Anomalous Ionic Conductance in Carbon Nanotube Nanochannels

Abstract

Simulations and experimental studies have reported an unusually high ionic conductance in carbon nanotube (CNT) 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 nanofluidic platform having a >30-nm wide, FIB-nanomachined silicon nitride nanopore (SiNx) in series with vertically-aligned sub-5 nm carbon nanotubes, 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. A few literature reports attributed this giant ionic current in CNTs to a strong electro-osmotic flow. To quantify electro-osmosis in CNT pores, we investigated translocation of neutral molecules in a single CNT nanochannel with the resistive pulse technique. Furthermore, we employed finite element analysis to elucidate the relationship between CNT pore characteristics (e.g., size, surface charge, slip length) and the resulting ionic transport and magnitude of the electro-osmotic component. We confirmed computationally that the unusually large conductance in our experimental platform results from the presence of a strong electro-osmotic coupling between the CNT channel and the SiNx nanopore in series with it. Finally, we apply first-principle simulations to show that cation-CNT interactions may help explain the origin of this electro-osmotic flow.