Electrostatic-Charge- and Electric-Field-Induced Smart Gating for Water Transportation

Inkjet-Printed Dielectric Layer for the Enhancement of Electrowetting Display Devices

Electrowetting with a dielectric layer is commonly preferred in practical applications. However, its potential is often limited by factors like the properties of the dielectric layer and its breakdown, along with the complexity of the deposition method. Fortunately, advancements in 3D inkjet printing offer a more adaptable solution for making patterned functional layers. In this study, we used a negative photoresist (HN-1901) to create a new dielectric layer for an electrowetting display on a 3-inch ITO glass using a Dimatix DMP-2580 inkjet printer. The resulting devices performed better due to their enhanced resistance to dielectric breakdown. We meticulously investigated the physical properties of the photoresist material and printer settings to achieve optimal printing. We also controlled the uniformity of the dielectric layer by adjusting ink drop spacing. Compared to traditional electrowetting display devices, those with inkjet-printed dielectric layers showed significantly fewer defects like bubbles and electrode corrosion. They maintained an outstanding response time and breakdown resistance, operating at an open voltage of 20 V. Remarkably, these devices achieved faster response times of ton 22.3 ms and toff 14.2 ms, surpassing the performance of the standard device.

Confined Water Dominates Ion/Molecule Transport in Hydrogel Nanochannels

Current artificial nanochannels rely more on charge interactions for intelligent mass transport. Nevertheless, popular charged nanochannels would lose their advantages in long-term applications. Confined water, an indispensable transport medium in biological nanochannels, dominating the transport process in the uncharged nanochannels perfectly provides a new perspective. Herein, we achieve confined-water-dominated mass transport in hydrogel nanochannels (HNCs) constructed by in situ photopolymerization of acrylic acid (PAA) hydrogel in anodic alumina (AAO) nanochannels. HNCs show selectivity to Na+ transport and a high transport rate of molecules after introducing Na+/Li+, compared with other alkali metal ions like Cs+/K+. The mechanism given by ATR-FTIR shows that the hydrogen-bonding structure of confined water in HNCs is destabilized by Na+/Li+, which facilitates mass transport, but is constrained by Cs+/K+, resulting in transport inhibition. This work elucidates the relationship between confined water and mass transport in uncharged nanochannels while also presenting a strategy for designing functional nanochannel devices.

Gas transport mechanisms through gas-permeable membranes in microfluidics: A perspective

Gas-permeable membranes (GPMs) and membrane-like micro-/nanostructures offer precise control over the transport of liquids, gases, and small molecules on microchips, which has led to the possibility of diverse applications, such as gas sensors, solution concentrators, and mixture separators. With the escalating demand for GPMs in microfluidics, this Perspective article aims to comprehensively categorize the transport mechanisms of gases through GPMs based on the penetrant type and the transport direction. We also provide a comprehensive review of recent advancements in GPM-integrated microfluidic devices, provide an overview of the fundamental mechanisms underlying gas transport through GPMs, and present future perspectives on the integration of GPMs in microfluidics. Furthermore, we address the current challenges associated with GPMs and GPM-integrated microfluidic devices, taking into consideration the intrinsic material properties and capabilities of GPMs. By tackling these challenges head-on, we believe that our perspectives can catalyze innovative advancements and help meet the evolving demands of microfluidic applications.

Interfacial Water-Dictated Oil Adhesion Based on Ion Modulation

Oil adhesion on ionic surfaces is ubiquitous in organisms and natural environments and is generally determined by surface chemical component and texture. However, when adhesion occurs, water molecules at the solid-liquid interface, acting as a bridge not only influenced by the structure and composition of the solid surface but also interacting with the neighboring oil molecules, play a crucial role but are always overlooked. Herein, we investigate the oil adhesion process on a carboxyl-terminated self-assembled monolayer surface (COOH-SAM) in ionic solutions and observe the interfacial water structure via surface-enhanced Raman scattering (SERS) in this system. It is found that the lower the tetracoordinated water content, the stronger the oil adhesion. Compared to monovalent ions, the strengthened binding of multivalent ions to the COOH-SAM surface makes the interfacial water more disordered, which eventually leads to a stronger oil adhesion. Notably, the amount of oil adhesion decreases with an increase in the thickness of the interfacial water region. The interfacial water-dictated oil adhesion has been demonstrated in capillary to simulate the water-driven oil recovery, providing a molecular-level explanation for enhanced oil recovery from low salinity water flooding and also indicating potential applications in intelligent microfluidic and seawater desalination.

Hydrophobic Gating and Spatial Confinement in Hierarchically Organized Block Copolymer-Nanopore Electrode Arrays for Electrochemical Biosensing of 4-Ethyl Phenol

Hydrophobic gating in biological transport proteins is regulated by stimulus-specific switching between filled and empty nanocavities, endowing them with selective mass transport capabilities. Inspired by these, solid-state nanochannels have been integrated into functional materials for a broad range of applications, such as energy conversion, filtration, and nanoelectronics, and here we extend these to electrochemical biosensors coupled to mass transport control elements. Specifically, we report hierarchically organized structures with block copolymers on tyrosinase-modified two-electrode nanopore electrode arrays (BCP@NEAs) as stimulus-controlled electrochemical biosensors for alkylphenols. A polystyrene-b-poly(4-vinyl)pyridine (PS-b-P4VP) membrane placed atop the NEA endows the system with potential-responsive gating properties, where water transport is spatially and temporarily gated through hydrophobic P4VP nanochannels by the application of appropriate potentials. The reversibility of hydrophobic voltage-gating makes it possible to capture and confine analyte species in the attoliter-volume vestibule of cylindrical nanopore electrodes, enabling redox cycling and yielding enhanced currents with amplification factors >100× when operated in a generator-collector mode. The enzyme-coupled sensing capabilities are demonstrated using nonelectroactive 4-ethyl phenol, exploiting the tyrosinase-catalyzed turnover into reversibly redox-active quinones, then using the quinone-catechol redox reaction to achieve ultrasensitive cycling currents in confined BCP@NEA sensors giving a limit-of-detection of ∼120 nM. The mass transport controlled sensing platform described here is relevant to the development of enzyme-coupled multiplex biosensors for sensitive and selective detection of biomarkers and metabolites in next-generation point-of-care devices.

