A Tunable Ionic Diode Based on a Biomimetic Structure‐Tailorable Nanochannel

A tunable ionic diode is presented that is based on biomimetic structure‐tailorable nanochannels, with precise ion‐transport characteristics from ohmic behavior to bidirectional rectification as well as gating properties. The forward/reverse directions of the ionic diode and the degree of rectification can be well‐regulated by combining the patterned surface charge and the sophisticated structure. This system creates an ideal platform for precise transportation of ions and molecules, and potential applications in analytical sciences are anticipated.     

Biomimetic Nanofluidic Diode Composed of Dual Amphoteric Channels Maintains Rectification Direction over a Wide pH Range

pH‐gated ion channels in cell membranes play important roles in the cell's physiological activities. Many artificial nanochannels have been fabricated to mimic the natural phenomenon of pH‐gated ion transport. However, these nanochannels show pH sensitivity only within certain pH ranges. Wide‐range pH sensitivity has not yet been achieved. Herein, for the first time, we provide a versatile strategy to increase the pH‐sensitive range by using dual amphoteric nanochannels. In particular, amphoteric polymeric nanochannels with carboxyl groups derived from a block copolymer (BCP) precursor and nanochannels with hydroxyl groups made from anodic alumina oxide (AAO) were used. Due to a synergistic effect, the hybrid nanochannels exhibit nanofluidic diode properties with single rectification direction over a wide pH range. The novel strategy presented here is a scalable, low‐cost, and robust alternative for the construction of large‐area membranes for nanofluidic applications, such as the separation of biomolecules.     

平行四面体纳米孔道中DNA超结构的组装和离子输运特性

<正>受生物膜离子通道结构和功能的启发,人工制备固体纳米孔道门控开关器件一直备受关注[1,2].基于仿生纳米孔道的非对称离子传输性质制备的离子二极管和场效应管装置对于构建离子电路和能量转换的纳米器件至关重要[3,4].然而,仿生制备的固体纳米孔道在离子传输过程中有漏电流的存在,严重影响了固体纳米孔道应用的灵敏度和信噪比[5].针对这一问题,研究者利用DNA分子的特殊识别和自组装的功能特性,相继构筑了基于DNA和纳米孔道的智能响应体系[6,7].但在之前的研究工作中,分[8]     

具有串/并联复合离子输运特性的仿生纳米通道的构筑与整流性能

人工构筑了基于分枝氧化铝纳米通道的串/并联复合的纳流体二极管体系, 其具有可调的离子整流性能. 在这种两级分枝结构的1-2-2, 1-2-3, 1-3-2和1-3-3型氧化铝纳米通道中, 若将每一个分枝节点等效为一个二极管, 那么其一级分枝节点相当于串联的1个二极管, 二级分枝节点相当于并联的多个二极管. 因此1-2-2和1-2-3型纳米通道的电路图可等效为并联的2个二极管与第3个二极管相串联, 1-3-2和1-3-3型纳米通道的电路图可等效为并联的3个二极管与第4个二极管相串联. 但由于1-2-2和1-2-3型以及1-3-2和1-3-3型的二级分枝的结构和数目不同, 可将这4种纳米通道等效为不同的串/并联复合特性的纳流体二极管体系, 并且表现出依次增大的离子整流. 即分枝氧化铝纳米通道内部一级分枝和二级分枝的结构或数目共同调控的表面电荷非对称性可以改变其离子整流性能. 进一步地, 具有代表性的1-2-2型分枝纳米通道的整流率随分枝通道长度的增加而增加, 这表明分枝部分对整个串/并联复合纳流体二极管的整流特性起到决定性的作用. 相比于以前的单个离子二极管体系, 这种具有串/并联复合特性的多级分枝氧化铝纳米通道将为构筑更复杂的仿生纳流体二极管的研究提供有价值的借鉴.     

