Agarose–guar gum assisted synthesis of processable polyaniline composite: morphology and electro-responsive characteristics

Abstract

The present communication reports the development of processable polyaniline (PANI) in the film form via agarose–guar gum (A–G) assisted in situ polymerization of aniline using potassium dichromate as an oxidant. The network structure of A–G not only provides the mechanical support to polyaniline, but also a microporous template, which allows the reinforcement of PANI into nanostructures with an interpenetrated polymer network between PANI and A–G, as evidenced by optical microscopy and SEM. The FTIR and TGA analysis confirms the formation of an agarose–guar gum–polyaniline composite (A–G–PANI) having hydrogen bonding interactions. The A–G–PANI film has better adherence property on the glass substrate as compared with PANI. The A–G–PANI composite shows appreciable DC conductivity in the range of 10⁻³–10⁻² S cm⁻¹ and electrochemical activity, having oxidation–reduction transitions corresponding to polyaniline. It exhibits both ionic and electronic conductivity as confirmed by EIS. The electro-responsive characteristics exhibited by the novel A–G–PANI composite make it a promising electrode material for the construction of chemical sensors and biosensors.

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Introduction

In recent years, the formation of composites between polyaniline (PANI) and other conventional polymers, which have good mechanical properties, is gaining considerable importance. This is in order to overcome the poor processability of PANI, which arises due to infusibility and poor solubility in many solvents.¹ Composites of polyaniline with various organic polymers, such as polyacrylamide, nylon, nafion, polyvinylalcohol, polystyrene and polyurethane, have been previously reported.² Biopolymers like cellulose and its derivatives, chitosan, starch, lignosulfate and acacia gum have also been utilized for composite formation.^3–7 Biopolymers are the preferred choice for this purpose because of their cost effective, ecofriendly and biocompatible nature.

Agarose (A) is a biopolymer consisting of alternating β-D-galactopyranose (1–3 linked) and 3,6-anhydro-α-L-galactopyranose (1–4 linked) units, forming a thermoreversible hydrogel with a 3-D network structure. The network structure of agarose is particularly important in various biological and biomedical applications. Guar gum (G) is a galactomannan, which acts as a thickening agent, swells in cold water, but does not form aggregates or a network structure. Studies have been conducted on the formation of an agarose and guar gum composite gel (A–G) and the advantages of the interaction between them.^8,9 Such a combination of biopolymers has unique properties, like an excellent membrane forming ability, high permeability towards water, good adhesion, biocompatibility, nontoxicity, high mechanical strength and the ability to form a water insoluble microporous structure. This suggests that biopolymer composite gel (A-G) can serve as a potential material for the synthesis of processable polyaniline in the film form. Agarose–guar gum composite gel membrane has been successfully used by our group for enzyme immobilization, which was suitable for construction of various biosensors.^10,11 With this background, we planned to synthesize a composite of these natural biopolymers with conducting polyaniline, thus imparting novel features to the biopolymer. This kind of approach will widen the scope of biopolymers for various applications.

Interestingly, it has been observed that templated-synthesis of PANI was generally undertaken in achieving control over the shape and size in the nanometer range and to obtain unique morphologies.^1,12,13 Biopolymer based templated-synthesis of polyaniline is still unnoticed and only a few reports are available. The formation of controlled size polyaniline using crosslinked carboxymethyl chitin as a biopolymer template has been reported.¹⁴ An agar gel template was also used for the preparation of a compact polyaniline film, where polyaniline was electrochemically synthesized.¹⁵

To the best of our knowledge, this is the first time an agarose–guar gum biopolymer has been used for polyaniline composite formation. The main focus is on the use of the 3-D network structure of agarose–guar gum, which will not only provide mechanical support, but also a microporous template for the synthesis of polyaniline. The influence of reaction conditions on the synthesis of the composite and the electrical conductivity of A–G–PANI films as a function of these conditions were investigated. The surface morphology of the composite and the interaction between the biopolymer and polyaniline were studied. Further, the electrical and electrochemical analysis of the composite was carried out to study the electro-responsive characteristics.

Experimental

Materials

Analytical grade agarose having medium EEO (Sigma Aldrich), guar gum (SRL-India), analytical grade hydrochloric acid (35%) and potassium dichromate were used as procured. Aniline (AR grade) was double distilled prior to use. The aniline monomer solution was prepared in hydrochloric acid and potassium dichromate solution was prepared in distilled water. Microscopic glass slides were used as substrates to give additional mechanical support for the deposition. The glass substrates were cleaned by sequentially washing with chromic acid and 10% NaOH solution. In this process, glass slides were kept in the chromic acid solution and heated in a water bath at 70 °C for 10 min, followed by rinsing with distilled water several times. The similar procedure was repeated with 10% NaOH washing. Cleaned slides were air dried in an oven and stored under dust free conditions.

