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Rapid, facile microwave-assisted synthesis of xanthan gum grafted polyaniline for chemical sensor
Accepted Manuscript Title: Rapid, Facile Microwave-assisted synthesis of Xanthan gum grafted polyaniline for chemical sensor Author: Sadanand Pandey James Ramontja PII: DOI: Reference:
S0141-8130(16)30363-4 http://dx.doi.org/doi:10.1016/j.ijbiomac.2016.04.055 BIOMAC 6024
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
29-2-2016 5-4-2016 19-4-2016
Please cite this article as: Sadanand Pandey, James Ramontja, Rapid, Facile Microwave-assisted synthesis of Xanthan gum grafted polyaniline for chemical sensor, International Journal of Biological Macromolecules http://dx.doi.org/10.1016/j.ijbiomac.2016.04.055 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Rapid, Facile Microwave-assisted synthesis of Xanthan gum grafted polyaniline for chemical sensor Sadanand Pandey*, James Ramontja 1
Department of Applied chemistry, University of Johannesburg, P.O Box 17011,
Doornfontien, Johannesburg 2028, South Africa
Corresponding Author: [email protected]; [email protected] ph.:+27-11-559-6644
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Graphical Abstract
2
Highlights
Polyaniline grafted xanthan gum was synthesized by microwave assisted technique.
Process is highly energy efficient and takes significantly less reaction time (∼50 sec).
Detailed characterization of grafted xanthan gum by UV-Vis, FTIR, XRD, SEM, and TGA.
Fabricated grafted sample film were then examined for the chemical sensor.
Chemiresistive NH3 sensors with superior room temperature sensing performance.
Abstract Grafting method, through microwave radiation procedure is extremely productive in terms of time consumption, cost effectiveness and environmental friendliness. In this study, conductive and thermally stable composite (mwXG-g-PANi) was synthesized by grafting of aniline (ANi) on to xanthan gum (XG) using catalytic weight of initiator, ammonium peroxydisulfate in the process of microwave irradiation in aqueous medium. The synthesis of mwXG-g-PANi were confirm by FTIR, XRD, TGA, and SEM. The influence of altering the microwave power, exposure time of microwave, concentration of monomer and the amount of initiator of graft polymerization were studied over the grafting parameters, for example, grafting percentage (%G) and grafting efficiency (%E). The maximum %G and %E achieved was 172 and 74.13 respectively. The outcome demonstrates that the microwave irradiation strategy can increase the reaction rate by 72 times over the conventional method. Electrical conductivity of XG and mwXG-g-PANi composite film was performed. The fabricated grafted sample film were then examined for the chemical sensor. The mwXG-g-PANi, effectively integrated and handled, are NH3 sensitive and exhibit a rapid sensing in presence of NH3 vapor. Chemiresistive NH3 sensors with superior room temperature sensing performance were produced with sensor response of 905 at 1ppb and 90% recovery within few second. Keywords: Biopolymer; Polyaniline; Graft copolymer; Microwave irradiation; Green technology; Chemical sensor 3
1. Introduction The detection of toxic gases has been a major focus of sensor research in recent years. Ammonia is a kind of gas with high toxicity. It is regulated by occupational safety and health administration (OSHA) and has a current permissible exposure limit (PEL) of 35 ppm over 15 minutes short term exposure limit (STEL). Besides, ammonia concentration at or above 2500 ppm exposures for 30 minutes are considered immediately dangerous to life and health (IDLH), and may cause induced chemical pneumonitis, burns (eyes, face and mouth), severe local edema, dyspnea, progressive cyanopsis and even death [1]. The human body naturally produces ammonia by various metabolic activities [2]. In medical sector, existence of excessive amount of ammonia in exhaled human breath can be treated as indications of several diseases related to dysfunctions of liver and kidneys [3, 4]. Hence, the detection of ammonia gas/vapour is of paramount importance in terms of both environmental as well as health monitoring sectors. A wide variety of materials, such as, metal oxides (ZnO, SnO2, TiO2, V2O5, In2O3 etc.), polymers and carbon based materials have been used as sensing elements for various ammonia gas sensor applications [5-7]. However, most of the currently available gas sensors based on these materials, often suffer from more than one drawback such as poor selectivity, influence of humidity, external stimulus such as Joule heating or UV illumination for response/recovery, operation at high temperature (200–500 °C) which lead to high power consumption. Therefore, new sensing materials with low detection limit, high sensitivity and reproducibility, fast response and recovery without any external stimulus, low cost and ecofriendly sensor are expected for gas detection at room temperature. Polyaniline (PANi) known as intrinsically conducting polymers (ICPs) has attracted specifically consideration in view of its ease of doping, its different chemical forms available depending on acid/base treatment and its substantial stability in ambient atmosphere. All these characteristics clarify, why PANi is contemplate as a moderately effectively prepared and processed ICP for an extensive variety of applications. PANi is pH sensitive and may fulfil the requirements for the development of chemical sensor. PANi has been utilized as a cost effective conducting polymer [8]. Biological and electronic properties of PANi were altered by doping distinctive types of inorganic nanomaterials [9]. Similarly biopolymer has been widely used as a part of sensors because of its 4
biodegradable, biocompatible and non-toxicity [10]. Independently both PANi and xanthan gum (XG) had indicated brilliant properties as conducting and biocompatible material, respectively. However XG has restricted conductivity while PANi has limited biocompatibility which has been a subject of in-depth research in last decade or so. There were a few endeavours to enhance the performance of biopolymer and PANi relies upon alteration in structural and surface chemistry by consolidating nano-metal-oxides [9, 11]. Synthesis of metal nano-particles itself is extremely complex process and requires several chemical processes. Thus, the utilization of chemicals for synthesis and nondegradable characteristics of metal oxides can have several environment issues when chemical wastes were disposed. Secondly, oxidative characteristics of nano-materials likewise restrain the stability amid storage and used sensing film in ambient atmosphere. Taking into consideration these advantage, the objectives of the present work was to develop an appropriate method for metal free conducting biopolymer matrixes by modification of carbohydrate polymers with PANi which may prompt the synthesis of multifunctional electrical conducting composite. It will be the favorable advantages such as technological applications, affinity with environment and biological frameworks, and is financially savvy. The biopolymer selected for the present study is XG. XG is derived from Xanthomonas campestris. The structural unit of XG biopolymer consist of backbone of β-(1– 4)-D-glucopyranose glucan along with side chains of β-(3-1)-α-linked D-mannopyranose-(21)-β-D-glucuronic acid-(4-1)-β-D-mannopyranose on alternating residues [12]. Previously reports shows that inorganic salt complexes of biopolymer such as gum arabica (GA) act as a superionic electrical conductor [13]. Mostly, addition of conducting polymers, for example, PANi into a flexible matrix of XG ought to result in great processability alongside the electrical conductivity, chemical stability toward dopants and thermal stability. It has been notice presently, that microwave irradiation is rising as effective device for the chemical processing [14] and in different fields of chemistry including polymers. Fundamental point of interest is that it brings about practically rapid 'in core' handling of materials in a homogeneous and specific way. Under microwave irradiation grafting of polyacrylamide, polyacrylonitrile onto potato starch [15, 16]; and poly (ethylacrylate) onto XG [17] have been performed in absence or at low concentration of initiator.
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In this context, the present work aimed to provides details study of the ammonium peroxydisulfate (APS) initiated synthesis of mwXG-g-PANi under microwave conditions. This study deal with, mwXG-g-PANi optimization, plausible mechanism, different characteristic properties of mwXG-g-PANi such as morphology, conducting, and thermal behavior of the mwXG-g-PANi were explored. Here, we have also reported, a rapid and highly sensitive mwXG-g-PANi composite based chemiresistive sensor for the room temperature detection of ammonia levels in a lower range from 1 parts-per-billion (ppb) to 100 ppb. The sensor is based on the mwXG-g-PANi composite of polysaccharide (xanthan gum, XG) and polyaniline (PANi). Sensor response, temporal response, reproducibility and stability studies reveal excellent ammonia sensing of the mwXG-g-PANi composite. The general schematic diagram of the present work was shown in (Scheme.1). 2. Experimental 2.1. Materials The biopolymer, XG from xanthomonas campestris (G1253, Sigma), monomer, aniline (≥99.5%, Sigma-Aldrich; 242284), initiator, APS (≥98.0%, Sigma-Aldrich; 248614), solvent, 1-Methyl-2-pyrrolidone (NMP) (Merck; 806072), hydrochloric acid (32% Merck; 100319) and Ammonium hydroxide solution (32.0%, Sigma-Aldrich; V000637) were used. 2.2. Graft copolymerization method for synthesis of mwXG-g-PANi composite During the grafting experiment, XG was dissolve in 25mL deionized (DI) water. A known amount of ANi and hydrochloric acid (HCl) solutions were included in the container. Further catalytic amount of APS was added in order to initiate the reaction of graft copolymerization. Further, the reaction mixture was exposed to microwave irradiation at definite microwave power and exposure time. After desired time period, the grafted sample was precipitated by pouring the reaction mixture into the NMP. After sought time period, the copolymer were dried in a vacuum oven at 60°C and weighed. The % grafting (%G), % efficiency (%E) and % homopolymer (%H) were calculated by the following (equations 1-3). [18]
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2.3. Analysis and characterizations FTIR spectra of copolymer were perform using Perkin–Elmer PE 1600 FTIR spectrophotometer (USA) in the range of 4000–400 cm-1. The powder XRD patterns of biopolymer and grafted samples was performed by using XRD (Rigaku Ultima IV, X-ray diffractometer) employing CuKα radiation of the wavelength of 1.5406 Å with visible slights at 45 kV/40 mA. The surface morphology of the grafted samples was examined by a scanning electron microscopy (SEM), (TESCAN, VEGA SEM) under a 20 kV electron acceleration voltage
by
carbon
coating
of
samples.