Highly-Efficient Ion Gating through Self-Assembled Two-Dimensional Photothermal Metal-Organic Framework Membrane

Biological ion channels regulate the ion flow across cell membrane via opening or closing of the pores in response to various external stimuli. Replicating the function of high ion gating effects with artificial porous materials has been challenging. Herein, we report that the self-assembled two-dimensional metal-organic framework (MOF) membrane can serve as an excellent nanofluidic platform for smart regulation of ion transport. The MOF membrane with good photothermal performance exhibits extremely high ion gating ratio (up to 10⁴), which is among the highest values in MOF membrane nanochannels for light-controlled ion gating reported so far. By repeatedly turning on and off the light, the nanofluidic device shows outstanding stability and reversibility that can be applied in the remote light-switching system. This work may spark promising applications of MOF membrane with variety of stimuli responsive properties in ion sieving, biosensing, and energy conversion.     

Ion transport regulation through triblock copolymer/PET asymmetric nanochannel membrane:Model system establishment and rectification mapping

Controlling ions transport across the membrane at different pH environments is essential for the physiological process and artificial systems.Many efforts have been devoted to pH-responsive ion gating,while rarely systems can maintain the rectification in pH-changing environments.Here,a composite nanochannel system is fabricated,which shows unidirectional rectification with high performance in a wide pH range.In the system,block copolymer(BCP) and polyethylene te rephthalate(PET) are employed for the amphoteric nanochannels fabrication.Based on the composite system,a model is built for the theoretical simulation.Thereafter,rectification mapping is conducted on the system,which can provide abundant info rmation about the relations between charge distribution and ions transport prope rties.The proposed rectification mapping can definitely help to design new materials with special ion transport properties,such as high-performance membranes used in the salinity gradient power generation field.     

A Tunable Ionic Diode Based on a Biomimetic Structure-Tailorable Nanochannel

A tunable ionic diode is presented that is based on biomimetic structure-tailorable nanochannels, with precise ion-transport characteristics from ohmic behavior to bidirectional rectification as well as gating properties. The forward/reverse directions of the ionic diode and the degree of rectification can be well-regulated by combining the patterned surface charge and the sophisticated structure. This system creates an ideal platform for precise transportation of ions and molecules, and potential applications in analytical sciences are anticipated.     

Smart DNA Hydrogel Integrated Nanochannels with High Ion Flux and Adjustable Selective Ionic Transport

Nanochannels based on smart DNA hydrogels as stimulus‐responsive architecture are presented for the first time. In contrast to other responsive molecules existing in the nanochannel in monolayer configurations, the DNA hydrogels are three‐dimensional networks with space negative charges, the ion flux and rectification ratio are significantly enhanced. Upon cyclic treatment with K⁺ ions and crown ether, the DNA hydrogel states could be reversibly switched between less stiff and stiff networks, providing the gating mechanism of the nanochannel. Based on the architecture of DNA hydrogels and pH stimulus, cation or anion transport direction could be precisely controlled and multiple gating features are achieved. Meanwhile, G‐quadruplex DNA in the hydrogels might be replaced by other stimulus‐responsive DNA molecules, peptides, or proteins, and thus this work opens a new route for improving the functionalities of nanochannel by intelligent hydrogels.     

Ion diode logics for pH control

Electronic control over the generation, transport, and delivery of ions is useful in order to regulate reactions, functions, and processes in various chemical and biological systems. Different kinds of ion diodes and transistors that exhibit non-linear current versus voltage characteristics have been explored to generate chemical gradients and signals. Bipolar membranes (BMs) exhibit both ion current rectification and water splitting and are thus suitable as ion diodes for the regulation of pH. To date, fast switching ion diodes have been difficult to realize due to accumulation of ions inside the device structure at forward bias--charges that take a long time to deplete at reverse bias. Water splitting occurs at elevated reverse voltage bias and is a feature that renders high ion current rectification impossible. This makes integration of ion diodes in circuits difficult. Here, we report three different designs of micro-fabricated ion bipolar membrane diodes (IBMDs). The first two designs consist of single BM configurations, and are capable of either splitting water or providing high current rectification. In the third design, water-splitting BMs and a highly-rectifying BM are connected in series, thus suppressing accumulation of ions. The resulting IBMD shows less hysteresis, faster off-switching, and also a high ion current rectification ratio as compared to the single BM devices. Further, the IBMD was integrated in a diode-based AND gate, which is capable of controlling delivery of hydroxide ions into a receiving reservoir.     

