Enzymatic Synthesis of Highly Electroactive Oligoanilines from a p-Aminodiphenylamine/Aniline Mixture with Anionic Vesicles as Templates

Abstract

Oligoanilines with characteristic properties of the electrically conductive emeraldine salt form of polyaniline (PANI-ES) are promising molecules for various applications. A mixture of such oligoanilines can be obtained, for example, enzymatically under mild conditions from the linear aniline dimer p-aminodiphenylamine (PADPA) with hydrogen peroxide (H2O2) and low amounts of horseradish peroxidase (HRP) in an aqueous pH = 4.3 suspension of anionic vesicles formed from AOT, the sodium salt of bis(2-ethylhexyl)sulfosuccinate. However, the simultaneous formation of undesired side products containing phenazine-type units or oxygen atoms is unsatisfactory. We have found that this situation can be improved considerably by using a mixture of PADPA and aniline instead of PADPA only but otherwise nearly identical conditions. The PANI-ES-like oligoaniline products that are obtained from the PADPA and aniline mixture were not only found to have much lower contents of phenazine-type units and not contain oxygen atoms but also were shown to be more electroactive in cyclic voltammetry measurements than the PANI-ES-like products obtained from PADPA only. The AOT vesicle suspension remained stable without product precipitation during and after the entire reaction so that it could be analyzed by in situ UV/visible/near-infrared, in situ electron paramagnetic resonance, and in situ Raman spectroscopy measurements. These measurements were complemented with ex situ high-performance liquid chromatography analyses of the deprotonated and reduced products formed from mixtures of PADPA and either fully or partially deuterated aniline. On the basis of the results obtained, a reaction mechanism is proposed for explaining this improved HRP-triggered, vesicle-assisted synthesis of electroactive PANI-ES-like products. The oligomeric products obtained can be further used, without additional special workup, for example, to coat electrodes for their possible application in biosensor devices.