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[Display omitted] • We synthesized carbon dots with quantum yields reaching 97.1%. • An inner filter effect immunosensor was setup for aflatoxin M 1 residue analysis. • Our immunosensor detection limit was 0.0186 ng/mL for aflatoxin M 1 in milk. • This immunosensor format is flexible for other chemical residue analysis. Here, a facile hydrothermal method was used to synthesize highly photoluminescent N -doped carbon dots, and the quantum yields reached 97.1%. Then, a label-free immunosensor based on the inner filter effect of carbon dots was developed for ultrasensitive detection of aflatoxin M 1 residues in milk. The detection limit was 0.0186 ng/mL (equivalents to 18.10 ng/kg), which satisfied the most stringent maximum tolerable limit value of 25 ng/kg. Besides, the immunosensor showed a good linear relationship from 0.003 ng/mL to 0.81 ng/mL, and the average recoveries ranged from 79.6% to 112.5% for spiked milk samples, with relative standard deviations ranging from 6.7% to 13.3%. Compared with other immunoassays, the inner filter effect-based immunosensor incorporating fluorescent detection into conventional enzymatic cascade amplification systems and could be a reliable on-site screening method for aflatoxin M 1 residue analysis. [ABSTRACT FROM AUTHOR]