Strong Electroosmotic Coupling Dominates Ion Conductance of 1.5 nm Diameter Carbon Nanotube Porins

Abstract

Extreme confinement in nanometer-sized channels can alter fluid and ion transport in significant ways, leading to significant water flow enhancement and unusual ion correlation effects. These effects are especially pronounced in carbon nanotube porins (CNTPs) that combine strong confinement in the inner lumen of carbon nanotubes with the high slip flow enhancement due to smooth hydrophobic pore walls. We have studied ion transport and ion selectivity in 1.5 nm diameter CNTPs embedded in lipid membranes using a single nanopore measurement setup. Our data show that CNTPs are weakly cation selective at pH 7.5 and become nonselective at pH 3.0. Ion conductance of CNTPs exhibits an unusual 2/3 power law scaling with the ion concentration at both neutral and acidic pH values. Coupled Navier−Stokes and Poisson−Nernst−Planck simulations and atomistic molecular dynamics simulations reveal that this scaling originates from strong coupling between water and ion transport in these channels. These effects could result in development of a next generation of biomimetic membranes and carbon nanotube-based electroosmotic pumps.

Bioinspired and biomimetic membranes for water purification and chemical separation: A review

“…2018; Chen et al, 2019;Yao et al, 2019;Sullivan et al, 2020). These special CNTs have subnanometer pores with a columnar length of a few nanometers that can be inserted into lipid bilayer membranes.…”

Ionic transport through carbon nanotube porins

“…The researchers further demonstrated that these membranes were capable of achieving similar gas separation and water filtration properties to the nanotube forest membranes grown by CDV. 43,53 The Hinds group also developed a CNT membrane platform based on microtomed slices of an epoxy resin block filled with randomly oriented long carbon nanotubes, 54 where sufficiently long CNTs were able to span the cross-section of the membrane and form well-defined pores. A key advantage of these hybrid techniques was that they alleviated researchers from the limitation of CNT pore sizes imposed by the vertically-aligned CNT "forest" synthesis method and enabled them to use smaller diameter pores.…”

“…72 Interestingly, the Marand group reported that membranes incorporating CNTs functionalized with chainlike zwitterion molecules rejected essentially all ions, giving a 98.6% rejection rate with NaCl. 53 The high levels of rejection were attributed to both size exclusion (from the bulky functional groups) and electrostatic repulsion (from the multiple charges) effects.…”

“…43,51,52 We further conducted pH-dependent studies with our CNTPs and found a significant difference between pH 3 and pH 5 in water and ion transport studies, which corresponds to the pK a of carboxylate groups. 8,52,53 Additionally, we were able to modify the CNTP ends using EDC coupling to attach either methyl groups or gold nanoparticles. 38,54,55 These results demonstrate that flexibility of the platform by showing how alternative functional groups can be incorporated, which can be used to tune the transport behavior of the CNTPs.…”

“…2.13, 2.14, and Table 2.1). 6,7,8,53 The painted lipid bilayer method is a well-established method used to form planar model lipid systems. In this setup, apertures with diameters ca.…”