Organic/Inorganic Hybrid Nanochannels Based on Polypyrrole‐Embedded Alumina Nanopore Arrays: pH‐ and Light‐Modulated Ion Transport

Inspired by the asymmetric structure and responsive ion transport in biological ion channels, organic/inorganic hybrid artificial nanochannels exhibiting pH‐modulated ion rectification and light‐regulated ion flux have been constructed by introducing conductive polymer into porous nanochannels. The hybrid nanochannels are achieved by partially modifying alumina (Al₂O₃) nanopore arrays with polypyrrole (PPy) layer using electrochemical polymerization, which results in an asymmetric component distribution. The protonation and deprotonation of Al₂O₃ and PPy upon pH variation break the surface charge continuity, which contributes to the pH‐tunable ion rectification. The ionic current rectification ratio is affected substantially by the pH value of electrolyte and the pore size of nanochannels. Furthermore, the holes (positive charges) in PPy layer induced by the cooperative effect of light and protons are used to regulate the ionic flux through the nanochannels, which results in a light‐responsive ion current. The magnitude of responsive ionic current could be amplified by optimizing this cooperation. This new type of stimuli‐responsive PPy/Al₂O₃ hybrid nanochannels features advantages of unique optical and electric properties from conducting PPy and high mechanical performance from porous Al₂O₃ membrane, which provide a platform for creating smart nanochannels system.     

Bioinspired Self‐Gating Nanofluidic Devices for Autonomous and Periodic Ion Transport and Cargo Release

The biochemical oscillatory reaction induced self‐gating process of biological ion channels is essential to life processes, characterized as autonomous, continuous, and periodic. However, few synthetic nanochannel systems can achieve such excellent self‐gating property. Their gating properties work greatly depending on the frequent addition of reactants or the supply of external stimuli. Herein, a novel bioinspired self‐gating nanofluidic device that can transport mass in a continuous and periodic manner is reported. This self‐gating device is constructed by using a fully closed‐system pH oscillator to control the gating processes of the artificial proton‐gated nanochannels. With cyclic oscillation of protons inside the nanochannel induced by the oscillatory chemical reactions of the pH oscillator, surface charge density and polarity of the nanochannels can be self‐regulated, resulting in an autonomous and periodic switching of the nanochannel conductance between high and low states as well as the selectivity between cation selective and anion selective states. Moreover, by using Rhodamine B and Ruthenium(II) compound as the cationic cargoes, periodic release of these charged molecules is also observed. Therefore, this work opens up a new avenue to build self‐gating nanofluidic devices, which may not only act as ion oscillators, but potentially find applications in controlled‐release fields as well.     

Multi-Control of Ion Transport in a Field-Effect Iontronic Device based on Sandwich-Structured Nanochannels

Biomimetic smart nanochannels can regulate ion transport behavior responsive to the external stimuli, having huge potential in nanofluidic devices, sensors and energy conversion. Field-effect nanofluidic diodes or transistors based on electric-responsive nanochannels are emerging owing to their advantages such as non-invasiveness, in situ, real time, and high efficiency. However, simultaneously realizing the voltage-control of the ion conductance and ion current rectification (ICR) properties is still a big challenge. Here, a field-effect iontronic device is developed based on ionomer/anodic aluminum oxide/conducting polymer sandwich-structured nanochannel to realize the multi-control of ion transport behaviors including ion conductance, ICR magnitude, and ICR direction by modulating the surface charge, wettability, and morphology of the nanochannel. The electroactive conducting polymer carries tunable surface charges responsive to the electric stimuli, leading to the regulation of ICR values. The complex three-segment structures lead to the reverse of ICR direction by reconfiguring the charge distribution along with the whole channel. The switching wettability between hydrophilic and hydrophobic results in the regulation of ion conductance. Furthermore, the field-effect iontronic device functions in a wide salinity range especially in hypersaline environment, due to the salinity-adaptive properties of the membrane. A new route is provided for designing more functional field-effect nanofluidic devices.     

