Photocurrent generation based on a light-driven proton pump in an artificial liquid membrane

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.

Harnessing Maxwell's demon to establish a macroscale concentration gradient

Maxwell's demon describes a thought experiment in which a 'demon' regulates the flow of particles between two adjoining spaces, establishing a potential gradient without appearing to do work. This seeming paradox led to the understanding that sorting entails thermodynamic work, a foundational concept of information theory. In the past centuries, many systems analogous to Maxwell's demon have been introduced in the form of molecular information, molecular pumps and ratchets. Here we report a functional example of a Maxwell's demon that pumps material over centimetres, whereas previous examples operated on a molecular scale. In our system, this demon drives directional transport of o-fluoroazobenzene between the arms of a U-tube apparatus upon light irradiation, transiting through an aqueous membrane containing a coordination cage. The concentration gradient thus obtained is further harnessed to drive naphthalene transport in the opposite direction.

Molecular Ratchets and Kinetic Asymmetry: Giving Chemistry Direction

Over the last two decades ratchet mechanisms have transformed the understanding and design of stochastic molecular systems-biological, chemical and physical-in a move away from the mechanical macroscopic analogies that dominated thinking regarding molecular dynamics in the 1990s and early 2000s (e.g. pistons, springs, etc), to the more scale-relevant concepts that underpin out-of-equilibrium research in the molecular sciences today. Ratcheting has established molecular nanotechnology as a research frontier for energy transduction and metabolism, and has enabled the reverse engineering of biomolecular machinery, delivering insights into how molecules 'walk' and track-based synthesisers operate, how the acceleration of chemical reactions enables energy to be transduced by catalysts (both motor proteins and synthetic catalysts), and how dynamic systems can be driven away from equilibrium through catalysis. The recognition of molecular ratchet mechanisms in biology, and their invention in synthetic systems, is proving significant in areas as diverse as supramolecular chemistry, systems chemistry, dynamic covalent chemistry, DNA nanotechnology, polymer and materials science, molecular biology, heterogeneous catalysis, endergonic synthesis, the origin of life, and many other branches of chemical science. Put simply, ratchet mechanisms give chemistry direction. Kinetic asymmetry, the key feature of ratcheting, is the dynamic counterpart of structural asymmetry (i.e. chirality). Given the ubiquity of ratchet mechanisms in endergonic chemical processes in biology, and their significance for behaviour and function from systems to synthesis, it is surely just as fundamentally important. This Review charts the recognition, invention and development of molecular ratchets, focussing particularly on the role for which they were originally envisaged in chemistry, as design elements for molecular machinery. Different kinetically asymmetric systems are compared, and the consequences of their dynamic behaviour discussed. These archetypal examples demonstrate how chemical systems can be driven inexorably away from equilibrium, rather than relax towards it.

Enhancing Sustainable Energy Conversion Efficiency by Incorporating Photoelectric Responsiveness into Multiporous Ionic Membrane

The evolution of porous membranes has revitalized their potential application in sustainable osmotic-energy conversion. However, the performance of multiporous membranes deviates significantly from the linear extrapolation of single-pore membranes, primarily due to the occurrence of ion-concentration polarization (ICP). This study proposes a robust strategy to overcome this challenge by incorporating photoelectric responsiveness into permselective membranes. By introducing light-induced electric fields within the membrane, the transport of ions is accelerated, leading to a reduction in the diffusion boundary layer and effectively mitigating the detrimental effects of ICP. The developed photoelectric-responsive covalent-organic-framework membranes exhibit an impressive output power density of 69.6 W m-2 under illumination, surpassing the commercial viability threshold by ≈14-fold. This research uncovers a previously unexplored benefit of integrating optical electric conversion with reverse electrodialysis, thereby enhancing energy conversion efficiency.

Elementary Reaction Steps That Precede or Follow a Unimolecular Reaction Step Can Obfuscate Interpretation of the Driving-Force Dependence to Its Rate Constant

Assessing the validity of a driving-force-dependent kinetic theory for a unimolecular elementary reaction step is difficult when the observed reaction rate is strongly influenced by properties of the preceding or following elementary reaction step. A well-known example occurs for bimolecular reactions with weak orbital overlap, such as outer-sphere electron transfer, where bimolecular collisional encounters that precede a fast unimolecular electron-transfer step can limit the observed rate. A lesser-appreciated example occurs for bimolecular reactions with stronger orbital overlap, including many proton-transfer reactions, where equilibration of an endergonic unimolecular proton-transfer step results in a relatively small concentration of reaction products, thus slowing the rate of the following step such that it becomes rate limiting. Incomplete consideration of these points has led to discrepancies in interpretation of data from the literature. Our reanalysis of these data suggests that proton-transfer elementary reaction steps have a nonzero intrinsic free energy barrier, implying, in the parlance of Marcus theory, that there is non-negligible nuclear reorganization. Outcomes from our analyses are generalizable to inner-sphere electron-transfer reactions such as those involved in (photo)electrochemical fuel-forming reactions.

