pubs.acs.org/Langmuir © 2010 American Chemical Society
Magnetic Enhancement of Phototaxing Catalytic Motors Neetu Chaturvedi,† Yiying Hong,‡ Ayusman Sen,‡ and Darrell Velegol*,† †
Department of Chemical Engineering and ‡Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 Received October 30, 2009. Revised Manuscript Received December 18, 2009 W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/Langmuir. n
We use the “stimulus-quench-fuse” (SQF) technique to fabricate micrometer-size colloidal heterodoublets. The doublets consist of silver and magnetic Dynabead microspheres, and the stimulus is a temporal lowering of the pH. The resulting asymmetric colloidal doublets behave as catalytic motors and show self-propulsion and phototaxis under ultraviolet (UV) light in the presence of hydrogen peroxide (H2O2), by the mechanism of diffusiophoresis. The magnetic heterodoublets show autonomous movement, in random directions, in the presence of H2O2 and UV light, but if an external magnetic field is also present, they align themselves and show a directed motion forming exclusion regions around them. The assembly process described in this Article can be adapted to a wide variety of materials providing a simple, quick, inexpensive, reliable, and scalable approach for the development of synthetic motors capable of performing directed motion and forming exclusion zones and patterns.
Introduction The fabrication of small scale synthetic motors is an important step in developing nanomachines and devices. Nanoscale and microscale devices have current or potential applications as roving sensors, display elements, drug delivery vectors, and other technologies.1-6 Currently a bottleneck exists in producing large quantities of catalytic nanomotors. In this Article, we present a simple, inexpensive, quick, and scalable method of fabricating colloidal heterodoublets which behave as catalytic motors. The heterodoublets are fabricated using the stimulus-quench-fuse (SQF) technique with pH as the stimulus, and the resulting motors show self-propulsion and collective phototaxis under ultraviolet (UV) light in the presence of hydrogen peroxide (H2O2). In addition, when an external magnetic field is applied, the motors show a directed motion that gives exclusion regions. Autonomous motors must generate or help to generate their own field that causes the movement. In contrast, various types of external fields have been used for colloid transport in fluids, including electrophoresis,7,8 diffusiophoresis,7-10 magnetophoresis,11 and thermophoresis.7,12,13 However, in 2002 Whitesides and co-workers showed the use of a catalytic reaction, namely, H2O2 decomposition, to power the motion of millimeter scale *To whom correspondence should be addressed: Mailing address: 108 Fenske Laboratory, The Pennsylvania State University, University Park, PA 16802. Telephone: (814) 865-8739. Fax: (814) 865-7846. E-mail: velegol@psu. edu. (1) Hamley, I. W. Angew. Chem., Int. Ed. 2003, 42, 1692–1712. (2) Lee, B. S.; Lee, S. C.; Holliday, L. S. Biomed. Microdevices 2003, 5, 269–280. (3) Goel, A.; Vogel, V. Nat. Nanotechnol. 2008, 3, 465–475. (4) Alivisatos, P. Nat. Biotechnol. 2004, 22, 47–52. (5) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4128–4158. (6) Lowe, C. R. Curr. Opin. Struct. Biol. 2000, 10, 428–434. (7) Anderson, J. L. Annu. Rev. Fluid Mech. 1989, 21, 61–99. (8) Wei, Y. K.; Keh, H. J. Langmuir 2001, 17, 1437–1447. (9) Lin, M. M. J.; Prieve, D. C. J. Colloid Interface Sci. 1983, 95, 327–339. (10) Ebel, J. P.; Anderson, J. L.; Prieve, D. C. Langmuir 1988, 4, 396–406. (11) Watarai, H.; Suwa, M.; Iiguni, Y. Anal. Bioanal. Chem. 2004, 378, 1693– 1699. (12) Zhang, K. J.; Briggs, M. E.; Gammon, R. W.; Sengers, J. V.; Douglas, J. F. J. Chem. Phys. 1999, 111, 2270–2282. (13) Piazza, R. J. Phys.: Condens. Matter 2004, 16, S4195–S4211. (14) Ismagilov, R. F.; Schwartz, A.; Bowden, N.; Whitesides, G. M. Angew. Chem., Int. Ed. 2002, 114, 674–676.
