A light-regulated host-guest-based nanochannel system inspired by channelrhodopsins protein

Super-Assembled Lamellar Conductive Heterochannels with Optical-Electrical Coupling Sensitivity for Smart Ion Transport

Artificial nanofluidic devices inspired bylight-driven ion transport in biological systems, leveraging the photoelectric effect, have attracted extensive attention for their potential in signal transduction and smart ion transport applications. However, effective separation of photogenerated carriers in traditional p-n junction interface can be hindered by energy band structure of different semiconductor materials. Here, we present a novel approach using conductive polypyrrole (PPy) to modify graphene oxide (GO), creating polypyrrole-graphene oxide (PyGO) functional lamellar conductive nanochannels with tailored channel-sized gradients and inherent optical-electrical coupling sensitivity via a facile super-assembly strategy. This design facilitates the PyGO own conductive lamellar channels and efficient separation of photogenerated carriers, resulting in significantly enhanced selective ion transport behavior. Coupling the conductivity and photosensitivity of PPy contributes to a peak power density of 14.1 W m-2 under a salinity differential of 0.5/0.01 M NaCl, which is 35.6 % higher than that under dark conditions. Additionally, combing the salinity gradients with optical-electrical coupling sensitivity of the nanofludic devices, we demonstrate the application of PyGO in a real-time detection device for monitoring ion concentrations in nutrient solutions, paving the way for smart irrigation systems in agriculture. This work presents a novel and effective strategy for light-driven ion transport with potential applications in energy conversion and beyond.

Light-Boosted Osmotic Energy Conversion and Ion Pumping through a Graphdiyne Oxide-Based Membrane

Two-dimensional (2D) membranes with engineered light-responsive ion transport dynamics have been explored to construct efficient nanofluidic platforms that show great potential in osmotic and solar-osmotic energy conversion. However, the power density is still limited by poor responsivity and the inevitable trade-off effect between ion selectivity and flux. Here, we observed light-pumping ion transport behavior in graphdiyne oxide (GDYO) with a unique carbon hybrid skeleton that provides sensitive photoelectric responsivity and high-speed cation pathways. Molecular dynamics simulations verify that the coexistent interaction effects between cations and the negatively charged sites in GDYO (i.e., oxygen-containing groups and electron-rich acetylenic bonds) could significantly promote cation transmembrane transport via an absorption-acceleration mechanism. Furthermore, the GDYO-based system, possessing a coupled photon-electron-ion transport behavior due to its inherent semiconductor properties, could subtly realize unidirectional ion movement, consuming luminous energy either from low concentration to high concentration or vice versa, flexibly promoting the osmotic power density by ∼195% to 11.91 W m-2.

Supramolecular Control of the Photoisomerization of a Coumarin-Based Photoswitch

The complex formation of the cationic stilbene-type photoswitch CP with the anionic macrocycles carboxylato-pillar[5]arene (WP5) and carboxylato-pillar[6]arene (WP6) has been investigated in aqueous solution by optical spectroscopic, NMR and isothermal calorimetric experiments and theoretical calculations. Subsequently, the photoisomerization reactions of the supramolecular complexes formed have been studied. CP consists of a 7-diethylamino-coumarin fluorophore and an N-methylpyridinium unit, which are connected via an ethene bridge. The trans isomer of CP is fluorescent, and its cis isomer is dark. The binding constants of the WP6 complexes of the two photoisomers of CP are larger by 2 orders of magnitude than those of the respective complexes with WP5, and trans-CP forms more stable complexes with the individual pillararenes than the cis isomer. As shown by NMR spectroscopic measurements and theoretical calculations, the two isomers of CP form external complexes with WP5 and inclusion complexes with WP6. On complexation with WP6, the quantum yields of both the trans-to-cis and cis-to-trans photoisomerization reactions of CP increase significantly, and the fluorescence quantum yield of trans-CP is also enhanced. These changes are due to the suppression of the TICT deactivation process, which is characteristic of 7-dialkylamino-coumarin derivatives.

Gadolinium-Sensitive Artificial Nanochannel Membrane for Information Encryption

Inspired from ion channels in the myelinated axon of Xenopus laevis found to be affected by gadolinium on axonal currents, we present a solid nanochannel membrane sensitive to gadolinium (Gd3+), which can be achieved via the use of the macrocyclic triacetic acid derivative in the host-guest chemistry approach. The macrocyclic nanochannel has good responsiveness toward Gd3+, even at the nanomolar concentration level, evidenced by discernible changes in rectification, ionic conductance, and XPS analyses. Notably, the Gd3+-sensitive nanochannel membrane can be switched by the addition of a diethylenetriaminepentaacetic acid (DTPA) derivative. Further studies have indicated that the gated behavior of Gd3+ in the nanochannel can be attributed to the strong binding strength between DO3A and Gd3+, which induces a surface charge reversal within the nanochannel. The mechanism has been confirmed through several experimental techniques, including isothermal titration calorimetry (ITC) experiments, fluorescence titration experiments, and finite element analysis. Based on its Gd3+ responsiveness of the constructed ion channel, we successfully developed an advanced multilevel information encryption application of the artificial solid nanochannel membrane. Furthermore, it is anticipated that a more effective encryption system will be built by utilizing the bionic ion channel system's ease of use and straightforward functionalization.

