ZnO-based microrockets with light-enhanced propulsion

Micro/Nanomotor-Driven Intelligent Targeted Delivery Systems: Dynamics Sources and Frontier Applications

Micro/nanomotors represent a promising class of drug delivery carriers capable of converting surrounding chemical or external energy into mechanical power, enabling autonomous movement. Their distinct autonomous propulsive force distinguishes them from other carriers, offering significant potential for enhancing drug penetration across cellular and tissue barriers. A comprehensive understanding of micro/nanomotor dynamics with various power sources is crucial to facilitate their transition from proof-of-concept to clinical application. In this review, micro/nanomotors are categorized into three classes based on their energy sources: endogenously stimulated, exogenously stimulated, and live cell-driven. The review summarizes the mechanisms governing micro/nanomotor movements under these energy sources and explores factors influencing autonomous motion. Furthermore, it discusses methods for controlling micro/nanomotor movement, encompassing aspects related to their structure, composition, and environmental factors. The remarkable propulsive force exhibited by micro/nanomotors makes them valuable for significant biomedical applications, including tumor therapy, bio-detection, bacterial infection therapy, inflammation therapy, gastrointestinal disease therapy, and environmental remediation. Finally, the review addresses the challenges and prospects for the application of micro/nanomotors. Overall, this review emphasizes the transformative potential of micro/nanomotors in overcoming biological barriers and enhancing therapeutic efficacy, highlighting their promising clinical applications across various biomedical fields.

Advanced materials for micro/nanorobotics

Autonomous micro/nanorobots capable of performing programmed missions are at the forefront of next-generation micromachinery. These small robotic systems are predominantly constructed using functional components sourced from micro- and nanoscale materials; therefore, combining them with various advanced materials represents a pivotal direction toward achieving a higher level of intelligence and multifunctionality. This review provides a comprehensive overview of advanced materials for innovative micro/nanorobotics, focusing on the five families of materials that have witnessed the most rapid advancements over the last decade: two-dimensional materials, metal-organic frameworks, semiconductors, polymers, and biological cells. Their unique physicochemical, mechanical, optical, and biological properties have been integrated into micro/nanorobots to achieve greater maneuverability, programmability, intelligence, and multifunctionality in collective behaviors. The design and fabrication methods for hybrid robotic systems are discussed based on the material categories. In addition, their promising potential for powering motion and/or (multi-)functionality is described and the fundamental principles underlying them are explained. Finally, their extensive use in a variety of applications, including environmental remediation, (bio)sensing, therapeutics, etc., and remaining challenges and perspectives for future research are discussed.

Ficin-Cyclodextrin-Based Docking Nanoarchitectonics of Self-Propelled Nanomotors for Bacterial Biofilm Eradication

Development of bioinspired nanomotors showing effective propulsion and cargo delivery capabilities has attracted much attention in the last few years due to their potential use in biomedical applications. However, implementation of this technology in realistic settings is still a barely explored field. Herein, we report the design and application of a multifunctional gated Janus platinum-mesoporous silica nanomotor constituted of a propelling element (platinum nanodendrites) and a drug-loaded nanocontainer (mesoporous silica nanoparticle) capped with ficin enzyme modified with β-cyclodextrins (β-CD). The engineered nanomotor is designed to effectively disrupt bacterial biofilms via H₂O₂-induced self-propelled motion, ficin hydrolysis of the extracellular polymeric matrix (EPS) of the biofilm, and controlled pH-triggered cargo (vancomycin) delivery. The effective synergic antimicrobial activity of the nanomotor is demonstrated in the elimination of Staphylococcus aureus biofilms. The nanomotor achieves 82% of EPS biomass disruption and a 96% reduction in cell viability, which contrasts with a remarkably lower reduction in biofilm elimination when the components of the nanomotors are used separately at the same concentrations. Such a large reduction in biofilm biomass in S. aureus has never been achieved previously by any conventional therapy. The strategy proposed suggests that engineered nanomotors have great potential for the elimination of biofilms.

