Pores Of Carbon Nanotubes Research Articles

Anomalous Ionic Conductance in Carbon Nanotube Nanochannels

Simulations and experimental studies have reported an unusually high ionic conductance in carbon nanotube (CNT) nanochannels. The origin of this phenomenon is, however, poorly-understood: literature reports often disagree in the magnitude of the different transport mode contributions to the measured ionic current and even in what ions are actually carrying the current; moreover, results obtained with single pore measurements differ frequently from those with membranes containing billions of open CNT channels, i.e. the average CNT behavior. Toward shedding light on these phenomena, we fabricated a novel platform having vertically-aligned sub-5nm carbon nanotubes as nanofluidic channels, the number of which can be controlled from one to billions. By employing this platform with only one or a few open CNTs, we observed giant ionic currents in CNT channels and a power-law increase of conductance with KCl concentration (G ∼ cn, n=0.1-0.4), a dependence that seems to be unique of CNT pores. To understand and quantify electro-osmotic flow in CNT pores (which has been proposed as the transport mode responsible for the giant ionic currents in CNTs), we investigated translocation of neutral molecules in a single CNT nanochannel with the resistive pulse technique. Furthermore, we used first-principle simulations to show that cation-CNT interactions may explain the origin of this electro-osmotic flow. Implications of our findings on the physics of electric-field-driven ionic and molecular transport in CNTs will be presented here.

Molecular dynamics simulation of seawater reverse osmosis desalination using carbon nanotube membranes

Reverse osmosis (RO) desalination of 35,000 ppm seawater consisting of sodium and chlorine ions with water was simulated using molecular dynamics. In this study, RO simulations were performed at different pressures using carbon nanotube (CNT) membranes having varying pore diameters (8–14 Å). Results showed that a CNT membrane allowed high water fluxes because of the presence of a low energy barrier for water–carbon interactions, with continuous and ordered water chains observed inside the CNTs. The investigations into the ion rejection capability of CNT membranes revealed that (6, 6) CNTs could provide high rejection rates (>95%). The rejection of ions was mainly due to the presence of a high energy barrier caused by removing the hydrating water molecules necessary for ions to pass through the narrow CNT pore. The desalination performance of (6, 6) CNT membranes and conventional polymeric membranes was then compared. It was found that CNT membranes have a higher normalized flux and comparable ion rejection, indicating their feasibility for effective seawater desalination. Overall, this study demonstrated the usefulness of molecular dynamics in investigating membrane processes and its potential role in improving desalination performance.

Sequential protein unfolding through a carbon nanotube pore.

An assortment of biological processes, like protein degradation and the transport of proteins across membranes, depend on protein unfolding events mediated by nanopore interfaces. In this work, we exploit fully atomistic simulations of an artificial, CNT-based nanopore to investigate the nature of ubiquitin unfolding. With one end of the protein subjected to an external force, we observe non-canonical unfolding behaviour as ubiquitin is pulled through the pore opening. Secondary structural elements are sequentially detached from the protein and threaded into the nanotube, interestingly, the remaining part maintains native-like characteristics. The constraints of the nanopore interface thus facilitate the formation of stable "unfoldon" motifs above the nanotube aperture that can exist in the absence of specific native contacts with the other secondary structure. Destruction of these unfoldons gives rise to distinct force peaks in our simulations, providing us with a sensitive probe for studying the kinetics of serial unfolding events. Our detailed analysis of nanopore-mediated protein unfolding events not only provides insight into how related processes might proceed in the cell, but also serves to deepen our understanding of structural arrangements which form the basis for protein conformational stability.

