Highly selective and high-performance osmotic power generators in subnanochannel membranes enabled by metal-organic frameworks

Regulating Nonlinear Ion Transport through a Solid-State Pore by Partial Surface Coatings

Using functional nanofluidic devices to manipulate ion transport allows us to explore the nanoscale development of blue energy harvesters and iontronic building blocks. Herein, we report on a method to alter the nonlinear ionic current through a pore by partial dielectric coatings. A variety of dielectric materials are examined on both the inner and outer surfaces of the channel with four different patterns of coated or uncoated surfaces. Through controlling the specific part of the surface charge, the pore can behave like a resistor, diode, and bipolar junction transistor. We use numerical simulations to find out the reason for the asymmetric ion transport in the pore and illustrate the relationship between specifically charged surfaces and electroosmotic flow. These findings help understand the role of the corresponding surface composition in ion transport, which provides a direct approach to modify the electroosmotic-flow-driven ionic current rectification in the channel-based device via dielectric coatings.

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.

Two-Dimensional Nanofluidic Membranes with Intercalated In-Plane Shortcuts for High-Performance Blue Energy Harvesting

Two-dimensional nanofluidic membranes offer great opportunities for developing efficient and robust devices for ionic/water-nexus energy harvesting. However, low counterion concentration and long pathway through limited ionic flux restrict their output performance. Herein, it is demonstrated that rapid diffusion kinetics can be realized in two-dimensional nanofluidic membranes by introducing in-plane holes across nanosheets, which not only increase counterion concentration but also shorten pathway length through the membranes. Thus, the holey membranes exhibited an enhanced performance relative to the pristine ones in terms of osmotic energy conversion. In particular, a biomimetic multilayered membrane sequentially assembled from pristine and holey sections offers an optimized combination of selectivity and permeability, therefore generating a power density up to 6.78 W m^-2 by mixing seawater and river water, superior to the majority of the state-of-the-art lamellar nanofluidic membranes. This work highlights the importance of channel morphologies and presents a general strategy for effectively improving ion transport through lamellar membranes for high-performance nanofluidic devices.

A cellulose-derived supramolecule for fast ion transport

Supramolecular frameworks have been widely synthesized for ion transport applications. However, conventional approaches of constructing ion transport pathways in supramolecular frameworks typically require complex processes and display poor scalability, high cost, and limited sustainability. Here, we report the scalable and cost-effective synthesis of an ion-conducting (e.g., Na+) cellulose-derived supramolecule (Na-CS) that features a three-dimensional, hierarchical, and crystalline structure composed of massively aligned, one-dimensional, and ångström-scale open channels. Using wood-based Na-CS as a model material, we achieve high ionic conductivities (e.g., 0.23 S/cm in 20 wt% NaOH at 25 °C) even with a highly dense microstructure, in stark contrast to conventional membranes that typically rely on large pores (e.g., submicrometers to a few micrometers) to obtain comparable ionic conductivities. This synthesis approach can be universally applied to a variety of cellulose materials beyond wood, including cotton textiles, fibers, paper, and ink, which suggests excellent potential for a number of applications such as ion-conductive membranes, ionic cables, and ionotronic devices.

Switchable Ion Current Saturation Regimes Enabled via Heterostructured Nanofluidic Devices Based on Metal-Organic Frameworks

The use of track-etched membranes allows further fine-tuning of transport regimes and thus enables their use in (bio)sensing and energy-harvesting applications, among others. Recently, metal-organic frameworks (MOFs) have been combined with such membranes to further increase their potential. Herein, the creation of a single track-etched nanochannel modified with the UiO-66 MOF is proposed. By the interfacial growth method, UiO-66-confined synthesis fills the nanochannel completely and smoothly, yet its constructional porosity renders a heterostructure along the axial coordinate of the channel. The MOF heterostructure confers notorious changes in the transport regime of the nanofluidic device. In particular, the tortuosity provided by the micro- and mesostructure of UiO-66 added to its charged state leads to iontronic outputs characterized by an asymmetric ion current saturation for transmembrane voltages exceeding 0.3 V. Remarkably, this behavior can be easily and reversibly modulated by changing the pH of the media and it can also be maintained for a wide range of KCl concentrations. In addition, it is found that the modified-nanochannel functionality cannot be explained by considering just the intrinsic microporosity of UiO-66, but rather the constructional porosity that arises during the MOF growth process plays a central and dominant role.