Wetting hysteresis induces effective unidirectional water transport through a fluctuating nanochannel

We propose a water pump that actively transports water molecules through nanochannels. Spatially asymmetric noise fluctuations imposed on the channel radius cause unidirectional water flow without osmotic pressure, which can be attributed to hysteresis in the cyclic transition between the wetting/drying states. We show that the water transport depends on fluctuations, such as white, Brownian, and pink noises. Because of the high-frequency components in white noise, fast switching of open and closed states inhibits channel wetting. Conversely, pink and Brownian noises generate high-pass filtered net flow. Brownian fluctuation leads to a faster water transport rate, whereas pink noise has a higher capability to overcome pressure differences in the opposite direction. A trade-off relationship exists between the resonant frequency of the fluctuation and the flow amplification. The proposed pump can be considered as an analogy for the reversed Carnot cycle, which is the upper limit of the energy conversion efficiency.

Learning from the Brain: Bioinspired Nanofluidics

The human brain completes intelligent behaviors such as the generation, transmission, and storage of neural signals by regulating the ionic conductivity of ion channels in neuron cells, which provides new inspiration for the development of ion-based brain-like intelligence. Against the backdrop of the gradual maturity of neuroscience, computer science, and micronano materials science, bioinspired nanofluidic iontronics, as an emerging interdisciplinary subject that focuses on the regulation of ionic conductivity of nanofluidic systems to realize brain-like functionalities, has attracted the attention of many researchers. This Perspective provides brief background information and the state-of-the-art progress of nanofluidic intelligent systems. Two main categories are included: nanofluidic transistors and nanofluidic memristors. The prospects of nanofluidic iontronics' interdisciplinary progress in future artificial intelligence fields such as neuromorphic computing or brain-computer interfaces are discussed. This Perspective aims to give readers a clear understanding of the concepts and prospects of this emerging interdisciplinary field.

Nonwoven Membranes with Infrared Light-Controlled Permeability

This study presents the development of the first composite nonwoven fiber mats (NWFs) with infrared light-controlled permeability. The membranes were prepared by coating polypropylene NWFs with a photothermal layer of poly(N-isopropylacrylamide) (PNIPAm)-based microgels impregnated with graphene oxide nanoparticles (GONPs). This design enables "photothermal smart-gating" using light dosage as remote control of the membrane's permeability to electrolytes. Upon exposure to infrared light, the GONPs trigger a rapid local increase in temperature, which contracts the PNIPAm-based microgels lodged in the pore space of the NWFs. The contraction of the microgels can be reverted by cooling from the surrounding aqueous environment. The efficient conversion of infrared light into localized heat by GONPs coupled with the phase transition of the microgels above the lower critical solution temperature (LCST) of PNIPAm provide effective control over the effective porosity, and thus the permeability, of the membrane. The material design parameters, namely the monomer composition of the microgels and the GONP-to-microgel ratio, enable tuning the permeability shift in response to IR light; control NWFs coated with GONP-free microgels displayed thermal responsiveness only, whereas native NWFs showed no smart-gating behavior at all. This technology shows potential toward processing temperature-sensitive bioactive ingredients or remote-controlled bioreactors.

Light-responsive nanochannels based on the supramolecular host–guest system

The light-responsive nanochannel of rhodopsin gained wider research interest from its crucial roles in light-induced biological functions, such as visual signal transduction and energy conversion, though its poor stability and susceptibility to inactivation in vitro have limited its exploration. However, the fabrication of artificial nanochannels with the properties of physical stability, controllable structure, and easy functional modification becomes a biomimetic system to study the stimulus-responsive gating properties. Typically, light-responsive molecules of azobenzene (Azo), retinal, and spiropyran were introduced into nanochannels as photo-switches, which can change the inner surface wettability of nanochannels under the influence of light; this ultimately results in the photoresponsive nature of biomimetic nanochannels. Furthermore, the fine-tuning of their stimulus-responsive properties can be achieved through the introduction of host–guest systems generally combined with a non-covalent bond, and the assembling process is reversible. These host–guest systems have been introduced into the nanochannels to form different functions. Based on the host–guest system of light-responsive reversible interaction, it can not only change the internal surface properties of the nanochannel and control the recognition and transmission behaviors but also realize the controlled release of a specific host or guest molecules in the nanochannel. At present, macrocyclic host molecules have been introduced into nanochannels including pillararenes, cyclodextrin (CD), and metal–organic frameworks (MOFs). They are introduced into the nanochannel through chemical modification or host–guest assemble methods. Based on the changes in the light-responsive structure of azobenzene, spiropyran, retinal, and others with macrocycle host molecules, the surface charge and hydrophilic and hydrophobic properties of the nanochannel were changed to regulate the ionic and molecular transport. In this study, the development of photoresponsive host and guest-assembled nanochannel systems from design to application is reviewed, and the research prospects and problems of this photo-responsive nanochannel membrane are presented.

