Kinetic Model of Gas Transport in Carbon Nanotube Channels

Carbon nanotubes represent a rare experimental realization of a nanofluidic channel, which has molecularly smooth walls and nanometer scale inner diameter. This unique combination of properties gives the carbon nanotube channel an ability to support enhanced transport of water and gases with flows often exceeding those of conventional channels by several orders of magnitude. Surprisingly, most of these transport enhancement phenomena can be explained using very simple mechanisms that hardly go beyond classical physics concepts. Here we present a simplified analytical model that uses classic kinetic theory formalism to describe gas transport in carbon nanotube channels and to highlight the role of surface defects and adsorbates in determining transport efficiency. We also extend this description to include the possibility of gas molecule diffusion along the nanotube walls. Our results show that in all cases the conditions at the nanotube channel walls play a critical role in determining the transport efficiency and that in some cases obtaining efficient transport has to involve optimization of flows from diffusion through the gas phase and along the nanotube surface

Electronic control of H⁺ current in a bioprotonic device with carbon nanotube porins - Fig 3

(a) Pd contact with SLB incorporating 0.8 nm diameter CNTPs with K-HEPES buffer at pH = 6.0, is semipermeable to H+, with CNTPs facilitating the rapid flow of H+ to the Pd/solution interface. (b) Pd contact with SLB incorporating Narrow CNTPs with K-HEPES buffer pH = 7.0, is still semipermeable to H+ but facilitating lower flow of H+ to the Pd/solution interface. (c) iH+ versus time plot for V = −250 mV and V = 50 mV. Gray trace SLB, red trace SLB+ CNTps (K-HEPES, pH = 6.0), black trace SLB+ CNTPs (K-HEPES, pH = 7.0). The iH+ for measurements K-HEPES pH = 6.0 is higher than K-HEPES pH = 7. We can hypothesize that at pH = 6.0 we have a driving force due to the lower pH across the membrane in addition to the applied voltage that expedite the flow of H+ while at pH = 7.0 we have only the applied voltage as a driving force to transport the H+ across. We did not observe any significant different between the iH+ at pH = 8.0 as compare to pH = 7.0 which might be due the buffer capacity of HEPES at different pH condition (Fig A in S1 File). (The data are collected from 3 different devices with different dimensions: SLB- K-HEPES pH = 7.0 : 3 different devices of 2 × 50 μm, Pd/SLB+CNTPs+Ca+2: 3 different devices of 2 × 50 μm. The error bars are the root mean square of the displacement of the data from the average value).</p

Electronic control of H⁺ current in a bioprotonic device with carbon nanotube porins - Fig 2

(a) Pd contact with SLB. The SLB blocks H+ from transferring from the solution to the Pd contact even with V = -250 mV (vs. Ag/AgCl). (b) Pd contact with SLB incorporating 0.8 nm diameter CNTPs is semipermeable to H+, with CNTPs facilitating the rapid flow of H+ to the Pd/solution interface. (c) Upon addition of Ca+2 to the bulk solution, H+ current through CNTPs becomes partially blocked. (d) iH+ vs. time plots recorded at V = −250 mV and V = 20 mV. Blue trace: SLB, red trace: SLB with CNTPs, gray trace SLB with CNTPs in presence of Ca+2 ions in the bulk solution. (The data are collected from 3 different devices with different dimensions: Pd / SLB: 3 different devices of 2 × 50 μm, Pd/SLB+CNTPs: 3 different devices of 2 × 50 μm, Pd/SLB+CNTPs+Ca+2: 3 different devices of 2 × 50 μm. The error bars are the root mean square of the displacement of the data from the average value).</p

Frictionless Sliding of Single-Stranded DNA in a Carbon Nanotube Pore Observed by Single Molecule Force Spectroscopy

Smooth inner pores of carbon nanotubes (CNT) provide a fascinating model for studying biological transport. We used an atomic force microscope to pull a single-stranded DNA oligomer from a carbon nanotube pore. DNA extraction from CNT pores occurs at a nearly constant force, which is drastically different from the elastic profile commonly observed during polymer stretching with atomic force microscopy. We show that a combination of the frictionless nanotube pore walls and an unfavorable DNA solvation energy produces this constant force profiles

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

Conductive AFM of SLB with CNTPs channels.

