Carbon dots as nanosensor for sensitive and selective detection of Hg2+ and l-cysteine by means of fluorescence “Off–On” switching

An “Off–On” approach for highly sensitive and selective detection of mercury ions (Hg 2+ ) and l -cysteine ( l -Cys) was demonstrated with the use of water-soluble carbon dots (CDs) as novel fluorescent probe. In this present sensing system, Hg 2+ cations can efficiently coordinate onto the surface of CDs by means of electrostatic interactions and remarkably quench the fluorescence signal of CDs as a result of the fluorescence charge transfer (turn-off). In addition, the resulting CDs-Hg 2+ system was observed to be sensitive to l -Cys due to the ability of l -Cys to coordinate with Hg 2+ and the formation of strong Hg 2+ -SR bond. When l -Cys was added, Hg 2+ combined with l -Cys rather than with CDs resulting in the remove of Hg 2+ from the surface of CDs and shielding of fluorescence quenching, and thus, a significant fluorescence restore of CDs was observed (turn-on). Under the optimized conditions, good linearity for detecting Hg 2+ was attained over the concentration range from 2 to 22   μM with a detection limit of 0.017   μM, while the concentrations of l -Cys ranging from 2 to 20   μM exhibited a linear relationship with the relative fluorescence intensity with a correlation coefficient of R 2   =   0.99329. Moreover, this CDs-based sensor was successfully applied for visual identification and quantitation of Hg 2+ and l -Cys in real water sample and fluorescence imaging of NIH-3T3 cells with satisfying resolution.