Nanoionics from Biological to Artificial Systems: An Alternative Beyond Nanoelectronics

Ion transport under nanoconfined spaces is a ubiquitous phenomenon in nature and plays an important role in the energy conversion and signal transduction processes of both biological and artificial systems. Unlike the free diffusion in continuum media, anomalous behaviors of ions are often observed in nanostructured systems, which is governed by the complex interplay between various interfacial interactions. Conventionally, nanoionics mainly refers to the study of ion transport in solid-state nanosystems. In this review, to extent this concept is proposed and a new framework to understand the phenomena, mechanism, methodology, and application associated with ion transport at the nanoscale is put forward. Specifically, here nanoionics is summarized into three categories, i.e., biological, artificial, and hybrid, and discussed the characteristics of each system. Compared with nanoelectronics, nanoionics is an emerging research field with many theoretical and practical challenges. With this forward-looking perspective, it is hoped that nanoionics can attract increasing attention and find wide range of applications as nanoelectronics.

Wetting-Induced Water Promoted Flow on Tunable Liquid-Liquid Interface-Based Nanopore Membrane System

Membrane separation provides effective methods for solving the global water crisis. Contemporary membrane systems depend on interfacial interactions between liquid and solid membrane matrixes. However, it may lead to a limiting permeate flux due to the large flow resistance at hydrophobic liquid-solid interfaces. Herein, the liquid-liquid interface with improved interface energy is reversibly introduced in membrane systems to boost wetting and reduce transport resistance. A series of interfaces were systematically explored to reveal mechanisms of wetting and boosted flow performances, which are further supported by simulations. Findings of this study highlight that interfacial liquids with lower surface energies, lower viscosities, and higher solubilities can effectively improve water flow without sacrificing rejection performance, achieving by transforming a solid-liquid interface into liquid-liquid interface interaction. It provides a concept to design advanced membrane systems for water purification (e.g., desalination and oil-water separation) and energy conversion processes.

Liquid Gating Meniscus-Shaped Deformable Magnetoelastic Membranes with Self-Driven Regulation of Gas/Liquid Release

Liquid gating membranes have been demonstrated to show unprecedented properties of dynamicity, stability, adaptivity, and stimulus-responsiveness. Most recently, smart liquid gating membranes have attracted increasing attention to bring some brand-new properties for real-world applications, and various environment-driven systems have been created. Here, a self-driven system of a smart liquid gating membrane is further developed by designing a new sytem based on a liquid gating magnetoelastic porous membrane with reversible meniscus-shaped deformations, and it is not subject to the complex gating liquid restriction of magnetorheological fluids. Compared with other systems, this magnetic-responsive self-driven system has the advantage that it provides a universal and convenient way to realize active regulation of gas/liquid release. Experiments and theoretical calculations demonstrate the stability, the nonfouling behavior, and the tunability of the system. In addition, this system can be used to perfectly open and close gas transport, and the gating pressure threshold for the liquid release can be reduced under the same conditions. Based on the above capabilities, combined with the fast and 3D contactless operation, it will be of benefit in fields ranging from visible gas/liquid mixture content monitoring and energy-saving multiphase separation, remote fluid release, and beyond.

Potential-induced wetting and dewetting in pH-responsive block copolymer membranes for mass transport control

Wetting and dewetting behavior in channel-confined hydrophobic volumes is used in biological membranes to effect selective ion/molecular transport. Artificial biomimetic hydrophobic nanopores have been devised utilizing wetting and dewetting, however, tunable mass transport control utilizing multiple transport modes is required for applications such as controllable release/transport, water separation/purification and energy conversion. Here, we investigate the potential-induced wetting and dewetting behavior in a pH-responsive membrane composed of a polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) when fabricated as a hierarchically-organized sandwich structure on a nanopore electrode array (NEA), i.e. BCP@NEA. At pH < pK_a(P4VP) (pK_a ∼ 4.8), the BCP acts as an anion-exchange membrane due to the hydrophilic, protonated P4VP cylindrical nanodomains, but at pH > pK_a(P4VP), the P4VP domains exhibit charge-neutral, hydrophobic and collapsed structures, blocking mass transport via the hydrophobic membrane. However, when originally prepared in a dewetted condition, mass transport in the BCP membrane may be switched on if sufficiently negative potentials are applied to the BCP@NEA architecture. When the hydrophobic BCP membrane is introduced on top of 2-electrode-embedded nanopore arrays, electrolyte solution in the nanopores is introduced, then isolated, by exploiting the potential-induced wetting and dewetting transitions in the BCP membrane. The potential-induced wetting/dewetting transition and the effect on cyclic voltammetry in the BCP@NEA structures is characterized as a function of the potential, pH and ionic strength. In addition, chronoamperometry and redox cycling experiments are used to further characterize the potential response. The multi-modal mass transport system proposed in this work will be useful for ultrasensitive sensing and single-molecule studies, which require long-time monitoring to explore reaction dynamics as well as molecular heterogeneity in nanoconfined volumes.