Stimuli-Responsive Ion Transport Regulation in Nanochannels by Adhesion-Induced Functionalization of Macroscopic Outer Surface

Responsive regulation of ion transport through nanochannels is crucial in the design of smart nanofluidic devices for sequencing, sensing, and water-energy nexus. Functionalization of the inner wall of the nanochannel enhances interaction with ions and fluid but restricts versatile chemical approaches and accurate characterizations of fluidic interfaces. Herein, we reveal a responsive regulating mechanism of ion transport through nanochannels by polydopamine (PDA)-induced functionalization on the macroscopic outer surface of nanochannels. Responsive molecules were codeposited with PDA on the outer surface of nanochannels and formed a valve of nanometer thickness to manually manipulate ion transport by changing its gap spacing, surface charge, and wettability under external stimulus. The response ratio can be up to 100-fold by maximizing the proportion of responsive molecules on the outer surface. Laminating the codepositions of different responsive molecules with PDA on the channel's outer surface produces multiple responses. A nearly universal adhesion of PDA with responsive molecules on the open outer surface induces nanochannels responsive to different external stimuli with variable response ratios and arbitrary combinations. The results challenge the primary role of functionalization on the nanoconfined interface of nanofluidics and open opportunities for developing new-style nanofluidic devices through the functionalization of macroscopic interface.

A Visible-Light Regulated ATP Transport in Retinal-Modified Pillar[6]arene Layer-by-Layer Self-Assembled Sub-Nanochannel

Natural light-responsive rhodopsins play a critical role in visual conversion, signal transduction, energy transmission, etc., which has aroused extensive interest in the past decade. Inspired by these gorgeous works of living beings, scientists have constructed various biomimetic light-responsive nanochannels to mimic the behaviors of rhodopsins. However, it is still challenging to build stimuli-responsive sub-nanochannels only regulated by visible light as the rhodopsins are always at the sub-nanometer level and regulated by visible light. Pillar[6]arenes have an open cavity of 6.7 Å, which can selectively recognize small organic molecules. They can be connected to ions of ammonium or carboxylate groups on the rims. Therefore, we designed and synthesized the amino and carboxyl-derived side chains of pillar[6]arenes with opposite charges. The sub-nanochannels were constructed through the electrostatic interaction of layer-by-layer self-assembled amino and carboxyl-derived pillar[6]arenes. Then, the natural chromophore of the retinal with visible light-responsive performance was modified on the upper edge of the sub-nanochannel to realize the visible light switched on and off. Finally, we successfully constructed a visible light-responsive sub-nanochannel, providing a novel method for regulating the selective transport of energy-donating molecules of ATP.

Nano-Confined Effect and Heterojunction Promoted Exciton Separation for Light-Boosted Osmotic Energy Conversion

The osmotic energy conversion properties of biomimetic light-stimulated nanochannels have aroused great interest. However, the power output performance is limited by the low light-induced current and energy conversion efficiency. Here, nanochannel arrays with simultaneous modification of ZnO and di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,20-bipyridyl-4,40-dicarboxylato) ruthenium (II) (N719) onto anodic aluminum oxide (AAO) to combine the nano-confined effect and heterojunction is designed, which demonstrate rectified ion transport behavior due to the asymmetric composition, structure and charge. High cation selectivity and ion flux contribute to the high power density of ≈7.33 W m-2 by mixing artificial seawater and river water. Under light irradiation, heterojunction promoted the production and separation of exciton, enhanced cation selectivity, and improved the utilization efficiency of osmotic energy, providing a remarkable power density of ≈18.49 W m-2 with an increase of 252% and total energy conversion efficiency of 30.43%. The work opens new insights into the biomimetic nanochannels for high-performance energy conversion.

Ion transport in nanofluidics under external fields

Nanofluidic channels with tailored ion transport dynamics are usually used as channels for ion transport, to enable high-performance ion regulation behaviors. The rational construction of nanofluidics and the introduction of external fields are of vital significance to the advancement and development of these ion transport properties. Focusing on the recent advances of nanofluidics, in this review, various dimensional nanomaterials and their derived homogeneous/heterogeneous nanofluidics are first briefly introduced. Then we discuss the basic principles and properties of ion transport in nanofluidics. As the major part of this review, we focus on recent progress in ion transport in nanofluidics regulated by external physical fields (electric field, light, heat, pressure, etc.) and chemical fields (pH, concentration gradient, chemical reaction, etc.), and reveal the advantages and ion regulation mechanisms of each type. Moreover, the representative applications of these nanofluidic channels in sensing, ionic devices, energy conversion, and other areas are summarized. Finally, the major challenges that need to be addressed in this research field and the future perspective of nanofluidics development and practical applications are briefly illustrated.

Enzyme-Instructed Host-Guest Assembly/Disassembly for Biomedical Applications

Compared with the normal assembly/disassembly approaches, enzyme-instructed host-guest assembly/disassembly strategies due to their superior biocompatibility and specificity for specific substrates, can more effectively and precisely release molecules at lesions for reflecting in vivo biological events. Specifically, due to the over-expression of enzymes in specific tissues, the assembly/disassembly processes can directly occur on the pathological sites (or regions of interest), thus these enzyme-instructed processes are widely and effectively used for disease treatment or precise bioimaging. Based on it, we introduce the concept and major strategies of enzyme-instructed host-guest assembly/disassembly, illustrate their importance in the diagnosis and treatment of diseases, and review their advances in biomedical applications. Further, the challenges of these strategies in the clinic and future tendencies are also prospected.

