CONSPECTUS: 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 selfnavigation 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. Shapeasymmetric 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.

Swarming micro/nanomotors can self-organize into cohesive groups to execute cooperative tasks. To date, research work has focused on the construction of egalitarian microswarms composed of similar individuals. The construction and collective behaviors of hierarchical leaderfollower-like microswarms are demonstrated. By inducing converging electrohydrodynamic flows under an AC electric field, dielectric microparticles with different sizes and dielectric properties can hierarchically self-organize into leader-follower-like microswarms under attractive electrohydrodynamic interactions, and show novel emergent collective behaviors. First, different from immobile single constituents or egalitarian clusters, the hierarchical microswarms autonomously move with tunable speed. Second, they exhibit multimode collective photoresponses emerging from different behaviors of the constituents in response to light signals. With a vertical UV signal, the photoresponsive followers tend to surround the leader and stop the microswarm. In response to sidewise UV signals, the constituents with stronger phototaxis would migrate to the position away from light stimuli, and thus the microswarms reorient parallel/antiparallel to the light direction and perform collective positive/negative phototaxis. Due to differential roles and huge design spaces of constituents, the hierarchical microswarms are envisioned to possess merits of high-efficiency, multiresponsiveness, and multifunctions, and may serve as intelligent micro/nanorobot systems for biomedicine and microengineering.

Synthetic chemotactic micro/nanomotors are envisioned to actively ‘seek out’ targets by following specific chemicals, but they are mainly powered by bioincompatible fuels and only show pseudochemotaxis (or advanced chemokinesis) due to their weak self-reorientation capabilities. Here we demonstrate that synthetic ZnO-based Janus micromotors can be powered by an alternative biocompatible fuel of CO2, and further provide the first example of self-reorientation-induced biomimetic chemotaxis using them. The ZnO-based micromotors are highly sensitive to the dissolved CO2 in water, which enables the corrosion of ZnO to continuously occur by providing H+ through hydration. Thus, they can autonomously move even in the water exposed to air based on self-diffusiophoresis. Furthermore, they can sense local CO2 gradient and perform positive chemotaxis by self-reorientations under the electroosmotic viscous torque. Our discovery opens a gate to develop intelligent micro/nanomotors powered by and sensitive to biocompatible atmospheric or endogenous gaseous chemicals for biomedical and environmental applications.

The bubble-propelled amorphous TiO2/Au Janus micromotors with the reversibly light-controlled motion state and speed have been demonstrated by utilizing the efficient photocatalytic H2O2 decomposition over the in situ H2O2 sensitized amorphous TiO2.

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The key principle of designing a micro/nanomotor is to introduce asymmetry to a micro/nanoparticle. However, micro/nanomotors designed based on external asymmetry and inherent chemical and geometrical asymmetry usually suffer from tedious small-scale preparation, high cost, and/ or complexity of external power and control devices, making them face insurmountable hurdles in practical applications. Herein, considering the possible distinct properties of different polymorphs, we propose a novel design strategy of simple-structured micromotors by introducing inherent asymmetry in crystalline phases. The inherent phase asymmetry can be easily introduced in spherical TiO 2 particles by adjusting the calcination temperature to control the phase transition and growth of primary grains. The as-designed anatase/rutile TiO 2 micromotors not only show efficient autonomous motions controlled by light in liquid media stemming from the asymmetric surface photocatalytic redox reactions but also have a promising application in environmental remediation due to their high photocatalytic activity in "on-the-fly" degradation of organic pollutants, facile large-scale fabrication, and low cost. The proposed design strategy may pave the way for the large-scale productions and applications of micro/nanomotors.