Dynamic Curvature Nanochannel‐Based Membrane with Anomalous Ionic Transport Behaviors and Reversible Rectification Switch

Biological nanochannels control the movements of different ions through cell membranes depending on not only those channels' static inherent configurations, structures, inner surface's physicochemical properties but also their dynamic shape changes, which are required in various essential functions of life processes. Inspired by ion channels, many artificial nanochannel‐based membranes for nanofluidics and biosensing applications have been developed to regulate ionic transport behaviors by using the functional molecular modifications at the inner surface of nanochannel to achieve a stimuli‐responsive layer. Here, the concept of a dynamic nanochannel system is further developed, which is a new way to regulate ion transport in nanochannels by using the dynamic change in the curvature of channels to adjust ionic rectification in real time. The dynamic curvature nanochannel‐based membrane displays the advanced features of the anomalous effect of voltage, concentration, and ionic size for applying simultaneous control over the curvature‐tunable asymmetric and reversible ionic rectification switching properties. This dynamic approach can be used to build smart nanochannel‐based systems, which have strong implications for flexible nanofluidics, ionic rectifiers, and power generators.     

Muscle‐Inspired Highly Anisotropic,Strong, Ion‐Conductive Hydrogels

Biological tissues generally exhibit excellent anisotropic mechanical properties owing to their well‐developed microstructures. Inspired by the aligned structure in muscles, a highly anisotropic, strong, and conductive wood hydrogel is developed by fully utilizing the high–tensile strength of natural wood, and the flexibility and high‐water content of hydrogels. The wood hydrogel exhibits a high–tensile strength of 36 MPa along the longitudinal direction due to the strong bonding and cross‐linking between the aligned cellulose nanofibers (CNFs) in wood and the polyacrylamide (PAM) polymer. The wood hydrogel is 5 times and 500 times stronger than the bacterial cellulose hydrogels (7.2 MPa) and the unmodified PAM hydrogel (0.072 MPa), respectively, representing one of the strongest hydrogels ever reported. Due to the negatively charged aligned CNF, the wood hydrogel is also an excellent nanofluidic conduit with an ionic conductivity of up to 5 × 10^?4 S cm^–1 at low concentrations for highly selective ion transport, akin to biological muscle tissue. The work offers a promising strategy to fabricate a wide variety of strong, anisotropic, flexible, and ionically conductive wood‐based hydrogels for potential biomaterials and nanofluidic applications.     

Tunable Ionic Transport Control inside a Bio‐Inspired Constructive Bi‐Channel Nanofluidic Device

Inspired by the cooperative functions of the asymmetrical ion channels in living cells, a constructive bi‐channel nanofluidic device that demonstrates the enhanced capability of multiple regulations over both the ion flux amount and the ionic rectification property is prepared. In this bi‐channel system, the construction routes of the two asymmetric conical nanochannels provide a way to efficiently transform the nanodevice into four different functional working modes. In addition, the variation of external pH conditions leads the nanodevice to the uncharged, semi‐charged and charged states, where the multistory ionic regulating function property is enhanced by the charged degree. This intelligent integration of the single functional nanochannels demonstrates a promising future for building more functional multi‐channel integrated nanodevices as well as expands the functionalities of the bio‐inspired smart nanochannels.     

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.     

Reversible Ion‐Conducting Switch in a Novel Single‐Ion Supramolecular Hydrogel Enabled by Photoresponsive Host–Guest Molecular Recognition

A novel ion‐conducting supramolecular hydrogel with reversible photoconductive properties in which the azobenzene motif, α‐cyclodextrin (α‐CD), and ionic liquid are grafted onto the gel matrix is reported. Host–guest interactions with different association constants between α‐CD and azobenzene or the anionic part of the ionic liquid can be readily tuned by photoinduced trans–cis isomerization of the azobenzene unit. When irradiated by 365 nm light, α‐CD prefers to form a complex with the anionic part of the ionic liquid, resulting in decreased ionic mobility and thus high resistance of the hydrogel. However, under 420 nm light irradiation, a more stable complex is again formed between α‐CD and trans‐azobenzene, thereby releasing the bound anions to regenerate the low‐resistive hydrogel. As such, remote control of the ionic conductivity of the hydrogel is realized by simple host–guest chemistry. With the incorporation of a logic gate, this hydrogel is able to reversibly switch an electric circuit on and off by light irradiation with certain wavelengths. The concept of photoswitchable ionic conductivity of a hydrogel mediated by competitive molecular recognition is potentially promising toward the fabrication of optoelectronic devices and applications in bioelectronic technology.     