Preparation of agarose–guar gum matrix

A measured amount of agarose powder was added to distilled water and stirred to obtain a uniform suspension of 2% (w/v). It was heated up to 80 °C in a water bath to obtain a transparent solution, which ensures complete dissolution of agarose. Guar gum powder was gently dissolved in distilled water without formation of clumps to get a 1% (w/v) solution at room temperature (25 ± 2 °C).

Agarose and guar gum solutions were mixed carefully in a 1 : 1 (v/v) proportion in order to avoid the formation of air bubbles. The solution of A–G (100 μl, total polymer concentration of 1.5%) was drop cast on a glass slide (1 cm²) to obtain a uniform film. On similar lines, agarose films (1.5% w/v) were also deposited on the glass substrate. The cast films were dried in a hot air oven at 45–50 °C and stored in an airtight container at room temperature to maintain a dust and moisture free environment. This agarose–guar gum biopolymer matrix has been used as a template for the synthesis of a conducting polymer. The thickness of the A–G film deposited on a glass slide of 1 cm² was found to be uniform and in the range of 17 ± 2 μm, as measured with a KLA-Tencor Profilometer (P-16⁺). Similarly, A–G films with thickness of 47 ± 3 μm were deposited by drop casting 4 ml of A–G solution on a glass slide of 7.5 × 2.5 cm².

Synthesis of agarose–guar gum–polyaniline (A–G–PANI) composite

Synthesis of polyaniline was carried out using the A–G biopolymer matrix template prepared as mentioned above. These A–G films (total polymer concentration of 1.5%) were kept in aniline monomer solution (0.05 M–0.6 M), prepared in 1 M HCl for 1 hour to achieve maximum swelling of the biopolymer, which was checked by weighing the films. The polymerization reaction was initiated by adding the potassium dichromate solution with a concentration equivalent to a molar ratio of 5 : 1 (aniline : oxidant). The polymerization was carried out for about one hour at a temperature of 24 ± 2 °C. The completion of polymerization was ensured by conductivity measurement (until no significant increase in conductivity was observed). After completion of the polymerization, A–G–PANI films were washed with 1 M HCl, followed by distilled water to remove unreacted aniline and water soluble products, respectively. The aniline monomer concentration was varied in the range of 0.05 M to 0.6 M to achieve different polyaniline contents in the composite. The polyaniline content in the composite was determined gravimetrically using the equation: PANI (weight%) = W_c − W_b/W_c × 100, where W_c and W_b are the dry weights of A–G–PANI composite and biopolymer (A–G), respectively. The influence of reaction conditions on the electrical conductivity of the films was determined using the four probe method. Polyaniline in the powder form was also synthesized under identical reaction conditions by adopting the previously reported method.¹⁶ Further, the films were deposited on a precleaned glass substrate by drop casting the suspension of polyaniline prepared in 1 M HCl.

Characterization

The electrical conductivity was measured by the four probe method (Jaldel-RM-3), at room temperature (25 ± 2 °C) and at 40% humidity. The four probe method gives sheet resistivity (R_s, ohms per square) and by measuring the thickness of the film (t, cm), the specific resistivity (ρ = R_s × t) and corresponding conductivity (σ = 1/ρ) of the films were calculated.

UV-visible spectra of A–G–PANI composite in DMSO were recorded by spectrophotometer (Shimadzu UV-C5300). The FTIR spectra of the films were obtained using a Bruker Tensor 37 FTIR spectrophotometer, with platinum ATR, considering air as a baseline.

The surface morphology of the films was studied by optical microscopy and SEM (Jeol-JSM 6360A). Samples were coated with platinum before imaging and an accelerating voltage of 20 kV was applied to facilitate the imaging. Optical microscopic images of films were taken using a Carl Zeiss Microscope equipped with a primostar digital camera (5 MP), at a total magnification of 100×. Images were processed by TS view software, using the standard calibration scale for the determination of pore size.

Thermogravimetric analysis was performed using a TG analytical system (Shimadzu-DTG-60H) in the temperature range of 25 °C–900 °C under a nitrogen gas atmosphere.

