Indirect Surface-Enhanced Raman Scattering Sensor for Direct Detection of Gaseous Elemental Mercury

Gaseous elemental mercury (GEM─Hg0(g)) is a highly toxic global pollutant with environmental and human health effect concerns. Monitoring the GEM released in gold mining operation regions is extremely necessary. Herein, we develop an indirect surface-enhanced Raman scattering (SERS)-miniaturized sensor for the direct detection of Hg0(g) using gold nanorods (AuNRs) and rhodamine 6G as a probe molecule. SERS spectra show a suppressed signal after exposure to Hg0(g) in different concentrations caused by the morphologic transition of the nanorods to spheres. Discrete dipole approximation (DDA) and density functional theory (DFT) simulations reveal that the energetic phenomena involved during the Au–Hg amalgam formation process can lead to drastic changes in the AuNR's plasmonic characteristic, suppressing the SERS signal. This effect is the key to achieving the high performance to detect Hg0(g) until 0.08 μg. Additionally, the SEM-EDS results confirmed the AuNR's morphological changes after exposure to Hg0(g), and principal component analysis and root-mean-square error reveal the high sensitivity, mainly for the lower amount of Hg0(g) (0.08–1.02 μg), corroborating the DDA and DFT simulations whose Au–Hg alloy formation on the AuNR's surface is energetically more favorable, occurring more quickly and efficiently than the diffusion process. Moreover, these results show nanorod structures are more efficient than spherical ones.