Micro/Nanomotors for Water Purification

Kinetic monitoring of molecular interactions during surfactant-driven self-propelled droplet motion by high spatial resolution waveguide sensing

HYPOTHESIS

Self-driven actions, like motion, are fundamental characteristics of life. Today, intense research focuses on the kinetics of droplet motion. Quantifying macroscopic motion and exploring the underlying mechanisms are crucial in self-structuring and self-healing materials, advancements in soft robotics, innovations in self-cleaning environmental processes, and progress within the pharmaceutical industry. Usually, the driving forces inducing macroscopic motion act at the molecular scale, making their real-time and high-resolution investigation challenging. Label-free surface sensitive measurements with high lateral resolution could in situ measure both molecular-scale interactions and microscopic motion.

EXPERIMENTS

We employ surface-sensitive label-free sensors to investigate the kinetic changes in a self-assembled monolayer of the trimethyl(octadecyl)azanium chloride surfactant on a substrate surface during the self-propelled motion of nitrobenzene droplets. The adsorption-desorption of the surfactant at various concentrations, its removal due to the moving organic droplet, and rebuilding mechanisms at droplet-visited areas are all investigated with excellent time, spatial, and surface mass density resolution.

FINDINGS

We discovered concentration dependent velocity fluctuations, estimated the adsorbed amount of surfactant molecules, and revealed multilayer coverage at high concentrations. The desorption rate of surfactant (18.4 s-1) during the microscopic motion of oil droplets was determined by in situ differentiating between droplet visited and non-visited areas.

Chemical Logic Gates on Active Colloids

Recent studies have shown that active colloidal motors using enzymatic reactions for propulsion hold special promise for applications in fields ranging from biology to material science. It will be desirable to have active colloids with capability of computation so that they can act autonomously to sense their surroundings and alter their own dynamics. It is shown how small chemical networks that make use of enzymatic chemical reactions on the colloid surface can be used to construct motor-based chemical logic gates. The basic features of coupled enzymatic reactions that are responsible for propulsion and underlie the construction and function of chemical gates are described using continuum theory and molecular simulation. Examples are given that show how colloids with specific chemical logic gates, can perform simple sensing tasks. Due to the diverse functions of different enzyme gates, operating alone or in circuits, the work presented here supports the suggestion that synthetic motors using such gates could be designed to operate in an autonomous way in order to complete complicated tasks.

Dual-functional metal-organic frameworks-based hydrogel micromotor for uranium detection and removal

Self-propelled micro/nanomotors have attracted great attention for environmental remediation, however, their use for radioactive waste detection and removal has not been addressed. Engineered micromotors that are able to combine fast detection and highly adsorptive capability are promising tools for radioactive waste management but remain challenging. Herein, we design self-propelled micromotors based on zeolite imidazolate framework (ZIF-8)-hydrogel composites via inverse emulsion polymerization and show their potential for efficient uranium detection and removal. The incorporation of magnetic ferroferric oxide nanoparticles enables the magnetic recycling and actuation of the single micromotors as well as formation of swarms of worm-like or tank-treading structure. Benefited from the enhanced motion, the micromotors show fast and high-capacity uranium adsorption (747.3 mg g-1), as well as fast uranium detection based on fluorescence quenching. DFT calculation confirms the strong binding between carboxyl groups and uranyl ions. The combination of poly(acrylic acid-co-acrylamide) with ZIF-8 greatly enhances the fluorescence of the micromotor, facilitating the high-resolution fluorescence detection. A low detection limit of 250 ppb is reached by the micromotors. Such self-propelled micromotors provide a new strategy for the design of smart materials in remediation of radioactive wastewater.

Clustering of chemically propelled nanomotors in chemically active environments

Synthetic nanomotors powered by chemical reactions have been designed to act as vehicles for active cargo transport, drug delivery, and a variety of other uses. Collections of such motors, acting in consort, can self-assemble to form swarms or clusters, providing opportunities for applications on various length scales. While such collective behavior has been studied when the motors move in a chemically inactive fluid environment, when the medium in which they move is a chemical network that supports complex spatial and temporal patterns, through simulation and theoretical analysis we show that collective behavior changes. Spatial patterns in the environment can guide and control motor collective states, and interactions of the motors with their environment can give rise to distinctive spatiotemporal motor patterns. The results are illustrated by studies of the motor dynamics in systems that support Turing patterns and spiral waves. This work is relevant for potential applications that involve many active nanomotors moving in complex chemical or biological environments.

3D Motion Manipulation for Micro- and Nanomachines: Progress and Future Directions

In the past decade, micro- and nanomachines (MNMs) have made outstanding achievements in the fields of targeted drug delivery, tumor therapy, microsurgery, biological detection, and environmental monitoring and remediation. Researchers have made significant efforts to accelerate the rapid development of MNMs capable of moving through fluids by means of different energy sources (chemical reactions, ultrasound, light, electricity, magnetism, heat, or their combinations). However, the motion of MNMs is primarily investigated in confined two-dimensional (2D) horizontal setups. Furthermore, three-dimensional (3D) motion control remains challenging, especially for vertical movement and control, significantly limiting its potential applications in cargo transportation, environmental remediation, and biotherapy. Hence, an urgent need is to develop MNMs that can overcome self-gravity and controllably move in 3D spaces. This review delves into the latest progress made in MNMs with 3D motion capabilities under different manipulation approaches, discusses the underlying motion mechanisms, explores potential design concepts inspired by nature for controllable 3D motion in MNMs, and presents the available 3D observation and tracking systems.