In Situ TEM Observation of Electrochemical Lithiation of Sulfur Confined within Inner Cylindrical Pores of Carbon Nanotubes

Lithium insertion into sulfur confined within 200 nm cylindrical inner pores of individual carbon nanotubes (CNTs) was monitored in situ in a transmission electron microscope (TEM). This electrochemical reaction was initiated at one end of the S‐filled CNTs. The material expansion during lithiation was accommodated by the expansion into the remaining empty pore volume and no fracture of the CNT walls was detected. A sharp interface between the initial and lithiated S was observed. The reaction front was flat, oriented perpendicular to the confined S cylinder, and propagated along the cylinder length. Lithiation of S in the proximity of conductive carbon proceeded at the same rate as the one in the center of the pore, suggesting the presence of electron pathways at the Li2S/S interface. Density of states calculations further confirmed this hypothesis. In situ electron diffraction showed a direct phase transformation of S into nanocrystalline Li2S without detectable formation of any intermediates, such as polysulfides and LiS. These important insights may elucidate some of the reaction mechanisms and guide the improvements in the design of C–S nanocomposites for high specific energy Li–S batteries. The proposed use of conductive CNTs with tunable pore diameter as cylindrical reaction vessels for in situ TEM studies of electrochemical reactions proved to be highly advantageous and may help to resolve the ongoing problems in battery technology.

Correlation and prediction of adsorption capacity and affinity of aromatic compounds on carbon nanotubes

Adsorption of 22 nonpolar and polar aromatic compounds on 10 carbon nanotubes (CNTs) with various diameters, lengths and surface oxygen-containing group contents was investigated to develop predictive correlations for adsorption, using the isotherm fitting of Polanyi theory-based Dubinin-Ashtakhov (DA) model. Adsorption capacity of aromatic compounds on CNTs is negatively correlated with melting points of aromatic compounds, and surface oxygen-containing group contents and surface area ratios of mesopores to total pores of CNTs, but positively correlated with total surface area of CNTs. Adsorption affinity is positively correlated with solvatochromic parameters of aromatic compounds, independent of tube lengths and surface oxygen-containing group contents of CNTs, but negatively correlated with surface area ratios of mesopores to total pores of CNTs. The correlations of adsorption capacity and adsorption affinity with properties of both aromatic compounds and CNTs clearly have physical significance, can be used successfully with DA model to predict adsorption of aromatic compounds on CNTs from the well-known physiochemical properties of aromatic compounds (i.e., solvatochromic parameters, melting points) and CNTs (i.e., surface area and total acidic group contents), and thus can facilitate the environmental application of CNTs as sorbents and environmental risk assessment of both aromatic contaminants and CNTs.

Nanoscale Bubble Valves on MWCNT Membranes for Chemical Energy Storage

Nanoscale Bubble Valves on MWCNT Membranes for Chemical Energy Storage

Nanofluidic Transport through Isolated Carbon Nanotube Channels: Advances, Controversies, and Challenges.

Owing to their simple chemistry and structure, controllable geometry, and a plethora of unusual yet exciting transport properties, carbon nanotubes (CNTs) have emerged as exceptional channels for fundamental nanofluidic studies, as well as building blocks for future fluidic devices that can outperform current technology in many applications. Leveraging the unique fluidic properties of CNTs in advanced systems requires a full understanding of their physical origin. Recent advancements in nanofabrication technology enable nanofluidic devices to be built with a single, nanometer-wide CNT as a fluidic pathway. These novel platforms with isolated CNT nanochannels offer distinct advantages for establishing quantitative structure-transport correlations in comparison with membranes containing many CNT pores. In addition, they are promising components for single-molecule sensors as well as for building nanotube-based circuits wherein fluidics and electronics can be coupled. With such advanced device architecture, molecular and ionic transport can be manipulated with vastly enhanced control for applications in sensing, separation, detection, and therapeutic delivery. Recent achievements in fabricating isolated-CNT nanofluidic platforms are highlighted, along with the most-significant findings each platform enables for water, ion, and molecular transport. The implications of these findings and remaining open questions on the exceptional fluidic properties of CNTs are also discussed.