Some of the physical properties of the films were analyzed by the following techniques. The swelling behavior of A–G and A–G–PANI films was studied by gravimetric analysis at room temperature (25 ± 2 °C), in distilled water for 1 hour. The swollen films were removed from distilled water and immediately weighed with a microbalance (Citizen-CY 54), after the removal of unabsorbed water from the surface. The swelling ratio was calculated by the equation (W_{swollen film} − W_{dried film})/(W_{dried film}), in which W is the weight of films in the swollen and dried form. The relative adherence of A–G–PANI films was ensured with the simple scotch tape press test. In this test, scotch tape (1 cm²) was pressed firmly on the surface of the glass substrate, where the film is deposited and later on the tape was stripped off the surface. Thus, the relative adherences of the A–G–PANI composite film and the PANI film, deposited on glass substrate, were examined. The mechanical properties, including tensile strength, Young's modulus and strain at break of A–G and A–G–PANI composite films, were determined by tensile test using a universal testing machine (Lloyd Instruments-LR 10K plus), with a test speed of 0.5 mm min⁻¹ and a load cell of 100 N. Sample size of 40 mm length, 10 mm width and thickness of 47 μm was used. Three specimens were used for each sample in the tensile test.

The electro-responsive characteristics of the A–G–PANI films were determined by I–V, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). For I–V measurement, the electrical contacts were made with room temperature curable silver paste. For the CV and EIS, films were deposited on precleaned ITO glass slides with a working window of 0.5 × 0.5 cm². I–V and CV were recorded using an Autolab potentiostat (3023). An EIS was taken with a Biologic galvanostat/potentiostat in the frequency range of 1 mHz–7 MHz, at the equivalent potential of 35 mV. An EIS was recorded with a three electrode system in a solution of Fe (CN₆)^−4/−3 (10 mM in 1 M HCl). Curve fitting was done by EC Lab software (10.21) using an appropriate Randles equivalent circuit.

Results and discussion

Synthesis of A–G–PANI composite

The scheme for the formation of the A–G–PANI composite via biopolymer assisted in situ polymerization of aniline is described in Fig. 1a. Agarose–guar gum forms a hydrogel with a 3-D network structure, which facilitates the absorption of the aniline monomer during the swelling period.^8,9,17 The agarose–guar gum has a large amount of free hydroxyl groups, which helps in the interaction with the aniline monomer via hydrogen bonding, that can lead to the uniform distribution of the monomer in the network of biopolymer.³ With the subsequent addition of an oxidant, aniline present in the agarose–guar gum matrix undergoes oxidative polymerization and forms polyaniline. The formation of polyaniline in the matrix of agarose–guar gum is demonstrated by the development of dark green coloration of films, with different intensities depending upon the amount of polyaniline in the composite as compared with only A–G film (Fig. 1b).

Fig. 1 (a) Schematic showing formation of A–G–PANI composite via biopolymer assisted in situ polymerization of aniline monomer. (b) Images of A–G and A–G–PANI composites with different PANI contents.

The weight% of polyaniline in A–G–PANI composites synthesized at various concentrations of aniline monomer is given in Table 1. The polyaniline content of 1.6 wt% to a maximum of 27.2 wt% was achieved, by increasing the concentration of the aniline monomer from 0.05 M to 0.5 M. Thus, the content of polyaniline in the A–G–PANI composite is modulated by varying the concentration of the aniline monomer. The higher content of PANI was not observed here for the A–G–PANI composite, as the agarose–guar gum hydrogel remains attached to the glass substrate, which tends to limit the swelling of the hydrogel.¹⁸ With the free standing membrane of A–G when used as the matrix for the in situ polymerization of aniline, the maximum polyaniline content was observed to be 90%. It is worth mentioning here that the film of hydrogen supported on the solid surface is of great importance, as it represents a ‘smart’ coating that can be microstructured and integrated into the microfabrication process, particularly in the field of biosensors.¹⁸

Table 1 PANI content and electrical conductivity of A–G–PANI composite films as a function of aniline monomer concentration