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objects at an air/water interface.14 Later, Sen and co-workers reported the autonomous movement of bimetallic (platinum/ gold) nanorods that catalyze the electrochemical decomposition of H2O2 in aqueous solutions.15-19 Other novel catalytic systems demonstrated autonomous motility at the micro- and nanoscale through several mechanisms, such as bubble propulsion,20,21 diffusiophoresis,22 or self-electrophoresis.15,23-29 For instance, we have reported the movement of platinum-gold nanorods toward higher H2O2 concentrations through “active diffusion”23 and negative phototaxis by silica-silver Janus particles under UV irradiation in the presence of H2O2 by the mechanism of selfdiffusiophoresis.30 Synthesis and assembly of magnetic colloidal suspensions have been reported in the literature. These particles have been used as building blocks for the formation of ordered patterns on surfaces via manipulation using magnetic fields.31 Paramagnetic chains (15) Paxton, W. F.; Baker, P. T.; Kline, T. R.; Wang, Y.; Mallouk, T. E.; Sen, A. J. Am. Chem. Soc. 2006, 128, 14881–14888. (16) Paxton, W. F.; Kistler, K. C.; Olmeda, C. C.; Sen, A.; St. Angelo, S. K.; Cao, Y.; Mallouk, T. E.; Lammert, P. E.; Crespi, V. H. J. Am. Chem. Soc. 2004, 126, 13424–13431. (17) Paxton, W. F.; Sen, A.; Mallouk, T. E. Chem.;Eur. J. 2005, 11, 6462–6470. (18) Kline, T. R.; Paxton, W. F.; Mallouk, T. E.; Sen, A. Angew. Chem., Int. Ed. 2005, 44, 744–746. (19) Catchmark, J. M.; Subramanian, S.; Sen, A. Small 2005, 1, 202–206. (20) Vicario, J.; Eelkema, R.; Browne, W. R.; Meetsma, A.; La Crois, R. M.; Feringa, B. L. Chem. Commun. 2005, 3936–3938. (21) Pantarotto, D.; Browne, W. R.; Feringa, B. L. Chem. Commun. 2008, 1533– 1535. (22) Kline, T. R.; Sen, A. Langmuir 2006, 22, 7124–7127. (23) Hong, Y.; Blackman, N. M. K.; Kopp, N. D.; Sen, A.; Velegol, D. Phys. Rev. Lett. 2007, 99, 178103(1)–178103(4). (24) Paxton, W. F.; Sundararajan, S.; Mallouk, T. E.; Sen, A. Angew. Chem., Int. Ed. 2006, 45, 5420–5429. (25) Sundararajan, S.; Lammert, P. E.; Zudans, A. W.; Crespi, V. H.; Sen, A. Nano Lett. 2008, 8, 1271–1276. (26) Ibele, M. E.; Wang, Y.; Kline, T. R.; Mallouk, T. E.; Sen, A. J. Am. Chem. Soc. 2007, 129, 7762–7763. (27) Ozin, G. A.; Manners, I.; Fournier-Bidoz, S.; Arsenault, A. Adv. Mater. 2005, 17, 3011–3018. (28) McDermott, J. J.; Velegol, D. Langmuir 2008, 24, 4335–4339. (29) Kline, T. R.; Paxton, W. F.; Wang, Y.; Velegol, D.; Mallouk, T. E.; Sen, A. J. Am. Chem. Soc. 2005, 127, 17150–17151. (30) Sen, A.; Ibele, M.; Hong, Y.; Velegol, D. Faraday Discuss. 2009, 143, 15–27. (31) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. Chem. Mater. 2001, 13, 109–116.
