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 decisively and may find new chemistry applications based on local temperature landscapes. The observed nanofabrication of core–shell particles is one such example.

A temperature gradient can induce solutes to migrate from a hot to a cold region, and vice versa, in solution. This process, termed thermophoresis, has been applied to manipulate, transport, and separate various macromolecules and colloids by exploiting a microscale temperature gradient. Here we describe using a single gold nanoparticle (AuNP) as an efficient nanoscale heating source to promote thermophoresis. Moreover, on introducing a substrate with high thermal conductivity such as sapphire, a strong local temperature gradient can be shaped in the medium near the AuNP under continuous wave laser illumination. We observed molecules such as polyethylene glycol and sodium dodecyl sulfate being transported toward the AuNP and attaching to its surface, forming a gold core−organic shell structure within several tens of seconds of illumination. Spectroscopically, because of the gradual increasing encapsulation, progressive red shifts with enhanced scattering intensities were seen for the localized surface plasmon resonance bands of the AuNP with increasing cycles of illumination. Postmortem scanning electron microscopy provided direct evidence of shell formation. Our technique is relevant to nanofabrication based upon optical heating at the nanometer scale.