Biocatalytic approach as alternative to chemical synthesis of polyaniline/carbon nanotube composite with enhanced electrochemical properties

Abstract

A new approach is proposed to biocatalytical synthesis of a thin homogeneous layer of conducting polyaniline (PANI) on the surface of MWCNTs using the laccase Trametes hirsuta. Aniline dimer, an enhancer of laccase-catalyzed aniline polymerization, has been adsorbed on MWCNTs. Phytic acid has been used as acidic dopant and gelator. The PANI/MWCNT composite has a core shell structure and shows a high specific capacitance of 554 F g⁻¹ and long-term cycling stability. The approach enables the production of nanostructured composite conducting hydrogel.

---

Nowadays biocatalysis is widely used for the production of various compounds and is an alternative to environmentally harmful chemical synthesis. Many natural polymers, like polysaccharides and lignin, proteins, DNA and others, are produced by enzyme-catalyzed syntheses. However, natural polymers have a serious disadvantage – they are non-conductive. A new class of intrinsically conducting polymers (CP) was discovered in the 1970s.¹ These materials combine the properties of organic polymers and electroconductivity typical of semiconductors or metals.²

Polyaniline (PANI) is a widely-used representative of this class of polymers. It is of tremendous interest for researchers due to its high environmental stability, the simplicity of its synthesis and unique electrical and optical properties.³ PANI is generally produced by chemical polymerization of aniline. However, the chemical synthesis has serious shortcomings, since the reaction requires large amounts of oxidants, such as ammonium peroxydisulfate, iron(III) chloride or potassium dichromate, proceeds in strongly acidic solutions and has an autocatalytic character.^4,5 Besides, benzidine is a toxic by-product of PANI synthesis in strongly acidic media.⁵

An alternative method of PANI production is biocatalysis with the use of peroxidase⁶ or fungal high redox-potential laccases.⁷ In contrast to chemical polymerization of aniline, the laccase-catalyzed reaction is kinetically controllable and proceeds in rather mild conditions (weakly acidic aqueous solution and room temperature) without toxic by-products. Atmospheric oxygen, which is reduced to water by a four-electron mechanism, is the oxidant in the enzymatic reaction. Thus, the enzyme-catalyzed synthesis of PANI meets the requirements of green chemistry.⁸

Recently, hybrid nanocomposites made of conducting PANI and carbon nanotubes (CNT) have aroused a considerable interest. The variety of properties of these composites enables their use as a functional basis for the development of a new generation of microelectronic devices, such as rechargeable batteries, supercapacitors, photovoltaic cells, light-emitting diodes (LED), sensors etc.⁹ PANI/CNT composites show properties of certain components with a synergistic effect. Incorporations of CNTs into a conducting polymer matrix promotes charge transfer processes on CP–CNT border due to the side-selective interaction between the polymer quinoid ring and CNTs. However, the polymeric phase which is not attached to the CNT surface worsens the macroscopic characteristics of the composite. All the techniques used to enhance the CNT–CP interaction reported in literature¹⁰ do not allow to solve the problem.

PANI/CNT composites are generally produced by in situ chemical polymerization of aniline in the presence of CNTs. This method of the composite production has all the shortcomings of PANI chemical synthesis described above. Various acids such as hydrochloric acid, toluenesulfonic acid, dodecylbenzensulfonic acid, camphorsulfonic acid, poly(4-styrenesulfonic acid) are used as dopants to obtain the conducting PANI salt in the emeraldine oxidation state. In some papers published recently, the authors used phytic acid as acidic dopant in the chemical synthesis of PANI¹¹ and PANI/MWCNT composite.¹²

Here, we describe a new approach to the production of PANI/MWCNT composite with a core shell structure. We have used phytic acid as dopant and a laccase from the fungus Trametes hirsuta. Atmospheric oxygen served as a terminal oxidant.

In our experiments, we have found that aniline dimer (AD) is an enhancer of the laccase-catalyzed polymerization of aniline (Fig. 1). We have assumed that AD adsorbed on MWCNTs will accelerate the enzymatic polymerization of aniline on the surface of carbon nanomaterial. Thus, we can minimize the production of PANI not bound to the surface of MWCNTs.

Fig. 1 The dependence of the laccase-catalyzed polymerization of aniline on the aniline dimer concentration, using poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) as template. Concentration of aniline dimer in a reaction medium, μM: (1) – 0; (2) – 2; (3) – 5; (4) – 10; (5) – 20. The values were calculated as the average of 5 measurements. Conditions: 0.1 M citrate buffer (pH 3.5), [aniline] = [PAMPS] = 25 mM, the specific activity of laccase in a reaction medium – 0.4 U ml⁻¹.

Acid treated MWCNTs were used to synthesize PANI/MWCNT composites. The enzymatic synthesis was carried out by two methods.

