Disposable screen-printed electrodes for the immunochemical detection of polychlorinated biphenyls

Three different electrochemical approaches for the detection of cr-naphthol (amperometry, chronoamperometry and differential pulse voltammetry), using screen-printed carbon electrodes, were compared with respect to measurable range, detection limit, reproducibility, sample throughput and suitability for in field use. The highest sensitivity (252 nA/mu M) with a calculated detection limit of 0.1 mu M, combined with a remarkable 'time for sample measurement' (5.5 s) was achieved with differential pulse voltammetry. The standard error over the whole measured range (0.5-100 mu M) was below 5%. The scan speed and the pulse amplitude of the differential pulse voltammetric method were optimised using a simplex method. The optimised electrochemical procedure was then used to evaluate the detection of low levels of alkaline phosphatase: this enzyme catalyses the hydrolysis of alpha-naphthol phosphate to alpha-naphthol. Using 2 min as incubation time for the enzyme reaction, a linear calibration curve in the range 1-10(-5) U/ml was obtained with a calculated detection limit of 2.1 x 10(-6)U/ml. The standard error for the 10(-2)U/ml enzyme standard was 6.8%. Finally a competitive enzyme immunoassay for polychlorinated biphenyls based on alkaline phosphatase was performed using the optimised electrochemical detection of alpha-naphthol. The assay measurable range was 0.01-10 mu g/ml, and the detection limit was 0.01 mu g/ml (B-x/B-o%=10).