Ion transport in nanofluidics under external fields

Interfacial Mesochannels as Cation Pump for Enhanced Osmotic Energy Harvesting

Membranes integrating 1D materials are rapidly emerging as highly promising platforms for osmotic energy harvesting. However, their power output is often constrained by insufficient ion selectivity. Herein, we demonstrate a cation pumping strategy by designing mesoporous silica coated multiwalled carbon nanotubes/aramid nanofiber (MCNTs@mSiO2/ANF) composite membranes as osmotic power generators. Cations can be initially enriched in the negatively charged and small-pore-sized (∼ 3 nm) interfacial mesopore channels, establishing a strong cation concentration gradient toward the interfiber nanochannels. The gradient continuously drives cations into the interfiber pores, facilitating charge separation, and improving ion selectivity. Additionally, the hydrophilic nature of the mesoporous silica shells promotes ion transport and contributes to high ion flux. Consequently, the fabricated MCNTs@mSiO2/ANF composite nanochannel membranes can deliver a notable power density of 8.24 W m-2 with an excellent ion selectivity of 0.91 under a 50-fold NaCl salinity gradient. Importantly, the membranes demonstrate long-term stability for osmotic energy capturing. When placed between natural seawater and river water, the composite membranes yield an impressive power density of 9.93 W m-2, surpassing that of the state-of-the-art 1D material-based membranes. This work paves the way for the practical applications of nanofiber-based membranes in sustainable osmotic energy conversion.

Clay-Based Nanofluidic Membrane with Enhanced Space Charge for Robust Osmotic Energy Harvesting

Converting the salinity gradient energy into electric energy through permselective membranes has great potential to alleviate the energy crisis. However, the competition between selectivity and permeability, along with the instability of traditional permselective membranes, limits their realistic applications. Herein, a robust clay-based nanofluidic membrane of aramid nanofiber@palygorskite/anodic alumina oxide (ANF@PAL/AAO) with a 3D interworking network has been fabricated for efficient osmotic energy harvesting. The 3D interconnected nanochannels stacked by needle-like PAL provide more and shorter paths for ion transport, thereby increasing the permeability. Moreover, the collaboration between the surface charge of PAL and the space charge brought by ANFs improves ion selectivity, further enhancing the energy conversion performance. Results show that the as-prepared ANF@PAL/AAO membrane displays a power output of 45 W m-2 at 500-fold NaCl gradient and can withstand acidity/alkalinity and high salinity environments. The present work paves a facile way for the application of clay-based nanofluidic devices in practical energy conversion.

Concentration Energy Ion Channels with Molecular-Structure Dual Recognition for Sustainable Environmental Monitoring

Pesticides are vital for crop and seafood production but leave persistent residues that pose risks to ecosystems and human health through bioaccumulation. The detection of pesticide residues requires highly sensitive and selective technologies. Herein, a nanochannel sensor capable of dual recognition of ionic charge and molecular conformation based on molecular imprinting technology (MIT) is presented, offering a significant improvement in selectivity and sensitivity over traditional nanopore sensors. The MIT-based nanochannels with imprinting sites tailored to pesticide molecules go beyond recognizing the molecular size and surface functional groups, enabling the detection of molecular configurations. In this research, the approach enables the detection of 10 pesticide molecules with detection limits (LODs) ranging from 12.9 to 26.9 pM, achieving two orders of magnitude lower than fluorescence-based methods. Density functional theory (DFT) and molecular dynamics (MD) simulations revealed hydrogen bonding as the dominant interaction in the imprinting process. This versatile nanochannel construction method, proposed for the first time, provides dual recognition capabilities and is expected to advance nanochannel sensing while promoting sustainable environmental development.

Artificial Light-Driven Ion Pumps

Nature's molecular machinery has long provided inspiration for the development of functional materials, with natural ion pumps exemplifying the efficient conversion of solar energy into directional ion transport. This process is crucial for cellular signaling, bioenergy conversion, and photosynthesis. Motivated by these biological systems, artificial light-driven ion pumps have emerged as transformative technologies for sustainable energy harvesting, desalination, and bioelectronic innovations. This review categorizes synthetic light-driven ion pumps into two primary mechanistic paradigms: (1) photoelectric-driven transport, which leverages photoinduced charge separation in semiconductor structures, and (2) molecular phototransduction, which utilizes light-induced isomerization or conformational changes in photoactive molecules. For each paradigm, we trace their biomimetic origins to natural ion transport mechanisms, followed by a detailed analysis of design strategies, operational principles, and material innovations. These innovations range from dynamic photoresponsive molecules and semiconductors to semiconductor heterostructures, all of which enable precise control over ion selectivity, flux, and energy conversion in a spatiotemporal manner. Finally, we discuss the emerging applications of light-driven ion pumps and the remaining challenges for their practical implementation.

