutraceuticals can overcome earlier mentioned drawbacks of nutraceuticals. For example, nanoencapsulation of nutraceutical improves bioavailability along with controlled release ­giving site specific nutraceutical delivery (Acosta 2009). CurQlife®-a patented product of Laila Pharmaceutical ltd. is the best known example of application of nanotechnology to overcome poor solubility and bioavailability of nutraceutical. It is a surfactant solution system with particle size between 100 nm and 250 nm which elicits sustained release of bioactive curcumin upto 58  h. The bioavailability of CurQlife® was found to be 20 fold higher in pre-clinical studies and 48 fold higher in clinical studies compared to unformulated curcumin (Sripathy et al. 2015).

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Although the global market statistics for nutraceutical are alluring, tremendous amount of challenges are yet to be dealt with to develop an effective nutraceutical product. Poor bioavailability of bioactive is the major challenge while developing a nutraceutical formulation. Most of the bioactives exhibit poor aqueous solubility leading to decrease in bioavailability which results to increase in dose of nutraceutical (Aklakur et al. 2015; Ranjan et al. 2014; Dasgupta et al. 2015; Jain et al. 2017). Degradation of bioactives has been considered another vital challenge in successful delivery of nutraceutical. For instance, polyphenols exhibit poor efficacy when incorporated into tablets and capsules due to incomplete absorption and enzymatic degradation (Acosta 2009). Furthermore, free form of antioxidants when delivered through conventional formulations leads to its degradation causing decreased efficacy (Mozafari et al. 2006). Probiotics also revealed poor oral bioavailability due to degradation of bacteria present in the formulation by gastric pH and bile salts (Keservani et al. 2016). Furthermore, some lipophilic bioactives like ω-3 fatty acid, β-carotene or lycopene are chemically unstable or rather sensitive to heat, light or oxygen (Mcclements 2015; Mcclements et al. 2009) while few others like carotenoids used in nutraceutical formulation being crystalline in nature adds concern regarding long term stability of the formulation (Mcclements et  al. 2009). Compatibility of bioactive lipids, proteins, peptides and amino acids with other food matrix is a major factor contributing towards formulation instability, loss of activity and side-effects (Cho et  al. 2004; Pedrosa Delgado et  al. 2006). Minerals being charged species form complexes or bonds with oppositely charged moieties, when incorporated into nutraceutical tend to result in aggregation of particles causing physical instability (McClements 2005). Also, they are known to cause chemical degradation of other compounds and tend to have lower bioavailability due to decreased absorption (Mcclements et al. 2009). McGhie et al. reported 15 different anthocyanin which are absorbed and excreted unmetabolised by humans due to their inability to retain at specific site or tissue (McGhie et al. 2003). One of the major obstacle of probiotic delivery is design of colon targeted delivery system without systemic absorption. Infact, challenges associated with oral delivery of these proteins include poor intestinal permeability and lack of efficient processing techniques which would retain their complete bioactivity and deliver active peptide at the site of action (Brayden and Baird 2013; Chatterton et al. 2006; Meisel 2005, 1997). Table 3.1 presents a brief summary of various bioactives in marketed nutraceutical and their drawback. This chapter comprises of brief overview regarding hurdles in delivery of a nutraceutical and nanotechnology as a means to overcome them.

3.2  Nanotechnology- Old Nutraceutical in New Form Nanoparticles result in enhanced pharmacodynamics, pharmacokinetics, site specific delivery and controlled release compared to bioactives they carry. Nutraceutical when loaded into nanocarriers are known to escalate the surface area to volume ratio

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Table 3.1  Bioactives in marketed nutraceutical along with their formulation issues Bioactive Coenzyme Q-10 (Ubiquinone)

Formulation Tablet, capsule, cream and lotion

Vitamin D

Sachet, tablet, syrup and emulsion.