An Ionic Diode Covalent Organic Framework Membrane for Efficient Osmotic Energy Conversion

Heterogeneous membranes that exhibit an ionic diode effect are promising candidates for osmotic energy conversion. However, existing heterogeneous membranes lack molecular-level designed ion channels, thereby limiting their power densities. Here, we demonstrate ionic diode covalent organic framework (COF) membranes with well-defined ion channels, asymmetric geometry and surface charge polarity as high-performance osmotic power generators. The COF diode membranes are comprised of heterojunctions combining a positively charged ultrathin COF layer and a negatively charged COF layer supported by a porous COF nanofiber scaffold, exhibiting an ionic diode effect that effectuates fast unidirectional ion diffusion and anion selectivity. Density functional theory calculations reveal that the differentiated interactions between anions and COF channels contributed to superior I^- transport over other anions. Consequently, the COF diode membranes achieved high output power densities of 19.2 and 210.1 W m^-2 under a 50-fold NaCl and NaI gradient, respectively, outperforming state-of-the-art heterogeneous membranes. This work suggests the great potential of COF diode membranes for anion transport and energy-related applications.

Transfer learning enhanced water-enabled electricity generation in highly oriented graphene oxide nanochannels

Harvesting energy from spontaneous water flow within artificial nanochannels is a promising route to meet sustainable power requirements of the fast-growing human society. However, large-scale nanochannel integration and the multi-parameter coupling restrictive influence on electric generation are still big challenges for macroscale applications. In this regard, long-range (1 to 20 cm) ordered graphene oxide assembled framework with integrated 2D nanochannels have been fabricated by a rotational freeze-casting method. The structure can promote spontaneous absorption and directional transmission of water inside the channels to generate considerable electric energy. A transfer learning strategy is implemented to address the complicated multi-parameters coupling problem under limited experimental data, which provides highly accurate performance optimization and efficiently guides the design of 2D water flow enabled generators. A generator unit can produce ~2.9 V voltage or ~16.8 μA current in a controllable manner. High electric output of ~12 V or ~83 μA is realized by connecting several devices in series or parallel. Different water enabled electricity generation systems have been developed to directly power commercial electronics like LED arrays and display screens, demonstrating the material's potential for development of water enabled clean energy.

Photo-controllable Ion-Gated Metal-Organic Framework MIL-53 Sub-nanochannels for Efficient Osmotic Energy Generation

By closing and opening ion channels, electric eels are able to convert ion concentration gradients into electricity. Inspired by electric eels, considerable artificial sub-nanoscale ion channels with high ion selectivity and transportation efficiency have been designed for harvesting the osmotic energy between ionic solutions of different salinities, but constructing smart ion-gated sub-nanochannels for effective ion transport is still a huge challenge. Herein, photo-controllable sub-nanochannels of metal-organic framework (MOF) NH₂-MIL-53 encapsulated with spiropyrans (SP-MIL-53) were fabricated by a facile in situ growth strategy. Interestingly, the highly ordered sub-nanochannels of SP-MIL-53 were switched on and off to efficiently regulate the ion flux by the light-driven isomerization of SP, which made it a smart ionic gate with a high on-off ratio of 16.2 in 10 mM KCl aqueous solution via UV irradiation. Moreover, the ion-gated sub-nanochannel membrane yielded a high power density of 8.3 W m^-2 under a 50-fold KCl concentration gradient in the open state. Density functional theory calculations revealed that K⁺ ions in SP-MIL-53 sub-nanochannels had a higher mobility constant (3.61 × 10^-2) with UV irradiation than without UV illumination (2.33 × 10^-22). This work provides an effective way to develop smart ion-gating sub-nanochannels for capturing salinity gradient power.

Adsorptive removal of ibuprofen to binary and amine-functionalized UiO-66 in the aquatic environment: synergistic/antagonistic evaluation

The removal of ibuprofen (IBP) from the aqueous solution by metal-organic frameworks such as UiO-66, UiO-66-NH₂, and a binary MOF (UiO-66@5%HKUST-1) was studied. MOFs were synthesized by the solvothermal method. The synthesized MOFs were characterized by Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and N₂ adsorption. BET results showed that binary MOF and UiO-66-NH₂ had a smaller surface area and were mesoporous compared to UiO-66, while UiO-66 was microporous. Quantitative investigations were conducted to understand the effect of binary and functional UiO-66 in adsorbing IBP and compared to UiO-66. The results showed that UiO-66 with 213 mg/g had the highest adsorption in comparison to other adsorbents. UiO-66-NH₂ showed the lowest adsorption (96 mg/g) due to a large decrease in the surface area. The binary MOF, despite a slight decrease in surface area (1277.6 m²/g), had lower adsorption than UiO-66 (147 mg/g) due to the antagonistic effects between the adsorbent and IBP. Furthermore, the pH of the solution had a great effect on the adsorption of IBP, and the results showed that increasing the pH values above 4 reduced the adsorption of IBP.