Versatile Multi-Wavelength Light-Responsive Metal-Organic Frameworks Micromotor through Porphyrin Metalation for Water Sterilization

Traditional metal-organic frameworks (MOFs) based micro/nanomotors (MOFtors) can achieve three-dimensional (3D) motion mainly depending on noble metal (e.g., Pt), toxic fuels (e.g., hydrogen peroxide), and surfactants, or under external magnetic fields. In this study, light-driven MOFtors are constructed based on PCN-224(H) and regulated their photothermal and photochemical properties responding to the light of different wavelengths through porphyrin metalation. The resulting PCN-224(Fe) MOFtors presented a strong 3D motion at a maximum speed of 1234.9 ± 367.5 µm s-1 under visible light due to the various gradient fields by the photothermal and photochemical effects. Such MOFtors exhibit excellent water sterilization performance. Under optimal conditions, the PCN-224(Cu) MOFtors presented the best antibacterial performance of 99.4%, which improved by 23.4% compared to its static counterpart and 43.7% compared to static PCN-224(H). The underlying mechanism demonstrates that metal doping could increase the production of reactive oxygen species (ROS) and result in a more positive surface charge under light, which are short-distance effective sterilizing ingredients. Furthermore, the motion of MOFtors appears very important to extend the short-distance effective sterilization and thus synergistically improve the antibacterial performance. This work provides a new idea for preparing and developing light-driven MOFtors with multi-responsive properties.

Micro/nanorobots for remediation of water resources and aquatic life

Nowadays, global water scarcity is becoming a pressing issue, and the discharge of various pollutants leads to the biological pollution of water bodies, which further leads to the poisoning of living organisms. Consequently, traditional water treatment methods are proving inadequate in addressing the growing demands of various industries. As an effective and eco-friendly water treatment method, micro/nanorobots is making significant advancements. Based on researches conducted between 2019 and 2023 in the field of water pollution using micro/nanorobots, this paper comprehensively reviews the development of micro/nanorobots in water pollution control from multiple perspectives, including propulsion methods, decontamination mechanisms, experimental techniques, and water monitoring. Furthermore, this paper highlights current challenges and provides insights into the future development of the industry, providing guidance on biological water pollution control.

Site-Selective Superassembly of a Multilevel Asymmetric Nanomotor with Wavelength-Modulated Propulsion Mechanisms

Micro-/nanomotors with advanced motion manipulation have recently received mounting interest; however, research focusing on the motion regulation strategies is still limited, as the simple construction and composition of micro-/nanomotors restrict the functionality. Herein, a multifunctional TiO₂-SiO₂-mesoporous carbon nanomotor is synthesized via an interfacial superassembly strategy. This nanomotor shows an asymmetric matchstick-like structure, with a head composed of TiO₂ and a tail composed of SiO₂. Mesoporous carbon is selectively grown on the surface of TiO₂ through surface-charge-mediated assembly. The spatially anisotropic distribution of the photocatalytic TiO₂ domain and photothermal carbon domain enables multichannel control of the motion, where the speed can be regulated by energy input and the directionality can be regulated by wavelength. Upon UV irradiation, the nanomotor exhibits a head-leading self-diffusiophoretic motion, while upon NIR irradiation, the nanomotor exhibits a tail-leading self-thermophoretic motion. As a proof-of-concept, this mechanism-switchable nanomotor is employed in wavelength-regulated targeted cargo delivery on a microfluidic chip. From an applied point of view, this nanomotor holds potential in biomedical applications such as active drug delivery and phototherapy. From a fundamental point of view, this research can provide insight into the relationship between the nanostructures, propulsion mechanisms, and motion performance.

Role of Bubble Evolution in the Bubble-Propelled Janus Micromotors

Bubble-propelled Janus micromotors have attracted extensive attention in recent years and have been regarded as powerful tools in the environmental and medical fields due to their excellent movement ability. The movement ability can mainly be attributed to the periodic growth, detachment, and/or collapse of the bubble. However, subjected to the experimental conditions, the mechanism of bubble evolution on the motion of the micromotor could not be elucidated clearly. In this work, a finite element method was employed for exploring the role of bubble evolution in bubble-propelled Janus micromotors, which emphasized the growth and collapse of bubbles. After the proposed model was verified by the scallop theorem, the influence of the growth and rapid collapse of bubbles on micromotors was investigated. Results show that the growth and collapse of a bubble can drive the micromotor to produce a displacement, but the displacement caused by a bubble collapse is significantly greater than that caused by bubble growth. The reasons for this phenomenon are analyzed and explained. In addition to the influence of bubble size, the collapse time of the bubble is also investigated.

Steering Micromotors via Reprogrammable Optoelectronic Paths

Steering micromotors is important for using them in practical applications and as model systems for active matter. This functionality often requires magnetic materials in the micromotor, taxis behavior of the micromotor, or the use of specifically designed physical boundaries. Here, we develop an optoelectronic strategy that steers micromotors with programmable light patterns. In this strategy, light illumination turns hydrogenated amorphous silicon conductive, generating local electric field maxima at the edge of the light pattern that attracts micromotors via positive dielectrophoresis. As an example, metallo-dielectric Janus microspheres that self-propelled under alternating current electric fields were steered by static light patterns along customized paths and through complex microstructures. Their long-term directionality was also rectified by ratchet-shaped light patterns. Furthermore, dynamic light patterns that varied in space and time enabled more advanced motion controls such as multiple motion modes, parallel control of multiple micromotors, and the collection and transport of motor swarms. This optoelectronic steering strategy is highly versatile and compatible with a variety of micromotors, and thus it possesses the potential for their programmable control in complex environments.

