(260a) Electrostatically Modulating the Transport Properties of Carbon Nanotube Membranes

A membrane structure consisting of a high density (~ 10 ¹⁰/cm²) of aligned multiwalled carbon nanotubes with inner pore diameters (~ 7 nm) spanning across a continuous polystyrene matrix was successfully fabricated. Integral to the membrane synthesis is a plasma oxidation process which opens up the CNT tips and introduces carboxylate functionality. ‘Gate-keeper' molecules can be placed at the entrance to the CNT to modulate the transport behaviour.

When functionalized with an anionic dye molecule, using well known carbodiimide mediated coupling between the accessible amine groups of the dye and surface carboxylic acid of the CNT membrane, 200-250 % increase of flow of cationic Ruthenium bi-pyridine [Ru-(bipy)3+2] was observed due to Coulombic attraction (vida infra). Screening by aqueous electrolyte (0.1(M) KCl), caused the transport rate to decline by 250-300 %. However, we also found 150-160% and 100-120 % decrease in the transport rates of neutral species with electrolyte screening. This indicates that both electrostatics and the degree of hydrophillicity of the CNT core entrance control the transport through CNT-spacer-dye membrane.

We also investigated the effect of increased charge density on the transport properties of CNT-spacer-dye membrane. Electrochemical grafting was performed at –0.6 v using 0.1(M) KCl, 0.1(M) HCl, 5 mM of 4-carboxy phenyl diazonium tetrafluoroborate salt in aqueous medium. Toluidine Blue (TBO) adsorption-desorption tests indicated ~ 4 times increase in surface carboxyl functional density on the membrane. Similar sequence of functionalisation (spacer-dye) was carried out on the membrane after electrochemical grafting. Unlike the previous case, we observed a decrease (~ 200 %) in the flow of Ruthenium bi-pyridine [Ru-(bipy)₃⁺²]. Screening of the charge in 0.1 (M) KCl increased the flow of the positively charged species. This is consistent with chemical grafting along the entire length of CNT core, where attractive surface charge will slow overall diffusion flux.

These studies indicate the importance of charge density and spatial location of the functional molecules inside the CNT in controlling the transport properties of CNT membranes.