Tunable catalytic tubular micro-pumps operating at low concentrations of hydrogen peroxide

Microswimming by oxidation of dibenzylamine

Chemiophoretic nano- and micromotors require a constant flow of product molecules to maintain a gradient that enables their propulsion. Apart from a smaller number of redox reactions that have been used, catalytic reactions are the main source of energy with the obvious benefit of making on-board fuel storage obsolete. However, the decomposition of H₂O₂ seems to strongly dominate the literature and although motion in H₂O through water splitting is becoming more popular, so far only a few different reactions have been used for propulsion of photocatalytic microswimmers. Here, we investigate the possibility of extending the range of possible fuelling reactions to organic reactions with high significance in organic synthesis - the oxidation of amines to imines. Herein, motion of the microswimmers is analysed at different amine concentrations and light intensities. The findings thereof are correlated with the reaction products identified and quantified by gas chromatography (GC).

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.

Oxygen Generation Using Catalytic Nano/Micromotors

Gaseous oxygen plays a vital role in driving the metabolism of living organisms and has multiple agricultural, medical, and technological applications. Different methods have been discovered to produce oxygen, including plants, oxygen concentrators and catalytic reactions. However, many such approaches are relatively expensive, involve challenges, complexities in post-production processes or generate undesired reaction products. Catalytic oxygen generation using hydrogen peroxide is one of the simplest and cleanest methods to produce oxygen in the required quantities. Chemically powered micro/nanomotors, capable of self-propulsion in liquid media, offer convenient and economic platforms for on-the-fly generation of gaseous oxygen on demand. Micromotors have opened up opportunities for controlled oxygen generation and transport under complex conditions, critical medical diagnostics and therapy. Mobile oxygen micro-carriers help better understand the energy transduction efficiencies of micro/nanoscopic active matter by careful selection of catalytic materials, fuel compositions and concentrations, catalyst surface curvatures and catalytic particle size, which opens avenues for controllable oxygen release on the level of a single catalytic microreactor. This review discusses various micro/nanomotor systems capable of functioning as mobile oxygen generators while highlighting their features, efficiencies and application potentials in different fields.

Enzyme-Powered Porous Micromotors Built from a Hierarchical Micro- and Mesoporous UiO-Type Metal-Organic Framework

Here, we report the design, synthesis, and functional testing of enzyme-powered porous micromotors built from a metal-organic framework (MOF). We began by subjecting a presynthesized microporous UiO-type MOF to ozonolysis, to confer it with mesopores sufficiently large to adsorb and host the enzyme catalase (size: 6-10 nm). We then encapsulated catalase inside the mesopores, observing that they are hosted in those mesopores located at the subsurface of the MOF crystals. In the presence of H₂O₂ fuel, MOF motors (or MOFtors) exhibit jet-like propulsion enabled by enzymatic generation of oxygen bubbles. Moreover, thanks to their hierarchical pore system, the MOFtors retain sufficient free space for adsorption of additional targeted species, which we validated by testing a MOFtor for removal of rhodamine B during self-propulsion.

Silver-Based Self-Powered pH-Sensitive Pump and Sensor

Nonmechanical nano/microscale pumps that provide precise control over flow rate without the aid of an external power source and that are capable of turning on in response to specific analytes in solution are needed for the next generation of smart micro- and nanoscale devices. Herein, a self-powered chemically driven silver micropump is reported that is based on the two-step catalytic decomposition of hydrogen peroxide, H₂O₂. The pumping direction and speed can be controlled by modulating the solution pH, and modeling and theory allow for the kinetics of the reaction steps to be connected to the fluid velocity. In addition, by changing the pH dynamically using glucose oxidase (GOx)-catalyzed oxidation of glucose to gluconic acid, the direction of fluid pumping can be altered in situ, allowing for the design of a glucose sensor. This work underscores the versatility of catalytic pumps and their ability to function as sensors.

Visible-Light-Driven Water-Fueled Ecofriendly Micromotors Based on Iron Phthalocyanine for Highly Efficient Organic Pollutant Degradation

The light-driven micromotor has been demonstrated to have great potential in the environmental remediation field. However, it is still challenging to develop highly efficient, ecofriendly, and visible-light-powered micromotors for organic pollutant degradation. In this paper, we report an ecofriendly micromotor based on iron phthalocyanine (FePc) and gelatin, which exhibits the visible-light-driven self-propulsion behavior using water fuel based on the photocatalytic reaction and self-diffusiophoresis mechanism. Fast motion behavior is observed which induces the rapid agitation of the solution. This, together with the excellent photocatalytic activity, makes the FePc-based micromotor highly efficient when utilized in the degradation of organic pollutants with a normalized reaction rate constant of 2.49 × 10^-2 L m^-2 s^-1, which is by far the fastest and is far superior than the stationary counterpart. The external fuel-free propulsion and the high efficiency in pollutant degradation make the current micromotor potentially attractive for environmental remediation.

