A membrane structure consisting of an aligned array of open ended carbon nanotubes (∼7nm internal diameter) spanning across an inert polymer matrix allows the diffusive transport of aqueous ionic species through carbon nanotube cores. The plasma oxidation process that opens carbon nanotubes tips inherently introduces carboxylic acid groups at the carbon nanotube tips, which allows for a limited amount of chemical functional at the carbon nanotube pore entrance. However for numerous applications, it was important to increase the density of carboxylic acid groups at the pore entrance for effective separation processes. Aqueous diazonium-based electrochemistry significantly increased the functional density of carboxylic acid groups. pH dependent dye adsorption-desorption and interfacial capacitance measurements indicate a 5 to 6 times increase in functional density. To further control the spatial location of the functional chemistry, a fast flowing inert liquid column inside the carbon nanotube core was found to restrict the diazonium grafting to the carbon nanotube tips only. This was confirmed by the increased flux of positively charged Ru (bipy)3+2 with anionic functionality. The electrostatic enhancement of ion diffusion was readily screened in 0.1M electrolyte solution consistent with the membrane pore geometry and increased functional density.

Enhanced Electrostatic Modulation of Ionic Diffusion through Carbon Nanotube Membranes by Diazonium Grafting Chemistry. Majumder, M., Keis, K., Zhan, X., Meadows, C., Cole, J., Hinds, B.J.: Journal of Membrane Science, 2008, 316[1-2], 89-96