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Ion Channel Based Nanopore Sensors: Ionic transport in carbon nanotubes

S. Joseph, R. J. Mashl, E. Jacobsson, N. R. Aluru
Beckman Institute of Advanced Science and Technology, US

Keywords: Nanofluidics, Single molecule detection, ion channel, biosensor

Transport of molecules through macromolecular pores is of considerable importance in many biological and nano-electromechanical systems. Ion channels found in cell membranes are crucial for shaping electrical signals and controlling flow of ions and fluids across cells. Channels have interesting properties like gating and selective permeability that is still being widely investigated. In recent years engineered ion channels have been developed to function as single molecule detection systems. Though they have significant advantages including biocompatibility, their lack of durability makes them reliable only in a lab setting. Therefore the question arises whether the functionality of ion channels can be incorporated into artificial nanotubes that could be bulk manufactured but is far less complex than a biological system. Carbon nanotubes being rigid and having exciting electrical and mechanical properties seem ideal but fundamental questions have to be addressed regarding transport of water and ions through them. Hummer et. al. showed by molecular dynamics simulations that water molecules enter nanotubes of diameter greater than 8.1A& even though carbon is hydrophobic. In this paper we explore for the first time the possibility of transport of an electrolytic solution (KCl) through a carbon nanotube by molecular dynamics simulations.It is observed that ion occupancy is very low in a (16,16) 13.4A& long carbon nanotube in a 1.85 M KCl solution. This is because the Van der Waals forces between the carbon nanotube and the ions are not strong enough to overcome the attractive force between the potassium and chloride ions in the bulk solution. When partial charges of +/-0.38 e were placed at the mouth atoms of the tube to create a dipole and when an external electric field was applied along the axis of the tube it is observed that the occupancy increases significantly. As the next step, to mimic a real ion-channel, functional groups were attached at the ends of the tube — CH2-COO— and CH2-NH3+ at either end respectively. The functionalized carbon nanotube was placed in a slab with similar properties of that of a lipid bilayer with 1.5 M KCl solution. An electric field of 0.15 V/nm was used to drive the ions through the tubes. The Chloride ion occupancy is much higher than potassium ion occupancy even though the net partial charges at the mouths are same in magnitude. The attractive force between the COO- and the K+ ions causes the K+ ions to “bind” to the COO— groups at the mouth reducing the K+ occupancy in the tube. Like ionic channels, the functionalized carbon nanotube also seems to exhibit selective permeability of anions over cations and further studies are being done to explain this phenomenon.

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