Effects of Hydrostatic Pressure on the Surface Plasmon Resonance of Gold Nanocrystals

Abstract

The surface plasmon resonances of gold nanospheres and nanorods have been measured as a function of hydrostatic pressure up to 17 GPa in methanol−ethanol 4:1 solvent and up to 10 GPa in paraffin. Both the sphere resonance and the longitudinal rod resonance exhibit redshifts, whereas the transverse rod mode shows an extremely weak redshift or blueshift depending on the nanorod aspect ratio. Solidification of the solvent around 11 GPa causes some aggregation of the particles, readily identified through broadening of the surface plasmon band and further redshifting. Spectra collected during loading and unloading cycles exhibit only minimal hysteresis if the pressure remains below 11 GPa. The surface plasmon shifts are the result of two competing effects. Compression of the conduction electrons in the metals increases the bulk plasma frequency, which causes a blueshift. However, the increase in the solvent density under hydrostatic load leads to an increase in the solvent refractive index, which in turn leads to a redshift. We find that after accounting for the solvent contribution, we can spectroscopically determine the bulk modulus of the gold nanoparticles with a precision of 10%. The value obtained of K 0 = 190 GPa is significantly higher than the value for bulk gold (167 GPa). Furthermore, we show that pressure-induced solidification causes a significant broadening and anomalous shift of the surface plasmon band that we attribute to aggregation and nanorod deformation.

Monodisperse Gold Nanorods for High-Pressure Refractive Index Sensing

“…In particular, the surface plasmon spectroscopy-based n(P) data are very similar to those obtained by combining Brillouin spectroscopy and sound velocity measurements under high-pressure conditions. Interestingly, the LSPR pressure shifts of aqueous AuNR dispersions are well described by a modified Mie-Gans model, in agreement with previous findings for AuNR dispersions in alcohol[4]. We also show that the analysis of the two main competing mechanisms responsible for LSPR -the compression of conduction electrons in the metal causes a blueshift while an increase in solvent density under pressure leads to a redshiftenables us to decouple both effects, thereby providing a direct approach to extract the variation of the refractive index of water n(P) from the LSPR pressure shift across the three investigated water phases: liquid, ice VI and ice VII.…”

“…is the nanorod depolarization or shape factor, = 2 is the dielectric function of the non-absorbing medium, which also yields a refractive index, , while in Eq.2 is a renormalization constant. The change in particle volume The Journal of Physical Chemistry Letters analysis of extinction spectra of gold nanocrystal colloids in ethanol-methanol mixtures over a range of hydrostatic pressures [4]. We found that the bulk modulus of gold for an AuNR of 0 = 190 GPa is significantly higher than that for bulk gold metal, 0 = 167 GPa [23].…”

“…3) The LSPR band broadens with increasing pressure, after water solidification. In particular, LSPR broadening becomes more prominent in ice phase VII, where solidification-induced AuNR aggregation or stress are likely responsible for said broadening [4,18]. In addition, the variation of the optical density at the LSPR maximum, as a function of pressure, shows anomalies once hydrostaticity is lost ( Figure 4).…”

“…nanospheres and nanorods), due to changes in their aspect ratio (AR) or nanoparticle size, as well as in the refractive index of the surrounding medium. Gold nanorod (AuNR) plasmonics has been recently exploited to investigate the nanoparticle's mechanical properties, through adequate models describing SPR shifts under hydrostatic and non-hydrostatic highpressure conditions [4]. This new methodology is noteworthy owing to the difficulty in applying well-defined loading forces to nanomaterials, as well as problems related to measuring deformations with suitable accuracy.…”

Individual Plasmonic Nanoprobes for Biosensing and Bioimaging: Recent Advances and Perspectives

“…Plasmonic nanoparticles (PNPs) have been the focus of research in fields as diverse as chemical and biological sensing and solar energy harvesting due to the unique optical properties originating from surface plasmon resonance (SPR). [ 1–8 ] PNPs have many desirable properties, including good conductivity, easy modification, low toxicity, and excellent biocompatibility. Furthermore, single PNPs are extremely sensitive to chemical composition, morphology, surface chemistry, and the localized surrounding medium.…”