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2008, 112, 2797-2801 Published on Web 02/01/2008
Chemical Locomotives Based on Polymer Supported Catalytic Nanoparticles Aditya Agrawal,† Krishna Kanti Dey,‡ Anumita Paul,§ Saurabh Basu,| and Arun Chattopadhyay*,‡,§ Department of Chemical Engineering, Centre for Nanotechnology, Department of Chemistry, and Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781 039, India ReceiVed: October 20, 2007; In Final Form: January 1, 2008
In this letter, we report the development of a polymer-based chemical locomotive. Pd nanoparticle coated polymer resin beads were used for catalytic decomposition of H2O2 in aqueous medium. The oxygen bubbles generated in this way stuck to the beads after their formation. When there were sufficient numbers of bubbles formed or when the bubbles were large, vertical movement of the bead with constant velocity could be observed. The velocity could be as much as 0.59 cm s-1 at the highest H2O2 concentration used. Further, the vertical velocity could systematically be controlled by changing the viscosity of the medium (by addition of glycerol). We observed that the velocity was inversely proportional to the viscosity of the medium. Appropriate theoretical analysis has also been reported. Once the vertical velocity was zero, horizontal motion and rotational motion could be observed. Further, a collection of such beads was used to propel large macroscopic objects providing linear and rotational motions.
Autonomous motion of objects at various length scales consisting of nanoscale components, powered by chemical and biochemical reactions, would open the door to the development of small-scale locomotives, which would operate without continuous and direct external intervention. Naturally occurring biological systems provide plenty of examples for biochemically driven autonomous movements of submicronscale and nanoscale objects. For example, the motion of the bacteria Listeria monocytogenes is propelled by polymerization of actin in the host cell.1 On the other hand, although external electric, magnetic, or electromagnetic field has the capability of moving objects in a medium, however, practical usage become limited at increasingly smaller dimensions. Also, autonomous nanoscale motions could be useful for self-assembly of molecular systems,2 energy conversions,3 micropumps,4 etc. Recent experiments suggest that catalytic chemical and biochemical reactions could be a convenient driving force for autonomous movements that also include involvement of nanoscale functional components. For example, pioneering works by Whitesides and co-workers demonstrated the autonomous movement of a millimeter-scale Pt containing poly dimethylsiloxane, where catalytic decomposition of H2O2 by Pt, producing O2 bubbles, led the propulsion.5 Similar bubble propulsion of silica microparticles driven by a manganese-based synthetic catalase chemically tethered to the microparticle has been reported by Feringa and co-workers.6 Through a series of elegant designs and experiments, the group led by Sen and Mallouk used interfacial tension gradients as the driving force * To whom correspondence should be addressed. E-mail: arun@ iitg.ernet.in. † Department of Chemical Engineering. ‡ Centre for Nanotechnology. § Department of Chemistry. | Department of Physics.
10.1021/jp710185j CCC: $40.75
for directional motion of nanoscale Pt-Au composite objects, where Pt catalytically decomposes H2O2.7 They have also demonstrated the ability to control the movement of a catalytic nanorod using an external magnetic field.8 On the other hand, Golestanian et al. have proposed a molecular machine whose working principle is based on diffusophoresis, which is based on asymmetric distribution of reaction products.9,10 Autonomous movements of synthetic motors powered by sunlight11 and catalytic reactions12 have also been reported. In previous reports, autonomous movements of chemical locomotives have been based on driving forces such as bubble propulsion, interfacial tension, and polymerization. However, in real systems, buoyancy would play a significant role in the propulsion of chemical locomotives especially when the dimensions of the objects become large. This, however, has not been specifically addressed in any of the reported studies. Also, there is no report on moving larger objects (hundreds of micrometers) by nanoscale functional components. Herein we report the development of a polymer-metal nanoparticle (NP) based catalytic chemical locomotive that has nanoscale functional components and which drives objects that are much larger than their dimensions. Also, we address the role of buoyancy force in the movement and use viscosity of the medium to control the velocity of the moving object. Finally, we show that even larger objects could be moved by systematic design and incorporation of a number of catalytic bodies. The experimental procedures are briefly described here. A total of 3 g of commercially available cation exchange resin microbeads (polystyrene divinylbenzene copolymer, AmberliteIR 120, Merck) was at first kept in 10 mL of 3 M HCl (Merck) solution for 1 h. The beads were then washed thoroughly with water to remove the excess HCl. That was confirmed by checking the pH of the resultant water for which a value of 6.8 was obtained. This step is necessary to replace the Na+ ions in © 2008 American Chemical Society
2798 J. Phys. Chem. C, Vol. 112, No. 8, 2008
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Figure 1. (A) SEM image of Pd nanoparticles formed over the surface of polymer resin beads. Dimension of the nanoparticles were found to vary from 30 to 146 nm. (B) Energy dispersive X-ray (EDX) profile of a coated resin bead.