Achieving Control in Micro-/Nanomotor Mobility

Unprecedented opportunities exist for the generation of advanced nanotechnologies based on synthetic micro/nanomotors (MNMs), such as active transport of medical agents or the removal of pollutants. In this regard, great efforts have been dedicated toward controlling MNM motion (e.g., speed, directionality). This was generally performed by precise engineering and optimizing of the motors' chassis, engine, powering mode (i.e., chemical or physical), and mechanism of motion. Recently, new insights have emerged to control motors mobility, mainly by the inclusion of different modes that drive propulsion. With high degree of synchronization, these modes work providing the required level of control. In this Minireview, we discuss the diverse factors that impact motion; these include MNM morphology, modes of mobility, and how control over motion was achieved. Moreover, we highlight the main limitations that need to be overcome so that such motion control can be translated into real applications.

Recent Advances in One-Dimensional Micro/Nanomotors: Fabrication, Propulsion and Application

Due to their tiny size, autonomous motion and functionalize modifications, micro/nanomotors have shown great potential for environmental remediation, biomedicine and micro/nano-engineering. One-dimensional (1D) micro/nanomotors combine the characteristics of anisotropy and large aspect ratio of 1D materials with the advantages of functionalization and autonomous motion of micro/nanomotors for revolutionary applications. In this review, we discuss current research progress on 1D micro/nanomotors, including the fabrication methods, driving mechanisms, and recent advances in environmental remediation and biomedical applications, as well as discuss current challenges and possible solutions. With continuous attention and innovation, the advancement of 1D micro/nanomotors will pave the way for the continued development of the micro/nanomotor field.

Halloysite-Based Nanorockets with Light-Enhanced Self-Propulsion for Efficient Water Remediation

Halloysite-based tubular nanorockets with chemical-/light-controlled self-propulsion and on-demand acceleration in velocity are reported. The nanorockets are fabricated by modifying halloysite nanotubes with nanoparticles of silver (Ag) and light-responsive α-Fe₂O₃ to prepare a composite of Ag-Fe₂O₃/HNTs. Compared to the traditional fabrication of tubular micro-/nanomotors, this strategy has merits in employing natural clay as substrates of an asymmetric tubular structure, of abundance, and of no complex instruments required. The velocity of self-propelled Ag-Fe₂O₃/HNTs nanorockets in fuel (3.0% H₂O₂) was ca. 1.7 times higher under the irradiation of visible light than that in darkness. Such light-enhanced propulsion can be wirelessly modulated by adjusting light intensity and H₂O₂ concentration. The highly repeatable and controlled "weak/strong" propulsion can be implemented by turning a light on and off. With the synergistic coupling of the photocatalysis of the Ag-Fe₂O₃ heterostructure and advanced oxidation in H₂O₂/visible light conditions, the Ag-Fe₂O₃/HNTs nanorockets achieve an enhanced performance of wastewater remediation. A test was done by the catalytic degradation of tetracycline hydrochloride. The light-enhanced propulsion is demonstrated to accelerate the degradation kinetics dramatically. All of these results illustrated that such motors can achieve efficient water remediation and open a new path for the photodegradation of organic pollutions.

Biodegradable Small-Scale Swimmers for Biomedical Applications

Most forms of biomatter are ephemeral, which means they transform or deteriorate after a certain time. From this perspective, implantable healthcare devices designed for temporary treatments should exhibit the ability to degrade and either blend in with healthy tissues, or be cleared from the body with minimal disruption after accomplishing their designated tasks. This topic is currently being investigated in the field of biomedical micro- and nanoswimmers. These tiny devices have the ability to move through fluids by converting physical or chemical energy into motion. Several architectures of these devices have been designed to mimic the motion strategies of nature's motile microorganisms and cells. Due to their motion abilities, these devices have been proposed as minimally invasive tools for precision healthcare applications. Hence, a natural progression in this field is to produce motile structures that can adopt, or even surpass, similar transient features as biological systems. The fate of small-scale swimmers after accomplishing their therapeutic mission is critical for the successful translation of small-scale swimmers' technologies into clinical applications. In this review, recent research efforts are summarized on the topic of biodegradable micro- and nanoswimmers for biomedical applications, with a focus on targeted therapeutic delivery.

