A simple fluorescent “on-off-on” nanosensor based on nitrogen-doped carbon dots for selective detection of Hg2+ and thiamine

A fluorescent "on-off-on" nanosensor based on nitrogen-doped carbon dots (N-CDs) was developed for the detection of Hg2+ and thiamine (vitamin B1). N-CDs were synthesized through a one-step microwave heating approach using d-fructose and l-lysine as precursors in aqueous media. The as-synthesized N-CDs exhibited a strong fluorescent emission peak at 440 nm when excited at 360 nm. The fluorescence emission intensity of synthesized N-CDs was quenched when adding Hg2+ (turn off) at concentrations between 1 and 10 μM, with limit of detection (LOD) 0.40 μM due to the formation of a complex between N-CDs and Hg2+. By contrast, quenched fluorescence of the N-CD/Hg2+ system was recovered by adding thiamine that effectively removed Hg2+ from the N-CD surface resulting in fluorescence turning on. The intensity of the restored fluorescence was proportional to the concentration of thiamine, ranging from 0.2 to 2.0 μM with LOD down to 30 nM. The "on-off-on" process of N-CD fluorescence probes was successfully applied to determine Hg2+ in real water and cosmetic samples and thiamine in grain food samples with accurate results.