Rectification of Concentration Polarization in Mesopores Leads To High Conductance Ionic Diodes and High Performance Osmotic Power

MOF composites for revolutionizing blue energy harvesting and next-gen soft electronics

Metal-organic frameworks (MOFs) are porous materials with highly ordered and crystalline structures, which have earned tremendous attention in the academic community in recent years owing to their high tunability in porosity and pore structure. By integrating MOFs with soft colloids or polymers to form MOF composites, the rigidity and brittle nature of MOFs can be compensated for, thus achieving synergistic effects for a wide variety of applications. In particular, the past decade has seen the advancement of MOF composites in the budding fields of blue energy harvesting and soft electronics, which have received growing interest in the past 5 years. This review focuses on the applications of MOF composites in these two fields, and starts by examining the nanoarchitectures of MOFs, followed by the fabrication of MOF composites. Furthermore, topical advances of MOF composites in blue energy harvesting and soft electronics are reviewed and summarized, and their challenges and future opportunities are discussed as the final touch. This article provides comprehensive review and valuable insights into the development of MOF composites, which may open up new avenues for blue energy harvesting and soft electronics to solve the imminent energy crisis and to advance the wearable technology in healthcare.

An innovative electrophoresis-coupled electrochemiluminescence immunosensor for rapid and sensitive detection of carcinoembryonic antigen

Electrochemiluminescence (ECL) immunosensor provides unique advantages for the sensitive biomarker detection. However, the lengthy detection duration and plentiful system interferences have hindered their development. Here, we present an innovative approach in which electrophoresis coupled with ECL immunoassay was implemented to construct an electric field-enhanced ECL immunosensor for efficient detection of carcinoembryonic antigen (CEA). The electrophoresis device can be regarded as an electric field-driven incubation system, with a working electrode as the anode and a platinum disk as the cathode. Upon applying direct voltage, CEA was swiftly transported to the electrode surface via an upward electric field force, drastically cutting the CEA incubation time from 60 min to just 5 min-a 12-fold reduction compared to traditional methods. Our method also achieved a broad linear detection range from 10-2 to 104 pg/mL, with a lower detection limit of 2.33 fg/mL. Additionally, we utilized the COMSOL Multiphysics-based numerical model and substantial experiment results, demonstrating that the incorporation of an electrophoresis system has allowed for rapid detection with high sensitivity, thereby boosting the overall efficiency of the ECL immunosensor. This study underscores the potential of the electric field-enhanced ECL immunosensor for broad application in the biodetection field.

Recent advances in the design principles of lithium selective membranes

The growing demand for lithium in energy storage applications has intensified the need for efficient lithium extraction technologies, with membrane processes emerging as a promising approach. Among various membrane technologies, nanostructured membranes with precisely engineered channels have shown exceptional potential for selective lithium extraction due to their ability to control ion transport at the molecular level. This review provides a comprehensive analysis of the fundamental design principles governing lithium-selective membranes, with a specific focus on nanochannel-based systems. We examine the critical parameters that influence lithium selectivity, including surface charge distribution, nanochannel dimensions, morphology, and wettability, while exploring how these factors interact with external driving forces to enable selective ion transport. This work extensively analyzes recent developments in nanochannel engineering and ion transport mechanisms, providing crucial insights into optimizing membrane selectivity and performance. By critically analyzing current challenges in scaling up these technologies and identifying promising research directions, this work provides a roadmap for developing next-generation lithium-selective membranes with enhanced efficiency and selectivity.

On the Physical Origins of Reduced Ionic Conductivity in Nanoconfined Electrolytes

Ion transport through nanoscale pores is at the heart of numerous energy storage and separation technologies. Despite significant efforts to uncover the complex interplay of ion-ion, ion-water, and ion-pore interactions that give rise to these transport processes, the atomistic mechanisms of ion motion in confined electrolytes remain poorly understood. In this work, we use machine learning-based molecular dynamics simulations to characterize ion transport with first-principles-level accuracy in aqueous NaCl confined to graphene slit pores. We find that ionic conductivity decreases as the degree of confinement increases, a trend governed by changes in both ion self-diffusion and dynamic ion-ion correlations. We show that the self-diffusion coefficients of our confined ions are strongly influenced by the overall electrolyte density, which changes nonmonotonically with slit height based on the layering of water molecules within the pore. We further observe a shift in the ions' diffusion mechanism toward more vehicular motion as the degree of confinement increases. Despite the ubiquity of ideal solution (Nernst-Einstein) assumptions in the field, we find that nonideal contributions to transport become more pronounced under confinement. This increase in nonideal ion correlations arises not simply from an increase in the fraction of associated ions, as is commonly assumed, but from an increase in ion pair lifetimes. By building a mechanistic understanding of confined electrolyte transport, this work provides insights that could guide the design of nanoporous materials optimized for efficient and selective ion transport.

Anodized Aluminum Oxide Membrane Ionic Memristors

Memory effect in ion transport (IT) at the solid-solution interface is uniquely attractive in that the conductance depends on or "memorizes" the previous states. Hysteretic and rectified transport properties offer exciting potential to developing advanced iontronics and neuromorphic functions, improving the efficiency of energy conversion and electrochemical processes, and overcoming the selectivity-throughput bottleneck in the enrichment of low abundant species for environment- and energy-friendly separations, among others. Herein, memory effects are discovered in the rectified electrokinetic IT through anodized aluminum oxide (AAO) membranes containing densely packed highly ordered nanochannels (1010 per cm2). Characteristic memristor responses of pinched current-potential loops are resolved in voltammetric experiments and successfully reproduced through finite element simulation. Excitatory and inhibitory conductance states are shown to arise from the enrichment and depletion of mobile charge carriers. Structurewise, the transport symmetry is broken by the barrier oxide layer (BOL) on the one end of the cylindrical nanochannels across the AAO membranes. Charge selectivity is attributed to the gradient(s) of the space charge density across the BOL characterized by depth profiling via X-ray photoelectron spectroscopy analysis. The space charge gradient(s) overcomes the fundamental limitation of widely exploited surface charge effects to enable intense rectification and hysteresis prevailing at very high ionic concentrations up to 1-2 M. A new strategy is developed for controlling the preferential IT direction and selectivity via counterion intercalation and extraction/exchange. Mechanistic understanding is further confirmed through parameter variations such as potential scan rate and ionic strength, which also demonstrates convenient controls of the related functions.

