Quantifying Photochemical Propulsion in Light-Powered Janus Micromotors

Active colloids that convert light into motion provide insight into nonequilibrium 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 average velocities 33.2 ± 2.99 μm s^-1 with instantaneous velocities up to ∼100 μm s^-1. Optical tweezers measurements on single Au-PS@TiO₂ Janus particles reveal transient propulsion forces with a median of 4.5-6.2 pN lasting on average 21-41 ms and reaching up to 20 pN under optical confinement. Simulations incorporating these transient forces reproduce the observed trajectories, confirming their role in driving active motion. Functionalization with long DNA polymers further enhances directional motion 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.