Carbon Nanotube Porins Research Articles

A Protonic Biotransducer for Controlling ATP Synthesis in Mitochondria

Controlling the flow of ions and electrons at the device/biological interface has become an important challenge in broad fields such as bioelectronics and medical biology. These devices provide a new way to translate bidirectionally between the ionic language of biology and the electronic language of circuitry. The current strategy to develop such ion and electron controlling devices is to use new functional materials at the device/biological interface. For example, silicon nanowire and carbon nanotube based transistors use ions to read out biological functions. Aluminum nanostraws and carbon nanotube porins can selectively deliver molecules such as cations, DNA, and nicotine into biological materials. Since conductive polymers allow the transport of both ions and electrons, there are many conductive polymer based examples including organic field effect transistors for biosensing and organic ion pumps for locally delivering ions and neurotransmitters into cells and the brain. Along with ions and small molecules, protons (H+) play an important role in biology. Examples include the homeostatic pH regulation in body and bacteria, acid sensing ion channels activating in neuron cells, proton activated bioluminescence in dinoflagellates, and pH responsive flagella in bacteria. Mitochondria in particular are a noteworthy organelle that utilize the transport of protons and electrons across a membrane to synthesize adenosine triphosphate (ATP) molecules. To translate H+ signals from biological environments into measureable electronic signals, Rolandi and co-workers developed a prototype bioprotonic device using a Pd/PdHx protode. In his group, the metallic protode was used to measure the protonic conductivity in biological materials such as chitosan and jelly from in the ampullae of Lorenzini of sharks, and to integrate electronic signals with an enzymatic flip flop circuit, ion channels, and light-sensitive bacteriorhodopsin. Gorodestky and co-workers have separately demonstrated high proton conductivity in reflectin squid proteins and Pd/PdHx based transistors. All attempts however focused on the interfacing between Pd-based metal and biological materials, even though the Pd protode may occur side reactions such as oxygen conversion (PdO, PdO2 and Pd(OH)x) at the potentials beginning at 0.9 V vs. NHE and toxic hydrogen peroxide production. Here, we develop an organic biotransducer using a high H+-coupling conductive polymer of sulfonated polyaniline (SPA) that monitors and modulates the pH in the vicinity of the SPA electrode, even in solutions with the high buffering capacity typical of mitochondrial environments (Fig.1). We integrate this biotransducer with mitochondria isolated from pig hearts to translate the H+ signal from the activated mitochondria into a measureable protonic current, and also to control the activity of ATP synthase through electrochemical pH modulation at the SPA surface. Figure 1

Electronic control of H+ current in a bioprotonic device with carbon nanotube porins.

Hybrid biotic abiotic devices can be used to interface electronics with biological systems for novel therapies or to increase device functionality beyond silicon. Many strategies exist to merge the electronic and biological worlds, one dominated by electrons and holes as charge carriers, the other by ions. In the biological world, lipid bilayers and ion channels are essential to compartmentalize the cell machinery and regulate ionic fluxes across the cell membrane. Here, we demonstrate a bioelectronic device in which a lipid bilayer supported on H+-conducting Pd/PdHx contacts contains carbon nanotubes porin (CNTP) channels. This bioelectronic device uses CNTPs to control of H+ flow across the lipid bilayer with a voltage applied to the Pd/PdHx contacts. Potential applications of these devices include local pH sensing and control.

Carbon Nanotube Porins in Block Copolymers as Fully Synthetic Mimics of Biological Membranes

Carbon Nanotube Porins in Block Copolymers as Fully Synthetic Mimics of Biological Membranes

Optical Sensing of Ion Flux through Biomimetic Carbon Nanotube Channels

Increasing demands for cheaper and more efficient water treatment methods have never been higher, as shortages of fresh water have become a growing pain in many regions worldwide. In theoretical research and simulations, carbon nanotubes have shown a potential in water treatment, desalination and energy conversion with high ion selectivity and enhanced water transport. In this research, we present an approach to investigate fluid transport across each single carbon nanotube channel by visualizing the ion and water flux under a Total Internal Reflection Fluorescence Microscopy (TIRF). Using a droplet interface bilayer (DIB) technique, we place an aqueous droplet (∼ 200 nanoliters) on an oil-agarose hydrogel interface, which brings together the lipid monolayers on the droplet and the interface to form a bilayer that mimics a cell membrane. We then incorporate the carbon nanotube channels in this bilayer. The ion and mass fluxes in each channel were characterized through the optical image of ion-sensitive fluorescence, which made it available to trace the flux through a large number of nanoscale channels individually instead of averaging them. The evanescent wave from TIRF also made it possible by eliminating the noise in the background and confining the illuminated region. We envision that our method will allow probing of fundamental transport properties through biomimetic carbon nanotube porins.

