Janus Micromotors Research Articles

Electrokinetic Janus micromotors moving on topographically flat chemical patterns

Ionic and molecular selectivity are considered unique for the nanoscale and not realizable in microfluidics. This is due to the scale-matching problem—a difficulty to match the dimensions of ions and electrostatic potential screening lengths with micron-sized confinements. Here, we demonstrate a microscale realization of ionic transport processes closely resembling those specific to ionic channels or in nanofluidic junctions, including selectivity, guidance and flow focusing. As a model system, we explore electrokinetic spherical Janus micromotors moving over charged surfaces with complex charge distribution and without any topographical wall. We discuss peculiarities of the long-range electrostatic interaction on the behavior of the system including interface crossing and reflection of positively charged particles from negatively charged interfaces. These results are crucial for understanding the electrokinetic transport of biochemical species under confinement, have the potential to increase the precision of lab-on-chip-based assays, as well as broadening use cases and control strategies of nano-/micromachinery.

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.

Enhanced and Robust Directional Propulsion of Light-Activated Janus Micromotors by Magnetic Spinning and the Magnus Effect.

Advances in the versatile design and synthesis of nanomaterials have imparted diverse functionalities to Janus micromotors as autonomous vehicles. However, a significant challenge remains in maneuvering Janus micromotors by following desired trajectories for on-demand motility and intelligent control due to the inherent rotational Brownian motion. Here, we present the enhanced and robust directional propulsion of light-activated Fe3O4@TiO2/Pt Janus micromotors by magnetic spinning and the Magnus effect. Once exposed to a low-intensity rotating magnetic field, the micromotors become physically actuated, and their rotational Brownian diffusion is quenched by the magnetic rotation. Photocatalytic propulsion can be triggered by unidirectional irradiation based on a self-electrophoretic mechanism. Thus, a transverse Magnus force can be generated due to the rotational motion and ballistic motion (photocatalytic propulsion) of the micromotors. Both the self-electrophoretic propulsion and the Magnus force are periodically changed due to the magnetic rotation, which results in an overall directed motion moving toward a trajectory with a deflection angle from the direction of incident light with enhanced speed, maneuverability, and steering robustness. Our study illustrates the admirable directional motion capabilities of light-driven Janus micromotors based on magnetic spinning and the Magnus effect, which unfolds a new paradigm for addressing the limitations of directionality control in the current asymmetric micromotors.

Three-dimensional hierarchical Ag/Mg(Ni)Al-layered double hydroxide Janus micromotor derived from lotus pollen for active removal of organic pollutant

Three-dimensional hierarchical Ag/Mg(Ni)Al-layered double hydroxide Janus micromotor derived from lotus pollen for active removal of organic pollutant

Transition metal dichalcogenide-based Janus micromotors for on-the-fly Salmonella detection

Janus micromotors encapsulating transition metal dichalcogenides (TMDs) and modified with a rhodamine (RhO)-labeled affinity peptide (RhO-NFMESLPRLGMH) are used here for Salmonella enterica endotoxin detection. The OFF–ON strategy relies on the specific binding of the peptide with the TMDs to induce fluorescence quenching (OFF state); which is next recovered due to selectively binding to the endotoxin (ON state). The increase in the fluorescence of the micromotors can be quantified as a function of the concentration of endotoxin in the sample. The developed strategy was applied to the determination of Salmonella enterica serovar Typhimurium endotoxin with high sensitivity (limits of detection (LODs) of 2.0 µg/mL using MoS2, and 1.2 µg/mL using WS2), with quantitative recoveries (ranging from 93.7 ± 4.6 % to 94.3 ± 6.6%) in bacteria cultures in just 5 min. No fluorescence recovery is observed in the presence of endotoxins with a similar structure, illustrating the high selectivity of the protocol, even against endotoxins of Salmonella enterica serovar Enteritidis with great similarity in its structure, demonstrating the high bacterial specificity of the developed method. These results revealed the analytical potential of the reported strategy in multiplexed assays using different receptors or in the design of portable detection devices.Graphical abstract

