Angstrom-Scale Capillaries—Ion Selectivity Beyond Steric Effects

Understanding ion transport in nano/angstrom scale channels has practical relevance in applications such as membrane desalination, blue energy, supercapacitors and batteries, as well as in understanding ionic flow through biological channels. Synthetic Å-channels are now a reality with the emergence of several cutting-edge bottom-up and top-down fabrication methods. In particular, the use of atomically thin 2D-materials and nanotubes as components to build fluidic conduits has pushed the limits of fabrication to the Å-scale. In this talk, I will discuss about angstrom (Å)-scale capillaries, which can be dubbed as “2D-nothing”. The Å-capillary is an antipode of graphene, created by what is left behind after extracting one-atomic layer out of a crystal [1]. What is intriguing here is, the dimensions of these channels being comparable to the size of a water molecule.<br/>The Å-capillaries have helped probe several intriguing molecular-scale phenomena experimentally, including: water flow under extreme atomic-scale confinement [1] complete steric exclusion of ions [3,5], specular reflection and quantum effects in gas reflections off a surface [2,7], voltage gating of ion flows [4] translocation of DNA [6]. I will present ionic flows induced by stimuli (electric, pressure, concentration gradient) and discuss the importance of ionic parameters that are often overlooked in the selectivity between ions. The mass production and the robustness of the nano/angstrofluidic devices for large-scale applications are still the main challenges. Toward the end of the talk, I will discuss strategies to scale the production of Å-capillaries.<br/><b>Acknowledgements</b><br/>B.R. acknowledges the funding from Royal Society university research fellowship and enhancement award RGF\EA\181000, and funding from the European Union’s H2020 Framework Programme/ERC Starting Grant agreement number 852674 - AngstroCAP.<br/>This work was done in collaboration with A.K. Geim, A. Keerthi, K. Gopinadhan, A. Esfandiar, S. Goutham, Y. You, F.C. Wang, S. J Haigh from University of Manchester, and with L. Bocquet, T. Mouterde, A. Poggioli, A. Siria, from Micromégas team, ENS Paris.<br/><u>References:</u><br/>[1] B. Radha et al., Molecular transport through capillaries made with atomic-scale precision. <b><i>Nature</i></b> 538, 222<br/>(2016).<br/>[2] A. Keerthi et al., Ballistic molecular transport through two-dimensional channels, <b><i>Nature</i></b> (2018), 558, 420.<br/>[3] A. Esfandiar et al., Size effect in ion transport through angstrom-scale slits. <b><i>Science</i></b> 358, 511 (2017).<br/>[4] T. Mouterde et al., Molecular streaming and voltage gated response in Angstrom scale channels. <b><i>Nature</i></b> 567,<br/>87 (2019).<br/>[5] K. Gopinadhan et al., Complete ion exclusion and proton transport through monolayer water. <b><i>Science</i></b> 363,<br/>145 (2019).<br/>[6] W. Yang et al., Translocation of dna through ultrathin nanoslits. <b><i>Advanced Materials</i></b> 2007682, (2021).<br/>[7] J. Thiruraman et al., Gas flows through atomic-scale apertures, <b><i>Science Advances </i></b>6, eabc7927, (2020)