pubs.acs.org/Langmuir © 2010 American Chemical Society
Rapid Fabrication of Bimetallic Spherical Motors Philip M. Wheat, Nathan A. Marine, Jeffrey L. Moran, and Jonathan D. Posner* Mechanical Engineering, Chemical Engineering, Arizona State University, Tempe, Arizona 85287 Received June 1, 2010. Revised Manuscript Received July 14, 2010 Catalytic bimetallic nanomotors can swim at 100 body lengths per second as well as pick up, haul, and release micrometer-scale cargo. The electrokinetic locomotion of bimetallic nanomotors is driven by the electrocatalytic decomposition of hydrogen peroxide. The motors are typically fabricated by electrodeposition-based template synthesis techniques that result in heterogeneous samples and require specialized knowledge of electrochemistry, a three-electrode potentiostat setup, cyanide-based chemistry, and porous membranes. This paper presents a rapid and facile method for fabrication of spherical bimetallic motors that only requires access to metal deposition equipment and commercially available microspheres. The resulting spherical motors swim at speeds comparable to rod-shaped motors with the same dimensions and composition. The spherical motors’ velocity increases with fuel concentration and decreasing diameter.
Introduction Synthetic nanomotors are being developed to mimic nanoscale biomotors present in biological systems. Efforts in this area range from synthetic modifications on existing biomotors1-5 to purely synthetic catalytic bimetallic nanomotors.6-8 Bimetallic nanomotors have been engineered to swim at 100 body lengths per second as well as pick up, haul, and release micrometer-scale cargo.9,10 Their motion can be controlled using external magnetic fields9,11 as well as chemical12-14 and thermal15 fields. Bimetallic *To whom correspondence should be addressed. E-mail: Jonathan.Posner@ asu.edu. (1) van den Heuvel, M. G. L.; Dekker, C. Motor proteins at work for nanotechnology. Science 2007, 317 (5836), 333-336. (2) Goel, A.; Vogel, V. Harnessing biological motors to engineer systems for nanoscale transport and assembly. Nat. Nanotechnol. 2008, 3 (8), 465-475. (3) Brunner, C.; Wahnes, C.; Vogel, V. Cargo pick-up from engineered loading stations by kinesin driven molecular shuttles. Lab Chip 2007, 7 (10), 1263-1271. (4) Doot, R. K.; Hess, H.; Vogel, V. Engineered networks of oriented microtubule filaments for directed cargo transport. Soft Matter 2007, 3 (3), 349-356. (5) Hess, H.; Bachand, G. D.; Vogel, V. Powering nanodevices with biomolecular motors. Chem—Eur. J. 2004, 10 (9), 2110-2116. (6) Paxton, W. F.; Kistler, K. C.; Olmeda, C. C.; Sen, A.; St Angelo, S. K.; Cao, Y. Y.; Mallouk, T. E.; Lammert, P. E.; Crespi, V. H. Catalytic nanomotors: Autonomous movement of striped nanorods. J. Am. Chem. Soc. 2004, 126 (41), 13424-13431. (7) Paxton, W. F.; Sen, A.; Mallouk, T. E. Motility of catalytic nanoparticles through self-generated forces. Chem—Eur. J. 2005, 11 (22), 6462-6470. (8) Fournier-Bidoz, S.; Arsenault, A. C.; Manners, I.; Ozin, G. A. Synthetic selfpropelled nanorotors. Chem. Commun. 2005, 4, 441-443. (9) Burdick, J.; Laocharoensuk, R.; Wheat, P. M.; Posner, J. D.; Wang, J. Synthetic nanomotors in microchannel networks: Directional microchip motion and controlled manipulation of cargo. J. Am. Chem. Soc. 2008, 130 (26), 8164. (10) Sundararajan, S.; Lammert, P. E.; Zudans, A. W.; Crespi, V. H.; Sen, A. Catalytic motors for transport of colloidal cargo. Nano Lett. 2008, 8 (5), 1271-1276. (11) Kline, T. R.; Iwata, J.; Lammert, P. E.; Mallouk, T. E.; Sen, A.; Velegol, D. Catalytically driven colloidal patterning and transport. J. Phys. Chem. B 2006, 110 (48), 24513-24521. (12) Calvo-Marzal, P.; Manesh, K. M.; Kagan, D.; Balasubramanian, S.; Cardona, M.; Flechsig, G. U.; Posner, J.; Wang, J. Electrochemically-triggered motion of catalytic nanomotors. Chem. Commun. 2009, 30, 4509-4511. (13) Ibele, M.; Mallouk, T. E.; Sen, A. Schooling Behavior of Light-Powered Autonomous Micromotors in Water. Angew. Chem., Int. Ed. 2009, 48 (18), 3308-3312. (14) Hong, Y.; Blackman, N. M. K.; Kopp, N. D.; Sen, A.; Velegol, D. Chemotaxis of nonbiological colloidal rods. Phys. Rev. Lett. 2007, 99 (17), 178103. (15) Balasubramanian, S.; Kagan, D.; Manesh, K. M.; Calvo-Marzal, P.; Flechsig, G. U.; Wang, J. Thermal Modulation of Nanomotor Movement. Small 2009, 5 (13), 1569-1574. (16) Moran, J. L.; Wheat, P. M.; Posner, J. D. Locomotion of Electrocatalytic Nanomotors due to Reaction Induced Charge Auto-Electrophoresis. Phys. Rev. E 2010, 81, 065302(R).
