Biomimicking TRPM8: A Conversely Temperature-Dependent Nonionic Retrorse Nanochannel for Ion Flow Control.

Ion channels play a crucial role in the transmembrane transport and signal transmission of substances. In animals, transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential melastatin 8 (TRPM8) serve as temperature-sensing units in sensory nerve endings. TRPV1 allows cells to sense heat, while TRPM8 enables them to detect cold, both serving to protect living organisms from harmful substances and environments. However, almost all studies on artificial nanochannels have mainly focused on TRPV1-like "forward nanochannels" thus far, which are incapable of "backward" responding to heat. So, we constructed an innovational TRPM8-inspired "retrorse nanochannel" through internal modification of poly(acrylamide-co-acrylonitrile) [P(AAm-co-AN)] with an upper critical solution temperature (UCST). Our results demonstrated that the internally modified nanochannels exhibited rapid, stable, and reversible heat-closing capability and converse temperature dependence within the typical temperature range of 25-40 °C. The biomimetic ion channel can effectively function as a facile, precise, and reversible thermal gate for controlling the transport of ions and substances. It also offers a promising microscopic technology for managing thermal effects on the substance, fluid, energy, and even signal delivery.

Gradient Quasi-Solid Electrolyte Enables Selective and Fast Ion Transport for Robust Aqueous Zinc-Ion Batteries.

The quasi-solid electrolytes (QSEs) attract extensive attention due to their improved ion transport properties and high stability, which is synergistically based on tunable functional groups and confined solvent molecules among the polymetric networks. However, the trade-off effect between the polymer content and ionic conductivity exists in QSEs, limiting their rate performance. In this work, the epitaxial polymerization strategy is used to build the gradient hydrogel networks (GHNs) covalently fixed on zinc anode. Then, it is revealed that the asymmetric distribution of negative charges benefits GHNs with fast and selective ionic transport properties, realizing a higher Zn2+ transference number of 0.65 than that (0.52) for homogeneous hydrogel networks (HHNs) with the same polymer content. Meanwhile, the high-density networks formed at Zn/GHNs interface can efficiently immobilize free water molecules and homogenize the Zn2+ flux, greatly inhibiting the water-involved parasitic reactions and dendrite growth. Thus, the GHNs enable dendrite-free stripping/plating over 1000 h at 8 mA cm-2 and 1 mAh cm-2 in a Zn||Zn symmetric cell, as well as the evidently prolonged cycles in various full cells. This work will shed light on asymmetric engineering of ion transport channels in advanced quasi-solid battery systems to achieve high energy and safety.

Skeleton engineering of rigid covalent organic frameworks to alter the number of binding sites for improved radionuclide extraction.

Crystalline porous covalent frameworks (COFs) have been considered as a platform for uranium extraction from seawater and nuclear waste. However, the role of rigid skeleton and atomically precise structures of COFs is often ignored in the design of defined binding configuration. Here, a COF with an optimized relative position of two bidentate ligands realizes full potential in uranium extraction. Compared with the para-chelating groups, the optimized ortho-chelating groups with oriented adjacent phenolic hydroxyl groups on the rigid skeleton endow an additional uranyl binding site, thereby increasing the total number of binding sites up to 150%. Experimental and theoretical results indicate that the uranyl capture is greatly improved via the energetically favored multi-site configuration and the adsorption capacity reaches up to 640 mg g-1, which exceeds that of most reported COF-based adsorbents with chemical coordination mechanism in uranium aqueous solution. This ligand engineering strategy can efficiently advance the fundamental understanding of designing the sorbent systems for extraction and remediation technology.

Construction of a hierarchical membrane with angstrom-scale ion channels for enhanced Li+/Mg2+ separation.

A biomimetic hierarchical membrane consisting of ZIF-8 and MXene with controllable morphology could be fabricated by the facile electrochemical deposition method, well-realizing Li+/Mg2+ sieving. This membrane could work stably in real brine with perm-selectivity of Li+/Mg2+ up to 47.4.

Engineering Multi-field-coupled Synergistic Ion Transport System Based on the Heterogeneous Nanofluidic Membrane for High-Efficient Lithium Extraction.