In one case, AD was preadsorbed on the surface of acid-treated MWCNTs (see ESI†). 10 mg of AD-modified MWCNTs was dispersed in 13 mM phytic acid solution (pH 2.8), containing 65 mM of aniline monomer. The polymerization was initiated by addition of the stock laccase solution. The final specific activity of laccase in the reaction medium was 0.2 U ml⁻¹. The reaction mixture was stirred for 12 h at room temperature. PANI/MWCNT composite was precipitated by centrifugation, washed several times with water and ethanol, and then dried to constant weight. The content of PANI in the composite was about 47 wt% (composite 1). In the other case, AD was added in the reaction medium. The AD concentration was 1 mM. The content of PANI in this composite was about 56 wt% (composite 2).

Both composites were characterized by the following parameters: the structure of repeating units of conducting PANI, morphology, electroconductivity, specific capacitance and electrochemical stability.

FTIR spectra of pristine PANI obtained by the enzymatic polymerization of aniline in the presence of AD and PANI/MWCNT composites are shown in Fig. 2. PANI and both composites exhibit characteristic absorption bands arising from the vibration mode of both C=N and C=C stretching of the quinoid diimine unit near 1576 cm⁻¹, while the band near 1497 cm⁻¹ is attributed to C=C aromatic ring stretching of the benzenoid diamine unit. The peak near 1300 cm⁻¹ belongs to C–N stretching of secondary aromatic amines. The 1,4-disubstituted aromatic rings vibrate near 800 cm⁻¹.¹³

Fig. 2 FTIR spectra of pristine PANI (1), PANI/MWCNT composite 1 (2) and composite 2 (3).

The conductivity of the PANI/MWCNT composites measured by the four-point probe method at room temperature was 5.4 S cm⁻¹ (composite 1) and 2.3 S cm⁻¹ (composite 2). These values were significantly higher than that of pristine PANI (0.2–0.3 S cm⁻¹).

The morphology of both PANI/MWCNT composites was studied by transmission electron microscopy (TEM). As evident from the TEM images shown in Fig. 3, aniline polymerization on the MWCNT resulted in a core–shell structure composite. The surface of acid treated MWCNTs was smooth (Fig. 3a), and their diameter was about 10–15 nm. The diameter of PANI/MWCNT composites increased to 20–25 nm (Fig. 3b and c). PANI in composite 1 generated a uniform thin and rough coating layer on the surface of MWCNTs. Besides, composite 1 had no polymeric phase not bound to the surface of MWCNTs.

Fig. 3 TEM images of acid treated MWCNTs (a) and PANI/MWCNT composite 1 (b) and composite 2 (c).

Both PANI/MWCNT composites were also studied by the method of cyclic voltammetry, using a three electrode electrochemical cell. The specific capacitance was 554 F g⁻¹ (composite 1) and 487 F g⁻¹ (composite 2) at a potential scan rate of 5 mV s⁻¹. It is noteworthy that acid treated MWCNTs had a rather low specific capacitance, not exceeding 45–47 F g⁻¹. Cyclic voltammograms were recorded at a varied potential scan rate. The specific capacitance of composite 1 decreased gradually from 554 to 420 F g⁻¹ with increasing the potential scan rate from 5 to 100 mV s⁻¹ (see ESI†).

The electrochemical stability of both PANI/MWCNT composites was determined by the change of their specific capacitance during repeated cycling. The stability of composite 1 was higher than that of composite 2 (Fig. 4). After 3000 cycles of the potential scanning in the range of −0.1 to +0.6 V, the specific capacitance of composite 1 decreased by 6%.

Fig. 4 The dependence of the specific capacitance of PANI/MWCNT composite 1 (1) and composite 2 (2) on CV cycle numbers in the voltage range of −0.1 to +0.6 V at a potential scan rate of 100 mV s⁻¹. The electrochemical stability of each composite was measured twice, and the measurements were averaged to give the final value reported.

In ref. 11 and 12, PANI-only and PANI/MWCNT conducting hydrogels were chemically fabricated using phytic acid as an acidic dopant. These hydrogels are rather promising materials for bioelectronics and energy storage devices. We have obtained PANI/MWCNT conducting hydrogel by laccase-catalyzed polymerization of aniline at high concentrations of aniline monomer and phytic acid (1.0 and 0.2 M, respectively) and a high specific activity of laccase (1.5 U ml⁻¹) in the reaction medium.

Conclusions

We have proposed a new approach to biocatalytical synthesis of PANI/MWCNT composite using AD as enhancer in laccase-catalyzed in situ polymerization of aniline and phytic acid as dopant. Preadsorption of AD on the surface of acid treated MWCNT enabled us to fabricate a composite with better morphology, high conductivity (5.4 S cm⁻¹), high specific capacitance (554 F g⁻¹) and long cycling stability, which may be explained by the absence of the polymer phase not-bound to the surface of MWCNTs. Enzyme-catalyzed PANI/MWCNT composite may be used as an electroactive material in the electrodes of supercapacitors and other electronic devices.

