New Strategies For Biocatalytic And Electrochemical Synthesis Of Polyaniline

Polyaniline (PANI) is a promising conducting polymer due to its excellent electrochemical performance and good chemical stability. It has broad applications in anticorrosion coatings, rechargeable batteries, sensors, electromagnetic shielding, antistatic protection and electroluminescent materials. Traditionally, conducting PANI is obtained by oxidizing aniline monomer chemically or electrochemically under strong acidic conditions. The existing problems are harsh conditions, complex by-products, poor solubility, which limit the application of PANI. How to get PANI with better performance under mild conditions is the goal of the scientific researchers.Biocatalysis is generally used under room temperature, atmospheric pressure and weak acidic conditions with high efficiency, good regioselectivity, less by-products and so on. The enzymatic aniline polymerization in amphiphilic molecular system may produce conducting and soluble polyaniline in mild conditions. What’s more, the naturally chiral of enzyme may induce chiral polyaniline formation which has potential application in chemical and biological sensors, chiral recognition, etc. The match of the conditions for aniline polymerization and the conditions for an enzyme activity is the key to achieve the above goal.Conductivity is a key performance index of PANI. The conductivity of PANI is related to its chain linearity. The better the linearity is, the better the conductivity will be. β-cyclodextrin is cyclic oligosaccharide which has an appearance of truncated cone. Its outer surface is hydrophilic, but its inner cavity is hydrophobic β-cyclodextrin can form an inclusion complex with aniline which may shield the ortho-position of aniline and the polymerization of aniline would be para-coupled.Electrochemical synthesis of polyaniline plays an important role in aniline polymerization. The PANI with different electrochemical properties could be obtained by changing the conditions for the electropolymerization. An electrolyte is a very important factor that affects the process of the polymerization. Room temperature ionic liquids (ILs) have been tried as electrolytes for the electrochemical synthesis of conducting polymers which are considered as green processes. Protons are indispensable to the aniline electropolymerization. It is a new attempt to synthesis a proton functionalized anilinium salt ionic liquids or anilinium salt deep eutectic solvents used for aniline electropolymerization at room temperature without solvent/electrolyte. Based on the above considerations, some new strategies to synthesis conducting PANI efficiently are tried in the following sections:1. HRP/H2O2 catalyzed synthesis of chiral, conducting and water soluble polyanilineEnzyme catalytic polymerization of aniline in amphiphilic system has incomparable advantages. Horseradish peroxidase (HRP)/H2O2-triggered polymerization of aniline in sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) micellar solution is first presented. Results show that the critical micelle concentration of AOT in the polymerization system is very low and AOT has a small effect on the activity of HRP, so it matches with HRP well for the end. As a template, the AOT micellar solution is essential for the formation of conducting polyaniline. In the present system, AOT also acts as a dopant and a dispersant, leading to the formation of conducting and water soluble polyaniline. Optimization of the pH and the H2O2 concentration results in more conducting polyaniline. During the investigation, it was found that HRP itself can induce the formation of chiral polyaniline without any other chiral inducer. In short, with the aid of AOT, one-step biosynthesis of chiral, conducting and water soluble polyaniline using HRP as both catalyst and chiral inducer is realized. Compared with other aniline polymerization systems reported elsewhere, the present system shows many advantages. It needs less templates and no foreign chiral inducer while the synthesized polyaniline has good dispersibility and high electrical conductivity.2. p-Cyclodextrin improves the linearity of polyaniline synthesized enzymaticallyThe conductivity of polyaniline is closely related to its linearity. β-cyclodextrin has hydrophobic inner cavity which can provide a micro-reactor for aniline polymerization. β-cyclodextrin can shield the ortho-position of aniline and the polymerization of aniline would be para-coupled. The influence of β-cyclodextrin (β-CD) on the laccase-catalyzed aniline polymerization in sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) micellar solution is first presented. Results show that the anionic AOT micelle is compatible with laccase and β-CD also has little negative effect on the expression of the catalytic activity of laccase. β-CD can improve the linearity or conductivity of polyaniline biosynthesized in AOT micellar solution. Component analysis shows that AOT is doped in polyaniline chain while β-CD does not thread on the polyaniline chain. Mechanism analysis indicates that the improvement of the linearity of polyaniline by β-CD is due to steric hindrance close to the ortho-position of aniline, which favors the para-coupled aniline polymerization.3. Electropolymerization in a novel proton functionalized room temperature ionic liquid anilinium acetateElectrochemical synthesis of polyaniline plays an important role in aniline polymerization. An electrolyte is a very important factor that affects the formation of PANI. Room temperature ionic liquids (ILs) have been tried as an electrolyte for the electrochemical synthesis of PANI. Protons are indispensable to the aniline electropolymerization. Based on this, a novel proton functionalized room temperature ionic liquid-anilinium acetate ([HANI]Ac) with high ionic conductivity and low viscosity has been synthesized and used for the first time for aniline electropolymerization without any additives. The electropolymerization process reveals that the onset potential for anilinium oxidation (Eonset) in neat [HANI]Ac is+0.45 V vs. SCE, which is lower than those in aqueous and nonaqueous (ionic liquid as solvent/electrolyte) solutions. The control experiments and theoretical calculations indicate that the low Eonset in neat [HANI]Ac should be attributed to charge balancing counter-ion Ac-. Compared with [HANI]Ac/[BMIM]PF6 binary system, the mass transfer resistance of anilinium in neat [HANI]Ac is small, therefore the electropolymerization can be kept on at a high rate in a period. The cyclic voltammograms and Fourier transform infrared spectra demonstrate that the obtained PANI is highly conductive. The PANI has moderate solubility in [HANI]Ac and its adherence to the electrode is poor. During the electropolymerization, the accumulated PANI diffuses from the electrode/solution interface to the bulk phase which leads to different current signals of PANI under different conditions. Stirring slightly increases the oxidation current signal of anilinium but greatly reduces the redox current signal of PANI. Under quiescent conditions, lowering the upper switching potential reduces the redox current signal of PANI. The potential scan rate has great effect not only on the current signal of PANI but also on the redox peak shape. The electropolymerization process of [HANI]Ac in the presence of triethylamine further demonstrates that proton deficiency can result in a significant reduction of the rate of aniline polymerization.4. Electropolymerization in proton functionalized anilinium salts/glycol deep eutectic solventsDeep eutectic solvents (DES) are considered to be new generation ionic liquids. Aniline electropolymerization in two DES systems (anilinium hydrochloride/glycol ([HANI]C1/(CH2OH)2) and anilinium nitrate/glycol ([HANI]NO3/(CH2OH)2)) is presented. Experiment results show that the mixing of solid anilinium salt with glycol (proper proportion) can reduce the melting point of anilinium salt, forming deep eutectic mixtures which are liquid at room temperature. The good conductivity and low viscosity show that those DES are suitable for aniline electropolymerization. The cyclic voltammograms (CVs) of electropolymerization process in [HANI]C1/(CH2OH)2 and [HANI]NO3/(CH2OH)2 show that the resulting PANI has two typical redox peaks. With the increase of CVs cycles, the PANI on the surface of the electrode increases constantly, but the reversibility of the redox peaks becomes worse. Component analysis shows that the resulting PANI is of conductive. The CVs of the PANI modified electrode in H2SO4 shows that the specific capacitances of the PANI obtained from [HANI]C1/(CH2OH)2 and [HANI]NO3/(CH2OH)2 are 341 F g-1 and 492 F g-1, respectively. These PANI-based capacitors are superior to that from H2SO4 aqueous solution due to their porosity of the formed nanofibres.