Omega-3 fatty acid

Capsule and Tablet

Quercetin

Tablet and capsule

Dibenzoylmethane

Used in Sunscreen cream as butyl methoxydibenzoyl-­ methane Capsule, tablet, ointment, energy drink, soap and cosmetic Tablet, capsule, cream, lotion, face scrub

Curcumin

Lycopene

Vitamin C

β-Carotene

Vitamin B12

Tablet, capsule, syrup, suspension, cream and lotion Tablet, capsule, syrup, cream, suspension and lotion

Tablet, capsule, syrup, and cream

Delivery issue Low aqueous solubility, intestinal permeability and low bioavailability Sensitive to Ultraviolet radiation High dose due to impaired oral absorption and unknown oral bioavailability dynamics Sensitive against relative humidity and temperature Poor aqueous solubility, low bioavailability and rapid gastric digestion Low aqueous solubility and bioavailability

References Liu and Artmann (2009)

Low aqueous solubility and bioavailability

Prasad et al. (2014)

Isomerization due to high temperature, light and oxygen Sensitive to light, temperature and pH

Shi et al. (2008b)

Oxidative stability is low during process because of increased thermal, mechanical or chemical stresses Low bioavailability Low bioavailability due to reduced absorption

Boon et al. (2010), Thrandur et al. (2009)

Shaker A. Mousa (2015) Torres-Giner et al. (2010) Nam et al. (2016) Ahmad (2007)

Oyetade et al. (2012)

Lindsay Helen (2010)

and manipulate the solubility of the bioactives leading to enhanced absorption and bioavailability (Acosta 2009). Nanotechnology has also been utilised to encapsulate bioactives improving their stability, quality, safety and nutrition (Huang and Yu 2010). Different nanocarriers including dendrimers, submicron emulsion, liposomes, polymeric micelles, polymeric nanoparticles and lipid nanocarriers have been exploited for the delivery of nutraceutical (Fernandes et al. 2014).

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3.2.1  Submicron Emulsion Microemulsions are solution like thermodynamically stable, isotropic, newtonian and non-viscous systems consisting of nanodroplets stabilized by surfactants (Kogan and Garti 2006; Spernath and Aserin 2006). Surfactant has equal affinity for both the phases in microemulsion and both phases are miscible with each other. During microemulsion formation equilibrium lyotropic liquid crystalline phases are formed which vanishes the surface tension between the two phases leading to self-­ assembled droplet structures (Mason et al. 2006). Microemulsion consist of bioactive, oil, water, surfactant and co-surfactant combination. Microemulsion formation is entropy driven process utilizing low energy and aims in formation of self-­ assembled structure (Rosano et al. 1988). The average diameter of microemulsion ranges from 20 to 200 nm (Ramli et al. 2015). Microemulsion can be water in oil type (w/o) when the dispersed phase is water and continuous phase is oil and vice versa in oil in water type (o/w) and novel U-type microemulsion. o/w type microemulsion remain stable even after dilution in gastric environment while the w/o type emulsions destabilize in aqueous environment (Spernath and Aserin 2006). Liu et al. developed w/o microemulsion for fluorescent labelled peptide 5-(and 6)-carboxytetramethylrhodamine labelled human immunodeficiency virus (HIV) transactivator protein (TAT). Proteins are easily deactivated by enzymes and acidic pH but they are hydrophilic. w/o microemulsion were encapsulated into enteric coated capsule for intestinal delivery (Liu et  al. 2012). Absorption of drug from microemulsion depends on various factors of the bioactive like particle size, partition coefficient between two phases, the presence of drug at interface, site or path of absorption and solubility (Spernath and Aserin 2006). Garti et al. observed a U-type microemulsion system in which the w/o microemulsion was diluted with water and upon excess dilution leads to the formation of o/w microemulsion with similar droplet size as that of w/o microemulsion (Garti et al. 2006), wherein the bioactive is solubilized at the interface of w/o microemulsion on dilution with water the system turns to bicontinuous phase which on further dilution turns into o/w microemulsion system with the drug more attached with its hydrophilic counterpart at the interface with reduced solubility (Garti et al. 2006). Thus, this difference in transition points between the three regions can be utilized for triggering the release of future bioactives(Spernath et al. 2006). Bioenhancers phase was used to increase solubilisation of phytochemicals like lutein, lycopene and phytosterols using (R+)-D-limonene, propylene glycol, ethanol, tween 60 and tween 80 (Garti et al. 2004). Microemulsion drug delivery system has been exploited for various administration routes like oral, transdermal and topical. Maleki and Dizaj prepared o/w microemulsion of Vitamin A palmitate to improve its solubility and hence its oral bioavailability. Sunflower oil was used as oil phase while Tween 80 and sucrose solution as surfactant and co-surfactant in aqueous phase. Non-ionic high HLB value surfactant was utilized in order to achieve a stable o/w microemulsion (Dizaj 2013). Ramli et al. prepared carboxymethycellulose based U-type w/o microemulsion for

