Old silver mirror in qualitative analysis with new shoots in quantification: Nitrogen-doped carbon dots (N-CDs) as fluorescent probes for “off-on” sensing of formalin in food samples

A novel fluorometric assay for selective and sensitive determination of formalin (FA) was developed based on nitrogen-doped carbon dots (N-CDs) coupled with silver mirror reaction. N-CDs was synthesized using the hydrothermal method with the ethylene glycol and ammonia solution as carbon and nitrogen precursors, respectively. The detection principle was based on "off-on" fluorescence switching. Specifically, the fluorescence signal of N-CDs was first turned off after incorporating the Ag+ and Tollens' reagents. Then, in the presence of FA, the Ag+ species on the N-CDs surface were reduced to Ag0 species and the fluorescence signal of N-CDs was switched back on. The fluorescence intensity due to the N-CDs signal linearly increased with the increasing FA concentrations in the range of 5-100 mg L-1, with the detection limit of 1.5 mg L-1. The proposed approach provides rapid, simple, sensitive, and selective detection of FA in various food samples.