Electrically Conductive Polyaniline/Silk Fibroin Composite for Ammonia and Acetaldehyde Sensing

Abstract

In this study, camphorsulphonic acid-doped conducting polyaniline/silk fibroin (Pani/SF) composites were prepared by in-situ polymerisation. The Pani/SF composite fibres were characterised by using Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, thermogravimetric analysis, scanning electron microscopy and digital photographs. Results indicated that the Pani was successfully coated on silk fibroin. The electrical properties of Pani/SF composite fibres were influenced by the extent of loading of aniline monomers. The as-prepared material was studied as a sensing material by observing the change in their electrical conductivity on exposure to ammonia and acetaldehyde, followed by ambient air at room temperature. It was found that the composite fibres showed good reversibility and conductivity decreased on exposure to higher concentration of ammonia and acetaldehyde vapours at room temperature.

1. Introduction

Conducting polymers are very versatile materials with potential applications in various evolving technologies such as electrodes in primary and secondary batteries,^1,2 energy storage devices,^3,4 microelectronics, ⁵ photocatalysts, ⁶ optoelectronic devices,^7,8 sensors and actuators ⁹ . Among all conducting polymers, polyaniline (Pani) drew widespread attention because it possesses unique properties such as low cost, high environmental stability, reversible and tailorable electrical properties by controlled charge-transfer processes.^10–12 Polyaniline can experience changes on adsorption and desorption of chemical species onto its surface, making it a potential sensing material for humidity, toxic solvents,^13–16 and organic vapours such as methanol, ethanol, chloroform, dichloromethane and hexane.^17,18

Silk fibroin (SF) is an attractive scaffold biomaterial because of its excellent mechanical properties, biocompatibility, biodegradability, tissue regeneration and various other biomedical applications. ¹⁹ Natural silk fibroin fibres (a filament core protein) is obtained by removing the outer sericin (glue-like coating of a nonfilamentous protein) from silk fibres with anhydrous sodium carbonate solution at appropriate temperature. ²⁰ A number of composites of silk fibroin have been reported for various potential applications. Ismail et al. ²¹ reported the use of a fibroin/polyaniline composite for electrochemical characterisation as a reactive sensor. Xia et al. ²² reported work on a silk fibroin/polyaniline (core/shell) coaxial fibre for application in cell proliferation. The development of gas sensors by conducting composite materials has drawn more attention over the last few years. ²³

Among various toxic compounds, acetaldehyde and ammonia are classified as potentially dangerous to the environment, even at low concentration. Thus, the development of new materials for the detection and determination of these toxic volatile compounds in the atmosphere are of immediate concern to environmental researchers. Herein we have tried to explore the ammonia and acetaldehyde sensing capabilities of Pani by modifying the non-conducting silk fibroin into Pani-coated conducting composite fibres (Pani/SF). To the best of our knowledge, a sensing application of Pani-modified SF fibres (Pani/SF) has not yet been reported elsewhere.

2. Experimental

2.1. Materials

In the preparation of Pani and Pani/SF composite fibres, the chemicals used were: aniline 99% (E. Merck, India), CSA (camphorsulphonic acid) ≥98% (TCI, Tokyo), cocoons for silk (Banaras, India), potassium persulphate (E. Merck, India) and methanol. Double-distilled water was used throughout the experiments.

2.2. Preparation of Silk Fibroin

Cocoons were degummed twice for 1.5 h in aqueous solution of 0.02 M Na₂CO₃ and then rinsed thoroughly with double-distilled water to extract the silk fibroin (SF). The extracted SF was then dried at 40°C for 24 h.

2.3. Preparation of Pani and Pani/SF Composites

Aniline (2.0 mL) was mixed with 100 mL of 0.1 M camphorsulphonic acid (CSA) under constant stirring for 1 h. A solution of potassium persulphate (PPS) was added dropwise and the mixture was allowed to react for 24 h. The final dark green coloured reaction mixture was then filtered, washed several times with double distilled water and methanol and then dried in an air oven at 70°C for 10 h.