Precisely Navigated Biobot Swarms of Bacteria Magnetospirillum magneticum for Water Decontamination

Hybrid biological robots (biobots) prepared from living cells are at the forefront of micro-/nanomotor research due to their biocompatibility and versatility toward multiple applications. However, their precise maneuverability is essential for practical applications. Magnetotactic bacteria are hybrid biobots that produce magnetosome magnetite crystals, which are more stable than synthesized magnetite and can orient along the direction of earth's magnetic field. Herein, we used Magnetospirillum magneticum strain AMB-1 (M. magneticum AMB-1) for the effective removal of chlorpyrifos (an organophosphate pesticide) in various aqueous solutions by naturally binding with organic matter. Precision control of M. magneticum AMB-1 was achieved by applying a magnetic field. Under a programed clockwise magnetic field, M. magneticum AMB-1 exhibit swarm behavior and move in a circular direction. Consequently, we foresee that M. magneticum AMB-1 can be applied in various environments to remove and retrieve pollutants by directional control magnetic actuation.

Collective Behaviors of Active Matter Learning from Natural Taxes Across Scales

Taxis orientation is common in microorganisms, and it provides feasible strategies to operate active colloids as small-scale robots. Collective taxes involve numerous units that collectively perform taxis motion, whereby the collective cooperation between individuals enables the group to perform efficiently, adaptively, and robustly. Hence, analyzing and designing collectives is crucial for developing and advancing microswarm toward practical or clinical applications. In this review, natural taxis behaviors are categorized and synthetic microrobotic collectives are discussed as bio-inspired realizations, aiming at closing the gap between taxis strategies of living creatures and those of functional active microswarms. As collective behaviors emerge within a group, the global taxis to external stimuli guides the group to conduct overall tasks, whereas the local taxis between individuals induces synchronization and global patterns. By encoding the local orientations and programming the global stimuli, various paradigms can be introduced for coordinating and controlling such collective microrobots, from the viewpoints of fundamental science and practical applications. Therefore, by discussing the key points and difficulties associated with collective taxes of different paradigms, this review potentially offers insights into mimicking natural collective behaviors and constructing intelligent microrobotic systems for on-demand control and preassigned tasks.

Controlled propulsion of micro/nanomotors: operational mechanisms, motion manipulation and potential biomedical applications

Inspired by natural mobile microorganisms, researchers have developed micro/nanomotors (MNMs) that can autonomously move by transducing different kinds of energies into kinetic energy. The rapid development of MNMs has created tremendous opportunities for biomedical fields including diagnostics, therapeutics, and theranostics. Although the great progress has been made in MNM research, at a fundamental level, the accepted propulsion mechanisms are still a controversial matter. In practical applications such as precision nanomedicine, the precise control of the motion, including the speed and directionality, of MNMs is also important, which makes advanced motion manipulation desirable. Very recently, diverse MNMs with different propulsion strategies, morphologies, sizes, porosities and chemical structures have been fabricated and applied for various uses. Herein, we thoroughly summarize the physical principles behind propulsion strategies, as well as the recent advances in motion manipulation methods and relevant biomedical applications of these MNMs. The current challenges in MNM research are also discussed. We hope this review can provide a bird's eye overview of the MNM research and inspire researchers to create novel and more powerful MNMs.

Cascade Catalytically Released Nitric Oxide-Driven Nanomotor with Enhanced Penetration for Antibiofilm

Nanodrugs are becoming increasingly important in the treatment of bacterial infection, but their low penetration ability to bacterial biofilm is still the main challenge hindering their therapeutic effect. Herein, nitric oxide (NO)-driven nanomotor based on L-arginine (L-Arg) and gold nanoparticles (AuNPs) loaded dendritic mesoporous silica nanoparticles (AG-DMSNs) is fabricated. AG-DMSNs have the characteristics of cascade catalytic reaction, where glucose is first catalyzed by the asymmetrically distributed AuNPs with their glucose oxidase (GOx)- mimic property, which results in unilateral production of hydrogen peroxide (H₂ O₂ ). Then, L-Arg is oxidized by the produced H₂ O₂ to release NO, leading to the self-propelled movement. It is found that the active movement of nanomotor promotes the AG-DMSNs ability to penetrate biofilm, thus achieving good biofilm clearance in vitro. More importantly, AG-DMSNs nanomotor can eliminate the biofilm of methicillin-resistant Staphylococcus aureus (MRSA) in vivo without causing damage to normal tissues. This nanomotor provides a new platform for the treatment of bacterial infections.

