Carbon Nanotube Porins Research Articles

Molecular transport enhancement in pure metallic carbon nanotube porins.

Nanofluidic channels impose extreme confinement on water and ions, giving rise to unusual transport phenomena strongly dependent on the interactions at the channel-wall interface. Yet how the electronic properties of the nanofluidic channels influence transport efficiency remains largely unexplored. Here we measure transport through the inner pores of sub-1 nm metallic and semiconducting carbon nanotube porins. We find that water and proton transport are enhanced in metallic nanotubes over semiconducting nanotubes, whereas ion transport is largely insensitive to the nanotube bandgap value. Molecular simulations using polarizable force fields highlight the contributions of the anisotropic polarizability tensor of the carbon nanotubes to the ion-nanotube interactions and the water friction coefficient. We also describe the origin of the proton transport enhancement in metallic nanotubes using deep neural network molecular dynamics simulations. These results emphasize the complex role of the electronic properties of nanofluidic channels in modulating transport under extreme nanoscale confinement.

Sonochemical Synthesis and Ion Transport Properties of Surfactant-Stabilized Carbon Nanotube Porins.

Carbon nanotube porins (CNTPs), short segments of carbon nanotubes stabilized by a lipid coating, are a promising example of artificial membrane channels that mimic a number of key behaviors of biological ion channels. While the lipid-assisted synthesis of CNTPs may facilitate their subsequent incorporation into lipid bilayers, it limits the applicability of these pores in other self-assembled membrane materials and also precludes the use of large-scale purified CNT feedstocks. Here we demonstrate that CNTPs can be synthesized by sonochemical cutting of long CNT feedstocks in the presence of different surfactants, producing CNTS with transport properties identical with those obtained by the lipid-assisted procedure. Our results open up a wide variety of synthetic routes for CNTP production.

Electroneutrality Breakdown and Perm-Selective End Groups in Carbon Nanotube Porins

Sub-nanometer carbon nanotube porins (n-CNTP) have been shown to achieve high water permeation with significant permselectivity precluding anion transport. Therefore, n-CNTPs are useful model systems to probe and optimize the nanoscale transport phenomena relevant for selective ionic separations. Surprisingly, these n-CNTPs demonstrate a unique conductance scaling behavior with ionic concentration, which has led to various continuum models being constructed in pursuit of determining the underpinning physics governing nanoscale ionic transport. Here, we formulate a continuum model to capture the conductance and permselectivity of n-CTNPs considering the breakdown of electroneutrality within the pore. Electroneutrality breakdown occurs when the ionic charge within the pore is not balanced by the charge on the pore walls. While electroneutrality is commonly assumed in the analysis of nanopore transport, we predict a departure from electroneutrality due to the extreme one-dimensional confinement in n-CNTPs, strongly influencing the conductance and selectivity of the pore. Furthermore, our analysis demonstrates the importance of the entrance groups in modulating ion transport through n-CNTPs. Lastly, we provide a framework for future refinements to incorporate additional microscopic physics into the phenomenological model.

Probing the Ion Transport Properties of Ultrashort Carbon Nanotubes Integrated with Supported Lipid Bilayers via Electrochemical Analysis.

Supported lipid bilayers (SLBs) are commonly used to investigate interactions between cell membranes and their environment. These model platforms can be formed on electrode surfaces and analyzed using electrochemical methods for bioapplications. Carbon nanotube porins (CNTPs) integrated with SLBs have emerged as promising artificial ion channel platforms. In this study, we present the integration and ion transport characterization of CNTPs in in vivo environments. We combine experimental and simulation data obtained from electrochemical analysis to analyze the membrane resistance of the equivalent circuits. Our results show that carrying CNTPs on a gold electrode results in high conductance for monovalent cations (K+ and Na+) and low conductance for divalent cations (Ca2+).

Co‐Assembly of Carbon Nanotube Porins into Biomimetic Peptoid Membranes (Small 21/2023)

Biomimetic Peptoid Membranes Co-assembly of carbon nanotube porins into peptoid sheets creates novel precision pore membrane nanostructures. The membrane matrix is made up of peptoids, self-assembled designed sequence polymers, while carbon nanotube porins, short fragments of carbon nanotubes inserted into the peptoid matrix, form precision pores. This new membrane could be advantageous for high-value precision separation applications. More details can be found in article number 2206810 by Marcel Baer, Aleksandr Noy, Chun-Long Chen, and co-workers.

Co-Assembly of Carbon Nanotube Porins into Biomimetic Peptoid Membranes.