Micro-Bio-Chemo-Mechanical-Systems: Micromotors, Microfluidics, and Nanozymes for Biomedical Applications

Wireless nano-/micromotors powered by chemical reactions and/or external fields generate motive forces, perform tasks, and significantly extend short-range dynamic responses of passive biomedical microcarriers. However, before micromotors can be translated into clinical use, several major problems, including the biocompatibility of materials, the toxicity of chemical fuels, and deep tissue imaging methods, must be solved. Nanomaterials with enzyme-like characteristics (e.g., catalase, oxidase, peroxidase, superoxide dismutase), that is, nanozymes, can significantly expand the scope of micromotors' chemical fuels. A convergence of nanozymes, micromotors, and microfluidics can lead to a paradigm shift in the fabrication of multifunctional micromotors in reasonable quantities, encapsulation of desired subsystems, and engineering of FDA-approved core-shell structures with tuneable biological, physical, chemical, and mechanical properties. Microfluidic methods are used to prepare stable bubbles/microbubbles and capsules integrating ultrasound, optoacoustic, fluorescent, and magnetic resonance imaging modalities. The aim here is to discuss an interdisciplinary approach of three independent emerging topics: micromotors, nanozymes, and microfluidics to creatively: 1) embrace new ideas, 2) think across boundaries, and 3) solve problems whose solutions are beyond the scope of a single discipline toward the development of micro-bio-chemo-mechanical-systems for diverse bioapplications.

Smartdust 3D-Printed Graphene-Based Al/Ga Robots for Photocatalytic Degradation of Explosives

Milli/micro/nanorobots are considered smart devices able to convert energy taken from different sources into mechanical movement and accomplish the appointed tasks. Future advances and realization of these tiny devices are mostly limited by the narrow window of material choices, the fuel requirement, multistep surface functionalization, rational structural design, and propulsion ability in complex environments. All these aspects call for intensive improvements that may speed up the real application of such miniaturized robots. 3D-printed graphene-based smartdust robots provided with a magnetic response and filled with aluminum/gallium molten alloy (Al/Ga) for autonomous motion are presented. These robots can swim by reacting with the surrounding environment without adding any fuel. Because their outer surface is coated with a hydrogel/photocatalyst (chitosan/carbon nitride, C₃ N₄ ) layer, these robots are used for the photocatalytic degradation of the picric acid as an explosive model molecule under visible light. The results show a fast and efficient degradation of picric acid that is attributed to a synergistic effect between the adsorption capability of the chitosan and the photocatalytic activity of C₃ N₄ particles. This work provides added insight into the large-scale fabrication, easy functionalization, and propulsion of tiny robots for environmental applications.

Light-Driven ZnO Brush-Shaped Self-Propelled Micromachines for Nitroaromatic Explosives Decomposition

Self-propelled micromachines have recently attracted lots of attention for environmental remediation. Developing a large-scale but template-free fabrication of self-propelled rod/tubular micro/nanomotors is very crucial but still challenging. Here, a new strategy based on vertically aligned ZnO arrays is employed for the large-scale and template-free fabrication of self-propelled ZnO-based micromotors with H₂ O₂ -free light-driven propulsion ability. Brush-shaped ZnO-based micromotors with different diameters and lengths are fully studied, which present a fast response to multicycles UV light on/off switches with different interval times (2/5 s) in pure water and slow directional motion in aqueous hydrogen peroxide solution in the absence of UV light. Light-induced electrophoretic and self-diffusiophoretic effects are responsible for these two different self-motion behaviors under different conditions, respectively. In addition, the pH of the media and the presence of H₂ O₂ show important effects on the motion behavior and microstructure of the ZnO-based micromotors. Finally, these novel ZnO-based brush-shaped micromotors are demonstrated in a proof-of-concept study on nitroaromatic explosive degradation, i.e., picric acid. This work opens a completely new avenue for the template-free fabrication of brush-shaped light-responsive micromotors on a large scale based on vertically aligned ZnO arrays.

Construction of dendritic Janus nanomotors with H2O2 and NIR light dual-propulsion via a Pickering emulsion

