Impact of PEG additives and pore rim functionalization on water transport through sub-1 nm carbon nanotube porins

Abstract

Carbon nanotubes represent one of the most interesting examples of a nanofluidic channel that combines extremely small diameters with atomically smooth walls and well-defined chemical functionalities at the pore entrance. In the past, sub-1 nm diameter carbon nanotube porins (CNTPs) embedded in a lipid membrane matrix demonstrated extremely high water permeabilities and strong ion selectivities. In this work, we explore additional factors that can influence transport in these channels. Specifically, we use stopped-flow transport measurements to focus on the effect of chemical modifications of the CNT rims and chaotropic polyethyleneglycol (PEG) additives on CNTP water permeability and Arrhenius activation energy barriers for water transport. We show that PEG, especially in its more chaotropic coiled configuration, enhances the water transport and reduces the associated activation energy. Removal of the static charges on the CNTP rim by converting -COOH groups to neutral methylamide groups also reduces the activation energy barriers and enhances water transport rates.

Water-ion permselectivity of narrow-diameter carbon nanotubes

“…At this size, a CNTP channel is still likely to transport some ions. Thus, further improvements of the salt rejection of CNTPs may require further functionalization ( 38 ) of the CNTP pore mouth or their sidewalls.…”

Single-file transport of water through membrane channels

“… 99 Moreover, the measured value exceeds the upper limit of water transport through a pore of ~15 kcal mol −1 (see the preceding section). In their rebuttal in Science 116 as well as in their contribution in the current Faraday Discussions volume, 117 the authors do not provide evidence that water or ions pass the nanotubes. On the contrary, the inhibitory effect of calcium on both the ion and water fluxes is compatible with the view that calcium binding to the lipid alters lipid packing, which may partly seal the nanotube induced leak for water and ions in the bilayer.…”