Micromotors with Spontaneous Multipattern Motion and Microvortex for Enhanced “On‐the‐Fly” Molecule Enrichment

“On‐the‐fly” molecule enrichment by micro/nanomotors obviously improves heterogeneous catalysis, trace detection, and environmental monitoring, yet faces challenges of the trade‐off between collection range and interaction time. Inspired by the versatile foraging process of predators, this work demonstrates that micromotors doing spontaneous multipattern motion with microvortex can greatly enhance “on‐the‐fly” enrichment, demonstrated by highly sensitive surface‐enhanced Raman scattering detection. It leverages an axis‐asymmetric bowl‐shaped structure and the nonlinear Ag–AgCl reaction, realizing alternating low‐velocity swinging forward and accelerated steering motions for prolonged interaction and large work area. Moreover, the bowl‐shaped microstructure bestows a micro‐vortex above the Ag side due to the competition of electric potential and pressure gradient, also extending interaction time during the acceleration. Consequently, it exhibits at least an order of magnitude larger enhancement of detection signals than the counterparts. This proof‐of‐concept study highlights the significance of motion mode and structure design in guiding flow field, offering substantial benefits for applications.


Introduction
Molecule enrichment, commonly referring to the collection of trace species from bulk solutions to the solid substrate surfaces, is a crucial link to carry out heterogeneous catalysis, trace detection, environmental monitoring, and biomedical applications. [1]t is usually achieved by chemical interactions such as covalent bonds [2] and antibody-antigen reactions, [3] as well as physical interactions, such as magnetic, [4] static, [5] and fluidic effects, depending on the proximity of molecules and efficient collision on substrate surfaces. [6]Passive substrates, achieving enrichment only through free diffusion or convection, usually exhibit a low efficiency in molecule enrichment, subject to the limitation of laminar flow. [7]lternatively, active substrates, also named as micro/nanomotors (MNMs), are able to transport in solutions, making built-in solution mixing and a large portion cover. [8]They show enhanced mass transfer and collection range, leading to "on-the-fly" molecule enrichment with a significantly increased efficiency for enhanced metal-ion sequestration, [9] chemical and biological warfare agents detection and removal, [10] biomolecule recognition, [11] and so on. [12]For example, a magnetic "rod-like" micromotor doing continuous rotation under a rotating magnetic field can enhance the enrichment of trace R6G via creating a microvortex for efficient solution mixing. [8]A matchlike silica-coated silver nanowire micromotor utilized light-induced translation to achieve "on-the-fly" crystal violet enrichment and detection. [13]Nevertheless, the so far developed "on-the-fly" molecule enrichments most depend on the simple and rapid mobility of MNMs, losing sight of the possibly reduced interaction time deriving from rapid motion. [14]n analogy to predators in versatile foraging processes including slow searching, rapid approaching, and sudden turning, [15] MNMs performing molecule enrichment may also need a multipattern motion with autonomous switch of velocity and/or direction.For example, they move slowly for prolonging interaction time, and steer swiftly for extending collection range and enhancing mass transfer.However, the multipattern motions of the so far developed MNMs all depend on the change of external inputs, such as "fuel" concentration, [16] light intensity, [17] and temperature. [18]In contrast, chemical-driven MNMs, such as those based on catalyzed decomposition of hydrogen peroxide, [19] catalyzed oxidation of glucose, [20] and reaction between active metal and water/acid, [21] are generally difficult to regulate the motion patterns in real-time once they are propelled.
