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Thermoresponsive microgels undergo a volume phase transition from a swollen state under good solvent conditions to a collapsed state under poor solvent conditions. The most prominent examples of such responsive systems are based on poly-(N-isopropylacrylamide). When cross-linked with N,N'-methylenebisacrylamide, such microgels typically possess a fuzzy-spherelike morphology with a higher cross-linked core and a loosely cross-linked fuzzy shell. Despite the efforts devoted to understanding the internal structure of microgels and their kinetics during collapse/swelling, the origins of the accompanying changes in light scattering intensity have barely been addressed. In this work, we study core-shell microgels that contain small gold nanoparticle cores with microgel shells of different thicknesses and cross-linker densities. All microgels are small enough to fulfill the Rayleigh-Debye-Gans criterion at all stages of swelling. Due to the high X-ray contrast of the gold cores, we can use absolute intensity small-angle X-ray scattering to determine the number density in the dilute dispersions. This allows us to extract polymer volume fractions of the microgels at different stages of swelling from form factor analysis of small-angle neutron scattering data. We match our findings to results from temperature-dependent absorbance measurements. The increase in absorbance during the shrinkage of the microgels is related to the transition from fuzzy spheres to hard sphere-like scattering objects with a rather homogeneous density profile. We provide a first attempt to model experimental spectra using finite difference time domain simulations that take into account the structural changes during the volume phase transition. Our findings significantly contribute to the understanding of the optical properties of thermoresponsive microgels. Further, we provide polymer volume fractions and microgel refractive indices as a function of the swelling state.

The collapse dynamics of gold–poly(N-isopropylacrylamide) core–shell microgels were measured by capacitor-discharge temperature-jump spectroscopy. Using a series of temperature jumps from 31 °C up to 38.9 °C, we could monitor a characteristic two-component volume phase transition by changes in optical density that occurred on a time scale of milliseconds. Kinetic data were compared for microgels over a range of polymer shell thicknesses and cross-linker densities. We show that the fast component of the two-component collapse is consistent with the rapid contraction of the loosely cross-linked outer corona of the polymeric microgel, where the polymer density is lowest. The slow component corresponds to subsequent rearrangement of the polymer chains. The lifetime of the fast component scales linearly with the overall change in microgel radius, and the dynamics are consistent with the collapse of long polymer chains in the outer corona. The lifetime of the slow polymer rearrangement is almost constant over all the tested parameters. The relative contribution of the slow component to the overall change in optical density is largest when the initial and final states of the transition are closer to the fully collapsed state of the microgels. The relative contribution of the fast component is largest when the microgel is initially more swollen.

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.

The extensive use of halomethanes (CH3X, X = F, Cl, Br, I) as refrigerants, propellants, and pesticides has drawn serious concern due to their adverse biological and atmospheric impact. However, there are currently no portable rapid and accurate monitoring systems for their detection. This work introduces an approach for the selective and sensitive detection of halomethanes using photoluminescence spectral shifts in cesium lead halide perovskite nanocrystals. Focusing on iodomethane (CH3I) as a model system, it is shown that cesium lead bromide (CsPbBr3) nanocrystals can undergo rapid (<5 s) halide exchange, but only after exposure to oleylamine to induce nucleophilic substitution of the CH3I and release the iodide species. The extent of the halide exchange is directly dependent on the CH3I concentration, with the photoluminescence emission of the CsPbBr3 nanocrystals exhibiting a redshift of more than 150 nm upon the addition of 10 ppmv of CH3I. This represents the widest detection range and the highest sensitivity to the detection of halomethanes using a low-cost and portable approach reported to date. Furthermore, inherent selectivity for halomethanes compared to other organohalide analogues is achieved through the dramatic differences in their alkylation reactivity.