Valence States Modulation Strategy for Picomole Level Assay of Hg2+ in Drinking and Environmental Water by Directional Self-Assembly of Gold Nanorods.

In this study, we present a valence states modulation strategy for picomole level assay of Hg2+ using directional self-assembly of gold nanorods (AuNRs) as signal readout. Hg2+ ions are first controllably reduced to Hg+ ions by appropriate ascorbic acid, and the reduced Hg+ ions react with the tips of the preadded AuNRs and form gold amalgam. Such Hg+ decorated AuNRs then end-to-end self-assemble into one-dimensional architectures by the bridging effects of lysine based on the high affinity of NH2-Hg+ interactions. Correspondingly, the AuNRs' longitudinal surface plasmon resonance is gradually reduced and a new broad band appears at 900-1100 nm region simultaneously. The resulting distinctly ratiometric signal output is not only favorable for Hg2+ ions detection but competent for their quantification. Under optimal conditions, the linear range is 22.8 pM to 11.4 nM, and the detection limit is as low as 8.7 pM. Various transition/heavy metal ions, such as Pb2+, Ti2+, Co2+, Fe3+, Mn2+, Ba2+, Fe2+, Ni2+, Al3+, Cu2+, Ag+, and Au3+, do not interfere with the assay. Because of ultrahigh sensitivity and excellent selectivity, the proposed system can be employed for assaying ultratrace of Hg2+ containing in drinking and commonly environmental water samples, which is difficult to be achieved by conventional colorimetric systems. These results indicate that the present platform possesses specific advantages and potential applications in the assay of ultratrace amounts of Hg2+ ions.