Chemotaxis is the ability of living systems to sense chemical gradients in their surroundings and react accordingly. Most prominent chemotactic behavior is exhibited by simple microorganisms that are able to migrate towards gradients of concentrations of the chemo-attractant, by activation of complex intracellular sensing cascades making use of specific chemoreceptors.1 Classical demonstrations of this phenomenon were described by Engelmann, who observed the accumulation of Bacterium termo, a rod-shaped bacterium, in oxygen-rich surroundings of cells undergoing photosynthesis.2 Yet, other bacteria can show completely different behavior, such as Spirillum tenue, which is repelled by high oxygen pressure, presenting an anti-chemotactic behavior.3
Increasing research in the field of artificial micro- and nanomotors has revealed enormous advances and several similarities not only to biological motors, but also to bacteria.4 Recently, several sources including magnetic field,5 light,6 temperature,7 electrical stimuli,8 or ultrasound waves9 have been employed for external control over the motion of microscopic motors. Enzymes and DNA used as motors showed attraction to chemical gradients, that is, chemotaxis.10 Sen et al. reported the chemotactic behavior of Janus motors powered by polymerization reactions.11 The same group studied the chemotaxis of bimetallic rods in a gradient of hydrogen peroxide solution when a H2O2-soaked agarose gel was placed on a cover slip containing the nanomotors suspension.12 The study was also performed using capillaries containing different concentrations of H2O2 as chemo-attractant, similar to the classical experiments on bacterial chemotaxis.13 The authors described that the motion of the self-propelled rods presented a slight bias directed towards the capillaries with higher H2O2 concentration. Recently, Solovev et al. reported that catalytic tubular microjets can be attracted to specific targets by using capillary forces.14 This effect takes place at the air–liquid interface and resembles the motion of water striders, induced by the meniscus climbing effect. There is thus the possibility that the capillaries used for the chemotaxis of nanorods may also have contributed to a physical attraction force (capillary force) in addition to the chemical attractive force provided by the H2O2. To date, there is a lack of chemotactic studies in which the attraction of artificial motors within the bulk liquid only originates from the chemical source, thereby ruling out any other external factor, such as capillary forces. Moreover, the movement of living organisms and the chemotactic motion of bacteria varies depending on their size and shape.15 Thus, a comparative study between artificial chemotaxis of two types of motors, with different shapes and size within the same system, is of great interest.
Herein, we present the chemotactic attraction of two types of catalytic motors (tubular microjets16 and Janus particles5h) towards high concentrations of hydrogen peroxide, which is used both as a fuel and as chemical attractant, in a microfluidic device where the capillary forces that act at the air–liquid interface can be neglected. We quantified the deviation angle (opening angle β) the motors experience once the intersection of the channels is reached for different concentrations of the chemicals. Remarkably, although driven by different propulsion mechanisms, both types of catalytic motors, tubular microjets and spherical Janus beads, orient and deviate towards higher hydrogen peroxide concentrations. We observed that spherical motors are more sensitive to the gradient of the fuel imposed in the system, which is probably governed by the processes of translational and rotational diffusion of the catalytically active motors. Their “turn” is different in magnitude, making the shape of the artificial motors an important parameter in the movement through chemotaxis.
We designed a three-inlet parallel flow device in a Ψ-channel geometry17 (Figure 1 A and Experimental Section) where colloidal micromotors flow through the central channel, whereas the aqueous solutions (with or without H2O2) flow in the other two side-channels.
