A New Strategy for Highly Efficient Separation between Monovalent Cations by Applying Opposite-Oriented Pressure and Electric Fields.

Biological ion channels exhibit excellent ion selectivity, but it has been challenging to design their artificial counterparts, especially for highly efficient separation of similar ions. Here, a new strategy to achieve high selectivity between alkali metal ions with artificial nanostructures is reported. Molecular dynamics (MD) simulations and experiments are combined to study the transportation of monovalent cations through graphene oxide (GO) nanoslits by applying pressure or/and electric fields. It is found that the ionic transport selectivity under the pressure driving reverses compared with that under the electric field driving. Moreover, MD simulations show that different monovalent cations can be separated with unprecedentedly high selectivity by applying opposite-oriented pressure and electric fields. This highly efficient separation originates from two distinctive ionic transporting modes, that is, hydration shells drive ions under pressure, but drag ions under the electric field. Hence, ions with different hydration strengths can be efficiently separated by tuning the net mobility induced by the two types of driving forces when the selected ions are kept moving while the other ones are immobilized. And nanoconfinement is confirmed to enhance the separation efficacy. This discovery paves a new avenue for separating similar ions without elaborately designing biomimetic nanostructures.

The Optimization of the Transition Zone of the Planar Heterogeneous Interface for High-Performance Seawater Desalination.

Reverse osmosis has become the most prevalent approach to seawater desalination. It is still limited by the permeability-selectivity trade-off of the membranes and the energy consumption in the operation process. Recently, an efficient ionic sieving with high performance was realized by utilizing the bi-unipolar transport behaviour and strong ion depletion of heterogeneous structures in 2D materials. A perfect salt rejection rate of 97.0% and a near-maximum water flux of 1529 L m-2 h-1 bar-1 were obtained. However, the energy consumption of the heterogeneous desalination setup is a very important factor, and it remains largely unexplored. Here, the geometric-dimension-dependent ion transport in planar heterogeneous structures is reported. The two competitive ion migration behaviours during the desalination process, ion-depletion-dominated and electric-field-dominated ion transport, are identified for the first time. More importantly, these two ion-transport behaviours can be regulated. The excellent performance of combined high rejection rate, high water flux and low energy consumption can be obtained under the synergy of voltage, pressure and geometric dimension. With the appropriate optimization, the energy consumption can be reduced by 2 orders of magnitude, which is 50% of the industrial energy consumption. These findings provide beneficial insight for the application and optimized design of low-energy-consumption and portable water desalination devices.

The Selective Transport of Ions in Charged Nanopore with Combined Multi-Physics Fields.

The selective transport of ions in nanopores attracts broad interest due to their potential applications in chemical separation, ion filtration, seawater desalination, and energy conversion. The ion selectivity based on the ion dehydration and steric hindrance is still limited by the very similar diameter between different hydrated ions. The selectivity can only separate specific ion species, lacking a general separation effect. Herein, we report the highly ionic selective transport in charged nanopore through the combination of hydraulic pressure and electric field. Based on the coupled Poisson-Nernst-Planck (PNP) and Navier-Stokes (NS) equations, the calculation results suggest that the coupling of hydraulic pressure and electric field can significantly enhance the ion selectivity compared to the results under the single driven force of hydraulic pressure or electric field. Different from the material-property-based ion selective transport, this method endows the general separation effect between different kinds of ions. Through the appropriate combination of hydraulic pressure and electric field, an extremely high selectivity ratio can be achieved. Further in-depth analysis reveals the influence of nanopore diameter, surface charge density and ionic strength on the selectivity ratio. These findings provide a potential route for high-performance ionic selective transport and separation in nanofluidic systems.

Harnessing Ionic Power from Equilibrium Electrolyte Solution via Photoinduced Active Ion Transport through van-der-Waals-Like Heterostructures.

