Bubble-propelled Micromotors Research Articles

Bubble-propelled micromotors for ammonia generation.

Micromotors have emerged as promising tools for environmental remediation, thanks to their ability to autonomously navigate and perform specific tasks at the microscale. In this study, we present the development of MnO2 tubular micromotors modified with laccase for enhanced oxidation of organic pollutants by providing an additional oxidative catalytic pathway for pollutant removal. These modified micromotors exhibit efficient ammonia generation through the catalytic decomposition of urea, suggesting their potential application in the field of green energy generation. Compared to bare micromotors, the MnO2 micromotors modified with laccase exhibit a 20% increase in rhodamine B degradation. Moreover, the generation of ammonia increased from 2 to 31 ppm in only 15 min, evidencing their high catalytic activity. To enable precise tracking of the micromotors and measurement of their speed, a deep-learning-based tracking system was developed. Overall, this work expands the potential applicability of bio-catalytic tubular micromotors in the energy field.

Biodegradable magnesium fuel-based Janus micromotors with surfactant induced motion direction reversal

The speed and motion directionality of bubble-propelled micromotors is dependent on bubble lifetime, bubble formation frequency and bubble stabilization. Absence and presence of bubble stabilizing agents should significantly influence speed and propulsion pattern of a micromotor, especially for fast-diffusing molecules like hydrogen. This study demonstrates a fully biodegradable Janus structured micromotor, propelled by hydrogen bubbles generated by the chemical reaction between hydrochloric acid and magnesium. Six different concentrations of hydrochloric acid and five different concentrations of the surfactant Triton X-100 were tested, which also cover the critical micelle concentration at a pH corresponding to an empty stomach. The Janus micromotor reverses its propulsion direction depending on the availability and concentration of a surfactant. Upon surfactant-free condition, the Janus micromotor is propelled by bubble cavitation, causing the micromotor to be pulled at high speed for short time intervals into the direction of the imploding bubble and thus backwards. In case of available surfactant above the critical micelle concentration, the Janus micromotor is pushed forward by the generated bubbles, which emerge at high frequency and form a bubble trail. The finding of the propulsion direction reversal effect demonstrates the importance to investigate the motion properties of artificial micromotors in a variety of different environments prior to application, especially with surfactants, since biological media often contain large amounts of surface-active components.

Integration of BiOI nanosheets into bubble-propelled micromotors for efficient water purification

Integration of 2D layered materials into micromotors is of great interest for scientists since 2D materials can endow micromotors with new characteristics or enhance their intrinsic properties. In this work, we develop the BiOI-based self-propelled micromotors (i.e., rGO/ZnO/BiOI/Co-Pi/Pt micromotors) with tubular and arc geometric shapes via template-assisted electrochemical deposition method. BiOI, constructed by nanosheets, is a visible-light-excited photocatalyst. The elaborately designed heterostructure is able to promote the charge separation and transport capacity of BiOI. Each micromotor can not only exhibit vigorous stirring-like movement for self-propulsion but also lead to enhanced photocatalytic dynamics for dye degradation. Using Rhodamine 6G as the dye pollutant model, BiOI-based micromotors can degrade 94% of dye molecules within 30 min under visible light illumination while their degradation efficiency is only 5.2% when in dark. The dye degradation mechanism is mainly based on the generation of reactive oxygen species during the photocatalysis process. The synergistic effect among i) “on-the-fly” chemistry, ii) the collective behavior of micromotors and iii) the visible-light-excited BiOI photocatalyst with synthetic nanoflake morphology and well-designed heterojunction make an efficient and active photocatalytic platform for water purification.