the commercially available resin beads by hydrogen ions. This was followed by keeping activated resin beads in 5 mL aqueous 0.1 M Pd(NO3)2 (Merck) solution for 2 h. The solution was then decanted which was followed by washing the beads with water to remove excess Pd(NO3)2. They were then treated with 5 mL of 10 mM NaBH4 solution for 2 h. This was followed by collecting the beads which were then air-dried. The beads were then ready for use in further experiments. The aqueous solutions mentioned above were made using Milli Q grade water (resistivity value of 18.2 MΩ cm). To study the effect of viscosity, 5.47 M glycerol (98% purified, purchased from Merck, India) solution was used as the stock solution. For observations of the movement of macroscopic objects, the grinded resin beads were glued to the surface of the macroscopic objects using styrofoam dissolved in toluene (Merck, India). Pd-coated resin beads were investigated by LEO 1430 VP scanning electron microscope. The videos were captured by Creative WebCam manufactured by Creative Technology Limited, Singapore. Composites of polymer-metal NPs offer the possibility of generation of scalable devices, where the NPs could act as the catalyst for chemical reaction needed for propulsion and the polymer would act not only as a host to the NPs providing stability but also would be supporting the flexibility in terms of size, shape, and ease of synthesis. Although, there have been reports of use of biopolymers and biosystems for autonomous movements,13-16 there is no report on the use of a composite for such motors. We have used a polymer-NP composite, where nanometer sized Pd NPs were deposited on micron-sized ionexchange polymer resin beads. The Pd NPs on the bead catalyzed the decomposition of aqueous H2O2 into water and O2 (eq 1).
2H2O2(l) f O2(g) + 2H2O(l)
(1)
The deposition of a large number of NPs on the spherical bead produced enough O2 bubbles to impart its motion. Since the beads are insoluble in water and have higher density they settle at the bottom of the container. The buoyancy force of the bubble grown on the beads drives them upward toward the top of the container. For a bead moving upward, we found that the viscous drag is two orders of magnitude smaller than the buoyancy force and in the present case the latter is the dominant propeller. The deposition of Pd NPs on the ion-exchange polymer resin beads has been followed using a previously developed method by Majumdar et al.17 The choice of ion-exchange resin beads as the polymer was based on their ion-exchange properties, and stability to work under various solvent, pH, thermal, and ionic conditions. The average diameter of these spherical resin beads
Figure 2. Time dependent vertical motion of Pd NP-coated polymer bead placed in 30 mL of 5% aqueous hydrogen peroxide solution. The numbers in the panels indicate the time of the shots.
has been measured to be 820 µm, and the average weight is 0.8 mg. The method of NP synthesis is based on ion exchange of salt of metal with commercially available resin beads followed by reduction of the metal ions using NaBH4 as the reducing agent. When the Pd2+-exchanged beads were reduced by NaBH4, growth of a large number of NPs on the beads with average diameter less than 90 nm could be observed on the surface of the beads. A typical scanning electron microscopy (SEM) view of the coated bead and corresponding energy dispersive X-ray (EDX) measurement shown in Figure 1A,B respectively, indicate the formation of Pd NPs. When the Pd NP-coated polymer bead was immersed in 5% aqueous H2O2 solution, bubble formation around the bead could be observed by naked eyes. Although, there was no movement initially, however, as the bubbles grew bigger vertical movement toward the top of the container could be observed. We observed that a maximum velocity of 0.59 cm s-1 could be achieved. Three still pictures of a video-shot of a typical bead movement are shown in Figure 2. As is clear from the figure, the bead moved by 1.42 cm in 4 s and in 8 s by 2.25 cm. Also, clear is the uniform velocity with which the bead moved upward. It was further observed that sometimes the bead quickly moved downward upon bursting of the bubble at the interface. On the other hand, movement in the plane of the interface was also observed, when the bead was at the air-water interface. The velocity in the plane of the interface was, however, much less and of about 0.049 cm s-1. The underlying mechanism of motion of the bead mentioned above is akin to the vertical motion of effervescent tablets (such as vitamin C tablets) when placed in water, although the mechanisms of the bubble formation are fundamentally different
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J. Phys. Chem. C, Vol. 112, No. 8, 2008 2799
Figure 3. Viscosity effect on motion of Pd NP-coated polymer bead placed in 30 mL of 5% aqueous hydrogen peroxide solution added with 100 µL of glycerol. The numbers in the panels indicate the time of the shots.