The Pani/SF composite fibres were prepared by in-situ oxidative polymerisation of aniline in the presence of CSA modified SF fibres using potassium persulphate as an oxidising agent. The extracted silk fibroin fibres were dipped in 100 mL of 0.1 M CSA aqueous solution and stirred for 1 h. The resultant SF fibres were filtered and dried at 40°C. Thus the CSA modified SF fibres (300 mg) each were added to different amounts of aniline solutions prepared in 0.1 M CSA, and stirred for 1 h. A solution of K₂S₂O₈ (6 g) in 100 mL of 0.1 M CSA was then added dropwise into the mixture at room temperature with constant stirring. The colour of the SF fibres changed from light yellow to greenish black, indicating the polymerisation of aniline on the silk fibroin fibres. The fibrous reaction mixture was then stirred for a further 24 h. The final Pani/SF composite fibres was then filtered, washed thoroughly with double-distilled water and methanol to remove excess acid, potassium persulphate and Pani oligomers. Thus-prepared Pani/SF composite fibres were dried at 50°C for 20 h in an air oven and were stored in desiccators for further experiments. For electrical conductivity measurements, 250 mg of material from each sample was pelletised at room temperature with the help of a hydraulic pressure machine at 80 kN load for 15 min. The preparation details are given in Table 1 .

Table 1

Proportions of the constituents in the preparation of Pani-SF composites

Sample I.D	Aniline (mL)	K2S2O8 (g)	SF fibres (mg)
Pani	2.0	6	0
Pani/SF-1	1.0	6	300
Pani/SF-2	2.0	6	300
Pani/SF-3	3.0	6	300
Pani/SF-4	5.0	6	300

3. Characterisation

In order to investigate the morphology, structure and chemical composition of Pani/SF fibres, a variety of methods were used including attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). The spectra were recorded using a Perkin-Elmer 1725 instrument. Scanning electron microscopy (SEM) studies were carried out on a JEOL, JSM, 6510-LV (Japan). Thermogravimetric analysis (TGA) was done by using a Perkin Elmer instrument in the temperature range from 35 to 700°C. The thermal stability in terms of DC electrical conductivity of Pani and Pani/SF composite fibres under isothermal and cyclic ageing conditions was also studied. For this study, a four-in-line probe with a temperature controller, PID-200 (Scientific Equipment, Roorkee, India) was used to measure the DC electrical conductivity and its temperature dependence. The equation used in the calculation of DC electrical conductivity was:

$$\sigma =[In2(2S/W)]/[2\pi S(V/I)]$$

(1)

where I, V, W and S are the current (A), voltage (V), thickness of the pellet (cm) and probe spacing (cm) respectively and σ is the conductivity (S/cm). ²⁴ In testing of isothermal stability, the pellets were heated at 50°C, 70°C, 90°C, 110°C and 130°C in an air oven and the DC electrical conductivity was measured at each particular temperature at an interval of 10 min in the accelerated ageing experiments. In testing of the stability under cyclic ageing condition, DC conductivity measurements were taken 5 times within the temperature range of 50–130°C.

4. Results and Discussion

4.1 Mechanism of Preparation of Pani/SF Composite Fibres

The mechanistic view of the polymerisation process seems to involve the anilinium cations (phenyl-NH₃⁺) getting trapped by Coulombic attraction between anilinium cations and the sulphonate group of camphorsulphonic acid adsorbed on the surface of the silk fibroin. Thus the SF fibres get uniformly surrounded by anilinium cations. This arrangement comes into contact with K₂S₂O₈, the anilinium cations become polymerised on the surface of the SF fibres, forming a uniform smooth layer of Pani (emeraldine salt). The uniform deposition of Pani on the CSA modified surface of silk fibroin may be supported by the interaction between negatively-charged sulphonate groups and polarons of Pani. The similar mechanism is mentioned by Youyi Xia et. al. in which SF fibres were modified by methyl orange (MO) and then aniline was polymerised on the surface of MO modified SF fibres. ²² The schematic presentation of coulombic attraction is given in Figure 1 .