3  Nanotechnology Based Delivery of Nutraceuticals Fig. 3.1 Pseduoternary phase diagram of oleic acid/ tween 20/ propylene glycol/ water for U-type microemulsion of Vitamin C (Ramli et al. 2015) (reproduced with permission)

69 T20/PG 10 20 30 40

Water 50 60 70 80 90 WATER

90 80 70 60 50 40 30 20 10 OA

10 20 30 40 50 60 70 80 90

(Km = 3.1)

Vitamin C for transdermal application. The formulation consisted of tween 20 and propylene glycol in the ratio of 3:1 and oleic acid: Surfactant/co-surfactant ratio as 1:9. The aqueous phase also consisted of carboxymethylcellulose. The pseudoternary phase diagram for finally selected excipients is as shown in Fig. 3.1 (Ramli et al. 2015). Microemulsion has been utilized as an efficient carrof pungent bioactive which causes gastric irritation potential and exhibit low half-life and limited bioavailablity. Zhu et al. formulated capsaicin o/w microemulsion using medium chain triglyceride as oil, Cremaphor EL and ethanol as surfactant and co-surfactant respectively using ultrasonication method. It is lipophilic compound which is insoluble in water and has very short half life of 7.06 min. The microemulsion was evaluated for gastric irritation potential, in vivo bioavailability studies and in vivo and in vitro correlation. Capsaicin microemulsion demonstrated no gastric irritation whereas the vacuoles and inflammatory cell infilteration can be seen in free Capsaicin treated rat but not in rat group treated with normal saline and Capsaicin microemulsion. The release of Capsaicin from microemulsion was faster than from its solution due to high hydrophilicity of microemulsion. Also, since the lipophilic bioactive is in solubilized form in oil it enhances the bioavailability of the bioactive. 80% of the bioactive was released in 2 hrs in hydrochloric acid (pH 1.2) or water while 20% released in 4 hrs in phosphate buffer pH 6.8. This demonstrated that the microemulsion can be utilized in controlled release of Capsaicin in intestine. The Tmax was delayed by 2.67-­ fold in microemulsion while the half-life of Capsaicin in microemulsion increased by 6.45-fold than free Capsaicin. IVIVC studies resulted in level A correlation for Capsaicin microemulsion release in hydrochloric acid (pH 1.2) while the in vitro release studies in phosphate buffer pH 6.8 did not show proper correlation with in vivo release pattern (Zhu et al. 2015). Similarly, carotenoids are highly lipophilic with poor solubility along with incomplete absorption during their gastrointestinal passage due to non-uniform particle size. US 2013/0004619 published on January 3, 2013 describes Winsor Type III microemulsion of carotenoids extracted from marigold prepared from R-(+)-limonene, tween 80, ethanol, glycerol and purified

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water to overcome low solubility and absorption limitation of carotenoids. The solubility of carotenoids in prepared microemulsion with 20% w/w and 35% w/w purified water was observed to be 6 times and 8.4 times higher compared to pure carotenoid (Pei-Yong Chow 2013). Patent no. US20070087104 describes a method of preparing o/w microemulsion using low HLB, medium HLB and high HLB emulsifier for highly insoluble bioactives. The formulation doesn’t require any co-­ emulsifier which leads to off-flavor. The patent demonstrated incorporation of Vitamin A palmitate, Vitamin E and β-carotene into one microemulsion system using Tween 80 (High HLB), triglyceryl monostearate (medium HLB) and glyceryl monooleate (low HLB) (Chanamai 2007). Despite the benefits of microemulsion, nanoemulsions are preferred due to lesser amount of surfactants required in later (Nasr 2015). Nanoemulsion are kinetically stable o/w or w/o emulsions with droplet size ranging from 10–100 nm (Gupta et al. 2016). They consist of lipophilic and hydrophilic phase stabilized by means of surfactant and co-surfactant (Komaiko and Mcclements 2016; Mason et  al. 2006; Dasgupta et al. 2018). They can be formulated using two different approaches namely high-energy approach and low energy approach. In high-energy approach, larger droplet size of macroemulsion is reduced by means of induced mechanical energy using high pressure homogenizer, sonicator or microfluidizer to form nanoemulsion. The input energy in high-energy method is of the order of 108–1010 Watt/Kg (Aboalnaja et al. 2016; Gupta et al. 2016). In low energy process, nanoemulsion was prepared either by diluting the macroemulsion or reducing the temperature until inversion point of the system which leads to the formation of small droplets with high surface area. Inversion point is the specific point of a system when the interfacial tension between oil and aqueous phase is minimum resulting in formation of smaller droplet without external energy input. The input energy for low energy system ranges from 103–105 watt/kg (Gupta et al. 2016). However, it has been observed that the rate of destabilization of nanoemulsion formed by high pressure homogenization is less compared to low energy process due to high polydispersity index of the later. Low energy method can be further divided into isothermal and thermal process. However, since thermal method will require change in temperature which is energy intensive, isothermal method is preffered (Komaiko and Mcclements 2016). Isothermal method consists of emulsion inversion method/phase inversion composition and phase inversion temperature. Both the methods have their own limitations like high-energy approach requires use of costly equipments while low-energy approach requires excess amount of surfactants (Aboalnaja et al. 2016). Dias et al. prepared nanoemulsion for copaiba oil by both high pressure homogenization and spontaneous emulsification process. Copaiba oil is known to contain many diterpenes and sesquiterpenes. β- Caryophyllene is the major component of Copaiba oil and is known to possess anti-cancer, anti-inflammatory, anti-­ulcer, anti-bronchitis effect. However, stabilization of the oil was a major concern hence topical nanoemulsion of this oil was prepared. Copaiba oil, medium chain triglycerides and span-80 consisted of hydrophobic component along with acetone and ethanol for spontaneous emulsification process. Tween 20 and purified water consisted of aqueous phase. The nanoemulsion prepared by high pressure homogenization showed better stability than with spontane-