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and quasi-one-dimemsional superstructures from suspensions of paramagnetic colloidal particles have been fabricated under an external magnetic field.32-34 Gold, nickel, and two-segment nickel-gold nanowires have been synthesized by electrodeposition into alumina templates.35 More importantly to our work, magnetic fields have been used to give directionality to the movement of catalytic nanoparticles. By incorporating ferromagnetic nickel segments into the platinum-gold nanowire structure, Kline et al. demonstrated that the motion of the magnetic nanorods could be controlled remotely by using a magnetic field.18 Shi et al. have developed a facile method to drive a glass fiber on a water surface induced by an external magnetic field.36 In the present work, we demonstrate a simple but powerful method for assembling silver, magnetic, and other colloidal particles into heterodoublets. Previously, in our lab, we have used the SQF technique28,37,38 by raising the ionic strength of the solution (stimulus), allowing diffusion-limited aggregation for a set time, and then quenching the aggregation by adding deionized water to lower the ionic strength. However, in the present assembly, we faced the key challenge that the magnetic particles were very stable against forming heterodoublets when using salt as the stimulus, even up to 1 M KCl. The resolution to the difficulty was to use low pH as the stimulus and to neutralize the solution in order to quench the aggregation. Using this technique, we are able to produce heterodoublet catalytic motors quickly and reliably. In addition, using this simple technique opens up many new possibilities for adding components to the motors to increase their functionality.
Materials and Methods Materials. Monodisperse 1 μm magnetic Dynabeads (10% w/v, carboxylic acid functionalized polystyrene latex microspheres) were purchased from Invitrogen Corporation (Carlsbad, CA). Silver particles of size 0.5-1.5 μm were synthesized using the recipe developed by Goia and Matijevic39 and modified by Velikov et al.40 Silver nitrate, ascorbic acid, gum Arabic, and potassium chloride (KCl) were purchased from Sigma-Aldrich Chemicals, USA. Hydrochloric acid and sodium hydroxide were purchased from EMD Chemicals Inc. (Gibbstown, NJ). The deionized (DI) water that was used for all experiments (Millipore Corp. Milli-Q system) had a specific resistance greater than 18 MΩ 3 cm (i.e., “equilibrium water”). The glass microslides and other glassware were obtained from VWR International. Coverwell imaging chambers were purchased from Grace Bio Labs. Instrumentation. The behavior of the particles was monitored and recorded using a Zeiss Axiovert 200 reflectance/transmission microscope equipped with a digital video camera connected to a PC and a HBO100 UV lamp (center wavelength of 360 nm, maximum intensity of 2.5 W cm-2 at the center). The electron microscopy images were obtained on a Hitachi S-3000H scanning electron microscope (SEM) with an accelerating voltage of 5 keV at the Penn State Material Characterization Laboratory. The (32) Furst, E.; Suzuki, C.; Fermigier, M.; Gast, A. P. Langmuir 1998, 14, 7334– 7336. (33) Vuppu, A.; Farcia, A. A.; Hayes, M. A. Langmuir 2003, 19, 8646–8653. (34) Wang, H.; Chen, Q.; Sun, L.; Qi, H.; Yang, X.; Zhou, S.; Xiong, J. Langmuir 2009, 25, 7135–7139. (35) Bauer, L. A.; Reich, D. H.; Meyer, G. J. Langmuir 2003, 19, 7043–7048. (36) Shi, F.; Liu, S.; Gao, H.; Ding, N.; Dong, L.; Tremel, W.; Knoll, W. Adv. Mater. 2009, 21, 1927–1930. (37) Yake, A. M.; Panella, R. A.; Snyder, C. E.; Velegol, D. Langmuir 2006, 22, 9135–9141. (38) Snyder, C. E.; Yake, A. M.; Feick, J. D.; Velegol, D. Langmuir 2005, 21, 4813–4815. (39) Goia, D. V.; Matijevic, E. New J. Chem. 1998, 22, 1203–1215. (40) Velikov, K. P.; Zegers, G. E.; van Blaaderen, A. Langmuir 2003, 19, 1384– 1389.