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.

Self-Assembled Phage-Based Colloids for High Localized Enzymatic Activity

Catalytically active colloids are model systems for chemical motors and active matter. It is desirable to replace the inorganic catalysts and the toxic fuels that are often used with biocompatible enzymatic reactions. However, compared to inorganic catalysts, enzyme-coated colloids tend to exhibit less activity. Here, we show that the self-assembly of genetically engineered M13 bacteriophages that bind enzymes to magnetic beads ensures high and localized enzymatic activity. These phage-decorated colloids provide a proteinaceous environment for directed enzyme immobilization. The magnetic properties of the colloidal carrier particle permit repeated enzyme recovery from a reaction solution, while the enzymatic activity is retained. Moreover, localizing the phage-based construct with a magnetic field in a microcontainer allows the enzyme-phage-colloids to function as an enzymatic micropump, where the enzymatic reaction generates a fluid flow. This system shows the fastest fluid flow reported to date by a biocompatible enzymatic micropump. In addition, it is functional in complex media including blood, where the enzyme-driven micropump can be powered at the physiological blood-urea concentrations.

Design and Fabrication of Tubular Micro/Nanomotors via 3D Laser Lithography

Catalytic tubular micro/nanomachines convert chemical energy from a surrounding aqueous fuel solution into mechanical energy to generate autonomous movements, propelled by the oxygen bubbles decomposed by hydrogen peroxide and expelled from the microtubular cavity. With the development of nanotechnology, micro/nanomotors have attracted more and more interest due to their numerous potential for in vivo and in vitro applications. Here, highly efficient chemical catalytic microtubular motors were fabricated via 3D laser lithography and their motion behavior under the action of driving force in fluids was demonstrated. The frequency of catalytically-generated bubbles ejection was influenced by the geometrical shape of the micro/nanomotor and surrounding chemical fuel environment, resulting in the variation in motion speed. The micro/nanomotors generated with a rocket-like shape displayed a more active motion compared with that of a single tubular micro/nanomotor, providing a wider range of practical micro-/nanoscale applications in the future.

Low-Voltage, Flexible, and Self-Encapsulated Ultracompact Organic Thin-Film Transistors Based on Nanomembranes

Organic thin-film transistors (OTFTs) are an ever-growing subject of research, powering recent technologies such as flexible and wearable electronics. Currently, many studies are being carried out to push forward the state-of-the-art OTFT technology to achieve characteristics that include high carrier mobility, low power consumption, flexibility, and the ability to operate under harsh conditions. Here, we tackle this task by proposing a novel OTFT architecture exploring the so-called rolled-up nanomembrane technology to fabricate low-voltage (<2 V), ultracompact OTFTs. As the OTFT gate electrode, we use strained nanomembranes, which allows all transistor components to be rolled-up and confined into a tubular-shaped tridimensional device structure with reduced footprint (ca. 90% of their planar counterpart), without any loss of electrical performance. Such an innovative architecture endows the OTFTs high mechanical flexibility (bending radius of <30 μm) and robustness-the devices can be reversibly deformed, withstanding more than 500 radial compression/decompression cycles. Additionally, the tubular device design possesses an inherent self-encapsulation characteristic that protects the OTFT active region from degradation by UV-light and hazardous vapors. The reported strategy is also shown to be compatible with different organic semiconductor materials. All of these characteristics contribute to further extending the potentialities of OTFTs, mainly toward rugged electronics.

Geometry Design, Principles and Assembly of Micromotors

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

Carbon dioxide bubble-propelled microengines in carbonated water and beverages

We demonstrate a new type of gaseous fuel for rolled-up tubular Ti/Cr microengine powered by carbon dioxide microbubbles in carbonated water and brewed beverages.