Nanofluidic ion transport through van der Waals heterostructures, composed of two or more types of reconstructed 2D nanomaterials, gives rise to fascinating opportunities for light-energy harvesting, due to coupling between the optoelectronic properties of the layered constituents and ion transport in between the atomic layers. Here, a photoinduced active ion transport phenomenon through transition metal dichalcogenides (TMDs)-based van-der-Waals-like multilayer heterostructures is reported for harnessing ionic power from equilibrium electrolyte solution. The binary heterostructure comprises sequentially stacked 2D-WS2 and 2D-MoS2 multilayers with sub-1 nm interlayer spacing. Upon visible-light illumination, a net ionic flow is initiated through the Janus membrane, suggesting a directional cationic transport from WS2 to MoS2 part. The transport mechanism is explained in terms of a photovoltaic effect due to type II band alignment of WS2 /MoS2 heterostructures. The driving mechanism can be generally applied to a variety of heterogeneous TMD membranes with type II semiconductor heterojunctions. In equilibrium ionic solutions, the maximum ionic photoresponse approaches ≈21 µA cm-2 and ≈45 mV under one sun equivalent excitation. Under optimized conditions, the harvested power density reaches 2 mW m-2 . The proof-of-concept demonstration of photonic-to-ionic power generation within angstrom-scale confinement anticipates potential for light-controlled ionic circuits, artificial photosynthesis, and biomimetic energy conversion.

Vertically-Oriented Ti3C2Tx MXene Membranes for High Performance of Electrokinetic Energy Conversion.

The electrokinetic effect to convert the mechanical energy from ambient has gained sustained research attention because it is free of moving parts and easy to be miniaturized for microscale applications. The practical application is constrained by the limited electrokinetic energy conversion performance. Herein, we report vertically oriented MXene membranes (VMMs) with ultrafast permeation as well as high ion selectivity, in which the permeation is several thousand higher than the largely researched horizontally stacked MXene membranes (HMMs). The VMMs can achieve a high streaming current of 8.17 A m-2 driven by the hydraulic pressure, largely outperforming all existing materials. The theoretical analysis and numerical calculation reveal the underlying mechanism of the ultrafast transport in VMMs originates from the evident short migration paths, the low energy loss during the ionic migration, and the large effective inlet area on the membrane surface. The orientation of the 2D lamella in membranes, the long-overlooked element in the existing literatures, is identified to be an essential determinant in the performance of 2D porous membranes. These understandings can largely promote the development of electrokinetic energy conversion devices and bring advanced design strategy for high-performance 2D materials.

Vertically Transported Graphene Oxide for High-Performance Osmotic Energy Conversion.

Reverse electrodialysis is a promising method to harvest the osmotic energy stored between seawater and freshwater, but it has been a long-standing challenge to fabricate permselective membranes with the power density surpassing the industry benchmark of 5.0 W m-2 for half a century. Herein, a vertically transported graphene oxide (V-GO) with the combination of high ion selectivity and ultrafast ion permeation is reported, whose permeation is three orders of magnitude higher than the extensively studied horizontally transported GO (H-GO). By mixing artificial seawater and river water, an unprecedented high output power density of 10.6 W m-2 is obtained, outperforming all existing materials. Molecular dynamics (MD) simulations reveal the mechanism of the ultrafast transport in V-GO results from the quick entering of ions and the large accessible area as well as the apparent short diffusion paths in V-GO. These results will facilitate the practical application of osmotic energy and bring an innovative design strategy for various systems involving ultrafast transport, such as filtration and catalysis.

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.

The development of a hydrogen-helium dual-beam ion implanter.

Multiple ion beam facilities are powerful tools to simulate the irradiation effects of neutrons on relevant nuclear materials. Since hydrogen and helium are often generated in neutron irradiated materials as transmutation products and they play important roles in the defect evolution, the triple beam accelerator and transmission electron microscope link in situ facility instead of a monobeam or dual-beam facility is necessary to simulate neutron irradiation. A hydrogen-helium dual-beam ion implanter has been developed for a triple ion beam in situ facility at Xiamen University. A Penning ion source has been developed to produce H2 + and He+ simultaneously. A special system consisting of two Wien filters and an Einzel lens has been proposed to purify the species from a single Penning source and allow for the measurement of the He+/H2 + ratio, without disturbing the accelerator settings. The detail of this dual-beam ion implanter will be introduced in detail.

On the Origin of Ionic Rectification in DNA-Stuffed Nanopores: The Breaking and Retrieving Symmetry.

