Real-time observation of dynamic structure of liquid-vapor interface at nanometer resolution in electron irradiated sodium chloride crystals

The dynamics and structure of the liquid and vapor interface has remained elusive for decades due to the lack of an effective tool for directly visualization beyond micrometer resolution. Here, we designed a simple liquid-cell for encapsulating the liquid state of sodium for transmission electron microscopic (TEM) observation. The real-time dynamic structure of the liquid-vapor interface was imaged and videoed by TEM on the sample of electron irradiated sodium chloride (NaCl) crystals, a well-studied sample with low melting temperature and quantum super-shells of clusters. The nanometer resolution images exhibit the fine structures of the capillary waves, composed of first-time observed three zones of structures and features, i.e. flexible nanoscale fibers, nanoparticles/clusters, and a low-pressure area that sucks the nanoparticles from the liquid to the interface. Although the phenomenons were observed based on irradiated NaCl crystals, the similarities of the phenomenons to predictions suggest our real-time ovserved  dynamic structure might be useful in validating long-debated theoretical models of the liquid-vapor interface, and enhancing our knowledge in understanding the non-equilibrium thermodynamics of the liquid-vapor interface to benefit future engineering designs in microfluidics.

Molecular Mean-Field Theory of Ionic Solutions: A Poisson-Nernst-Planck-Bikerman Model

We have developed a molecular mean-field theory—fourth-order Poisson–Nernst–Planck–Bikerman theory—for modeling ionic and water flows in biological ion channels by treating ions and water molecules of any volume and shape with interstitial voids, polarization of water, and ion-ion and ion-water correlations. The theory can also be used to study thermodynamic and electrokinetic properties of electrolyte solutions in batteries, fuel cells, nanopores, porous media including cement, geothermal brines, the oceanic system, etc. The theory can compute electric and steric energies from all atoms in a protein and all ions and water molecules in a channel pore while keeping electrolyte solutions in the extra- and intracellular baths as a continuum dielectric medium with complex properties that mimic experimental data. The theory has been verified with experiments and molecular dynamics data from the gramicidin A channel, L-type calcium channel, potassium channel, and sodium/calcium exchanger with real structures from the Protein Data Bank. It was also verified with the experimental or Monte Carlo data of electric double-layer differential capacitance and ion activities in aqueous electrolyte solutions. We give an in-depth review of the literature about the most novel properties of the theory, namely Fermi distributions of water and ions as classical particles with excluded volumes and dynamic correlations that depend on salt concentration, composition, temperature, pressure, far-field boundary conditions etc. in a complex and complicated way as reported in a wide range of experiments. The dynamic correlations are self-consistent output functions from a fourth-order differential operator that describes ion-ion and ion-water correlations, the dielectric response (permittivity) of ionic solutions, and the polarization of water molecules with a single correlation length parameter.

Electro-Osmotic Vortices Promote the Capture of Folded Proteins by PlyAB Nanopores

Biological nanopores are emerging as powerful tools for single-molecule analysis and sequencing. Here, we engineered the two-component pleurotolysin (PlyAB) toxin to assemble into 7.2 × 10.5 nm cylindrical nanopores with a low level of electrical noise in lipid bilayers, and we addressed the nanofluidic properties of the nanopore by continuum simulations. Surprisingly, proteins such as human albumin (66.5 kDa) and human transferrin (76-81 kDa) did not enter the nanopore. We found that the precise engineering of the inner surface charge of the PlyAB induced electro-osmotic vortices that allowed the electrophoretic capture of the proteins. Once inside the nanopore, two human plasma proteins could be distinguished by the characteristics of their current blockades. This fundamental understanding of the nanofluidic properties of nanopores provides a practical method to promote the capture and analysis of folded proteins by nanopores.

