Since the first molecular motors were synthesized in the late 1990s, the catalog has expanded to include self-propelling micromotors that roll, walk, or swim. Building such devices is challenging enough, but controlling their direction of motion has also proved difficult. Now chemist Baohu Dai and his colleagues at the University of Hong Kong have created a tree-shaped micromotor that exploits the motion of charges inside and outside the device to swim toward or away from a light source. Each device is about 10 μm long and consists of titanium dioxide nanowires (yellow in the illustration) sprouting from one end of a silicon wire (pink). Ordinarily the microswimmers succumb to Brownian motion (see the article by Dean Astumian and Peter Hänggi, Physics Today, November 2002, page 33). But the randomness is overcome when the miniature trees are immersed in an aqueous solution of hydrogen peroxide and illuminated with 365 nm UV light. That combination spurs a redox reaction in which the rod end serves as the cathode and consumes electrons and protons that are supplied by the dissociation of H2O2 on the branched (anode) end. While the electrons travel through the silicon wire, the protons migrate in the surrounding solution, dragging fluid in one direction so that the motor is propelled in the opposite direction. Dai and colleagues demonstrated that they could alter the swimmers’ direction of motion either by tailoring the surface charge of the silicon wire beforehand or by illuminating one side of the TiO2 tree to produce a torque. The motors may find use as micro-sized pumps. A potential future experiment would be to monitor the collective behavior of many of the devices. Bathing a swarm of microswimmers in light would cause their ion gradients to overlap and result in complex interactions between individuals that may resemble the chemical-fueled communication and motion of slime molds. (B. Dai et al., Nat. Nanotechnol., in press, doi:10.1038/nnano.2016.187.)