in the two cases. Catalytic decomposition of H2O2 by the Pd NPs, that are present on the bead surface, leads to formation of tiny oxygen bubbles, which remain stuck to the surface. A control experiment suggests that Pd NPs are essential for the production of bubbles as the Pd2+-coated bead did not lead to discernible bubble formation. The bubbles adhere to the bead due to capillary forces, although some of them might actually get detached immediately after their formation because of insufficient force of adhesion. However, when the number of bubbles is small and the sizes of the bubbles are diminutive then the buoyancy force is not sufficient to lift the bead from the bottom of the container. Movement of the small bubbles leads to their coalescence on the bead surface, while newer bubbles continue to be produced. Our observations indicate that when the bubbles coalesce on top of the beads the beads start to move upward. On the other hand, when the bubbles are formed at the sides of the beads they need to move to the top in order to put the bead into motion. It is important to mention here that when the bubbles grow at the intervening region between the top of the glass surface of the container and the bottom part of the bead the movement of the bead is generally inhibited, unless larger bubbles form which takes a rather long time. We were particularly interested in finding ways to control the motion of the beads. This could possibly be achieved by controlling the growth and coalescence of bubbles. An important parameter in the dynamics of gas bubbles in a liquid medium is viscosity and its change can effectively be used as a tool to control the dynamics of the beads. In our experiments, when glycerol is added to the medium, the vertical velocity started going down with increasing concentration of glycerol. For example, when 40 µL of glycerol was added to 30 mL of 5% H2O2 solution, the velocity (vertical) went down to 0.27 cm s-1 which is nearly half of that without the presence of glycerol (0.59 cm s-1). A photographic representation of the timedependent movement of a bead in the presence of glycerol is shown in Figure 3, which clearly shows a decrease in the velocity in the upward direction. At lower viscosities, the coalescence of smaller bubbles is favored, while at higher viscosity it is not the case. In the following we provide an explanation for this. The viscosity of the solution as a function of surface tension is described by the following empirical relation18
ln γ ) ln A +
B η
(2)
Here A and B are constants, γ is the surface tension, and η is the viscosity. As a consequence of increase in viscosity, the force of adhesion between the bubbles and the bead diminishes
Figure 4. Schematic representation of various forces acting on a resin bead containing a bubble on top.
