Enzymatic Synthesis Of Conducting Polyaniline In Bicontinuous Microemulsions As Templates

Polyaniline(PANI)is a chemically stable and highly conductive polymer that has been applied in many fields.The chemical or electrochemical synthesis of PANI is a conventional method.It requires a strong oxidant or a higher oxidation potential,in addition to a strongly acidic medium,and moreover the selectivity of the reaction is not satisfactory.By comparison,the enzyme-assisted oxidative polymerization of aniline under mild conditions can be carried out with good selectivity.So the enzymatic method has attracted great attention.It has been shown that the biosynthesis of conducting PANI can be carried out without a template;however,in the presence of a template,a high quality PANI with high molecular weight and good conductivity could be obtained.So far,templates tried mostly for the enzymatic polymerization of aniline are various aggregates formed via self-assembly by strong acid-based surfactants,such as micelles,vesicles,etc.and microemulsions have been rarely tried,especially bicontinuous microemulsions,which.have large interfacial area and zero averaged interfacial curvature.Therefore,the use of bicontinuous microemulsion as a template for the synthesis of conducting PANI synthesis may be beneficial to the linear growth of PANI and spins hops between PANI chains.There has been no report in this respect,so it is worthwhile to try to do so.To this end,the following two works have been carried out:1.Horseradish peroxidase-catalyzed oxidative polymerization of aniline in bicontinuous microemulsion stabilized by AOT/SDSA bicontinuous microemulsion system consisting of sodium bis(2-ethylhexyl)sulfosuccinate(AOT),sodium dodecylsulfate(SDS),an aqueous solution(a 0.1 M disodium hydrogenphosphate/citric acid buffer solution of pH=4.3),and decane was investigated in detail for applications as reaction medium for the horseradish peroxidase(HRP)/hydrogen peroxide-mediated synthesis of the conductive form of polyaniline(PANI).To formulate the optimal single-phase microemulsion with the phase inversion temperature being close to room temperature,the T-? fishlike phase diagram of the AOT/SDS-buffer-decane pseudoternary system was determined.The effects of the surfactant concentration(?).the oil-to-water ratio(a)and the temperature(T)on the conductivity of the resulting PANI and the underlying mechanism were investigated using UV-vis-NIR,ESR and small-angle X-ray scattering(SAXS)techniques.Results show that the bicontinuous microemulsion plays a template role in the PANI biosynthesis,and the PANI conductivity also depends on the activity and stability of the solubilized HRP,which are determined by the microdomain size of the microemulsions and the reaction temperature.As far as ?is concerned,it is found that as ? increases,the microdomain size of the bicontinuous microemulsion as well as the activity of the solubilized HRP decreases significantly,leading to a great decrease of the PANI conductivity.Compared with ?,the change of a has small effect on the PANI conductivity.As the a-dependent microdomain size changes little with increasing a.the small change of the PANI conductivity should be ascribed mainly to the HRP activity.For T,over the range of 20??35?,it has a slight effect on the microdomain size of the bicontinuous microemulsion,but it has a marked effect on the enzymatic properties of HRP.As the temperature increases,the activity of HRP increases,but its thermal stability decreases greatly.Therefore,the thermal stability of HRP is the major factor causing that the PANI conductivity decreases significantly with increasing T.The present work deepens the understanding of the template effect of anionic surfactant aggregates in the biosynthesis of conducting PANI.2.Improvement of enzymatic synthesis of conducting polyaniline in anionic surfactant AOT stabilized bicontinuous microemulsion by adding zwitterionic surfactant SB-12HRP mediated synthesis of PANI was investigated in detail in the bicontinuous microemulsion stabilized by AOT and dodecyl sulfobetaine(SB-12).The addition of zwitterionic surfactant SB-12 not only reduces the phase inversion temperature of the anionic surfactant AOT stabilized rnicroemulsion.but also enhances the catalytic performance of the solubilized HRP.Compared with the previously reported AOT/SDS stabilized bicontinuous microemulsion,the present AOT/SB-12 stabilized bicontinuous microemulsion is much more suitable for the expression of the catalytic activity of HRP;moreover,the conductivity of the resulting PANI is improved greatly(a 15-fold increase in conductivity).The present results indicate further that the catalytic activity of HRP and its stability are crucial for the biosynthesis of PANI in bicontinuous microemulsions.To understand the underlying mechanism of the medium effects,ESR,UV-vis-NIR and SAXS spectroscopies were used to investigate the effects of the oil-to-water ratio(a)and the surfactant concentration(?)of the present bicontinuous microemulsion on the conductivity of resulting PANI.Results show that the bicontinuous microemulsion stabilized by both AOT and SB-12 still has a template role in the biosynthesis of PANI and the PANI conductivity is mainly determined by the microstructure of the template and by the microstructure-dependent activity and stability of the solubilized HRP.Lower a and y values are beneficial to the enzymatic synthesis of high-quality PANI.The present study is of great help to develop new strategies for the biosynthesis of high quality PANI.