Schimadzu
UV-1208
models
UV–Vis
spectrophotometer (Japan) was used for chemical structure and conjugation measurement. Apart from this UV-Vis also measure the electronic transition and doping–dedoping behavior of mwXG-g-PANi. Thermal stability of copolymer was determined using thermogravimetric analyzer (TGA) (Perkin Elmer model 4000, USA).The pH measurements were made with OHAUS starter 2100 (USA). Microwave oven LG (Model No. MS-283MC; 1200 W) having 2450 MHz microwave frequency and a power output from 0 to 1200 W was used with constant alteration for utilization. Finally, the sensing properties of the mwXG-g-PANi film are tested against increasing ammonia concentration by monitoring the changes in current through DC current-voltage (I-V) measurement using Keithley 237. 2.4. Measurement of sensing properties The synthesised mwXG-g-PANi (172 %G) sample was crush in a smooth agate and mortar. The sensing film of XG and mwXG-g-PANi (172 %G) were deposited on glass substrate (10 x 10 mm) using the drop coating technique and dried on hot plate at 60 °C for 20 min. The thicknesses of mwXG-g-PANi composite films were found to be ~200µm. The schematic diagram of the chemiresistor type sensor is shown in (Figure. 1a).The digital photograph of the mwXG-g-PANi film with electrical leads is shown in (Figure 1b). In order to measure the ammonia response, the current of the mwXG-g-PANi composite films were measured in air ambient and in ammonia atmosphere. For current measurement, two electrodes of copper wire contact was prepared by using silver paste, separated by 6 mm, onto mwXG-g-PANi composite films. The current was measured using Keithley 237 (Keithley, USA). Thereafter these sensors were fixed into the homemade gas-sensing unit as shown in our previous work [19, 20]. A voltage of 20 V applied across the two electrodes and the current was 7
observed. The changes in current–voltage characteristics of the sensing films were utilized for detection of NH3. In the test, a fixed (20 V DC) voltage was applied, and the DC current was recorded. Sensor response ∆G/Go value defined as the relative variation in conductance due to the introduction of analyte [21]. ΔG/G0 = (G − G0)/G0………………………………………. (4) Where G0 is the conductance in the dried air, and G is the conductance in measuring NH3 mixed with dried air, respectively. The variation of current with time (temporal response) has been monitored for the sensing study. In all the experiments, sensors were first exposed to dried air (ambient atmosphere) to obtain the baseline, then to a desired concentration of NH3, and then back to air (ambient atmosphere) which completed one cycle. 3. Results and discussions 3.1. Synthesis of the mwXG-g-PANi composite The different parameters for optimization of mwXG-g-PANi was performed (Figure.1). 3.1.1 Effect of microwave power The effect of progress in MW power of grafting reaction has been examined by varying the MW power (%) from 20 to 100, keeping other parameter constant {such as [APS] 5 x 10-3M; [HCl] 1 x 10-1M; [ANi] 5 x 10-2M and [XG] 2.0 g/L}. It perceive that %G and %E increases from 31 to 102 and 13.36 to 44.82 respectively on increasing the MW power (%) from 20 to 80 because of more availability of MW energy at high MW power, causing generation of more monomer and macro radicals that resulted in high %G (Figure. 2a). On further increase in MW power (%) above 80, grafting decreases. This behavior may be explained due to the fact that on increasing the MW power (%), there is increment in homopolymer formation or can be due to decomposition of graft copolymer at high MW power (%). 3.1.2 Effect of exposure time The effect of progress in MW exposure time of grafting reaction by altering the time period from 10 to 90 sec at MW power (80%) were studied, keeping other parameter constant {such as [APS] 5 x 10-3M; [HCl] 1 x 10-1M[ANi]; 5 x 10-2M and [XG] 2.0 g/L}. It has been observed that %G and %E increases from 62 to 113 and 26.72 to 48.70 respectively on increasing the MW exposure time from 10 to 50 min. Further enhancement in time duration of microwave causes slowdown in %G (Figure. 2b). Because on increasing the exposure 8
time, propagation of grafting chains takes place due to availability of more active species, which accounts for higher %G. On further increment in time period, the mutual annihilation of growing grafted chain occurs, which results in decrement of grafting parameters and increment in homopolymer formation. 3.1.3 Effect of APS concentration The impact of APS on grafting parameters by varying the concentration of APS from 5 x 103
M to 45 x 10-3M were studied, keeping other parameter constant {such as MW power (80%),
MW exposure time (50s), [HCl] 1 x 10-1M, [ANi] 5 x 10-2M and [XG] 2.0 g/L}. It has been found that the %G and %E increment from 112 to 160 and 48.27 to 68.96 respectively on expanding the concentration of APS throughout the cited range i.e., from 5 x 10-3M to 45 x 10-3 (Figure. 2c). It may be due to the fact that progressive reduction of APS producing primary free radicals, which attack on the XG molecules creating more active sites to which monomer addition takes place. 3.1.4 Effect of hydrogen ion concentration To observe the effect of hydrogen ion concentration, the reaction has been conveyed at different concentration of HCl from 1 x 10-1M to 2 M, keeping other parameter constant {such as MW power(80%), MW exposure time (50s), [APS] 45 x 10-3M, [ANi] 5 x 10-2M and [XG] 2.0 g/L} and results are demonstrated in (Figure. 2d). The %G and %E parameters increment from 160 to 172 and 68.96 to 74.13 respectively consistently on expanding the concentration of hydrogen ion from 0.1M to 1.5M; however, beyond that grafting parameters become constant. This finding may be due to protonation of the aniline monomer which accelerates the generation of PANi ion radicals. 3.1.5 Effect of ANi concentration The effect of ANi concentration on graft copolymerization has been studied over by altering its concentration from 5 x 10-2M to 25 x 10-2M, keeping other parameter constant {such as MW power(80%), MW exposure time (50s), [APS] 45 x 10-3M, [HCl] 1.5 x 10-1M and [XG] 2.0 g/L} and results have been exhibited in (Figure. 2e). Result shows the %G parameters increase from 172 to 390, but %E decreases from 74.13 to 26.63 respectively on increasing the concentration of ANi from 5 x 10-2 to 25 x 10-2M, the increase in %G is because of more prominent accessibility of monomer molecules at the close proximity to the polymeric 9
backbone. The monomer molecules, which are at immediate vicinity of reaction sites become acceptors of XG macroradicals (XGO*) resulting in chain initiation and thereafter themselves becomes free radical donors to neighboring molecules leading to lowering of termination. 3.1.6 Effect of XG concentration The grafting of ANi onto XG was perform at different concentrations of XG. As the concentration of XG increased from 2 to 10g/L, keeping other parameter constant {such as MW power (80%), MW exposure time (50s), [APS] 45 x 10-3M [HCl] 1.5 x 10-1M and [ANi] 5 x 10-2M} (Fig. 1f). The %G and %E have been found to decrease from 172 to 90 and 74.13 to 38.79 respectively. This may be due to increase in concentration of XG, causes increase in viscosity of reaction medium, results in the obstruction in the movement of free radicals thereby, decreases the %G and %E. It is also observed that, the final temperature of the reaction medium after microwave irradiation using MW power 80% for 50s is 86°C. While the boiling point of ANi is 184°C. Thus the presence of final reaction temperature of 86°C confirm there is no loss in ANi concentration after completion of reaction because of its high boiling point. 3.2 Characterization of the mwXG-g-PANi composite 3.2.1 Effect of pH on mwXG-g-PANi by UV-Vis Spectra The pH influences the protonation-deprotonation equilibrium of conducting emeraldine salt and non-conducting emeraldine base forms of polyaniline. From the literature, it suggest that two distinct absorption bands located between 315-345 and 610-650 nm [22]. It is the typical absorption spectrum of the blue polyaniline base. The distinct absorption bands varies depending on preparation and/or processing of PANi. For the green protonated polyaniline form, which is obtained from the base by treatment with acids, three absorption bands are usually observed at 325-360 nm, 400-430 nm, and 780-826nm [23]. In the present section, the spectrophotometric response to a change of pH of mwXGg-PANi is investigated in detail. Figure. 3a shows UV-Vis absorption spectra of mwXG-gPANi at pH = 1, 3 7, 9 and 10. A composite mwXG-g-PANi showed a three absorption bands at 330, 430 and 820 nm which are typical of the protonated form of grafted PANi dispersion in an acidic region. The band at 330nm correspond to the overlapping of glucopyranose components of xanthan gum and (π*← π transitions of benzenoid rings of 10
grafted PANi with bands at 430 nm (assigned to π*←π transition in polaron/bipolaron of grafted PANi) and at 820 nm (assigned polaron transitions of grafted PANi) [23]. It was observed that absorption band at 430 nm (assigned to the polaron transition) is found only at pH < 7 and it disappears in alkaline solutions (Figure. 3a). The peak at 630 nm is typical of grafted PANi base in alkaline media (pH = 9,11). It can be seen that the 430nm peak disappear and new peak at 630nm appear at basic pH. The absorption changes in the spectra are usually attributed to the unprotonated quinoid diimine structure [22]. At pH > 9 an increase of the conjugation length and a decrease of the band gap take place. This demonstrates the transformation of protonated benzoid structures into unprotonated quinoid structures in the media of different pH, which affects its electronic absorption spectra [22]. XG indicated (Figure. 3a) a broad absorption band at 310 nm (due to glucopyranose components). Besides, the characteristic peaks of glucopyranose and PANi was essentially observed and it supports the grafting of PANi on to XG. Along these lines, UV-Vis spectra affirmed the chemical structure, conjugation, electronic transition and doping-dedoping behavior of mwXG-g-PANi. 3.2.2 FTIR spectroscopy Structural changes of mwXG-g-PANi (172 %G) were affirmed by FTIR spectroscopy (Figure. 3b). The range of XG showcases a wide band at 3323.16 cm−1, a typical stretch for the OH group. A band corresponding to aliphatic C-H stretching appears at 2916.65 cm−1. Bands at 1642.23 cm−1 represent (C=O stretching carboxylate group of typical saccharides), respectively. The polysaccharide likewise displayed another peak at 1015 cm−1, due to stretching of the C-O bond [17]. The FTIR spectrum of mwXG-g-PANi in (Figure. 3b) demonstrates all significant peaks correspond to XG and PANI. The accompanying key characteristic bands were observed: 3323.16 cm−1 (overlapping of O-H stretching), 2916.65 cm−1. (aliphatic C-H stretching), 1642.23 cm -1 (C=O stretching of carbonyl group of typical saccharide, 1576.21 and 1487.13 cm−1 (non-symmetric vibration mode of C=C in benzenoid and quinoid ring system)[24], 1215.67 cm-1 (C-N stretching vibration mode in benzenoid ring system), and 1310.90 cm−1(C-N stretching vibration of secondary aromatic amine)[25]; 1120.13 cm−1 (BNH+= Q vibration, indicating grafted PANi is conductive and in the form of emeraldine 11
salt)[26]. 863.21 and 819.35 cm−1 (aromatic ring and out of plane C-H deformation vibrations for 1,4-disubstituted aromatic ring system), and at 3013 cm−1 (C-H stretching vibrations of aromatic ring). It was observed that in mwXG-g-PANi, the peak at 3200- 3500 cm-1 are of quite reduced intensity, (due to overlapping of O–H stretching of XG and N–H stretching of ANi groups at PANi grafts). This reduction in broad and intense peak affirm that’s the appreciable amounts of O–H and N–H at XG have been grafted with PANi Chain. The absorption band of the B-NH+=Q bending vibration of pure PANi was observed at 1227.75 cm-1 but shifted to 1120.13 cm-1 in the XG-g-PANi due to the steric effect of XG. The absorption peaks at 1642.23 cm-1 for XG has been shifted to 1701.12 cm-1 in the mwXG-g-PANi due to C=O stretching of carbonyl group, typical saccharide absorption. The absorption peaks at 2916.65cm-1 for XG has been shifted to 3013.67cm-1. The intense sharp peak at 1015cm-1 is drastically reduce in grafted copolymer. From the FTIR data it is clear that the grafted copolymer mwXG-g-PANi had characteristic peaks of PANi and of XG, which could be a strong evidence of grafting. 3.2.3 X-ray Diffraction (XRD) Analysis X-ray diffraction tests can give a lot of data on structural aspects. The XRD patterns of mwXG-g-PANi (172 %G) are indicated in (Figure. 3c, d). It can be seen that immaculate XG demonstrates a typical amorphous pattern (Figure. 3c), while on account of the mwXG-gPANi, the XRD pattern demonstrates the semi crystalline structure (Figure. 3d). In the XRD pattern of mwXG-g-PANi, Bragg diffraction peak at 2 θ = 16.30°, 20.12° and 25.56° relate to the emeraldine PANI. The peaks centered at 2 θ = 16.30° and 25.56° are attributed to the periodicity parallel and perpendicular to the PANi chains [27]. The peak at 2θ = 20.12° also represents the characteristic distance between the ring planes of benzene rings in adjacent chains or the close-contact inter-chain distance [28]. The crystallinity is because of the vicinity of PANi. By and large polymers are thought to be amorphous however PANI is indicating crystalline structure due to its fiber nature and planar nature of benzenoid and quinoid functional groups. From the XRD pattern of mwXG-g-PANi, it is clear that PANi is grafted onto XG and the grafted PANi has high ordered crystal structure which is expected to exhibit high electrical conductivity.
