The Sen Group

Catalytically-Driven Nanomotors

Ayusman Sen
Department of Chemistry
The Pennsylvania State University
University Park, Pennsylvania 16802, USA
E-mail: asen@psu.edu

Nanoscale moving systems are currently the subject of intense interest due, in part, to their potential applications in nanomachinery, nanoscale assembly, robotics, tribology, fluidics, and chemical/biochemical sensing. Catalyzed motion on the nanoscale is ubiquitous in biology - as the basis for cell motility, cell division, intracellular transport, ATP synthesis, and muscular movement. We have now described the first examples of catalytically driven motion at the nanoscale outside the biological systems. These motors are autonomous in that they do not require external electric, magnetic, gravitational, or optical fields to provide the energy needed for propulsion. Instead, the input energy is supplied locally and chemically.

Because catalyzed movement on the nanoscale is (outside of living systems, and artificial systems that use biological motor proteins) a new phenomenon, there is much to be learned, and a good possibility that unexpected applications will arise from exploratory research. Freed from the constraint of biological reactions, one can envision using much simpler fuels than ATP, and much more accessible catalysts than enzymes. By analogy to biological systems, we can project some “obvious” applications of catalytic nanomotors, including: (a) engines for micro/nanoscopic machines, (b) chemotactic static and roving sensors, (c) delivery vehicles for molecules and nanoparticles, and (d) formation of patterns or arrays by autonomous local deposition of materials.

At the nanoscale, surface forces dominate over inertia and can be harnessed to move small objects. We have shown that nano- and micro-objects with spatially defined catalytic zones exhibit linear or rotational non-Brownian movement when placed in an aqueous “fuel” solution.^1,2 A large number of metals and metal complexes catalyze reactions that can be used to generate chemical gradients at the surface of nano- and microscale objects. By appropriate design, these gradients can be translated into anisotropic body and/or surface forces. Depending on the shape of the object and the placement of the catalyst, different kinds of motion can be achieved. The resulting nanomotors can, in principle, be tethered or coupled to other objects to act as the “engines” of nanoscale assemblies.

In work reported by the Penn State group, striped platinum/gold nanorods, 370 nm in diameter with 1 μm long platinum and gold segments, were synthesized electrochemically in alumina membranes, freed using a previously reported procedure,³ and characterized by TEM and dark-field optical microscopy. In the latter, the gold and the platinum segments were clearly distinguishable, allowing the direction of motion to be monitored.

A suspension of rods in aqueous hydrogen peroxide was prepared and a known volume (25 μL) of this mixture was placed in a sealed well on a clean glass slide and topped with a glass cover slip. Rods remained suspended in the fluid above the slide due to surface charge repulsions between the rods and glass. The nanorods move along their long axis with the platinum end forward (Fig. 1, Video at http://research.chem.psu.edu/axsgroup/supporting_information.html).¹ Forward movement is preferred because the drag force is minimized in this direction and the catalytic reaction takes place on only one end. The direction of movement is opposite to that expected if oxygen generated at the platinum end impelled the rod by momentum recoil or through a pressure increase. The dimensions of the nanorods are similar to bacteria and their average speed (10 body lengths/sec) is also comparable to multi-flagellar bacteria, such as Bacillus cereus. Control experiments establish that purely Brownian motion occurs in the absence of the hydrogen peroxide fuel.


Fig. 1. Trajectory plots of three 2 μm long platinum/gold rods identified (left) over the next 5 sec. (right) in 2.5% aqueous peroxide. Scale on x and y axes are in microns. See real-time video at http://research.chem.psu.edu/axsgroup/supporting_information.html.

The Penn State group has used external magnetic fields to further control the directionality rod movement.⁴ Striped 1.5 μm x 400 nm platinum/nickel/gold/nickel/gold nanorods (respective segment sizes (nm): platinum, 550; nickel, 100; gold, 200; nickel, 100; gold, 350) have ferromagnetic nickel segments which can be magnetized and used to control the direction of rod movement. These rods have a calculated magnetic moment of 1.3×10^-15 A·m², comparable to that observed for magnetotactic bacteria. The axial velocity is essentially unaltered by the magnetic field, thereby demonstrating that the field only aligns the rods and neither impedes nor enhances the axial motion. Fig. 2 shows the trajectory path of a nanorod spelling PSU and demonstrates the micron-scale control of the rods in the presence of a magnetic field while they are moving autonomously in 5% hydrogen peroxide.

The nanorods are a simple system for performing controlled experiments in conjunction with theoretical modeling to understand the basic principles of catalytically driven movement. However, more complex designs are needed to achieve controlled motion and, depending upon the application, larger applied forces than those that can be achieved in a cylindrical geometry.


Figure 2. The trajectory path of a Pt/Ni/Au/Ni/Au nanorod tracing the letters “PSU” in 5 wt% H₂O₂.

One form of controlled motion is rotation, which is important for gear and propeller or turbine systems. Gear systems are particularly interesting, since they can change the direction of motion and trade off torque versus speed.