Photochemically Activated Motors: From Electrokinetic to Diffusion Motion Control

Self-propelled micro/nanomotors that can transform chemical energy from the surrounding environment into mechanical motion are cutting edge nanotechnologies with potential applications in biomedicine and environmental remediation. These applications require full understanding of the propulsion mechanisms to improve the performance and controllability of the motors. In this work, we demonstrate that there are two competing chemomechanical mechanisms at semiconductor/metal (Si/Pt) micromotors in a pump configuration under visible light exposure. The first propulsion mechanism is driven by an electro-osmotic process stemmed from a photoactivation reaction mediated by H₂O₂, which takes place in two separated redox reactions at the Si and Pt interfaces. One reaction involves the oxidation of H₂O₂ at the silicon side, and the other the H₂O₂ reduction at the metal side. The second mechanism is not light responsive and is triggered by the redox decomposition of H₂O₂ exclusively at the Pt surface. We show that it is possible to enhance/suppress one mechanism over the other by tuning the surface roughness of the micromotor metal. More specifically, the actuation mechanism can be switched from light-controlled electrokinetics to light-insensitive diffusio-osmosis by only increasing the metal surface roughness. The different actuation mechanisms yield strikingly different fluid flow velocities, electric fields, and light sensitivities. Consequently, these findings are very relevant and can have a remarkable impact on the design and optimization of photoactivated catalytic devices and, in general, on bimetallic or insulating-metallic motors.

Harnessing catalytic pumps for directional delivery of microparticles in microchambers

The directed transport of microparticles in microfluidic devices is vital for efficient bioassays and fabrication of complex microstructures. There remains, however, a need for methods to propel and steer microscopic cargo that do not require modifying these particles. Using theory and experiments, we show that catalytic surface reactions can be used to deliver microparticle cargo to specified regions in microchambers. Here reagents diffuse from a gel reservoir and react with the catalyst-coated surface. Fluid density gradients due to the spatially varying reagent concentration induce a convective flow, which carries the suspended particles until the reagents are consumed. Consequently, the cargo is deposited around a specific position on the surface. The velocity and final peak location of the cargo can be tuned independently. By increasing the local particle concentration, highly sensitive assays can be performed efficiently and rapidly. Moreover, the process can be repeated by introducing fresh reagent into the microchamber.

Targeted delivery of microparticles is desirable for rapid, sensitive biological assays or self-assembly process. Here Das et al. use catalytic reactions on the surface of microfluidic chambers to generate unidirectional flows that carry and deposit microparticles to selective regions of the chamber.

Microfluidic pumping by micromolar salt concentrations

An ion-exchange-resin-based microfluidic pump is introduced that utilizes trace amounts of ions to generate fluid flows. We show experimentally that our pump operates in almost deionized water for periods exceeding 24 h and induces fluid flows of μm s^-1 over hundreds of μm. This flow displays a far-field, power-law decay which is characteristic of two-dimensional (2D) flow when the system is strongly confined and of three-dimensional (3D) flow when it is not. Using theory and numerical calculations we demonstrate that our observations are consistent with electroosmotic pumping driven by μmol L^-1 ion concentrations in the sample cell that serve as 'fuel' to the pump. Our study thus reveals that trace amounts of charge carriers can produce surprisingly strong fluid flows; an insight that should benefit the design of a new class of microfluidic pumps that operate at very low fuel concentrations.

Enzyme Catalysis To Power Micro/Nanomachines

Enzymes play a crucial role in many biological processes which require harnessing and converting free chemical energy into kinetic forces in order to accomplish tasks. Enzymes are considered to be molecular machines, not only because of their capability of energy conversion in biological systems but also because enzymatic catalysis can result in enhanced diffusion of enzymes at a molecular level. Enlightened by nature's design of biological machinery, researchers have investigated various types of synthetic micro/nanomachines by using enzymatic reactions to achieve self-propulsion of micro/nanoarchitectures. Yet, the mechanism of motion is still under debate in current literature. Versatile proof-of-concept applications of these enzyme-powered micro/nanodevices have been recently demonstrated. In this review, we focus on discussing enzymes not only as stochastic swimmers but also as nanoengines to power self-propelled synthetic motors. We present an overview on different enzyme-powered micro/nanomachines, the current debate on their motion mechanism, methods to provide motion and speed control, and an outlook of the future potentials of this multidisciplinary field.