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3.2.4 Thermal analysis The impact of PANi on the thermal stability of XG was mulled over with the assistance of the TGA technique (Figure. 3e). TGA of XG demonstrated that the degradation of XG started at about 200°C. The rate of weight reduction expanded with an increment in the temperature. About 10% weight reduction happened until at 100°C, and this was because of the desorption of water. About 50% weight reduction happened at 300°C; after that, the rate of weight reduction diminished, thus it displayed a solitary step degradation process. Around 70% XG degraded at 750°C. The degradation of mwXG-g-PANi began at 150°C. In any case, a 3.2% weight reduction was observed at 100°C, which may have been due to the loss of absorbed water. The graft copolymer showed a two-stage degradation process, i.e., from 242.88 to 346.34°C and from 346.34 to 750°C. 26% weight reduction was observed at 300°C. Therefore the affirmation of grafting is clear from the thermal analysis, which demonstrates that, the mwXG-g-PANi has more thermal stability than the XG. 3.2.5 Morphological analysis of mwXG-g-PANi The SEM images at two distinct magnification of 1kx and 4kx of XG (Figure. 4a, b) and mwXG-g-PANi (Figure. 4c, d) are exhibited. From the Figure 4, SEM images reveal some variations in the morphological structure of XG and mwXG-g-PANi sample acquired from the graft copolymerization onto XG. The grafting presents colossal changes on the surface of the carbohydrate particles. It can be easily be seen in SEM images, that after copolymerization of xanthan gum, the XG particles show irregular morphology, as a lobule of the XG is changed into porous and fluffy. The plain polymer has a smooth surface while the grafted copolymer has rough irregular surface morphology. Consequently, the morphology supports the grafting of PANi on to the XG. 3.3. Mechanism for graft copolymerization The substantial number of hydroxyl groups situated at gum macromolecule carry on as though they are fasten to a stationary flatboat (gum macromolecule) and their confined rotation (Galema, 1997)[14] along these lines will be seen under microwaves, bringing about dielectric heating which cause bond breakage and therefore producing radical locales. Moreover, microwaves are additionally have a particular impact in bringing down of the energy of activation of the reactions and these two truths quicken the fast graft 13
copolymerization. In the above's light a possible oxidative radical system for the PANi grafting on to XG under microwave irradiation has been proposed. As already reported [15,16], where we have investigated free radical copolymerization of vinyl monomers with carbohydrate polymers utilizing peroxydisulfate, a chain mechanism is included because of formation of sulfate ion radicals (SO4-*), which are surely understood ion chain carriers for the graft copolymerization (Scheme 2). In the meantime peroxydisulfate invigorates the oxidative polymerization reaction of aniline through a medium of cationic radicals and form PANi and PANi radicals [29] (Scheme 3). At last XG macro radicals and PANi cation radicals are consolidated to form mwXG-g-PANi graft copolymer. SO4-* is the primary radicals produced from the APS by the lessening of one electron, indicated in Scheme 2. At the same time, APS produce SO42- ions by the decrease of two electrons and go about as oxidant. They start the oxidative polymerization of aniline, as the polymerization of monomer is accounted for to be speedier than the H abstraction from the biopolymer backbone [15]. The macro radicals XGO* may be produced by the abstraction of H by the developing PANi ion radical (PANi*) in the acid medium, which may include onto the XGO* macro radicals creating new radical XGO-PANi* and these chains will develop and joined with other XGOPANi* chains to give a graft copolymer (Scheme 4). 3.4 Influence of the mwXG-g-PANi composite on response of NH3 sensor We have examined the NH3 by increasing the concentration of 25% ammonia solution in the range of 1–100 ppb and monitoring the changes in current. The applied voltage used in ammonia sensing experiments is 20V because the composite film has very low electrical conductivity, hence, we used higher voltage to have appreciable current above the background noise. Both the sensor response and the temporal response of the mwXG-g-PANi sensor have been evaluated. The room temperature I-V characteristics of mwXG-g-PANi (172 %G) from -20V to 20V are shown in Figure.5a. However, at the higher density of PANi deposits, the XG are almost continuously covered with PANi forming a film-like structure and consequently, a linear Ohmic behavior is observed (Figure.5a). The values of electrical conductivities at room temperature of thin films of XG and mwXG-g-PANi are 7.34 x 10-9 and 31.3 ῼ-1cm-1, respectively. Temporal response for different NH3 concentrations ranging from 1ppb to 100ppb is shown in (Figure. 5b). The constant baseline current of 0.63A at 20V was observed. The change in current with increase in the NH3 concentration follows a linear behaviour as shown 14
in (Figure. 5c). It increases with the increase in NH3 concentration. The response ∆G/Go of the mwXG-g-PANi (172 %G) film at different NH3 concentration are shown in (Figure. 5d). It shows the dependence of sensor response on the NH3 concentration. It can be noted that the sensor response increases linearly with the increase in NH3 concentration. The Chemiresistive NH3 sensor response ∆G/Go value for 1 ppb is found to be ~905 which is extremely high and sensitive at room temperature. This means that the low detection limit of the sensor is at the level of ppb. Response and recovery time are the basic parameter of the sensors. Response and recovery time are defined as the time taken for the sensor to attain 90% of maximum change in current on exposure to analyte (in gas/vapour phase) is the response time (Tres). The time taken by the sensor to get back 90% of the original current is the recovery time (Trec). A small value of response and recovery time are the indicative of a good sensor. The sensing response time and recovery time of the mwXG-g-PANi film sensor is about few seconds (i.e Tres=16s, Trec=10s in 1ppb; Tres=20s, Trec=18s in 50ppb and Tres=25s, Trec=23s in 100ppb) which confirm the fast response of sensor. The above results indicate that by monitoring the change in the conductance of the mwXG-g-PANi film, the presence of NH3 molecules could be monitored. This demonstrates that mwXG-g-PANi film could be a good candidate for chemical sensors in detecting vapour NH3. 3.5 Long term stability of sensor Having an appreciation of the sensors’ tolerance and life-time is important for both the manufacturer and the end-user because these characteristics contribute to the quality enhancement of a product. Usually in the present article we have reported the long term stability of few months of mwXG-g-PANi composite sensor. The stability of the sensor towards ammonia sensing has been studied over a period of 3 months (Figure not shown). Preliminary results had shown that the sensor response ∆G/Go of the sensor at 50 ppb concentration of ammonia decreases by ~2% in 3 months, display an incredible stability of the sensor. In contrast as far as industry is concerned, the long term stability starts only after one year of operation. Thus the claim of long term stability is irrelevant on commercial point of view. Thus stability test of more than 12 months need to be exploited in the near future in order to better understand the stability of our sensor.