A Bio‐inspired Potassium and pH Responsive Double‐gated Nanochannel

Bio‐inspired nanochannels have emerged as an interface to mimic the functionalities of biological nanochannels. One remaining challenge is to develop double‐gated nanochannels with dual response, which can regulate the ion transport direction by alternately opening and closing the two gates. In this work, a bio‐inspired potassium and pH responsive double‐gated nanosystem is presented, constructed through immobilizing C‐quadruplex and G‐quadruplex DNA molecules onto the top and bottom tip side of a cigar‐shaped nanochannel, respectively. It is demonstrated that the two gates of the nanochannel can be opened and closed alternately/simultaneously. This phenomenon results from the attached DNA conformational transition caused by adjusting the concentrations of potassium ion and proton. This design is believed to be the first example of dual‐responsive double‐gated nanosystem, and paves a new way to investigate more intelligent bio‐inspired nanofluidic system.     

Fabrication of Bio‐Inspired 2D MOFs/PAA Hybrid Membrane for Asymmetric Ion Transport

Biological ion channels are known as membrane proteins which can turn on and off under environmental stimulus to regulate ion transport and energy conversion. Rapid progress made in biological ion channels provides inspiration for developing artificial nanochannels to mimic the structures and functions of ion transport systems and energy conversion in biological ion channels. Due to the advantages of abundant pore channels, metal–organic frameworks (MOFs) have become competitive materials to control the nanofluidic transport. Herein, a facile in situ synthesis method is developed to prepare hybrid nanochannels constructed by 2D MOFs and porous anodic aluminum (PAA). The introduction of asymmetries in the chemical composition and surface charge properties gives the hybrid outstanding ion current rectification properties and excellent ion selectivity. A power density of 1.6 W m^?2 is achieved by integrating it into a salinity‐gradient‐driven device. With advantages of facile fabrication method and high ion selectivity, the prepared 2D MOFs/PAA hybrid membrane offers a promising candidate for power conversion and water desalination.     

Synthetic Asymmetric‐Shaped Nanodevices with Symmetric pH‐Gating Characteristics

Synthetic stimuli‐gated nanodevices displaying intelligent ion transport properties similar to those observed in biological ion channels have attracted increasing interests for their wide potential applications in biosensors, nanofluidics, and energy conversions. Here, bioinspired asymmetric shaped nanodevices are reported that can exhibit symmetric and linear pH‐gating ion transport features based on polyelectrolyte‐asymmetric‐functionalized asymmetric hourglass‐shaped nanochannels. The pH‐responsive polymer brushes grafted on the inner channel surface are acted as a gate that open and close in response to external pH changing to linearly and symmetrically regulate transmembrane ionic currents of the channel. A complete experimental characterization of the pH‐dependent ion transport behaviors of the nanodevice and a comprehensive discussion of the experimental results in terms of theoretical simulation are also presented. Both experimental and theoretical data shown in this work demonstrate the feasibility of using the asymmetric chemical modification method to achieve symmetric pH gating behaviors inside the asymmetric nanochannels, and lay the foundation to build diverse stimuli‐gated artificial asymmetric shaped ion channels with symmetric gating ion transport features.     

Ionic Current Rectification in Soft‐Matter Diodes with Liquid‐Metal Electrodes

A soft‐matter‐based diode composed of hydrogel and liquid metal (eutectic gallium indium, EGaIn) is presented. The ability to control the thickness, and thus resistivity, of an oxide skin on the metal enables rectification. First, a simple model system with liquid‐metal/electrolyte‐solution/Pt interfaces is characterized. The electrically insulating oxide skin on the EGaIn electrode is reduced or oxidized further depending on the direction of the bias, thereby allowing unidirectional ionic current. The forward current of the diode increases as the conductivity of the electrolyte increases, whereas backward current depends on the pH of the medium in contact with the insulating oxide layer on the EGaIn electrode. As a result, the diode shows a higher rectification ratio (defined as the ratio of forward to backward current measured at the same absolute bias) with more conductive electrolyte at neutral pH. Replacement of the liquid electrolyte solution with a hydrogel improves the structural stability of the soft diode. The rectification performance also improves due to the increased ionic conductivity by the gel. Finally, a diode composed entirely of soft materials by replacing the platinum electrode with a second liquid‐metal electrode is presented. Contacting each liquid metal with a polyelectrolyte gel featuring different pH values provided asymmetry in the device, which is necessary for rectification. A hydrogel layer infused with a strong basic polyelectrolyte removes the insulating oxide layer, allowing one interface with the EGaIn electrode to be conductive regardless of the direction of bias. Thus, the oxide layer at the other interface rectifies the current.     