Enzyme-Photocatalyst Tandem Microrobot Powered by Urea for Escherichia coli Biofilm Eradication

Urinary-based infections affect millions of people worldwide. Such bacterial infections are mainly caused by Escherichia coli (E. coli) biofilm formation in the bladder and/or urinary catheters. Herein, the authors present a hybrid enzyme/photocatalytic microrobot, based on urease-immobilized TiO₂ /CdS nanotube bundles, that can swim in urea as a biocompatible fuel and respond to visible light. Upon illumination for 2 h, these microrobots are able to remove almost 90% of bacterial biofilm, due to the generation of reactive radicals, while bare TiO₂ /CdS photocatalysts (non-motile) or urease-coated microrobots in the dark do not show any toxic effect. These results indicate a synergistic effect between the self-propulsion provided by the enzyme and the photocatalytic activity induced under light stimuli. This work provides a photo-biocatalytic approach for the design of efficient light-driven microrobots with promising applications in microbiology and biomedicine.

Trapping and detecting nanoplastics by MXene-derived oxide microrobots

Nanoplastic pollution, the final product of plastic waste fragmentation in the environment, represents an increasing concern for the scientific community due to the easier diffusion and higher hazard associated with their small sizes. Therefore, there is a pressing demand for effective strategies to quantify and remove nanoplastics in wastewater. This work presents the “on-the-fly” capture of nanoplastics in the three-dimensional (3D) space by multifunctional MXene-derived oxide microrobots and their further detection. A thermal annealing process is used to convert Ti₃C₂T_x MXene into photocatalytic multi-layered TiO₂, followed by the deposition of a Pt layer and the decoration with magnetic γ-Fe₂O₃ nanoparticles. The MXene-derived γ-Fe₂O₃/Pt/TiO₂ microrobots show negative photogravitaxis, resulting in a powerful fuel-free motion with six degrees of freedom under light irradiation. Owing to the unique combination of self-propulsion and programmable Zeta potential, the microrobots can quickly attract and trap nanoplastics on their surface, including the slits between multi-layer stacks, allowing their magnetic collection. Utilized as self-motile preconcentration platforms, they enable nanoplastics’ electrochemical detection using low-cost and portable electrodes. This proof-of-concept study paves the way toward the “on-site” screening of nanoplastics in water and its successive remediation.

Nanoplastic water pollution represents an increasing concern. Here, photogravitactic MXene-derived microrobots are programmed to trap nanoplastics in the layered structure and magnetically transfer them to low-cost electrodes for further detection.

A Robot Platform for Highly Efficient Pollutant Purification

In this study, we propose a highly efficient robot platform for pollutant adsorption. This robot system consists of a flapping-wing micro aircraft (FWMA) for long-distance transportation and delivery and cost-effective multifunctional Janus microrobots for pollutant purification. The flapping-wing micro air vehicle can hover for 11.3 km with a flapping frequency of approximately 15 Hz, fly forward up to 31.6 km/h, and drop microrobots to a targeted destination. The Janus microrobot, which is composed of a silica microsphere, nickel layer, and hydrophobic layer, is used to absorb the oil and process organic pollutants. These Janus microrobots can be propelled fast up to 9.6 body lengths per second, and on-demand speed regulation and remote navigation are manageable. These Janus microrobots can continuously carry oil droplets in aqueous environments under the control of a uniform rotating magnetic field. Because of the fluid dynamics induced by the Janus microrobots, a highly efficient removal of Rhodamine B is accomplished. This smart robot system may open a door for pollutant purification.

Pick up and dispose of pollutants from water via temperature-responsive micellar copolymers on magnetite nanorobots

Nano/micromotor technology is evolving as an effective method for water treatment applications in comparison to existing static mechanisms. The dynamic nature of the nano/micromotor particles enable faster mass transport and a uniform mixing ensuring an improved pollutant degradation and removal. Here we develop thermosensitive magnetic nanorobots (TM nanorobots) consisting of a pluronic tri-block copolymer (PTBC) that functions as hands for pollutant removal. These TM nanorobots are incorporated with iron oxide (Fe3O4) nanoparticles as an active material to enable magnetic propulsion. The pickup and disposal of toxic pollutants are monitored by intermicellar agglomeration and separation of PTBC at different temperatures. The as-prepared TM nanorobots show excellent arsenic and atrazine removal efficiency. Furthermore, the adsorbed toxic contaminants on the TM nanorobots can be disposed by a simple cooling process and exhibit good recovery retention after multiple reuse cycles. This combination of temperature sensitive aggregation/separation coupled with magnetic propulsion opens a plethora of opportunities in the applicability of nanorobots in water treatment and targeted pollutant removal approaches.

Shape Engineering of TiO2 Microrobots for "On-the-Fly" Optical Brake

Hybrid microrobots have recently attracted attention due to their ability to combine different energy sources and/or external stimuli for propulsion and performing desired tasks. Despite progresses in the past, on-demand speed modulation for hybrid microrobots has not been analyzed in detail. Herein, the influence of surface properties and crystallite size on the propulsion mechanism of Pt/TiO₂ chemical/light-driven hybrid microrobots is investigated. The morphology of urchin-like Pt/TiO₂ microrobots leads to "on-the-fly" optical brake behavior under UV irradiation. In contrast, smooth Pt/TiO₂ microrobots demonstrate accelerated motion in the same conditions. The comparison between two types of microrobots also indicates the significance of a high surface area and a high crystallite size to increase their speed. The results demonstrate the profound impact of surface features for next-generation smart micro/nanorobots with on-demand reaction capability in dynamically changing environments.