Figure 1

Schematic representation of possible interaction between Pani and CSA modified SF fibres in Pani/SF composite fibres

4.2. ATR-FTIR Analytical Studies

ATR-FTIR spectra of the SF and Pani/SF-4 composite fibres are shown in Figure 2 . Figure 2(a) shows the spectra of pure SF fibres in which the peaks observed at 1636 cmr ¹ and 1540 cmr ¹ correspond to amide I and amide II vibrations which may be attributed to the β sheet structure of silk fibroin. ²⁵ In the spectra of Pani/SF-4 composite fibres, the four major peaks are observed. The peaks observed at 1569 cmr ¹ and 1488 cmr ¹ may be due to C=C stretching mode of quinonoid and benzenoid rings respectively. The peaks at around 1292 cmr ¹ and 800 cmr ¹ may be attributed to C–N bond stretching and C–H bond out-of-plane bending respectively, as shown in Figure 2(b) . ²⁶ There are some extra peaks observed in Pani/SF-4 spectra, which may be related to presence of camphorsulphonic acid in Pani. The peak at around 1726 cmr ¹ may be attributed to the characteristic peak of CSA due to stretching of CO bonds; also the peak at 620 cmr ¹ may be related to S–O bond stretching of the sulphonate group. ²⁷ All these results indicated and supported the presence of CSA doped Pani coated on the SF fibres.

Figure 2

ATR-FTIR spectra of: (a) SF fibres and (b) Pani/SF-4 fibres

4.3 Thermogravimetric Analysis (TGA)

The thermal stability of the Pani/SF-4 composite fibres was investigated and shown in Figure 3 . The initial weight loss may be due to removal of low weight oligomers of Pani and absorbed water on the surface of Pani. Afterwards, the sharp degradation of the Pani/SF-4 composite fibres is observed at about 295.4°C, ²² which is higher than that of pristine SF fibres (281.3°C). ²⁸ This degradation pattern of Pani/ SF-4 fibres suggesting that Pani/SF-4 composite fibres resist degradation under thermo-oxidative conditions.

Figure 3

TGA thermogram of Pani/SF-4 composite fibres

4.4 Scanning Electron Microscopic (SEM) Studies

The morphology of SF fibres and Pani/SF-4 composite fibres were studied by SEM at different magnifications as presented in Figures 4 . Figure 4 (a) and 4 (b) show SEM images of SF fibres, which seem to be very rough-surfaced. The change from rough-surfaced SF fibres to smooth-surfaced Pani/SF-4 composite fibres observed in Figure 4 (c) and 4 (d), indicating the growth of a smooth Pani layer on SF fibres. The SF fibres were modified by CSA, helping uniform adsorption of anilinium molecules onto the fibres and providing uniform coatings. Thus the Pani/SF-4 composite fibres exhibit smooth-surfaced morphology.

Figure 4

SEM micrographs of: (a and b) SF fibres, (c and d) Pani/SF-4 composite fibres at different magnifications

4.5 Digital Photographic Studies

Figures 5(a) and 5(b) show the digital photographs of SF and Pani/SF-4 composite fibres respectively. Evidently, it may be seen that the light yellow colour of the original SF fibres changed to deep green, suggesting that the SF fibres had been successfully covered with Pani.

Figure 5

Digital photographs of: (a) SF fibres and (b) Pani/SF-4 composite fibres

4.6. Electrical Conductivity Studies

For the electrical conductivity measurements and sensing experiments, 250 mg material from each sample was pelletised at room temperature with the help of a hydraulic pressure machine at 80 kN force for 15 min. The initial DC electrical conductivity Pani and Pani/SF composite fibres were measured by the standard four-in-line probe method. The variations in DC electrical conductivity in Pani/SF composites with loading of different amounts of aniline monomer are shown in Figure 6 . In Pani/SF-4 composite fibres, the highest electrical conductivity was found with highest loading (5 mL) of aniline monomer. ²⁴ The electrical conductivity of CSA doped Pani/SF composite fibres increases with increase in aniline monomer content in the composites materials varied from 10⁻⁶ to 10⁻² S/cm. Therefore, Pani/SF-4 was selected for temperature dependence of electrical conductivity studies and as a chemical sensing material. The DC electrical conductivity in CSA doped Pani/SF composite fibres was found to be lower than that of pristine CSA doped Pani. In all Pani/SF composite fibres, the silk fibroin was functionalised by CSA having negatively charged functional sulphonate group (SO₃^-) which interacts strongly with positively charged nitrogen of polyaniline which induce the formation of conductive polymer. Based on the mechanism involved in the formation, the SO₃^- groups interact with the polarons of the Pani, thus decreasing their mobility, resulting in a decrease in electrical conductivity in Pani/SF composite fibres.

