Advanced Functional Materials

By Ashutosh Tiwari and Lokman Uzun

Because of their unique properties (size, shape, and surface functions), functional materials are gaining significant attention in the areas of energy conversion and storage, sensing, electronics, photonics, and biomedicine.  Within the chapters of this book written by well-known researchers, one will find the range of methods that have been developed for preparation and functionalization of organic, inorganic and hybrid structures which are the necessary building blocks for the architecture of various advanced functional materials. The book discusses these innovative methodologies and research strategies, as well as provides a comprehensive and detailed overview of the cutting-edge research on the processing, properties and technology developments of advanced functional materials and their applications.

Specifically, Advanced Functional Materials:

• Compiles the objectives related to functional materials and provides detailed reviews of fundamentals, novel production methods, and frontiers of functional materials, including metalic oxides, conducting polymers, carbon nanotubes, discotic liquid crystalline dimers, calixarenes, crown ethers, chitosan and graphene.
• Discusses the production and characterization of these materials, while mentioning recent approaches developed as well as their uses and applications for sensitive chemiresistors, optical and electronic materials, solar hydrogen generation, supercapacitors, display and organic light-emitting diodes, functional adsorbents, and antimicrobial and biocompatible layer formation.

This volume in the Advanced Materials Book Series includes twelve chapters divided into two main areas: Part 1: Functional Metal Oxides: Architecture, Design and Applications and Part 2: Multifunctional Hybrid Materials: Fundamentals and Frontiers

• Language: English
• Publisher: Wiley
• Release date: May 8, 2015
• ISBN: 9781118998984

Ashutosh Tiwari

Professor Ashutosh Tiwari is Director at Institute of Advanced Materials, Sweden; Secretary General, International Association of Advanced Materials; Chairman and Managing Director of VBRI Sverige AB and AAA Innotech Pvt. Ltd; Editor-in-Chief, Advanced Materials Letters and Docent in the Applied Physics with the specialization of Biosensors and Bioelectronics from Linköping University, Sweden. Prof. Tiwari has several national and international affiliations including in the United States of America, Europe, Japan, China and India. His research focus is on the design and advanced applications of cutting-edge advanced materials for new age devices. He has more than 200 peer-reviewed primary research publications in the field of materials science and nanotechnology and has edited or authored over 50 books.

Book preview

Preface

Functional materials are gaining significant attention in the areas of energy conversion and storage, sensing, electronics, photonics, and biomedicine. The parameters such as size, shape, and surface functions are critical to control the properties for different applications and, because of their unique properties, functional materials are very effective. A range of methods have been developed for preparation and functionalization of organic, inorganic, and hybrid structures, which are the necessary building blocks for the top-down as well as bottom-up architecture of various advanced functional materials. They possess unique physico-chemical properties such as large surface areas, good conductivity and mechanical strength, high thermal stability, and desirable flexibility, which together make a new type of materials phenomenon.

This book compiles the objectives related to functional materials and provides detailed reviews of fundamentals, novel production methods, and frontiers of functional materials, including metallic oxides, conducting polymers, carbon nanotubes, discotic liquid crystalline dimers, calixarenes, crown ethers, chitosan, and graphene. After discussing the production and characterization of these materials, their uses and applications for sensitive chemiresistors, optical and electronic materials, solar hydrogen generation, supercapacitors, display and organic light-emitting diodes (OLED), functional adsorbents, and antimicrobial and biocompatible layer formation are highlighted.

This volume in the Advanced Materials Book Series includes 13 chapters divided into two main areas. In Part 1, Functional Metal Oxides: Architecture, Design and Applications, distinguished researchers present recent efficient strategies such as nanocasting, spray pyrolysis, sol–gel, and wet chemical methods to develop functional metal oxides in respect to architecture, design, and applications; meanwhile, they summarize their uses as chemiresistors for sensitive detection of toxic chemicals, high-performance fuel cell electrodes of mesoporous materials through nanocasting route, spray-pyrolyzed thin-film solar cells, biomedical agent, solar hydrogen generators, and supercapacitors.

Part 2, Multifunctional Hybrid Materials: Fundamentals and Frontiers, includes several hybrid materials such as discotic liquid crystalline dimers, supramolecular nanoassemblies, carbon-based hybrid materials, organometal halide Perovskites, novel-architecture copolymers by gamma radiation, graphene, chitosan, and antimicrobial biopolymers. In this part, prominent authors present fundamental approaches for production of multifunctional hybrid materials while designating their frontier applications such as OLEDs, flexible displays, artificial receptors for detection platform, supercapacitors for advanced electrodes, photovoltaic applications, stimuli-responsive carriers for the sustained and targeted drug delivery, selective and efficient adsorbent materials, and antimicrobial surface.

The book is written for a wide readership, including university students and researchers from diverse backgrounds such as chemistry and chemical engineering, materials science and nanotechnology engineering, physics, life sciences, agriculture and biotechnology, petroleum and natural gas technology, forensic science, and biomedical engineering. It can be used not only as a textbook for undergraduate and graduate students but also as a review and reference book for researchers in the materials science, bioengineering, medical, physics, forensics, agriculture, biotechnology, and nanotechnology arenas. We hope that the chapters of this book will provide the reader with valuable insight into functional materials in respect to the fundamentals of architecture, design, and applications.

Editors

Ashutosh Tiwari, PhD, DSc

Lokman Uzun, PhD

January 12, 2015

Part 1

FUNCTIONAL METAL OXIDES: ARCHITECTURE, DESIGN, AND APPLICATIONS

Chapter 1

Development of Toxic Chemicals Sensitive Chemiresistors Based on Metal Oxides, Conducting Polymers and Nanocomposites Thin Films

Sadia Ameen¹, M. Shaheer Akhtar², Hyung-Kee Seo¹, and Hyung-Shik Shin*,¹

¹Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea

²New & Renewable Energy Material Development Center (NewREC), Chonbuk National University, Jeonbuk, Republic of Korea

*Corresponding author: [e-mail]

Abstract

Semiconductor materials in nanoscale are gaining a significant attention in the areas of energy conversion and storage, sensing, electronics, photonics, and biomedicine. The parameters such as size, shape, and surface characteristics are significant to control the properties for different applications and thus, semiconducting nanostructured materials of one dimension (1D) are effectively used to fabricate a variety of chemosensors. Particularly, the properties of semiconducting nanostructured materials are altered to achieve high flexibility in various applications. On the other hand, the conducting polymers like polypyrrole, polythiophene, polyaniline, and polyfuran are p-type semiconductors of unique electronic properties, low-energy optical transitions, low ionization potential, and high electron affinity, which are promising for the application of conductometric polymer sensors. The harmful chemicals such as volatile and non-volatile organics are extensively detected by the sensor technology. The sensitivity, selectivity, and stability are the most important aspects of investigation for a variety of sensors. Among several sensors, the electrochemical method provides the advantages of high sensitivity, wide linear range, economical, rapid response, portability, and ease of operating procedure.