The global carbon neutrality strategy brings a wave of rechargeable lithium-ion batteries technique development and induces an ever-growing consumption and demand for lithium (Li). Among all the Li exploitation, extracting Li from spent LIBs would be a strategic and perspective approach, especially with the low energy consumption and eco-friendly membrane separation method. However, current membrane separation systems mainly focus on monotonous membrane design and structure optimization, and rarely further consider the coordination of inherent structure and applied external field, resulting in limited ion transport. Here, we propose a heterogeneous nanofluidic membrane as a platform for coupling multi-external fields (i.e., light-induced heat, electrical, and concentration gradient fields) to construct the multi-field-coupled synergistic ion transport system (MSITS) for Li-ion extraction from spent LIBs. The Li flux of the MSITS reaches 367.4 mmol m-2 h-1, even higher than the sum flux of those applied individual fields, reflecting synergistic enhancement for ion transport of the multi-field-coupled effect. Benefiting from the adaptation of membrane structure and multi-external fields, the proposed system exhibits ultrahigh selectivity with a Li+/Co2+ factor of 216,412, outperforming previous reports. MSITS based on nanofluidic membrane proves to be a promising ion transport strategy, as it could accelerate ion transmembrane transport and alleviate the ion concentration polarization effect. This work demonstrated a collaborative system equipped with an optimized membrane for high-efficient Li extraction, providing an expanded strategy to investigate the other membrane-based applications of their common similarities in core concepts.

Tunable Ion Transport in Two-Dimensional Nanofluidic Channels.

Layered two-dimensional (2D) materials with interlayer channels at the nanometer scale offer an ideal platform to control ion transport behaviors, including high-precision separation, ultrafast diffusion, and tunable permeation flux, which show great potential for energy conversion and storage, water treatment, catalysis, biosynthesis, and sensing. Recent advances in controlling the structure and functionality of 2D nanofluidic channels sustainably open doors for more revolutionary applications. In this Perspective, we first present a brief introduction to the fundamental mechanisms for ion transport in 2D nanofluidic channels and an overview of state-of-the-art assembly technologies of nanochannel membranes. We then point out new avenues for developing advanced nanofluidics, combining molecular-level cross-linking, and surface modification in nanoconfinement. Finally, we outline the potential applications of these 2D nanofluidic channel membranes and their technical challenges that need to be addressed to afford for practical applications.

Heterogeneous Electrospinning Nanofiber Membranes with pH-regulated Ion Gating for Tunable Osmotic Power Harvesting.

Biological ion channels existing in organisms are critical for many biological processes. Inspired by biological ion channels, the heterogeneous electrospinning nanofiber membranes (HENM) with functional ion channels are constructed by electrospinning technology. The HENM successfully realizes ion-gating effects, which can be used for tunable energy conversions. Introduction of pyridine and carboxylic acid groups into the HENM plays an important role in generating unique and stable ion transport behaviors, in which gates become alternative states of open and close, responding to symmetric/asymmetric pH stimulations. Then we used the HENM to convert osmotic energy into electric energy which reach a maximum value up to 12.34 W m-2 and the output power density of HENM-based system could be regulated by ion-gating effects. The properties of the HENM provide widespread potentials in application of smart nanofluidic devices, energy conversion, and water treatment.

Surfactant-Assisted Sulfonated Covalent Organic Nanosheets: Extrinsic Charge for Improved Ion Transport and Salinity-Gradient Energy Harvesting.

Charge-governed ion transport is the vital property of nanofluidic channels for salinity-gradient energy harvesting and other electrochemical energy conversion technologies. 2D nanofluidic channels constructed by nanosheets exhibit great superiority in ion selectivity, but a high ion transport rate remains challenging due to the insufficiency of intrinsic surface charge density in nanoconfinement. Herein, extrinsic surface charge into nanofluidic channels composed of surfactant-assisted sulfonated covalent organic nanosheets (SCONs), which enable tunable ion transport behaviors, is demonstrated. The polar moiety of surfactant is embedded in SCONs to adjust in-plane surface charges, and the aggregation of nonpolar moiety results in the sol-to-gel transformation of SCON solution for membrane fabrication. The combination endows SCON/surfactant membranes with considerable water-resistance, and the designable extrinsic charges promise fast ion transport and high ion selectivity. Additionally, the SCON/surfactant membrane, serving as a power generator, exhibits huge potential in harvesting salinity-gradient energy where corresponding output power density can reach up to 9.08 W m-2 under a 50-fold salinity gradient (0.5 m NaCl|0.01 m NaCl). The approach to extrinsic surface charge provides new and promising insight into regulating ion transport behaviors.