Artificial micro/nanomotors with a dual-propulsion property have attracted considerable attention recently due to their attractive performances in complex fluidic environments. In this work, we successfully constructed Janus nanomotors with H₂O₂ and NIR light dual-propulsion by employing dendritic porous silica nanoparticles (DPSNs) as carriers via a Pickering emulsion and electrostatic self-assembly. The aminopropyl-modified DPSNs (DPSNs-NH₂) with positive charge were semiburied in paraffin wax microparticles in order to achieve electrostatic adsorption of Pt nanoparticles (NPs) with negative charge on the exposed surface for H₂O₂ propulsion, followed by electrostatic adsorption of negatively charged CuS NPs with excellent NIR light absorption on the other exposed surface of the eluted DPSNs-NH₂@Pt for NIR light propulsion. Center-radial large mesopores facilitate the high density loading of Pt NPs and CuS NPs for efficient propulsion. Compared with the commonly used sputtering approach, this Pickering emulsion method can realize relatively large-scale fabrication of Janus NPs. DPSNs-NH₂@Pt@CuS Janus nanomotors can be effectively driven not only by self-diffusiophoresis, which results from the decomposition of H₂O₂ catalyzed by Pt NPs, but also by self-thermophoresis, which is generated from thermal gradients caused by the photothermal effect of CuS NPs. Moreover, the motion speed of the nanomotors can be conveniently modulated by regulating the H₂O₂ concentration and NIR light intensity. This work provides a novel exploration into the construction of dual-propulsion nanomotors, which are supposed to have significant potential in biomedical and intelligent device applications.

Active Micromotor Systems Built from Passive Particles with Biomimetic Predator-Prey Interactions

Inspired by chasing-escaping behaviors of predator and swarming prey in nature, here we demonstrate a concept to create active micromotor systems from two species of passive microparticles with biomimetic predator-prey interactions. In this concept, the biomimetic predator-prey interactions are established in a binary particle system comprising the diffusiophoretic attractive microparticles (prey particles) and the diffusiophoretic repulsive ones (predator particles). In the absence of additional chemical fuels and external fields, the predator particles are attracted by and constantly chase the swarming prey particles, which, in response, escape from the former and show dynamic group reconfigurations because of the local repulsion. Based on this concept, various synthetic active micromotor systems have been demonstrated, including active ZnO-TiO₂, Ag₃PO₄-TiO₂, and ZnO-AgBr micromotor systems. As the predator and prey particles are powered by each other through the biomimetic predator-prey interactions, the concept proposed here provides an advanced method to develop not only a class of single micromotors powered by passive particles or "solid fuels" but also micromotor swarms capable of manipulating "moving cargo". In addition, it also illustrates a proof-of-concept implementation of intelligent micro/nanomotor systems composed of heterogeneous individuals with complementary or cooperative functions.

Advanced Hybrid GaN/ZnO Nanoarchitectured Microtubes for Fluorescent Micromotors Driven by UV Light

The development of functional microstructures with designed hierarchical and complex morphologies and large free active surfaces offers new potential for improvement of the pristine microstructures properties by the synergistic combination of microscopic as well as nanoscopic effects. In this contribution, dedicated methods of transmission electron microscopy (TEM) including tomography are used to characterize the complex hierarchically structured hybrid GaN/ZnO:Au microtubes containing a dense nanowire network on their interior. The presence of an epitaxially stabilized and chemically extremely stable ultrathin layer of ZnO on the inner wall of the produced GaN microtubes is evidenced. Gold nanoparticles initially trigger the catalytic growth of solid solution phase (Ga_{1- x} Zn_x )(N_{1- x} O_x ) nanowires into the interior space of the microtube, which are found to be terminated by AuGa-alloy nanodots coated in a shell of amorphous GaO_x species after the hydride vapor phase epitaxy process. The structural characterization suggests that this hierarchical design of GaN/ZnO microtubes could offer the potential to exhibit improved photocatalytic properties, which are initially demonstrated under UV light irradiation. As a proof of concept, the produced microtubes are used as photocatalytic micromotors in the presence of hydrogen peroxide solution with luminescent properties, which are appealing for future environmental applications and active matter fundamental studies.

Magnetic Fields Enhanced the Performance of Tubular Dichalcogenide Micromotors at Low Hydrogen Peroxide Levels

Propulsion at the microscale has attracted significant research interest. In this work, a numerical simulation to explain the speed boost of up to 34 % experienced by transition metal dichalcogenides (TMD) based micromotors under the effect of applied magnetic fields is described. The simulations show that, when an external magnetic field is applied, the flow regime changes from turbulent to laminar. This causes an increase in the residence time of the fuel over the catalyst surface, which enhances the oxygen production. The more efficient generation and growth of the bubbles lead to an increase of the capillary force exerted by them. Interestingly, the effect is more pronounced as the level of fuel decrease. The validity of the model is also proven by comparing both theoretical and experimental results. Interestingly, the speed enhancement in magnetic mode depends on geometrical factors only, as a similar phenomenon was observed in a variety of microjets with a variable surface roughness. The understanding of such phenomena will open new avenues for understanding and controlling the motion behavior of high-towing-force catalytic micromotors.