The discovery of ionic current rectification (ICR) phenomena in synthetic nanofluidic systems elicits broad interest from interdisciplinary fields of chemistry, physics, materials science, and nanotechnology; and thus, boosts their applications in, for example, chemical sensing, fluidic pumping, and energy related aspects. So far, it is generally accepted that the ICR effect stems from the broken symmetry either in the nanofluidic structures, or in the environmental conditions. Although this empirical regularity is supported by numerous experimental and theoretical results, great challenge still remains to precisely figure out the correlation between the asymmetric ion transport properties and the degree of symmetry breaking. An appropriate and quantified measure is therefore highly demanded. Herein, taking DNA-stuffed nanopores as a model system, we systematically investigate the evolution of dynamic ICR in between two symmetric states. The fully stuffed and fully opened nanopores are symmetric; therefore, they exhibit linear ion transport behaviors. Once the stuffed DNA superstructures are asymmetrically removed from one end of the nanopore via aptamer-target interaction, the nanofluidic system becomes asymmetric and starts to rectify ionic current. The peak of ICR is found right before the breakthrough of the stuffed DNA forest. After that, the nanofluidic system gradually retrieves symmetry, and becomes non-rectified. Theoretical results by both the coarse-grained Poisson-Nernst-Planck model and the 1D statistic model excellently support the experimental observations, and further establish a quantified correlation between the ICR effect and the degree of asymmetry for different molecular filling configurations. Based on the ICR properties, we develop a proof-of-concept demonstration for sensing ATP, termed the ATP balance. These findings help to clarify the origin of ICR, and show implications to other asymmetric transport phenomena for future innovative nanofluidic devices and materials.

Concentration-gradient-dependent ion current rectification in charged conical nanopores.

Ion current rectification (ICR) in negatively charged conical nanopores is shown to be controlled by the electrolyte concentration gradient depending on the direction of ion diffusion. The degree of ICR is enhanced with the increasing forward concentration difference. An unusual rectification inversion is observed when the concentration gradient is reversely applied. A numerical simulation based on the coupled Poisson and Nernst-Planck (PNP) equations is proposed to solve the ion distribution and ionic flux in the charged and structurally asymmetric nanofluidic channel with diffusive ion flow. Simulation results qualitatively describe the diffusion-induced ICR behavior in conical nanopores suggested by the experimental data. The concentration-gradient-dependent ICR enhancement and inversion is attributed to the cooperation and competition between geometry-induced asymmetric ion transport and the diffusive ion flow. The present study improves our understanding of the ICR in asymmetric nanofluidic channels associated with the ion concentration difference and provides insight into the rectifying biological ion channels.

Current rectification in temperature-responsive single nanopores.

Herein we demonstrate a fully abiotic smart single-nanopore device that rectifies ionic current in response to the temperature. The temperature-responsive nanopore ionic rectifier can be switched between a rectifying state below 34 degrees C and a non-rectifying state above 38 degrees C actuated by the phase transition of the poly(N-isopropylacrylamide) [PNIPAM] brushes. On the rectifying state, the rectifying efficiency can be enhanced by the dehydration of the attached PNIPAM brushes below the LCST. When the PNIPAM brushes have sufficiently collapsed, the nanopore switches to the non-rectifying state. The concept of the temperature-responsive current rectification in chemically-modified nanopores paves a new way for controlling the preferential direction of the ion transport in nanofluidics by modulating the temperature, which has the potential to build novel nanomachines with smart fluidic communication functions for future lab-on-chip devices.

A biomimetic potassium responsive nanochannel: G-quadruplex DNA conformational switching in a synthetic nanopore.

Potassium is especially crucial in modulating the activity of muscles and nerves whose cells have specialized ion channels for transporting potassium. Normal body function extremely depends on the regulation of potassium concentrations inside the ion channels within a certain range. For life science, undoubtedly, it is significant and challenging to study and imitate these processes happening in living organisms with a convenient artificial system. Here we report a novel biomimetic nanochannel system which has an ion concentration effect that provides a nonlinear response to potassium ion at the concentration ranging from 0 to 1500 microM. This new phenomenon is caused by the G-quadruplex DNA conformational change with a positive correlation with ion concentration. In this work, G-quadruplex DNA was immobilized onto a synthetic nanopore, which undergoes a potassium-responsive conformational change and then induces the change in the effective pore size. The responsive ability of this system can be regulated by the stability of G-quadruplex structure through adjusting potassium concentration. The situation of the grafting G-quadruplex DNA on a single nanopore can closely imitate the in vivo condition because the G-rich telomere overhang is attached to the chromosome. Therefore, this artificial system could promote a potential to conveniently study biomolecule conformational change in confined space by the current measurement, which is significantly different from the nanopore sequencing. Moreover, such a system may also potentially spark further experimental and theoretical efforts to simulate the process of ion transport in living organisms and can be further generalized to other more complicated functional molecules for the exploitation of novel bioinspired intelligent nanopore machines.