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nanomotors propel themselves by electrocatalytically decomposing hydrogen peroxide7,16,17 through a mechanism we recently described as reaction induced charge autoelectrophoresis (RICA).16 Bimetallic nanomotors in an aqueous hydrogen peroxide solution catalyze peroxide oxidation at one of the metal surfaces (anode), generating protons, electrons, and oxygen molecules. The electrons conduct through the motor to the other metal surface (cathode) and complete the reduction reaction by combining with protons, peroxide, and oxygen to generate water. The asymmetric reactions result in an excess and depletion of protons in the surrounding electrolyte at the anode and cathode ends, respectively. The proton imbalance results in asymmetric free charge density, which generates an electric dipole and field pointing from the anode to the cathode. In addition, the particle’s negative surface charge attracts cations from the bulk solution which form a positively charged diffuse screening layer surrounding the particle. The self-generated electric field couples with the charge density induced by both the reactions and the diffuse layer to produce an electrical body force that drives fluid from the anode to the cathode. The fluid motion results in locomotion of the motor in the direction of the anode. Net motion of the nanomotor requires some native charge, or zeta potential; if the particle were instead positively charged with identical reactions taking place on its surface, it would move in the opposite direction. The nanomotor velocity is linearly dependent on the reaction flux density and the native surface charge.16 The common method for producing bimetallic nanomotors is electrodeposition-based template synthesis.18,19 This method produces heterogeneous samples and requires specialized knowledge of electrochemistry, a three-electrode setup and potentiostat, and cyanide-based chemistry. In addition, the most common templates, porous alumina anodiscs (Whatman, cat. no. 6809-6022; Maidstone, U.K.), have not been commercially available for some (17) Wang, Y.; Hernandez, R. M.; Bartlett, D. J.; Bingham, J. M.; Kline, T. R.; Sen, A.; Mallouk, T. E. Bipolar electrochemical mechanism for the propulsion of catalytic nanomotors in hydrogen peroxide solutions. Langmuir 2006, 22 (25), 10451-10456. (18) Martin, C. R. Nanomaterials - a Membrane-Based Synthetic Approach. Science 1994, 266 (5193), 1961-1966. (19) Nicewarner-Pena, S. R.; Freeman, R. G.; Reiss, B. D.; He, L.; Pena, D. J.; Walton, I. D.; Cromer, R.; Keating, C. D.; Natan, M. J. Submicrometer metallic barcodes. Science 2001, 294 (5540), 137-141. (20) Hillebrenner, H.; Buyukserin, F.; Stewart, J. D.; Martin, C. R. Template synthesized nanotubes for biomedical delivery applications. Nanomedicine 2006, 1 (1), 39-50.