In this chapter, we have briefly discussed the semiconducting metal oxides nanostructures like TiO2, ZnO, conducting polymers, and nano composites for the efficient detection of harmful and toxic chemicals. The chapter includes brief literature surveys, properties, and the latest research advancements for the development of various metal oxide nanostructures, nanocomposites, and conducting polymer-based nanomaterials as efficient electrode for detecting harmful chemical through the effective electrochemical technique. The modified electrodes with different inorganic, organic nanomaterials and nanocomposites are reviewed for the reliable and effective detection of harmful chemicals by electrochemical and current–voltage (I–V) characteristics.

Keywords: Semiconductor materials, nanoscale aspects, conducting polymer, nanocomposites thin films, chemiresistors, chemosensors

1.1 Introduction

In recent years, numerous intensive research efforts in the field of nanotechnology have shown great potential. There has been a significant improvement for the synthesis of desired organic/inorganic nanomaterials for the applications in areas of energy conversion, sensing, electronics, photonics, and biomedicine. With the development of nanoscience and nanotechnology, one-dimensional (1D) semiconducting nanostructured materials like nanotubes, nanorods, nanosheets, nanoballs, and other nanostructured materials have been widely applied for the fabrication of varieties of chemosensors. It is generally accepted that 1D nanostructure are ideal systems for exploring a large number of novel phenomena at nanoscale and investigating the size and dimensionality dependence of structure properties for potential applications [1]. Among the inorganic semiconductor nanomaterials, 1D metal oxide nanostructures are the focus of current research efforts in nanotechnology since they are the most common minerals on earth due to their special shapes, compositions, and chemical, and physical properties [2]. On the other hand, the nanocomposites generally contain more than one single component and achieve the properties which are different from those of single component nanomaterials and thus, could be widely used for the effective fabrication of chemiresistors to detect the harmful toxic chemicals. The conducting polymers like polypyrrole (PPy), polythiophene, polyindol, polyaniline (PANI), and polyfuran are known p-type semiconductors with unique electronic properties, low energy optical transitions, low ionization potential and high electron affinity [3] and thus, widely used as sensitive materials for conductometric polymer sensors. The conducting polymers could be easily synthesized through simple chemical or electrochemical process and their conductivities could be altered by modifying the electronic structures through doping or de-doping procedures [4] and therefore, conducting polymers could suitably work as an effective working electrode and might offer the fast response toward the detection of various harmful chemicals. The sensor technology is popularly known for the detection of harmful chemicals and the sensitivity, selectivity, and stability are the most important aspect of investigation of a variety of sensors. Up to now, efforts have been made by controlling the sensors structures [5, 6], sensor fabrication techniques [7], and surface modification [8] to detect the toxic chemicals. Among several sensors like fluorescence based chemical sensors [9], chemically modified electrode chemical sensors [10] and chemiluminescence based sensors [11], the electrochemical method provides the advantages of high sensitivity, wide linear range, economical, rapid response, portability and ease of operating procedure [12, 13]. However, the electrochemical method is still a challenge to enhance the electron transfer rate over the surface of working electrode for sensors. Therefore, the modifications of the electrodes with different inorganic and organic nanomaterials could be promising for the reliable and effective detection of harmful chemicals by electrochemical and current–voltage (I–V) characteristics. In this chapter, we have briefly discussed the semiconducting metal oxides nanostructures like TiO2, ZnO, conducting polymers, and nanocomposites for the efficient detection of harmful and toxic chemicals. The preparation methods, morphologies, the physical and chemical properties of metal oxides, nanocomposites, and conducting polymers have shown the significant impacts on the optical, electrical, and electronic properties of the nanomaterials and their performances for detecting the harmful chemicals. The chapter briefly surveys several metal oxides, nanocomposites and conducting polymers in terms of their processing, functionality, and applications in sensing the harmful chemicals. With addition, the recent literatures have been reviewed on the basis of morphology, structure, and physiochemical properties of TiO2 and ZnO nanostructured semiconductors with brief description on the recent literatures of their sensing applications. TiO2 and ZnO nanostructures based chemiresistors have shown comparable sensing performances. It has been noticed that the sensing performance are considerably affected by the preparation, morphology, and the electrical properties of semiconducting metal oxide.

1.2 Semiconducting Metal Oxide Nanostructures for Chemiresistor

1.2.1 Prospective Electrode of TiO2 Nanotube Arrays for Sensing Phenyl Hydrazine

The inorganic metal oxide materials in nanoscale have recently received a great deal of interest owing to their unique structures, electrical, and catalytic properties [14, 15]. The nanostructures of titania (TiO2) are one of most versatile metal oxides, exhibiting exotic inert surface and the optical properties. The tailoring of multidimensional TiO2 nanostructures to 1D is playing a significant role for determining the physiological and electrical properties. One-dimensional TiO2 nanomaterials are reported to display the large surface to volume area as compared to the bulk materials [16, 17]. Among various 1D TiO2 nanostructures, TiO2 NTs generally exhibits the large surface area [18], outstanding charge transport properties [19], excellent electronic, mechanical, and chemical stability properties [20]. In particular, TiO2 NTs with highly uniform morphology and unique-orientated growth properties are promising for the applications in gas sensors and biosensors [21]. Recently, TiO2 nanostructures with high surface area are extensively utilized for the detection of harmful chemicals through sensing. Kwon et al. studied the enhanced ethanol sensing properties over the surface of TiO2 NTs electrode based sensors [22]. Chen et al. fabricated a room temperature hydrogen sensor with TiO2 NT arrays based electrode [23]. Ameen et al. [24] have grown TiO2 NT arrays on Ti foil substrate by simple electrochemical anodic oxidation and utilized as the working electrode for the fabrication of a highly sensitive, reliable, and reproducible chemical sensor for the detection of harmful phenyl hydrazine chemical.