Samples^a	Agarose–guar gum concentration (%)	Aniline monomer concentration (M)	PANI (wt%)	Electrical conductivity (S cm^−1)
a C represents A–G–PANI composite.b Conductivity of A–G film was measured using a Keithely electrometer-6517 B.
A–G^b	1.5	—	—	2.5 ± 2 × 10^−9
A–G–PANI–C1	1.5	0.05	1.6%	4.1 ± 2 × 10^−6
A–G–PANI–C2	1.5	0.1	2.3%	2.3 ± 1.5 × 10^−4
A–G–PANI–C3	1.5	0.2	2.6%	2.9 ± 1.3 × 10^−3
A–G–PANI–C4	1.5	0.3	3.7%	1.3 ± 2 × 10^−2
A–G–PANI–C5	1.5	0.4	14%	3.4 ± 1 × 10^−2
A–G–PANI–C6	1.5	0.5	27.2%	5.4 ± 1.2 × 10^−2
A–G–PANI–C7	1.5	0.6	26.7%	3.1 ± 1.5 × 10^−2

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Electrical conductivity of A–G–PANI composites

The electrical conductivity of the composite films as a function of aniline monomer concentration is shown in Table 1. As demonstrated, the aniline monomer concentration determines the polyaniline content in the composite, and thus the electrical conductivity of the A–G–PANI films. The conductivity of the A–G–PANI composite films varies by four orders of magnitude in the range of 10⁻⁶–10⁻² S cm⁻¹ with different polyaniline content. The A–G–PANI composites with a PANI content of 1.6 wt% and 2.6 wt% exhibits conductivity in the order of 10⁻⁶ and 10⁻³ S cm⁻¹, respectively. The maximum conductivity of 5.4 ± 1.2 × 10⁻² S cm⁻¹ was obtained with a PANI content of 27.2 wt%. The increase in the conductivity of the A–G–PANI composites as a function of PANI wt% can be explained on the basis of the effective medium theory (Springett model).^19,20 The conductivity value of the developed A–G–PANI composite is more or less comparable with previously reported biopolymer–polyaniline composites. The conductivities of PANI/chitosan and substituted PANI/chitosan composites were found to be in the range between 7.73 × 10⁻⁵, 1.68 × 10⁻⁴, 6.84 × 10⁻⁶ and 1.53 × 10⁻⁴ S cm⁻¹.²¹ Similarly, the conductivities of PANI/rice straw fiber composites with different polyaniline loadings were in the range of 1.56 × 10⁻¹⁰ (2.5 wt% PANI), 4.56 × 10⁻⁷ (5 wt% PANI), 2.5 × 10⁻⁵ (10 wt% PANI) S cm⁻¹ and conductivity in the order of 10⁻³ S cm⁻¹ for PANI/lignosulfate composites was observed by other workers.^22,6 The conductivities of cellulose–PANI blends were reported to be 2.43 × 10⁻⁴ (4 wt% PANI), 1.84 × 10⁻³ (7.8 wt% PANI), 2.71 × 10⁻³ (9.6 wt% PANI) and 3.0 × 10⁻² (21.2 wt% PANI) S cm⁻¹.⁴

The electrical conductivity of A–G–PANI films was also found to be influenced by the aniline to oxidant molar ratio and the HCl concentration, which tends to affect the polymerization process of polyaniline. The aniline to potassium dichromate ratio of 5 and 1 M HCl concentration was optimized to give the highest conductivity of A–G–PANI composite (see ESI Fig. S1†).

UV-visible spectral analysis

The A–G film did not exhibit any absorption peaks in the UV-visible range, whereas the UV-visible spectrum of the A–G–PANI composite film showed absorption peaks at 330 and 630 nm, corresponding to characteristic peaks of polyaniline (ESI Fig. S2†). The peak at 330 nm is assigned to the π–π* transition within the benzoid ring and the peak at 630 nm corresponds to the π–π* transition within a quinoid ring of polyaniline.^23,24 Furthermore, the ratio of peak intensity at 630 nm and 330 nm (A₆₃₀/A₃₃₀) is related to the oxidation state of polyaniline.²⁵ In the present study, this ratio for A–G–PANI composite was found to be 0.5, thereby suggesting an emeraldine state of polyaniline.

FTIR spectra

FTIR analysis was carried out to confirm polyaniline in the A–G–PANI composite and to determine the change in the molecular structure of agarose–guar gum biopolymer, after in situ synthesis of polyaniline. The FTIR spectrum of the A–G film showed bands corresponding to the characteristic bands of agarose and guar gum (Fig. 2a). The band at 929 cm⁻¹ can be assigned to the stretching vibration of the C–O–C bridge of 3,6-anhydro-l-galactopyrose in agarose and the band at 1034 cm⁻¹ is due to glycosidic bond stretching in agarose and guar gum. The band at 1640 cm⁻¹ corresponds to C=O stretching vibration, characteristic of the saccharide structure and the band at 3307 cm⁻¹ is related to –OH stretching in agarose and guar gum.^26,27

Fig. 2 FTIR spectra of (a) A–G film (b) A–G–PANI composite and (c) PANI powder.

