Giant Conductance and Anomalous Concentration Dependence in Sub-5 nm Carbon Nanotube Nanochannels

Abstract

Simulations and experimental studies have reported an unusually high ionic conductance in carbon nanotubes (CNTs) nanochannels. The origin of this phenomenon is, however, poorly-understood: literature reports often disagree in the magnitude of the different transport mode contributions to the measured ionic current and even in what ions are actually carrying the current; moreover, results obtained with single pore measurements differ frequently from those with membranes containing billions of open CNT channels, i.e. the average CNT behavior. Toward shedding light on these phenomena, we fabricated a novel platform having vertically-aligned sub-5nm carbon nanotubes as nanofluidic channels, the number of which can be controlled from one to billions. By employing this platform with only one or a few open CNTs, we observed giant ionic currents in CNT channels and a power-law increase of conductance with KCl concentration (G ∼ cn, n=0.1-0.4), a dependence that seems to be unique of CNT pores. To understand and quantify electro-osmotic flow in CNT pores (which has been proposed as the transport mode responsible for the giant ionic currents in CNTs), we investigated translocation of neutral molecules in a single CNT nanochannel with the resistive pulse technique. Furthermore, we studied the effect of solution pH, matrix type, and CNT diameter on the magnitude and the on the electrolyte concentration based power law dependence of the conductance. Implications of our findings on the physics of electric-field-driven ionic and molecular transport in CNTs will be presented here.