Complete steric exclusion of ions and proton transport through confined monolayer water

Abstract

Biological membranes allow permeation of water molecules but can reject even smallest ions. Behind these exquisite separation properties are protein channels with angstrom-scale constrictions (e.g., aquaporins). Despite recent progress in creating nanoscale pores and capillaries, they still remain distinctly larger than protein channels. We report capillaries made by effectively extracting one atomic plane from bulk crystals, which leaves a two-dimensional slit of a few Å in height. Water moves through these capillaries with little resistance whereas no permeation could be detected even for such small ions as Na + and Cl -. Only protons can diffuse through monolayer water inside the capillaries. The observations improve our understanding of molecular transport at the atomic scale and suggest further ways to replicate the impressive machinery of living cells.It has long been an aspirational goal to create artificial structures and devices with separation properties similar to those of biological membranes 1-5 . The latter utilize a number of separation mechanisms but it is believed that angstrom-scale constrictions within protein channels 6,7 play a key role in steric (size) exclusion of ions with the smallest hydration diameters D H  7 Å, typically present in biofluids and seawater 8,9 . Such constrictions are particularly difficult to replicate artificially because of the lack of fabrication tools capable to operate with such precision and, also, because the surface roughness of materials is typically much larger than the required angstrom scale 1 . Nonetheless, several artificial systems with nanometer and sub-nanometer dimensions were recently demonstrated, including narrow carbon and boron-nitride nanotubes 5,10,11 , graphene oxide laminates 12,13 and atomic-scale pores in graphene and MoS 2 monolayers 3,4,14 . The resulting devices exhibited high selectivity with respect to certain groups of ions (for example, they blocked large ions but allowed small ones 12,13 or rejected anions but allowed cations and vice versa 2,3,5 ). Most recently, van der Waals assembly of two-dimensional (2D) crystals 15 was used to make slit-like channels of several Å in height 16,17 . They were atomically smooth and chemically inert and exhibited little ( 10 -4 C cm -2 ) surface charge 17 . The channels allowed fast water permeation 16 and blocked large ions with a complete cutoff for diameters larger than 10 Å (ref. 17 ). Small ions (for example, those in seawater with D H of 7 Å) still permeated through those channels with little hindrance, showing that an angstrom-scale confinement comparable to that in aquaporins 6,7 is essential for steric exclusion of small-diameter ions. In this report, we describe 2D channels with the height h of about 3.4 Å (ref. 18), which are twice smaller than any hydrated ion (smallest ions are K + and Clwith D H  6.6 Å) 8,19 but sufficiently large to allow water inside (effective size of water molecules is 2.8 Å). The achieved confinement matches the size of protein constrictions in biological ...

Ultrahigh Ionic Exclusion through Carbon Nanomembranes

“…To achieve it, nature utilizes aquaporins, membrane proteins with 0.3 nm wide channels that efficiently transport water molecules in a single‐file motion across cell membranes but block all ionic species . This collective movement of water in natural nanoconduits stimulates the development of artificial membranes to provide clean water for mankind, and the key is to create similarly sized channels . Commercially used osmosis membranes are mostly derived from polymers whose chains are often randomly arranged leading to a broad pore size distribution .…”

Graphene oxide quantum dots embedded polysulfone membranes with enhanced hydrophilicity, permeability and antifouling performance

“…Recently, graphene oxide (GO) has become a promising membrane material owing to its impressive separation performance [22][23][24][25]. GO membranes possess ultrafast water transport properties and extraordinary water adsorption characteristics [26,27].…”

Solvation‐Involved Nanoionics: New Opportunities from 2D Nanomaterial Laminar Membranes

“…This contributes to the establishment of a robust theoretical framework where quantitative analysis is allowed. Indeed, this requirement has been well reflected by the persistent effort toward the manufacturing of nanochannels with precisely reduced sizes and well‐defined geometries . Understanding and proper interpretation of experimental results can only be made possible if the channel structures are quantifiable.…”

“…Due to the atomic thickness of the 2D monolayer materials, the extracting method is capable of constructing nanochannels with precision at unprecedented angstrom‐level . Such a well‐defined nanochannel is perfectly suited for fundamental study of ion transport involved in angstrom‐scale confinement …”

“…This strategy allows the cost‐effective production of 2D‐NLMs with tunable nanochannels at large scale . Given the ease of synthesis, high‐level structural tunability and versatility, 2D‐NLMs have been extensively explored for confined molecular and ion transport by many research groups including Gao, Geim, Gogotsi, Guo, Huang, Jin, Li, Mi, Nair, Ren, Zhu, and more. The following section will lay out some of the typical attributes of 2D‐NLMs obtained by cascading to correlate with the advances in solvation‐involved nanoionics research.…”