Artificial ion channels with ion permeability and selectivity comparable to their biological counterparts are highly desired for efficient separation, biosensing, and energy conversion technologies. In the past two decades, both nanoscale and sub-nanoscale ion channels have been successfully fabricated to mimic biological ion channels. Although nanoscale ion channels have achieved intelligent gating and rectification properties, they cannot realize high ion selectivity, especially single-ion selectivity. Artificial angstrom-sized ion channels with narrow pore sizes <1 nm and well-defined pore structures mimicking biological channels have accomplished high ion conductivity and single-ion selectivity. This review comprehensively summarizes the research progress in the rational design and synthesis of artificial subnanometer-sized ion channels with zero-dimensional to three-dimensional pore structures. Then we discuss cation/anion, mono-/di-valent cation, mono-/di-valent anion, and single-ion selectivities of the synthetic ion channels and highlight their potential applications in high-efficiency ion separation, energy conversion, and biological therapeutics. The gaps of single-ion selectivity between artificial and natural channels and the connections between ion selectivity and permeability of synthetic ion channels are covered. Finally, the challenges that need to be addressed in this research field and the perspective of angstrom-scale ion channels are discussed.

Angstrom-scale ion channels towards single-ion selectivity.

Cost and efficiency perspectives of ceramic membranes for water treatment.

Advances in graphene oxide membranes for water treatment

Robust zirconia ceramic membrane with exceptional performance for purifying nano-emulsion oily wastewater

Transition metal dichalcogenide membranes exhibit good antiswelling properties but poor water desalination property. Here, a one‐step covalent functionalization of MoS2 nanosheets for membrane fabrication is reported, which is accomplished by simultaneous exfoliating and grafting the lithium‐ion‐intercalated MoS2 in organic iodide water solution. The lithium intercalation amount in MoS2 is optimized so that the quality of the produced 2D nanosheets is improved with homogeneous size distribution. The lamellar MoS2 membranes are tested in reverse osmosis (RO), and the functionalized MoS2 membrane exhibits rejection rates of >90% and >80% for various dyes (Rhodamine B, Crystal Violet, Acid Fuchsin, Methyl Orange, and Evans Blue) and NaCl, respectively. The excellent ion‐sieving performance and good water permeability of the functionalized MoS2 membranes are attributed to the suitable channel widths that are tuned by iodoacetamide. Furthermore, the stability of the functionalized MoS2 membranes in NaCl and dye solutions is also confirmed by RO tests. Molecular dynamics simulation shows that water molecules tend to form a single layer between the amide‐functionalized MoS2 layers but a double layer between the ethanol‐functionalized MoS2 (MoS2‐ethanol) layers, which indicates that a less packed structure of water between the MoS2‐ethanol layers leads to lower hydrodynamic resistance and higher permeation.

Simultaneous Electrochemical Exfoliation and Covalent Functionalization of MoS2 Membrane for Ion Sieving

It remains challenging for graphene oxide (GO) membranes to achieve highly efficient performance and sufficient stability for aqueous molecule/ion precise separations. Herein, a molecular-level rational structure design protocol was proposed to develop ceramic-based graphene oxide framework (GOF) membranes with significantly enhanced sieving performance and stability for efficient removal of salts and micropollutants. Via a molecular cross-linking strategy, interlayered nanochannels between GO nanosheets can be rationally designed, featuring precisely tailorable channel size, promising surface chemistries and remarkably robust stability suitable for aqueous separation. Due to a significantly decreased nanochannel size, cross-linking of TU (thiourea) molecule significantly improved monovalent salt rejection (95.6% for NaCl), outperforming existing state-of-the-art GO-based, commercial organic nanofiltration and emerging two-dimensional MoS2 membranes, while moderately decreasing water permeability. In comparison, the GOF membranes cross-linked with MPD (m-phenylenediamine) exhibited a simultaneous increase in permeability and rejection for both salts and micropollutants (21.0% and 53.3% enhancement for chloramphenicol (CAP) solution), breaking their conventional trade-off issue. Cross-linking mechanism indicates that more robust nanochannels were formed by stronger covalent bonds via dehydration condensation between amine (TU/MPD) and carboxyl groups (GO), and nucleophilic addition between amine (TU/MPD) and epoxy groups (GO). Molecule/ion separation mechanism involved size sieving (steric hindrance), electrostatic interaction, Donnan effect, and partial dehydration effect. This work provides a novel protocol for rationally designing size and surface chemistry of highly robust GO nanochannels at a subnanometer level to construct water-stable functional GOF membranes with enhanced sieving performance for water treatment applications.

Cross-linked Graphene Oxide Framework Membranes with Robust Nano-Channels for Enhanced Sieving Ability.

Through a simple process, a hierarchical NiCoMoS@NiCoAl hydrotalcite (LDH) core-shell structure is prepared by growing the ultrathin NiCoAl-LDH nanosheets onto NiCoMoS nanoneedles arrayed on Ni foam (NF). After coating NiCoAl-LDH...

3D hierarchical core-shell structural NiCoMoS@ NiCoAl hydrotalcite for high-performance supercapacitors

Free-standing 2D nanoassemblies are a new class of advanced functional materials that can integrate the unique properties of their individual components. Herein, we report a facile template-free solvothermal strategy to manipulate the assembly of structurally distinct building blocks into free-standing 2D quasi-nanosheets. The obtained 2D nanoassemblies are magnetic in nature with thickness in the range of tens of nanometers and lateral dimensions of several micrometers. They showed exceptionally high stability and can even remain intact in any solvent for months without any significant disassembly. The magnetic quasi-nanosheets were further used to remove polystyrene (PS), a model microplastic pollutant, from water. The robust phosphomolybdic acid (PMA) cluster-based hybrid exhibited outstanding PS removal performance because of the unique quasi-nanosheet architecture and multiple coordination sites of the redox-active PMA clusters. Overall, our developed co-assembly and subsequent water purification strategy effectively tackles various limitations associated with traditional assembly and water purification strategies.

PMA-FeCo mixed-oxide magnetic quasi-nanosheets.

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Cross-linked Graphene Oxide Framework Membranes with Robust Nano-Channels for Enhanced Sieving Ability.

3D hierarchical core-shell structural NiCoMoS@ NiCoAl hydrotalcite for high-performance supercapacitors

PMA-FeCo mixed-oxide magnetic quasi-nanosheets.