Absorption spectrum of surface-bound cap-shaped gold particles.

A surface-adsorbed monolayer of cap-shaped gold particles upon submicrometer-sized polystyrene spheres exhibits pronounced absorption in the visible region. When the surrounding refractive index was altered by immersion in a fluid, the direction of the shift in the absorption spectrum was dependent on the incidence angle of the irradiation. When a thiol molecule, known to adsorb selectively on gold upon polystyrene, was added, the resultant shift in the absorption spectrum's peak was consistently toward longer wavelengths. Consequently, at certain incidence angles, a change in the refractive index of the surrounding fluid produces no shift, whereas thiol adsorption results in a clear shift, apparently reflecting the different spatial regions in which the refractive index is altered by these two procedures.