Nanostructured polyaniline films on silicon for sensitive sensing of ammonia

Sensors and Actuators A 198 (2013) 107–112

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Nanostructured polyaniline films on silicon for sensitive sensing of ammonia Amit L. Sharma a,∗ , Kamal Kumar b , Akash Deep a a b

Central Scientific Instruments Organisation (CSIR-CSIO), Sector 30-C, Chandigarh 160030, India Department of Physics, Government Degree College, Ranikhet, Uttrarakhand, India

a r t i c l e

i n f o

Article history: Received 20 December 2012 Received in revised form 16 April 2013 Accepted 16 April 2013 Available online 28 April 2013 Keywords: Polyaniline Silicon Sensor Nanostructured Ammonia

a b s t r a c t Silicon (Si)–nanopolyaniline (PAni) conducting platform has been electrochemically developed for the sensitive sensing of ammonia. The nano PAni films are formed through the assembly of nano granules of the polymer. The average height of the film is in the range of 50 nm. The electrical conductivity of the Si–PAni is influenced in the presence of ammonia. The response of this conductometric sensor is almost linear within 5–50 ppm of ammonia. The response and the recovery times are observed to be 10 and 60 s, respectively for 10 ppm of ammonia. The response time of the sensor for other concentrations (viz. 20, 30, 40, 50 and 60 ppm ammonia) is also in the range of 10 ± 2 s. However, the recovery of the sensor takes slightly longer time (70 ± 3 s) in case of higher concentrations (≥40 ppm) of ammonia. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Conducting polymers have been widely used in the development and fabrication of biosensors for several applications [1–3] owing to their special physical, optical and electrical properties. Major advantages of using conducting polymers lie in their diversity and ease of synthesis coupled with excellent transducing properties. The use of conducting polymers offers the recognition of even the smallest changes in the substrate’s electrical and optical properties when exposed to different types of gases or liquids. The conducting polymer based sensors can be of importance particularly for toxic gases [4,5]. Ammonia is one such gas whose sensitive detection may be of importance in different industrial processes, fertilizer and food technology industry, clinical diagnosis, and environmental pollution monitoring. Ammonia is a toxic gas with exposure limit values of 25 ppm for a period of 8 h and of 35 ppm for a period of 10 min. Some of the more notable techniques employed for the detection of ammonia gas include quartz crystal microbalance [6], zeolite film [7], laser-based photoacoustic [8], nanofibers [9,10], piezoresistive SiO2 cantilever [11], and silicon process technology [12]. Polyaniline (PAni) is a popular polymer due to its electrical conductivity, stability and significant redox behaviour. PAni may be synthesized either chemically or electrochemically. The chemical synthesis generally involves the mixing of aniline with acid (HCl, H2 SO4 or perchloric acid) in presence of a suitable oxidant, such

∗ Corresponding author. Tel.: +91 172 2637311x308; fax: +91 172 2657082. E-mail address: amit [email protected] (A.L. Sharma). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.04.026

as ammonium peroxydisulfate and camphorsulfonic acid [13–15]. The electrochemical synthesis of PANi may be performed using galvanostatic, potentiostatic or potentiodynamic methods [16]. PAni has been reported for applications in light-emitting diodes [17], rechargeable batteries [18], photovoltaic cells [19] and sensors [20–22]. PAni has also been used to form nanocomposites with improved mechanical strength and sensor characteristics [23–25], which find applications in the detection of various vapours such as humidity and liquefied petroleum gas as well as different gas species such as H2 , CO and NO2 . Some of the recently published reports indicate the viability of Si–PAni heterojunctions for electroluminescent devices and applications [26,27]. Despite of the great technological importance of Si–PAni support, there are few reports citing the sensing applications of this platform [28]. Present work explores the utility of Si–nano PAni substrate for the sensitive detection of ammonia. Polymer films have been electrochemically deposited and characterized with a number of techniques. It may be pertinent to mention here that owing to a large surface-to-volume ratio, the proposed biosensor inherently carries desired enhancements in the sensitivity and response time. Si–nano PAni substrate may also offer the fabrication of robust devices, enhanced polymer porosity, improved sensitivity and prolonged sensor life. 2. Experimental Aniline and all other chemicals and solvents were high purity products from Merck/Sigma Aldrich. The boron doped p-type 1 1 1 silicon wafers (resistivity 10−3 – 40  cm) were purchased from Sigma–Aldrich. Stock and working solutions were