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Sorvall Biofuge Primo centrifuge was from Kendro Laboratory Products, and it was equipped with a swing-bucket rotor. The magnetic field was applied using strong Neodymium bar magnets. Zeta Potentials. The particle ζ potentials were measured on a Brookhaven Instruments ZetaPALS (phase analysis light scattering) ζ potential analyzer. For the Dynabeads, ζ = -33 ( 0.5 mV; and for the silver particles, ζ = -43 ( 1.6 mV. These ζ potential measurements were taken in 10 mM KCl solution at pH 5.2.
Fabrication of Heterodoublets. a. Synthesis of Silver Particles. Silver (Ag) particles were synthesized using the recipe
developed by Goia and Matijevic39 and modified by Velikov et al.,40 involving the reduction of silver ions to metal silver by ascorbic acid. The diameter of the silver particles ranged from 0.5 to 1.5 μm. The silver particle suspension, with a volume fraction of 0.002, was later used for making doublets. When the silver is irradiated with UV light in the presence of H2O2, the following unbalanced reaction takes place. The thermal heat of reaction can be neglected in comparison with the UV absorption. This effect will be presented later in detail. Ag þ HOOH f Agþ þ HOO - þ H2 O
ð1Þ þ
The difference in the diffusion coefficients between Ag and OOH- establishes a diffusion-induced electric field that moves the silver particles by the mechanism of diffusiophoresis.22 b. Fabrication of Silver-Dynabead Doublets. Heterodoublets made from like-charged colloidal particles have been fabricated in our lab using the salting out-quenching-fusing technique by aggregating them in higher ionic strength.28,37,38 Silver and Dynabeads are both negatively charged colloids, and higher ionic strength was required to aggregate them into heterodoublets. However, the Dynabeads were too stable to be aggregated in high salt concentrations. This was one of the major challenges that we faced in the fabrication of silver-Dynabead doublets. The resolution was the stimulus-quench-fuse technique, in which the aggregation was stimulated by lowering the pH of the solution to 3, which reassociated the carboxyl charges on the Dynabead particles and thus lowered the negative charge. The aggregation was quenched by neutralizing the solution back to a pH of 6.5. An amount of 8 μL of 1 μm carboxylic acid functionalized Dynabeads was mixed with 80 μL of 0.5-1.5 μm silver particles in 2 mL of DI water. Hydrochloric acid was added to make the pH of the solution equal to 3. The tube was rolled for 10 min, after which the aggregation was quenched by adding an equivalent amount of sodium hydroxide base to make the solution pH approximately equal to 6.5, thereby forming mostly doublets with very few larger particle aggregates. The solution was then rinsed with DI water by repeated cycles of centrifugation. The particle solution was centrifuged at 1800 rpm for 4 min; the supernatant was drawn off and replaced with DI water. This procedure was repeated three times, finally obtaining a 2 mL suspension of silver-Dynabead doublets in DI water, as shown in the SEM images of Figure 1. The heterodoublets can also be fused at this time by heating them above the glass transition temperature of the polystyrene latex. However, we did not need to do so for our experiments; they held together in a stable manner. The yield of the doublets in the solution was roughly 20%, with mostly singlets and less than 10% of larger particle aggregates, as found before with the salting out-quenching-fusing technique.37 The solution with singlets, doublets, and larger particle aggregates was used as-is for the experiments. Diffusiophoretic Movement of Heterodoublets. A silverDynabead doublet suspension (with a volume fraction of roughly 0.001) was made using the stimulus-quench-fuse technique as described in the previous section. An amount of 270 μL of the asprepared doublet suspension was mixed with 3 μL of 30% H2O2 DOI: 10.1021/la904133a
