Modification-free colorimetric and visual detection of Hg²⁺ based on the etching from core-shell structural Au-Ag nanorods to nanorices

A simple and rapid colorimetric approach for Hg²⁺ detection is achieved by etching silver-coated gold nanobipyramids (Au NBs@Ag nanoparticles) based on the redox reaction between Hg²⁺ and silver nanoshell of the Au NBs@Ag nanoparticles. The decrease of the silver nanoshell thickness leads to first blue shift and then red shift of the longitudinal localized surface plasmon resonance (LSPR) peak and accompanies a vivid color change. The blue shift is more suitable for colorimetric detection which has the obvious multicolor change from yellow-green to green and then to blue-gray; and could be employed to quantitatively detect the concentrations of Hg²⁺. This nanoprobe with the longitudinal LSPR peak at 730 nm exhibits a linear range of 0.1–20 μM with a limit of detection (LOD) of 22 nM based on blue shift and has the most obvious color change with the lowest LOD of 0.8 μM by direct observation. In this case, the Au NBs@Ag nanoparticles are gradually etched from the nanorods to nanorices as the Hg²⁺ concentration increases. Compared with other techniques, this method could avoid tedious procedures and is time-saving. Furthermore, this nanoprobe has good selectivity, and is well demonstrated in real samples with satisfied results.