Direct observation of mercury amalgamation on individual gold nanorods using spectroelectrochemistry

Abstract

We report on the use of a spectroelectrochemical method for the investigation of mercury amalgamation on gold nanorods. Hg2+ was electrochemically reduced at gold nanorod electrodes, and the consequent optical changes resulting from deposition of mercury were monitored by dark-field microscopy. Nanorods displayed marked scattering color transitions from red to green and longitudinal surface plasmon resonance (LSPR) blue-shifts up to 197 nm. Corresponding cathodic voltammograms showed strong reduction peaks at potentials which were attributed to Hg2+ reduction. The highest optical blue-shifts were observed for working electrodes constituted by well-separated gold nanorods deposited on ITO substrates, which also showed transition from diffusion-limited to steady-state electrochemical behavior. Theoretical simulations were carried out to support experimental results and to obtain further insight into the diffusional behavior of mercury reduction at nanorod electrodes. Real time observation of the amalgamation process was performed by monitoring the evolution of spectral response from single nanorods in the presence of Hg2+ and subjected to either linear sweep voltammetry or chronoamperometry. The analysis confirmed a direct correlation between the occurrence of spectral changes and the formation of an amalgam resulting from deposition and subsequent diffusion of reduced mercury into the nanorods. This work shows the potential of this method for elucidation of reaction mechanisms occurring at nanoscale electrodes and for sensitive detection of mercury for environmental applications.