Confining Spirocyclic Fluorescein in an Asymmetric Solid-State Nanochannel: A Simple and Versatile Design Concept for Fabricating Integrated Nanofluidic Diodes with Adjustable Surface Chemistry

Using small molecules to integrate multifunctional surfaces within a nanopore is an effective way to endow smart responsibilities of nanofluidic diodes. However, the complex synthesis of the small molecules hinders their further application in achieving multifunctional surfaces. Here, a simple and versatile design concept is reported for fabricating bioinspired integrated nanofluidic diodes with adjustable surface chemistry by confining a spirocyclic fluorescein derivative, 6-aminofluorescein (6-AF), within an asymmetric track-etched nanopore. The pH-dependent open-close of lactone ring in 6-AF allows facile fabrication of a pH-gated nanofluidic diode, confirmed with finite element simulations. This pH-gated nanofluidic diode also shows high specificity for sensing 3-nitropropionic acid (3-NPA), indicating its potential applications in food safety. Moreover, three functional nanofluidic diodes are successfully constructed via a regioselective Vilsmeier reaction between 6-AF and N-methylformanilide, the electrophilic addition reaction between 6-AF and propargyl bromide, and a highly controllable reduction process between 6-AF and NaBH4/I2. The combination of asymmetric nanopores with small molecules not only expands traditional fluorescent spirocyclic molecules to the realm of electrochemistry but also offers valuable insights for the achievement of novel fluorescence-electrochemical coupling detection methods. Besides, the introduction of spirocyclic small molecules to asymmetric nanopores serves as an inspiration source to explore new design concepts for nanofluidic devices.

Engineered Ionic Rectifier with Steep Channel Gradient from Angstrom-Scale to Mesoscale Based on Ultrathin MXene-Capped Single Conical Mesochannel: A Promising Platform for Efficient Osmotic Energy Generation

Ionic rectifier that mimics the directional ion transport in biological ion channels has been shown with potential toward boosting osmotic energy conversion performance. However, the achieved power by existing rectifying devices is still limited, because they are constructed based on tiny nanoscale channels, which experience high resistance. Here, a novel high-performance ionic rectifier (abbreviated as MXene@MC) with steep channel gradient from angstrom-scale to mesoscale is reported by capping an ultrathin 2D Ti3C2Tx MXene laminate on an asymmetric conical mesochannel (MC). The device can strongly rectify ionic current (with a high ratio of 7.3-fold) even in high 0.5 m electrolyte solution, and thus a single channel can achieve an ultra-large osmotic conductance of 0.596 µS. These features enable MXene@MC as an ultrahigh performance osmotic energy generator, achieving an unprecedented osmotic power of 343 pW under a 1000-fold salinity gradient at neutral pH. Notably, simulations are also provided to demonstrate the findings of the proposed ionic rectifier and efficient osmotic energy conversion. This study unravels the underlying physics of ion transport induced by the apparent structural asymmetry of ion-selective channels, thereby providing a promising platform for further development of high-performance osmotic energy generators.

Negative memory capacitance and ionic filtering effects in asymmetric nanopores

The pervasive model for a solvated, ion-filled nanopore is often a resistor in parallel with a capacitor. For conical nanopore geometries, here we propose the inclusion of a Warburg-like element, which is necessary to explain otherwise anomalous observations such as negative capacitance and low-pass filtering of translocation events (we term this phenomenon as Warburg filtering). The negative capacitance observed here has long equilibration times and memory (that is, mem-capacitance) at negative voltages. We used the transient occlusion of the pore using λ-DNA and 10 kbp DNA to test whether events are being attenuated by purely ionic phenomena when there is sufficient amplifier bandwidth. We argue here that both phenomena can be explained by the inclusion of the Warburg-like element, which is mechanistically linked to concentration polarization and activation energy to generate and maintain localized concentration gradients. We conclude the study with insights into the transduction of molecular translocations into electrical signals, which is not simply based on pulse-like resistance changes but instead on the complex and nonlinear storage of ions that enter dis-equilibrium during molecular transit.

Ice-Confined Synthesis of Stacked Polymer Nanospheres as Osmotic Power Generation Membranes

Osmotic power extracts electricity from salinity gradients and provides a viable route toward clean energy. To improve the energy conversion efficiency, common strategies rely on fabricating precisely controlled nanopores to meet the requirements of high ionic conductivity and selectivity. We report ion transport through the free-volume networks in stacked polymer nanospheres for osmotic power harvesting. Such nanospheres, composed of coiled poly(acrylic acid) molecules, are synthesized at an ice-liquid interface where they self-assemble into continuous membranes with controlled thicknesses and morphologies. We achieve a rival power density of a few thousand watts per square meter, attributed to the fast and selective ion transport through the nanostructured membranes. The selectivity is further found to originate from the membranes' tunable charging states determined by the association/dissociation equilibrium of the residual groups and the presence of translocation ions. Our work suggests polymer membranes absent of straight-through pores as a new platform for efficient osmotic energy generation.

A switchable and facile ionic diode modulated by polyethylene glycol

We introduce a switchable ionic diode modulated by PEG, enabling dynamic control of ion transport and reversible ion flow switching. This system achieves tunable current rectification over two orders of magnitude, simplifying fabrication and offering versatile, scalable solutions for high-performance ionic devices in energy harvesting, nanofluidics, and ionic circuits.

The Effect of Electrolyte Properties on Ionic Transport through Solid-State Nanopores: Experiment and Simulation

Nanopore membranes enable versatile technologies that are employed in many different applications, ranging from clean energy generation to filtration and sensing. Improving the performance can be achieved by conducting numerical simulations of the system, for example, by studying how the nanopore geometry or surface properties change the ionic transport behavior or fluid dynamics of the system. A widely employed tool for numerical simulations is finite element analysis (FEA) using software, such as COMSOL Multiphysics. We found that the prevalent method of implementing the electrolyte in the FEA can diverge significantly from physically accurate values. It is often assumed that salt molecules fully dissociate, and the effect of the temperature is neglected. Furthermore, values for the diffusion coefficients of the ions, as well as permittivity, density, and viscosity of the fluid, are assumed to be their bulk values at infinite dilution. By performing conductometry experiments with an amorphous SiO2 nanopore membrane with conical pores and simulating the pore system with FEA, it is shown that the common assumptions do not hold for different mono- and divalent chlorides (LiCl, NaCl, KCl, MgCl2, and CaCl2) at concentrations above 100 mM. Instead, a procedure is presented where all parameters are implemented based on the type of salt and concentration. This modification to the common approach improves the accuracy of the numerical simulations and thus provides a more comprehensive insight into ion transport in nanopores that is otherwise lacking.

Highly Anion-Conductive Viologen-Based Two-Dimensional Polymer Membranes as Nanopower Generators

Two-dimensional polymers (2DPs) and their layer-stacked 2D covalent organic frameworks (2D COFs) membranes hold great potential for harvesting sustainable osmotic energy. The nascent research has yet to simultaneously achieve high ionic flux and selectivity, primarily due to inefficient ion transport dynamics. This is directly related to ultrasmall pore size (<3 nm), much smaller than the duple Debye length in the diluted electrolyte (6-20 nm), as well as low charge density (<4.5 mC m-2). Here, we introduce a π-conjugated viologen-based 2DP (V2DP) membrane possessing a large pore size of 4.5 nm, strategically enhancing the overlapping of the electric double layer, coupled with an exceptional positive surface charge density (~6 mC m-2). These characteristics enable the membrane to facilitate high anion flux while maintaining ideal selectivity. Notably, V2DP membranes realize an impressive current density of 5.5×103 A m-2, surpassing benchmarks set by previously reported nanofluidic membranes. In the practical application scenario involving the mixing of artificial seawater and river water, the V2DP membranes exhibit a considerable ion transference number of 0.70 towards Cl-, contributing to an outstanding power density of ~55 W m-2. Theoretical calculations reveal the important role of the large quantity of anion transport sites, which act as binding sites evenly located in the positively charged N-containing pyridine rings. These binding sites enable kinematic coupling and decoupling between anions and the V2DP skeleton, establishing a continuous Cl- ion transport pathway. This work demonstrates the great promise of large-area ultrathin 2DP membranes featuring highly organized charged ion transport networks when applied for osmotic energy conversion.

