Microwave prepared nitrogen and sulfur co-doped carbon quantum dots for rapid determination of ascorbic acid through a turn off–on strategy

[Display omitted] •A simple and eco-friendly method for determination of ascorbic acid in C-VIT drops.•Highly fluorescent nitrogen and sulfur co-doped carbon quantum dots were prepared.•The native fluorescence was quenched by ferric ions, then restored by ascorbic acid.•The method is easy, rapid, green, reliable, cost-effective and sensitive. A simple and eco-friendly microwave method was applied for the preparation of highly fluorescent nitrogen and sulfur co-doped carbon quantum dots (NS-CQDs) and used for the determination of ascorbic acid (ASC) in pharmaceutical dosage forms. The prepared NS-CQDs had bright blue fluorescence at a maximum emission wavelength of 440 nm, after excitation with 350 nm, with a quantum yield of 62.5 %. The developed NS-CQDs were prepared from citric acid and l-cysteine in one minute. The native fluorescence of NS-CQDs was quenched by ferric ions due to the formation of non-fluorescent CQDs/ Fe3+ complex. The quenched fluorescence could be restored by the addition of ASC due to the reducing properties of ASC which converts Fe3+ to Fe2+. The method was found linear over the concentration range of 2.0–100 μg/mL, with a limit of detection was 0.6 μg/mL and a coefficient of determination of 0.9965. The proposed method was cross-validated and statistically compared with a reported HPLC method. The results indicated that the developed method was greener, according to the analytical eco-scale and the green analytical procedure index (GAPI). The prepared NS-CQDs were used for spectrofluorometric determination of ASC in pharmaceutical dosage forms, with percentage recoveries ranging between 98 and 102 %, and relative standard deviations less than 2 %. The method was easy, rapid, reliable, and sensitive and did not require expensive reagents or sophisticated equipment.