Chemical Functional Groups Regulate Ion Concentrations and pHs in Nanopores

Understanding ion behaviors in functionalized nanopores is essential to deciphering reactions in both natural and engineered systems, such as sediments, biological ion channels, and membranes. While many efforts have shown the modified ion behaviors in the functionalized nanopores, a direct measurement and analysis to show how chemical functional groups affect ion concentrations in nanopores are critically needed. Herein, we present a plasmonic nanosensor that can measure the local concentrations of protons, anions (phosphate, nitrate, sulfate, and arsenate), and cations (mercury, lead, and copper) in functionalized nanopores, and we compare their concentrations in nanopores with the corresponding bulk concentrations. Notably, chemical functional groups induced ion concentrations differently in nanopores. In pristine nanopores and methyl- and phenyl-functionalized nanopores, we discovered an unexpected concurrence of an enhanced anion concentration and a suppressed cation concentration. In addition, the nanopore pH is dependent on bulk solution compositions and can be lower by 2.5 units, even when the bulk solution is well-buffered. In contrast, for hydrophilic (amine, thiol, and carboxyl) nanopores, pH depended on the pKa of the functional groups, and the heavy metal concentrations depended on chemical interactions with the functional groups. Our findings provide a better understanding of water chemistry in nanopores and can help precisely control ions in nanopores to benefit the design of membrane-based desalination techniques, CO2 storage, and porous catalysts.

Vacancy engineering in tungsten oxide nanofluidic membranes for high-efficiency light-driven ion transport

Bioinspired light-driven ion transport has shown great potential in solar energy harvesting. To achieve efficiencies comparable to biological counterparts, effective coregulation of permselectivity and photoresponsivity is crucial. Herein, vacancy engineering has been proven to be a powerful strategy for considerably increasing the efficiency of light-driven ion transport in tungsten oxide (WO3-x) nanofluidic membranes by enhancing the negative surface charges and narrowing bandgaps. The enhancement in light-driven ion transport can be attributed to the efficient redistribution of surface charges due to the effective separation of photogenerated carriers. At an optimized vacancy concentration, WO2.66 membrane (WO2.66M) delivers an ionic photocurrent of 0.8 μA cm-2 in a 10-4 M KCl electrolyte, which is four times higher than that generated by the original WO2.85 membrane (WO2.85M). Following this strategy, uphill ion transport and photoenhanced osmotic energy conversion are successfully achieved in the WO3-x nanofluidic membrane system. This study shows that atomic vacancy engineering is an efficient approach to increase the light-driven ion transport dynamics of nanofluidics, providing an efficient strategy to enhance light-driven ion transport for potential applications in power harvesting and ion separation.

Photothermal-Enhanced Ion Transport in Robust 2D Hybrid Nanofluidic Membranes for Osmotic Energy Conversion

Multifunctional 2D membranes with interstitial nanofluidic channels are of great significance for controllable ion transport and osmotic energy conversion. Herein, the robust photothermal-responsive 2D hybrid membranes based on the near-parallel laminar stacking of black phosphorus (BP) and montmorillonite (MMT) nanosheets reinforced by cellulose nanofibers (CNF) are developed. The resultant hybrid membrane exhibits cationic selectivity and surface-charge-governed ion transport properties. The photothermal effect of BP nanosheets increases the surface temperature of the hybrid membrane under illumination, which contributes to enhanced ion transport. This photothermal-enhanced ion transport boosts the maximum power density of osmotic energy conversion from 4.84 to 5.31 W·m-2 by 9.7% at a 50-fold concentration gradient under 400 mW·cm-2 simulated sunlight. This work reveals the integration of the photothermal effect of BP nanosheets in 2D nanofluidic membranes, providing a possible route to enhance the osmotic energy conversion performance by renewable light energy.

