Rational ion transport management mediated through membrane structures

Abstract Unique membrane structures endow membranes with controlled ion transport properties in both biological and artificial systems, and they have shown broad application prospects from industrial production to biological interfaces. Herein, current advances in nanochannel‐structured membranes for manipulating ion transport are reviewed from the perspective of membrane structures. First, the controllability of ion transport through ion selectivity, ion gating, ion rectification, and ion storage is introduced. Second, nanochannel‐structured membranes are highlighted according to the nanochannel dimensions, including single‐dimensional nanochannels (i.e., 1D, 2D, and 3D) functioning by the controllable geometrical parameters of 1D nanochannels, the adjustable interlayer spacing of 2D nanochannels, and the interconnected ion diffusion pathways of 3D nanochannels, and mixed‐dimensional nanochannels (i.e., 1D/1D, 1D/2D, 1D/3D, 2D/2D, 2D/3D, and 3D/3D) tuned through asymmetric factors (e.g., components, geometric parameters, and interface properties). Then, ultrathin membranes with short ion transport distances and sandwich‐like membranes with more delicate nanochannels and combination structures are reviewed, and stimulus‐responsive nanochannels are discussed. Construction methods for nanochannel‐structured membranes are briefly introduced, and a variety of applications of these membranes are summarized. Finally, future perspectives to developing nanochannel‐structured membranes with unique structures (e.g., combinations of external macro/micro/nanostructures and the internal nanochannel arrangement) for mediating ion transport are presented.


INTRODUCTION
Ion transport plays a significant role in biological activities, ranging from the basic uptake of ionic mineral elements to the high-level conversion of biological energy and sensitive transmission of nerve signals. Recently, great efforts have been made to explore the protein structures and underlying ion transport mechanisms of biological ion channel proteins, including cations (e.g., ATPase proton pump, [1] TrkH potassium-ion channel, [2] rhodopsin sodiumion pump, [3] CorA magnesium ion channel, [4] TRPV6 calcium ion channel, [5] and sCtr1 copper ion transporter [6] )  of  F I G U R E  Representative membrane structures for rational ion transport management, including nanochannel-structured membranes with single-dimensional nanochannels (i.e., 1D, 2D, and 3D) and mixed-dimensional nanochannels (i.e., 1D/1D, 1D/2D, 1D/3D, 2D/2D, 2D/3D, and 3D/3D), ultrathin membranes, and sandwich-like membranes, and their broad applications in the fields of ion separation, water purification, energy storage and conversion, sensors, and bioelectronics biological ion channel proteins [14][15][16][17][18][19][20][21] or artificial components with channels [22,23] in phospholipid bilayers. Reconstructed EcClC proteins embedded in lipid bilayers can function as H + and Cl − pumps with improved and controllable transport properties because of the superimposing or counterimposing gradients of H + and Cl − ions. [14] However, this type of reconstituted ion channel usually requires sophisticated technology, limiting further applications.
At present, research on nanochannel-structured membranes for ion transport is mainly focused on selecting artificial raw materials (i.e., components) and constructing nanochannels with specific geometric parameters (e.g., shape and size) and interface properties (e.g., charge, wettability, and recognition). Typical examples of nanochannel-structured membranes with single-dimensional nanochannels (i.e., 1D, 2D, and 3D) are provided according to the nanochannel dimensions. For instance, the threading/dethreading states between positively charged azobenzene and negatively charged pillararene on conical nanochannel walls made of polyethylene terephthalate (PET) can be reversibly switched under light irradiation, which contributes to the change in surface charge and achieves ion-selective transport. [24] As a representative reconstructed layered material, negatively charged graphene oxide (GO) with 2D nanochannels 1 nm high showed surface-charge-controlled ion transport at high salt concentrations, exhibiting the potential for developing flexible and large-scale nanochannel-structured membranes. [25] A metal-organic framework (MOF) is a typical porous crystalline material with uniform pores. In situ-grown UiO-66-X MOFs in PET nanochannels showed ultrahigh F − conductivity and F − /Cl − selectivity owing to the specific interactions between F − and F − binding sites in channels consisting of angstrom-scale windows and nanoscale cavities. [26] With the development of nanochannel-structured membranes, a diversity of membrane structures can be constructed, such as nanochannel-structured (e.g., single-and mixeddimensional nanochannels), ultrathin (e.g., sub-micrometer and monolayer), and sandwich-like (e.g., three-layer and multilayer) membranes ( Figure 1). 1D nanochannels, which are typically simplified nanochannel models, are widely used for transporting specific ions, taking advantage of suitable raw materials, controllable geometrical parameters, and selective surface modifications. [27,28] By adjusting the interlayer spacing by physical or chemical methods, membranes with 2D nanochannels consisting of restacked nanosheets showed controlled ion transport properties. [29][30][31] However, continuous 3D nanochannels can shorten the ion diffusion pathways and facilitate ion transport. [32] Typical heterogeneous membranes with mixed-dimensional nanochannels (i.e., 1D/1D, 1D/2D, 1D/3D, 2D/2D, 2D/3D, and 3D/3D) generally exhibit unique ion transport properties in contrast to the intrinsic ones because of their asymmetric characteristics such as asymmetric components, geometric parameters (e.g., shape and size), and interface properties (e.g., charge and wettability). [33,34] Recently, ultrathin membranes with vertically ordered channels were constructed that showed both high permeability and selectivity owing to the short ion transport distance. [35,36] Furthermore, sandwich-like membranes with a more delicate design of nanochannels and combination structures were applied to precisely tune the ion transport behaviors. [37,38] Notably, combining the external macro/micro/nanostructure and the internal nanochannel arrangement adjustment, traditional membranes are endowed with peculiar structures (e.g., anti-T [39] and vertically aligned [40] MXenes) and ion transport properties (e.g., directional ion transport). In addition, natural plants (e.g., grass stems [41,42] and woods [43][44][45][46] ) with abundant ion channels have also been used to manipulate ion transport after further chemical treatments. Thus, rational ion transport management can be achieved by mediating the membrane structures.
The controllability of ion transport in nanochannelstructured membranes is mainly reflected in the different ion transport properties (e.g., ion selectivity, ion gating, ion rectification, and ion storage). With respect to construction methods, superwettable interface-based reactions and assemblies [47,48] show great potential for fabricating ultrathin and mixed-dimensional membranes. For instance, ultrathin membranes have been fabricated by superspreading liquids on superhydrophilic surfaces. [49][50][51] Membranes with microscale lateral lengths were also developed through triphase interface-mediated epitaxial growth on superhydrophobic surfaces, [52,53] exhibiting the potential for constructing mixed-dimensional membranes. Significantly, the applications of nanochannel-structured membranes are gradually changing from general industrial production to bioelectronics (e.g., electron batteries for detecting ionic signals in biosystems [41] ). Very recently, Jiang et al. reported a quantumconfined ion superfluid related to biosystems, which shows a new path to investigating controlled ion transport across the nanochannels in both biological membranes and artificial counterparts. [54][55][56][57] Therefore, it is necessary to systematically review recent advances in nanochannel-structured membranes with unique structures for controlling ion transport to further promote their application in the fields of ion separation, water purification, energy storage and conversion, sensors, and bioelectronics.
Herein, we provide a comprehensive overview of rational ion transport management mediated through unique membrane structures (e.g., nanochannel, ultrathin, and sandwichlike structures) ( Figure 2). First, we introduce the controllability of ion transport through ion selectivity, ion gating, ion rectification, and ion storage. Second, we highlight membranes with nanochannels of different dimensions and their heterogeneous membranes. Third, we discuss representative ultrathin membranes and sandwich-like membranes. Then, various stimulus-responsive nanochannels are highlighted. Next, we briefly discuss typical construction methods of nanochannel-structured membranes, followed by a summary of various applications in different fields. Finally, future perspectives in constructing nanochannel-structured membranes with specific structures for mediating ion transport are presented.

 CONTROLLABILITY OF ION TRANSPORT
To clarify the significant role of the unique structures of nanochannel-structured membranes for regulating ion transport, it is necessary to first introduce ion transport properties. In this section, the controllability of ion transport in nanochannel-structured membranes is briefly discussed in terms of ion selectivity, ion gating, ion rectification, and ion storage.

. Ion selectivity
Specific ions can be transported across the nanochannels by adjusting the nanochannel size, charge, and wettability of the nanochannel walls and by specific recognition between target ions and recognition sites. More specifically, ions with sizes smaller than the nanochannel diameters can pass through the nanochannels based on the size effect. [58][59][60][61][62][63] When the nanochannel diameters are similar to the Debye length, the strong electrostatic interactions between the ions and the nanochannel interfaces can lead to anion-selective transport in positively charged nanochannels and cation-selective transport in negatively charged ones. [27,64,65] Hydrophilic and hydrophobic nanochannels tend to transport polar and nonpolar matter, respectively. [66][67][68] Taking advantage of the specific recognition between the target ions and molecules with binding sites on the inner walls, selective transport of specific cations [69,70] and anions [71][72][73] can be achieved. The performance of ion selectivity can be quantitatively reflected by the water flux, interception, and ionic conductance of these membranes.