The grown TiO2 on Ti substrate displays highly ordered and self-assembled NT arrays, as shown in Figure 1.1(a). At high magnification (Figure 1.1(b)), a uniform and closely packed TiO2NT arrays are seen. Moreover, the grown TiO2 NT arrays present distinguishable diameter distribution. The average diameter and wall thickness of TiO2 NT arrays are observed as 100 ± 20 nm and 20 ± 5 nm, respectively. The inset of Figure 1.1(a) depicts the cross-sectional FESEM image of grown TiO2 NT arrays which exhibits the average length of ~15 µm. Figure 1.1(c, d) shows the transmission electron microscopy, high resolution (HR) TEM, and selected area electron patterns (SAED) of TiO2 NT arrays. A hollow tubular morphology could be seen in the TEM image of grown TiO2 NT arrays (Figure 1.1(c)), which is consistent with the FESEM results. The TiO2 NT arrays exhibit the average diameter and wall thickness of 100 ± 20 and 20 ± 5 nm, respectively. The SAED pattern of TiO2 NT arrays (inset of Figure 1.1(c)) shows polycrystalline phases in the anatase TiO2. The HRTEM image of TiO2 NT arrays (Figure 1.1(d)) shows well-resolved lattice fringes of crystalline TiO2 NT arrays with plane spacing of ~0.35 nm, which corresponds to anatase TiO2 (101). These observations considerably deduce the good crystallinity of grown TiO2 NT arrays. The phase composition and structural properties of TiO2 NT arrays are investigated by Raman scattering spectroscopy and the corresponding mapping, as shown in Figure 1.1(e, f). The grown TiO2 NT arrays obtain three active Raman modes at ~396.1, ~518.2, and ~637.4 cm–1 which correspond to the active Raman modes of anatase phase with symmetries of B1g(1), (B1g(2) + Ag(1)), and Eg3, respectively, and match with the Raman modes of anatase TiO2 [25, 26]. Consequently, Raman spectrum does not exhibit any Raman mode at ~445 cm–1, indicating that no rutile phase exist in the grown TiO2 NT arrays. The inset of Figure 1.1(e) shows the Raman mapping in the range of ~390–460 cm–1 and reveals the larger dark part which corresponds to the peak at ~396.1 cm–1 whereas Figure 1.1(f) shows the Raman mapping in the range of ~550–650 cm–1 which exhibits highly uniform surface, suggesting that the grown TiO2 NT arrays are in good quality of anatase TiO2 phase.

Figure 1.1 FESEM images of TiO2 NT arrays at low (a) and high (b) magnifications. Inset of (a) shows cross section image of TiO2 NT arrays, TEM image (c), and HRTEM image (d) of TiO2 NT arrays. Inset of (c) shows SAED patterns of TiO2 NT arrays, (e) Raman spectrum and inset of (e) shows the corresponding Raman mapping images in 390–460 cm–1, and (f) the corresponding Raman mapping images 550–650 cm–1 of TiO2 NT arrays.

Reprinted with permission from Ameen, App. Phys. Lett. 103(2013) 061602. @ 2013, American Institute of Physics Ltd.

The typical amperometric plot is shown in Figure 1.2(a). In the beginning, the electrochemical experiment is performed in PBS without phenyl hydrazine for stabilizing the background current. The successive addition of phenyl hydrazine has shown the linear increase in the current, exhibiting a linear relationship between the current and phenyl hydrazine concentrations. Figure 1.2(b) depicts the linear plot of current versus concentration of phenyl hydrazine for TiO2 NT arrays electrode which again confirms the linear relationship between current and concentration of phenyl hydrazine. These observations infer that the TiO2 NT arrays electrode is highly effective catalyst to detect the sensing response of phenyl hydrazine at very low concentration which might attribute to the good electrocatalytic, and direct electron transfer or fast electron exchange behavior of TiO2 NT arrays. The current (I)–voltage (V) characteristics are measured for evaluating the sensing properties (sensitivity, detection limit, and correlation coefficient) of the fabricated phenyl hydrazine chemical sensor over TiO2 NT arrays electrode. The current response is measured from 0.0–2.5 V, and the time delaying and response times are 1.0 and 10 s, respectively. The detailed sensing parameters of the fabricated phenyl hydrazine chemical sensor based on TiO2 NT arrays electrode are evaluated by measuring a series of I–V characteristics with various concentrations of phenyl hydrazine (0.25 µM–0.10 mM). From Figure 1.2(c), the current has continuously increased with the increase of the phenyl hydrazine concentrations from 0.25 µM–0.10 mM, suggesting the good sensing response toward phenyl hydrazine chemical by TiO2 NT arrays electrode. The enhancement in current might due to the better electrocatalytic behavior, generation of large number of ions, and the increase of ionic strength of the solution with the addition of phenyl hydrazine. The sensing parameters such as sensitivity, detection limit, and correlation coefficient are calculated by a calibration curve of current versus phenyl hydrazine concentration of the fabricated phenyl hydrazine chemical sensor. Figure 1.2(d) presents the plot of calibration current versus phenyl hydrazine concentration which reveals that the current increases linearly up to the phenyl hydrazine concentration of ~1 µM and afterwards achieve a saturation level in the calibrated plot. The saturation point occurs due to the less availability of free active sites on the surface of TiO2 NT arrays electrode for phenyl hydrazine chemical at higher concentration (>10 µM) in PBS. The fabricated phenyl hydrazine chemical sensor based on TiO2 NT arrays electrode exhibits significantly high and reproducible sensitivity of ~40.9 µA.mM–1.cm–2 and the detection limit of ~0.22 µM with correlation coefficient (R) of ~0.98601 and short response time of 10 s. Importantly, the fabricated phenyl hydrazine chemical sensor based on TiO2 NT arrays electrode displays a good linearity in the range of 0.25 µM–1 µM. Thus, TiO2 NT arrays electrode provides suitable surface for the oxidation of phenyl hydrazine and determines the sensing responses by increasing the current values. The reusability and reproducibility of the fabricated phenyl hydrazine chemical sensor based on TiO2 NT arrays electrode were elucidated by measuring the sensing responses with the I–V characteristics for three consecutive weeks. The sensing parameters or properties showed the negligible drops in the fabricated phenyl hydrazine chemical sensor based on TiO2 NT arrays electrode, which deduces the long term stability of the fabricated phenyl hydrazine chemical sensor. Thus, TiO2 NT arrays with anatase phase and good crystal quality are promising and effective working electrode for the detection of phenyl hydrazine chemical or other hazardous chemicals.

Figure 1.2 (a) Typical amperometric plot and (b) linear plot of current versus concentration of phenyl hydrazine of TiO2 NT arrays based chemical sensor. (c) The I–V characteristics and (d) the calibration curve of current versus phenyl hydrazine concentration of TiO2 NT arrays electrode based chemical sensor at different phenyl hydrazine concentrations (0.25 µM–0.10 mM) in 10ml of 0.1M PBS, and (e) schematic illustration of proposed mechanism of phenyl hydrazine chemical sensors over the surface of TiO2 NT arrays based electrode.

Reprinted with permission from Ameen, App. Phys. Lett. 103 (2013) 061602. @ 2013, American Institute of Physics Ltd.

