Ion-Directed Assembly of Gold Nanorods: A Strategy for Mercury Detection

Abstract

Water-soluble gold nanorods (Au NRs) have been functionalized with an <i>N</i>-alkylaminopyrazole ligand, 1-[2-(octylamino)­ethyl]-3,5-diphenylpyrazole (PyL), that has been demonstrated able to coordinate heavy metal ions. The <i>N</i>-alkylaminopyrazole functionalized Au NRs have been characterized by electron microscopy and spectroscopic investigation and tested in optical detection experiments of different ions, namely, Zn²⁺, Cd²⁺, Hg²⁺, Cu²⁺, Pb²⁺, and As³⁺. In particular, the exposure of the functionalized NRs to increasing amounts of Hg²⁺ ions has resulted in a gradual red-shift and broadening of the longitudinal plasmon band, up to 900 nm. Interestingly, a significantly different response has been recorded for the other tested ions. In fact, no significant shift in the longitudinal plasmon band has been observed for any of them, while a nearly linear reduction in the plasmon band intensity versus ion concentration in solution has been detected. The very high sensitivity for Hg²⁺ with respect to other investigated ions, with a limit of detection of 3 ppt, demonstrates that the functionalization of Au NRs with PyL is a very effective method to be implemented in a reliable colorimetric sensing device, able to push further down the detection limit achieved by applying similar strategies to spherical Au NPs