1：Experimental Design and Method Selection:

The study employed a one-step solvothermal method to synthesize carbon dots using sodium citrate, carbamide, and anhydrous calcium chloride as precursors in toluene solvent. The method was designed to achieve homogeneous carbon dots with high crystallinity and yellow fluorescence without complex separation processes. Control experiments were conducted by varying the addition of anhydrous calcium chloride and changing the solvent to water to investigate the formation mechanism.

2：Sample Selection and Data Sources:

Precursors included sodium citrate, carbamide, and anhydrous calcium chloride from Sinopharm Chemical Reagent Co. Ltd., and toluene as the solvent. Organic solvents for testing (ethyl alcohol, acetonitrile, tetrahydrofuran, dioxane) were chromatographic grade from Sigma-Aldrich. Ultrapure water was prepared using a Millipore-Q water system.

3：List of Experimental Equipment and Materials:

Equipment included Transmission Electron Microscopy (TEM) (JEM-2100F, 200 kV), Atomic Force Microscope (AFM) (Dimension Icon), X-ray Photoelectron Spectroscopy (XPS) (Thermo Fisher Scientific Escalab 250XI), Raman spectrometer (Horiba LabRaman HR Evolution), multi-function fluorescence spectrophotometer (FS5), Teflon-lined stainless steel autoclave, cylindrical filtration membrane filter (0.22 μm), dialysis membranes (Spectra/Por? 7, 132,105), and lyophilizers. Materials were as specified in the sample selection.

4：22 μm), dialysis membranes (Spectra/Por? 7, 132,105), and lyophilizers. Materials were as specified in the sample selection. Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: Synthesis involved mixing precursors in toluene in an autoclave, heating at 200°C for 12 h, followed by purification through filtration and dialysis. For water detection, carbon dots were dispersed in organic solvents, and fluorescence emission spectra were recorded before and after adding water, with intensity changes measured.

5：Data Analysis Methods:

Data analysis included size distribution from TEM and AFM images, crystallinity assessment from Raman spectra (ID/IG ratio), chemical structure analysis from FTIR and XPS, optical properties from fluorescence and UV-Vis spectra, and quantum yield measurement using an integration sphere. Statistical analysis involved linear regression for calibration curves and standard deviation for detection limits.