1.2.2 Aligned ZnO Nanorods with Porous Morphology as Potential Electrode for the Detection of p-Nitrophenylamine

Aniline and its derivatives are the main constituents in various manufacturing industries of polymers, dyes, pesticides, pharmaceutical, rubber chemicals, explosives, and paintings [27, 28]. The excess release of aniline and its derivatives severely disturbs the water ecosystem and clean environment because it easily adsorbs on the colloidal organic matter and moves from soil to the groundwater [29]. The metal oxide semiconductors such as SnO2 [30], TiO2 [31], ZnO [32], Fe2O3 [33], In2O3 [34], CeO2 [35], and WO3 [36] have been utilized as efficient electron mediator electrode materials for the detection of harmful chemicals. ZnO semiconductors with wide band gap of ~3.37 eV are the most exotic and versatile materials owing to its high exciton, binding energy, biocompatibility, better electrochemical activities, non-toxicity, chemical and photochemical stability and high electron communication features [37–39]. In particular, ZnO nanomaterials based electrode shows good electrochemical activities toward chemicals, biomolecules and gases owing to their high electron transfer characteristics with high electrochemical and photochemical stability [40, 41]. The changes in the surface and morphological properties of ZnO materials display significant impact on the performances of electrochemical system [42, 43]. Recently, 1D ZnO nanostructures such as nanobelts (NBs), nanowires (NWs), nanotubes (NTs), and NRs are extensively studied due to their sufficiently high surface-to-volume ratio and good electrical characteristics which are essential for fabricating various electrochemical and photoelectrochemical devices [44, 45]. Among 1D ZnO nanomaterials, aligned ZnO NRs are the highly explored material especially, in the fabrication of optoelectronic, electrochemical, photoelectrochemical and solar devices because of its unique electronic and high surface-to-volume ratio [46]. Recently, J.J. Hassan fabricated the high-sensitivity room temperature hydrogen gas sensor based on oblique and vertical ZnO nanorod arrays [47]. Ameen et al. reported the synthesis of vertically aligned ZnO NRs by low temperature solution process for the detection of hydrazine chemical via chemical sensor based on aligned ZnO NRs and demonstrated the high sensitivity [44]. Y. Lv et al. prepared ZnO NRs by a simple solution route for the fabrication of triethylamine gas sensor. Umar et al. fabricated the electrochemical sensor for the detection of hydroquinone chemical by Ce doped ZnO NRs electrode [48]. The synthesis of aligned nanoporous ZnO NRs thin film for the selective detection of p-nitrophenylamine (p-NPA) chemical was reported by Ameen et al. [49]. The synthesized aligned nanoporous ZnO NRs were characterized in terms of morphological, structural, optical, electrochemical and sensing properties. The fabricated aligned nanoporous ZnO NRs electrode based chemical sensor showed the rapid detection of p-NPA with high sensitivity of ~184.26 µA.mM–1.cm–2 and good reproducibility.

Figure 1.3 shows the FESEM images of aligned nanoporous ZnO NRs thin film. The cross section of ZnO thin film, as shown in Figure 1.3(a) describes highly dense and well aligned nanoporous ZnO NRs with the average length of ~3–4 µm. The low magnification surface image, as shown in Figure 1.3(b, c) displays uniform growth of aligned ZnO NRs with nanoporous surface. The average diameter of each hexagonal ZnO NRs is estimated as ~200–300 nm, as shown in the inset of Figure 1.3(c). To check the elemental composition, the EDX spectrum (Figure 1.3(d)) reveals that the grown aligned nanoporous ZnO NRs exhibit good aspect ratio of Zn and O elements in terms of atomic weight percentage. No other peak has been seen in the EDX spectrum, indicating the purity of the synthesized aligned nanoporous ZnO NRs.

Figure 1.3 (a) Cross-section view and (b, c) surface view FESEM images and (d) the EDX spectrum of aligned nanoporous ZnO NRs. Inset shows the surface image at high magnification of aligned nanoporous ZnO NRs.

Reprinted with permission from Ameen, Appl. Catal. A: Gen. 470 (2014) 271. @ 2014, Elsevier Ltd.

The UV-DRS and room temperature PL analysis were used to investigate the optical properties of grown aligned nanoporous ZnO NRs thin film. Figure 1.4(a) shows the UV-DRS spectrum and derived band energy plot of aligned nanoporous ZnO NRs thin film (as shown in the inset of Figure 1.4(a)). An intense absorption edge at ~384 nm near UV region is recorded by the grown aligned nanoporous ZnO NRs thin film, which usually originates from a charge transfer process from the valence band to conduction band of ZnO [50–52]. From UV-DRS graph, the band gap of aligned nanoporous ZnO NRs thin film is calculated as ~3.23 eV which is approximately same to the band gap of bulk ZnO nanomaterials [53, 54]. This indicates that the grown aligned nanoporous ZnO NRs thin film actively absorbs the UV light as normal bulk ZnO materials. Figure 1.4(b) depicts the room temperature PL spectrum of grown aligned nanoporous ZnO NRs thin film. Two emission PL peaks are observed in which an intensive sharp UV emission at ~383.2 nm occurs due to the free exciton emission from the wide band gap of aligned nanoporous ZnO NRs thin film, while a broader green emission at ~530.1 nm is attributed to the recombination of electrons in single occupied oxygen vacancies and structural defects in the aligned nanoporous ZnO NRs [55]. Generally, the broad green emission is originated from the increased concentration of singly ionized oxygen vacancies and generation of non-stoichiometric phase structure in the grown ZnO nanomaterials. The intense UV emission with weak broad green emission suggests the good optical properties and high quality of aligned nanoporous ZnO NRs grown on FTO substrate. This result is in consistent with the UV-DRS studies.

Figure 1.4 UV-DRS (a) and room temperature PL spectra (b) of aligned nanoporous ZnO NRs.

Reprinted with permission from Ameen, Appl. Catal. A: Gen. 470 (2014) 271. @ 2014, Elsevier Ltd.

The cyclovoltametry (CV) analysis was performed to elucidate the electrochemical properties of aligned nanoporous ZnO NRs thin film electrode toward p-NPA chemical. The CV measurements were performed using three electrode systems comprising of aligned nanoporous ZnO NRs thin film as working electrode, Pt wire as cathode and reference Ag/AgCl electrode. The electrochemical system without p-NPA chemical in phosphate buffer solution (PBS, pH = 7.0) depicted very low oxidation or anodic current, while a significant increment in the oxidation current occured after the addition of p-NPA (5 µM) in PBS. This change indicated that the aligned nanoporous ZnO NRs electrode executed quick sensing response toward p-NPA chemical which might result from the good electrocatalytic surfaces of aligned nanoporous ZnO NRs. Moreover, the typical CV of aligned nanoporous ZnO NRs electrode with a series of p-NPA concentrations (5–150 µM) in 0.1 M PBS at the scan rate of 50 mVs–1 was performed. With the increase of p-NPA concentration, the oxidation current substantially increased and reached to the highest current of ~239.8 µA at highest concentration of p-NPA (150 µM), which was 4 factors higher than the oxidation current at lowest p-NPA concentration (5 µM). In general, the high height of the oxidation peak is related to the faster electron transfer reaction in the electrochemical system and represented the high electrocatalytic behavior of the electrode [56]. Herein, aligned nanoporous ZnO NRs electrode exhibited increased oxidation current which might due to the high electrocatalytic behavior toward p-NPA chemical in PBS.

