Dehydration as a Universal Mechanism for Ion Selectivity in Graphene and Other Atomically Thin Pores

Emerging Advances around Nanofluidic Transport and Mass Separation under Confinement in Atomically Thin Nanoporous Graphene

Membrane separation stands as an environmentally friendly, high permeance and selectivity, low energy demand process that deserves scientific investigation and industrialization. To address intensive demand, seeking appropriate membrane materials to surpass trade-off between permeability and selectivity and improve stability is on the schedule. 2D materials offer transformational opportunities and a revolutionary platform for researching membrane separation process. Especially, the atomically thin graphene with controllable porosity and structure, as well as unique properties, is widely considered as a candidate for membrane materials aiming to provide extreme stability, exponentially large selectivity combined with high permeability. Currently, it has shown promising opportunities to develop separation membranes to tackle bottlenecks of traditional membranes, and it has been of great interest for tremendously versatile applications such as separation, energy harvesting, and sensing. In this review, starting from transport mechanisms of separation, the material selection bank is narrowed down to nanoporous graphene. The study presents an enlightening overview of very recent developments in the preparation of atomically thin nanoporous graphene and correlates surface properties of such 2D nanoporous materials to their performance in critical separation applications. Finally, challenges related to modulation and manufacturing as well as potential avenues for performance improvements are also pointed out.

Cs+ conductance in graphene membranes with Ångström-scale pores: the role of pore entrance geometry

Recently, 2D materials, such as graphene, have been successfully implemented as artificial conduits of molecular sizes. The extreme precision with which these structures can be fabricated provides an unprecedented framework for the development of highly specific and efficient devices. In this work, we study the electrophoretic transport of Cs+ ions in a graphene membrane with effective pore heights of 3.4 Å by conducting molecular dynamics simulations. The entrance of the pore is systematically modified to investigate the effect of pore geometry on ionic conductance. Simulation results suggest a significant correlation between ionic conductance and entrance geometry, with a variation of the conductance up to 100% across the studied cases. To explain the observed correlation, two mechanisms involving an intimate relationship between ion dehydration and edge functional groups are proposed. The present study provides theoretical insights that can aid the design of graphene-based membranes with tunable ionic transport properties.

The Interplay of Solvation and Polarization Effects on Ion Pairing in Nanoconfined Electrolytes

The nature of ion-ion interactions in electrolytes confined to nanoscale pores has important implications for energy storage and separation technologies. However, the physical effects dictating the structure of nanoconfined electrolytes remain debated. Here we employ machine-learning-based molecular dynamics simulations to investigate ion-ion interactions with density functional theory level accuracy in a prototypical confined electrolyte, aqueous NaCl within graphene slit pores. We find that the free energy of ion pairing in highly confined electrolytes deviates substantially from that in bulk solutions, observing a decrease in contact ion pairing but an increase in solvent-separated ion pairing. These changes arise from an interplay of ion solvation effects and graphene's electronic structure. Notably, the behavior observed from our first-principles-level simulations is not reproduced even qualitatively with the classical force fields conventionally used to model these systems. The insight provided in this work opens new avenues for predicting and controlling the structure of nanoconfined electrolytes.

Enhancing ion selectivity by tuning solvation abilities of covalent-organic-framework membranes

Understanding the molecular-level mechanisms involved in transmembrane ion selectivity is essential for optimizing membrane separation performance. In this study, we reveal our observations regarding the transmembrane behavior of Li+ and Mg2+ ions as a response to the changing pore solvation abilities of the covalent-organic-framework (COF) membranes. These abilities were manipulated by adjusting the lengths of the oligoether segments attached to the pore channels. Through comparative experiments, we were able to unravel the relationships between pore solvation ability and various ion transport properties, such as partitioning, conduction, and selectivity. We also emphasize the significance of the competition between Li+ and Mg2+ with the solvating segments in modulating selectivity. We found that increasing the length of the oligoether chain facilitated ion transport; however, it was the COF membrane with oligoether chains containing two ethylene oxide units that exhibited the most pronounced discrepancy in transmembrane energy barrier between Li+ and Mg2+, resulting in the highest separation factor among all the evaluated membranes. Remarkably, under electro-driven binary-salt conditions, this specific COF membrane achieved an exceptional Li+/Mg2+ selectivity of up to 1352, making it one of the most effective membranes available for Li+/Mg2+ separation. The insights gained from this study significantly contribute to advancing our understanding of selective ion transport within confined nanospaces and provide valuable design principles for developing highly selective COF membranes.