Borate-driven ionic rectifiers based on sugar-bearing single nanochannels

Recently, much scientific effort has been centered on the control of the ionic transport properties of solid state nanochannels and the rational design and integration of chemical systems to induce changes in the ionic transport by means of interactions with selected target molecules. Here, we report the fabrication of a novel nanofluidic device based on solid-state nanochannels, which combines silane chemistry with both track-etched and atomic layer deposition (ALD) technologies. Nanodevice construction involves the coating of bullet-shaped single-pore nanochannels with silica (SiO₂) by ALD and subsequent surface modification by reaction between silanol groups exposed on pore walls and N-(3-triethoxysilylpropyl)-gluconamide, in order to create a gluconamide-decorated nanochannel surface. The formation of a boroester derivative resulting from the selective reaction of borate with the appended saccharides leads to important changes in the surface charge density and, concomitantly, in the iontronic properties of the nanochannel. Furthermore, we propose a binding model to rationalize the specific interaction saccharide-borate in the surface. Besides, this unique nanodevice exhibits a highly selective and reversible response towards borate/fructose exposure. On the basis of the surface charge variation resulting from borate binding, the nanochannel can reversibly switch between "ON" and "OFF" states in the presence of borate and fructose, respectively. In addition, this work describes the first report of the functionalization of PET/SiO₂ nanochannels by the ALD technique. We believe that this work provides a promising framework for the development of new nanochannel-based platforms suitable for multiple applications, such as water quality monitoring or directed molecular transport and separation.

Surface Charge Regulated Asymmetric Ion Transport in Nanoconfined Space

The asymmetric ion transport in the nanoconfined space, similar to that of natural ion channels, has attracted broad interest in sensor, energy conversion, and other related fields. Among these systems, the surface charge plays an important role in regulating ion transport behaviors. Herein, this surface charge-regulated asymmetric ion transport behavior is systematically explored in the nanoconfined space and the influence on the performance of nanofluidic energy conversion system. The ion transport behaviors in the nanoconfined space are classified into pure diffusion, electrical double layer, and the polarization controlled state. The asymmetric solution environment or surface charge distribution induces asymmetric ion transport behavior which is largely controlled by the low concentration side. The ion-selectivity and the energy conversion performance can be effectively enhanced by improving the local apparent surface charge (more active sites and higher charge strength) or introducing a selective layer with dense surface charge on the low concentration side. These material design concepts for asymmetric ion transport are further supported by both simulation and experiment. The results provide a significant comprehension for ion behaviors in nanoconfined space and the development of high-performance energy storage and conversion systems.

Layer-by-layer assembly in nanochannels: assembly mechanism and applications

Layer-by-layer (LbL) assembly is a versatile technology to construct multifunctional nanomaterials using various supporting substrates, enabled by the large selection freedom of building materials and diversity of possible driving forces. The fine regulation over the film thickness and structure provides an elegant way to tune the physical/chemical properties by mild assembly conditions (e.g. pH, ion strength). In this review, we focus on LbL in nanochannels, which exhibit a different growth mechanism compared to "open", convex substrates. The assembly mechanism in nanochannels is discussed in detail, followed by the summary of applications of LbL assemblies liberated from nanochannel templates which can be used as nanoreactors, drug carriers and transporting channels across cell membranes. For fluidic applications, robust membrane substrates are required to keep in place nanotube arrays for membrane-based separation, purification, biosensing and energy harvesting, which are also discussed. The good compatibility of LbL with crossover technologies from other fields allows researchers to further extend this technology to a broader range of research fields, which is expected to result in an increased number of applications of LbL technology in the future.

Evaporation-driven transport-control of small molecules along nanoslits

Understanding and controlling the transport mechanisms of small molecules at the micro/nanoscales is vital because they provide a working principle for a variety of practical micro/nanofluidic applications. However, most precedent mechanisms still have remaining obstacles such as complicated fabrication processes, limitations of materials, and undesired damage on samples. Herein, we present the evaporation-driven transport-control of small molecules in gas-permeable and low-aspect ratio nanoslits, wherein both the diffusive and advective mass transports of solutes are affected by solvent evaporation through the nanoslit walls. The effect of the evaporation flux on the mass transport of small molecules in various nanoslit-integrated micro/nanofluidic devices is characterized, and dynamic transport along the nanoslit is investigated by conducting numerical simulations using the advection-diffusion equation. We further demonstrate that evaporation-driven, nanoslit-based transport-control can be easily applied to a micro/nanofluidic channel network in an independent and addressable array, offering a unique working principle for micro/nanofluidic applications and components such as molecule-valves, -concentrators, -pumps, and -filters.

Transport Phenomena in Nano/Molecular Confinements

The transport of fluid and ions in nano/molecular confinements is the governing physics of a myriad of embodiments in nature and technology including human physiology, plants, energy modules, water collection and treatment systems, chemical processes, materials synthesis, and medicine. At nano/molecular scales, the confinement dimension approaches the molecular size and the transport characteristics deviates significantly from that at macro/micro scales. A thorough understanding of physics of transport at these scales and associated fluid properties is undoubtedly critical for future technologies. This compressive review provides an elaborate picture on the promising future applications of nano/molecular transport, highlights experimental and simulation metrologies to probe and comprehend this transport phenomenon, discusses the physics of fluid transport, tunable flow by orders of magnitude, and gating mechanisms at these scales, and lists the advancement in the fabrication methodologies to turn these transport concepts into reality. Properties such as chain-like liquid transport, confined gas transport, surface charge-driven ion transport, physical/chemical ion gates, and ion diodes will provide avenues to devise technologies with enhanced performance inaccessible through macro/micro systems. This review aims to provide a consolidated body of knowledge to accelerate innovation and breakthrough in the above fields.