(a) The current map for the Pd contact with SLB incorporating CNTPs. The hot spot (green spot) correspond to higher current (red trace) that represent CNT and the background (purple area) correspond to negligible amount of current (black trace) which represent SLB membrane. (b) In the IV curve the red trace collected from the green spot and the back trace collected from the purple area. The green spot has i ~ 1.78 nA ± 0.09 nA and purple area has i ~ 5.86 pA ± 0.98 pA. This current most likely represents the electron conductivity of CNT. (The data are collected from 3 different areas of the AFM image for both green spot and purple area. The error bars are the root mean square of the displacement of the data from the average value).</p

A bioprotonic device with integrated carbon nanotube porins (CNTPs) supports proton current across the SLB through the CNTPs when a negative voltage (<i>-V</i>) is applied on the Pd contact.

When H+ reach the surface of the Pd contact, they are reduced to H by an incoming electron and diffuse into the Pd to form palladium hydride (PdHx). The current density at the contact (–iH+), measures the rate of H+ flux along the CNTPs.</p

Layer-by-Layer Electrostatic Self-Assembly of Polyelectrolyte Nanoshells on Individual Carbon Nanotube Templates

Carbon nanotubes have been featured prominently in the nanotechnology research for some time, yet robust strategies for noncovalent chemical modification of the nanotube surface are still missing. Such strategies are essential for the creation of functional device architectures. Here, we present a new general procedure for carbon nanotube modification based on polyelectrolyte layer-by-layer assembly. We have built multilayer structures around individual carbon nanotube bridges by first modifying the nanotube surface with a pyrene derivative followed by layer-by-layer deposition of polyelectrolyte macroions on the nanotube. Transmission electron microscopy and scanning confocal fluorescence microscopy images confirm the formation of nanometer-thick amorphous polymer nanoshells around the nanotubes. These multilayer polyelectrolyte shells on individual carbon nanotubes introduce nearly unlimited opportunities for the incorporation of various functionalities into nanotube devices, which, in turn, opens up the possibility of building more complex multicomponent structures

Growth Kinetics of Vertically Aligned Carbon Nanotube Arrays in Clean Oxygen-free Conditions

Vertically aligned carbon nanotubes (CNTs) are an important technological system, as well as a fascinating system for studying basic principles of nanomaterials synthesis; yet despite continuing efforts for the past decade many important questions about this process remain largely unexplained. We present a series of parametric ethylene chemical vapor deposition growth studies in a “hot-wall” reactor using ultrapure process gases that reveal the fundamental kinetics of the CNT growth. Our data show that the growth rate is proportional to the concentration of the carbon feedstock and monotonically decreases with the concentration of hydrogen gas and that the most important parameter determining the rate of the CNT growth is the production rate of active carbon precursor in the gas phase reaction. The growth termination times obtained with the purified gas mixtures were strikingly insensitive to variations in both hydrogen and ethylene pressures ruling out the carbon encapsulation of the catalyst as the main process termination cause

Laser-Assisted Simultaneous Transfer and Patterning of Vertically Aligned Carbon Nanotube Arrays on Polymer Substrates for Flexible Devices

We demonstrate a laser-assisted dry transfer technique for assembling patterns of vertically aligned carbon nanotube arrays on a flexible polymeric substrate. A laser beam is applied to the interface of a nanotube array and a polycarbonate sheet in contact with one another. The absorbed laser heat promotes nanotube adhesion to the polymer in the irradiated regions and enables selective pattern transfer. A combination of the thermal transfer mechanism with rapid direct writing capability of focused laser beam irradiation allows us to achieve simultaneous material transfer and direct micropatterning in a single processing step. Furthermore, we demonstrate that malleability of the nanotube arrays transferred onto a flexible substrate enables post-transfer tailoring of electric conductance by collapsing the aligned nanotubes in different directions. This work suggests that the laser-assisted transfer technique provides an efficient route to using vertically aligned nanotubes as conductive elements in flexible device applications