Catalytic Micromotors Research Articles

A Viscosity-Based Model for Bubble-Propelled Catalytic Micromotors.

Micromotors have shown significant potential for diverse future applications. However, a poor understanding of the propelling mechanism hampers its further applications. In this study, an accurate mechanical model of the micromotor has been proposed by considering the geometric asymmetry and fluid viscosity based on hydrodynamic principles. The results obtained from the proposed model are in a good agreement with the experimental results. The effects of the semi-cone angle on the micromotor are re-analyzed. Furthermore, other geometric parameters, like the length-radius aspect ratio, exert great impact on the velocity. It is also observed that micromotors travel much slower in highly viscous solutions and, hence, viscosity plays an important role.

Correction: Visible-light controlled catalytic Cu2O-Au micromotors.

Correction for 'Visible-light controlled catalytic Cu2O-Au micromotors' by Dekai Zhou, et al., Nanoscale, 2017, DOI: 10.1039/c6nr08088j.

On the peculiar bubble formation, growth, and collapse behaviors in catalytic micro-motor systems

Bubbles often play a critical role in micro-systems involving Janus catalytic micro-motors (JCMs). Here, we examine some peculiar behaviors of the formation, growth, and collapse of the bubbles observed in recent experiments, in which JCM-laden droplets were dispensed on solid substrates and mixed with droplets of hydrogen peroxide solution. First, no oxygen bubble is visible near isolated JCMs when their size is smaller than a certain threshold, but bubbles can form and grow between a circular ring of small JCMs without touching any JCMs. Using analytical modeling and numerical simulations, we show that the lack of bubble formation near small, isolated JCMs originates from the low supersaturation of oxygen near their surface, which is caused by the efficient dissipation of oxygen molecules generated on their surface toward the bulk solution. In contrast, a cluster of small JCMs can collectively produce high enough oxygen supersaturation near the cluster to nucleate a bubble. Second, the radius of these bubbles grows following a power law of $$R \sim t^{0.7}$$ , rather than the typical $$R \sim t^{1/2}$$ or $$R \sim t^{1/3}$$ laws for the growth of bubbles driven by simple diffusion or direct gas injection into the bubble. Our numerical simulations showed that this anomalous growth law is a result of the cooperative action of the oxygen supersaturation-driven bubble growth and the mutual motion between the JCMs and the growing bubble. Finally, once a bubble grows to its maximal size, it collapses far more rapidly than the time scale expected for bubbles that contain non-condensable gas and exist in bulk liquids. Our scale analysis and numerical simulations show that this rapid collapse can be explained by the coalescence of the bubble with the air–liquid interface of the liquid film.

Tailored magnetic carbon allotrope catalytic micromotors for ‘on-chip’ operations

Carbon allotrope micromotors are proposed as active components in lab-on-a-chip systems. Highly rough carbon black tubular engines are used for fluorescence detection operations. The potential of ultrafast lectin carbon nanonotube micromotors with an inner anti-biofouling layer for selective transport of sugar modified particles (as cell mimics) in human plasma is illustrated.

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.

Manganese Oxide Based Catalytic Micromotors: Effect of Polymorphism on Motion.

Manganese oxide (MnO2) has recently emerged as a promising alternate material for the fabrication of self-propelled micromotors. Platinum (Pt) has been traditionally used as a catalytic material for this purpose. However, the high cost associated with Pt restricts its widespread use toward practical applications where large amounts of material are required. MnO2 exists in different crystalline forms (polymorphs), which govern its catalytic behavior. In spite of this, the recent reports on MnO2 based micromotors have seldom reported on the polymorphic form involved. In the present work, we synthesized six different types of MnO2 based micromotors, which represent different geometrical designs (i.e., spherical, rod-like, and tube-like microparticles) and polymorphs, and characterized their motion behavior in different chemical environments. Out of all micromotors tested, the hollow spherical MnO2 microparticles reached the maximum velocity of ∼1600 μm s-1, which represents the fastest MnO2 based catalytic micromotor reported until date. The findings of this study will have a profound impact on the design and application of the next-generation synthetic micro- and nanomotors based on MnO2 as a low-cost and environment friendly material.