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connections were imprinted to supply the power (point distance of 1.5 cm) and to measure the resistance (point distance of 1.0 cm). Prior to the experiments, the chamber was evacuated and flushed with nitrogen to create oxygen-free environment. A known volume of aqueous ammonia was introduced in to the test chamber and change in the film resistance was noted as it attains a constant value. After every test, sensor’s original resistance was restored by sucking out residual ammonia from the chamber followed by circulation of the air. The sensitivity ‘S’ of the sensor was estimated according to the following expression. S= Fig. 1. Schematic of experimental arrangement for the sensing of ammonia.

prepared in deionized (DI) water. Electrochemical polymerization of aniline and the cyclic voltammetry studies were carried out with a three-electrode configured potentiostat–galvanostat system (model Autolab Metrohm B.V.). Desired concentration of the aniline solution (0.1 M) was prepared in 1 M HCl. Dry N2 gas was purged in to this monomer solution for about 30 min in order to remove any dissolved O2 . Si wafer, Pt wire and Ag/AgCl electrode were used as working, counter and reference electrodes, respectively. Before electrochemical experiments, the p-Si(1 1 1) wafers were etched with 1% (v/v) aqueous HF solution (20 s) and 40% (v/v) aqueous NH4 F solution (for 4 min) in order to obtain H-terminated silicon surface. The PAni coated silicon wafers were immersed in an aqueous solution of 0.5 M H2 SO4 in order to remove aniline monomer and oligomer PAni from the polymeric film. The PAni coated wafers were then rinsed with deionized water for several times and finally dried overnight in a vacuum oven at 50 ◦ C. Cyclic voltammograms (CV) of the PAni film were studied in aniline free 1 M HCl at a scan rate of 10 mV/s. Successful polymerization of aniline into PAni was verified by attenuated total reflectance (ATR) – fourier transform infrared (FTIR, instrument make: Nicolet iS10) and Raman (instrument make: Invia, Renishaw, laser wavelength: 514 nm) spectroscopy. Morphological parameters of the polymer deposits were characterized by Field Emission Scanning Electron (FESEM, Hitachi, 4300 SE/N) and Atomic Force (AFM, Park Systems, XE-NSOM) microscopes. Response of the sensor for ammonia was investigated with the help of a constant current source and a multimeter. Different concentrations (1–100 ppm) of the aqueous ammonia (in deionized water) were prepared in closed glass containers of 1 L volume. The related experiments were carried out in a home-made test chamber of 2 L capacity (Fig. 1). Ammonia was carried to the chamber through a glass tube. An outlet was provided to attach a suction pump. A fan was hoarded to deliver uniform distribution of gas inside the chamber. The sensing electrode was suspended in the chamber with the help of a thin solid support. Required

R Ri

(1)

where Ri is the resistance of Si–PAni film in the absence of ammonia and R is the change in resistance observed after introducing ammonia in to the test chamber. 3. Results and discussion 3.1. Electrochemical deposition of PAni PAni was electrochemically deposited on Si surface (10 mm × 10 mm exposed Si surface) using cyclic voltammetry (CV) technique for duration of 5 min. The recorded current–voltage curve is shown in Fig. 2a. The increase in the redox current was gradual with the increasing number of the deposition cycles, thus indicating continuous growth of the polymer film on Si surface. During the process, a consistent increase in the charge flow with respect to the reaction time highlighted the sequential inclusion and expulsion of the anions (counter ions). Cyclic voltammetry (CV) studies of the deposited PAni films were carried out at different scan rates (20–100 V s−1 ) within the potential range of −0.2 to 1.0 V (vs. Ag/AgCl) (Fig. 2b). The CV plot is characterized by three peaks at 0.24, 0.60 and 0.82 V, associated with the oxidation of monomer to radical cation, presence of phenazine rings, and the oxidation of radical cation to radical dication, respectively. An increase in the scan rate causes the oxidations peaks to shift towards higher potential, while the reduction peaks reposition towards the lower potential. This observation, along with the appearance of the peaks (sharp and higher anodic peaks; smaller and broader cathodic peaks) highlights the conducting nature of the oxidized polymer film and the insulating nature of the reduced polymer film. PAni acts as electrically conductive material in the protonated form of emeraldine salt. The protonation in the presence of HCl leads to the conversion of N sites to N+ Cl− . Since HCl is dissociated, the formation of N+ Cl− H+ groups is realized throughout the polymer chain. Hopping of valance electrons makes the product electrically conductive [29].

