We report gas and water flow measurements through microfabricated membranes in which aligned carbon nanotubes with diameters of less than 2 nanometers serve as pores. The measured gas flow exceeds predictions of the Knudsen diffusion model by more than an order of magnitude. The measured water flow exceeds values calculated from continuum hydrodynamics models by more than three orders of magnitude and is comparable to flow rates extrapolated from molecular dynamics simulations. The gas and water permeabilities of these nanotube-based membranes are several orders of magnitude higher than those of commercial polycarbonate membranes, despite having pore sizes an order of magnitude smaller. These membranes enable fundamental studies of mass transport in confined environments, as well as more energy-efficient nanoscale filtration.

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...

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