1：Experimental Design and Method Selection:

The study utilized a fluorescence sensor based on N-GQDs for thiourea detection, with Hg2t acting as a fluorescence switch. The mechanism involves the quenching of N-GQD fluorescence by Hg2t and its restoration upon thiourea addition due to the strong binding between thiourea and Hg2t.

2：2t. Sample Selection and Data Sources:

2. Sample Selection and Data Sources: The samples included tap water, lake water, orange juice, and lemon juice, which were prepared and diluted to remove interfering substances.

3：List of Experimental Equipment and Materials:

Instruments used included a transmission electron microscope (Tecnai G2 F20), luminescence spectrometer (LS-55), X-ray diffraction spectrometer (Ultima IV), FT-IR spectrometer (Nicolet 380), XPS (ESCALAB 250Xi), and Raman spectrometer (Renishaw). Materials included citric acid, urea, mercury dichloride, thiourea, and various other chemicals.

4：Experimental Procedures and Operational Workflow:

N-GQDs were synthesized hydrothermally. The fluorescence quenching by Hg2t and recovery by thiourea were measured under optimized conditions (pH 7.0, Hg2t concentration of 3.5 mM).

5：0, Hg2t concentration of 5 mM). Data Analysis Methods:

5. Data Analysis Methods: The fluorescence intensity changes were analyzed to determine thiourea concentrations, with detection limits and linear ranges calculated.