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ous emulsification process. This could be because although the droplet size of the system prepared by both methods were similar, the PDI of later was 0.3 while that of the prior was 0.2 (Dias et al. 2014). Nanoemulsion has many advantages like ability to incorporate wide variety of bioactives with lipophilic, hydrophilic or ampiphilic property which enables nanoemulsion as reliable formulation strategy for nutraceuticals, biocompatible excipients are utilized, a range of physicochemical and biological effects can be obtained with this system, protection of sensitive bioactive, resistance from gastric degradation and sustaining the release profile (Aboalnaja et al. 2016). For example, Ganriel Davidov Pardo and David Julian McClement formulated Resveratrol nanoemulsion to protect it from chemical degradation by using grape seed oil, orange oil and tween 80 as surfactant which have GRAS status and are biocompatible(Davidov-­ Pardo and McClements 2015). Sari et  al. developed curcumin nanoemulsion to enhance the bioavailability of curcumin by increasing aqueous solubility and reducing the degradation in gastric environment. Medium chain triglyceride was utilized as carrier oil while tween 80 and whey protein-70 as surfactants. The nanoemulsion were prepared by high energy process utilizing ultrasonication technique. In vitro evaluation of nanoemulsion by digestion in simulated gastric fluid and simulated intestinal fluid demonstrated that 90% of Curcumin was encapsulated in nanoemulsion droplet even after digestion with simulated gastric fluid. While, treatment with simulated intestinal fluid caused release of curcumin in 3 h. Whey protein-70 known to contain β-lactoglobulin were resistant to the enzymatic action of pepsin which prevented the nanoemulsion from destabilization while, simulated intestinal fluid containing lipase enzyme and bile salts which displaced the whey protein-70 layer and lipase causing degradation of oil caused release of curcumin. Thus, curcumin was protected due to nanoemulsion from degradation in gastric environment (Sari et al. 2015). Nanoemulsions has also been used to enhance the solubility and bioavailability of poorly soluble bioactives. N-oleoylethanolamine exhibit poor aqueous solubility and bioavailability. Miguel Wulff-Perez et  al. synthesized nanoemulsion of N-oleoylethanolamine for obesity which led to improved bioavailability. N-oleoylethanolamine loaded nanoemulsion comprised of sunflower oil, poloxamer 188 and water. The in vivo toxicity studies were performed on male Wistar rats. The rats were divided into three groups. Each group received intra-peritoneal injection of saline, tween 20 (10% in saline-control) and nanoemulsion (10% in saline). Tween-20 caused fatty liver while nanoemulsion did not affect the liver. Further, one-way ANOVA revealed that tween-20 produced effect on both, cholesterol and triglycerides which increased after the treatment. Although, the level of cholesterol and triglycerides were higher compared to control in nanoemulsion injected group but the levels were comparatively lower than tween-20 which is known to cause toxicity (Alen 2014). The results are as shown in the Table 3.2. Careful selection of excipients used in nanoemulsion is vital to predict in vivo behaviour like use of triblock polymer as surfactant can provide protection from degradation in gastric environment while, causing controlled release of bioactive in intestine. Also, such nanoemulsion can escape the reticuloendothelial clearance (Alen 2014).

