Horseradish peroxidase-catalyzed oxidative polymerization of aniline in bicontinuous microemulsion stabilized by AOT/SDS

Abstract

A 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-γ fishlike phase diagram of the AOT/SDS-buffer-decane pseudoternary system was determined. The effects of the surfactant concentration (γ), the oil-to-water ratio (α) 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 γ is concerned, it is found that as γ 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 γ, the change of α has small effect on the PANI conductivity. As the α-dependent microdomain size changes little with increasing α, the small change of the PANI conductivity should be ascribed mainly to the HRP activity. For T, over the range of 20 °C–35 °C, 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. To the best of our knowledge, this work is the first study of the use of bicontinuous microemulsions as reaction media for the enzymatic synthesis of a conducting polymer.