Dual-Functional Super-Assembled Mesoporous Carbon-Titania/AAO Hetero-Channels for Bidirectionally Photo-Regulated Ion Transport

Photo-regulated nanofluidic devices have attracted great attention in recent years due to their adjustable ion transport in real time. However, most of the photo-responsive nanofluidic devices can only adjust the ionic current unidirectionally, and cannot simultaneously increase or decrease the current signal intelligently by one device. Herein, a mesoporous carbon-titania/ anodized aluminum hetero-channels (MCT/AAO) is constructed by super-assembly strategy, which exhibits dual-function of cation selectivity and photo response. The polymer and TiO2 nanocrystals jointly build the MCT framework. Polymer framework with abundant negatively charged sites endows MCT/AAO with excellent cation selectivity, and TiO2 nanocrystals are responsible for the photo-regulated ion transport. High photo current densities of 1.8 mA m-2 (increase) and 1.2 mA m-2 (decrease) are realized by MCT/AAO benefiting from the ordered hetero-channels. Significantly, MCT/AAO can also achieve the bidirectionally adjustable osmotic energy by alternating the configurations of concentration gradient. Theoretical and experimental results reveal that the superior photo-generated potential is responsible for the bidirectionally adjustable ion transport. Consequently, MCT/AAO performs the function of harvesting ionic energy from the equilibrium electrolyte solution, which greatly expands its practical application field. This work provides a new strategy in constructing dual-functional hetero-channels toward bidirectionally photo-regulated ionic transport and energy harvesting.

Highly Selective Transport and Enrichment of Lithium Ions through Bionic Ion Pair Receptor Nanochannels

Inspired by ion pair cotransport channels in biological systems, a bionic nanochannel modified with lithium ion pair receptors is constructed for selective transport and enrichment of lithium ions (Li⁺). NH₂-pillar[5]arene (NP5) is chosen as ion pair receptors, and the theoretical simulation and NMR titration experiments illustrate that NP5 has good affinity for the ion pair of LiCl through a strong host-guest interaction at the molecular level. Due to the confinement effect and ion pair cooperation recognition, an NP5-based receptor was introduced into an artificial PET nanochannel. An I-V test indicated that the NP5 channel realized the highly selective recognition for Li⁺. Meanwhile, transmembrane transport and COMSOL simulation experiments proved that the NP5 channel achieved the transport and enrichment of Li⁺ through the cooperative interaction between NP5 and LiCl. Moreover, the receptor solution of transmembrane transport LiCl in the NP5 channel was used to cultivate wheat seedlings, which obviously promoted their growth. This nanochannel based on the ion pair recognition will be much useful for practical applications like metal ion extraction, enrichment, and recycle.

A multi-stimuli-responsive nanochannel inspired by biological disulfide bond

Disulfide bonds exist widely in channel protein and play an essential role in matter exchange and signal transduction (e.g., rhodopsin, canonical transient receptor potential 5 (TRPC5)). The research on disulfide bond in nanochannel is significant for the cognition of their biological functions. However, the fragility of biological channel limits the in-situ study and practical application. Herein, an innovative biologically-inspired artificial nanochannel based on disulfide bond (NCDS) with excellent durability, adjustable surface property is proposed. The constructed NCDS has a multi-response to UV-light, thiol (e.g., cysteine (Cys)) or pH stimulation, and can obtain reversibility after regulation by hydrogen peroxide (H₂O₂) or H⁺. The biomimetic NCDS shows great potential in biosensor and intelligent response design. This study also shines new light to channel protein based on disulfide bond that despite the nanochannel has specificity, it will be modulated by the change of nature environment, such as UV-light and chemical microenvironment (e.g., redox state and pH), which might be the reason of some disease.

Switchable biomimetic nanochannels for on-demand SO2 detection by light-controlled photochromism

In contrast to the conventional passive reaction to analytes, here, we create a proof-of-concept nanochannel system capable of on-demand recognition of the target to achieve an unbiased response. Inspired by light-activatable biological channelrhodopsin-2, photochromic spiropyran/anodic aluminium oxide nanochannel sensors are constructed to realize a light-controlled inert/active-switchable response to SO₂ by ionic transport behaviour. We find that light can finely regulate the reactivity of the nanochannels for the on-demand detection of SO₂. Pristine spiropyran/anodic aluminium oxide nanochannels are not reactive to SO₂. After ultraviolet irradiation of the nanochannels, spiropyran isomerizes to merocyanine with a carbon‒carbon double bond nucleophilic site, which can react with SO₂ to generate a new hydrophilic adduct. Benefiting from increasing asymmetric wettability, the proposed device exhibits a robust photoactivated detection performance in SO₂ detection in the range from 10 nM to 1 mM achieved by monitoring the rectified current.