A pH-tunable nanofluidic diode: electrochemical rectification in a reconstituted single ion channel

We report pH-dependent electrochemical rectification in a protein ion channel (the bacterial porin OmpF) reconstituted on a planar phospholipid membrane. The measurements performed at single-channel level show that the electric current is controlled by the protein fixed charge and it can be tuned by adjusting the local pH. Under highly asymmetric pH conditions, the channel behaves like a liquid diode. Unlike other nanofluidic devices that display also asymmetric conductance, here the microscopic charge distribution of the system can be explored by using the available high-resolution (2.4 A) channel crystallographic structure. Continuum electrostatics calculations confirm the hypothesized bipolar structure of the system. The selective titration of the channel residues is identified as the underlying physicochemical mechanism responsible for current rectification.     

Insight into Ion Transfer through the Sub‐Nanometer Channels in Zeolitic Imidazolate Frameworks

A crack‐free sub‐nanometer composite structure for the study of ion transfer was constructed by in situ growth of ZIF‐90 [Zn(ICA)₂, ICA=Imidazole‐2‐carboxaldehyde] on the tip of a glass nanopipette. The potential‐driven ion transfer through the sub‐nanometer channels in ZIF‐90 is strongly influenced by the pH of the solution. A rectification ratio over 500 is observed in 1 m KCl solution under alkaline conditions (pH 11.58), which is the highest value reported under such a high salt concentration. Fluorescence experiments show the super‐high rectification ratio under alkaline conditions results from the strong electrostatic interaction between ions and the sub‐nanometer channels of ZIF‐90. In addition to providing a general pathway for further study of mass‐transfer process through sub‐nanometer channels, the approach enable all kinds of metal–organic frameworks (MOFs) to be used as ionic permselectivity materials in nanopore‐based analysis.     

A hydrophobic entrance enhances ion current rectification and induces dewetting in asymmetric nanopores

Hydrophobic interactions and local dewetting of hydrophobic cavities have been identified as a key mechanism for ionic gating in biological voltage-gated channels in a cell membrane. Hydrophobic interactions are responsible for rectification of the channels, i.e. the ability to transport ions more efficiently in one direction compared to the other. We designed single polymer nanopores with a hydrophobic gate on one side in the form of a single layer of C10 or C18 thiols. This nanoporous system behaves like an ionic diode whose direction of rectification is regulated by the pH of the electrolyte. In addition, reversible dewetting of the hydrophobic region of the pore was observed as voltage-dependent ion current fluctuations in time between conducting and non-conducting states. The observations are in accordance with earlier molecular dynamics simulations, which predicted the possibility of spontaneous and reversible dewetting of hydrophobic pores.     

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.     

Zn2+ and EDTA Cooperative Switchable Nanofluidic Diode Based on Asymmetric Modification of Single Nanochannel

Design and fabrication of smart switchable nanofluidic diodes remains a challenge in the life and materials sciences. Here, we present the first example of a novel Zn²⁺/EDTA switchable nanofluidic diode system based on the control of one‐side of the modified hourglass‐shaped nanochannel with salicylaldehyde Schiff base (SASB). The nanofluidic diode can be turned on in the response of Zn²⁺ and turned off in response to EDTA solution with good reversibility and recyclability.     

Highly rectified ion transport through 2D WSe2/MoS2 bi-layered membranes

Through a two-step vacuum-filtration process, WSe₂ and MoS₂ nanosheets were sequentially deposited onto a polymeric nanoporous support, forming WSe₂/MoS₂ bi-layered heterostructure. Highly rectified ion transport phenomenon is observed through the heterogeneous 2D layered membranes.     

Theoretical investigation into molecular diodes integrated in series using the non-equilibrium Green's function method

We have conducted a theoretical study on the electronic transport behaviour of two molecular diodes connected in series. The single diode is composed of o-nitrotoluene and o-aminotoluene connecting via a σ-bridge, and the tandem diode is two single diodes connecting via a π-bridge. It was found that the rectification ratio was greatly improved due to the electronic coupling in the tandem diode. The rectification ratio of the tandem molecular diode can be 20 times higher than that of the single diode, which is quite different from a traditional diode. In addition, we also found that the high rectification ratio correlates with the intramolecular coupling of the tandem system. When long conjugated wires are employed in two single diodes, the rectification ratio is reduced.     