Nanofluidic Membranes to Address the Challenges of Salinity Gradient Energy Harvesting: Roles of Nanochannel Geometry and Bipolar Soft Layer

Researchers are looking for new, clean, and accessible sources of energy due to rising global warming caused by the usage of fossil fuels and the irreversible harm that this does to the environment. Water salinity is one of the newest and most accessible renewable energy sources, which has sparked a lot of interest. Reverse electrodialysis (RED) has been utilized in the past to turn saline water into electricity. NRED, a reverse electrodialysis method utilizing nanofluidics, has gained popularity as nanoscale research advances. Developing and evaluating NRED systems is time-consuming and expensive due to the method's novelty; thus, modeling is required to identify the best locations for implementation and to comprehend its workings. In this work, we examined the influence of bipolar soft layer and nanochannel geometry on ion transfer and power production simultaneously. To achieve this, the two trumpet and cigarette geometries were coated with a bipolar soft layer so that both negative (type (I)) and positive (type (II)) charges could be positioned in the nanochannel's small aperture. After that, at steady state conditions, the Poisson-Nernst-Planck (PNP) and Navier-Stokes (NS) equations were solved concurrently. The findings revealed that altering the nanochannel coating from type (I) to type (II) alters the channel's selectivity from cations to anions. An approximately 22-fold improvement in energy conversion efficiency was achieved by raising the concentration ratio from 10 to 100 for the type (I) trumpet nanochannel. Type (I) cigarette geometry is advised for maximum power output at low and medium concentration ratios, whereas type (I) trumpet geometry is recommended for the maximum power production at high concentration ratios.

Fabrication of a Reusable Carbon Dot/Gold Nanoparticle/Metal-Organic Framework Film for Fluorescence Detection of Lead Ions in Water

Solid-state sensing platforms are desirable for the development of reusable sensors to promote public health measures such as testing for drinking water contamination. A bioinspired metal-organic framework (MOF)-based material has been developed by imitating metal-protein interactions in biological systems to attain high sensitivity and selectivity to Pb²⁺ through fluorescence sensing. A zirconium terephthalate-type framework (also known as NH₂-UiO-66) was modified with both gold nanoparticles and thiol-functionalized carbon dots to give HS-C/Au(x)/UiO-66 composites with different Au content (x) and were subsequently adapted into films that show extraordinary sensitivity to Pb²⁺. The HS-C/Au(1.4)/UiO-66 film that consists of 1.4 wt % Au shows a quenching response with the limit of detection of 80 parts per trillion and sustained performance for five cycles. Moreover, the fluorescence response of the HS-C/Au(x)/UiO-66 film to Pb²⁺ can be reversed from emission quenching to enrichment of fluorescence by increasing the Au content. The performance of the HS-C/Au(x)/UiO-66 film as a solid-state sensor demonstrates its potential for application in reusable sensing devices to ensure public safety from Pb²⁺ contamination in drinking water.

New insight into the co-adsorption of oxytetracycline and Pb(II) using magnetic metal-organic frameworks composites in aqueous environment: co-adsorption mechanisms and application potentials

The present study aimed to investigate the co-adsorption and application of water stabilized Fe₃O₄@ZIF-8 composite with magnetic cubic crystal structure. This new material was successfully prepared by facile modification strategy and rational design, which was used for simultaneous adsorption of oxytetracycline (OTC) and Pb(II) in aqueous solution. The co-adsorption behavior and mechanism of the composite for OTC and Pb(II) were systematically investigated by characterization techniques and batch experiments, and its application potential was effectively evaluated. The results showed that the synthesized Fe₃O₄@ZIF-8 composite innovatively retained the cubic crystal structure of ZIF-8 and was successfully loaded on the surface of Fe₃O₄ particles with small particle size to form a core-shell structure. The Fe₃O₄@ZIF-8 composite possessed a large specific surface area (1722 m²/g), magnetic separation performance (13.4 emu/g), and rich functional groups. The co-adsorption of OTC and Pb(II) on Fe₃O₄@ZIF-8 had fast reaction kinetics (equilibrium within 90 min) and large adsorption capacity (310.29 mg/g and 276.06 mg/g respectively). The adsorption process for both contaminants followed pseudo-second order kinetics and Langmuir isotherm models and had synergistic and competitive effects at the same time. π-π stacking and electrostatic interaction were the main mechanisms of adsorption. Fe₃O₄@ZIF-8 had good adsorption performance after cyclic adsorption for 4 times and it performed well in the treatment of real waste water. This study provided a new sight for the control of combined pollution of OTC and Pb(II) and proved Fe₃O₄@ZIF-8 composites have great application potentials for complex wastewater treatment.