The A–G–PANI composite film showed the FTIR spectrum resembling that of the A–G biopolymer (Fig. 2b). For the A–G–PANI composite film, bands are observed at 929 cm⁻¹, 1035 cm⁻¹, 1637 cm⁻¹, and 3222 cm⁻¹ corresponding to the agarose–guar gum, along with the appearance of new bands at 1235 cm⁻¹, 1300 cm⁻¹, 1491 cm⁻¹ and 1577 cm⁻¹. These bands correspond to the characteristic bands of polyaniline (Fig. 2c), thereby confirming the development of the A–G–PANI composite. Bands at 1491 cm⁻¹ and 1577 cm⁻¹ belong to the C=C stretching vibration in the benzoid and quinoid rings in polyaniline, whereas bands at 1300 cm⁻¹ and 1235 cm⁻¹ are due to C–N stretching of the quinoid and benzoid rings in polyaniline.²⁸

Moreover, the broadening of the –OH band in the region of 3000–3700 cm⁻¹ for the A–G–PANI composite as compared with the A–G biopolymer indicates the formation of hydrogen bonding between the amino N–H of polyaniline and the –OH group of the agarose–guar gum biopolymer. Such an interaction was previously reported between polyaniline and other polymers, such as nanofibrillated cellulose, polyacrylonitrile, chitosan and poly(N-vinyl-2-pyrrolidone).^3,21,29,30 In the case of substituted polyaniline/chitosan composites, broadening of the band between 2878–3435 cm⁻¹ was reported due to hydrogen–oxygen interaction between the N–H and –OH groups of PANI and chitosan.²¹ Similarly, the decrease in the peak intensity characteristic of the –OH bands of iota-carrageenan in the region 2500–3500 cm⁻¹ was reported due to hydrogen bonding interaction between –OH and N–H groups of iota-carrageenan and PANI in the composite.³¹ The weakening of the OH peak at around 3400 cm⁻¹ in a nanofibrillated cellulose–polyaniline composite as compared to only nanofibrillated cellulose has been ascribed to hydrogen bonding interaction between them.³

Morphology of A–G–PANI composite: optical microscopy and SEM analysis

Optical images of the agarose–guar gum film and the only agarose film showed the distinct morphological differences between them. An extended network (Fig. 3b) is observed for A–G film as compared with the compact network of agarose (Fig. 3a). The A–G film has a larger pore size and wider pore size distribution (Fig. 3d) as compared with the agarose film (Fig. 3c). The pore size of the A–G film is found to be in the range of 13.5 ± 6 μm, hence, it can offer a template for the growth of polyaniline.

Fig. 3 Optical image of (a) agarose film (1.5%) (b) A–G film (total polymer concentration 1.5%) at 100× magnification. (c) and (d) indicate the pore size (μm) distribution of agarose and A–G films.

SEM images revealed an uneven surface for the A–G film (Fig. 4a), while agglomerated polyaniline particles with spherical morphology (0.4 ± 0.1 μm) were observed for the PANI powder (Fig. 4b). The SEM image of the A–G–PANI composite indicated the growth of nanostructured polyaniline inside the pores of the A–G film, with particle size in the range of 70 ± 20 nm (Fig. 4c and d). It also showed the linkages between these polyaniline particles, which resemble a network of polyaniline (Fig. 4c and d). Based on optical microscopy and SEM analysis, formation of an interpenetrated network between agarose–guar gum and PANI can be proposed. A similar morphology was observed for poly(acrylate-aniline) composite.^32,33 As observed, the agarose–guar gum, being a porous structure, has a strong absorption capacity, which causes the absorption of a large amount of aniline monomer in the matrix. On subsequent addition of an oxidant, aniline present in the pores of the biopolymer matrix undergoes oxidative polymerization and forms polyaniline. During the process of polymerization, polyaniline may integrate at some point, as the pores of A–G are randomly oriented. This is an interpenetrated network of polymer, in which the biopolymer (A–G) is in the form of a 3-D network, whereas polyaniline remains in the chain configuration, which is either free or interlinked.³⁴ The large pore size, wide pore size distribution and lower cross-linking density of agarose–guar gum film might have played a role in the development of such morphology.