Membranes: Carbon Nanotube Porins in Amphiphilic Block Copolymers as Fully Synthetic Mimics of Biological Membranes (Adv. Mater. 51/2018)

Membranes: Carbon Nanotube Porins in Amphiphilic Block Copolymers as Fully Synthetic Mimics of Biological Membranes (Adv. Mater. 51/2018)

Carbon Nanotube Porins in Amphiphilic Block Copolymers as Fully Synthetic Mimics of Biological Membranes

Biological membranes provide a fascinating example of a separation system that is multifunctional, tunable, precise, and efficient. Biomimetic membranes, which mimic the architecture of cellular membranes, have the potential to deliver significant improvements in specificity and permeability. Here, a fully synthetic biomimetic membrane is reported that incorporates ultra-efficient 1.5 nm diameter carbon nanotube porin (CNTPs) channels in a block-copolymer matrix. It is demonstrated that CNTPs maintain high proton and water permeability in these membranes. CNTPs can also mimic the behavior of biological gap junctions by forming bridges between vesicular compartments that allow transport of small molecules.

Silicon Nanoribbon pH Sensors Protected by a Barrier Membrane with Carbon Nanotube Porins.

Limited biocompatibility and fouling propensity can restrict real-world applications of a large variety of biosensors. Biological systems are adept at protecting and separating vital components of biological machinery with semipermeable membranes that often contain defined pores and gates to restrict transmembrane transport only to specific species. Here we use a similar approach for creating fouling-resistant pH sensors. We integrate silicon nanoribbon transistor sensors with an antifouling lipid bilayer coating that contains proton-permeable carbon nanotube porin (CNTP) channels and demonstrate robust pH detection in a variety of complex biological fluids.

Response to Comment on "Enhanced water permeability and tunable ion selectivity in subnanometer carbon nanotube porins".

Horner and Pohl argue that high water transport rates reported for carbon nanotube porins (CNTPs) originate from leakage at the nanotube-bilayer interface. Our results and new experimental evidence are consistent with transport through the nanotube pores and rule out a defect-mediated transport mechanism. Mechanistic origins of the high Arrhenius factor that we reported for narrow CNTPs at pH 8 require further investigation.

Comment on "Enhanced water permeability and tunable ion selectivity in subnanometer carbon nanotube porins".

Tunuguntla et al (Reports, 25 August 2017, p. 792) report that permeation of single-file water occurs faster through carbon nanotubes than through aquaporins. We show that this conclusion violates fundamental thermodynamic laws: Because of its much lower activation energy, aquaporin-mediated water transport must be orders of magnitude faster. Leakage at the nanotube-membrane interface may explain the discrepancy.

Ionic Transport through 1.5 NM Diameter Carbon Nanotube Porins

Ionic Transport through 1.5 NM Diameter Carbon Nanotube Porins

Impact of PEG additives and pore rim functionalization on water transport through sub-1nm carbon nanotube porins.

Carbon nanotubes represent one of the most interesting examples of a nanofluidic channel that combines extremely small diameters with atomically smooth walls and well-defined chemical functionalities at the pore entrance. In the past, sub-1 nm diameter carbon nanotube porins (CNTPs) embedded in a lipid membrane matrix demonstrated extremely high water permeabilities and strong ion selectivities. In this work, we explore additional factors that can influence transport in these channels. Specifically, we use stopped-flow transport measurements to focus on the effect of chemical modifications of the CNT rims and chaotropic polyethyleneglycol (PEG) additives on CNTP water permeability and Arrhenius activation energy barriers for water transport. We show that PEG, especially in its more chaotropic coiled configuration, enhances the water transport and reduces the associated activation energy. Removal of the static charges on the CNTP rim by converting -COOH groups to neutral methylamide groups also reduces the activation energy barriers and enhances water transport rates.