Fig. 2. (a) Growth curve of PAni on Si surface; (b) cyclic voltammogram of PAni–Si nanodeposits.

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Fig. 3. FE-SEM investigations of the electrodeposited PAni film.

3.2. Spectroscopic and microscopic studies The use of Si as substrate has helped in obtaining nanostructured thin film as it was possible to control the growth due to a limited conductivity of the base. A run of 5 successive cycles gave fairly smooth and homogenous nanostructured PAni films as evidenced from the FE-SEM and AFM investigations (Figs. 3 and 4). AFM line and region analyses have indicated the film thickness to be in the range of 45–50 nm. Quantitative analyses by AFM images to depict the height roughness Ra and Rpv , can be used to describe the surface morphology. Ra is defined as the mean value of the surface height relative to the centre plane, and Rpv is the peak to valley height within the given area. Analysis of AFM scan indicated the Rpv values of 52.587 nm, while the surface roughness was computed to be 10.862 nm. The root mean square roughness (Rq ) was observed to be 9.623 nm. Material surface features usually exhibit a fractal character right after the growth. Fractal dimension (Df ) has been computed by using cube counting method implemented within the AFM data analysis software XEI (version 1.7.6). In the related algorithm, a cubic lattice with lattice constant l is superimposed on the z-expanded surface. Initially l is set at X/2 (where X is length of edge of the surface), resulting in a lattice of eight cubes. S(l) refers to the number of all cubes that contain at least one pixel of the image. The lattice constant l is then reduced stepwise by a factor of 2 and the process repeated until l equals to the distance between two adjacent pixels. The slope of a plot of log S(l) vs. log(1/l) gives the fractal dimension Df. The fractal dimension provides useful information about the degree of fragmentation of the surfaces. The occurrence of fractal character in the deposited PAni on Si has been studied (Fig. 4). It can be seen that the data-points

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follow a roughly linear trend, which means that the fractal character is preserved in the deposited PAni films. The fractal dimension is estimated to be 2.326. This Df value highlights that the fragmented PAni surface has significant degree of porosity. Considering the FESEM information and AFM topography, along with the data of line analysis, it can be concluded that the monolayer of PAni is electrodeposited as nano granules. Growth of PAni as globular nanostructures on the p-Si(1 1 1) wafers can be explained from the fact that the taken platform {p-Si(1 1 1)} inherently tends to absorb atmospheric O2 . It results in to the development of inhomogeneous thin oxide layer with number of defects. These defects effectively serve as the active sites for the assembly of PAni in nanostructured pattern. In all the FE-SEM and AFM investigations, the PAni film is characterized by a globular structure and higher surface roughness which could be attributed to kinetically controlled 2D or 3D growth of the polymer islands. Similar to chemical polymerization, the electrochemical growth of PAni is preferably carried out in acidic medium in order to prevent the formation of short conjugation oligomeric material. Aniline cation radicals produced during the induction period are adsorbed on the silicon surface and PAni chains grow from these primary nucleation centres. The oxidation of the aniline monomer is an irreversible process, which occurs at higher positive potentials than the redox potential of polyaniline. When conditions favour, the coupling of the anilinium radicals takes place followed by the elimination of two protons. The species undergoes re-aromatization to form into dimer; and subsequently into the oligomer. The chain propagation is achieved by further coupling of the radical cations of the above oligomer and anilinium. The electroneutrality conditions are satisfied through the doping of counter anions originating from the electrolyte [30,31]. The involved nucleation and growth process can further be elaborated by the metal deposition theory [32,33]. According to the theory, nucleation may take place via two kinds of processes: instantaneous and progressive. The polymer growth may occur through one- (1D), two- (2D), and three- (3D) dimensional processes. The PAni nano globules are formed by progressive nucleation, with 3D growth mechanism. During the process, the mass transfer controls the polymer growth and the 3D growth of the PAni nanoglobules is governed according to the following equation [34]: d=

Q · Mw z·F ·A·

(2)

where d is the diameter/film thickness (cm), Q is the charge (C), Mw is the molecular weight of aniline (93.13 g mol−1 ), z is the number of electron transferred per aniline unite (0.5), F is Faraday’s constant (96485 C mol−1 ), A is the surface area of electrode (0.0706 cm2 ) and  is the density of aniline(1.02 g cm−3 ). Under the given experimental conditions, the total charge passed varied from 2.0 mC to 2.8 mC. Solving the equation for the collected experimental data gives the average values of d in the range of 37–52 nm. The theoretical

Fig. 4. AFM topography of the electrodeposited PAni film on Si surface and estimation of fractal dimension.