Light‐Gating Titania/Alumina Heterogeneous Nanochannels with Regulatable Ion Rectification Characteristic

Bioinspired artificial nanochannels exhibiting ion transport properties similar to biological ion channels have been attracting some attention for biosensors, separation technologies, and nanofluidic diodes. Herein, an easily available artificial heterogeneous nanochannel shows both ion gating and ion rectification characteristics when irradiated by ultraviolet light. The fabrication of heterogeneous nanochannels includes the coating of an anatase TiO₂ porous layer on an alumina porous supporter, followed by a chemical modification with octadecyltrimethoxysilane (OTS) molecules. The irreversible decomposition of OTS molecules by TiO₂ photocatalysis under ultraviolet light results in a change of surface wettability and an asymmetric distribution of surface negative charges simultaneously, which contributes to the ion gating and ion rectification. The asymmetric distribution of negative charges in the TiO₂ porous layer can be controlled by the irradiation time of ultraviolet light, which regulates the ion rectification characteristic.     

Engineered Smart Gating Nanochannels for High Performance in Formaldehyde Detection and Removal

Bioinspired nanochannels for smart mass transport control have shown great potential for various applications in nanofluids, biosensing, and separation. Here, a nanochannel‐based smart responsive platform exhibiting high formaldehyde (HCHO) sensitivity is designed and successfully fabricated by functionalizing the inner pore surface with ethylenediamine (EDA). By employing the nucleophilic addition reaction between HCHO and EDA immobilized on the nanochannels, the artificial nanochannels can switch from an open state to a closed state with the increase in HCHO. This is because the surface charge density and the wettability of the nanochannels change along with the HCHO immobilization. Meanwhile, the EDA‐functionalized platform can hold a large amount of HCHO due to the abundant nanochannels of the membrane, so it presents a significant ability to remove HCHO in complex matrices. Also, the cultivation of mesenchymal stem cells in media containing HCHO can achieve excellent vitality in the presence of the EDA‐functionalized nanochannels materials. This work paves an avenue for designing and developing bioinspired nanochannel based platform for harmful compounds detection and removal.     

Ionic Diode Based on an Asymmetric-Shaped Carbon Black Nanoparticle Membrane

Bioinspired ionic diodes are widely explored to mimic the controllable ion transport of biological ion channels. However, due to their vertical structures, the integration of conventional ionic diodes into complex ionic circuits is still a challenge. Here, a horizontal ionic diode is developed based on an asymmetric nanochannel network membrane (NCNM) constructed from carbon black nanoparticles. The rectification of ionic current is achieved through the asymmetric concentration polarization of ions at two ends of the asymmetric NCNM. The rectification ratio of the NCNM ionic diode can be modified flexibly by changing the working fluid and the geometry of the NCNM. It is found that with the presence of cationic surfactant in the working fluid, the rectification ratio increases more than 30 times from 3.03 to 109.77. Advanced functions of the developed ionic component, including working as an ionic transistor for current switching and integrating into an ionic diode bridge on a single nanofluidic chip for rectifying alternating current signals, are also demonstrated in this paper. The horizontally arranged NCNM ionic diode possesses the advantages of easy fabrication and integration that can be practically applied in the development of ionic electronics and biocomputing.     