The effect of particle size on the dynamics of self-electrophoretic Janus micromotors, sputtering distribution, and rectifying voltage

Micromotors have gained increasing attention in last two decades, due to their controllable self-propulsion maneuverability at micro scales. However, the effect of particle size on micromotors is still vague and interwoven due to the micromotors’ size, polydisperse distribution, and structural variations. Microfluidic technology is used to clarify the phenomenon, due to its precise micro scale fabrication. Herein, we fabricated a H2O2-driven self-electrophoretic Janus micromotor and systematically explored the effects of particle size on their dynamics, the sputtering distribution, and the rectifying voltages. Their speeds were found to be inversely proportional to their sizes. When the particle size was fixed, their speeds increased as coating thickness increased until its hemisphere was fully coated, and this critical coating thickness was also proportional to the sizes of the micromotors. As further investigation goes on, we noticed that electrical voltage to rectify micromotor was proportional to its size too. To summarize, our results showed that larger micromotors moved more slowly, required a thicker metal coating to reach full speeds, and needed higher voltages to be rectified. Through all of these investigations, we believed that microfluidic technology is a valuable tool, which can systematically probe micromotors dynamics and clarify our understanding of micromotors behaviors.

Surface Roughening of Pt-Polystyrene Spherical Janus Micromotors for Enhanced Motion Speed.

Spherical Janus micromotors (SJMs) have attracted much attention, and their high-speed motion is highly desired due to their various potential applications. However, the conventional template-deposition method often leads to an active Pt coating with a smooth surface, which is unbeneficial to speed enhancement in terms of catalytic reaction. Here, a facile surface roughening method is presented to fabricate the Pt-polystyrene (PS) SJMs with rough Pt surface (or Ptr-PS SJMs) by plasma-etching the PS colloidal monolayer and then depositing Pt. The Ptr-PS SJMs can exhibit directional motion pushed by the Pt in the various H2O2 solutions, and they show much higher motion speeds than the Pt-PS SJMs with smooth Pt surfaces at the same H2O2 concentration. The Pt-pushed motion is related to the locally asymmetric catalytic reaction of the Pt coating on PS. The speed is also associated with the surface roughness of the Pt coating. The Pt film with a rough surface causes enhanced motion speed due to the improvement of reaction catalytic activity. This work presents a new route to enhancing the motor motion speed, which is of significance in designing micromotors with high-speed motion.

A Platform for Stop-Flow Gradient Generation to Investigate Chemotaxis.

The ability of artificial microswimmers to respond to external stimuli and the mechanistical details of their origins belong to the most disputed challenges in interdisciplinary science. Therein, the creation of chemical gradients is technically challenging, because they quickly level out due to diffusion. Inspired by pivotal stopped flow experiments in chemical kinetics, we show that microfluidics gradient generation combined with a pressure feedback loop for precisely controlling the stop of the flows, can enable us to study mechanistical details of chemotaxis of artificial Janus micromotors, based on a catalytic reaction. We find that these copper Janus particles display a chemotactic motion along the concentration gradient in both, positive and negative direction and we demonstrate the mechanical reaction of the particles to unbalanced drag forces, explaining this behaviour.

Upstream Rheotaxis of Catalytic Janus Spheres.