Bioinspired Light-Driven Proton Pump: Engineering Band Alignment of WS2 with PEDOT:PSS and PDINN

Bioinspired two-dimensional (2D) nanofluidic systems for photo-induced ion transport have attracted great attention, as they open a new pathway to enabling light-to-ionic energy conversion. However, there is still a great challenge in achieving a satisfactory performance. It is noticed that organic solar cells (OSCs, light-harvesting device based on photovoltaic effect) commonly require hole/electron transport layer materials (TLMs), PEDOT:PSS (PE) and PDINN (PD), respectively, to promote the energy conversion. Inspired by such a strategy, an artificial proton pump by coupling a nanofluidic system with TLMs is proposed, in which the PE- and PD-functionalized tungsten disulfide (WS2) multilayers construct a heterogeneous membrane, realizing an excellent output power of ≈1.13 nW. The proton transport is fine-regulated due to the TLMs-engineered band structure of WS2. Clearly, the incorporating TLMs of OSCs into 2D nanofluidic systems offers a feasible and promising approach for band edge engineering and promoting the light-to-ionic 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.

Efficient and cold-tolerant moisture-enabled power generator combining ionic diode and ionic hydrogel

The ionic diode structure has become one of the attractive structures in the field of moisture-based power generation. However, existing devices still suffer from poor moisture trapping, low surface charge, and inefficient ion separation, resulting in low output power. Moreover, water freezes at low temperatures (<0 °C), limiting the ionic diode structure to generate electricity in cold environments. In this paper, a moisture-enabled power generator has been designed and fabricated, which assembles a negatively charged ionic hydrogel film and a positively charged anodized aluminum oxide (AAO) film to construct a heterojunction. The hydrogel polymer network is modified with a large number of sulfonate groups that dissociate to provide nanoscale pores with high surface charge to improve the rectification ratio. And the lithium chloride (LiCl) salt with high hydration ability is added to the hydrogel as a moisture-trapping and anti-freezing component. Usually salt ions reduce the Debye length, so that the ion transport is finally not controlled by the electric double layer (EDL) and the rectification fails. Interestingly, due to the natural affinity of the hydrogel polymer network for LiCl, LiCl is locked on the hydrogel side and does not easily enter the AAO pores to change the distribution of EDL within the nanochannel. As a result, the device rectification ratio is almost independent of the amount of LiCl addition, demonstrating an excellent balance of high output power and high freeze resistance. Ultimately, the device exhibits excellent power generation performance in the -20 °C to 60 °C temperature range and 15% to 93% RH humidity range. Typically, under high humidity (93% RH) at room temperature (25 °C), it provides an open-circuit voltage of 1.25 V and a short-circuit current of 300 μA cm-2, with an on-load output power of up to 71.35 μW cm-2. Under medium humidity (50% RH) at low temperature (-20 °C), it provides an open-circuit voltage of 1.11 V and a short-circuit current of 15 μA cm-2.

Sustainable lithium extraction enabled by responsive metal-organic frameworks with ion-sieving adsorption effects

Metal-organic frameworks (MOFs) are superior ion adsorbents for selectively capturing toxic ions from water. Nevertheless, they have rarely been reported to have lithium selectivity over divalent cations due to the well-known flexibility of MOF framework and the similar physiochemical properties of Li+ and Mg2+. Herein, we report an ion-sieving adsorption approach to design sunlight-regenerable lithium adsorbents by subnanoporous MOFs for efficient lithium extraction. By integrating the ion-sieving agent of MOFs with light-responsive adsorption sites of polyspiropyran (PSP), the ion-sieving adsorption behaviors of PSP-MOFs with 6.0, 8.5, and 10.0 Å windows are inversely proportional to their pore size. The synthesized PSP-UiO-66 with a narrowest window size of 6.0 Å shows high LiCl adsorption capacity up to 10.17 mmol g-1 and good Li+/Mg2+ selectivity of 5.8 to 29 in synthetic brines with Mg/Li ratio of 1 to 0.1. It could be quickly regenerated by sunlight irradiation in 6 min with excellent cycling performance of 99% after five cycles. This work sheds light on designing selective adsorbents using responsive subnanoporous materials for environmentally friendly and energy-efficient ion separation and purification.

Bioinspired light-driven chloride pump with helical porphyrin channels

Halorhodopsin, a light-driven chloride pump, utilizes photonic energy to drive chloride ions across biological membranes, regulating the ion balance and conveying biological information. In the light-driven chloride pump process, the chloride-binding chromophore (protonated Schiff base) is crucial, able to form the active center by absorbing light and triggering the transport cycle. Inspired by halorhodopsin, we demonstrate an artificial light-driven chloride pump using a helical porphyrin channel array with excellent photoactivity and specific chloride selectivity. The helical porphyrin channels are formed by a porphyrin-core star block copolymer, and the defects along the channels can be effectively repaired by doping a small number of porphyrins. The well-repaired porphyrin channel exhibits the light-driven Cl- migration against a 3-fold concentration gradient, showing the ion pumping behavior. The bio-inspired artificial light-driven chloride pump provides a prospect for designing bioinspired responsive ion channel systems and high-performance optogenetics.

