Sulfur and nitrogen co-doped carbon dots (NSCDs) were quickly synthesized by the microwave-assisted method from triammonium citrate and thiourea. NSCDs showed a quantum yield of 11.5% with excitation and emission bands at 355 and 432 nm, respectively. Also, a fluorescence quenching was observed in the presence of Pb(II) ions, and the as-synthesized CDs were used as a sensitive probe for detecting Pb(II) in water and food samples. The results showed the optimal conditions for Pb(II) determination were CDs concentration of 0.02 mg mL− 1 at pH 6.0–7.0 and an incubation time of 20 min. The relative fluorescence intensity of NSCDs was proportional to Pb(II) concentrations in the range of 0.029–2.40 and 2.40–14.4 μmol L− 1 with a correlation coefficient (R2) of 0.998 and 0.955, respectively, and a detection limit of 9.2 × 10– 3 μmol L− 1. Responses were highly repeatable, with a standard deviation below 3.5%. The suggested method demonstrates the potential of a green, fast, and low-cost approach for Pb(II) determination in water, tea, and rice samples with satisfying results.

N, S co-doped fluorescent carbon dots synthesized by microwave irradiation: a sensitive probe for Pb (II) ions detection in food samples
    Abstract
    1 Introduction
    2 Materials and methods
        2.1 Instrumentation
        2.2 Chemicals and reagents
        2.3 Preparation of carbon dots
        2.4 Quantum yield calculation
        2.5 Fluorescence measurement of Pb(II) ions
        2.6 Effects of coexisting ions
        2.7 Treatment of real samples
    3 Results and discussion
        3.1 Synthesis and characterization of CDs
        3.2 Fluorogenic detection of Pb(II)
        3.3 Analytical features
        3.4 Interference study
        3.5 Analysis of food samples
    4 Conclusion
    Anchor 19
    Acknowledgements 
    References

• Elias Aboobakri(Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, P.O. Box: 9177948974, Mashhad, Iran)
• Tahereh Heidari(Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, P.O. Box: 9177948974, Mashhad, Iran)
• Moslem Jahani(Department of Food Chemistry, Research Institute of Food Science and Technology (RIFST), Mashhad, Iran)