Sensing Thermophoretic\nForces by Nanoplasmonic Actuators\nwith Interferometric Scattering Readout

Noble metal nanoparticles (NPs) represent nanoscale,\noptically\naddressable heat sources whose temperature gradients give rise to\nthermophoretic forces that can act back on the NPs. Herein we investigate\n20 nm Ag NPs bound via molecular tethers to a 20 nm thin Au film as\nnanoplasmonic actuators that generate a local temperature gradient\nand simultaneously act as optical sensors of forces that induce their\ndisplacement from their equilibrium position. Forces of sufficient\nmagnitude to affect the NP–film distance modulate the interferometric\nscattering (iSCAT) signal of the individual NPs and become detectable\ndue to the distance-dependent damping of the NP scattering in the\nvicinity of the metal film. With total incident power densities within\na range between 1.40 and 4.80 kW cm<sup>–2</sup>, the experiments\nreveal a continuous decay in the NP iSCAT signal, consistent with\na decrease in the NP–film separation due to an attractive thermophoretic\nforce.