Cis-Trans Reisomerization Preceding Reprotonation of the Retinal Chromophore Is Common to the Schizorhodopsin Family: A Simple and Rational Mechanism for Inward Proton Pumping

The creation of unidirectional ion transporters across membranes represents one of the greatest challenges in chemistry. Proton-pumping rhodopsins are composed of seven transmembrane helices with a retinal chromophore bound to a lysine side chain via a Schiff base linkage and provide valuable insights for designing such transporters. What makes these transporters particularly intriguing is the discovery of both outward and inward proton-pumping rhodopsins. Surprisingly, despite sharing identical overall structures and membrane topologies, these proteins facilitate proton transport in opposite directions, implying an underlying rational mechanism that can transport protons in different directions within similar protein structures. In this study, we unraveled this mechanism by examining the chromophore structures of deprotonated intermediates in schizorhodopsins, a recently discovered subfamily of inward proton-pumping rhodopsins, using time-resolved resonance Raman spectroscopy. The photocycle of schizorhodopsins revealed the cis-trans thermal isomerization that precedes reprotonation at the Schiff base of the retinal chromophore. Notably, this order has not been observed in other proton-pumping rhodopsins, but here, it was observed in all seven schizorhodopsins studied across the archaeal domain, strongly suggesting that cis-trans thermal isomerization preceding reprotonation is a universal feature of the schizorhodopsin family. Based on these findings, we propose a structural basis for the remarkable order of events crucial for facilitating inward proton transport. The mechanism underlying inward proton transport by schizorhodopsins is straightforward and rational. The insights obtained from this study hold great promise for the design of transmembrane unidirectional ion transporters.

Photo-Modulated Ionic Polymer as an Adaptable Electron Transport Material for Optically Switchable Pixel-Free Displays

In addition to electrically driven organic light-emitting diode (OLED) displays that rely on complicated and costly circuits for switching individual pixel illumination, developing a facile approach that structures pixel-free light-emitting displays with exceptional precision and spatial resolution via external photo-modulation holds significant importance for advancing consumer electronics. Here, optically switchable organic light-emitting pixel-free displays (OSPFDs) are presented and fabricated by judiciously combining an adaptive photosensitive ionic polymer as electron transport materials (ETM) with external photo-modulation as the switching mode while ensuring superior illumination performance and seamless imaging capability. By irradiating the solution-processed OSPFDs with light at specific wavelengths, efficient and reversible tuning of both electron transport and electroluminescence is achieved simultaneously. This remarkable control is achieved by altering the energetic matching within OSPFDs, which also exhibits a high level of universality and adjustable flexibility in the three primary color-based light-emitting displays. Moreover, the ease of creating and erasing desired pixel-free emitting patterns through a non-invasive photopatterning process within a single OSPFD is demonstrated, thereby rendering this approach promising for commercial displaying devices and highly precise pixelated illuminants.

Efficient photo-driven ion pump through slightly reduced vertical graphene oxide membranes

Solar energy can be harvested using biological light-driven ion pumps for the sustainability of life. It remains a significant challenge to develop high-performance artificial light-driven ion pumps for solar energy harvesting in all solid-state materials. Here, we exploit the benefits of short channel lengths and efficient light absorption to demonstrate efficient photo-driven ion transport in slightly reduced vertical graphene oxide membranes (GOMs). Remarkably, this photo-driven ion pump exhibits excellent ability, countering a 10-fold electrolyte concentration gradient. We propose a plausible mechanism where light illumination enhances the electric potential of ion channels on GOMs triggered by the separation of photoexcited charge carriers between the sp2 and sp3 carbon clusters. This results in the establishment of an electric potential difference across the effective ion channels composed of sp3 carbon clusters, thus driving the directional transport of cations from the illuminated side to the non-illuminated side. The promising results of this study provide new possibilities for the application of vertical 2D nanofluidic membranes in areas such as artificial photosynthesis, light harvesting, and water treatment.

Monodirectional Photocycle Drives Proton Translocation

Photoisomerization of retinal is pivotal to ion translocation across the bacterial membrane and has served as an inspiration for the development of artificial molecular switches and machines. Light-driven synthetic systems in which a macrocyclic component transits along a nonsymmetric axle in a specific direction have been reported; however, unidirectional and repetitive translocation of protons has not been achieved. Herein, we describe a unique protonation-controlled isomerization behavior for hemi-indigo dyes bearing N-heterocycles, featuring intramolecular hydrogen bonds. Light-induced isomerization from the Z to E isomer is unlocked when protonated, while reverse E → Z photoisomerization occurs in the neutral state. As a consequence, associated protons are displaced in a preferred direction with respect to the photoswitchable scaffold. These results will prove to be critical in developing artificial systems in which concentration gradients can be effectively generated using (solar) light energy.

