Ion Selectivity Research Articles

Layered Bio-Inorganic MXene Membranes: A Green Approach for Uranium Extraction from Seawater Using Genetically Modified E. coli.

With the growing demand for clean energy, efficient uranium extraction technologies are needed, especially from seawater, where uranium reserves are huge. Here, we developed a composite membrane by inserting Escherichia coli engineered with super uranyl-binding protein (SUP) within a two-dimensional (2D) MXene (Ti3C2Tx) layer. SUP endowed the bioinorganic hybrid membrane with ultrahigh selectivity for uranyl ions, while the engineered E. coli improved the mechanical strength and economy of the membranes. Experimental results showed that the membranes achieved precise recognition of uranyl ions and excellent ion screening performance (SFU/V ≈ 43, SFNa/U ≈ 158). Excellent separation performance and cyclic stability tests demonstrated the industrial application potential of the membrane. This method offers a green and sustainable solution, combining biological engineering and nanomaterial innovation, providing an environmentally friendly and efficient approach for uranium extraction from seawater, marking a significant advancement in the field of clean energy resource development.

Nanofiltration Membrane with Enhanced Ion Selectivity Based on a Precision-Engineered Ultrathin Polyethylene Supporting Layer.

Nanofiltration (NF) technology is increasingly used in the water treatment and separation fields. However, most research has focused on refining the selective layer while overlooking the potential role of the supporting layer. With expertise in ultrathin polymer films, particularly in the production of polyethylene (PE) membranes, we explore the possibility of improving NF membrane performance by precisely controlling the structure and surface properties of the ultrathin supporting layer in this work. Here, we introduced an innovative NF membrane that used a submicrometer ultrathin PE membrane produced through a biaxial stretching process, which is significantly thinner than commercial PE membranes available on the market. The core innovations are as follows: first, we focused on precise control of the supporting layer rather than just the selective layer, achieving significant enhancements in overall NF membrane performance; second, the ultrathin PE supporting layer served as a tunable interface for interfacial polymerization, offering possibilities for structural control of the selective layer and advancing membrane performance innovations. The resulting NF membrane boasts an overall thickness of ∼630 nm, which represents the thinnest NF membrane documented to date. This ultrathin NF membrane showed an ultrahigh Cl-/SO42- selectivity of 338.03, placing it at the forefront of existing literature. This study sheds light on the important role of the supporting layer in the preparation of selective layers. We believe that this approach has the potential to contribute to the development of ultrathin, high-performance NF membranes.

Enhancement of Synaptic Behavior in Organic Electrochemical Transistors via the Introduction of Layer‐by‐Layer Grown Metal‐Organic Framework

AbstractOrganic electrochemical transistors (OECTs) are promising for neuromorphic architectures as they can generate multiple electrical states through the control of ion transport. However, conventional OECTs face limitations in mimicking a fully functional biological synapse due to their inability to achieve long‐term plasticity. In this study, a metal‐organic framework (MOF)‐enhanced OECT (MOECT) is fabricated by introducing MOF into the ion‐organic semiconductor (OSC) layer. MOFs are synthesized using the layer‐by‐layer (LBL) method, and additional cross‐linked OSC is introduced to prevent damage to the semiconductor layers during synthesis. The synthesized MOF layers hinder the rediffusion of ions in OECT, allowing ions to remain in the OSC for an extended period. In this study, the MOECT showed a change in current depending on the doping level, recording a current state 4.4 × 107 times higher than that of pristine OECT. Ultimately, the developed MOECTs are applied as synaptic transistors. MOECTs show 14% higher excitatory postsynaptic current (EPSC) after 130 s compared to pristine OECT, thereby strengthening the long‐term plasticity characteristics of neuromorphic devices. This method enhances the performance of synaptic transistors by introducing MOF, offering various possibilities through the selection of different MOF structures and ions, indicating it is a methodological approach with high potential.

Synthesis and Evaluation of the Extraction Efficiency of Pristine Zeolite Na-A to Remove ReO4- Ions (Surrogate of 99TcO4-) from a Simulated Low-Level Waste Solution.