A Mechanically Robust, Extreme Environment-Stable, and Fast Ion Transport Nanofluidic Fiber

Constructing mechanically strong and environmentally stable nanofluidic fibers with excellent ion transport remains a challenge. Herein, we design a mechanically robust and stable aramid nanofiber/carboxylated aramid nanofiber (ANF/cANF) hybrid nanofluidic fiber with a high ionic conductivity via a wet spinning-induced orientation strategy. Benefiting from the oriented structure and strong interfacial interactions of the filaments, the ANF/cANF nanofluidic fiber exhibits a high tensile strength of 276.8 MPa. Carboxylation and oriented nanochannels dramatically reduce the charge transfer resistance, resulting in a high ionic conductivity. As a result, the ANF/cANF nanofluidic fiber obtains a 5-fold increase in ionic conductivity compared to that of the disordered fiber. Notably, the nanofluidic fiber maintains its structural integrity and mechanical properties after 90 days of immersion in water. Additionally, it retains its favorable surface-charge-dominated ion transport capabilities even under extreme conditions, including exposure to acids, alkalis, and ethanol, as well as after treatments at high (150 °C) and low (-196 °C) temperatures.

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.

Dual Ionic Signal Detection: Modulation of Surface Charge of Nanofluidic Iontronics by Dual-Split Gate Voltages

Nanofluidic iontronics, including the field-effect ionic diode (FE-ID) and field-effect ionic transistor (FE-IT), represent emerging nanofluidic logic devices that have been employed in sensitive analyses. Making analyte recognitions in predefined nanofluidic devices has been verified to improve the sensitivity and selectivity using a single ionic signal, such as ionic current amplification, rectification, and Coulomb blockade. However, the detection of analytes in complex systems generally necessitates more diverse signals beyond just ionic currents. Here, we demonstrated that dual ionic signals, steady ionic switching ratio, and transient response time (ts) act as detection signals modulated by dual-split gate voltages along the nanochannel for the detection of charged analytes. With an increase in gate voltage, the switching ratio decreases in both FE-ID and FE-IT, whereas the response time exhibits an exponential increase specifically in the FE-ID. Moreover, the response time shows no significant correlation with the external transmembrane voltage in the FE-IT. These results contribute to the optimization of reconfigurable iontronics through gate voltage modulation, providing a theoretical foundation for multiple ionic signal detection.

Characterization of Diffusioosmotic Ion Transport for Enhanced Concentration-Driven Power Generation via Charge Heterogeneity in Nanoporous Membranes

Nanoscopic mass/ion transport through heterogeneous nanostructures with various physicochemical environments occurs in both natural and artificial systems. Concentration gradient-driven mass/ion transport mechanisms, such as diffusioosmosis (DO), are primarily governed by the structural and electrical features of the nanostructures. However, these phenomena under various electrical and chemical conditions have not been adequately investigated. In this study, we fabricated a pervaporation-based particle-assembled membrane (PAM)-integrated micro-/nanofluidic device that facilitates easy tuning of the surface charge heterogeneity in nanopores/nanochannels. The nanochannels in the device consisted of two heterogeneous and in-series PAMs. The device was used to quantitatively measure electric signals generated by DO within the nanochannels with a single electrolyte or a combination of two electrolytes. Then, we characterized ion transport by changing surface charge heterogeneity and applying various electrolytic conditions, characterizing the concentration-driven power generation under these conditions. We found that not only does the charge heterogeneity provide additional resistance to ion transport but also the manipulation of the heterogeneity enables the effective modulation of ion transport and optimization of concentration-driven power generators regarding ion selectivity. In conjunction with the surface charge heterogeneity, the electrolytic conditions significantly affected the net flux of ion transport by enhancing or even negating the ion selectivity. Hence, we anticipate that both the platform and results will provide a deeper understanding of ion transport in nanostructures within complex environments by optimizing and improving practical concentration-driven applications, such as energy conversion/harvesting, molecular focusing/separation, and ionic diodes and memristors.