. Ion gating
Owing to the changes in the nanochannel size, charge, and wettability of the nanochannel walls, nanochannels usually

. Ion rectification
Under the effects of the asymmetric shape, [83,84] charge, [85] and the components [67] of nanochannels or asymmetric external stimuli (e.g., pH, [86,87] pressure, [88] concentration, [89,90] and temperature gradients [91] ), specific ions show preferential transport in one direction at a bias, whereas they are blocked in the opposite direction at an inverse bias. For example, neg-atively charged conical nanochannels exhibit ion rectification owing to their asymmetric structure. When the charge on the nanochannel walls was converted to positive, the ion rectification direction was inverted. [65] The performance of ion rectification can be visually illustrated by asymmetric I-V curves and quantitatively compared using the rectification ratio.

. Ion storage
3D interconnected nanochannels can facilitate inserting and extracting electrolyte ions from electrode materials, which is usually adopted in designing electrochemical energy storage devices with superior performance (e.g., ultrahigh energy and power density, suitable cycling stability). For example, electrodes made of holey 2D nanosheets can shorten the ion transport pathways and increase the rate of ion transport, taking advantage of the ultrathin thickness and continuous 3D nanochannels. [92] Recently, they have been widely used to construct electrochemical capacitors, lithium-ion batteries, and sodium-ion batteries. The corresponding electrochemical performance can be accurately analyzed using cyclic voltammetry (CV) curves, charge/discharge (CD) curves, and specific capacitances. As mentioned above, the controllability of ion transport is mainly accomplished through ion selectivity, ion gating, ion rectification, and ion storage, which are in turn determined by internal factors such as the components, geometric parameters (e.g., shape and size), and interface properties (e.g., charge and wettability) of these membranes and external factors such as the pH, pressure, concentration, and temperature of the electrolyte solutions. Controllable ion transport mediated by the unique membrane structures (e.g., nanochannel, ultrathin, and sandwich-like structures) is discussed in the following sections.

