Determination of Salicylhydroxamic Acid in Human Urine Using S-Doped Carbon Quantum Dots in a Fluorescence “On-Off-On” Mode

Sulfur-doped carbon quantum dots (S-CQDs) were synthesized via a one-pot hydrothermal method using ascorbic acid and thioglycolic acid as precursors. The S-CQDs showed a uniform size distribution, excellent photostability, and bright blue fluorescence with a fluorescence quantum yield of 32.07%. The as-prepared S-CQDs were used as fluorescent probes for the rapid and accurate determination of salicylhydroxamic acid (SHA) in human urine via an “on-off-on” responsive mode. The sensing mechanism involves two steps: initially, Fe3+ quenches the fluorescence of S-CQDs through photoinduced electron transfer (PET); subsequently, SHA competitively coordinates with the Fe3+ on the surface of S-CQDs, leading to the recovery of the fluorescence. Under optimized conditions, this method enabled the determination of SHA in artificial urine over a concentration range of 0.1–120 mg/L, with a limit of detection (LOD) of 0.033 mg/L. The spiked recoveries of SHA in urine samples ranged from 99.55% to 109.28%, with coefficients of variation (CVs) between 2.79% and 6.19%. Meanwhile, the low cytotoxicity of S-CQDs was verified by CCK-8 assays. These results confirm the high accuracy and precision of the proposed method, making it suitable for SHA monitoring in pharmacokinetic studies, pharmaceutical quality control, and clinical diagnostics.