In this work, we design a bowl-shaped Janus micromotor (BJM) based on Ag-AgCl oscillatory reactions that can spontaneously carry out a multipattern motion including alternatively swinging forward and accelerated steering with microvortex to facilitate significantly enhanced "on-the-fly" molecule enrichment.As shown in Scheme 1, the BJM is composed of a hollow polystyrene (PS) bowl with silver (Ag) half-coated on the inside and outside surface, and is driven by the Ag-AgCl nonlinear chemical reactions under constant UV light irradiation in an aqueous solution of H 2 O 2 and KCl mixture.It can periodically switch the motion patterns between swinging forward (Phase I) and accelerated steering (Phase II).This multipattern motion ensures a relatively long time for interaction and a large range cover for collection, achieving highly efficient enrichment.Meanwhile, with the competition of negative curvature-induced pressure gradient and asymmetric reaction-generated electric potential gradient, there is a microvortex above the Ag side of the motor, prolonging the interaction time of molecules and substrates even during the acceleration.As a result, the as-developed motors can act as highly sensitive surface-enhanced Raman scattering (SERS) probes, demonstrating enhanced "on-the-fly" molecule enrichment by more than an order of magnitude.This result gives a distinct implication that chemical-driven MNMs can overcome the intrinsic hard-to-modulated problem without external input assistance.The BJM was obtained by self-collapse of the Janus Ag/PS hollow spherical particle (Figure S1, Supporting Information) following our recently reported work, [22] as depicted in Figure S2, Supporting Information (See Experimental Sections).The SEM and TEM images in Figure 1a,b show the hollow bowl-shaped morphology of the BJM with the outer and inner diameter of 1.2 AE 0.16 μm and 735 AE 20 nm (Figure S3, Supporting Information), indicating its asymmetry in geometry.From SEM and EDS shown in Figure 1c-e, the C element is distributed in the whole bowl, while the Ag element is only detected upon asymmetrically convex and concave surfaces.This indicates the Ag element is distributed only on one side of the geometric symmetry plane of the BJM.In view of this, the as-prepared BJM has no symmetry axis regarding composition and geometry, revealing the axis-asymmetric feature.

Results and Discussion
Given the nonlinearity arising from the oxidation of Ag and photocatalytic decomposition of AgCl, the as-prepared BJM can move in 1.7 wt% H 2 O 2 and 600 μM KCl aqueous solution under UV light irradiation (500 mW cm À2 ).The motion behavior was recorded by an optical microscope and all micrographs were taken on the X-Y plane.Figure 1f, taken from Video S1, Supporting Information, presents the moving trajectory of a typical BJM in 121 frames (f ) with unequivocal color-indicated instantaneous velocities.To avoid the unclear identification caused by the overlap, the trajectory is divided into two sections of 0-91 and 92-121 f.It can be found that the locomotion of the as-prepared BJM is accompanied by oscillating changes in moving velocity and direction, and its redirection often occurs as the moving velocity increases.According to the moving states, this multipattern motion can be defined as two phases.Phase I is "swing forward," in which the BJM moves forward at a low velocity accompanied by a random swing, while Phase II is a sudden acceleration period, displaying an obvious steering with increasing velocities and negative gravitaxis.Subsequently, a decay in velocity occurs and the BJM returns to Phase I. Figure 1g is the analysis of instantaneous translation linear velocities (ν) and rotation angular velocities (ω) in 121 f, further indicating the oscillatory of the motion arising from a time-variable driving Scheme 1. Schematic illustration of a typical axis-asymmetric Ag-based bowl-shaped Janus micromotor (BJM), showing a time-periodically varying autonomous multipattern motion behavior with a microvortex for enhanced "on-the-fly" molecule enrichment.
force.The simultaneous increases of ν and ω suggest that the BJM is redirected together with increasing the driving force.
