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Direct Observation of the Direction of Motion for Spherical Catalytic Swimmers Stephen J. Ebbens* and Jonathan R. Howse Department of Chemical and Biological Engineering, University of Sheffield, Sheffield, U.K.
bS Supporting Information ABSTRACT: Nonconductive Janus particle swimmers made by coating fluorescent polymer beads with hemispheres of platinum have been fully characterized using video microscopy to reveal that they undergo propulsion in hydrogen peroxide fuel away from the catalytic platinum patch. The platinum coating shadows the fluorescence signal from half of each swimmer to allow the orientation to be observed directly and correlated quantitatively with the resulting swimming direction. The observed swimmer direction is consistent with both the bubble release and diffusiophoretic propulsion mechanisms.
’ INTRODUCTION Swimming devices is the general term used to describe synthetic objects that have been engineered to exhibit enhanced motion either at a solution air interface or fully immersed within a solution as considered here. The highly miniaturized micrometer and nanoscale forms of these devices offer the potential to operate as microfluidic transport systems1,2 and deliver targeted drug payloads.3 Two classes of such devices have been demonstrated to date; the first are driven by deformations and require actuation by external fields and the second, the focus of the study here, are fully autonomous and powered by the asymmetric surface catalytic decomposition of solution-borne fuel molecules. A range of these catalytic devices have been produced, using different materials, with assorted dimensions and geometries.4 Although much progress has been made in enhancing the performance of these devices, for example, in achieving high translation speeds5 and controlling trajectories,6 the fundamental mechanisms producing these behaviors remains the subject of debate, particularly for nonconducting spherical swimmer types.7,8 One of the fundamental mechanistic indicators for each device type is the direction of motion relative to the asymmetrical catalyst distribution. The importance of understanding this fundamental property has been illustrated for conducting bimetallic nanorods composed of platinum and another conducting metal, such as gold. Here a systematic study of the effect of changing the device composition with the travel direction was used to support a swimming mechanism relying on electron transport between their two halves during a coupled electrochemistry redox process.9 This study took advantage of the high reflectivity and the obvious contrast between the two segments of the metallic rod swimmers and the dampening of their Brownian rotation because of the high aspect ratio that allowed reflection mode optical microscopy to observe the travel r 2011 American Chemical Society
direction. However, for the related class of devices that have spherical geometry and only one conductive component, such as micrometer-sized spherical silica or polymer beads coated with a hemisphere of catalyst,10 an equivalent rationalization of the direction of motion has not, to date, been undertaken. This has, in part, been due to the difficulties in observing spherical swimmers’ orientation due to their symmetrical shape, with micrometer-scale sizes approaching the resolution limits of conventional optical microscopy, and the poor intrinsic optical contrast between the materials the devices are made from and their rapidly randomized trajectories. Two main mechanisms have been proposed to explain the motion of these nonconductive devices; the first proposes nanobubbles detaching from the surface of the catalytic patch, thus imparting momentum to the device. For some larger-scale swimming devices, this mechanism is clearly responsible for the propulsion as a trail of bubbles can be observed, correlating to the catalytic patch position.11 13 The dominance of this mechanism on the mesoscopic scale is likely to be due to the inability of the surrounding water to dissolve the large local concentrations of oxygen generated by the reduction of the hydrogen peroxide fuel. However, on smaller scales, where instantaneous local oxygen concentrations remain lower, a direct observation of these bubbles has so far not been possible. Instead, evidence for the mechanism relies on indirect observations such as the variation of velocity with surface tension.8 However, the key feature of bubble release is that it will explain only the propulsion of devices that move away from the catalyst, a feature that has not been confirmed as yet for small spherical swimmers. Received: August 23, 2011 Revised: September 15, 2011 Published: September 19, 2011 12293
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Figure 1. Schematic representation of a fluorescent Janus particle swimmer. The dark hemisphere (left) represents the platinum-coated half, and the bright-green area (right) corresponds to the uncoated fluorescing hemisphere. The swimmer is powered by asymmetrically decomposing hydrogen peroxide fuel into water and oxygen, which is thought to result in propulsion by self-diffusiophoresis (as shown here) or nanobubble detachment. (Inset) Typical fluorescence microscopy image, with an outline drawn to guide the eye.
