The channel surface plays a pivotal role in governing ion transport behavior. Specifically, the capacity to differentiate between cation and anion movement is regulated by electrostatic interactions mediated by surface charges. These electrostatic charges are driven by the dissociation of surface groups, isomorphic doping, or native crystallographic defects^[64]. Once established, these charges interact with the surrounding electrolyte to form an electric double layer (EDL) near the channel walls, which selectively attracts counter-ions and repels co-ions. In nanofluidic systems, this ion selectivity is quantitatively characterized by the Debye screening length (λ_D), representing the effective thickness of the EDL^[65]. When the channel width (d) is comparable to or smaller than λ_D (d ≤ λ_D), EDL overlap occurs, leading to robust co-ion exclusion and preferential counter-ion transport. For ionic concentrations from 1 M to 1 μM, λ_D varies from approximately 1 to 100 nm. This range guides the design of charged nanochannels with dimensions matched to this scale to exploit EDL overlap for improved selectivity^[66]. To allow quantitative assessment of the selective transport processes, Zhou et al. calculated the potential of mean force (PMF) to quantify ion entry energetics into a nanochannel with bare edges [Figure 3A]^[67]. The modularity of 2D organic frameworks enables the precise design of adaptive surface charges. For example, integrating dipolar benzothiadiazole units into COF nanochannels enables dynamic charge regulation via ion-dipole interactions^[68]. Exposure to multivalent anions amplifies the negative surface charge, strengthening EDL overlap and enhancing cation perm-selectivity, whereas multivalent cations can reversibly invert the membrane polarity. This highlights how tailored pore chemistry actively modulates electrostatic interactions for tunable ion selectivity.

Notably, ion selectivity persists even when the d exceeds λ_D, extending to micrometer scales, indicating other mechanisms are involved. In such regimes, selectivity is no longer dictated by EDL overlap, but governed by the Dukhin number (Du), which evaluates the competition between ion-selective surface conductance (G_surface) and non-selective bulk conductance (G_bulk)^[69,70]:

$$ D u=\frac{G_{\text {surface }}}{G_{\text {bulk }}}=\frac{\left|\sigma_{s}\right|}{e c r}=\frac{l_{D u}}{d} $$

where σ_s is the surface charge density, c denotes the bulk salt concentration, e is the elementary charge, and l_Du is the Dukhin length. Both the Du and l_Du describe the relative significance of surface and bulk contributions to conductance. When d ≤ l_Du, surface conductance predominates, resulting in ion-selective transport. Impressively, experimental evidence from micropore systems further confirms that the Du can attain values as high as 10 in low-concentration electrolytes, validating the dominant surface conduction in micrometer-scale channels^[71]. This indicates that surface conduction remains significant and non-negligible, and even surpasses bulk conduction, especially in dilute solutions. Crucially, this mechanism is expected to guide the design of 2D organic frameworks. The equation implies that maximizing σ_s sustains a high Du even in enlarged pores. By densely grafting charged functional groups onto 2D pore walls, researchers can exploit surface conduction to simultaneously achieve high ion flux (via wider channels) and precise selectivity, breaking the conventional permeability-selectivity trade-off^[72].

Ion selectivity in nanochannels can be achieved by utilizing the effective ionic size in solution. However, this strategy is inadequate for hydrated ions with comparable diameters^[73]. Enhanced selectivity occurs when confinement dimensions match or fall below those of hydrated ions^[74]. In 2D organic framework membranes, intrinsic pore structures and interlayer spacings define the effective channel length for ion transport. To enter these spaces, ions must overcome a free-energy barrier from partial dehydration or hydration shell reorganization at the channel entrance^[75].

Shortening the channel length generally enhances ion permeability^[76]. Membranes with single-atomic or molecular-layer thickness minimize mass transfer resistance, substantially increasing ion flux^[77-79]. For example, Yang et al. designed monolayer COF membranes with well-defined pore structures. When integrated into salinity-gradient power generators using artificial seawater and river water, they delivered output power densities exceeding 200 W cm^-2[46]. In such ultrathin membranes, ion transport occurs perpendicular to the surface. In contrast to longer channels, where selectivity primarily arises from interactions between ions and the inner channel walls, the extremely short channel length precludes reliance on wall-mediated effects^[64]. Instead, discrimination depends critically on a well-defined charged region surrounding the pore entrance. This suggests that an excessively high density of ion channels may decrease the selectivity of ultrathin membranes, as the interpore regions may become insufficient to sustain effective charge separation^[80]. To investigate ion binding and dehydration in sub-nanometer channels of varying sizes, Mo et al. designed functionalized UiO-66-(X)₂ membranes with tunable channel dimensions (X = NH₂, SH, OH, and OCH₃ (OMe)^[81]. Changing the functional group from OH to OMe left monovalent cation conductance largely unchanged, while significantly decreasing divalent cation conductance [Figure 3B-D]. This indicates that as the channel size decreases, monovalent cation transport is largely unaffected, whereas divalent cation transport is significantly hindered.