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Figure 2. Schematic of a system undergoing diffusiophoresis. Cations diffuse faster than anions, demonstrated in the schematic by the fact that at any location the cations tend to be on the right-hand side more. This gives rise to an electric field generated in r direction, which moves a negatively charged particle, with a velocity U, in f direction. Figure 1. SEM images of the heterodoublets fabricated from silver particles and carboxylic acid functionalized Dynabead particles. and 87 μL of DI water to prepare 360 μL of particle suspension in 0.25% H2O2. Initial movements and the phototactic response of the doublets in H2O2 were recorded with video microscope equipment upon UV irradiation. In the general case, the velocity of a particle undergoing diffusiophoresis is the sum of a diffusiophoretic component (Udp) and an electroosmotic component (the electric field acting on the electric double layer of the underlying substrate, given by Udo). Electrical and osmotic pressure mechanisms together contribute to Udp. The electrical mechanism arises from an electric field caused by a concentration gradient of ions with different mobilities; the pressure mechanism arises from an interfacial pressure gradient as a result of attraction/repulsion of solutes from the surface. In an unbounded solution of a symmetrically charged binary electrolyte with a uniform concentration gradient rn, the diffusiophoretic velocity of a charged particle is 2 3 ! εkT 4 Dþ -D 2kT 2 5rn Udp ¼ ln 1 -γ ð2Þ ζ Zeη Dþ þ D - p Ze n0 where Dþ and D- are the diffusion coefficients of the cation and anion, respectively, Z is the absolute value of the valences of ions, e is the charge of a proton, k is the Boltzmann constant, T is the absolute temperature, ε is the dielectric permittivity of the solution, η is the viscosity of the solution, ζp is the zeta potential of the particle, γ = tanh(Zeζp/4kT), and n0 is the bulk concentration of ions at the particle location, as if the particle was not there.7 The electroosmotic component is given by a similar equation, with the particle zeta potential replaced by the wall zeta potential.7 Figure 2 shows a schematic of a system undergoing diffusiophoresis. As the ions diffuse from higher concentration to lower concentration, the positive ions (which have a higher diffusion coefficient) tend to diffuse faster than the negative ions. If the positive ions were to continue to diffuse faster, we would have a macroscopic charge separation, which in general does not happen. To maintain local electroneutrality in the fluid, however, an electric field is spontaneously produced by the ions. The electric field keeps the ions transporting at the same rate down the concentration gradient. This same electric field also drives the movement of the particle, giving the first term of the diffusiophoretic velocity given in eq 2. The concentration of the ion (n, #/m3) can be estimated by approximating a single heterodoublet as a system having a continuous source of ions, from which the diffusing ions are continuously generated at a constant rate q. Thus, at a point distant r from the source at time t, n ¼
q r erfc pffiffiffiffiffiffiffiffiffi 4πDr 2 ðDtÞ
ð3Þ
(41) Crank, J. The Mathematics of Diffusion; Oxford University Press: New York, 1975.
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where q is the rate of generation of the ion (#/s), and D is the diffusion coefficient.41 The diffusion coefficients for the cations (Dþ is 1.65 10-9 m2/s)42 and anions (D- is 0.3 10-9 m2/s)22 are known at 25 °C. For our system, ζp (overall average zeta potential of a heterodoublet) is -40 mV at 10 mM KCl. Based on a calculation from eq 3, rn/n0 is estimated as 2.3 104 m-1 with the assumption that a single heterodoublet acts as a stationary system from which the diffusing ions are continuously generated. We use a time of 1 s, which is roughly the time in which a heterodoublet travels one diameter distance. The diffusiophoretic speed of a heterodoublet is estimated as 1.1 μm/s, using eq 2. While calculating the speed, the gradients of ionic strength caused by other particles are neglected. Furthermore, when the initial ionic strength for the species starts near zero (e.g., for DI water), the production rate (q) cancels in taking rn/n0, since n0 = n in that case.
Magnetic-Diffusiophoretic Movement of Magnetic Doublets. In order to observe the magnetic effect of silverDynabead doublets, control experiments were performed with silver singlets and Dynabead singlets. The suspension of silver particles in 0.1% H2O2 (volume fraction roughly 0.001) was prepared by mixing 300 μL of particle suspension with 2 μL of 30% H2O2 and 298 μL of DI water. The movement of silver particles in H2O2 and UV light was recorded in the presence of an external magnetic field. A similar procedure was repeated for Dynabead singlets and silver-Dynabead doublets. Long-Term Phototactic Response of Doublets. To observe the effect of thermal diffusion on particles, the phototactic response of silver particles in DI water (volume fraction roughly 0.001) was recorded over longer periods of UV irradiation. A similar procedure was repeated for Dynabeads. The long-term phototactic response of silver-Dynabead doublets was investigated by observing their behavior in 0.25% H2O2 over longer periods of UV irradiation.