Tuning Pore Size in Graphene in the Angstrom Regime for Highly Selective Ion-Ion Separation

Zero-dimensional pores spanning only a few angstroms in size in two-dimensional materials such as graphene are some of the most promising systems for designing ion-ion selective membranes. However, the key challenge in the field is that so far a crack-free macroscopic graphene membrane for ion-ion separation has not been realized. Further, methods to tune the pores in the Å-regime to achieve a large ion-ion selectivity from the graphene pore have not been realized. Herein, we report an Å-scale pore size tuning tool for single layer graphene, which incorporates a high density of ion-ion selective pores between 3.5 and 8.5 Å while minimizing the nonselective pores above 10 Å. These pores impose a strong confinement for ions, which results in extremely high selectivity from centimeter-scale porous graphene between monovalent and bivalent ions and near complete blockage of ions with the hydration diameter, DH, greater than 9.0 Å. The ion diffusion study reveals the presence of an energy barrier corresponding to partial dehydration of ions with the barrier increasing with DH. We observe a reversal of K+/Li+ selectivity at elevated temperature and attribute this to the relative size of the dehydrated ions. These results underscore the promise of porous two-dimensional materials for solute-solute separation when Å-scale pores can be incorporated in a precise manner.

Spiers Memorial Lecture: Towards understanding of iontronic systems: electroosmotic flow of monovalent and divalent electrolyte through charged cylindrical nanopores

In many practical applications, ions are the primary charge carrier and must move through either semipermeable membranes or through pores, which mimic ion channels in biological systems. In analogy to electronic devices, the "iontronic" ones use electric fields to induce the charge motion. However, unlike the electrons that move through a conductor, motion of ions is usually associated with simultaneous solvent flow. A study of electroosmotic flow through narrow pores is an outstanding challenge that lies at the interface of non-equilibrium statistical mechanics and fluid dynamics. In this paper, we will review recent works that use dissipative particle dynamics simulations to tackle this difficult problem. We will also present a classical density functional theory (DFT) based on the hypernetted-chain approximation (HNC), which allows us to calculate the velocity of electroosmotic flows inside nanopores containing 1 : 1 or 2 : 1 electrolyte solution. The theoretical results will be compared with simulations. In simulations, the electrostatic interactions are treated using the recently introduced pseudo-1D Ewald summation method. The zeta potentials calculated from the location of the shear plane of a pure solvent are found to agree reasonably well with the Smoluchowski equation. However, the quantitative structure of the fluid velocity profiles deviates significantly from the predictions of the Smoluchowski equation in the case of charged pores with 2 : 1 electrolyte. For low to moderate surface charge densities, the DFT allows us to accurately calculate the electrostatic potential profiles and the zeta potentials inside the nanopores. For pores with 1 : 1 electrolyte, the agreement between theory and simulation is particularly good for large ions, for which steric effects dominate over the ionic electrostatic correlations. The electroosmotic flow is found to depend very strongly on the ionic radii. In the case of pores containing 2 : 1 electrolyte, we observe a reentrant transition in which the electroosmotic flow first reverses and then returns to normal as the surface change density of the pore is increased.

Tailoring Monovalent Ion Sieving in Graphene-Oxide Membranes with High Flux by Rationally Intercalating Crown Ethers

Two-dimensional membranes have shown promising potential for ion-selective separation due to their well-defined interlayer channels. However, the typical "trade-off" effect of throughput and selectivity limits their developments. Herein, we report a precise tailoring of monovalent cation sieving technology with enhanced water throughput via the intercalation of graphene-oxide membranes with selective crown ethers. By tuning the lamellar spacing of graphene oxide, a critical interlayer distance (∼11.04 Å) is revealed to maximize water flux (53.4 mol m-2 h-2 bar-1) without sacrificing ion selectivity. As a result, the elaborately enlarged interlayer distance offers improved water permeance. Meanwhile, various specific cations with remarkably high selectivity can be separated in mixed solutions because of the strong chelation with crown ethers. This work opens up a new avenue for high-throughput and precise regulation of ion separations for various application scenarios.

Ion transport in two-dimensional flexible nanoporous membranes

Ion transport is a fundamental mechanism in living systems that plays a role in cell proliferation, energy conversion, and maintaining homeostasis. This has inspired various nanofluidic applications such as electricity harvesting, molecular sensors, and molecular separation. Two dimensional (2D) nanoporous membranes are particularly promising for these applications due to their ultralow transport barriers. We investigated ion conduction across flexible 2D membranes via extensive molecular dynamics simulations. We found that the microscopic fluctuations of these membranes can significantly increase ion conductance, for example, by 320% in Cu-HAB with 0.5 M KCl. Our analysis of ion dynamics near the flexible membranes revealed that ion hydration is destabilized when the membrane fluctuated within a specific frequency range leading to improved ion conduction. Our results show that the dynamic coupling between the fluctuating membrane and ions can play a crucial role in ion conduction across 2D nanoporous membranes.