. D Nanochannel-structured membranes
To mimic biological ion channels embedded in plasma membranes to finely modulate ion transport, constructing artificial counterparts has been carried out in many studies. As a representative simplified nanochannel model, 1D nanochannels with various raw materials, controllable geometrical parameters, and selective surface modifications have been widely used for studying tunable ion transport. Here, we introduce 1D nanochannel-structured membranes for ion transport based on their composition (e.g., organic and inorganic membranes) (Figures 4 and 5, respectively). First, representative 1D nanochannel-structured organic membranes with various geometrical parameters include PET, polyimide (PI), polycarbonate (PC), short cyclodextrin nanotubes (CDNTs), and covalent organic frameworks (COFs) are discussed. PET is usually used to prepare symmetric (e.g., cylindrical [93] and cigar-shaped [94][95][96] ) and asymmetric (e.g., bullet-, [83] funnel-, [85,97] hourglass-, [75,98] and coneshaped [24,79,[99][100][101][102][103] ) nanochannels. For instance, PET membranes with cylindrical nanochannels can provide pathways for proton transport across a transmembrane concentration gradient. In one study, these membranes exhibited a continuous and stable current and were used for constructing respiration-based biocells with superior performance. [93] By fine adjustment of the symmetric geometry degree λ (i.e., the ratio of the tip-side conical length to the base-side conical length) and the surface charge on the inner walls, the funnel-shaped nanochannels showed switchable ion transport properties between bidirectional rectification and ion gating ( Figure 4A). [85] In addition, Siwy et al. constructed an asymmetric cone-shaped PI nanochannel with a large opening approximately 2.4 μm in diameter and an estimated small opening approximately 2 nm in diameter by a track-etching method. [104] The prepared nanochannel stably rectified the ion current owing to the asymmetric charge distribution at the nanochannel interface. In addition, asymmetric (e.g., cone- [105,106] and hourglass-shaped [64] ) PC nanochannels showed stimulus-responsive ion rectification owing to the change in surface charge. Taking advantage of the adsorption of multivalent ions (e.g., trivalent iron ions) on negatively charged hourglass-shaped nanochannels with a 208 nm base diameter and a 0.91 nm tip diameter, the corresponding local charge was inverted and a bipolar junction was formed, leading to an ion current rectification (ICR) ratio higher than 650 (Figure 4B). [64] Furthermore, the ICR ratio could be reversibly tuned between high and low through the adsorption and desorption of multivalent ions. CDNTs with controlled lengths, diameters, and number of cyclodextrins and selective chemical modifications were used as ion nanochannels and inserted into lipid bilayers to transport ions. [107,108] As shown by the I-V curves of the CDNTs ( Figure 4C), their conductance was found to be 0.077 ± 0.005 nS. [108] In addition, a COF F I G U R E  Representative 1D nanochannel-structured organic membranes. (A) Asymmetric funnel-shaped PET (left) and I-V curves with different symmetric geometry degree λ values (right). Reproduced with permission. [85] Copyright 2017, Wiley-VCH. (B) Symmetric hourglass-shaped polycarbonate (PC) (left) and reversible switching of ion current rectification (ICR) ratios (right). Reproduced with permission. [64] Copyright 2019, American Chemical Society. (C) Short cyclodextrins nanotubes (CDNTs) (left) and I-V curve of the CDNTs (right). Reproduced with permission. [108] Copyright 2015, American Chemical Society. (D) Covalent organic framework (COF) with ordered 1D nanochannels (left) and ion conductivities of COFs with (red) and without (black) oligo(ethylene oxide) chains as a function of temperature (right). Reproduced with permission. [109] Copyright 2018, American Chemical Society F I G U R E  Representative 1D nanochannel-structured inorganic membranes. (A) Symmetric cylinder-shaped anodic aluminum oxide (AAO) (left) and pH-modulated ion rectification property (right). Reproduced with permission. [110] Copyright 2013, Wiley-VCH. (B) Non-covalent functionalization of ordered and perpendicular mesochannels of silica (SiO 2 ) (left) and the distinct ion transport regions of anion selective, ambipolar, and cation selective (right). Reproduced with permission. [116] Copyright 2014, Wiley-VCH. (C) Silicon nitride (Si 3 N 4 ) nanopore (left) and ICR ratios at different pH and salt gradient values (right). Reproduced with permission. [90] Copyright 2019, American Chemical Society. (D) Asymmetric cone-shaped carbon nitride (C 3 N 4 ) nanotubes (left) and photocurrent responses with tip-side irradiation (right). Reproduced with permission. [124] Copyright 2019, Wiley-VCH with oligo(ethylene oxide) chains exhibited enhanced lithiumion transport through a vehicle mechanism in contrast to a COF without oligo(ethylene oxide) chains on the ordered 1D nanochannel walls ( Figure 4D), [109] which provides new inspiration for developing ion conductors.
Many inorganic materials such as anodic aluminum oxide (AAO), carbon nanotubes (CNTs), silica (SiO 2 ), silicon nitride (Si 3 N 4 ), and carbon nitride (C 3 N 4 ) have been used to construct 1D nanochannel-structured membranes. Common symmetric (i.e., cylinder-shaped [110,111] ) and asymmetric (e.g., cone- [112] and hourglass-shaped [113] ) nanochannels can be fabricated by AAO. For instance, a cylindrical nanochannelstructured AAO decorated with 3-aminopropyltrimethoxysilane (APTMS) at the desired positions was constructed using a two-step anodization method ( Figure 5A). [110] Because of the asymmetric charge distribution induced by the protonation or deprotonation of the modified amine and the intrinsic hydroxyl groups on the inner walls, the prepared AAO nanochannels exhibited pH-modulated ion rectification. Inspired by the structure of biological ion channel proteins (i.e., KcsA), researchers designed a single-walled CNT decorated with carbonyl oxygen atoms in a special arrangement along the inner wall for the selective separation of Na + and K + ions. [114] The simulation results showed that the hydration structure of the ions in the confined nanochannel led to a remarkable sieving ability, which could be finely adjusted by changing the positions of the carbonyl oxygen atoms. Furthermore, various nanochannel structures have been found in SiO 2 membranes, including conical [115] and cylindrical (i.e., SBA-15 [116][117][118] ) nanochannels and mesochannels with 3D cubic structures (i.e., SBA-16 [119,120] ). For instance, by adjusting the extent of positively charged acceptors (e.g., viologen) bound to a negatively charged donor (e.g., pyranine), the charge in the confined mesochannels (<10 nm) of silica films will invert, leading to controllable ion transport from anion-selective to ambipolar to cation-selective ( Figure 5B). [116] Moreover, the cylindrical nanochannel-structured mesoporous silica showed surface charge-governed proton transport at low proton concentrations. [118] When a lower gate voltage (1 V) was applied to the membranes, the corresponding proton conduction was enhanced two-to fourfold. For example, the surface charge of cylindrical Si 3 N 4 nanopores with a diameter of 10 nm and a length of 30 nm could be modulated through both the applied voltage and the salt gradient under a constant pH value, showing great application potential in physiological research ( Figure 5C). [90] In addition, the ion transport properties of cylindrical [121][122][123] and conical [124] C 3 N 4 nanotubes were also investigated. For example, asymmetric cone-shaped C 3 N 4 nanotubes with a 70-80 nm base diameter and a 15-20 nm tip diameter were used to fabricate highly sensitive and stable ionic photodetectors, where the surface charge gradient along the nanochannels induced by unilateral illumination led to controlled ion transport ( Figure 5D). [124] According to the material composition, 1D nanochannelstructured membranes with ion transport properties are mainly classified as either organic (e.g., PET, PI, PC, CDNTs, and COF) or inorganic (e.g., AAO, CNTs, SiO 2 , Si 3 N 4 , C 3 N 4 ). Controlled ion transport is achieved through the precise mediation of geometrical parameters and selective surface modifications at specific positions. However, the raw materials used to construct 1D nanochannels are still limited. Very recently, Hu et al. performed excellent work on ionic conductive wood with aligned nanochannels. Positively charged artificial wood (i.e., cationic wood) consisting of cellulose nanofibers was used to transport ions through the aligned nanochannels between these nanofibers, which not only exhibited excellent mechanical properties (5.5-and 20times improvement in wet and dry conditions, respectively) but also dramatically enhanced ion conductance (25-times improvement at a low KCl concentration). [44] Furthermore, it is meaningful to achieve synchronous enhancement of the ability to transport ions from a single nanochannel to multiple nanochannels. [125,126] Thus, it is necessary to seek new materials to design 1D nanochannels with fine geometrical parameters (e.g., diameter, length, curvature, and density) and surface modifications and to promote their application in ion transport-related areas.
Since the fabrication of 2D nanofluidic channels based on restacked GO nanosheets have been reported, many studies have focused on the use of 2D nanochannel-structured membranes for controllable ion transport. Taking GO membranes as an example, both chemical (e.g., cross-linking with intercalated molecules [128][129][130] ) and physical (e.g., physical confinement [61,131,132] and ion intercalation [133] ) methods have been applied to mediate the interlayer spacing. When layered GO membranes are physically embedded in epoxy, their swelling is mechanically restricted ( Figure 6A). [61] Then, the interlayer spacing d can be finely tuned between 6.4 and 9.8 Å, exhibiting a smaller channel size than that of hydrated ions. Through this strategy, accurate ion sieving based on size selection can be achieved. Multilayered C 3 N 4 membranes with controlled thicknesses from 140 nm to 1 μm showed surfacecharge-controlled ion transport ( Figure 6B). [134] Anion intercalation can also be used to tune the interlayer spacing for selective permeation. [135] For MXenes, both the interlayer spacing and the arrangement structure (e.g., vertically F I G U R E  Representative 2D nanochannel-structured inorganic membranes. (A) Graphene oxide (GO) under physical confinement (left) and permeation rates through GO with adjustable interlayer spacing (right). Reproduced with permission. [61] Copyright 2017, Macmillan Publishers Limited. (B) C 3 N 4 (left) and conductance-salt concentration curves at different pH values (right). Reproduced with permission. [134] Copyright 2018, Wiley-VCH. (C) Vertically aligned MXenes (left) and cyclic voltammograms of MXenes with different arrangement forms (right). Reproduced with permission. [40] Copyright 2018, Macmillan Publishers Limited. (D) Vertically aligned MoS 2 on the AAO inner walls (left) and comparison of the rate performance between metallic MoS 2 nanotubes and 2H MoS 2 nanosheets (right). Reproduced with permission. [142] Copyright 2018, Wiley-VCH. (E) Boron nitride (BN) with angstrom-scale channels (left) and pressure-driven streaming current under various voltage. (ΔP and ΔV represent pressure and voltage, respectively) (right). Reproduced with permission. [143] Copyright 2019, Macmillan Publishers Limited. (F) Kaolinite (left) and diffusion current-concentration difference curves at different pH values (right). Reproduced with permission. [144] Copyright 2017, Wiley-VCH. (G) MMT (left) and permeance toward to methylene blue (MB), methyl orange (MO), and rhodamine B (RB) (right). Reproduced with permission. [146] Copyright 2019, The Royal Society of Chemistry. (H) Vanadyl phosphate (VOPO 4 ) intercalated with triethylene glycol (TEG) and tetrahydrofuran (THF) (left) and rate performance of the pure, TEG-, and THF-intercalated VOPO 4 (right). Reproduced with permission. [147] Copyright 2017, American Chemical Society aligned and anti-T MXenes) were controlled through chemical [136] and physical [39,40,137,138] methods to investigate their ion transport properties. For instance, vertically aligned 2D titanium carbide (Ti 3 C 2 T x ) MXene membranes enabled directional ion transport through fast transport pathways, which can be used as thickness-independent capacitance for electrochemical energy storage ( Figure 6C). [40] This type of membrane also has great application potential in the fields of separation and catalysis. In addition, a typical layered Ti 3 C 2 MXene membrane could be bent owing to its mechanical flexibility while maintaining the surface charge-governed ion transport characteristics. [139] MoS 2 with expanded interlayer spacing provided a high-efficiency electron/ion transport pathway for rapid Li + and K + transport. [140,141] Recently, Zhu et al. prepared vertically aligned metallic MoS 2 on AAO inner walls (i.e., metallic MoS 2 nanotubes) using a solvothermal method. [142] As the electrode material in lithium-ion batteries, the metallic MoS 2 nanotubes exhibited remarkable rate performance (589 mA h g −1 ) at a high current density (20 A g −1 ) because of the fast electrolyte ion transport induced by the porous and aligned structure ( Figure 6D). The nanofluidic channels between individual BN nanosheets for ion transport have also been studied in recent years. For instance, BN membranes with angstrom-scale channels constructed through van der Waals assembly showed adjustable pressure-driven ion transport under various voltages, which can contribute to understanding the underlying principles of mechanosensitive ion nanochannels in biological and artificial membranes (Figure 6E). [143] Furthermore, Jiang et al. reported reconstituted kaolinite membranes with channels with sub-nanometer (6.8 Å) and nanoscale (13.8 Å) widths, demonstrating obvious cation-selective transport controlled by the surface charge ( Figure 6F). [144] Other 2D nanochannel-structured membranes made of MMT [145,146] and VOPO 4 [147] nanosheets have also attracted the attention of researchers. For example, lamellar MMT membranes (water permeance of 429 L m −2 h −1 atm −1 , thickness of 2.5 μm) decorated with a cationic surfactant showed high separation efficiency for cationic and anionic dyes ( Figure 6G). [146] VOPO 4 membranes have expanded interlayer spacing through controlled intercalation of organic molecules such as triethylene glycol (TEG) and tetrahydrofuran (THF), which can be used to improve sodium-ion storage ( Figure 6H). [147] Representative 2D nanochannel-structured inorganic membranes have been introduced based on restacked nanosheets, which include GO, C 3 N 4 , MXenes, MoS 2 , BN, kaolinite, MMT, and VOPO 4 . To clarify the regulation of the 2D nanochannel structure for ion transport, the corresponding strategies (e.g., physical or chemical methods) for adjusting the interlayer spacing and special arrangement of the nanosheets were discussed. It is meaningful to develop 2D nanochannel-structured membranes using other layered inorganic and organic materials and to investigate the corresponding ion transport characteristics. For example, Hu et al. reported that multiple confined spacings (∼1 nm) consisting of graphite flakes and cellulose nanofibers exhibited higher cationic conductivity owing to abundant nanochannels and a negatively charged surface. [148] Layered ionic liquids with nanoconfined nanochannels were constructed through complexation between surfactants and ionic liquids, showing temperature-mediated ion transport along the parallel and normal directions. [149] Recently, 2D nanochannel-structured membranes with special surface and nanochannel arrangement structures have attracted the attention of researchers. For instance, anti-T [39] MXenes showed ion transport in the vertical direction because of the specific surface structure and vertically aligned [40] structure, exhibiting rapid ion transport owing to the efficient ion transport pathways resulting from the arrangement structure. It is still a challenge to further fabricate 2D nanochannel-structured membranes by mediating the surface and nanochannel arrangement structures, as well as by choosing novel raw materials.