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3.6 Reproducibility of the sensor The reproducibility of the sensor has been tested for ammonia samples at three different concentration (1ppb, 5ppb and 100ppb). The relative standard deviation (RSD) of the current and sensor response with three different ammonia concentrations (fig.5c, d) are found to be ~2% for ten successive measurements. This study confirms excellent reproducibility/reversibility of the sensor. 3.7 Sensor response with water We have investigated the response of the sensor with DI water. No sensing is observed with water alone (Figure not shown). We believe that DI water used for dilution in order to prepare different concentration of ammonia does not show any impact on sensing. Because, the vapor phase diagram of ammonia-water shows that ammonia having low boiling point always contribute more vapor of ammonia above the aqueous ammonia solution for a wide range of ammonia concentration kept under equilibrium at room temperature. When it is not kept under closed equilibrium condition, the tendency of escaping ammonia above the liquid is more than water vapor. Hence, we believe that with the increase in ammonia concentration in the analyte, the water vapor content above the liquid reduces and hence, the response of the sensor towards the analyte vapor is almost the ammonia vapor rather than a combination of both. Moreover, the sensor is futile for the sensitivity towards water vapor. 3.8 Plausible Mechanism for ammonia sensing The sensing mechanism of PANi on ammonia gas and inorganic semiconductor oxides is different. Fig.5b shows the temporal response of mwXG-g-PANi sensor exposed to different concentration of ammonia. The current (µA) of sensor shows enhancement from 63.5 µA to 93 µA when expose to ammonia vapour from 1ppb to 100ppb respectively. In comparison with the other reported PANi sensor in literature (30-31), the present response exhibit an inverse nature which is quite unexpected. The change in relative variation in conductance is found to be linearly proportional to concentration of ammonia (Fig.5d). Although, the exact mechanism of sensing is not very clear at this stage, the plausible phenomena for increase in the current on exposure to ammonia can be interpreted by considering the interaction of NH3 with XG. In the ammonia vapour environment NH3 can 16
attack two adsorption sites. (i) N+-H sites originated from small HCl content present during polymerization reaction; and (ii) XG molecules trapped in the PANi chain. At the latter site NH3 molecule may take up a hydrogen from XG forming NH4+ species which create –O- in the PANi chain simultaneously leaving one site for conduction. This explain the increment in current imparted by self-doping in the presence of XG on exposure to ammonia. An unusual response in magnitude observed in the present case suggest a significant role of the incorporated XG in the PANi chain. 4. Conclusion A facile and effective method to construct the chemical sensors based on mwXG-g-PANi for NH3 detection has been demonstrated. The mwXG-g-PANi sensor were synthesized by in situ polymerization using microwave irradiation, and it shows high stability. The sensing properties of the obtained sensors were investigated. The results indicated that mwXG-gPANi sensor had high sensitivity, stability and good reproducibility to NH3. It is found to exhibit high sensitivity over a broad range of concentration from 1 ppb up to 100ppb. The response is highly reversible over up to 10 cycles. The sensor exhibits stable response up to 90 days suggesting long term stability of sensing material. The chemiresistive sensors allowed rapid and quantitative detection of trace NH3 concentrations at room temperature.
Conflict of interest The authors declare no competing financial interest. Acknowledgements The authors Dr. Sadanand Pandey is grateful to National Research foundation (NRF), South Africa (Grant No: 80428) for its liberal budgetary support. The authors also acknowledge University of Johannesburg (UJ), (South Africa) and NTPC (India) for Lab and instruments facilities. We additionally says thanks to Mr. Mufamadi Nndwakhulu and Mr. Edward Malenga, Department of Extraction & Engineering Metallurgy, UJ for providing SEM and XRD characterizations of our samples.
17
References [1]. G. Clayton, F. Clayton. Patty’s industrial hygiene and toxicology. 3rd rev. ed. New York. John Wiley and sons. Eds (1981). [2]. B. Timmer, W. Olthuis, A.v.d. Berg, Sens. Actuators B: Chem. 107 (2005) 666–677. [3]. T.H. Risby, S. Solga, Applied Physics B 85 (2006) 421–426. [4]. J.L. Hurtado, C.R. Lowe, ACS Appl. Mater. Interfaces 6 (2014) 8903–8908. [5]. S Pandey, GK Goswami, KK Nanda, Int J Biol Macromol. 51 (2012), 583-589. [6]. G. Chen, et al. Sci. Rep. 2, (2012) 343. [7]. Dan, Y. et al. Nano Letters 9(2009), 1472–1475. [8]. J. Huang, Pure Appl. Chem. 78 (2006)15–27. [9]. J.W. Schultze, H. Karabalut, Electrochim. Acta. 50 (2005)1739–1743. [10]. X.L. Luo, J.J. Xu, Y. Du, H.Y. Chen, Anal. Biochem. 334 (2004) 284–289. [11]. K.J. Feng, Y.H. Yang, Z.J. Wang, J.H. Jiang, G.L. Shen, R.Q. Yu, Talanta 70 (2006) 561–565. [12]. F. GarcõÂa-Ochoaa,, V.E. Santosa, J.A. Casasb, E. GoÂmez, Biotechnol. Adv. 18 (2000) 549 – 579. [13]. H. Mallik, N. Gupta, A. Sarkar, Mater Sci Eng C 20 (2002) 215–218. [14]. S.A. Galema, Chem. Soc. Rev. 26 (1997) 233–238. [15]. V. Singh, A. Tiwari, S. Pandey, S.K. Singh, Starch/ Starke, 58 (2006) 536–543. [16]. V. Singh, A. Tiwari, S. Pandey, S.K. Singh, Express Polym Lett. 1 (2007) 51–58. [17]. S. Pandey, S.B. Mishra, Carbohydr Polym. 90 (2012) 370–379. [18]. V.D. Athawale, V. Lele, Carbohydr Polym. 35 (1998) 21–27. [19]. S. Pandey, G.K. Goswami, K.K. Nanda, Sci. Rep. 3 (2013) 2082. [20]. S. Pandey, K.K. Nanda, ACS Sensors 1 (2016) 55-62. [21]. Z.H. Wang, Adv. Mater. 15 (2003) 432-436. [22]. M. Wan, J. Polym. Sci., Polym. Chem., 30 (1992) 543-549. [23]. S.C. Yang, R.J. Cushman, D. Zhang, Synth. Met. 29 (1989) E401-408. [24]. J. Bhadra, D. Sarkar, Bull. Mater. Sci. 33 (2010) 519-523. [25]. H. Swaruparani, S. Basavaraja, C. Basavaraja, S.H. Do and A. Venkataraman, J. Appl. Polym. Sci. 117 (2010) 1350⎼1360. [26]. D.B. Mahesh, S. Basavaraja, D.S. Balaji, V. Shivakumar, L. Arunkumar and A. Venkataraman, Polymer Composites, 30 (2009) 1668⎼1677. [27]. H.C. Pant, M.K. Patra, S.C. Negi, A. Bhatia, S.R. Vadera, N. Kumar, Bull. Mater. Sci. 29 (2006) 379–384. [28]. J.P Pouget, C.H. Hsu, A.G. MacDiarmid, A.J. Epstein, Synth. Met. 69 (1995) 119–120. [29]. Y. Ding, A.B. Padias, H.K. Jr. Hall, J. Polym. Sci., Part A: Polym. Chem. 37(14) (1999) 2569–2579. [30]. G.Bidan, Sens Actuators. B 6 (1992) 45-46 [31]. H. Hirata, L. Sun, Sens Actuators. A 40 (1994) 159-163.
18
Lists of captions. Figure. 1. Sensor device structure (a) schematic cross sectional view of sensor with sensing film of mwXG-g-PANi onto glass substrate, (b) Top view of prepared sensor with ohmic contacts. Figure. 1. Optimization of mwXG-g-PANi composite. Effect of (a). Microwave power; (b). Exposure time; (c) APS concentration; (d). Hydrogen ion concentration; (e) Aniline concentration; and (f) XG concentration. Figure. 3. Characterization of XG and mwXG-g-PANi composite. (a) Effect of pH on UVVis spectra. (b). FTIR. (c) XRD and (d) TGA. Figure. 4. Morphology analysis of (a). XG (1kx); (b) XG (4kx); (c) mwXG-g-PANi (1kx); and (d) mwXG-g-PANi (4kx). Figure 5 Representative room temperature NH3 sensing behavior (a) I-V characteristic of a sensor made of the mwXG-g-PANI film (b) Temporal response curve of our mwXG-g-PANI sensor in 1 ppb to 100 ppb range. (c) Log-log plot of current versus different NH3 concentration. Solid red line is its linear fit. (d) Log-log plot of sensor response with different NH3 concentration. The red solid line is the linear fit. Vertical error bars represent the standard deviation from the mean. Scheme 1. Schematic diagram of the present work. Scheme 2. Generation of primary radicals under microwave irradiation. Scheme 3. Formation of secondary radicals under microwave irradiation. Scheme 4. General mechanism for microwave-accelerated graft copolymerization of PANi on to XG
19
(a)
(b)
Figure.1
20
(a) 120
120
(b)
%G %E
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80
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21
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Figure. 3 22
Figure. 4
23
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Figure 5
24
Scheme 1.
25
(Oxidising action of APS) O O
-
O
O O NH 4+
-
S
O NH 4 +
S
MW
O
HO
O
O O
S
O O
S
O
-
O
O
ammoniumperoxydisulphate
peroxodisulphate ion
-2e
O
-
O
-
S
O
O
sulphate ion (APS act as initiator) O O HO
S
O O
S O
O
peroxodisulphat e ion
-
O
O
O
-e
-
O
S
O
MW
-
+
O
S
O
O
sulphate ion
sulphate ion
Scheme 2.
26
O
-
NH4+ -e
H 2N
APS
-2H+
H3N +
NH2
NH2+
-2e
-H+ NH
NH NH+
+
H2N
NH
NH -2e NH 2
-H+
NH
NH
+ NH3+ NH2
NH
NH2+ NH
NH NH
Polyaniline ion radical
Scheme 3.
27
OH
SO4-*
+
MW
O*
Initiation
Alkoxy macroradical
Xanthan gum (XG)
-H + +
OH
M*
O* + MW
PANi ion radical
XG f ree radical site
MnH
Homopolymer
-H+ O* +
M*
O
M*
MW Propogation
O
O
M*
+
*M
-2H +
MW
M
M
O
O Termination
Graft copolymer (mwXG-g-PANi)
Scheme 4.
28
S0141-8130(16)30363-4 http://dx.doi.org/doi:10.1016/j.ijbiomac.2016.04.055 BIOMAC 6024
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
29-2-2016 5-4-2016 19-4-2016
Please cite this article as: Sadanand Pandey, James Ramontja, Rapid, Facile Microwave-assisted synthesis of Xanthan gum grafted polyaniline for chemical sensor, International Journal of Biological Macromolecules http://dx.doi.org/10.1016/j.ijbiomac.2016.04.055 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Rapid, Facile Microwave-assisted synthesis of Xanthan gum grafted polyaniline for chemical sensor Sadanand Pandey*, James Ramontja 1
Department of Applied chemistry, University of Johannesburg, P.O Box 17011,
Doornfontien, Johannesburg 2028, South Africa
Corresponding Author: [email protected]; [email protected] ph.:+27-11-559-6644
1
Graphical Abstract
2
Highlights
Polyaniline grafted xanthan gum was synthesized by microwave assisted technique.
