Stochastic transport through carbon nanotubes in lipid bilayers and live cell membranes

Abstract

There is much interest in developing synthetic analogues of biological membrane channels with high efficiency and exquisite selectivity for transporting ions and molecules. Bottom-up and top-down methods can produce nanopores of a size comparable to that of endogenous protein channels, but replicating their affinity and transport properties remains challenging. In principle, carbon nanotubes (CNTs) should be an ideal membrane channel platform: they exhibit excellent transport properties and their narrow hydrophobic inner pores mimic structural motifs typical of biological channels. Moreover, simulations predict that CNTs with a length comparable to the thickness of a lipid bilayer membrane can self-insert into the membrane. Functionalized CNTs have indeed been found to penetrate lipid membranes and cell walls, and short tubes have been forced into membranes to create sensors, yet membrane transport applications of short CNTs remain underexplored. Here we show that short CNTs spontaneously insert into lipid bilayers and live cell membranes to form channels that exhibit a unitary conductance of 70-100 picosiemens under physiological conditions. Despite their structural simplicity, these 'CNT porins' transport water, protons, small ions and DNA, stochastically switch between metastable conductance substates, and display characteristic macromolecule-induced ionic current blockades. We also show that local channel and membrane charges can control the conductance and ion selectivity of the CNT porins, thereby establishing these nanopores as a promising biomimetic platform for developing cell interfaces, studying transport in biological channels, and creating stochastic sensors.

Nanoscopic Characterization of DNA within Hydrophobic Pores: Thermodynamics and Kinetics

“…It now becomes clear that, although molecular mobility is anisotropic, favouring DNA translocation along the nanopore main axis, it is mostly due to entropic reasons, because the kinetic energies (velocities) are identical independently of the particular direction in space. The in vitro experiments of Geng et al 5 showed that porin-embedded nanotubes are able to transport ssDNA at an average speed of 1.5 nucleotides per millisecond towards the cellular internal volume; on the other hand, Pei and Gao 16 …”

“…Owing to several appealing features, such as large surface areas, well-defined physico-chemical properties and the hydrophobic nature of their pristine structure, CNTs are considered ideal candidates to be used as nanopores for biomolecular confinement. Present day potential applications span different purposes and objectives, such as intracellular penetration via endocytosis and delivery of biological cargoes [3][4][5] , ultrafast nucleotide sequencing [6][7][8] and gene and DNA delivery to cells 5,9,10 . The remarkable experimental work by Geng et al 5,11 has shown that carbon nanotubes can spontaneously penetrate the lipid bilayer of a liposome, and the corresponding hybrid incorporated into live mammalian cells to act as a nanopore through which water, ions and DNA are delivered to the cellular interior.…”

“…Present day potential applications span different purposes and objectives, such as intracellular penetration via endocytosis and delivery of biological cargoes [3][4][5] , ultrafast nucleotide sequencing [6][7][8] and gene and DNA delivery to cells 5,9,10 . The remarkable experimental work by Geng et al 5,11 has shown that carbon nanotubes can spontaneously penetrate the lipid bilayer of a liposome, and the corresponding hybrid incorporated into live mammalian cells to act as a nanopore through which water, ions and DNA are delivered to the cellular interior. For an efficient and costeffective industrial fabrication of SWCNT-based technology for DNA encapsulation/delivery, the interactions between the solid and the biomolecule need to be thoroughly understood in order to render the DNA/SWCNT device able to be used under physiological conditions, T = 310K and [NaCl] = 134 mM.…”

Taking Electrons out of Bioelectronics: From Bioprotonic Transistors to Ion Channels

“…Differences in methods of communication (electrons versus ions) between traditional electronic devices and biological systems result in a challenge at the interface 7, 8. Silicon nanowire and carbon nanotube transistors integrated with enzymes, antibodies, and lipid bilayers use ions to gate electronic currents and record biological reactions in the intracellular and extracellular space 2, 3, 9, 10, 11, 12, 13, 14, 15, 16. Organic polymers with mixed electronic and ionic conductivity integrated in electrodes and electrochemical transistors transduce ionic to electronic currents and amplify small biological signals3, 17, 18, 19, 20, 21, 22 In addition, organic iontronics locally deliver ions and neurotransmitters in the extracellular space to affect cell and tissue function 23, 24, 25, 26.…”

High-speed atomic force microscopy and its future prospects

“…A short carbon nanotube (CNT) may be able to be used as an SICM probe. CNTs can be inserted into lipid membranes and transport ions across the membranes (Geng et al 2014). The inner diameter of CNTs is very small.…”