Scattering of Visible Light by Au-PNIPAM Core-Shell Microgels

Core-shell microgels based on PNIPAM are an important class of soft colloidal materials that undergo a volume phase transition in aqueous environment in response to temperature changes. At low temperatures the microgels are swollen by large amounts of water, and as the temperature is raised they adopt increasingly collapsed states. The shrinkage with increasing temperature is accompanied by structural changes as well as a significant increase in light scattering that can be probed by UV/vis extinction measurements. However, it remains unexplored how the structural characteristics of microgels at various states of collapse are reflected in the corresponding UV/vis extinction cross-section spectra. In this work, we systematically investigated three optical models against experimental data for five Au-PNIPAM microgels that contain small gold nanoparticle cores. This series of microgels covered different size regimes and cross-linker densities. The optical models took a limited set of structural parameters as inputs and simulated the optical spectra of the microgels at any given temperature. Although the spectra were reasonably well replicated by simple homogeneous sphere models, we found that a model that incorporated the fuzzy morphology of the core-shell microgels, and used recursive T-matrix-assisted Mie theory to calculate the corresponding extinction cross section, gave the best agreement to experimental data.