Process is highly energy efficient and takes significantly less reaction time (∼50 sec).
Detailed characterization of grafted xanthan gum by UV-Vis, FTIR, XRD, SEM, and TGA.
Fabricated grafted sample film were then examined for the chemical sensor.
Chemiresistive NH3 sensors with superior room temperature sensing performance.
Abstract Grafting method, through microwave radiation procedure is extremely productive in terms of time consumption, cost effectiveness and environmental friendliness. In this study, conductive and thermally stable composite (mwXG-g-PANi) was synthesized by grafting of aniline (ANi) on to xanthan gum (XG) using catalytic weight of initiator, ammonium peroxydisulfate in the process of microwave irradiation in aqueous medium. The synthesis of mwXG-g-PANi were confirm by FTIR, XRD, TGA, and SEM. The influence of altering the microwave power, exposure time of microwave, concentration of monomer and the amount of initiator of graft polymerization were studied over the grafting parameters, for example, grafting percentage (%G) and grafting efficiency (%E). The maximum %G and %E achieved was 172 and 74.13 respectively. The outcome demonstrates that the microwave irradiation strategy can increase the reaction rate by 72 times over the conventional method. Electrical conductivity of XG and mwXG-g-PANi composite film was performed. The fabricated grafted sample film were then examined for the chemical sensor. The mwXG-g-PANi, effectively integrated and handled, are NH3 sensitive and exhibit a rapid sensing in presence of NH3 vapor. Chemiresistive NH3 sensors with superior room temperature sensing performance were produced with sensor response of 905 at 1ppb and 90% recovery within few second. Keywords: Biopolymer; Polyaniline; Graft copolymer; Microwave irradiation; Green technology; Chemical sensor 3
1. Introduction The detection of toxic gases has been a major focus of sensor research in recent years. Ammonia is a kind of gas with high toxicity. It is regulated by occupational safety and health administration (OSHA) and has a current permissible exposure limit (PEL) of 35 ppm over 15 minutes short term exposure limit (STEL). Besides, ammonia concentration at or above 2500 ppm exposures for 30 minutes are considered immediately dangerous to life and health (IDLH), and may cause induced chemical pneumonitis, burns (eyes, face and mouth), severe local edema, dyspnea, progressive cyanopsis and even death [1]. The human body naturally produces ammonia by various metabolic activities [2]. In medical sector, existence of excessive amount of ammonia in exhaled human breath can be treated as indications of several diseases related to dysfunctions of liver and kidneys [3, 4]. Hence, the detection of ammonia gas/vapour is of paramount importance in terms of both environmental as well as health monitoring sectors. A wide variety of materials, such as, metal oxides (ZnO, SnO2, TiO2, V2O5, In2O3 etc.), polymers and carbon based materials have been used as sensing elements for various ammonia gas sensor applications [5-7]. However, most of the currently available gas sensors based on these materials, often suffer from more than one drawback such as poor selectivity, influence of humidity, external stimulus such as Joule heating or UV illumination for response/recovery, operation at high temperature (200–500 °C) which lead to high power consumption. Therefore, new sensing materials with low detection limit, high sensitivity and reproducibility, fast response and recovery without any external stimulus, low cost and ecofriendly sensor are expected for gas detection at room temperature. Polyaniline (PANi) known as intrinsically conducting polymers (ICPs) has attracted specifically consideration in view of its ease of doping, its different chemical forms available depending on acid/base treatment and its substantial stability in ambient atmosphere. All these characteristics clarify, why PANi is contemplate as a moderately effectively prepared and processed ICP for an extensive variety of applications. PANi is pH sensitive and may fulfil the requirements for the development of chemical sensor. PANi has been utilized as a cost effective conducting polymer [8]. Biological and electronic properties of PANi were altered by doping distinctive types of inorganic nanomaterials [9]. Similarly biopolymer has been widely used as a part of sensors because of its 4
biodegradable, biocompatible and non-toxicity [10]. Independently both PANi and xanthan gum (XG) had indicated brilliant properties as conducting and biocompatible material, respectively. However XG has restricted conductivity while PANi has limited biocompatibility which has been a subject of in-depth research in last decade or so. There were a few endeavours to enhance the performance of biopolymer and PANi relies upon alteration in structural and surface chemistry by consolidating nano-metal-oxides [9, 11]. Synthesis of metal nano-particles itself is extremely complex process and requires several chemical processes. Thus, the utilization of chemicals for synthesis and nondegradable characteristics of metal oxides can have several environment issues when chemical wastes were disposed. Secondly, oxidative characteristics of nano-materials likewise restrain the stability amid storage and used sensing film in ambient atmosphere. Taking into consideration these advantage, the objectives of the present work was to develop an appropriate method for metal free conducting biopolymer matrixes by modification of carbohydrate polymers with PANi which may prompt the synthesis of multifunctional electrical conducting composite. It will be the favorable advantages such as technological applications, affinity with environment and biological frameworks, and is financially savvy. The biopolymer selected for the present study is XG. XG is derived from Xanthomonas campestris. The structural unit of XG biopolymer consist of backbone of β-(1– 4)-D-glucopyranose glucan along with side chains of β-(3-1)-α-linked D-mannopyranose-(21)-β-D-glucuronic acid-(4-1)-β-D-mannopyranose on alternating residues [12]. Previously reports shows that inorganic salt complexes of biopolymer such as gum arabica (GA) act as a superionic electrical conductor [13]. Mostly, addition of conducting polymers, for example, PANi into a flexible matrix of XG ought to result in great processability alongside the electrical conductivity, chemical stability toward dopants and thermal stability. It has been notice presently, that microwave irradiation is rising as effective device for the chemical processing [14] and in different fields of chemistry including polymers. Fundamental point of interest is that it brings about practically rapid 'in core' handling of materials in a homogeneous and specific way. Under microwave irradiation grafting of polyacrylamide, polyacrylonitrile onto potato starch [15, 16]; and poly (ethylacrylate) onto XG [17] have been performed in absence or at low concentration of initiator.
5
In this context, the present work aimed to provides details study of the ammonium peroxydisulfate (APS) initiated synthesis of mwXG-g-PANi under microwave conditions. This study deal with, mwXG-g-PANi optimization, plausible mechanism, different characteristic properties of mwXG-g-PANi such as morphology, conducting, and thermal behavior of the mwXG-g-PANi were explored. Here, we have also reported, a rapid and highly sensitive mwXG-g-PANi composite based chemiresistive sensor for the room temperature detection of ammonia levels in a lower range from 1 parts-per-billion (ppb) to 100 ppb. The sensor is based on the mwXG-g-PANi composite of polysaccharide (xanthan gum, XG) and polyaniline (PANi). Sensor response, temporal response, reproducibility and stability studies reveal excellent ammonia sensing of the mwXG-g-PANi composite. The general schematic diagram of the present work was shown in (Scheme.1). 2. Experimental 2.1. Materials The biopolymer, XG from xanthomonas campestris (G1253, Sigma), monomer, aniline (≥99.5%, Sigma-Aldrich; 242284), initiator, APS (≥98.0%, Sigma-Aldrich; 248614), solvent, 1-Methyl-2-pyrrolidone (NMP) (Merck; 806072), hydrochloric acid (32% Merck; 100319) and Ammonium hydroxide solution (32.0%, Sigma-Aldrich; V000637) were used. 2.2. Graft copolymerization method for synthesis of mwXG-g-PANi composite During the grafting experiment, XG was dissolve in 25mL deionized (DI) water. A known amount of ANi and hydrochloric acid (HCl) solutions were included in the container. Further catalytic amount of APS was added in order to initiate the reaction of graft copolymerization. Further, the reaction mixture was exposed to microwave irradiation at definite microwave power and exposure time. After desired time period, the grafted sample was precipitated by pouring the reaction mixture into the NMP. After sought time period, the copolymer were dried in a vacuum oven at 60°C and weighed. The % grafting (%G), % efficiency (%E) and % homopolymer (%H) were calculated by the following (equations 1-3). [18]
6
2.3. Analysis and characterizations FTIR spectra of copolymer were perform using Perkin–Elmer PE 1600 FTIR spectrophotometer (USA) in the range of 4000–400 cm-1. The powder XRD patterns of biopolymer and grafted samples was performed by using XRD (Rigaku Ultima IV, X-ray diffractometer) employing CuKα radiation of the wavelength of 1.5406 Å with visible slights at 45 kV/40 mA. The surface morphology of the grafted samples was examined by a scanning electron microscopy (SEM), (TESCAN, VEGA SEM) under a 20 kV electron acceleration voltage
by
carbon
coating
of
samples.