Giant Osmotic Energy Conversion through Vertical-Aligned Ion-Permselective Nanochannels in Covalent Organic Framework Membranes

Nanofluidic membranes have been demonstrated as promising candidates for osmotic energy harvesting. However, it remains a long-standing challenge to fabricate high-efficiency ion-permselective membranes with well-defined channel architectures. Here, we demonstrate high-performance osmotic energy conversion membranes based on oriented two-dimensional covalent organic frameworks (COFs) with ultrashort vertically aligned nanofluidic channels that enabled efficient and selective ion transport. Experiments combined with molecular dynamics simulations revealed that exquisite control over channel orientation, charge polarity, and charge density contributed to high ion selectivity and permeability. When applied to osmotic energy conversion, a pair of 100 nm thick oppositely charged COF membranes achieved an ultrahigh output power density of 43.2 W m^-2 at a 50-fold salinity gradient and up to 228.9 W m^-2 for the Dead Sea and river water system. The achieved power density outperforms the state-of-the-art nanofluidic membranes, suggesting the great potential of oriented COF membranes in the fields of advanced membrane technology and energy conversion.

Anomalous thermo-osmotic conversion performance of ionic covalent-organic-framework membranes in response to charge variations

Increasing the charge density of ionic membranes is believed to be beneficial for generating high output osmotic energy. Herein, we systematically investigated how the membrane charge populations affect permselectivity by decoupling their effects from the impact of the pore structure using a multivariate strategy for constructing covalent-organic-framework membranes. The thermo-osmotic energy conversion efficiency is improved by increasing the membrane charge density, affording 210 W m⁻² with a temperature gradient of 40 K. However, this enhancement occurs only within a narrow window, and subsequently, the efficiency plateaued beyond a threshold density (0.04 C m⁻²). The complex interplay between pore-pore interactions in response to charge variations for ion transport across the upscaled nanoporous membranes helps explain the obtained results. This study has far-reaching implications for the rational design of ionic membranes to augment energy extraction rather than intuitively focusing on achieving high densities.

The development of ionic membranes with a high charge population is critical for realizing efficient thermo-osmotic energy conversion. Here, the authors demonstrated that the thermo-osmotic energy conversion efficiency can be improved by increasing the membrane charge density but this enhancement only occurs within a narrow window.

Large-Area Covalent Organic Polymers Membrane via Sol-Gel Approach for Harvesting the Salinity Gradient Energy

Many materials with nanofluidic channels are exploited to achieve salinity gradient energy conversion. However, most materials are fragile, difficult to process, or only prepared into a limited size, which greatly restricts their practical application in the future. Herein, a covalent organic polymers membrane with high mechanical property and stability is fabricated, which can keep integrity in harsh conditions for up to 1 month. In addition, by using the sol-gel approach, a large-area membrane with an area of 26 × 26 cm is expediently fabricated in lab conditions. When the membrane is applied to salinity gradient energy conversion, the maximum output power density is up to 6.21 W m^-2 . This work provides a simple method for the fabrication of large-area membrane for salinity gradient energy conversion in future real-world applications.

Thermo-Osmotic Energy Conversion Enabled by Covalent-Organic-Framework Membranes with Record Output Power Density

A vast amount of energy can be extracted from the untapped low-grade heat from sources below 100 °C and the Gibbs free energy from salinity gradients. Therefore, a process for simultaneous and direct conversion of these energies into electricity using permselective membranes was developed in this study. These membranes screen charges of ion flux driven by the combined salinity and temperature gradients to achieve thermo-osmotic energy conversion. Increasing the charge density in the pore channels enhanced the permselectivity and ion conductance, leading to a larger osmotic voltage and current. A 14-fold increase in power density was achieved by adjusting the ionic site population of covalent organic framework (COF) membranes. The optimal COF membrane was operated under simulated estuary conditions at a temperature difference of 60 K, which yielded a power density of ≈231 W m^-2 , placing it among the best performing upscaled membranes. The developed system can pave the way to the utilization of the enormous supply of untapped osmotic power and low-grade heat energy, indicating the tremendous potential of using COF membranes for energy conversion applications.

Bio-Inspired Salinity-Gradient Power Generation With UiO-66-NH2 Metal-Organic Framework Based Composite Membrane

Salinity-gradient directed osmotic energy between seawater and river water has been widely considered as a promising clean and renewable energy source, as there are numerous river estuaries on our planet. In the past few decades, reverse electrodialysis (RED) technique based on cation-selective membranes has been used as the key strategy to convert osmotic energy into electricity. From this aspect, developing high-efficiency anion-selective membranes will also have great potential for capturing osmotic energy, however, remains systematically unexplored. In nature, electric eels can produce electricity from ionic gradients by using their “sub-nanoscale” protein ion channels to transport ions selectively. Inspired by this, here we developed a UiO-66-NH₂ metal-organic framework (MOF) based anion-selective composite membrane with sub-nanochannels, and achieved high-performance salinity-gradient power generation by mixing artificial seawater (0.5 M NaCl) and river water (0.01 M NaCl). The UiO-66-NH₂ metal-organic framework based composite membranes can be easily and economically fabricated with dense structure and long-term working stability in saline, and its performance of power generation can also be adjusted by pH to enhance the surface charge density of the MOF sub-nanochannels. This study will inspire the exploitation of MOFs for investigating the sub-nanochannel directed high-performance salinity-gradient energy harvesting systems based on anion-selective ion transport.