Zeolite Na-A was synthesized through hydrothermal, alkali-fusion, and sonochemical methods, using kaolin as an economically viable precursor. The synthesized zeolite Na-A samples were characterized using XRD, FT-IR, SEM, and Raman spectroscopy. Specific surface area and pore size distribution analyses (by BJH and DFT models) were conducted using a BET surface area analyzer. Additionally, thermal degradation studies were performed by TG-DTA to check the thermal stability of zeolite Na-A at high temperatures. Furthermore, kaolin-derived zeolite Na-A was employed for the extraction of perrhenate ions (ReO4-), which are a nonradioactive surrogate for pertechnetate ions (99TcO4-) without any postsynthetic modifications (using toxic surfactants, etc.) utilizing in situ modification of the solution medium for the first time. The sonochemically synthesized zeolite Na-A demonstrated superior sorption performance for the solid-phase extraction of ReO4- ions from the simulated low-level waste solution. The adsorption process was found to follow pseudo-second-order kinetics. The Langmuir isotherm model fit perfectly with the experimental data (R2 = 0.997) and exhibited a maximum sorption capacity of 926.8 mg/g at pH ∼11, showing superior sorption capacities compared to those of the numerous materials reported earlier. XPS confirmed the speciation of extracted rhenium as NH4Re(VII)O4, providing critical insights into the adsorption mechanisms and validating the suitability of the sonochemically synthesized zeolites toward ReO4- sorption. Furthermore, Raman studies of ReO4- adsorbed zeolite Na-A reflect the absence of characteristic breathing bands, indicating the closure of the pore openings due to the occupancy of adsorbate moieties within the pores. This study not only highlights the utilization of sonochemically synthesized zeolite Na-A as an efficient, benign, and cost-effective adsorbent for 99TcO4- removal from nuclear waste but also emphasizes its potential sustainable applications in various other industrial processes such as wastewater treatment, catalysis, gas separation, pollution control, and resource recovery from industrial effluents and in the pharmaceutical industry for selective ion removal.

PH-Independent Selective Formation of VO2+ Motif Incorporating a Family of Hydrazone Ligands: Synthesis, Structure, and Luminescence-Based Sensing Studies toward Selective Metal Ions.

Two new dioxidovanadium(V) compounds with the general formula [VVO2(L)] (L = hydrazone ligand) were synthesized using three different methods (acidic, neutral, and basic media) with either VIVOSO4. 5H2O, [VIVO(acac)2], or NH4VVO3 as the starting material and hydrazone ligands. In both cases, five coordinated V5+ ions adopted distorted square pyramidal geometry through the coordination of two oxido ligands and one hydrazone ligand. In compound 1, the presence of free hydroxyl groups stabilized the molecular species through intramolecular O-H•••N type hydrogen bond interactions. In compound 2, the free amino groups are involved in both intramolecular N-H•••N type and intermolecular N-H•••O type hydrogen bond interactions. Compound 1 showed highly selective luminescence turn-on behavior, along with a 33-nm blue shift in the presence of Al3+ ions in an aqueous medium. The experimental limit of detection (LOD) was found to be 66 nM. Whereas compound 2 showed luminescence quenching behavior in the presence of Fe3+, Al3+, and Cr3+ ions. The LODs were observed to be 312, 408, and 280 nM for Fe3+, Al3+, and Cr3+ ions, respectively. The luminescence response mechanisms of both compounds in the presence of metal ions have been correlated with molecular-level interactions.

A High-Performance Polyimide Composite Membrane with Flexible Polyphosphazene Derivatives for Vanadium Redox Flow Battery.

A sulfonated polyimide, S-F-abSPI, with alkyl sulfonic acid side chains, and a polyphosphonitrile derivative, poly[4-methoxyphenoxy (4-fluorophenoxy) phosphazene] (PFMPP), were designed and synthesized. Composite modification of the S-F-abSPI membrane was carried out using PFMPP, resulting in the preparation of composite membranes with different composite ratios, which were then subjected to performance testing and characterization. Experimental results revealed a significant enhancement in the proton conductivity of the S-F-abSPI membrane, reaching 0.116 S cm-1, slightly higher than that of the N212 membrane. The S-F-abSPI/1% PFMPP composite membrane exhibited the optimal comprehensive performance, with a surface resistance as low as 0.54 Ω cm2, comparable to that of the N212 membrane. At a high current density of 200 mA cm-2 during charge-discharge, the composite membrane achieved a voltage efficiency (VE) of 83.12% and an energy efficiency (EE) of 81.95%. Cycling tests over 200 cycles demonstrated the composite membrane's excellent long-term cycling stability. The alkyl sulfonic acid side chains enhanced the proton conductivity of the membrane, while electrostatic potential distribution calculations indicated strong interactions between PFMPP and the base membrane, enhancing the membrane's mechanical strength, reducing vanadium ion permeability, and improving chemical stability and vanadium ion selectivity. This composite membrane holds promise for high-performance VRFB applications.