resulting in the formation of large number of smaller bubbles, which get detached from the bead soon after their nucleation. Thus if bubble coalescence is minimized by increasing the viscosity then this would possibly lead to controlled motion of the beads. The total buoyancy force owing to the tiny bubbles attached to the bead surface is however small, and hence would be insufficient to lift the coated beads. In addition, at higher values of viscosity we should be able to see a lot of small oxygen bubbles detaching from the bead surface. Thus as the bubbles get detached early the bead loses its vertical velocity and hence slows down. We have observed such decrease in the vertical velocity of the beads in our experiment. Interestingly, horizontal movement and rotational movement of the bead can be observed at sufficiently higher concentration of glycerol (400 µL in 30 mL solution). This can be explained by the following. As the concentration of glycerol is increased coalescence of bubbles decreases and as a result large number of smaller bubbles comes out of the bead. At any moment there will be larger number of smaller bubbles on the side of the bead than at the top or bottom of the bead. These bubbles coming out simultaneously from the sides lead the bead to rotate or walk along the surface of the glass as this does not require overcoming gravitational forces (which is required for vertical movement). Thus, changing viscosity of solution can act as a control for the motion of the beads, which can have wide applications. We shall present a simple theoretical model and explain the observations mentioned above. Consider a polymer bead with radius Rp and a single bubble, with radius Rb, attached to the top of the bead. Also, for simplicity one can ignore the change in size of the bubble during the movement of the bead. Further, while calculating the weight of the system Fg, the weight of the bubble can be neglected as its density is much less than that of the bead. Since the dimensions of the bead and the bubble together are much less than that of the liquid in the container, the later can be assumed to be infinite in extent so that the edge effects can safely be neglected and that the viscous drag can be calculated using Stokes formula, Fv ) 6πηrV, where we have approximated
r)
RP + R b 2
Furthermore, viscosity of a binary liquid mixture can roughly be estimated from the empirical expression, η ) x12η1 + x22η2
2800 J. Phys. Chem. C, Vol. 112, No. 8, 2008
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ηV ) constant
Figure 5. Dependence of the vertical velocity of a resin bead on the viscosity of H2O2 solution when mixed with various amounts of glycerol.
+ x1x2η′, where η is the viscosity of the mixture, xi and ηi are respectively the mole fraction and viscosity of species i in the mixture and η′ is a composition independent cross coefficient, nearly equal to η1 + η2.19 Since the bubble does not get detached from the bead during the motion, we may assume that the force resulting from the excess pressure inside the gas bubble FP, is balanced by the capillary force FC. Now, the system will move with a uniform velocity in the upward direction when Fb ≈ Fv + Fg, which gives
4 6πηrV ) Fb - πRP3FPg 3
(3)
where FP is the density of the material of the resin bead. Thus one arrives at the following expression:
4 F - πR F g) ( 3 ηV ) 3
b
P
6πr
P
(4)
Further, if we consider the force of buoyancy (Fb) to be approximately the same irrespective of viscosity (as the change is small in our experiments) of the solution then
(5)
Thus if we plot the velocity of the system (bead plus bubble) as a function of viscosity, which is controlled by adding chosen amount of glycerol we should get an inverse relationship (rectangular hyperbola). In order to pursue this, we had measured the vertical velocity of the bead (plus bubble) as a function of viscosity by adding certain amount of glycerol. The results are shown in Figure 5. It is important to mention here that the above relationship is found to be different from other scaling laws observed in cases like phoretic locomotion, signifying the difference in underlying mechanisms behind these motions.20 Further, when glycerol is added into the solution, interfacial tension at the solid-liquid interface is reduced, and it was observed that the detachment of the bubbles from the beads’ surface was favored compared to their coalescence. Overall, the lack of enough bubbles and their coalescence prevented the generation of sufficient buoyancy force to lift the bead. Our final aim in these studies was to find a way of using these Pd NP-coated resin beads for moving macroscopic objects using the same catalytic reaction as the driving force. In order to achieve this, the beads were grinded into smaller pieces of diameters on the order of 10 µm or so. They were then dispersed in water and coated with Pd NPs as before. They were collected by centrifugation and then glued on the parts of macroscopic objects before using them for studies. For example, as shown in Figure 6, the top surface of a lever of 3 cm length was coated with approximately 80 mg of beads and then kept immersed in aqueous H2O2. In this way vertical movement of the lever was achieved with a velocity of 1.62 cm s-1. Further, when the same beads were used to coat a rotating device, as shown in Figure 7, rotational motion with angular velocity 0.86 rad s-1 could be observed. To summarize, we have been able to develop a polymersupported chemical locomotive with Pd NPs on the polymer beads as the functional components, where the movement is driven by buoyancy. Also, we have been able to work out the effect of viscosity on the velocity and have shown that by controlling the viscosity of the solution the vertical velocity can be reduced leading to horizontal motion. We hope the present
Figure 6. Motion of a thin strip of aluminum (∼3 cm × 1 cm × 1 mm) containing Pd NP-coated polymer resin beads in the presence of 5% H2O2. The numbers in the panels indicate the time of the shots.