Results and Discussion Diffusiophoresis of Single Heterodoublets in H2O2 and UV Light Only. When silver-Dynabead doublets were placed in 0.25% H2O2 and irradiated with UV light, an ion gradient (and hence a temporary electric field pointing toward the center) was built up due to the release of Agþ and OOH- ions in the solution. Therefore, silver-Dynabead doublets moved in their self-generated ion gradient along their axis with the silver particle leading, as shown in Figure 3 (see video 1 showing the diffusiophoretic movement of a silver-Dynabead heterodoublet in 0.25% H2O2 and UV light). The silver particle leads because the overall ion strength gradient is such that the ionic strength increases from the silver toward the Dynabead, and therefore, the particle moves with the silver leading, since both silver and Dynabead have a negative zeta potential. Because of Brownian rotation, and some(42) CRC Handbook of Chemistry and Physics, 87th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2006.
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Figure 4. Path of silver-Dynabead heterodoublet in H2O2 and UV light, observed with an optical microscope and tracked using Physvis. The path of the heterodoublet is tracked before and after UV irradiation. The boxed region is the path of the doublet in the absence of UV light from t = 0 s to t = 40 s, and the curve outside the boxed region is the path of the doublet in the presence of UV light after t = 40 s.
Figure 3. Time-lapse optical microscopy images of a silverDynabead heterodoublet in 0.25% H2O2 and UV light, showing the trajectory over 3 s, with the silver particle leading: (a) t = 0 s and (b) t = 1 s. Note that as the heterodoublet moves to the right, the larger silver particle is on the right. For (c) t = 2 s and (d) t = 3 s, as the heterodoublet moves upward, the silver particle leads upward. This trend is readily visible in video 1 in the HTML version. Scale Bar = 5 μm.
times because of a slight asymmetry in the particle shapes, a rotational motion exists. As a result, the doublet makes random turns during its translational movement. Similar silver-leading autonomous motion was observed in the case of a silver coating deposited on one side of a microsphere,30 which indicates that the motion is independent of the relative shape and size of the two components. The diffusiophoretic movement can be turned on and off by turning on and off the UV light. Figure 4 shows the path of a silver-Dynabead heterodoublet in the absence and presence of UV light. The doublet performed only thermal Brownian motion in the absence of UV light from t = 0 s to t = 40 s (represented in red); however, it moved much quicker than the Brownian motion in the presence of UV light after t = 40 s (represented in black). The observed speed of the silver-Dynabead doublet was roughly 3 μm/s. Based on a calculation from eqs 2 and 3, the diffusiophoretic speed was estimated as 1.1 μm/s. Thus, the experimental results are within a factor of 3 of predictions from the theory, given the expected gradients of ionic strength caused by other particles. Magnetic-Diffusiophoretic Movement of Heterodoublets. a. Silver Singlets in External Magnetic Field (Simple Clustering). The movement of silver particles in H2O2 and UV light was observed in the presence of an external magnetic field. When the silver particles were placed in H2O2 in the presence of magnetic field, they performed thermal Brownian motion. When the UV light was turned on, the particles moved and formed aggregates as shown in Figure 5. They showed the same behavior in the absence of magnetic field. Thus, the behavior of silver singlets in H2O2 and UV light is independent of the magnetic field. We hypothesize that the particles move by diffusiophoresis, either due to surface heterogeneities in the particle that give asymmetric catalysis or perhaps due to a solute redistribution suggested by Golestanian.43 No movement was seen in the absence of H2O2 while in the presence of UV light, which suggests that a thermal gradient is not responsible for the motion. With time, the silver (43) Golestanian, R. Phys. Rev. Lett. 2009, 102, 188305(1)–188305(4).
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Figure 5. Optical microscopy images of silver singlets clustering in 0.1% H2O2 under an external magnetic field (v direction): (a) UV off and (b) UV on for 8 s.
