Immunosensor for the determination of okadaic acid based on screen-printed electrode

A disposable immunosensor for okadaic acid (OA), using a screen-printed electrode (SPE), was developed and characterised. Detection of the product, p-aminophenol, resulting from the reaction catalysed by alkaline phosphatase (AP), was carried out using an amperometric three-electrode system poised at a voltage of + 300 mV versus Ag/AgCl. Alkaline phosphatase was used as a label for the antigen, OA, and two kinds of alkaline phosphatase preparation were studied for the conjugation of okadaic acid. The calibration curve for okadaic acid obtained from the conjugate created from low-activity AP, 969 units/mg, was unsatisfactory in terms of sensitivity, but a high-activity conjugate delivered the required sensitivity and limit of detection. Studies on the stability of the sensor with alpha-OA antibody and OA-AP conjugate showed that the current response decreased drastically after one day. Stabilisation strategies have been formulated to overcome this problem. The calibration curve obtained with the high activity conjugate was linear up to 40 ng/ml of okadaic acid with a minimum concentration of analyte detected of 5 ng/ml and a detection limit of 2 ng/ml.