Compared with previously reported MNMs performing simply continuous movements for "on-the-fly" enrichment, the BJM doing multipattern motion is supposed to have an advantage that Phase I of swinging forward can prolong the interaction time of the substrates and molecules, and Phase II of acceleration steering can enlarge the enrichment range and enhance the mass transfer.Thus, the "on-the-fly" enrichment efficiency is expected to be enhanced on the BJM substrates.Surface-enhanced Raman scattering (SERS) is a detection technology widely used in biological and chemical sensing, and the amplification of the detected signals greatly depends on the effective enrichment capacity of molecules on the substrate surface. [9,23]For its signal acquisition, a common environmental pollutant of rhodamine 6G (R6G) is chosen as a model analyte. [24]o validate the enhanced "on-the-fly" enrichment ability of the BJM with the multipattern motion, the spherical Janus Ag/PS micromotor (SJM) exhibiting an oscillatory velocity but no distinct steering (Figure S4 and Video S2, Supporting Information) was also used for comparison.In detail, the motor-based SERS probes were predispersed in a mixed aqueous solution of 1.7 wt% H 2 O 2 and 600 μM KCl in the presence of UV light irradiation for 10 min before drying for detection.Figure 1h presents the Raman spectra of 10 À6 M R6G when the active and inactive BJMs, as well as active the SJM are used as probes, respectively.They have characteristic Raman peaks at 612 cm À1 (P1: C─C─C in-plane vibration), 773 cm À1 (P2: C─H bonds out-of-plane vibration), 1,365 cm À1 (P3: aromatic C─C stretching vibration), and 1,650 cm À1 (P4: aromatic C─C stretching vibration). [25]Compared with the inactive probe, the active motors show significantly enhanced Raman signals due to "on-the-fly" enrichment.By comprising the intensities of the peaks at 612 cm À1 , the active BJM after 10 min collection exhibits 4 and 16 times higher signal intensity than the active SJM and inactive BJM, respectively.This strongly demonstrates that the multipattern can significantly enhance "on-the-fly" molecule enrichment.In this case, trace R6G (%10 À6 M) has ignorable influences on the motion of the BJMs.

Mechanism Analysis and Impact Factors of the Multipattern Motion
To elucidate the enhanced molecule enrichment of BJMs, deep understanding of motion mechanisms and induced fluid behavior is necessary.The as-prepared BJMs in a mixed H 2 O 2 and KCl aqueous solution illuminated by UV light will undergo Ag-AgCl nonlinear chemical reaction involving the oxidation of metal Ag and photodecomposition of AgCl [26] Ag þ Therefore, Equation ( 1) is an ion-consumed reaction, while Equation ( 2) is an ion-generated reaction.The driving force of the BJM originates from H þ and Cl À ions diffusioninduced built-in electric field as the diffusivity of H þ (9.31 Â 10 À9 m 2 s À1 ) [27] is bigger than that of Cl À (2.24 Â 10 À9 m 2 s À1 ). [28]26a] Accordingly, as shown in Figure 1h, the motion of the BJM would alternate between a long swinging stage (>36 f, Phase I) and a short time fast steering (<4 f, Phase II).This is further elucidated by combining numerical stimulation of the driving mechanism of the BJM.Since the redirection of the BJM occurs phenomenally accompanied by increasing ν, we established the theoretical model based on the ion-generated reaction of AgCl photodecomposition (Equation ( 2)). Figure 2a shows that the electrical potential near the PS side is higher than that near the Ag side.As a result, an electrical field established points from PS to Ag.This endows an off-centroid driving force on the surface of the BJM, and makes the BJM do steering when the driving force can offset the molecular thermal motion, as illustrated in Figure 2b.At the initial stage, Equation (1) occurs at a slower reaction rate, [26a,29]  inducing a small driving force to be offset by molecule thermal fluctuations, thus resulting in swinging forward with a low velocity. [30]28a] Their diffusions cause ion-diffusiophoresis to propel the BJM.Due to its axis-asymmetric structure, a larger off-centroid driving force causes a pronounced acceleration and the ensuing steering.With regenerating Ag, Equation (1) is reactivated.The BJM returns to a long, low-velocity swinging forward phase.Thus, the unique motion behavior is a result of a synergy of the driving force and Brownian force.