Ultrafast rectifying counter-directional transport of proton and metal ions in metal-organic framework-based nanochannels

Bioinspired control of ion transport at the subnanoscale has become a major focus in the fields of nanofluidics and membrane separation. It is fundamentally important to achieve rectifying ion-specific transport in artificial ion channels, but it remains a challenge. Here, we report a previously unidentified metal-organic framework nanochannel (MOF NC) nanofluidic system to achieve unidirectional ultrafast counter-directional transport of alkaline metal ions and proton. This highly effective ion-specific rectifying transport behavior is attributed to two distinct mechanisms for metal ions and proton, elucidated by theoretical simulations. Notably, the MOF NC exhibits ultrafast proton conduction stemming from ultrahigh proton mobility, i.e., 11.3 × 10^-7 m² /V·s, and low energy barrier of 0.075 eV in MIL-53-COOH subnanochannels. Furthermore, the MOF NC shows excellent osmotic power-harvesting performance in reverse electrodialysis. This work expects to inspire further research into multifunctional biomimetic ion channels for advanced nanofluidics, biomimetics, and separation applications.

Biomimetic Nanochannels: From Fabrication Principles to Theoretical Insights

Biological nanochannels which can regulate ionic transport across cell membranes intelligently play a significant role in physiological functions. Inspired by these nanochannels, numerous artificial nanochannels have been developed during recent years. The exploration of smart solid-state nanochannels can lay a solid foundation, not only for fundamental studies of biological systems but also practical applications in various fields. The basic fabrication principles, functional materials, and diverse applications based on artificial nanochannels are summarized in this review. In addition, theoretical insights into transport mechanisms and structure-function relationships are discussed. Meanwhile, it is believed that improvements will be made via computer-guided strategy in designing more efficient devices with upgrading accuracy. Finally, some remaining challenges and perspectives for developments in both novel conceptions and technology of this inspiring research field are stated.

Asymmetric Electrokinetic Energy Conversion in Slip Conical Nanopores

Ion current rectification (ICR) phenomena in asymmetric nanofluidic structures, such as conical-shaped nanopores and funnel-shaped nanochannels, have been widely investigated in recent decades. To date, the effect of asymmetric nanofluidic structures on electrokinetic power generation driven by the streaming current/potential has not been explored. Accordingly, this study employed a numerical model based on the Poisson equation, Nernst–Planck equation, and Navier–Stokes equation to investigate the electrokinetic energy conversion (EKEC) in a conical nanopore while considering hydrodynamic slippage. The results indicated that the asymmetric characteristics of streaming current (short-circuit current), streaming potential (open-circuit voltage), maximum power generation, maximum conversion efficiency, and flow rate were observed in conical nanopores under the forward pressure bias (tip-to-base direction) and reverse pressure bias (base-to-tip direction) once the nonequilibrium ion concentration polarization (ICP) became considerable. The rectification behaviors in the streaming current, maximum power, and maximum conversion efficiency were all shown to be opposite to those of the well-known ICR in conical nanopores. In other words, the reverse pressure bias revealed a higher EKEC performance than the forward pressure bias. It was concluded that the asymmetric behavior in EKEC is attributed to the asymmetric electrical resistance resulting from asymmetric ion depletion and ion enrichment. Particularly, it was found that the decrease in electrical resistance (i.e., the change in electrical resistance dominated by the ion enrichment) observed in the reverse pressure bias enhanced the maximum power and maximum conversion efficiency. The asymmetric EKEC characteristics became more significant with increasing slip length, surface charge density, cone angle, and pressure bias, especially at lower salt concentrations. The present findings provide useful information for the future development of EKEC in engineered membranes with asymmetric nanopores.

Light-Regulated Nanofluidic Ionic Diodes with Heterogeneous Channels Stemming from Asymmetric Growth of Metal-Organic Frameworks

Nanofluidic ionic diodes have attracted much attention, because of the unique property of asymmetric ion transport and promising applications in molecular sensing and biosensing. However, it remains a challenge to fabricate diode-like nanofluidic system with molecular-size pores. Herein, we report a new and facile approach to construct nanofluidic ionic diode by in situ asymmetric growth of metal-organic frameworks (MOFs) in nanochannels. We implement microwave-assisted strategy to obtain asymmetric distribution of MOFs in porous anodic aluminum oxide with barrier layer on one side. After etching the barrier layer and modifying with positively charged molecules, the nanofluidic device possesses asymmetric geometry and surface charge, performing the ionic current rectification (ICR) behavior in different electrolyte concentrations. Moreover, the ICR ratio is readily regulated with visible light illumination mainly due to the enhancement of surface charge of MOFs, which is further confirmed by finite element simulation. This study provides a reliable way to build the nanofluidic platform for investigating the asymmetric ion transport through the molecular-size pores, which is envisaged to be important for molecular sensing based on ICR with molecular-size pores.