Molecular dynamics simulations of carbon nanotube porins in lipid bilayers

Artificial channels made of carbon nanotube (CNT) porins are promising candidates for applications in filtration and molecular delivery devices. Their symmetric shape and high mechanical, chemical, and thermal stability ensure well-defined transport properties, and at the same time make them ideal model systems for more complicated membrane protein pores. As the technology to produce and tune CNTs advances, simulations can aid in the design of customized membrane porins. Here we concentrate on CNTs embedded in lipid membranes. To derive design guidelines, we systematically studied the interaction of CNT porins with their surrounding lipids. For our simulations, we developed an AMBER- and Lipid14-compatible parameterization scheme for CNTs with different chirality and with functional groups attached to their rim, and a flexible coarse-grained description for open-ended CNTs fitting to the MARTINI lipid model. We found that the interaction with lipid acyl chains is independent of the CNT chirality and the chemical details of functional groups at the CNT rims. The latter, however, are important for the interactions with lipid head groups, and for water permeability. The orientation and permeability of the pore are mainly determined by how well its hydrophobicity pattern matches the membrane. By identifying the factors that determine the structure both of isolated CNTs in lipid membranes and of CNT clusters, we set the foundation for a targeted design of CNT-membrane systems.

Enhanced water permeability and tunable ion selectivity in subnanometer carbon nanotube porins.

Fast water transport through carbon nanotube pores has raised the possibility to use them in the next generation of water treatment technologies. We report that water permeability in 0.8-nanometer-diameter carbon nanotube porins (CNTPs), which confine water down to a single-file chain, exceeds that of biological water transporters and of wider CNT pores by an order of magnitude. Intermolecular hydrogen-bond rearrangement, required for entry into the nanotube, dominates the energy barrier and can be manipulated to enhance water transport rates. CNTPs block anion transport, even at salinities that exceed seawater levels, and their ion selectivity can be tuned to configure them into switchable ionic diodes. These properties make CNTPs a promising material for developing membrane separation technologies.

Real-time dynamics of carbon nanotube porins in supported lipid membranes visualized by high-speed atomic force microscopy.

In-plane mobility of proteins in lipid membranes is one of the fundamental mechanisms supporting biological functionality. Here we use high-speed atomic force microscopy (HS-AFM) to show that a novel type of biomimetic channel-carbon nanotube porins (CNTPs)-is also laterally mobile in supported lipid membranes, mimicking biological protein behaviour. HS-AFM can capture real-time dynamics of CNTP motion in the supported lipid bilayer membrane, build long-term trajectories of the CNTP motion and determine the diffusion coefficients associated with this motion. Our analysis shows that diffusion coefficients of CNTPs fall into the same range as those of proteins in supported lipid membranes. CNTPs in HS-AFM experiments often exhibit 'directed' diffusion behaviour, which is common for proteins in live cell membranes.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Single-Channel Measurements of Conductance through Sub-Nanometer Carbon Nanotube Porins

Carbon nanotubes (CNTs) have narrow hydrophobic inner pores, highly reminiscent of protein channels, that enable fast transport of water and ions. Their rigid structure and chemical robustness allow for diverse ex vivo applications. We have recently reported biomimetic membrane channels cased on carbon nanotubes—carbon nanotube porins (CNTPs). We have shown that CNTPs self-insert into lipid membranes and form stable pores. In this study, we used electrophysiological single-channel measurements to quantify ion conductance and selectivity of 0.8 nm diameter CNTPs. We report the conductance of single CNTP channels, as well as the scaling of the conductance with the ion concentration. We also used the reversal potential measurements to characterize the selectivity of the ion transport and found that CNTPs are highly selective to cations over anions. We will discuss the mechanism of this selectivity, and demonstrate ways to control it. These findings are important and relevant for developing CNTPs into a potential candidate nanopore for water treatment applications. Our studies on intrinsic characterization of CNTP transport properties will also enable researchers to consider this new material for a variety of biophysical applications.

Tunable Ion Selectivity in Sub-Nanometer Diameter Carbon Nanotube Porins

Carbon nanotube porins (CNTPs), ultra-short carbon nanotubes (5-20nm) that can self-insert into lipid bilayers, allow single-channel planar lipid bilayer measurements of ionic transport through carbon nanotube (CNT) pores. CNTs with sub-nanometer diameters have been predicted to display ultra-fast water transport and high levels of ionic exclusion resulting from one-dimensional water wire transport through their hydrophobic cores. Here, we show that ultra-short 0.8-nm-diameter CNTPs, can spontaneously insert into lipid bilayers to form highly cation selective channels that exhibit low ionic conductance. Using reversal potential in various asymmetric salt conditions, we observe that these CNTPs exhibit large permselectivity values, corresponding to a high selectivity for both potassium and sodium over chloride. We also show evidence that neutralization of charges on the CNTs enhances ion selectivity. We also show that by using a solid-state nanopore to act as a support and form a completely solvent-free bilayer system, we can observe extremely stable ionic currents through these nanotubes, in contrast to previous reports with solvent containing bilayers. These results establish CNTPs as a promising biomimetic platform for developing cell interfaces, creating stochastic sensors, and water desalination.