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Table 3.2  Hepatic Triglyceride and Cholesterol level of male Wistar rats after treatment with saline, tween 20 and N-oleylethanolamine nanoemulsion (Alen 2014). (Reproduced with permission) Biological parameters Triglycerides (mg/dl) Cholesterol (mg/dl HDL cholesterol (mg/l)

Treatment (n=8 animals/ group) Saline Tween 20 122.0 ± 6.1 141.0 ± 5.2 79.50 ± 1.12 88.17 ± 0.48 24.0 ± 0.9 25.0 ± 0.9

Nanoemulsion 105.0 ± 0.9 83.83 ± 1.40 25.0 ± 1.2

Bioavailability of a bioactive are influenced by various attributes which include low solubility, high absorption or biotransformation of bioactive (Aboalnaja et al. 2016). For example, resveratrol derived from grapes is a natural anti-oxidant. It exhibit various beneficial effects including anti-oxidant, anti-atherosclerotic, cardioprotective and chemoprotective. However, it has high first pass metabolism and poor aqueous solubility leading to poor bioavailability. Sessa et al. formulated o/w nanoemulsion of resveratrol using high pressure homogenization technique with peanut oil which protected the drug from degradation to as low as 15–25% and enhanced drug uptake through the CaCo-2 cell compared to Resveratrol-DMSO formulation. The permeability of nanoemulsion lies between 1 × 10−7 cm/s (poorly permeable) and 1 × 10−5 cm/s (highly permeable) (Donsì et al. 2011). Nanoemulsion has been explored for delivery of bioactives via oral (Tabibiazar et al. 2015), parenteral (Rahali et al. 2010), topical (Dias et al. 2014), transdermal (Mostafa et al. 2015) and nasal (Nasr 2015). Rao et al. formulated nanoemulsion of β-carotene for oral delivery. β-carotene is known to reduce the risk of various chronic diseases like macular degeneration, cancer and cardiovascular disease but it is insoluble in water and slightly soluble in oils at ambient temperature which causes its precipitation in GIT resulting in poor bioavailability. Incorporation of β-carotene into digestible carrier corn oil led to enhancement of its bioaccesibility and bioavailability. This was due to the fact that, corn oil contains triacylglycerol, when in small intestine lipase action leads to breakdown of triacylglycerol into monoacylglycerol and fatty acids. These fatty acids along with bile salts and phospholipids forms free micelles in which β-carotene gets solubilized leading to enhanced bioaccesibility and bioavailability. Here, bioaccesibility was with reference to the fraction of drug which solubilizes into the formed micelles. Thus, the nanoemulsion of β-carotene was formed using D-Limonene, corn oil, sucrose monopalmitate and lysolecithin using high pressure homogenization method. The formed nanoemulsion was evaluated in vitro for simulated gastrointestinal tract model. The nanoemulsion when in mouth led to increased droplet size due to flocculation. Also, in simulated gastric fluid the droplet size further increased due to increased aggregation. In simulated intestinal fluid the droplet size reduced due to lipid oxidation of digestible oil forming free fatty acids. Thus, the bioavailability of β-carotene increased from 5% to 76% by increasing the concentration of corn oil from 0% to 100% respectively (Rao et al. 2013). Lipid emulsion for parenteral nutrition are usually provided separately and then mixed with other electrolyte, amino acids and sugars prior to administration due to

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stability issues of the mixture and complexity of oils used in parenteral nutrition. Rahali et al. prepared lipid nanocapsules by phase inversion temperature method for Olive oil and Soyabean oil in 80/20 ratio which is usually taken for parenteral nutrition. The formulation was made using Labrafac as surfactant. The nanocapsules showed significant improvement in the stability of final mixture which is made before administration (Rahali et al. 2010). Kong et  al. synthesized Hyaluronic acid based Vitamin E nanoemulsion for enhanced permeation through the skin. Vitamin E being highly lipophilic, absorbed into the lipophilic stratum corneum. However, it does not partition from stratum corneum in the epidermis thus lowering its bioavailability. Nanoemulsion were formulated using oil/water/surfactant system and solvent evaporation method. Skin permeation studies resulted in sufficient amount of flux for nanoemulsion while no flux for Vitamin E based ethanol solution. Fluorescence studies demonstrated presence of nanoemulsion in deeper dermis of skin as shown in Fig. 3.2. Hyaluronic acid being hydrophilic could not pass through stratum corneum which is highly lipophilic. Thus, the diffusion occurs through follicular pathway as can be seen in fluorescence studies (Kong et al. 2011). Maha Nasr. prepared Hyaluronic acid based