Self-Diffusiophoresis of Janus Catalytic Micromotors in Confined Geometries.

The self-diffusiophoresis of Janus catalytic micromotors (JCMs) in confined environment is studied using direct numerical simulations. The simulations revealed that, on average, the translocation of a JCM through a short pore is moderately slowed down by the confinement. This slowdown is far weaker compared to the transport of particles through similar pores driven by forces induced by external means or passive diffusiophoresis. Pairing of two JCMs facilitates the translocation of the one JCM entering the pore first but slows down the second JCM. Depending on its initial orientation, a JCM near the entrance of a pore can exhibit different rotational motion, which determines whether it can enter the pore. Once a JCM enters a narrow pore, it can execute a self-alignment process after which it becomes fully aligned with the pore axis and moves to the center line of the pore. Analysis of these results showed that, in addition to hydrodynamic effect, the translation and rotation of JCM is also affected by the "chemical effects", i.e., the modification of the chemical species concentration around a JCM by confining walls and neighboring JCMs. These chemical effects are unique to the self-diffusiophoresis of JCMs and should be considered in design and operations of JCMs in confined environment.

Locomotive Studies on Electrodeposited Bimetallic Microtubular Motors for pH Sensing

In order to harness the potential of self-propelled miniaturized machines, there has been a sustained effort to increase locomotion in chemically-powered micromotors. Catalytic microdevices, propelled by hydrogen peroxide decomposition to oxygen bubbles, has been the most ubiquitous of them all. The microtubular motors were achieved through a templated-electrodeposition approach with a silver-sputtered polycarbonate membrane, serving as a working electrode in a three-electrode setup. Bimetallic components were accomplished with the sequential electrodeposition of copper, followed by platinum, which serves as the inner catalytic lining. The use of pH to alter the mobility of micromotors is demonstrated in this work through the manipulation of hydrogen peroxide chemistry in different acidity/alkalinity. The addition of sodium hydroxide to increase the pH of the solution led to a consequent increase in activity of micromotors. Meanwhile, introduction of hydrochloric acid led to corrosion of the microstructures, culminating in locomotive changes. Such dramatic changes in activity and velocities of the micromotors allow the usage of this behavior for pH detection. This concept was illustrated in the electrosynthesized tubular bimetallic Cu/Pt micromotors. Alteration of pH shall serve as a useful general strategy for manipulating hydrogen peroxide decomposition in oxygen-bubble propelled catalytic micromotors.

Aptamer-Modified Graphene-Based Catalytic Micromotors: Off–On Fluorescent Detection of Ricin

An aptamer-based catalytic micromotor sensing strategy for “Off-On” real-time fluorescent detection of the ricin B toxin is described. This approach relies on self-propelled reduced graphene-oxide (rGO)/platinum (Pt) micromotors, modified with a specific ricin B aptamer tagged with a fluorescein-amidine (FAM) dye, whose fluorescence is quenched due to π–π interactions with the rGO surface. The continuous movement of the motor in the sample accelerates the specific binding of the ricin B toxin to the aptamer–dye conjugate and leads to real-time fluorescent “On” detection. Coupling the “Off-On” fluorescent switching properties of the aptamer modified-rGO/Pt micromotors with their inherent mixing capabilities thus leads to high speed, simplicity, and sensitivity advantages, thus addressing the limitations of current ricin detection strategies. The new micromotor strategy represents an attractive route for detecting biological threats in a variety of biodefense applications.

Self-propelled manganese oxide-based catalytic micromotors for drug delivery

A novel self-propelled drug delivery vehicle was developed to capture and transport an anticancer drug through electrostatic interactions.

Wastewater Mediated Activation of Micromotors for Efficient Water Cleaning.