Giant Conductance and Anomalous Concentration Dependence in Sub-5 nm Carbon Nanotube Nanochannels

Simulations and experimental studies have reported an unusually high ionic conductance in carbon nanotubes (CNTs) nanochannels. The origin of this phenomenon is, however, poorly-understood: literature reports often disagree in the magnitude of the different transport mode contributions to the measured ionic current and even in what ions are actually carrying the current; moreover, results obtained with single pore measurements differ frequently from those with membranes containing billions of open CNT channels, i.e. the average CNT behavior. Toward shedding light on these phenomena, we fabricated a novel platform having vertically-aligned sub-5nm carbon nanotubes as nanofluidic channels, the number of which can be controlled from one to billions. By employing this platform with only one or a few open CNTs, we observed giant ionic currents in CNT channels and a power-law increase of conductance with KCl concentration (G ∼ cn, n=0.1-0.4), a dependence that seems to be unique of CNT pores. To understand and quantify electro-osmotic flow in CNT pores (which has been proposed as the transport mode responsible for the giant ionic currents in CNTs), we investigated translocation of neutral molecules in a single CNT nanochannel with the resistive pulse technique. Furthermore, we studied the effect of solution pH, matrix type, and CNT diameter on the magnitude and the on the electrolyte concentration based power law dependence of the conductance. Implications of our findings on the physics of electric-field-driven ionic and molecular transport in CNTs will be presented here.

Temperature-dependent gas transport performance of vertically aligned carbon nanotube/parylene composite membranes.

A novel composite membrane consisting of vertically aligned carbon nanotubes (CNTs) and parylene was successfully fabricated. Seamless filling of the spaces in CNT forests with parylene was achieved by a low-pressure chemical vapor deposition (CVD) technique and followed with the Ar/O2 plasma etching to expose CNT tips. Transport properties of various gases through the CNT/parylene membranes were explored. And gas permeances were independent on feed pressure in accordance with the Knudsen model, but the permeance values were over 60 times higher than that predicted by the Knudsen diffusion kinetics, which was attributed to specular momentum reflection inside smooth CNT pores. Gas permeances and enhancement factors over the Knudsen model firstly increased and then decreased with rising temperature, which confirmed the existence of non-Knudsen transport. And surface adsorption diffusion could affect the gas permeance at relatively low temperature. The gas permeance of the CNT/parylene composite membrane could be improved by optimizing operating temperature.

A study of interaction potentials for H 2 adsorption in Single Walled Nano Tubes: a possible way to more realistic predictions

A comparative analysis of interaction potentials, classified according to the parametrization method, namely Lorentz-Berthelot rules, semi-empirical or ab initio calculations, found their energy depths to scale, respectively, to ca 30K, ca 40K, and ca 60K. We draw the Potential Energy Surfaces (PESs) for a hydrogen probe molecule inside a Carbon Nano-Tube (CNT): it is shown that the adsorption energy increases with the hard radius of the interaction potential and decreases as the CNT pore enlarges. This is valid just for low-medium pressures, when hydrogen repulsions are negligible. If not, adsorption is driven by H2-H2 hard radius despite all other parameters. Monte Carlo (MC) simulations, following the Gibbs Ensemble (GE) in high density conditions, confirm that the thermodynamic equilibrium of an order-disorder phase transition show no changes throughout any of the studied potentials. We also analyse, in the Grand Canonical (GC) ensemble, the geometric and structural characteristics of square lattice bundles of Single Walled Nano Tubes (SWNTs) with regard to their influence on adsorption storage. To do so, we develop a method for independently simulate inner or outer adsorption in infinitely long nanotube lattice systems. Our results suggest a pressure range for convenient H2 storage and enlighten the influence of CNT size on adsorption performance. In addition, larger CNTs are capable to host further hydrogen layers, but only at very high pressures.