Surface-charge governed ionic blockade in angstrom-scale latent-track channels

When channels are scaled down to the size of hydrated ions, Coulomb interactions are enhanced in confinement, resulting in new phenomena. Herein, we found blockade of ionic transport in latent-track angstrom-scale channels governed by surface charge, fundamentally different from Coulomb blockade or Wien effects. The channels are non-conductive at low voltage, blocked by cations bound at the surface in confinement; however, they change to conductive with increasing voltage due to the release of bound ions. The increase in surface charge density gradually causes the conduction to be ohmic. Using Kramers' escape framework, we rationalized an analytical equation to describe the experimental results, uncovering new fundamental insights into ion transport in the smallest channels.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Designing Nanofluidic Diode from a Hybrid-Bilayer Covalent Organic Framework: Molecular Simulation Investigation

Nanofluidic diodes are potentially useful in many important applications such as sensing, electronics, and energy conversion. However, the manufacturing of controllable nanopores for nanofluidic diodes is technically challenging. Herein, a nanofluidic diode is designed from a highly programmatic covalent organic framework (COF). Through molecular simulation, remarkable diode behavior is observed in a hybrid-bilayer COF but not in its constituent single-layer COFs. The rectification effect of ion current in the hybrid-bilayer COF is attributed to an asymmetric electrostatic potential across the COF nanopore. Furthermore, a synergistic effect of counterion is unraveled in the hybrid-bilayer COF, and the presence of counterion is found to reduce the entry barrier and facilitate ion transport. The performance of the hybrid-bilayer COF as a nanofluidic diode is comprehensively investigated by varying salt concentration, layer number, interlayer spacing, and slipping. This proof-of-concept simulation study demonstrates the feasibility of the hybrid-bilayer COF as a nanofluidic diode and the finding may stimulate the development of new nanofluidic platforms.

Field-Induced Hydration Shell Reorganization Enables Electro-osmotic Flow in Nanochannels

Electro-osmotic flow is the motion of fluid driven by an applied electric field, for which an electric double layer near a charged surface is deemed essential. Here, we find that electro-osmotic flow can occur in electrically neutral nanochannels in the absence of definable electric double layers through extensive molecular dynamics simulations. An applied electric field is shown to cause an intrinsic channel selectivity between cations and anions, by reorienting the hydration shells of these confined ions. The ion selectivity then results in a net charge density in the channel that induces the unconventional electro-osmotic flow. The flow direction is amenable to manipulation by the field strength and the channel size, which will inform ongoing efforts to develop highly integrated nanofluidic systems capable of complex flow control.

Unveiling the impact of imidazole derivative with mechanistic insights into neutralize interfacial polymerized membranes for improved solute-solute selectivity

Domestic wastewater (DWW) contains a reservoir of nutrients, such as nitrogen, potassium, and phosphorus; however, emerging micropollutants (EMPs) hinder its applications in resource recovery. In this study, a novel class of nanofiltration (NF) membranes was developed; it enabled the efficient removal of harmful EMP constituents while preserving valuable nutrients in the permeate. Neutral (IM-N) and positively charged (IM-P) imidazole derivative compounds have been used to chemically functionalize pristine polyamide (PA) membranes to synchronously inhibit the hydrolysis of residual acyl chloride and promote their amination. Owing to their distinct properties, these IM modifiers can custom-build the membrane physicochemical properties and structures to benefit the NF process in DWW treatment. The electroneutral NF membrane exhibited ultrahigh solute-solute selectivity by minimizing the Donnan effects on ion penetration (K, N, and P ions rejection < 25%) while imposing remarkable size-sieving obstruction against EMPs (rejection ratio > 91%). Moreover, the hydrophilic IM-modifier synergistically led to enhanced water permeance of 9.2 L m^-2 h^-1 bar^-1, reaching a 2-fold higher magnitude than that of the pristine PA membrane, along with excellent antifouling/antibacterial fouling properties. This study may provide a paradigm shift in membrane technology to convert wastewater streams into valuable water and nutrient resources.

Engineering Polymeric Nanofluidic Membranes for Efficient Ionic Transport: Biomimetic Design, Material Construction, and Advanced Functionalities

Design elements extracted from biological ion channels guide the engineering of artificial nanofluidic membranes for efficient ionic transport and spawn biomimetic devices with great potential in many cutting-edge areas. In this context, polymeric nanofluidic membranes can be especially attractive because of their inherent flexibility and benign processability, which facilitate massive fabrication and facile device integration for large-scale applications. Herein, the state-of-the-art achievements of polymeric nanofluidic membranes are systematically summarized. Theoretical fundamentals underlying both biological and synthetic ion channels are introduced. The advances of engineering polymeric nanofluidic membranes are then detailed from aspects of structural design, material construction, and chemical functionalization, emphasizing their broad chemical and reticular/topological variety as well as considerable property tunability. After that, this Review expands on examples of evolving these polymeric membranes into macroscopic devices and their potentials in addressing compelling issues in energy conversion and storage systems where efficient ion transport is highly desirable. Finally, a brief outlook on possible future developments in this field is provided.

Coupling solute interactions with functionalized graphene membranes: towards facile membrane-level engineering

Optimizing ion transport through nanoporous graphene membranes with intricate engineering at nanoscale levels finds applications ranging from ion segregation to desalination. Such membrane-level engineering often requires futuristic and state-of-the-art micro- and nanofabrication infrastructure making it less accessible to widespread applications. In this study, the effective membrane pore size is modulated using macroscopic membrane functionalization, which, when combined with the solute concentration, can prove to be facile nanoscale engineering towards achieving selectivity. By performing robust molecular dynamics (MD) simulations of aqueous NaCl solution through a nanoporous graphene membrane, we demonstrate that varying membrane wettability influences the structural organization of ions and water molecules both in the vicinity and inside the nanopore, which is manifested in the form of altered permeation characteristics. Moreover, the disparate solvation characteristics of the ionic species in conjunction with the variable van der Waals interactive forces affect the ion-selective nature (Cl^- over Na⁺) of the membrane. The relative hydrophilization, resulting from the effective functionalization of the nanoporous graphene membrane, not only allows greater control over the permeation characteristics of ions and water molecules mediated by an altered depletion ratio but also gives rise to the ion-selective nature of the membrane, thus providing a sound understanding of the transport properties of ion-water solutions through nanoporous materials.