Schimadzu
UV-1208
models
UV–Vis
spectrophotometer (Japan) was used for chemical structure and conjugation measurement. Apart from this UV-Vis also measure the electronic transition and doping–dedoping behavior of mwXG-g-PANi. Thermal stability of copolymer was determined using thermogravimetric analyzer (TGA) (Perkin Elmer model 4000, USA).The pH measurements were made with OHAUS starter 2100 (USA). Microwave oven LG (Model No. MS-283MC; 1200 W) having 2450 MHz microwave frequency and a power output from 0 to 1200 W was used with constant alteration for utilization. Finally, the sensing properties of the mwXG-g-PANi film are tested against increasing ammonia concentration by monitoring the changes in current through DC current-voltage (I-V) measurement using Keithley 237. 2.4. Measurement of sensing properties The synthesised mwXG-g-PANi (172 %G) sample was crush in a smooth agate and mortar. The sensing film of XG and mwXG-g-PANi (172 %G) were deposited on glass substrate (10 x 10 mm) using the drop coating technique and dried on hot plate at 60 °C for 20 min. The thicknesses of mwXG-g-PANi composite films were found to be ~200µm. The schematic diagram of the chemiresistor type sensor is shown in (Figure. 1a).The digital photograph of the mwXG-g-PANi film with electrical leads is shown in (Figure 1b). In order to measure the ammonia response, the current of the mwXG-g-PANi composite films were measured in air ambient and in ammonia atmosphere. For current measurement, two electrodes of copper wire contact was prepared by using silver paste, separated by 6 mm, onto mwXG-g-PANi composite films. The current was measured using Keithley 237 (Keithley, USA). Thereafter these sensors were fixed into the homemade gas-sensing unit as shown in our previous work [19, 20]. A voltage of 20 V applied across the two electrodes and the current was 7
observed. The changes in current–voltage characteristics of the sensing films were utilized for detection of NH3. In the test, a fixed (20 V DC) voltage was applied, and the DC current was recorded. Sensor response ∆G/Go value defined as the relative variation in conductance due to the introduction of analyte [21]. ΔG/G0 = (G − G0)/G0………………………………………. (4) Where G0 is the conductance in the dried air, and G is the conductance in measuring NH3 mixed with dried air, respectively. The variation of current with time (temporal response) has been monitored for the sensing study. In all the experiments, sensors were first exposed to dried air (ambient atmosphere) to obtain the baseline, then to a desired concentration of NH3, and then back to air (ambient atmosphere) which completed one cycle. 3. Results and discussions 3.1. Synthesis of the mwXG-g-PANi composite The different parameters for optimization of mwXG-g-PANi was performed (Figure.1). 3.1.1 Effect of microwave power The effect of progress in MW power of grafting reaction has been examined by varying the MW power (%) from 20 to 100, keeping other parameter constant {such as [APS] 5 x 10-3M; [HCl] 1 x 10-1M; [ANi] 5 x 10-2M and [XG] 2.0 g/L}. It perceive that %G and %E increases from 31 to 102 and 13.36 to 44.82 respectively on increasing the MW power (%) from 20 to 80 because of more availability of MW energy at high MW power, causing generation of more monomer and macro radicals that resulted in high %G (Figure. 2a). On further increase in MW power (%) above 80, grafting decreases. This behavior may be explained due to the fact that on increasing the MW power (%), there is increment in homopolymer formation or can be due to decomposition of graft copolymer at high MW power (%). 3.1.2 Effect of exposure time The effect of progress in MW exposure time of grafting reaction by altering the time period from 10 to 90 sec at MW power (80%) were studied, keeping other parameter constant {such as [APS] 5 x 10-3M; [HCl] 1 x 10-1M[ANi]; 5 x 10-2M and [XG] 2.0 g/L}. It has been observed that %G and %E increases from 62 to 113 and 26.72 to 48.70 respectively on increasing the MW exposure time from 10 to 50 min. Further enhancement in time duration of microwave causes slowdown in %G (Figure. 2b). Because on increasing the exposure 8
time, propagation of grafting chains takes place due to availability of more active species, which accounts for higher %G. On further increment in time period, the mutual annihilation of growing grafted chain occurs, which results in decrement of grafting parameters and increment in homopolymer formation. 3.1.3 Effect of APS concentration The impact of APS on grafting parameters by varying the concentration of APS from 5 x 103
M to 45 x 10-3M were studied, keeping other parameter constant {such as MW power (80%),
MW exposure time (50s), [HCl] 1 x 10-1M, [ANi] 5 x 10-2M and [XG] 2.0 g/L}. It has been found that the %G and %E increment from 112 to 160 and 48.27 to 68.96 respectively on expanding the concentration of APS throughout the cited range i.e., from 5 x 10-3M to 45 x 10-3 (Figure. 2c). It may be due to the fact that progressive reduction of APS producing primary free radicals, which attack on the XG molecules creating more active sites to which monomer addition takes place. 3.1.4 Effect of hydrogen ion concentration To observe the effect of hydrogen ion concentration, the reaction has been conveyed at different concentration of HCl from 1 x 10-1M to 2 M, keeping other parameter constant {such as MW power(80%), MW exposure time (50s), [APS] 45 x 10-3M, [ANi] 5 x 10-2M and [XG] 2.0 g/L} and results are demonstrated in (Figure. 2d). The %G and %E parameters increment from 160 to 172 and 68.96 to 74.13 respectively consistently on expanding the concentration of hydrogen ion from 0.1M to 1.5M; however, beyond that grafting parameters become constant. This finding may be due to protonation of the aniline monomer which accelerates the generation of PANi ion radicals. 3.1.5 Effect of ANi concentration The effect of ANi concentration on graft copolymerization has been studied over by altering its concentration from 5 x 10-2M to 25 x 10-2M, keeping other parameter constant {such as MW power(80%), MW exposure time (50s), [APS] 45 x 10-3M, [HCl] 1.5 x 10-1M and [XG] 2.0 g/L} and results have been exhibited in (Figure. 2e). Result shows the %G parameters increase from 172 to 390, but %E decreases from 74.13 to 26.63 respectively on increasing the concentration of ANi from 5 x 10-2 to 25 x 10-2M, the increase in %G is because of more prominent accessibility of monomer molecules at the close proximity to the polymeric 9
backbone. The monomer molecules, which are at immediate vicinity of reaction sites become acceptors of XG macroradicals (XGO*) resulting in chain initiation and thereafter themselves becomes free radical donors to neighboring molecules leading to lowering of termination. 3.1.6 Effect of XG concentration The grafting of ANi onto XG was perform at different concentrations of XG. As the concentration of XG increased from 2 to 10g/L, keeping other parameter constant {such as MW power (80%), MW exposure time (50s), [APS] 45 x 10-3M [HCl] 1.5 x 10-1M and [ANi] 5 x 10-2M} (Fig. 1f). The %G and %E have been found to decrease from 172 to 90 and 74.13 to 38.79 respectively. This may be due to increase in concentration of XG, causes increase in viscosity of reaction medium, results in the obstruction in the movement of free radicals thereby, decreases the %G and %E. It is also observed that, the final temperature of the reaction medium after microwave irradiation using MW power 80% for 50s is 86°C. While the boiling point of ANi is 184°C. Thus the presence of final reaction temperature of 86°C confirm there is no loss in ANi concentration after completion of reaction because of its high boiling point. 3.2 Characterization of the mwXG-g-PANi composite 3.2.1 Effect of pH on mwXG-g-PANi by UV-Vis Spectra The pH influences the protonation-deprotonation equilibrium of conducting emeraldine salt and non-conducting emeraldine base forms of polyaniline. From the literature, it suggest that two distinct absorption bands located between 315-345 and 610-650 nm [22]. It is the typical absorption spectrum of the blue polyaniline base. The distinct absorption bands varies depending on preparation and/or processing of PANi. For the green protonated polyaniline form, which is obtained from the base by treatment with acids, three absorption bands are usually observed at 325-360 nm, 400-430 nm, and 780-826nm [23]. In the present section, the spectrophotometric response to a change of pH of mwXGg-PANi is investigated in detail. Figure. 3a shows UV-Vis absorption spectra of mwXG-gPANi at pH = 1, 3 7, 9 and 10. A composite mwXG-g-PANi showed a three absorption bands at 330, 430 and 820 nm which are typical of the protonated form of grafted PANi dispersion in an acidic region. The band at 330nm correspond to the overlapping of glucopyranose components of xanthan gum and (π*← π transitions of benzenoid rings of 10
grafted PANi with bands at 430 nm (assigned to π*←π transition in polaron/bipolaron of grafted PANi) and at 820 nm (assigned polaron transitions of grafted PANi) [23]. It was observed that absorption band at 430 nm (assigned to the polaron transition) is found only at pH < 7 and it disappears in alkaline solutions (Figure. 3a). The peak at 630 nm is typical of grafted PANi base in alkaline media (pH = 9,11). It can be seen that the 430nm peak disappear and new peak at 630nm appear at basic pH. The absorption changes in the spectra are usually attributed to the unprotonated quinoid diimine structure [22]. At pH > 9 an increase of the conjugation length and a decrease of the band gap take place. This demonstrates the transformation of protonated benzoid structures into unprotonated quinoid structures in the media of different pH, which affects its electronic absorption spectra [22]. XG indicated (Figure. 3a) a broad absorption band at 310 nm (due to glucopyranose components). Besides, the characteristic peaks of glucopyranose and PANi was essentially observed and it supports the grafting of PANi on to XG. Along these lines, UV-Vis spectra affirmed the chemical structure, conjugation, electronic transition and doping-dedoping behavior of mwXG-g-PANi. 3.2.2 FTIR spectroscopy Structural changes of mwXG-g-PANi (172 %G) were affirmed by FTIR spectroscopy (Figure. 3b). The range of XG showcases a wide band at 3323.16 cm−1, a typical stretch for the OH group. A band corresponding to aliphatic C-H stretching appears at 2916.65 cm−1. Bands at 1642.23 cm−1 represent (C=O stretching carboxylate group of typical saccharides), respectively. The polysaccharide likewise displayed another peak at 1015 cm−1, due to stretching of the C-O bond [17]. The FTIR spectrum of mwXG-g-PANi in (Figure. 3b) demonstrates all significant peaks correspond to XG and PANI. The accompanying key characteristic bands were observed: 3323.16 cm−1 (overlapping of O-H stretching), 2916.65 cm−1. (aliphatic C-H stretching), 1642.23 cm -1 (C=O stretching of carbonyl group of typical saccharide, 1576.21 and 1487.13 cm−1 (non-symmetric vibration mode of C=C in benzenoid and quinoid ring system)[24], 1215.67 cm-1 (C-N stretching vibration mode in benzenoid ring system), and 1310.90 cm−1(C-N stretching vibration of secondary aromatic amine)[25]; 1120.13 cm−1 (BNH+= Q vibration, indicating grafted PANi is conductive and in the form of emeraldine 11
salt)[26]. 863.21 and 819.35 cm−1 (aromatic ring and out of plane C-H deformation vibrations for 1,4-disubstituted aromatic ring system), and at 3013 cm−1 (C-H stretching vibrations of aromatic ring). It was observed that in mwXG-g-PANi, the peak at 3200- 3500 cm-1 are of quite reduced intensity, (due to overlapping of O–H stretching of XG and N–H stretching of ANi groups at PANi grafts). This reduction in broad and intense peak affirm that’s the appreciable amounts of O–H and N–H at XG have been grafted with PANi Chain. The absorption band of the B-NH+=Q bending vibration of pure PANi was observed at 1227.75 cm-1 but shifted to 1120.13 cm-1 in the XG-g-PANi due to the steric effect of XG. The absorption peaks at 1642.23 cm-1 for XG has been shifted to 1701.12 cm-1 in the mwXG-g-PANi due to C=O stretching of carbonyl group, typical saccharide absorption. The absorption peaks at 2916.65cm-1 for XG has been shifted to 3013.67cm-1. The intense sharp peak at 1015cm-1 is drastically reduce in grafted copolymer. From the FTIR data it is clear that the grafted copolymer mwXG-g-PANi had characteristic peaks of PANi and of XG, which could be a strong evidence of grafting. 3.2.3 X-ray Diffraction (XRD) Analysis X-ray diffraction tests can give a lot of data on structural aspects. The XRD patterns of mwXG-g-PANi (172 %G) are indicated in (Figure. 3c, d). It can be seen that immaculate XG demonstrates a typical amorphous pattern (Figure. 3c), while on account of the mwXG-gPANi, the XRD pattern demonstrates the semi crystalline structure (Figure. 3d). In the XRD pattern of mwXG-g-PANi, Bragg diffraction peak at 2 θ = 16.30°, 20.12° and 25.56° relate to the emeraldine PANI. The peaks centered at 2 θ = 16.30° and 25.56° are attributed to the periodicity parallel and perpendicular to the PANi chains [27]. The peak at 2θ = 20.12° also represents the characteristic distance between the ring planes of benzene rings in adjacent chains or the close-contact inter-chain distance [28]. The crystallinity is because of the vicinity of PANi. By and large polymers are thought to be amorphous however PANI is indicating crystalline structure due to its fiber nature and planar nature of benzenoid and quinoid functional groups. From the XRD pattern of mwXG-g-PANi, it is clear that PANi is grafted onto XG and the grafted PANi has high ordered crystal structure which is expected to exhibit high electrical conductivity.