Proton-Coupled Photochromic Hemithioindigo: Toward Photoactivated Chemical Sensing and Imaging

We report photoswitchable fluorescent hemithioindigos (HTIs) where the metastable E isomers were stabilized by the proton-bridged intramolecular hydrogen bond. Titration experiments and computational analysis indicated that the E isomers were much more basic than the Z isomers, which enabled photoactivated colorimetric and fluorescent pH response in solvents and polypropylene films. The HTIs exhibited reversibly switchable fluorescence with the Z isomers being the most fluorescent. Moreover, the HTIs were lysosomotropic and the kinetic fluorescence evolution during photoswitching was able to differentiate subcellular compartments with different pH. The combination of photoenhanced basicity, switchable fluorescence, and proton-coupled photochromism lay the groundwork for a broad range of chemical and biological applications.

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.

Light-driven proton transmembrane transport enabled by bio-semiconductor 2D membrane: A general peptide-induced WS2 band shifting strategy

Light-driven proton directional transport is important in living beings as it could subtly realize the light energy conversion for living uses. In the past years, 2D materials-based nanochannels have shown great potential in active ion transport due to controllable properties, including surface charge distribution, wettability, functionalization, electric structure, and external stimuli responsibility, etc. However, to fuse the inorganic materials into bio-membranes still faces several challenges. Here, we proposed peptide-modified WS2 nanosheets via cysteine linkers to realize tunable band structure and, hence, enable light-driven proton transmembrane transport. The modification was achieved through the thiol chemistry of the -SH groups in the cysteine linker and the S vacancy on the WS2 nanosheets. By tuning the amino residues sequences (lysine-rich peptides, denoted as KFC; and aspartate-rich peptides, denoted as DFC), the ζ-potential, surface charge, and band energy of WS2 nanosheets could be rationally regulated. Janus membranes formed by assembling the peptide-modified WS2 nanosheets could realize the proton transmembrane transport under visible light irradiation, driven by a built-in potential due to a type II band alignment between the KFC-WS2 and DFC-WS2. As a result, the proton would be driven across the formed nanochannels. These results demonstrate a general strategy to build bio-semiconductor materials and provide a new way for embedding inorganic materials into biological systems toward the development of bioelectronic devices.

Designs and Applications of Multi-stimuli Responsive FRET Processes in AIEgen-Functionalized and Bi-fluorophoric Supramolecular Materials

Materials capable of displaying strong ratiometric fluorescence with Förster resonance energy transfer (FRET) processes have attracted much research interest because of various chemosensor and biomedical applications. This review highlights several popular strategies in designing FRET-OFF/ON mechanisms of ratiometric fluorescence systems. In particular, the developments of organic and polymeric FRET materials featuring aggregation-induced emission-based luminogens (AIEgens), supramolecular assemblies, photochromic molecular switches and surfactant-induced AIE/FRET mechanisms are presented. AIEgens have been frequently employed as FRET donor and/or acceptor fluorophores to obtain enhanced ratiometric fluorescences in solution and solid states. Since AIE effects and FRET processes rely on controllable distances between fluorophores, many interesting fluorescent properties can be designed by regulating aggregation states in polymers and supramolecular systems. Photo-switchable fluorophores, such as spiropyran and diarylethene, provide drastic changes in fluorescence spectra upon photo-induced isomerizations, leading to photo-switching mechanisms to activate/deactivate FRET processes. Supramolecular assemblies offer versatile platforms to regulate responsive FRET processes effectively. In rotaxane structures, the donor-acceptor distance and FRET efficiency can be tuned by acid/base-controlled shuttling of the macrocycle component. The tunable supramolecular interactions are strongly influenced by external factors (such as pH values, temperatures, analytes, surfactants, UV-visible lights, etc.), which induce the assembly and disassembly of host-guest systems and thus their FRET-ON/FRET-OFF behavior. In addition, the changes in donor or acceptor fluorescence profiles upon detections of analytes can also sufficiently alter the FRET behavior and result in different ratiometric fluorescence outputs. The strategies and examples provided in this review offer the insights and toolkits for future FRET-based material developments.

Photoinduced movement: how photoirradiation induced the movements of matter

Pioneered by the success on active transport of ions across membranes in 1980 using the regulation of the binding properties of crown ethers with covalently linked photoisomerizable units, extensive studies on the movements by using varied interactions between moving objects and environments have been reported. Photoinduced movements of various objects ranging from molecules, polymers to microscopic particles were discussed from the aspects of the driving for the movements, materials design to achieve the movements and systems design to see and to utilize the movements are summarized in this review.

Bioinspired Artificial Ion Pumps

Ion pumps are important membrane-spanning transporters that pump ions against the electrochemical gradient across the cell membrane. In biological systems, ion pumping is essential to maintain intracellular osmotic pressure, to respond to external stimuli, and to regulate physiological activities by consuming adenosine triphosphate. In recent decades, artificial ion pumping systems with diverse geometric structures and functions have been developing rapidly with the progress of advanced materials and nanotechnology. In this Review, bioinspired artificial ion pumps, including four categories: asymmetric structure-driven ion pumps, pH gradient-driven ion pumps, light-driven ion pumps, and electron-driven ion pumps, are summarized. The working mechanisms, functions, and applications of those artificial ion pumping systems are discussed. Finally, a brief conclusion of underpinning challenges and outlook for future research are tentatively discussed.

