Engineering of buoyancy-propelled metal-organic-framework based micro/nanomotors

Abstract

Artificial micro/nanomotors (MNMs), inspired by mobile biomolecular entities, have demonstrated great potential as miniaturized robots performing diverse tasks from environmental remediation to biological treatment owing to their great mobility and versatility. The reported MNMs can be propelled using various power sources, including magnetic field, electric field, ultrasound, light, and chemical reaction. MNMs that operate on chemical reactions are usually equipped with higher velocity due to the superior energy conversion efficiency, which dominantly present as bubble propelled systems. However, the majority of the bubble propelled MNMs utilize bubble ejection and detachment force, which result in swarming and linear motion for Janus and tubular motors, respectively. It is still challenging for chemical propelled MNMs to have absolute control in direction without external field. A crafty design to circumvent this limitation is to develop biocatalytic MNMs with bubble buoyancy propulsion. This thesis focuses on the design, fabrication, and applications of submarine-like buoyancy-propelled MNMs that move in the vertical direction. I fabricated buoyancy propelled nanomotors with one-pot synthesis and provided the first work characterizing detailed motion behavior with electrochemistry. With coupled biocatalytic cascade reaction converting glucose as the fuel to oxygen bubbles, the nanomotor was propelled by buoyancy, which dominated the initiative collision at the electrode surface. Four representative electrical impact signals were observed and corresponded to four types of motion patterns. The corresponding relationship was confirmed with a numerical simulation. The integration of MNMs and electrochemistry provided a new dimension to characterize and understand the complex dynamics of the self-propelled nanoparticles. I further investigated the buoyancy propelled MNMs in biomedical applications. The pH-sensitive polymer incorporated micromotor exhibited regulated vertical motion via hydrophilic/hydrophobic phase shifting in different pH environments, and the system was proved to be applicable for anti-cancer drug delivery in a proof-of-concept three-dimensional cell culture. The proposed micromotor opens up new avenues in autonomous robotic fabrication for in vivo drug delivery in complex media. I also investigated the buoyancy propelled MNMs in water remediation. The buoyancy propelled nanomotor exhibited reversible vertical motion in low concentrations of H2O2, which induced the convection of the micro-environment and increased the pollutants to get in contact with the absorbents. The proposed nanomotor showed efficient removal of both inorganic heavy metal ions and organic per- and poly-fluoroalkyl substances (PFAS) in complex environments. At last, the buoyancy propelled MNMs were studied for the vertically spatial separation of targeted cancer cells in mixed samples. With the aid of antibody surface modification, the buoyancy-propelled nanomotors can autonomously attach to the targeted cells and endow the cancer cells with vertical motion. With a customized glass tube, the floated cells can be easily separated. The proposed nanomotor exhibited great isolating efficiency with facile operations, which broadened the development of cell separation methods towards biocompatible nanostructures. The findings presented in this thesis open up new avenues for the development of buoyancy propelled MNMs in diverse applications.