Rapid and Sensitive Electrochemical Detection of Carbaryl Based on Enzyme Inhibition and Thiocholine Oxidation Mediated by a Ruthenium(III) Complex

In the electrochemical detection method for pesticides that measures their inhibitory effects on acetylcholinesterases (AChEs), the direct electrooxidation of the enzyme product (thiocholine, SCh) is slow at conventional electrodes. To overcome this limitation, an electron mediator is required to lower the applied potential and facilitate the transfer of electrons between the enzyme product and electrode. In this study, [Ru(NH3)5py]3+ is introduced as an electron mediator in inhibition-based pesticide detection. To obtain a better signal-to-background ratio, [Ru(NH3)5py]3+, which undergoes a fast outer-sphere reaction, is combined with low-electrocatalytic indium-tin-oxide (ITO) electrodes at which many interfering species undergo slow redox reactions. AChE is immobilized onto an avidin-modified ITO electrode via the direct adsorption of avidin onto ITO followed by the biospecific binding of biotinylated AChE to the avidin. SCh is generated from acetylthiocholine by AChE. Subsequently, SCh converts [Ru(NH3)5py]3+ to [Ru(NH3)5py]2+, which is then oxidized at the ITO electrode. This procedure allows the sensitive detection of carbaryl at a low applied potential of 0.15 V vs Ag/AgCl. The calculated detection limit for carbaryl is approximately 0.3 pM. This simple and sensitive pesticide sensor is thus very promising and should be extendable to the onsite environmental monitoring of other pesticides.