Massively Enhanced Charge Selectivity, Ion Transport, and Osmotic Energy Conversion by Antiswelling Nanoconfined Hydrogels

Developing a nanofluidic membrane with simultaneously enhanced ion selectivity and permeability for high-performance osmotic energy conversion has largely been unexplored. Here, we tackle this issue by the confinement of highly space-charged hydrogels within an orderedly aligned nanochannel array membrane. The nanoconfinement effect endows the hydrogel-based membrane with excellent antiswelling property. Furthermore, experimental and simulation results demonstrate that such a nanoconfined hydrogel membrane exhibits massively enhanced cation selectivity and ion transport properties. Consequently, an amazingly high power density up to ∼52.1 W/m2 with an unprecedented energy conversion efficiency of 37.5% can be reached by mixing simulated salt-lake water (5 M NaCl) and river water (0.01 M NaCl). Both efficiency indexes surpass those of most of the state-of-the-art nanofluidic membranes. This work offers insights into the design of highly ion-selective membranes to achieve ultrafast ion transport and high-performance osmotic energy harvesting.

Enhancing Ionic Selectivity and Osmotic Energy by Using an Ultrathin Zr-MOF-Based Heterogeneous Membrane with Trilayered Continuous Porous Structure

Designing a nanofluidic membrane with high selectivity and fast ion transport property is the key towards high-performance osmotic energy conversion. However, most of reported membranes can produce power density less than commercial benchmark (5 W/m2), due to the imbalance between ion selectivity and permeability. Here, we report a novel nanoarchitectured design of a heterogeneous membrane with an ultrathin and dense zirconium-based UiO-66-NH2 metal-organic framework (MOF) layer and a highly aligned and interconnected branched alumina nanochannel membrane. The design leads to a continuous trilayered pore structure of large geometry gradient in the sequence from angstrom-scale to nano-scale to sub-microscale, which enables the enhanced directional ion transport, and the angstrom-sized (~6.6-7 Å) UiO-66-NH2 windows render the membrane with high ion selectivity. Consequently, the novel heterogeneous membrane can achieve a high-performance power of ~8 W/m2 by mixing synthetic seawater and river water. The power density can be largely upgraded to an ultrahigh ~17.1 W/m2 along with ~48.5 % conversion efficiency at a 50-fold KCl gradient. This work not only presents a new membrane design approach but also showcases the great potential of employing the zirconium-based MOF channels as ion-channel-mimetic membranes for highly efficient blue energy harvesting.

Impact of surface charge density modulation on ion transport in heterogeneous nanochannels

The PNP nanotransistor, consisting of emitter, base, and collector regions, exhibits distinct behavior based on surface charge densities and various electrolyte concentrations. In this study, we investigated the impact of surface charge density on ion transport behavior within PNP nanotransistors at different electrolyte concentrations and applied voltages. We employed a finite-element method to obtain steady-state solutions for the Poisson-Nernst-Planck and Navier-Stokes equations. The ions form a depletion region, influencing the ionic current, and we analyze the influence of surface charge density on the depth of this depletion region. Our findings demonstrate that an increase in surface charge density results in a deeper depletion zone, leading to a reduction in ionic current. However, at very low electrolyte concentrations, an optimal surface charge density causes the ion current to reach its lowest value, subsequently increasing with further increments in surface charge density. As such, atV app = + 1 V andC 0 = 1 mM , the ionic current increases by 25% when the surface charge density rises from 5 to 20  mC . m - 2 , whereas atC 0 = 10 mM , the ionic current decreases by 65% with the same increase in surface charge density. This study provides valuable insights into the behavior of PNP nanotransistors and their potential applications in nanoelectronic devices.

Unipolar Ionic Diode Nanofluidic Membranes Enabled by Stepped Mesochannels for Enhanced Salinity Gradient Energy Harvesting

Developing ionic diode membranes featuring asymmetric structures is in high demand for salinity gradient energy harvesting. These membranes offer benefits in mitigating ion concentration polarization, thereby promoting ion permeability. However, most reported works focus on the role of heterogeneous charge-based bipolar ionic diode membranes for ion concentration polarization suppression, with comparatively less attention given to maintaining ion selectivity. Herein, unipolar ionic diode nanofluidic mesoporous silica membranes featuring stepped mesochannels were developed via a micellar sequential oriented interfacial self-assembly strategy as a salinity gradient energy harvester. Due to the asymmetric mesochannels and unipolar structure (both sides carry negative charge), the ionic diode membranes exhibit a strong rectification ratio of ∼15.91 to facilitate unidirectional ion transport while maintaining excellent cation selectivity (cation transfer number of ∼0.85). Besides, the vertically aligned mesochannels significantly reduce ion transport resistance, generating a high ionic flux. Consequently, the unipolar ionic diode nanofluidic membranes demonstrate a power output of 5.88 W/m2 between artificial sea and river water. The unipolar feature gives notable enhancements of 296% and 144% in power output compared to the symmetric membrane and bipolar ionic diode membrane, respectively. This work opens up new routes for designing ionic diode membranes for salinity gradient energy harvesting.

A nanofluidic spiking synapse

Currently, the nanofluidic synapse can only perform basic neuromorphic pulse patterns. One immediate problem that needs to be addressed to further its capability of brain-like computing is the realization of a nanofluidic spiking device. Here, we report the use of a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate membrane to achieve bionic ionic current-induced spiking. In addition to the simulation of various electrical pulse patterns, our synapse could produce transmembrane ionic current-induced spiking, which is highly analogous to biological action potentials with similar phases and excitability. Moreover, the spiking properties could be modulated by ions and neurochemicals. We expect that this work could contribute to biomimetic spiking computing in solution.

Nano-Confined Effect and Heterojunction Promoted Exciton Separation for Light-Boosted Osmotic Energy Conversion

The osmotic energy conversion properties of biomimetic light-stimulated nanochannels have aroused great interest. However, the power output performance is limited by the low light-induced current and energy conversion efficiency. Here, nanochannel arrays with simultaneous modification of ZnO and di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,20-bipyridyl-4,40-dicarboxylato) ruthenium (II) (N719) onto anodic aluminum oxide (AAO) to combine the nano-confined effect and heterojunction is designed, which demonstrate rectified ion transport behavior due to the asymmetric composition, structure and charge. High cation selectivity and ion flux contribute to the high power density of ≈7.33 W m-2 by mixing artificial seawater and river water. Under light irradiation, heterojunction promoted the production and separation of exciton, enhanced cation selectivity, and improved the utilization efficiency of osmotic energy, providing a remarkable power density of ≈18.49 W m-2 with an increase of 252% and total energy conversion efficiency of 30.43%. The work opens new insights into the biomimetic nanochannels for high-performance energy conversion.

