N-CQDs@Fe³⁺ Nanoprobe for Off-On Fluorescent Sensing of Tiopronin: A Simple, Sensitive, and Cost-Effective Strategy toward Pharmaceutical Quality Control

Blue fluorescent nitrogen-doped carbon quantum dots (N-CQDs) with excellent water solubility were synthesized through a one-step hydrothermal process, utilizing citric acid as the carbon source and diethylenetriamine as the nitrogen source. Dynamic light scattering (DLS) measurements revealed that the average particle size of the N-CQDs was 9.8 nm. The presence of water-soluble functional groups, including amino and carboxyl groups on the surface of the N-CQDs, was confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FT-IR). In the presence of Fe³⁺ ions, the N-CQDs served as effective fluorophores, undergoing selective binding and static fluorescence quenching. Upon addition of tiopronin (TPN), competitive coordination between TPN and Fe³⁺ induced the release of Fe³⁺ from the N-CQD surface, resulting in significant fluorescence recovery. Optimal experimental conditions were established by fine-tuning the choice of solvent, pH, and the concentrations of both N-CQDs and Fe ³⁺ . Within the TPN concentration range of 0.2 to 0.7 mmol/L, the N-CQDs@Fe ³⁺ fluorescence system demonstrated remarkable sensitivity and selectivity. A strong linear relationship was observed between the fluorescence value lg ( F – F ₀ ) and TPN concentration, described by the linear equation y = 1.4203x + 2.1880, with an R² value of 0.9993 and a limit of detection (LOD) of 80 µmol/L. 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 not only simple and rapid but also reliable and sensitive, without requiring expensive reagents or complex equipment.