We present wastewater-mediated activation of catalytic micromotors for the degradation of nitroaromatic pollutants in water. These next-generation hybrid micromotors are fabricated by growing catalytically active Pd particles over thin-metal films (Ti/Fe/Cr), which are then rolled-up into self-propelled tubular microjets. Coupling of catalytically active Pd particles inside the micromotor surface in the presence of a 4-nitrophenol pollutant (with NaBH4 as reductant) results in autonomous motion via the bubble-recoil propulsion mechanism such that the target pollutant mixture (wastewater) is consumed as a fuel, thereby generating nontoxic byproducts. This study also offers several distinct advantages over its predecessors including no pH/temperature manipulation, limited stringent process control and complete destruction of the target pollutant mixture. The improved intermixing ability of the micromotors caused faster degradation ca. 10 times higher as compared to its nonmotile counterpart. The high catalytic efficiency obtained via a wet-lab approach has promising potential in creating hybrid micromotors comprising of multicatalytic systems assembled into one entity for sustainable environmental remediation and theranostics.

Marangoni Flow Induced Collective Motion of Catalytic Micromotors

A new collective motion of non-bubble-propelled spherical Janus catalytic micromotors has been observed. When the local concentration of micromotors is high, bubbles start to form between the motors. As the bubble grows, micromotors move collectively toward the center of the bubble regardless of the orientations of their catalyst surface, eventually become aggregated, and captured around the perimeter of the bubble. It is suggested that this collective motion of the micromotors, too fast for the diffusiophoresis, can be caused by the entrainment of micromotors by the evaporation-induced Marangoni flow near the bubble. Numerical simulations confirmed that the direction and strength of such Marangoni flow are consistent with the fast, collective motion of micromotors observed experimentally.

Influence of pH on the Motion of Catalytic Janus Particles and Tubular Bubble-Propelled Micromotors.

Self-propelled miniaturized machines harness the chemical potential of their environment for movement. Locomotion of chemically powered micromotors have been hugely dependent on the surroundings. The use of pH to alter the mobility of micromotors is demonstrated in this work through the manipulation of hydrogen peroxide chemistry in different acidity/alkalinity. The sequential addition of sodium hydroxide to increase the pH of the solution led to a consequent increase in activity of micromotors. Meanwhile, addition of hydrochloric acid compromised the structural integrity of the microstructures, culminating in locomotive changes. Such dramatic changes in activity and velocities of the micromotors allow the usage of this behavior for pH detection. This concept was illustrated with Janus silver micromotors and tubular bimetallic Cu/Pt micromotors. Alteration of pH serves as a useful general strategy for increasing hydrogen peroxide decomposition for enhanced oxygen-bubble propulsion in catalytic micromotors.

Micromotor‐Based Biomimetic Carbon Dioxide Sequestration: Towards Mobile Microscrubbers

AbstractWe describe a mobile CO2 scrubbing platform that offers a greatly accelerated biomimetic sequestration based on a self‐propelled carbonic anhydrase (CA) functionalized micromotor. The CO2 hydration capability of CA is coupled with the rapid movement of catalytic micromotors, and along with the corresponding fluid dynamics, results in a highly efficient mobile CO2 scrubbing microsystem. The continuous movement of CA and enhanced mass transport of the CO2 substrate lead to significant improvements in the sequestration efficiency and speed over stationary immobilized or free CA platforms. This system is a promising approach to rapid and enhanced CO2 sequestration platforms for addressing growing concerns over the buildup of greenhouse gas.

Micromotor-Based Biomimetic Carbon Dioxide Sequestration: Towards Mobile Microscrubbers.

We describe a mobile CO2 scrubbing platform that offers a greatly accelerated biomimetic sequestration based on a self-propelled carbonic anhydrase (CA) functionalized micromotor. The CO2 hydration capability of CA is coupled with the rapid movement of catalytic micromotors, and along with the corresponding fluid dynamics, results in a highly efficient mobile CO2 scrubbing microsystem. The continuous movement of CA and enhanced mass transport of the CO2 substrate lead to significant improvements in the sequestration efficiency and speed over stationary immobilized or free CA platforms. This system is a promising approach to rapid and enhanced CO2 sequestration platforms for addressing growing concerns over the buildup of greenhouse gas.

Vapor-Driven Propulsion of Catalytic Micromotors.