Engineering solid nanochannels with macrocyclic host-guest chemistry for stimuli responses and molecular separations

Biological channels in the cell membrane play a critical role in the regulation of signal transduction and transmembrane transport. Researchers have been committed to building biomimetic nanochannels to imitate the above significant biological processes. Unlike the fragile feature of biological channels, numerous solid nanochannels have aroused extensive interests for their controllable chemical properties on the surface and superior mechanical properties. Surface functionalization has been confirmed to be vital to determine the properties of solid nanochannels. Macrocyclic hosts (e.g., the crown ethers, cyclodextrins, calix[n]arenes, cucurbit[n]urils, pillar[n]arenes, and trianglamine) can be tailored to the interior surface of the nanochannels with the performance of stimuli response and separation. Macrocycles have good reversibility and high selectivity toward specific ions or molecules, promoting functionalies of solid nanochannels. Hence, the combination of macrocyclic hosts and solid nanochannels is conducive to taking both advantages and achieving applications in functional nanochannels (e.g., membranes separations, biosensors, and smart devices). In this review, the most recent advances in nanochannel membranes decorated by macrocyclic host-guest chemistry are briefed. A variety of macrocyclic hosts-based responsive nanochannels are organized (e.g., the physical stimuli and specific molecules or ions stimuli) and nanochannels are separated (e.g., water purifications, enantimerseparations, and organic solvent nanofiltration), respectively. Hopefully, this review can enlighten on how to effectively build functional nanochannels and facilitate their practical applications in membrane separations.

MXenes Functionalized with Macrocyclic Hosts: From Molecular Design to Applications

Two-dimensional (2D) MXene has aroused wide attention for its excellent physical and chemical properties. The interlayer engineering formed by layer-by-layer stacking of MXene nanosheets can be employed for molecular sieving and water purification by incorporating specific groups onto the exterior surface of MXene. Macrocyclic hosts exhibiting unique structural features and recognition ability can construct smart devices for external stimuli with reversible features between macrocycles and guests. On that basis, macrocyclic hosts can be anchored to MXene to provide numerous insights into their compositions and intercalation states. In this review, the MXene prepared based on macrocyclic hosts from molecular design to applications is highlighted. Various MXenes functionalized with macrocyclic hosts are empowered in functional membrane (including water purification, organic solvent nanofiltration, and electromagnetic shielding), photocatalysis, sensing, and adsorption (interactions with specific guest). Hopefully, this review can bring new inspiration to the design of multifunctional MXene-based materials and improving its practical applications.

Supramolecular Biopharmaceutical Carriers Based on Host-Guest Interactions

Traditional drugs have the disadvantages of poor permeability and low solubility, which makes the utilization of pesticides lower and brings many side effects. With the continuous development of supramolecular chemistry in recent years, it has also played an irreplaceable role in the field of pharmaceutical science. Supramolecular macrocycles, such as crown ethers, cyclodextrins, calixarenes, pillararenes and cucurbiturils, are potentially good candidates for drug carriers due to their biocompatibility, hydrophobic cavity and ease of derivatization. The encapsulation of drugs based on host-guest interaction has the advantage of being adjustable and reversible as well as improving the low availability of drugs. Here, the recent advances in methods and strategies for drug encapsulation and release based on supramolecular macrocycles with host-guest interactions have been systematically summarized, laying a bright foundation for the development of novel nanopesticide preparations in the future and pointing out future directions of novel biopesticide research.

An H2S-Regulated Artificial Nanochannel Fabricated by a Supramolecular Coordination Strategy

Hydrogen sulfide (H₂S), as the third gasotransmitter, has an important impact on physiological and pathological activities. Herein, we fabricated an artificial nanochannel with a conductance value of 2.01 nS via a supramolecular coordination strategy. Benefiting from the unique H₂S-mediated covalent reaction, the nanochannel biosensor could change from ON to OFF states with the addition of H₂S. Furthermore, this nanochannel directed the ion transport, showing a high rectification ratio as well as gating ratio. Subsequently, theoretical simulations were conducted to help to reveal the possible mechanism of the functionalized nanochannel. This study can provide insights for better understanding the process of H₂S-regulated biological channels and fabricating gas gated nanofluids.

Light-responsive nanochannels based on the supramolecular host–guest system

The light-responsive nanochannel of rhodopsin gained wider research interest from its crucial roles in light-induced biological functions, such as visual signal transduction and energy conversion, though its poor stability and susceptibility to inactivation in vitro have limited its exploration. However, the fabrication of artificial nanochannels with the properties of physical stability, controllable structure, and easy functional modification becomes a biomimetic system to study the stimulus-responsive gating properties. Typically, light-responsive molecules of azobenzene (Azo), retinal, and spiropyran were introduced into nanochannels as photo-switches, which can change the inner surface wettability of nanochannels under the influence of light; this ultimately results in the photoresponsive nature of biomimetic nanochannels. Furthermore, the fine-tuning of their stimulus-responsive properties can be achieved through the introduction of host–guest systems generally combined with a non-covalent bond, and the assembling process is reversible. These host–guest systems have been introduced into the nanochannels to form different functions. Based on the host–guest system of light-responsive reversible interaction, it can not only change the internal surface properties of the nanochannel and control the recognition and transmission behaviors but also realize the controlled release of a specific host or guest molecules in the nanochannel. At present, macrocyclic host molecules have been introduced into nanochannels including pillararenes, cyclodextrin (CD), and metal–organic frameworks (MOFs). They are introduced into the nanochannel through chemical modification or host–guest assemble methods. Based on the changes in the light-responsive structure of azobenzene, spiropyran, retinal, and others with macrocycle host molecules, the surface charge and hydrophilic and hydrophobic properties of the nanochannel were changed to regulate the ionic and molecular transport. In this study, the development of photoresponsive host and guest-assembled nanochannel systems from design to application is reviewed, and the research prospects and problems of this photo-responsive nanochannel membrane are presented.