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Fig. 5. (a) FTIR spectra of deposited PAni; (b) Raman spectra of deposited PAni.

findings agree with the experimental data and explain the observed size of the PAni structure. The above range of the PAni nanostructures has also been realized in some of the previous studies [35]. The FTIR investigations on deposited PAni (Fig. 5a) confirm the polymerization in the backdrop of typically observed vibrational frequencies [36–39]. The peak at 1292 cm−1 corresponds to the C N of rings 3 and 4. Another band at 1498 cm−1 corresponds to the C N of the ring 3. The C N of the ring 4 is observed at 1529 cm−1 . Bands observed at 1596 and 1650 cm−1 represents two vibrational modes of C N of the ring 2. Other vibrational modes at 1572, 1340 and 1164 cm−1 are further indicative of the formation of rings 1, 2, 3 and 4. Raman spectroscopy uses a laser as radiation source with higher energy than the corresponding vibrational transitions. However, due to the scattering of the incident light (the Raman process), the vibrational frequencies (Raman bands) can be probed. Raman studies of PAni using laser lines in the visible region provide useful information. Raman spectrum (Fig. 5b) of the PAni is characterized with signature bands between 1300 and 1400 cm−1 . In particular, the peak at 1325 cm−1 can be assigned to a C N•+ polaronic stretch. The signal at 1490 cm−1 indicates the presence of C N quinoid groups. The amine group is characterized by ␯CN stretch at 1215 cm−1 . The ␤CH band at 1182 cm−1 is another attribute of the oxidized polymer, which may also be used as a qualitative measure of the degree of oxidation of the polymer chain. The band at 1575 cm−1 can be correlated to the angular deformation of NH group. Absence of bands at 1618 (␯CC stretch of the benzene ring) and 1181 cm−1 (␤CH angular deformation of the benzene ring) ruled out the co-presence of reduced PAni. In this regard, the band at 1325 cm−1 can be associated to ␯C N of polarons with different conjugation lengths and the presence of charged phenazine-like rings in the oxidized form of PAni.

3.3. Response of Si–PAni films to ammonia Response of the Si–PAni sensor towards different concentrations (1–100 ppm) is given in Fig. 6. All the values are an average of three determinations. The value of standard deviations was in the range ±0.2. The sensitivity of the sensor showed direct correlation with the ammonia concentration up to 50 ppm, suggesting that the Si–PAni electrode can be usefully employed for the sensitive sensing of ammonia. The increase in the resistance of Si–PAni film upon its exposure to the ammonia is explained by loss of a proton from the polymer film to form NH4 + ions. The pattern is observed up to a certain concentration, beyond which the polymer film encounters proton deficient condition and the sensor’s response does not show any further variation. Si–PAni electrode almost completely regained the original conductivity upon the evacuation of ammonia from the chamber.

Fig. 6. Response of the Si–PAni sensor towards 1–100 ppm ammonia.

The proposed ammonia sensor is basically consisted of a sensing resistor (Si substrate) and an ammonia sensing film (nano polyaniline) in parallel. A very thin silicon dioxide layer is located between the sensing resistor and the PAni film. Fig. 7 depicts the energy band diagram of the proposed sensor. During the PAni film growth on Si, the equalization of Fermi levels of the two materials is achieved by the transfer of majority carriers from Si to PAni. At equilibrium, the semiconductor surface gets depleted of the majority carriers. During the formation of this depletion layer, the entire band structure in the semiconductor bulk “shifts” downward, and the bands bend so that the band edge energy remains constant [40]. Simultaneously, the contact of the PAni layer with air produces holes on the surface, thus causing upward bending of the polymer’s valance edge band. The exposure of ammonia with the sensing film is associated with the reduction of H+ holes present on the surface of PAni according to the following equation: PAniH + NH3 ↔ PAni + NH4 +