Robust and cost-effective membrane-based separations are essential to solving many global crises, such as the lack of clean water. Even though the current polymer-based membranes are widely used for separations, their performance and precision can be enhanced by using a biomimetic membrane architecture that consists of highly permeable and selective channels embedded in a universal membrane matrix. Researchers have shown that artificial water and ion channels, such as carbon nanotube porins (CNTPs), embedded in lipid membranes can deliver strong separation performance. However, their applications are limited by the relative fragility and low stability of the lipid matrix. In this work,wedemonstrate that CNTPs can co-assemble into twodimension(2D) peptoid membrane nanosheets, opening up a way to produce highly programmable synthetic membranes with superior crystallinity and robustness. A combination of molecular dynamics (MD) simulations, Ramanspectroscopy, X-ray diffraction(XRD), and atomicforcemicroscopy(AFM) measurements to verify the co-assembly of CNTP and peptoids are used and show that it does not disrupt peptoid monomer packing within the membrane. These results provide a new option for designing affordable artificial membranes and highly robust nanoporous solids.

Nanofluidics: Transport in chirality-controlled carbon nanotube porins (CNTPs) and fluorescence ultrashort nanotubes (FUNs).

Carbon nanotubes (CNTs) with different chirality are great models to study the influence of channel electronic properties on ion transport through nanofluidic channels. We prepared short chirality-separated pure (6,5), (7,4), and (7,5)/(8,4) CNT porins (CNTPs), which have different electronic properties but are almost identical in diameter. Single channel conductance from potassium transport through these CNTPs indicates that electronic properties have a weak effect on ion transport. Sodium and lithium ion transport through different chirality CNTPs have also been studied. Chemical functionalities and defects on the CNTP walls can also influence transport properties. To address that possibility, we measured ion transport through fluorescence ultrashort nanotubes (FUNs) - functionalized channels with defective walls dispersed by surfactants such as CHAPS and SDOC. These channels have significantly lower conductance compared to the pristine (6,5) CNTPs. Finally, we discuss other factors that influence conductance in these nanofluidic channels.

Breakdown of the Nernst-Einstein relation in carbon nanotube porins.

For over 100 years, the Nernst-Einstein relation has linked a charged particle's electrophoretic mobility and diffusion coefficient. Here we report experimental measurements of diffusion and electromigration of K+ ions in narrow 0.8-nm-diameter single-walled carbon nanotube porins (CNTPs) and demonstrate that the Nernst-Einstein relation in these channels breaks down by more than three orders of magnitude. Molecular dynamics simulations using polarizable force fields show that K+ ion diffusion in CNTPs in the presence of a single-file water chain is three orders of magnitude slower than bulk diffusion. Intriguingly, the simulations also reveal a disintegration of the water chain upon application of electric fields, resulting in the formation of distinct K+-water clusters, which then traverse the CNTP at high velocity. Finally, we show that although individual ion-water clusters still obey the Nernst-Einstein relation, the overall relation breaks down because of two distinct mechanisms for ion diffusion and electromigration.

Nanoporous Membranes of Densely Packed Carbon Nanotubes Formed by Lipid-Mediated Self-Assembly.

Nanofiltration technology faces the competing challenges of achieving high fluid flux through uniformly narrow pores of a mechanically and chemically stable filter. Supported dense-packed 2D-crystals of single-walled carbon nanotube (CNT) porins with ∼1 nm wide pores could, in principle, meet these challenges. However, such CNT membranes cannot currently be synthesized at high pore density. Here, we use computer simulations to explore lipid-mediated self-assembly as a route toward densely packed CNT membranes, motivated by the analogy to membrane-protein 2D crystallization. In large-scale coarse-grained molecular dynamics (MD) simulations, we find that CNTs in lipid membranes readily self-assemble into large clusters. Lipids trapped between the CNTs lubricate CNT repacking upon collisions of diffusing clusters, thereby facilitating the formation of large ordered structures. Cluster diffusion follows the Saffman-Delbrück law and its generalization by Hughes, Pailthorpe, and White. On longer time scales, we expect the formation of close-packed CNT structures by depletion of the intervening shared annular lipid shell, depending on the relative strength of CNT-CNT and CNT-lipid interactions. Our simulations identify CNT length, diameter, and end functionalization as major factors for the self-assembly of CNT membranes.

An Ideal Artificial Water Channel—Carbon Nanotube Porins (Cntps) for Preparing Highly Permeable Reverse Osmosis Membrane with Excellent Antifouling Capacity

An Ideal Artificial Water Channel—Carbon Nanotube Porins (Cntps) for Preparing Highly Permeable Reverse Osmosis Membrane with Excellent Antifouling Capacity

Putting together the puzzle of ion transfer in single-digit carbon nanotubes: mean-field meets ab initio.

