Chemisch betriebene Mikro- und Nanomotoren

Chemically Driven Rotatory Molecular Machines

Molecular machines are at the frontier of biology and chemistry. The ability to control molecular motion and emulating the movement of biological systems are major steps towards the development of responsive and adaptive materials. Amazing progress has been seen for the design of molecular machines including light-induced unidirectional rotation of overcrowded alkenes. However, the feasibility of inducing unidirectional rotation about a single bond as a result of chemical conversion has been a challenging task. In this Review, an overview of approaches towards the design, synthesis, and dynamic properties of different classes of atropisomers which can undergo controlled switching or rotation under the influence of a chemical stimulus is presented. They are categorized as molecular switches, rotors, motors, and autonomous motors according to their type of response. Furthermore, we provide a future perspective and challenges focusing on building sophisticated molecular machines.

Enzyme-Powered Micro/Nanomotors for Cancer Treatment

The incidence and lethal rate of cancers are rapidly rising recently, however current treatments of cancers, such as surgical resection, radiotherapy, chemotherapy and targeted therapy, usually require long treatment period and have more side effects and high recurrence rate. Enzyme-powered micro/nanomotors (EMNMs), with powerful self-propulsion, enhanced permeability and good biocompatibility, have shown great potential in crossing biological barrier and targeted drug transportation for cancer treatment; moreover, advanced approaches based on EMNMs such as photothermal therapy and starvation therapy have also been widely explored in cancer treatment. Although there are several review works discussing the progress of micro/nanomotors for biomedical applications, there is not one review paper with the focus on the cancer treatment based on EMNMs. Therefore, in this review, we try to concisely and timely summarize the recent progress of cancer treatment based on enzyme-driven micro/nanomotors, such as brain tumors, bladder cancer, breast cancer and others. Finally, the challenges and outlook of cancer therapy based on EMNMs are discussed, hoping to provide fundamental guidance for the future development.

Generic Rules for Distinguishing Autophoretic Colloidal Motors

Distinguishing the operating mechanisms of nano- and micromotors powered by chemical gradients, i.e. "autophoresis", holds the key for fundamental and applied reasons. In this article, we propose and experimentally confirm that the speeds of a self-diffusiophoretic colloidal motor scale inversely to its population density but not for self-electrophoretic motors, because the former is an ion source and thus increases the solution ionic strength over time while the latter does not. They also form clusters in visually distinguishable and quantifiable ways. This pair of rules is simple, powerful, and insensitive to the specific material composition, shape or size of a colloidal motor, and does not require any measurement beyond typical microscopy. These rules are not only useful in clarifying the operating mechanisms of typical autophoretic micromotors, but also in predicting the dynamics of unconventional ones that are yet to be experimentally realized, even those involving enzymes.

2D Active Nanobots Based on Soft Nanoarchitectonics Powered by an Ultralow Fuel Concentration

Enzyme catalysis to power micro/nanomotors has received tremendous attention because of the vast potential in applications ranging from biomedicine to environmental remediation. However, the current design is mainly based on a complex three-dimensional (3D) architecture, with limited accessible surface areas for the catalytic sites, and thus require a higher fuel concentration to achieve active motion. Herein we report for the first time an enzyme-powered 2D nanobot, which was designed by a facile strategy based on soft nanoarchitectonics for active motion at an ultralow fuel concentration (0.003% H 2 O 2 ). The 2D nanobot exhibited efficient positive chemotactic behavior and the ability to swim against gravity by virtue of solutal buoyancy. As a proof-of-concept, the 2D nanobots showed an excellent capability for "on-the-fly" removal of methylene blue (MB) dye with an efficiency of 85%.

Au/Pt-Egg-in-Nest Nanomotor for Glucose-Powered Catalytic Motion and Enhanced Molecular Transport to Living Cells

Nanostructures converting chemical energy to mechanical work by using benign metabolic fuels, have huge implications in biomedical science. Here, we introduce Au/Pt-based Janus nanostructures, resembling to "egg-in-nest" morphology (Au/Pt-ENs), showing enhanced motion as a result of dual enzyme-relay-like catalytic cascade in physiological biomedia, and in turn showing molecular-laden transport to living cells. We developed dynamic-casting approach using silica yolk-shell nanoreactors: first, to install a large Au-seed fixing the silica-yolk aside while providing the anisotropically confined concave hollow nanospace to grow curved Pt-dendritic networks. Owing to the intimately interfaced Au and Pt catalytic sites integrated in a unique anisotropic nest-like morphology, Au/Pt-ENs exhibited high diffusion rates and displacements as the result of glucose-converted oxygen concentration gradient. High diffusiophoresis in cell culture media increased the nanomotor-membrane interaction events, in turn facilitated the cell internalization. In addition, the porous network of Au/Pt-ENs facilitated the drug-molecule cargo loading and delivery to the living cells.

