Colorimetric determination of mercury(II) using gold nanoparticles and double ligand exchange

A colorimetric assay is described for highly selective and sensitive determination of Hg(II) ions by using gold nanoparticles (AuNPs) functionalized with dithioerythritol (DETL). This method relies on the unique optical properties of DETL-functionalized AuNPs as well as the thiophilicity of both AuNPs and Hg(II). In the presence of DETL, the AuNPs aggregate due to ligand exchange between thiol groups of DETL and the citrate ions on the surface of AuNPs. This induces a color change from red to blue. On addition of Hg(II), the thiol groups preferably interact with Hg(II) rather than with AuNPs. Thus, the DETL is released from the surface of the AuNPs and binds to Hg(II). This triggers the redispersion of the AuNPs. The ratio of absorbances at 650 and 525nm drops linearly in two Hg(II) concentration ranges (viz. from 0.1 to 0.5M, and from 0.5 to 5M). The ions Cu(II), Pb(II), and Cd(II) do not interfere even in the absence of masking agents. The detection limit is as low as 24nM.