Nature employs channel proteins to selectively pass water across cell membranes, which inspires the search for bio-mimetic analogues. Carbon nanotube porins (CNTPs) are intriguing mimics of water channels, yet ion transport in CNTPs still poses questions. As an alternative to continuum models, here we present a molecular mean-field model that transparently describes ion coupling, yet unlike continuum models, computes ab initio all required thermodynamic quantities for the KCl salt and H+ and OH- ions present in water. Starting from water transfer, the model considers the transfer of free ions, along with ion-pair formation as a proxy of non-mean-field ion-ion interactions. High affinity to hydroxide, suggested by experiments, making it a dominant charge carrier in CNTPs, is revealed as an exceptionally favorable transfer of KOH pairs. Nevertheless, free ions, coexisting with less mobile ion-pairs, apparently control ion transport. The model well explains the observed effects of salt concentration and pH on conductivity, transport numbers, anion permeation and its activation energies, and current rectification. The proposed approach is extendable to other sub-nanochannels and helps design novel osmotic materials and devices.

An Ideal Artificial Water Channel—Carbon Nanotube Porins (Cntps) for Preparing Highly Permeable Reverse Osmosis Membrane with Excellent Antifouling Capacity

An Ideal Artificial Water Channel—Carbon Nanotube Porins (Cntps) for Preparing Highly Permeable Reverse Osmosis Membrane with Excellent Antifouling Capacity

Electric Field Induced Biomimetic Transmembrane Electron Transport Using Carbon Nanotube Porins (Small 32/2021)

Carbon Nanotube Porins In article number 2102517, Frankie J. Rawson and co-workers report the use of carbon nanotube porins (CNTPs) as voltage dependant-anion channel mimics. CNTPs are inserted into the membrane of neuronal cells. On applying an electric field, gold ions are reduced at the membrane interface containing the CNTPs via low voltage bipolar electrochemistry. Image credit: Dr. Leonora Martínez Núñez.

Electric Field Induced Biomimetic Transmembrane Electron Transport Using Carbon Nanotube Porins.

Cells modulate their homeostasis through the control of redox reactions via transmembrane electron transport systems. These are largely mediated via oxidoreductase enzymes. Their use in biology has been linked to a host of systems including reprogramming for energy requirements in cancer. Consequently, the ability to modulate membrane redox systems may give rise to opportunities to modulate underlying biology. The current work aims to develop a wireless bipolar electrochemical approach to form on-demand electron transfer across biological membranes. To achieve this goal, it is shown that by using membrane inserted carbon nanotube porins (CNTPs) that can act as bipolar nanoelectrodes, one can control electron flow with externally applied electric fields across membranes. Before this work, bipolar electrochemistry has been thought to require high applied voltages not compatible with biological systems. It is shown that bipolar electrochemical reaction via gold reduction at the nanotubes can be modulated at low cell-friendly voltages, providing an opportunity to use bipolar electrodes to control electron flux across membranes. The authors provide new mechanistic insight into this newly describe phenomena at the nanoscale. The results presented give rise to a new method using CNTPs to modulate cell behavior via wireless control of membrane electron transfer.

Electrostatic gating of ion transport in carbon nanotube porins: A modeling study.

Carbon nanotube porins (CNTPs) are biomimetic membrane channels that demonstrate excellent biocompatibility and unique water and ion transport properties. Gating transport in CNTPs with external voltage could increase control over ion flow and selectivity. Herein, we used continuum modeling to probe the parameters that enable and further affect CNTP gating efficiency, including the size and composition of the supporting lipid membrane, slip flow in the carbon nanotube, and the intrinsic electronic properties of the nanotube. Our results show that the optimal gated CNTP device consists of a semiconducting CNTP inserted into a small membrane patch containing an internally conductive layer. Moreover, we demonstrate that the ionic transport modulated by gate voltages is controlled by the charge distribution along the CNTP under the external gate electric potential. The theoretical understanding developed in this study offers valuable guidance for the design of gated CNTP devices for nanofluidic studies, novel biomimetic membranes, and cellular interfaces in the future.

Membrane fusion and drug delivery with carbon nanotube porins

Drug delivery mitigates toxic side effects and poor pharmacokinetics of life-saving therapeutics and enhances treatment efficacy. However, direct cytoplasmic delivery of drugs and vaccines into cells has remained out of reach. We find that liposomes studded with 0.8-nm-wide carbon nanotube porins (CNTPs) function as efficient vehicles for direct cytoplasmic drug delivery by facilitating fusion of lipid membranes and complete mixing of the membrane material and vesicle interior content. Fusion kinetics data and coarse-grained molecular dynamics simulations reveal an unusual mechanism where CNTP dimers tether the vesicles, pull the membranes into proximity, and then fuse their outer and inner leaflets. Liposomes containing CNTPs in their membranes and loaded with an anticancer drug, doxorubicin, were effective in delivering the drug to cancer cells, killing up to 90% of them. Our results open an avenue for designing efficient drug delivery carriers compatible with a wide range of therapeutics.

