Coherence Resonance in a Single-Walled Carbon Nanotube Ion Channel

Oscillations in Carbon Nanotube Conductivity Theoretical studies have suggested that protons can be conducted rapidly in water trapped inside carbon nanotubes. C. Y. Lee et al. (p. 1320) connected two aqueous reservoirs with opened, single-walled carbon nanotubes, half a millimeter long but only 1.5 nanometers wide, and observed a high, stable proton current under electroosmotic conditions arising from a single nanotube. The addition of alkali cations caused random pore blocking and oscillations in ion current, resembling events seen in biological ion channels. Opened, water-filled carbon nanotubes can exhibit oscillations in proton conductivity when alkali ions are present. Biological ion channels are able to generate coherent and oscillatory signals from intrinsically noisy and stochastic components for ultrasensitive discrimination with the use of stochastic resonance, a concept not yet demonstrated in human-made analogs. We show that a single-walled carbon nanotube demonstrates oscillations in electroosmotic current through its interior at specific ranges of electric field that are the signatures of coherence resonance. Stochastic pore blocking is observed when individual cations partition into the nanotube obstructing an otherwise stable proton current. The observed oscillations occur because of coupling between pore blocking and a proton-diffusion limitation at the pore mouth. The result illustrates how simple ionic transport can generate coherent waveforms within an inherently noisy environment and points to new types of nanoreactors, sensors, and nanofluidic channels based on this platform.