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Far- and near-field properties of gold nanoshells studied by photoacoustic and surface-enhanced Raman spectroscopies.

Gold nanoshells, with a silica core and different core and shell dimensions, have been extensively investigated. Optical far-field properties, namely extinction and absorption, have been separately determined by means of spectrophotometry and photoacoustic spectroscopy, respectively, in the 440-900 nm range. The enhancement factor for surface-enhanced Raman scattering, which is related to near-field effects, has been measured from 568 to 920 nm. The absorption contribution to extinction decreases as the overall diameter increases. Moreover, absorption and scattering display different spectral distributions, the latter being red shifted. The Surface Enhanced Raman Scattering enhancement profile, measured using thiobenzoic acid as a Raman probe, is further shifted to the red. The latter result suggests that the enhancement is dominated by the presence of hot spots, which are possibly related to the surface roughness of gold nanoshell particles.