Chemistry pumps: a review of chemically powered micropumps

Lab-on-a-chip devices have over recent years attracted a significant amount of attention in both the academic circle and industry, due to their promise in delivering versatile functionalities with high throughput and low sample amount. Typically, mechanical or electrokinetic micropumps are used in the majority of lab-on-a-chip devices that require powered fluid flow, but the technical challenges and the requirement of external power associated with these pumping devices hinder further development and miniaturization of lab-on-a-chip devices. Self-powered micropumps, especially those powered by chemical reactions, have been recently designed and can potentially address some of these issues. In this review article, we provide a detailed introduction to four types of chemically powered micropumps, with particular focus on their respective structures, operating mechanisms and practical usefulness as well as limitations. We then discuss the various functionalities and controllability demonstrated by these micropumps, ending with a brief discussion of how they can be improved in the future. Due to the absence of external power sources, versatile activation methods and sensitivity to environmental cues, chemically powered micropumps could find potential applications in a wide range of lab-on-a-chip devices.

Self-Propelling Hydrogel/Emulsion-Hydrogel Soft Motors for Water Purification

We fabricate a kind of catalytic self-propelling hydrogel soft motor (H-motor) via a facile injection loading method with low energy consumption. The factors influencing the practicability of H-motors, including locomotive ability and reusability, are investigated. The succession of rapid bubble evolution and propulsion endows the millimeter-sized columnar H-motors with length/diameter of 1 a remarkable speed of 3.84 mm s(-1) in 10% (w/w) hydrogen peroxide (H2O2) solution. Moreover, the H-motors maintain undiminished propulsion capability and functionality even after repeated loading for 6 times. Additionally, we also fabricate emulsion-hydrogel soft motors (E-H-motors) templated from the oil/water (O/W) emulsion for the first time, which exhibit a faster speed of 4.33 mm s(-1) under the same conditions. It can be ascribed to the additional liberation of low-boiling oil phase stored in the emulsion-hydrogels caused by catalytic reaction heat, which is appropriate for larger propulsive situations. The stabilized, efficient, and reusable H-motors are selected for industrial effluents purification to fit the imperious demands about the disposal of organic pollutants in water. The synergy effect between catalytic degradation and enhanced intermixing of the fluid flow around the miniaturized soft motors gives rise to an effective and exhaustive removal of organic contaminants.

Synthetic Nano- and Micromachines in Analytical Chemistry: Sensing, Migration, Capture, Delivery, and Separation

Synthetic nano- and microscale machines move autonomously in solution or drive fluid flows by converting sources of energy into mechanical work. Their sizes are comparable to analytes (sub-nano- to microscale), and they respond to signals from each other and their surroundings, leading to emergent collective behavior. These machines can potentially enable hitherto difficult analytical applications. In this article, we review the development of different classes of synthetic nano- and micromotors and pumps and indicate their possible applications in real-time in situ chemical sensing, on-demand directional transport, cargo capture and delivery, as well as analyte isolation and separation.

Lab-in-a-tube systems as ultra-compact devices

In this Focus article, I will give an overview on the current and future interests of our multidisciplinary research group. One of our main interests is to develop highly integrated on-chip components towards ultra-compact devices for biosensing technologies (lab-in-a-tube). Our other activities are focused in developing self-powered devices that can generate either motion of a fluid or autonomous propulsion. We are particularly interested in three-dimensional (3D) nanofabrication technologies and stimuli responsive soft materials.

Chemically powered micro- and nanomotors

Chemically powered micro- and nanomotors are small devices that are self-propelled by catalytic reactions in fluids. Taking inspiration from biomotors, scientists are aiming to find the best architecture for self-propulsion, understand the mechanisms of motion, and develop accurate control over the motion. Remotely guided nanomotors can transport cargo to desired targets, drill into biomaterials, sense their environment, mix or pump fluids, and clean polluted water. This Review summarizes the major advances in the growing field of catalytic nanomotors, which started ten years ago.

Sequential tasks performed by catalytic pumps for colloidal crystallization

Gold-platinum catalytic pumps immersed in a chemical fuel are used to manipulate silica colloids. The manipulation relies on the electric field and the fluid flow generated by the pump. Catalytic pumps perform various tasks, such as the repulsion of colloids, the attraction of colloids, and the guided crystallization of colloids. We demonstrate that catalytic pumps can execute these tasks sequentially over time. Switching from one task to the next is related to the local change of the proton concentration, which modifies the colloid ζ potential and, consequently, the electric force acting on the colloids.