Figure 6. Optical microscopy images of Dynabead singlets assembling into chains in 0.1% H2O2 under an external magnetic field (v direction): (a) UV off and (b) UV on for 20 s.
particles tend to form aggregates. The explanation for these aggregates is not entirely clear at present. It is possible that a temporary and local increase in ionic strength enables van der Waals attractions to hold the particles in a secondary energy minimum. We do not yet rule out a convective mechanism, although we cannot determine a possible path by which diffusiophoresis could cause aggregates to form. b. Dynabeads in External Magnetic Field (Magnetic Chains). The movement of Dynabeads in H2O2 and UV light was observed in the presence of an external magnetic field. Magnetic beads self-assembled in H2O2 into a chainlike structure with the application of magnetic field, as shown in Figure 6a. When the UV light was turned on, the Dynabeads remained aligned and did not show any change in their behavior as shown in Figure 6b, suggesting that the behavior of Dynabeads depends on the magnetic field and not on H2O2 and UV light. These chainlike structures were very sensitive to the external magnetic field. They align in the direction of the magnetic field and could be manipulated by changing the orientation of the field as shown in Figure 7. DOI: 10.1021/la904133a
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Figure 7. Optical microscopy images of magnetic colloidal chains formed from Dynabead singlets in DI water under an external magnetic field. The red arrow represents the direction of the magnetic field. Scale bar = 20 μm.
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Figure 9. Time-lapse optical microscopy images of silverDynabead heterodoublet suspension in 0.1% H2O2 under an external magnetic field. Magnetic field is applied in v direction. (a) UV off, (b) UV on, t = 3 s, (c) UV on, t = 8 s, and (d) UV on, t = 13 s. Scale bar = 20 μm.
Figure 8. Reflection microscopy image of the heterogeneous chain formed by the as-prepared silver-Dynabead doublet suspension in 0.1% H2O2 under an external magnetic field (f direction). The chain consists of Dynabeads (represented by dull particles) and silver particles (represented by bright particles).
c. Silver-Dynabead Heterodoublets in External Magnetic Field and UV Light (Exclusion Zones). When the silverDynabead doublets were placed in 0.1% H2O2 and an external magnetic field was applied, heterogeneous magnetic chains were formed in the solution. Figure 8 shows a reflection microscopy image of such a chain. Only silver microspheres are visible in the reflective mode; however, the rough structure of the chain suggests that it is constructed with both of the particles. The as-prepared silver-Dynabead doublet suspension in H2O2 also contains silver and Dynabead singlets performing Brownian motion. When the UV light was turned on, freely suspended particles in the solution moved away (they appear almost to have “exploded”) from the heterogeneous magnetic chains, creating exclusion zones around the chains. (See videos 2 and 3 in the HTML version showing silver-Dynabead heterodoublet suspension in 0.1% H2O2 under an external magnetic field. Magnetic field is applied in v direction. UV light is turned on at t = 3 s, after which the magnetic chains form exclusion regions.) Figure 9 shows the time lapse optical microscopy image of the phenomenon. Figure 9a shows two magnetic chains with a uniform distribution of particles around them in the absence of UV light. When the UV light was turned on, as in Figure 9b, the particles in the solution were exploded away from the chains, creating exclusion zones around them. The silver particles in the chain generate a high concentration of ions in H2O2 and UV light, and the gradient of ions moves the freely suspended particles in the solution away from the magnetic chain. The chains broke into two parts where a silver particle existed between two magnetic particles, which is where magnetic attraction is reduced. The chains also showed some horizontal displacement as the ion gradient works not only, by diffusiophoresis, on the 6312 DOI: 10.1021/la904133a
Figure 10. (a-d) Time-lapse optical microscopy images of silver and Dynabead singlets showing their phototactic behavior under UV irradiation: (a) silver particles at 0 min, (b) silver particles after 100 min, (c) Dynabeads at 0 min, and (d) Dynabeads after 100 min. Scale bar in (a) and (b) = 20 μm. Scale bar in (c) and (d) = 100 μm.
free particles but also on the negatively charged chains (Figure 9c and d). When the UV light was turned off, silver particles in the suspension, which initially moved away from the chains, now diffused back toward the chains and restored their original state of Brownian motion. This phenomenon was clearly observable with the switching on and off of the UV light. It was also repeatable, with particles moving away from chains in the presence of UV light and coming back toward the chains in the absence of UV light. No hysteresis was evident. Long-Term Patterns of Singlets and Heterodoublets. a. Silver Singlets and Dynabead Singlets in UV. Colloidal particles in a suspension show thermal diffusion with a bias, along with motion due to applied fields.44 This effect was investigated by observing the phototactic response of silver (44) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: New York, 1989.
