Graphene quantum dots as on-off-on fluorescent probes for chromium(VI) and ascorbic acid

We report that graphene quantum dots (GQDs) are viable fluorescent probes for the determination of chromium(VI) and ascorbic acid in an on-off-on mode. The fluorescence of GQDs is strongly quenched by Cr(VI) mainly due to an inner filter effect and static quenching. This shifts the system to the off status. The quenching mechanism of this fluorescent system was investigated in some detail. Fluorescence intensity is inversely proportional to the concentration of Cr(VI) in the 0.05 to 500 mu M concentration range with a 3.7 nM detection limit. The fluorescence of GQDs-Cr(VI) system is converted back to on by adding ascorbic acid which will reduce yellow Cr(VI) ion, thereby eliminating the inner filter effect and static quenching. The relative intensity of restored fluorescence is directly proportional to the concentration of ascorbic acid in the 1.0 to 500 mu M range, and the limit of detection is 0.51 mu M. There are almost no interferences to commonly encountered other substances. The methods were applied to the determination of Cr(VI) in spiked tape, lake and river waters, and of ascorbic acid in a tablet and human urine. Both gave satisfactory results.