(3)

The above reaction causes the bending of valance band edge of the reduced PAni as well as the shifting of band away from the Fermi level. The positive charge appearing in the semiconductor surface as a consequence of gas adsorption causes an upward band bending and depletion layer is broadened. The phenomenon accounts for the observed increase in the polymer resistivity. The resistivity of the Si may also be influenced upon the sensor’s exposure with the analyte gas. It occurs because of the reduction in the accumulated electrons at the interface. Consequently, the conduction band edge rearranges to reduce the band bending. As a result, the conduction band edge moves furtherer from the Fermi level and the resistance of Si increases. The above phenomenon accounts for the observed reduction in the sensor’s conductivity upon exposure to ammonia. NH4 + ions quickly decompose to yield NH3 and H+ . The redoping of the copolymer chain with the available H+ ions restores of the original level of the conductivity and reverses the band bending.

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Fig. 7. Energy band diagram of sensor in air and ammonia (Ec – conduction band, EF – Fermi level, EFi – intrinsic Fermi level, Ev – valence band).

synthesized PAni films, thereby offering an interesting alternative. The sensor can be effectively used for the online and continuous detection of ammonia. Acknowledgements Financial support from the Council of Scientific and Industrial Research (CSIR) (OMEGA Projects: PSC0202/1.2 and PSC0202/2.2.5), India is gratefully acknowledged. We thank the Director, CSIO-CSIR Chandigarh for providing required facilities. Fig. 8. Response and recovery time of the Si–PAni sensor during successive tests.

References

Fig. 9. Response and recovery time of the Si–PAni sensor with respect to different ammonia concentrations (5, 10, 20, 30, 40, 50, and 60 ppm).

Si–PAni sensor has also been investigated for the response and recovery times. For this, the electrode was exposed with 10 ppm of ammonia. The experiment was run for several successive cycles. The obtained data are depicted in Fig. 8. The average response and recovery times are found to be 10 and 60 s, respectively. These values were observed to remain more or less unchanged during 48 numbers of successive investigations, thereby indicating that the interaction of the gas with the sensor did not induce any irreversible change in the polymer’s structure. The response and recovery time of the sensor was also checked with respect to the ammonia concentrations. The data are given in Fig. 9. The response time remained more or less constant. However, the recovery time of the sensor increased to around 70 s during the detection of 40 ppm (and more) of ammonia. 4. Conclusions Nanostructured Si–PAni substrate has been demonstrated as a potential platform for the detection of ammonia. The choice of Si as a substrate has helped in growing nanostructured PAni films which show excellent detection of ammonia with quick response and recovery times. The sensor is expected to be more stable and easier to fabricate than other chemically or electrochemically