Dynamic Response of Ionic Current in Conical Nanopores

Ionic current rectification (ICR) of charged conical nanopores has various applications in fields including nanofluidics, biosensing, and energy conversion, whose function is closely related to the dynamic response of nanopores. The occurrence of ICR originates from the ion enrichment and depletion in conical pores, whose formation is found to be affected by the scanning rate of voltages. Here, through time-dependent simulations, we investigate the variation of ion current under electric fields and the dynamic formation of ion enrichment and depletion, which can reflect the response time of conical nanopores. The response time of nanopores when ion enrichment forms, i.e., at the "on" state is significantly longer than that with the formation of ion depletion, i.e., at the "off" state. Our simulation results reveal the regulation of response time by different nanopore parameters including the surface charge density, pore length, tip, and base radius, as well as the applied conditions such as the voltage and bulk concentration. The response time of nanopores is closely related to the surface charge density, pore length, voltage, and bulk concentration. Our uncovered dynamic response mechanism of the ionic current can guide the design of nanofluidic devices with conical nanopores, including memristors, ionic switches, and rectifiers.

Geometrically Scalable Iontronic Memristors: Employing Bipolar Polyelectrolyte Gels for Neuromorphic Systems

Iontronics that are capable of mimicking the functionality of biological systems within an artificial fluidic network have long been pursued for biomedical applications and ion-based intelligence systems. Here, we report on facile and robust realization of iontronic bipolar memristors featuring a three-layer polyelectrolyte gel structure. Significant memristive hysteresis of ion currents was successfully accomplished, and the memory time proved geometrically scalable from 200 to 4000 s. These characteristics were enabled by the ion concentration polarization-induced rectification ratio within the polyelectrolyte gels. The memristors exhibited memory dynamics akin to those observed in unipolar devices, while the bipolar structure notably enabled prolonged memory time and enhanced the ion conductance switching ratio with mesoscale (10-1000 μm) geometry precision. These properties endow the devices with the capability of effective neuromorphic processing with pulse-based input voltage signals. Owing to their simple fabrication process and superior memristive performance, the presented iontronic bipolar memristors are versatile and can be easily integrated into small-scale iontronic circuits, thereby facilitating advanced neuromorphic computing functionalities.

Ion-Concentration-Hopping Heterolayer Gel for Ultrahigh Gradient Energy Conversion

Conventional solid ion channel systems relying on single one- or two-dimensional confined nanochannels enabled selective and ultrafast convective ion transport. However, due to intrinsic solid channel stacking, these systems often face pore-pore polarization and ion concentration blockage, thereby restricting their efficiency in macroscale ion transport. Here, we constructed a soft heterolayer-gel system that integrated an ion-selective hydrogel layer with a water-barrier organogel layer, achieving ultrahigh cation selectivity and flux and effectively providing high-efficiency gradient energy conversion on a macroscale order of magnitude. Specifically, the hydrogel layer featured an unconfined 3D network, where the fluctuations of highly hydrated polyelectrolyte chains driven by thermal dynamics enhanced cation selectivity and mitigated transfer energy barriers. Such chain fluctuation mechanisms facilitated ion-cluster internal transmission, thereby enhancing ion concentration hopping for more efficient ion-selective transport. Compared to the existing rigid nanochannel-based gradient energy conversion systems, such a heterogel-based power generator exhibited a record power density of 192.90 and 1.07 W/m2 at the square micrometer scale and square centimeter scale, respectively (under a 500-fold artificial solution). We anticipate that such heterolayer gels would be a promising candidate for energy separation and storage applications.

Negative Differential Resistance in Conical Nanopore Iontronic Memristors

Emerging ion transport dynamics with memory effects at nanoscale solution-substrate interfaces offers a unique opportunity to overcome the bottlenecks in traditional computational architectures, trade-offs in selectivity and throughput in separation, and electrochemical energy conversions. Negative differential resistance (NDR), a decrease in conductance with increasing potential, constitutes a new function from the perspective of time-dependent instead of steady-state nanoscale electrokinetic ion transport but remains unexplored in ionotronics to develop higher-order complexity and advanced capabilities. Herein, NDR is introduced in hysteretic and rectified ion transport through single conical nanopipettes (NPs) as ionic memristors. Deterministic and chaotic behaviors are controlled via an electric field as the sole stimulus. The NDR arises fundamentally from the availability and redistribution of the ionic charges during the hysteretic and rectified transport at asymmetric nanointerfaces. The elucidated mechanism is generalizable, and the drastically simplified operations enable tunable state-switching dynamics with higher-order complexity besides the first-order synaptic functions in multiple excitatory and inhibitory states.

Addressing Challenges in Ion-Selectivity Characterization in Nanopores

Ion selectivity is the basis for designing smart nanopore/channel-based devices, e.g., ion separators and biosensors. Quantitative characterization of ion selectivities in nanopores often employs the Nernst or Goldman-Hodgkin-Katz (GHK) equation to interpret transmembrane potentials. However, the direction of the measured transmembrane potential drop is not specified in these equations, and selectivity values calculated using absolute values of transmembrane potentials do not directly reveal the ion for which the membrane is selective. Moreover, researchers arbitrarily choose whether to use the Nernst or GHK equation and overlook the significant differences between them, leading to ineffective quantitative comparisons between studies. This work addresses these challenges through (a) specifying the transmembrane potential (sign) and salt concentrations in terms of working and reference electrodes and the solutions in which they reside when using the Nernst and GHK equations, (b) reporting of both Nernst-selectivity and GHK-selectivity along with solution compositions and transmembrane potentials when comparing different nanopores/channels, and (c) performing simulations to define an ideal selectivity for nanochannels. Experimental and modeling studies provide significant insight into these fundamental equations and guidelines for the development of nanopore/channel-based devices.

Single-Layer Hexagonal Boron Nitride Nanopores as High-Performance Ionic Gradient Power Generators

Atomically thin two-dimensional (2D) materials have emerged as promising candidates for efficient energy harvesting from ionic gradients. However, the exploration of robust 2D atomically thin nanopore membranes, which hold sufficient ionic selectivity and high ion permeability, remains challenging. Here, the single-layer hexagonal boron nitride (hBN) nanopores are demonstrated as various high-performance ion-gradient nanopower harvesters. Benefiting from the ultrathin atomic thickness and large surface charge (also a large Dukhin number), the hBN nanopore can realize fast proton transport while maintaining excellent cation selectivity even in highly acidic environments. Therefore, a single hBN nanopore achieves the pure osmosis-driven proton-gradient power up to ≈3 nW under 1000-fold ionic gradient. In addition, the robustness of hBN membranes in extreme pH conditions allows the ionic gradient power generation from acid-base neutralization. Utilizing 1 m HCl/KOH, the generated power can be promoted to an extraordinarily high level of ≈4.5 nW, over one magnitude higher than all existing ionic gradient power generators. The synergistic effects of ultrathin thickness, large surface charge, and excellent chemical inertness of 2D single-layer hBN render it a promising membrane candidate for harvesting ionic gradient powers, even under extreme pH conditions.

