Light-controlled propulsion of catalytic microengines.

Control over the autonomous motion of artificial nano/ micromachines is essential for real biomedical and nanotechnological applications. Consequently, a complete nanomachine should be able to be turned on and off at will. Developments over the last few years on synthetic catalytic nano/microengines and motors have enabled the harvesting of chemical energy from local molecules and transforming it into an effective autonomous motion. Several impressive applications have recently reported the use of artificial micromachines for the detection of biomolecules with roving nanomotors, transport of animal cells in a fluid, and other microcargo delivery. Recently, the use of a light source has been implemented to propel microparticle-based motors generated by a selfdiffusiophoretic mechanism. Despite this interesting approach, the motion of the particles is limited by the dissolution of the materials and to the ultraviolet (UV) spectrum. Moreover, a reversible method to start and stop the propulsion of micromotors by a visible-light source remains a challenge. Here we report the tuning of the propulsion power of Ti/ Cr/Pt catalytic microengines (m-engines) through illumination of a solution by a white-light source. We show that light suppresses the generation of microbubbles, stopping the engines if they are fixed-to or self-propelled above a platinum-patterned surface. The m-engines are reactivated by dimming the light source that illuminates the fuel solution. The illumination of the solution with visible light in the presence of Pt diminishes the concentration of hydrogen peroxide fuel and degrades the surfactant, consequently reducing the motility of the microjets. Electrochemical measurements and analysis of the surface tension support our findings. We also study the influence of different wavelengths over the visible spectrum (500–750 nm) on the formation of microbubbles. Rolled-up Ti/Cr/Pt catalytic m-engines with diameters of 5–10 mm and a length of 50 mm were prepared as described previously elsewhere and in the Experimental Section. Microengines were immersed into solutions of aqueous H2O2 (2.5% v/v) as fuel and benzalkonium chloride (ADBAC) (0.5% v/v), as the surfactant, to determine the influence of white light on the mobility of the m-engines. At lower concentrations of both chemicals, the generation of microbubbles is significantly reduced. Thus, the motility of the catalytic m-engines is controlled by a small change in the fuel (H2O2 and/or surfactant) concentration. These conditions allow us to investigate a concentration range close to the metastable state, that is, where the probability of stopping the m-engines is high. Figure 1A