Flexible fabrication of lipophilic-hydrophilic micromotors by off-chip photopolymerization of three-phase immiscible flow induced Janus droplet templates

Self-propelled microparticles are promising for lots of applications ranging from analytical detection to water treatment. Herein, we present an effective approach to fabricate lipophilic-hydrophilic micromotors via the photocuring of three-phase immiscible flow induced droplet templates. In the microfluidic system, two immiscible inner fluids, the lipophilic 1, 6-Hexanediol diacrylate (HDDA), and the hydrophilic poly (ethylene glycol) diacrylate (PEGDA), are simultaneously injected into a theta-shaped cylindrical capillary from two separate inlets, and they are emulsified into Janus drops when encountering the outer immiscible silicone oil. Because of the immiscible feature of droplet templates, off-chip photopolymerization strategy has been used, which can significantly decrease the blocking chance of microdevice. And also, the lipophilic-hydrophilic structure of droplets is convenient for the loading of cargos with different characteristics. More importantly, the size and configuration of droplet templates can be flexibly regulated by changing the flow rates of three different phases. Accordingly, multifunctional micromotors can be fabricated by adding different nanoparticles and materials into the HDDA or PEGDA phase first and then photocuring the droplets. Taking the bubble-propelled micromotors for example, we prepare microswimmers by loading Ag, TiO2 and Fe3O4 nanoparticles into the PEGDA phase. The swimming behaviors of micromotors in H2O2 solution are systematically investigated, finding that the proportion of PEGDA phase and the concentration of H2O2 both positively affect the moving speed. Furthermore, the applicability of motor particles on water treatment is successfully demonstrated by using neutral red solution as the model pollutant. And the micromotors can be recycled using magnets after the catalytic degradation process. Therefore, this micromotor generation technique and this kind of micromotor can be attractive for many applications.

Asparaginase immobilized, magnetically guided, and bubble-propelled micromotors

In this study, tubular poly(3,4-ethylene dioxythiophene)-polypyrrole/Ni/ Pt (PEDOT-PPy-COOH/Ni/Pt) micromotors were prepared and modified with L-asparaginase, which is an anti-leukemic drug. Inner Pt layer of the micromotor decomposes H2O2 to produce O2 and allows the motor for self-propulsion, while the Ni segment enables remote control of the motors via a magnetic field. These tiny motors have self-propulsion capability and can be easily controlled by an external magnetic field. By immobilizing asparaginase, these enzyme-carrying motors demonstrated improved activity for the L-asparagine hydrolysis. At the end of the immobilization process, the enzyme showed improved thermal and pH stability, ascending protease resistance, longer storage time, and reusability. Kinetic parameters of free and immobilized asparaginase were also evaluated. It was shown that affinity for asparaginase towards its substrate did not change significantly upon immobilization. Besides Vmax value of immobilized L-asparaginase was found to be higher than that of free enzyme. Stability against protease trypsin was also tested and it was demonstrated that immobilized L-asparaginase was more stable for trypsin than free L-asparaginase. Finally, artificial serum experiments were also constructed to show the possible in vivo usage, and it was realized that L-asparaginase immobilized micromotor exhibited higher activity than free enzyme and control static micromotor experiments.

Multi-response biocompatible Janus micromotor for ultrasonic imaging contrast enhancement

The acid-base environment of the biological digestive system is formed with the help of hydrogen ions and bicarbonate. At the same time, the chloride capacity in the blood is not ignorable. These ionic environments provide a new horizon for the medical application of bubble-propelled micromotors. Herein, a Janus Mg-based micromotor is reported as a novel ultrasonic contrast agent and its potential advantages in medical imaging are explored. With the capability of being powered by the magnetic field, these micromotors could be navigated to the corresponding ionic environment, and then they would rapidly generate continuous bubbles. These bubbles achieve an efficient and guided propulsion of micromotor, also provide support for contrast-enhanced ultrasound imaging. Besides owning the dual functions of imaging and targeted transport, the micromotors present aggregation response under the action of ultrasound, which is helpful for local reinforcement of the imaging effect. Such multi-response biocompatible Janus micromotor provides a new perspective for medical imaging and diagnosis.

Bubble-propelled micromotors based on hierarchical MnO2 wrapped carbon nanotube aggregates for dynamic removal of pollutants.