12
3.2.4 Thermal analysis The impact of PANi on the thermal stability of XG was mulled over with the assistance of the TGA technique (Figure. 3e). TGA of XG demonstrated that the degradation of XG started at about 200°C. The rate of weight reduction expanded with an increment in the temperature. About 10% weight reduction happened until at 100°C, and this was because of the desorption of water. About 50% weight reduction happened at 300°C; after that, the rate of weight reduction diminished, thus it displayed a solitary step degradation process. Around 70% XG degraded at 750°C. The degradation of mwXG-g-PANi began at 150°C. In any case, a 3.2% weight reduction was observed at 100°C, which may have been due to the loss of absorbed water. The graft copolymer showed a two-stage degradation process, i.e., from 242.88 to 346.34°C and from 346.34 to 750°C. 26% weight reduction was observed at 300°C. Therefore the affirmation of grafting is clear from the thermal analysis, which demonstrates that, the mwXG-g-PANi has more thermal stability than the XG. 3.2.5 Morphological analysis of mwXG-g-PANi The SEM images at two distinct magnification of 1kx and 4kx of XG (Figure. 4a, b) and mwXG-g-PANi (Figure. 4c, d) are exhibited. From the Figure 4, SEM images reveal some variations in the morphological structure of XG and mwXG-g-PANi sample acquired from the graft copolymerization onto XG. The grafting presents colossal changes on the surface of the carbohydrate particles. It can be easily be seen in SEM images, that after copolymerization of xanthan gum, the XG particles show irregular morphology, as a lobule of the XG is changed into porous and fluffy. The plain polymer has a smooth surface while the grafted copolymer has rough irregular surface morphology. Consequently, the morphology supports the grafting of PANi on to the XG. 3.3. Mechanism for graft copolymerization The substantial number of hydroxyl groups situated at gum macromolecule carry on as though they are fasten to a stationary flatboat (gum macromolecule) and their confined rotation (Galema, 1997)[14] along these lines will be seen under microwaves, bringing about dielectric heating which cause bond breakage and therefore producing radical locales. Moreover, microwaves are additionally have a particular impact in bringing down of the energy of activation of the reactions and these two truths quicken the fast graft 13
copolymerization. In the above's light a possible oxidative radical system for the PANi grafting on to XG under microwave irradiation has been proposed. As already reported [15,16], where we have investigated free radical copolymerization of vinyl monomers with carbohydrate polymers utilizing peroxydisulfate, a chain mechanism is included because of formation of sulfate ion radicals (SO4-*), which are surely understood ion chain carriers for the graft copolymerization (Scheme 2). In the meantime peroxydisulfate invigorates the oxidative polymerization reaction of aniline through a medium of cationic radicals and form PANi and PANi radicals [29] (Scheme 3). At last XG macro radicals and PANi cation radicals are consolidated to form mwXG-g-PANi graft copolymer. SO4-* is the primary radicals produced from the APS by the lessening of one electron, indicated in Scheme 2. At the same time, APS produce SO42- ions by the decrease of two electrons and go about as oxidant. They start the oxidative polymerization of aniline, as the polymerization of monomer is accounted for to be speedier than the H abstraction from the biopolymer backbone [15]. The macro radicals XGO* may be produced by the abstraction of H by the developing PANi ion radical (PANi*) in the acid medium, which may include onto the XGO* macro radicals creating new radical XGO-PANi* and these chains will develop and joined with other XGOPANi* chains to give a graft copolymer (Scheme 4). 3.4 Influence of the mwXG-g-PANi composite on response of NH3 sensor We have examined the NH3 by increasing the concentration of 25% ammonia solution in the range of 1–100 ppb and monitoring the changes in current. The applied voltage used in ammonia sensing experiments is 20V because the composite film has very low electrical conductivity, hence, we used higher voltage to have appreciable current above the background noise. Both the sensor response and the temporal response of the mwXG-g-PANi sensor have been evaluated. The room temperature I-V characteristics of mwXG-g-PANi (172 %G) from -20V to 20V are shown in Figure.5a. However, at the higher density of PANi deposits, the XG are almost continuously covered with PANi forming a film-like structure and consequently, a linear Ohmic behavior is observed (Figure.5a). The values of electrical conductivities at room temperature of thin films of XG and mwXG-g-PANi are 7.34 x 10-9 and 31.3 ῼ-1cm-1, respectively. Temporal response for different NH3 concentrations ranging from 1ppb to 100ppb is shown in (Figure. 5b). The constant baseline current of 0.63A at 20V was observed. The change in current with increase in the NH3 concentration follows a linear behaviour as shown 14
in (Figure. 5c). It increases with the increase in NH3 concentration. The response ∆G/Go of the mwXG-g-PANi (172 %G) film at different NH3 concentration are shown in (Figure. 5d). It shows the dependence of sensor response on the NH3 concentration. It can be noted that the sensor response increases linearly with the increase in NH3 concentration. The Chemiresistive NH3 sensor response ∆G/Go value for 1 ppb is found to be ~905 which is extremely high and sensitive at room temperature. This means that the low detection limit of the sensor is at the level of ppb. Response and recovery time are the basic parameter of the sensors. Response and recovery time are defined as the time taken for the sensor to attain 90% of maximum change in current on exposure to analyte (in gas/vapour phase) is the response time (Tres). The time taken by the sensor to get back 90% of the original current is the recovery time (Trec). A small value of response and recovery time are the indicative of a good sensor. The sensing response time and recovery time of the mwXG-g-PANi film sensor is about few seconds (i.e Tres=16s, Trec=10s in 1ppb; Tres=20s, Trec=18s in 50ppb and Tres=25s, Trec=23s in 100ppb) which confirm the fast response of sensor. The above results indicate that by monitoring the change in the conductance of the mwXG-g-PANi film, the presence of NH3 molecules could be monitored. This demonstrates that mwXG-g-PANi film could be a good candidate for chemical sensors in detecting vapour NH3. 3.5 Long term stability of sensor Having an appreciation of the sensors’ tolerance and life-time is important for both the manufacturer and the end-user because these characteristics contribute to the quality enhancement of a product. Usually in the present article we have reported the long term stability of few months of mwXG-g-PANi composite sensor. The stability of the sensor towards ammonia sensing has been studied over a period of 3 months (Figure not shown). Preliminary results had shown that the sensor response ∆G/Go of the sensor at 50 ppb concentration of ammonia decreases by ~2% in 3 months, display an incredible stability of the sensor. In contrast as far as industry is concerned, the long term stability starts only after one year of operation. Thus the claim of long term stability is irrelevant on commercial point of view. Thus stability test of more than 12 months need to be exploited in the near future in order to better understand the stability of our sensor.
15
3.6 Reproducibility of the sensor The reproducibility of the sensor has been tested for ammonia samples at three different concentration (1ppb, 5ppb and 100ppb). The relative standard deviation (RSD) of the current and sensor response with three different ammonia concentrations (fig.5c, d) are found to be ~2% for ten successive measurements. This study confirms excellent reproducibility/reversibility of the sensor. 3.7 Sensor response with water We have investigated the response of the sensor with DI water. No sensing is observed with water alone (Figure not shown). We believe that DI water used for dilution in order to prepare different concentration of ammonia does not show any impact on sensing. Because, the vapor phase diagram of ammonia-water shows that ammonia having low boiling point always contribute more vapor of ammonia above the aqueous ammonia solution for a wide range of ammonia concentration kept under equilibrium at room temperature. When it is not kept under closed equilibrium condition, the tendency of escaping ammonia above the liquid is more than water vapor. Hence, we believe that with the increase in ammonia concentration in the analyte, the water vapor content above the liquid reduces and hence, the response of the sensor towards the analyte vapor is almost the ammonia vapor rather than a combination of both. Moreover, the sensor is futile for the sensitivity towards water vapor. 3.8 Plausible Mechanism for ammonia sensing The sensing mechanism of PANi on ammonia gas and inorganic semiconductor oxides is different. Fig.5b shows the temporal response of mwXG-g-PANi sensor exposed to different concentration of ammonia. The current (µA) of sensor shows enhancement from 63.5 µA to 93 µA when expose to ammonia vapour from 1ppb to 100ppb respectively. In comparison with the other reported PANi sensor in literature (30-31), the present response exhibit an inverse nature which is quite unexpected. The change in relative variation in conductance is found to be linearly proportional to concentration of ammonia (Fig.5d). Although, the exact mechanism of sensing is not very clear at this stage, the plausible phenomena for increase in the current on exposure to ammonia can be interpreted by considering the interaction of NH3 with XG. In the ammonia vapour environment NH3 can 16
attack two adsorption sites. (i) N+-H sites originated from small HCl content present during polymerization reaction; and (ii) XG molecules trapped in the PANi chain. At the latter site NH3 molecule may take up a hydrogen from XG forming NH4+ species which create –O- in the PANi chain simultaneously leaving one site for conduction. This explain the increment in current imparted by self-doping in the presence of XG on exposure to ammonia. An unusual response in magnitude observed in the present case suggest a significant role of the incorporated XG in the PANi chain. 4. Conclusion A facile and effective method to construct the chemical sensors based on mwXG-g-PANi for NH3 detection has been demonstrated. The mwXG-g-PANi sensor were synthesized by in situ polymerization using microwave irradiation, and it shows high stability. The sensing properties of the obtained sensors were investigated. The results indicated that mwXG-gPANi sensor had high sensitivity, stability and good reproducibility to NH3. It is found to exhibit high sensitivity over a broad range of concentration from 1 ppb up to 100ppb. The response is highly reversible over up to 10 cycles. The sensor exhibits stable response up to 90 days suggesting long term stability of sensing material. The chemiresistive sensors allowed rapid and quantitative detection of trace NH3 concentrations at room temperature.