Bioinspired Ultrafast-Responsive Nanofluidic System for Ion and Molecule Transport with Speed Control

The design of an intelligent nanofluidic system for regulating the transport of substances such as ions and molecules is significant for applications in biological sensing, drug delivery, and energy harvesting. However, the existing nanofluidic system faces challenges in terms of an uncontrollable transport speed for molecules and ions and also a complex preparation processes, low durability, and slow response rate. Herein, we demonstrate the use of a bioinspired ferrofluid-based nanofluid that can facilitate multilevel ultrafast-responsive ion and molecule transport with speed control. Specifically, we reversibly deform bulk ferrofluids using a magnet and wet/dewet the outer surface of superhydrophilic nanochannels for building a smart transport system. By changing the direction and strength of the external magnetic field, a speed control, ultrafast-responsive molecular transport (<0.1 s), and controlled current gating ratio are achieved owing to the different pattern changes of ferrofluids on the outer surface of nanochannels. We also illustrate a practical application of this strategy for antibacterial devices to control the transport of drug molecules in a programmed manner. These results suggest that molecule transport can be further complexified and quantified through an intelligent nanofluidic system.

Water in Nanopores and Biological Channels: A Molecular Simulation Perspective

This Review explores the dynamic behavior of water within nanopores and biological channels in lipid bilayer membranes. We focus on molecular simulation studies, alongside selected structural and other experimental investigations. Structures of biological nanopores and channels are reviewed, emphasizing those high-resolution crystal structures, which reveal water molecules within the transmembrane pores, which can be used to aid the interpretation of simulation studies. Different levels of molecular simulations of water within nanopores are described, with a focus on molecular dynamics (MD). In particular, models of water for MD simulations are discussed in detail to provide an evaluation of their use in simulations of water in nanopores. Simulation studies of the behavior of water in idealized models of nanopores have revealed aspects of the organization and dynamics of nanoconfined water, including wetting/dewetting in narrow hydrophobic nanopores. A survey of simulation studies in a range of nonbiological nanopores is presented, including carbon nanotubes, synthetic nanopores, model peptide nanopores, track-etched nanopores in polymer membranes, and hydroxylated and functionalized nanoporous silica. These reveal a complex relationship between pore size/geometry, the nature of the pore lining, and rates of water transport. Wider nanopores with hydrophobic linings favor water flow whereas narrower hydrophobic pores may show dewetting. Simulation studies over the past decade of the behavior of water in a range of biological nanopores are described, including porins and β-barrel protein nanopores, aquaporins and related polar solute pores, and a number of different classes of ion channels. Water is shown to play a key role in proton transport in biological channels and in hydrophobic gating of ion channels. An overall picture emerges, whereby the behavior of water in a nanopore may be predicted as a function of its hydrophobicity and radius. This informs our understanding of the functions of diverse channel structures and will aid the design of novel nanopores. Thus, our current level of understanding allows for the design of a nanopore which promotes wetting over dewetting or vice versa. However, to design a novel nanopore, which enables fast, selective, and gated flow of water de novo would remain challenging, suggesting a need for further detailed simulations alongside experimental evaluation of more complex nanopore systems.

Electric Field Induced Wetting of a Hydrophobic Gate in a Model Nanopore Based on the 5-HT3 Receptor Channel

In this study we examined the influence of a transmembrane voltage on the hydrophobic gating of nanopores using molecular dynamics simulations. We observed electric field induced wetting of a hydrophobic gate in a biologically inspired model nanopore based on the 5-HT3 receptor in its closed state, with a field of at least ∼100 mV nm-1 (corresponding to a supra-physiological potential difference of ∼0.85 V across the membrane) required to hydrate the pore. We also found an unequal distribution of charged residues can generate an electric field intrinsic to the nanopore which, depending on its orientation, can alter the effect of the external field, thus making the wetting response asymmetric. This wetting response could be described by a simple model based on water surface tension, the volumetric energy contribution of the electric field, and the influence of charged amino acids lining the pore. Finally, the electric field response was used to determine time constants characterizing the phase transitions of water confined within the nanopore, revealing liquid-vapor oscillations on a time scale of ∼5 ns. This time scale was largely independent of the water model employed and was similar for different sized pores representative of the open and closed states of the pore. Furthermore, our finding that the threshold voltage required for hydrating a hydrophobic gate depends on the orientation of the electric field provides an attractive perspective for the design of rectifying artificial nanopores.

Improved Ion Transport and High Energy Conversion through Hydrogel Membrane with 3D Interconnected Nanopores

To mimic and use the functions of the ion transport system that are central to biological processes, bioinspired ion-selective membranes are developed and show great potential in a variety of fields. However, the practical applications of them are now limited due to low pore density, low conductivity, or scale-up difficulty. Herein, we demonstrate a 2-hydroxyethyl methacrylate phosphate (HEMAP) hydrogel membrane with 3D interconnected nanopores and space charged through simple photopolymerization. The HEMAP hydrogel membrane exhibits high conductance and outstanding ion selectivity, and the membrane-based osmotic power generator shows the excellent output power density up to 5.38 W/m². Both experimentally and theoretically, the 3D interconnected structure is revealed to play a key role in enhancing charge-governed ion transport and energy conversion. This work highlights the advantages of 3D interconnected nanopores in ion diffusion and shows the potential of our designed hydrogel membrane in osmotic energy conversion, water desalination, and sensors.

Nanoconfined Fluids: What Can We Expect from Them?