Cultivation-Free Typing of Bacteria Using Optical DNA Mapping

A variety of pathogenic bacteria can infect humans, and rapid species identification is crucial for the correct treatment. However, the identification process can often be time-consuming and depend on the cultivation of the bacterial pathogen(s). Here, we present a stand-alone, enzyme-free, optical DNA mapping assay capable of species identification by matching the intensity profiles of large DNA molecules to a database of fully assembled bacterial genomes (>10 000). The assay includes a new data analysis strategy as well as a general DNA extraction protocol for both Gram-negative and Gram-positive bacteria. We demonstrate that the assay is capable of identifying bacteria directly from uncultured clinical urine samples, as well as in mixtures, with the potential to be discriminative even at the subspecies level. We foresee that the assay has applications both within research laboratories and in clinical settings, where the time-consuming step of cultivation can be minimized or even completely avoided.

Solvation-Involved Nanoionics: New Opportunities from 2D Nanomaterial Laminar Membranes

Nanoporous laminar membranes composed of multilayered 2D nanomaterials (2D-NLMs) are increasingly being exploited as a unique material platform for understanding solvated ion transport under nanoconfinement and exploring novel nanoionics-related applications, such as ion sieving, energy storage and harvesting, and in other new ionic devices. Here, the fundamentals of solvation-involved nanoionics in terms of ionic interactions and their effect on ionic transport behaviors are discussed. This is followed by a summary of key requirements for materials that are being used for solvation-involved nanoionics research, culminating in a demonstration of unique features of 2D-NLMs. Selected examples of using 2D-NLMs to address the key scientific problems related to nanoconfined ion transport and storage are then presented to demonstrate their enormous potential and capabilities for nanoionics research and applications. To conclude, a personal perspective on the challenges and opportunities in this emerging field is presented.

Thermal Dependence of the Mesoscale Ionic Diode: Modeling and Experimental Verification

Mesoscale ionic diodes, which can rectify ionic current at conditions at which their pore size is larger than 100 nm and thus over 100 times larger than the Debye length, have been recently discovered with potential applications in ionic circuits as well as osmotic power generation. Compared with the conventional nanoscale ionic diodes, the mesoscale ionic diodes can offer much higher conductance, ionic current resolution, and power generated. However, the thermal response, which has been proven playing a crucial role in nanofluidic devices, of the mesoscale ionic diode remains significantly unexplored. Here, we report the thermal dependence of the mesoscale ionic diode comprising a conical pore with a tip opening diameter of ∼400 nm. To capture its underlying physics more accurately, our model takes into account the practical equilibrium chemistry reaction of functional carboxyl groups on the pore surface. Modeling results predict that in the mesoscale ionic diode prepared currents increase but the performance decreases with the increase of temperature, which is consistent with our experimental data and indicates that the ion transport properties apparently depend on the presence of highly mobile hydroxide ions. The results gathered can provide important guidance for the design of new mesoscale ionic diodes, enriching their applications in thermoelectric power and thermoresponsive chemical sensors.

Electric-Field-Induced Ionic Sieving at Planar Graphene Oxide Heterojunctions for Miniaturized Water Desalination

Layered graphene oxide membranes (GOMs) offer a unique platform for precise sieving of small ions and molecules due to controlled sub-nanometer-wide interlayer distance and versatile surface chemistry. Pristine and chemically modified GOMs effectively block organic dyes and nanoparticles, but fail to exclude smaller ions with hydrated diameters less than 9 Å. Toward sieving of small inorganic salt ions, a number of strategies are proposed by reducing the interlayer spacing down to merely several angstroms. However, one critical challenge for such compressed GOMs is the extremely low water flux (<0.1 Lm^-2 h^-1 bar^-1 ) that prevents these innovative nanomaterials from being used in real-world applications. Here, a planar heterogeneous graphene oxide membrane (PHGOM) with both nearly perfect salt rejection and high water flux is reported. Horizontal ion transport through oppositely charged GO multilayer lateral heterojunction exhibits bi-unipolar transport behavior, blocking the conduction of both cations and anions. Assisted by a forward electric field, salt concentration is depleted in the near-neutral transition area of the PHGOM. In this situation, deionized water can be extracted from the depletion zone. Following this mechanism, a high rejection rate of 97.0% for NaCl and water flux of 1529 Lm^-2 h^-1 bar^-1 at the outlet via an inverted T-shaped water extraction mode are achieved.

