Optimisation of a reagentless laccase electrode for the detection of the inhibitor azide

Here we report on the optimisation of a reagentless enzyme sensor for the detection of azide based on the mediated reduction of O₂ by a laccase enzyme co-immobilised in a redox hydrogel on electrode surfaces. The sensor response is shown to be influenced by the enzyme loading, the electrolyte pH and ionic strength. The response of the sensor is stable, decreasing by only 25% over a sixteen-hour period. Reproducible inhibition curves for the determination of azide levels from cyclic voltammetric scans can be obtained by normalisation of the sensor response. The resulting enzyme inhibition biosensor can detect levels of azide as low as 2.5 μM under these conditions. Constant potential amperometric detection at the laccase enzyme electrode in a flow injection set-up yields a peak current for inhibition of the mediated reduction of O₂. Reproducible peak currents and areas (8.0 and 6.3% RSD, respectively, for n = 11) are obtained for repeated injections of 100 μM azide. Reproducible response curves can be obtained by injection of a 25 mM azide sample and assuming that the peak height and peak area obtained represent 100% inhibition of the enzyme.