Conjugated microporous polymer membranes for light-gated ion transport

Inspired by the light-gated ion channels in cell membranes that play important roles in many biological activities, herein, we developed an artificial light-gated ion channel membrane out of conjugated microporous polymers. Through bottom-up design of the monomer molecular structure and by the electropolymerization method, the membrane pore size and thickness were precisely controlled on the molecular level. The obtained membrane exhibited uniform pore size and highly sensitive light-switchable response. The photoisomerization of the polymer chain resulted in a reversible "on and off" light control over the pore size and subsequently led to light-gated ion transport across the membrane for a series of ions including hydrogen, potassium, sodium, lithium, calcium, magnesium, and aluminum ions.

An Ultraviolet/Visible Light Regulated Protein Transport Gate Constructed by Pillar[6]arene-based Host-Guest System

Protein transport is an interesting and intrinsic life feature that is highly relevant to physiology and disease in living beings. Herein, inspired by nature, based on the supramolecular host-guest interaction, we have introduced the classical azobenzene light switches and L-phenylalanine derived pillar[6]arene (L-Phe-P6) into the artificial nanochannel to construct light-responsive nanochannels that could regulate protein transport effectively under the control of ultraviolet (UV) and visible (Vis) light. The light-controlled distribution of L-Phe-P6 in the channel led to the difference in surface charges in the nanochannel, which eventually brought the difference in protein transport. This research may not only provide a convenient theoretical model for biological research, but also a flexible light-responsive protein transport model, which will play a crucial role in light-controlled release of protein drugs and so on.

Biomimetic Nanochannels: From Fabrication Principles to Theoretical Insights

Biological nanochannels which can regulate ionic transport across cell membranes intelligently play a significant role in physiological functions. Inspired by these nanochannels, numerous artificial nanochannels have been developed during recent years. The exploration of smart solid-state nanochannels can lay a solid foundation, not only for fundamental studies of biological systems but also practical applications in various fields. The basic fabrication principles, functional materials, and diverse applications based on artificial nanochannels are summarized in this review. In addition, theoretical insights into transport mechanisms and structure-function relationships are discussed. Meanwhile, it is believed that improvements will be made via computer-guided strategy in designing more efficient devices with upgrading accuracy. Finally, some remaining challenges and perspectives for developments in both novel conceptions and technology of this inspiring research field are stated.

Polarization-sensitive optoionic membranes from chiral plasmonic nanoparticles

Optoelectronic effects differentiating absorption of right and left circularly polarized photons in thin films of chiral materials are typically prohibitively small for their direct photocurrent observation. Chiral metasurfaces increase the electronic sensitivity to circular polarization, but their out-of-plane architecture entails manufacturing and performance trade-offs. Here, we show that nanoporous thin films of chiral nanoparticles enable high sensitivity to circular polarization due to light-induced polarization-dependent ion accumulation at nanoparticle interfaces. Self-assembled multilayers of gold nanoparticles modified with L-phenylalanine generate a photocurrent under right-handed circularly polarized light as high as 2.41 times higher than under left-handed circularly polarized light. The strong plasmonic coupling between the multiple nanoparticles producing planar chiroplasmonic modes facilitates the ejection of electrons, whose entrapment at the membrane-electrolyte interface is promoted by a thick layer of enantiopure phenylalanine. Demonstrated detection of light ellipticity with equal sensitivity at all incident angles mimics phenomenological aspects of polarization vision in marine animals. The simplicity of self-assembly and sensitivity of polarization detection found in optoionic membranes opens the door to a family of miniaturized fluidic devices for chiral photonics.

Light-Controlled Ionic/Molecular Transport through Solid-State Nanopores and Nanochannels

Biological nanochannels perfectly operate in organisms and exquisitely control mass transmembrane transport for complex life process. Inspired by biological nanochannels, plenty of intelligent artificial solid-state nanopores and nanochannels are constructed based on various materials and methods with the development of nanotechnology. Specially, the light-controlled nanopores/nanochannels have attracted much attention due to the unique advantages in terms of that ion and molecular transport can be regulated remotely, spatially and temporally. According to the structure and function of biological ion channels, light-controlled solid-state nanopores/nanochannels can be divided into light-regulated ion channels with ion gating and ion rectification functions, and light-driven ion pumps with active ion transport property. In this review, we present a systematic overview of light-controlled ion channels and ion pumps according to the photo-responsive components in the system. Then, the related applications of solid-state nanopores/nanochannels for molecular sensing, water purification and energy conversion are discussed. Finally, a brief conclusion and short outlook are offered for future development of the nanopore/nanochannel field.

Photoresponsive Solid Nanochannels Membranes: Design and Applications

Light stimuli have notable advantages over other environmental stimuli, such as more precise spatial and temporal regulation, and the ability to serve as an energy source to power the system. In nature, photoresponsive nanochannels are important components of organisms, with examples including the rhodopsin channels in optic nerve cells and photoresponsive protein channels in the photosynthesis system of plants. Inspired by biological channels, scientists have constructed various photoresponsive, smart solid-state nanochannels membranes for a range of applications. In this review, the methods and applications of photosensitive nanochannels membranes are summarized. The authors believe that this review will inspire researchers to further develop multifunctional artificial nanochannels for applications in the fields of biosensors, stimuli-responsive smart devices, and nanofluidic devices, among others.