Bio-Inspired Subnanofluidics: Advanced Fabrication and Functionalization

Biological ion channels can realize high-speed and high-selective ion transport through the protein filter with the sub-1-nanometer channel. Inspired by biological ion channels, various kinds of artificial subnanopores, subnanochannels, and subnanoslits with improved ion selectivity and permeability are recently developed for efficient separation, energy conversion, and biosensing. This review article discusses the advanced fabrication and functionalization methods for constructing subnanofluidic pores, channels, tubes, and slits, which have shown great potential for various applications. Novel fabrication methods for producing subnanofluidics, including top-down techniques such as electron beam etching, ion irradiation, and electrochemical etching, as well as bottom-up approaches starting from advanced microporous frameworks, microporous polymers, lipid bilayer embedded subnanochannels, and stacked 2D materials are well summarized. Meanwhile, the functionalization methods of subnanochannels are discussed based on the introduction of functional groups, which are classified into direct synthesis, covalent bond modifications, and functional molecule fillings. These methods have enabled the construction of subnanochannels with precise control of structure, size, and functionality. The current progress, challenges, and future directions in the field of subnanofluidic are also discussed.

Enhanced Rectification Performance in Bipolar Janus Graphene Oxide Channels by Lateral Electric Fields

Improving the ionic rectification in nanochannels enables versatile applications such as biosensors, energy harvesting, and fluidic diodes. While previous work mostly focused on the effect of channel geometry and surface charge, in this work via a series of molecular dynamics simulations, we find a striking phenomenon that the ionic current rectification (ICR) ratio in Janus graphene oxide (GO) channels can be tremendously promoted by lateral electric fields. First, under a given axial electric field, an additional lateral electric field can improve the ICR ratio by several times to an order, depending on the channel symmetry. The symmetric channel has an obviously greater ICR ratio because it maintains a more pronounced ion transport disparity at opposite axial fields. The underlying mechanism for the function of the lateral electric field is that it promotes the lateral migration of ions and thus amplifies the ion-residue electrostatic interaction at opposite axial fields, enlarging the ion dynamical difference. Furthermore, for different axial electric fields, the ICR ratio can always be improved by lateral electric fields (up to two orders), suggesting that the ICR improvement is universal. Our results demonstrate that applying a lateral electric field could be a new method to improve the rectification performance of nanochannels, providing valuable guidance for the design of efficient ionic diode devices.

Single Idiosyncratic Ionic Generator Working in Iso-Osmotic Solutions Via Ligand Confined Assembled in Gaps Between Nanosheets

Numerous reported bioinspired osmotic energy conversion systems employing cation-/anion-selective membranes and solutions with different salinity are actually far from the biological counterpart. The iso-osmotic power generator with the specific ionic permselective channels (e.g., K+ or Na+ channels) which just allow specific ions to get across and iso-osmotic solutions still remain challenges. Inspired by nature, we report a bioinspired K+ -channel by employing a K+ selective ligand, 1,1,1-tris{[(2'-benzylaminoformyl)phenoxy]methyl}ethane (BMP) and graphene oxide membrane. Specifically, the K+ and Na+ selectivity of the prepared system could reach up to ≈17.8, and the molecular dynamics simulation revealed that the excellent permselectivity of K+ mainly stemmed from the formed suitable channel size. Thus, we assembled the K+ -selective iso-osmotic power generator (KSIPG) with the power density up to ≈15.1 mW/m2 between equal concentration solutions, which is higher than traditional charge-selective osmotic power generator (CSOPG). The proposed strategy has well shown the realizable approach to construct single-ion selective channels-based highly efficient iso-osmotic energy conversion systems and would surely inspire new applications in other fields, including self-powered systems and medical materials, etc.

Ionic Transistors

Biological voltage-gated ion channels, which behave as life's transistors, regulate ion transport precisely and selectively through atomic-scale selectivity filters to sustain important life activities. By this inspiration, voltage-adaptable ionic transistors that use ions as signal carriers may provide an alternative information processing unit beyond solid-state electronic devices. This review provides a comprehensive overview of the first generation of biomimetic ionic transistors, including their operating mechanisms, device architecture development, and property characterizations. Despite its infancy, significant progress has been made in the applications of ionic transistors in fields such as DNA detection, drug delivery, and ionic circuits. Challenges and prospects of full exploitation of ionic transistors for a broad spectrum of practical applications are also discussed.

Bioinspired light-driven chloride pump with helical porphyrin channels

Halorhodopsin, a light-driven chloride pump, utilizes photonic energy to drive chloride ions across biological membranes, regulating the ion balance and conveying biological information. In the light-driven chloride pump process, the chloride-binding chromophore (protonated Schiff base) is crucial, able to form the active center by absorbing light and triggering the transport cycle. Inspired by halorhodopsin, we demonstrate an artificial light-driven chloride pump using a helical porphyrin channel array with excellent photoactivity and specific chloride selectivity. The helical porphyrin channels are formed by a porphyrin-core star block copolymer, and the defects along the channels can be effectively repaired by doping a small number of porphyrins. The well-repaired porphyrin channel exhibits the light-driven Cl- migration against a 3-fold concentration gradient, showing the ion pumping behavior. The bio-inspired artificial light-driven chloride pump provides a prospect for designing bioinspired responsive ion channel systems and high-performance optogenetics.

Desalination Performance in Janus Graphene Oxide Channels: Geometric Asymmetry vs Charge Polarity

Improving the desalination performance of membranes is always in the spotlight of scientific research; however, Janus channels with polarized surface charge as nanofiltration membranes are still unexplored. In this work, using molecular dynamics simulations, we demonstrate that Janus graphene oxide (GO) channels with appropriate geometry and surface charge can serve as highly efficient nanofiltration membranes. We observe that the water permeability of symmetric Janus GO channels is significantly superior to that of asymmetric channels without sacrificing much ion rejection, owing to weakened ion blockage and electrostatic effects. Furthermore, in symmetric Janus GO channels, the transport of water and ions is sensitive to the charge polarity of the channel inner surface, which is realized by tuning the ratio of cationic and anionic functionalization. Specifically, with the increase in cationic functionalization, the water flux decreases monotonously, while ion rejection displays an interesting maximum behavior that indicates desalination optimization. Moreover, the trade-off between water permeability and ion rejection suggests that the Janus GO channels have an excellent desalination potential and are highly tunable according to the specific water treatment requirements. Our work sheds light on the key role of channel geometry and charge polarity in the desalination performance of Janus GO channels, which paves the way for the design of novel desalination devices.