. D Nanochannel-structured membranes
Representative 3D nanochannel-structured membranes have been fabricated using various raw materials, including MOFs, COFs, conjugated microporous polymers (CMPs), block copolymers (BCPs), cellulose nanofibers, ethoxylated trimethylolpropane triacrylate (ETPTA), carbon fibers, and SiO 2 nanoparticles. The common characteristic of these membranes is the continuous 3D nanochannels, which shorten the ion diffusion pathways and facilitate ion transport. Thus, we present the diversity of 3D nanochannel-structured membranes with respect to the material compositions ( Figure 7). Here, typical 3D nanochannel-structured membranes (e.g., MOFs, COFs, CMPs, BCPs, cellulose nanofibers, ETPTA, carbon fibers, and SiO 2 nanoparticles) with tunable ion transport properties are discussed. As typical polycrystalline microporous framework membranes, MOFs commonly exhibit interconnected and well-defined pores on the angstrom scale. Hence, MOFs are widely used for the selective transport of alkali metal ions and alkali earth metal ions with or without the introduction of polyelectrolytes (e.g., poly(sodium vinyl sulfonated-co-acrylic acid and polystyrene sulfonate). [60,[150][151][152][153][154] For instance, uniform Mg-MOF-74 membranes were used as the separators for the selective transport of Mg 2+ while prohibiting the transport of other solvents and counterions ( Figure 7A). [150] The corresponding ionic conductivity of this kind of MOF membrane with a thickness of 202 nm can reach approximately 3.17 × 10 −6 S cm −1 at room temperature. In addition, MOFs modified with polystyrene sulfonate showed fast and ideal lithiumion selectivity owing to the size-sieving effects and the affinity differences of different ions to the sulfonate groups. [154] COF is another representative polycrystalline microporous material. Wang et al. investigated the Li + transport in 3D (i.e., CD-COF) and 1D (i.e., COF-5, COF-300, and EB-COF) channelstructured COFs after incorporating them with polyethylene glycol (PEG) in anionic, neutral, and cationic states. [155] The results indicated enhanced ion conductivity of 1.78 × 10 −3 S cm −1 at 120 • C in cationic COFs because the local motion of PEG chains in the channels promoted Li + transport. Organic nanosheets (e.g., MOFs and COFs) can also be used for ion separation. [31,36,156,157] For example, a nanosheet-stacked COF 9 membrane with carboxylate-modified nanopores 2.8 nm in diameter was prepared by Hoberg et al., which exhibited high water flux (∼2260 L m −2 h −1 bar −1 ) and efficient cation selectivity ( Figure 7B). [58] CMP is an organic porous polymer that is characterized by both π-conjugated skeletons and intrinsic nanopores, which can be applied in areas such as ion separation and detection. [158][159][160] For example, CMP with carbazole groups (i.e., TPBCz-CMP) was used to detect redox-active ions through redox-induced fluorescence quenching because the carbazole groups in the CMP nanopore walls were inclined to be oxidized. [159] In addition, interconnected 3D nanochannels in BCPs can be constructed through the microphase separation method and used to modulate the transport of specific ions. [161,162] Seo et al. designed poly(styrene-co-divinylbenzene) (i.e., P(S-co-DVB)) membranes with 3D continuous mesopores and obtained controllable ion permeability by mediating the pore size and sulfonic acid content ( Figure 7C). [162] Very recently, Hu et al. constructed interconnected 3D nanochannels using cellulose nanofibers, which exhibited charge-adjusted sodiumion transport ( Figure 7D). [45] After further chemical treatment, the cellulose nanofibers showed enhanced zeta potential (−45 mV) and ionic conductivity (1.0 × 10− 3 S cm −1 ) in contrast to the untreated ones. A new class of battery F I G U R E  Representative 3D nanochannel-structured membranes. (A) MOF membranes as the separators (left) and electrochemical impedance spectra obtained with and without MOF on Au-coated Si wafers (right). Reproduced with permission. [150] Copyright 2019, Wiley-VCH. (B) Layered COF membranes with efficient water transport pathways (left) and cation transport through the membranes (right). Reproduced with permission. [58] Copyright 2018, American Chemical Society. (C) Block copolymer (BCP) with continuous pore structure (left) and ion concentration-time curves across the membranes with different sulfonic acid content (right). Reproduced with permission. [162] Copyright 2018, American Chemical Society. (D) Interconnected 3D nanochannels consisting of cellulose nanofibers (left) and ionic conductivity of the treated and untreated cellulose nanofibers (right). Reproduced with permission. [45] Copyright 2018, American Chemical Society. (E) Ethoxylated trimethylolpropane triacrylate (ETPTA) with 3D inverse opal structure (left) and discharge C-rate capability (right). Reproduced with permission. [163] Copyright 2014, American Chemical Society. (F) Mesoporous carbon fibers (left) and ion diffusion resistance of the prepared carbon fiber electrodes (right). Reproduced under the terms of the Creative Commons CC BY license. [166] Copyright 2019, The Author(s) separators was fabricated using ETPTA with a 3D inverse opal structure, which facilitated rapid ion transport (Figure 7E). [163] The special 3D nanochannels endowed the separator with a high discharge C-rate capability (60 mAh g −1 when cathode/anode = 3.5/1.7 mg cm −2 ) compared to common PP/PE/PP separators. Carbon (e.g., mesoporous membranes [164,165] and porous fibers [166,167] ) ( Figure 7F) and SiO 2 (e.g., self-assembled nanoparticles [168,169] ) with 3D nanochannels are common inorganic materials for adjusting ion transport.
Inorganic nanosheets, including stacked porous nanosheets (e.g., holey graphene frameworks [HGFs], [170,171] porous carbon, [172][173][174] and TaS 2 [175] ) and hybrid nanosheets (e.g., MXene/rGO [176,177] ) have also been used to construct 3D nanochannels ( Figure 8). Duan et al. designed a highperformance supercapacitor electrode using 3D HGFs with hierarchical pores ( Figure 8A). [171] The CV and CD curves exhibited typical rectangular and triangular shapes, indicating efficient ion transport in the electrode. 3D porous MXene/rGO aerogels have good electron conductivity and fast Li + transport capability. As anodes in fuel cells, they showed better rate and cycling performance than the rGO aerogel ones ( Figure 8B). [176] In this section, typical membranes with interconnected 3D nanochannels were summarized, such as MOFs, COFs, CMPs, BCPs, cellulose nanofibers, ETPTA, carbon fibers, and SiO 2 nanoparticles. Inorganic nanosheets (e.g., stacked porous nanosheets and hybrid nanosheets) for developing 2D nanochannels can also be used to design 3D nanochannels, which provides inspiration for the novel construction of nanochannel-structured membranes. Interestingly, natural plants can be used for ion transport after chemical treatment because of their abundant 3D channels. For instance, carbon membranes derived from natural wood were used as electrodes in metal-air batteries, which showed superior specific capacity owing to the hierarchical pores for fast ion transport. [178] Therefore, in addition to materials with intrinsic pores, holey or hybrid layered materials and natural plants can also be used to construct 3D porous membranes.

2D/2D heterogeneous membranes
2D/2D heterogeneous membranes are constructed by using one type of lamellar material with different modifications [203][204][205][206] or two types of lamellar materials. [207] Guo et al. investigated the preferential directions of proton transport under the driving forces of electric field, concentration difference, and hydraulic pressure using a 2D/2D membrane consisting of reconstructed negatively charged GO (n-GO) and positively charged GO (p-GO) ( Figure 9D). [206] The results indicated that protons preferred to travel from the n-GO side to the p-GO side under the electric field while traveling in the reverse direction under other driving forces. A MoS 2 /WSe 2 membrane showed a high ICR ratio (∼35) owing to the asymmetric characteristics (i.e., composition, interlayer spacing, and surface charge) in the separate MoS 2 and WSe 2 layers. [207] 3.4.5 2D/3D heterogeneous membranes A 2D/3D heterogeneous membrane can be developed by combining restacked nanosheets and continuous 3D porous materials. For example, Jiang et al. designed a typical 2D/3D heterogeneous membrane taking advantage of laminar GO with a negative charge and a 3D porous polymer (i.e., PPSU-Pyx) with a tunable charge density ( Figure 9E). [208] The adjustable surface charge and wettability under the pH stimulus endowed the heterogeneous membrane with a reversible nano-gating property.