The Penn State group has recently demonstrated catalytic rotational movement of a free gear (~100 μm diameter, Fig. 3).² The individual teeth were coated with platinum on one face of each tooth. This geometry generates interfacial tension forces across each tooth. The inset of Fig. 3 shows a perspective rendering of the gear tooth. The large arrow shows the expected direction of movement based on the model described above.


Fig. 3. Photograph of a microfabricated gear with platinum catalysts on the gear teeth.

The platinum/gold gear was fabricated by a combination of optical lithography, evaporation, and electroplating on a Si substrate. The gears were released from the surface by wet chemical etching of the sacrificial 0.25 μm silica, and were suspended in 1% aqueous hydrogen peroxide. The gear suspension was placed on a silicon wafer with a periodic array of posts etched into the surface at ~100 μm spacing, and were observed under an optical microscope. Fig. 4 shows still images of the gear at different rotation angles (see real-time video of gear movement at http://research.chem.psu.edu/axsgroup/supporting_information.html). The gear (which is the same size as the gear in Fig. 3 without the lettering in the center) rotates at ~1 sec^-1, corresponding to a linear velocity of ~300 μm/sec at the platinum-coated gear tooth. This is more than an order of magnitude faster than nanorod movement, consistent with the expected scaling with surface area.

Fig. 4. Gear at 0 (a), 90 (b), 180 (c) and 270 degrees (d).

A second area of catalytically-induced movement involves fluid flow. By Galilean invariance, one should expect immobilized catalytic objects to generate flow in the surrounding fluid, with similar scaling of device size and fluid velocity. Recently, we microfabricated silver and gold catalytic surfaces designed to pump fluids (Fig. 5).⁵ The fluid pumping was confirmed by observing the convective-fluid flow behavior and pattern formation of tracer particles suspended in dilute hydrogen peroxide solutions. Remarkably, the pumping/patterning effect only occurred when the silver catalyst and the gold were in electrical contact. Furthermore, tracer particles with differing electrical charges exhibited different behavior in response to the catalytically generated force.


Figure 5. Top: Pattern formation of 1.8 μm negatively charged polystyrene spheres (which appear dark in the image) on a silver-patterned gold surface in 0.1% H₂O₂ solution after approximately 5 min. The large circles in the image are oxygen bubbles. Scale bar represents 100μm. Bottom: Movement of positively-charged spheres towards a Ag disk (a-b) and away from Ag into a higher plane of focus (c, notice that the substrate is out of focus). The convection-driven periodic motion of 2 μm Au rods is illustrated in a plot of the radial displacement of one rod vs. time (bottom trace, d-f). Scale bar represents 10 μm.

In conclusion, we have demonstrated that one can build nanomotors “from scratch” that mimic biological motors by using catalytic reactions to create forces based on chemical gradients. These motors are autonomous in that they do not require external electric, magnetic, or optical fields as energy sources. Instead, the input energy is supplied locally and chemically. A very large number of metals, metal complexes, and enzymes catalyze reactions that can be used to generate chemical gradients at the surface of nanoscale objects. By appropriate design, these gradients can be translated into anisotropic body and/or surface forces. Depending on the shape of the object and the placement of the catalyst, different kinds of motion can be achieved. The resulting nanomotors can, in principle, be tethered or coupled to other objects to act as the “engines” of nanoscale assemblies. Finally, by anchoring the catalysts on a surface, it is possible to induce fluid flow in the surrounding medium. Thus, it is possible to use the stored chemical energy of fuels to induce sustained flow in a variety of fluid microsystems.

1.	W. F. Paxton, K. C. Kistler, C. C. Olmeda, A. Sen, S. K. St. Angelo, Y. Cao, T. E. Mallouk, P. Lammert, V. H. Crespi, “Autonomous Movement of Striped Nanorods,” J. Am. Chem. Soc., 2004, 126, 13424-13431.
2.	J. Catchmark, S. Subramanian, A. Sen, “Directed Rotational Motion of Microscale Objects Using Interfacial Tension Gradients Continually Generated via Catalytic Reactions,” Small, 2005, 1, 202-206.
3.	B. R. Martin, D. J. Dermody, B. D. Reiss, M. Fang, L. A. Lyon, M. J. Natan, T. E. Mallouk, “Orthogonal Self-Assembly of Colloidal Gold-Platinum Nanorods,” Adv. Mater., 1999, 11, 1021-1025.
4.	T. R. Kline, W. F. Paxton, T. E. Mallouk, A. Sen, “Catalytic Nanomotors: Remote-Controlled Autonomous Movement of Striped Metallic Nanorods,” Angew. Chem., 2005, 44, 744-746.
5.	T. R. Kline, W. F. Paxton, Y. Wang, D. Velegol, T. E. Mallouk, A. Sen, “Catalytic Micropumps: Microscopic Convective Fluid Flow and Pattern Formation,” J. Am. Chem. Soc., 2005, 127, 17150-17151.