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 in situ precipitation of photothermal Fe₃O₄ 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 Fe₃O₄@poly(glycidyl methacrylate)/polystyrene (Fe₃O₄@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 Fe₃O₄ 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, Fe₃O₄@PGS is exploited as a platform for performing "chemistry-on-the-fly" using both the catalytic efficiency of Fe₃O₄ NPs and an immobilized enzyme (lipase). It is found that the propulsion increases the catalytic efficiency of Fe₃O₄ NPs for rhodamine B degradation by over 10 times under sunlight. Moreover, it is proved to accelerate the enzymatic reactions of lipase on Fe₃O₄@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.