DETECTION OF ORGANOPHOSPHATE AND CARBAMATE PESTICIDES USING DISPOSABLE BIOSENSORS BASED ON CHEMICALLY MODIFIED ELECTRODES AND IMMOBILIZED CHOLINESTERASE

An amperometric biosensor for the detection of organophosphate and carbamate pesticides was constructed as a disposable strip containing a cobalt phthalocyanine-modified carbon composite electrode and a cross-linked cholinesterase layer. With butyrylthiocholine as substrate, enzymatically produced thiocholine was oxidized at + 250 mV. The steady-state current, I(ss), was used as a measure of the enzyme activity. In the presence of pesticides, an irreversible inhibition of cholinesterase occurred, resulting in a decrease in the rate of current change, dI/dt. The ratio (dI/dt)/I(ss) was used for evaluations. The influence of the cholinesterase loading and the cholinesterase to glutaraldehyde ratio on the biosensor response was studied and the measuring conditions (pH, temperature, substrate concentration) were optimized. Detection limits of 0.30, 1.2 and 11 nmol 1-1 for paraoxon, dichlorvos and carbaryl, respectively, were achieved. The time of inhibition varied from a few seconds (high pesticide concentrations) to 6 min required for reliable measurements at levels close to the detection limit. When the analysis was performed by 10 min preincubation of the biosensor or free cholinesterase with sample, paraoxon concentrations above 3.0 nmol l-1 could be detected.