The orientation of nanochannels can also affect the channel length, which can be divided into horizontal and vertical orientations. Vertically oriented membranes, where ions permeate perpendicular to the laminated sheets, exhibit permeability up to three orders of magnitude higher than horizontally oriented ones. This ultrafast transport in vertically aligned channels is attributed to rapid ion entry, increased accessible surface area, and shortened diffusion pathways^[82,83]. In 2D organic frameworks, nanochannel regulation is achieved primarily by designing molecular nodes and linkers within the porous nanosheets. Selecting building units with dimensions matching the molecular size of target species enables precise control over channel size and ion selectivity^[84]. Naren et al. revealed that lithium ions (Li⁺) migrate within the COF cavities via two distinct pathways: an interlayer vertical channel and an intralayer parallel channel [Figure 3E]^[85]. The activation barrier for vertical migration is only 0.76 eV, less than half the 1.56 eV required for the parallel route, as the enlarged pore aperture relaxes steric constraints imposed by the 0.42 nm interlayer spacing. This enables Li⁺ to hop directly through the pore interior rather than between adjacent sheets [Figure 3F]^[85].

Membranes designed for cation separation, desalination, and proton conduction rely fundamentally on precise size exclusion. Under extreme confinement, the transport mechanism is dictated by the relative size between the channel diameter (D_C) and the ion’s hydration diameter (D_H). In the regime where D_C > D_H, ions translocate freely with their primary solvation shells intact^[86]. However, as D_C scales down (D_C ≤ D_H), permeation forces ions to partially or fully shed their hydration shells. This dehydration process imposes a steep thermodynamic energy barrier, shifting transport into an activated translocation regime where permeation rates drop exponentially. In conventional 2D membranes, this steric exclusion implies that larger hydrated ions are either completely blocked or diverted through tortuous interlayer pathways, severely lowering permeability. In contrast, adequately sized species can directly traverse the intrinsic pores of 2D organic framework nanosheets, significantly enhancing permeability. For example, ideal desalination channels are engineered between the diameter of a water molecule (~ 0.28 nm) and typical hydrated ions (0.7-0.9 nm)^[87]. Consistent with this, Corry et al. demonstrated that the precise cut-off size for NaCl rejection strictly falls within the 0.6-0.8 nm range^[88]. Consequently, tailoring channel dimensions to exploit this hydration-dependent energy barrier, often in combination with specific pore functionalization to further modulate transport^[89], remains the primary strategy for achieving precise ion sieving.

When ions pass through neutrally charged pores, their hydration shells size plays a decisive role in ion transport. Zhao et al. demonstrated that K⁺ and Cl^- cannot permeate neutral pores with a 2 Å diameter, whereas both ions readily traverse 8 Å pores^[90]. To allow ions to pass while retaining their hydration shells, pore diameters must exceed approximately 7 Å. In larger neutral pores, ionic transport is primarily limited by access resistance at the pore entrances and exits, well described by continuum electrokinetic theory^[91,92]. Additionally, ion diffusion in 2D nanochannels is position-dependent. Liao et al. used molecular-dynamics simulations with multiple force fields to reveal a linear correlation between the diffusivity of ions with small first hydration shell (HS) radius (≤ rHS of K⁺) and their distance from the channel wall (d_ion-wall)^[93]. When d_ion-wall is large, D_channel/D_bulk > 1. In contrast, for large rHS ions such as K⁺, Rb⁺ and Cs⁺, D_channel/D_bulk remains constant and is insensitive to d_ion-wall [Figure 3G and H]^[93]. Quantitative analysis further elucidated the underlying physics by linking fundamental quantities, involving the water free-energy landscape around the ion, water residence time in the hydration shell, and the water-ion friction coefficient. As ion position can be tuned by external electric or magnetic fields, this mechanism offers a promising route to regulate ionic transport in 2D nanochannels for various applications^[29,94].