Fig. 4 SEM images of (a) A–G film, (b) PANI powder and (c) and (d) A–G–PANI film at different magnifications.

Thermogravimetric analysis

The thermal stability of A–G–PANI composite film (14 wt% PANI) was determined by TGA using PANI and A–G film as a reference. The thermogram of A–G film (Fig. 5a) indicated thermal degradation of this biopolymer in three stages. It showed an initial 31% of weight loss up to 141 °C, due to dehydration, whereas the rapid decomposition of the biopolymer started in the range of 250 °C–320 °C with a weight loss of 40% and final degradation to constituent elements (C, H, O) occurred at 478 °C.^35,36 PANI is considerably more thermally stable, with an initial weight loss of 35% up to 200 °C, due to dopant (Cl⁻) losses (Fig. 5c). Polymer decomposition begins at 360 °C with the residual weight of 52% at 478 °C. The complete degradation of PANI was observed at 940 °C.³⁷ Composite film of A–G–PANI (14 wt% PANI) had thermal degradation intermediates of A–G and polyaniline. It showed only 28% weight loss up to 200 °C, because of moisture and dopant losses, which is considerably less as compared to the constituent polymers. This may be due to the formation of an interpenetrated network structure, preventing the loss of small molecules like dopants.³⁷ The composite film had a gradual polymer degradation with weight loss of only 12% in the range of 250 °C–320 °C; this weight loss was found to be lower as compared with the A–G film. The complete degradation of the A–G–PANI composite was achieved at 882 °C (Fig. 5b). The increased thermal stability of the A–G–PANI film as compared to the A–G film is due to the formation of a composite with an interpenetrated polymer network. These results are in agreement with previous reports, indicating the enhanced thermal stability of PANI–chitosan due to composite formation.^4,38

Fig. 5 TGA thermograms of (a) A–G film, (b) A–G–PANI composite film (14 wt% PANI) and (c) PANI powder.

Physical characterization

Swelling study of A–G and A–G–PANI films was carried out to understand the cross linking density of the polymeric matrix. The results of the swelling study showed that the swelling ratio of the A–G–PANI film (1.7 ± 0.5) was less as compared to the A–G film (3.6 ± 0.4), indicating an increased cross-linking density. This is due to the development of an interpenetrated network between the polyaniline and the agarose–guar gum.

The simple scotch tape press test was used to qualitatively estimate the relative adhesiveness of the A–G–PANI films and the PANI film deposited on a glass substrate.³⁹ It was observed that the PANI film was almost detached from the glass surface, whereas the A–G–PANI composite films up to 14 wt% PANI remained firmly attached on the glass substrate, confirming better adhesive properties of A–G–PANI composite films (ESI Fig. S3†). The formation of a stable composite film of polyaniline using biopolymer assisted synthesis has demonstrated the processability of polyaniline. Moreover, we have also observed that A–G–PANI composite films prepared by mixing of polyaniline in molten agarose–guar gum composite gel showed complete loss of film forming ability with different polyaniline content (0.1%, 1%, 5% w/v), thereby indicating the advantage of in situ polymerization in the biopolymer matrix, which is exploited in the present report for the development of the A–G–PANI composite.

The mechanical properties of the A–G and A–G–PANI composite films with different polyaniline content are presented in Table 2. The agarose–guar gum film has high tensile strength, Young's modulus and strain at break (26.9 Mpa, 1.0 Gpa and 8.8%, respectively). The high mechanical properties of the A–G biopolymer are due to strong intra- and intermolecular hydrogen bonding formed during the gelation process.⁴⁰ The Young's modulus of the composite films with different PANI content remains more or less the same as that of the A–G film, within the experimental errors. The tensile strength and strain at break are less for the A–G–PANI composite films as compared with the A–G film, indicating that the A–G–PANI composite films are more brittle than the pure A–G film. However, the tensile strength among the A–G–PANI composites with different polyaniline contents show some variation. Tensile strength increases with PANI content up to 14 wt%, then it decreases. This may be due to a more uniform distribution of PANI in the biopolymer matrix with increasing PANI content. The similar kind of variation was reported for chitosan/PANI blend; tensile strength decreases initially with the addition of 10% PANI, whereas it increases further with the addition of 20–40% PANI and again decrease at 50%. The increase in tensile strength was reported to be due to the more uniform distribution of PANI in the chitosan matrix.⁴ The strain at break for the A–G–PANI composites decreases substantially with increasing PANI content in the composite. Thus, the scotch tape test and tensile test of the A–G–PANI composites demonstrated the processability of PANI, by formation of a stable composite film with better mechanical properties. These results suggest that the processability of polyaniline can be improved by composite development with biopolymers like agarose–guar gum.

