Single-Walled Carbon Nanotubes: Mimics of Biological Ion Channels

Abstract

Here we report on the ion conductance through individual, small diameter single-walled carbon nanotubes. We find that they are mimics of ion channels found in natural systems. We explore the factors governing the ion selectivity and permeation through single-walled carbon nanotubes by considering an electrostatic mechanism built around a simplified version of the Gouy-Chapman theory. We find that the single-walled carbon nanotubes preferentially transported cations and that the cation permeability is size-dependent. The ionic conductance increases as the absolute hydration enthalpy decreases for monovalent cations with similar solid-state radii, hydrated radii, and bulk mobility. Charge screening experiments using either the addition of cationic or anionic polymers, divalent metal cations, or changes in pH reveal the enormous impact of the negatively charged carboxylates at the entrance of the single-walled carbon nanotubes. These observations were modeled in the low-to-medium concentration range (0.1-2.0 M) by an electrostatic mechanism that mimics the behavior observed in many biological ion channel-forming proteins. Moreover, multi-ion conduction in the high concentration range (>2.0 M) further reinforces the similarity between single-walled carbon nanotubes and protein ion channels.

Keywords: Ion channel; multi-ion conduction; nanofluidic device; single-walled carbon nanotubes.

Figure 1

Ion-selective single-walled carbon nanotube-based biomimetic devices. Structural comparison of (a) a biological ion channel with (b) a small-diameter single-walled carbon nanotube. The ionic conductance through both nanochannels is strongly affected by the presence of fixed charges and/or electronegative groups on the pore through an electrostatic mechanism. (a) Side view of the potassium channel MthK (PDB ID: 1LNQ). A ring of eight negative charges is formed at the entrance of the intracellular vestibule of CaK channels, which increases the local concentration of K⁺. (b) Model representation of a (10,0) carbon nanotube with a ring of carboxyl groups (oxygen atoms shown in red) present at the channel entrance. The residues along the K-channel and the entrance of the SWCNT are colored for a better view. (c) Schematic of the SWCNT devices studied here. Ionic current–voltage measurements were performed across the two fluidic reservoirs patterned in a PMMA resist through two Ag/AgCl electrodes. The two fluidic reservoirs are connected through the interior of one carbon nanotube (1.5 ± 0.4 nm on average diameter and 20 μm in length) laying on the surface of the Si/SiO₂ substrate. Images are not drawn to scale.

Figure 2

Ion permeability through SWCNTs (Device 1). (a) Effect of ion size on conductance. Ionic conductance is plotted as a function of monovalent equivalent concentration of electrolytes with varying ion sizes. Comparing the conductance–concentration curves of KCl (black squares), K₃[Fe(CN)₆] (blue diamonds), and [Ru(bpy)₃]Cl₂ (red circles) suggests that cations are the major charge carriers through carbon nanotubes. (b) Reversal potential (E_rev) as a function of KCl concentration in reservoir 2. Concentration of KCl in reservoir 1 was fixed at 10 mM, while in reservoir 2 [KCl] was varied between 10 mM and 1.0 M. E_rev was always positive on the more dilute side confirming the cation selectivity of these channels. The solid line is the best fit of data according to the GHK equation yielding a permeability ratio of 121 ± 5 for cations over anions in KCl. (c) Effect of cation enthalpy of hydration. Conductance comparison among KNO₃ (red circles), NH₄NO₃ (blue diamonds), and TlNO₃ (black squares) electrolytes with various concentrations at a pH of ∼6.0. Cations with lower absolute value of enthalpy of hydration exhibit higher conductance.

Figure 3

Probing the electrostatic mechanism and conduction across SWCNTs (Device 1). (a) Reversal potential modulation of the surface potential was accomplished using electrolyte screening or electrostatic adsorption of polycations. The KCl concentration in reservoir 2 was always equal to 2 times the concentration of KCl in reservoir 1. Reversal potential as a function of KCl concentration in reservoir 1 before (black squares) and after (red circles) addition of a small amount of polycation to reservoir 1. (b) Conductance variation, with respect to the solution with pH = 7.0. Data recorded as we symmetrically reduced the pH of 100 mM KCl solutions from 8.0 to 2.0. An approximate pK_a of ∼5 is observed for the channel functional groups. (c) Ionic conductance dependence on concentration of LiCl (black squares), NaCl (red circles), and KCl (blue diamonds) electrolytes in the low to medium concentration range. Solid lines are the best fit of data according to the electrostatic model described in the main text. Inset: same ionic conductance data as in (c) plotted on a linear concentration axis. (d) Effect of cation valency on conductance. Conductance modulation with respect to 200 mM KCl divalent-free solutions as divalent salt is added. MgCl₂ (black squares), CaCl₂ (red circles), SrCl₂ (blue triangles), and BaCl₂ (green triangles).

Figure 4

Conductance reduction at high ion concentrations. Ionic conductance dependence of (a) Device 1 and (b) Device 2 on the concentration of LiCl (black squares), NaCl (red circles), and KCl (blue diamonds) electrolytes in the low to high concentration range. (c) Ionic conductance dependence on LiCl concentration in four CNT devices with different conductivity levels.