Preparation of High‐Performance Ionogels with Excellent Transparency,Good Mechanical Strength,and High Conductivity

Ionogels offer great potential for diverse electric applications. However, it remains challenging to fabricate high‐performance ionogels with both good mechanical strength and high conductivity. Here, a new kind of transparent ionogel with both good mechanical strength and high conductivity is designed via locking a kind of free ionic liquid (IL), i.e., 1‐ethyl‐3‐methylimidazolium dicyanamide ([EMIm][DCA]), into charged poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid) (PAMPS)‐based double networks. On the one hand, the charged PAMPS double network provides good mechanical strength and excellent recovery property. On the other hand, the free [EMIm][DCA] locked in the charged double network through electrostatic interaction offers ionic conductivity as high as ≈1.7–2.4 S m^?1 at 25 °C. It is demonstrated that the designed ionogel can be successfully used for a flexible skin sensor even under harsh conditions. Considering the rationally designed chemical structures of ILs and the diversity of charged polymer networks, it is envisioned that this strategy can be extended to a broad range of polymer systems. Moreover, functional components such as conducting polymers, 0D nanoparticles, 1D nanowires, and 2D nanosheets can be introduced into the polymer systems to fabricate diverse novel ionogels with unique functions. It is believed that this design principle will provide a new opportunity to construct next‐generation multifunctional ionogels.     

Direct‐Write Formation and Dissolution of Silver Nanofilaments in Ionic Liquid‐Polymer Electrolyte Composites

Materials with reconfigurable optical properties are candidates for applications such as optical cloaking and wearable sensors. One approach to fabricate these materials is to use external fields to form and dissolve nanoscale conductive channels in well‐defined locations within a polymer. In this study, conductive atomic force microscopy is used to electrochemically form and dissolve nanoscale conductive filaments at spatially distinct points in a polyethylene glycol diacrylate (PEGDA)‐based electrolyte blended with varying amounts of ionic liquid (IL) and silver salt. The fastest filament formation and dissolution times are detected in a PEGDA/IL composite that has the largest modulus (several GPa) and the highest polymer crystal fraction. This is unexpected because filament formation and dissolution events are controlled by ion transport, which is typically faster within amorphous regions where polymer mobility is high. Filament kinetics in primarily amorphous and crystalline regions are measured, and two different mechanisms are observed. The formation time distributions show a power‐law dependence in the crystalline regions, attributable to hopping‐based ion transport, while amorphous regions show a normal distribution. The results indicate that the timescale of filament formation/dissolution is determined by local structure, and suggest that structure could be used to tune the optical properties of the film.     

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.     

Carbon Nanotube and Graphene‐based Bioinspired Electrochemical Actuators

Bio‐inspired actuation materials, also called artificial muscles, have attracted great attention in recent decades for their potential application in intelligent robots, biomedical devices, and micro‐electro‐mechanical systems. Among them, ionic polymer metal composite (IPMC) actuator has been intensively studied for their impressive high‐strain under low voltage stimulation and air‐working capability. A typical IPMC actuator is composed of one ion‐conductive electrolyte membrane laminated by two electron‐conductive metal electrode membranes, which can bend back and forth due to the electrode expansion and contraction induced by ion motion under alternating applied voltage. As its actuation performance is mainly dominated by electrochemical and electromechanical process of the electrode layer, the electrode material and structure become to be more crucial to higher performance. The recent discovery of one dimensional carbon nanotube and two dimensional graphene has created a revolution in functional nanomaterials. Their unique structures render them intriguing electrical and mechanical properties, which makes them ideal flexible electrode materials for IPMC actuators in stead of conventional metal electrodes. Currently although the detailed effect caused by those carbon nanomaterial electrodes is not very clear, the presented outstanding actuation performance gives us tremendous motivation to meet the challenge in understanding the mechanism and thus developing more advanced actuator materials. Therefore, in this review IPMC actuators prepared with different kinds of carbon nanomaterials based electrodes or electrolytes are addressed. Key parameters which may generate important influence on actuation process are discussed in order to shed light on possible future research and application of the novel carbon nanomateials based bio‐inspired electrochemical actuators.     

