Physics Tomorrow Letters

Introduction

Although much experimental, theoretical, and molecular modeling effort has been devoted over the past years to understanding water and ion (electrolyte) transport through carbon nanotubes (CNTs) [1–3], the origin of the electric charge localized on the surface of industrially important CNT based nanofluidic systems still remains unclear (see Ref. [4] and references therein). It has already been proposed that this surface charge could arise from functional groups at the CNT entrances [5, 6] and/or the specific adsorption of ions, such as OH− [7]. Although the above cited studies lead to the conclusion that this surface charge plays a key role in governing ion transport in CNTs, it is difficult to regulate it directly and one is left to making inferences, for example by studying the variation of ionic conductance G with the pH or salt concentration, cs, of the external bulk reservoirs bounding the CNT. Intriguing results have been obtained, including a power law behavior, G ∝ c α s , with 1/2 ≤ α ≤ 1, which could be interpreted as the manifestation of an underlying surface charge regulation mechanism [8–10]. Through a simplified feasibility study we propose in this work that by biasing a CNT incorporated in a nanofluidic system via an applied gate voltage, Vg, and taking into account explicitly the quantum capacitance (QC) of the quasi-1D CNT structure as well as the nonlinear capacitance of the confined electrolyte ion the pore, it should be possible to quantify the CNT surface charge density σQ and establish a link between the intrinsic CNT electronic properties and ion transport through the same structure, such as the electrolyte conductance through the CNT. A major conclusion it that these intrinsic electronic properties will depend significantly, under certain conditions, on whether the CNT is metallic (M) or semiconducting (SC).