Quantification of Excited-State Brønsted-Lowry Acidity of Weak Photoacids Using Steady-State Photoluminescence Spectroscopy and a Driving-Force-Dependent Kinetic Theory

Photoacids and photobases constitute a class of molecules that upon absorption of light undergoes a reversible change in acidity, i.e. pK_a. Knowledge of the excited-state pK_a value, pK_a*, is critical for predicting excited-state proton-transfer behavior. A reasonable approximation of pK_a* is possible using the Förster cycle analysis, but only when the ground-state pK_a is known. This poses a challenge for the study of weak photoacids (photobases) with less acidic (basic) excited states (pK_a* (pK_b*) > 7), because ground-state pK_a (pK_b) values are >14, making it difficult to quantify them accurately in water. Another method to determine pK_a* relies on acid-base titrations with photoluminescence detection and Henderson-Hasselbalch analysis. This method requires that the acid dissociation reaction involving the thermally equilibrated electronic excited state reaches chemical quasi-equilibrium, which does not occur for weak photoacids (photobases) due to slow rates of excited-state proton transfer. Herein, we report a method to overcome these limitations. We demonstrate that liquid water and aqueous hydroxide are unique proton-accepting quenchers of excited-state photoacids. We determine that Stern-Volmer quenching analysis is appropriate to extract rate constants for excited-state proton transfer in aqueous solutions from a weak photoacid, 5-aminonaphthalene-1-sulfonate, to a series of proton-accepting quenchers. Analysis of these data by Marcus-Cohen bond-energy-bond-order theory yields an accurate value for pK_a* of 5-aminonaphthalene-1-sulfonate. Our method is broadly accessible because it only requires readily available steady-state photoluminescence spectroscopy. Moreover, our results for weak photoacids are consistent with those from previous studies of strong photoacids, each showing the applicability of kinetic theories to interpret driving-force-dependent rate constants for proton-transfer reactions.

Spontaneous and Selective Potassium Transport through a Suspended Tailor-Cut Ti3C2Tx MXene Film

Biological ion pumps selectively transport target ions against the concentration gradient, a process that is crucial to maintaining the out-of-equilibrium states of cells. Building an ion pump with ion selectivity has been challenging. Here we show that a Ti₃C₂T_x MXene film suspended in air with a trapezoidal shape spontaneously pumps K⁺ ions from the base end to the tip end and exhibits a K⁺/Na⁺ selectivity of 4. Such a phenomenon is attributed to a range of properties of MXene. Thanks to the high stability of MXene in water and the dynamic equilibrium between evaporation and swelling, the film keeps a narrow interlayer spacing of ∼0.3 nm when its two ends are connected to reservoirs. Because of the polar electrical structure and hydrophilicity of the MXene nanosheet, K⁺ ions experience a low energy barrier of ∼4.6 k_BT when entering these narrow interlayer spacings. Through quantitative simulations and consistent experimental results, we further show that the narrow spacings exhibit a higher energy barrier to Na⁺, resulting in K⁺/Na⁺ selectivity. Finally, we show that the spontaneous ion transport is driven by the asymmetric evaporation of the interlayer water across the film, a mechanism that is similar to pressure driven streaming current. This work shows how ion transport properties can be facilely manipulated by tuning the macroscopic shape of nanofluidic materials, which may attract interest in the interface of kirigami technologies and nanofluidics and show potential in energy and separation applications.

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

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

Light-Powered Ion Pumping in a Cation-Selective Conducting Polymer Membrane

The simulation of the ion pumping against a proton gradient energized by light in photosynthesis is of significant importance for the energy conversion in a non-biological environment. Herein, we report light-powered ion pumping in a polystyrene sulfonate anion (PSS) doped polypyrrole (PPy) conducting polymer membrane (PSS-PPy) with a symmetric geometry. This PSS-PPy conducting polymer membrane exhibits a cationic selectivity and a light-responsive surface-charge-governed ion transport attributed to the negatively charged PSS groups. An asymmetric visible irradiation on one side of the PSS-PPy membrane induces a built-in electric field across the membrane due to the intrinsic photoelectronic property of PPy, which drives the cationic transport against the concentration gradient, demonstrating an ion-pumping effect. This work is a prototype that uses a geometry-symmetric conducting polymer membrane as a light-powered artificial ion pump for active ion transport, which exhibits potential applications in nanofluidic energy conversion.

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.

Angstrom-scale ion channels towards single-ion selectivity

Artificial ion channels with ion permeability and selectivity comparable to their biological counterparts are highly desired for efficient separation, biosensing, and energy conversion technologies. In the past two decades, both nanoscale and sub-nanoscale ion channels have been successfully fabricated to mimic biological ion channels. Although nanoscale ion channels have achieved intelligent gating and rectification properties, they cannot realize high ion selectivity, especially single-ion selectivity. Artificial angstrom-sized ion channels with narrow pore sizes <1 nm and well-defined pore structures mimicking biological channels have accomplished high ion conductivity and single-ion selectivity. This review comprehensively summarizes the research progress in the rational design and synthesis of artificial subnanometer-sized ion channels with zero-dimensional to three-dimensional pore structures. Then we discuss cation/anion, mono-/di-valent cation, mono-/di-valent anion, and single-ion selectivities of the synthetic ion channels and highlight their potential applications in high-efficiency ion separation, energy conversion, and biological therapeutics. The gaps of single-ion selectivity between artificial and natural channels and the connections between ion selectivity and permeability of synthetic ion channels are covered. Finally, the challenges that need to be addressed in this research field and the perspective of angstrom-scale ion channels are discussed.

