Sensitive Detection of Organophosphorus Pesticides Using a Needle Type Amperometric Acetylcholinesterase-based Bioelectrode. Thiocholine Electrochemistry and Immobilised Enzyme Inhibition

An acetylcholinesterase (AChE) based amperometric bioelectrode for a selective detection of low concentrations of organophosphorus pesticides has been developed. The amperometric needle type bioelectrode consists of a bare cavity in a PTFE isolated Pt-Ir wire, where the AChE was entrapped into a photopolymerised polymer of polyvinyl alcohol bearing styrylpyridinium groups (PVA-SbQ). Cyclic voltammetry, performed at Pt and AChE/Pt disk electrodes, confirmed the irreversible, monoelectronic thiocholine oxidation process and showed that a working potential of +0.410 V vs. Ag/AgCl, KCl sat was suitable for a selective and sensitive amperometric detection of thiocholine. The acetylthiocholine detection under enzyme kinetic control was found in the range of 0.01-0.3 U cm −2 of immobilised AChE. The detection limit, calculated for an inhibition ratio of 10%, was found to reach 5 μM for dipterex and 0.4 μM for paraoxon. A kinetic analysis of the AChE-pesticide interaction process using Hanes-Woolf or Lineweaver-Burk linearisations and secondary plots allowed identification of the immobilised enzyme inhibition process as a mixed one (non/uncompetitive) for both dipterex and paraoxon. The deviation from classical Michaelis Menten kinetics induced from the studied pesticides was evaluated using Hill plots.