Colorimetric and turn-on fluorescence determination of mercury (II) by using carbon dots and gold nanoparticles.

A colorimetric and turn-on fluorometric assay with high sensitivity and selectivity is described for the optical detection of mercury (II) ions (Hg), based on carbon dots with -SH (SN-CDs) and gold nanoparticles (AuNPs). On addition of Hg, the color of the system (SN-CDs/AuNPs) changes from red to blue. A new absorption peak appears at 700 nm, and its absorbance increases with the concentration of Hg, while at 530 nm, the absorbance of AuNPs decreases. Taking the ratio of absorbance at 700 and 530 nm as a signal, a colorimetric method with linear detection range of 0.5-4.0 μM was established for the determination of Hg. Meanwhile, citrate ions on the surface of AuNPs can reduce Hg to Hg, and through the strong affinity of Hg and gold, gold-mercury alloys were formed to occupy the surface of AuNPs, so that the SN-CDs were re-free and the fluorescence of SN-CDs was restored. Consequently, a fluorometric method was founded in the linear detection range from 0.5 to 15.0 μM of mercury (II). This dual-mode (colorimetric and turn-on fluorometric) method was applied successfully for determination of Hg in real water samples.