Published on Web 07/21/2010
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Figure 1. Schematic of the fabrication method. Fluorescent polystyrene microspheres are deposited on a glass substrate and then (a) halfcoated with one metal. The spheres are then resuspended in water and (b) deposited again with random orientation. (c) The exposed uppersurfaces are coated again. This process is repeated until the sphere is completely coated with the first metal, and then the spheres are halfcoated with a second metal. The fluorescent micrographs show the progressive coverage of 5 μm fluorescent spheres after (e) 1, (f) 2, (g) 3, and (h) 4 cycles.
time, requiring researchers to fabricate their own porous templates20 or consider alternative fabrication techniques entirely. McDermott and Velegol fabricated bimetallic motors from colloidal doublets by controlled aggregation of spheres of two unique metals.21 This method avoids the use of template synthesis but results in large nanomotors with considerable settling velocity. Although the doublets tended to settle to the bottom of the chamber, they were shown to be capable of autonomous motion in hydrogen peroxide solutions at speeds up to 15 μm/s. A particle’s settling velocity depends on its volume and density. Large metallic particles and rods tend to settle and can become fixed to the bottom surface of the chamber and remain stationary regardless of H2O2 concentration. Here we present a facile and repeatable technique to fabricate homogeneous samples of spherical Janus bimetallic catalytic motors that are nearly neutrally buoyant and avoid the use of specialized equipment, templates, and cyanide chemistry. The process described here only requires access to a sputter coater or evaporator, the desired metals, and polystyrene microspheres. Similar patchy22,23 and half-coated24 Janus spheres have been fabricated using metal deposition. Unlike these previous methods, our process yields spheres completely coated in metal. In this work, the two metals deposited on the particle are in electrical contact with each other, completing the electrical circuit required for the electrochemically driven motion of the catalytic motors. We find that the spherical catalytic motors swim with speeds comparable to cylindrical motors of the same dimension and that the swimming speed is a function of the motor diameter and solution peroxide concentration.
Experimental Methodology The spherical bimetallic motors are fabricated by sputter coating the entire surface of spherical particles with one metal and then half-coating the metal coated sphere with a second metal. In this work, we use fluorescent polystyrene spheres (F = 1.05 g/cm3, (21) McDermott, J. J.; Velegol, D. Simple fabrication of metallic colloidal doublets having electrical connectivity. Langmuir 2008, 24 (8), 4335-4339. (22) Ye, S.; Carroll, R. L. Design and Fabrication of Bimetallic Colloidal “Janus” Particles. ACS Appl. Mater. Interfaces 2010, 2 (3), 616-620. (23) Pawar, A. B.; Kretzschmar, I. Patchy Particles by Glancing Angle Deposition. Langmuir 2008, 24 (2), 355-358. (24) Gangwal, S.; Cayre, O. J.; Bazant, M. Z.; Velev, O. D. Induced-Charge Electrophoresis of Metallodielectric Particles. Phys. Rev. Lett. 2008, 100 (5), 058302.
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Duke Scientific Inc., Fremont, CA). The particle fluorescence aids in the interpretation of the coating process. We choose polystyrene spheres because they yield motors with small density differences from the bulk solution and minimize settling velocities. We use gold and platinum metals so that our results can be compared to the Au-Pt cylindrical nanomotors results,7,25 but the same process can be used with nearly any metal that can be deposited on spheres. First, a 1% volume fraction aqueous dispersion of spheres is deposited onto a 2.5 2.5 cm2 square glass substrate. The solvent evaporates at room temperature, forming a monolayer of spheres. The upper hemispheres are coated with 20 nm of gold using a sputter coater (Cressington 108 auto, Cressington Scientific Instruments, Watford WD19 4BX, England, U.K.). The sputter coater only coats the upper hemisphere due to the directionality of the sputter coater.26-28 The half-coated spheres are resuspended in an aqueous solution and then deposited into a monolayer on a clean glass slide. The second deposition results in a monolayer of spheres that have random orientations relative to their coated hemispheres. Each time the spheres are redeposited, they are randomly oriented, allowing for previously unexposed portions of the spheres to be coated. This process is illustrated in Figure 1a-c. We repeat this process seven or eight times to ensure the entire surface is coated with the first metal. We inspect the coating of the spheres after each deposition with epifluorescence microscopy using an inverted microscope (Nikon TE2000, Japan), a 40 objective with a 1.5 multiplier, NA = 0.6, and metal halide illumination (EXFO X-Cite 120, Ontario, Canada). The fluorescence micrographs are shown in Figure 1e-h. We consider the spheres fully coated once we no longer observe any fluorescence from the spheres. The fully Au coated spheres are redeposited on a clean substrate and finally coated with 20 nm of platinum, resulting in a Janus sphere that is half coated with gold and half platinum as shown in Figure 1d. The completed Janus spheres are then resuspended and are ready for use as spherical nanomotors. This method can be used with any material that can be sputtered or evaporated onto spheres. (25) Laocharoensuk, R.; Burdick, J.; Wang, J. Carbon-nanotube-induced acceleration of catalytic nanomotors. ACS Nano 2008, 2 (5), 1069-1075. (26) Takei, H.; Shimizu, N. Gradient sensitive microscopic probes prepared by gold evaporation and chemisorption on latex spheres. Langmuir 1997, 13 (7), 1865-1868. (27) Tien, C. L.; Cunnington, G. R. Recent Advances in High-Performance Cryogenic Thermal Insulation. Cryogenics 1972, 12 (6), 419. (28) Paunov, V. N.; Cayre, O. J. Supraparticles and “Janus” particles fabricated by replication of particle monolayers at liquid surfaces using a gel trapping technique. Adv. Mater. 2004, 16 (9-10), 788.
