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...

Advances in the synthesis and scalable manufacturing of single-walled carbon nanotubes (SWCNTs) remain critical to realizing many important commercial applications. Here we review recent breakthroughs in the synthesis of SWCNTs and highlight key ongoing research areas and challenges. A few key applications that capitalize on the properties of SWCNTs are also reviewed with respect to the recent synthesis breakthroughs and ways in which synthesis science can enable advances in these applications. While the primary focus of this review is on the science framework of SWCNT growth, we draw connections to mechanisms underlying the synthesis of other 1D and 2D materials such as boron nitride nanotubes and graphene.

Nanofluidics in carbon nanotubes Extremely high aspect ratios, molecularly smooth hydrophobic graphitic walls, and nanoscale inner diameters of carbon nanotubes give rise to the unique phenomenon of ultra-efficient transport of water and gas through these ultra-narrow molecular pipes. Water and gas molecules move through nanotube pores orders of magnitude faster than through other pores of comparable size. The proposed water transport mechanism has a distinct similarity to the transport mechanisms of biological ion channels. Molecular dynamics simulations and experimental measurements of water transport underscore the importance of nanotube structure in enabling ultra-efficient transport through the pore.

Small-diameter carbon nanotubes (CNTs) are shown to enable exceptionally fast transport of water vapor under a concentration gradient driving force. Thanks to this property, membranes having sub-5 nm CNTs as conductive pores feature outstanding breathability while maintaining a high degree of protection from biothreats by size exclusion.

A nanopore decorated with crown ether and DNA is selective to potassium ions over sodium ions at concentrations up to 1 M.

We have investigated growth kinetics of multiwall carbon nanotube (MWCNT) arrays produced by catalytic thermal decomposition of ethylene gas in hydrogen, water, and argon mixture. The MWCNT growth rate exhibits a nonmonotonic dependence on total pressure and reaches a maximum at approximately 750 Torr of total pressure. Water concentrations in excess of 3000 ppm lead to the decrease in the observed growth rate. Optimal pressure and water concentration combination results in a reliable growth of well-aligned MWCNT arrays at a maximum growth rate of approximately 30 microm/min. These MWCNT arrays can reach heights of up to 1 mm with typical standard deviations for the array height of less than 8% over a large number of process runs spread over the time of 8 months. Nanotube growth rate in this optimal growth region remains essentially constant until growth reaches an abrupt and irreversible termination. We present a quantitative model that shows how accumulation of the amorphous carbon patches at the catalyst particle surface and the carbon diffusion to the growing nanotube perimeter causes this abrupt growth cessation. The influence of the partial pressures of ethylene and hydrogen on the ethylene decomposition driving force explains the nonlinear behavior of the growth rate as a function of total process pressure.

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The selectivity of ion transport in nanochannels is of primary importance for a number of physical, chemical, and biological processes ranging from fluid separation to ion-channel-regulated cellular processes. Fundamental understanding of these phenomena requires model nanochannels with well-defined and controllable structural properties. Carbon nanotubes provide an ideal choice for nanofluidic studies because of their simple chemistry and structure, the atomic scale smoothness and chemical inertness of the graphitic walls, and the tunability of their diameter and length. Here, we investigate the selectivity of single and, for the first time, binary salt mixtures transport through narrow carbon nanotubes that act as the only pores in a silicon nitride membrane. We demonstrate that negatively charged carboxylic groups are responsible for the ion rejection performance of carbon nanotube pores and that ion permeation of small salts can be tuned by varying solution pH. Investigation of the effect of solution composition and ion valences for binary electrolytes with common cation in a pressure-driven flow reveals that the addition of slower diffusing multivalent anions to a solution of faster diffusing monovalent anions favors permeation of the monovalent anion. Larger fractions and valences of the added multivalent anions lower the rejection of the monovalent anion. In some cases, we observe negative rejection at low monovalent ion content.