3D/3D heterogeneous membranes
It is feasible to prepare 3D/3D heterogeneous membranes on a large scale because of the simple fabrication methods. [82,209,210] For instance, a 3D microscale porous hydrogel with a negative charge and nanoscale porous CP with a positive charge (e.g., Hydrogel/CP) were used to obtain 3D/3D heterogeneous membranes. [82] The asymmetries in chemical composition, structure, and surface charge polarity in 3D/3D heterogeneous membranes can lead to ion rectification properties, which can be controlled by external stimuli (e.g., electricity and pH). An asymmetric porestructured separator consisting of nanoporous terpyridinemodified cellulose nanofibers (TYP-CNF) and macroporous polyvinylpyrrolidone/polyacrylonitrile (PVP/PAN) was designed after considering both the leakage current and ion transport rate, and it exhibited a high discharge rate capability ( Figure 9F). [210] As mentioned above, heterogeneous membranes with mixed-dimensional nanochannels, that is, 1D/1D, 1D/2D 1D/3D, 2D/2D, 2D/3D, and 3D/3D membranes, generally show more controllable ion transport properties through the design of asymmetries in chemical composition, structure, and surface charge polarity and the induction of various external stimuli. It is still necessary to investigate the combination forms of different nanochannel-structured materials for precisely tunable ion transport.

 ULTRATHIN MEMBRANES
With decreasing membrane thickness, the mass transport resistance is significantly reduced, and the corresponding permeation flux can be enhanced. [211] Thus, it is urgent to develop ultrathin membranes, especially sub-micrometer and monolayer membranes, with superior mechanical and ion transport properties. [35,212] Ion transport pathways can be further decreased by introducing ordered nanopores or subnanopores into the ultrathin membranes. In this section, we provide typical examples of ultrathin membranes, including sub-micrometer (e.g., polyamide, carbon, SiO 2 ) and monolayer (e.g., graphene, MoS 2 , and BN) membranes ( Figure 10). Typical examples of ultrathin membranes include submicrometer (e.g., polyamide, carbon, SiO 2 ) and monolayer (e.g., graphene, MoS 2 , and BN) membranes. Polyamide membranes are commonly used for desalination via reverse osmosis. To enhance membrane permeance, many studies have focused on decreasing the thickness. [213][214][215][216] For instance, transferable ultrathin polyamide nanofilms with thicknesses of less than 8 nm were prepared at the oil-water interface by interfacial polymerization. [213] When the thickness was decreased to approximately 6 nm, the corresponding water permeance reached 2.7 L m −2 h −1 bar −1 ( Figure 10A). Carbon nanomembranes with thicknesses of approximately 1.2 nm and channel diameters of approximately 0.7 nm exhibited both high water permeance (∼1.1 × 10 −4 mol m −2 s −1 Pa −1 ) and good selectivity because the hydrogen-bonded water inside the channel enlarged the sub-nanopores and contributed to fast water permeation ( Figure 10B). [217] Ultrathin SiO 2 membranes with ordered nanochannels are widely used for selective separation. [59,218,219] For instance, Su et al. prepared SiO 2 membranes (thickness of 10-120 nm) with vertically aligned nanochannels (pore size of ∼2.3 nm and porosity of 16.7%) that showed both high permeation flux and fine selectivity based on size and charge ( Figure 10C). [59] Monolayer membranes made of graphene, MoS 2 , and BN have also been applied for controllable ion transport. For single-layer graphene, the 2D confined space [220] between the graphene layer and the single-crystal Pt (111) surface and the 1D single sub-nanometer pore [221][222][223] were both used to modulate ion transport. Karnik et al. studied the diverse ion F I G U R E   Representative ultrathin membranes (e.g., sub-micrometer and monolayer membranes). Sub-micrometer membranes include (A) ultrathin polyamide nanofilms (left) and water permeance and permeability versus thickness (right), (B) sub-nanometer channel-structured carbon membranes with thickness of ∼1.2 nm (left) and permeances of various vapors and gases (right), and (C) ultrathin SiO 2 membranes with ordered and perpendicular nanochannels (left) and permeation flux curves of fluorescein anion across the SiO 2 membrane, dialysis membrane and PC membrane (right). (A) Reproduced with permission. [213] Copyright 2018, Wiley-VCH. (B) Reproduced with permission. [217] Copyright 2018, American Chemical Society. (C) Reproduced with permission. [59] Copyright 2015, American Chemical Society. Monolayer membranes include (D) a single-layer graphene membrane with isolated sub-nanometer pores (left) and I-V curves with linear, activated, and saturated features (right), (E) a single-layer MoS 2 membrane with a sub-nanometer pore for single-ion transport (left) and I-V curves under varied ion concentrations (right), and (F) molecular dynamics simulations for monolayer BN membranes with sub-nanometer pores (left) and rejection rate versus water permeability for different BN membranes (right). (D) Reproduced with permission. [222] Copyright 2015, Macmillan Publishers Limited. (E) Reproduced with permission. [224] Copyright 2016, Macmillan Publishers Limited. (F) Reproduced with permission. [227] Copyright 2018, American Chemical Society transport behaviors through a single-layer graphene membrane with isolated pores less than 2 nm in size, which may have been caused by ion dehydration and electrostatic interactions ( Figure 10D). [222] Recently, ion transport across single-layer MoS 2 with sub-nanometer pores [224,225] and nanopores [226] has also attracted the attention of researchers. For instance, a single-layer MoS 2 membrane (thickness of 0.65 nm) with sub-nanometer pores (diameters of 0.6 nm) was used to study single-ion transport behavior ( Figure 10E). [224] The measured I-V curves exhibited a striking nonlinear characteristic, with an apparent gap as a result of the synergistic effect of Coulomb blockade and dehydration. Ghoufi et al. reported the ultrahigh water permeability and ion rejection (∼100%) of monolayer BN membranes with sub-nanometer pores through molecular dynamics simulations ( Figure 10F). [227] Most recently, Lozada-Hidalgo et al. reported selective proton transport through monolayer BN membranes with defect-free characteristics, showing potential applications for ion separation. [228] In this section, representative ultrathin membranes with nano or sub-nanochannels were introduced, which included sub-micrometer (e.g., polyamide, carbon and SiO 2 ,) and monolayer (e.g., graphene, MoS 2 , and BN) membranes. To achieve both high permeability and selectivity, novel materials must be screened to construct ultrathin membranes with vertically ordered channels to shorten the ion transport distance. For example, single-layer MOFs and COFs with uniform pore sizes are ideal materials for efficient ion separation because of their short ion transport pathways.

 SANDWICH-LIKE MEMBRANES
To precisely tune ion transport behaviors, sandwich-like membranes with more delicate designing of nanochannels and combination structures have been prepared. In this section, we introduce sandwich-like membranes with different combination forms ( Figure 11) gathered from recent research (Figures 12 and 13). Sandwich-like membranes can be classified into two types: three-layer sandwich-like membranes comprising (i) two thin layers A and C combined with 1D nanochannels B and (ii) two laminar layers A and C combined F I G U R E   Various types of sandwich-like membranes. Three-layer sandwich-like membranes, including (i) two thin layers A and C combined with 1D nanochannels B and (ii) two laminar layers A and C combined with 1D nanochannels B. Multilayer sandwich-like membranes, including (iii) two kinds of nanosheets A and C combined with 1D nanochannels B, (iv) restacked by three kinds of nanosheets A, B, and C, (v) sandwich-like nanosheets A decorated with mesoporous nanosheets B, and (vi) nanosheets A combined with 3D nanopores B with 1D nanochannels B, and multilayer sandwich-like membranes comprising (iii) two kinds of nanosheets A and C combined with 1D nanochannels B, (iv) restacked by three kinds of nanosheets A, B and C, (v) sandwich-like nanosheets A decorated with mesoporous nanosheets B, and (vi) nanosheets A combined with 3D nanopores B ( Figure 11). Recent studies on sandwich-like membranes for ion transport are discussed from the perspective of three-layer and multilayer sandwich-like membranes. Through the combination of 1D nanochannel-structured AAO and two layers of different inorganic oxides with thicknesses of several hundred nanometers and different isoelectric points, a typical three-layer sandwich-like WO 3 /AAO/NiO membrane was constructed that exhibited efficient modulation of the ionic transport owing to the opposite charges on the exterior surfaces ( Figure 12A). [37] Another example is a sandwich-like electric double-layer separation (EDLS) membrane consisting of an N-functionalized graphene sheet hydrogel (GS) layer, a 1D porous separator, and an N-functionalized GS (CN-GS) layer ( Figure 12B). [38] The lamellar structure of the CN-GS layer selectively allowed ions to be transported along the 2D nanochannels under the control of the surface potential. Compared with three-layer sandwich-like membranes, multilayer sandwich-like membranes are more common in mediating ion transport. For instance, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), rGO, and CNTs were used to construct sandwich-like electrodes (PTCDA/rGO/CNTs) with interconnected micro/nano channels, which showed high ion diffusion coefficients and superior rate performances in lithium/sodium-ion batteries ( Figure 13A). [229] Taking advantage of GO, MMT, and sulfonated polyvinyl alcohol (SPVA), Jiang et al. designed sandwich-like composite membranes (GO/MMT/SPVA) with both high ionic conductivity (326 mS cm −1 ) and high mechanical properties (tensile strength of 250 MPa), which were owing to the continuous 2D nanochannels and special brick-and-mortar structure, respectively ( Figure 13B). [230] Very recently, sandwich-like hybrid nanomembranes (PPy/rGO/PPy) were fabricated as supercapacitors with superior volumetric capacitance (102 F cm −3 ) using rGO decorated with polypyrrole (PPy) on both sides