Fluid flow is ubiquitous in many environments that form habitats for microorganisms. Therefore, it is not surprising that both biological and artificial microswimmers show responses to flows that are determined by the interplay of chemical and physical factors. In particular, to deepen the understanding of how different systems respond to flows, it is crucial to comprehend the influence played by swimming pattern. The tendency of organisms to navigate up or down the flow is termed rheotaxis. Early theoretical studies predicted a positive rheotactic response for puller-type spherical Janus micromotors. However, recent experimental studies have focused on pusher-type Janus particles, finding that they exhibit cross-stream migration in externally applied flows. To study the response to the flow of swimmers with a qualitatively different flow pattern, we introduce Cu@SiO2 micromotors that swim toward their catalytic cap. On the basis of experimental observations, and supported by flow field calculations using a model for self-electrophoresis, we hypothesize that they behave effectively as a puller-type system. We investigate the effect of externally imposed flow on these spherically symmetrical Cu@SiO2 active Janus colloids, and we indeed observe a steady upstream directional response. Through a simple squirmer model for a puller, we recover the major experimental observations. Additionally, the model predicts a "jumping" behavior for puller-type micromotors at high flow speeds. Performing additional experiments at high flow speeds, we capture this phenomenon, in which the particles "roll" with their swimming axes aligned to the shear plane, in addition to being dragged downstream by the fluid flow.

Self-propelled Janus micromotors for pH-responsive release of small molecule drug

Oral administration of drugs has received significant attention in the last 1, 2 decades, as it is the most comfortable and painless administration route, which leads to high patient compliance. Although oral delivery of drugs shows positive effects in patients, several hurdles must be overcome such as enzymatic degradation, large pH-variations found throughout the gastrointestinal (GI) tract, and continuous mucus secretion. We have studied biocompatible non-toxic micromotors as promising man-made microdevices for targeted furosemide release. Micromotors are micro-scale autonomous entities that can perform various tasks in the GI tract as a result of their impressive motion abilities. Their directional motion makes them ideal candidates to bring the necessary drug dosage to where it is needed. Herein, the micromotors were loaded for the first time with furosemide and coated with pH-sensitive polymer Eudragit® L100 using ultrasonic spray coating technique in order to achieve targeted drug delivery and pH-responsive release. Coherent anti-Stokes Raman scattering spectroscopy (CARS) was used for visualizing drug loading efficiency on the surface of micromotors. In vitro drug release was evaluated under acidic and neutral pH chemical environments and obtained results confirm their function as pH-sensitive microdevices for a drug release.

Three-Dimensional Hierarchical HRP-MIL-100(Fe)@TiO2@Fe3O4 Janus Magnetic Micromotor as a Smart Active Platform for Detection and Degradation of Hydroquinone.

A novel multifunctional Janus magnetic micromotor was designed and constructed by using MIL-100(Fe)@TiO2@Fe3O4 multicore-shells modified with horseradish peroxidase (HRP) as a smart active platform to realize detection and degradation of hydroquinone (HQ). The obtained micromotor showed a unique three-dimensional (3D) hierarchical architecture with highly exposed active sites and could autonomously move at a speed of 140 ± 7.0 μm·s-1 by O2 bubbles generated from the catalytic decomposition of H2O2 fuel. Benefiting from the combination of active self-propulsive motion, high peroxidase-like activity, tuned heterojunctions with matching band structures, and a 3D hierarchical structure, an effective platform involving dynamically sensitive detection and quick removal of HQ from water was established by using the multifunctional HRP-integrated MIL-100(Fe)@TiO2@Fe3O4 Janus micromotor. The proposed multifunctional Janus magnetic micromotor had advantages of simple and feasible fabrication, sensitive detection and effective photo-Fenton degradation of HQ in a wide pH range of 4-7, and magnetic recycling, revealing potential for environmental remediation applications.

Chemical design of self-propelled Janus droplets

Chemical design of self-propelled Janus droplets

Photosensitive Polymeric Janus Micromotor for Enzymatic Activity Protection and Enhanced Substrate Degradation.