MOFs are porous crystalline materials composed of metal nodes coordinated by organic linkers. Unlike 3D counterparts, 2D MOFs feature layered structures with in-plane connectivity^[113]. They offer large surface areas, tunable pore structures, and well-defined topological networks, including honeycomb (hcb), four-eight square (fes), square (sql), kagomé (kgm), and hexagonal (hxl) [Figure 6A]^[114]. Rational design often relies on transition metals with predictable coordination geometries, such as Cu²⁺ and Zn²⁺, combined with rigid organic ligands. Porphyrin derivatives are effective building blocks for 2D networks due to their structural rigidity, high connectivity, and planar conjugated macrocyclic frameworks^[115,116]. Additionally, combining trigonal ligands with square-planar metal centers enables hexagonal layered MOFs^[117]. These tailored 2D MOFs exhibit unique functionalities, such as mixed ionic-electronic conductors. For instance, Roh et al. showed that grafting ionophilic ethylene glycol (EG) groups onto a conductive MOF (cMOF) backbone enables high ionic conductivity (1.1 × 10^-4 S/cm) [Figure 6B and C]^[118]. This exemplifies how side-chain engineering can decouple and control ionic and electronic transport^[118].

2D COFs are an emerging class of porous crystalline materials featuring layered architectures formed via covalent bonding between π-conjugated building blocks^[119]. Unlike graphene oxide (GO), graphene, molybdenum disulfide (MoS₂), or boron nitride (BN), 2D COFs possess intrinsic nanopores serving as nanochannels for efficient molecular and ion transport^[120]. Realizing functional 2D COFs for nanofluidics requires precise pore engineering to establish structure-property relationships. This typically involves two strategies: (1) tailoring the pore chemistry by selecting linkages and framework units; (2) controlling the pore size, geometry, and arrangement by assembling building units with defined shapes. For the first strategy, choosing dynamic covalent bonds, such as B-O or C=N linkages, critically affects the stability and crystallinity^[121]. Additionally, incorporating substituent groups, metal clusters, or heteroatoms via predesign or post-synthetic modification enables customization. Regarding the second strategy, common geometries include trigonal, tetragonal, hexagonal, and rhombic ones, which are formed by combinations such as C₃ + C₂, C₄ + C₂, C₂ + C₂, or 'T'+’C₂^[119]. For instance, a COF-DT membrane with pH-responsive gating behavior was fabricated by designing a C₃ + C₂ topology functionalized with hydroxyl groups [Figure 6D-F]^[122].

2D HOFs are porous materials assembled via hydrogen bonding and other noncovalent interactions, including π-π stacking and van der Waals forces^{[123 124]}. While sharing some common structural features with MOFs and COFs, 2D HOFs are constructed via weak, reversible hydrogen bonds, endowing them with distinctive properties like self-healing, mild synthesis conditions, and solution processability. The synthesis of HOFs typically proceeds under ambient conditions via solvent evaporation-induced recrystallization, facilitated by high precursor solubility in organic solvents. Moreover, hydrogen bond flexibility and reversibility contribute to the regeneration of HOFs^[125]. In 2D HOFs, stability is often enhanced by incorporating building blocks with extended π-conjugated systems. These frameworks benefit from synergistic in-plane hydrogen bonding and out-of-plane π-π stacking. Typically, organic units assemble via hydrogen bonds into 2D layers, which then stack via π-π interactions to form 3D architectures with 1D open channels aligned along the stacking direction. Among reported topologies, the 2D sql is most commonly used for stable 2D HOFs^[126,127]. Liu et al. synthesized PFC-1 from 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H₄TBAPy), a building block with a π-conjugated pyrene core and four benzoic acid arms. Each H₄TBAPy unit connects to four neighbors via cyclic carboxylic acid dimers, forming 2D sql hydrogen-bonded sheets^[123]. These sheets stack in an AA mode along the [100] direction via π-π interactions (interlayer spacing ≈ 3.34 Å), generating 1D square channels (~ 18 × 23 Å) within a 3D open framework [Figure 6G]. The material exhibits a BET surface area of 2,122 m² g^-1 and retains crystallinity after immersion in concentrated HCl for 117 days [Figure 6H]^[123].