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Figure 2. (a) Scanning electron microscope image of 5 μm diameter polystyrene sphere coated with gold and then half coated with nickel. (b) Energy-dispersive X-ray spectroscopy compositional analysis of the same microsphere.
We use scanning electron microscopy (SEM) (NOVA 200 Nanolab, FEI Hillsboro, OR) and energy-dispersive X-ray spectroscopy (EDS) (NSS212E, Thermo Fisher Scientific Inc., Waltham, MA) to, respectively, image the shape and compositions of 5 μm Au/Ni spheres. The SEM and EDS images are shown in Figure 2. Here, we use nickel in place of the platinum because it provides more highly resolved compositional analysis using EDS. In this case, the nickel is first evaporated onto the deposited spheres (Cressington 308R Evaporator, Cressington Scientific Instruments, England, U.K.). The SEM image in Figure 2a shows a smooth, upper nickel surface and a lower, rough gold surface. Analysis of several spheres shows that they are nearly spherical (with some small defects) and have a hemispherical coating of the second metal. Transmission optical microscopy is used to observe the swimming nanomotors. We use an inverted microscope (Nikon TE2000, Japan) with a 40 objective (NA = 0.6) and 100 W halogen illumination (Nikon TE2 PS 100W, Japan). The images are captured using a CCD camera (Coolsnap HQ, Photometrics, Tucson, AZ). Experiments are performed in chambered glass wells with an area of 0.8 cm2 (cat. no. 12-565-18, Thermo Fisher Scientific Inc., Waltham, MA). During the experiments, the chambers are sealed to prevent evaporation-induced convection. Each particle is tracked for 100 frames, and the time averaged velocity is calculated from the sphere path. The sphere centers are calculated in MATLAB from the intensity weighted centers of the spheres in each frame. Particle centers at each time are paired using an optical flow algorithm.
Results and Discussion Figure 3 depicts the paths of a representative 3 μm diameter nanomotor at hydrogen peroxide concentrations of 1%, 2%, 3%, and 4%. Each path represents an elapsed time of 11.7 s. These nanomotors clearly exhibit an advective velocity in excess of the migration that would be observed for a Brownian sphere. As we increase the peroxide concentration (from (a) to (d)) the path length increases (over a fixed time) suggesting an increase in swimming velocity. In Figure 4, we show measurements of the motor swimming velocity as a function of H2O2 concentration for spheres of 2, 3, and 5 μm diameters. The velocities achieved by the spherical nanomotors are comparable to previously published results for rod-shaped Au/Pt nanomotors in H2O2. Typical maximum speeds for rod-shaped Au/Pt nanomotors 2 μm long and 220 nm in diameter have been reported to be between 7 and 8 μm/s.6,25 Figure 4 shows velocities of about 15 μm/s attained by the 2 μm spheres. Figure 3 shows that the spherical motors exhibit directed motion that is relatively straight. The rotational diffusivities of spheres are less than those of rods with equivalent lengths. The rotational diffusivity of a spheroid particle (approximating a rod) is given by 3kT lnð2a=bÞ - 1=2 Dθ, rod ¼ 3 8πηa 13054 DOI: 10.1021/la102218w
Figure 3. Representative traces for 3 μm Au-Pt bimetallic microspheres in aqueous solutions containing (a) 1%, (b) 2%, (c) 3%, and (d) 4% H2O2.