Ionotronics Based on Horizontally Aligned Carbon Nanotubes

Controlled ion transport through ion channels of cell membranes regulates signal transduction processes in biological systems and has also inspired the thriving development of ionic electronics (ionotronics or iontronics) and biocomputing. However, for constructing highly integrated ionic electronic circuits, the integration of natural membrane‐spanning ion channel proteins or artificial nanomembrane‐based ionic diodes into planar chips is still challenging due to the vertically arranged architecture of conventional nanomembrane‐based artificial ionic diodes. Here, a new design of ionic diode is reported, which allows chip‐scale integration of ionotronics, based on horizontally aligned nanochannels made from multiwalled carbon nanotubes (MWCNTs). The rectification of ion transport through the MWCNT nanochannels is enabled by decoration of oppositely charged polyelectrolytes on the channel entrances. Advanced ionic electronic circuits including ionic logic gates, ionic current rectifiers, and ionic bipolar junction transistors (IBJT) are demonstrated on planar nanofluidic chips by stacking a series of ionic diodes fabricated from the same bundles of MWCNTs. The horizontal arrangement and facile chip‐scale fabrication of the MWCNT ionic diodes may enable new designs of complex but monolithic ionotronic systems. The MWCNT ionic diode may also prove to be an excellent platform for investigation of electrokinetic ion transport in 1D carbon materials.     

Facile Preparation of a Patterned,Aminated Polymer Surface by UV‐Light‐Induced Surface Aminolysis

Introducing amine functional groups on polymer surfaces is extremely important for studying various processes that involve polymer surfaces. We report a novel and extremely simple method for preparing a tertiary‐amine‐terminated poly(ethylene terephthalate) (PET) surface by using a UV‐light‐induced surface aminolysis reaction. X‐ray photoelectron spectroscopy and attenuated total‐reflection infrared spectroscopy give direct evidence of the incorporation of tertiary amine functionalities and the possible reaction mechanism behind this technique. Tertiary amines are easily protonated, so we have developed an extremely simple method for immobilizing and patterning biomolecules on a soft surface by the electrostatic self‐assembly of proteins, such as immunoglobulin (IgG) and horseradish peroxide (HRP), onto a patterned, aminated surface. An enzyme–substrate reaction, which is followed optically by observing the resulting precipitation on the surface, is used to reveal the patterned immobilization of HRP, where 3‐amino‐9‐ethylcarbazole, as a substrate for HRP, is deposited on the aminated surface after HRP adsorption. Fluorescein isothiocyanate‐labeled IgG (FITC‐IgG) has been immobilized electrostatically onto the ordered aminated spots, and the fluorescence intensity ratio of the IgG‐immobilized region (inside the spot) to the background (outside the spot) is about 5:1, as calculated from a fluorescence image and fluorescence spectra obtained by microlaser confocal Raman spectroscopy. We have found that the background intensity is mainly caused by the autofluorescence of virgin PET, and after subtracting this value from the measured intensity inside and outside the spot, respectively, a much higher intensity ratio between the spot and the background is obtained (about 22:1). The patterned immobilization of FITC‐IgG has been further proven by examining the change in intensity inside the spot after photobleaching the fluorophore.     

Protein‐Enabled Layer‐by‐Layer Syntheses of Aligned,Porous‐Wall,High‐Aspect‐Ratio TiO2 Nanotube Arrays

An aqueous, protein‐enabled (biomimetic), layer‐by‐layer titania deposition process is developed, for the first time, to convert aligned‐nanochannel templates into high‐aspect‐ratio, aligned nanotube arrays with thin (34 nm) walls composed of co‐continuous networks of pores and titania nanocrystals (15 nm ave. size). Alumina templates with aligned open nanochannels are exposed in an alternating fashion to aqueous protamine‐bearing and titania precursor‐bearing (Ti(IV) bis‐ammonium‐lactato‐dihydroxide, TiBALDH) solutions. The ability of protamine to bind to alumina and titania, and to induce the formation of a Ti–O‐bearing coating upon exposure to the TiBALDH precursor, enables the layer‐by‐layer deposition of a conformal protamine/Ti–O‐bearing coating on the nanochannel surfaces within the porous alumina template. Subsequent protamine pyrolysis yields coatings composed of co‐continuous networks of pores and titania nanoparticles. Selective dissolution of the underlying alumina template through the porous coating then yields freestanding, aligned, porous‐wall titania nanotube arrays. The interconnected pores within the nanotube walls allow enhanced loading of functional molecules (such as a Ru‐based N719 dye), whereas the interconnected titania nanoparticles enable the high‐aspect‐ratio, aligned nanotube arrays to be used as electrodes (as demonstrated for dye‐sensitized solar cells with power conversion efficiencies of 5.2 ± 0.4%).     

