Some particles can catalyse a chemical reaction in which the release of a gas molecule propels them in solution. A common example of such a reaction is the disproportionation of hydrogen peroxide to water and oxygen by some metals, such as Pt and Ag. If a Janus particle is made so that the catalytic centre is on one side only, the particle will move with a preferred direction. Chen et al. have now shown that a communication system between two different kinds of swimmers can be designed so that one swimmer sends a chemical signal to a second one to increase the propagation speed of the latter.
The microparticle that sends the message is composed of polystyrene half-coated with Ni, Au and Ag layers. The Ni layer serves as a magnetic element to externally guide the particle. The receiver is a SiO2 Janus particle half-coated with a Pt layer. In the presence of hydrogen peroxide, metallic Ag dissolves in solution to give Ag+ and oxygen superoxide. If the sender is guided near the receiver, Ag+ will deposit on top of metallic Pt and reduce to metallic Ag by the action of the oxygen superoxide to form small nanoparticles. In turn, the newly formed Pt/Ag interface catalyses the decomposition of hydrogen peroxide, propelling itself away. Because a Pt/Ag surface catalyses the reaction much faster than Pt alone, the receiver experiences an acceleration of about three to six times, depending on the concentration of hydrogen peroxide in the solution.