In summary, while MOFs, COFs, and HOFs share substantial surface areas, tunable pore structures, and an excellent capacity for surface functionalization, their distinct intrinsic bonding interactions fundamentally drive differences in structural stabilities and ion transport mechanisms^[128,129]. Assembled via coordination bonds, MOF membranes provide exceptional chemical diversity. Their specific transport pathways and selectivity are heavily governed by controllable host-guest interactions at open metal centers and versatile organic linkers^[130]. COF membranes are bridged by robust covalent bonds, granting them high chemical stability essential for practical precision separations. 2D COFs typically feature rigid, vertically aligned one-dimensional channels that minimize transport resistance, enabling ultrafast ion flow and precise steric sieving to overcome the permeability-selectivity trade-off^[131]. Meanwhile, HOFs are assembled through non-covalent interactions, resulting in highly biocompatible and dynamic pore structures that naturally govern their transport behaviors. Specifically, continuous hydrogen-bonded networks formed by Brønsted acid-base pairs serve as intrinsic conduits for efficient proton conduction, while specific functional sites like unprotonated carboxylic groups endow the membranes with remarkable ion selectivity^[132,133].

Despite the inherent advantages described above, specific dynamic linkages within COFs and the fundamentally weaker non-covalent networks of HOFs can still face degradation under extreme, long-term operational conditions. To address these bottlenecks, the chemical durability of imine-linked COFs prone to hydrolytic cleavage can be secured through stepwise post-synthetic functionalization into amide and pyridine N-oxide groups, which successfully preserves structural crystallinity even after 24-h exposure to 10 M strong acids and bases^[134]. Alternatively, a linkage-encoding strategy embeds fully conjugated enaminone bonds into the backbone, allowing the membrane to maintain its macroscopic morphology, long-range order completely, and vertically aligned nanochannels after 24-hour immersion in 12 M sulfuric acid at 110 °C or 5 M potassium hydroxide, drastically outperforming conventional imine or incompletely tautomerized ketoenamine networks^[135]. For HOFs, permanent porosity can be stabilized against activation collapse by incorporating cooperative hydrogen bonding and π-π interactions to lock the organic building units^[133]. For instance, incorporating metal ions into porphyrin frameworks yields robust architectures that maintain structural integrity under 270 °C thermal stress and remain crystalline in concentrated hydrochloric acid for 24 h^[136]. These organic framework membranes exhibit distinct intrinsic advantages and limitations, which directly determine their optimal application scenarios [Table 2]^[137-140].

Solution processing is ideal for fabricating membranes from organic framework materials with good solvent dispersibility and stability^[141]. Among solution-based methods, vacuum filtration is particularly efficient and scalable for producing 2D organic framework membranes. For instance, Jian et al. exfoliated 2D monolayer nanosheets of an aluminum tetra-(4-carboxyphenyl) porphyrin framework (Al-MOF)^[142]. Then they assembled an ultrathin, continuous Al-MOF membrane by vacuum filtering a dilute nanosheet suspension onto an anodic aluminum oxide (AAO) support^[142]. In polar solvents, HOFs evolve into small particles while retaining intrinsic crystalline order due to surface polar groups (e.g., -NH₂, -COOH) that do not participate in the internal hydrogen-bonding network. These exposed groups interact effectively with polar solvents like N-methylpyrrolidone (NMP), Dimethyl sulfoxide (DMSO), and N, N-dimethylformamide (DMF), enabling homogeneous dispersion and high-quality membrane fabrication^[133,143]. However, solution processing via vacuum filtration requires large nanosheets to exceed substrate pore sizes for effective retention, which induces rapid, forced deposition and misaligned assembly. Although individual flakes preserve their intrinsic crystallinity, the macroscale structure suffers from disordered amorphous voids between layers^[144]. To address this limitation, Duan et al. used hydrophilic polymers, polyvinyl alcohol (PVA) and polyethylene glycol (PEG), to form multiple-site hydrogen bonding with HOF monomers, namely, 1,2,4,5-tetrakis(4-carboxyphenyl)benzene (TCPB)^[145]. The flexible polymer chains promoted tight stacking during vacuum filtration, enhancing the crystallinity and stability of the resulting lamellar membranes [Figure 7A and B]^[145].

