Amperometric Detection of Parathion and Methyl Parathion with a Microhole-ITIES

Abstract

An amperometric sensor featuring a microhole-liquid/gel interface for the detection of both parathion and methyl parathion is developed on the basis of their different kinetics behavior when interacting with the enzyme organophosphorus hydrolase (OPH). OPH hydrolyzes parathion and methyl parathion producing a common product of para-nitrophenol and either diethylthio- or dimethylthio- phosphoric acid, respectively, of which all can release protons depending upon their pKa values. The detection method for both organophosphate (OP) compounds is designed to measure the current associated with the transfer of protons released from the products of OPH hydrolysis across a polarized microhole-water/polyvinylchloride-nitrophenyloctylether (PVC-NOPE) gel interface. The selective transfer of protons across the interface is tailored by the use of a proton selective ligand, ETH 1778, in the gel layer. A disposable proton selective sensor that can quantitatively analyze the OP compounds is also fabricated using simple polydimethylsiloxane microfabrication. Cyclic voltammetry and differential pulse stripping voltammetry are first utilized to characterize the transfer of protons across the microhole-water/PVC-NPOE gel interface initiated by the OPH reaction with parathion and methyl parathion and to establish a detection limit for each OP compound. In order to sequentially detect parathion and methyl parathion using a single proton selective strip-sensor, a novel time-resolved detection methodology is developed based on the different catalytic kinetics of OPH with each OP analyte; the maximum peak current for the preconcentrated protons transferring back from the organic to water phase assisted by ETH 1778 increases proportionally to the concentration of each OP agent. Since the maximum peak currents for both OP analytes are observed at different reaction times it was possible to demonstrate the multiplexed analysis of both parathion and methyl parathion down to 0.5 mu M using a single sensor.