Voltage-activated transport of ions through single-walled carbon nanotubes

Abstract

Ionic transport through single-walled carbon nanotubes (SWCNTs) is promising for many applications but remains both experimentally challenging and highly debated. Here we report ionic current measurements through microfluidic devices containing one or several SWCNTs of diameter of 1.2 to 2 nm unexpectedly showing a linear or a voltage-activated I-V dependence. Transition from an activated to a linear behavior, and stochastic fluctuations between different current levels were notably observed. For linear devices, the high conductance confirmed with different chloride salts indicates that the nanotube/water interface exhibits both a high surface charge density and flow slippage, in agreement with previous reports. In addition, the sublinear dependence of the conductance on the salt concentration points toward a charge-regulation mechanism. Theoretical modelling and computer simulations show that the voltage-activated behavior can be accounted for by the presence of local energy barriers along or at the ends of the nanotube. Raman spectroscopy reveals strain fluctuations along the tubes induced by the polymer matrix but displays insufficient doping or variations of doping to account for the apparent surface charge density and energy barriers revealed by ion transport measurements. Finally, experimental evidence points toward environment-sensitive chemical moieties at the nanotube mouths as being responsible for the energy barriers causing the activated transport of ions through SWCNTs within this diameter range.

Role of charge regulation and flow slip in the ionic conductance of nanopores: An analytical approach

“…Later Secchi et al [5] showed that the CNT conductance at low c s varies with c s and the pH of the solution and extracted an exponent α = 1/3. A similar observation has been made recently by Yazda et al [7], but with a different power law. The theoretical explanation proposed in the literature is charge regulation [5,25], i.e.…”

“…Experiments on single-walled CNTs have been performed recently for radii between 0.6 and 1 nm and using KCl [7]. For those that showed a linear I − V curve, the conductance was measured as a function of c s .…”

“…Note that in this paper we assumed that the nanopore is uniformly charged. The eventual presence of charge defects leads to non-linear I − V curves such as observed in [7]. Furthermore, we adopted the bulk values for the ionic mobilities and treated the ions at the mean-field level (thereby neglecting the effects of excluded volume, ionic correlations, and dielectric exclusion [33,35]).…”

“…behavior at low c s . Note that, compared to the CNTs used in Ref [7],. these CNTs have much larger radii, and are multi-walled.To model their data, Secchi et al simplified the problem by adopting several assumptions (including the neglect of electro-osmosis) that enable them to uncover the intermediate scaling regime in c 1/3 s , which we have shown to be visible only for very highly charged nanopores in the absence of slip.…”

Switching on microglia with electro-conductive multi walled carbon nanotubes

“…CNTs have recently been regarded as interesting candidates for highly selective and permeable solid-state ion channels, mimicking biological ion channels, due to their unique structure. Recent data, demonstrating the electrophoretic transport of ions through CNTs [53] support and strengthen our hypothesis that electrochemical interactions, due to the generation of ionic currents, could take place between cell membranes and electro-conductive CNTs. Taken together, our results show, for the first time, that electroconductive MWCNTs can change microglia phenotype and function, without been cytotoxic.…”