Water pollution is currently an urgent public health and environmental issue. Bubble-propelled micromotors might offer an effective approach for dealing with environmental contamination. Herein, we present the synthesis of multi-walled carbon nanotube (MWCNT)/manganese dioxide (MnO2) micromotors based on MWCNT aggregates as microscale templates by a simple one-step hydrothermal procedure. The morphology, composition, and structure of the obtained MWCNT/MnO2 micromotors were characterized in detail. The MnO2 nanoflakes formed a catalytic layer on the MWCNT backbone, which promoted effective bubble evolution and propulsion at remarkable speeds of 359.31 μm s−1. The bubble velocity could be modulated based on the loading of MnO2 nanoflakes. The rapid movement of these MWCNT/MnO2 catalytic micromotors resulted in a highly efficient moving adsorption platform, which considerably enhanced the effectiveness of water purification. Dynamic adsorption of organic dyes by the micromotors increased the degradation rate to approximately 4.8 times as high as that of their corresponding static counterparts. The adsorption isotherms and adsorption kinetics were also explored. The adsorption mechanism was well fitted by the Langmuir model, following pseudo-second-order kinetics. Thus, chemisorption of Congo red at the heterogeneous MnO2 wrapped microimotor surface was the rate determining step. The high propulsion speed and remarkable decontamination efficiency of the MWCNT/MnO2 micromotors indicate potential for environmental contamination applications.

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.

Self-propelled micromachines for analytical sensing: a critical review.

Self-propelled micromotors are micro- and nanoscale devices that move autonomously in solution by converting a specific stimulus into mechanical work. The broad scope of operations and applications along with the ultra-small dimensions have opened new possibilities to solve complex analytical challenges. Herein we give a critical overview of early developments and future prospects of such tiny moving objects for different analytical sensing and biosensing strategies. From early electrophoretic propelled nanomotors, which were limited to low viscous media, to bubble-propelled micromotors, the field has evolved into sophisticated all-in-one analytical systems with built-in sensing capabilities. Current progress for in vivo biosensing and integration into analytical instrumentation towards fully functional devices will be also covered. We hope that this review provides the reader with some general knowledge and future prospects of self-propelled micromachines as a new paradigm in analytical chemistry. Graphical abstract.

Microfluidic Fabrication of Bubble-Propelled Micromotors for Wastewater Treatment.

Bubble-propelled micromotors with controllable shapes and sizes have been developed by a microfluidic method, which serves for effective wastewater treatment. Using the emulsion from microfluidics as the template, monodisperse micromotors can be fabricated in large quantities based on phase separation and UV-induced monomer polymerization. By adjusting the volume ratio of the two immiscible oils (ethoxylated trimethylolpropane triacrylate/paraffin oil) in the initial emulsion, the geometry of the resulting micromotor can be precisely controlled from nearly spherical, hemispherical to crescent-shaped. The size of the micromotor can be manipulated by varying the fluid flow parameters. In addition, by incorporating functional nanoparticles into the asymmetric structure, the micromotor can be functionalized flexibly for water remediation. In this research, Fe3O4 and MnO2 nanoparticles were successfully loaded on Janus micromotors. Fe3O4 nanoparticles can act as catalysts for pollutant degradation and also control the movement direction of micromotors. MnO2 nanoparticles on the concave of the micromotor catalyzed H2O2 to produce bubble propulsion motion in solution, which further enhanced the degradation of pollutants. Consequently, the obtained micromotor demonstrated effective degradation of methylene blue and can be easily recovered by magnets. Furthermore, this simple and flexible strategy offers a synthetic way for anisotropic Janus particles, which will broaden their potential application.

Motion of Enzyme-Powered Microshell Motors.

Microshells are attractive in constructing bubble-propelled micromotors due to the lower energy consumption for bubbles forming on a concave surface. In this work, enzyme-powered microshell motors were fabricated on multimetallic (Au/Ag/Au) microshells along with the modification of catalase on its concave surface. The catalase triggered the decomposition of hydrogen peroxide to oxygen gas, hence propelling the autonomous motion of microshell motors. A size-dependent motion behaviour was observed for the microshell motors in the form of slow tremble and fast translation motion for a size smaller and larger than 5 μm, respectively, according to the size, generation efficiency and ejection mechanism of bubbles and the intensity of Brownian motion. In addition, the effect of fuel concentration on the motion speed of microshells was dependent on whether the bubble generation was affected by the limited mass transfer in the microshell space. These findings play an important role for the design of microshell motors.

