“On–off-on” fluorescence sensing of ascorbic acid based on a Fe3+-modulated carbon quantum dots–rhodamine 6G dual-emission system: application to pharmaceutical quality control

Reliable methods for quantifying ascorbic acid (AA) in pharmaceutical formulations are essential for quality control. In this study, nitrogen-doped carbon quantum dots (CQDs) with a quantum yield of 31.57% and abundant surface functional groups (–NH 2 , –OH, –COOH) were synthesized via a one-pot melting method using citric acid and tetraethylenepentamine. The CQDs were physically mixed with rhodamine 6G (Rh6G) to construct a dual-emission system (CQDs-Rh6G) that produces two well-separated fluorescence peaks under single-wavelength excitation. Upon addition of Fe 3+ , both emissions are effectively quenched via a static quenching mechanism (with a minor inner-filter effect), and the quenched fluorescence can be specifically restored by AA. The differential ratio ((F CQDs − F Rh6G )/F CQDs ) shows a linear response to AA in the range of 5.0–70.0 μM, with a detection limit of 2.70 μM. The method was successfully applied to commercial Vitamin C tablets and injections, with recoveries of 95.83%–105.10% and relative standard deviation < 4%, all falling within the acceptance criteria of the Chinese Pharmacopoeia (2020). While the system responds to total reducing equivalents rather than AA selectively, its simplicity, cost-effectiveness, and reliable performance in excipient-limited formulations make it a practical tool for routine pharmaceutical quality control.