Origin of Giant Ionic Currents in Carbon Nanotube Channels

Abstract

Fluid flow inside carbon nanotubes is remarkable: transport of water and gasses is nearly frictionless and the small channel size results in selective transport of ions. Very recently, devices have been fabricated in which one narrow single-walled carbon nanotube spans a barrier separating electrolyte reservoirs. Ion current through these devices is about two orders of magnitude larger than predicted from the bulk resistivity of the electrolyte. Electroosmosis can drive these large excess currents if the tube is both charged and transports anions or cations preferentially. By building a nanofluidic field-effect transistor with a gate electrode embedded in the fluid barrier, we show that the tube carries a negative charge and the excess current is carried by cations. The magnitude of the excess current, and its control by a gate electrode is correctly predicted by the Poisson-Nernst-Planck-Stokes equations.

Diameter-dependent ion transport through the interior of isolated single-walled carbon nanotubes

“…5d, builds upon these studies to reproduce the transport maximum that we observe experimentally. We note that previous transport models proposed to explain similar effects 21 do not explicitly contain the diameter dependencies in all parameters. First, we assume a fitted diameter-independent linear charge density on the SWNT, similar to that of previous works 21 , that results in a net positive proton concentration in the SWNT interior.…”

“…We note that previous transport models proposed to explain similar effects 21 do not explicitly contain the diameter dependencies in all parameters. First, we assume a fitted diameter-independent linear charge density on the SWNT, similar to that of previous works 21 , that results in a net positive proton concentration in the SWNT interior. With the much larger aspect ratio, we expect a more linear electric field throughout most of the length of our device than previous studies 21 , resulting in convective proton transport as well as diffusive proton transport if the SWNT is not blocked with a larger cation.…”

“…First, we assume a fitted diameter-independent linear charge density on the SWNT, similar to that of previous works 21 , that results in a net positive proton concentration in the SWNT interior. With the much larger aspect ratio, we expect a more linear electric field throughout most of the length of our device than previous studies 21 , resulting in convective proton transport as well as diffusive proton transport if the SWNT is not blocked with a larger cation. If the protons are unable to interact directly with the SWNT walls, the energy being dissipated at steady state from the electric field must be dissipated to the surrounding fluid in the SWNT.…”

“…Examples include silicon-based nanopores [2][3][4][5][6][7] , which achieve selectivity primarily through size-exclusion of macromolecules, and biological nanopores [8][9][10][11][12] , which manipulate their shape or present a specific pore opening to allow the passage of only specific ions such as potassium 8 , calcium 9 or sodium 9 . Singlewalled carbon nanotubes (SWNTs) are promising candidates for nanopore studies [13][14][15][16][17][18][19][20][21] as they can match the well-defined diameter and high aspect ratios of traditional silicon nanopores with sub-2 nm diameters and hydrophobic interior that allows for ion selectivity. These properties potentially allow for interesting desalination 22 and DNA-sequencing applications 13 , as well as the ability to probe basic fluid structure properties at the smallest of possible scales.…”

“…In follow-up work, we were able to further demonstrate pore-blocking phenomena through multiple parallel SWNTs resulting in a stochastic three-state system 32 . Other groups have studied transport through larger collections of SWNTs 22,28,29 or with much shorter lengths 1,21 , but stochastic pore-blocking was not observed under these conditions.…”