Single-Metal Hybrid Micromotor

Multimode stimuli-regulated propulsions are extremely useful for artificial micro-/nanomotors in performing specialized tasks in different microscopic environments. However, it is still a great challenge to develop a simple and efficient micro/nanosystem which can operate in complicated environments, either with fuel or without fuel. Here, we report a novel hybrid micromotor which only needs one metal with a special structure: micro-spherical shell with a hole. Since we attractively combine the inherently catalytic properties of Pt for chemical propulsion with a designed concave structure for acoustic propulsion, the micromotors can not only move rapidly in H₂O₂ fueled environment due to the chemical reaction between Pt and H₂O₂ but also can exhibit excellent acoustic propulsion in a fuel-free environment due to the non-uniform stress caused by ultrasound. In addition, the attractive group motion behavior of the motors, including aggregation, group migration, and dispersion, is easily realized by acoustic field regulation. The brand-new single-metal hybrid micromotors with a dual driving mode, flexible propulsion regulation, and efficient group motion regulation, which are essential for making micro-/nanomotors compatible with different surrounding environments, are expected to advance the field of artificial nanomachines.

Bubble-propelled plasmon-reinforced Pt-ZnIn2S4 micromotors for stirring-free photocatalytic water purification

With a combination of photocatalysts, plasmonic metal NPs, and strong motion ability, mass-produced bubble-propelled Pt-ZnIn2S4 micromotors presented the potential for replacing conventional mechanical stirring in photocatalytic water purification.

A Practical Guide to Analyzing and Reporting the Movement of Nanoscale Swimmers

The recent invention of nanoswimmers-synthetic, powered objects with characteristic lengths in the range of 10-500 nm-has sparked widespread interest among scientists and the general public. As more researchers from different backgrounds enter the field, the study of nanoswimmers offers new opportunities but also significant experimental and theoretical challenges. In particular, the accurate characterization of nanoswimmers is often hindered by strong Brownian motion, convective effects, and the lack of a clear way to visualize them. When coupled with improper experimental designs and imprecise practices in data analysis, these issues can translate to results and conclusions that are inconsistent and poorly reproducible. This Perspective follows the course of a typical nanoswimmer investigation from synthesis through to applications and offers suggestions for best practices in reporting experimental details, recording videos, plotting trajectories, calculating and analyzing mobility, eliminating drift, and performing control experiments, in order to improve the reliability of the reported results.

Nickel Sulfide Microrockets as Self-Propelled Energy Storage Devices to Power Electronic Circuits "On-Demand"

Miniaturized energy storage devices are essential to power the growing number and variety of microelectronic technologies. Here, a concept of self-propelled microscale energy storage elements that can move, reach, and power electronic circuits is reported. Microrockets consisting of a nickel sulfide (NiS) outer layer and a Pt inner layer are prepared by template-assisted electrodeposition, and designed to store energy through NiS-mediated redox reactions and propel via the Pt-catalyzed decomposition of H₂ O₂ fuel. Scanning electrochemical microscopy allows visualizing and studying the energy storage ability of a single microrocket, revealing its pseudocapacitive nature. This proves the great potential of such technique in the field of micro/nanomotors. On-demand delivery of energy storage units to electronic circuits has been demonstrated by releasing microrockets on an interdigitated array electrode as an example of electronic circuit. Owing to their self-propulsion ability, they reach the active area of the electrode and, in principle, power its functions. These autonomously moving energy storage devices will be employed for next-generation electronics to store and deliver energy in previously inaccessible locations.

Sulfur-encapsulated zeolite micromotors for the selective removal of cesium from high-salt water with accelerated cleanup times

Bubble-propelled sulfur-encapsulated NaX zeolite (S-NaX) micromotors were developed for the selective removal of cesium from high-salt conditions with accelerated cleanup times. NaX was first modified with sulfur to provide additional Lewis acid-base interactions with Cs⁺ for enhanced Cs⁺ selectivity, and then Pt was half-deposited on S-NaX for bubble propulsion via the catalytic decomposition of H₂O₂. The average velocity of the resulting S-NaX/Pt micromotors in 5 wt% H₂O₂ is 39.7 ± 17.1 μm/s, which is higher than that of a previously reported Cs adsorbent micromotor (35.4 μm/s). The Cs⁺ ion-exchange kinetics of the S-NaX micromotor is 1.32 times higher than that of the NaX micromotor in a 5 wt% H₂O₂ solution where the molar ratio of Na⁺ to Cs⁺ is 200, even though the sulfur in the S-NaX micromotor causes an adverse effect on the propulsion speed due to the sulfur poisoning effect. Moreover, the S-NaX micromotor in simulated groundwater also exhibited excellent Cs⁺ removal performance with distribution coefficient (K_d) values at least 3.2 times higher than those of the nonpropelled S-NaX and NaX micromotor, demonstrating the great potential for the treatment of radioactive Cs⁺-contaminated water.