Self-powered glucose-responsive micropumps

A self-powered polymeric micropump based on boronate chemistry is described. The pump is triggered by the presence of glucose in ambient conditions and induces convective fluid flows, with pumping velocity proportional to the glucose concentration. The pumping is due to buoyancy convection that originates from reaction-associated heat flux, as verified from experiments and finite difference modeling. As predicted, the fluid flow increases with increasing height of the chamber. In addition, pumping velocity is enhanced on replacing glucose with mannitol because of the enhanced exothermicity associated with the reaction of the latter.

Self-powered enzyme micropumps

Non-mechanical nano- and microscale pumps that function without the aid of an external power source and provide precise control over the flow rate in response to specific signals are needed for the development of new autonomous nano- and microscale systems. Here we show that surface-immobilized enzymes that are independent of adenosine triphosphate function as self-powered micropumps in the presence of their respective substrates. In the four cases studied (catalase, lipase, urease and glucose oxidase), the flow is driven by a gradient in fluid density generated by the enzymatic reaction. The pumping velocity increases with increasing substrate concentration and reaction rate. These rechargeable pumps can be triggered by the presence of specific analytes, which enables the design of enzyme-based devices that act both as sensor and pump. Finally, we show proof-of-concept enzyme-powered devices that autonomously deliver small molecules and proteins in response to specific chemical stimuli, including the release of insulin in response to glucose.

DNA polymerase as a molecular motor and pump

DNA polymerase is responsible for synthesizing DNA, a key component in the running of biological machinery. Using fluorescence correlation spectroscopy, we demonstrate that the diffusive movement of a molecular complex of DNA template and DNA polymerase enhances during nucleotide incorporation into the growing DNA template. The diffusion coefficient of the complex also shows a strong dependence on its inorganic cofactor, Mg2+ ions. When exposed to gradients of either nucleotide or cofactor concentrations, an ensemble of DNA polymerase complex molecules shows collective movement toward regions of higher concentrations. By immobilizing the molecular complex on a patterned gold surface, we demonstrate the fabrication of DNA polymerase-powered fluid pumps. These miniature pumps are capable of transporting fluid and tracer particles in a directional manner with the pumping speed increasing in the presence of the cofactor. The role of DNA polymerase as a micropump opens up avenues for designing miniature fluid pumps using enzymes as engines.

Effect of surfactants on the performance of tubular and spherical micromotors - a comparative study

The development of artificial micromotors is one of the greatest challenges of modern nanotechnology. Even though many kinds of motors have been published in recent times, systematic studies on the influence of components of the fuel solution are widely missing. Therefore, the autonomous movement of Pt-microtubes and Pt-covered silica particles is comparatively observed in the presence and absence of surfactants in the medium. One representative of each of the three main surfactant classes - anionic (sodium dodecyl sulfate, SDS), cationic (benzalkonium chloride, BACl) and non-ionic (Triton X) - has been chosen and studied.

Self-propelled micromotors for cleaning polluted water

We describe the use of catalytically self-propelled microjets (dubbed micromotors) for degrading organic pollutants in water via the Fenton oxidation process. The tubular micromotors are composed of rolled-up functional nanomembranes consisting of Fe/Pt bilayers. The micromotors contain double functionality within their architecture, i.e., the inner Pt for the self-propulsion and the outer Fe for the in situ generation of ferrous ions boosting the remediation of contaminated water.The degradation of organic pollutants takes place in the presence of hydrogen peroxide, which acts as a reagent for the Fenton reaction and as main fuel to propel the micromotors. Factors influencing the efficiency of the Fenton oxidation process, including thickness of the Fe layer, pH, and concentration of hydrogen peroxide, are investigated. The ability of these catalytically self-propelled micromotors to improve intermixing in liquids results in the removal of organic pollutants ca. 12 times faster than when the Fenton oxidation process is carried out without catalytically active micromotors. The enhanced reaction-diffusion provided by micromotors has been theoretically modeled. The synergy between the internal and external functionalities of the micromotors, without the need of further functionalization, results into an enhanced degradation of nonbiodegradable and dangerous organic pollutants at small-scale environments and holds considerable promise for the remediation of contaminated water.