Peptide Assembled in a Nano-confined Space as a Molecular Rectifier for the Availability of Ionic Current Modulation

Bioinspired nanochannels have emerged as a powerful tool for bioengineering and biomedical research due to their robust mechanical and controllable chemical properties. Inspired by inward-rectifier potassium (K⁺) channels, herein, the charged peptide assembly has been introduced into a nano-confined space for the modulation of ion current rectification (ICR). Peptide-responsive reaction-triggered sequence changes can contribute to polarity conversion of the surface charge; therefore, ICR reversal (ICRR) is generated. Compared with other responsive elements, natural charged peptides show the merit of controllable charge polarity. By electrochemically monitoring the ICRR as an output signal, one can utilize the peptide assembly-mediated ICRR to construct an ionic sensory platform. In addition, a logic gate has been established to demonstrate the availability of an ionic sensory platform for inhibitor screening. As peptide nanoassemblies may also have various structures and functions due to their diverse properties, the ionic modulation system can provide alternatives for the assay of peptide-associated biotargets with biomedical applications.

Dual pH-/Photo-Responsive Color Switching Systems for Dynamic Rewritable Paper

Smart color switching materials that can change color with a fast response and a high reversibility have attracted increasing attention in color-on-demand applications. However, most of them can only respond to a single stimulus from their external environment, which dramatically limits their broad applications. To address this problem, we report a new strategy in developing a dual pH-/photo-responsive color switching system by coupling the pH-dependent and redox-driven color switchable neutral red (NR) with photoreductive TiO_2-x nanoparticles. The biodegradable TiO_2-x nanoparticles/NR/agarose gel film shows a rapid color switching between yellow and red upon stimulation with acidic/basic vapors in more than 20 cycles because of the protonation and deprotonation process of NR. Moreover, the film shows interesting photoreversible color switching properties under both acidic and basic conditions, including a fast response time and a high reversibility. Taking advantage of the excellent dual pH-/photo-responsive color switching properties, we demonstrated the potential applications of the TiO_2-x nanoparticles/NR/agarose gel film in dynamic rewritable paper, in which the created patterns by photo-printing produce dynamic color changing upon applying an acidic or a basic vapor. We believe that the result will enable a new path for the development of dual- and even multi-responsive color switching systems, broadening their new applications.

Facile Surface Functionalization of MXene by Pillar[5]arene for Enhanced Electrochemical Performance

A simple strategy was used to prepare functional two-dimensional materials via combination of pillar[5]arene (P5) and MXene. Electrochemical results of MXene-P5 exhibits high supramolecular recognition, enrichment capability, and high electrochemical...

Porphyrin Derivative Based Tandem Response Nanochannels Triggered by Zn2+ and NO

We describe tandem response solid nanochannels tailored with a porphyrin derivative that show Zn2+ and NO gating behaviors with good selectivity, stability, and reversibility. EXAFS and fluorescence experiments showed that...

Recent advances in polymeric nanostructured ion selective membranes for biomedical applications

Nano structured ion-selective membranes (ISMs) are very attractive materials for a wide range of sensing and ion separation applications. The present review focuses on the design principles of various ISMs; nanostructured and ionophore/ion acceptor doped ISMs, and their use in biomedical engineering. Applications of ISMs in the biomedical field have been well-known for more than half a century in potentiometric analysis of biological fluids and pharmaceutical products. However, the emergence of nanotechnology and sophisticated sensing methods assisted in miniaturising ion-selective electrodes to needle-like sensors that can be designed in the form of implantable or wearable devices (smartwatch, tattoo, sweatband, fabric patch) for health monitoring. This article provides a critical review of recent advances in miniaturization, sensing and construction of new devices over last decade (2011-2021). The designing of tunable ISM with biomimetic artificial ion channels offered intensive opportunities and innovative clinical analysis applications, including precise biosensing, controlled drug delivery and early disease diagnosis. This paper will also address the future perspective on potential applications and challenges in the widespread use of ISM for clinical use. Finally, this review details some recommendations and future directions to improve the accuracy and robustness of ISMs for biomedical applications.

Construction of A High-Flux Protein Transport Channel Inspired by the Nuclear Pore Complex

Inspired by the nuclear pore complex (NPC), herein we have established a biomimetic high-flux protein delivery system via the ingenious introduction of pillar[5]arene-based host-guest system into one side of artificial hour-glass shaped nanochannel. With a transport flux of 660 lysozymes per minute, the system provides efficient high-flux protein transport at a rate which is significantly higher than that of an unmodified nanochannel and conventional bilateral symmetrical modified nanochannels. In view of these promising results, the use of artificial nanochannel to improve protein transport not only presents a new potential chemical model for biological research and better understanding of protein transport behavior in the living systems, but also provides a high-flux protein transporter device, which may have applications in the design of protein drug release systems, protein separation systems and microfluidics in the near future.

