An artificial sodium-selective subnanochannel

Abstract

Single-ion selectivity with high precision has long been pursued for fundamental bioinspired engineering and applications such as in ion separation and energy conversion. However, it remains a challenge to develop artificial ion channels to achieve single-ion selectivity comparable to their biological analogs, especially for high Na⁺/K⁺ selectivity. Here, we report an artificial sodium channel by subnanoconfinement of 4'-aminobenzo-15-crown-5 ethers (15C5s) into ~6-Å-sized metal-organic framework subnanochannel (MOFSNC). The resulting 15C5-MOFSNC shows an unprecedented Na⁺/K⁺ selectivity of tens to 10² and Na⁺/Li⁺ selectivity of 10³ under multicomponent permeation conditions, comparable to biological sodium channels. A co-ion-responsive single-file transport mechanism in 15C-MOFSNC is proposed for the preferential transport of Na⁺ over K⁺ due to the synergetic effects of size exclusion, charge selectivity, local hydrophobicity, and preferential binding with functional groups. This study provides an alternative strategy for developing potential single-ion selective channels and membranes for many applications.

Fig. 1.. Fabrication of artificial sodium channel by assembly of 15C5 into UiO-66-(COOH)₂–based MOFSNC embedded in the single PET NC for biomimetic Na⁺-selective transport.

(A) Schematic illustration of a UiO-66-(COOH)₂–based MOFSNC and the crystal structure of UiO-66-(COOH)₂ (i). EDC, N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride; NHSS, N-hydroxysulfosuccinimide sodium salt. (B) Schematic illustration of the 15C5-MOFSNC fabricated by grafting 15C5s into the MOFSNC and the crystal structure of 15C5-UiO-66-(COOH)₂ with one 15C5 in an octahedral cavity (ii). There is a gradient layer structure in the 15C5-MOFSNC: (1) a thinner 15C5-UiO-66-(COOH)₂ layer close to the base side; (2) UiO-66-(COOH)₂ layer with negligible 15C5 in the middle zone, and (3) a thicker 15C5-UiO-66-(COOH)₂ layer close to the tip side. (C) Ultraselective Na⁺ transport by 15C5-MOFSNC to mimic the function of biological sodium channels and the ring of the grafted 15C5s as the selective binding sites for Na⁺ (iii).

Fig. 2.. Characterization of the artificial sodium channel.

(A) SEM images of the MOFSNC: tip side surface (i), cross section (ii), and base side surface (iii). (B) PXRD of the MOFSNC and PET NC film with the 2θ range of 5° to 11° magnified as (i) and (ii). a.u., arbitrary unit. (C) Water contact angle of tip and base sides of MOFSNC and 15C5-MOFSNC. The enlarged contact angle on the top surface of 15C5-MOFSNC indicates the enhanced hydrophobicity. (D) The N₂ isotherms and (E) PSDs of MOF and MOF-15C5 crystals. The reduced BET surface and pore size of MOF-15C5 indicated the narrowed aperture size of MOFSNC-15C5 compared to MOFSNC.STP, Standard Temperature and Pressure. (F) Magnified view of a single 15C5 molecule (“licorice” style) binding with a Na⁺ ion (orange ball) confined within an octahedral cavity of UiO-66-(COOH)₂ observed by MD simulations. The volume enclosed by gray surfaces is the accessible volume in UiO-66-(COOH)₂ supercell (V_MOF). The volume enclosed by green surfaces is the volume occupied by water molecules (V_H2O). The gray region without overlapping green regions represents the volume occupied by 15C5 and cations. The filling of 15C5 molecule reduces water molecule numbers in the octahedra cavity, indicating the improved hydrophobicity of MOF channels.

Fig. 3.. Ion permeation rate and selectivity of MOFSNC and 15C5-MOFSNC.

(A) Schematics of the experimental setup for ion permeation under multicomponent conditions where a transmembrane voltage is applied to drive ion transport from the feed (base side) to the permeate side (tip side). (B and C) Permeation rate and selectivity of the MOFSNC and 15C5-MOFSNC in multicomponent solutions with an applied voltage bias of 1.0 V.

Fig. 4.. Ultrahigh Na⁺ selectivity of 15C5-MOFSNC.

(A) Permeation rates of 15C5-MOFSNC with applied voltage bias of 0.25 to 4.0 V. (B) Ion selectivity of 15C5-MOFSNC with applied voltage bias of 0.25 to 4.0 V. (C) The variation of ion selectivity with pH value. (D) Cycling stability of 15C5-MOFSNC ultrahigh ion selectivity.

Fig. 5.. Understanding of the ion selectivity mechanisms of 15C5-MOFSNC.

(A) I-V curves of the 15C5-MOFSNC. (B) I-V curves of the MOFSNC. (C) Permeation rates of 15C5-MOFSNC and MOFSNC in single-component solutions with an applied voltage bias of 1.0 V. (D) Schematic diagram of retarded Li⁺ transport in 15C5-MOFSNC.

Fig. 6.. Biomimetic ion selectivity mechanisms of 15C5-MOFSNC.

(A) Preferential transport for Na⁺ (i) and K⁺ (ii) in 15C5-MOF layers of 15C5-MOFSNC in single-component solutions. (B) Preferential transport for Na⁺ against K⁺ in 15C5-MOF layers of 15C5-MOFSNC in mixed solutions. The coordination of Na⁺-functional groups in the triangle window plane is highlighted in the dashed rectangle. Preferential binding of the 15C5 with Na⁺ leads to the exclusion of both K⁺ and Li⁺ under the multicomponent solution, which functions as the biological Na⁺ selectivity filter. (C) Biological sodium channel with the selectivity filter (i) as the inspiration for the biomimetic design of artificial sodium channels. The configuration of amino acids (ii) is highlighted to demonstrate the essential role of Na⁺-functional group coordination for Na⁺/K⁺ selectivity.

Fig. 7.. Theoretical simulations of Na⁺/K⁺ selectivity in the 15C5-MOFSNC.

(A) Calculated ion concentrations in 1D model for the 15C5-SNC system MOFSNC for mixture ion permeation experiment using the mPNP theory (Materials and Methods). When the binding energy of 15C5 with Na⁺ is zero (φ_Na⁺ = 0 eV), the cations Na⁺ and K⁺ have the same concentration. At φ_Na⁺ = 0.15 eV, the preferable binding to 15C5 groups enhances the Na⁺ concentration. The markedly dropped K⁺ concentration should be attributed to electrostatic repulsion from the adsorbed Na⁺ ions. (B) The average Na⁺ and K⁺ concentration in the 15C5-MOFSNC channel as a function of φ_Na⁺ for both pure (i.e., single component) and mixed (i.e., multi-component) ion permeation cases. (C) The dependence of ion fluxes on φ_Na⁺. The fluxes are normalized to that of Na⁺ in the single-component 1 M NaCl solution at φ_Na⁺ = 0. The relative order of Na⁺ and K⁺ fluxes is inverted for the mixture ion permeation case compared with the single-ion permeation case, which is consistent with experimental results. (D) The calculated Na⁺/K⁺ selectivity increased with the increase of φ_Na⁺ and 15C5 density. Note that the dashed vertical line in (B) represents the lower limit of φ_Na⁺ values in experiments via our analysis.