[1] A. Morrin, F. Wilbeer, O. Ngamna, S.E. Moulton, A.J. Killard, G.G. Wallace, et al., Novel biosensor fabrication methodology based on processable conducting polyaniline nanoparticles, Electrochemistry Communications 7 (2005) 317–322. [2] B.D. Malhotra, A. Chaubey, S.P. Singh, Prospects of conducting polymers in biosensors, Analytica Chimica Acta 578 (2006) 59–74. [3] A. Mulchandani, N.V. Myung, Conducting polymer nanowires-based label-free biosensors, Current Opinion in Biotechnology 22 (2011) 502–508. [4] S. Virji, J.D. Fowler, C.O. Baker, J. Huang, R.B. Kaner, B.H. Weiller, Polyaniline nanofiber composites with metal salts: chemical sensors for hydrogen sulfide, Small 1 (2005) 624–627. [5] P. Boeker, G. Horner, S. Rösler, Monolithic sensor array based on a quartz microbalance transducer with enhanced sensitivity for monitoring agricultural emissions, Sensors and Actuators B 70 (2000) 37–42. [6] R. Moos, R. Müller, C. Plog, A. Knezevic, H. Leye, E. Irion, et al., Selective ammonia exhaust gas sensor for automotive applications, Sensors and Actuators B 83 (2002) 181–189. [7] M.B. Pushkarsky, M.E. Webber, O. Baghdassarian, L.R. Narasimhan, C.K.N. Patel, Laser-based photoacoustic ammonia sensors for industrial applications, Applied Physics B 75 (2002) 391–396. [8] J. Huang, R.B. Kaner, Nanofiber formation in the chemical polymerization of aniline: a mechanistic study, Angewandte Chemie-International Edition 43 (2004) 5817–5821. [9] Y. Chen, P. Xu, X. Li, Self-assembling siloxane bilayer directly on SiO2 surface of micro-cantilevers for long-term highly repeatable sensing to trace explosives, Nanotechnology 21 (2010) 265501. [10] X. Li, D.-W. Lee, Integrated microcantilevers for high-resolution sensing and probing, Measurement Science & Technology 23 (2012) 022001. [11] E.J. Connolly, B. Timmer, H.T.M. Pham, J. Groeneweg, P.M. Sarro, W. Olthuis, et al., A porous SiC ammonia sensor, Sensors and Actuators B 109 (2005) 44–46. [12] M. Gautam, A.H. Jayatissa, Adsorption kinetics of ammonia sensing by graphene films decorated with platinum nanoparticles, Journal of Applied Physics 111 (2012) 094317–0943179. [13] J. Huang, S. Virji, B.H. Weiller, R.B. Kaner, Nanostructured polyaniline sensors, Chemistry: A European Journal 10 (6) (2004) 1314–1319. [14] J. Huang, R.B. Kaner, A general chemical route to polyaniline nanofibers, Journal of the American Chemical Society 126 (3) (2003) 851–855. [15] A.Z. Sadek, W. Wlodarski, K. Kalantar-Zadeh, C. Baker, R.B. Kaner, Doped and dedoped polyaniline nanofiber based conductometric hydrogen gas sensors, Sensors and Actuators A 139 (1–2) (2007) 53–57. [16] S. Bhandari, N.K. Singha, D. Khastgir, Electrochemical synthesis of nanostructured polyaniline: Heat treatment and synergistic effect of simultaneous dual doping, Journal of Applied Polymer Science (2012), http://dx.doi.org/10.1002/app.38803. [17] A.G. MacDiarmid, L.S. Yang, W.S. Huang, B.D. Humphrey, Polyaniline, Electrochemistry and application to rechargeable batteries, Synthetic Metals 18 (1987) 393–398. [18] D. Verma, V. Dutta, Role of novel microstructure of polyaniline–CSA thin film in ammonia sensing at room temperature, Sensors and Actuators B 134 (2008) 373–376.

112

A.L. Sharma et al. / Sensors and Actuators A 198 (2013) 107–112

[19] F. Arslan, S. Ustabas¸, H. Arslan, An amperometric biosensor for glucose determination prepared from glucose oxidase immobilized in polyaniline–polyvinylsulfonate film, Sensors 11 (2011) 8152–8163. [20] V.V.R. Sai, S. Mahajan, A.Q. Contractor, S. Mukherji, Immobilization of antibodies on polyaniline films and its application in a piezoelectric immunosensor, Analytical Chemistry 78 (2006) 8368–8373. [21] M.-C. Liu, C.-L. Dai, C.-H. Chan, C.-C. Wu, Manufacture of a polyaniline nanofiber ammonia sensor integrated with a readout circuit using the CMOS-MEMS technique, Sensors 9 (2009) 869–880. [22] A.L. Sharma, Electrochemical synthesis of poly(aniline-co-fluoroaniline) films and their application as humidity sensing material, Thin Solid Films 517 (2009) 3350–3356. [23] M.K. Ram, Ö. Yavuz, V. Lahsangah, M. Aldissi, CO gas sensing from ultrathin nano-composite conducting polymer film, Sensors and Actuators B 106 (2005) 750–757. [24] S.S. Joshi, T.P. Gujar, V.R. Shinde, C.D. Lokhande, Fabrication of n-CdTe/ppolyaniline heterojunction-based room temperature LPG sensor, Sensors and Actuators B 132 (2008) 349–355. [25] P. Kumar, S. Adhikari, P. Banerji, Fabrication and characterization of polyaniline/porous silicon heterojunction, Synthetic Metals 160 (2010) 1507–1512. [26] Q. Liu, M.H. Nayfeh, S.-T. Yau, Supercapacitor electrodes based on polyaniline–silicon nanoparticle composite, Journal of Power Sources 195 (2010) 3956–3959. [27] J. Tang, X. Jing, B. Wang, F. Wang, Infrared spectra of soluble polyaniline, Synthetic Metals 24 (1988) 231–238. [28] A. Deep, A.L. Sharma, P. Kumar, L.M. Bharadwaj, Nanostructured polyaniline–silicon substrate for protein biosensing, Sensors and Actuators B 171–172 (2012) 210–215. [29] V.V. Chabukswar, S. Pethkar, A.A. Athawale, Acrylic acid doped polyaniline as an ammonia sensor, Sensors and Actuators B 77 (2001) 657–663. [30] A. Hussain, A. Kumar, Electrochemical synthesis and characterization of chloride doped polyaniline, Bulletin of Materials Science 26 (3) (2003) 329–334. [31] G. Wallace, G. Spinks, L. Kane-Maguire, P. Teasdale, Conductive Electroactive Polymers, Taylor & Francis Group/CRC Press, Boca Raton, FL, 2009. [32] J. Heinze, B. Frontana-Uribe, S. Ludwigs, Electrochemistry of conducting polymers-persistent models and new concepts, Chemical Reviews 110 (8) (2010) 4724–4771. [33] J. Kankare, Electronically conducting polymers: basic methods of synthesis and characterization, in: D. Wise, G. Wnek, D. Trantolo, J. Cooper, D. Gresser (Eds.), Electrical and Optical Polymer Systems: Fundamentals: Methods, and Applications, Marcel Dekker, New York, 1998, pp. 167–199. [34] M.H. P-Azar, B. Habibi, Electropolymerization of aniline in acid media on the bare and chemically pre-treated aluminium electrodes: a comparative