Efficient Light-Driven Ion Pumping for Deep Desalination via the Vertical Gradient Protonation of Covalent Organic Framework Membranes

Traditional desalination methods face criticism due to high energy requirements and inadequate trace ion removal, whereas natural light-driven ion pumps offer superior efficiency. Current synthetic systems are constrained by short exciton lifetimes, which limit their ability to generate sufficient electric fields for effective ion pumping. We introduce an innovative approach utilizing covalent-organic framework membranes that enhance light absorption and reduce charge recombination through vertical gradient protonation of imine linkages during acid-catalyzed liquid-liquid interfacial polymerization. This technique creates intralayer and interlayer heterojunctions, facilitating interlayer hybridization and establishing a robust built-in electric field under illumination. These improvements enable the membranes to achieve remarkable ion transport across extreme concentration gradients (2000:1), with a transport rate of approximately 3.2 × 1012 ions per second per square centimeter and reduce ion concentrations to parts per billion. This performance significantly surpasses that of conventional reverse osmosis systems, representing a major advancement in solar-powered desalination technology by substantially reducing energy consumption and secondary waste.

The Role of Photobiomodulation to Modulate Ion Channels in the Nervous System: A Systematic Review

Photobiomodulation (PBM) is a safe and effective neurotherapy that modulates cellular pathways by altering cell membrane potentials, leading to beneficial biological effects such as anti-inflammatory and neuroregenerative responses. This review compiles studies from PubMed up to March 2024, investigating the impact of light at wavelengths ranging from 620 to 1270 nm on ion channels. Out of 330 articles screened, 19 met the inclusion criteria. Research indicates that PBM can directly affect various ion channels by influencing neurotransmitter synthesis in neighboring cells, impacting receptors like glutamate and acetylcholine, as well as potassium, sodium channels, and transient receptor potential channels. The diversity of studies hampers a comprehensive meta-analysis for evaluating treatment strategies effectively. This systematic review aims to explore the potential role of optoelectronic signal transduction in PBM, studying the neurobiological mechanisms and therapeutic significance of PBM on ion channels. However, the lack of uniformity in current treatment methods underscores the necessity of establishing standardized and reliable therapeutic approaches.

Aptamer-Conjugated Covalent-Organic Framework Nanochannels for Selective and Sensitive Detection of Aflatoxin B1

Sensitive and selective detection of trace aflatoxin B1 (AFB1) in foods is of great importance to guarantee food safety and quality but still challenging because of its trace amount and the interference from the complex food matrix. Here, we report the integration of aptamer (Apt) and an ordered 2D covalent organic framework (COF) to solid-state anodic aluminum oxide (AAO) nanochannels (Apt/COF/AAO) for selective and sensitive detection of trace AFB1. The high specificity of Apt for AFB1 led to a selective change in the surface charge of Apt/COF/AAO and in turn the current change of the nanochannel, permitting the selective and sensitive determination of trace AFB1 in complex food samples. The developed nanofluidic sensor gave a wide linear range (1-500 pg mL-1), low detection limit (0.11 pg mL-1), and good precision (relative standard deviation of 1.5% for 11 replicate determinations of 100 pg mL-1). In addition, the developed sensor was successfully used for the detection of AFB1 in food samples with the recovery of 86.9%-102.5%. The coupling of Apt-conjugated 2D COF with an AAO nanochannel provides a promising way for sensitive and selective determination of food contaminants in complex samples.

Illuminating Biomimetic Nanochannels: Unveiling Macroscopic Anticounterfeiting and Photoswitchable Ion Conductivity via Polymer Tailoring

Artificial photomodulated channels represent a significant advancement toward practical photogated systems because of their remote noncontact stimulation. Ion transport behaviors in artificial photomodulated channels, however, still require further investigation, especially in multiple nanochannels that closely resemble biological structures. Herein, we present the design and development of photoswitchable ion nanochannels inspired by natural channelrhodopsins (ChRs), utilizing photoresponsive polymers grafted anodic aluminum oxide (AAO) membranes. Our approach integrates spiropyran (SP) as photoresponsive molecules into nanochannels through surface-initiated atom transfer radical polymerization (SI-ATRP), creating a responsive system that modulates ionic conductivity and hydrophilicity in response to light stimuli. A key design feature is the reversible ring-opening photoisomerization of spiropyran groups under UV irradiation. This transformation, observable at the molecular level and macroscopically, allows the surface inside the nanochannels to switch between hydrophobic and hydrophilic states, thus efficiently modulating ion transport via changing water wetting behaviors. The patternable and erasable polySP-grafted AAO, based on a controllable and reversible photochromic effect, also shows potential applications in anticounterfeiting. This study pioneers achieving macroscopic anticounterfeiting and photoinduced photoswitching through reversible surface chemistry and expands the application of polymer-grafted structures in multiple nanochannels.