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Figure 11. Time-lapse optical microscopy images of the silverDynabead doublets and higher order aggregates showing their phototactic process in 0.25% H2O2 under UV illumination: (a) 0 min, (b) 10 min, (c) 60 min, and (d) 120 min. Scale bar = 20 μm.
singlets and Dynabead singlets (control experiment) upon UV irradiation. At time t = 0 min, there was a uniform distribution of particles both for silver particles and Dynabeads, as shown in Figure 10a and c. Gradually, both silver particles and Dynabeads started moving toward the UV illuminated region, that is, toward the center of the UV field due to the thermal diffusion effect. The colloidal particles present in the center region were heated due to UV illumination and transferred the heat to the surrounding medium which raised the temperature and lowered the viscosity of the fluid. This increased the diffusion coefficient of the particles in that particular region of the fluid, and thus, the particles moved toward the region of higher diffusion coefficient,44,45 that is, toward the UV illuminated region, as seen in Figure 10b and d, which illustrates the state of the silver singlets and Dynabead singlets, respectively, after 100 min of UV illumination. b. Pattern Formation by Silver-Dynabead Heterodoublets in UV. The phototactic effect was also investigated by recording the behavior of as-prepared doublet suspension in 0.25% H2O2 under UV light over longer periods of time. The combination of (45) Ermak, D. L.; McCammon, J. A. J. Chem. Phys. 1978, 69, 1352–1360.
Langmuir 2010, 26(9), 6308–6313
Article
several particles (singlets, doublets, and higher order aggregates) working together gives a longer-range concentration gradient that drives the particles to move collectively. Each particle both contributes to the gradient and is affected by the gradient. Diffusiophoresis predicts the movement of silver-Dynabead doublets opposite to the direction of the self-generated electric field, that is, away from the center, showing negative phototaxis.30 Thermal diffusion arising from the light absorption on the particle surface predicts positive phototaxis.44,45 Figure 11 shows the time-lapse images of the particles in 0.25% H2O2 under UV light. The experiment started with a uniform distribution of particles at t = 0 min; the particles moved away from the center initially in the first 10 min as seen in Figure 11b, suggesting that the diffusiophoretic effect dominated the thermal diffusion effect. By the end of 10-15 min, much of the H2O2 fuel has been consumed by the silver particles and so the thermal diffusion effect dominated the diffusiophoretic effect, explaining the movement of the particles from the outer region back toward the center region, as illustrated in Figure 11c and d.
Conclusions This work describes the simple, inexpensive, quick, reliable, and scalable SQF approach for the fabrication of microscale catalytic motors using particles of various material compositions, including metal and magnetic polymer. The catalytic motors are capable of performing autonomous movement by converting the chemical free energy of their local environment to mechanical energy. The initial diffusiophoretic movement and the long-term phototactic movement of the catalytic motors were studied. The multistimulus input, requiring both diffusiophoresis and magnetic stimuli, brings out interesting behavior in micro/nanomotors. For instance, the motors show autonomous movement, in random directions, in the presence of H2O2 and UV light, but if an external magnetic field is also present, they show directed movement, forming exclusion regions around them. The magnetophototactic response shown by the magnetic colloidal chains opens a new path for their use as delivery vehicles, as supports in bioscience applications,25 and as core units in patterned assemblies. Acknowledgment. The authors gratefully acknowledge support from the National Science Foundation through CBET Grant No. 0651611 and MRSEC Grant No. DMR-08-20404.
DOI: 10.1021/la904133a
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