Plasmonic-Heating-Induced Nanoscale Phase Separation of Free Poly(N-isopropylacrylamide) Molecules

The localized-surface-plasmon resonance (LSPR) of gold nanoparticles (Au NPs) depends sensitively on the environmental refractive index. We found that LSPRs of single Au NPs supported on a transparent substrate can monitor the phase transition/separation of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) in aqueous solution. Both reversible and irreversible phase separations were observed. Plasmonic-heating-induced redshifts and subsequent cooling-induced recovery were observed on a glass substrate. Besides reversible redshifts at lower laser intensities, permanent redshifts were evident at higher intensities after illumination on a sapphire substrate, which has a much higher thermal conductivity than glass. The permanent redshifts were caused by the formation of a PNIPAM shell around the NP. For this permanent aggregation, temperature shaping around the Au NP by the substrate was found decisive and may find new chemistry applications based on local temperature landscapes. The observed nanofabrication of core–shell particles is one such example