Synergistic regulation of nonbinary molecular switches by protonation and light

We report a molecular switching ensemble whose states may be regulated in synergistic fashion by both protonation and photoirradiation. This allows hierarchical control in both a kinetic and thermodynamic sense. These pseudorotaxane-based molecular devices exploit the so-called Texas-sized molecular box (cyclo[2]-(2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene); 1⁴⁺, studied as its tetrakis-PF₆ ^- salt) as the wheel component. Anions of azobenzene-4,4'-dicarboxylic acid (2H⁺•2) or 4,4'-stilbenedicarboxylic acid (2H⁺•3) serve as the threading rod elements. The various forms of 2 and 3 (neutral, monoprotonated, and diprotonated) interact differently with 1⁴⁺, as do the photoinduced cis or trans forms of these classic photoactive guests. The net result is a multimodal molecular switch that can be regulated in synergistic fashion through protonation/deprotonation and photoirradiation. The degree of guest protonation is the dominating control factor, with light acting as a secondary regulatory stimulus. The present dual input strategy provides a complement to more traditional orthogonal stimulus-based approaches to molecular switching and allows for the creation of nonbinary stimulus-responsive functional materials.

Excited State Proton Transfers in Hybrid Compound Based on Indoline Spiropyran of the Coumarin Type and Azomethinocoumarin in the Presence of Metal Ions

Spectral-luminescence properties of a hybrid compound containing a coumarin-type spiropyran and an azomethinocoumarin fragment in toluene-acetonitrile solution in the presence of Li⁺, Ca²⁺, Zn²⁺ and Mg²⁺ ions are reported. Two excited state proton transfers can occur in the hybrid compound—the transfer of a proton from the OH group of the 7-hydroxy coumarin tautomer to the N atom of the C=N bond of the azomethine fragment leading to green ESIPT fluorescence with a maximum at 540 nm and from the OH group of the 7-hydroxy coumarin tautomer to the carbonyl group of the pyrone chromophore, which leads to the formation of the 2-hydroxyl-tautomer T of coumarin with blue fluorescence with a maximum at 475 nm. Dependence of these excited state proton transfers on the metal nature and irradiation with an external UV source is discussed.

Light-Modulated Cationic and Anionic Transport across Protein Biopolymers*

Light is a convenient source of energy and the heart of light-harvesting natural systems and devices. Here, we show light-modulation of both the chemical nature and ionic charge carrier concentration within a protein-based biopolymer that was covalently functionalized with photoacids or photobases. We explore the capability of the biopolymer-tethered photoacids and photobases to undergo excited-state proton transfer and capture, respectively. Electrical measurements show that both the photoacid- and photobase-functionalized biopolymers exhibit an impressive light-modulated increase in ionic conductivity. Whereas cationic protons are the charge carriers for the photoacid-functionalized biopolymer, water-derived anionic hydroxides are the suggested charge carriers for the photobase-functionalized biopolymer. Our work introduces a versatile toolbox to photomodulate both protons and hydroxides as charge carriers in polymers, which can be of interest for a variety of applications.

Light-driven directional ion transport for enhanced osmotic energy harvesting

Light-driven ion (proton) transport is a crucial process both for photosynthesis of green plants and solar energy harvesting of some archaea. Here, we describe use of a TiO₂/C₃N₄ semiconductor heterojunction nanotube membrane to realize similar light-driven directional ion transport performance to that of biological systems. This heterojunction system can be fabricated by two simple deposition steps. Under unilateral illumination, the TiO₂/C₃N₄ heterojunction nanotube membrane can generate a photocurrent of about 9 μA/cm², corresponding to a pumping stream of ∼5500 ions per second per nanotube. By changing the position of TiO₂ and C₃N₄, a reverse equivalent ionic current can also be realized. Directional transport of photogenerated electrons and holes results in a transmembrane potential, which is the basis of the light-driven ion transport phenomenon. As a proof of concept, we also show that this system can be used for enhanced osmotic energy generation. The artificial light-driven ion transport system proposed here offers a further step forward on the roadmap for development of ionic photoelectric conversion and integration into other applications, for example water desalination.

