Photochromic Spatiotemporal Control of Bubble-Propelled Micromotors by a Spiropyran Molecular Switch

Controlling the environment in which bubble-propelled micromotors operate represents an attractive strategy to influence their motion, especially when the trigger is as simple as light. We demonstrate that spiropyrans, which isomerize to amphiphilic merocyanines under UV irradiation, can act as molecular switches that drastically affect the locomotion of the micrometer-sized engines. The phototrigger could be either a point or a field source, thus allowing different modes of control to be executed. A whole ensemble of micromotors was repeatedly activated and deactivated by just altering the spiropyran-merocyanine ratio with light. Moreover, the velocity of individual micromotors was altered using a point irradiation source that caused only localized changes in the environment. Such selective manipulation, achieved here with an optical microscope and a photochromic additive in the medium, reveals the ease of the methodology, which can allow micro- and nanomotors to reach their full potential of not just stochastic, but directional controlled motion.