Sensing Thermophoretic Forces by Nanoplasmonic Actuators with Interferometric Scattering Readout

Noble metal nanoparticles (NPs) represent nanoscale, optically addressable heat sources whose temperature gradients give rise to thermophoretic forces that can act back on the NPs. Herein we investigate 20 nm Ag NPs bound via molecular tethers to a 20 nm thin Au film as nanoplasmonic actuators that generate a local temperature gradient and simultaneously act as optical sensors of forces that induce their displacement from their equilibrium position. Forces of sufficient magnitude to affect the NP-film distance modulate the interferometric scattering (iSCAT) signal of the individual NPs and become detectable due to the distance-dependent damping of the NP scattering in the vicinity of the metal film. With total incident power densities within a range between 1.40 and 4.80 kW cm^-2, the experiments reveal a continuous decay in the NP iSCAT signal, consistent with a decrease in the NP-film separation due to an attractive thermophoretic force.