Fig. 3.2  Fluorescence studies demonstrating penetration of HA-GMS nanoemulsion into rat skin after 10 h exposure. A) Margin area of treated skin sample. B) Central area of treated skin sample. C) Magnification of follicular area (Kong et al. 2011) (reproduced with permission)

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nanoemulsion for Resveratrol and Curcumin by spontaneous emulsification method for trans-nasal delivery. Resveratrol and Curcumin are effective in neurodegenerative disorders as they inhibit the formation of β-amyloid fibres and reduction of cognitive defects. Also, Resveratrol is known to decrease reactive oxygen species and lipid peroxide levels. Hyaluronic acid acted as mucoadhesive agent. However, both the drugs have low bioavailability due to poor aqueous solubility and degradation due to high temperature, light or oxygen (Nasr 2015). Flavanoids are another group of bioactives which are insoluble in water and almost all other solvents which can be utilized for pharmaceutical or cosmetic use. Patent no. US 2012/0213842 describes enhancement of bioavailability of flavonoids by incorporating them into previously formed placebo o/w or w/o nanoemulsion with the aid of sonication or high pressure homogenization at high temperature of 120 °F - 170 °F (Birbara 2010). Tocopherol is a well-known anti-oxidant which has very low permeability and thus leads to low bioavailability and increase in dose. Patent no. US20090306198 describes a novel microfludized nanoemulsion formulation with fourfold improvement in cell penetration ability of Tocopherol with 110 nm droplet size. Soybean oil was utilized as oil phase and soy lecithin as surfactant. This nanoemulsion lead to decrease in tumor size by 77% compared to only 11% by plain Tocopherol. Tumor apoptosis studies revealed 31% apoptosis as against 8% in plain Tocopherol (Robert Nicolosi 2009). Due to the advantages and ease of preparation along with industrial scalability of nanoemulsion, nutraceutical based nanoemulsion of Vitamin A, D, E and K with brand name Vitalipid manufactured by Fresenius Kabi, Europe has reached the market for parentral nutrition (Patel and Rumit 2016). Another commercial product as a source of calories and essential fatty acids derived from soyabean and egg-yolk named Intralipid® manufactured by Baxter Healthcare corporation is an example of nanoemulsion for Intravenous administration (Drugs.com). Self-emulsifying drug delivery system (SEDDS) is defined as system consisting of bioactive dissolved in oil stabilized by surfactant and co-surfactant which on reaching the GIT emulsifies due to the gastric aqueous content and its motility. SEDDS could be microemulsion (SMEDDS) or nanoemulsion (SNEDDS). This system came into being because the microemulsion and nanoemulsion had various drawback like Ostwald ripening, poor palatability, inability to delivery through hard/soft gelatin capsule, high water content and hydrolysis of drug due to lipid content (Rehman et  al. 2016). The phase diagram of this system consists of oil phase, aqueous phase, multiphase system which are turbid and liquid crystalline phase which could be transparent which produce coloured bifringence or viscous which produce white bifringence pattern. The liquid crystalline phase region is the self-emulsification zone of this system (Susan et al. 1992). This system is renowned for its property to enhance and reproduce the plasma profile of bioactives. The improvement in plasma profile is a function of solubilisation, emulsification and dispersion of drug in gastrointestinal tract. This system when in gastric environment and when in contact with the fluid leads to formation of fine droplets which causes rapid emptying from stomach and distribute throughout the gastrointestinal tract. The fine droplets leads to large increase in surface area which improves the parti-