Macro-Meso-Microporous Metal-Organic Frameworks: Template-Assisted Spray Drying Synthesis and Enhanced Catalysis

Hierarchically porous metal-organic frameworks (HP-MOFs) are promising in many applications. However, most previous studies focus on HP-MOFs with two kinds of pore structures. Herein, a strategy for efficient construction of HP-MOFs possessing macro-meso-micropores using template-assisted spray drying followed by etching process is proposed. Taking ZIF-8 as an example, using polystyrene (PS) templates, the complete HP-ZIF-8 with all the three categories of pores can be easily fabricated. The close arrangement of intrinsic microporous nanosized ZIF-8 (N-ZIF-8) in the spray drying process results in the creation of mesopores, while the macropores are further generated after the removal of PS templates. The structures of macropores and mesopores can be easily adjusted by altering the size and proportion of PS and the size of N-ZIF-8, respectively. Furthermore, this method is extended to the preparation of HP-HKUST-1. As a proof-of-concept, HP-ZIF-8 displays excellent catalytic properties in Knoevenagel reaction owing to its unique pore features. Compared with conventional microsized ZIF-8 (M-ZIF-8) with similar size, HP-ZIF-8 achieves the significantly increased conversion of benzaldehyde from 55% to 100% within 3 h, and shows better recycling performance than N-ZIF-8.

Designing Angstrom-Scale Asymmetric MOF-on-MOF Cavities for High Monovalent Ion Selectivity

Biological ion channels feature angstrom-scale asymmetrical cavity structures, which are the key to achieving highly efficient separation and sensing of alkali metal ions from aqueous resources. The clean energy future and lithium-based energy storage systems heavily rely on highly efficient ionic separations. However, artificial recreation of such a sophisticated biostructure has been technically challenging. Here, a highly tunable design concept is introduced to fabricate monovalent ion-selective membranes with asymmetric sub-nanometer pores in which energy barriers are implanted. The energy barriers act against ionic movements, which hold the target ion while facilitating the transport of competing ions. The membrane consists of bilayer metal-organic frameworks (MOF-on-MOF), possessing a 6 to 3.4-angstrom passable cavity structure. The ionic current measurements exhibit an unprecedented ionic current rectification ratio of above 100 with exceptionally high selectivity ratios of 84 and 80 for K⁺ /Li⁺ and Na⁺ / Li⁺ , respectively (1.14 Li⁺ mol m^-2 h^-1 ). Furthermore, using quantum mechanics/molecular mechanics, it is shown that the combined effect of spatial hindrance and nucleophilic entrapment to induce energy surge baffles is responsible for the membrane's ultrahigh selectivity and ion rectification. This work demonstrates a striking advance in developing monovalent ion-selective channels and has implications in sensing, energy storage, and separation technologies.

Angstrom-scale ion channels towards single-ion selectivity

Artificial ion channels with ion permeability and selectivity comparable to their biological counterparts are highly desired for efficient separation, biosensing, and energy conversion technologies. In the past two decades, both nanoscale and sub-nanoscale ion channels have been successfully fabricated to mimic biological ion channels. Although nanoscale ion channels have achieved intelligent gating and rectification properties, they cannot realize high ion selectivity, especially single-ion selectivity. Artificial angstrom-sized ion channels with narrow pore sizes <1 nm and well-defined pore structures mimicking biological channels have accomplished high ion conductivity and single-ion selectivity. This review comprehensively summarizes the research progress in the rational design and synthesis of artificial subnanometer-sized ion channels with zero-dimensional to three-dimensional pore structures. Then we discuss cation/anion, mono-/di-valent cation, mono-/di-valent anion, and single-ion selectivities of the synthetic ion channels and highlight their potential applications in high-efficiency ion separation, energy conversion, and biological therapeutics. The gaps of single-ion selectivity between artificial and natural channels and the connections between ion selectivity and permeability of synthetic ion channels are covered. Finally, the challenges that need to be addressed in this research field and the perspective of angstrom-scale ion channels are discussed.