Engineering Bio-inspired Self-assembled Nanochannels for Smart Ion Transport.

Highly efficient biological ion channels with sophisticated transport characteristics in living organisms have inspired the design of artificial channels that are functionally comparable to those of their natural counterparts and applicable on a much larger scale. Self-assembly currently offers a facile approach for producing nanoconfined ion channels that exhibit smart ion-transport properties, including ion selectivity, gating, and rectification, and have shown great potential for various applications. In this Minireview, we give an overview of strategies for engineering bio-inspired self-assembled ion channels. We focus on emerging channel assemblies based on different fabrication processes such as supramolecular assembly, nanosystem-based fabrication, and polymer-based integration. The applications of these bio-inspired channels in the exploration of physiological events, detection of molecules/ions, ion separation, and energy conversion are concisely presented. Finally, future developments and challenges of this booming research field are proposed.

Biomimetic KcsA channels with ultra-selective K+ transport for monovalent ion sieving.

Ultra-selective and fast transport of K+ are of significance for water desalination, energy conversion, and separation processes, but current bottleneck of achieving high-efficiency and exquisite transport is attributed to the competition from ions of similar dimensions and same valence through nanochannel communities. Here, inspired by biological KcsA channels, we report biomimetic charged porous subnanometer cages that enable ultra-selective K+ transport. For nanometer to subnanometer scales, conically structured double-helix columns exhibit typical asymmetric transport behaviors and conduct rapid K+ with a transport rate of 94.4 mmol m-2 h-1, resulting in the K+/Li+ and K+/Na+ selectivity ratios of 363 and 31, respectively. Experiments and simulations indicate that these results stem from the synergistic effects of cation-π and electrostatic interactions, which impose a higher energy barrier for Li+ and Na+ and lead to selective K+ transport. Our findings provide an effective methodology for creating in vitro biomimetic devices with high-performance K+ ion sieving.

Covalent organic frameworks embedded in polystyrene membranes for ion sieving.

A mixed matrix membrane composed of COF-300 and polystyrene (PS) with controllable thickness and porosity achieves ion sieving performance, which is dependent on the regular pore size and surface functional groups of COF-300. Hence, the selectivity of the COF-300/PS membrane for K+/Li+ and Mg2+/Li+ reached 31.5 and 14.7, respectively.

Ionic Crosslinking-Induced Nanochannels: Nanophase Separation for Ion Transport Promotion.

Charge-governed ion transport is crucial to numerous industries, and the advanced membrane is the essential component. In nature, the efficient and selective ion transport is mainly governed by the charged ion channels located in cell membrane, indicating the architecture with functional differentiation. Inspired by this architecture, a membrane by ionic crosslinking sulfonated poly(arylene ether ketone) and imidazolium-functionalized poly(arylene ether sulfone) is designed and fabricated to make full use of the charges. This ionic crosslinking is designed to realize nanophase separation to aggregate the ion pathways in the membrane, which results in excellent ion selectivity and high ion conductivity. With the excellent ion transport behavior, ionic crosslinking membrane shows great potential in osmotic energy conversion, which maximum power density can be up to 16.72 W m-2 . This design of ionic crosslinking-induced nanophase separation offers a roadmap for ion transport promotion.

Ultra-Sensitive and Selective Electrochemical Bio-Fluid Biopsy for Oral Cancer Screening.

The early diagnosis of recurrence and metastasis is critically important for decreasing the morbidity and mortality associated with oral cancers. Although liquid biopsy methods hold great promise that provide a successive "time-slice" profile of primary and metastatic oral cancer, the development of non-invasive, rapid, simple, and cost-effective liquid biopsy techniques remains challenging. In this study, an ultrasensitive and selective electrochemical liquid biopsy is developed for oral cancer screening based on tracking trace amounts of cancer biomarker by functionalized asymmetric nano-channels. Detection via antigen-antibody reactions is assayed by evaluating changes in ionic current. Upon the recognition of cancer biomarker antigens in bio-fluids, the inner wall of nano-channel immobilized with the corresponding antibodies undergoes molecular conformation transformation and surface physicochemical changes, which significantly regulate the ion transport through the nano-channel and help achieve sensitivity with a detection limit of 10-12 g mL-1 . Furthermore, owing to the specificity of the monoclonal antibody for the antigen, the nano-channel exhibits high selectivity for the biomarker than for structurally similar biological molecules present in bio-fluids. The effectiveness of this technique is confirmed through the diagnosis of clinical cases of oral squamous cell carcinoma. This study presents a novel diagnostic tool for oral cancer detection in bio-fluids.