The second mechanism suggests that the motion is caused by a self-generated local concentration gradient of dissolved oxygen, termed self-diffusiophoresis.7 This mechanism has been applied in detail to predict features of the propulsion for two distinct geometries, individual spheres with asymmetric catalyst coverage14,15 and two sphere swimmers comprising a “plain” inactive sphere joined to a uniformly catalytic sphere.16 In these analyses, the predicted direction of motion relative to the catalytically active material depends on the interaction of the surrounding fluid molecules with both the catalytic and noncatalytic surfaces of the devices.14 Although the former case was experimentally realized soon after the initial prediction of propulsion, the case of joined spheres has been experimentally realized only recently. In this later case, the device shape asymmetry (because the two spheres were different sizes) allowed the straightforward observation of the direction of motion that was toward the platinum-coated cap.17 In this context, it is clearly important to develop a method capable of revealing the direction of travel for individual nonconductive spherical swimmers to ensure that future mechanistic proposals remain consistent with the observed behavior. To achieve this, we have made swimming devices comprising fluorescent polymer beads coated with the metal catalyst, an approach inspired by a previous description of “Brownian Moon” particles,18 to extract meaningful spatial and directional information about individual single-conductive component spherical swimmers (Figure 1). In our application, the metal coating acts as a mask for the fluorescent emission, allowing the asymmetrical coating to be observed and providing a catalyst for the reduction of the hydrogen peroxide fuel to produce propulsion. When viewed under a microscope, the fluorescent image of such nanoswimmers reveals the geometry and asymmetry of the sphere, enabling the orientation of the uncoated and platinumcoated hemispheres to be determined as the devices undergo propulsive motion.
’ EXPERIMENTAL SECTION Fluorescent (1.99 μm, λEX = 475 nm, λEM > 515 nm) microspheres were purchased from Brookhaven Instruments (U.K.) and coated with a
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Figure 2. Trajectory (45 s section) for a fluorescent swimmer (white) overlaid with captured images of the particle at that particular location. (Inset) Definitions of particle orientation (θ1) and step angle (θ2) that were measured as shown relative to the horizontal dashed line.
Figure 3. Comparison of particle orientation and step direction. hemisphere of 5-nm-thick platinum using vacuum evaporation and resuspended in a 5% peroxide fuel solution as previously described.10 Videos of the fluorescent nanoswimmers were recorded using an Andor Luca-S EMCCD camera at 10 fps and a spatial resolution of 3.8 pixels per micrometer connected to a Nikon Microscope equipped with a 20 magnification objective. Labview software was used to analyze the resulting images to reveal the x, y position of the swimmer in each frame, and shape recognition was used to quantify the orientation of the swimmer. To simplify focusing, swimmers were allowed to settle into an electrostatic suspension at the bottom of a 1 mm path length cuvette.