To investigate the detailed sensing parameters of aligned nanoporous ZnO NRs electrode, I–V measurements was performed in 10 ml of 0.1 M PBS solution without and with a series of p-NPA concentrations at room temperature. Figure 1.5(a) shows the I–V characteristics of aligned nanoporous ZnO NRs electrode without and with p-NPA (5 µM) in 10 ml of 0.1 M PBS solution. It is seen that the low current value is detected without p-NPA, but it is drastically increased after the addition of p-NPA, suggesting the quick sensing response to p-NPA chemical. Furthermore, Figure 1.5(b) exhibits the typical I–V response of aligned nanoporous ZnO NRs thin film electrode at various concentrations of p-NPA (5–150 µM) into 0.1 M PBS solution (pH = 7.0)). A continuous increase in the current with the increase in the concentrations of p-NPA from 5–150 µM is seen which suggests the generation of large number of ions and the increase of ionic strength of the solution with the addition of different concentrations of p-NPA. On the basis of these results, highly active electrocatalytic surface of the grown aligned nanoporous ZnO NRs electrode favors the efficient detection of p-NPA. The sensitivity and other sensing parameters are evaluated by the plot of calibrated current and the wide range of p-NPA concentrations, as shown in Figure 1.5(c). The current increases with the increase in the concentrations of p-NPA up to ~40 µM and then, the saturation starts after increasing the concentration of p-NPA. The fabricated p-NPA chemical sensor based on aligned nanoporous ZnO NRs electrode shows considerably high sensitivity of ~184.26 µA.mM–1.cm–2 and reasonable limit of detection (LOD) of ~53.7 µM with response time of 10 s. A good linear dynamic range (LDR) from 5–20 µM and the correlation efficient of R = ~0.97569 are obtained. Importantly, the fabricated p-NPA chemical sensor based on aligned porous ZnO NRs electrode demonstrates highly reliable and reproducible sensitivity. An illustration of sensing response for p-NPA chemical over the surface of aligned nanoporous ZnO NRs electrode is presented in Figure 1.5(d). In the beginning, p-NPA chemical is chemisorbed over the surface of ZnO NRs electrode due to presence of active sites. These active sites on ZnO NRs generates by the easy interaction of atmospheric oxygen or oxygenated species through the transfer of electrons from the conduction band of ZnO to the adsorbed oxygen atoms [57]. The oxygenated species are generally present in the form of HO–, O–, O2–, etc. After the addition of p-NPA, the oxygenated species (like HO–) on ZnO NRs get combined with p-NPA and oxidizes into CO2 and H2O after passing through several intermediate reactions as presented in Figure 1.5(d). The aligned porous ZnO NRs electrode exhibits reasonably good surface area of ~74.8 m²/g, which might helpful in the large generation of active sites over the surface of ZnO NRs. The quick and enhanced sensing properties could be explained by the nanoporous surface of aligned nanoporous ZnO NRs electrode which might provide large number of active sites surface for the detection of p-NPA. The high sensitivity and the good linearity of aligned nanoporous ZnO NRs electrode based chemical sensor are attributed to the excellent adsorption ability and high electrocatalytic and electrochemical activities of aligned nanoporous ZnO NRs thin film, as described in CV and amperometry results. The stability of p-NPA chemical sensor was analyzed by measuring the sensing performances via the I–V characteristics for three consecutive weeks. It was seen that no significant fall was observed in the sensing parameters of the fabricated p-NPA chemical sensor, confirming the long term stability of the fabricated aligned nanoporous ZnO NRs electrode based chemical sensor.

Figure 1.5 (a) The I–V characteristics of aligned nanoporous ZnO NR electrode without and with p-NPA (5 µM) in 10 ml of 0.1 M PBS solution, (b) I–V responses of aligned nanoporous ZnO NR electrode at various concentrations of p-NPA (5 µM–150 µM) in 0.1 M PBS (pH =7.0), (c) plot of calibrated current and the wide range of p-NPA concentrations, and (d) proposed mechanism of p-NPA during the electrochemical reaction.

Reprinted with permission from Ameen, Appl. Catal. A: Gen. 470 (2014) 271. @ 2014, Elsevier Ltd.

1.2.3 ZnO Nanotubes as Smart Chemiresistor for the Effective Detection of Ethanolamine Chemical

Unique wide band gap semiconductor called ZnO is highly explored and promising nanomaterial for a series of applications in field-effect transistors, lasers, photodiodes, sensors, and photovoltaics [58–60]. Due to excellent dielectric, ferroelectric, piezoelectric and pyroelectric properties, the ZnO nanomaterial find numerous applications particularly in photocatalysis, chemical sensors, memory resistors and photovoltaics [61, 62]. ZnO nanostructures in one dimension (1D) such as nanorods, nanobelts, nanowires and nanotubes show high surface-to-volume ratio, slow electron-hole recombination rate and good electrical characteristics [63]. Recently, the electrochemical method is adopted as the most acceptable technique to fabricate high performance chemical sensor. This system displays several advantages of high sensitivity, wide linear range, economical, rapid response, portability, and ease of operating procedure [64], but still facing the major problems of electron loss and low electron transfer rate over the surface of sensor’s working electrode. Effective modifications of the working electrodes are necessitated to increase the electron transfer rate and the stability of chemical sensors for the reliable detection of harmful chemicals [65]. Ameen et al. fabricated a hydrazine chemical sensor using vertically aligned ZnO NRs electrode and demonstrated the high sensitivity toward hydrazine chemical [66]. Similarly, Ibrahim et al. reported the Ag doped ZnO nanoflowers based sensor for the detection of phenyl hydrazine chemical via electrochemical system [67]. So far, various chemical sensors are fabricated for the detection of harmful chemicals such as ethanol, propanol, phenol and nitroaniline, etc [68, 69]. In continuation, the ethanolamine chemical is one of the most useable chemicals at laboratory level but its excess discharge is a big threat to the environment. The inhalation or insertion of ethanolamine might affect the respiratory system, creates drowsiness, skin irritation and fatal to central nervous system. Ameen et al. [70] synthesized the aligned ZnO nanotubes (NTs) thin films on FTO substrate by low temperature solution method and extensively utilized as smart and effective working electrode for ethanolamine chemical sensor. The fabricated chemical sensor based on aligned ZnO NTs thin film electrode showed the rapid detection of ethanolamine with the high sensitivity of ~37.4 × 10–4 mA.mM–1.cm–2, good linearity of ~0.05 mM–1 mM and the detection limit of ~19.5 µM.

