A highly sensitive and selective on-off-on fluorescent sensor based on nitrogen doped graphene quantum dots for the detection of Hg2+ and paraquat

A simple and rapid fluorescence on-off-on approach for sensing Hg2+ and paraquat was designed by using nitrogen doped graphene quantum dots (N-GQDs) and N-GQDs/Hg2+ system, respectively. It was found that the fluorescence of N-GQDs solution was quenched in the presence of Hg2+ ions (turn off), and the quenched NGQDs/Hg2+ system was recovered after addition of paraquat (turn on), due to the higher affinity of paraquat and Hg2+ than that of N-GQDs and Hg2+ that resulting into the release of Hg2+ from the surface of N-GQDs and the fluorescence recovery of N-GQDs. The developed fluorescence on-off-on approach exhibited high selectivity and sensitivity for detection of Hg2+ and paraquat. Under the optimum experimental conditions, good linearities were achieved over the range of 0-4.31 mu M for Hg2+ and 0.05-2.0 mu gmL(-1) for paraquat, respectively. The detection limits for Hg2+ and paraquat were 23 nM and 19 mu g L-1, respectively, and the recoveries of Hg2+ and paraquat spiked in real samples ranged from 95% to 105.8%. Results of this study suggested that the highly selective and sensitive fluorescence on-off-on approach can be used to the detection of Hg2+ and paraquat in real environmental water samples.