Ion‐Current Rectification in Nanopores and Nanotubes with Broken Symmetry

This article focuses on ion transport through nanoporous systems with special emphasis on rectification phenomena. The effect of ion‐current rectification is observed as asymmetric current–voltage (I–V) curves, with the current recorded for one voltage polarity higher than the current recorded for the same absolute value of voltage of opposite polarity. This diode‐like I–V curve indicates that there is a preferential direction for ion flow. Experimental evidence that ion‐current rectification is inherent to asymmetric, e.g., tapered, nanoporous systems with excess surface charge is provided and discussed. The fabrication and operation of asymmetric polymer nanopores, gold nanotubes, glass nanocapillaries, and silicon nanopores are presented. The possibility of tuning the direction and extent of rectification is discussed in detail. Theoretical models that have been developed to explain the ion‐current rectification effect are also presented.     

Downscaling and Charge Transport in Nanostructured Ferroelectric Memory Diodes Fabricated by Solution Micromolding

Ferroelectric polymer memory diodes are interface devices where charge injection into the organic semiconductor is controlled by the stray electric field of the ferroelectric polymer. Key to high current density and current modulation is the areal density of well‐defined interfaces. Here, bistable diodes are fabricated by using the soft lithography method solution micromolding. First, the semiconducting polymer poly(9,9‐dioctylfluorene) is patterned into linear gratings. Subsequently, bilinear arrays are obtained by backfilling with the ferroelectric polymer poly(vinylidenefluoride‐co‐trifluoroethylene). The lateral feature size is scaled down from 2 μm to 500 nm. Comprising memory diodes show rectifying J–V characteristics with an On‐current density larger than 10³ A m^?2 and an On/Off current ratio exceeding 10³. The charge transport is explained by 2D numerical simulations. Since the dependence of polarization on electric field is explicitly taken into account, entire J–V characteristics can be quantitatively described. The simulations reveal that rectifying J–V characteristics are inherently related to the concave shape of the patterned ferroelectric polymer. It is argued that the exponential increase in current density with decreasing feature size can be due to confinement of the semiconductor. High On‐current density combined with downscaling, rectification, and simple fabrication yield new opportunities for low‐cost integration of high‐density solution‐processed memories.     

Tunable Nanochannels along Graphene Oxide/Polymer Core–Shell Nanosheets to Enhance Proton Conductivity

Simultaneous manipulation of topological and chemical structures to induce ionic nanochannel formation within solid electrolytes is a crucial but challenging task for the rational design of high‐performance electrochemical devices including proton exchange membrane fuel cell. Herein, a novel generic approach is presented for the construction of tunable ion‐conducting nanochannels via direct assembly of graphene oxide (GO)/poly(phosphonic acid) core–shell nanosheets prepared by surface‐initiated precipitation polymerization. Using this simple and rapid approach to engineer GO/polymer nanosheets at the molecular‐level, ordered and continuous nanochannels with interconnected hydrogen‐bonded networks having a favorable water environment can be created. The resulting membranes exhibit proton conductivities up to 32 mS cm^?1 at 51% relative humidity, surpassing state‐of‐the‐art Nafion membrane and all previously reported GO‐based materials.     

Titania Nanotubes: Protein‐Enabled Layer‐by‐Layer Syntheses of Aligned,Porous‐Wall,High‐Aspect‐Ratio TiO2 Nanotube Arrays (Adv. Funct. Mater. 9/2011)