Inside Front Cover: Conducting‐Polymer Nanotubes for Controlled Drug Release (Adv. Mater. 4/2006)

Martin and co‐workers report on p. 405 that nanotubes formed from the conducting polymer poly(3,4‐ethylenedioxythiophene) (PEDOT), as shown on the inside cover, can be used for the controlled release of anti‐inflammatory drugs. The fabrication process includes electrospinning of a biodegradable polymer, either poly(L ‐lactide) or poly(lactide‐co‐glycolide), into which the required drug is incorporated, followed by electrochemical deposition of the conducting polymer around the drug‐loaded electrospun nanofibers. Drug release from the nanotubes is achieved by external electrical stimulation of the nanotubes.     

Physicochemical Properties of Surface‐Initiated Polymer Films in the Modification and Processing of Materials

This article reviews the physicochemical aspects of surface‐initiated polymer films used to modify planar and non‐planar surfaces and to produce micro‐ and nanoscale patterned features. Particular emphasis is placed on the molecular composition of the polymer and its effect on surface and bulk properties of ultrathin films. Recent advances in the use of responsive polymer films that exhibit dramatically altered properties upon changes in solvent, temperature, or ionic strength are reviewed. The uses of surface‐initiated polymer films to modify materials' properties and impact applications in chromatography, nanoparticle‐templated synthesis, and carbon nanotube dispersion are highlighted.     

Stabilization of Polymer‐Hydrogel Capsules via Thiol–Disulfide Exchange

Polymer hydrogels are used in diverse biomedical applications including drug delivery and tissue engineering. Among different chemical linkages, the natural and reversible thiol–disulfide interconversion is extensively explored to stabilize hydrogels. The creation of macro‐, micro‐, and nanoscale disulfide‐stabilized hydrogels commonly relies on the use of oxidizing agents that may have a detrimental effect on encapsulated cargo. Herein an oxidization‐free approach to create disulfide‐stabilized polymer hydrogels via a thiol–disulfide exchange reaction is reported. In particular, thiolated poly(methacrylic acid) is used and the conditions of polymer crosslinking in solution and on colloidal porous and solid microparticles are established. In the latter case, removal of the core particles yields stable, hollow, disulfide‐crosslinked hydrogel capsules. Further, a procedure is developed to achieve efficient disulfide crosslinking of multilayered polymer films to obtain stable, liposome‐loaded polymer‐hydrogel capsules that contain functional enzymatic cargo within the liposomal subcompartments. This approach is envisaged to facilitate the development of biomedical applications of hydrogels, specifically those including fragile cargo.     

High‐Quality‐Factor Mid‐Infrared Toroidal Excitation in Folded 3D Metamaterials

With unusual electromagnetic radiation properties and great application potentials, optical toroidal moments have received increasing interest in recent years. 3D metamaterials composed of split ring resonators with specific orientations in micro‐/nanoscale are a perfect choice for toroidal moment realization in optical frequency considering the excellent magnetic confinement and quality factor, which, unfortunately, are currently beyond the reach of existing micro‐/nanofabrication techniques. Here, a 3D toroidal metamaterial operating in mid‐infrared region constructed by metal patterns and dielectric frameworks is designed, by which high‐quality‐factor toroidal resonance is observed experimentally. The toroidal dipole excitation is confirmed numerically and further demonstrated by phase analysis. Furthermore, the far‐field radiation intensity of the excited toroidal dipoles can be adjusted to be predominant among other multipoles by just tuning the incident angle. The related processing method expands the capability of focused ion beam folding technologies greatly, especially in 3D metamaterial fabrication, showing great flexibility and nanoscale controllability on structure size, position, and orientation.     

Superwettability‐Based Interfacial Chemical Reactions

Superwetting interfaces arising from the cooperation of surface energy and multiscale micro/nanostructures are extensively studied in biological systems. Fundamental understandings gained from biological interfaces boost the control of wettability under different dimensionalities, such as 2D surfaces, 1D fibers and channels, and 3D architectures, thus permitting manipulation of the transport physics of liquids, gases, and ions, which profoundly impacts chemical reactions and material fabrication. In this context, the progress of new chemistry based on superwetting interfaces is highlighted, beginning with mass transport dynamics, including liquid, gas, and ion transport. In the following sections, the impacts of the superwettability‐mediated transport dynamics on chemical reactions and material fabrication is discussed. Superwettability science has greatly enhanced the efficiency of chemical reactions, including photocatalytic, bioelectronic, electrochemical, and organic catalytic reactions, by realizing efficient mass transport. For material fabrication, superwetting interfaces are pivotal in the manipulation of the transport and microfluidic dynamics of liquids on solid surfaces, leading to the spatially regulated growth of low‐dimensional single‐crystalline arrays and high‐quality polymer films. Finally, a perspective on future directions is presented.     