Table 2 Mechanical properties of A–G and A–G–PANI composite films

Samples	Tensile strength (MPa)	Young's modulus (Gpa)	Strain at break (%)
A–G	26.9 ± 5	1.0 ± 0.8	8.8 ± 5
A–G–PANI (1.6 wt%)	5.5 ± 2	1.0 ± 0.1	4.7 ± 0.3
A–G–PANI (2.3 wt%)	7.9 ± 1.6	1.2 ± 0.2	2.7 ± 1.7
A–G–PANI (2.6 wt%)	12.4 ± 5	1.1 ± 0.5	1.7 ± 0.2
A–G–PANI (14 wt%)	22 ± 6	1.2 ± 0.4	1.3 ± 0.1
A–G–PANI (27.2 wt%)	6 ± 5	1.2 ± 0.2	1.1 ± 0.1
PANI	—	—	—

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Electro-responsive characteristics

The electro responsive characteristics of the A–G–PANI composite were studied by I–V measurements, cyclic voltammetry and electrochemical impedance spectroscopy (Fig. 6–8).

Fig. 6 I–V plot of (a) A–G film (b) A–G–PANI composite film.

Current–voltage relationship (I–V measurements)

According to the I–V plot, current values obtained at any potential for the A–G–PANI film (Fig. 6b) were found to be more as compared with the A–G film (Fig. 6a), indicating the enhanced electrical conductivity of the A–G–PANI composite. The electrical conductivity of the A–G–PANI film is due to proton hopping along the PANI chains in the A–G–PANI composite.⁴¹

Cyclic voltammetry analysis

The cyclic voltammogram of the A–G film did not show any oxidation or reduction peaks (Fig. 7[I] a), indicating the absence of electrochemical activity. The cyclic voltammogram of the A–G–PANI composite film (Fig. 7[I] b) showed reversible redox electrochemical activity having distinct oxidation and reduction peaks corresponding to the transition of polyaniline into its different oxidation states at scan rate of 20 mV s⁻¹. In the voltammogram of the A–G–PANI composite during the direct scan (I), Epa1 corresponds to the transition from the leucoemeraldine state to the emeraldine state and Epa2 corresponds to the transition from the emeraldine state to the perigranine state. In the inverse scan (II), the reduction process is observed. An Epc2 corresponding to the conversion from the perigranine to the emeraldine state, whereas the emeraldine to leucoemeraldine conversion takes place at Epc1.⁴² This electrochemical behavior observed for the composite film of A–G–PANI was similar to the standard electrochemical behavior of polyaniline. The voltammogram and characteristic peak potentials (Epa1 and 2; Epc1 and 2) obtained for the composite are slightly different than the standard peak potentials observed for only polyaniline prepared under similar conditions (see ESI Fig. S4 and Table S1†). This behavior can be attributed to composite development and the interaction that occurred between polyaniline and the agarose guar gum biopolymer.

Fig. 7 [I] Cyclic voltammograms of (a) A–G film, (b) A–G–PANI composite film in 1 M HCl at a scan rate of 20 mV s⁻¹. [II] Cyclic voltammogram of A–G–PANI composite at different pH and inset is polyaniline conversion from salt to base form.

Moreover, the redox electrochemical activity of the A–G–PANI composite film was found to vary with respect to solution pH (Fig. 7[II]). This phenomenon is due to the pH dependent transition of the emeraldine salt to the emeraldine base (Fig. 7[II] inset), thereby leading to a different charge density at the interface of the A–G–PANI composite and the aqueous phase. This pH responsive ability of A–G–PANI composite can be exploited for the development of a pH transducer, which is an essential component of many chemical and biochemical sensors.