Super-assembly of freestanding graphene oxide-aramid fiber membrane with T-mode subnanochannels for sensitive ion transport

Biomimetic nacre-like membranes composed of two-dimensional lamellar sheets and one-dimensional nanofibers exhibit high mechanical strength and excellent stability. Thus, they show substantial application in the field of membrane science and water purification. However, the limited techniques for the assembly of two-dimensional lamellar membranes and one-dimensional nanofibers hamper their development and application. Herein, we developed a nacre-like and freestanding graphene oxide/aramid fiber membrane with abundant T-mode subnanochannels by introducing aramid fibers into graphene oxide interlamination via the super-assembly interaction between graphene oxide and aramid fibers. Benefiting from the presence of stable and adjustable sub-nanometer-size ion transport channels, the graphene oxide/aramid fiber composite membrane exhibited excellent mono/divalent ion selectivity of 3.51 (K⁺/Mg²⁺), which is superior to that of the pure graphene oxide membrane. The experimental results suggest that the mono/divalent ion selectivity is ascribed to the subnanochannels in the graphene oxide/aramid fiber composite membrane, electrostatic repulsion interaction and strong interaction between the divalent metal ion and carboxyl groups. Moreover, the composite membrane exhibited remarkable charge selectivity with a K⁺/Cl^- ratio of up to ∼158, indicating that this graphene oxide/aramid fiber composite membrane has great potential for application in energy conversion. This study provides an avenue to prepare freestanding and nacre-like composite membranes with abundant T-mode ion transport channels for ion recognition and energy conversion, which also shows great application prospects in the field of membrane science and water purification.

Porous Ti3C2Tx MXene Membranes for Highly Efficient Salinity Gradient Energy Harvesting

Extracting osmotic energy through nanoporous membranes is an efficient way to harvest renewable and sustainable energy using the salinity gradient between seawater and river water. Despite recent advances of nanopore-based membranes, which have revitalized the prospect of blue energy, their energy conversion is hampered by nanomembrane issues such as high internal resistance or low selectivity. Herein, we report a lamellar-structured membrane made of nanoporous Ti3C2Tx MXene sheets, exhibiting simultaneous enhancement in permeability and ion selectivity beyond their inherent trade-off. The perforated nanopores formed by facile H2SO4 oxidation of the sheets act as a network of cation channels that interconnects interplanar nanocapillaries throughout the lamellar membrane. The constructed internal nanopores lower the energy barrier for cation passage, thereby boosting the preferential ion diffusion across the membrane. A maximum output power density of the nanoporous Ti3C2Tx MXene membranes reaches up to 17.5 W·m-2 under a 100-fold KCl gradient at neutral pH and room temperature, which is as high as by 38% compared to that of the pristine membrane. The membrane design strategy employing the nanoporous two-dimensional sheets provides a promising approach for ion exchange, osmotic energy extraction, and other nanofluidic applications.

Tailoring biocompatibility of composite scaffolds of collagen/guar gum with metal-organic frameworks

Metal-organic frameworks (MOFs) are microporous materials with high potential for biomedical applications. They are useful as drug delivery systems, antibacterials, and biosensors. Recently, composite materials comprised of polymer matrixes and MOFs have gained relevance in the biomedical field due to their high potential as materials to accelerate wound healing. In this work, we studied the potential applications of composite hydrogels containing MgMOF74, CaMOF74, and Zn(Atz)(Py). The composite hydrogels are biodegradable, being completely degraded after 15 days by the action of collagenase and papain. The composites showed high biocompatibility reaching cell viabilities up to 165.3 ± 8.6% and 112.3 ± 12.8% for porcine fibroblasts and human monocytes, respectively. The composites did not show hemolytic character and they showed antibacterial activity against Escherichia coli reaching up to 84 ± 5% of inhibition compared with amoxicillin (20 ppm). Further, the immunological assays revealed that the composites produce a favorable cell signaling stimulating the secretion of the TGF-β and MCP-1 cytokines and maintaining the secretion of TNF-α in normal levels. Finally, the composites showed potential to be used as controlled drug delivery systems reaching a release efficiency of 30.5 ± 2.5% for ketorolac. Finally, results revealed that ColGG-Zn(Atz)(Py) was the best formulation evaluated.

Pore engineering of biomolecule-based metal–organic framework nanocarriers for improving loading and release of paclitaxel

There has been growing interest in employing metal–organic frameworks (MOFs) incorporated with biomolecules, known as b-MOFs, in biomedical applications.

Nanoarchitectonics of the cathode to improve the reversibility of Li-O2 batteries

The strategic design of the cathode is a critical feature for high-performance and long-lasting reversibility of an energy storage system. In particular, the round-trip efficiency and cycling performance of nonaqueous lithium-oxygen batteries are governed by minimizing the discharge products, such as Li₂O and Li₂O₂. Recently, a metal-organic framework has been directly pyrolyzed into a carbon frame with controllable pore volume and size. Furthermore, selective metallic catalysts can also be obtained by adjusting metal ions for outstanding electrochemical reactions. In this study, various bimetallic zeolitic imidazolate framework (ZIF)-derived carbons were designed by varying the ratio of Zn to Co ions. Moreover, carbon nanotubes (CNTs) are added to improve the electrical conductivity further, ultimately leading to better electrochemical stability in the cathode. As a result, the optimized bimetallic ZIF-carbon/CNT composite exhibits a high discharge capacity of 16,000 mAh·g^-1, with a stable cycling performance of up to 137 cycles. This feature is also beneficial for lowering the overpotential of the cathode during cycling, even at the high current density of 2,000 mA·g^-1.