An In Situ Cross‐Linked Nonaqueous Polymer Electrolyte for Zinc‐Metal Polymer Batteries and Hybrid Supercapacitors

This work reports the facile synthesis of nonaqueous zinc‐ion conducting polymer electrolyte (ZIP) membranes using an ultraviolet (UV)‐light‐induced photopolymerization technique, with room temperature (RT) ionic conductivity values in the order of 10^?3 S cm^?1. The ZIP membranes demonstrate excellent physicochemical and electrochemical properties, including an electrochemical stability window of >2.4 V versus Zn|Zn²⁺ and dendrite‐free plating/stripping processes in symmetric Zn||Zn cells. Besides, a UV‐polymerization‐assisted in situ process is developed to produce ZIP (abbreviated i‐ZIP), which is adopted for the first time to fabricate a nonaqueous zinc‐metal polymer battery (ZMPB; VOPO₄|i‐ZIP|Zn) and zinc‐metal hybrid polymer supercapacitor (ZMPS; activated carbon|i‐ZIP|Zn) cells. The VOPO₄ cathode employed in ZMPB possesses a layered morphology, exhibiting a high average operating voltage of ≈1.2 V. As compared to the conventional polymer cell assembling approach using the ex situ process, the in situ process is simple and it enhances the overall electrochemical performance, which enables the widespread intrusion of ZMPBs and ZMPSs into the application domain. Indeed, considering the promising aspects of the proposed ZIP and its easy processability, this work opens up a new direction for the emergence of the zinc‐based energy storage technologies.     

Ultrahigh‐Areal‐Capacitance Flexible Supercapacitor Electrodes Enabled by Conformal P3MT on Horizontally Aligned Carbon‐Nanotube Arrays

Nanocarbon electronic conductors combined with pseudocapacitive materials, such as conducting polymers, display outstanding electrochemical properties and mechanical flexibility. These characteristics enable the fabrication of flexible electrodes for energy‐storage devices; that is, supercapacitors that are wearable or can be formed into shapes that are easily integrated into vehicle parts. To date, most nanocarbon materials such as nanofibers are randomly dispersed as a network in a flexible matrix. This morphology inhibits ion transport, particularly under the high current density necessary for devices requiring high power density. Novel flexible densified horizontally aligned carbon nanotube arrays (HACNTs) with controlled nanomorphology for improved ion transport are introduced and combined with conformally coated poly(3‐methylthiophene) (P3MT) conducting polymer to impart pseudocapacitance. The resulting P3MT/HACNT nanocomposite electrodes exhibit high areal capacitance of 3.1 F cm^?2 at 5 mA cm^?2, with areal capacitance remaining at 1.8 F cm^?2 even at a current density of 200 mA cm^?2. The asymmetric supercapacitor cell also delivers more than 1–2 orders of magnitude improvement in both areal energy and power density over state‐of‐the‐art cells. Furthermore, little change in cell performance is observed under high strain, demonstrating the mechanical and electrochemical stability of the electrodes.     

The Enhancing Ionic Current Rectification in Single Graphene Conical Nanochannel

Enhanced ionic current rectification induced by the concentration gradient has been reported recently owing to the development of asymmetric nanofluidic system, in particular graphene nanochannels. However, the reasons for the enhancement of ionic current rectification of graphene conical nanochannels are not well understood. Herein, a finite element model of a single graphene conical nanochannel is established to explore the fundamental mechanisms of the ionic current rectification. The model proves that the highest cation concentration caused by graphene is almost 42 times higher than the bulk ion concentration. Compared with the nongraphene model, the graphene with higher surface charge density can absorb more cation in the tip (the small pore) of graphene conical nanochannel. Besides, the nanoscale corner consisting of graphene membrane and the wall of the conical nanochannel is a semihermetic reservoir for cation accumulation. It demonstrates that the enhancement of ionic current rectification is mainly ascribed to the unique structure of the graphene nanochannel and the graphene membrane with a high surface charge density. This work provides a new mechanism explanation about the influence of graphene in ionic current rectification.     