Ion Transport in Intelligent Nanochannels: A Comparative Analysis of the Role of Electric Field

This research delves into investigating ion transport behavior within nanochannels, enhanced through modification with a negatively charged polyelectrolyte layer (PEL), aimed at achieving superior control. The study examines two types of electric fields─direct current and alternating current with square, sinusoidal, triangular, and sawtooth waveforms─to understand their impact on ion transport. Furthermore, the study compares symmetric (cylindrical) and asymmetric (conical) nanochannel geometries to assess the influence of overlapping electrical double layers (EDLs) in generating specific electrokinetic behaviors such as ionic current rectification (ICR) and ion selectivity. The research employs the finite element method to solve the coupled Poisson-Nernst-Planck and Navier-Stokes equations under unsteady-state conditions. By considering factors such as electrolyte concentration, soft layer charge density, and electric field type, the study evaluates ion transport performance in charged nanochannels, investigating effects on concentration polarization, electroosmotic flow (EOF), ion current, rectification, and ion selectivity. Notably, the study accounts for ion partitioning between the PEL and electrolyte to simulate real conditions. Findings reveal that conical nanochannels, due to improved EDL overlap, significantly enhance ion transport and related characteristics compared to cylindrical ones. For instance, under ηε = ηD = 0.8, ημ = 2, C0 = 20 mM, and NPEL/NA = 80 mol m-3 conditions, the average EOF for conical and cylindrical geometries is 0.1 and 0.008 m/s, respectively. Additionally, the study explores ion selectivity and rectification based on the electric field type, unveiling the potential of nanochannels as ion gates or diodes. In cylindrical nanochannels, the ICR remains at unity, with lower ion selectivity across waveforms compared to conical channels. Furthermore, rectification and ion selectivity trends are identified as Rf,square > Rf,DC > Rf,triangular > Rf,sinusoidal > Rf,sawtooth and Ssawtooth > Ssinusoidal > Striangular > SDC > Ssquare for conical nanochannels. Our study of ion transport control in nanochannels, guided by tailored electric fields and unique geometries, offers versatile applications in the field of Analytical Chemistry. This includes enhanced sample separation, controlled drug delivery, optimized pharmaceutical analysis, and the development of advanced biosensing technologies for precise chemical analysis and detection. These applications highlight the diverse analytical contributions of our methodology, providing innovative solutions to challenges in chemical analysis and biosensing.

Surface diffusion enhanced ion transport through two-dimensional nanochannels

Fast ion permeation in nanofluidic channels has been intensively investigated in the past few decades because of their potential uses in separation technologies and osmotic energy harvesting. Mechanisms governing ion transport at this ultimately small spatial regime remain to be understood, which can only be achieved in nanochannels that are controllably fabricated. Here, we report the fabrication of two-dimensional nanochannels with their top and bottom walls consisting of atomically flat graphite and mica crystals, respectively. The distinct wall structures and properties enable us to investigate interactions between ions and interior surfaces. We find an enhanced ion transport within the channels that is orders of magnitude faster than that in the bulk solutions. The result is attributed to the highly dense packing of adsorbed cations at mica surfaces, where they diffuse in-plane. Our work provides insights into surface effects on ion transport at the nanoscale.

Essence of the Giant Reduction of Power Density in Osmotic Energy Conversion in Porous Membranes: Importance of Testing Area

Harvesting osmotic energy through nanofluidic devices with diverse materials has received considerable attention in recent years. Often, a small testing area on a membrane was chosen to assess its power performance by calculating power density as output power per effective area. Since the choice of this testing area is arbitrary, and it is usually quite small, the result obtained can be too optimistic. There is a need to come up with a common standard so that the performance of a device/membrane can be assessed reasonably. In this study, we systematically investigate the power density as a function of testing area in nanoporous anodic-aluminum-oxide membranes. Through changing the aperture size of substrates, we clearly show that the obtained power density decreases drastically with increasing testing area. For instance, the power density acquired from the testing area of μm2-scale can be five orders of magnitude larger than that from the pristine membrane of cm2-scale. We also advance simulations by building a 3D model to simulate osmotic-driven ion transport in the multichannel system. The result of modeling agrees with our experimental observation that the power density decreases with increasing number of channels, and the ionic concentration profile reveals that the concentration polarization becomes serious as the number of channels increases. Our result highlights the importance of effective area on testing the power performance in nanofluidic devices.

Ultra-mechanosensitive Chloride Ion Transport through Bioinspired High-Density Elastomeric Nanochannels

Mechanosensitive ion channels play crucial roles in physiological activities, where small mechanical stimuli induce the membrane tension, trigger the ion channels' deformation, and are further transformed into significant electrochemical signals. Artificial ion channels with stiff moduli have been developed to mimic mechanosensory behaviors, exhibiting an electrochemical response by the high-pressure-induced flow. However, fabricating flexible mechanosensitive channels capable of regulating specific ion transporting upon dramatic deformation has remained a challenge. Here, we demonstrate bioinspired high-density elastomeric channels self-assembled by polyisoprene-b-poly4-vinylpyridine, which exhibit ultra-mechanosensitive chloride ion transport resulting from nanochannel deformation. The PI-formed continuous elastic matrix can transmit external forces into internal tensions, while P4VP forms transmembrane chloride channels that undergo dramatic deformation and respond to mechanical stimuli. The integrated and flexible chloride channels present a dramatic and stable electrochemical signal toward a low pressure of 0.2 mbar. This research first demonstrates the artificial mechanosensory chloride channels, which could provide a promising avenue for designing flexible and responsive channel systems.

Harvesting Blue Energy Based on Salinity and Temperature Gradient: Challenges, Solutions, and Opportunities

Greenhouse gas emissions associated with power generation from fossil fuel combustion account for 25% of global emissions and, thus, contribute greatly to climate change. Renewable energy sources, like wind and solar, have reached a mature stage, with costs aligning with those of fossil fuel-derived power but suffer from the challenge of intermittency due to the variability of wind and sunlight. This study aims to explore the viability of salinity gradient power, or "blue energy", as a clean, renewable source of uninterrupted, base-load power generation. Harnessing the salinity gradient energy from river estuaries worldwide could meet a substantial portion of the global electricity demand (approximately 7%). Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are more prominent technologies for blue energy harvesting, whereas thermo-osmotic energy conversion (TOEC) is emerging with new promise. This review scrutinizes the obstacles encountered in developing osmotic power generation using membrane-based methods and presents potential solutions to overcome challenges in practical applications. While certain strategies have shown promise in addressing some of these obstacles, further research is still required to enhance the energy efficiency and feasibility of membrane-based processes, enabling their large-scale implementation in osmotic energy harvesting.

A Single-Entity Method for Actively Controlled Nucleation and High-Quality Protein Crystal Synthesis

Lack of controls and understanding in nucleation, which proceeds crystal growth and other phase transitions, has been a bottleneck challenge in chemistry, materials, biology, and other fields. The exemplary needs for better methods for biomacromolecule crystallization include (1) synthesizing crystals for high-resolution structure determinations in fundamental research and (2) tuning the crystal habit and thus the corresponding properties in materials and pharmaceutical applications. Herein, a deterministic method is established capable of sustaining the nucleation and growth of a single crystal using the protein lysozyme as a prototype. The supersaturation is localized at the interface between a sample and a precipitant solution, spatially confined by the tip of a single nanopipette. The exchange of matter between the two solutions determines the supersaturation, which is controlled by electrokinetic ion transport driven by an external potential waveform. Nucleation and subsequent crystal growth disrupt the ionic current limited by the nanotip and are detected. The nucleation and growth of individual single crystals are measured in real time. Electroanalytical and optical signatures are elucidated as feedbacks with which active controls in crystal quality and method consistency are achieved: five out of five crystals diffract at a true atomic resolution of up to 1.2 Å. As controls, those synthesized under less optimized conditions diffract poorly. The crystal habits during the growth process are tuned successfully by adjusting the flux. The universal mechanism of nano-transport kinetics, together with the correlations of the diffraction quality and crystal habit with the crystallization control parameters, lay the foundation for the generalization to other materials systems.

