Ion exclusion by sub-2-nm carbon nanotube pores

Abstract

Biological pores regulate the cellular traffic of a large variety of solutes, often with high selectivity and fast flow rates. These pores share several common structural features: the inner surface of the pore is frequently lined with hydrophobic residues, and the selectivity filter regions often contain charged functional groups. Hydrophobic, narrow-diameter carbon nanotubes can provide a simplified model of membrane channels by reproducing these critical features in a simpler and more robust platform. Previous studies demonstrated that carbon nanotube pores can support a water flux comparable to natural aquaporin channels. Here, we investigate ion transport through these pores using a sub-2-nm, aligned carbon nanotube membrane nanofluidic platform. To mimic the charged groups at the selectivity region, we introduce negatively charged groups at the opening of the carbon nanotubes by plasma treatment. Pressure-driven filtration experiments, coupled with capillary electrophoresis analysis of the permeate and feed, are used to quantify ion exclusion in these membranes as a function of solution ionic strength, pH, and ion valence. We show that carbon nanotube membranes exhibit significant ion exclusion that can be as high as 98% under certain conditions. Our results strongly support a Donnan-type rejection mechanism, dominated by electrostatic interactions between fixed membrane charges and mobile ions, whereas steric and hydrodynamic effects appear to be less important.biomimetic platform ͉ ion channel ͉ ion transport ͉ nanofiltration I on transport across cellular membranes is essential to many of life's processes, such as electrical signaling in nerves, muscles, and synapses or cell's maintenance of homeostatic balance. Biological systems achieve rapid, selective, and ultraefficient transmembrane mass transport by employing a large variety of specialized protein channels of nanometer or subnanometer size (1). High-resolution x-ray structures, protein sequencing, targeted mutations, and biophysical characterizations have provided new insight on the link between nanochannel protein architecture, transport rates, selectivity, and gating properties. Interestingly, these studies have shown that membrane nanochannels share several common features. For example, aquaporins (2, 3), proton channels (4, 5), and ion channels (6-11) all have relatively narrow and hydrophobic pore regions. By contrast, the selectivity filter regions of membrane ion channels are enriched with charged residues.Despite the enormous progress made in recent decades, the complex macromolecular nature of these biological machines still complicates our understanding of the underlying mechanisms responsible for fast mass transport, selectivity, gating, and the functional role of hydrophobic pore lining and charged functionalities. Thus, it is important to create simplified, biomimetic nanochannels that could help to clarify the physics of ion permeation at the nanoscale, as well as create the next generation of membranes that employ efficient molecul...

Characterization and evaluation of carbon nanotube Bucky-Paper membranes for direct contact membrane distillation

“…For Knüdsen numbers, K n > 1, the molecule -pore wall collisions dominate leading to Knüdsen type flow: For isoporous membranes with circular pores the permeance is given by [7][8][9][10][11][12][13][14][15][16][17]:…”

“…This has stimulated intense interest in CNT based membranes of various structures [8][9][10][11]. In this work we focus on CNT Bucky-Paper (BP) membranes, which is a paper-like structure of CNTs processed by vacuum filtration [12].…”

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

“…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. For this reason, a large number of molecular simulations have demonstrated interesting diameterdependent phenomena such as ion selectivity 15,22 , rapid proton conduction 18,23 that can be caused by the unique water structures and geometric confinements [24][25][26] , and altered water-ice phase behaviour 27 .…”

Two-dimensional covalent triazine framework as an ultrathin-film nanoporous membrane for desalination

“…Despite CNTs' high water permeability, 5 membranes based on CNTs exhibit poor salt rejection, and it remains challenging to fabricate membranes with dense, well--aligned nanotubes. 6,7 Zeolite membranes, on the other hand, possess three--dimensional networks that can effectively reject salt ions 8 but show relatively low water permeability. 9 Recently, nanoporous one--atom--thick graphene, the thinnest possible membrane made from matter, has drawn considerable attention and been shown computationally to provide significantly enhanced permeability compared to polyamide while maintaining excellent ability to reject salt.…”