Figure 6

Initial DC electrical conductivities of Pani and Pani/SF composite fibres

5. Stability Studies

5.1 Stability under Isothermal Ageing

The stability of Pani and Pani/SF-4 composite fibres in terms of DC electrical conductivity was studied under isothermal ageing conditions as shown in Figure 7 . The representation of relative electrical conductivity was calculated by the equation:

$${\sigma }_{r,t}=\frac{{\sigma }_t}{{\sigma }_O}$$

(2)

where σ_r,t is the relative electrical conductivity, σ_t = electrical conductivity at time t, σ_o = electrical conductivity at time zero. The comparative study of relative electrical conductivity with respect to time at different temperatures was done by analysing the stability of prepared materials in terms of DC electrical conductivity. The electrical conductivity of each of the samples was measured for temperature (50, 70, 90, 110, 130°C) versus time at intervals of 10 min up to 40 min. It can be understood From the Figure 7 (a) , that the stability of relative DC electrical conductivity of Pani is very fair at 50 and 70°C and from 90 to 130°C seems to be unstable. In Figure 7 (b) the relative DC electrical conductivity of Pani/SF-4 composite fibres is seems to be very stable at 50, 70, and 90°C. Thus it is inferred that the fibrous Pani/SF-4 material is sufficiently stable under ambient conditions in terms of DC electrical conductivity retention up to 90°C under isothermal ageing condition.

Figure 7

Relative electrical conductivity of: (a) Pani, (b) Pani/SF-4 composite fibres under isothermal ageing conditions

5.2 Stability under Cyclic Ageing Conditions

The stability in the term DC electrical conductivity retention of Pani and Pani/SF-4 composite fibres was also studied by a cyclic ageing method within the temperature range of 50 to 130°C, as shown in Figure 8 . The electrical conductivity was recorded for successive cycles and observed to be gradually increased in Pani, while it decreased in Pani/SF-4 composite fibres for each cycle. The relative electrical conductivity was calculated using the following equation:

$${\sigma }_r=\frac{{\sigma }_T}{{\sigma }_{50}}$$

(3)

where σ_r is relative electrical conductivity, σ_T is electrical conductivity at temperature T (°C) and σ₅₀ is electrical conductivity at 50°C.

Figure 8

Relative electrical conductivities of: (a) Pani, (b) Pani/SF-4 composite fibres under cyclic ageing conditions

As each cycle ran from temperature 50 to 130°C the relative electrical conductivity of Pani seemed to increase, maybe due to the increment of number of charge carriers which may be attributed to the formation of greater number of polarons/bipolarons. From Figure 8 (a) it may observed that the electrical conductivity of Pani for all of the cycles is similar and following the same trend with gain in the conductivity. In Pani/SF-4 composite fibres just opposite trend of Pani relative electrical conductivity is observed. The relative DC electrical conductivity found to decrease as temperature increases as shown in Figure 8 (b) . The decrease in electrical conductivity may be attributed to the removal of moisture, loss of low molecular weight oligomers of aniline and earlier degradation of coated polyaniline on silk fibroin under cyclic ageing conditions.

6. Chemical Sensing Studies

Acetaldehyde is a highly reactive volatile organic compound which may be responsible for dizziness, headache and even cancer if someone is exposed to its vapour for prolonged periods. Although ammonia solution does not usually cause problems for humans and other mammals, it is highly toxic to aquatic animals even at very low concentration. Thus both the compounds are classified as dangerous for the environment and the detection of acetaldehyde and ammonia vapours are important in environmental monitoring and chemical control. The acetaldehyde and ammonia vapours sensitivities of Pani/SF-4 fibres were monitored by measuring the changes in the electrical conductivity at room temperature, using a four-in-line probe electrical conductivity device as shown in Figure 9 .