Motile Micropump Based on Synthetic Micromotors for Dynamic Micropatterning

Micropump systems show great potential on the micropatterning process as a result of remarkable performance and functionality. However, existing micropumps cannot be employed as direct writing tools to perform the complex micropatterning process because of their lacking motility and controllability. Here, we propose a motile micropump system based on the combination of a water-driven ZnO/Ni/polystyrene Janus micromotor with a traditional immobilized micropump. This novel motile micropump system can translate the trajectory of Janus micromotors into predefined micropatterns by pumping away passive silica particles around the micromotor under the effect of diffusiophoresis. The resolution and efficiency of the micropatterning process can be regulated by controlling the diameters of Janus micromotors. Diverse surface micropatterns can be fabricated though remote magnetic control of the motile micropump system. Such ability to transform the versatile motile micropump into predetermined surface micropatterns creates new opportunities for mask-free micropatterning.

Biocompatibility of artificial micro/nanomotors for use in biomedicine

The advent of micro/nanomotors (MNMs) has shed light on the innovation of active biomedical systems or devices that might bring revolutionary solutions to traditional biomedical strategies. In spite of development beyond expectation over the last decade with a fair number of proof-of-concept demonstrations, the in vivo practical application of MNMs for clinical use is still in its infancy. The biocompatibility of MNMs is the first consideration before realizing practicality, taking into account the complicated interactions between the self-propelled MNMs and biological systems. Therefore, in this review, we focused on the biocompatibility of MNMs with regard to the fabrication materials and propulsion mechanisms by means of in-depth discussions on the advantages and limitations of MNMs for operating under physiological conditions. The future prospective and suggestions on the development of MNMs toward practical biomedical applications will also be proposed.

Visible-Light-Driven Single-Component BiVO4 Micromotors with the Autonomous Ability for Capturing Microorganisms

Light-driven micro/nanomotors represent the next generation of automotive devices that can be easily actuated and controlled by using an external light source. As the field evolves, there is a need for developing more sophisticated micromachines that can fulfill diverse tasks in complex environments. Herein, we introduce single-component BiVO₄ micromotors with well-defined micro/nanostructures that can swim both individually and as collectively assembled entities under visible-light irradiation. These devices can perform cargo loading and transport of passive particles as well as living microorganisms without any surface functionalization. Interestingly, after photoactivation, the BiVO₄ micromotors exhibited an ability to seek and adhere to yeast cell walls, with the possibility to control their attachment/release by switching the light on/off, respectively. Taking advantage of the selective motor/fungal cells attachment, the fungicidal activity of BiVO₄ micromotors under visible illumination was also demonstrated. The presented star-shaped BiVO₄ micromotors, obtained by a hydrothermal synthesis, contribute to the potential large-scale fabrication of light-powered micromotors. Moreover, these multifunctional single-component micromachines with controlled self-propulsion, collective behavior, cargo transportation, and photocatalytic activity capabilities hold promising applications in sensing, biohybrids assembly, cargo delivery, and microbiological water pollution remediation.

Simple-Structured Micromotors Based on Inherent Asymmetry in Crystalline Phases: Design, Large-Scale Preparation, and Environmental Application

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 TiO₂ particles by adjusting the calcination temperature to control the phase transition and growth of primary grains. The as-designed anatase/rutile TiO₂ 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.

Metal-Free Visible-Light Photoactivated C3N4 Bubble-Propelled Tubular Micromotors with Inherent Fluorescence and On/Off Capabilities

Photoactivated micromachines are at the forefront of the micro- and nanomotors field, as light is the main power source of many biological systems. Currently, this rapidly developing field is based on metal-containing segments, typically TiO₂ and precious metals. Herein, we present metal-free tubular micromotors solely based on graphitic carbon nitride, as highly scalable and low-cost micromachines that can be actuated by turning on/off the light source. These micromotors are able to move by a photocatalytic-induced bubble-propelled mechanism under visible light irradiation, without any metal-containing part or biochemical molecule on their structure. Furthermore, they exhibit interesting properties, such as a translucent tubular structure that allows the optical visualization of the O₂ bubble formation and migration inside the microtubes, as well as inherent fluorescence and adsorptive capability. Such properties were exploited for the removal of a heavy metal from contaminated water with the concomitant optical monitoring of its adsorption by fluorescence quenching. This multifunctional approach contributes to the development of metal-free bubble-propelled tubular micromotors actuated under visible light irradiation for environmental applications.