Six-Degree-of-Freedom Steerable Visible-Light-Driven Microsubmarines Using Water as a Fuel: Application for Explosives Decontamination

Micro/nanomotors are capable of a wide variety of tasks related, i.e., to biomedical or environmental applications. Light-driven semiconductor-based micromotors are especially appealing, as they can split surrounding water via light irradiation, and therefore, they can move infinitely. However, their motion is typically limited to in-plane motion with four degrees of freedom (4DoF) or even pseudo-1D motion with 2DoF. Herein, magnetically steerable tubular TiO₂ /Fe₃ O₄ /CdS micromotors, termed microsubmarines, with 6DoF motion, based on a fuel-free design where surrounding water acts as fuel upon visible light irradiation, are presented, with an average velocity of 7.9 µm s^-1 . Besides, the generation of radicals via such water splitting aids the photocatalytic chemicals degradation with the potential to use solar radiation. A light-induced self-electrophoretic mechanism is responsible for the self-propulsion and can be used to predict the motion direction based on the structure and composition. Finally, the TiO₂ /Fe₃ O₄ /CdS microsubmarines are tested in a proof-of-concept application of high-energy explosive, e.g., picric acid, photocatalytic degradation, with the best performance owing to the versatility of 6DoF motion, the surface coating with amorphous TiO₂ layer, and UV light. The results can help optimize light-active micromotor design for potential national security and environmental application, hydrogen evolution, and target cargo delivery.

Micro/nano-machines for spilled-oil cleanup and recovery: A review

High-efficiency, safe and economically viable nano-engineered platforms for oil spill cleanup and recovery are of great importance. This review takes account of the concept of nanomotors and micromotors and their most advancements in use for oil spill treatment. The fundamental facets of artificial micro- and nano-machines/nanobots/nanomotors (MNMs) are first documented, followed by the most recent influencing developments in chemical engineering approaches toward their specific utilizations. The surface chemistry of these MNMs, their behaviors in different water matrices and their roles in the removal of oil are examined, revealing great rooms for improvement. The strategies for surface and structural modification of these tiny machines toward enhancing their reactivity in the removal of oil and coupled tasking are discussed in details, highlighting the significance of fit-for-duty design and tailored fabrication. The engineering limitations and practical implementation barriers of this emerging technology and how it can be overcome are also considered. Finally, some engineering boundaries and perspectives of this fast-evolving field are proposed at the end.

Polypyrrole-Based Nanorobots Powered by Light and Glucose for Pollutant Degradation in Water

Novel photoactive and enzymatically active nanomotors were developed for efficient organic pollutant degradation. The developed preparation route is simple and scalable. Light-absorbing polypyrrole nanoparticles were equipped with a bi-enzyme [glucose oxidase/catalase (GOx/Cat)] system enabling the simultaneous utilization of light and glucose as energy sources for jet-induced nanoparticle movement and active radical production. The GOx utilizes glucose to produce hydrogen peroxide, which is subsequently degraded by Cat, resulting in the generation of active radicals and/or oxygen bubbles that propel the particles. Uneven grafting of GOx/Cat molecules on the nanoparticle surface ensures inhomogeneity of peroxide creation/degradation, providing the nanomotor random propelling. The nanomotors were tested for their ability to degrade chlorophenol, under various experimental conditions, that is, with and without simulated sunlight illumination or glucose addition. In all cases, degradation was accelerated by the presence of the self-propelled nanoparticles or light illumination. Light-induced heating also positively affects enzymatic activity, further accelerating nanomotor diffusion and pollutant degradation. In fact, the chemical and photoactivities of the nanoparticles led to more than 95% removal of chlorophenol in 1 h, without any external stirring. Finally, the quality of the purified water and the extent of pollutant removal were checked using an eco-toxicological assay, with demonstrated significant synergy between glucose pumping and sunlight illumination.

Propulsion Gait Analysis and Fluidic Trapping of Swinging Flexible Nanomotors

Micro- and nanomachines as feasible agents to exploit the microworld have attracted extensive research interest, particularly in the manipulation of soft nanorobots at small scales. Herein, we propose a model for regulating the motion of a swinging flexible nanomotor (SFN) driven by an oscillating magnetic field. Multisegments of an SFN are synthesized from nickel, gold, and porous silver. The coupling of magnetic actuation and the swinging pattern of SFNs are studied to reveal their mobility. Additionally, an optimal frequency occurs from the coupling of magnetic torque and structural deformation, rather than the simply considered step-out phenomenon. Meanwhile, a fluidic trapping region is formulated alongside the SFN. Such a trapping region is demonstrated by trapping a living neutrophil and accomplishing in vitro transportation using fluidic mediation. On-demand cargo delivery can be realized using a programmable magnetic field, and SFNs can be recycled with ease after manipulation owing to environmental concerns. In this study, we demonstrated the properties of SFNs that are useful bases for their customization and control. These flexible nanomotors may have the potential to promote drug delivery and biomedical operations in noncontact modes.

Recent development of autonomously driven micro/nanobots for efficient treatment of polluted water

Autonomously propelled micro/nanobots are one of the most advanced and integrated structures which have been fascinated researchers owing to its exceptional property that enables them to be carried out user-defined tasks more precisely even on an atomic scale. The unique architecture and engineering aspects of these manmade tiny devices make them viable options for widespread biomedical applications. Moreover, recent development in this line of interest demonstrated that micro/nanobots would be very promising for the water treatment as these can efficiently absorb or degrade the toxic chemicals from the polluted water based on their tunable surface chemistry. These auto propelled micro/nanobots catalytically degrade toxic pollutants into non-hazardous compounds more rapidly and effectively. Thus, for the last few decades, nanobots mediated water treatment gaining huge popularity due to its ease of operation and scope of guided motion that could be monitored by various external fields and stimuli. Also, these are economical, energy-saving, and suitable for large scale water treatment, particularly required for industrial effluents. However, the efficacy of these bots hugely relies on its design, characteristic of materials, properties of the medium, types of fuel, and surface functional groups. Minute variation for one of these things may lead to a change in its performance and hinders its dynamics of propulsion. It is deemed that nanobots might be a smart choice for using these as the new generation devices for treating industrial effluents before discharging it in the water bodies, which is a major concern for human health and the environment.