Nanofluidic osmotic power generators - advanced nanoporous membranes and nanochannels for blue energy harvesting

The increase of energy demand added to the concern for environmental pollution linked to energy generation based on the combustion of fossil fuels has motivated the study and development of new sustainable ways for energy harvesting. Among the different alternatives, the opportunity to generate energy by exploiting the osmotic pressure difference between water sources of different salinities has attracted considerable attention. It is well-known that this objective can be accomplished by employing ion-selective dense membranes. However, so far, the current state of this technology has shown limited performance which hinders its real application. In this context, advanced nanostructured membranes (nanoporous membranes) with high ion flux and selectivity enabling the enhancement of the output power are perceived as a promising strategy to overcome the existing barriers in this technology. While the utilization of nanoporous membranes for osmotic power generation is a relatively new field and therefore, its application for large-scale production is still uncertain, there have been major developments at the laboratory scale in recent years that demonstrate its huge potential. In this review, we introduce a comprehensive analysis of the main fundamental concepts behind osmotic energy generation and how the utilization of nanoporous membranes with tailored ion transport can be a key to the development of high-efficiency blue energy harvesting systems. Also, the document discusses experimental issues related to the different ways to fabricate this new generation of membranes and the different experimental set-ups for the energy-conversion measurements. We highlight the importance of optimizing the experimental variables through the detailed analysis of the influence on the energy capability of geometrical features related to the nanoporous membranes, surface charge density, concentration gradient, temperature, building block integration, and others. Finally, we summarize some representative studies in up-scaled membranes and discuss the main challenges and perspectives of this emerging field.

Bioinspired Graphene Oxide Membranes with pH-Responsive Nanochannels for High-Performance Nanofiltration

Tunable gating graphene oxide (GO) membranes with high water permeance and precise molecular separation remain highly desired in smart nanofiltration devices. Herein, bioinspired by the filtration function of the renal glomerulus, we report a smart and high-performance graphene oxide membrane constructed via introducing positively charged polyethylenimine-grafted GO (GO-PEI) to negatively charged GO nanosheets. It was found that the additional GO-PEI component changed the surface charge, improved the hydrophilicity, and enlarged the nanochannels. The glomerulus-inspired graphene oxide membrane (G-GOM) shows a water permeance up to 88.57 L m^-2 h^-1 bar^-1, corresponding to a 4 times enhancement compared with that of a conventional GO membrane due to the enlarged confined nanochannels. Meanwhile, owing to the electrostatic interaction, it can selectively remove positively charged methylene blue at pH 12 and negatively charged methyl orange at pH 2, with a removal rate of over 96%. The high and cyclic water permeance and highly selective organic removal performance can be attributed to the synergic effect of controlled nanochannel size and tunable electrostatic interaction in responding to the environmental pH. This strategy provides insight into designing pH-responsive gating membranes with tunable selectivity, representing a great advancement in smart nanofiltration with a wide range of applications.

Phase-change mesoporous Prussian blue nanoparticles for loading paclitaxel and chemo-photothermal therapy of cancer

Complete treatment of cancer remains a major challenge today. Herein, a biocompatible drug delivery system named as PCM + PTX@mPBs/PEG was constructed. In this system, Paclitaxel (PTX) was blended with phase-change material (PCM) and loaded in mesoporous Prussian blue nanoparticles (mPBs), and chelated with polyethylene glycol at surface. The blank PCM@mPBs/PEG had uniform particle size distribution, large pore size to load drug, excellent photothermal efficiency and good biocompatibility. After loading PTX, PCM + PTX@mPBs/PEG was demonstrated with a high loading capacity and the drug presented temperature-responsive release characteristics. In addition, PTX can be released under the exposure of an NIR laser. In vitro cell experiments showed that nanoparticles can be exposed to near-infrared irradiation to increase uptake in cells, which enhanced anticancer activity. After tail vein injection of PCM + PTX@mPBs/PEG suspension in tumor-bearing mice, PCM + PTX@mPBs/PEG can accumulate at the tumor site through passive transport. The tumor was effectively suppressed by phototherapy and chemotherapy with few side effects. In summary, compared with photothermal therapy or chemotherapy alone, the prepared PCM + PTX@mPBs/PEG showed synergistic photothermal and chemotherapeutic effects on cancer treatment of mice.