Linking Structure and Optical Properties of Plasmonic Nanoparticles on Tunable Spherical Surfaces.

The complexation of plasmonic nanoparticles (NPs) and thermoresponsive microgels is widely recognized as a powerful route to realize hybrid systems with tunable optical properties for different applications. At the same time, it provides a unique experimental platform to investigate the physics of NP organization on curved two-dimensional surfaces, a fundamental problem with implications spanning from biology to materials science yet unexplored at the nanoscale. However, a microscopic description of the mechanisms governing the spatial organization of the NPs and their rearrangement across the microgel volume phase transition (VPT) is lacking so far. Combining small-angle X-ray scattering and state-of-the-art simulations, we uncover how the microgel VPT controls NP-NP interactions, showing that temperature-induced microgel collapse drives a redistribution of NPs toward the periphery, with a tendency to order on the spherical surface. Moreover, we quantitatively reproduce both the structural and optical experimental data through a simple toy model, ultimately establishing for the first time a direct link between the interparticle distance and plasmon coupling. Our study paves the way for experimentally investigating phase transitions on tunable curved surfaces at the nanoscale, achieving fine control of their plasmonic response.