Figure 2c, taken from Video S3, Supporting Information, shows the dependence of the motion mode on the concentration of H 2 O 2 , and Figure 2d presents the curve of average peak velocity (ν peak ) at different 2 O 2 concentrations.In pure water, the BJM undergoes molecular thermal motion.In contrast, in the presence of H 2 O 2 , even if its concentration is as low as 0.17 wt%, it displays a dominant motion of translation accompanied by swinging forward at a low ν peak , but without steering.With the H 2 O 2 concentration increasing from 0.33 to 3.3 wt%, the ν peak increases and reaches a maximum of 43.9 AE 6.7 μm s À1 at 1.7 wt% of H 2 O 2 , which can be explained by the increased driving reaction rate.At the H 2 O 2 concentration of 3.3 wt%, the trajectories still show an evident steering together with acceleration.Such steering is induced by the increased driving force acting on the off-centroid of the BJM.Further increasing the H 2 O 2 concentration to 5.0 wt% gives rise to the further decrease of ν peak , while the steering in its trajectory becomes unobvious.This result further demonstrates that the motion pattern transformation of the BJM depends on the change of the driving force deriving from the nonlinear chemical reaction of Ag-AgCl and displays a velocity-dependent feature.Through analyzing the motion behavior of the BJM at different H 2 O 2 concentrations (Figure 2), the critical velocity for steering is evaluated to be 12.2 AE 4.2 μm s À1 (v threshold ).The velocity decrease in high supply of H 2 O 2 can be ascribed to the effect of ion concentration on the colloid stability. [31]This is indirectly supported by the fact that the BJM sinks to the bottom when the H 2 O 2 concentration is further increased to 10 wt%.

Microvortex Formation Mechanism and its Positive Effect for Enrichment
Additionally, the competition of the generated electric potential gradient and the pressure gradient resulting from negative curvature can form a secondary vortex flow on the concave of the BJM.To simplify the model, the motor is assumed to be immobile.Figure 3a shows the fluid behavior around a BJM, indicating an obvious microvortex on the active Ag side of the BJM and a relatively increased fluid velocity, which is in sharp contrast to the case of an SJM (Figure 3d).This is significant for enhanced "on-the-fly" molecule enrichment through increasing the contact between molecules and probes when the BJM moves with a high velocity, being similar to the deposits of mud caused by the natural secondary flow.
The formation of the microvortex is reasonably attributed to the contribution of the axis-asymmetric structure.In general, the motors moving in a low Reynolds number liquid environment (Re ( 1), where viscous force is dominant and inertia effect is neglected, are governed by Stokes equation [32] À∇p þ μ∇ 2 u ¼ F where p, μ, u are pressure, viscosity, and fluid velocity, and F is the additional force, namely, electric field force (i.e., ) in the present system.It indicates that the fluid motion is determined by pressure and electric field force. [33]Therefore, to elucidate the formation of the microvortex, the distributions of pressure and electric field around a BJM and an SJM are also simulated, respectively.All data are normalized to highlight the effect of structure.Points A-F in Figure 3b,c,e,f are representative positions indicating the characteristic pressures or fluid velocities.Specifically, points A(F), B, C(E), and D represent the locations with the lowest fluid velocity, the largest negative, zero, and the largest positive pressures, respectively.Different from the electrical potential, pressure, and fluid velocity around an SJM that have symmetric distributions along its boundary, those around a BJM have an obvious asymmetric feature.From position B to D in the BJM, the negative curvature in depression creates a large pressure gradient opposite to the electric potential gradient (from position B to D), which prevents the fluid from moving along the boundary, but rather in the direction of the pressure gradient (Figure 3b,c).Consequently, the fluid flows and decelerates along the boundary from B to D close to zero (Figure 3a), and the external fluid would replenish to form a distinctive microvortex on the region of zero pressure (point E). [34] In contrast, the SJM has too small pressure gradient to offset the effect of electric gradient (Figure 3f,g), and its axissymmetric feature results in the fluid slides along the boundary of the spherical particle symmetrically.