Enabling Rapid Charging Lithium Metal Batteries via Surface Acoustic Wave-Driven Electrolyte Flow

Both powerful and unstable, practical lithium metal batteries have remained a difficult challenge for over 50 years. With severe ion depletion gradients in the electrolyte during charging, they rapidly develop porosity, dendrites, and dead Li that cause poor performance and, all too often, spectacular failure. Remarkably, incorporating a small, 100 MHz surface acoustic wave device (SAW) solves this problem. Providing acoustic streaming electrolyte flow during charging, the device enables dense Li plating and avoids porosity and dendrites. SAW-integrated Li cells can operate up to 6 mA cm^-2 in a commercial carbonate-based electrolyte; omitting the SAW leads to short circuiting at 2 mA cm^-2 . The Li deposition is morphologically dendrite-free and close to theoretical density when cycling with the SAW. With a 245 µm thick Li anode in a full Li||LFP (LiFePO₄ ) cell, introducing the SAW increases the uncycled Li from 145 to 225 µm, decreasing Li consumption from 41% to only 8%. A closed-form model is provided to explain the phenomena and serve as a design tool for integrating this chemistry-agnostic approach into batteries whatever the chemistry within.

Tracking and Analyzing the Brownian Motion of Nano-objects Inside Hollow Core Fibers

Tracking and analyzing the individual diffusion of nanoscale objects such as proteins and viruses is an important methodology in life science. Here, we show a sensor that combines the efficiency of light line illumination with the advantages of fluidic confinement. Tracking of freely diffusing nano-objects inside water-filled hollow core fibers with core diameters of tens of micrometers using elastically scattered light from the core mode allows retrieving information about the Brownian motion and the size of each particle of the investigated ensemble individually using standard tracking algorithms and the mean squared displacement analysis. Specifically, we successfully measure the diameter of every gold nanosphere in an ensemble that consists of several hundreds of 40 nm particles, with an individual precision below 17% (±8 nm). In addition, we confirm the relevance of our approach with respect to bioanalytics by analyzing 70 nm λ-phages. Overall these features, together with the strongly reduced demand for memory space, principally allows us to record thousands of frames and to achieve high frame rates for high precision tracking of nanoscale objects.

An Electrically Actuated, Carbon-Nanotube-Based Biomimetic Ion Pump

Single-walled carbon nanotubes (SWCNTs) are well-established transporters of electronic current, electrolyte, and ions. In this work, we demonstrate an electrically actuated biomimetic ion pump by combining these electronic and nanofluidic transport capabilities within an individual SWCNT device. Ion pumping is driven by a solid-state electronic input, as Coulomb drag coupling transduces electrical energy from solid-state charge along the SWCNT shell to electrolyte inside the SWCNT core. Short-circuit ionic currents, measured without an electrolyte potential difference, exceed 1 nA and scale larger with increasing ion concentrations through 1 M, demonstrating applicability under physiological (∼140 mM) and saltwater (∼600 mM) conditions. The interlayer coupling allows ionic currents to be tuned with the source-drain potential difference and electronic currents to be tuned with the electrolyte potential difference. This combined electronic-nanofluidic SWCNT device presents intriguing applications as a biomimetic ion pump or component of an artificial membrane.

Dimension-reconfigurable bubble film nanochannel for wetting based sensing

Dimensions and surface properties are the predominant factors for the applications of nanofluidic devices. Here we use a thin liquid film as a nanochannel by inserting a gas bubble in a glass capillary, a technique we name bubble-based film nanofluidics. The height of the film nanochannel can be regulated by the Debye length and wettability, while the length independently changed by applied pressure. The film nanochannel behaves functionally identically to classical solid state nanochannels, as ion concentration polarizations. Furthermore, the film nanochannels can be used for label-free immunosensing, by principle of wettability change at the solid interface. The optimal sensitivity for the biotin-streptavidin reaction is two orders of magnitude higher than for the solid state nanochannel, suitable for a full range of electrolyte concentrations. We believe that the film nanochannel represents a class of nanofluidic devices that is of interest for fundamental studies and also can be widely applied, due to its reconfigurable dimensions, low cost, ease of fabrication and multiphase interfaces.

