Manipulating Nanoparticles in Solution with Electrically Contacted Nanotubes Using Dielectrophoresis Lifeng Zheng, † Shengdong Li, † James P. Brody, †,‡ and Peter J. Burke* ,†,‡ Department of Electrical Engineering and Computer Science and Biomedical Engineering, University of California, Irvine, California 92697 Received February 4, 2004. In Final Form: July 13, 2004 Dielectrophoresis is an electronic analogue 1,2 of optical tweezers 3 based on the same physical principle: an ac electric field induces a dipole moment on an object in solution, which then experiences a force proportional to the gradient of the field intensity. For both types of tweezers, this force must compete with thermal Brownian 4 motion to be effective, which becomes increasingly difficult as the particle size approaches the nanometer scale. Here we show that this restriction can be overcome by using the large electric field gradient in the vicinity of a carbon nanotube to electronically manipulate nanoparticles down to 2 nm in diameter. I. Introduction The physical principles of self-assembly that give rise to complicated three-dimensional structures on the na- nometer scale in both biological and synthetic systems have been studied extensively. 5,6 The forces at work include noncovalent inter- and intramolecular interactions, i.e., hydrogen bonding, van der Waals, metal-ligand interac- tions, π-π stacking, and hydrophobic vs hydrophilic interactions. These bottom-up principles of self-assembly, while efficient and economical, generally are passive; i.e., they are controlled only by macroscopic quantities such as temperature, pH, and solvent concentration. It would be a distinct advantage if this assembly process could be actively, electronically controlled, especially with nano- meter spatial resolution. In this regard, top-down ap- proaches to the manipulation of matter have been suc- cessful at the nanometer scale only with atomic force microscopy (AFM)/scanned probed technologies, which are difficult to scale to a massively parallel environment, such as that envisioned in the nascent field of molecular electronics. In a separate, related research theme, the use of electric fields generated by an external voltage source to actively manipulate the locations of nanometer scale objects and large molecules such as DNA and proteins is well-known from conventional, established techniques such as gel electrophoresis. Here, the electrodes used are typically macroscopic in size, i.e., many centimeters. A recent variant on this research theme is the integration of microelectronic fabrication techniques such as photo- lithography to fabricate electrodes with dimensions on the order of millimeters or hundreds of micrometers. 7,8 In these electrophoresis techniques, charged species respond via the Coulomb force to dc electric fields. As a result, a limitation of the technique is that neutral species are unaffected and hence cannot be manipulated. One available technique to electronically manipulate the position of both neutral and charged species in solution is to use ac electric fields, a technique called dielectro- phoresis. 1 The physical principles of dielectrophoresis are well-established. If a polarizable object is placed in an electric field, there will be an induced positive charge on one side of the object and an induced negative charge (of the same magnitude as the induced positive charge) on the other side of the object. The positive charge will experience a pulling force; the negative charge will experience a pushing force. However, in a nonuniform field, the electric field will be stronger on one side of the object and weaker on the other side of the object. Hence, the pulling and pushing forces will not cancel, and there will be a net force on the object. This is the dielectrophoresis (DEP) force. The key physical insight in this paper is that we use carbon nanotubes as the electrode to generate the electric field gradient; the nanotubes are electrically contacted by lithographically defined metal electrodes, which are far away from the region of interest, so that the fields from the metal electrodes are numerically insignificant com- pared to the fields generated by the nanotube itself. Since the electric field gradient in the vicinity of a nanotube is large, nanoparticles as small as 2 nm in diameter can be manipulated despite the large tendency for random, thermal Brownian motion important for such small particles. This is an order of magnitude smaller than previous nanoparticles that were manipulated with lithographically defined electrodes, and represents the first use of nanotube electrodes in dielectrophoresis. Because this allows an electronic link to the nanometer world, this technology may find applications as a com- ponent of massively parallel, actively controlled na- nomanufacturing platforms and, generally speaking, may provide a bridge between top-down and bottom-up approaches to nanotechnology. * To whom correspondence may be addressed. E-mail: pburke@uci.edu. † Department of Electrical Engineering and Computer Science. ‡ Biomedical Engineering. (1) Pohl, H. A. Dielectrophoresis: The Behavior of Neutral Matter in Nonuniform Electric Fields; Cambridge University Press: Cambridge, 1978. (2) Burke, P. J. Nanodielectrophoresis: Electronic Nanotweezers. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific: Stevenson Ranch, CA, 2004. (3) Ashkin, A.; Dziedzic, J. M.; Bjorkholm, J. E.; Chu, S. Opt. Lett. 1986, 11, 288-290. (4) Einstein, A. Ann. Phys. 1905, 17, 549-560. (5) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312-1319. (6) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1155-1196. (7) Cheng, J.; Sheldon, E. L.; Wu, L.; Uribe, A.; Gerrue, L. O.; Carrino, J.; Heller, M. J.; O’Connell, J. P. Nat. Biotechnol. 1998, 16, 541-546. (8) Huang, Y.; Ewalt, K. L.; Tirado, M.; Haigis, T. R.; Forster, A.; Ackley, D.; Heller, M. J.; O’Connell, J. P.; Krihak, M. Anal. Chem. 2001, 73, 1549-1559. 8612 Langmuir 2004, 20, 8612-8619 10.1021/la049687h CCC: $27.50 © 2004 American Chemical Society Published on Web 08/24/2004