Fenton regeneration of humic acid-spent carbon nanotubes

Carbon nanotubes (CNTs) are known as efficient sorbents for removal of organic compounds from aquatic solutions. In the present study, the efficiency of Fenton solution for regeneration of CNTs exhausted with humic acid was studied. CNTs were produced using chemical vapor deposition method. The molar ratios of Fe2+ to H2O2 as 0.5:10, 1:10, and 2:10 were used. The saturation cycles were continued up to five stages. Results of Fenton regeneration showed higher regeneration efficiency (65.4%) for 2:10M ratio comparing to other molar ratios after five cycles of regeneration. Slight decrease in adsorption capacity after each regeneration cycle was observed which may be due to the deposition of decomposed residues in CNT pores, which blocked the carbon porosity and decreased the specific surface area. In conclusion, the Fenton regeneration especially at higher Fe2+:H2O2 molar ratios showed a possibility as an alternative to chemical and thermal regenerations of CNTs.

Osmotically-Driven Transport through Carbon Nanotube Pores

Ion and water transport through biological membranes by osmosis is essential in many biological processes and the complex nature of biological pores and channels involved in these processes is not fully understood. We are developing simplified robust models of ion channels based on smooth, narrow and hydrophobic pores of carbon nanotubes. Incorporation of these pores into liposomes gives us a convenient system to study osmotically-induced transport using dynamic light scattering. We will present our measurements of the transport of charged and uncharged chemical species through the carbon nanotube pores and also discuss the role of the charges at the nanotube pore ends in regulating the ion transport selectivity.

A Simple, Single-Carbon-Nanotube Nanofludic Platform for Fundamental Transport Studies

Fundamental understanding of ionic and molecular transport phenomena in a simple model nanopore is critical for elucidating function mechanisms of much more complex biological systems, and for advancing technological areas such as membrane separation, energy harvesting/storage, and single molecule detection. For this goal, carbon nanotubes (CNTs) offer key advantages as model nanofluidic channels due to their simple chemical composition and structure (known with atomic precision), robustness, facile length and diameter control, and straightforward local functionalization at their open rim. CNTs have also very interesting fluidics properties such as enhanced pressure-driven fluid transport rates, unusually high electroosmotic flow, and ionic selectivity.Here, we present our work toward developing and validating a novel nanofluidic platform featuring an individual carbon nanotube (CNT) as the flow channel in an advanced Coulter Counter. To fabricate the CNT nanofluidic device, vertically aligned single-walled CNTs are synthesized directly on a suspended silicon nitride membrane and then bound in a solid matrix before an individual CNT is opened by focused ion beam milling. Single-molecule translocation studies with small molecular size analytes suggest the successful fabrication of a Coulter Counter with a-few-nm wide CNT nanochannel. Our initial ionic conductivity studies indicate a power-law increase of conductance with KCl concentration in CNT channels, a dependence that seems to be unique of CNT pores.

Analysis of carbon nanotube shortening and composite strengthening in carbon nanotube/aluminum composites fabricated by multi-pass friction stir processing

Carbon nanotubes (CNTs) were dispersed into Al matrix by multi-pass friction stir processing (FSP) to fabricate 4.5vol.% CNT/2009Al composites. The maximum strength of the composites was obtained with three-pass FSP and was attributed to the combined effect of CNT cluster reduction, grain refinement, and CNT shortening. A CNT shortening model is proposed to describe the CNT length change with FSP passes. The model indicates that the reciprocal of the CNT length has a linear relationship with the duration of the mechanical effect, which was verified by our and other investigators’ experimental results on various mechanical processes. Based on the concept of the load transfer efficiency of CNTs in the Al matrix, a universal strength model considering the microstructural parameters – the aspect ratio of CNTs, grain size, and concentration of CNT clusters and pores – is proposed to predict the strengthening of the CNT/2009Al composites with and without CNT clusters. The predictions are in good agreement with the experimental results.