Nanoconfined fluids (NCFs), which are confined in nanospaces, exhibit distinctive nanoscale effects, including surface effects, small-size effects, quantum effects, and others. The continuous medium hypothesis in fluid mechanics is not valid in this context because of the comparable characteristic length of spaces and molecular mean free path, and accordingly, the classical continuum theories developed for the bulk fluids usually cannot describe the mass and energy transport of NCFs. In this Perspective, we summarize the nanoscale effects on the thermodynamics, mass transport, flow dynamics, heat transfer, phase change, and energy transport of NCFs and highlight the related representative works. The applications of NCFs in the fields of membrane separation, oil and gas production, energy harvesting and storage, and biological engineering are especially indicated. Currently, the theoretical description framework of NCFs is still missing, and it is expected that this framework can be established by adopting the classical continuum theories with the consideration of nanoscale effects.

Electrowetting-Mediated Transport to Produce Electrochemical Transistor Action in Nanopore Electrode Arrays

Understanding water behavior in confined volumes is important in applications ranging from water purification to healthcare devices. Especially relevant are wetting and dewetting phenomena which can be switched by external stimuli, such as light and electric fields. Here, these behaviors are exploited for electrochemical processing by voltage-directed ion transport in nanochannels contained within nanopore arrays in which each nanopore presents three electrodes. The top and middle electrodes (TE and ME) are in direct contact with the nanopore volume, but the bottom electrode (BE) is buried beneath a 70 nm silicon nitride (SiN_x ) insulating layer. Electrochemical transistor operation is realized when small, defect-mediated channels are opened in the SiN_x . These defect channels exhibit voltage-driven wetting that mediates the mass transport of redox species to/from the BE. When BE is held at a potential maintaining the defect channels in the wetted state, setting the potential of ME at either positive or negative overpotential results in strong electrochemical rectification with rectification factors up to 440. By directing the voltage-induced electrowetting of defect channels, these three-electrode nanopore structures can achieve precise gating and ion/molecule separation, and, as such, may be useful for applications such as water purification and drug delivery.

Electrical Field Regulation of Ion Transport in Polyethylene Terephthalate Nanochannels

Rectified ion transport in nanochannels is the basis of ion channels in biological cells and has inspired emerging nanochannel applications in ion separation, Coulter counters, and biomolecule detection and nanochannel energy harvesters. In this work we fabricated a polyethylene terephthalate (PET) conical nanochannel using latent ion track etching technique and then systematically studied the ion transport and influence of cation species on the nanochannel surface with cyclic I-V measurement. We discovered the electrical regulation of the reversible and irreversible modification of the nanochannel transportation by bivalent and trivalent cations, revealing the existence of the switching threshold voltage which can control the current rectification in bivalent solution. The proposed mechanism of the transport state transition in the PET nanochannel mimics behaviors of voltage-gated biological ion channels. These findings provide new insight into the understanding of the ion channel signaling and translocation control of charged particles in nanochannel applications.

Redox-Driven Reversible Gating of Solid-State Nanochannels

The design of an electrochemically addressable nanofluidic diode is proposed, which allows tunable and nanofluidic operations via redox gating under electrochemical control. The fabrication process involves the modification of an asymmetric gold-coated solid-state nanopore with a thin layer of a redox polymer, poly(vinylferrocene) (PVFc). The composite nanochannel acts as a gate electrode by changing the electrochemical state and, consequently, the conversion/switching of ferrocene into ferricenium units upon the application of different voltages. It is shown that the electrochemical input accurately controls the surface charge density of the nanochannel walls with a predictable concomitant effect on the rectification properties. PVFc-based nanofluidic devices are able to discriminate the passage of anionic species through the nanochannel in a qualitative and quantitative manner by simply switching the redox potential of the PVFc layer. Experimental data confirmed that a rapid and reversible modulation of the ionic transport regimes can be easily attained by changing the applied potential. This applied potential plays the role of the gate voltage (V_g) in field-effect transistors (FET), so these nanofluidic channels behave as ionic FETs. Depending on the V_g values, the iontronic behavior can be switched between ohmic and diode-like regimes. We believe that this system illustrates the potential of redox-active polymers integrated into nanofluidic devices as plausible, simple, and versatile platforms to create electrochemically addressable nanofluidic devices for multiple applications.

Highly Switchable Adhesion of N-Doped Graphene Interfaces for Robust Micromanipulation

We demonstrated an N-doped graphene interface with highly switchable adhesion and robust micromanipulation capability triggered by external electric signals. Upon applying a small dc or ac electrical bias, this nanotextured surface can collect environmental moisture to form a large number of water bridges between the graphene and target surface, which lead to a drastic change in adhesive force. Turning on and off the electrical bias can control this graphene interface as a robust micro/nanomanipulator to pick up and drop off various micro/nano-objects for precise assembling. Molecular dynamics simulation reveals that the electrically induced electric double layer and ordered icelike structures at the graphene-water interface strengthen the water bridges and consequently enhance force switchability. In addition to the micro-/nanomanipulation, this switchable adhesion may have many technical implications such as climbing robots, sensors, microfluidic devices, and advanced drug delivery.

Wettability and Applications of Nanochannels

Wettability in nanochannels is of great importance for understanding many challenging problems in interface chemistry and fluid mechanics, and presents versatile applications including mass transport, catalysis, chemical reaction, nanofabrication, batteries, and separation. Recently, both molecular dynamic simulations and experimental measurements have been employed to study wettability in nanochannels. Here, wettability in three types of nanochannels comprising 1D nanochannels, 2D nanochannels, and 3D nanochannels is summarized both theoretically and experimentally. The proposed concept of "quantum-confined superfluid" for ultrafast mass transport in nanochannels is first introduced, and the mostly studied 1D nanochannels are reviewed from molecular simulation to water wettability, followed by reversible switching of water wettability via external stimuli (temperature and voltage). Liquid transport and two confinement strategies in nanochannels of melt wetting and liquid wetting are also included. Then, molecular simulation, water wettability, liquid transport, and confinement in nanochannels are introduced for 2D nanochannels and 3D nanochannels, respectively. Based on the wettability in nanochannels, broad applications of various nanochannels are presented. Finally, the perspective for future challenges in the wettability and applications of nanochannels is discussed.