Two-Dimensional Nanofluidic Membranes toward Harvesting Salinity Gradient Power.

The salinity gradient between seawater and river water has been identified as a promising, clean, renewable, and sustainable energy source that can be converted into electricity using ion-selective membranes in a reverse electrodialysis (RED) configuration. However, the major hindrance to current salinity gradient power (SGP) conversion is its poor energy efficiency due to the use of low-performance membrane processes, which affords power for neither miniaturized devices nor industrial-level applications. Nanofluidics, which combines strong confinement and surface charge effects at the nanoscale, contributes to novel transport properties, including excellent ion selectivity and high ion throughput; thus, nanofluidics may lead to technological breakthroughs and act as an emerging platform for harnessing SGP. Recently, two-dimensional (2D) materials have provided impressive energy extraction performance and further insight into fundamental transport mechanisms and theoretical feasibility. To reach the commercialization benchmark and real-world applications, an array of nanopores and channels that can be scaled up to industrial sizes is in high demand; additionally, it remains challenging to develop macroscale nanofluidic membranes that meet the "selectivity versus throughput" dual requirement. In the first section, we start with our understanding of the underlying mechanism of ion-channel interactions and transport characteristics in nanofluidic channel systems from the microscale to the macroscale. We review our recent efforts in this field by constructing a heterojunction with asymmetric ion transport behavior that generates rectification of the ion flux and creates an osmotic diode, which is composed of two nanofluidic layers with opposite polar charges and different chemical compositions. Another efficient way to improve the performance of the system is introducing charged functional materials intercalated into laminar 2D nanosheets. The intercalated nanofluidic material can be explained by two classical models to account for the synergistic effects that (i) improve the stability and mechanical properties of 2D materials with a fixed interlayer spacing and (ii) provide space charge for modulating ion diffusion; both of these effects contribute to its considerable energy conversion performance. Further, layer-by-layer membranes are superior to traditional membranes consisting of a simple stack because they retain their repulsion effect toward co-ions, largely strengthening the efficiency of ion separation and conversion. In particular, we highlight our views on the role of the 2D phase structure (e.g., semiconductor 2H phase and metallic 1T phase) in which the two phases differ from each other in physical and chemical properties, including ionic conductance, surface charge, and wetting, thereby presenting a state-of-the-art avenue for controlling ion transport. In view of the nature of 2D materials, we also report improved osmotic energy harvesting by exploiting the photoinduced heat gradient and electrons that increase ion mobility and surface charge, respectively. Finally, we point out specific research topics in which a combined project can certainly come into the limelight. For example, we discuss the combination of SGP with desalination systems and water splitting. We expect that this Account will stimulate further efforts toward functionalized 2D nanoporous materials and facilitate interdisciplinary efforts in chemistry, material engineering, environmental science, and nanotechnology.

Biomimetic Nanocomposite Membranes with Ultrahigh Ion Selectivity for Osmotic Power Conversion.

Ion transport in nanoconfinement exhibits significant features such as ionic rectification, ionic selectivity, and ionic gating properties, leading to the potential applications in desalination, water treatment, and energy conversion. Two-dimensional nanofluidics provide platforms to utilize this phenomenon for capturing osmotic energy. However, it is challenging to further improve the power output with inadequate charge density. Here we demonstrate a feasible strategy by employing Kevlar nanofiber as space charge donor and cross-linker to fabricate graphene oxide composite membranes. The coupling of space charge and surface charge, enabled by the stabilization of interlayer spacing, plays a key role in realizing high ion selectivity and the derived high-performance osmotic power conversion up to 5.06 W/m2. Furthermore, the output voltage of an ensemble of the membranes in series could reach 1.61 V, which can power electronic devices. The system contributes a further step toward the application of energy conversion.

Metallic Two-Dimensional MoS2 Composites as High-Performance Osmotic Energy Conversion Membranes.

