Quantifying photochemical propulsion in light-powered Janus micromotors

Abstract

Active colloids that convert light into motion provide insight into non-equilibrium chemical systems and routes toward microscale engines. Here we quantify the propulsion dynamics and force generation of light-activated Au-TiO2 Janus particles and their optically trappable polystyrene (PS) core-shell Au-PS@TiO2 analogues. By varying particle size, metal thickness, fuel concentration, and illumination wavelength, we show how photochemical energy conversion at the Au-TiO2 interface governs propulsion, yielding instantaneous velocities up to ~100 μm s⁻¹. Optical tweezers measurements on single Au-PS@TiO2 Janus particles reveal transient propulsion forces up to 20 pN lasting hundreds of milliseconds. Simulations incorporating these transient forces reproduce the observed trajectories, confirming their role in driving active motion. Functionalization with long DNA polymers further enhances propulsion by reducing rotational diffusion. These results establish a single-particle framework for quantifying active forces in photocatalytic Janus particles and offer design principles for light-powered micromotors.