The Exterior of Single-Walled Carbon Nanotubes as a Millimeter-Long Cation-Preferring Nanochannel

The exterior of single-walled carbon nanotubes is shown to facilitate preferential migration of cations over a millimeter length scale. Applying an electric field to droplets of NaCl placed at both ends of the nanotubes causes the transport of a cation-enriched solution along the nanotubes in the direction of the electric field, while the anion-enriched solution countermigrates along the adjacent substrate. This phenomenon is confirmed by Kelvin probe force microscopy and mass spectrometry imaging of individual nanotubes as well as formation of bright and dark lines along the nanotubes in scanning electron microscopy (SEM). Blocking the exterior of the nanotubes prevents formation of both the bright/dark lines in SEM and flow of current through the nanotubes, confirming the insignificance of interior ion transport and electron current. The cation-preferring transport results in the formation of positively charged salt crystals along the nanotubes (with a cation-to-anion ratio of 0.59:0.41 for KCl) followed by the subsequent shrinkage and growth of crystals in the direction of cation flux. Molecular dynamics simulations show that the cation−π interaction is responsible for such cation preference observed during transport. The loss of cation preference upon covalent functionalization of the nanotubes further supports this mechanism. Utilizing the short-range cation−π interaction as a transport mechanism suggests broader applications in areas where charge-specific transport is desired.