Immobilizing enzymes into microcarriers is a strategy to improve their long-term stability and reusability, hindered by (UV) light irradiation. However, in such approaches, enzyme-substrate interaction is mediated by diffusion, often at slow kinetics. In contrast, enzyme-linked self-propelled motors can accelerate this interaction, frequently mediated by the convection mechanism. This work reports on a new photosensitive polymeric Janus micromotor (JM) for UV-light protection of enzymatic activity and efficient degradation of substrates accelerated by the JMs. The JMs were assembled with UV-photosensitive modified chitosan, co-encapsulating fluorescent-labeled proteins and enzymes as models and magnetite and platinum nanoparticles for magnetic and catalytic motion. The JMs absorbed UV light, protecting the enzymatic activity and accelerating the enzyme-substrate degradation by magnetic/catalytic motion. Immobilizing proteins in photosensitive JMs is a promising strategy to improve the enzyme's stability and hasten the kinetics of substrate degradation, thereby enhancing the enzymatic process's efficiency.

Upconversion-nanoparticle-functionalized Janus micromotors for efficient detection of uric acid.

We report enzyme-powered upconversion-nanoparticle-functionalized Janus micromotors, which are prepared by immobilizing uricase asymmetrically onto the surface of silicon particles, to actively and rapidly detect uric acid. The asymmetric distribution of uricase on silicon particles allows the Janus micromotors to display efficient motion in urine under the propulsion of biocatalytic decomposition of uric acid and simultaneously detect uric acid based on the luminescence quenching effect of the UCNPs modified on the other side of SiO2. The efficient motion of the motors greatly enhances the interaction between UCNPs and the quenching substrate and improves the uric acid detection efficiency. Overall, such a platform using uric acid simultaneously as the detected substrate and motion fuel offers considerable promise for developing multifunctional micro/nanomotors for a variety of bioassay and biomedical applications.

Optimization of the Drag Forces of Shell Janus Micromotor: A Study Based on Hydrodynamical Analysis and Numerical Simulation

Micromotors are widely used in cell operation, drug delivery and environmental decontamination due to their small size, low energy consumption and large propelling power. Compared to traditional Janus micromotor, the shell Janus micromotor has better motion performance. However, the structural optimization of its motion performance is still unclear. The main factor restricting the motion performance of shell Janus micromotors is the drag forces. In the current work, theoretical analysis and numerical simulation were applied to analyze the drag forces of shell Janus micromotors. This study aims to design the optimum structure of shell Janus micromotors with minimum drag forces and obtain the magnitude of drag forces considering both the internal and external fluids of the shell Janus micromotors. Moreover, the influence of the motor geometry and Reynolds number on the drag coefficient was analyzed using numerical simulations. The results provide guidance for the optimum flow velocity, opening diameter and shell thickness to achieve minimum drag force.

Smartphone-Based Janus Micromotors Strategy for Motion-Based Detection of Glutathione.

Herein, we describe a Janus micromotor smartphone platform for the motion-based detection of glutathione. The system compromises a universal three-dimensional (3D)-printed platform to hold a commercial smartphone, which is equipped with an external magnification optical lens (20–400×) directly attached to the camera, an adjustable sample holder to accommodate a glass slide, and a light-emitting diode (LED) source. The presence of glutathione in peroxide-rich sample media results in the decrease in the speed of 20 μm graphene-wrapped/PtNPs Janus micromotors due to poisoning of the catalytic layer by a thiol bond formation. The speed can be correlated with the concentration of glutathione, achieving a limit of detection of 0.90 μM, with percent recoveries and excellent selectivity under the presence of interfering amino acids and proteins. Naked-eye visualization of the speed decrease allows for the design of a test strip for fast glutathione detection (30 s), avoiding previous amplification strategies or sample preparation steps. The concept can be extended to other micromotor approaches relying on fluorescence or colorimetric detection for future multiplexed schemes.

Janus particles and motors: unrivaled devices for mastering (bio)sensing.

