Spectroscopic evidence for electrochemical effect of mercury ions on gold nanoparticles

We demonstrated an interesting phenomenon that the electrochemical reduction of mercury ions (Hg²⁺) caused distinctly different morphology of gold nanorods (Au NRs) depending on the concentration of Hg²⁺. Specifically, in the case of low concentration, mercury formed through electrochemical reduction only deposited onto the gold surface, and then could be reversibly removed from the gold surface by electrochemical stripping process without causing obvious changes of Au NRs in shape and size. But in the case of high concentration, the reduced Hg not only deposited onto the gold surface but also entered into the interior of Au NRs making them change into gold nanospheres (Au NSs) in an irreversible manner due to the alloying effect of gold with Hg. In this situation, even though the most of Hg was removed, the Au NSs can no long return to the previous rod shape. The changing trend of Au NRs was characterized well by measuring the corresponding UV-Visible spectra. The formation of the Au/Hg nanoalloy was confirmed by high-resolution transmit electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS) results. The present study is helpful for having better understanding of the mechanism of electrochemical mercury-analysis by Au nanoparticles, and necessary to optimize the Au nanoparticles-based Hg sensing strategies.