Hydrothermal synthesis of fluorescent carbon dots from gardenia fruit for sensitive on-off-on detection of Hg2+ and cysteine.

Nitrogen and sulfur co-doped carbon dots (N/S-CDs) were prepared by a simple hydrothermal method using gardenia fruit as precursor. The N/S-CDs are nearly spherical particles with a size of 2.1 nm and possess excellent fluorescence stability in a wide pH range and high NaCl concentrations, as well as under UV light irradiation. The absolute quantum yield (QY) without any surface modification reaches up to 10.7%. Meanwhile, the N/S-CDs can be quenched by Hg2+ (turn-off), while the quenched fluorescence can be recovered (turn-on) by introducing cysteine (Cys), with linear ranges of 2-20 μM for Hg2+and 0.1-2.0 μM for Cys. On the basis of the on-off-on sensing mode, the N/S-CDs can be used to detect Hg2+ and Cys. Hence, the N/S-CDs would be a promising sensor in environmental and biological analysis.