pH-tunable ion selectivity in carbon nanotube pores.

The selectivity of ion transport in nanochannels is of primary importance for a number of physical, chemical, and biological processes ranging from fluid separation to ion-channel-regulated cellular processes. Fundamental understanding of these phenomena requires model nanochannels with well-defined and controllable structural properties. Carbon nanotubes provide an ideal choice for nanofluidic studies because of their simple chemistry and structure, the atomic scale smoothness and chemical inertness of the graphitic walls, and the tunability of their diameter and length. Here, we investigate the selectivity of single and, for the first time, binary salt mixtures transport through narrow carbon nanotubes that act as the only pores in a silicon nitride membrane. We demonstrate that negatively charged carboxylic groups are responsible for the ion rejection performance of carbon nanotube pores and that ion permeation of small salts can be tuned by varying solution pH. Investigation of the effect of solution ...