Figure 7. Rotational motion of a thin strip of aluminum sheet (5 cm × 2 cm × 2 mm) containing Pd NP-coated polymer resin beads. The beads were attached at the two curved and opposite ends of the sheet. The numbers in the panels indicate the time of the shots.
Letters study would spring newer horizons in chemical locomotion where flexible polymer and catalytic metal NPs would be used to drive objects at various scales using chemical reaction as the source of motion. Acknowledgment. We thank the Department of Science and Technology, (DST Nos. SR/S5/NM-01/2005 and 2/2/2005-S.F.) and Council of Scientific and Industrial Research (CSIR, 01(1947)/04/EMR-II), Government of India for financial support. We are thankful to the Central Instrument Facility, IIT Guwahati for help in carrying out SEM analysis. References and Notes (1) Pantaloni, D.; Clainche, C. L.; Carlier, M.-F. Science 2001, 292, 1502. (2) Hess, H. Soft Matter 2006, 2, 669. (3) Gazeau, F.; Baravian, C.; Bacri, J.-C.; Perzynski, R.; Shliomis, M. I.; Phys. ReV. E 1997, 56, 614. (4) Kline, T. R.; Paxton, W. F.; Wang, Y.; Velegol, D.; Mallouk, T. E.; Sen, A. J. Am. Chem. Soc. 2005, 127, 17150. (5) Ismagilov, R. F.; Schwartz, A.; Bowden, N.; Whitesides, G. M. Angew. Chem., Int. Ed. 2002, 41, 652. (6) Vicario, J.; Eelkema, R.; Browne, W. R.; Meetsma, A.; La Crois, R. M.; Feringa, B. L. Chem. Commun. 2005, 3936.
J. Phys. Chem. C, Vol. 112, No. 8, 2008 2801 (7) Paxton, W. F.; Kistler, K. C.; Olmeda, C. C.; Sen, A.; St. Angelo, S. K.; Cao, Y.; Mallouk, T. E.; Lammert, P.; Crespi, V. H. J. Am. Chem. Soc. 2004, 126, 13424. (8) (a) Kline, T. R.; Paxton, W. F.; Mallouk, T. E.; Sen, A. Angew. Chem. 2005, 117, 754. (b) Kline, T. R.; Paxton, W. F.; Mallouk, T. E.; Sen, A. Angew. Chem., Int. Ed. 2005, 44, 744. (9) Golestanian, R.; Liverpool, T. B.; Ajdari, A. Phys. ReV. Lett. 2005, 94, 220801. (10) Howse, J. R.; Jones, R. A. L.; Ryan, A. J.; Gough, T.; Vafabakhsh, R.; Golestanian, R. Phys. ReV. Lett. 2007, 99, 048102. (11) Balzani, V.; Clemente-Leo´n, M.; Credi, A.; Ferrer, B.; Venturi, M.; Flood, A. H.; Stoddart, J. F. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 1178. (12) Fournier-Bidoz, S.; Arsenault, A. C.; Manners, I.; Ozin G. A. Chem. Commun, 2005, 441. (13) Cameron, L. A.; Footer, M. J.; van Oudenaarden, A.; Theriot, J. A. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 4908. (14) Bohm, K. J.; Stracke, R.; Muhlig, P.; Unger, E. Nanotechnology 2001, 12, 238. (15) Jia, L.; Moorjani, S. G.; Jackson, T. N.; Hancock, W. O. Biomed. MicrodeVices 2004, 6, 67. (16) Soong, R. K.; Bachand, G. D.; Neves, H. P.; Olkhovets, A. G.; Craighead, H. G.; Montemagno, C. D. Science 2000, 290, 1555. (17) Majumdar, G.; Goswami, M.; Sarma, T. K.; Paul, A.; Chattopadhyay, A. Langmuir 2005, 21 (5), 1663. (18) Queimada, A. J.; Marrucho, I. M.; Stenby, E. H.; Coutinho, J. A. P. Fluid Phase Equilib. 2004, 161 and the reference therein. (19) Saksena, M. P.; Harminder; Kumar, S. J. Phys. C: Solid State Phys. 1975, 8, 2376 and the reference therein. (20) Golestanian, R.; Liverpool, T. B.; Ajdari, A. New J. Phys. 2007, 9, 126.