F I G U R E   Three-layer sandwich-like membranes for ion transport. (A) WO 3 /AAO/NiO membranes (left) and I-V curves at varied pH values (right).
Reproduced under the terms of the Creative Commons CC BY license. [37] Copyright 2018, The Author(s). (B) Sandwich-like membranes consisting of N-functionalized graphene sheet hydrogel (GS) layer, 1D porous separator, and N-functionalized GS (CN-GS) layer (left) and variation of ion concentrations during the separation process (right). Reproduced with permission. [38] Copyright 2015, American Chemical Society ( Figure 13C). [231] The cylindrical mesopores of PPy can facilitate rapid ionic diffusion in the plane. Fan et al. also constructed supercapacitors with superior specific capacitance of 345 F g −1 at 2 mV s −1 using sandwich-like carbon with confined nanochannels between GO nanosheets (Figure 13D). [232] In this system, the nanochannels enabled fast electrolyte ion transport while the GO nanosheets functioned as conductive and structural supports.
In this section, the reported sandwich-like membranes were classified into two types (i.e., three-layer and multilayer sandwich-like membranes) based on their combination forms, and simplified models were also presented ( Figure 11). More specifically, three-layer sandwich-like membranes include: (i) two thin layers A and C combined with 1D nanochannels B and (ii) two laminar layers A and C combined with 1D nanochannels B, whereas the multilayer ones include (iii) two kinds of nanosheets A and C combined with 1D nanochannels B, (iv) restacked by three kinds of nanosheets A, B and C, (v) sandwich-like nanosheets A dec-orated with mesoporous nanosheets B, and (vi) nanosheets A combined with 3D nanopores B. Owing to the fine structural adjustment, these membranes can mediate ion transport with specific requirements. To date, there have been few studies on sandwich-like membranes, in contrast to membranes with single-and mixed-dimensional nanochannels. Therefore, it is meaningful to construct sandwich-like membranes with more delicate structures and further investigate the corresponding ion transport properties.

 STIMULUS-RESPONSIVE NANOCHANNELS
Applying external stimuli to target nanochannels is a major strategy for achieving dynamically controllable ion transport. Therefore, it is necessary to investigate the corresponding responsiveness to various stimuli (e.g., pH, ions, molecules, light, temperature, electricity, magnetism, and mechano). . Reproduced with permission. [230] Copyright 2017, Wiley-VCH. (C) Sandwich-like hybrid nanomembranes (PPy/rGO/PPy) composed of rGO decorated with polypyrrole (PPy) on both sides (left) and areal (blue) and volumetric (red) capacitance versus scan rate (right). Reproduced with permission. [231] Copyright 2019, Wiley-VCH. (D) Sandwich-like membranes (i.e., C-GMOF) made of graphene and mesoporous carbon (left) and specific capacitance versus scan rate (right). Reproduced with permission. [232] Copyright 2016, Elsevier Ltd F I G U R E   Single-responsive nanochannels. (A) pH-Responsive hourglass-shaped PET nanochannels through adjusting the shape asymmetries and poly(acrylic acid) (PAA) locations (left) and I-V curves of the nanochannels modified with PAA on the small base sides at pH 10 (right). Reproduced with permission. [75] Copyright 2015, American Chemical Society. (B) Ion-responsive funnel-shaped nanochannels modified with 18-crown-6 units (left) and reversibly controlled ion rectification after alternately adding Pb 2+ and ethylenediamine tetraacetic acid (EDTA) (right). Reproduced with permission. [235] Copyright 2018, The Royal Society of Chemistry. (C) Inversion of surface charge in single conical PET nanochannel modified with polyethylenimine (left) and I-V curves of the nanochannels before and after adding ATP (right). Reproduced with permission. [238] Copyright 2010, The Royal Society of Chemistry. (D) Lamellar GO membranes with 2D nanochannels (left) and pumping rates of varied ions against the concentration gradient (right). Reproduced under the terms of the Creative Commons CC BY license. [239] Copyright 2019, The Author(s). (E) Temperature-responsive single conical PET nanochannel (left) and reversible switching of the rectifying state and the non-rectifying state (right). Reproduced with permission. [242] Copyright 2010, Wiley-VCH. (F) Lamellar GO (left) and reversibly modulating ion transport with voltages of 0.2 and −0.5 V (right). Reproduced with permission. [78] Copyright 2018, Macmillan Publishers Limited. (G) Magnetism-responsive nanochannels consisting of the superhydrophilic AAO and magnetically controlled ferrofluid-(left) and magnetic-modulated gating (right). Reproduced with permission. [247] Copyright 2018, Wiley-VCH. (H) Curved CNTs (left) and various ion transport behaviors (right). Reproduced with permission. [249] Copyright 2019, Wiley-VCH Here, we introduce typical stimulus-responsive nanochannels from two aspects, namely, single-responsive and dualresponsive nanochannels (Figures 14 and 15, respectively).

. Dual-responsive nanochannels
Ion transport across the nanochannels is usually simultaneously affected by different factors, and it is necessary to investigate the ion transport properties under dual and multiple stimuli. Here, we present the research on dual-responsive nanochannels, including ions and pH, light and pH, temperature and pH, electricity and pH, and temperature-and electricity-responsive nanochannels ( Figure 15). In 2014, Jiang et al. designed K + -and pH-responsive cigar-shaped PET nanochannels through the heterogeneous modification of Cquadruplex (C4) and G-quadruplex (G4) DNA molecules on the top and bottom tips, respectively ( Figure 15A). [80] Under the influence of K + and H + concentrations, the nanochannels opened and closed alternately or simultaneously through the conformational transition of the C4 and G4 DNA molecules. Hourglass-shaped AAO nanochannels decorated with both di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′bipyr-idyl-4,4′-dicarboxylato) ruthenium(II) (N719) and (3-aminopropyl)triethoxysilane (APTES) also exhibited dualresponsive ion transport owing to the redistribution of surface charge caused by pH and light ( Figure 15B). [81] The conical PI nanochannels showed dual-responsive properties after decorating with temperature-and pH-responsive poly(Nisopropyl acrylamide-co-acrylic acid) [P(NIPAAm-co-AAc)] ( Figure 15C). [253] More specifically, the temperature-induced conformational transition of P(NIPAAm-co-AAc) led to the opening and closing of the nanochannels, showing reversible switching between high and low ion conduction. The surface charge on the asymmetric nanochannel walls, adjusted by the pH, resulted in pH-dependent ion rectification. In addition, electricity and pH-responsive 3D/3D heterogeneous membranes consisting of an electricity-responsive PPy layer with nanopores and a pH-responsive poly(acrylamide-co-acrylic acid) (P(AAm-co-AA)) hydrogel layer with micropores ( Figure 15D) were fabricated. [82] These membranes showed dual-responsive ion rectification because of the asymmetric chemical composition, structure, and surface charge induced by electrical and pH stimuli. Furthermore, 2D MMT intercalated with a quaternary ammonium bilayer showed temperature-and electricity-responsive ion transport owing to the adjustments of the temperature-dependent phase state of the bilayers and the electrically dependent surface polarity ( Figure 15E). [145] Common stimulus-responsive nanochannels, such as single-responsive (e.g., pH, ions, molecules, light, temperature, electricity, magnetism, and mechano) and dual-responsive (e.g., ions and pH, light and pH, temperature and pH, electricity and pH, and temperature and electricity) nanochannels have been summarized in this section. Through precisely adjustable stimuli, the ion transport characteristics can be tuned dynamically and reversibly, which can establish the basis for further investigation of the sophisticated ion transport behaviors under multiple stimuli in both artificial and biological systems.

