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 (Hg²⁺), based on carbon dots with -SH (SN-CDs) and gold nanoparticles (AuNPs). On addition of Hg²⁺, 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 Hg²⁺, 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 Hg²⁺. Meanwhile, citrate ions on the surface of AuNPs can reduce Hg²⁺ to Hg⁰, and through the strong affinity of Hg⁰ 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 Hg²⁺ in real water samples.