Selective and high-sensitive label-free detection of ascorbic acid by carbon nitride quantum dots with intense fluorescence from lone pair states

Abstract

Label-free detection of ascorbic acid (AA) with high sensitivity and specificity based on the effects of AA on fluorescence quenching of carbon nitride quantum dots (CNQDs) is described. The CNQDs with a high fluorescence quantum yield of 21% and good dispersibility in water are prepared by reacting g-C3N4 sheets with ethylenediamine (EN). Fluorescence at 510 nm from the CNQDs is quenched gradually by addition of AA. As the AA concentration increases, the activity of the lone pair (LP) state of the CNQDs diminishes resulting in reduced fluorescence from the CNQDs and the mechanism of the static quenching effect is discussed. Since the CNQDs have a large specific surface area and abundant amino groups and AA exists in the anionic form at the physiological pH, the electrostatic interaction between CNQDs and AA inhibits excitation and emission of the LP states in the CNQDs. Owing to steric effects and hydrogen bonding, the CNQDs constitute a sensitive and selective detection platform for AA in a wide range from 0.5 to 200 μM with a detection limit of 150 nM (signal-to-noise ratio of 3). More importantly, the strategy can successfully be applied to the detection of AA concentrations in serum samples. This simple method which provides quantitative AA determination has large potential in clinical and health-related applications and the mechanism provides insights into intracellular AA monitoring.