In addition to vacuum filtration, Sun et al. pioneered solution-processing HOF membranes by optimizing precipitation conditions^[146]. They constructed a flexible and stable HOF (UPC-HOF-6) using a triangular building block, 4′,4′′,4′′′-nitrilotris[([1,1′-biphenyl]-4-diaminotriazine)] (NBP-DAT). The UPC-HOF-6 was dissolved in DMSO at various concentrations and coated onto AAO substrates^[146]. To elucidate the solution processability of well-defined HOFs, Yin et al. investigated the aggregation of a porous and crystalline HOF material (HOF-BTB) dispersed in solvent [Figure 7C]^[143]. Cryo-electron microscopy showed that the nanoscale fragments (10-150 nm) retained the bulk crystal structure. On the basis of these insights, a highly crystalline and continuous HOF membrane was successfully fabricated on macroporous AAO disks [Figure 7D], demonstrating the broad applicability of the described approach for different HOFs, solvents, and substrates^[143]. Despite its versatility, solution processing requires meticulous optimization to minimize residual solvent-induced defects that could cause pore blockage and compromise membrane performance.

Interfacial polymerization (IP) is a facile, scalable method for fabricating large-area thin films for 2D COF membranes. This technique exploits reactions at liquid-liquid interfaces to generate continuous membranes with controlled thickness and morphology. Dey et al. pioneered a bottom-up interfacial crystallization strategy for COF membrane fabrication, wherein an aldehyde linker was dissolved in dichloromethane (organic phase) and then reacted with an amine monomer in water (aqueous phase) [Figure 8A]^[147]. Slow diffusion at the interface yielded crystalline Tp-Bpy membranes, enabling simultaneous control over crystallization kinetics and morphology [Figure 8B]^[147]. Similarly, Zhu et al. utilized IP to coordinate metal ions within a COF film, reorienting random nanochannels into axially aligned arrays that boosted ion selectivity and permeability for osmotic energy and nanofluidic applications^[148]. Besides separating the two monomers into distinct phases, Matsumoto et al. co-dissolved both amine and aldehyde monomers in a single organic phase with a Lewis acid catalyst Sc(OTf)₃ in the aqueous phase^[149]. IP at the phase boundary yielded continuous 2D imine-linked COF films with lateral dimensions limited only by the vessel size. By tuning monomer concentration, film thickness was controlled from 10 µm down to 2.5 nm^[149].

The IP method has been extensively employed in the construction of COF membranes. In MOF synthesis, it has mainly focused on low-valent metal cations that are readily activated at low temperatures, such as Zeolitic Imidazolate Framework-8 (ZIF-8)^[150]. Synthesizing high-valent cluster-based MOF membranes (e.g., Zr, Cr, Fe) remains challenging due to high energy barriers and complex formation mechanisms^[151]. To overcome this difficulty, Sun et al. employed pre-synthesized metal clusters (e.g., Zr-clusters), circumventing energy-intensive in situ formation. Combined with roll-to-roll processing on flexible polymer supports, the preformed metal cluster interfacial polymerization (PMIP) enables continuous fabrication of Zr-fum-MOF membranes at ambient temperature [Figure 8C and D]. IP is advantageous for producing large-area, flexible membranes compatible with diverse polymeric substrates^[152]. However, the conventional interfacial polymerization method often yields membranes with a low crystalline fraction, typically ranging from 16.47% to 73.90%, which constrains the precision of molecular sieving^[153]. Furthermore, scaling up conventional interfacial polymerization compromises macroscale uniformity. To resolve this, Zhang et al. developed a reverse-phase microemulsion strategy using an ionic liquid network to prearrange monomers, enabling concurrent polymerization and crystallization within seconds. Combined with a scraping-assisted process, this method yielded highly crystalline COF membranes at a meter scale of 0.4 by 1.0 meters^[154].

In situ growth, including solvothermal and seed-assisted secondary growth, is widely used for synthesizing organic framework membranes, particularly those based on MOFs and COFs^[19]. A significant advancement in solvothermal synthesis began in 2009 with the work of Liu et al., who successfully fabricated the first MOF-5 membrane via in situ solvothermal growth on a porous alumina substrate^[155]. The resulting polycrystalline membrane exhibited continuous morphology, demonstrating the feasibility of directly growing MOF layers on supports. This approach was then extended to various MOF systems and adapted for 2D COFs. For instance, Caro et al. functionalized ceramic tubes with 3-aminopropyltriethoxysilane (APTES), followed by reacting with 1,3,5-triformylbenzene (TFB) to create a surface-anchored ligand layer [Figure 9A]. Vertical placement of these tubes in an autoclave with p-phenylenediamine (PDA) yielded well-intergrown, 400 nm thick tubular COF-LZU1 membranes [Figure 9B]^[156]. While in situ solvothermal synthesis produces highly crystalline and coherent membranes, anisotropic growth kinetics often lead to challenges in controlling crystal orientation^[19,157]. This misalignment generates defect pores exceeding 1 nm, which surpass the size of the intrinsic fundamental unit pores typically below 1 nm^[158].

