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Dielectrophoresis is an electronic analogue1,2 of optical tweezers3 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 Brownian4 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.

Using the tools of modern molecular biology, we probe the interaction of nanotubes on silicon chips with proteins via combinatorial phage display methods. By screening against a large library of random peptides, we find that over half of the single-walled carbon nanotube (SWNT)-binding peptides show a motif of SXWWXXW, where S is serine, W is tryptophan, and X is anything. In a helical wheel diagram, this peptide is amphiphilic, where the hydrophobic and aromatic tryptophan side groups are concentrated on one face of an R-helix. This theme is robust and occurs in all of the SWNT-binding peptides. Surprisingly, the other aromatic amino acids seem less likely to show up in the screen, indicating a special role of tryptophan in binding to SWNTs. By elucidating the physical principles underlying the interaction between SWNTs and peptides and proteins, this work lays the foundation for the eventual human (or computer) nanoengineered, precise, and economical manipulation of nanotubes using peptides and proteins for nanotube sorting, assembly into electronic components, and understanding the effect of biological function.

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