Organophosphorus Hydrolase-Based Amperometric Sensor: Modulation of Sensitivity and Substrate Selectivity

The detection of organophosphate (OP) insecticides with nitrophenyl substituents is reported using an enzyme electrode composed of organophosphorus hydrolase (OPH) and albumin co-immobilized to a nylon net and attached to a carbon paste electrode. The mechanism for this biosensor involves catalysis of the OP followed by the electrochemical detection of the product, p-nitrophenol. The application of small amounts of mineral oil or silicon oil to the surface of the immobilized enzyme increased the stability and response signals of these sensors. In the case of silicon oil, the response for parathion was five times higher than without oil. In addition, the application of either mineral or silicon oil to the enzyme-coated membranes increased the relative selectivity of the sensor for parathion over paraoxon, most likely reflecting the partition of the substrates between the aqueous and oil phases. The enzyme activity was stabilized for more than two months by the application of either oil to the electrode surface. Compounds evaluated using this technique included parathion, EPN, fenitrothion, and paraoxon. The detection limits for parathion and paraoxon are 15 nM and 20 nM, respectively. These detection limits are substantially lower than those reported for optical assays (e.g., μM range) and lower than those reported using several OPH electrodes (e.g., high nM range). Several characteristics that make this enzyme electrode attractive for further development as a potential field analytical method include; its simplicity and stability, as well as the relatively small amount of enzyme required (i.e., 3 International Units).