Ultrahigh Ionic Exclusion through Carbon Nanomembranes

The collective "single-file" motion of water molecules through natural and artificial nanoconduits inspires the development of high-performance membranes for water separation. However, a material that contains a large number of pores combining rapid water flow with superior ion rejection is still highly desirable. Here, a 1.2 nm thick carbon nanomembrane (CNM) made from cross-linking of terphenylthiol (TPT) self-assembled monolayers is reported to possess these properties. Utilizing their extremely high pore density of 1 sub-nm channel nm^-2 , TPT CNMs let water molecules rapidly pass, while the translocation of ions, including protons, is efficiently hindered. Their membrane resistance reaches ≈10⁴ Ω cm² in 1 m Cl^- solutions, comparable to lipid bilayers of a cell membrane. Consequently, a single CNM channel yields an ≈10⁸ higher resistance than pores in lipid membrane channels and carbon nanotubes. The ultrahigh ionic exclusion by CNMs is likely dominated by a steric hindrance mechanism, coupled with electrostatic repulsion and entrance effects. The operation of TPT CNM membrane composites in forward osmosis is also demonstrated. These observations highlight the potential of utilizing CNMs for water purification and opens up a simple avenue to creating 2D membranes through molecular self-assembly for highly selective and fast separations.

Electrodiffusioosmosis-Induced Negative Differential Resistance in pH-Regulated Mesopores Containing Purely Monovalent Solutions

Negative differential resistance (NDR) refers to a unique electrical property where current decreases with increasing voltage. Herein, we report experimental evidence showing that the NDR effect can be observed in mesopores that feature charged pore walls and are subjected to a KCl concentration gradient. NDR in our system originates from the solution and ion flows driven by the synergistic effects of electroosmosis [electroosmotic flow (EOF)] and diffusioosmosis, the so-called electrodiffusioosmosis. Experiments reveal that in addition to the ion current rectification, the mesopores considered here exhibit the NDR phenomenon that is dependent on the magnitude and direction of the salinity gradient and on pH. The NDR behavior can be observed only at conditions at which the EOF and diffusioosmosis occur in the opposite directions: diffusioosmosis fills the tip opening with a high concentration solution, while EOF brings a low concentration solution to the pore. All experimental findings are supported by our numerical model, which takes into account the interfacial site reactions of acidic and basic functional groups on the entire pore membrane surfaces. Our results provide an important insight into how liquid pH, salinity gradients, interfacial site reactions, and pore geometries can influence the current-voltage characteristics of mesopores, enriching transport modes that can be induced by voltage.

Light-Powered Directional Nanofluidic Ion Transport in Kirigami-Made Asymmetric Photonic-Ionic Devices

Nacre-mimetic 2D nanofluidic materials with densely packed sub-nanometer-height lamellar channels find widespread applications in water-, energy-, and environment-related aspects by virtue of their scalable fabrication methods and exceptional transport properties. Recently, light-powered nanofluidic ion transport in synthetic materials gained considerable attention for its remote, noninvasive, and active control of the membrane transport property using the energy of light. Toward practical application, a critical challenge is to overcome the dependence on inhomogeneous or site-specific light illumination. Here, asymmetric photonic-ionic devices based on kirigami-tailored graphene oxide paper are fabricated, and directional nanofluidic ion transport properties therein powered by full-area light illumination are demonstrated. The in-plane asymmetry of the graphene oxide paper is essential to the generation of photoelectric driving force under homogeneous illumination. This light-powered ion transport phenomenon is explained based on a modified carrier diffusion model. In asymmetric nanofluidic structures, enhanced recombination of photoexcited charge carriers at the membrane boundary breaks the electric potential balance in the horizontal direction, and thus drives the ion transport in that direction under symmetric illumination. The kirigami-based strategy provides a facile and scalable way to fabricate paper-like photonic-ionic devices with arbitrary shapes, working as fundamental elements for large-scale light-harvesting nanofluidic circuits.