Microfluidics offers a high degree of control over the chemical environment where analytes, microorganisms, or particles are spatially localized. In addition, it enables the impact of various undesirable factors, such as capillary forces that may interfere with the results of the analysis, to be decreased. Since the fluid flows and channel geometries can be precisely controlled, microfluidics seems an excellent tool for systematic quantitative analysis where gradients at the microscale are involved, for example, chemotaxis.18
We performed control experiments to evaluate and optimize the flow profiles formed by the merging of three channels inside of the microfluidic chip. These experiments were realized by labeling hydrogen peroxide solution flowing in the upper inlet i1 (Figure 1 B) with the fluorescent dye (Rhodamine 6G) and at same time pumping unlabeled water into two other inlets (i2 and i3). The profiles of Rhodamine 6G as model molecule were examined by fluorescent microscopy which allows 1) visualization of the interface between the fluorescent solution containing hydrogen peroxide and pure water and 2) determination of the evolution of the chemo-attractant profile across the channel upon changing the injection rates. Note that, however, the flow profiles of hydrogen peroxide at various flow speeds would be slightly different from that of Rhodamine 6G. We investigated the evolution of the hydrogen peroxide gradient across the channel with respect to the changes of the total injection rate Qtotal (range of 10–180 μL h−1), which is representing a sum of the flow rates Q1, Q2, and Q3 of the liquids, injected through inlets i1, i2 and i3, respectively. As expected, a decrease of the flow speed allows for stronger interdiffusion and leads to spreading of the hydrogen peroxide into channel i2. To maintain a controlled profile of the flows, we kept the total three inlet flows Qtotal constant at 140 μL h−1.
Catalytic motors, dispersed in the aqueous solution are injected through the central channel i2 into the microfluidic device. Various concentrations of hydrogen peroxide 0, 5, 10, and 15 % wt/wt and pure water were pumped through the inlets i1 and i3 at the equal injection rates Q1 and Q3, respectively. To prevent the aggregation of the motors (microjets, Janus particles) and their adhesion to the microchannel walls, 1 wt % of sodium dodecyl sulfate (SDS) was added into all the solutions. Once the catalytic motors reach the intersection area and “feel” the gradient of the fuel molecules they experience chemotactic attraction towards higher concentrations of hydrogen peroxide. This results in the deviation of the direction of the particles motion from the central part (parallel to the channel axis, i2) to the upper part of the channel (towards i1), containing more peroxide molecules. This deviation is quantified by tracking of the trajectories of the catalytic motors and estimating the opening angle β, determined as an angle between the vectors, tangential to the trajectories of the particle and parallel to the center line of the channel (see Figure 2 A). To investigate and rule out the effect of lateral flows caused by the inflow channels, we pumped non-catalytic reference particles (polystyrene, 3 μm) through the middle channel (i2), 10 % H2O2 through i1, and water through i3. In this negative control experiment, we neither observed a change in their motion along the channel nor deviation towards the chemo-attractant (Figure 2 B). In addition, to further exclude the possible variation on the channel flows influencing the deviation of the motors, we injected catalytic motors through the central channel i2 and water through i1 and i3, all of them at the same initial flow rates. Results showed that catalytic motors maintain straight motion and only few of them present a small deviation (opening angles β less than 1 degree) of their motion (Figure 2 C), which could be attributed to the local inhomogeneities of the flow profile in the channel or small asymmetry of the tubular microjets.
These control experiments indicate that the flowing conditions do not influence the chemotactic motion of catalytic motors and that the chemical composition (H2O2 aqueous solution versus deionized (DI) water solution) does not influence the migration of the particles towards one channel or another. We also inverted the channel configuration flowing H2O2 through i3 and water through i1 with the same results, that is, micromotors deviating towards the gradient of H2O2 (data not shown).