Polyamide-Crystalline Covalent Organic Framework Dual-Layer Nanofiltration Membrane with Improved Ion Selectivity

Polyamide-Crystalline Covalent Organic Framework Dual-Layer Nanofiltration Membrane with Improved Ion Selectivity

Sodium-Selective Channelrhodopsins

Channelrhodopsins (ChRs) are light-gated ion channels originally discovered in algae and are commonly used in neuroscience for controlling the electrical activity of neurons with high precision. Initially-discovered ChRs were non-selective cation channels, allowing the flow of multiple ions, such as Na+, K+, H+, and Ca2+, leading to membrane depolarization and triggering action potentials in neurons. As the field of optogenetics has evolved, ChRs with more specific ion selectivity were discovered or engineered, offering more precise optogenetic manipulation. This review highlights the natural occurrence and engineered variants of sodium-selective channelrhodopsins (NaChRs), emphasizing their importance in optogenetic applications. These tools offer enhanced specificity in Na+ ion conduction, reducing unwanted effects from other ions, and generating strong depolarizing currents. Some of the NaChRs showed nearly no desensitization upon light illumination. These characteristics make them particularly useful for experiments requiring robust depolarization or direct Na+ ion manipulation. The review further discusses the molecular structure of these channels, recent advances in their development, and potential applications, including a proposed drug delivery system using NaChR-expressing red blood cells that could be triggered to release therapeutic agents upon light activation. This review concludes with a forward-looking perspective on expanding the use of NaChRs in both basic research and clinical settings.

Real-Time Wireless Detection of Heavy Metal Ions Using a Self-Powered Triboelectric Nanosensor Integrated with an Autonomous Thermoelectric Generator-Powered Robotic System.

The integration of the Internet of Things (IoT) with advanced sensing technologies is transforming environmental monitoring and public health protection. In this study, a fully self-powered and automated chemical sensing system is developed and integrated with a robotic hand for "touch and sense" detection of toxic heavy metal ions (Pb2⁺, Cr⁶⁺, As3⁺) in aquatic environments. The system combines a self-powered solid-liquid triboelectric nanosensor (SL-TENS) with a thermoelectric generator (TEG), which harnesses ambient heat to power the robotic hand, eliminating the need for external power sources. The robotic hand is controlled wirelessly via an exo-hand, minimizing the risk of exposure during remote monitoring. The sensing component uses copper oxide nanowires (CuO NWs) coated with ion-selective membranes (ISMs) to enhance triboelectric output and enable highly selective ion detection. The system demonstrates effective real-time, on-site detection in lake water and data transmitted wirelessly to the user. This innovative approach provides a highly safe and efficient method for detecting hazardous pollutants in difficult-to-access areas, offering significant potential for wireless and real-time environmental monitoring and hazard prevention, thus contributing to the safeguarding of human health. This study presents a novel advancement in the field of IoT-enabled environmental monitoring systems.

Numerical Simulation of Autoresonant Ion Oscillations in an Anharmonic Electrostatic Trap.

This work presents the results of modeling the ion dynamics in the ART-MS (Autoresonant Trap Mass Spectrometry) device in the quasi-static approximation. This instrument utilizes an anharmonic, purely electrostatic trap for ion confinement and a radio frequency (RF) voltage source with decrementally varying frequency for selective ion extraction. The autoresonant interaction between the oscillatory motion of the ion and the RF voltage increases the amplitude of some confined ions, allowing their selective extraction. Numerical modeling shows that the extraction of ions with a given mass occurs not only at the fundamental frequency but also at its harmonics. This effect reduces the selective properties of devices of this type because along with the main mass component for a given frequency, it is possible to enter the detector channel of ions with another mass, for which this frequency corresponds to the second or higher harmonics, even a superposition of some of these harmonics of different ions.

Selective ion transport through hydrated micropores in polymer membranes.