Engineering Smart Nanofluidic Systems for Artificial Ion Channels and Ion Pumps: From Single-Pore to Multichannel Membranes

Biological ion channels and ion pumps with intricate ion transport functions widely exist in living organisms and play irreplaceable roles in almost all physiological functions. Nanofluidics provides exciting opportunities to mimic these working processes, which not only helps understand ion transport in biological systems but also paves the way for the applications of artificial devices in many valuable areas. Recent progress in the engineering of smart nanofluidic systems for artificial ion channels and ion pumps is summarized. The artificial systems range from chemically and structurally diverse lipid-membrane-based nanopores to robust and scalable solid-state nanopores. A generic strategy of gate location design is proposed. The single-pore-based platform concept can be rationally extended into multichannel membrane systems and shows unprecedented potential in many application areas, such as single-molecule analysis, smart mass delivery, and energy conversion. Finally, some present underpinning issues that need to be addressed are discussed.

On-Chip Stretching, Sorting, and Electro-Optical Nanopore Sensing of Ultralong Human Genomic DNA

Solid-state nanopore sensing of ultralong genomic DNA molecules has remained challenging, as the DNA must be controllably delivered by its leading end for efficient entry into the nanopore. Herein, we introduce a nanopore sensor device designed for electro-optical detection and sorting of ultralong (300+ kilobase pair) genomic DNA. The fluidic device, fabricated in-silicon and anodically bonded to glass, uses pressure-induced flow and an embedded pillar array for controllable DNA stretching and delivery. Extremely low concentrations (50 fM) and sample volumes (∼1 μL) of DNA can be processed. The low height profile of the device permits high numerical aperture, high magnification imaging of DNA molecules, which remain in focus over extended distances. We demonstrate selective DNA sorting based on sequence-specific nick translation labeling and imaging at high camera frame rates. Nanopores are fabricated directly in the assembled device by laser etching. We show that uncoiling and stretching of the ultralong DNA molecules permits efficient nanopore capture and threading, which is simultaneously and synchronously imaged and electrically measured. Furthermore, our technique provides key insights into the translocation behavior of ultralong DNA and promotes the development of all-in-one micro/nanofluidic platforms for nanopore sensing of biomolecules.

Water Desalination with Two-Dimensional Metal-Organic Framework Membranes

Providing fresh and drinkable water is a grand challenge the world is facing today. Development in nanomaterials can create possibilities of using energy-efficient nanoporous materials for water desalination. In this work, we demonstrated that ultrathin conductive metal-organic framework (MOF) is capable of efficiently rejecting ions while giving access to high water flux. Through molecular dynamic simulation, we discovered perfect ion rejection rate by two-dimensional (2D) multilayer MOF. The naturally porous structure of 2D MOF enables significantly 3-6 orders of magnitude higher water permeation compared to that of traditional membranes. Few layers MOF membranes show 1 order of magnitude higher water flux compared to that of single-layer nanoporous graphene or molybdenum disulfide (MoS₂) without the requirement of drilling pores. The excellent performance of 2D MOF membranes is supported by water permeation calculations, water density/velocity profiles at the pore, and the water interfacial diffusion near the pore. Water desalination performance of MOF offers a potential solution for energy-efficient water desalination.

Highly Efficient Ionic Photocurrent Generation through WS2 -Based 2D Nanofluidic Channels

The unique feature of nacre-like 2D layered materials provides a facile, yet highly efficient way to modulate the transmembrane ion transport from two orthogonal transport directions, either vertical or horizontal. Recently, light-driven active transport of ionic species in synthetic nanofluidic systems attracts broad research interest. Herein, taking advantage of the photoelectric semiconducting properties of 2D transition metal dichalcogenides, the generation of a directional and greatly enhanced cationic flow through WS₂ -based 2D nanofluidic membranes upon asymmetric visible light illumination is reported. Compared with graphene-based materials, the magnitude of the ionic photocurrent can be enhanced by tens of times, and its photo-responsiveness can be 2-3.5 times faster. This enhancement is explained by the coexistence of semiconducting and metallic WS₂ nanosheets in the hybrid membrane that facilitates the asymmetric diffusion of photoexcited charge carriers on the channel wall, and the high ionic conductance due to the neat membrane structure. To further demonstrate its application, photonic ion switches, photonic ion diodes, and photonic ion transistors as the fundamental elements for light-controlled nanofluidic circuits are further developed. Exploring new possibilities in the family of liquid processable colloidal 2D materials provides a way toward high-performance light-harvesting nanofluidic systems for artificial photosynthesis and sunlight-driven desalination.