Carbon nanotube porin diffusion in mixed composition supported lipid bilayers

Carbon nanotube porins (CNTPs), short pieces of carbon nanotubes capable of self-inserting into a lipid bilayer, represent a simplified model of biological membrane channels. We have used high-speed atomic force microscopy (HS-AFM) and all-atom molecular dynamics (MD) simulations to study the behavior of CNTPs in a mixed lipid membrane consisting of DOPC lipid with a variable percentage of DMPC lipid added to it. HS-AFM data reveal that the CNTPs undergo diffusive motion in the bilayer plane. Motion trajectories extracted from the HS-AFM movies indicate that CNTPs exhibit diffusion coefficient values broadly similar to values reported for membrane proteins in supported lipid bilayers. The data also indicate that increasing the percentage of DMPC leads to a marked slowing of CNTP diffusion. MD simulations reveal a CNTP-lipid assembly that diffuses in the membrane and show trends that are consistent with the experimental observations.

Strong Differential Monovalent Anion Selectivity in Narrow Diameter Carbon Nanotube Porins

Inner pores of carbon nanotubes combine extremely fast water transport and ion selectivity that could potentially be useful for high-performance water desalination and separation applications. We used dye-quenching halide assays and stopped-flow spectrometry to determine intrinsic permeability of three small monovalent halide anions (chloride, bromide, iodide) and one pseudohalide anion (thiocyanate) through narrow 0.8 nm diameter carbon nanotube porins (CNTPs). These measurements revealed unexpectedly strong differential ion selectivity with permeabilities of different ions varying by up to 2 orders of magnitude. Removal of the negative charge from the nanotube entrance increased anion permeability by only a relatively small factor, indicating that electrostatic repulsion was not a major determinant of CNTP selectivity. First principle molecular dynamics simulations revealed that the origin of this strong differential ion selectivity is partial dehydration of anions upon entry into the narrow CNTP channels.

Lipid coating and end functionalization govern the formation and stability of transmembrane carbon nanotube porins

Carbon nanotube (CNT) based transmembrane porins have attracted a lot of recent interest due to their excellent transport properties for water, ions and biomolecules. In experiments, CNTs with a length of about 10 nm can penetrate and form stable transmembrane channels across lipid membranes, while in sharp contrast, similar 10 nm long pristine CNTs were found parallelly clamped inside the hydrophobic core of a lipid bilayer in simulations. This apparent paradox has motivated the present study, where we performed all-atom molecular dynamics simulations to show that lipid molecules initially coated on the CNTs play an essential role in the formation and stabilization of the CNT porins. We demonstrate that, as a lipid coated CNT is inserted into the membrane, lipid molecules arrange themselves into a bicelle on the CNT outside the membrane, providing mechanical support to stabilize the transmembrane configuration of the CNT porin. Further analysis shows that the lipid coating density and end-functionalization strongly influence the CNT insertion into a lipid membrane. These findings shed light on the mechanism of formation and stabilization of CNT porins and provide a theoretical basis to design related transmembrane transport systems.

Strong Electroosmotic Coupling Dominates Ion Conductance of 1.5 nm Diameter Carbon Nanotube Porins.

Extreme confinement in nanometer-sized channels can alter fluid and ion transport in significant ways, leading to significant water flow enhancement and unusual ion correlation effects. These effects are especially pronounced in carbon nanotube porins (CNTPs) that combine strong confinement in the inner lumen of carbon nanotubes with the high slip flow enhancement due to smooth hydrophobic pore walls. We have studied ion transport and ion selectivity in 1.5 nm diameter CNTPs embedded in lipid membranes using a single nanopore measurement setup. Our data show that CNTPs are weakly cation selective at pH 7.5 and become nonselective at pH 3.0. Ion conductance of CNTPs exhibits an unusual 2/3 power law scaling with the ion concentration at both neutral and acidic pH values. Coupled Navier-Stokes and Poisson-Nernst-Planck simulations and atomistic molecular dynamics simulations reveal that this scaling originates from strong coupling between water and ion transport in these channels. These effects could result in development of a next generation of biomimetic membranes and carbon nanotube-based electroosmotic pumps.