Controlling the rate and direction of electroosmotic flow in template-prepared carbon nanotube membranes

We describe here carbon nanotube membranes (CNMs) prepared by doing chemical vapor deposition of graphitic carbon into the pores of microporous alumina template membranes. This approach yields a freestanding membrane containing a parallel array of carbon nanotubes (outside diameter ∼200 nm, wall thickness ∼40 nm) that spans the complete thickness of the membrane (60 μm). The electroosmotic flow (EOF) can be driven across these CNMs by allowing the membrane to separate two electrolyte solutions and using an electrode in each solution to pass a constant ionic current through the nanotubes. The as-synthesized CNM has anionic surface charge and as a result, the EOF is in the direction of cation migration across the membrane. In addition to this result, the key findings of this correspondence are: (1) that by using an electrochemical derivatization method, carboxylate groups can be covalently attached to the carbon nanotube walls; this enhances the anionic surface charge density, resulting in a corresponding increase in the EOF rate; (2) that electrochemical derivatization can also be used to attach cationic ammonium sites to the nanotube walls to yield CNMs that show EOF in the opposite direction of the as-synthesized or carboxylated membranes.