An “On–Off–On” Fluorescence Sensing System Based on Nitrogen-Doped Carbon Quantum Dots (N-CQDs) and Ferric Ions (Fe3+) for the Determination of Captopril (CAP) in Real Samples

In this study, nitrogen-doped carbon quantum dots (N-CQDs) are synthesized from citric acid and tetraethylenepentamine via a one-step melting method. Leveraging the fluorescence quenching of N-CQDs by Fe 3+ through a combined static quenching and inner filter effect mechanism, an “on–off–on” fluorescent sensing platform was constructed for the detection of captopril (CAP). The method relies on the competitive chelation of Fe 3+ by the thiol and carboxyl groups of CAP, which displaces the metal ions from the N-CQD surface and restores the quenched fluorescence. Under optimized conditions, the sensor exhibited a detection limit of 0.0620 mmol/L and a linear response spanning 0.1–1 mmol/L. The method was successfully applied to the determination of CAP in commercial tablet formulations, with recoveries meeting the acceptance criteria of the Chinese Pharmacopoeia (2020 Edition). The proposed approach offers a simple, rapid, and sensitive strategy for the quantitative analysis of CAP in pharmaceutical products.