Double-decrease of the fluorescence of CdSe/ZnS quantum dots for the detection of zinc(II) dimethyldithiocarbamate (ziram) based on its interaction with gold nanoparticles

A double-decrease strategy is described for ultrasensitive determination of the fungicide and vulcanization additive ziram. The assay principle is inspired by the interaction of ziram with gold nanoparticles (AuNPs). In this process, zinc ions are released, and ziram adsorption induces the aggregation of the AuNPs. The aggregated AuNPs decrease the intensity of the fluorescence of CdSe/ZnS quantum dots (QDs) capped with 3-mercaptopropionic acid via an inner filter effect. This is a result of the overlap between the absorption band of aggregated AuNPs (peaking at 680 nm) and the yellow emission of QDs (peaking at 608 nm). Zinc also exerts another decrease effect on the fluorescence of the CdSe/ZnS QDs, probably via a static quenching mechanism. Based on this double-decrease effect, ultrahigh sensitivity is achieved for ziram. The fluorescence response of the QDs (Ex / Em = 380/608 nm) is immediate. The relative fluorescence intensity is proportional to the ziram concentration within a wide range of 5 nM to 4 μM in two consecutive linear ranges. The limit of detection is as low as ~2 nM (signal-to-noise ratio of 3), which is much lower than the maximum residue limit defined by the EU pesticide database. It is also found that a similarly high sensitivity is obtained for another fungicide ferbam.