Carbonate Micromotors for Treatment of Construction Effluents

Concrete in construction has recently gained media coverage for its negative CO₂ footprint, but this is not the only problem associated with its use. Due to its chemical composition, freshly poured concrete changes the pH of water coming in contact with the surface to very alkaline values, requiring neutralization treatment before disposal. Conventional methods include the use of mineral acid or CO₂ pumps, causing high costs to building companies. In this paper, we present a micromotor based remediation strategy, which consists of carbonate particles half-coated with citric acid. To achieve this half coverage spray coating is used for the first time to design Janus structures. The motors propel diffusiophoretically due to a self-generated gradient formed as the acid coverage dissolves. The locally lower pH contributes to the dissolution of the carbonate body. These motors have been employed to study neutralization of diluted concrete wash water (CWW) at microscopic scale and we achieve visualization of the pH changes occurring in the vicinity of motors using anthocyanine as pH indicator dye. The effect of citric acid-carbonates hybrid on neutralization of real CWW on macroscopic scale has also been studied. In addition, all employed chemicals are cheap, non-toxic and do not leave any solid residues behind.

Self-propelled torus colloids

Suspensions of chemically powered self-propelled colloidal particles are examples of active matter systems with interesting properties. While simple spherical Janus particles are often studied, it is known that geometry is important and recent experiments have shown that chemically active torus-shaped colloids behave differently from spherical colloids. In this paper, coarse-grained microscopic simulations of the dynamics of self-diffusiophoretic torus colloids are carried out in bulk solution in order to study how torus geometric factors influence their active motion. The concentration and velocity fields are key ingredients in self-diffusiophoretic propulsion, and the forms that these fields take in the colloid vicinity are shown to be strong functions of torus geometric parameters such as the torus hole size and thickness of the torus tube. This work utilizes a method where self-diffusiophoretic torus colloids with various geometric and dynamical characteristics can be built and studied in fluid media that include chemical reactions and fluid flows. The model can be used to investigate the collective properties of these colloids and their dynamics in confined systems, topics that are of general importance for applications that use colloidal motors with complex geometries.

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.

Parameters Optimization of Catalytic Tubular Nanomembrane-Based Oxygen Microbubble Generator

A controllable generation of oxygen gas during the decomposition of hydrogen peroxide by the microreactors made of tubular catalytic nanomembranes has recently attracted considerable attention. Catalytic microtubes play simultaneous roles of the oxygen bubble producing microreactors and oxygen bubble-driven micropumps. An autonomous pumping of peroxide fuel takes place through the microtubes by the recoiling microbubbles. Due to optimal reaction–diffusion processes, gas supersaturation, leading to favorable bubble nucleation conditions, strain-engineered catalytic microtubes with longer length produce oxygen microbubbles at concentrations of hydrogen peroxide in approximately ×1000 lower in comparison to shorter tubes. Dynamic regimes of tubular nanomembrane-based oxygen microbubble generators reveal that this depends on microtubes’ aspect ratio, hydrogen peroxide fuel concentration and fuel compositions. Different dynamic regimes exist, which produce specific bubble frequencies, bubble size and various amounts of oxygen. In this study, the rolled-up Ti/Cr/Pd microtubes integrated on silicon substrate are used to study oxygen evolution in different concentrations of hydrogen peroxide and surfactants. Addition of Sodium dodecyl sulfate (SDS) surfactants leads to a decrease of bubble diameter and an increase of frequencies of bubble recoil. Moreover, an increase of temperature (from 10 to 35 °C) leads to higher frequencies of oxygen bubbles and larger total volumes of produced oxygen.

Wafer-Scale Fabrication of Micro- to Nanoscale Bubble Swimmers and Their Fast Autonomous Propulsion by Ultrasound

Fuel-free, biocompatible swimmers with dimensions smaller than one micrometer have the potential to revolutionize the way we study and manipulate microscopic systems. Sub-micrometer, metallic Janus particles can be propelled rapidly and autonomously by acoustically induced fluid streaming, but their operation at acoustic pressure nodes limits their utility. In contrast, bubble-based microswimmers have an "on board" resonant cavity that enables them to operate far from the source of acoustic power. So far, they have been fabricated by direct writing techniques that limit both their minimum dimensions and the number that can be produced. Consequently, the size scaling of the properties of bubble swimmers has not been explored experimentally. Additionally, 3D autonomous motion has not yet been demonstrated for this type of swimmer. We describe here a method for fabricating bubble swimmers in large numbers (>10⁹) with sizes ranging from 5 μm to 500 nm without direct writing or photolithographic tools. These swimmers follow a previously proposed scaling theory and reveal useful phenomena that enable their propulsion in different modes in the same experiment: with magnetic steering, autonomously in 3D, and in frequency-specific autonomous modes. These interesting behaviors are relevant to possible applications of autonomously moving micro- and nanorobots.