Programmable Dynamic Shapes with a Swarm of Light-Powered Colloidal Motors

We report robust control over the dynamic assembly, disassembly, and reconfiguration of light-activated molybdenum disulfide (MoS₂ ) colloidal motor swarms with features not possible in equilibrium systems. A photochemical reaction produces chemical gradients across the MoS₂ colloidal motors to drive them to move. Under illumination of a gradient light, these colloidal motors display a positive phototactic motion. Mesoscale simulations prove that the self-diffusiophoresis induced by the locally consumed oxygen gradient across MoS₂ colloidal motors dominates the phototactic process. By programming the structured illumination, the collective migration and well-defined shapes of colloidal motor swarms can be externally regulated. The successful realization of programmable swarm transformation of colloidal motors like the emergent behaviors of living systems in nature provides a direct proof-of-concept for active soft materials and systems, with adaptive and interactive functions.

A Journey of Nanomotors for Targeted Cancer Therapy: Principles, Challenges, and a Critical Review of the State-of-the-Art

A nanomotor is a miniaturized device that converts energy stored in the environment into mechanical motion. The last two decades have witnessed a surge of research interests in the biomedical applications of nanomotors, but little clinical translation. To accelerate this process, targeted cancer therapy is used as an example to describe a "survive, locate, operate, and terminate" (SLOT) mission of a nanomotor, where it must 1) survive in the unfriendly in vivo environment, 2) locate its target as well as be located by human operators, 3) carry out specific operations, and 4) terminate after the mission is completed. Along this journey, the challenges presented to a nanomotor, including to power, navigate, steer, target, release, control, image, and communicate are discussed, and how state-of-the-art nanomotors meet or fall short of these requirements is critically reviewed. These discussions are then condensed into a table for easy reference. In particular, it is argued that chemically powered nanomotors are intrinsically ill-positioned for targeted cancer therapy, while nanomotors powered by magnetic fields or ultrasound show more promises. Following this argument, a tentative nanomotor design is then presented in the end to conform to the SLOT guideline, and to inspire practical, functional nanorobots that are yet to come.

Light-Driven Locomotion of Bubbles

Remotely controlling the movement of small objects is a challenging research topic, which can realize the transportation of materials. In this study, remote locomotion control of particle-stabilized bubbles on a planar water surface by near-infrared laser or sunlight irradiation is demonstrated. A light-induced Marangoni flow was utilized to induce the locomotion of the bubbles on water surface, and the timing and direction of the locomotion can be controlled by irradiation timing and direction on demand. The velocity, acceleration, and force of the bubbles were analyzed. It was also confirmed that the bubbles can work as light-driven towing engines to pull other objects. Furthermore, it was demonstrated that the bubbles can work as an adhesive to bond two solid substrates by application of compressive stress under water. Such remote transport of the materials, pulling of the objects by light, and controlling the release of gas on demand should open up a wide field of conceivable applications.

Fabrication of Template-Less Self-Propelled Micromotors Based on A Metal-Sandwiched Polytryptophan Body: An Experimental and DFT Study

The diverse capabilities of self-propelled micro/nanomotors open up significant opportunities for various environmental and biomedical applications. Here, a synchronized two-lobed bubble exhaust drives micromotor comprising a metal (cobalt and gold) sandwiched polytryptophan body (Au/poly-Trp/Co) in a non-curved direction. The autonomous motion is achieved through the decomposition of chemical fuel to result in a kayak-like system. The ejected oxygen bubbles from the interfacial cobalt/polytryptophan layer, as well as the inert nature of the metal segments (Au-Co), were considered for some computational studies of the electronic properties of the composite and physical phenomena at the kayak/electrolyte interfaces, and confirmed the role of Co-Trp in the fuel decomposition. It is believed that the autonomous motion is the combined result of bubble recoil force, self-electrophoresis, and perturbation in the interfacial hydrogen-bond network of the poly-Trp body and water molecules. The velocity of the micromotor in the range 23±4 to 157±17 μm s^-1 at different concentrations of H₂ O₂ from 1 % to 10 %. Depending on the method of fragmentation, it is possible to have both single and multiple motorized kayaks with lengths of 1.5 and 6 μm, respectively, that can be tailored for environmental applications.