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tioning behaviour of the bioactive causing enhanced absorption and permeation of bioactive (Charman Susan et al. 1992). For proper formation of this system the surfactant and co-surfactant ratio has to be appropriately optimized. In the self-­ emulsification region of ternary phase diagram the nano- or microemulsion is formed by mere dilution and shaking the flask. This is possible because the surfactant assembled over the emulsion droplet strongly reduces the interfacial area. Also, the co-surfactant increases the fluidity by penetration between the surfactant molecule. Such observations were made by Taha et al. during their research on preparation of all-trans-retinol acetate SNEDDS for decreased variation in absorption pattern (Taha et al. 2004). Selection of various components on SNEDDS has direct impact on the final formulation. Oil used in SNEDDS has direct impact on the drug solubility, nanodroplet size and nanoemulsion formation. Medium chain and small chain triglycerides have the ability of nano-emulsification while long chain triglycerides leads to direct lymphatic transport. Selection of surfactant has a great impact on droplet size and nano-emulsification region. Co-surfactant have a profound impact on self-emulsification time and drug loading (Date et al. 2010). Liquid SNEDDS have certain drawbacks like interaction with capsule shell, instability during long term storage leading to precipitation during processing or storing at room temperature. Thus solid SNEDDS came into being with combined advantage of SNEDDS and solid dosage forms (Susan et al. 1992). Various techniques were utilized to convert liquid SNEDDS to solid SNEDDS (s-SNEDDS), the selection method depends upon the property of bioactive like solubility, compatibility with excipients, heat stability etc. (Date et al. 2010). s-SNEDDS are converted from liquid SNEDDS by various methods like adsorption, spray drying, freeze drying, extrusion spheronization and melt granulation (Tang et  al. 2008). Yoo et  al. developed lutein based s-SNEDDS by adsorption method for enhanced bioavailability of lutein by preparing SNEDDS using medium chain triglyceride, labrasol and Transcutol HP or lutrol E 400. The optimized SNEDDS was then adsorbed over Aerosil 200 and then evaluated for its dissolution profile. The dissolution profile was unaffected by Aerosil 200 and there was no interaction between other components of SNEDDS and adsorbent, further there was no aggregation or coalescence of nanodroplet (Yoo et al. 2010). While, Akhter et al. formulated Coenzyme Q 10 (Co Q-10) s-SNEDDS for enhanced bioavailability using Lauroglycol FCC as oil, Labrasol as surfactant and Transcutol P as co-surfactant. The formulated l-SNEDDS was converted to s-SNEDDS using Aerosil 300 by spray drying technique. In vitro drug release of s-SNEDDS was 97.5  ±  4.5% while that of marketed was 57.96 ± 0.46%. The Tmax was reduced while Cmax was 3.4 fold higher and AUC fivefold higher than marketed formulation. Stability studies resulted in only 0.66% degradation of bioactive as compared to 3.13% of marketed formulation at 40 °C ± 2 °C (Akhter et al. 2014). The stability of l-SNEDDS can also be improved by formulating semi-solid SNEDDS using blend of low and high HLB surfactant. For instance, Coenzyme Q-10 (Co Q-10) semi-solid SNEDDS were prepared using Cremaphor EL (High HLB) and Capmul MCM-C8 (Low HLB co-surfactant) due to low solubility of bioactive in oil leading to lesser drug loading. Also, many bioactives in l-SNEDDS suffer from irreversible precipitation of bioactive and excipients with

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time. The bioactive was blended in appropriate amount of lemon oil which led to recrystallization of Co Q-10  with the potential for irreversible precipitation and separation of bioactive during supersaturation or fluctuation in storage temperature. The semi-solid SNEDDS also enhances the dissolution of bioactive (Nazzal et al. 2002). Capsanthin (a type of carotenoid) suffers from poor absorption through gastrointestinal tract. Patent US2013/0004619 describes novel solid SNEDDS forming Winsor type III microemulsion prepared from R-(+)-Limonene, tween 80, glycerol and ethanol. The bioavailability of Capsanthin was observed to be 191% in bird serum and 20% increase in Capsanthin deposition in bird egg compared to free capsanthin (Pei-Yong Chow 2013). Large size of macromolecule and faster degradation due to enzymes of environmental factors are major drawback of proteins and peptides which leads to poor bioavailability. Lipids being biodegradable when in GIT leads to free fatty acid formation forming micelles leading to solubilisation and protection of bioactive (Pouton 2000). SNEDDS are thus advantageous for delivery of proteins and peptides. However, the major drawback with o/w emulsions is that on dilution inversion to w/o occurs leading to expulsion of bioactive. Rao et al. attempted to formulate stable o/w microemulsion for β- lactamase. The protein was dispersed in ampiphilic  soy phosphatidylcholine which was then mixed with oil, surfactant and co-­ surfactant mixture leading to the formation of SNEDDS (Venkata et al. 2008b). In vitro evaluation of these SNEDDS on MDCK cells revealed that the cell uptake of these SNEDDS was higher compared to solution of β- lactamase (Venkata et  al. 2008a). In vivo evaluation in rats resulted in increased Cmax and bioavailability which was 2.8 times and 2.6 times higher than β-lactamase solution respectively. The Tmax was between 4 h and 6 h and even at the end of 24 h small traces were detected (Venkata et al. 2008c). To summarize, submicron emulsion are advantageous to enhance the bioavailability and solubility of lipophilic bioactives. Due to their submicron size, they are highly permeable through the biological membrane. However, they have certain drawback like large amount of surfactant utilized in microemulsion, requirement of sophisticated instruments for development of nanoemulsion, inability to incorporate large dose of bioactive, destabilization at higher temperature and relative humidity, rancidity of oil used in submicron emulsion and toxicity due to large amount of surfactant in microemulsion. SNEDDS/SMEDDS possess the property to reduce poor palatability, high amount of water and hydrolysis of bioactive.  s-SNEDDS have highest stability amongst other sub-micron emulsion.

