Development of an amperometric sulfite biosensor based on sulfite oxidase with cytochrome c, as electron acceptor, and a screen-printed transducer

An amperometric biosensor for sulfite has been developed. The enzyme sulfite oxidase (SOD) and electron acceptor cytochrome c are mixed into the carbon ink that is deposited onto the working electrode of a screen-printed strip. A silver–silver chloride electrode is printed alongside the working electrode and serves as reference/counter electrode. The electrochemical behaviour of the biosensor surface in plain buffer has been investigated by cyclic voltammetry. In the voltage range −0.5 to +0.5 V, a well-defined anodic peak appeared at −0.15 V and a less well-defined anodic peak at about +0.2 V. In the presence of SO32−, the cyclic voltammogram obtained with the biosensor exhibited an increase in magnitude of the more positive peak; this was considered to result from the electrocatalytic oxidation of SO32− involving SOD and the heme (Fe2+/Fe3+) centre of cytochrome c. Amperometry in stirred solution was used to construct a hydrodynamic voltammogram for SO32− using the biosensor; this exhibited a single wave with a plateau beginning at +0.3 V. This wave corresponds to the electrocatalytic response observed by cyclic voltammetry. The pH and concentration of buffer components have been optimised for the determination of SO32− by amperometry in stirred solution. Using these conditions, a detection limit of 4 ppm was obtained. The stability of the biosensors was examined after storage in 0.05 M phosphate buffer pH 7.4 at 4°C; it was found that the initial response was retained for at least 45 days. The proposed biosensors were evaluated on samples of unspiked and spiked estuarine, river and tap waters. The recovery and precision data indicated that the devices could be expected to give reliable data in these waters.