1：Experimental Design and Method Selection:

A one-pot hydrothermal method was used for the synthesis of N, S-CQDs using L-cysteine as the single precursor.

2：Sample Selection and Data Sources:

L-cysteine was purchased from Aladdin Ltd. (Shanghai, China). All solutions were prepared using Milli-Q deionized water.

3：List of Experimental Equipment and Materials:

Stainless steel autoclave with a 50 mL Teflon liner, Tecnai-G2 F30 transmission electron microscope, Rigaku D/Max-2400 X-ray diffractometer, Thermo Nicolet Nexus FTIR model 670 spectrometer, ESCALAB 250xi photoelectron spectrometer, TU-1901 dual beam UV-Vis spectrophotometer, FLs920 steadystate/transientstate spectro xsort.

4：Experimental Procedures and Operational Workflow:

L-cysteine was dissolved in deionized water, transferred to an autoclave, heated at 180 °C for 12 h, centrifuged, and dialyzed to obtain N, S-CQDs.

5：Data Analysis Methods:

PL spectra were analyzed for Hg2+ ion detection, and the Stern-Volmer equation was used to analyze fluorescence quenching data.