A Ratiometric Fluorescence Assay for Detection of Ascorbic Acid Based on N,S Co-Doped Carbon Dots Combined With Ce⁴

Hence, N,S-CDs with photoluminescent property were simply synthesized via a one-step hydrothermal method. Combined with the commercial reagent Ce⁴⁺, a ratiometric fluorescence assay for ascorbic acid (AA) detection was established. Ce⁴⁺, possessing oxidization, could directly oxidize o-phenylenediamine (OPD) to form the yellow fluorescent product oxOPD. Under the excitation wavelength of 370 nm, oxOPD had a maximum fluorescence emission at 562 nm. Meanwhile, due to the occurrence of the inner filter effect (IFE), oxOPD quenched the fluorescence of N,S-CDs. However, ascorbic acid (AA) inhibited the oxidation of Ce⁴⁺, causing the fluorescence of oxOPD at 562 nm to decrease, accompanied by an increase in the fluorescence belonging to N,S-CDs at 450 nm. Thus, a Ce⁴⁺-assisted ratiometric fluorescence method was established for AA detection. The two fluorescence output signals in this method had opposite changing trends, which could reduce system errors and improve the accuracy. This method was successfully applied to the determination of AA in drugs and fruits.