Intelligent Micro/nanomotors with Taxis.

Abstract

Micro/nanomotors (MNMs) are micro/nanoscale devices that can convert energy from their surroundings into autonomous motion. With this unique ability, they may revolutionize application fields ranging from active drug delivery to biological surgeries, environmental remediation, and micro/nanoengineering. To complete these applications, MNMs are required to have a vital capability to reach their destinations. Employing external fields to guide MNMs to the targets is common and effective way. However, in application scenarios where targets are generally unknown or dynamically change, MNMs must possess the capability of self-navigation or self-targeting. Taking advantage of tactic movements toward or away from signal sources, numerous intelligent MNMs with self-navigation or self-targeting have been demonstrated and attracted much attention during the past few years. In this Account, we elucidate the intelligent response mechanisms of such tactic MNMs, which are summarized as two main models. One is that local vector fields, including those of chemical concentration gradients, gravity, flows, and magnetic fields existing in systems, achieve the overall alignment of asymmetric MNMs via aligning torques, directing the MNMs to swim toward or away from the signal sources. Another is that isotropic MNMs may produce propulsion forces with direction solely determined by the local vector field regardless of their Brownian rotations. Then we discuss and highlight the recent progress in tactic MNMs, including chemotactic, phototactic, rheotactic, gravitactic, and magnetotactic motors. Artificial chemotactic MNMs can be designed with different morphologies and compositions if asymmetric reactions are associated with chemical concentration gradients. In these systems, asymmetric phoretic slip flows are induced, leading to torques that enable the anisotropic particles to align and exhibit chemotaxis. For phototactic MNMs, light irradiation establishes asymmetric fields surrounding the motors via light-induced chemical reactions or physical effects to generate phototactic motion. Shape-asymmetric MNMs reorient in natural fluid flows because of torques applied by the flows, inducing rheotactic movements. MNMs with either the centroid or magnetic components distributed asymmetrically maintain orientation under the torque triggered by gravity or magnetic forces, generating tactic motions. In the end, we envision the future development of synthetic tactic MNMs, including enhancement of the sensitivity of motors to target signals, increasing the diversity of chemical motor systems, and combining multiple mechanisms to endow the tactic motors with multiple functionality. By highlighting the current achievements and offering our perspective on tactic MNMs, we look forward to inspiring the emergence of the next generation of intelligent MNMs with taxis.