Conflict of interest The authors declare no competing financial interest. Acknowledgements The authors Dr. Sadanand Pandey is grateful to National Research foundation (NRF), South Africa (Grant No: 80428) for its liberal budgetary support. The authors also acknowledge University of Johannesburg (UJ), (South Africa) and NTPC (India) for Lab and instruments facilities. We additionally says thanks to Mr. Mufamadi Nndwakhulu and Mr. Edward Malenga, Department of Extraction & Engineering Metallurgy, UJ for providing SEM and XRD characterizations of our samples.
17
References [1]. G. Clayton, F. Clayton. Patty’s industrial hygiene and toxicology. 3rd rev. ed. New York. John Wiley and sons. Eds (1981). [2]. B. Timmer, W. Olthuis, A.v.d. Berg, Sens. Actuators B: Chem. 107 (2005) 666–677. [3]. T.H. Risby, S. Solga, Applied Physics B 85 (2006) 421–426. [4]. J.L. Hurtado, C.R. Lowe, ACS Appl. Mater. Interfaces 6 (2014) 8903–8908. [5]. S Pandey, GK Goswami, KK Nanda, Int J Biol Macromol. 51 (2012), 583-589. [6]. G. Chen, et al. Sci. Rep. 2, (2012) 343. [7]. Dan, Y. et al. Nano Letters 9(2009), 1472–1475. [8]. J. Huang, Pure Appl. Chem. 78 (2006)15–27. [9]. J.W. Schultze, H. Karabalut, Electrochim. Acta. 50 (2005)1739–1743. [10]. X.L. Luo, J.J. Xu, Y. Du, H.Y. Chen, Anal. Biochem. 334 (2004) 284–289. [11]. K.J. Feng, Y.H. Yang, Z.J. Wang, J.H. Jiang, G.L. Shen, R.Q. Yu, Talanta 70 (2006) 561–565. [12]. F. GarcõÂa-Ochoaa,, V.E. Santosa, J.A. Casasb, E. GoÂmez, Biotechnol. Adv. 18 (2000) 549 – 579. [13]. H. Mallik, N. Gupta, A. Sarkar, Mater Sci Eng C 20 (2002) 215–218. [14]. S.A. Galema, Chem. Soc. Rev. 26 (1997) 233–238. [15]. V. Singh, A. Tiwari, S. Pandey, S.K. Singh, Starch/ Starke, 58 (2006) 536–543. [16]. V. Singh, A. Tiwari, S. Pandey, S.K. Singh, Express Polym Lett. 1 (2007) 51–58. [17]. S. Pandey, S.B. Mishra, Carbohydr Polym. 90 (2012) 370–379. [18]. V.D. Athawale, V. Lele, Carbohydr Polym. 35 (1998) 21–27. [19]. S. Pandey, G.K. Goswami, K.K. Nanda, Sci. Rep. 3 (2013) 2082. [20]. S. Pandey, K.K. Nanda, ACS Sensors 1 (2016) 55-62. [21]. Z.H. Wang, Adv. Mater. 15 (2003) 432-436. [22]. M. Wan, J. Polym. Sci., Polym. Chem., 30 (1992) 543-549. [23]. S.C. Yang, R.J. Cushman, D. Zhang, Synth. Met. 29 (1989) E401-408. [24]. J. Bhadra, D. Sarkar, Bull. Mater. Sci. 33 (2010) 519-523. [25]. H. Swaruparani, S. Basavaraja, C. Basavaraja, S.H. Do and A. Venkataraman, J. Appl. Polym. Sci. 117 (2010) 1350⎼1360. [26]. D.B. Mahesh, S. Basavaraja, D.S. Balaji, V. Shivakumar, L. Arunkumar and A. Venkataraman, Polymer Composites, 30 (2009) 1668⎼1677. [27]. H.C. Pant, M.K. Patra, S.C. Negi, A. Bhatia, S.R. Vadera, N. Kumar, Bull. Mater. Sci. 29 (2006) 379–384. [28]. J.P Pouget, C.H. Hsu, A.G. MacDiarmid, A.J. Epstein, Synth. Met. 69 (1995) 119–120. [29]. Y. Ding, A.B. Padias, H.K. Jr. Hall, J. Polym. Sci., Part A: Polym. Chem. 37(14) (1999) 2569–2579. [30]. G.Bidan, Sens Actuators. B 6 (1992) 45-46 [31]. H. Hirata, L. Sun, Sens Actuators. A 40 (1994) 159-163.
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Lists of captions. Figure. 1. Sensor device structure (a) schematic cross sectional view of sensor with sensing film of mwXG-g-PANi onto glass substrate, (b) Top view of prepared sensor with ohmic contacts. Figure. 1. Optimization of mwXG-g-PANi composite. Effect of (a). Microwave power; (b). Exposure time; (c) APS concentration; (d). Hydrogen ion concentration; (e) Aniline concentration; and (f) XG concentration. Figure. 3. Characterization of XG and mwXG-g-PANi composite. (a) Effect of pH on UVVis spectra. (b). FTIR. (c) XRD and (d) TGA. Figure. 4. Morphology analysis of (a). XG (1kx); (b) XG (4kx); (c) mwXG-g-PANi (1kx); and (d) mwXG-g-PANi (4kx). Figure 5 Representative room temperature NH3 sensing behavior (a) I-V characteristic of a sensor made of the mwXG-g-PANI film (b) Temporal response curve of our mwXG-g-PANI sensor in 1 ppb to 100 ppb range. (c) Log-log plot of current versus different NH3 concentration. Solid red line is its linear fit. (d) Log-log plot of sensor response with different NH3 concentration. The red solid line is the linear fit. Vertical error bars represent the standard deviation from the mean. Scheme 1. Schematic diagram of the present work. Scheme 2. Generation of primary radicals under microwave irradiation. Scheme 3. Formation of secondary radicals under microwave irradiation. Scheme 4. General mechanism for microwave-accelerated graft copolymerization of PANi on to XG
19
(a)
(b)
Figure.1
20
(a) 120
120
(b)
%G %E
80
80
%G&%E
100
%G&%E
100
60 40
%G %E
60 40
20
20
0 20
40
60
80
100
0
120
0
20
Microwave Power (%)
(c)
%G %E
160
%G&%E
%G&%E
100
180
120 100 80 60
140 120 100 80
40 0.00
0.01
0.02
0.03
0.04
60 0.0
0.05
0.5
1.0
APS (mM/L)
1.5
2.0
2.5
HCl (M/L)
320 %G %E
280
(f)
180 %G %E
160
240
140
200
%G&%E
%G&%E
80
200
(d)
%G %E
140
(e)
60
Exposure time (s)
180 160
40
160 120 80
120 100 80 60
40
40
0 0.05
0.10
0.15
0.20
Aniline (M/L) Figure.2
0.25
0.30
2
4
6
8
10
XG (g/L)
21
(b) 100
Absorbance (a.u)
XG mwXG-g-PANi pH1 pH9 pH11
pH3
pH7 pH 11
200
pH 1
400
(c)
600
800
XG mwXG-g-PANi
96
Transmittance (%)
(a)
1000
3323
92
1215 1642 2917
88
1487
84 80 76
1310 1576 863 819
72 500
3013
1120 1015
1000 1500 2000 2500 3000 3500 4000
Wavenumber (cm-1)
Wavelength (nm)
(d)
mwXG-g-PANi
XG
Intensity (a.u)
Intensity (a.u)
0
30
60
2degree
(e)100
90
Weight reduction (%)
90
0
30
60
90
2degree
XG mwXG-g_PANI
80 70 60 50 40 30 20 0
200
400
600
800
Temperature (oC)
Figure. 3 22
Figure. 4
23
(a)
Current (A)
0.5
Current (A)
(b)
mwXG-g-PANi
0.0
-0.5 -20
-10
0
10
Baseline 1 ppb 50 ppb 100 ppb
90
60
30 0.63A 20V
0
20
0
V(V)
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100 150 200 250 300 350
time (s)
(c) 100
(d) 1400 1200
80
G/Go
Current (A)
1300
60
1100 1000 900
1E-3
0.01
NH3 concn (ppm)
0.1
1E-3
0.01
NH3 Concn (ppm)
0.1
Figure 5
24
Scheme 1.
25
(Oxidising action of APS) O O
-
O
O O NH 4+
-
S
O NH 4 +
S
MW
O
HO
O
O O
S
O O
S
O
-
O
O
ammoniumperoxydisulphate
peroxodisulphate ion
-2e
O
-
O
-
S
O
O
sulphate ion (APS act as initiator) O O HO
S
O O
S O
O
peroxodisulphat e ion
-
O
O
O
-e
-
O
S
O
MW
-
+
O
S
O
O
sulphate ion
sulphate ion
Scheme 2.
26
O
-
NH4+ -e
H 2N
APS
-2H+
H3N +
NH2
NH2+
-2e
-H+ NH
NH NH+
+
H2N
NH
NH -2e NH 2
-H+
NH
NH
+ NH3+ NH2
NH
NH2+ NH
NH NH
Polyaniline ion radical
Scheme 3.
27
OH
SO4-*
+
MW
O*
Initiation
Alkoxy macroradical
Xanthan gum (XG)
-H + +
OH
M*
O* + MW
PANi ion radical
XG f ree radical site
MnH
Homopolymer
-H+ O* +
M*
O
M*
MW Propogation
O
O
M*
+
*M
-2H +
MW
M
M
O
O Termination
Graft copolymer (mwXG-g-PANi)
Scheme 4.
28