Ramya Tunuguntla

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

Go with the flow Enhanced water transport occurs in a number of narrow pore channels, such as biological aquaporins. Tunuguntla et al. thoroughly characterized molecular transport through narrow carbon nanotubes (CNTs) (see the Perspective by Siwy and Fornasiero). In contrast to previous studies, the authors focused on water and ion transport through relatively short (∼10-nm) fragments of CNTs embedded in lipid bilayer membranes. Strong confinement generated highly accelerated water flow compared with that observed in biological water transporters. A key factor in the transport rate was the tunable rearrangement of intermolecular hydrogen bonding. Furthermore, by changing the charges at the mouth of the nanotube, the authors were able to alter the ion selectivity. Science, this issue p. 792; see also p. 753 Small-diameter carbon nanotubes show promising transport properties and selectivity for water purification applications. 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.

Ultrafast proton transport in sub-1-nm diameter carbon nanotube porins

Proton transport plays an important role in many biological processes due to the ability of protons to rapidly translocate along chains of hydrogen-bonded water molecules. Molecular dynamics simulations have predicted that confinement in hydrophobic nanochannels should enhance the rate of proton transport. Here, we show that 0.8-nm-diameter carbon nanotube porins, which promote the formation of one-dimensional water wires, can support proton transport rates exceeding those of bulk water by an order of magnitude. The transport rates in these narrow nanotube pores also exceed those of biological channels and Nafion. With larger 1.5-nm-diameter nanotube porins, proton transport rates comparable to bulk water are observed. We also show that the proton conductance of these channels can be modulated by the presence of Ca(2+) ions. Our results illustrate the potential of small-diameter carbon nanotube porins as a proton conductor material and suggest that strong spatial confinement is a key factor in enabling efficient proton transport.

Osmotically-driven transport in carbon nanotube porins.

We report the measurements of transport of ions and uncharged species through carbon nanotube (CNT) porins--short segments of CNTs inserted into a lipid bilayer membrane. Rejection characteristics of the CNT porins are governed by size exclusion for the uncharged species. In contrast, rejection of ionic species is governed by the electrostatic repulsion and Donnan membrane equilibrium. Permeability of monovalent cations follows the general trend in the hydrated ion size, except in the case of Cs(+) ions.

Bioelectronic Light‐Gated Transistors with Biologically Tunable Performance

Light-activated bioelectronic silicon nanowire transistor devices are made by fusing proteoliposomes containing a bacteriorhodopsin (bR) proton pump onto the nanowire surface. Under green-light illumination, bR pumps protons toward the nanowire, and the pH gradient developed by the pump changes the transistor output. Furthermore, co-assembly of small biomolecules that alter the bilayer permeability to other ions can upregulate and downregulate the response of field-effect transistor devices.

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’.

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.

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.

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.

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.

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.