To approve the positive effect of the microvortex on enrichment, we use small inert particles instead of solute molecules for dynamic mass distribution simulations to characterize the solute molecules distribution around a BJM.Simultaneously, the distribution around an SJM is also simulated for comparison.As shown in Figure 3g,h, taken from Video S4, Supporting Information, the fluid first flows along either BJM or SJM surface from inactive PS to active Ag.The particles around an SJM are rapidly pushed away from the Ag surface by the flow field, while those around a BJM are first concentrated on the Ag side with slower movements due to the action of microvortex.This picturesquely demonstrates that the dispersed particles around a BJM have a relatively long interaction time and many more opportunities to contact the active surface for more excellent collection than that around an SJM.Although it is difficult to directly observe the microvortex due to the short acceleration time, high speed, and apparent negative gravitaxis during acceleration, the formation of the microvortex around a BJM can be indirectly evidenced by the positive effect of the fixed BJM on the SERS sensing.As shown in Figure S6, Supporting Information, the fixed BJM induces a higher Raman signal compared to the fixed SJM, %1.7 times higher.This indirectly confirms the existence of the microvortex, as they have both excluded the effect of motion modes.

Enhanced Efficiency and Generality of the BJM for Molecule Enrichment
For SERS sensing and detection, a high sensitivity detection in the shortest possible time is crucial.Therefore, the Raman signals changing over time and at a much lower analyte concentration were further investigated.Figure 4a shows the detection results of 10 À6 M R6G in 30 s.For inactive BJM, there is no Raman signal.With active probes, Raman signals are detected with recognizable intensity.Moreover, the Raman signal upon the active BJM is further enhanced by about twice by comparing the characteristic peak at 610 cm À1 (P1).This suggests that benefiting from the enlarged collection range and the prolonged interaction time, the BJM doing multipattern motion has significantly enhanced the "on-the-fly" enrichment ability and can achieve efficient detection in 30 s.With the enrichment time increasing, the Raman signals are gradually boosted for all three probes, and the active BJM system shows the fastest rising rate (Figure 4b).The decomposition of R6G occurs on light-excited Ag/AgCl composite, [35] as evidenced by the decreased signal intensities when prolonging the detection time (Figure S7, Supporting Information).However, the qualitative detection is basically not affected when the enrichment time is reasonably regulated as 5-10 min.This is because the R6G concentration is too low for the rapid decomposition while the BJMs have enhanced enrichment efficiency from the multipattern movement.When the concentration of R6G is further decreased to 5 Â 10 À10 M, which is lower than the Chinese industry testing limit (8.6 Â 10 À10 M), [36] the active BJM probe can still display a clearly resolved Raman signal (Figure S8, Supporting Information).This strongly demonstrates the positive effect of the multipattern motion behavior and microvortex on molecule enrichment (Figure 4c).In addition, the BJMs can also achieve the detection of trace amounts of benzidine (1 Â 10 À6 M) (Figure 4c), confirming the generality of the BJMs regarding the enhancement of "on-the-fly" molecule enrichment.

Conclusion
In summary, we have demonstrated a significantly enhanced "on-the-fly" enrichment strategy by designing a micromotor doing spontaneous multipattern motion by combining the particular axis-asymmetric bowl-shaped structure and the Ag-AgCl nonlinear chemical reaction.The micromotor is composed of hollow PS bowls with Ag half-coated.It can alternatively perform motions between swinging forward at a low velocity and steering only at the acceleration stage, simultaneously improving the interaction time, collection range, and mass transfer during enrichment.Additionally, there is a distinctive microvortex above the active Ag side of the concave surface resulting from the large pressure gradient formed by the negative curvature in depression, enabling the reduction of flow velocity to facilitate molecule enrichment in the acceleration stage.Consequently, as a SERS probe, the BJM manifests highly sensitivity for sensing R6G and benzidine, more sensitive by at least one order of magnitude than the counterparts.The result herein demonstrates the structure-directed motion for designing novel and functional micro/nanomotors that can advance the fields, such as adsorption, sensing, and catalysis, while the BJM reported indicates a proof-of-concept of motion mode-guided active substrate for highly enhanced enrichment.
