Halogen–copper redox chemistry as a driving force for spherical Janus microswimmers

Most Janus particle microswimmers use catalytic reaction mechanisms, including hydrogen peroxide or hydrazine degradation, to generate gradients for self-propulsion. However, several alternative reaction mechanisms can also lead to activity, including the redox reactions of noble metals, enzymes and glucose, often using spherical Cu@SiO2 Janus particles. Copper, as a very versatile metal, is involved in reactions, as an oxidant or as an electrode, for example in dilute halogen solutions. This so-called nanobattery system works well for rod-shaped bimetallic and shape-asymmetric rods. To allow comparability with simulations, we present the adaptation of this propulsion reaction for Janus colloids. Altogether, this study provides an overview of the chemical process and swimming behaviour of spherical Cu@SiO2 Janus particles in dilute halogen solutions. We combine an experimental approach that includes the determination of the effect of particle size and cap thickness, supported by simulations of these systems.