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 Ag 0 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., (Copyright © 2021 Elsevier B.V. All rights reserved.)