Nonenzymatic Electrochemical Superoxide Sensor

The electrochemistry of nitroblue tetrazolium chloride (NBTC) was studied in aqueous solutions of pH 6.97 on a glassy carbon macroelectrode and at a carbon fibre microelectrode; values of near unity for the reduction transfer coefficient and 7.2 x 10(-6) cm(2) s(-1) for the diffusion coefficient were generated. The electrochemically produced diformazan was shown to adsorb on the glassy carbon surface if the potential was held at -0.35 V [vs. saturated calomel electrode (SCE)]. A carbon paste electrode, fabricated by using dioctyl phthalate and graphite powder, was used as a nonenzymatic sensor. The sensitivity of the diformazan oxidation signal to the presence of superoxide was exploited to detect superoxide, which is voltammetrically visible at about +0.69 V (vs. SCE). The paste electrode was first immersed in aqueous superoxide solutions. It was subsequently equilibrated with NBTC by immersing it into aqueous NBTC solutions. The reduction of NBTC (by superoxide) thus took place in the paste, which allowed quantification of the superoxide in the aqueous phase by means of the diformazan oxidation signal. Values for the practical limit of detection and the sensor sensitivity, 0.059 nm and 1.79 mu A nm(-1) respectively, were obtained.