Designing with Iontronic Logic Gates─From a Single Polyelectrolyte Diode to an Integrated Ionic Circuit

This article presents the implementation of on-chip iontronic circuits via small-scale integration of multiple ionic logic gates made of bipolar polyelectrolyte diodes. These ionic circuits are analogous to solid-state electronic circuits, with ions as the charge carriers instead of electrons/holes. We experimentally characterize the responses of a single fluidic diode made of a junction of oppositely charged polyelectrolytes (i.e., anion and cation exchange membranes), with a similar underlying mechanism as a solid-state p- and n-type junction. This served to carry out predesigned logical computations in various architectures by integrating multiple diode-based logic gates, where the electrical signal between the integrated gates was transmitted entirely through ions. The findings shed light on the limitations affecting the number of logic gates that can be integrated, the degradation of the electrical signal, their transient response, and the design rules that can improve the performance of iontronic circuits.

Nanopores: synergy from DNA sequencing to industrial filtration - small holes with big impact

Nanopores in thin membranes play important roles in science and industry. Single nanopores have provided a step-change in portable DNA sequencing and understanding nanoscale transport while multipore membranes facilitate food processing and purification of water and medicine. Despite the unifying use of nanopores, the fields of single nanopores and multipore membranes differ - to varying degrees - in terms of materials, fabrication, analysis, and applications. Such a partial disconnect hinders scientific progress as important challenges are best resolved together. This Viewpoint suggests how synergistic crosstalk between the two fields can provide considerable mutual benefits in fundamental understanding and the development of advanced membranes. We first describe the main differences including the atomistic definition of single pores compared to the less defined conduits in multipore membranes. We then outline steps to improve communication between the two fields such as harmonizing measurements and modelling of transport and selectivity. The resulting insight is expected to improve the rational design of porous membranes. The Viewpoint concludes with an outlook of other developments that can be best achieved by collaboration across the two fields to advance the understanding of transport in nanopores and create next-generation porous membranes tailored for sensing, filtration, and other applications.

Surface charge density governs the ionic current rectification direction in asymmetric graphene oxide channels

Charged asymmetric channels are extensively investigated for the design of artificial biological channels, ionic diodes, artificial separation films, etc. These applications are attributed to the unique ionic current rectification phenomenon, where the surface charge density of the channel has a deep influence. In this work, we use molecular dynamics simulations to study the rectification phenomenon in asymmetric graphene oxide channels. A fascinating finding is that the ionic current rectification direction reverses from the negative to positive electric field direction with an increase in surface charge density. Specifically, at low charge density, the ionic flux reaches greater values in the negative electric field due to the enrichment of cations and anions, which provides a sufficient electrostatic shielding effect inside the channel and increases the possibility of ion release by the residues. However, at high charge density, the extremely strong residue attraction induces a Coulomb blockade effect in the negative electric field, which seriously impedes the ion transport and eventually leads to a smaller ionic current. Consequently, this ionic current order transition ultimately results in the rectification reversion phenomenon, providing a new route for the design of some novel nanofluidic devices.

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.

Enhancing the osmotic energy conversion of a nanoporous membrane: influence of pore density, pH, and temperature

Salinity gradient power, which converts Gibbs free energy of mixing to electric energy through an ion-selective pore, has great potential. Towards practical use, developing membrane-scaled nanoporous materials is desirable and necessary. Unfortunately, the presence of a significant ion concentration polarization (ICP) lowers appreciably the power harvested, especially at a high pore density. To alleviate this problem, we suggest applying an extra pressure difference ΔP across a membrane containing multiple nanopores, taking account of the associated power consumption. The results gathered reveal that the application of a negative pressure difference can improve the power harvested due to the enhanced selectivity. In addition, if the pore density of a membrane is high, raising its pore length is necessary to make the energy harvested economic. For example, if the pore length is 2000 nm and the pore density is 2.5 × 10⁹ pores per cm², an increment in the power density of 213 mW m^-2 can be obtained by applying ΔP = -1 bar at pH 11 and 323 K, where a net positive power density can be retrieved. The performance of the system considered under various conditions is examined in detail, along with associated mechanisms.

Chemiosomotic flow in a soft conical nanopore: harvesting enhanced blue energy

The salinity gradient energy or the 'blue energy' is one of the most promising inexpensive and abundant sources of clean energy, having immense capabilities to serve modern-day society. In this article, we overlay an extensive analysis of reverse electrodialysis (RED) for harvesting salinity gradient energy in a single conical nanochannel, grafted with a pH-tunable polyelectrolyte layer (PEL) on the inner surfaces. We primarily focus on the distinctiveness of the solution pH of the connecting reservoirs. In spite of acquiring a maximum power density of ∼1.2 kW m^-2 in the chosen configuration, we notice a counter-intuitive patterning of the ion transport for a certain span of pH, leading to diminishing power. To this end, we discuss the possible strategic avenues essentially to achieve a higher amount of power density. In order to achieve a desirable outcome within that pH zone, we employ two separate approaches intending to counter the underlying physics. Results reveal a great enhancement in the power density as well as in the efficiency even under the framework of both strategies proposed herein. Moreover, as shown, the window of solution pH has increased by three times, implicating the maximum power density mentioned above. We expect that the strategic procedure of augmented energy harvesting as discussed in this analysis can be of importance from the perspective of fabricating state-of-the-art nanodevices aimed at blue energy harvesting.

Breakdown of the Nernst-Einstein relation in carbon nanotube porins

For over 100 years, the Nernst-Einstein relation has linked a charged particle's electrophoretic mobility and diffusion coefficient. Here we report experimental measurements of diffusion and electromigration of K⁺ ions in narrow 0.8-nm-diameter single-walled carbon nanotube porins (CNTPs) and demonstrate that the Nernst-Einstein relation in these channels breaks down by more than three orders of magnitude. Molecular dynamics simulations using polarizable force fields show that K⁺ ion diffusion in CNTPs in the presence of a single-file water chain is three orders of magnitude slower than bulk diffusion. Intriguingly, the simulations also reveal a disintegration of the water chain upon application of electric fields, resulting in the formation of distinct K⁺-water clusters, which then traverse the CNTP at high velocity. Finally, we show that although individual ion-water clusters still obey the Nernst-Einstein relation, the overall relation breaks down because of two distinct mechanisms for ion diffusion and electromigration.

