Ion sieving in graphene oxide membranes via cationic control of interlayer spacing

Effective Separation of CO2 Using Metal-Incorporated rGO Membranes

Graphene-based materials, primarily graphene oxide (GO), have shown excellent separation and purification characteristics. Precise molecular sieving is potentially possible using graphene oxide-based membranes, if the porosity can be matched with the kinetic diameters of the gas molecules, which is possible via the tuning of graphene oxide interlayer spacing to take advantage of gas species interactions with graphene oxide channels. Here, highly effective separation of gases from their mixtures by using uniquely tailored porosity in mildly reduced graphene oxide (rGO) based membranes is reported. The gas permeation experiments, adsorption measurement, and density functional theory calculations show that this membrane preparation method allows tuning the selectivity for targeted molecules via the intercalation of specific transition metal ions. In particular, rGO membranes intercalated with Fe ions that offer ordered porosity, show excellent reproducible N₂ /CO₂ selectivity of ≈97 at 110 mbar, which is an unprecedented value for graphene-based membranes. By exploring the impact of Fe intercalated rGO membranes, it is revealed that the increasing transmembrane pressure leads to a transition of N₂ diffusion mode from Maxwell-Stefan type to Knudsen type. This study will lead to new avenues for the applications of graphene for efficiently separating CO₂ from N₂ and other gases.

Coordination-Driven Hierarchical Assembly of Hybrid Nanostructures Based on 2D Materials

2D materials have received tremendous scientific and engineering interests due to their remarkable properties and broad-ranging applications such as energy storage and conversion, catalysis, biomedicine, electronics, and so forth. To further enhance their performance and endow them with new functions, 2D materials are proposed to hybridize with other nanostructured building blocks, resulting in hybrid nanostructures with various morphologies and structures. The properties and functions of these hybrid nanostructures depend strongly on the interfacial interactions between 2D materials and other building blocks. Covalent and coordination bonds are two strong interactions that hold high potential in constructing these robust hybrid nanostructures based on 2D materials. However, most 2D materials are chemically inert, posing problems for the covalent assembly with other building blocks. There are usually coordination atoms in most of 2D materials and their derivatives, thus coordination interaction as a strong interfacial interaction has attracted much attention. In this review, recent progress on the coordination-driven hierarchical assembly based on 2D materials is summarized, focusing on the synthesis approaches, various architectures, and structure-property relationship. Furthermore, insights into the present challenges and future research directions are also presented.

Electric-Field-Induced Ionic Sieving at Planar Graphene Oxide Heterojunctions for Miniaturized Water Desalination

Layered graphene oxide membranes (GOMs) offer a unique platform for precise sieving of small ions and molecules due to controlled sub-nanometer-wide interlayer distance and versatile surface chemistry. Pristine and chemically modified GOMs effectively block organic dyes and nanoparticles, but fail to exclude smaller ions with hydrated diameters less than 9 Å. Toward sieving of small inorganic salt ions, a number of strategies are proposed by reducing the interlayer spacing down to merely several angstroms. However, one critical challenge for such compressed GOMs is the extremely low water flux (<0.1 Lm^-2 h^-1 bar^-1 ) that prevents these innovative nanomaterials from being used in real-world applications. Here, a planar heterogeneous graphene oxide membrane (PHGOM) with both nearly perfect salt rejection and high water flux is reported. Horizontal ion transport through oppositely charged GO multilayer lateral heterojunction exhibits bi-unipolar transport behavior, blocking the conduction of both cations and anions. Assisted by a forward electric field, salt concentration is depleted in the near-neutral transition area of the PHGOM. In this situation, deionized water can be extracted from the depletion zone. Following this mechanism, a high rejection rate of 97.0% for NaCl and water flux of 1529 Lm^-2 h^-1 bar^-1 at the outlet via an inverted T-shaped water extraction mode are achieved.