Bubble-Propelled Hierarchical Porous Micromotors from Evolved Double Emulsions

Bubble-propelled micromotors with well-engineered hierarchical porous structures are developed by a simple and flexible strategy for efficient water decontamination. Controllably and spontaneously evolved double emulsions from microfluidics are used as templates for continuous fabrication of the micromotors in one-step. The micromotors possess interconnected hierarchical porous structures with two well-aligned microscale pores incorporated in nanoporous matrix. The opening hole of one microscale pore at the bottom of micromotor is decorated with Fe3O4@Ag nanoparticles to decompose H2O2 for bubble-propelled motion. Due to their hydrophobic and magnetic hierarchical porous matrix, the micromotors can be used for capture of oil pollutants in water, and then easy recovery by using a magnetic field. Moreover, the micromotors can be flexibly functionalized by simply incorporating functional nanoparticles into the hierarchical porous structures for efficient water decontamination. This is demonstrated by modifyi...

Facile Fabrication of Bubble-Propelled Micromotors Carrying Nanocatalysts for Water Remediation

A facile and flexible approach is developed to fabricate bubble-propelled mesoporous micromotors carrying nanocatalysts for efficient water remediation. The micromotors are prepared by simply coating the hemispherical surface of Fe3O4-nanoparticle-containing mesoporous SiO2 microparticles with a polydopamine layer for decorating Ag nanoparticles, based on the versatile adhesion and reduction properties of polydopamine. The Fe3O4 nanoparticles can produce OH radicals from H2O2 for pollutant degradation via Fenton reaction, whereas the mesoporous SiO2 matrix provides large surface area with anchored Fe3O4 nanoparticles for improved degradation performance. Moreover, the Ag nanoparticles can decompose H2O2 into oxygen bubbles for powering the movement of micromotors to further enhance the pollutant degradation. The micromotors that synergistically integrate these functions enable efficient degradation of pollutants, such as methylene blue demonstrated in this work, for water remediation. This approach offers...

Zerovalent-Iron/Platinum Janus Micromotors with Spatially Separated Functionalities for Efficient Water Decontamination

Multifunctional zerovalent-iron/platinum (ZVI/Pt) Janus bubble-propelled micromotors with high decontamination efficiency and efficient self-propulsion properties were fabricated by the asymmetric deposition of catalytic platinum (Pt) in one hemisphere of ZVI microspheres. In the ZVI/Pt micromotors–hydrogen peroxide (H2O2) system, ZVI acts as a heterogeneous Fenton-like catalyst for the degradation of organic pollutants, while simultaneously the hemispheric Pt layer catalytically decomposes H2O2 into water and oxygen, thereby resulting in an oxygen-bubble propulsion system. The ZVI/Pt Janus micromotors were bubble-propelled at a high speed of over 200 μm/s in the presence of 5% H2O2. In addition, complete oxidative degradation of methylene blue (MB) occurred in the presence of 5% H2O2 after 60 min of treatment, whereas ZVI microspheres removed only 12% of MB in 60 min. The magnetic controllable and reusable properties of the ZVI/Pt micromotors make the water purification process more attractive and feasib...

Influence of Asymmetry and Driving Forces on the Propulsion of Bubble-Propelled Catalytic Micromotors

Bubble-propelled catalytic micromotors have recently been attracting much attention. A bubble-propulsion mechanism has the advantage of producing a stronger force and higher speed than other mechanisms for catalytic micromotors, but the nature of the fluctuated bubble generation process affects the motions of the micromotors, making it difficult to control their motions. Thus, understanding of the influence of fluctuating bubble propulsion on the motions of catalytic micromotors is important in exploiting the advantages of bubble-propelled micromotors. Here, we report experimental demonstrations of the bubble-propelled motions of propeller-shaped micromotors and numerical analyses of the influence of fluctuating bubble propulsion on the motions of propeller-shaped micromotors. We found that motions such as trochoid-like motion and circular motion emerged depending on the magnitude or symmetricity of fluctuations in the bubble-propulsion process. We hope that those results will help in the construction and application of sophisticated bubble-propelled micromotors in the future.

