Generalized\nand Scalable Synthesis of Manganese Dioxide-Based\nTubular Micromotors for Heavy Metal Ion Removal

Synthetic\nnano- and micromachines hold immense promise in biomedicine\nand environmental science. Currently, bubble-driven tubular micro/nanomotors\nhave garnered increasing attention owing to their exceptional high-speed\nself-propulsions. However, complex and low-yield preparation methods\nhave hindered their widespread applications. Herein, we present a\ngeneralized, scalable, and low-cost electrospinning-based strategy\nto fabricate MnO<sub>2</sub>-based composite tubular micromotors (MnO<sub>2</sub>-TMs) for efficient heavy metal ion removal. The inherent\nflexibility of precursor nanofibers derived from diverse matrix materials\nenables the creation of MnO<sub>2</sub>-TMs with a wide range of morphologies.\nIn response to morphology changes, the MnO<sub>2</sub>-TMs, based\non a bubble-propelled mechanism, exhibit multimodal motion patterns,\nincluding linear, circular, and spiral to stochastic swinging. To\nelucidate the underlying morphology-to-motion relationship, we conducted\nsystematic simulations of fluid dynamics around the MnO<sub>2</sub>-TMs. Furthermore, by incorporation of Fe<sub>3</sub>O<sub>4</sub> nanoparticles, the capabilities of MnO<sub>2</sub>-TMs can be expanded\nto include magnetic manipulation for directional navigation and efficient\nretrieval. Benefiting from these attributes, MnO<sub>2</sub>-TMs excel\nin removing heavy metal ions from water. The developed MnO<sub>2</sub>–MnWO<sub>4</sub>@Fe<sub>3</sub>O<sub>4</sub> TMs exhibit\nprominent adsorption capacities of 586.5 mg g<sup>–1</sup> for\nCu<sup>2+</sup> and 156.4 mg g<sup>–1</sup> for Pb<sup>2+</sup>. Notably, the magnetic property facilitates rapid separation and\nretrieval of the micromotors, and the absorbed ions can be simply\nrecovered by pH adjustment. This work establishes a general framework\nfor developing MnO<sub>2</sub>-based tubular micro/nanomotors to address\nenvironmental challenges.