Roles of Anion-Cation Coupling Transport and Dehydration-Induced Ion-Membrane Interaction in Precise Separation of Ions by Nanofiltration Membranes

Nanofiltration (NF) membranes are playing increasingly crucial roles in addressing emerging environmental challenges by precise separation, yet understanding of the selective transport mechanism is still limited. In this work, the underlying mechanisms governing precise selectivity of the polyamide NF membrane were elucidated using a series of monovalent cations with minor hydrated radius difference. The observed selectivity of a single cation was neither correlated with the hydrated radius nor hydration energy, which could not be explained by the widely accepted NF model or ion dehydration theory. Herein, we employed an Arrhenius approach combined with Monte Carlo simulation to unravel that the transmembrane process of the cation would be dominated by its pairing anion, if the anion has a greater transmembrane energy barrier, due to the constraint of anion-cation coupling transport. Molecular dynamics simulations further revealed that the distinct hydration structure was the primary origin of the energy barrier difference of cations. The cation having a larger incompressible structure after partial dehydration through subnanopores would induce a more significant ion-membrane interaction and consequently a higher energy barrier. Moreover, to validate our proposed mechanisms, a membrane grafting modification toward enlarging the energy barrier difference of dominant ions achieved a 3-fold enhancement in ion separation efficiency. Our work provides insights into the precise separation of ionic species by NF membranes.

Enhanced Ion Rejection in Carbon Nanotubes by a Lateral Electric Field

Reverse osmosis membranes hold great promise for dealing with global water scarcity. However, the trade-off between ion selectivity and water permeability is a serious obstacle to desalination. Herein, we introduce an effective strategy to enhance the desalination performance of the membrane. A series of molecular dynamics simulations manifest that an additional lateral electric field significantly promotes ion rejection in carbon nanotubes (CNTs) under the drive of longitudinal pressure. Specifically, with the increase in the electric field, the ion flux shows a deep linear decay, while the water flux decreases only slightly, resulting in a linear increase in ion rejection. The energy barriers of ions around the CNT inlet are obtained by calculating the potentials of mean force to explain enhanced ion rejection. The lateral electric field uniformly raises the energy barriers of ions by pushing them away from the CNT inlet, corresponding to the enhanced ion velocity in the field direction. Furthermore, with the increase in CNT diameter, there is a significant increase in the flux of both ions and water; however, the lateral electric field can also obviously enhance the ion rejection in wider CNTs. Consequently, the enhancement of ion rejection by lateral electric fields should be universal for different CNT diameters, which opens a new avenue for selective permeation and may have broad implications for desalination devices with large pore sizes.

A New Strategy for Highly Efficient Separation between Monovalent Cations by Applying Opposite-Oriented Pressure and Electric Fields

Biological ion channels exhibit excellent ion selectivity, but it has been challenging to design their artificial counterparts, especially for highly efficient separation of similar ions. Here, a new strategy to achieve high selectivity between alkali metal ions with artificial nanostructures is reported. Molecular dynamics (MD) simulations and experiments are combined to study the transportation of monovalent cations through graphene oxide (GO) nanoslits by applying pressure or/and electric fields. It is found that the ionic transport selectivity under the pressure driving reverses compared with that under the electric field driving. Moreover, MD simulations show that different monovalent cations can be separated with unprecedentedly high selectivity by applying opposite-oriented pressure and electric fields. This highly efficient separation originates from two distinctive ionic transporting modes, that is, hydration shells drive ions under pressure, but drag ions under the electric field. Hence, ions with different hydration strengths can be efficiently separated by tuning the net mobility induced by the two types of driving forces when the selected ions are kept moving while the other ones are immobilized. And nanoconfinement is confirmed to enhance the separation efficacy. This discovery paves a new avenue for separating similar ions without elaborately designing biomimetic nanostructures.

The Influence of Nanoconfinement on Electrocatalysis

The use of nanoparticles and nanostructured electrodes are abundant in electrocatalysis. These nanometric systems contain elements of nanoconfinement in different degrees, depending on the geometry, which can have a much greater effect on the activity and selectivity than often considered. In this Review, we firstly identify the systems containing different degrees of nanoconfinement and how they can affect the activity and selectivity of electrocatalytic reactions. Then we follow with a fundamental understanding of how electrochemistry and electrocatalysis are affected by nanoconfinement, which is beginning to be uncovered, thanks to the development of new, atomically precise manufacturing and fabrication techniques as well as advances in theoretical modeling. The aim of this Review is to help us look beyond using nanostructuring as just a way to increase surface area, but also as a way to break the scaling relations imposed on electrocatalysis by thermodynamics.