Ionic Diode Characteristics at a Polymer of Intrinsic Microporosity (PIM) | Nafion “Heterojunction” Deposit on a Microhole Poly(ethylene‐terephthalate) Substrate

Ionic diode phenomena occur at asymmetric ionomer | aqueous electrolyte microhole interfaces. Depending on the applied potential, either an “open” or a “closed” diode state is observed switching between a high ion flow rate and a low ion flow rate. Physically, the “open” state is associated mainly with conductivity towards the microhole within the ionomer layer and the “closed” state is dominated by restricted diffusion‐migration access to the microhole interface opposite to the ionomer. In this report we explore a “heterojunction” based on an asymmetric polymer of intrinsic microporosity (PIM) | Nafion ionomer microhole interface. Improved diode characteristics and current rectification are observed in aqueous NaCl. The effects of creating the PIM | Nafion micro‐interface are investigated and suggested to lead to novel sensor architectures.     

The Influence of Divalent Anions on the Rectification Properties of Nanofluidic Diodes: Insights from Experiments and Theoretical Simulations

During the last decade, the possibility of generating synthetic nanoarchitectures with functionalities comparable to biological entities has sparked the interest of the scientific community related to diverse research fields. In this context, gaining fundamental understanding of the central features that determine the rectifying characteristics of the conical nanopores is of mandatory importance. In this work, we analyze the influence of mono‐ and divalent salts in the ionic current transported by asymmetric nanopores and focus on the delicate interplay between ion exclusion and charge screening effects that govern the functional response of the nanofluidic device. Experiments were performed using KCl and K₂SO₄ as representative species of singly and doubly charged species. Results showed that higher currents and rectification efficiencies are achieved by doubly charged salts. In order to understand the physicochemical processes underlying these effects simulations using the Poisson‐Nernst‐Planck formalism were performed. We consider that our theoretical and experimental account of the effect of divalent anions in the functional response of nanofluidic diodes provides further insights into the critical role of electrostatic interactions (ion exclusion versus charge screening effects) in presetting the ionic selectivity to anions as well as the observed rectification properties of these chemical nanodevices.     

Continuously Tunable Ion Rectification and Conductance in Submicrochannels Stemming from Thermoresponsive Polymer Self‐Assembly

A biomimetic conical submicrochannel (tip side ca. 400 nm) with functions of continuously tunable ion rectification and conductance based on thermoresponsive polymer layer‐by‐layer (LbL) self‐assembly is presented. These self‐assembled polymers with different layers exhibited a capability to regulate the effective channel diameter, and different ion rectifications/conductance were achieved. By controlling temperature, the conformation and wettability of the assembled polymers were reversibly transformed, thus the ion rectification/conductance could be further adjusted subtly. Owing to the synergistic effect, the ion conductance could be tuned over a wide range spanning three orders of magnitude. Moreover, the proposed system can be applied for on‐demand on‐off molecule delivery, which was important for disease therapy. This study opens a new door for regulating channel size according to actual demand and sensing big targets with different size with one channel.     

A facile route for the fabrication of large-scale gate-all-around nanofluidic field-effect transistors with low leakage current

Active modulation of ions and molecules via field-effect gating in nanofluidic channels is a crucial technology for various promising applications such as DNA sequencing, drug delivery, desalination, and energy conversion. Developing a rapid and facile fabrication method for ionic field-effect transistors (FET) over a large area may offer exciting opportunities for both fundamental research and innovative applications. Here, we report a rapid, cost-effective route for the fabrication of large-scale nanofluidic field-effect transistors using a simple, lithography-free two-step fabrication process that consists of sputtering and barrier-type anodization. A robust alumina gate dielectric layer, which is formed by anodizing sputtered aluminium, can be rapidly fabricated in the order of minutes. When anodizing aluminium, we employ a hemispherical counter electrode in order to give a uniform electric field that encompasses the whole sputtered aluminium layer which has high surface roughness. In consequence, a well-defined thin layer of alumina with perfect step coverage is formed on a highly rough aluminium surface. A gate-all-around nanofluidic FET with a leak-free gate dielectric exhibits outstanding gating performance despite a large channel size. The thin and robust anodized alumina gate dielectric plays a crucial role in achieving such excellent capacitive coupling. The combination of a gate-all-around structure with a leak-free gate dielectric over a large area could yield breakthroughs in areas ranging from biotechnology to energy and environmental applications.