Molecular Dynamics Study on Lipid Wrapped Carbon Nanotube as an Artificial Membrane Channel

Carbon nanotube (CNT) is an ideal membrane channel because of its hydrophobic inner pore and prominent transport properties like typical biological channels. These CNT porins can transport water, small ions, and DNA. CNT-membrane interaction simulation shows that CNTs having a length similar to the thickness of lipid bilayer can be inserted into membrane and sustain angle almost perpendicular to membrane. Much longer ones can also be inserted into membrane but they are buried inside hydrophobic region of membrane (1). However, recent experimental research elucidated that longer CNTs wrapped by lipids can be inserted into lipid-bilayer and maintain their angle within 15 degrees (2).In this study, we performed molecular dynamics simulation for investigating interaction between lipids wrapped CNT and lipid-bilayer. Martini coarse-grained force field was used to reduce computational cost significantly. Also, wrapping process was analyzed by using the coarse-grained carbon nanotube (CG-CNT) and the 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) in various CNT length and surface adsorption density, respectively. Furthermore, we performed insertion simulation to see how these properties affect insertion process and finally compared CG-CNT's insertion angles with experimental data. As a result, CNT insertion takes longer time in general as the density of wrapped lipid increases. Also, we realized that CNT can maintain its insertion angle almost perpendicular to lipid-bilayer at a certain critical density.Finally, its insertion mechanism is fully discussed with respect to interaction energy between the wrapped CG-CNT and lipid membrane.(1) Lelimousin, M., and M. S. P. Sansom. 2013. Small 9:3639-3646.(2) Geng, J., K. Kim, J. F. et al. 2014. Nature 514:612-+.

High-Yield Synthesis and Optical Properties of Carbon Nanotube Porins

Carbon nanotube porins (CNTPs) are a convenient membrane-based model system for studying nanofluidic transport that replicates a number of key structural features of biological membrane channels. We present a generalized approach for CNTP synthesis using sonochemistry-assisted segmenting of carbon nanotubes. Prolonged tip sonication in the presence of lipid molecules debundles and fragments long carbon nanotube aggregates into stable and water-soluble individual CNTPs with lengths in the range 5–20 nm. We discuss the main parameters that determine the efficiency and the yield of this process, describe the optimized conditions for high-yield CNTP synthesis, and demonstrate that this methodology can be adapted for synthesis of CNTPs of different diameters. We also present the optical properties of CNTPs and show that a combination of Raman and UV–vis–NIR spectroscopy can be used to monitor the quality of the CNTP synthesis. Overall, CNTPs represent a versatile nanopore building block for creating higher-ord...

Synthesis, lipid membrane incorporation, and ion permeability testing of carbon nanotube porins.

Carbon nanotube porins (CNTPs) are 10- to 20-nm-long segments of lipid-stabilized single-walled carbon nanotubes (CNTs) that can be inserted into phospholipid membranes to form nanometer-scale-diameter pores that approximate the geometry and many key transport characteristics of biological membrane channels. We describe protocols for CNTP synthesis by ultrasound-assisted cutting of long CNTs in the presence of lipid amphiphiles, and for validation of CNTP incorporation into a lipid membrane using a proton permeability assay. In addition, we describe protocols for measuring conductance of individual CNTPs in planar lipid bilayers and plasma membranes of live cells. The protocol for the preparation and testing of the CNTPs in vesicle systems takes 3 d, and single CNTP conductance measurements take 2-5 h. The CNTPs produced by this cutting protocol remain stable and active for at least 10-12 weeks.

Structure of Carbon Nanotube Porins in Lipid Bilayers: An in Situ Small-Angle X-ray Scattering (SAXS) Study.

Carbon nanotube porins (CNTPs), small segments of carbon nanotubes capable of forming defined pores in lipid membranes, are important future components for bionanoelectronic devices as they could provide a robust analog of biological membrane channels. In order to control the incorporation of these CNT channels into lipid bilayers, it is important to understand the structure of the CNTPs before and after insertion into the lipid bilayer as well as the impact of such insertion on the bilayer structure. Here we employed a noninvasive in situ probe, small-angle X-ray scattering, to study the integration of CNT porins into dioleoylphosphatidylcholine bilayers. Our results show that CNTPs in solution are stabilized by a monolayer of lipid molecules wrapped around their outer surface. We also demonstrate that insertion of CNTPs into the lipid bilayer results in decreased bilayer thickness with the magnitude of this effect increasing with the concentration of CNTPs.