’ RESULTS Figure 2 displays an extract from a typical trajectory of a fluorescent nanoswimmer, together with a selection of overlaid captured images. The mean-squared displacement analysis of such trajectories has been shown to allow the quantification of the propulsion velocity, excluding contributions due to Brownian displacment.10 For the trajectory in Figure 2, the mean propulsion velocity determined in this way is 3.4 μm s 1, consistent with the velocities previously reported for such swimmers.10 In the corresponding overlaid images, the asymmetry of the swimmer is clearly revealed, with the bright region corresponding to the fluorescence signal escaping the bead and the dark region indicating the platinum-masked section. The overlaid images display the correlation between the propulsion direction (θ1, see Figure 2 inset) and the orientation of the swimmer (θ2), with 12294
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Langmuir changes in the trajectory corresponding to the rotation of the position of the catalytic patch. The swimming direction is clearly revealed to be away from the catalytic patch. This correlation is also clearly seen in captured videos (Supporting Information). To subject these findings to a more rigorous analysis, it is necessary to compare the swimmers' orientation quantitatively with the resulting displacement step direction. It should be noted that the correlation between the swimmer orientation and the resulting translation is not expected to be perfect because of the contribution of Brownian displacements to the particles' positions after each observed “step”. Also, the Brownian rotation of the actual swimmer is a continuous process, so a small degree of rotation of the propulsive force will occur between the captured frames. The swimmer orientation (θ1) was determined using shaperecognition software and plotted against the subsequent translational step angle (θ2) as determined from the particle’s position in consecutive images (Figure 3). This graph clearly displays that the particles' orientation determines the subsequent step direction throughout the trajectory, with additional stochastic deviations reflecting the ubiquitous Brownian effects that dominate at these length scales. This observation was verified for numerous additional swimmers. For particles undergoing pure Brownian motion, rotational and translational diffusion occur independently: coupling is established only by adding a propulsive force. This is illustrated in videos of control unfuelled swimmers observed in water (Supporting Information). A further observation apparent from Figure 2 and the supplementary videos is that the swimming particles’ rotation perpendicular to the swimming plane is constrained during propulsion so that particles present only hemispherical “phases”. In contrast, control unfuelled “moon” particles undergo unconstrained rotation exhibiting phases from completely obscured through a variety of crescent shapes to a full fluorescing sphere. This phenomenon is currently the subject of further investigation and is thought to originate from an interaction between the speeding swimmer and the nearby planar boundary.
’ DISCUSSION The manufacture of Janus swimmers from fluorescent polymer beads combined with quantitative image analysis has allowed their direction of motion to be unambiguously determined for the first time. This has extended the previous application of a shadowing method that inferred the rotational rates of colloids as a function of viscosity by looking at the flickering rate of the fluorescence intensity.18 Here improved optics, high frame rates, and a sensitive camera have allowed the determination of the orientation of a rapidly moving colloid by directly observing the crescent orientation. The direction of motion away from the catalyst is consistent with both the bubble release mechanism and diffusiophoresis. Although the bubble release mechanisms can only cause motion away from the catalyst, the details of the diffusiophoretic model actually allow for propulsion in either direction relative to the catalytically active component, depending on the interaction of the device surfaces with the surrounding solvent.14 The analytical theory describing diffusiophoresis for individual spherical Janus swimmers suggests that the experimentally observed “positive” swimming direction is due to the sum of the surface mobilities of both hemispheres being positive. However, the surface mobility is not an easily measured physical parameter, encompassing contributions due to surface chemistry,
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roughness, and solvent properties, so this correlation may be difficult to confirm experimentally. In a related study, equivalently sized two-particle dumbbell swimmers made from a platinum particle attached to an inactive silica particle were recently observed to move toward the platinum cap in the opposite direction to that reported here.17 It consequently appears that the diffusiophoretic models' prediction of motion in either direction relative to the catalyst, depending on surface solvent interactions, can potentially provide a unified explanation for both of these systems. Unfortunately, the diffusiophoresis simulation applied to the two-particle swimmers used a different parameter set to describe the fluid device interactions, hampering a direct comparison between the two experimental findings. It is clear that the design of future experiments to look at the correlation of the propulsion direction and magnitude with device surface chemistry and roughness, using the methodology reported here, would be greatly assisted by a consistent theoretical approach for both geometries. The reported absence of a diffusiophoretic or bubble-release propulsion mechanism contributing to nanorod swimmers made from two conductive materials also remains puzzling.9 This conclusion was based on the report that no propulsion was observed if the two conducting materials in these devices are separated by an insulator. However, this reliance on a conductive path is clearly not observed for the various existing singleconductive-component swimmers discussed here and elsewhere. In fact, diffusiophoresis is theoretically expected to contribute a propulsive force to rod-shaped swimmers, albeit reduced in magnitude by a factor related to the aspect ratio. The diffusiophoresis model also has the potential to explain the materialdependent change in the direction of motion reported for rod swimmers, currently used to support an electrochemical mechanism. It consequently appears that the potential for diffusiophoresis models to explain swimming device behavior for a variety of geometries should be further investigated as part of the important task of unifying the mechanistic understanding, which will underlie the successful engineering of swimming devices capable of realizing nanotransport applications.