where a and b are the major and minor radii of the spheroid (halflength and radius of the rod), respectively.29,30 The diffusivity of a sphere of radius a is31 Dθ, sphere ¼
kT 8πηa3
Evaluating these for a rod with radius b = 150 nm and half-length a = 1 μm (typical dimensions for a bimetallic nanorod motor), we obtain Dθ,rod = 1.0271 rad2/s, while for a sphere with radius a = 1 μm (the smallest sphere radius in this work) we obtain Dθ,sphere = 0.1638 rad2/s, an order of magnitude smaller. The rotational diffusivity decreases further for larger spheres due to the inverse proportionality between diffusivity and radius cubed, so it is still smaller than that of the bimetallic rods. We found that the rotational diffusivity of a sphere is always less than that of a rod if the diameter of the sphere is equal to the length of the rod. Therefore, we should expect the spheres to perform more directed motion than the rods. As predicted using a simple scaling analysis16,32 and demonstrated experimentally,13,25 there is a linear relationship between velocity and H2O2 concentration. The slope of this relationship can be described by an effective mobility of the motors, given as the ratio of a motor’s velocity to the concentration of peroxide in which it is immersed. Defined in this way, the mobility is approximately constant for a given sphere diameter and exhibits a roughly linear decrease with increasing diameter. The ButlerVolmer equation, which governs the reaction flux at the catalyst surface, predicts that the flux for both the rods and spheres should be equivalent when used in the same solution. From our simple scaling analysis, we expect an increase in the characteristic velocity with length at fixed hydrogen peroxide concentrations.16 This can be inferred from quadratic dependence of surface area (29) Rose, K. A.; Meier, J. A.; Dougherty, G. M.; Santiago, J. G. Rotational electrophoresis of striped metallic microrods. Phys. Rev. E 2007, 75 (1), 011503. (30) Doi, M.; Edwards, S. F.The Theory of Polymer Dynamics; Oxford University Press: New York, 1988. (31) Berg, H. C.Random Walks in Biology; Princeton University Press: Princeton, NJ, 1993. (32) Prevo, B. G.; Velev, O. D. Controlled, rapid deposition of structured coatings from micro- and nanoparticle suspensions. Langmuir 2004, 20 (6), 2099-2107.
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Figure 4. Average bimetallic spherical micromotor speeds versus H2O2 concentration for (0) 2 μm, (4) 3 μm, and (O) 5 μm diameter spheres (the error bars correspond to a single standard deviation).
and linear dependence of the Stokes drag on particle size. We see the converse experimental result suggesting that the viscous interactions with the solid substrate they swim along may be important. Modeling the nanomotors as a function of size and near an interface will be the focus of future work.
Summary We have introduced a facile method for fabricating bimetallic nanomotors. These motors can be produced with a wide range of (33) Jiang, P.; McFarland, M. J. Large-scale fabrication of wafer-size colloidal crystals, macroporous polymers and nanocomposites by spin-coating. J. Am. Chem. Soc. 2004, 126 (42), 13778-13786.
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core materials, including those with relatively low density. The buoyancy of the particles could be made more neutral or positive by adding solute to the hydrogen peroxide solutions; however, some solute molecules could interfere with the surface reactions or alter the properties of the electric double layer, which plays an important role in the particle locomotion. The spherical nanomotors have velocities comparable to cylindrical ones with equivalent half-lengths. Low density motors have lower settling velocities, enabling studies over a larger range of motors sizes. We observe a linear increase in motor velocity with concentration, consistent with previous work. The nanomotor velocity decreases with the sphere radius. This fabrication technique may be improved by using a more uniform particle monolayer. This would reduce the number of coating steps and prevent poor half coatings of the second metal by reduction in the number of occluded spheres by neighboring spheres. For example, a flow coating deposition process could be used to deposit the monolayers before each coating step.32 Spin coating has also been used extensively to produce highly regular colloidal monolayers;33 however, a considerable fraction of the spheres may be lost on each pass of this process. Acknowledgment. The authors gratefully acknowledge the use of facilities within the Center for Solid State Science (CSSS) at Arizona State University. P.M.W. is particularly grateful for the assistance of Grant Baumgardner and Karl Weiss for training and assistance in coating samples and SEM imaging. This work was sponsored by a NSF grant (CBET-0853379). P.M.W. and J.L.M. were supported by NSF Graduate Research Fellowships (NSF GRF).
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