Thermally controlled permeation of ionic molecules through synthetic nanopores functionalized with amine-terminated polymer brushes

We present temperature-dependent ionic transport through an array of nanopores (cylindrical and conical) and a single conical nanopore functionalized with amine-terminated poly(N-isopropylacrylamide) [PNIPAAM-NH(2)] brushes. For this purpose, nanopores are fabricated in heavy ion irradiated polyethylene terephthlate (PET) membranes by a controlled chemical track-etching technique, which leads to the generation of carboxyl (COOH) groups on the pore surface. End-functionalized polymer chains are immobilized onto the inner pore walls via a 'grafting-to' approach through the covalent linkage of surface COOH moieties with the terminal amine groups of the PNIPAAM molecules by using carbodiimide coupling chemistry. The success of the chemical modification reaction is corroborated by measuring the permeation flux of charged analytes across the multipore membranes in an aqueous solution, and for the case of single conical pore by measuring the current-voltage (I-V) characteristics, which are dictated by the electrostatic interaction of the charged pore surface with the mobile ions in an electrolyte solution. The effective nanopore diameter is tuned by manipulating the environmental temperature due to the swelling/shrinking behaviour of polymer brushes attached to the inner nanopore walls, leading to a decrease/increase in the ionic transport across the membrane. This process should permit the thermal gating and controlled release of ionic drug molecules through the nanopores modified with thermoresponsive polymer chains across the membrane.     

Soft but Powerful Artificial Muscles Based on 3D Graphene–CNT–Ni Heteronanostructures

Bioinspired soft ionic actuators, which exhibit large strain and high durability under low input voltages, are regarded as prospective candidates for future soft electronics. However, due to the intrinsic drawback of weak blocking force, the feasible applications of soft ionic actuators are limited until now. An electroactive artificial muscle electro‐chemomechanically reinforced with 3D graphene–carbon nanotube–nickel heteronanostructures (G–CNT–Ni) to improve blocking force and bending deformation of the ionic actuators is demonstrated. The G–CNT–Ni heteronanostructure, which provides an electrically conductive 3D network and sufficient contact area with mobile ions in the polymer electrolyte, is embedded as a nanofiller in both ionic polymer and conductive electrodes of the ionic actuators. An ionic exchangeable composite membrane consisting of Nafion, G–CNT–Ni and ionic liquid (IL) shows improved tensile modulus and strength of up to 166% and 98%, respectively, and increased ionic conductivity of 0.254 S m^?1. The ionic actuator exhibits enhanced actuation performances including three times larger bending deformation, 2.37 times higher blocking force, and 4 h durability. The electroactive artificial muscle electro‐chemomechanically reinforced with 3D G–CNT–Ni heteronanostructures offers improvements over current soft ionic actuator technologies and can advance the practical engineering applications.     

Silver‐Quantum‐Dot‐Modified MoO3 and MnO2 Paper‐Like Freestanding Films for Flexible Solid‐State Asymmetric Supercapacitors

Free‐standing paper‐like thin‐film electrodes have great potential to boost next‐generation power sources with highly flexible, ultrathin, and lightweight requirements. In this work, silver‐quantum‐dot‐ (2–5 nm) modified transition metal oxide (including MoO₃ and MnO₂) paper‐like electrodes are developed for energy storage applications. Benefitting from the ohmic contact at the interfaces between silver quantum dots and MoO₃ nanobelts (or MnO₂ nanowires) and the binder‐free nature and 0D/1D/2D nanostructured 3D network of the fabricated electrodes, substantial improvements on the electrical conductivity, efficient ionic diffusion, and areal capacitances of the hybrid nanostructure electrodes are observed. With this proposed strategy, the constructed asymmetric supercapacitors, with Ag quantum dots/MoO₃ “paper” as anode, Ag quantum dots/MnO₂ “paper” as cathode, and neutral Na₂SO₄/polyvinyl alcohol hydrogel as electrolyte, exhibit significantly enhanced energy and power densities in comparison with those of the supercapacitors without modification of Ag quantum dots on electrodes; present excellent cycling stability at different current densities and good flexibility under various bending states; offer possibilities as high‐performance power sources with low cost, high safety, and environmental friendly properties.