Mercury Speciation with Fluorescent Gold Nanocluster as a Probe

Fluorescent nanoparticles are widely used for sensing biologically significant species. However, there are only rare reports of the discrimination or speciation of metal species. In this work, we report for the first time the speciation of mercury (Hg²⁺) and methylmercury (CH₃Hg⁺) by taking advantage of the fluorescence feature of folic-acid-capped gold nanoclusters (FA-AuNCs). FA-Au NCs exhibit an average size of 2.08 ± 0.15 nm and a maximum emission at λ_ex/λ_em = 280/440 nm with a quantum yield of 27.3%. It is interesting that Hg²⁺ causes a significant quench of the fluorescence of FA-Au NCs, whereas CH₃Hg⁺ leads to a remarkable fluorescence enhancement. On the basis of this discriminative fluorescent response between Hg²⁺ and CH₃Hg⁺, a novel nanosensor for the speciation of CH₃Hg⁺ and Hg²⁺ was developed, providing limits of detection (LODs) of 28 nM for Hg²⁺ and 25 nM for CH₃Hg⁺ within 100-1000 nM. This sensing system is highly selective to mercury. Its practical applications were further demonstrated by the analysis of CH₃Hg⁺ and the speciation of mercury (CH₃Hg⁺ and Hg²⁺) in environmental water and fish samples.