We injected different concentrations of hydrogen peroxide acting as chemo-attractant through the inlet i1 to investigate the effect of the fuel gradient profile on the chemotactic behavior of the catalytic objects. While both catalytic objects feel attracted towards the fuel, spherical particles reveal larger opening angles β than tubular microjets. A systematic study on different concentrations of attractant from 5 % to 15 % wt/wt shows that tubular microjets only slightly increased the mean opening angle from 3°, 4° to 5° (Figure 3 A–C) whereas spherical Janus motors deviate with average angles of 8°, 10°, and 14° respectively (Figure 3 D–F). All the plots were derived from a set of at least 50 motors (n>50). As shown in Figure 3, spherical motors experience broader dispersion on the opening angles than the tubular microjets, indicating that the shape and geometry of the motors influence their chemotactic behavior. These differences are illustrated in Figure 4 where, upon increasing peroxide concentration, the mean deviation angle of both types of motors increases, with larger angles for the Janus particles for all concentrations. Also, the standard deviations are always larger for Janus motors than for tubular microjets. We attribute this variation to the difference in rotational diffusion caused by the different particle size/shape of the spherical particle (5 μm diameter) and tubular microjets (50 μm in length; Figure 5 A). The fluid around the motors induces an interfacial drag force provoking the spherical particles to rotate more easily than the microtubes. Moreover, the catalytic sites preferably rotate around the symmetry axis of the micro-object, along their trajectory of motion and give rise to motions in different directions which in turn leads to larger standard deviations in the experiments. In contrast, the microtubes containing catalytic sites in their hollow interior, have three axes of symmetry with non-identical coefficients of rotational diffusion τR. Thus, motion of the microtubes is not so much affected by the fluid flowing around them and thus, their motion is straighter in nature over the investigated region where the chemical gradient is generated. The exact mechanism of enhanced diffusion of the tubular microjets towards the gradient of chemo-attractant is still not well understood.
However, the phenomenon can be qualitatively described by assuming that, when the catalytic micromotors are immersed in a gradient of concentration of fuel and turn towards higher concentrations they make longer “runs” than if they turn towards lower concentration of fuel. Consequently, a net chemotactic displacement is observed. These results are in agreement with the observations from Sen and co-workers on chemotactic motion of self-propelled nanorods in bulk.12
The self-propelled micromotors are subject to Brownian diffusion, which is dependent on the size and symmetry.19 The displacement of the particle is defined by the processes of the translational and rotational diffusion with coefficients: DT=kB T/6πηR0 and DR=kB T/8πηR03, respectively.20 Where, kB T is the thermal energy, η is the viscosity of water, and R0 is the radius of the catalytic objects. The entering of the area containing fuel-molecules induces the local chemical reaction near the catalytic side of the micromotors. This leads to the enhancement of the fluctuations of the particles owing to the increase of the translational component of the diffusion similar to the results reported by Paxton et al. and Howse et al.21
However, the degree of deviation of both types of catalytic micromotors towards the higher concentrations of hydrogen peroxide is governed by the processes of rotational diffusion, which cause random reorientations of the catalytic object. The important parameter is the characteristic time of rotational diffusion τR=1/DR=8πηR03/kB T, which is very different for spherical Janus particles and tubular microjets when placed in water at T=300 K, that is, τRJanus≈102 s and τRTube≈105 s (along the long axis of the tube, length of the microtube is 50 μm), respectively. Furthermore, we found a trend of a nonlinear monotonic decrease of τR with an increase of concentration of H2O2 in solution, similar to that found by Howse et al. (data not shown).21b Subsequently, spherical Janus motors experience much stronger affinity to reorient within the gradient of hydrogen peroxide, compared to the tubular microjets. In addition, once the concentration of the peroxide increases, the difference in the chemotactic behavior (deviation towards higher fuel concentration, opening angle) of the spherical motors and tubular microjets becomes more visible. This fact is the direct consequence of the decrease of τR for the catalytic objects in the solution containing fuel.
In conclusion, we presented a controllable way of studying chemotactic behavior of two families of artificial catalytic micromotors, known to be driven by distinct mechanisms. Spherical Janus particles and tubular microjets move towards the gradient of fuel in microfluidic channels without the influence of capillary forces. The chemotactic motion was found to be dependent on the concentration of chemo-attractant but also on the size and shape of the artificial micromotors. Different types of micromotors deviate in different angles, which could lead to possible applications, such as separation of objects at micron- and nanoscale. The chemotactic effect using microfluidics has been extensively studied in bio-organisms, such as bacteria. Herein, we presented an extension of that phenomenon into man-made micromotors, demonstrating that these can sense the environment and be directed towards desired locations. Further studies could lead to the understanding of collective behaviors of catalytic motors, the attraction or repulsion to other molecules rather than hydrogen peroxide, such pollutants in water.