Photothermal Responses in Phase-Separated PNIPAM-Nanoparticle Composite Droplets

Liquid-liquid phase separation (LLPS) refers to the spontaneous formation of two distinct liquid phases in a homogenous mixture of polymer or macromolecular components. We have observed the formation of liquid droplets via LLPS in a suspension of Poly(N-isopropylacrylamide) microgels mixed with gold nanoparticles and polyacrylamide. The phase-separated droplets demonstrate a photothermal response when exposed to a high-intensity LED due to the surface plasmon resonance and absorption of light by the gold nanoparticles. We noted two varieties of response depending on the overall size of the phase-separated droplets and the concentration of the gold nanoparticles inside the droplets. The larger droplets with high nanoparticle concentrations shrink their size under light – a behavior consistent with the phase transition of PNIPAM microgels above the lower critical solution temperature. On the other hand, the smaller droplets and those with lower nanoparticle concentrations exhibit a reversible expansion that is proportional to the intensity of the incident light. These observations have implications for future medical and engineering applications, such as drug delivery and microfluidic control systems.