Capability of Au nano-rhombic dodecahedra in a label-free colorimetric assay: application in the determination of S 2- and Hg 2 .

Anisotropic nanoparticles possess high sensitivity to the environmental refractive index that is strongly dependent on their active facets. As a polyhedron exclusively enclosed by active facets, Au nano-rhombic dodecahedra (AuNRDs) with multiple tips and edges have been less explored with respect to their capability in surface plasmon resonance (SPR) sensing. Here, we report on the use of AuNRDs in the colorimetric detection of S 2- and Hg 2+ . Upon Ag coating, the AuNRDs can probe S 2- with high sensitivity and selectivity due to the chemical transformation of Ag into Ag 2 S. Furthermore, Hg 2+ can be detected by the SPR of the AuNRDs based on the formation of HgS from the Ag 2 S shell on the AuNRDs. These changes in the morphology and composition of the AuNRDs have been confirmed by electron microscopy and elemental mapping. The promising colorimetric response of Ag-coated AuNRDs to S 2- is visible through the naked eye and the change in the SPR of the AuNRDs shifted linearly with an increasing concentration of S 2- in the range of 0.5-15 μM, with a detection limit of 0.26 μM. The change in the SPR of the Ag 2 S-coated AuNRDs also indicates a dependence on the concentration of Hg 2+ in the range from 0 to 100 μM, with a detection limit of 38 nM. The limits of detection are lower or close to the maximum allowable levels of S 2- and Hg 2+ for drinking water defined by national or international administrations. These seminal results unveil new opportunities for the AuNRDs and other anisotropic nanostructures with active facets in SPR-based sensing applications.