Ultraviolet Analysis of Gold Nanorod and Nanosphere Solutions

The localized surface plasmon resonances of gold nanoparticles have been widely studied at visible and near-infrared frequencies but less so in the ultraviolet. The spectral extinction measurements of gold nanospheres at UV wavelengths reported here closely match calculated spectra down to 200 nm using the measured dielectric function of gold. The nanosphere volume attenuation coefficient, αv, is size-dependent in the 200–300 nm wavelength range where the dielectric function follows the Drude free electron model (as it does for wavelengths larger than 500 nm). In the interband transition region, the extinction is more closely proportional to mass and therefore has a value of αv that is largely independent of size. Gold nanorod solutions exhibit very strong UV extinction below 220 nm due to charge transfer to solvent (c.t.t.s.) excitations of the bromide counterion to the cationic surfactant. The gold nanorod UV properties depend on the alignment between the optical polarization and nanorod structure. For polarization along each major axis, the spectral extinction is similar to that of a sphere of nanorod size in that dimension.