Estimation of magnetohydrodynamic radiative nanofluid flow over a porous non-linear stretching surface: application in biomedical research

The present study investigates the effects of thermal radiation and chemical reaction on magnetohydrodynamic flow, heat, and mass transfer characteristics of nanofluids such as Cu-water and Ag-water over a non-linear porous stretching surface in the presence of viscous dissipation and heat generation. Using similarity transformation, the governing boundary layer equations of the problem are transformed into non-linear ordinary differential equations and solved numerically by the shooting method along with the Runge-Kutta-Fehlberg fourth-fifth-order integration scheme. The influences of various parameters on velocity, temperature, and concentration profiles of the flow field are analysed and the results are plotted graphically. A backpropagation neural network is applied to predict the skin friction coefficient, Nusselt number, and Sherwood number and these results are presented through graphs. The present numerical results are compared with the existing results and are found to be in good agreement. The results of artificial neural network and the obtained numerical values agree well with an error <5%.

Strong Electroosmotic Coupling Dominates Ion Conductance of 1.5 nm Diameter Carbon Nanotube Porins

Extreme confinement in nanometer-sized channels can alter fluid and ion transport in significant ways, leading to significant water flow enhancement and unusual ion correlation effects. These effects are especially pronounced in carbon nanotube porins (CNTPs) that combine strong confinement in the inner lumen of carbon nanotubes with the high slip flow enhancement due to smooth hydrophobic pore walls. We have studied ion transport and ion selectivity in 1.5 nm diameter CNTPs embedded in lipid membranes using a single nanopore measurement setup. Our data show that CNTPs are weakly cation selective at pH 7.5 and become nonselective at pH 3.0. Ion conductance of CNTPs exhibits an unusual 2/3 power law scaling with the ion concentration at both neutral and acidic pH values. Coupled Navier-Stokes and Poisson-Nernst-Planck simulations and atomistic molecular dynamics simulations reveal that this scaling originates from strong coupling between water and ion transport in these channels. These effects could result in development of a next generation of biomimetic membranes and carbon nanotube-based electroosmotic pumps.

Unraveling the Anomalous Surface-Charge-Dependent Osmotic Power Using a Single Funnel-Shaped Nanochannel

Nanofluidic osmotic power, which converts a difference in salinity between brine and fresh water into electricity with nanoscale channels, has received more and more attention in recent years. It is long believed that to gain high-performance osmotic power, highly charged channel materials should be exploited so as to enhance the ion selectivity. In this paper, we report counterintuitive surface-charge-density-dependent osmotic power in a single funnel-shaped nanochannel (FSN), violating the previous viewpoint. For the highly charged nanochannel, the performance of osmotic power decreases with a further increase in its surface charge density. With increasing pH (surface charge density), the FSN enables a local maximum power density as high as ∼3.5 kW/m² in a 500 mM/1 mM KCl gradient. This observation is strongly supported by our rigorous model where the equilibrium chemical reaction between functional carboxylate ion groups on the channel wall and protons is taken into account. The modeling reveals that for a highly charged nanochannel, a significant increase in the surface charge density amplifies the ion concentration polarization effect, thus weakening the effective salinity ratio across the channel and undermining the osmotic power generated.