Realizing small-flake graphene oxide membranes for ultrafast size-dependent organic solvent nanofiltration

Membranes for organic solvent nanofiltration (OSN) or solvent-resistant nanofiltration (SRNF) offer unprecedented opportunities for highly efficient and cost-competitive solvent recovery in the pharmaceutical industry. Here, we describe small-flake graphene oxide (SFGO) membranes for high-performance OSN applications. Our strategy exploits lateral dimension control to engineer shorter and less tortuous transport pathways for solvent molecules. By using La³⁺ as a cross-linker and spacer for intercalation, the SFGO membrane selective layer was stabilized, and size-dependent ultrafast selective molecular transport was achieved. The methanol permeance was up to 2.9-fold higher than its large-flake GO (LFGO) counterpart, with high selectivity toward three organic dyes. More importantly, the SFGO-La³⁺ membrane demonstrated robust stability for at least 24 hours under hydrodynamic stresses that are representative of realistic OSN operating conditions. These desirable attributes stem from the La³⁺ cross-linking, which forms uniquely strong coordination bonds with oxygen-containing functional groups of SFGO. Other cations were found to be ineffective.

Direct Growth of Wafer-Scale, Transparent, p-Type Reduced-Graphene-Oxide-like Thin Films by Pulsed Laser Deposition

Reduced graphene oxide (rGO) has attracted significant interest in an array of applications ranging from flexible optoelectronics, energy storage, sensing, and very recently as membranes for water purification. Many of these applications require a reproducible, scalable process for the growth of large-area films of high optical and electronic quality. In this work, we report a one-step scalable method for the growth of reduced-graphene-oxide-like (rGO-like) thin films via pulsed laser deposition (PLD) of sp² carbon in an oxidizing environment. By deploying an appropriate laser beam scanning technique, we are able to deposit wafer-scale uniform rGO-like thin films with ultrasmooth surfaces (roughness <1 nm). Further, in situ control of the growth environment during the PLD process allows us to tailor its hybrid sp²-sp³ electronic structure. This enables us to control its intrinsic optoelectronic properties and helps us achieve some of the lowest extinction coefficients and refractive index values (0.358 and 1.715, respectively, at 2.236 eV) as compared to chemically grown rGO films. Additionally, the transparency and conductivity metrics of our PLD grown thin films are superior to other p-type rGO films and conducting oxides. Unlike chemical methods, our growth technique is devoid of catalysts and is carried out at lower process temperatures. This would enable the integration of these thin films with a wide range of material heterostructures via direct growth.

Selectivity mechanism of magnesium and calcium in cation-binding pocket structures of phosphotyrosine

Magnesium (Mg^{2+}) and calcium (Ca^{2+}) are of essential importance in biological activity, but the molecular understanding of their selectivity is still lacking. Here, based on density functional theory calculations and ab initio molecular dynamics simulations, we show that Mg^{2+} binds more tightly to phosphotyrosine (pTyr) and stabilizes the conformation of pTyr, while Ca^{2+} binds more flexibly to pTyr with less structural stability. The key for the selectivity is attributed to the cation-π interactions between the hydrated cations and the aromatic ring together with the synergic interaction between the cations and the side groups in pTyr to form a cation-binding pocket structure, which we refer as side-group-synergetic hydrated cation-π interaction. The existence and relative strength of the cation-π interactions in the pocket structures as well as their structural stability have been demonstrated experimentally with ultraviolet (UV) absorption spectra and ^{1}H NMR spectra. The findings offer insight into understanding the selectivity of Mg^{2+} and Ca^{2+} in a variety of biochemical and physiological essential processes.

Molecular insights into the dispersion stability of graphene oxide in mixed solvents: Theoretical simulations and experimental verification

HYPOTHESIS

Improving the dispersion stability of graphene oxide (GO) suspensions is of great importance in many potential applications of GO, such as GO-based laminated membranes used for separation, printable electronics, and aqueous liquid crystals.

EXPERIMENTS

Molecular dynamics (MD) simulations and quantum chemistry (QC) calculations along with complementary experiments were performed to study the dispersion stability of GO in the mixtures of water and polar organic solvents (dimethyl sulfoxide (DMSO), ethanol, and acetone).

FINDINGS

GO exhibits better dispersion stability in a solvent mixture than in pure water. The MD simulations uncover the underlying mechanism that mixed solvent layers are formed steadily on the surface of GO sheets and screen the interactions between them. QC calculations reveal that both DMSO and water form hydrogen bonds with the oxidized regions of GO. X-ray diffraction experiments confirm that the GO sheets are intercalated by DMSO and water molecules. Furthermore, the optimal ratio of the organic solvent to water is determined to achieve the best dispersion stability of GO through MD simulations. And such ratio is also verified by ultraviolet absorption spectral experiments. Thus, our findings provide a facile method to prepare GO suspensions with high dispersion stability.

