Plasmonic Sensing of Refractive Index and Density in Methanol–Ethanol Mixtures at High Pressure

Abstract

The localized surface plasmon resonance (LSPR) of gold nanospheres dispersed in methanol–ethanol 4:1 was measured as a function of pressure up to 60 GPa. The LSPR exhibits an intense red-shift with pressure in the range of 0–10 GPa, followed by a slower blue-shift at higher pressures. This is because an increase in the solvent refractive index with pressure leads to a red-shift of the LSPR peak wavelength while an increase in the electron density of the gold nanospheres with pressure leads to a blue-shift. Solvent solidification at 10 GPa and associated nonhydrostatic effects have a negligible influence on the LSPR shifts in the case of nanospheres. Here we show that both the LSPR shifts and changes in the nanospheres absorption coefficient can be explained on the basis of Gans’ model, and this enables the solvent refractive index and the density of the solvent to be determined across the hydrostatic pressure range from 0 to 60 GPa. Interestingly, plasmonic sensing shows no evidence of crystallization or glass phase transitions in MeOH–EtOH 4:1 within the explored pressure range.