The morphological properties are shown in Figure 1.6(a, b). The vertically arranged tubular morphology of ZnO on FTO substrate is observed (Figure 1.6(a)). At high magnification image (Figure 1.6(b)), each ZnO NT exhibits the hexagonal hollow morphology with the average diameter of ~700–900 nm and the outer thickness of ~60–100 nm. It is also seen that the aligned ZnO NTs are in the form of nanograins which have few developed free surfaces as well as grain boundaries and interfaces. The presence of these grain boundaries in aligned ZnO NTs might define the good optical and physical properties of nanograined ZnO NTs [71]. The aligned ZnO NTs thin film is further characterized by X-rays diffraction patterns to investigate the crystalline nature of grown nanomaterials, shown in Figure 1.6(c). The aligned ZnO NTs thin film shows the diffraction peaks at 31.8° (100), 34.5° (002), 36.3° (101), 47.8° (102), 56.7° (110), 63.1° (103), and 65.6° (200), corresponding to the wurtzite structure of ZnO crystals [72] and are well indexed to JCPDS No. 36–1451. The diffraction peak at 34.5° (002) displays much higher intensity, suggesting the preferential growth direction due to the instability of polar (002) plane [73]. The other diffraction peaks are associated to FTO substrate [74]. The occurrence of these grain boundaries in ZnO NTs is explained by the formation of invisible amorphous superficial and intergranular layers in between ZnO NTs [62]. The structural and purity of aligned ZnO NTs thin film are further analyzed by Fourier transform infrared, as shown in Figure 1.6(d). A strong IR band at ~540 cm–1 is observed which corresponds to the typical Zn-O group of ZnO nanomaterials [75]. The IR peaks at ~3380, ~1643, and ~890 cm–1 are associated to OH stretching mode, water scissoring vibration and bending vibration of nitrate, respectively [76]. Therefore, the grown NTs are pure and well crystalline ZnO along with few impurities.

Figure 1.6 FESEM images at low (a) and high (b) magnifications, XRD patterns (c) and FTIR spectrum (d) of aligned ZnO NTs thin film.

Reprinted with permission from Ameen, Mater. Lett. 106 (2013) 254. @ 2013, Elsevier Ltd.

The sensing behavior of aligned ZnO NTs electrode was demonstrated toward the concentration of ethanolamine chemical. Figure 1.7(a) depicts the illustration of the fabricated ethanolamine chemical sensor based on aligned ZnO NTs electrode. Typically, the sensor comprises of two electrode electrochemical system using aligned ZnO NTs as working electrode and Pt wire as a cathode in a fixed amount of phosphate buffer solution (PBS, 0.1 M, 10 ml, pH 7) as an electrolyte. A low current is detected when the electrochemical system does not contain ethanolamine chemical, as shown in Figure 1.7(a). After the addition of ethanolamine (0.05 mM), a drastic increase in the current is observed, suggesting that aligned ZnO NTs electrode shows quick sensing response toward the low concentration (0.05 mM) of ethanolamine. The detail sensing parameters of the fabricated ethanolamine chemical sensor were investigated by performing a series of I–V characteristics (Figure 1.7(b)) with various ethanolamine concentrations ranging from 0.05–10 mM in 0.1 M PBS. The current increases continuously with the increase of ethanolamine concentrations from 0.05–10 mM. These increments represent the generation of large number of ions and the increase of ionic strength of the solution with the addition of different concentrations of ethanolamine chemical. Figure 1.7(c) shows the plot of calibrated current versus ethanolamine concentrations to evaluate the sensitivity of the fabricated ethanolamine chemical sensor. The current increases linearly with the increase of ethanolamine concentrations up to 1 mM and afterwards, a saturation level is recorded in the calibrated plot which might due to the unavailability of free active sites over aligned ZnO NTs electrode for ethanolamine adsorption at the higher concentration (>1 mM). The fabricated aligned ZnO NTs electrode based ethanolamine chemical sensor shows significantly high, reliable and reproducible sensitivity of ~37.4 × 10–4 mA.mM–1.cm–2, and the detection limit of ~19.5 µM with good linearity of ~0.05–1 mM, correlation coefficient (R) of ~0.9850 and a short response time (10 s). To investigate the reproducibility of the sensor, the I–V characteristics of the fabricated ethanolamine chemical sensor were measured for three consecutive weeks and found that the sensor did not show any significant decrease in the sensitivity and other sensing parameters. These results deduced the long term stability of the fabricated ethanolamine chemical sensor based on aligned ZnO NTs electrode. Moreover, the high sensitivity is determined by the presence of large number of oxygenated species on the surface of aligned ZnO NTs electrode which might considerably create large number of active sites for the conversion of ethanolamine to less harmful products. Thus, the aligned ZnO NTs electrode could be a promising electrode for fabricating the advanced chemical sensor to detect the harmful chemicals.

Figure 1.7 Schematic illustration of the fabricated ethanolamine chemical sensor (a), the I–V characteristics of aligned ZnO NTs thin film modified chemical sensor in ethanolamine concentrations in 10 ml PBS solution (b) and corresponding calibration curve of current versus ethanolamine concentrations (c).

Reprinted with permission from Ameen, Mater. Lett. 106 (2013) 254. @2013, Elsevier Ltd.

1.3 Conducting Polymers Nanostructures for Chemiresistors

1.3.1 Sea-Cucumber-Like Hollow Polyaniline Spheres as Efficient Electrode for the Detection of Aliphatic Alcohols

Sensor technology is popularly known for the detection of volatile organic compounds (VOCs) such as alcohols, ethers, esters, halocarbons, ammonia, NO2, and warfare agent stimulants [77, 78]. Among various chemicals, ethanol is the commonly used highly corrosive essential chemical which is easily miscible with water or several other organic solvents [79, 80]. In particular, ethanol is extensively used chemical in perfumes or fragrances, colorings and medicinal industries and as feeding solvent for the synthesis of several organic products on a large scale [81]. Due to aforementioned causes, a reliable, simple, economical, highly sensitive, rapid and accurate method or technology is required for the early detection of ethanol. Conjugated polymers offer unique electronic properties owing to their good electrical conductivity, low energy optical transitions, low ionization potential and high electron affinity [82]. PANI shows unique electrical and electronic properties which could be easily changed either by the oxidation of PANI chain or by the protonation of imine nitrogen polymer backbone [83, 84]. S.S. Barkadea et al. prepared PANI/Ag nanocomposites electrode for analyzing ethanol vapor sensing [85]. A. Choudhury et al. fabricated the modified electrode of PANI thin film and demonstrated the sensing properties of alcohol vapor [84]. Additionally, M.H.H. Jumali and co-workers studied the influence of PANI on ZnO thin films for methanol sensing properties [86]. Ameen et al. [87] reported the sea-cucumber-like hollow PANI sphere through template free method, where the existence of the hydrogen bond between OH group of salicyclic acid and amine group of polymer chain was a driving force to form self-assembled hollow PANI spheres. The fabricated ethanol chemical sensor based on sea-cucumber-like hollow PANI spheres electrode exhibited an ultrahigh sensitivity of ~426.5 µA.cm–2. mM–1 with a correlation coefficient (R) of ~0.90157.