Influence of Host-Guest Interaction between Chiral Selectors and Probes on the Enantioseparation Properties of Graphene Oxide Membranes

Graphene oxide (GO)-based membranes have displayed superior performances in the chiral resolution compared with conventional polymer-based and inorganic membranes. However, the effect of the host-guest interaction between chiral selectors and probes on the enantioseparation properties of GO-based membranes remains to be established. In this work, l-phenylalanine (l-Phe, as the chiral selector)-modified GO-based (l-Phe-GO) membranes were fabricated, and their enantioseparation performances toward various enantiomers, that is, d- and l-phenylalanine (d- and l-Phe), d- and l-methionine (d- and l-Met), N-acyl-d-phenylalanine (N-acyl-d-Phe) and N-acyl-l-phenylalanine (N-acyl-l-Phe), and N-acyl-d-methionine (N-acyl-d-Met) and N-acyl-l-methionine (N-acyl-l-Met), were detected. Results show that (i) l-Phe is preferential to transport d-enantiomers relative to l-enantiomers; (ii) as far as d-enantiomers are concerned, the d-Phe-like enantiomers move faster than d-Met-like ones through the l-Phe-GO membrane owing to their different host-guest interactions. The strength of interactions between chiral selectors and probes was further confirmed from both experimental and theoretical standpoints. In the former case, the enantioselective adsorption of l-Phe-GO nanosheets toward the aforementioned enantiomers demonstrates that l-Phe delivers a higher adsorption capacity to d-enantiomers relative to l-enantiomers, and meanwhile, d-Phe-like enantiomers are better than d-Met-like enantiomers in the adsorption capacity. In the latter case, the chiral separation mechanism is clarified using the periodical density functional theory (DFT) calculation, indicating that l-Phe interacts with d-enantiomers more strongly than l-enantiomers. Especially, our calculations unveil that the difference in the interaction strength is principally dominated by the nonstereoselective interactions between chiral probes and the GO surface. Therefore, our findings suggest that the nonstereoselective weak interaction can be employed to improve the enantioselectivity of GO-based membranes.

Unexpected large impact of small charges on surface frictions with similar wetting properties

Generally, the interface friction on solid surfaces is regarded as consistent with wetting behaviors, characterized by the contact angles. Here using molecular dynamics simulations, we find that even a small charge difference (≤0.36 e) causes a change in the friction coefficient of over an order of magnitude on two-dimensional material and lipid surfaces, despite similar contact angles. This large difference is confirmed by experimentally measuring interfacial friction of graphite and MoS₂ contacting on water, using atomic force microscopy. The large variation in the friction coefficient is attributed to the different fluctuations of localized potential energy under inhomogeneous charge distribution. Our results help to understand the dynamics of two-dimensional materials and biomolecules, generally formed by atoms with small charge, including nanomaterials, such as nitrogen-doped graphene, hydrogen-terminated graphene, or MoS₂, and molecular transport through cell membranes.

Cation-controlled wetting properties of vermiculite membranes and its promise for fouling resistant oil-water separation

Manipulating the surface energy, and thereby the wetting properties of solids, has promise for various physical, chemical, biological and industrial processes. Typically, this is achieved by either chemical modification or by controlling the hierarchical structures of surfaces. Here we report a phenomenon whereby the wetting properties of vermiculite laminates are controlled by the hydrated cations on the surface and in the interlamellar space. We find that vermiculite laminates can be tuned from superhydrophilic to hydrophobic simply by exchanging the cations; hydrophilicity decreases with increasing cation hydration free energy, except for lithium. The lithium-exchanged vermiculite laminate is found to provide a superhydrophilic surface due to its anomalous hydrated structure at the vermiculite surface. Building on these findings, we demonstrate the potential application of superhydrophilic lithium exchanged vermiculite as a thin coating layer on microfiltration membranes to resist fouling, and thus, we address a major challenge for oil–water separation technology.

Manipulation of surface energy and wetting properties of solids may impact a variety of processes, including membrane fouling. Here the authors tune properties of vermiculite laminates from superhydrophilic to hydrophobic by cation exchange, and demonstrate potential for fouling resistant oil–water separation.

Enantioseparation processes and mechanisms in functionalized graphene membranes: Facilitated or retarded transport?

