Polymer-modulated optical properties of gold sols

The optical properties of a series of gold nanoparticles (D ≈ 2.3-8 nm) grafted with a stabilizing polymer with a wide range of chain lengths (M _w ≈ 6.5K-29.5K) have been studied quantitatively in different dielectric solvents. Mie-Drude dipolar theory was applied to model the localized surface plasmon resonance (SPR) peak position, as well as the peak width of the absorption spectra, using the dielectric function of gold. The modeled spectra yielded information on the dielectric function of the polymer shell, μ _s, from which the average polymer concentration in the shell was deduced. Combining information from optical modeling and structural properties obtained from small-angle neutron scattering (SANS) on the polymer shell thickness and from transmission electron microscopy (TEM) on the gold core size, the SPR peak shifts and their attenuated sensitivity to solvent refractive index were characterized. The SPR behaviors for all of the gold colloids with different core sizes and graft chain lengths were thus expressed as a function of the effective polymer volume fraction, p, of the composite nanoparticle. It was found that sensitivity to the solvent dielectric property decreased until, for p > 0.9 (corresponding to an effective shell thickness on the order of the core radius), the SPR mode was 'frozen' in by the polymer shell and lost sensitivity to the solvent.