Unexpected Self-Propulsion Behaviors of Copper-Based Active Janus Particles

This work investigates the self-propulsion behavior of copper-based Janus particles in aqueous environments, with a focus on understanding their motion both in the presence and absence of hydrogen peroxide. Unexpectedly, copper-based particles demonstrated directed motion in deionized water, even in the absence of hydrogen peroxide. A novel analysis method based on clustering in the MSD fitting parameter space was developed to identify subtle deviations from purely diffusive motion. The results revealed a time-dependent increase in particle activity, which appears to be correlated with the oxidation state of the copper cap. Experiments in hydrogen peroxide confirmed active propulsion at low concentrations (0.05–0.40%), while higher concentrations led to particle immobilization due to the formation of thick oxide layers. These findings challenge previously accepted models based on catalytic oxygen production and instead suggest that copper oxidation plays a predominant role in the propulsion mechanism. Finally, conditions for encapsulating active particles inside Giant Unilamellar Vesicles (GUVs) were explored, identifying a glucose–sucrose solution as a promising internal medium. Although active behavior within GUVs remains difficult to detect, the results lay the groundwork for future development of cell-inspired micromotors.