Preparation of BJMs: 150 μL, 2 mg mL À1 of hollow PS spherical particle suspension was drop-casted on a piece of clean glass side (2.5 cm Â 2.5 cm) and then freeze-dried them to obtain a monolayer.Hollow Janus PS-Ag spherical particles were prepared by sputtering an Ag layer for 40 s, followed by sonication and resuspension in ethanol to fill the cavities of the hollow particles with ethanol.The as-prepared BJMs were obtained by redispersing the hollow Janus PS-Ag spherical particles into deionized water and subsequently stirring for 24 h.Self-collapse starting from the PS-Ag junction is caused by the bombardment of sputtering atoms as well as the difference in mechanical properties (E PS = 3.2 GPa, [37] E Ag = 85 GPa [38] ) and thermal properties (T PS = 212 °C, T Ag = 961.93°C) of PS and Ag. [39]Figure S2, Supporting Information shows a typical BJM morphology.We figure out the structure parameters from the top views and side views of 10 BJMs, a typical BJM morphology is given in Figure S2, Supporting Information.The outer and inner diameters are 1.2 AE 0.16 μm and 735 AE 20 nm, respectively.
Characterization: Scanning electron microscopy (SEM) images were acquired on a Zeiss Ultra Plus field-emission SEM.A 5 kV acceleration voltage was used for measurement.Elemental mapping and line-scanning were acquired with EDS detector X-Max 50 from Oxford Instruments at 15 kV.In addition, transmission electron microscopy (TEM) images were acquired on a JEM-1400Plus high-resolution at 120 kV.Raman spectra were collected by using a confocal Horiba LabRAM HR Evolution Raman spectrometer with 514 nm laser.
Particle Image Velocimetry Analysis: The motion of the BJM was observed by a Leica optical inverted microscope equipped with a 100Â oil immersion objective.The BJM was placed on a thin sheet of glass, and videos were recorded by a CCD camera at a frame rate of about 18 fps.Its motion was triggered by a UV light with a wavelength of 365 nm and a power intensity of 500 mW cm À2 .All optical micrographs were taken on the X-Y plane where particles were settled on, slightly above the bottom substrate.
Motor tracking was achieved by Video Spot Tracker (version 08.11) software.In short, each particle was first distinguished from the background via gray difference by an artificial set, and their changes of coordinates were counted to calculate the instantaneous velocities.
Data Analysis for Angular Velocity: To calculate the instantaneous angular velocity in Figure 2b, the BJMs in optical micrographs were identified by Python, and then we selected the PS-Ag interface by gray values as baselines to calculate orientation angles (θ) between them with the horizontal line, following the instantaneous angular velocities (ω ¼ θ 2 Àθ 1 t 2 Àt 1 ) are obtained.Herein, anticlockwise angles are considered as positive, and clockwise angles are negative.
Numerical Simulation: The theoretical modal is based on the laminar flow module, electrostatic module, dilute transfer module, and particle tracking module on COMSOL Multiphysics (5.5a).To simplify the model, chemical reaction Equation ( 2) is represented by stable ion flux of H þ , Cl À on Ag side.Ion diffusion results in concentration gradient of products around a BJM, which is solved by dilute transfer module where J i is ionic flux, u is flow velocity, φ is electrostatic potential, D is ionic diffusion coefficient, c i is ionic concentration, and z i is ionic charge.The electrical potential around a BJM was calculated by a variable, space charge density ρ e , which depends on ion concentrations distribution solved by dilute modules, governing equation of electrostatic module as where ε 0 is the relative dielectric constant, ε r is absolute dielectric constant, and φ is electric potential.Electrical potential gradient around a BJM also attributed to the mass transfer of ions by z i FD i ∇:ðc i ∇φÞ RT in Equation ( 4), the BJM surface was set as the zero-charge boundary condition, and the electric potential and initial value were set to zero.