3.2.2  Lipid Nanoparticles Lipid nanoparticles were introduced in 1991 by Rainer H. Müller as an alternative to nanoemulsion, liposomes and polymeric micelles. These are known to eliminate the drawback like instability, costly phospholipids and drug leakage and toxicity and lack of industrially feasible method of nanoemulsion, liposomes and polymeric

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nanoparticles respectively. Lipid nanoparticles were formed by replacing the oil of nanoemulsion with solid lipid leading to solid lipid nanoparticles (SLN) (Müller et al. 2000). SLN can be prepare by various techniques like high shear homogenization coupled with ultrasonication, high pressure homogenization, solvent evaporation/emulsification method and microemulsion technique depending upon the availability of instruments, desired particle size of the nanoparticles and route of administration of the formulation (Mäder and Mehnert 2001). Rao et al. formulated SLN of a natural antioxidant-Bixin obtained from the plant Bixa Orellana using Trimyrisitin and glyceryl monostearate as solid lipid stabilized by soy and egg lecithin by high shear homogenization coupled with ultrasonication. Bixin is poorly water soluble and exhibit poor bioavailability. It showed improvement in bioavailability when incorporated into SLN.  The particle size of the SLN ranged from 135.5–352.8 nm with PDI 0.185–0.572. The entrapment efficiency was 99% with loading efficiency 17.96%. In vivo histopathological studies revealed that Bixin loaded SLN demonstrated higher hepatoprotection against Paracetamol induced hepatotoxicity compared with blank-SLN and free Bixin solution (Rao et al. 2014). SLN due to their nano-size and use of biocompatible excipients are known to enhance tumor concentration of boactive. They lead to enhanced residence time and targeting to tumor cells. Incorporation of anti-cancer bioactive into SLN is favourable because bioactives with different physicochemical properties can be encapsulated along with enhance stability and pharmacokinetic of bioactive along with reduced in vitro toxicity (Joshi and Müller 2009). Zhang et al. synthesized SLN of curcumin for tumor targeting along with enhanced stability and aqueous solubility of curcumin. Two different formulation with lipid to curcumin ratio 2:1 and 4:1 were synthesized by sol-gel method. The entrapment efficiency was found to be 62% and 75% with surface charge −5.3 mV and − 11.6 mV for SLN to curcumin ratio 2:1 and 4: 1 respectively. To confirm for enhanced solubility of Curcumin, the free curcumin solution when incubated with phosphate buffer saline (pH -7.4) at 37 ° C for 6 h reduced the concentration of Curcumin while the concentration of Curcumin in phosphate buffer saline remained >80%. SLN: curcumin (2: 1) formulation inhibited non-small cell lung cancer cells (NCL-H1299) with decreased IC-50 value of 32 μM from 185 μM and on A549 cells from 78 μM to 18 μM. Similarly, for SLN to Curcumin formulation of 4:1 the IC-50 value decreased to 9 μM and 4  μM for NCL-1299 and A549 cells respectively. Pharmacokinetic studies upon intraperitoneal injection in mice revealed 26.4 fold increase in AUC. Tissue distribution was analysed using nude mice having A549 xenografts. The curcumin concentration in tumor cells was 18.2  pmol/mg protein and 2.6 fold higher in lungs compared to plain curcumin upon oral administration. The level of curcumin upon intraperitoneal administration increased to 4.3 fold and 32.6 fold in tumor cell and lungs respectively. The tumor volume was reduced by 65.3% (p 

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