Characterization of an electro-synthesized methoxylated polypyrrole film used as permselective barrier in amperometric biosensors by X-ray photoelectron and Fourier transform infrared spectroscopy

Poly(pyrrole) films overoxidized in NaOH/methanolic solutions and used as anti-interferent barrier in a bilayer based amperometric biosensors have been characterized by X-ray photoelectron (XPS) and Fourier transform infrared (FT-IR) reflectance spectroscopy. The two techniques were complementary from the point of view of the analyzed region of the film: XPS sampled the outermost layers (ca. 100 Angstrom) while FT-IR, even able to sample the whole film, gave a response arising mainly from the innermost layers near the electrode surface. The top layers of such a film was found to exhibit some of the functionalities (mainly hydroxylic) present in permselective overoxidized poly(pyrrole) (PPYox) prepared from aqueous phosphate buffer. Methoxy groups were found only in the bulk of the film. Since methoxylation and hydroxylation occurred mainly in the beta-position to pyrrole nitrogen, the double bonds conjugation was not extensively affected and the film maintained a remarkable electronic conductivity (sigma(DC)=27.4 S/cm). Moreover, based on FT-IR evidences, a beta-multimethoxylation that would destroy conjugation of pyrrole chains could be excluded. The nature of the electrode material was crucial: the methoxylation reaction occurred only on platinum and not on glassy carbon or graphite. These findings suggested that the functionalization started at the electrode-polymer interface, probably due to methoxy species produced near the electrode surface as intermediate of methanol oxidation. (C) 1999 Elsevier Science B.V. All rights reserved.