The negative BJM is driven by a self-built electric field, as the motor was fixed in the model, we set the BJM surface as the electro-osmosis boundary condition to simulate the electric force applied in the BJM ðu • ∇uÞ ¼ À∇p þ μ∇ 2 u, ∇⋅u ¼ 0 (6)   where U eo is the electro-osmotic speed of the BJM, ζ is the zeta potential of the BJM, and the tangential electric field E ta was derived from the electrostatic module, the zeta potential on the surface of the BJM is À29 mV.The governing equation of fluid field is the Stokes equation, as follows The couple between fluid field and mass transfer is in u∇c i , and the attribution of mass transfer to fluid field is p, effected by chemical potential μ, related to ion concentration c i . [40]he boundary conditions at the outer edge of the simulation domain are chosen to represent the bulk, constant concentration of ionic species, and a no-stress boundary condition for fluid.At the BJM's surface, we prescribe a uniform charge density.We solve for transient state for the BJM, solutions show the system reaches a stable state after 10 ms, and results in discussion all in 10 ms.More information on the simulation setup and all simulation parameters can be found in Supporting Information.
To decipher the dynamic enrichment ability, we simulate molecular motion in flow caused by the BJM dq dt ¼ τ p m p F drag (8)   where q, m p are the position, mass of molecular, respectively, F drag is the force caused by flow.The boundary condition at the BJM's surface is set as stick boundary, which means to fix the particle position at the instant the wall is struck.The molecules are released in the simulation domain randomly initially.Surface-Enhanced Raman Scattering Measurements: For a typical test, 10 μL BJMs were added into 10 μL 1 Â 10 À8 M R6G aqueous solution on the Si substrate, and then 20 μL 5 wt% H 2 O 2 and 600 μM KCl solution were transferred into the above solution.After being irradiated by UV light with an intensity of 500 mW cm À2 for 10 min for active molecule enrichment, the BJM was collected and dried for Raman measurements.

Figure 1 .
Figure 1.Characterization, typical motion behavior, and enhanced "on-the-fly" enrichment demonstration of the BJM.a) SEM images, b) TEM image, and c-e) EDS analysis.Therein, (c) is the Ag (green) and C (red) elemental line scan profiles, and (d) and (e) are Ag and C elemental mapping images, respectively.Scale bars: 400 nm.f ) The typical moving trajectory over 121 frames.The coded different colors record the instantaneous translation linear velocities (ν).g) The typical instantaneous ν and rotation angular velocity (ω) in 121 frames.Herein, the anticlockwise direction is positive.h) Raman spectra of 1 Â 10 À6 M R6G when using different probes.

Figure 2 .
Figure 2. Numerical simulation and experimental validation for the multipattern motion mechanism of the BJM.a) Simulated electrical potential (color-coded), electric field distribution (arrows), and the radial outward F drive around a BJM.b) Schematics for the motion behavior.When dominated by Equation (1), the BJM is only propelled by a small F drive that can be offset by the molecular thermal motion, resulting in a swinging forward behavior (Phase I).When dominated by Equation (2), the BJM is subjected to a large F drive , resulting in acceleration steering (Phase II).c) Representative motion trajectories and d) average peak velocities (ν peak ) of the BJM moving at different H 2 O 2 concentrations.The ν peak is calculated based on 30 micromotors.

Figure 3 .
Figure 3.The comparison of flow fields, pressures, and dynamic particle distributions around a BJM and an SJM.a) Simulated fluid velocity (color-coded) and streamline, as well as b) pressure distribution around a BJM.c) Electric potential (black line), pressure (red line), and fluid velocity (color-coded bar at the top) along the boundary of a BJM.d) Simulated fluid velocity (color-coded) and streamline, as well as e) pressure distribution around an SJM.f ) Electrical potential (black line), pressure (red line), and fluid velocity (color-coded bar at the top) along the boundary of an SJM.The white arrows in the color-coded bars of (c,f ) present the flow direction.Dynamically simulated particle distributions around g) a BJM and h) an SJM after moving 10 ms, respectively.Scale bar: 250 nm.All data are normalized.

Figure 4 .
Figure 4.The enrichment efficiency changes over time and the enrichment generality of the BJM.a) Raman spectra of 1 Â 10 À6 M R6G obtained in 30 s. b) The Raman signal intensity based on the characteristic peak of P1 changing with time upon three probes.c) Raman spectra of 1 Â 10 À6 M benzidine using the active and inactive BJM as probes, respectively.