Figure 9

Schematic representation of acetaldehyde and ammonia vapours sensor unit by four-in-line probe technique

The electrical conductivity of 250 mg pelletised Pani/SF-4 composite fibres showed significant response on exposure to different concentrations (0.1, 0.2, 0.3, and 0.5 M) of aqueous acetaldehyde at room temperature, as shown in Figure 10 . The sensitivity of the material was investigated by the two parameters; response time and sensing intensity. The pellet was attached with four probes and placed in a closed chamber of acetaldehyde vapour for 60 s and after that the pellet was removed from chamber and exposed to air for next 60 s. It can be seen that the electrical conductivity decreased for the first 60 s for all of the different concentration of solutions of acetaldehyde and recovered in next 60 s on exposing to fresh air. Relatively substantial changes in electrical conductivity and fast recovery observed in an atmosphere of 0.5 M aqueous acetaldehyde vapour.

Figure 10

Effect on the DC electrical conductivity of Pani/SF-4 on exposure to different concentrations (0.1 M, 0.2 M, 0.3 M, 0.5 M) of acetaldehyde with respect to time

The reversibility response of Pani/SF-4 composite fibres towards 0.5 M aqueous acetaldehyde was investigated as shown in Figure 11 . The reversibility was determined by first keeping the sample in acetaldehyde vapour for 10 s, followed by 10 s in open air, for a total duration of 120 s. The conductivity after each cycle never recovered upon exposure to air due to two process occurring simultaneously, i.e. electrical neutralisation of Pani backbone and the desorption of acetaldehyde.

Figure 11

Reversible electrical conductivity response of Pani/SF-4 composite fibres in the vapour of 0.5 M acetaldehyde solution

The similar experiment was done in presence of different concentrations (0.1, 0.2, 0.3, and 0.5 M) of ammonia solution at room temperature, as shown in Figure 12 . It may be seen that the electrical conductivity decreased on exposure to ammonia solutions (0.1, 0.2, 0.3, and 0.5 M) for first 60 s and regained in next 60 s on exposing to fresh air. The electrical conductivity response of Pani/SF-4 in 0.5 M ammonia solution was found to be better than other ammonia solutions.

Figure 12

Effect on the DC electrical conductivity of Pani/SF-4 composite fibres on exposure to the vapour of different concentrations (0.1 M, 0.2 M, 0.3 M, 0.5 M) of ammonia with respect to time

The reversibility of the response of Pani/SF-4 composite fibres towards 0.5 M ammonia solution was also investigated, as shown in Figure 13 . The similar experiment was done in the absence and in the presence of ammonia vapour for a total of 120 s by first keeping the sample in ammonia vapour for 10 s followed by 10 s in fresh air. It is observed that the conductivity after each cycle never reverted to its original value upon exposure to air. The reason for not recovering electrical conductivity of Pani/SF-4 composite fibres was the same as was in case of acetaldehyde. If the air exposure time is increased, the conductivity may recover due to complete desorption of ammonia from the polymer surface.

Figure 13

Reversible electrical conductivity response of Pani/SF-4 composite fibres for 0.5 M ammonia solution

6.1 Proposed Mechanism for Sensing

The sensing mechanism of ammonia and acetaldehyde vapours through the DC electrical conductivity response of the composite fibres may be explained by simple adsorption and desorption affecting the conduction mechanism at room temperature. In Pani/ SF-4 composite fibres, the DC electrical conductivity decreases after exposure to acetaldehyde and ammonia vapours, which may be attributed to a decrease in mobility of the charge carriers in Pani. The decrease in mobility of charge carriers may be due to interaction of lone pairs of oxygen and nitrogen of acetaldehyde and ammonia respectively with positive charged polarons concentrated at the nitrogens of polyaniline.^24,29 Thus the mobility of the charge-carrying polarons decreases, resulting in a decrease in the electrical conductivity. When the adsorbed vapours of ammonia and acetaldehyde on the surface of polyaniline get released from the surface under ambient conditions, it leads to an increase in the electrical conductivity.

Scheme 1.

Proposed interactions of Pani/SF-4 composite fibres with acetaldehyde and ammonia

7. Conclusions

Pani/SF composite fibres were successfully prepared by an in-situ polymerisation method and confirmed by FTIR and TGA analysis. SEM analysis confirmed the uniform deposition of Pani on CSA-modified SF fibres. Pani/SF composite fibres showed lower conductivity but greater thermal stability in terms of their DC electrical conductivity under isothermal and cyclic ageing conditions than pristine Pani. The results highlighted excellent reversible acetaldehyde and ammonia sensing, indicating that the Pani/SF-4 composite fibres could be utilised as a sensor material.