Surface Charge‐Reversible Tubular Micromotors for Extraction of Nucleic Acids in Microsystems

Extraction of nucleic acids in microsystems is of significance for biomedical applications, but the current extraction methods generally require sophisticated microchannels and external equipment, hindering their practical applications. In this work, we have demonstrated a simple, versatile and efficient approach to extract nucleic acids in microsystems by developing cationic branched polyethyleneimine (PEI)‐functionalized tubular micromotors. The as‐developed tubular micromotors are fabricated by a two‐step process combining the template‐assisted electrodeposition and carbodiimide chemistry, and contain an inner catalytic Pt layer, a middle magnetic Ni layer and an outer cationic PEI layer. They exhibit autonomous bubble‐propelled motion in aqueous hydrogen peroxide solutions, which can be guided by an external magnetic field, and the surface charges can be reversibly modulated by changing the pH value of the solution. Consequently, the as‐developed tubular micromotors can selectively absorb nucleic acids from acidic solutions and desorb them into alkaline solutions, leading to the extraction of nucleic acids with high efficiency without external stirring. Furthermore, they can be operated in a microchannel chip without the aid of a pumping system. Our results indicate that this PEI‐functionalized tubular micromotor platform provides a novel, simple and versatile microsystem nucleic acid extraction technology, holding considerable promise for important practical applications.     

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.     

Dual Effect of Manganese Oxide Micromotors: Catalytic Degradation and Adsorptive Bubble Separation of Organic Pollutants

Manganese oxide (MnO₂) based micromotors exhibiting a dual effect, that is, catalytic degradation and adsorptive bubble separation, were employed for water remediation. The dual effect of MnO₂ microparticles led to a greater than 90 % of decolorization of non‐biodegradable organic dyes in just 1 h, without the need for external agitation or bubble generation. These findings suggest high potential of MnO₂ micromotors for decontamination of organic pollutants from wastewaters or natural water reserves.     

Micro/Nanomotors for Water Purification

Self-propelled micro/nanomotors are synthetic machines that can convert different sources of energy into motion; at the same time, they are able to serve innovative environmental applications, for example, water purification. The self-propelled micro and nanomachines can rapidly zoom through the solution, carrying catalytic surface or chemical to remove or degrade pollutants in a much faster fashion than that of static systems, which depend on diffusion and fluxes. This review highlights the recent progress of micro/nanomotors in water pollutant detection and pollutant removal applications.     

Periodic Oscillatory Motion of a Self-Propelled Motor Driven by Decomposition of H2O2 by Catalase

A self-propelled motor driven by the enzymatic reaction of catalase adsorbed onto a filter paper floating on an aqueous solution of H₂O₂ was used to study nonlinear behavior in the motor's motion. An increase in the concentration of H₂O₂ resulted in a change from no motion to irregular oscillatory motion, periodic oscillatory motion, and continuous motion. The mechanisms underlying oscillation and mode bifurcation are discussed based on experimental results on O₂ bubble formation and growth on the underside of the motor.     

Catalytic Polymer Multilayer Shell Motors for Separation of Organics

A catalytic polymer multilayer shell motor has been developed, which effects fast motion‐based separation of charged organics in water. The shell motors are fabricated by sputtering platinum onto the exposed surface of silica templates embedded in Parafilm, followed by layer‐by‐layer assembly of polyelectrolyte multilayers to the templates. The catalytic shell motors display high bubble propulsion with speeds of up to 260 μm s^?1 (13 body lengths per second). Moreover, the polyelectrolyte multilayers assembled at high pH (pH>9.0) adsorb approximately 89 % of dye molecules from water, owing to the electrostatic interaction between the positively charged polymers and the anionic dye molecules, and subsequently release them at neutral pH in a microfluidic device. The efficient propulsion coupled with the effective adsorption behavior of the catalytic shell motors in a microfluidic device results in accelerated separation of organics in water and thus holds considerable promise for water analysis.     

Chemical locomotion

Research into the autonomous motion of artificial nano- and microscale objects provides basic principles to explore possible applications, such as self-assembly of superstructures, roving sensors, and drug delivery. Although the systems described have unique propulsion mechanisms, motility in each case is made possible by the conversion of locally available chemical energy into mechanical energy. The use of catalysts onboard can afford nondissipative systems that are capable of directed motion. Key to the design of nano- and micromotors is the asymmetric placement of the catalyst: its placement in an environment containing a suitable substrate translates into non-uniform consumption of the substrate and distribution of reaction products, which results in the motility of the object. These same principles are exploited in nature to effect autonomous motion.     

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.     

Smart Microdevices Laying “Breadcrumbs” to Find the Way Home: Chemotactic Homing TiO2/Pt Janus Microrobots

Synthetic microrobots or micromotors are known to show “intelligent” behavior such as magnetotaxis, phototaxis, chemotaxis, active detection, and chemical communication. Herein, we present the concept of micromotors laying “breadcrumbs”; that is, these micromachines can move/return to a home position without external guidance after their external energy input is stopped. As a demonstration, TiO₂/Pt Janus micromotors that move forward with UV light can return back following the previous path when the UV light is turned off. Such autonomy of motion opens the door for truly independent applications of micromotors in the “deliver‐and‐return” fashion.