Off-on-off detection of the activity of acetylcholine esterase and its inhibitors using MoOx quantum dots as a photoluminescent probe

The authors describe a fluorometric off-on-off method for the determination of the activity of acetylcholine esterase (AChE). Molybdenum oxide quantum dots (QDs) are used as a fluorescent probe having excitation/emission peaks at 405/525 nm. It is found that the photoluminescence of such QDs is quenched by Cu(II) ion but that it can be restored by acetylcholine which is formed by AChE-catalyzed hydrolysis of acetylthiocholine. The effect is due to the strong affinity between Cu(II) and the thiol group of acetylthiocholine. Hence, quenching is increasingly reversed with increasing concentrations of AChE. However, fluorescence is not restored if the activity of AChE is inhibited by an inhibitor. Under optimal conditions, a linear relationship is founded in the 0.05 to 15 mU.mL(-1) AChE activity range, with a limit of detection of 35 mu U.mL(-1) (applying the 3 sigma/k criterion). Three organophosphate pesticides and propazine were used to validate the assay. All showed strong inhibition, with IC50 values (the concentration required for 50% inhibition to occur) to be 52, 221, 48 and 9 nM for diazinon, chlorpyriphos, monocrotophos and propazine. Benefitting from its sensitivity, specificity and simplicity, the assay in our perception provides a valuable tool for detection of AChE activity and in screening for its inhibitors.