Vesicles as soft templates for the enzymatic polymerization of aniline.

The feasibility of using surfactant vesicles as soft templates for the peroxidase-triggered polymerization of aniline was investigated. It was found that mixed anionic vesicles (diameter approximately 80 nm) composed of sodium dodecylbenzenesulfonate (SDBS) and decanoic acid (1:1, molar ratio) are promising templates. In the presence of the vesicles and horseradish peroxidase/hydrogen peroxide (H2O2) as initiator system, aniline polymerizes under optimized conditions at pH=4.3 to the desired conductive emeraldine form of polyaniline (PANI). The optimal polymerization conditions were elaborated, and some of the chemical and physicochemical aspects of the reaction system were investigated. After addition of aniline and peroxidase to the vesicles, aniline is only loosely associated with the vesicles, as shown by NOESY-NMR and zeta potential measurements. In contrast, the peroxidase strongly binds to the vesicle surface, as shown by fluorescence measurements using TNS (2-(p-toluidino)naphthalene-6-sulfonate) as vesicle membrane probe. This binding of the enzyme to the vesicle surface indicates that the polymerization reaction is initiated predominantly on the surface of the vesicles. Cryo-transmission electron microscopy indicates that the polymerization product remains associated with the vesicles on their surface. For short reaction times (30 s<t<60 s), it is shown that oligoanilines containing an excess of oxidized units are obtained, as shown by VIS/NIR spectroscopy and MALDI-TOF mass spectrometry. For longer reaction times (1 min<t<30 min), the relative amount of over oxidized units in PANI decreases until polymers are obtained which have a VIS/NIR spectrum that is typical for the emeraldine salt form of PANI (lambdamax approximately 1000 nm). The appearance of stable unpaired electrons during the reaction was demonstrated by EPR measurements, in full support of the in situ formation of the conductive emeraldine salt form of PANI. At the end of the reaction (after 1 h), the PANI formed remains homogenously dispersed in the aqueous solution thanks to the presence of the vesicles. No precipitation occurs on a time scale of at least several weeks. FTIR and 13C NMR measurements of the product isolated from the reaction mixture confirm the formation of the emeraldine form of PANI. If the polymerization reaction is carried out in the absence of vesicles but under otherwise identical reaction conditions, the outcome of the reaction is very different, i.e., no indication at all for the formation of the conductive form of PANI.