Role of Nanomaterials in the Treatment of Wastewater: A Review

Water is an essential part of life and its availability is important for all living creatures. On the other side, the world is suffering from a major problem of drinking water. There are several gases, microorganisms and other toxins (chemicals and heavy metals) added into water during rain, flowing water, etc. which is responsible for water pollution. This review article describes various applications of nanomaterial in removing different types of impurities from polluted water. There are various kinds of nanomaterials, which carried huge potential to treat polluted water (containing metal toxin substance, different organic and inorganic impurities) very effectively due to their unique properties like greater surface area, able to work at low concentration, etc. The nanostructured catalytic membranes, nanosorbents and nanophotocatalyst based approaches to remove pollutants from wastewater are eco-friendly and efficient, but they require more energy, more investment in order to purify the wastewater. There are many challenges and issues of wastewater treatment. Some precautions are also required to keep away from ecological and health issues. New modern equipment for wastewater treatment should be flexible, low cost and efficient for the commercialization purpose.

Light-Triggered Catalytic Performance Enhancement Using Magnetic Nanomotor Ensembles

Micro/nanomachines have attracted extensive attention in the biomedical and environmental fields for realizing functionalities at small scales. However, they have been rarely investigated as active nanocatalysts. Heterogeneous nanocatalysts have exceptional reusability and recyclability, and integration with magnetic materials enables their recovery with minimum loss. Herein, we propose a model active nanocatalyst using magnetic nanomotor ensembles (MNEs) that can degrade contaminants in an aqueous solution with high catalytic performance. MNEs composed of a magnetite core coated with gold nanoparticles as the nanocatalyst can rotate under the action of a programmable external field and carry out rapid reduction of 4-nitrophenol (4-NP). The hydrogen bubbles generated in the catalytic reaction provide random perturbations for the MNEs to travel in the reaction solution, resulting in uniform processing. The reduction can be further boosted by irradiation with near-infrared (NIR) light. Magnetic field induces the rotation of the MNEs and provides microstirring in the catalysis. Light enhances the catalytic activity via the photothermal effect. These MNEs are also capable of moving to the targeted region through the application of a programmable magnetic field and then process the contaminant in the targeted region. We expect that such magnetic MNEs may help better in applying active heterogeneous nanocatalysts with magnetic field and light-enhanced performance in industrial applications due to their advantages of low material cost and short reaction time.

Radioactive Uranium Preconcentration via Self-Propelled Autonomous Microrobots Based on Metal-Organic Frameworks

Self-propelled micromachines have recently attracted attention for environmental remediation, yet their use for radioactive waste management has not been addressed. Engineered micromotors that are able to combine highly adsorptive capabilities together with fast autonomous motion in liquid media are promising tools for the removal of nuclear waste, which is one of the most difficult types to manage. Herein, we fabricate self-propelled micromotors based on metal-organic frameworks (MOFs) via template-based interfacial synthesis and show their potential for efficient removal of radioactive uranium. A crucial challenge of the MOF-based motors is their stability in the presence of fuel (hydrogen peroxide) and acidic media. We have ensured their structural stability by Fe doping of zeolitic imidazolate framework-8 (ZIF-8). The implementation of magnetic ferroferric oxide nanoparticles (Fe₃O₄ NPs) and catalytic platinum nanoparticles (Pt NPs) results in the magnetically responsive and bubble-propelled micromotors. In the presence of 5 wt % H₂O₂, these micromotors are propelled at a high speed of ca. 860 ± 230 μm·s^-1 (i.e., >60 body lengths per second), which is significantly faster than that of other microrod-based motors in the literature. These micromotors demonstrate a highly efficient removal of uranium (96%) from aqueous solution within 1 h, with the subsequent recovery under magnetic control, as well as stable recycling ability and high selectivity. Such self-propelled magnetically recoverable micromotors could find a role in the management and remediation of radioactive waste.

Recyclable nanographene-based micromachines for the on-the-fly capture of nitroaromatic explosives

It has been more than a decade since nano/micromachines (NMMs) have received the particular attention of scientists in different research fields. They are able to convert chemical energy into mechanical motion in their surrounding environment. Herein, a powerful, efficient and fast strategy of using nanosized reduced graphene oxide flake (n-rGO)-based self-propelled tubular micromachines for the removal of nitroaromatic compounds (NACs) is described. This method relies on the integration of the rGO as a well-known adsorbent of aromatic compounds with chemically powered engines for the removal of explosive compounds such as 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol (TNP) and 2,4-dinitrotoluene (DNT). Nanographene oxide reduced electrochemically inside the pores of the polycarbonate membrane to form an outer layer (n-rGO, adsorbent layer) of the micromachines. Subsequent electrodeposition of nickel (Ni, magnetic layer) and platinum (Pt, catalytic layer) resulted in the formation of n-rGO/Ni/Pt micromachines. Notably, the bubble-propelled micromachines were able to remove nitroaromatic compounds with high efficiency (∼90-92%) compared to the efficiency of magnetic-guided (22-42%) and static (2.5-7%) micromachines. Most importantly, the micromachines were regenerated and reused several times. The regeneration is based on an electrochemical method in which electron injection into the machine causes the expulsion of contaminants from the outer layer of the micromachines within a few seconds. The integration of the powerful self-propulsion, high adsorbent capacity of rGO and the introduced ultrafast regeneration procedure are beneficial for the realization of an active platform for water remediation.