Building intercalation structure for high ionic conductivity via aliovalent substitution

Two-dimensional (2D) materials, which possess robust nanochannels, high flux and allow scalable fabrication, provide new platforms for nanofluids. Highly efficient ionic conductivity can facilitate the application of nanofluidic devices for modern energy conversion and ionic sieving. Herein, we propose a novel strategy of building an intercalation crystal structure with negative surface charge and mobile interlamellar ions via aliovalent substitution to boost ionic conductivity. The Li2xM1-xPS3 (M = Cd, Ni, Fe) crystals obtained by the solid-state reaction exhibit distinct capability of water absorption and apparant variation of interlayer spacing (from 0.67 to 1.20 nm). The assembled membranes show the ultrahigh ionic conductivity of 1.20 S/cm for Li0.5Cd0.75PS3 and 1.01 S/cm for Li0.6Ni0.7PS3. This facile strategy may inspire the research in other 2D materials with higher ionic transport performance for nanofluids.

Host-Guest Recognition Boosts Biomimetic Mono/Multivalent Cation Separation

Biomimetic ion permselective membranes with ultrahigh ion permeability and selectivity represent a research frontier in ion separation, yet the successful fabrication of such membranes remains a formidable challenge. Here, we demonstrate a 4-sulfocalix[4]arene (4-SCA)-modified graphene oxide (GO) membrane that shows extraordinary performance in separating mono-from multivalent cations, as well as having reversible pH-responsiveness. The resulting 4-SCA-modified GO (SCA-GO) membrane preferentially transports potassium ions (K⁺) over radionuclide cations (Co²⁺, UO₂²⁺, La³⁺, Eu³⁺, and Th⁴⁺). The ion selectivities are an order of magnitude higher than that of the unmodified GO membrane. Theoretical calculations and experimental investigations demonstrate that the much-improved ion selectivity arises from the specific recognition between 4-SCA and radionuclide cations. The transport of multivalent radionuclides is impeded by a binding-obstructing mechanism from the host-guest interactions. Interestingly, the host-guest interactions are responsive to the protonation/deprotonation transformation of the 4-SCA. Therefore, the SCA-GO membrane mimics pH-regulated ion selective behavior found in biological ion channels. Our strategy of designing a biomimetic permselective GO membrane may allow efficient nuclear wastewater treatment and, more importantly, deepen our understanding of biomimetic ion transport mechanisms.

Mixed Matrix Membrane with Penetrating Subnanochannels: A Versatile Nanofluidic Platform for Selective Metal Ion Conduction

Biological ion channels penetrated through cell membrane form unique transport pathways for selective ionic conductance. Replicating the success of ion selectivity with mixed matrix membranes (MMMs) will enable new separation technologies but remains challenging. Herein, we report a soft substrate-assisted solution casting method to develop MMMs with penetrating subnanochannels for selective metal ion conduction. The MMMs are composed of penetrating Prussian white (PW) microcubes with subnanochannels in dense polyimide (PI) matrices, achieving selective monovalent metal ion conduction. The ion selectivity of K⁺ /Mg²⁺ is up to 14.0, and the ion conductance of K⁺ can reach 45.5 μS with the testing diameter of 5 mm, which can be further improved by increasing the testing area. Given the diversity of nanoporous materials and polymer matrices, we expect that the MMMs with penetrating subnanochannels could be developed into a versatile nanofluidic platform for various emerging applications.

Light-Enhanced Osmotic Energy Harvester Using Photoactive Porphyrin Metal-Organic Framework Membranes

High ion selectivity and permeability, as two contradictory aspects for the membrane design, highly hamper the development of osmotic energy harvesting technologies. Metal-organic frameworks (MOFs) with ultra-small and high-density pores and functional surface groups show great promise in tackling these problems. Here, we propose a facile and mild cathodic deposition method to directly prepare crack-free porphyrin MOF membranes on a porous anodic aluminum oxide for osmotic energy harvesting. The abundant carboxyl groups of the functionalized porphyrin ligands together with the nanoporous structure endows the MOF membrane with high cation selectivity and ion permeability, thus a large output power density of 6.26 W m^-2 is achieved. The photoactive porphyrin ligands further lead to an improvement of the power density to 7.74 W m^-2 upon light irradiation. This work provides a promising strategy for the design of high-performance osmotic energy harvesting systems.

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.

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.

Porphyrin-Containing MOFs and COFs as Heterogeneous Photosensitizers for Singlet Oxygen-Based Antimicrobial Nanodevices

Visible-light irradiation of porphyrin and metalloporphyrin dyes in the presence of molecular oxygen can result in the photocatalytic generation of singlet oxygen (¹O₂). This type II reactive oxygen species (ROS) finds many applications where the dye, also called the photosensitizer, is dissolved (i.e., homogeneous phase) along with the substrate to be oxidized. In contrast, metal-organic frameworks (MOFs) are insoluble (or will disassemble) when placed in a solvent. When stable as a suspension, MOFs adsorb a large amount of O₂ and photocatalytically generate ¹O₂ in a heterogeneous process efficiently. Considering the immense surface area and great capacity for gas adsorption of MOFs, they seem ideal candidates for this application. Very recently, covalent-organic frameworks (COFs), variants where reticulation relies on covalent rather than coordination bonds, have emerged as efficient photosensitizers. This comprehensive mini review describes recent developments in the use of porphyrin-based or porphyrin-containing MOFs and COFs, including nanosized versions, as heterogeneous photosensitizers of singlet oxygen toward antimicrobial applications.

Porphyrin-based MOFs as heterogeneous photocatalysts for the eradication of organic pollutants and toxins

Water and air pollution are among the major environmental challenges of this era. Waste management, economic sustainable development and renewable energy are unavoidable concomitant considerations. Over the past five years, nanosized metal-organic frameworks (nano-MOFs) have been developed for the elimination of pollutants in wet media and air-born toxins using the highly efficient reactive oxygen species (ROS) of type I (H2O2, •OH, O[Formula: see text] and of type II (1O[Formula: see text]. The ROS are catalytically and efficiently generated through photosensitization, and porphyrins and metalloporphyrins are pigments of choice for this purpose. This short review summarizes the fundamentals of ROS generation by porphyrin-based nano-MOFs (mainly through the formation of ROS type II) and their composites (leading to ROS type I), which includes energy and electron transfer processes, and their applications in these environmental issues.