Direct observation of chemical reactions in metallic nanoparticles is of great importance to have a deeper understanding of these processes for their uses in nanoparticle-based sensing and catalytic applications. Recently, scattering-based dark-field (DF) microscopy has been incorporated into electrochemical cells for enabling optical investigation of electrochemical processes on nanoparticle surface (Figure 1). In this spectroelectrochemical approach, electrodes constituted by low-density nanoparticles deposited on ITO substrates were used to inject electrons into nanoparticles whose optical response was then monitored by DF microscopy.
In the present study, we used spectroelectrochemistry (electrochemistry + DF microscopy and spectroscopy) to characterize mercury amalgamation on gold nanorods coated with mesoporous silica shell (AuNRs@mSiO2O) at the single particle level. Mercury (Hg2+) was electrochemically reduced at AuNR electrodes, and the consequent optical changes resulting from deposition of mercury were monitored by scattering-based DF microscopy. Mesoporous silica shell allowed for approaching of mercury on the AuNR surface. Single AuNRs@mSiO2O showed longitudinal surface plasmon resonance (LSPR) blue-shifts as well as a linewidth broadening in their scattering spectra, caused by the reduction (or amalgamation reaction) of mercury on the Au nanoparticle surface. Furthermore, reduction peaks were observed at potentials which were attributed to Hg2+ reduction under linear sweep voltammetry.