Ultrasound regeneration of multi wall carbon nanotubes saturated by humic acid

The main purpose of this study was to evaluate the efficiency of ultrasound technique for regeneration of multi wall carbon nanotubes exhausted by humic acid. Carbon nanotubes (CNTs) were prepared by chemical vapor deposition method in the presence of Co–Mg/MgO as the catalyst. Results of ultrasound regeneration (in frequencies of 130 and 35 kHz) showed higher regeneration efficiency at 130 kHz after five cycles of regeneration. The slight decrease in adsorption capacity after each regeneration cycle may be due to the deposition of decomposed residues in CNT pores, which blocked carbon porosity and decreased specific surface area. In conclusion, ultrasonic regeneration showed the possibility as an alternative to chemical and thermal regeneration of CNTs.

Mesoporous Calcium Carbonate as a Phase Stabilizer of Amorphous Celecoxib – An Approach to Increase the Bioavailability of Poorly Soluble Pharmaceutical Substances

The bioavailability of crystalline pharmaceutical substances is often limited by their poor aqueous solubility but it can be improved by formulating the active substance in the amorphous state that is featured with a higher apparent solubility. Although the possibility of stabilizing amorphous drugs inside nano-sized pores of carbon nanotubes and ordered mesoporous silica has been shown, no conventional pharmaceutical excipients have so far been shown to possess this property. This study demonstrates the potential of using CaCO3 , a widely used excipient in oral drug formulations, to stabilize the amorphous state of active pharmaceutical ingredients, in particular celecoxib. After incorporation of celecoxib in the vaterite particles, a five to sixfold enhancement in apparent solubility of celecoxib is achieved due to pore-induced amorphization. To eliminate the possibility of uncontrolled phase transitions, the vaterite particles are stored in an inert atmosphere at 5 °C throughout the study. Also, to demonstrate that the amorphization effect is indeed associated with vaterite mesopores, accelerated stress conditions of 100% relative humidity are employed to impose transition from mesoporous vaterite to an essentially non-porous aragonite phase of CaCO3 , which shows only limited amorphization ability. Further, an improvement in solubility is also confirmed for ketoconazole when formulated with the mesoporous vaterite. Synthesis of the carrier particles and the incorporation of the active substances are carried out simultaneously in a one-step procedure, enabling easy fabrication. These results represent a promising approach to achieve enhanced bioavailability in new formulations of Type II BCS drugs.

Self-Assembling Organic Nanotubes with Precisely Defined, Sub-nanometer Pores: Formation and Mass Transport Characteristics

The transport of molecules and ions across nanometer-scaled pores, created by natural or artificial molecules, is a phenomenon of both fundamental and practical significance. Biological channels are the most remarkable examples of mass transport across membranes and demonstrate nearly exclusive selectivity and high efficiency with a diverse collection of molecules. These channels are critical for many basic biological functions, such as membrane potential, signal transduction, and osmotic homeostasis. If such highly specific and efficient mass transport or separation could be achieved with artificial nanostructures under controlled conditions, they could create revolutionary technologies in a variety of areas. For this reason, investigators from diverse disciplines have vigorously studied small nondeformable nanopores. The most exciting studies have focused on carbon nanotubes (CNTs), which have exhibited fast mass transport and high ion selectivity despite their very simple structure. However, the limitations of CNTs and the dearth of other small (≤2 nm) nanopores have severely hampered the systematic investigation of nanopore-mediated mass transport, which will be essential for designing artificial nanopores with desired functions en masse. Researchers can overcome the difficulties associated with CNT and other artificial pores by stacking macrocyclic building blocks with persistent shapes to construct tunable, self-assembling organic pores. This effort started when we discovered a highly efficient, one-pot macrocyclization process to efficiently prepare several classes of macrocycles with rigid backbones containing nondeformable cavities. Such macrocycles, if stacked atop one another, should lead to nanotubular assemblies with defined inner pores determined by their constituent macrocycles. One class of macrocycles with aromatic oligoamide backbones had a very high propensity for directional assembly, forming nanotubular structures containing nanometer and sub-nanometer hydrophilic pores. These self-assembling hydrophilic pores can form ion channels in lipid membranes with very large ion conductances. To control the assembly, we have further introduced multiple hydrogen-bonding side chains to enforce the stacking of rigid macrocycles into self-assembling nanotubes. This strategy has produced a self-assembling, sub-nanometer hydrophobic pore that not only acted as a transmembrane channel with surprisingly high ion selectivity, but also mediated a significant transmembrane water flux. The stacking of rigid macrocycles that can be chemically modified in either the lumen or the exterior surface can produce self-assembling organic nanotubes with inner pores of defined sizes. The combination of our approach with the availability and synthetic tunability of various rigid macrocycles should produce a variety of organic nanopores. Such structures would allow researchers to systematically explore mass transport in the sub-nanometer regime. Further advances should lead to novel applications such as biosensing, materials separation, and molecular purifications.

