1：Experimental Design and Method Selection:

The study designed a colorimetric sensor using S-doped carbon dots functionalized gold nanoparticles (S-CDs@Au NPs). The method is based on the aggregation of S-CDs@Au NPs in the presence of dopamine and Fe3+ ions, causing a red shift in the localized surface plasmon resonance (LSPR) peak from 520 nm to 670 nm. The absorbance ratio A670/A520 is used as the analytical signal. Parameters such as reaction time, pH, Au NPs concentration, and Fe3+ concentration were optimized using the univariable method.

2：Sample Selection and Data Sources:

Samples included synthetic dopamine solutions, dopamine ampoule (pharmaceutical), human urine, and human serum. Real samples were spiked with known concentrations of dopamine and pretreated (e.g., dilution for urine, protein removal for serum using acetonitrile and centrifugation).

3：List of Experimental Equipment and Materials:

Equipment: Biowave II diode array spectrophotometer (WPA, Cambridge, UK) with 1.0 cm glass cuvettes for absorbance measurement; Metrohm pH meter (model 780, Switzerland) for pH measurement; Zeiss EM10C transmission electron microscope (Oberkochen, Germany) for TEM analysis; Dynamic light scattering (DLS) particle size analyzer (Malvern, UK) for particle size; Brucker Vector 22 spectrometer (Karlsruhe, Germany) for FT-IR spectra. Materials: Chloroauric acid (HAuCl4.4H2O), phenylamine-4-sulfonic acid, chitosan, dopamine, uric acid, threonine, urea, glucose, folic acid, oxalate, histidine, tyrosine, glycine, tryptophan, aspartic acid, lysine, Fe3+ ions, NaCl, KCl, Na2CO3, CaCl2, ZnCl2, NH4Cl, MgCl2, deionized water from Mill-Q system.

4：0 cm glass cuvettes for absorbance measurement; Metrohm pH meter (model 780, Switzerland) for pH measurement; Zeiss EM10C transmission electron microscope (Oberkochen, Germany) for TEM analysis; Dynamic light scattering (DLS) particle size analyzer (Malvern, UK) for particle size; Brucker Vector 22 spectrometer (Karlsruhe, Germany) for FT-IR spectra. Materials:

4. Experimental Procedures and Operational Workflow: Synthesis of Au NPs using chitosan as reducing/stabilizing agent; Synthesis of S-CDs via hydrothermal method; Functionalization of Au NPs with S-CDs; DA determination: Mix Au NPs, S-CDs, adjust pH to ~4.0, add Fe3+ and DA, measure absorbance after 3 minutes; Real sample preparation: Dilution for urine, protein precipitation for serum using acetonitrile and centrifugation.

5：4H2O), phenylamine-4-sulfonic acid, chitosan, dopamine, uric acid, threonine, urea, glucose, folic acid, oxalate, histidine, tyrosine, glycine, tryptophan, aspartic acid, lysine, Fe3+ ions, NaCl, KCl, Na2CO3, CaCl2, ZnCl2, NH4Cl, MgCl2, deionized water from Mill-Q system. Experimental Procedures and Operational Workflow:

5. Data Analysis Methods: Calibration curve constructed using absorbance ratio A670/A520 vs. DA concentration; Detection and quantification limits calculated as 3Sb/m and 10Sb/m; Relative standard deviations (RSDs) for precision; Recovery tests for accuracy in real samples.