A Bubble-Dragged Catalytic Polymer Microrocket

We report the bubble dragged microrocket consisting of functionalized multilayer polymer covered asymmetrically by platinum nanoparticles. The microrocket is pushed back during bubble growth over a small step and dragged forward over a big step during bubble explosion. Each bubble explosion induced a shock wave of gas which propagates in water at ultrafast speed. The bubble dragged microrocket can move along an approximate straight line instead of a fluctuating circle which is the trajectory of a bubble-pushed microrocket in most cases, which makes it a promising candidate for drug delivery and simulating rod-shaped bacteria.

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.

Turn-Number-Dependent Motion Behavior of Catalytic Helical Carbon Micro/Nanomotors

Helical micro/nanomotors (MNMs) can be propelled by external fields to swim through highly viscous fluids like complex biological environments, which promises miniaturized robotic tools to perform assigned tasks at small scales. However, the catalytic propulsion method, most widely adopted to drive MNMs, is seldom studied to actuate helical MNMs. Herein, we report catalytic helical carbon MNMs (CHCM) by sputtering Pt onto helical carbon nano-coils (HCNC) that are in bulk prepared by a thermal chemical vapor deposition method. The Pt-triggered H₂ O₂ decomposition can drive the MNMs by an electrokinetic mechanism. The MNMs demonstrate versatile motion behaviors including both directional propulsion and rotation depending on the turn number of the carbon helix. Besides, due to the ease of surface functionalization on carbon and other properties such as biocompatibility and photothermal effect, the helical carbon MNMs promise multifunctional applications for biomedical or environmental applications.

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.

Wireless Coupling of Conducting Polymer Actuators with Light Emission

Combining the actuation of conducting polymers with additional functionalities is an interesting fundamental scientific challenge and increases their application potential. Herein we demonstrate the possibility of direct integration of a miniaturized light emitting diode (LED) in a polypyrrole (PPy) matrix in order to achieve simultaneous wireless actuation and light emission. A light emitting diode is used as a part of an electroactive surface on which electrochemical polymerization allows direct incorporation of the electronic device into the polymer. The resulting free-standing polymer/LED hybrid can be addressed by bipolar electrochemistry to trigger simultaneously oxidation and reduction reactions at its opposite extremities, leading to a controlled deformation and an electron flow through the integrated LED. Such a dual response in the form of actuation and light emission opens up interesting perspectives in the field of microrobotics.

Photocatalytically Powered Matchlike Nanomotor for Light-Guided Active SERS Sensing

Surface enhanced Raman spectroscopy (SERS) is a powerful optical sensing technique that can detect analytes of extremely low concentrations. However, the presence of enough SERS probes in the detection area and a close contact between analytes and SERS probes are critical for efficient acquisition of a SERS signal. Presented here is a light-powered micro/nanomotor (MNM) that can serve as an active SERS probe. The matchlike AgNW@SiO₂ core-shell structure of the nanomotors work as SERS probes based on the shell-isolated enhanced Raman mechanism. The AgCl tail serves as photocatalytic nanoengine, providing a self-propulsion force by light-induced self-diffusiophoresis. The phototactic behavior was utilized to achieve enrichment of the nanomotor-based SERS probes for on-demand biochemical sensing. The results demonstrate the possibility of using photocatalytic nanomotors as active SERS probes for remote, light-controlled, and smart biochemical sensing on the micro/nanoscale.

Light-Activated Active Colloid Ribbons

We report a dynamic self-organization of self-propelled peanut-shaped hematite motors from non-equilibrium driving forces where the propulsion can be triggered by blue light. They result in one-dimensional, active colloid ribbons with a positive phototactic characteristic. The motion of colloid motors is ascribed to the diffusion-osmotic flow in a chemical gradient by the photocatalytic decomposition of hydrogen peroxide fuel. We show that self-propelled peanut-shaped colloids readily form one-dimensional, slithering ribbon structures under the out-of-equilibrium collisions. This self-organization intrinsically results from the competition among the osmotically driven motion, the phoretic attraction and the inherent magnetic moments. The giant size number fluctuation in colloid ribbons is observed above a critical point 4.1 % of the surface density of colloid motors. Such phototactic colloid ribbons may provide a model system to understand the emergence of function in biological systems and have potential to construct bioinspired active materials based on different active building blocks.