Synthetic azobenzene-containing metal-organic framework ion channels toward efficient light-gated ion transport at the subnanoscale

Artificial nanochannels with diverse responsive properties have been widely developed to replicate the smart gating functionalities of biological ion channels. However, in these traditional nanochannels, common responsive molecules are usually too small to efficiently block the large channels under the closed states, leading to weak gating performances. Herein, we report carboxylated azobenzene-coordinated metal-organic-framework (AZO-MOF) ion channels with impressive light-gating properties. The AZO-MOF ion channels were synthesized by the confined growth of AZO-MOFs, composed of light-responsive AZO-containing ligands, non-responsive ligands and metal clusters, into ion-track-etched polymer nanochannels. The AZO-MOF ion channels with an appropriate number of AZO ligands showed a well-maintained crystalline and three-dimensional porous structure, including nanoscale cavities and subnanoscale windows for LiCl conduction. Meanwhile, the AZO-containing ligands bend and stretch upon light irradiation to open and close the pathways, thus gating the ion flux through the AZO-MOF ion channels with high on-off ratios up to 40.2, which is ∼2.3-30 times those of AZO-encapsulated MOF ion channels and AZO-modified nanochannels. This work suggests ways to achieve subnanoscaled gating of ion transport by angstrom-porous MOFs coordinated by stimuli-responsive ligands.

Rational ion transport management mediated through membrane structures

Unique membrane structures endow membranes with controlled ion transport properties in both biological and artificial systems, and they have shown broad application prospects from industrial production to biological interfaces. Herein, current advances in nanochannel-structured membranes for manipulating ion transport are reviewed from the perspective of membrane structures. First, the controllability of ion transport through ion selectivity, ion gating, ion rectification, and ion storage is introduced. Second, nanochannel-structured membranes are highlighted according to the nanochannel dimensions, including single-dimensional nanochannels (i.e., 1D, 2D, and 3D) functioning by the controllable geometrical parameters of 1D nanochannels, the adjustable interlayer spacing of 2D nanochannels, and the interconnected ion diffusion pathways of 3D nanochannels, and mixed-dimensional nanochannels (i.e., 1D/1D, 1D/2D, 1D/3D, 2D/2D, 2D/3D, and 3D/3D) tuned through asymmetric factors (e.g., components, geometric parameters, and interface properties). Then, ultrathin membranes with short ion transport distances and sandwich-like membranes with more delicate nanochannels and combination structures are reviewed, and stimulus-responsive nanochannels are discussed. Construction methods for nanochannel-structured membranes are briefly introduced, and a variety of applications of these membranes are summarized. Finally, future perspectives to developing nanochannel-structured membranes with unique structures (e.g., combinations of external macro/micro/nanostructures and the internal nanochannel arrangement) for mediating ion transport are presented.

An ion transport switch based on light-responsive conformation-dependent G-quadruplex transmembrane channels

A light-responsive ion transport switch has been developed based on conformation-dependent azobenzene-incorporated lipophilic G-quadruplex channels, which provides a new smart approach for the selective transport of K⁺ ions across the lipid membrane.

Enantioselective Antiport in Asymmetric Nanochannels

Enantioselective sensing and separation are major challenges. Nanochannel technologies are energy-saving and efficient for membrane separation. Herein, inspired by biological antiporter proteins, artificial nanochannels with antiporter behavior were fabricated for chiral sensing and separation. Tyrosine enantiomers were incorporated into hourglass-shaped nanochannels via stepwise modifications to fabricating multiligand-modified asymmetric channels. Chiral distinction of naproxen enantiomers was amplified in the l-Tyr/d-Tyr channels, with an enantioselectivity coefficient of 524, which was over 100-fold that of one-ligand-modified nanochannels. Furthermore, transport experiments evidenced the spontaneous antiport of naproxen enantiomers in the l-Tyr/d-Tyr channels. The racemic naproxen sample was separated via the chiral antiport process, with an enantiomeric excess of 71.2%. Further analysis using electro-osmotic flow experiments and finite-element simulations confirmed that the asymmetric modified multiligand was key to achieving separation of the naproxen enantiomers. We expect these multiligand-modified asymmetric nanochannels to provide insight into mimicking biological antiporter systems and offer an approach to energy-efficient and robust enantiomer separation.

Multifunctional Pillar[n]arene-Based Smart Nanomaterials

The construction of smart nanomaterials from host macrocycles that are responsive to specific stimuli has gained significant attention in recent years. The application of pillar[n]arenes has been of particular interest given their ease of functionalization and tunability of the intrinsic cavity electronic properties that allows them to encapsulate a great variety of guests and complex with metal ions with high selectivity via noncovalent interactions, endowing them with captivating properties and functions. Herein, we present the most recent advances in the design and functionalization of pillar[n]arene-based smart nanomaterials, and their applications for sensing, catalysis, drug delivery, and artificial transmembrane channels.

DNA Assembly-Based Stimuli-Responsive Systems

Stimuli-responsive designs with exogenous stimuli enable remote and reversible control of DNA nanostructures, which break many limitations of static nanostructures and inspired development of dynamic DNA nanotechnology. Moreover, the introduction of various types of organic molecules, polymers, chemical bonds, and chemical reactions with stimuli-responsive properties development has greatly expand the application scope of dynamic DNA nanotechnology. Here, DNA assembly-based stimuli-responsive systems are reviewed, with the focus on response units and mechanisms that depend on different exogenous stimuli (DNA strand, pH, light, temperature, electricity, metal ions, etc.), and their applications in fields of nanofabrication (DNA architectures, hybrid architectures, nanomachines, and constitutional dynamic networks) and biomedical research (biosensing, bioimaging, therapeutics, and theranostics) are discussed. Finally, the opportunities and challenges for DNA assembly-based stimuli-responsive systems are overviewed and discussed.