Further research aimed at enhancing the specific capacitance and stability of PANI/MWCNT composite are now in progress.

Acknowledgements

This work was supported by the Russian Foundation for Basic Research (project no. 15-08-00142-a).

References

1. H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang and A. J. Heeger, J. Chem. Soc., Chem. Commun., 1977, 16, 578.
2. (a) A. F. Diaz, J. F. Rubinson and H. B. Mark, Adv. Polym. Sci., 1988, 84, 113; (b) A. G. MacDiamid, Synth. Met., 2002, 125, 11.
3. (a) A. A. Syed and M. K. Dinesan, Talanta, 1991, 38, 815; (b) A. G. MacDiarmid, J. C. Chiang, A. F. Richter and A. J. Epstein, Synth. Met., 1987, 18, 285; (c) S. Bhadra, D. Khastgir, N. K. Singha and J. H. Lee, Prog. Polym. Sci., 2009, 34, 783.
4. (a) K. Tzou and R. V. Gregory, Synth. Met., 1992, 47, 267; (b) J. Stejskal, P. Kratochvil and M. Spirkova, Polymer, 1995, 36, 4135.
5. F. Lux, Polymer, 1994, 35, 2915.
6. W. Liu, J. Kumar, S. Tripathy, K. J. Senecal and L. Samuelson, J. Am. Chem. Soc., 1999, 121, 71.
7. (a) A. V. Karamyshev, S. V. Shleev, O. V. Koroleva, A. I. Yaropolov and I. Y. Sakharov, Enzyme Microb. Technol., 2003, 33, 556; (b) A. V. Streltsov, O. V. Morozova, N. A. Arkharova, V. V. Klechkovskaya, I. N. Staroverova, G. P. Shumakovich and A. I. Yaropolov, J. Appl. Polym. Sci., 2009, 114, 928; (c) G. P. Shumakovich, I. S. Vasil'eva, O. V. Morozova, V. G. Khomenkov, I. N. Staroverova, I. A. Budashov, I. N. Kurochkin, J. A. Boyeva, V. G. Sergeyev and A. I. Yaropolov, J. Appl. Polym. Sci., 2010, 117, 1544.
8. (a) M. Mogharabi and M. A. Faramarzi, Adv. Synth. Catal., 2014, 356, 897; (b) S. Witayakran and A. J. Ragauskas, Adv. Synth. Catal., 2009, 351, 1187.
9. (a) C. Oueiny, S. Berlioz and F.-X. Perrin, Prog. Polym. Sci., 2014, 39, 707; (b) S. Bocchini, A. Chiolerio, S. Porro, D. Accardo, N. Garino, K. Bejtka, D. Perrone and C. F. Pirri, J. Mater. Chem. C, 2013, 1, 5101; (c) H. Acar, S. Çınar, M. Thunga, M. R. Kessler, N. Hashemi and R. Montazami, Adv. Funct. Mater., 2014, 24, 4135; (d) A. Guiseppi-Elie, Biomaterials, 2010, 31, 2701; (e) H. Zhang, B. He, Q. Tang and L. Yu, J. Power Sources, 2015, 275, 489.
10. (a) M. Cochet, W. K. Maser, A. M. Benito, M. A. Callejas, M. T. Martínez, J.-M. Benoit, J. Schreiber and O. Chauvet, Chem. Commun., 2001, 16, 1450; (b) M. Holzinger, J. Abraham, P. Whelan, R. Graupne, L. Ley, F. Hennrich, M. Kappes and A. Hirsch, J. Am. Chem. Soc., 2003, 125, 8566; (c) V. V. Abalyaeva, V. R. Bogatyrenko, I. V. Anoshkin and O. N. Efimov, Polym. Sci., Ser. B, 2010, 52, 252; (d) N. Boulanger, J. Yu and D. R. Barbero, Nanoscale, 2014, 6, 11633; (e) B.-W. Lee, C.-H. Park, J. H. Song and Y.-J. J. Kim, Nanosci. Nanotechnol., 2011, 11, 6089.
11. L. Pan, G. Yu, D. Zhai, H. R. Lee, W. Zhao, N. Liu, H. Wang, B. C.-K. Tee, Y. Shi, Y. Cui and Z. Bao, PNAS, 2012, 109, 9287.
12. P.-Y. Chen, N.-M. D. Courchesne, M. N. Hyder, J. Qi, A. M. Belcher and P. T. Hammond, RSC Adv., 2015, 5, 37970.
13. (a) G. Louarn, M. Lapkowski, S. Quillard, A. Pron, J. P. Buisson and S. Lefrant, J. Chem. Phys., 1996, 100, 6998; (b) C. H. Lim and Y. J. Yoo, Process Biochem., 2000, 36, 233.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12352j

---