Giant gateable thermoelectric conversion by tuning the ion linkage interactions in covalent organic framework membranes

Efficient energy conversion using ions as carriers necessitates membranes that sustain high permselectivity in high salinity conditions, which presents a significant challenge. This study addresses the issue by manipulating the linkages in covalent-organic-framework membranes, altering the distribution of electrostatic potentials and thereby influencing the short-range interactions between ions and membranes. We show that a charge-neutral covalent-organic-framework membrane with β-ketoenamine linkages achieves record permselectivity in high salinity environments. Additionally, the membrane retains its permselectivity under temperature gradients, providing a method for converting low-grade waste heat into electrical energy. Experiments reveal that with a 3 M KCl solution and a 50 K temperature difference, the membrane generates an output power density of 5.70 W m-2. Furthermore, guided by a short-range ionic screening mechanism, the membrane exhibits adaptable permselectivity, allowing reversible and controllable operations by finely adjusting charge polarity and magnitude on the membrane's channel surfaces via ion adsorption. Notably, treatment with K3PO4 solutions significantly enhances permselectivity, resulting in a giant output power density of 20.22 W m-2, a 3.6-fold increase over the untreated membrane, setting a benchmark for converting low-grade heat into electrical energy.

On-Water Surface Synthesis of Two-Dimensional Polymer Membranes for Sustainable Energy Devices

ConspectusIon-selective membranes are key components for sustainable energy devices, including osmotic power generators, electrolyzers, fuel cells, and batteries. These membranes facilitate the flow of desired ions (permeability) while efficiently blocking unwanted ions (selectivity), which forms the basis for energy conversion and storage technologies. To improve the performance of energy devices, the pursuit of high-quality membranes has garnered substantial interest, which has led to the exploration of numerous candidates, such as polymeric membranes (e.g., polyamide and polyelectrolyte), laminar membranes (e.g., transition metal carbide (MXene) and graphene oxide (GO)) and nanoporous 2D membranes (e.g., single-layer MoS2 and porous graphene). Despite impressive progress, the trade-off effect between ion permeability and selectivity remains a major scientific and technological challenge for these membranes, impeding the efficiency and stability of the resulting energy devices.Two-dimensional polymers (2DPs), which represent monolayer to few-layer covalent organic frameworks (COFs) with periodicity in two directions, have emerged as a new candidate for ion-selective membranes. The crystalline 2DP membranes (2DPMs) are typically fabricated either by bulk crystal exfoliation followed by filtration or by direct interfacial synthesis. Recently, the development of surfactant-monolayer-assisted interfacial synthesis (SMAIS) method by our group has been pivotal, enabling the synthesis of various highly crystalline and large-area 2DPMs with tunable thicknesses (1 to 100 nm) and large crystalline domain sizes (up to 120 μm2). Compared to other membranes, 2DPMs exhibit well-defined one-dimensional (1D) channels, customizable surface charge, ultrahigh porosity, and ultrathin thickness, enabling them to overcome the permeability-selectivity trade-off challenge. Leveraging these attributes, 2DPMs have established their critical roles in diverse energy devices, including osmotic power generators and metal ion batteries, opening the door for next-generation technology aimed at sustainability with a low carbon footprint.In this Account, we review our achievements in synthesizing 2DPMs through the SMAIS method and highlight their selective-ion-transport properties and applications in sustainable energy devices. We initially provide an overview of the SMAIS method for producing highly crystalline 2DPMs by utilizing the programmable assembly and enhanced reactivity/selectivity on the water surface. Subsequently, we discuss the critical structural parameters of 2DPMs, including pore sizes, charged sites, crystallinity, and thickness, to elucidate their roles in selective ion transport. Furthermore, we present the burgeoning landscape of energy device applications for 2DPMs, including their use in osmotic power generators and as electrode coating in metal ion batteries. Finally, we conclude persistent challenges and future prospects encountered in synthetic chemistry, material science, and energy device applications within this rapidly evolving field.