Molybdenum disulfide (MoS2) has shown large promise in harvesting osmotic energy. However, the current investigations generally focus on proof-of-concept nanoscale single-pore devices with a semiconductor phase structure. Exploration of the application viability of MoS2 in a more robust macroscopic-scale two-dimensional (2D) nanofluidic membrane and acquisition of fundamentals of how the phase structure influences the power generation process are highly demanded. Here, we demonstrate that robust and stable composite membranes made up of 2D metallic MoS2 can act as high-performance osmotic power generators. Both experiment and simulation reveal that the higher electron density of metallic MoS2 increases the affinity of cations to the surface, which renders the system excellent ion selectivity and high ionic flux and greatly promotes transmembrane ion diffusion. When natural river water and seawater are mixed, the power density can achieve about 6.7 W m-2. This work shows the great potential of metallic MoS2 in nanofluidic energy devices.

Improved Ion Transport in Hydrogel-Based Nanofluidics for Osmotic Energy Conversion.

In nature, ultrafast signal transfer based on ion transport, which is the foundation of biological processes, commonly works in a hydrogel-water mixed mechanism. Inspired by organisms' hydrogel-based system, we introduce hydrogel into nanofluidics to prepare a hydrogel hybrid membrane. The introduction of a space charged hydrogel improves the ion selectivity evidently. Also, a power generator based on the hydrogel hybrid membrane shows an excellent energy conversion property; a maximum power density up to 11.72 W/m2 is achieved at a 500-fold salinity gradient. Furthermore, the membrane shows excellent mechanical properties. These values are achievable, which indicates our membrane's huge potential applications in osmotic energy conversion.

Biomimetic Nacre-Like Silk-Crosslinked Membranes for Osmotic Energy Harvesting.

As an approach to harvesting sustainable energy from ambient conditions, the osmotic energy between river water and seawater contributes to solving global issues such as the energy shortage and environmental pollution. Current attempts based on a reverse electrodialysis technique are limited mainly due to the economically unviable power density and inadequate mass transportation of membrane materials. Here, we demonstrate a benign strategy for designing a multilayer graphene oxide-silk nanofiber-graphene oxide biomimetic nacre-like sandwich as an osmotic power generator. Enhanced interfacial bonding endows the composite membranes with long-term stability in saline, and meanwhile, the two-dimensional nanofluidic channel configuration also reduces the ion transport resistance and provides large storage spaces for ions. Thus, the output power density of the proposed membrane-based generator achieves a value of up to 5.07 W m-2 by mixing seawater and river water. Furthermore, we experimentally and theoretically demonstrate that the thermal-field drives the increased output power density due to the advances in ionic movement range and activity of electrode reaction, showing the promise of strengthened thermo-osmotic energy conversion.

Tailoring A Poly(ether sulfone) Bipolar Membrane: Osmotic-Energy Generator with High Power Density.

Osmotic energy, obtained through different concentrations of salt solutions, is recognized as a form of a sustainable energy source. In the past years, membranes derived from asymmetric aromatic compounds have attracted attention because of their low cost and high performance in osmotic energy conversion. The membrane formation process, charging state, functional groups, membrane thickness, and the ion-exchange capacity of the membrane could affect the power generation performance. Among asymmetric membranes, a bipolar membrane could largely promote the ion transport. Here, two polymers with the same poly(ether sulfone) main chain but opposite charges were synthesized to prepare bipolar membranes by a nonsolvent-induced phase separation (NIPS) and spin-coating (SC) method. The maximum power density of the bipolar membrane reaches about 6.2 W m-2 under a 50-fold salinity gradient, and this result can serve as a reference for the design of bipolar membranes for osmotic energy conversion systems.

Robust sulfonated poly (ether ether ketone) nanochannels for high-performance osmotic energy conversion.

The membrane-based reverse electrodialysis (RED) technique has a fundamental role in harvesting clean and sustainable osmotic energy existing in the salinity gradient. However, the current designs of membranes cannot cope with the high output power density and robustness. Here, we construct a sulfonated poly (ether ether ketone) (SPEEK) nanochannel membrane with numerous nanochannels for a membrane-based osmotic power generator. The parallel nanochannels with high space charges show excellent cation-selectivity, which could further be improved by adjusting the length and charge density of nanochannels. Based on numerical simulation, the system with space charge shows better conductivity and selectivity than those of a surface-charged nanochannel. The output power density of our proposed membrane-based device reaches up to 5.8 W/m2 by mixing artificial seawater and river water. Additionally, the SPEEK membranes exhibit good mechanical properties, endowing the possibility of creating a high-endurance scale-up membrane-based generator system. We believe that this work provides useful insights into material design and fluid transport for the power generator in osmotic energy conversion.