Acylhydrazone Subunits as a Proton Cargo Delivery System in 7-Hydroxyquinoline

The reimagined concept of long-range tautomeric proton transfer using crane subunits is shown by designing and synthesising two new acylhydrazones containing a 7-hydroxyquinoline (7-OHQ) platform. The acylhydrazone subunits attached to the 7-OHQ at the 8th position act as crane arms for delivering proton cargo to the quinoline nitrogen. Light-induced tautomerization to their keto forms leads to Z/E isomerization of the C=C axle bond, followed by proton delivery to the quinoline nitrogen by the formation of covalent or hydrogen bonds. The axle's being either an imine or ketimine bond is the structural difference between the studied compounds. The -CH₃ group in the latter provides steric strain, resulting in different proton transport pathways. Both compounds show long thermal stability in the switched state, which creates a tuneable action of bidirectional proton cargo transport by using different wavelengths of irradiation. Upon the addition of acid, the quinoline nitrogen is protonated; this results in E/Z configuration switching of the acylhydrazone subunits. This was proven by single-crystal X-ray structure analysis and NMR spectroscopy.

Proton-Transfer Dynamics of Photoacidic Merocyanines in Aqueous Solution

Photoacids attract increasing scientific attention, as they are valuable tools to spatiotemporally control proton‐release reactions and pH values of solutions. We present the first time‐resolved spectroscopic study of the excited state and proton‐release dynamics of prominent merocyanine representatives. Femtosecond transient absorption measurements of a pyridine merocyanine with two distinct protonation sites revealed dissimilar proton‐release mechanisms: one site acts as a photoacid generator as its pK_a value is modulated in the ground state after photoisomerization, while the other functions as an excited state photoacid which releases its proton within 1.1 ps. With a pK_a drop of 8.7 units to −5.5 upon excitation, the latter phenolic site is regarded a super‐photoacid. The 6‐nitro derivative exhibits only a phenolic site with similar, yet slightly less photoacidic characteristics and both compounds transfer their proton to methanol and ethanol. In contrast, for the related 6,8‐dinitro compound an intramolecular proton transfer to the ortho‐nitro group is suggested that is involved in a rapid relaxation into the ground state.

Tautomeric influence on the photoinduced birefringence of 4-substituted phthalimide 2-hydroxy Schiff bases in PMMA matrix

The photoinduced birefringence of two 4-substituted phthalimide 2-hydroxy Schiff bases, containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties has been investigated in PMMA matrix. Their optical behaviour as nanocomposite films was revealed by combined use of DFT quantum chemical calculations (in ground and excited state) and experimental optical spectroscopy (UV-Vis and fluorescence). The results have shown that solid-state reversible switching takes place by enol/keto tautomerization and Z/E isomerization. Birefringence study was performed in the PMMA nanocomposite films using pump lasers at λ_rec = 355 nm and λ_rec = 442 nm. Fast response time and high stability of anisotropy up to 58% for (4) and 95% for (5) after turning off the excitation laser, was observed, which makes these materials appropriate candidates for cutting-edge optical information technology materials. The possibility for multiple cycles of recording, reading and optical erasure of the photoinduced birefringence at λ_rec = 442 nm in 5 has been demonstrated. The obtained results have shown that the maximum value of the measured birefringence is close to the anisotropic characteristics of the most frequently used azo materials.

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

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

Transport Phenomena in Nano/Molecular Confinements

The transport of fluid and ions in nano/molecular confinements is the governing physics of a myriad of embodiments in nature and technology including human physiology, plants, energy modules, water collection and treatment systems, chemical processes, materials synthesis, and medicine. At nano/molecular scales, the confinement dimension approaches the molecular size and the transport characteristics deviates significantly from that at macro/micro scales. A thorough understanding of physics of transport at these scales and associated fluid properties is undoubtedly critical for future technologies. This compressive review provides an elaborate picture on the promising future applications of nano/molecular transport, highlights experimental and simulation metrologies to probe and comprehend this transport phenomenon, discusses the physics of fluid transport, tunable flow by orders of magnitude, and gating mechanisms at these scales, and lists the advancement in the fabrication methodologies to turn these transport concepts into reality. Properties such as chain-like liquid transport, confined gas transport, surface charge-driven ion transport, physical/chemical ion gates, and ion diodes will provide avenues to devise technologies with enhanced performance inaccessible through macro/micro systems. This review aims to provide a consolidated body of knowledge to accelerate innovation and breakthrough in the above fields.

Light-Driven Active Ion Transport

Solar energy can be harvested by biological systems to regulate the directional transport of protons and ions across cells and organelles. Structural and functional bio-mimic photo-active ion nanofluidic conductors, usually in the forms of ion channels and ion pumps, have been increasingly applied to realize active ion transport. However, progress in attaining effective light-driven active transport of ions (protons) has been constrained by the inherent limitations of membrane materials and their chemical and topological structures. Recent advances in the construction of photo-responsive physical ion pump in all-solid-state membranes could potentially lead to new classes of membrane-based materials for active ion transport. In this concept, the development of the state-of-the-art technologies for manufacturing artificial light-driven active ion transport systems are presented and discussed, which mainly involves the utilization of solar energy to realize two types of active ion transport, chemically and physically active ion transport. Afterward, we summarize the key factors towards culminating highly effective and selective membranes for active ion transport. To conclude, we highlight the promising application perspectives of this light-driven active ion transport technique in the field of energy conversion, bio-interfaces and water treatment.