Despite considerable advancements in in situ solvothermal membrane fabrication, further reducing the membrane thickness remains challenging due to scarce nucleation sites^[159]. To address this limitation, seed-assisted secondary growth techniques have been developed to enhance flux. In 2023, Pu et al. introduced negatively charged COF nanosheets as porous substrates that are capable of selectively adsorbing Zn²⁺ ions^[160], and then deprotonated 2-methylimidazole (2-MIM) ligands coordinated with surface-enriched Zn²⁺, initiating heterogeneous nucleation [Figure 9C]. This strategy produced ZIF-8@COF nanosheet seeds, yielding ZIF-8 membranes as thin as 100 nm [Figure 9D]. While seed-assisted secondary growth enables ultrathin, high-permeance membranes, it requires precise control over seed-layer uniformity and orientation, increasing procedural complexity. Furthermore, secondary growth parameters (for example, temperature, duration, precursor concentration) critically influence the membrane quality, which requires meticulous optimization^[157,161].

Liquid-air interface synthesis is an effective strategy for fabricating membranes by guiding molecular assembly at the interface^[162]. This method typically involves the self-assembly of amphiphilic monomers at the interface, followed by in situ polymerization or cross-linking to form large-area, ordered ultrathin membranes with well-defined nanochannels^[163]. To promote polymer chain alignment and crystallization (e.g., within microscale droplets at the interface), the following factors are critical: a positive spreading coefficient for rapid, large-area film expansion, and abundant interfacial binding sites to enrich polymer chains via interfacial interactions. Satisfying these conditions enables tightly packed, highly oriented polymer arrangements within short time scales, thereby enhancing crystallinity at the interface^[164,165]. Zhang et al. fabricated highly crystalline COF films via a gas-liquid interface method. The films exhibit a highly ordered tetragonal lattice with single crystal domains exceeding 10⁴ nm^2[166].

The Langmuir-Blodgett (LB) technique utilizes amphiphilic molecules with hydrophilic heads and hydrophobic tails. These molecules spontaneously assemble into monolayer or multilayer films at the water-air interface, where lateral compression induces ordered molecular packing. The resulting films are then transferred onto solid substrates to form uniform layers^[167-169] [Figure 10A and B]. Leveraging interfacial confinement and directional interactions, the LB technique precisely regulates molecular conformation and orientation. For example, Mao et al. fabricated a cationic AB-stacked COF bilayer using molecular design and the LB method^[170]. The structure features highly ordered sub-2 nm pores, an atomic thickness of 1.3 nm, exceptional pore utilization, and a high surface charge density of 4.4 mC m^-2[170]. Similarly, Makiura et al. demonstrated room-temperature bottom-up fabrication of NAFS-1, a preferentially oriented ultrathin porphyrinic MOF nanofilm with long-range crystalline order in both in-plane and out-of-plane directions^[171]. Integrating layer-by-layer and LB methods with metal-coordinated pyridine linkers drives interdigitated growth, offering a versatile solution-based route for well-controlled MOF nanofilms in sensors, catalysts and fuel-cell electrodes^[171]. Despite its molecular-level precision, the LB technique is constrained by limited monomer selection, low collapse pressures, and low transfer yields^[169,172].

Similar to the LB method, Lu et al. developed a supramolecular nanocrystal membrane (SNM) using interfacial hydrogen bonding in nanoconfined spaces^[173]. Mediated by polyethylene glycol (PEG), the interaction guided the ordered arrangement of polycaprolactone (PCL) chains into a continuous nanofilm [Figure 10C]. The resulting SNM features large crystal domains (100-150 nm) and a thickness of 6 nm [Figure 10D]. These domains contain abundant, ordered sub-nanometer channels that form continuous water pathways and align water molecules, thereby increasing the energy barrier for sodium ion transport while enhancing water flux^[173]. A comprehensive comparison of these fabrication strategies is summarized in Table 3^{[141,145,150,157,159,168,172]}.