Multiwavelength Phototactic Micromotor with Controllable Swarming Motion for "Chemistry-on-the-Fly".

Artificial nano/micromotors that represent the next-generation automotive microdevices hold considerable promise in various potential applications. However, it is a great challenge to design light-powered micro/nanomotors with effective propulsion that can fulfill diverse tasks. Herein, a multilight-responsive micromotor is fabricated by precipitation of photothermal FeO nanoparticles (NPs) onto different microparticles. The composites exhibit phototactic swarming movement by irradiation at 320-550 nm, which can be reversibly and remotely manipulated by irradiation position, "on/off" switch, and light intensity. The micromotor made of FeO@poly(glycidyl methacrylate)/polystyrene (FeO@PGS) core-shell particles presents a propulsion speed as high as 270 μm/s under ultraviolet (UV) irradiation. Using an array of experimental methods and numerical simulations, thermal convection mechanism is proposed for the propulsion. Namely, under light irradiation, the photogenerated heat on FeO NPs decreases the density of the irradiated spot, leading to the swarming motion of the composite particles propelled by a "hydrodynamic drag" toward the light spot. Then, FeO@PGS is exploited as a platform for performing "chemistry-on-the-fly" using both the catalytic efficiency of FeO NPs and an immobilized enzyme (lipase). It is found that the propulsion increases the catalytic efficiency of FeO NPs for rhodamine B degradation by over 10 times under sunlight. Moreover, it is proved to accelerate the enzymatic reactions of lipase on FeO@PGS in both aqueous and organic systems by more than 50%. Such a multiwavelength phototactic swimmer paves the way to the design of advanced micromotors for various applications, such as drug delivery, microsurgery, and sensing.