Membranes for Osmotic Power Generation by Reverse Electrodialysis

In recent years, the utilization of the selective ion transport through porous membranes for osmotic power generation (blue energy) has received a lot of attention. The principal of power generation using the porous membranes is same as that of conventional reverse electrodialysis (RED), but nonporous ion exchange membranes are conventionally used for RED. The ion transport mechanisms through the porous and nonporous membranes are considerably different. Unlike the conventional nonporous membranes, the ion transport through the porous membranes is largely dictated by the principles of nanofluidics. This owes to the fact that the osmotic power generation via selective ion transport through porous membranes is often referred to as nanofluidic reverse electrodialysis (NRED) or nanopore-based power generation (NPG). While RED using nonporous membranes has already been implemented on a pilot-plant scale, the progress of NRED/NPG has so far been limited in the development of small-scale, novel, porous membrane materials. The aim of this review is to provide an overview of the membrane design concepts of nanofluidic porous membranes for NPG/NRED. A brief description of material design concepts of conventional nonporous membranes for RED is provided as well.

Enhanced Ionic Current Rectification through Innovative Integration of Polyelectrolyte Bilayers and Charged-Wall Smart Nanochannels

The tools utilized by humans continue to shrink and speed up. Lab-on-a-chip (LOC) is one of the most recent techniques for decreasing the size of chemical systems. Today, LOCs have made substantial strides in developing nanomaterial fabrication techniques. Controlling and regulating the fluid and ion mobility in these systems is crucial. Layer-by-layer (LBL) soft layers are one of the most effective strategies for controlling fluid flow in channels. In light of the present constraints for developing these systems and the high expense of experimental investigations, it is vital to employ modeling to minimize costs and comprehend their underlying ideas and operations. In this study, we examined the influence of the LBL soft layer's presence in the charged nanochannels on the ion transport parameters. To examine the effect of the coating length of the LBL soft layer, we first examined three lengths of coating: one with a length greater than half (type (I)), one with a length equal to half (type (II)), and one with a length less than half (type (III)) of the nanochannel length. Then, by solving Poisson-Nernst-Planck and Navier-Stokes equations, we determined the influences of pH, soft layer charge density (N_PEL/N_A), bulk concentration (C₀), and hard surface charge density (σ) on the ionic current rectification (R_f) and selectivity (S) of the nanochannel. The maximum rectification of 30.65 was achieved using a nanochannel of type (III) and σ = +10 mC/m². The current results demonstrate a promising hybrid architecture consisting of an LBL soft layer and a smart charged nanochannel for enhanced rectification.

Ion Current Rectification and Long-Range Interference in Conical Silicon Micropores

Fluidic devices exhibiting ion current rectification (ICR), or ionic diodes, are of broad interest for applications including desalination, energy harvesting, and sensing, among others. For such applications a large conductance is desirable, which can be achieved by simultaneously using thin membranes and wide pores. In this paper we demonstrate ICR in micrometer sized conical channels in a thin silicon membrane with pore diameters comparable to the membrane thickness but both much larger than the electrolyte screening length. We show that for these pores the entrance resistance is key not only to Ohmic conductance around 0 V but also for understanding ICR, both of which we measure experimentally and capture within a single analytic theoretical framework. The only fit parameter in this theory is the membrane surface potential, for which we find that it is voltage dependent and its value is excessively large compared to the literature. From this we infer that surface charge outside the pore strongly contributes to the observed Ohmic conductance and rectification by a different extent. We experimentally verify this hypothesis in a small array of pores and find that ICR vanishes due to pore-pore interactions mediated through the membrane surface, while Ohmic conductance around 0 V remains unaffected. We find that the pore-pore interaction for ICR is set by a long-ranged decay of the concentration which explains the surprising finding that the ICR vanishes for even a sparsely populated array with a pore-pore spacing as large as 7 μm.

Laser-Assisted Scalable Pore Fabrication in Graphene Membranes for Blue-Energy Generation

The osmotic energy from a salinity gradient (i. e. blue energy) is identified as a promising non-intermittent renewable energy source for a sustainable technology. However, this membrane-based technology is facing major limitations for large-scale viability, primarily due to the poor membrane performance. An atomically thin 2D nanoporous material with high surface charge density resolves the bottleneck and leads to a new class of membrane material the salinity gradient energy. Although 2D nanoporous membranes show extremely high performance in terms of energy generation through the single pore, the fabrication and technical challenges such as ion concentration polarization make the nanoporous membrane a non-viable solution. On the other hand, the mesoporous and micro porous structures in the 2D membrane result in improved energy generation with very low fabrication complexity. In the present work, we report femtosecond (fs) laser-assisted scalable fabrication of μm to mm size pores on Graphene membrane for blue energy generation for the first time. A remarkable osmotic power in the order of μW has been achieved using mm size pores, which is about six orders of magnitudes higher compared to nanoporous membranes, which is mainly due to the diffusion-osmosis driven large ionic flux. Our work paves the way towards fs laser-assisted scalable pore creation in the 2D membrane for large-scale osmotic power generation.

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.

Electrodiffusioosmosis induced negative differential resistance in micro-to-millimeter size pores through a graphene/copper membrane

Negative differential resistance (NDR) is one of the nonlinear transport phenomena in which ionic current decreases with the increase in electromotive potential. Electro-osmosis, diffusio-osmosis, and surface charge density of pores are the driving forces for observing NDR in nanoscale ion transport. Here, we report electrodiffusioosmosis induced NDR using micro to millimeter size pores in a two-dimensional (2D) graphene-coated copper (Gr/Cu) membrane. Along with NDR, we also observed ion current rectification (ICR), in which there is preferential one-directional ion flow for equal and opposite potentials. The experimentally observed NDR effect has been validated by performing ion transport simulations using Poisson-Nernst-Planck (PNP) equations and Navier-Stokes equations with the help of COMSOL Multiphysics considering salinity gradient across the membrane. Charge polarization induced electro-osmotic flow (EOF) dominates over diffusio-osmosis, causing the backflow of low concentration/conductivity solution into the pore, thereby causing NDR. This finding paves the way toward potential applications in ionic tunnel diodes as rectifiers, switches, amplifiers, and biosensors.

Metal-Organic Cage as Single-Molecule Carrier for Solid-State Nanopore Analysis

The ability to detect biomolecules at the single-molecule level is at the forefront of biological research, precision medicine, and early diagnosis. Recently, solid-state nanopore sensors have emerged as a promising technique for label-free and precise diagnosis assay. However, insufficient sensitivity and selectivity for small analytes are a great challenge for clinical diagnosis applications via solid-state nanopores. Here, for the first time, a metal-organic cage, PCC-57, is employed as a carrier to increase the sensitivity and selectivity of solid-state nanopores based on the intrinsic interaction of the nanocage with biomolecules. Firstly, it is found that the carrier itself is undetectable unless bound with the target analytes and used oligonucleotides as linkers to attach PCC-57 and target analytes. Secondly, two small analytes, oligonucleotide conjugated angiopep-2 and polyphosphoric acid, are successfully distinguished using the molecular carrier. Finally, selectivity of nanopore detection is achieved by attaching PCC-57 to oligonucleotide-tailed aptamers, and the human alpha-thrombin sample is successfully detected. It is believed that the highly designable metal-organic cage could serve as a rich carrier repository for a variety of biomolecules, facilitating single-molecule screening of clinically relevant biomolecules based on solid-state nanopores in the future.