Multiscale Interface Effect on Homogeneous Dielectric Structure of ZrO₂/Teflon Nanocomposite for Electrowetting Application

Electrowetting-on-dielectric is a preferred option in practical applications of the electrowetting phenomenon but limited by dielectric and breakdown performances of the dielectric layer. In the present work, a ceramic/polymer nanocomposite as a novel dielectric layer is developed to intensify the overall electrowetting performances by multiscale interface effect. Hereinto, surface fluoro-modified ZrO₂ nanoparticles (mZrO₂) are dispersed well in AF 1600 matrix to form a mZrO₂@AF 1600 nanocomposite. The small addition of mZrO₂ improves the dielectric constant of the nanocomposite, and the experimental value is larger than the theoretical value calculated by Maxwell⁻Garnett model, but fits well with the Rahaman⁻Khastgir model. The molecular dynamics simulations with the explicit model further verify the interfacial effect. Meanwhile, double contact angle modulation and higher breakdown field strength (Eb) are obtained. For the three-layer sandwich structure, both the top and bottom AF 1600 layer decrease the surface roughness for better electrowetting reproducibility and wider wettability modulation. The Forlani⁻Minnaja theory related to the empirical relationship between Eb and thickness of dielectric layer fit well with the monolayer structure, but cannot be applied in multi-layer structures. A new relationship is proposed to guide the design of dielectric multi-layers with high breakdown field strength.

Nanoscale Phase Measurement for the Shale Challenge: Multicomponent Fluids in Multiscale Volumes

Hydrocarbon recovery from shale reservoirs provides an increasing share of world energy. These resources are multicomponent fluid mixtures within multiscale geometries, and understanding their associated phase-change thermodynamics presents an array of challenges for experimentalists, theorists, operators, and policy makers. Here, we quantify hydrocarbon mixture phase behavior via direct imaging of connected channels spanning 4 orders of magnitude (10 nm to 10 μm) with supporting density functional theory. The methane/propane mixture dew point shifts, with early condensation of heavy components in nanopores because of a combination of capillarity and competitive surface adsorption. The bubble point in nanoconfinement is found to be deeply suppressed (∼3-fold), to below the bulk dew point of the original mixture, because of the exchange of mixture components with larger connected volumes. The trapping of the heaviest components of hydrocarbon mixtures within the smallest connected pores has implications for shale operations, reserve estimation, and ultimately energy security.

Electrically controlled water permeation through graphene oxide membranes

Controlled transport of water molecules through membranes and capillaries is important in areas as diverse as water purification and healthcare technologies^1-7. Previous attempts to control water permeation through membranes (mainly polymeric ones) have concentrated on modulating the structure of the membrane and the physicochemical properties of its surface by varying the pH, temperature or ionic strength^3,8. Electrical control over water transport is an attractive alternative; however, theory and simulations^9-14 have often yielded conflicting results, from freezing of water molecules to melting of ice^14-16 under an applied electric field. Here we report electrically controlled water permeation through micrometre-thick graphene oxide membranes^17-21. Such membranes have previously been shown to exhibit ultrafast permeation of water^17,22 and molecular sieving properties^18,21, with the potential for industrial-scale production. To achieve electrical control over water permeation, we create conductive filaments in the graphene oxide membranes via controllable electrical breakdown. The electric field that concentrates around these current-carrying filaments ionizes water molecules inside graphene capillaries within the graphene oxide membranes, which impedes water transport. We thus demonstrate precise control of water permeation, from ultrafast permeation to complete blocking. Our work opens up an avenue for developing smart membrane technologies for artificial biological systems, tissue engineering and filtration.

Magnetic Gated Biomimetic Artificial Nanochannels for Controllable Ion Transportation Inspired by Homing Pigeon

The homing pigeon-inspired artificial nanochannel can be modulated by moderate magnetic field in a fast and noncontacting way. The ionic current, as well as rectifying ability and conductance is controlled by the magnetic field reversibly through elastic deformation of the nanochannel. Different gating effects are obtained at the two sides of the asymmetrically conical nanochannel due to the different response models. The magnetic gated nanochannel system also exhibits an excellent stability and a quick response in a noncontacting way, which may be promising in electronic devices related to biological or healthcare applications.

Highly Efficient Gating of Electrically Actuated Nanochannels for Pulsatile Drug Delivery Stemming from a Reversible Wettability Switch

Many ion channels in the cell membrane are believed to function as gates that control the water and ion flow through the transitions between an inherent hydrophobic state and a stimuli-induced hydration state. The construction of nanofluidic gating systems with high gating efficiency and reversibility is inspired by this hydrophobic gating behavior. A kind of electrically actuated nanochannel is developed by integrating a polypyrrole (PPy) micro/nanoporous film doped with perfluorooctanesulfonate ions onto an anodic aluminum oxide nanoporous membrane. Stemming from the reversible wettability switch of the doped PPy film in response to the applied redox potentials, the nanochannels exhibit highly efficient and reversible gating behaviors. The optimized gating ratio is over 10⁵ , which is an ultrahigh value when compared with that of the existing reversibly gated nanochannels with comparable pore diameters. Furthermore, the gating behavior of the electrically actuated nanochannels shows excellent repeatability and stability. Based on this highly efficient and reversible gating function, the electrically actuated nanochannels are further applied for drug delivery, which achieves the pulsatile release of two water-soluble drug models. The electrically actuated nanochannels may find potential applications in accurate and on-demand drug therapy.

