The nitrogen–sulfur co-doped carbon dots (CDs) were synthesized using scallion leaves via an environmentally friendly and facile hydrothermal method, which did not require surface passivating agents. When the hemin concentration was increased from 0.5 to 10.0 μM, the fluorescence emission intensity of the CDs was also linearly quenched. The detection limit was estimated to be 0.1 μM. The fluorescence sensing mechanism was systematically explored by a series of characterization methods. The

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... ts revealed that the fluorescence sensing quenching mechanism was ascribed to the inner filter effect and photo-induced electron transfer. The proposed mechanism was further verified by our theoretical calculations. The proposed sensor was successfully applied for the determination of hemin in human serum samples.