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-..
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