The morphology of synthesized PANI nanostructures is studied by FESEM images, as shown in Figure 1.8. The synthesized PANI nanostructures possess sea-cucumber-like morphology with a hollow open mouth which is much similar to the real image of sea-cucumber (Holothuroids), as shown in inset of Figure 1.8(a). The average diameter of each sea-cucumberlike hollow PANI sphere is in the range of ~1–2 µm (Figure 1.8(b)). Noticeably, the shell of PANI sphere is of thickness ~100–200 nm, consisting uniformly distributed nanofibers which assemble into the morphology of sea-cucumber-like hollow PANI spheres. From Figure 1.8(c, d), the FESEM images of sea-cucumber-like hollow PANI sphere electrode (pellet) displays the highly compact and uniform morphology with few voids. The high magnification image of the electrode depicts the non-damaging hollow PANI spheres, indicating the stability of sea-cucumber-like hollow PANI morphology.

Figure 1.8 FESEM images at low (a) and high resolution (b) of sea-cucumber-like hollow PANI spheres and surface morphology of sea-cucumber-like hollow PANI spheres electrode at (c) low and (d) high magnification.

Reprinted with permission from Ameen, RSC Adv. 3 (2013) 10460. @ 2013, RSC Pub.

Figure 1.9 shows the TEM images of sea-cucumber-like hollow PANI spheres. The morphology of PANI nanostructures has not changed under the electron beam and exhibits the similar sea-cucumber-like hollow spheres morphology, as shown in Figure 1.9(a). The single hollow sphere is of the average diameter of ~1–2 µm and the shell of the hollow spheres is comprised of sea-cucumber-like nanostructures, as shown in Figure 1.9(b). The element mapping images of sea-cucumber-like hollow PANI spheres have been analyzed to elucidate the elemental compositions of the synthesized PANI nanostructures. The elements mapping images of sea-cucumber-like hollow PANI spheres for carbon (C), nitrogen (N), and sulfur (S) elements are shown in Figure 1.10. The sea-cucumber-like hollow PANI sphere exhibits the major distribution of C and N element which indicates that the synthesized PANI is composed of C and N atoms. However, some assays of S are also detected which might present due to the use of oxidant during polymerization. Thus, the majorly distributed C and N elements in the synthesized PANI confirm the formation of sea-cucumber-like hollow PANI spheres.

Figure 1.9 TEM images at low (a) and high resolution (b) of sea-cucumber-like hollow PANI spheres.

Reprinted with permission from Ameen, RSC Adv. 3 (2013) 10460. @ 2013, RSC Pub.

Figure 1.10 Elemental mapping images of (a) surface view, (b) mixed view, (c) C, (d) N, and (e) S elements of sea-cucumber-like hollow PANI spheres.

Reprinted with permission from Ameen, RSC Adv. 3 (2013) 10460. @ 2013, RSC Pub.

The Raman scattering spectrum of sea-cucumber-like hollow PANI spheres is shown in Figure 1.11. The synthesized sea-cucumber-like hollow PANI spheres obtain the Raman bands at 1170, 1477, and 1597 cm–1, corresponding to the C–H bending vibration of semi quinonoid rings (cation-radical segments), N–H deformation vibration associated with the semiquinonoid structures and C=C stretching vibration in the quinonoid ring, respectively [88, 89]. A Raman band at 1361 cm–1 provides the information of C–N+• vibration of delocalized polaronic structures in the synthesized sea-cucumber-like hollow PANI spheres [90]. These structural characterizations reveal that the synthesized PANI nanostructures possess the typical structure of PANI. Figure 1.11(b, c) shows the Raman mapping by selecting the two ranges of ~1150–1175 cm–1 and ~1470–1500 cm–1. A Raman shifts at ~1170 cm–1 in the range of ~1150–1175 cm–1 is assigned to the C–H bending vibration of the semiquinonoid rings (shown in Figure 1.11(a)), as expressed by the light blue color in the mapping image (Figure 1.11(b)). The Raman mapping image in the range of ~1470–1500 cm–1 (Figure 1.11(c)) significantly exhibits the N–H deformation vibration which is associated with the semiquinonoid structures in sea-cucumber-like hollow PANI spheres. The corresponding Raman mapping clearly indicates the composition of sea-cucumber-like hollow PANI spheres with uniform distribution of C–H bending vibration and N–H deformation vibration of semiquinonoid rings in sea-cucumberlike hollow PANI spheres.

Figure 1.11 Raman spectrum (a) and its corresponding Raman mapping in 1150–1175 cm–1 (b) and 1470–1500 cm–1 (c) of sea-cucumber-like hollow PANI spheres.

Reprinted with permission from Ameen, RSC Adv. 3 (2013) 10460. @ 2013, RSC Pub.

Figure 1.12 shows the CV analysis to investigate the electrocatalytic activity of sea-cucumber-like hollow PANI spheres electrode toward the detection of ethanol. Figure 1.12(a) shows the typical cyclic voltammogram (CV) of sea-cucumber-like hollow PANI spheres electrode without and with ethanol in 0.1 M phosphate buffer solution (PBS, pH=7.0) at the scan rate of 100 mVs–1. The CV in 0.1 M PBS (pH=7.0) without ethanol exhibits very low redox current density in the range of ~0.6 to 1.0 V. The introduction of ethanol in 0.1 M PBS (pH=7.0) increases the redox current density. It is noticed that the oxidation peak of CV is prominent with maximum anodic current ~5.56 × 10–4 A at 0.33 V, indicating the oxidation process of ethanol over the sea-cucumber-like hollow PANI spheres electrode. However, the electrochemical response of ethanol is reversible and the cathodic peak (Ic) of ~ –4.65 × 10–4 A at ~ –0.04 V is observed. Thus, the unique morphology of sea-cucumber-like hollow PANI spheres significantly involves the high electron transfer process via high electrocatalytic activity of electrode which might efficiently detect ethanol. Figure 1.12(b) shows CV responses of sea-cucumber-like hollow PANI spheres electrode at various scan rates from 10–200 mVs–1 in 0.1 M PBS buffer solution with 25 µM ethanol. The anodic current linearly increases with the increase of scan rates from 10–200 mVs–1 which suggests the oxidation process through the controlled diffusion. In other words, the increased anodic current at highest scan rate also suggests the favorable electro-oxidation of ethanol with the potential sweep. Moreover, the high anodic current is usually attributed to a faster electron transfer reaction in the electrochemical system by the high electrocatalytic behavior of the electrode [91]. Herein, the electrode based on unique sea-cucumber-like hollow PANI spheres might accomplish the faster electron transfer reaction and the electrocatalytic activity at high scan rate of 200 mVs–1.

Figure 1.12 (a) Typical CV curve without and with ethanol (25 µM) in 10 ml of 0.1 M PBS solution and (b) CV sweep curves at different scan rates of sea-cucumber-like hollow PANI spheres.

Reprinted with permission from Ameen, RSC Adv. 3 (2013) 10460. @ 2013, RSC Pub.