Electrochemical impedance spectroscopic analysis

Electrochemical impedance spectroscopy is an effective tool to obtain an insight into the electrochemical reaction occurring at the surface of the modified electrode. The EIS curves for (a) ITO, (b) ITO/A–G, (c) ITO/A–G–PANI composite film (2.3 wt% PANI) and (d) ITO/A–G–PANI composite film (14 wt% PANI) are shown in Fig. 8(I) and (II) with the appropriate Randles equivalent circuit. In Randles circuit, it was assumed that the resistance to charge transfer (R_ct) and the Warburg impedance (W) were both in parallel to the double layer capacitance (C_dl). This parallel combination of R_ct and C_dl gives rise to a semicircle in the complex plane plot of the imaginary (Z′′) against the real (Z′) part of impedance.^43,44 It is well established that the comparison of R_ct is an important parameter in stating the effectiveness of the coated electrode material for the charge transfer from the solution to the electrode. It was observed that ITO/A–G modified electrode has the lowest R_ct value (49.76 ± 0.2 Ohm) due to the excellent ionic conduction offered by a three-dimensional network of the neutral biopolymer (agarose–guar gum). R_ct values were found to be higher for the A–G–PANI composite films as compared with the A–G film (see ESI Table S2†). Perhaps this was expected as PANI exhibits capacitive behavior on the surface of the electrode.^45,46 Higher R_ct is observed for the composite film with 14 wt% PANI (R_ct = 2.169 × 10⁶ ± 0.3 Ohm), as compared with the composite film with 2.3 wt% PANI (R_ct = 230.2 ± 0.1 Ohm), due to a greater amount of polyaniline. It should be noted that the excellent ionic conductivity offered by the agarose–guar gum film becomes significant in accelerating the charge transfer ability. A similar kind of decreased R_ct and accelerated electron transfer was observed with graphene–cadmium sulfate quantum dots-agarose modified electrode.⁴⁷

Fig. 8 Nyquist plot of [I] (a) ITO, (b) ITO/A–G and (c) ITO/A–G–PANI with 2.3 wt% PANI [II] (d) ITO/A–G–PANI with 14 wt% PANI. The inset is a Randles equivalent circuit diagram.

The Nyquist diagrams of the A–G–PANI composite films (Fig. 8) show a large semicircle in the wider frequency region, indicating that the electronic conductivity of the A–G–PANI composites is due to both ionic and electrical conductions, offered by the combination of an ionic conducting biopolymer (A–G) and an electrically conducting polymer (polyaniline). These detailed studies on the AC and DC conductivity of A–G–PANI composites with different polyaniline content form the necessary base to modulate the properties of the composite according to the requirements during the construction of electrical and electrochemical sensors.

Conclusions

A novel composite of agarose–guar gum–polyaniline was successfully prepared via biopolymer assisted synthesis of polyaniline. The incorporation of PANI in A–G film has conferred electrical conductivity, electrochemical activity and enhanced thermal stability to biopolymers, whereas PANI has acquired improved morphology and processability. The A–G–PANI films were characterized by UV-visible and FTIR spectroscopy, SEM and TGA, which confirmed the development of a composite with an interpenetrated network structure between A–G and PANI. The composite films are electrically conducting (10⁻³–10⁻² S cm⁻¹), and the electrical conductivity is dependent on polyaniline content in the A–G–PANI composite. I–V, CV and EIS studies have provided insight into the electrical and electrochemical behavior of the composite and has also demonstrated the processability of A–G–PANI film as a stable electrode material. A–G–PANI films are electrochemically active and have both ionic and electronic conductivity. The A–G–PANI composite exhibits better mechanical properties combined with appreciable electrical properties, which can potentially be used as an electrode material in the development of electrochemical sensors and biosensor design.

Acknowledgements

Authors C. Vaghela and M. Karve gratefully acknowledge U.G.C, New Delhi, for the Meritorious Fellowship and Emeritus Fellowship, respectively. Authors are thankful to Ganesh Markad, Department of Chemistry, University of Pune, for the help during EIS measurements and Ajinkya Trimukhe and Dr R. R. Deshmukh, Department of Physics, Institute of Chemical Technology for providing UTM facility.

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Footnote

† Electronic supplementary information (ESI) available: Effect of varying (a) aniline : oxidant molar ratio (b) HCl concentration on the conductivity of A–G–PANI films. UV-visible spectra of (a) A–G film and (b) A–G–PANI composite film in DMSO. Photographs showing an adhesion test using scotch tape pressed on the surface of (a) A–G–PANI composite films with different PANI content and (b) PANI and A–G–PANI composite films, deposited on glass slide. Cyclic voltammogram of polyaniline in 1 M HCl at a scan rate of 20 mV s⁻¹. Comparison of the standard cathodic peak potentials and anodic peak potentials of PANI powder and A–G–PANI film. The values of each component in the Randles equivalent circuit for each electrode/material/electrolyte system. See DOI: 10.1039/c4ra08688k

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