Indirect solvent assisted tautomerism in 4-substituted phthalimide 2-hydroxy-Schiff bases

The paper presents the synthesis and characterization of two 4-substituted phthalimide 2-hydroxy-Schiff bases containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties. The structural differences of 2-hydroxyaryl substituents, resulting in different enol/keto tautomeric behaviour, depending on the solvent environment were studied by absorption UV-Vis spectroscopy. Compound 5 is characterized by a solvent-dependent tautomeric equilibrium (K_T in toluene = 0.12, acetonitrile = 0.22 and MeOH = 0.63) while no tautomerism is observed in 4. Ground state theoretical DFT calculations by using continuum solvation in MeOH indicate an energy barrier between enol/keto tautomer 5.6 kcal mol^-1 of 4 and 0.63 kcal mol^-1 of 5, which confirms the experimentally observed impossibility of the tautomeric equilibrium in the former. The experimentally observed specific solvent effect in methanol is modeled via explicit solvation. The excited state intramolecular proton transfer (ESIPT) was investigated by steady state fluorescence spectroscopy. Both compounds show a high rate of photoconversion to keto tautomers hence keto emissions with large Stokes shifts in five alcohols (MeOH, EtOH, 1-propanol, 1-butanol, and 1-pentanol) and various aprotic solvents (toluene, dichlormethane, acetone, AcCN). According to the excited state TDDFT calculations using implicit solvation in MeOH, it was found that enol tautomers of 4 and 5 are higher in energy compared to the keto ones, which explains the origin of the experimentally observed keto form emission.

Nanochannels regulating ionic transport for boosting electrochemical energy storage and conversion: a review

Electrochemical power sources, as one of the most promising energy storage and conversion technologies, provide great opportunities for developing high energy density electrochemical devices and portable electronics. However, uncontrolled ionic transport in electrochemical energy conversion, typically undesired anion transfer, usually causes some issues degrading the performance of energy storage devices. Nanochannels offer an effective strategy to solve the ionic transport problems for boosting electrochemical energy storage and conversion. In this review, the advantages of nanochannels for electrochemical energy storage and conversion and the construction principle of nanochannels are introduced, including ion selectivity and ultrafast ion transmission of nanochannels, which are considered as two critical factors to achieve highly efficient energy conversion. Recent advances in applications of nanochannels in lithium secondary batteries (LSBs), electrokinetic energy conversion systems and concentration cells are summarized in detail. Nanochannels exist in the above systems in two typical forms: functional separator and electrode protective layer. Current research on nanochannel-based LSBs is still at the early stage, and deeper and broader applications are expected in the future. Finally, the remaining challenges of nanochannel fabrication, performance improvement, and intelligent construction are presented. It is envisioned that this paper will provide new insights for developing high-performance and versatile energy storage electronics based on nanochannels.

Enlightening Materials with Photoswitches

Incorporating molecular photoswitches into various materials provides unique opportunities for controlling their properties and functions with high spatiotemporal resolution using remote optical stimuli. The great and largely still untapped potential of these photoresponsive systems has not yet been fully exploited due to the fundamental challenges in harnessing geometrical and electronic changes on the molecular level to modulate macroscopic and bulk material properties. Herein, progress made during the past decade in the field of photoswitchable materials is highlighted. After pointing to some general design principles, materials with an increasing order of the integrated photoswitchable units are discussed, spanning the range from amorphous settings over surfaces/interfaces and supramolecular ensembles, to liquid crystalline and crystalline phases. Finally, some potential future directions are pointed out in the conclusion. In view of the exciting recent achievements in the field, the future emergence and further development of light-driven and optically programmable (inter)active materials and systems are eagerly anticipated.

Electric-Field-Induced Ionic Sieving at Planar Graphene Oxide Heterojunctions for Miniaturized Water Desalination

Layered graphene oxide membranes (GOMs) offer a unique platform for precise sieving of small ions and molecules due to controlled sub-nanometer-wide interlayer distance and versatile surface chemistry. Pristine and chemically modified GOMs effectively block organic dyes and nanoparticles, but fail to exclude smaller ions with hydrated diameters less than 9 Å. Toward sieving of small inorganic salt ions, a number of strategies are proposed by reducing the interlayer spacing down to merely several angstroms. However, one critical challenge for such compressed GOMs is the extremely low water flux (<0.1 Lm^-2 h^-1 bar^-1 ) that prevents these innovative nanomaterials from being used in real-world applications. Here, a planar heterogeneous graphene oxide membrane (PHGOM) with both nearly perfect salt rejection and high water flux is reported. Horizontal ion transport through oppositely charged GO multilayer lateral heterojunction exhibits bi-unipolar transport behavior, blocking the conduction of both cations and anions. Assisted by a forward electric field, salt concentration is depleted in the near-neutral transition area of the PHGOM. In this situation, deionized water can be extracted from the depletion zone. Following this mechanism, a high rejection rate of 97.0% for NaCl and water flux of 1529 Lm^-2 h^-1 bar^-1 at the outlet via an inverted T-shaped water extraction mode are achieved.