The electrochemical behavior of the modified electrode based on seacucumber-like hollow PANI spheres was further characterized by electrochemical impedance (EIS). Figure 1.13 shows the EIS plots of the sea-cucumber-like hollow PANI spheres electrode in 0.1 M PBS (pH = 7.0) with the different concentrations of ethanol (25 µM–10 mM) at a frequency range from 100 kHz–1 Hz. Figure 1.13(a–e) shows the EIS behavior, where the depressed semicircle in the high frequency region is observed which typically attributes to the parallel combination of the charge transfer resistance (RCT) of the electrochemical reaction and the double layer capacitance (Cdl) at the interface of the PANI film/electrolyte [92]. It is reported that signal response for sensing is determined by the values of RCT at the interfaces of the PANI electrode and different concentrations of ethanol in PBS [93]. The sea-cucumber-like PANI hollow spheres electrode shows the significant decrease in the RCT with the increased concentration of ethanol in PBS. The recorded RCT values are in the following order of ~135.9 kΩ (25 µM) > ~40.12 kΩ (50 µM) > ~13.07 kΩ (75 µM) > ~6.636 kΩ (0.1 mM) > ~0.667 kΩ (10 mM). In general, the higher RCT value results to the low charge transfer rate at the interface of PANI electrode/electrolyte in the electrochemical system [94]. Moreover, the RCT also depends on dielectric and the insulating features at the electrode/electrolyte interface [95]. At higher ethanol concentration (10 mM), the sea-cucumber-like hollow PANI electrode displays the smallest RCT value of ~0.667 kΩ, indicating the higher charge transfer rate and high sensing response toward ethanol chemical. In other words, the unique morphology of PANI and the concentration of ethanol significantly favor the high ions transport or charge transfer at the electrode and the solution (ethanol in PBS) interfaces.

Figure 1.13 EIS plots of sea-cucumber-like hollow PANI spheres based electrode at different concentration of ethanol.

Reprinted with permission from Ameen, RSC Adv. 3 (2013) 10460. @ 2013, RSC Pub.

Furthermore, the detailed sensing behavior of the fabricated ethanol sensor with sea-cucumber-like hollow PANI spheres electrode was further elucidated by measuring a series of current (I)–voltage (V) characteristics with various concentration of ethanol ranging from 25 µM–10 mM in 0.1 M PBS. The current successively increased with the increase of ethanol concentration from 25 µM–10 mM in PBS, which might explain by the generation of large number of ions with the addition of different ethanol concentration, resulting in the increased ionic strength of the solution. The calibration curve of current versus concentration showed the sensitivity of the fabricated ethanol sensor with sea-cucumber-like hollow PANI spheres electrode. The current increased with the increase of ethanol concentration up to ~1 mM and then reaches a saturation level, which was consistent with CV and EIS results. The saturation point occured due to the unavailability of free active sites over the surface of sea-cucumber-like hollow PANI spheres electrode for the adsorption of ethanol chemical at the higher concentration (>1 mM). Using the calibration curve, the sensitivity of ethanol was evaluated by taking the slope and divided by an active area of the electrode (0.5 cm²). The high and the reproducible sensitivity of ~426.5 µA.mM–1.cm–2 with a correlation coefficient (R) of ~0.90157 was obtained by the sea-cucumber-like hollow PANI spheres electrode based chemical sensor toward the detection of ethanol. The fabricated ethanol chemical sensor showed the reasonable detection limit of ~515.7 µM and a short response time (10 s). A good linearity in the range of 25 µM–0.1 mM was achieved with the sea-cucumber-like hollow PANI spheres electrode based ethanol chemical sensor. For stability and reproducibility or reversibility, the fabricated ethanol chemical sensor was monitored by measuring the I–V characteristics for three consecutive weeks. No significant fall was detected in the sensing parameters or properties, suggesting the long term stability or durability of the fabricated ethanol chemical sensor based on sea-cucumber-like hollow PANI spheres electrode. This remarkably high sensitivity might explain due to the unique hollow morphology of PANI nanostructures, good optical/electronic behaviors, strong electrocatalytic activity and strong adsorptive properties of electrode toward the ethanol chemical. Furthermore, the sensing response of sea-cucumber-like hollow PANI spheres electrode to ethanol chemical was determined by steady state current–time measurements. At first, the electrochemical experiment was performed in PBS (10 ml) solution without ethanol to stabilize the background current. Thereafter, the ethanol solution (3 µM) was added successively drop by drop in 10 ml of PBS by the peristaltic pump after every 30 s. The steady state current–time responses of sea-cucumber-like hollow PANI spheres electrode to ethanol chemical showed that after every addition of ethanol chemical into PBS in the amperometric measurements significantly increases the current.

In order to check the selectivity of alcohols, a series of experiments have been performed using various alcohols such as methanol, propanol and butanol and the sensing parameters of each system were determined. Figure 1.14(a) shows the I–V characteristics of different alcohol sensors at 25 µM in 10 ml PBS. Among different alcohols, the ethanol chemical sensor exhibits the maximum current of ~2.62 × 10–5 A, indicating the highest sensing response with sea-cucumber-like hollow PANI spheres electrode. Figure 1.14(b) represents the comparative studies of sensitivities and the detection limit toward different alcohols in PBS over the sea-cucumber-like hollow PANI spheres electrode. The order of sensitivities of different alcohols are as ethanol (~426.5 µA.mM–1.cm–2) > propanol (~335.5 µA.mM–1.cm–2) > butanol (~331.6 µA.mM–1.cm–2) > methanol (~246.2 µA.mM–1.cm–2) and the detection limits are in the order of ethanol (~515.7 µM) > methanol (~379.6 µM) > propanol (~222.4 µM) > butanol (~213.1 µM). On the other hand, the ethanol chemical sensing responses are also determined in different buffer solutions such as acetic acid and citric acid buffers to elucidate the suitability of PBS. The ethanol chemical sensor in acetic acid and citric acid buffers demonstrate the relatively low sensitivities of ~299.8 and ~349.0 µA. mM–1.cm–2 whereas, the sensitivity of ~426.5 µA.mM–1.cm–2 in PBS is highest toward ethanol chemical. This experiment clearly suggests that ethanol and phosphate buffer is highly suitable chemical and buffer to obtain the good sensing response of alcohols. Inclusively to check the selectivity other than aliphatic alcohols, the sensing performances of sea-cucumber-like hollow PANI spheres electrode toward amine (ethylamine) and thiol (ethanethiol) chemicals have been evaluated. The ethylamine and ethanethiol chemical sensors show lower sensitivities of ~80.7 and ~103.6 s µA.mM–1.cm–2 in phosphate buffer as compared to aliphatic alcohols chemical sensors. Thus, the sea-cucumber-like hollow PANI spheres electrode is a promising active electrocatalytic electrode for the effective detection of ethanol chemical.

Figure 1.14 (a) I–V characteristics of different alcohols (methanol, ethanol, propanol, and butanol) sensors at 25 µM in 10 ml PBS and (b) histograms of sensitivity and detection limit versus different alcohol sensors based on sea-cucumber-like hollow PANI spheres electrode.

Reprinted with permission from Ameen, RSC Adv. 3 (2013) 10460. @ 2013, RSC