N-Doped CQDs as a Fluorescence Probe for Tiopronin Detection in Pharmaceutical Formulations via Static Quenching.

In recent years, fluorescent sensors based on carbon quantum dots (CQDs) have been widely reported for pharmaceutical detection owing to their numerous advantages. In this study, a Fe³⁺-mediated fluorescence “off–on” sensor based on nitrogen-doped carbon quantum dots (N-CQDs) was developed for the selective detection of tiopronin (TPN). Briefly, highly fluorescent N-CQDs were synthesized via a one-step hydrothermal method using citric acid monohydrate and diethylenetriamine. XPS, FT-IR, UV-Vis, fluorescence spectroscopy, and DLS confirmed that the N-CQDs exhibit uniform size, aqueous stability, excitation-independent fluorescence, and a high quantum yield of 62.99%. Fe³⁺ selectively bound to N-CQDs to form a complex, resulting in fluorescence quenching. Subsequently, upon the addition of TPN, the fluorescence of N-CQDs showed a proportional recovery. During this process, the competitive coordination between TPN and Fe³⁺ led to the release of Fe³⁺ from the N-CQDs, thereby restoring the quenched fluorescence. The limit of detection (LOD) of TPN was 80 μmol/L, with a linear range of 0.2–0.7 mmol/L and an R² value of 0.9993. The content of TPN in enteric-coated tablets was quantified using this method, yielding recovery rates ranging from 92.59% to 104.22%. This method is simple, rapid, and reliable, offering good sensitivity without the need for expensive reagents or complex equipment.