ZnO/ZnO2/Pt Janus Micromotors Propulsion Mode Changes with Size and Interface Structure: Enhanced Nitroaromatic Explosives Degradation under Visible Light

Self-motile mesoporous ZnO/Pt-based Janus micromotors accelerated by bubble propulsion that provide efficient removal of explosives and dye pollutants via photodegradation under visible light are presented. Decomposition of H₂O₂ (the fuel) is triggered by a platinum catalytic layer asymmetrically deposited on the nanosheets of the hierarchical and mesoporous ZnO microparticles. The size-dependent motion behavior of the mesoporous micromotors is studied; the micromotors with average size ∼1.5 μm exhibit enhanced self-diffusiophoretic motion, whereas the fast bubble propulsion is detected for micromotors larger than 5 μm. The bubble-propelled mesoporous ZnO/Pt Janus micromotors show remarkable speeds of over 350 μm s^-1 at H₂O₂ concentrations lower than 5 wt %, which is unusual for Janus micromotors based on dense materials such as ZnO. This high speed is related to efficient bubble nucleation, pinning, and growth due to the highly active and rough surface area of these micromotors, whereas the ZnO/Pt particles with a smooth surface and low surface area are motionless. We discovered new atomic interfaces of ZnO₂ introduced into the ZnO/Pt micromotor system, as revealed by X-ray diffraction (XRD), which contribute to enhance their photocatalytic activity under visible light. Such coupling of the rapid movement with the high catalytic performance of ZnO/Pt Janus micromotors provides efficient removal of nitroaromatic explosives and dye pollutants from contaminated water under visible light without the need for UV irradiation. This paves the way for real-world environmental remediation efforts using microrobots.

High-Motility Visible Light-Driven Ag/AgCl Janus Micromotors

Visible light-driven nano/micromotors are promising candidates for biomedical and environmental applications. This study demonstrates blue light-driven Ag/AgCl-based spherical Janus micromotors, which couple plasmonic light absorption with the photochemical decomposition of AgCl. These micromotors reveal high motility in pure water, i.e., mean squared displacements (MSD) reaching 800 µm² within 8 s, which is 100× higher compared to previous visible light-driven Janus micromotors and 7× higher than reported ultraviolet (UV) light-driven AgCl micromotors. In addition to providing design rules to realize efficient Janus micromotors, the complex dynamics revealed by individual and assemblies of Janus motors is investigated experimentally and in simulations. The effect of suppressed rotational diffusion is focused on, compared to UV light-driven AgCl micromotors, as a reason for this remarkable increase of the MSD. Moreover, this study demonstrates the potential of using visible light-driven plasmonic Ag/AgCl-based Janus micromotors in human saliva, phosphate-buffered saline solution, the most common isotonic buffer that mimics the environment of human body fluids, and Rhodamine B solution, which is a typical polluted dye for demonstrations of photocatalytic environmental remediation. This new knowledge is useful for designing visible light driven nano/micromotors based on the surface plasmon resonance effect and their applications in assays relevant for biomedical and ecological sciences.