Molecular investigation of the wettability of rough surfaces using molecular dynamics simulation

In the present study, a computational investigation on the effect of surface roughness on the wettability behavior of water nanodroplets has been performed via molecular dynamics simulation.

Effect of an external electric field on capillary filling of water in hydrophilic silica nanochannels

Development of functional nanofluidic devices requires understanding the fundamentals of capillary driven flow in nanochannels.

Probing Laser-Induced Heterogeneous Microenvironment Changes in Room-Temperature Ionic Liquids

Modulating the heterogeneous microenvironment in room-temperature ionic liquids (RTILs) by external stimuli is an important approach for understanding and designing external field-induced chemical reactions in natural and applied systems. Here, we report for the first time the redistribution of oxygen molecules related to microstructure changes in RTILs induced by an external laser field, which is probed simultaneously by the triplet-state dynamics of porphyrin. A remarkably long-lived triplet state of porphyrin is observed with changes of microstructures after irradiation, suggesting that charge-shifted O₂ molecules are induced by the external field and/or rearranged intrinsic ions move from nonpolar domains into the polar domains of RTILs through electrostatic interactions. The results suggest that heterogeneous systems like ionic liquids in the presence of external stimuli can be designed for reaction systems associated with not only O₂ but also for CO₂ , CS₂ , etc. and many other similar solvent molecules for many promising applications.

Biomimetic Voltage-Gated Ultrasensitive Potassium-Activated Nanofluidic Based on a Solid-State Nanochannel

In living organism, voltage-gated potassium channels play a crucial role and are largely responsible for various vital movements. For life science, it is significant and challenging to imitate and control the potassium ion transportation with a convenient artificial system. Here, we reported a voltage-gated ultrasensitive potassium-activated nanofluidic system using a 4'-aminobenzo-18-crown-6 molecule-functionalized funnel-shaped solid-state nanochannel. The switchlike property between open and closed states can be tuned freely by reversible immobilization and the release of potassium ions. By virtue of good reversibility and excellent stability, this system can potentially be applied in controlled drug release and biosensors.

Electrowetting Performances of Novel Fluorinated Polymer Dielectric Layer Based on Poly(1H,1H,2H,2H-perfluoroctylmethacrylate) Nanoemulsion

In electrowetting devices, hydrophobic insulating layer, namely dielectric layer, is capable of reversibly switching surface wettability through applied electric field. It is critically important but limited by material defects in dielectricity, reversibility, film forming, adhesiveness, price and so on. To solve this key problem, we introduced a novel fluorinated polyacrylate—poly(1H,1H,2H,2H-perfluoroctylmethacrylate (PFMA) to construct micron/submicron-scale dielectric layer via facile spray coating of nanoemulsion for replacing the most common Teflon AF series. All the results illustrated that, continuous and dense PFMA film with surface relief less than 20 nm was one-step fabricated at 110 °C, and exhibited much higher static water contact angle of 124°, contact angle variation of 42°, dielectric constant of about 2.6, and breakdown voltage of 210 V than Teflon AF 1600. Particularly, soft and highly compatible polyacrylate mainchain assigned five times much better adhesiveness than common adhesive tape, to PFMA layer. As a promising option, PFMA dielectric layer may further facilitate tremendous development of electrowetting performances and applications.

Inorganic Nanowires-Assembled Layered Paper as the Valve for Controlling Water Transportation

Layered materials with open interlayer channels enable various applications such as tissue engineering, ionic and molecular sieving, and electrochemical devices. However, most reports focus on the two-dimensional nanosheets-assembled layered materials, whose interlayer spacing is limited at the nanometer scale. Herein, we demonstrate that one-dimensional inorganic nanowires are the ideal building blocks for the construction of layered materials with open interlayer channels as well, which has not aroused much attention before. It is found that the relatively long inorganic nanowires are capable of assembling into free-standing layered paper with open interlayer channels during the filtration process. The spacings of interlayer channels between adjacent layers are up to tens of micrometers, which are much larger than those of the two-dimensional nanosheets-assembled layered materials. But the closed interlayer channels are observed when the relatively short inorganic nanowires are used as building blocks. The mechanism based on the relationship between the structural variation and the nanowires used is proposed, including the surface charge amplified effect, surface charge superimposed effect, and pillarlike supporting effect. According to the proposed mechanism, we have successfully fabricated a series of layered paper sheets whose architectures (including interlayer channels of cross section and pores on the surface) show gradient changes. The as-prepared layered paper sheets are employed as the valves for controlling water transportation. Tunable water transportation is achieved by the synergistic effect between in-plane interlayer channels (horizontal transportation) from the open to the closed states, and through-layer pores (vertical transportation) without surface modification or intercalation of any guest species.

Nano Day: Celebrating the Next Decade of Nanoscience and Nanotechnology

Nanoscience and nanotechnology are poised to contribute to a wide range of fields, from health and medicine to electronics, energy, security, and more. These contributions come both directly in the form of new materials, interfaces, tools, and even properties as well as indirectly by connecting fields together. We celebrate how far we have come, and here, we look at what is to come over the next decade that will leverage the strong and growing base that we have built in nanoscience and nanotechnology.