Design strategies and structure simplification methods of self-propelled micro-/nanomotors

Abstract

Self-propelled micro-/nanomotors (MNMs), which are defined as micro-/nanodevices capable of converting various energy into autonomous motion, can be used to pick up, transport, and release various cargoes within a liquid medium. They have important potential applications, for example, in drug delivery, biosensors, protein and cell separation, microsurgeries and environment remediation. This review comprehensively introduces the design strategies and structures of self-propelled MNMs along with an outlook for their future development. It starts with the summary of the propulsion mechanisms of self-propelled MNMs of bubble recoiling and self-phoresis induced by the asymmetric release of products or heat. For bubble recoiling propulsion, the continuous momentum change is caused by a jet of bubbles, while for self-phoresis propulsion, the MNMs move in a local electric field, concentration gradient, surface tension gradient, or temperature gradient, etc. After systematically and in-depth understanding these propulsion mechanisms, it has been pointed out that the key to design self-propelled MNMs is to construct an asymmetric field across micro-/ nanoparticles. Following this clue, the structures evolution and simplification methods of self-propelled MNMs are reviewed. Janus structures and multilayer-tubular structures, which are prepared through asymmetric modification process, electrochemical synthesis, template-assisted method, rolled-up nanotech, etc., have been firstly proposed to construct asymmetric fields across micro-/nanoparticles for their propulsion. However, the complicated structure and preparation process hinder the application of MNMs. Anisotropic single-component irregular particles, tubes and bowl-like MNMs, which are obtained by dry spinning method, “growing-bubble”-templated self-assembly, etc., have been subsequently achieved by utilizing their anomalous morphology and the nucleation preference of bubble molecules on different surfaces. This kind of MNMs show somewhat simple structure and can be easily fabricated, but the motion direction is still difficult to control because of the Brownian motion. Isotropic semiconducting MNMs have been recently developed by taking advantage of the limited light penetration depth in the isotropic photoresponsive particles, of which the motion is independent of the rotational Brownian motion. This suggests a remarkable breakthrough in design strategy of MNMs due to the simple isotropic structure of the motor and the controllability in both motion direction and speed by light. Besides the evolution of self-propelled MNMs from the complicated structure to the simplified one, some remarkable progresses have also been made on the motion control, functionalization, etc. For example, the speed and state of MNMs can so far be easily adjusted by the concentration of fuels, the intensity of external fields, etc. The direction can be controlled accurately by magnetic field, electric field, light, etc. Numerous complex tasks can also be performed effectively, such as protein separation, drug delivery, environmental detection and remediation, etc. Lastly, an outlook is also provided on the future development and main challenges of self-propelled MNMs. The future development of MNMs should be focused on improving energy conversion efficiency through optimization of structures, exploring new propulsion mechanisms and endowing MNMs with environmental responses for self-navigation, detection, and specific operations. In this way, MNMs will approach to the practical applications in biomedicine, environment treatment, microengineering, etc.