1：Experimental Design and Method Selection:

The study employs a FRET-based ratiometric fluorescence sensing approach using CDs as energy donors and NAC-AuNPs as energy acceptors, assembled via electrostatic interaction. The method is chosen for its ability to reduce background effects and provide built-in correction using two emission wavelengths.

2：Sample Selection and Data Sources:

L-Cys solutions of various concentrations (1.0-110 μM) are used as samples. Human blood serum samples are collected from a clinical laboratory and diluted for real-sample application.

3：0-110 μM) are used as samples. Human blood serum samples are collected from a clinical laboratory and diluted for real-sample application. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: Key materials include CDs synthesized from 4-aminobenzoic acid and m-phenylenediamine, NAC-AuNPs synthesized from HAuCl4·3H2O and NAC, and L-Cys. Equipment includes a JEOL JEM-2100 TEM, TU-1901 UV-vis spectrophotometer, FP-8300 fluorescence spectrometer, and FLS 920 fluorescence lifetime spectrometer.

4：Experimental Procedures and Operational Workflow:

CDs and NAC-AuNPs are synthesized via hydrothermal and reduction methods, respectively. For L-Cys detection, CDs, NAC-AuNPs, and L-Cys are mixed in phosphate buffer (pH 7.0), incubated for 3 min, and fluorescence spectra are recorded at 414 nm excitation. Parameters like incubation time and pH are optimized.

5：0), incubated for 3 min, and fluorescence spectra are recorded at 414 nm excitation. Parameters like incubation time and pH are optimized. Data Analysis Methods:

5. Data Analysis Methods: Fluorescence intensities at 539 nm and 630 nm are measured, and the ratio I539/I630 is plotted against L-Cys concentration for linear regression analysis. Detection limit is calculated as 3δ/k. Selectivity is tested against other amino acids and ions.