An aqueous, protein‐enabled (biomimetic), layer‐by‐layer titania deposition process is developed, for the first time, to convert aligned‐nanochannel templates into high‐aspect‐ratio, aligned nanotube arrays with thin (34 nm) walls composed of co‐continuous networks of pores and titania nanocrystals (15 nm ave. size). Alumina templates with aligned open nanochannels are exposed in an alternating fashion to aqueous protamine‐bearing and titania precursor‐bearing (Ti(IV) bis‐ammonium‐lactato‐dihydroxide, TiBALDH) solutions. The ability of protamine to bind to alumina and titania, and to induce the formation of a Ti–O‐bearing coating upon exposure to the TiBALDH precursor, enables the layer‐by‐layer deposition of a conformal protamine/Ti–O‐bearing coating on the nanochannel surfaces within the porous alumina template. Subsequent protamine pyrolysis yields coatings composed of co‐continuous networks of pores and titania nanoparticles. Selective dissolution of the underlying alumina template through the porous coating then yields freestanding, aligned, porous‐wall titania nanotube arrays. The interconnected pores within the nanotube walls allow enhanced loading of functional molecules (such as a Ru‐based N719 dye), whereas the interconnected titania nanoparticles enable the high‐aspect‐ratio, aligned nanotube arrays to be used as electrodes (as demonstrated for dye‐sensitized solar cells with power conversion efficiencies of 5.2 ± 0.4%).     

An Electrical Rectifier Based on Au Nanoparticle Array Fabricated Using Direct‐Write Electron Beam Lithography

Close‐packed arrays of Au nanoparticles are produced in patterned regions by electron beam (e‐beam) lithography using a highly sensitive direct–write resist, N⁺AuCl₄^?(C₈H₁₇)₄Br. While the e–beam causes dewetting of the resist to nucleate Au nanoparticles, the following step of thermolysis aids particle growth and removal of the organic part. Thus formed arrays contain Au nanoparticles. Such arrays are patterned into ≈10 μm wide stripes between Au contact pads on SiO₂/Si substrates to realize electrical rectification. Under forward bias, the device exhibits a threshold voltage of +4.3 V and a high current rectification ratio of 3 × 10⁵, which are stable over many repetitive measurements. The threshold voltage of the rectifier can be reduced by applying an electric stress or by varying the electron dosage used for array formation. The nanoparticle rectifier element could be transferred onto flexible substrates such as PDMS, where the nanoparticle coupling is influenced by swelling of the substrate. Obviously, the nanoparticle size, shape, and the spacing in array are all important for the rectifier device performance. Based on the electrical measurements the mechanism of rectification is found to be due to switching of electrical conduction with applied bias, from short–distance tunneling to F–N type tunneling followed by transient filament formation.     

Chemical Lithography by Surface‐Induced Photoreaction of Nitro Compounds

Searching for systems of self‐assembled monolayers (SAMs) that can be used as templates for chemical lithography, we found that nitro groups on aromatic SAMs are selectively converted on Ag to amino groups by irradiation with a visible laser. 4‐nitrobenzenethiol on Ag was thus converted to 4‐aminobenzenethiol by irradiating it with an Ar⁺ laser. This was evident from surface‐enhanced Raman scattering (SERS) as well as from a coupling reaction forming amide bonds. The surface‐induced photoreaction allowed us to prepare patterned binary monolayers on Ag that showed different chemical reactivities. Using the binary monolayers as a lithographic template, we induced site‐specific chemical reactions, such as the selective growth of biominerals on either the nitro‐ or amine‐terminated regions by adjusting the crystal‐growth conditions. We also demonstrated that patterned, amine‐terminated monolayers can be fabricated even on gold by using silver nanoparticles as photoreducing catalysts.     

Multifunctional Lateral Transition‐Metal Disulfides Heterojunctions

The intrinsic spin‐dependent transport properties of two types of lateral VS₂|MoS₂ heterojunctions are systematically investigated using first‐principles calculations, and their various nanodevices with novel properties are designed. The lateral VS₂|MoS₂ heterojunction diodes show a perfect rectifying effect and are promising for the applications of Schottky diodes. A large spin‐polarization ratio is observed for the A‐type device and pure spin‐mediated current is then realized. The gate voltage significantly tunes the current and rectification ratio of their field‐effect transistors. In addition, they all demonstrate a sensitive photoresponse to blue light, and could be used as photodetector and photovoltaic device. Moreover, they generate an effective thermally driven current when a temperature gratitude appears between the two terminals, suggesting them as potential thermoelectric materials. Hence, the lateral VS₂|MoS₂ heterojunctions show a multifunctional nature and have various potential applications in spintronics, optoelectronics, and spin caloritronics.