Applying Transition-State Theory to Explore Transport and Selectivity in Salt-Rejecting Membranes: A Critical Review

Membrane technologies using reverse osmosis (RO) and nanofiltration (NF) have been widely implemented in water purification and desalination processes. Separation between species at the molecular level is achievable in RO and NF membranes due to a complex and poorly understood combination of transport mechanisms that have attracted the attention of researchers within and beyond the membrane community for many years. Minimizing existing knowledge gaps in transport through these membranes can improve the sustainability of current water-treatment processes and expand the use of RO and NF membranes to other applications that require high selectivity between species. Since its establishment in 1949, and with growing popularity in recent years, Eyring's transition-state theory (TST) for transmembrane permeation has been applied in numerous studies to mechanistically explore molecular transport in membranes including RO and NF. In this review, we critically assess TST applied to transmembrane permeation in salt-rejecting membranes, focusing on mechanistic insights into transport under confinement that can be gained from this framework and the key limitations associated with the method. We first demonstrate and discuss the limited ability of the commonly used solution-diffusion model to mechanistically explain transport and selectivity trends observed in RO and NF membranes. Next, we review important milestones in the development of TST, introduce its underlying principles and equations, and establish the connection to transmembrane permeation with a focus on molecular-level enthalpic and entropic barriers that govern water and solute transport under confinement. We then critically review the application of TST to explore transport in RO and NF membranes, analyzing trends in measured enthalpic and entropic barriers and synthesizing new data to highlight important phenomena associated with the temperature-dependent measurement of the activation parameters. We also discuss major limitations of the experimental application of TST and propose specific solutions to minimize the uncertainties surrounding the current approach. We conclude with identifying future research needs to enhance the implementation and maximize the benefit of TST application to transmembrane permeation.

Highly Ion-Permselective Porous Organic Cage Membranes with Hierarchical Channels

Membranes of high ion permselectivity are significant for the separation of ion species at the subnanometer scale. Here, we report porous organic cage (i.e., CC3) membranes with hierarchical channels including discrete internal cavities and cage-aligned external cavities connected by subnanometer-sized windows. The windows of CC3 sieve monovalent ions from divalent ones and the dual nanometer-sized cavities provide pathways for fast ion transport with a flux of 1.0 mol m^-2 h^-1 and a mono-/divalent ion selectivity (e.g., K⁺/Mg²⁺) up to 10³, several orders of magnitude higher than the permselectivities of reported membranes. Molecular dynamics simulations illustrate the ion transport trajectory from the external to internal cavity via the CC3 window, where ions migrate in diverse hydration states following the energy barrier sequence of K⁺ < Na⁺ < Li⁺ ≪ Mg²⁺. This work sheds light on ion transport properties in porous organic cage channels of discrete frameworks and offers guidelines for developing membranes with hierarchical channels for efficient ion separation.

The coming of age of water channels for separation membranes: from biological to biomimetic to synthetic

Water channels are one of the key pillars driving the development of next-generation desalination and water treatment membranes. Over the past two decades, the rise of nanotechnology has brought together an abundance of multifunctional nanochannels that are poised to reinvent separation membranes with performances exceeding those of state-of-the-art polymeric membranes within the water-energy nexus. Today, these water nanochannels can be broadly categorized into biological, biomimetic and synthetic, owing to their different natures, physicochemical properties and methods for membrane nanoarchitectonics. Furthermore, against the backdrop of different separation mechanisms, different types of nanochannel exhibit unique merits and limitations, which determine their usability and suitability for different membrane designs. Herein, this review outlines the progress of a comprehensive amount of nanochannels, which include aquaporins, pillar[5]arenes, I-quartets, different types of nanotubes and their porins, graphene-based materials, metal- and covalent-organic frameworks, porous organic cages, MoS₂, and MXenes, offering a comparative glimpse into where their potential lies. First, we map out the background by looking into the evolution of nanochannels over the years, before discussing their latest developments by focusing on the key physicochemical and intrinsic transport properties of these channels from the chemistry standpoint. Next, we put into perspective the fabrication methods that can nanoarchitecture water channels into high-performance nanochannel-enabled membranes, focusing especially on the distinct differences of each type of nanochannel and how they can be leveraged to unlock the as-promised high water transport potential in current mainstream membrane designs. Lastly, we critically evaluate recent findings to provide a holistic qualitative assessment of the nanochannels with respect to the attributes that are most strongly valued in membrane engineering, before discussing upcoming challenges to share our perspectives with researchers for pathing future directions in this coming of age of water channels.