Up to date, functionalized graphene-based membranes have exhibited a promising potential in the enantioseparation. However, since precisely controlling the interlayer distance of two-dimensional materials is a great challenge in practical experiments, the transport mechanism of chiral guests in such membranes, together with various critical parameters that play a controlling role in the transport behaviors of the preferentially binding enantiomer in narrow channels, remains to be explored. The molecular dynamics (MD) simulation, especially using the steered MD (SMD) method, might be an alternative way to investigate the enantioseparation processes and mechanisms of layered membranes with different interlayer distances. In this work, D-alanine modified graphene sheets with different interlayer distances were built as membrane models, whereas D- and L-phenylalanine were selected as chiral probes. The effect of the interlayer distance and the applied external force on the enantioseparation performance was examined. Results show that such two parameters exert a significant influence on the enantioseparation performance: (a) Increasing the interlayer distance would result in a conversion from the retarded to the facilitated mechanism at a proper external force (medium); (b) both the large and small driving forces would only lead to the appearance of the retarded transport for the preferential enantiomer, unlike the moderate force; (c) the interaction energy of L-phenylalanine with D-isomer selector decreases with the rising interlayer distances studied in this work, regardless of what the external force is. Our findings can provide guidance on the practical applications in the membrane-based chiral separation.

Nanofiber Based Organic Solvent Anion Exchange Membranes for Selective Separation of Monovalent anions

Present anion exchange membranes are generally constructed by simple and positively charged polymers with insufficient organic solvent resistance, and exhibit a low selectivity in the separation of anions. Here, dissolving poly(paraphenylene terephthalamide) nanofibers into small nanofibers and performing a reaction with quaternary ammonium groups in the one-dimensional small nanofibers, high-performance anion exchange membranes were successfully fabricated. By increasing the 2,3-epoxypropyl trimethylammonium chloride content, the synthesized amide nanofiber quaternary ammonium membranes (ANF#QA) exhibited a higher anion exchange capacity (as high as 1.75 mmol·g^-1) and achieved a high electrochemical performance. In electrodialysis, the ANF#QA-10 membrane showed an exceptional Cl^- selectivity in dilute and concentrated cells. Due to the dense structure and presence of carboxyl groups on the nanofibers, the ANF#QA membranes exhibited a selective separation of monovalent anions. After 48 h of immersion in aqueous acetone solutions, the final ANF#QA-10 membrane exhibited high desalination and concentration efficiency as the initial membrane. This work highlights the promising use of positive charges on small nanofibers, and proposes the design of a special anion exchange membrane, which can be used for electrodialysis in organic solvent solutions, and to selectively separate monovalent anions.

Phenanthroline-Based Polyarylate Porous Membranes with Rapid Water Transport for Metal Cation Separation

The selective separation of ions in terms of extremely similar size and properties remains an important challenge in water purification. We innovated a kind of porous nanofilm via interfacial polymerization using rigid heterocyclic ligands to achieve high valent cation selectivity and rapid water/ion transport. The interconnected microporosity and uniformly distributed cation-affinitive sites of the ultrathin membranes enabled water permeation (7.5 L m^-2 h^-1 bar^-1), ion permeance of Na⁺ (1.5 mol m^-2 h^-1 bar^-1), and Mg²⁺/Na⁺ permselectivity (2.1) during nanofiltration. The forward osmosis exhibited a prominent water flux of 95 LMH at 1 M NaCl draw solution, which expanded various applications. The polyarylate membranes comprising 4,7-diphenyl-1,10-phenanthroline showed a higher water permeation and ion selectivity than the planar monomers, e.g., resorcinol. A distinct fluorescence responsiveness existed between membranes and cations for the interaction characterization. Host-guest nuclear magnetic resonance (NMR) spectroscopy and solid-state nuclear magnetic resonance spectroscopy characterized the preferential affinitive of divalent/high-valent cations in the interconnected microporous powders; an ultraviolet spectrophotometer characterized the light responsiveness of the porous nanofilms. Such an active membrane has potential applications in selective separation and adsorption of cations, photocatalytic materials, photosensors, and other fields.