[35]

[36]

[37]

[38] [39] [40]

characterization of the polyaniline deposited electrodes, Electrochimica Acta 52 (2007) 4222–4230. R. Arsat, X.F. Yu, Y.X. Li, W. Wlodarski, K. Kalantar-Zadeh, Hydrogen gas sensor based on highly ordered polyaniline nanofibers, Sensors and Actuators B 137 (2) (2009) 529–532. S.G. Pawar, M.A. Chougule, S.L. Patil, B.T. Raut, P.R. Godse, S. Sen, et al., Room temperature ammonia gas sensor based on polyaniline–TiO nanocomposite, IEEE Sensors Journal 11 (2011) 3417–3423. S. Chakraborty, S. Bandyopadhyay, R. Ameta, R. Mukhopadhyay, A.S. Deuri, Application of FTIR in characterization of acrylonitrile-butadiene rubber (nitrile rubber), Polymer Testing 26 (2007) 38–41. S. Srinivasan, P. Pramanik, Optical properties of chemically prepared polyemeraldine, Synthetic Metals 63 (1994) 199–204. L. Wen, N.M. Kocherginsky, Doping-dependent ion selectivity of polyaniline membranes, Synthetic Metals 106 (1999) 19–27. L. Mao-Chen, D. Ching-Liang, C. Chih-Hua, W. Chyan-Chyi, Manufacture of a polyaniline nanofiber ammonia sensor integrated with a readout circuit using the CMOS-MEMS technique, Sensors 9 (2009) 869–880.

Biographies Dr. Amit L. Sharma did his Ph.D. in Physics in 2002 from the Delhi University, India. He has research experience in the fields of conducting polymers, biosensing and thin layer substrate fabrication. Dr. Sharma is working as a Senior Scientist at the Optical Devices and Systems division of CSIO-CSIR, Chandigarh, India. His present research activities include the development of nanostructures and thin layers for biosensor and optical filer applications. Dr. Kamal Kumar completed his Ph.D. in Physics in 2010 from the Gurukula Kangri University, Hardwar, India. His research interests include amorphous nanostructures, thin films and metal oxide nanoparticles. He is currently working as a Lecturer at the Government Degree College, Ranikhet, Uttrarakhand, India. Dr. Akash Deep completed his Ph.D. in Chemistry in 2004 from the Indian Institute of Technology, Roorkee, India. After doing 3 years of his postdoctoral research in the field of analytical and environmental chemistry, Dr. Deep joined the Central Scientific Instruments Organisation (CSIR-CSIO), Chandigarh, India in 2008 as a scientist. Dr. Deep is currently involved in the development of bionanosensors for environmental and clinical applications. His research interests also include the synthesis and applications of quantum dots, conducting polymers, metal organic frameworks and their nanocomposites.