Chemically-powered micromotors offer exciting opportunities in diverse fields, including therapeutic delivery, environmental remediation, and nanoscale manufacturing. However, these nanovehicles require direct addition of high concentration of chemical fuel to the motor solution for their propulsion. We report the efficient vapor-powered propulsion of catalytic micromotors without direct addition of fuel to the micromotor solution. Diffusion of hydrazine vapor from the surrounding atmosphere into the sample solution is instead used to trigger rapid movement of iridium-gold Janus microsphere motors. Such operation creates a new type of remotely-triggered and powered catalytic micro/nanomotors that are responsive to their surrounding environment. This new propulsion mechanism is accompanied by unique phenomena, such as the distinct off-on response to the presence of fuel in the surrounding atmosphere, and spatio-temporal dependence of the motor speed borne out of the concentration gradient evolution within the motor solution. The relationship between the motor speed and the variables affecting the fuel concentration distribution is examined using a theoretical model for hydrazine transport, which is in turn used to explain the observed phenomena. The vapor-powered catalytic micro/nanomotors offer new opportunities in gas sensing, threat detection, and environmental monitoring, and open the door for a new class of environmentally-triggered micromotors.

Micromotor‐Based Energy Generation

AbstractA micromotor‐based strategy for energy generation, utilizing the conversion of liquid‐phase hydrogen to usable hydrogen gas (H2), is described. The new motion‐based H2‐generation concept relies on the movement of Pt‐black/Ti Janus microparticle motors in a solution of sodium borohydride (NaBH4) fuel. This is the first report of using NaBH4 for powering micromotors. The autonomous motion of these catalytic micromotors, as well as their bubble generation, leads to enhanced mixing and transport of NaBH4 towards the Pt‐black catalytic surface (compared to static microparticles or films), and hence to a substantially faster rate of H2 production. The practical utility of these micromotors is illustrated by powering a hydrogen–oxygen fuel cell car by an on‐board motion‐based hydrogen and oxygen generation. The new micromotor approach paves the way for the development of efficient on‐site energy generation for powering external devices or meeting growing demands on the energy grid.

Micromotor‐Based Energy Generation

A micromotor-based strategy for energy generation, utilizing the conversion of liquid-phase hydrogen to usable hydrogen gas (H2), is described. The new motion-based H2-generation concept relies on the movement of Pt-black/Ti Janus microparticle motors in a solution of sodium borohydride (NaBH4) fuel. This is the first report of using NaBH4 for powering micromotors. The autonomous motion of these catalytic micromotors, as well as their bubble generation, leads to enhanced mixing and transport of NaBH4 towards the Pt-black catalytic surface (compared to static microparticles or films), and hence to a substantially faster rate of H2 production. The practical utility of these micromotors is illustrated by powering a hydrogen-oxygen fuel cell car by an on-board motion-based hydrogen and oxygen generation. The new micromotor approach paves the way for the development of efficient on-site energy generation for powering external devices or meeting growing demands on the energy grid.

Precise Localization and Control of Catalytic Janus Micromotors Using Weak Magnetic Fields

We experimentally demonstrate the precise localization of spherical Pt-Silica Janus micromotors (diameter 5 μm) under the influence of controlled magnetic fields. First, we control the motion of the Janus micromotors in two-dimensional (2D) space. The control system achieves precise localization within an average region-of-convergence of 7 μm. Second, we show that these micromotors provide sufficient propulsion force, allowing them to overcome drag and gravitational forces and move both downwards and upwards. This propulsion is studied by moving the micromotors in three-dimensional (3D) space. The micromotors move downwards and upwards at average speeds of 19.1 μm/s and 9.8 μm/s, respectively. Moreover, our closed-loop control system achieves localization in 3D space within an average region-of-convergence of 6.3 μm in diameter. The precise motion control and localization of the Janus micromotors in 2D and 3D spaces provides broad possibilities for nanotechnology applications.

Bubble-propelled trimetallic microcaps as functional catalytic micromotors

We report on the preparation of trimetallic (Pt/Ni/Au) microcaps using silica particles as templates.