Plasmonic-Heating-Induced Nanoscale Phase Separation of Free Poly(<i>N</i>‑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(<i>N</i>-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 decisively and may find new chemistry applications based on local temperature landscapes. The observed nanofabrication of core–shell particles is one such example

Localized Phase Separation of Thermoresponsive Polymers Induced by Plasmonic Heating

Optical excitation-induced heating of a single gold nanoparticle potentially offers a high-temperature field confined to the immediate neighborhood of the particle. In this study, we applied darkfield microscopy imaging and Rayleigh scattering spectroscopy to pursue phase separation of aqueous thermoresponsive poly­(<i>N</i>-isopropylacrylamide) and poly­(vinyl methyl ether) adjacent to a gold nanoparticle that was heated by continuous wave laser illumination. Gold nanoparticles were supported on transparent substrates of glass or sapphire. From the imaging study, we observed that a 1–10 μm microdroplet covering the nanoparticle formed and grew in time scales of seconds to a few tens of seconds. The growth was triggered by the illumination, and the droplet collapsed when the laser was blocked. At the same time, we observed scattering spectral changes characterized by a progressive redshift in the localized surface plasmon resonance (LSPR) band and an increasing scattering intensity in the region of wavelengths shorter than the LSPR band with increasing laser intensity. The scattering spectral changes were interpreted by the encapsulation of the nanoparticle by a polymer-rich droplet with increasing sizes. The present study revealed that thermoresponsive polymers were attracted to a hot gold nanoparticle and formed a microdroplet under illumination with a wavelength near the LSPR. Our findings demonstrate the potential of plasmonic heating to manipulate polymer migration and accumulation, which may find applications in protein crystallization