Photochromic Spatiotemporal Control of Bubble-Propelled Micromotors by a Spiropyran Molecular Switch.

Controlling the environment in which bubble-propelled micromotors operate represents an attractive strategy to influence their motion, especially when the trigger is as simple as light. We demonstrate that spiropyrans, which isomerize to amphiphilic merocyanines under UV irradiation, can act as molecular switches that drastically affect the locomotion of the micrometer-sized engines. The phototrigger could be either a point or a field source, thus allowing different modes of control to be executed. A whole ensemble of micromotors was repeatedly activated and deactivated by just altering the spiropyran-merocyanine ratio with light. Moreover, the velocity of individual micromotors was altered using a point irradiation source that caused only localized changes in the environment. Such selective manipulation, achieved here with an optical microscope and a photochromic additive in the medium, reveals the ease of the methodology, which can allow micro- and nanomotors to reach their full potential of not just stochastic, but directional controlled motion.

The gating effect by thousands of bubble-propelled micromotors in macroscale channels.

Increasing interest in the utilization of self-propelled micro-/nanomotors for environmental remediation requires the examination of their efficiency at the macroscale level. As such, we investigated the effect of micro-/nanomotors' propulsion and bubbling on the rate of sodium hydroxide dissolution and the subsequent dispersion of OH(-) ions across more than 30 cm, so as to understand how these factors might affect the dispersion of remediation agents in real systems which might require these agents to travel long distances to reach the pollutants. Experimental results showed that the presence of large numbers of active bubble-propelled tubular bimetallic Cu/Pt micromotors (4.5 × 10(4)) induced a gating effect on the dissolution and dispersion process, slowing down the change in pH of the solution considerably. The retardation was found to be dependent on the number of active micromotors present in the range of 1.5 × 10(4) to 4.5 × 10(4) micromotors. At lower numbers (0.75 × 10(4)), however, propelling micromotors did speed up the dissolution and dispersion process. The understanding of the combined effects of large number of micro-/nanomotors' motion and bubbling on its macroscale mixing behavior is of significant importance for future applications of these devices.

Frontispiece: Iridium-Catalyst-Based Autonomous Bubble-Propelled Graphene Micromotors with Ultralow Catalyst Loading

Frontispiece: Iridium-Catalyst-Based Autonomous Bubble-Propelled Graphene Micromotors with Ultralow Catalyst Loading

Bubble-propelled micromotors for enhanced transport of passive tracers.

Fluid convection and mixing induced by bubble-propelled tubular microengines are characterized using passive microsphere tracers. Enhanced transport of the passive tracers by bubble-propelled micromotors, indicated by their mean squared displacement (MSD), is dramatically larger than that observed in the presence of catalytic nanowires and Janus particle motors. Bubble generation is shown to play a dominant role in the effective fluid transport observed in the presence of tubular microengines. These findings further support the potential of using bubble-propelled microengines for mixing reagents and accelerating reaction rates. The study offers useful insights toward understanding the role of the motion of multiple micromotors, bubble generation, and additional factors (e.g., motor density and fuel concentration) upon the observed motor-induced fluid transport.

Influence of real-world environments on the motion of catalytic bubble-propelled micromotors

Self-propelled autonomous micromachines have recently been tasked to carry out various roles in real environments. In this study, we expose the microjets to various types of water that are present in the real world, examples include tap water, rain water, lake water and sea water, and we sought to investigate their behaviors under real world conditions. We observed that the viability and mobility of the catalytic bubble jet engines are strongly influenced by the type of environmental water sample. Amongst the four water samples tested, the seawater sample exhibits the strongest influence, completely disabling any motions arising from the microjets. The motion of the microjets is also reduced in tap water, which contains large quantities of inorganic ions that have been purposely introduced into tap water via processing in water treatment plants. Lake water and rain water samples exhibited the least influence on the microjet's motion. All of the four water samples were also characterized by determining their ion compositions and conductivities, and we will show that there is a distinct correlation between the reduced mobility of the microjets with the ion content of the water found in real environments.