Photocatalytic Micro/Nanomotors: From Construction to Applications

Synthetic micro/nanomotors (MNMs) are a particular class of micrometer or nanometer scale devices with controllable motion behavior in solutions by transferring various energies (chemical, optical, acoustic, magnetic, electric, etc.) into mechanical energy. These tiny devices can be functionalized either chemically or physically to accomplish complex tasks in a microcosm. Up to now, MNMs have exhibited great potential in various fields, ranging from environmental remediation, nanofabrication, to biomedical applications. Recently, light-driven MNMs as classic artificial MNMs have attracted much attention. Under wireless remote control, they can perform reversible and repeatable motion behavior with immediate photoresponse. Photocatalytic micro/nanomotors (PMNMs) based on photocatalysts, one of the most important light-driven MNMs, can utilize energy from both the external light source and surrounding chemicals to achieve efficient propulsion. Unlike other kinds of MNMs, the PMNMs have a unique characteristic: photocatalytic property. On one hand, since photocatalysts can convert both optical and chemical energy inputs into mechanical propulsion of PMNMs via photocatalytic reactions, the propulsion generated can be modulated in many ways, such as through chemical concentration or light intensity. In addition, these PMNMs can be operated at low levels of optical and chemical energy input which is highly desired for more practical scenarios. Furthermore, PMNMs can be operated with custom features, including go/stop motion control through regulating an on/off switch, speed modulation through varying light intensities, direction control through adjusting light source position, and so forth. On the other hand, as superoxide radicals can be generated by photocatalytic reactions of activated photocatalysts, the PMNMs show great potential in environment remediation, especially in organic pollutant degradation. In order to construct more practical PMNMs for future applications and further extend their application fields, the ideal PMNMs should be operated in a fully environmentally friendly system with strong propulsion. In the past decade, great progress in the construction, motion regulation, and application of PMNMs has been achieved, but there are still some challenges to realize the perfect system. In this Account, we will summarize our recent efforts and those of other groups in the development toward attractive PMNM systems. First, we will illustrate basic principles about the photocatalytic reactions of photocatalysts and demonstrate how the photocatalytic reactions affect the propulsion of PMNMs. Then, we will illustrate the construction strategies for highly efficient and biocompatible PMNMs from two key aspects: (1) Improvement of energy conversion efficiency to achieve strong propulsion of PMNMs. (2) Expansion of the usable wavelengths of light to operate PMNMs in environment-friendly conditions. Next, potential applications of PMNMs have been described. In particular, environment remediation has taken major attention for the applications of PMNMs due to their photocatalytic properties. Finally, in order to promote the development of PMNMs which can be operated in fully green environments for more practical applications, an outlook of key challenges and opportunities in construction of ideal PMNMs is presented.

Cu@TiO2 Janus microswimmers with a versatile motion mechanism

We report novel metal-capped TiO2 photochemically-active colloids endowed with a 'hybrid drive': directional motion is achieved in water upon UV illumination, as well as in dilute peroxide solutions upon illumination with UV or visible light. From the different behaviours of nearby particles, we infer that distinct reaction pathways affect the local composition and flow of the solution.

Fuel-Free Light-Powered TiO2/Pt Janus Micromotors for Enhanced Nitroaromatic Explosives Degradation

Nitroaromatic explosives such as 2,4,6-trinitrotoluene (2,4,6-TNT) and 2,4-dinitrotoluene (2,4-DNT) are two common nitroaromatic compounds in ammunition. Their leakage leads to serious environmental pollution and threatens human health. It is important to remove or decompose them rapidly and efficiently. In this work, we present that light-powered TiO₂/Pt Janus micromotors have high efficiency for the "on-the-fly" photocatalytic degradation of 2,4-DNT and 2,4,6-TNT in pure water under UV irradiation. The redox reactions, induced by photogenerated holes and electrons on the TiO₂/Pt Janus micromotor surfaces, produce a local electric field that propels the micromotors as well as oxidative species that are able to photodegrade 2,4-DNT and 2,4,6-TNT. Furthermore, the moving TiO₂/Pt Janus micromotors show an efficient degradation of nitroaromatic compounds as compared to the stationary ones thanks to the enhanced mixing and mass transfer in the solution by movement of these micromotors. Such fuel-free light-powered micromotors for explosive degradation are expected to find a way to environmental remediation and security applications.

Geometry Design, Principles and Assembly of Micromotors

Discovery of bio-inspired, self-propelled and externally-powered nano-/micro-motors, rotors and engines (micromachines) is considered a potentially revolutionary paradigm in nanoscience. Nature knows how to combine different elements together in a fluidic state for intelligent design of nano-/micro-machines, which operate by pumping, stirring, and diffusion of their internal components. Taking inspirations from nature, scientists endeavor to develop the best materials, geometries, and conditions for self-propelled motion, and to better understand their mechanisms of motion and interactions. Today, microfluidic technology offers considerable advantages for the next generation of biomimetic particles, droplets and capsules. This review summarizes recent achievements in the field of nano-/micromotors, and methods of their external control and collective behaviors, which may stimulate new ideas for a broad range of applications.