 CONSTRUCTION METHODS OF NANOCHANNEL-STRUCTURED MEMBRANES
The common methods for constructing nanochannelstructured membranes with specific structures (e.g., nanochannel, ultrathin, and sandwich-like structures) have been introduced in previous reviews [27][28][29]33,36,74,156,254,255] . Therefore, we will briefly list the typical fabrication approaches for designing nanochannel-structured membranes using top-down and bottom-up approaches in this section (Table 1).
In this section, representative approaches for constructing nanochannels were briefly listed in terms of top-down and bottom-up approaches. Top-down approaches, including various etching methods (e.g., track etching, anodization, metal nanoparticle-assisted etching, and reactive ion etching) and chemical delignification and pyrolysis, are used to fabricate 1D and 3D nanochannels, respectively. Bottom-up approaches, including different deposition methods (e.g., CVD, ALD, and magnetron sputtering), vacuum filtration, coating, spinning, assembly, and various polymerization methods (e.g., VDP and interfacial polymerization) are mainly used to develop various layered 2D, heterogeneous, and sandwich-like membranes. Natural plants also provide nanochannels for ion transport. For instance, the natural ion channels in grass stems were applied to construct an electron battery after immersion in saturated salt solutions, which provides new inspiration for designing artificial membranes with nanochannels. [41] The nanochannels in positively charged cellulose nanofibers consisting of chemically treated wood enabled rapid chloride ion transport. [44] The 3D printing approach is being used more often to obtain membranes with multiscale structures to mediate ion transport for application in energy storage. [138,[274][275][276] For example, multidimensional MXenes with internal nanochannels and external microstructures were fabricated using the 3D printing method. These multidimensional MXenes worked as high-performance supercapacitors owing to the abundant ion transport pathways. [138] Hu et al. further designed macroscale BN rods composed of vertically aligned BN nanosheets, thus exhibiting the potential of using a 3D printing approach for constructing multiscale structures. [274] Superwettable interfaces (e.g., superhydrophilic and superhydrophobic interfaces) also provide efficient platforms for constructing TA B L E  Construction methods of nanochannel-structured membranes with specific structures for controlling ion transport

Methods
Feature Ref.
(E) Short single-wall CNTs for DNA2-Bzim translocation (left) and current signature (right). Reproduced under the terms of the Creative Commons CC-BY-NC-SA license. [310] Copyright 2013, The Author(s). (F) Electron batteries with natural ionic cables (i.e., grass stems) for transporting ions to interact with the targeted biosystems. Reproduced under the terms of the Creative Commons CC BY license. [41] Copyright 2017, The Author(s) nanochannel-structured membranes (e.g., mixeddimensional nanochannels, ultrathin, and sandwich-like membranes). [47,48,[277][278][279] For instance, taking advantage of the superspreading of liquids on superhydrophilic interfaces, [49][50][51] Jiang et al. obtained membranes with controllable thickness and low roughness, which could be used to prepare uniformly ultrathin MOF and COF membranes and corresponding heterogeneous and sandwich-like membranes. In addition, triphase interface-mediated epitaxial growth on superhydrophobic surfaces may be used for designing these membranes efficiently because of the controlled growth rates in the lateral direction. [52] With the development of preparation technology, nanochannel-structured membranes with single-or mixed-dimensional nanochannels and even specific combinations can be more precisely obtained for modulating ion transport.

 APPLICATIONS OF MEMBRANES WITH UNIQUE STRUCTURES
The ultimate goal of constructing nanochannel-structured membranes with controllable ion transport properties is to promote corresponding applications in various fields such as ion separation, water purification, energy storage and conversion, sensors, and bioelectronics. Hence, we discuss the repre-sentative applications of these membranes with unique structures (e.g., nanochannel, ultrathin, and sandwich-like structures) in this section ( Figure 16).

. Ion separation
Owing to the introduction of nanochannel structures in the desired membranes, selective ion separation can be achieved by mediating the nanochannel shape and size, charge, wettability, and specific recognition of the nanochannel walls. Membranes such as 1D CNTs [114,280] and polyhydrazide nanotubes, [281] 2D layered GO, [61,282,283] MOFs, [60,154,284] and COFs [58] consisting of interconnected and uniform pores and ultrathin [285] and sandwich-like structures [38] have been used for controllable ion sieving. For instance, stacked graphene and GO membranes with sub-nanometer interlayer spacing showed selective transport of alkali and alkaline earth cations across the membrane owing to the synergistic effect of cation-π interactions and ion dehydration. [282] Recently, Wang et al. reported the rapid selective transport of alkali metal cations (selectivity of ∼4.6, for LiCl/RbCl) through MOFs (e.g., ZIF-8) with uniform nanopores as a result of the ion selectivity of angstrom-scale windows and the rapid ion conduction through the nanoscale cavities in the pores ( Figure 16A). [60] Reconstituted nanoporous COF membranes with carboxylate-modified pore walls (diameter of 2.8 nm) exhibited excellent water permeance (∼2260 L m −2 h −1 bar −1 ) and cation selectivity. [58] Notably, novel artificial water nanochannels with diameters of approximately 3 Å and a cluster structure were made of unimolecular peptide-appended hybrid [4]arene. [286] Combined with the water-wire pathways in the matrix, these artificial water nanochannels exhibited remarkable permselectivity over water/salt, which provides inspiration for constructing the next generation of selective separation membranes.

. Sensors
Nanochannel-structured membranes have already shown great application potential in sensing external ions, [159,308,309] molecules, [310][311][312][313][314][315] and stimuli. [45,241] For example, Wu et al. constructed nanochannels characterized by enhanced ion transport and selective detection of modified 5hydroxymethylcytosine in single-stranded DNA (i.e., DNA2-Bzim) through embedded short single-wall CNTs in a lipid bilayer ( Figure 16E). [310] Asymmetric glass nanoslits (width of 70 nm) with a triangular structure were also applied for the sensitive and selective detection of troponin T. [311] Temperature-sensing PC nanochannels 50 nm in diameter were obtained by decorating a wax composite with reversible temperature-controlled expansion and contraction, which could be used to detect the body temperature within a narrow range. [241] Cellulose nanofibers with 3D interconnected nanochannels (1-2 nm) showed rapid sodium-ion transport and could be used to construct ultrasensitive humidity sensors. [45] A reversible and sensitive microsensor was prepared to monitor the real-time pH change in a living rat brain, successfully taking advantage of transient ion transport, which provides a new approach for in vivo neurochemical recording with high spatiotemporal resolution. [316]

. Bioelectronics
In nature, ion transport is ubiquitous in biological activities, such as the transmission of nerve signals and the functioning of enzymes. Significantly, the gradually rising field of bioelectronics provides an efficient way to further study real-time ion transport in biological systems. For example, organic electrochemical transistors (OECTs) consisting of gate, source, and drain electrodes and CP layers showed large transconductance, high volumetric capacitance, long-term mechanical stability, and good biocompatibility. They are therefore considered promising bioelectronics that can be used to stimulate nerves and record real-time signals. [317][318][319][320][321][322][323] This corresponding process involves both the normal ion transport from the electrolyte solutions to the CP layers and horizontal electron transport between the source and drain electrodes along the CP layers simultaneously. The representative CP layer is made of poly(3,4-ethyle-nedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) because of its high electrical conductivity and good biocompatibility. After inducing uniform nanopores, the normal ion transport from the electrolyte solution to the PEDOT:PSS layer can be further enhanced. [324] Recently, Hu et al. proposed the concept of an electron battery through which ions can be transported in external ionic cables to interact with the targeted biosystems ( Figure 16F). [41] More specifically, the authors constructed ionic cables by immersing grass stems in saturated salt solutions. The cables showed both stable ionic conductivity and superior mechanical strength because the interconnected channels in the stems were maintained for transporting ions in different deformed states. Furthermore, a calcium wave was observed after stimulating single-layer living cells with a controlled ionic current provided by the electron battery. In addition, bio-inspired synapses developed using nanochannels connected with immiscible ionic liquids and KCl solutions demonstrated voltage-mediated movement of the interfaces, showing promising applications in neuromorphic devices. [325] Most recently, Jiang et al. presented controlled ion transport related to biosystems (i.e., quantum-confined ion superfluid [54][55][56][57] ), which will open a new path for studying controlled ion transport across the nanochannel-structured membranes in both biological systems and artificial counterparts. The corresponding process is enthalpy-driven ordered fluid in confined nanochannels without energy loss, which can explain the phenomena of nerve signal transmission. In detail, Na + enter into the cell when the Na + channels open, then K + exit from the cell when the K + channels open, leading to the rapid ion transport to achieve ultrafast neural signal transmission. This process can also be simplified by the model of the movement of multiple-single pendulums with close contact. What is more, two main approaches for investigating the quantum-confined ion superfluid were clarified, including THz response for detecting the ions in biological nanochannels and artificial counterparts remotely. [55] Taking advantage of no energy loss and highly ordered during the transport process in the confined nanochannels, quantumconfined superfluid-based catalysis with low reaction barrier, superior efficiency and selectivity was proposed. [56] Thus, it is believed that advanced bioelectronics with ultrafast signal transmission and nanoreactors with high efficiency can be prepared with the development of membranes with unique structures.
In this section, the typical applications of nanochannelstructured membranes were discussed in detail, including ion separation, water purification, energy storage and conversion, sensors, and bioelectronics. It is believed that membranes constructed with fine structures (e.g., nanochannels, ultrathin, and sandwich-like structures) will pave the way for promoting corresponding applications in various fields. For instance, research on bioelectronics is gradually emerging as a hot topic as the next stage of studying controlled ion transport through nanochannel-structured membranes. Significantly, the concept of quantum-confined ion superfluids proposed in recent years will also promote the development of nanochannel-structured membranes with controllable ion transport properties in both biological and artificial systems. [54][55][56][57][326][327][328]

