Construction of a nitrogen-doped carbon quantum dot-silver nanoparticle composite fluorescence sensing system for the highly selective detection of thiram in soil and pears.

The widespread application of thiram poses potential threats to the ecological environment and human health. Existing detection methods rely on large-scale instruments and make it difficult to meet the requirements for rapid on-site detection. Therefore, in this study, nitrogen-doped carbon quantum dots (N-CQDs) were prepared by a one-step hydrothermal method using citric acid and melamine; the average particle size was 4.6 nm, and they exhibited good stability in aqueous solution. At the same time, silver nanoparticles (AgNPs) were synthesized by reducing nitric acid and sodium borohydride, with an average particle size of 7.6 nm. A fluorescence sensing system of N-CQDs/AgNPs was constructed based on the internal filter effect (IFE) for the highly selective detection of thiram. AgNPs can effectively quench the fluorescence of N-CQDs through the IFE; when thiram is present, the absorption spectrum of AgNPs shifts to the red, and the overlapping area between the fluorescence emission spectrum of N-CQDs and that of AgNPs increases, resulting in further fluorescence quenching. This system shows a good linear relationship in the concentration range of 0.007-5 µg mL-1 (R 2 = 0.99298), and the detection limit is as low as 0.002 µg mL-1. This method is highly sensitive, selective, simple to operate, and capable of rapid detection, providing a new strategy for the on-site rapid detection of thiram residues in agricultural products and environmental samples.