The response of screen-printed enzyme electrodes containing cholinesterases to organo-phosphates in solution and from commercial formulations

Electrodes containing a layer of cobalt phthalocyanine, mixed with graphite ink, and a layer of acetyl-or butyryl cholinesterase were constructed by screen-printing, except for a thin layer of polyurethane applied by an airbrush. When the electrodes were made the anodes of electrochemical cells, the generation of current by the addition of enzyme substrate was inhibited by standard solutions of organo-phosphate pesticides. Cholinesterase activity was inhibited by paraoxon and dichlorvos concentrations as low as 10(-8) M. The relationship between the logarithm of concentration and the inhibition quotient (ratio between initial current and the rate of change of the inhibited current) was curvilinear. Malathion only caused an inhibition of current generated by electrodes containing acetyl cholinesterase. Inhibition by malathion required a period of pre-incubation in the pesticide before addition of enzyme substrate, and was evident down to, and including, a concentration of 10(-7) M. The addition of diluted commercial formulations of some organo-phosphate pesticides caused rapid inhibition of electrode function. Nearly linear relationships were observed between the logarithm of the degree of dilution of commercial formulations containing P=O groups and the natural logarithms of the inhibition quotients. Inhibition by formulations containing malathion did not require pre-incubation of the electrodes in the mixture (only electrodes containing acetyl cholinesterase were tested). The relationship between the logarithm of the degree of dilution and the natural logarithm of the inhibition quotient was curvilinear. The responses of the screen-printed electrodes to commercial formulations of pesticides indicate that they may be a suitable platform for further development of sensors capable of detecting the presence of these pesticides in the environment. (C) 1997 Elsevier Science Limited.