Ion-conducting polymer membranes are essential in manyseparation processes and electrochemical devices, including electrodialysis1, redox flow batteries2, fuel cells3 and electrolysers4,5. Controlling ion transport and selectivity in these membranes largely hinges on the manipulation of pore size. Although membrane pore structures can be designed in the dry state6, they are redefined upon hydration owing to swelling in electrolyte solutions. Strategies to control pore hydration and a deeper understanding of pore structure evolution are vital for accurate pore size tuning. Here we report polymer membranes containing pendant groups of varying hydrophobicity, strategically positioned near charged groups to regulate theirhydration capacity and pore swelling. Modulation of thehydrated micropore size (less thantwo nanometres) enables direct control over water and ion transport across broad length scales, as quantified by spectroscopic and computational methods. Ion selectivity improves in hydration-restrained pores created by more hydrophobic pendant groups. These highly interconnected ion transport channels, with tuned pore gate sizes, show higher ionic conductivity and orders-of-magnitude lower permeation rates of redox-active species compared with conventional membranes, enabling stable cycling of energy-dense aqueous organic redox flow batteries. This pore size tailoringapproach provides a promising avenue to membranes with precisely controlled ionic and molecular transportfunctions.

Highly Resistive Biomembranes Coupled to Organic Transistors enable Ion‐Channel Mediated Neuromorphic Synapses

AbstractOrganic electrochemical transistors (OECTs) functionalized with lipid membranes could enable new hybrid synapses for sensing and neuromorphic computing in biological media. However, prior attempts to pair these components resulted in low quality membranes formed on the OECT surface. We present a new method for forming a highly‐resistive phospholipid bilayer coupled to a poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) OECT that avoids the disruptive effects of the rough polymer surface. Transistor coupled droplet bilayers (TCDBs) formed from diphytanoyl phosphatidylcholine lipids exhibit an average specific resistance that is 1000X higher than values reported for solid‐supported lipid membranes assembled on PEDOT:PSS. High membrane resistance and the addition of voltage‐activated ionophores enable us to demonstrate that selective ion transport and spontaneous membrane resealing in response to dynamic gate voltage imparts selective programming and memory‐storage capability to an OECT without chemical modification of the PEDOT:PSS. These capabilities enable paired‐pulse facilitation and depression with writing speeds and memory retentions that are both >10X higher than previously reported with membrane‐coated OECTs. The high resistance of the TCDB establishes a basis for hybrid biomolecular synapses that can integrate stimuli‐responsive membranes and organic electronics for future applications in sensing, signal processing, and neuromorphic computing at the edge of biology.

Structural studies of ion channels: achievements, problems and perspectives

The superfamily of membrane proteins known as P-loop channels encompasses potassium, sodium, and calcium channels, as well as TRP channels and ionotropic glutamate receptors. An increasing number of crystal and cryo-EM structures are uncovering both general and specific features of these channels. Fundamental folding principles, the arrangement of structural segments, key residues that influence ionic selectivity, gating, and binding sites for toxins and medically relevant ligands have now been firmly established. The advent of AlphaFold2 (AF2) models represents another significant step in computationally predicting protein structures. Comparison of experimental P-loop channel structures with their corresponding AF2 models shows consistent folding patterns in experimentally resolved regions. Despite this remarkable progress, many crucial structural details, particularly important for predicting the outcomes of mutations and designing new medically relevant ligands, remain unresolved. Certain methodological challenges currently hinder the direct assessment of such details. Until the next methodological breakthrough occurs, a promising approach to analyzing ion channel structures in greater depth involves integrating various experimental and theoretical methods.

Guanidinium‐Based Covalent Organic Framework Membranes for Superior Mono‐ and Divalent Cations Separation

Biological membranes exhibit extraordinary efficiency in processing ion permeability and selectivity. However, creating artificial membranes for an ideal ion separation is still challenging due to the subtle distinctions in valence and size among different ions. In the realm of biological recognition, the ultimate selectivity of ion channels is considered to stem from the specific binding interactions and appropriate charge density. Designing artificial membranes to achieve similar performance not only helps the understanding of complex ion transport in bioprocesses but also facilitates critical industrial separations. Inspiring by the remarkable performance in biological systems, a guanidinium‐based covalent organic framework membrane is designed, which exhibits an excellent capability to recognize mono‐/divalent cations, achieving K+/Mg2+ selectivity up to 202 in a mixed salt solution. Furthermore, the membrane displays rapid ion transport owing to the uniform sub‐2 nm channels. The experimental results and molecular dynamics simulations illustrate that the charge‐assisted hydrogen bonding sites and the Cl− counter ions within 1D channels play critical roles in cations sieving. Specifically, divalent ions passing through the positively charged channels need to overcome higher energy barriers than monovalent ions. These findings offer promising avenues for the development of advanced multifunctional membranes for efficient ion separation and sustainable water‐related separations.