Thermal Transport in Graphene Oxide Films: Theoretical Analysis and Molecular Dynamics Simulation

As a derivative material of graphene, graphene oxide films hold great promise in thermal management devices. Based on the theory of Fourier formula, we deduce the analytical formula of the thermal conductivity of graphene oxide films. The interlaminar thermal property of graphene oxide films is studied using molecular dynamics simulation. The effect of vacancy defect on the thermal conductance of the interface is considered. The interfacial heat transfer efficiency of graphene oxide films strengthens with the increasing ratio of the vacancy defect. Based on the theoretical model and simulation results, we put forward an optimization model of the graphene oxide film. The optimal structure has the minimum overlap length and the maximum thermal conductivity. An estimated optimal overlap length for the GO (graphene-oxide) films with degree of oxidation 10% and density of vacancy defect 2% is 0.33 μm. Our results can provide effective guidance to the rationally designed defective microstructures on engineering thermal transport processes.

Osmotic Water Permeation through a Carbon Nanotube

Osmosis are essential for not only the application of nanofluidic devices but also the understanding of working principles of biological transmembrane proteins. Despite considerable experimental interests, comprehensive simulation work is still lacking, possibly because of the periodic boundary condition that inevitably leads to the spontaneous exchange of two side reservoirs. To eliminate this disadvantage, herein we design a simple model system by introducing a dipalmitoylphosphatidylcholine bilayer into a common carbon-nanotube (CNT)-based setup, which allows long-time osmotic simulations. Interestingly, the osmotic water flux exhibits an excellent linear relation with the concentration gradient and an Arrhenius relation with the temperature, which highly coincides with recent experimental observations. Furthermore, the osmotic flux can be quantitatively comparable to not only the experimental CNTs and protein channels but also the theoretical estimation from the Hagen-Poiseuille equation. The designed simulation model could open a new window for future studies on osmosis.

Metal-Based Nanocatalysts via a Universal Design on Cellular Structure

Metal-based nanocatalysts supported on carbon have significant prospect for industry. However, a straightforward method for efficient and stable nanocatalysts still remains extremely challenging. Inspired by the structure and comptosition of cell walls and membranes, an ion chemical bond anchoring, an in situ carbonization coreduction process, is designed to obtain composite catalysts on N-doped 2D carbon (C-N) loaded with various noble and non-noble metals (for example, Pt, Ru, Rh, Pd, Ag, Ir, Au, Co, and Ni) nanocatalysts. These 2 nm particles uniformly and stably bond with the C-N support since the agglomeration and growth are suppressed by anchoring the metal ions on the cell wall and membrane during the carbonization and reduction reactions. The Pt@C-N exhibits excellent catalytic activity and long-term stability for the hydrogen evolution reaction, and the relative overpotential at 100 mA cm-2 is only 77 mV, which is much lower than that of commercial Pt/C and Pt single-atom catalysts reported recently.

Reduced Holey Graphene Oxide Membranes for Desalination with Improved Water Permeance

Reduced graphene oxide (r-GO) membranes with narrow channels exhibit salt rejections comparable to conventional nanofiltration (NF) membranes. However, their water permeances are much lower because of the high tortuosity for water permeation. Herein, we report a facile solution-processable approach to create in-plane nanopores on GO nanosheets before reduction, dramatically decreasing the tortuosity and increasing water permeance while retaining the salt rejection. Specifically, holey GO (HGO) nanosheets were prepared via chemical etching using hydrogen peroxide followed by the deposition on a porous support by vacuum filtration and then reduction via exposure to hydriodic acid solutions to generate the reduced HGO (r-HGO) membrane. The generation of nanopores increases the water permeance from 0.4 L m^-2 h^-1 bar^-1 (LMH/bar) to 6.6 LMH/bar with Na₂SO₄ rejection greater than 98.5%, and the membranes were robust under strong cross-flow shearing force for 36 h. Both water permeance and Na₂SO₄ rejection of these r-HGO membranes for the first time simultaneously reach the level of the commercial polyamide-based NF membranes. Given their good antibacterial properties and resistance to aggressive chemical washing, the r-HGO membranes show promise as next-generation NF membranes for desalination.

Two-dimensional nanochannel membranes for molecular and ionic separations

This review highlights the construction and regulation of two-dimensional nanochannel membranes (2DNCMs) as well as their applications in molecular and ionic separations.

Effects of cationic concentration on controlling the interlayer spacings for highly effective ion rejection via graphene oxide membranes

The interlayer spacings of graphene oxide membranes can be finely controlled using different concentrations of cations.

