Gated ion transport in disjoint carbon nanotubes by a water bridge

Abstract

Nanofluidic ion transport in carbon nanotubes (CNTs) has numerous applications, including membrane separation, energy conversion, and ionic circuits. To imitate the natural gated ion channel systems, CNTs require a more complex structure beyond a single intact tube. In this study, we present molecular dynamics simulations demonstrating the gated ion transport in two disjoint CNTs with a nanogap. The observation of nonlinear current-voltage characteristics, featuring onset and plateau voltages for ion transport, indicates that the nanogap serves as a gate. Ions cannot transport through the disjoint CNTs below the onset voltage. Interestingly, a water bridge forms occasionally within the nanogap above the onset voltage, allowing ions to transport through coupling with water molecules. Above the plateau voltage, a stable water bridge forms and ions can transport in disjoint CNTs with the same efficiency as in the intact CNT. The formation of water bridges is due to that the electric stress on polarized water molecules near the nanogap overcomes the surface tension. Furthermore, the gated ion transport behavior is dependent on the nanogap length, CNT diameter, and CNT wettability, but not on the ion concentration. This work offers a viable approach for developing CNT-based nanofluidic devices with finely tuned ion transport efficiency.