Visual sensing of Hg^sup 2+^ using unmodified Au@Ag core-shell nanoparticles

In this work, we have developed a novel visual Hg^sup 2+^ sensor in aqueous solution using unmodified Au@Ag core-shell nanoparticles at room temperature, based on a redox reaction between Ag-shell and Hg^sup 2+^. The prepared Au@Ag core-shell nanoparticles exhibited good monodispersity. In the presence of Hg^sup 2+^, Hg^sup 2+^ was reduced into Hg (0) by Ag-shell, and deposited on the surface of Au-core to form Au-Hg alloys and caused nanoparticles aggregation, leading to the color changes of the solution from yellow to purplish red, and the surface plasmon resonance spectra of Au@Ag core-shell nanoparticles red shift. Under the optimal conditions, the visual sensor could selectively detect Hg^sup 2+^ as low as 0.4 µM with the naked eye and 5.0 nM by UV-vis spectra analysis methods. The designed sensor had several advantages: (1) the nanoparticles surface need not be functionalized. (2) The color change of the solution was in 2 s and could easily be observed with naked eyes. The visual sensor had been applied to detection of Hg^sup 2+^ in tap and lake water, which obtained satisfied result.