Electrochemical sensor for superoxide anion radical using polymeric iron porphyrin complexes containing axial 1-methylimidazole ligand as cytochrome c mimics

A needle-type electrochemical sensor for the facile detection of superoxide anion radical (O₂^-.) was devised using an electrodeposited film of a polymeric porphyrin complex attached to a carbon microelectrode which was placed in a stainless steel 18G needle tube as an auxiliary electrode. The sensing element was prepared by means of electropolymerization of bromoiron(III) mesotetra (3-thienyl)porphyrin in the presence of 1-methylimidazole in CH₂Cl₂ containing 100 mM tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte, which gave a smooth film of the corresponding polymer. The film was electrochemically active to give a redox response near -0.05 V versus Ag/ AgCl due to the iron(II /III) couple. The microsensor was applied to detect O₂ ^-. produced by xanthine oxidation catalyzed by xanthine oxidase. The amperometric response for O₂^-. was monitored at an electrode potential of 0.5 V versus the auxiliary electrode in a 10 mM phosphate buffer. The microsensor displayed a high catalytic activity for the oxidation of O₂^-. and showed a linear relationship between the current and the O₂ ^-. concentration. Axial coordination of an imidazole ligand to the iron porphyrin center enhanced selectivity for O₂^-. by impeding the undesired coordination of H₂O₂ that resulted from the dismutation of O₂^-..