LAUSR

DOI: 10.1002/adom.201900951 Recently it was demonstrated that gold nanoparticles (NPs) can be reversibly dis/assembled with light using a poly(N-isopropylacrylamide) (pNIPAM) shell.[13,14] The pNIPAM undergoes a lower critical solution temperature (LCST) phase change at Tc = 32 °C between a hydrated and dehydrated state. Above 32 °C the pNIPAM shell shrinks as water is expelled and the coating increases in hydrophobicity. The particles then agglomerate due to strong interparticle cohesion forces. When the temperature is cooled below 32 °C the pNIPAM rapidly swells, forcing the agglomerated particles to separate. The relatively large optical absorption cross-section of small Au NPs allows efficient local heating above the LCST with focused light. These Au@pNIPAM core– shell nanoparticles thus provide a promising platform to exploit the mechanical work done during the phase change of pNIPAM with light. Other authors have also shown the potential of Au@ pNIPAM core–shell nanoparticles as sensors.[15,16] Here we show that light-responsive artificial chromatophores can be created by loading microdroplets with Au@pNIPAM nanoparticles. The microdroplets both limit the size of the nanoparticle clusters and can localize them to the oil–water interface. This results in dramatic color changes during few degree Celsius heating and cooling, and highly repeatable cycling. Intriguingly, the preferential assembly of nanoparticle clusters at the oil–water interface generates locomotion, shown to arise through two separate mechanisms. Under strong laser illumination, high pressure microbubbles are rapidly formed at the interface that expand and ballistically propel the droplets on ms timescales. Under weaker illumination, the localization of NPs to the liquid interface increases the local surface tension and creates a gradient across the microdroplet that drives Marangoni flows. This results in interfacial shear, which causes droplets to swim. This significant advance in artificial nanoscale actuation delivers microscale function, analogous to the pervasive use of nanoscale motor proteins in biology.