The morphologies of Lennard-Jones liquid encapsulated by carbon nanotubes

Abstract Using molecular dynamics simulations, we studied the morphologies of Lennard-Jones liquid encapsulated in carbon nanotubes (CNTs) for a wide range of liquid–CNT interaction, system size and temperature. The morphology of liquid is found to be sensitive to the filling ratio of liquid (a ratio of liquid volume to the available volume of CNT pore) and the liquid–CNT interaction. The ‘phase diagram’, namely by the morphologies versus the liquid–CNT interaction and the filling ratio, is obtained. In most cases, the liquid inside CNTs forms a thin liquid shell attached to a carbon wall when the filling ratio is small. With the increasing of the filling ratio, liquid tends to form droplet. As the filling ratio increases further, liquids form a cylinder with finite length. Finally, the whole inner space of CNT was filled with liquid when the filling ratio is large enough. Current studies could shed light on the adsorption and flow of liquid inside CNTs.

Computational Design of a Carbon Nanotube Fluorofullerene Biosensor

Carbon nanotubes offer exciting opportunities for devising highly-sensitive detectors of specific molecules in biology and the environment. Detection limits as low as 10−11 M have already been achieved using nanotube-based sensors. We propose the design of a biosensor comprised of functionalized carbon nanotube pores embedded in a silicon-nitride or other membrane, fluorofullerene-Fragment antigen-binding (Fab fragment) conjugates, and polymer beads with complementary Fab fragments. We show by using molecular and stochastic dynamics that conduction through the (9, 9) exohydrogenated carbon nanotubes is 20 times larger than through the Ion Channel Switch ICS™ biosensor, and fluorofullerenes block the nanotube entrance with a dissociation constant as low as 37 pM. Under normal operating conditions and in the absence of analyte, fluorofullerenes block the nanotube pores and the polymer beads float around in the reservoir. When analyte is injected into the reservoir the Fab fragments attached to the fluorofullerene and polymer bead crosslink to the analyte. The drag of the much larger polymer bead then acts to pull the fluorofullerene from the nanotube entrance, thereby allowing the flow of monovalent cations across the membrane. Assuming a tight seal is formed between the two reservoirs, such a biosensor would be able to detect one channel opening and thus one molecule of analyte making it a highly sensitive detection design.

Effect of catalyst confinement and pore size on Fischer-Tropsch synthesis over cobalt supported on carbon nanotubes

This paper studies the impact of structure of cobalt catalysts supported on carbon nanotubes (CNT) on the activity and product selectivity of Fischer-Tropsch synthesis (FTS) reaction. Three types of CNT with average pore sizes of 5, 11, and 17 nm were used as the supports. The catalysts were prepared by selectively impregnating cobalt nanoparticles either inside or outside CNT. The TPR results indicated that the catalyst with Co particles inside CNT was easier to be reduced than those outside CNT, and the reducibility of cobalt oxide particles inside the CNT decreased with the cobalt oxide particle size increasing. The activity of the catalyst with Co inside CNT was higher than that of catalysts with Co particles outside CNT. Smaller CNT pore size also appears to enhance the catalyst reduction and FTS activity due to the little interaction between cobalt oxide with carbon and the enhanced electron shift on the non-planar carbon tube surface.