Tailoring CO2-Activated Ion Nanochannels Using Macrocyclic Pillararenes

Gas-responsive nanochannels have great relevance for applications in many fields. Inspired by CO₂-sensitive ion channels, herein we present an approach for designing solid-state nanochannels that allow controlled regulation of ion transport in response to alternate CO₂/N₂ stimuli. The pillar[5]arene (P5N) bearing diethylamine groups can convert into the water-soluble host P5C, containing cationic tertiary ammonium salt groups after absorbing CO₂. Subsequently, the nanochannel walls are tailored using P5N-based host-guest chemistry. The ion transport rate of K⁺ in the P5N nanochannels under CO₂ was 1.66 × 10^-4 mol h^-1 m^-2, whereas that under N₂ was 7.98 × 10^-4 mol h^-1 m^-2. Notably, there was no significant change to the ion current after eight cycles, which may indicate the stability and repeatability of CO₂-activated ion nanochannels. It is speculated that the difference in ion conductance resulted from the change in wettability and surface charge within the nanochannels in response to the gas stimuli. Achieving CO₂-activated ion transport in solid-state nanochannels opens new avenues for biomimetic nanopore systems and advanced separation processes.

A layer-by-layer assembled D/L-arginine-calix[4]arene-Si-surface for macroscopic enantio-selective discrimination of (R)/(S)-ibuprofen

Chiral arginine was introduced by layer-by-layer assembly onto a calix[4]arene-diacid modified silica surface to control the adsorption of different kinds of ibuprofen droplets. The droplet of (S)-ibuprofen slid off rapidly, whereas the droplet of (R)-ibuprofen absorbed on the modified surface.

Smart pH-Modulated Two-Way Photoswitch Based on a Polymer-Modified Single Nanochannel

In this article, we have demonstrated a smart pH-modulated two-way photoswitch that can reversibly switch ion transport under alternating light exposure over a wide pH range. This photoswitch was prepared by functionalizing the interior of a single conical glass nanochannel with a poly-spiropyran-linked methacrylate (P-SPMA) polymer through surface-initiated atom transfer radical polymerization. The P-SPMA polymer brushes comprise functional groups that are responsive to light and pH, which can cause configuration and charge changes to affect the properties of the nanochannel wall. The SPMA polymer-modified nanochannel not only reversibly controlled ion transport under alternating light irradiation but also efficiently and flexibly regulated the direction and extent of the ion transport based on the pH. This two-way photoswitch exhibits the considerable potential of photoresponsive polymers for the advancement of "intelligent" bionic nanochannel devices for ion screening and optical sensing in various applications.

A Visible-Light-Regulated Chloride Transport Channel Inspired by Rhodopsin

Inspired by the light-regulating capabilities of naturally occurring rhodopsin, we have constructed a visible-light-regulated Cl^- -transport membrane channel based on a supramolecular host-guest interaction. A natural retinal chromophore, capable of a visible-light response, is used as the guest and grafted into the artificial channel. Upon introduction of an ethyl-urea-derived pillar[6]arene (Urea-P6) host, threading or de-threading of the retinal and selective bonding of Cl^- can be utilized to regulate ion transport. Based on the visible-light responsiveness of the host-guest interaction, Cl^- transport can be regulated by visible light between ON and OFF states. Visible-light-regulated Cl^- transport as a chemical model permits to understand comparable biological ion-selective transport behaviors. Furthermore, this result also supplies a smart visible-light-responsive Cl^- transporter, which may have applications in natural photoelectric conversion and photo-controlled delivery systems.

A reversible single-molecule ligand-gating ion transportation switch of ON–OFF–ON type through a photoresponsive pillar[6]arene channel complex

A reversible pseudo-single-ligand-gated ion transportation switch of ON–OFF–ON type was achieved through host–guest complexation with pillar[6]arene (P[6]) as the ion channel, and a photoresponsive azobenzene as the dual-role (open and close) ligand.

A reversible pseudo-single-ligand-gated ion transportation switch of ON–OFF–ON type through pillar[6]arene and photoresponsive azobenzene as dual-role ligand.

Photoswitchable Conjugated Oligoelectrolytes for a Light-Induced Change of Membrane Morphology

The synthesis of a new conjugated oligoelectrolyte (COE), namely DSAzB, is described, which contains a conjugated core bearing a diazene moiety in the center of its electronically delocalized structure. Similar to structurally related phenylenevinylene-based COEs, DSAzB readily intercalates into model and natural lipid bilayer membranes. Photoinduced isomerization transforms the linear trans COE into a bent or C-shape form. It is thereby possible to introduce DSAzB into the bilayer of a cell and disrupt its integrity by irradiation with light. This leads to controlled permeabilization of membranes, as demonstrated by the release of calcein from DMPG/DMPC vesicles and by propidium iodide influx experiments on S. epidermidis. Both experiments support that the permeabilization is selective for the light stimulus, highly efficient, and repeatable. Target-selective and photoinduced actions demonstrated by DSAzB may have broad applications in biocatalysis and related biotechnologies.