Tannic acid-derivatized graphitic carbon nitride quantum dots as an "on-off-on" fluorescent nanoprobe for ascorbic acid via copper(II) mediation.

A microwave-assisted hydrothermal route was employed to prepare fluorescent tannic acid (TA)-derivatized graphitic carbon nitride quantum dots. The resulting dots display blue fluorescence (best measured at excitation/emission wavelengths of 350/452 nm) with a quantum yield as high as ~44%. The incorporated TA imparts a fluorescence switching behavior in that very low concentrations of Cu(II) can quench the fluorescence, while (AA) can restore it. It is presumed that AA causes Cu(II) to be transformed to Cu(I). Based on these findings, a fluorometric method was designed for AA detection. The probe allows AA to be detected with a 50 pM limit of detection and a linear analytical range that extends from 0.1 to 200 nM of AA. Real and spiked samples were successfully assayed by the probe to demonstrate its analytical applicability. Graphical abstract Schematic presentation of fluorescent graphitic carbon nitride quantum dots functionalized with tannic acid. Their fluorescence is quenched by Cu2+ and recovered by ascorbic acid (AA). This is exploited in an assay with a picomolar detection limit.