Two-dimensional material membranes for critical separations

In this review, we summarize the separation mechanisms and materials adopted for the fabrication of 2D material membranes as well as their applications in critical separations.

Atomistic insights into the separation mechanism of multilayer graphene membranes for water desalination

Graphene-based membranes have been extensively explored owing to their excellent separation properties. In this paper, multiple factors regarding desalination performance were investigated by molecular dynamics (MD) simulations. These factors include the interlayer spacing distance (H), the gap width (dG), offset (O), and the number of gaps and layers in a multilayer graphene membrane (MGM). It is found that salt rejection is influenced significantly by the interlayer spacing distance owing to the largest free energy between ions and graphene sheets as well as the relatively larger size of the hydration layer around the ions. The optimal desalting parameter (dG = 1 nm, H = 0.8 nm) was selected; MGM systems based on the optimized parameter exhibited excellent salt rejection for NaCl, MgCl2 and CaCl2 solutions. These results can provide some ideas for the future design of graphene-based membranes.

Theory and simulation developments of confined mass transport through graphene-based separation membranes

The perspectives of graphene-based membranes based on confined mass transport from simulations and experiments for water desalination.

Kinetic Ionic Permeation and Interfacial Doping of Supported Graphene

Due to its outstanding electrical properties and chemical stability, graphene finds widespread use in various electrochemical applications. Although the presence of electrolytes strongly affects its electrical conductivity, the underlying mechanism has remained elusive. Here, we employ terahertz spectroscopy as a contact-free means to investigate the impact of ubiquitous cations (Li⁺, Na⁺, K⁺, and Ca²⁺) in aqueous solution on the electronic properties of SiO₂-supported graphene. We find that, without applying any external potential, cations can shift the Fermi energy of initially hole-doped graphene by ∼200 meV up to the Dirac point, thus counteracting the initial substrate-induced hole doping. Remarkably, the cation concentration and cation hydration complex size determine the kinetics and magnitude of this shift in the Fermi level. Combined with theoretical calculations, we show that the ion-induced Fermi level shift of graphene involves cationic permeation through graphene. The interfacial cations located between graphene and SiO₂ electrostatically counteract the substrate-induced hole doping effect in graphene. These insights are crucial for graphene device processing and further developing graphene as an ion-sensing material.

Ultrahigh permeance of a chemical cross-linked graphene oxide nanofiltration membrane enhanced by cation-π interaction

Cross-linking with large flexible molecules is a common method to improve the stability and control the interlayer spacing of graphene oxide (GO) membranes, but it still suffers from the limitation of low water flux. Herein, a novel high flux GO membrane was fabricated using a pressure-assisted filtration method, which involved a synergistic chemical cross-linking of divalent magnesium ions and 1,6-hexanediamine (HDA) on a polyethersulfone (PES) support. The membrane cross-linked with magnesium ions and HDA (GOHDA-Mg2+ ) exhibited a high water flux up to 144 L m-2 h-1 bar-1, about 7 times more than that of cross-linked GO membranes without adding magnesium ions (GOHDA), while keeping excellent rejection performance. The GOHDA-Mg2+ membrane also showed an outstanding stability in water for a long time. The effects of magnesium ions on the GOHDA-Mg2+ membrane were analyzed using several characterization methods, including Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The results indicated that magnesium ions not only promoted reasonable cross-linking, but also improved the stacking of GO sheets to give lower mass transfer resistance channels for water transport in the membranes, resulting in the ultrahigh permeance of the GO membranes.

2D Nanosheets and Their Composite Membranes for Water, Gas, and Ion Separation

Two-dimensional nanosheets have shown great potential for separation applications because of their exceptional molecular transport properties. Nanosheet materials such as graphene oxides, metal-organic frameworks, and covalent organic frameworks display unique, precise, and fast molecular transport through nanopores and/or nanochannels. However, the dimensional instability of nanosheets in harsh environments diminishes the membrane performance and hinders their long-term operation in various applications such as gas separation, water desalination, and ion separation. Recent progress in nanosheet membranes has included modification by crosslinking and functionalization that has improved the stability of the membranes, their separation functionality, and the scalability of membrane formation while the membranes' excellent molecular transport properties are retained. These improvements have enhanced the potential of nanosheet membranes in practical applications such as separation processes.