Monodisperse Gold Nanorods for High-pressure Refractive-index Sensing.

The effects of hydrostatic pressure on the surface plasmon resonances (SPR) of aqueous dispersions of monodisperse gold nanorods were determined up to 9 GPa. The ultranarrow longitudinal SPR band of monodisperse nanorods allows us to monitor a gradual redshift with pressure, which shows abrupt jumps at the liquid to ice phase VI and ice phase VII transitions. Despite solidifying at low pressure (about 1 GPa), water displays a regime of quasi-hydrostaticity in said phase VI and VII, up to ca. 5 GPa. Above this pressure, non-hydrostatic effects manifest themselves through broadening of the SPR bands, but barely any effect is observed on the position of the SP mode. The variation in the SPR peak wavelength with pressure allowed us to determine the pressure dependence of the refractive index of water. Unlike Brillouin scattering or interferometric techniques, this plasmon spectroscopy based method leads to a more direct determination of the refractive index, which is well described empirically by Murnaghan-type equations in the three explored phases. We report herein the obtained analytical functions providing the pressure dependence of refractive index in the liquid, ice VI and ice VII phases of water.