Cation-Gated Ion Transport at Nanometer Scale for Tunable Power Generation

Gated ion channels in biological cell membranes allow efficient tuning of cross-membrane ion transport with enhanced permeation and selectivity, converting ionic signals into various forms of electrical signals and energies on demands, which functionalities though are still difficult to achieve in artificial membranes. Here, we report cation-gated ion transport through synthesized porous aromatic films containing nanometer-scale ionic channels together with -NH₂ groups at interiors. Ion selectivity and permeability is greatly tuned by gating cations, up to 2 orders of magnitude, and as a consequence, the membrane efficiently produces switchable electricity output from salinity gradients. The results are attributed to positively charged cations binding at -NH₂ groups, which screens the intrinsic negative surface charge at channels' interiors and inverts charge polarity there. Our work adds understanding to ion gating effects at nanoscale and offers strategies of developing smart membranes and their heterostructures for separation, energy conversion, cell membrane mimics, and related technologies.

Promoting Electroosmotic Water Flow through a Carbon Nanotube by Weakening the Competition between Cations and Anions in a Lateral Electric Field

Understanding the electroosmotic flow through a nanochannel is essential to the design of novel nanofluidic devices, ranging from desalination to nanometer water pumps. Nonetheless, the competition between cation and anion in electric fields inevitably leads to a limited pumping of water, and thus weakening their competition could be a new avenue for the fundamental control of water transport. In this work, through a series of molecular dynamics simulations, we find a surprising phenomenon in which under the drive of a traditional longitudinal electric field, an additional lateral electric field can significantly weaken the competitive transport of a cation and anion through a carbon nanotube, which spontaneously leads to a massive increase in electroosmotic water flux. Specifically, with the increase in the lateral electric field, the anion flux exhibits an almost linear reduction, and the cation flux is stable and can even be enhanced. As a result, the net water flux along the cation direction increases significantly. The key to this unexpected phenomenon lies in the size and mobility difference between the cation and anion. The anion is larger and has greater mobility and is thus more susceptible to the lateral electric field, which ultimately leads to the reduction of its flux. For different ion types and CNT lengths, we can observe similar electropumping phenomenon, where the friction force induced by the lateral electric field becomes nontrivial for long CNTs. Our results provide a new route to tune the competitive transport of cations and anions and should be useful for the design of novel electroosmotic pumps.

One-pot synthesis of magnetic Prussian blue for the highly selective removal of thallium(I) from wastewater: Mechanism and implications

Thallium (Tl) often enters the environment via mineral exploitation and utilization. The main restriction of Tl removal is the interference of high concentrations of coexisting ions in wastewater, therefore, enhancing the selectivity for Tl is essential to its treatment. Magnetic Prussian blue particles (Fe₃O₄@PB), an ion-sieving material with an open structure, were synthesized through a "one-pot" method at room temperature for the highly selective removal of Tl⁺. The removal percentage of Tl⁺ was over 92% even when the concentration of coexisting ions (e.g. Zn²⁺, Cd²⁺, Cu²⁺, and Pb²⁺) were 10,000 times higher than the initial concentration of Tl⁺. The maximal experimental removal capacity was 528 mg Tl/g Fe₃O₄@PB, and the removal percentage remained steady at pH 3-10. The high selectivity of Fe₃O₄@PB for Tl⁺ is attributed to the fact that hydrated Tl⁺ has a smaller hydrated diameter and a lower hydration free energy than other coexisting ions, while the rapid adsorption kinetics of Tl⁺ results from the negative surface charge and the network of nanocapillaries of the Fe₃O₄@PB. Overall, a new low-cost material that is easy to synthesize and has superior Tl⁺ removal capacity with extremely high selectivity for Tl⁺ was obtained for effective magnetic removal of thallium from wastewater.

Machine learning reveals key ion selectivity mechanisms in polymeric membranes with subnanometer pores

Designing single-species selective membranes for high-precision separations requires a fundamental understanding of the molecular interactions governing solute transport. Here, we comprehensively assess molecular-level features that influence the separation of 18 different anions by nanoporous cellulose acetate membranes. Our analysis identifies the limitations of bulk solvation characteristics to explain ion transport, highlighted by the poor correlation between hydration energy and the measured permselectivity (R² = 0.37). Entropy-enthalpy compensation, spanning 40 kilojoules per mole, leads to a free-energy barrier (∆G^‡) variation of only ~8 kilojoules per mole across all anions. We apply machine learning to elucidate descriptors for energetic barriers from a set of 126 collected features. Notably, electrostatic features account for 75% of the overall features used to describe ∆G^‡, despite the relatively uncharged state of cellulose acetate. Our work presents an approach for studying ion transport across nanoporous membranes that could enable the design of ion-selective membranes.

Asymmetric transport and desalination in graphene channels

An asymmetric desalination phenomenon occurs in graphene channels with different geometries.