Segmental-porosity regulation of nanochannel membranes with anomalously improved ion selectivity for enhanced osmotic power generation

Nanochannel-based osmotic energy conversion can directly convert salinity gradient energy between solutions with different ion concentrations into electric energy. In the energy conversion process, the high-concentration side of the nanochannel membrane has high ion permeability but low ion selectivity, whereas the low-concentration side exhibits opposite properties, which is the inherent limitation of osmotic power. However, the relationship between ion selectivity and permeability is unclear, which hinders the improvement of the output power density. This paper reports a segmental-porosity regulation method to break through the inherent constraints of the ion selectivity–permeability trade-off. The nanochannel membrane was divided into three regions along the ion transport direction as upstream, midstream, and downstream. The porosities of these three regions were separately regulated to optimize the structures for ion selectivity–permeability matching. When a high porosity was adopted for the downstream region and a small and mediate porosity combination was used for the upstream and midstream regions, the highest output power density was achieved, which was 78.44% higher than that of a straight nanochannel with homogeneous porosity under a 100-fold concentration ratio. The outstanding osmotic power generation performance of the optimized segmental structure was attributed to the alleviated ion concentration polarization and unexpectedly improved ion selectivity. Finally, the process and underlying mechanisms of segmental-porosity regulation are also proposed. This paper reports a novel methodology for designing the structure and porosity of nanochannel membranes to enhance osmotic power generation.

Lysine-regulated turing structure membrane via interfacial polymerization for enhanced Li+/Mg2+ separation

The development of advanced membranes with precise ion selectivity is essential for efficient Li+/Mg2+ separation. In this study, we synthesized a novel Lys-TMC membrane via a nucleophilic addition reaction between Lysine and trimesoyl chloride (TMC), yielding in a negatively charged surface interconnected by amide bonds. We systematically investigated the effects of varying Lys:TMC ratios on the membrane’s chemical structure, surface morphology, and ion separation performance. Our results revealed that the degree of cross-linking increases initially with the Lysine content, reaching an optimum at a Lys:TMC ratio of 10:1, where the membrane exhibited prominent Turing patterns on its surface. These patterns were critical for achieving effective ion separation. In contrast, excessive Lysine led to spontaneous polymerization, resulting in irregular ‘stone-like’ structures and the loss of Turing patterns. The optimally cross-linked Lys-TMC membrane displayed superior Li+/Mg2+ separation performance, highlighting the crucial role of controlled cross-linking and Turing structure formation in enhancing ion selectivity. This work provides a mechanistic understanding of the relationship between Turing structure formation and ion sieving performance, offering a promising strategy for designing advanced separation membranes.

Synthesis and enhanced adsorption for F− by three novel high entropy LDHs using TEA-assisted hydrothermal technology

Nowadays the design, preparation and application of high entropy LDHs (HE-LDHs) has already become research hotspots. In this paper, three types of high entropy ZnCoCrMgZr-LDHs (HE-LDHs-1, 2 and 3) were successfully prepared for the first time using TEA as alkali source by simple hydrothermal synthesis, and characterized by XRD, TG, SEM, X-ray photoelectron spectroscopy, BET and zeta potential. The effect of triethanolamine (TEA) on the formation of high entropy LDHs and their adsorption behaviour for F− were investigated. Research showed that high content of TEA can maintain and provide more hydroxyl groups required for the formation of LDHs, while the introduction of high valence Zr4+ breaks the regular arrangement of LDHs themselves. Three types of high entropy LDHs have excellent adsorption performance for F− with the maximum adsorption capacities of 113.78 mg/g, 142.12 mg/g and 128.24 mg/g, respectively. They also have strong resistance to anionic interference. The adsorption process is the transition from physical adsorption to chemical adsorption in the F− adsorption process from HE-LDHs-1 to 3. The adsorption isotherm models of LDHs-1 exhibit high R2 values which shown that their F− adsorption behaviour is the result of the combined effect of physical and chemical adsorption. The adsorption behaviour of LDHs-2 and 3 is more in line with the quasi second-order kinetic model and Langmuir model, indicating that the F− adsorption process is a single-layer adsorption based on chemical reaction-controlled kinetics. The lower temperatures are more conducive to the process for all three adsorbents adsorption process generates heat. FLDHs can still re-adsorb the dye CR after four cycles of adsorbing fluoride ions. HE-LDHs exhibit high adsorption, selectivity, and good reuse performance for fluoride ions, making them promising for wastewater treatment applications.

