Shape-controlled synthesis of liquid metal nanodroplets for photothermal therapy

The capping agents for liquid metal (LM) nanodroplets in aqueous solutions are restricted to thiol-containing and positively-charged molecules or macromolecules. However, both thiolate-metal complex and electrostatic interaction are liable to detachment upon strong mechanical forces such as sonication, leading to limited stability and applications. To address this, we utilized ultrasmall water soluble melanin nanoparticles (MNPs) as the capping agent, which exhibited strong metal binding capability with the oxide layer of gallium based LMs and resulted in enhanced stability. Interestingly, shape-controlled synthesis of LM nanodroplets can be achieved by the incorporation of MNPs. Various EGaIn nanostructures including nanorice, nanosphere and nanorod were obtained by simply tuning the feed ratio, sonication time, and suspension temperature. Among these shapes, EGaIn nanorice has the best photothermal conversion efficiency, which could be leveraged for photothermal therapy.