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 Hg 2+ and paraquat was designed by using nitrogen doped graphene quantum dots (N-GQDs) and N-GQDs/Hg 2+ system, respectively. It was found that the fluorescence of N-GQDs solution was quenched in the presence of Hg 2+ ions (turn off), and the quenched N-GQDs/Hg 2+ system was recovered after addition of paraquat (turn on), due to the higher affinity of paraquat and Hg 2+ than that of N-GQDs and Hg 2+ that resulting into the release of Hg 2+ 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 Hg 2+ and paraquat. Under the optimum experimental conditions, good linearities were achieved over the range of 0–4.31 μM for Hg 2+ and 0.05–2.0 μg mL −1 for paraquat, respectively. The detection limits for Hg 2+ and paraquat were 23 nM and 19 μg L −1 , respectively, and the recoveries of Hg 2+ 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 Hg 2+ and paraquat in real environmental water samples.