A rapid and sensitive paper-based sensor for sulfide ion detection in Chinese liquors

The accurate monitoring of sulfide ion concentrations is vital for ensuring food safety, but current detection methods are often cumbersome and confined to laboratories. This study introduces an innovative paper-based colorimetric sensor that leverages the peroxidase-like activity of cobalt tetrasulfonate phthalocyanine (CoTsPc) for rapid, sensitive, and selective sulfide ion quantification. The underlying mechanism relies on the interaction between cobalt and sulfide ions, modulating the catalytic efficiency of CoTsPc and triggering a visible color change when 3,3′,5,5′-tetramethylbenzidine (TMB) and hydrogen peroxide are present. By controlling solvent evaporation, the reaction kinetics are standardized, enabling reproducible and accurate colorimetric measurements. Experimental results show that adding sulfide ions to the TMB-H₂O₂ reaction system causes a gradual decrease in color intensity on the paper substrate. This change verifies effectiveness of the sensor, highlighting its potential for reliable sulfide ion detection in food safety applications, while providing a simple and portable alternative to conventional methods.

Machine learning framework for selective and sensitive metal ion sensing with nitrogen-doped graphene quantum dots heterostructure

This study introduces a machine learning (ML) framework to optimize photodetector performance for sensor applications. Using the data from the fabricated photodetector with the heterostructure of nitrogen-doped graphene quantum dot and gold nanoparticles (Au@N-GQDs), various supervised ML models (more than 20 models) are trained and tested for the selection and refinement of the most effective algorithm for our work. Depending on the best-performed ML model, the optimized working wavelength of the photodetector is found for the detection of metal ions. Remarkably, the ML-based sensor shows a high level of selectivity and sensitivity in nM level towards Fe3+ ions in Brahmaputra river water. A strong alignment between model predictions and experimental outcomes validates the efficacy of the proposed ML-based framework. Moreover, data visualization techniques such as heatmaps, classification algorithms, and confusion matrices are introduced to identify the trends in the database. The mechanistic insight of the sensor performance towards Fe3+ ion sensing is further explained with heatmap analysis and experimental verification, which emphasizes the role of photo-induced charge transfer and Fe–O bond formation between metal ions and Au@N-GQDs due to the high electron affinity of Fe3+ ions.

Coordination, hydration, and diffusion of vanadyl cations in negatively charged polymer membranes

Charged polymer membranes that selectively transport ions are crucial for advancing electrochemical technologies for water purification, resource recovery, and energy generation/storage. Recent studies suggest that the ion selectivity trends of such membranes are due to ion dehydration at the membrane/solution interface, where ions that require less energy to shed their hydration can partition more favorably into the membrane. However, direct evidence of ion dehydration in polymer membranes at relevant conditions is scarce, with claims based primarily on correlations between ion hydration energies and membrane transport properties. The objective of this study was to investigate the electronic environment, local coordination, and hydration of vanadyl counter-ion in negatively charged membranes with broadly varying water content via x-ray absorption spectroscopy. The results highlight that vanadyl counter-ions maintain similar oxidation state, electronic state, and coordination number in the membranes as in aqueous solutions of vanadyl sulfate and that vanadyl dehydration is unlikely in these membranes. However, as the membrane water content decreases, the vanadyl diffusion coefficient decreases and the activation energy of diffusion increases. We attribute these significant changes in membrane transport properties to increased Coulombic interactions between vanadyl and the fixed charge groups, resulting from a decreased dielectric constant of the membrane, rather than to ion dehydration at the membrane/solution interface. This study represents significant progress in understanding the mechanisms that govern ion transport in charged polymer membranes over a broad range of water content, highlighting the critical role of Coulombic interactions between the fixed charges and mobile ions.