Controlled Focused Ion Beam Milling of Composite Solid State Nanopore Arrays for Molecule Sensing

Various nanoscale fabrication techniques are elaborated to form artificial nanoporous/nanochannel membranes to be applied for biosensing: one of the most prevalent is the micro-electromechanical systems (MEMS) compatible focused ion beam (FIB) milling. This technique can be easily adopted in micro- and nanomachining process sequences to develop composite multi-pore structures, although its precision and reproducibility are key points in the case of these thick multi-layered membranes. This work is to demonstrate a comprehensive characterisation of FIB milling to improve the reliability of the fabrication of solid state nanopore arrays with precisely predetermined pore geometries for a targeted molecule type to be recognised. The statistical geometric features of the fabricated nanopores were recorded as the function of the process parameters, and the resulting geometries were analysed in detail by high resolution scanning electron microscope (SEM), transmission electron microscope (TEM) and ion scanning microscopy. Continuous function of the pore diameter evolution rate was derived from the experimental results in the case of different material structures, and compared to former dissentient estimations. The additional metal layer was deposited onto the backside of the membrane and grounded during the ion milling to prevent the electrical charging of dielectric layers. The study proved that the conformity of the pore geometry and the reliability of their fabrication could be improved significantly. The applicability of the developed nanopore arrays for molecule detection was also considered by characterising the pore diameter dependent sensitivity of the membrane impedance modulation based measurement method.

Ionic thermal up-diffusion in nanofluidic salinity-gradient energy harvesting

Advances in nanofabrication and materials science give a boost to the research in nanofluidic energy harvesting. Contrary to previous efforts on isothermal conditions, here a study on asymmetric temperature dependence in nanofluidic power generation is conducted. Results are somewhat counterintuitive. A negative temperature difference can significantly improve the membrane potential due to the impact of ionic thermal up-diffusion that promotes the selectivity and suppresses the ion-concentration polarization, especially at the low-concentration side, which results in dramatically enhanced electric power. A positive temperature difference lowers the membrane potential due to the impact of ionic thermal down-diffusion, although it promotes the diffusion current induced by decreased electrical resistance. Originating from the compromise of the temperature-impacted membrane potential and diffusion current, a positive temperature difference enhances the power at low transmembrane-concentration intensities and hinders the power for high transmembrane-concentration intensities. Based on the system's temperature response, we have proposed a simple and efficient way to fabricate tunable ionic voltage sources and enhance salinity-gradient energy conversion based on small nanoscale biochannels and mimetic nanochannels. These findings reveal the importance of a long-overlooked element—temperature—in nanofluidic energy harvesting and provide insights for the optimization and fabrication of high-performance nanofluidic power devices.

Electroosmotic Flow of Viscoelastic Fluid in a Nanochannel Connecting Two Reservoirs

: Electroosmotic flow (EOF) of viscoelastic fluid with Linear Phan-Thien-Tanner (LPTT) constitutive model in a nanochannel connecting two reservoirs is numerically studied. For the first time, the influence of viscoelasticity on the EOF and the ionic conductance in the micro-nanofluidic interconnect system, with consideration of the electrical double layers (EDLs), is investigated. Regardless of the bulk salt concentration, significant enhancement of the flow rate is observed for viscoelastic fluid compared to the Newtonian fluid, due to the shear thinning effect. An increase in the ionic conductance of the nanochannel occurs for the viscoelastic fluid. The enhancement of the ionic conductance is significant under the overlapping EDLs condition.

Voltage-Gated Ion Transport in Two-Dimensional Sub-1 nm Nanofluidic Channels

Voltage-gated nanofluidic systems have shown a wide range of potential applications in biosensors, energy harvest, and separation. Two-dimensional (2D) nanofluidic membranes fabricated with electrically conductive nanosheets have high ion conductivity and voltage-gated ion transport behaviors. However, the voltage-gating effect of the sub-nanometer-sized 2D channel membranes has not been well-investigated. In this work, a high-performance voltage-gated 2D nanofluidic device is constructed by assembling MXene nanosheets into a laminar membrane with sub-1 nm interlayer channels. By applying external voltage to the membrane, the ion conductivity of the device is enhanced by positive voltages and reduced by negative voltages, exhibiting a high voltage-gating on-off ratio of ∼10. The on-off ratio is found to be dependent on ion concentration and ion species. This work demonstrates that 2D membranes with interlayer spacings comparable to those of hydrated ion diameters can achieve high and tunable voltage-gating function, which provides a strategy to construct devices for highly efficient on-demand ion transport.