Imaging the proton concentration and mapping the spatial distribution of the electric field of catalytic micropumps

Catalytic engines can use hydrogen peroxide as a chemical fuel in order to drive motion at the microscale. The chemo-mechanical actuation is a complex mechanism based on the interrelation between catalytic reactions and electro-hydrodynamics phenomena. We studied catalytic micropumps using fluorescence confocal microscopy to image the concentration of protons in the liquid. In addition, we measured the motion of particles with different charges in order to map the spatial distributions of the electric field, the electrostatic potential and the fluid flow. The combination of these two techniques allows us to contrast the gradient of the concentration of protons against the spatial variation in the electric field. We present numerical simulations that reproduce the experimental results. Our work sheds light on the interrelation between the different processes at work in the chemomechanical actuation of catalytic pumps. Our experimental approach could be used to study other electrochemical systems with heterogeneous electrodes.

Dual stimuli-responsive, rechargeable micropumps via "host-guest" interactions

We demonstrate a supramolecular approach to the fabrication of self-powered micropumps based on "host-guest" molecular recognition between α- and β-cyclodextrin and trans-azobenzene. Both hydrogels and surface coatings based on host-guest partners were used as scaffolds to devise the micropumps. These soft micropumps are dual stimuli-responsive and can be actuated either by light or by introducing guest molecules. Furthermore, the micropumps can be recharged through reversible host-guest interaction.

Small-scale heat detection using catalytic microengines irradiated by laser

We demonstrate a novel approach to modulating the motion speed of catalytic microtubular engines via laser irradiation/heating with regard to small-scale heat detection. Laser irradiation on the engines leads to a thermal heating effect and thus enhances the engine speed. During a laser on/off period, the motion behaviour of a microengine can be repeatable and reversible, demonstrating a regulation of motion speeds triggered by laser illumination. Also, the engine velocity exhibits a linear dependence on laser power in various fuel concentrations, which implies an application potential as local heat sensors. Our work may hold great promise in applications such as lab on a chip, micro/nano factories, and environmental detection.

Transition between collective behaviors of micromotors in response to different stimuli

We report a Ag(3)PO(4) microparticle system showing collective behaviors in aqueous medium. Transition between two emergent patterns, namely "exclusion" and "schooling", can be triggered by shift in chemical equilibrium upon the addition or removal of NH(3) or in response to UV light. The transitions are consistent with a self-diffusiophoresis mechanism resulting from ion gradients. The reported system is among the few examples of nanomotors that are based on a reversible nonredox reaction and demonstrates new design principles for micro/nanomotors. Potential applications of the reported system in logic gates, microscale pumping, and hierarchical assembly have been demonstrated.

Self-generated diffusioosmotic flows from calcium carbonate micropumps

Calcium carbonate particles, ubiquitous in nature and found extensively in geological formations, behave as micropumps in an unsaturated aqueous solution. The mechanism causing this pumping is diffusioosmosis, which drives flows along charged surfaces. Our calcium carbonate microparticles, roughly ∼10 μm in size, self-generate ionic gradients as they dissolve in water to produce Ca(2+), HCO(3)(-), and OH(-) ions that migrate into the bulk. Because of the different diffusion coefficients of these ions, spontaneous electric fields of roughly 1-10 V/cm arise in order to maintain electroneutrality in the solution. This electric field drives the diffusiophoresis of charged tracers (both positive and negative) as well as diffusioosmotic flows along charged substrates. Here we show experimentally how the directionality and speed of the tracers can be engineered by manipulating the tracer zeta potential, the salt gradients, and the substrate zeta potential. Furthermore, because the salt gradients are self-generated, here by the dissolution of solid calcium carbonate microparticles another manipulated variable is the placement of these particles. Importantly, we find that the zeta potentials on surfaces vary with both time and location because of the adsorption or desorption of Ca(2+) ions; this change affects the flows significantly.

Triggered "on/off" micropumps and colloidal photodiode

We discuss a set of smart micropumps that sense their surrounding environment and respond accordingly. First we show that crystallites of a photoacid generator function as micropumps in the presence of UV light via diffusiophoresis and can be turned "on/off" in a controlled manner. The pump can be restarted multiple times simply by re-illumination. The electroosmotic component was distinguished from the diffusiophoretic component and compared. We also demonstrate patterning. Second, we show that a polymeric imine can also work as a micropump in acidic environment wherein the velocity can be controlled by controlling the pH and, in turn, the ion gradient; the highest velocities are achieved at the lowest pH. Finally, we combined the photoacid and polyimine pumps to create a colloidal photodiode, where we attain both spatial and temporal control over colloidal transport and obtain amplification along with rectification.

