Abstract: 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 behavior 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 optimized 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.

Abstract: An electrochemical biosensor for cows milk progesterone has been developed and used in a competitive immunoassay under thin-layer, continuous-flow conditions. Single-use biosensors were fabricated by depositing anti-progesterone monoclonal antibody (mAb) onto screen-printed carbon electrodes (SPCEs). Three operational steps could be identified: (1) Competitive binding of sample/conjugate (alkaline-phosphatase-labelled progesterone, AP-prog) mixture, (2) establishment of a steady-state amperometric baseline current and (3), measurement of an amperometric signal in the presence of enzyme substrate (1-naphthyl phosphate, 1-NP). In the thin-layer cell, the enzyme product, 1-naphthol, showed electrochemical behavior consistent with bulk conditions and gave a linear amperometric response under continuous-flow conditions (Eapp=+0.3 V vs. Ag/AgCl) over the range 0.1-1.0 µg/ml. After pre-incubating biosensors with progesterone standards, signal generation within the cell (substrate CONCENTRATION=5 mM) was recorded amperometrically as rate (nA/s) or maximum current (imax, nA). Response values for milk standards were approximately 50% of those prepared in buffer. In both cases, calibration plots over the range 0-50 ng/ml progesterone were obtained. By conducting sample binding under flowing conditions, only 7% of the previous response was obtained, even at a substrate concentration of 50 mM, resulting in low signal:noise ratio. Using a stop-flow arrangement (i.e. quiescent sample binding, followed by continuous flow), low-noise amperograms were obtained at [1-NP]=5 mM. Calibration plots were obtained over the range 0-25 ng/ml, with a coefficient of variation of 12.5% for five replicate real milk samples.