’ CONCLUSION We have demonstrated a method to ascertain the direction of propulsion for micrometer-scale spherical autonomous swimmers using fluorescent “moon” particles. The application of this method has shown that such Janus particle swimmers move away from their catalytic patch, in the opposite direction to the closely related cojoined two sphere swimmers that have been reported elsewhere. A diffusiophoretic model for propulsion appears to offer the potential to explain both of these observations. However, more work is required to ascertain in more detail the physical factors that determine the parameters in the diffusiophoretic model and so aid the extension of this propulsion model to other swimmer geometries such as rods. ’ ASSOCIATED CONTENT
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Supporting Information. Videos 1 and 2 containing realtime fluorescence microscopy for representative swimming devices observed in hydrogen peroxide fuel and water, respectively. This material is available free of charge via the Internet at http:// pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail: s.ebbens@sheffield.ac.uk.
’ ACKNOWLEDGMENT We acknowledge funding by the EPSRC (grant no. EP/ G04077X/1). ’ REFERENCES (1) Burdick, J.; Laocharoensuk, R.; Wheat, P. M.; Posner, J. D.; Wang, J. J. Am. Chem. Soc. 2008, 130, 8164–8165. (2) Sundararajan, S.; Lammert, P. E.; Zudans, A. W.; Crespi, V. H.; Sen, A. Nano Lett. 2008, 8, 1271–1276. (3) Ghalanbor, Z.; Mareshi, S. A.; Ranjbar, B. Med. Hypotheses 2005, 65, 198–199. (4) Ebbens, S. J.; Howse, J. R. Soft Matter 2010, 6, 726–738. (5) Demirok, U. K.; Laocharoensuk, R.; Manesh, K. M.; Wang, J. Angew. Chem., Int. Ed. 2008, 47, 9349–9351. (6) Ebbens, S.; Jones, R.; Ryan, A.; Golestanian, R.; Howse, J. Phys. Rev. E 2010, 82, 6–9. (7) Golestanian, R.; Liverpool, T.; Ajdari, A. Phys. Rev. Lett. 2005, 94, 1–4. (8) Gibbs, J. G.; Zhao, Y.-P. Appl. Phys. Lett. 2009, 94, 163104. (9) Wang, Y.; Hernandez, R. M.; Bartlett, D. J.; Bingham, J. M.; Kline, T. R.; Sen, A.; Mallouk, T. E. Langmuir 2006, 22, 10451–10456. (10) Howse, J.; Jones, R.; Ryan, A.; Gough, T.; Vafabakhsh, R.; Golestanian, R. Phys. Rev. Lett. 2007, 99, 8–11. (11) Pantarotto, D.; Browne, W. R.; Feringa, B. L. Chem. Commun. 2008, 1533–1535. (12) Vicario, J.; Eelkema, R.; Browne, W. R.; Meetsma, A.; La Crois, R. M.; Feringa, B. L. Chem. Commun. (Cambridge, U.K.) 2005, 3936 3938. (13) Ismagilov, R. F.; Schwartz, A.; Bowden, N.; Whitesides, G. M. Angew. Chem., Int. Ed. 2002, 41, 652–654. (14) Golestanian, R.; Liverpool, T. B.; Ajdari, A. New J. Phys. 2007, 9, 126–126. (15) Golestanian, R. Phys. Rev. Lett. 2009, 102, 1–4. (16) R€uckner, G.; Kapral, R. Phys. Rev. Lett. 2007, 98, 13–16. (17) Valadares, L. F.; Tao, Y.-G.; Zacharia, N. S.; Kitaev, V.; Galembeck, F.; Kapral, R.; Ozin, G. A. Small 2010, 6, 565–572. (18) Behrend, C. J.; Anker, J. N.; Kopelman, R. Appl. Phys. Lett. 2004, 84, 154–156.
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