Janus particles are a unique type of materials combining two different functionalities in a single unit. This allows the combination of different analytical properties leading to new analytical capabilities, i.e., enhanced fluid mixing to increase sensitivity with targeting capturing abilities and unique advantages in terms of multi-functionality and versatility of modification, use, and operation both in static and dynamic modes. The aim of this conceptual review is to cover recent (over the last 5 years) advances in the use of Janus microparticles and micromotors in (bio)-sensing. First, the role of different materials and synthetic routes in the performance of Janus particles are described. In a second main section, electrochemical and optical biosensing based on Janus particles and motors are covered, including in vivo and in vitro methodologies as the next biosensing generation. Current challenges and future perspectives are provided in the conclusions section.Graphical abstract

Investigating Time-Dependent Active Motion of Janus Micromotors using Dynamic Light Scattering.

Advances in fabrication methods have positioned Janus micromotors (JMs) as candidates for use as autonomous devices in applications across diverse fields, spanning drug delivery to environmental remediation. While the design of most micromotors is straightforward, the non-steady state active motion exhibited by these systems is complex and difficult to characterize. Traditionally, JM active motion is characterized using optical microscopy single particle tracking for systems confined in 2D. Dynamic light scattering (DLS) offers an alternative high-throughput method for characterizing the 3Dactive motion in bulk JM dispersions with additional capabilities to quantify time-dependent behavior for a broader range of JM sizes. Here, the active motion of spherical JMs is examined by DLS and it is demonstrated that the method enables decoupling of the translational and rotational diffusion. Systematic studies quantifying the time-dependent diffusive properties as a function of fuel concentration, JM concentration, and time after fuel addition are presented. The analyses presented in this work position DLS to facilitate future advances of JM systems by serving as a fast-screening characterization method for active motion.

Propulsion Mechanisms of Light‐Driven Plasmonic Colloidal Micromotors

Colloidal micromotors are important candidates for a wide spectrum of applications, ranging from medicine to environmental remediation. Thus far, the propulsion force determination has been obtained from the colloidal motor motion speed and surrounding viscosity via the Stokes drag. Herein, a precise force measurement method and detailed analysis of the fundamental propulsion mechanisms of colloidal Janus micromotors propelled by thermophoretic and steam bubble force vectors, revealing findings uninvestigated to date, are presented. Optical tweezers provide fast and high‐precision force measurements in all three orthogonal dimensions simultaneously. Colloidal Janus micromotors are compared with isotropic hot Brownian reference microparticles, which have no defined force vector that propels them perpendicular to the direction of the laser beam. Janus micromotors display a defined laser power intensity‐dependent thermophoretic propulsion, as well as bubble force‐based propulsion, after surpassing the threshold value for the water boiling. The steam bubble propulsion force vector and the thermophorethic force vectors sum up for the Janus micromotor propulsion direction. On the contrary, the bubble force counteracts photophoretic force in propagation direction of light. Moreover, the thermal‐based reduction of viscosity around the Janus colloidal motor contributes significantly to its speed and guidance abilities.

γ-Fe2O3@Ag-mSiO2[sbnd]NH2 magnetic Janus micromotor for active water remediation

Micromotors have hold great promise for environmental remediation. Herein, we reported a novel γ-Fe2O3@Ag-mSiO2NH2 magnetic Janus micromotor for dynamic removal of heavy metal ions and organic pollutants from water. The as-prepared Janus micromotors displayed unique core-shell structure composed of amino-functionalized meso‑SiO2 shell and γ-Fe2O3 magnetic core, and could be propelled by O2 bubbles generated by Ag-catalyzed decomposition of H2O2. This Janus micromotor could move with a high velocity of 203.06 ± 10.6 μm s−1 (about 253.8 body lengths s–1) in 10 wt.% H2O2 solution. γ-Fe2O3@Ag-mSiO2NH2 magnetic Janus micromotor showed much higher removal capacities for Cu2+ (15.59 mg g−1) and doxycycline (31.45 mg g−1) than its static control samples due to the synergy of self-propulsive movement and amino-functionalized porous core-shell structure. Furthermore, the magnetic γ-Fe2O3 cores enabled the remote magnetic control on the movement direction and magnetic recycling. This work provides a new strategy for environmental pollutants treatment.