Capability of Au nano-rhombic dodecahedra in a label-free colorimetric assay: application in the determination of S2- and Hg2+

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 S2- and Hg2+. Upon Ag coating, the AuNRDs can probe S2- with high sensitivity and selectivity due to the chemical transformation of Ag into Ag2S. Furthermore, Hg2+ can be detected by the SPR of the AuNRDs based on the formation of HgS from the Ag2S 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 S2- is visible through the naked eye and the change in the SPR of the AuNRDs shifted linearly with an increasing concentration of S2- in the range of 0.5-15 mu M, with a detection limit of 0.26 mu M. The change in the SPR of the Ag2S-coated AuNRDs also indicates a dependence on the concentration of Hg2+ in the range from 0 to 100 mu M, with a detection limit of 38 nM. The limits of detection are lower or close to the maximum allowable levels of S2- and Hg2+ 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.