 CONCLUSION AND PERSPECTIVE
In nature, the common biological activities (e.g., uptake of ionic mineral elements, conversion of biological energy, and transmission of nerve signals) are strongly associated with the controllable transport of specific ions mediated by biological ion channel proteins across the plasma membranes, which have attracted the attention of researchers. Therefore, diverse artificial membranes with unique structures have been constructed to investigate finely tuned ion transport in artificial and biological systems, showing wide application prospects from industrial production to biological interfaces. Herein, we highlight representative nanochannel-structured membranes with controllable ion transport characteristics from the point of view of membrane structures (e.g., nanochannel and combination structures). First, we introduced the controllability of ion transport (e.g., ion selectivity, ion gating, ion rectification, and ion storage). We then presented nanochannel-structured membranes with singleand mixed-dimensional nanochannels. Third, we discussed typical ultrathin and sandwich-like membranes. Then, we summarized the stimulus-responsive nanochannels in detail. We briefly listed the construction methods of nanochannelstructured membranes and reviewed their broad applications in different fields.
The primary aims to construct membranes with different structures, and the advantages and disadvantages were discussed. The initial aims to construct 1D nanochannelstructured membranes is to investigate the fundamental principles of ion transport taking advantage of the simplified nanochannels and precise controllability for experimental measurements and analysis in theory. However, the construction of these 1D nanochannels requires sophisticated instruments and is rather expensive and time-consuming, which hinders the practical applications. [27,74] Whereas, 2D nanochannel-structured membranes composed of stacked lamellar materials show several advantages, including facile and scalable fabrication methods (e.g., vacuum filtration), easy chemical modification in bulk solutions, adjustable channel size. [29] Moreover, these membranes can transport ions vertically and horizontally, showing potential in preparing nanofluidic devices on chips. 3D nanochannel-structured membranes have no strict limitations for raw materials and usually can be obtained in large scale via simple fabrication approaches. [82] Thus, they have drawn great attention for the simplicity. Inspired by the junction channels between the adjacent cells, heterogeneous membranes are readily constructed through the simple hybridization of homogeneous membranes with different dimensions, which exhibit novel functions that differ from those of the single materials. [33] However, the bulk 3D nanochannel-structured membranes are usually thick, which can increase ion transport resistance and reduce permeation flux significantly. It is urgently required to design ultrathin membranes covered with straight and short pathways. [36] Therefore, it is still a great challenge to prepare free-standing ultrathin membranes with suitable stability and optimized permeability and selectivity. Compared to the low efficiency of modification on the inner surfaces of the targeted nanochannels, sandwich-like membranes consisting of layers with mediated charges on the exterior surfaces of the middle nanochannels were successfully constructed via simple magnetron sputtering or electrochemical polymerization. [37,329,330] The corresponding ion transport properties can also be finely controlled via the change of the charges on outside layers. In the practical applications, we can choose the suitable membranes with specific structures according to the controllability of ion transport and the feasibility of operation.
There is still a long way to go to construct nanochannelstructured membranes for precisely adjusted ion transport. It is necessary to discover new raw materials to fabricate the desired membranes with specific nanochannels and search for new modifying molecules to endow the nanochannels with specific recognition and/or sensitive stimuli responsiveness. For instance, molecular machines [331][332][333] can be further induced into nanochannel-structured membranes to mediate ion transport properties. It is also urgently required to achieve fine membrane structures (e.g., nanochannel, ultrathin, and sandwich-like structures) to provide efficient ion transport pathways. First, the basic geometric parameters of single-dimensional nanochannels, including the nanochannel size, shape, and density, need to be mediated accurately and repeatedly through physical and/or chemical methods. For example, a single-layer MoS 2 membrane with sub-nanometer pores [224] and BN with angstrom-scale channels [143] were used to investigate the single-ion transport behavior and voltage-controlled ion transport, respectively. Second, mixed-dimensional heterogeneous membranes and sandwich-like membranes with clear and suitable combination interfaces usually allow for more precise adjustment toward ion transport, resulting in higher requirements for the construction of these membranes. Taking advantage of triphase interface-mediated epitaxial growth on superhydrophobic surfaces, [52,334] these kinds of multiple nanochannel-structured membranes can be developed owing to the controlled growth rates in the lateral direction. Third, to address the trade-off between permeability and selectivity, ultrathin membranes with vertically arranged nanochannels (e.g., single-layer MOFs and COFs) are ideal candidates. Through the superspreading of liquids on superwettable interfaces, membranes with controllable thickness and low roughness have been obtained, [49][50][51] which may provide inspiration for developing uniformly ultrathin MOF and COF membranes and the corresponding heterogeneous and sandwichlike membranes. Finally, combining external structures (e.g., geometric structures [335][336][337] and surface macro/micro/nano structures [39,274,291] ) and internal structures (e.g., nanochannel arrangement [40,294,338] ) is another feasible strategy for controlling ion transport. For instance, GO membranes with asymmetric geometric structures such as non-uniform thickness, [335] Kirigami shapes, [336] partial bilayers, [337] anti-T, [39] and vertically aligned [40] MXenes with special surface and arrangement structures showed more controllable ion transport. Recently, Hu et al. designed macroscale BN rods consisting of vertically aligned BN nanosheets through a 3D printing technique, which can be used to investigate the potential role of multiscale structures in mediating ion transport. [274] Interestingly, various micro/nanostructures on biological surfaces can also be considered in the development of nanochannel-structured membranes. [339] Furthermore, natural nanochannels in organisms can be used to transport ions after special treatment. For instance, cellulose nanofibers with abundant aligned nanochannels [44] in cationic wood showed rapid chloride ion transport, and hierarchical porous carbon [178,296] derived from natural wood functioned as high-performance electrode materials because of their efficient ion transport pathways. In addition, natural ionic cables made of grass stems [41] can transport ions to interact with biosystems in different deformed states. Thus, much effort is required to precisely construct membranes with unique nanochannels and sandwich-like structures as well as with ultrathin thickness to mediate ion transport, which will promote applications in various fields such as ion separation, water purification, energy storage and conversion, sensors, and bioelectronics.
Notably, the concept of quantum-confined ion superfluid was proposed for the explanation of ultrafast neural signal transmission in life system, such as the transformation of biological information in smell, vision, and audition. [54] It is suggested that conventional action potential principle cannot explain the phenomena of nerve signal transmission, because large amount of energy was required for ion diffusion. On the contrary, the corresponding process is enthalpy-driven ordered fluid in confined nanochannels without energy loss. For instance, Na + enter into the cell with the open of Na + channels, then K + exit from the cell with the open of K + channels, resulting in rapid ion transport through the membrane to achieve ultrafast neural signal transmission. This process is similar to the fast movement of multiple-single pendulums with close contact. Moreover, two approaches were given to further explore the quantum-confined ion superfluid. [55] First, THz response is a feasible way to detect the ions in the confined nanochannels in life system. Second, artificial counterparts can be adapted to investigate the THz response remotely through the controlled design of membrane structure. Beneficial from the quantum-confined superfluid, mass (e.g., ions and molecules) transport in the confined nanochannels is a quantum way, showing no energy loss and is highly ordered. In this context, quantum-confined superfluid-based catalysis was also put forward to achieve reduced reaction barrier, superior efficiency, and selectivity under non-harsh conditions. [56] With the development of membranes with optimized structure, not only fundamental understanding of ultrafast signal transmission in biological system can be promoted, but also highly efficient information transformation and chemical reactions in artificial counterparts can be achieved.

A C K N O W L E D G M E N T S
This work was supported by the National Natural Science Foundation of China (52003012, 22002005 and 21988102), and the China Postdoctoral Science Foundation (2019M660401).