Motion analysis of light-powered autonomous silver chloride nanomotors

Powered by UV light, nano/micrometer-sized silver chloride particles exhibit autonomous movement and form "schools" in aqueous solution, i.e. regions in which the number density of particles is significantly higher than the global average. In this paper, the silver chloride particles in such a system are classified by their proximity to other AgCl particles--be they isolated, coupled or schooled--and their motion paths are tracked and analyzed. By plotting time-averaged mean squared displacements of each particle over various time intervals from 0.1 s to 15.0 s, we discover different diffusive behaviors for the three classes of silver chloride particles.

Lab-in-a-tube: ultracompact components for on-chip capture and detection of individual micro-/nanoorganisms

A review of present and future on-chip rolled-up devices, which can be used to develop lab-in-a-tube total analysis systems, is presented. Lab-in-a-tube is the integration of numerous rolled-up components into a single device constituting a microsystem of hundreds/thousands of independent units on a chip, each individually capable of sorting, detecting and analyzing singular organisms. Such a system allows for a scale-down of biosensing systems, while at the same time increasing the data collection through a large, smart array of individual biosensors. A close look at these ultracompact components which have been developed over the past decade is given. Methods for the capture of biomaterial are laid out and progress of cell culturing in three-dimensional scaffolding is detailed. Rolled-up optical sensors based on photoluminescence, optomechanics, optofluidics and metamaterials are presented. Magnetic sensors are introduced as well as electrical components including heating, energy storage and resistor devices.

Cargo-towing synthetic nanomachines: towards active transport in microchip devices

This review article discusses the use of synthetic catalytic nano motors for cargo manipulations and for developing miniaturized lab-on-chip systems based on autonomous transport. The ability of using chemically-powered artificial nanomotors to capture, transport and release therapeutic payloads or nanostructured biomaterials represents one of the next major prospects for nanomotor development. The increased cargo-towing force of such self-propelled nanomotors, along with their precise motion control within microchannel networks, versatility and facile functionalization, pave the way to new integrated functional lab-on-a-chip powered by active transport and perform a series of tasks. Such use of cargo-towing artificial nanomotors has been inspired by on-chip kinesin molecular shuttles. Functionalized nano/microscale motors can thus be used to pick a selected nano/microscale chemical or biological payload target at the right place, transport and deliver them to a target location in a timely manner. Key challenges for using synthetic nanomachines for driving transport processes along microchannel networks are discussed, including loading and unloading of cargo and precise motion control, along with recent examples of related cargo manipulation processes and guided transport in lab-on-a-chip formats. The exciting research area of cargo-carrying catalytic man-made nanomachines is expected to grow rapidly, to lead to new lab-on-a-chip formats and to provide a wide range of future microchip opportunities.

Self-propelled nanotools

We describe nanoscale tools in the form of autonomous and remotely guided catalytically self-propelled InGaAs/GaAs/(Cr)Pt tubes. These rolled-up tubes with diameters in the range of 280-600 nm move in hydrogen peroxide solutions with speeds as high as 180 μm s(-1). The effective transfer of chemical energy to translational motion has allowed these tubes to perform useful tasks such as transport of cargo. Furthermore, we observed that, while cylindrically rolled-up tubes move in a straight line, asymmetrically rolled-up tubes move in a corkscrew-like trajectory, allowing these tubes to drill and embed themselves into biomaterials. Our observations suggest that shape and asymmetry can be utilized to direct the motion of catalytic nanotubes and enable mechanized functions at the nanoscale.

Dynamics of catalytic tubular microjet engines: dependence on geometry and chemical environment

Strain-engineered tubular microjet engines with various geometric dimensions hold interesting autonomous motions in an aqueous fuel solution when propelled by catalytic decomposition of hydrogen peroxide to oxygen and water. The catalytically-generated oxygen bubbles expelled from microtubular cavities propel the microjet step by step in discrete increments. We focus on the dynamics of our tubular microjets in one step and build up a body deformation model to elucidate the interaction between tubular microjets and the bubbles they produce. The average microjet velocity is calculated analytically based on our model and the obtained results demonstrate that the velocity of the microjet increases linearly with the concentration of hydrogen peroxide. The geometric dimensions of the microjet, such as length and radius, also influence its dynamic characteristics significantly. A close consistency between experimental and calculated results is achieved despite a small deviation due to the existence of an approximation in the model. The results presented in this work improve our understanding regarding catalytic motions of tubular microjets and demonstrate the controllability of the microjet which may have potential applications in drug delivery and biology.