Manganese-doped ZnS quantum dots as a phosphorescent probe for use in the bi-enzymatic determination of organophosphorus pesticides

We present a sensitive and selective method for the determination of organophosphorus pesticides (OPs) based on the inhibition of the enzyme acetylcholinesterase (AChE). It is making use of quantum dots QDs of the type Mn: ZnS that display long-lived phosphorescence emission and act as optical probes for hydrogen peroxide (H2O2). In this assay, acetylcholine (ACh) is first hydrolyzed by AChE, and the enzyme choline oxidase (ChOx) further oxidizes choline under the formation of H2O2 which quenches the phosphorescence of the QDs. If, however, OPs are added to the solution, the rate of enzymatic hydrolysis by AChE is retarded. This reduces the rate of production of H2O2 and results in a reduced quenching efficiency. The slow decay time of the phosphorescence of the QDs also allows time-resolved luminescence intensity to be measured. This can eliminate background fluorescence from the sample and therefore improves analytical accuracy and the signal-to-noise ratio. Under optimized conditions, there is a linear relationship between luminescence intensity and the concentration of paraoxon in the 1 pM to 1 mu M range, with an similar to 0.1 pM limit of detection which is much lower than that of most existing methods. The phosphorescent probe was applied to determine OPs in spiked real samples.