Li+ Selectivity of Carboxylate Graphene Nanopores Inspired by Electric Field and Nanoconfinement

The regulation of the ion selectivity by electric field and ion association on the Li⁺ selectivity of carboxyl functionalized graphene nanopores are investigated by molecular dynamics simulation. Carboxylate graphene nanopores of sub-2 nm exhibit excellent Li⁺ selectivity under the electric field of 1.0 V nm^-1 . The results show that ion association inspired by electric field may be a key factor affecting ion selectivity of sub-2 nm nanopores. The ion association of Mg²⁺ and Cl^- can be promoted obviously near the nanopores under the electric field of 1.0 V nm^-1 . The migrating of Mg²⁺ can be retarded by stable clusters of Mg²⁺ and Cl^- formed near nanopores. The degree of association of Li⁺ with Cl^- is relatively low and the disassociation of the Li⁺ cluster is easier so that Li⁺ can more easily pass through the nanopores. These results gain insight into the effect of ion association inspired by electric field and nanoconfinement of graphene nanopore on Mg²⁺ /Li⁺ separation, and provide helpful information for the application of nanoporous materials in extraction of Li⁺ ion from salt-lake brine.

The Selective Transport of Ions in Charged Nanopore with Combined Multi-Physics Fields

The selective transport of ions in nanopores attracts broad interest due to their potential applications in chemical separation, ion filtration, seawater desalination, and energy conversion. The ion selectivity based on the ion dehydration and steric hindrance is still limited by the very similar diameter between different hydrated ions. The selectivity can only separate specific ion species, lacking a general separation effect. Herein, we report the highly ionic selective transport in charged nanopore through the combination of hydraulic pressure and electric field. Based on the coupled Poisson–Nernst–Planck (PNP) and Navier–Stokes (NS) equations, the calculation results suggest that the coupling of hydraulic pressure and electric field can significantly enhance the ion selectivity compared to the results under the single driven force of hydraulic pressure or electric field. Different from the material-property-based ion selective transport, this method endows the general separation effect between different kinds of ions. Through the appropriate combination of hydraulic pressure and electric field, an extremely high selectivity ratio can be achieved. Further in-depth analysis reveals the influence of nanopore diameter, surface charge density and ionic strength on the selectivity ratio. These findings provide a potential route for high-performance ionic selective transport and separation in nanofluidic systems.

Enthalpic and Entropic Selectivity of Water and Small Ions in Polyamide Membranes

While polyamide reverse osmosis and nanofiltration membranes have been extensively utilized in water purification and desalination processes, the molecular details governing water and solute permeation in these membranes are not fully understood. In this study, we apply transition-state theory for transmembrane permeation to systematically break down the intrinsic permeabilities of water and small ions in loose and tight polyamide nanofiltration membranes into enthalpic and entropic components using an Eyring-type equation. We analyze trends in these components to elucidate molecular phenomena that induce water-salt, monovalent-divalent, and monovalent-monovalent selectivity at different pH values. Our results suggest that in pores that are either too small or contain an electrostatically repelling mouth, the thermal activation of ions in the form of ion dehydration is less likely, promoting entropically driven selectivity with steric exclusion of hydrated ions. Instead, larger uncharged pores enable ion dehydration, inducing enthalpic selectivity that is driven by differences in the ion hydration properties. We also demonstrate that electrostatic interactions between cations and intrapore carboxyl groups hinder salt permeability, increasing the enthalpic barrier of the transport. Last, permeation tests of monovalent cations in the loose and tight polyamide membranes expose opposite rejection trends that further support the phenomenon of ion dehydration in large subnanopores.

Rectification Correlation between Water and Ions through Asymmetric Graphene Channels

Rectification phenomena occurring in asymmetric channels are essential for the design of novel nanofluidic devices such as nanodiodes. Previous studies mostly focus on ion current rectification, while its correlations with water dynamics are rarely explored. In this work, we analyze the transport of water and ions through asymmetric graphene channels under the drive of electric fields using molecular dynamics simulations. A key observation is that the water flux also exists in the rectification phenomenon that follows the ion flux behaviors because of their dynamical coupling relation in electric fields, and both their rectification ratios exhibit maximum behaviors with the change of the channel opening ratio. This is because the ion dehydration is highly asymmetric for small opening ratios. In addition, the cations and anions have distinct rectification ratios that are strongly dependent on the field strength, where the values for anions can even be 1-2 orders larger. This can be attributed to their different hydration shell and dehydration processes in the graphene channel. The translocation time of ions displays a power law relation with the field strength, in agreement with the prediction by Langevin dynamics. Due to the exclude-volume effect, the occupancy of water and ions shows a clear competition and thus changes in an opposite trend with the field strength. Our results demonstrate the rectification correlations between water and ions, and tuning the geometry of graphene channels provides a simple and robust new route to achieve high rectification ratios.

Two-dimensional materials as solid-state nanopores for chemical sensing

Solid-state nanopores as a versatile alternative to biological nanopores have grown tremendously over the last two decades. They exhibit unique characteristics including mechanical robustness, thermal and chemical stability, easy modifications and so on. Moreover, the pore size of a solid-state nanopore could be accurately controlled from sub-nanometers to hundreds of nanometers based on the experimental requirements, presenting better adaptability than biological nanopores. Two-dimensional (2D) materials with single layer thicknesses and highly ordered structures have great potential as solid-state nanopores. In this perspective, we introduced three kinds of substrate-supported 2D material solid-state nanopores, including graphene, MoS2 and MOF nanosheets, which exhibited big advantages compared to traditional solid-state nanopores and other biological counterparts. Besides, we suggested the fabrication and modulation of 2D material solid-state nanopores. We also discussed the applications of 2D materials as solid-state nanopores for ion transportation, DNA sequencing and biomolecule detection.

