Scattering of Visible\nLight by Au–PNIPAM Core–Shell\nMicrogels

Abstract

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