Two-dimensional monolayer salt nanostructures can spontaneously aggregate rather than dissolve in dilute aqueous solutions

It is well known that NaCl salt crystals can easily dissolve in dilute aqueous solutions at room temperature. Herein, we reported the first computational evidence of a novel salt nucleation behavior at room temperature, i.e., the spontaneous formation of two-dimensional (2D) alkali chloride crystalline/non-crystalline nanostructures in dilute aqueous solution under nanoscale confinement. Microsecond-scale classical molecular dynamics (MD) simulations showed that NaCl or LiCl, initially fully dissolved in confined water, can spontaneously nucleate into 2D monolayer nanostructures with either ordered or disordered morphologies. Notably, the NaCl nanostructures exhibited a 2D crystalline square-unit pattern, whereas the LiCl nanostructures adopted non-crystalline 2D hexagonal ring and/or zigzag chain patterns. These structural patterns appeared to be quite generic, regardless of the water and ion models used in the MD simulations. The generic patterns formed by 2D monolayer NaCl and LiCl nanostructures were also confirmed by ab initio MD simulations. The formation of 2D salt structures in dilute aqueous solution at room temperature is counterintuitive. Free energy calculations indicated that the unexpected spontaneous salt nucleation behavior can be attributed to the nanoscale confinement and strongly compressed hydration shells of ions.

Aqueous solutions under nanoscale confinement exhibit interesting physicochemical properties. This work reports evidence on the spontaneous formation of two-dimensional alkali chloride crystalline/non-crystalline nanostructures in dilute aqueous solution under nanoscale confinement by computer simulations.

In Situ Characterization of Dehydration during Ion Transport in Polymeric Nanochannels

The transport of hydrated ions across nanochannels is central to biological systems and membrane-based applications, yet little is known about their hydrated structure during transport due to the absence of in situ characterization techniques. Herein, we report experimentally resolved ion dehydration during transmembrane transport using modified in situ liquid ToF-SIMS in combination with MD simulations for a mechanistic reasoning. Notably, complete dehydration was not necessary for transport to occur across membranes with sub-nanometer pores. Partial shedding of water molecules from ion solvation shells, observed as a decrease in the average hydration number, allowed the alkali-metal ions studied here (lithium, sodium, and potassium) to permeate membranes with pores smaller than their solvated size. We find that ions generally cannot hold more than two water molecules during this sterically limited transport. In nanopores larger than the size of the solvation shell, we show that ionic mobility governs the ion hydration number distribution. Viscous effects, such as interactions with carboxyl groups inside the membrane, preferentially hinder the transport of the mono- and dihydrates. Our novel technique for studying ion solvation in situ represents a significant technological leap for the nanofluidics field and may enable important advances in ion separation, biosensing, and battery applications.

High-throughput molecular simulations reveal the origin of ion free energy barriers in graphene oxide membranes

Graphene oxide (GO) membranes are highly touted as materials for contemporary separation challenges including desalination, yet understanding of the interplay between their structure and salt rejection is limited. K⁺ ion permeation through hydrated GO membranes was investigated by combining structurally realistic molecular models and high-throughput molecular dynamics simulations. We show that it is essential to consider the complex GO microstructure to quantitatively reproduce experimentally-derived free energy barriers to K⁺ permeation for membranes with various interlayer distances less than 1.3 nm. This finding confirms the non-uniformity of GO nanopores and the necessity of the high-throughput approach for this class of material. The large barriers arise due to significant dehydration of K⁺ inside the membrane, which can have as few as 3 coordinated water molecules, compared to 7 in bulk solution. Thus, even if the membranes have an average pore size larger than the ion's hydrated diameter, the significant presence of pores whose size is smaller than the hydrated diameter creates bottlenecks for the permeation process.

Field-Dependent Dehydration and Optimal Ionic Escape Paths for C2N Membranes

Most analytic theories describing electrostatically driven ion transport through water-filled nanopores assume that the corresponding permeation barriers are bias-independent. While this assumption may hold for sufficiently wide pores under infinitely small bias, transport through subnanometer pores under finite bias is difficult to interpret analytically. Given recent advances in subnanometer pore fabrication and the rapid progress in detailed computer simulations, it is important to identify and understand the specific field-induced phenomena arising during ion transport. Here we consider an atomistic model of electrostatically driven ion permeation through subnanoporous C2N membranes. We analyze probability distributions of ionic escape trajectories and show that the optimal escape path switches between two different configurations depending on the bias magnitude. We identify two distinct mechanisms contributing to field-induced changes in transport-opposing barriers: a weak one arising from field-induced ion dehydration and a strong one due to the field-induced asymmetry of the hydration shells. The simulated current-voltage characteristics are compared with the solution of the 1D Nernst-Planck model. Finally, we show that the deviation of simulated currents from analytic estimates for large fields is consistent with the field-induced barriers and the observed changes in the optimal ion escape path.