Metal-organic frameworks in drug delivery: engineering versatile platforms for therapeutic applications

Recently, metal-organic frameworks (MOFs) have attracted much attention as versatile materials for drug delivery and personalized medicine. MOFs are porous structures made up of metal ions coupled with organic ligands. This review highlights the synthesis techniques used to design MOFs with specific features such as surface area and pore size, and the drug encapsulation within MOFs not only improves their stability and solubility but also allows for controlled release kinetics, which improves therapeutic efficacy and minimizes adverse effects. Furthermore, it discusses the challenges and potential advantages of MOF-based drug delivery, such as MOF stability, biocompatibility, and scale-up production. With further advancements in MOF synthesis, functionalization techniques, and understanding of their interactions using biological systems, MOFs can have significant promise for expanding the area of personalized medicine and improving patient outcomes.

Cathodic Deposition-Assisted Synthesis of Thin Glass MOF Films for High-Performance Gas Separations

Glass metal-organic framework (MOF) films can be fabricated from their crystalline counterparts through a melt-quenching process and are prospective candidates for gas separation because of the elimination of the grain boundaries in crystalline MOF films. However, current techniques are limited to producing glass MOF films with a thickness of tens of micrometers, which leads to ultralow gas permeances. Here, we report a novel cathodic deposition-assisted synthesis of glass ZIF-62 films with a thickness as low as ~1 μm. Electrochemical analyses and deposition experiments suggest that the cathodic deposition can lead to pure crystalline ZIF-62 films with a controllable thickness of ~2 μm to ~15 μm. Accordingly, glass ZIF-62 films with a thickness of ~1 μm to ~10 μm can be obtained after a thermal treatment. The fabricated defect-free glass ZIF-62 film measuring 2 μm in thickness shows a remarkable CO2/N2 and CO2/CH4 selectivity of 31.4 and 33.4, respectively, with a CO2 permeance which is over 30 times higher than the best-performing glass ZIF-62 films in literature.

Metal-Organic Frameworks and Their Composites for Chronic Wound Healing: From Bench to Bedside

Chronic wounds are characterized by delayed and dysregulated healing processes. As such, they have emerged as an increasingly significant threat. The associated morbidity and socioeconomic toll are clinically and financially challenging, necessitating novel approaches in the management of chronic wounds. Metal-organic frameworks (MOFs) are an innovative type of porous coordination polymers, with low toxicity and high eco-friendliness. Documented anti-bacterial effects and pro-angiogenic activity predestine these nanomaterials as promising systems for the treatment of chronic wounds. In this context, the therapeutic applicability and efficacy of MOFs remain to be elucidated. It is, therefore, reviewed the structural-functional properties of MOFs and their composite materials and discusses how their multifunctionality and customizability can be leveraged as a clinical therapy for chronic wounds.

Ligand-Oxidation-Based Anodic Synthesis of Oriented Films of Conductive M-Catecholate Metal-Organic Frameworks with Controllable Thickness

Effective control over the crystallization of metal-organic framework (MOF) films is of great importance not only for the performance study and optimization in related applications but also for the fundamental understanding of the involved reticular chemistry. Featuring many technological advantages, electrochemical synthesis has been extensively reported for many MOF materials but is still challenged by the production of dense oriented films with a large-range tuning of thickness. Here, we report a ligand-oxidation-based anodic strategy capable of synthesizing oriented films of two-dimensional (2D) and three-dimensional (3D) conductive M-catecholate MOFs (2D Cu3(HHTP)2, 2D Zn3(HHTP)2, 2D Co3(HHTP)2, 3D YbHHTP, and 2D Cu2TBA) with tunable thicknesses up to tens of micrometers on commonly used electrodes. This anodic strategy relies on the oxidation of redox-active catechol ligands and follows a stepwise electrochemical-chemical reaction mechanism to achieve effective control over crystallizing M-catecholate MOFs into films oriented in the [001] direction. Benefiting from the electrically conductive nature, Cu3(HHTP)2 films could be thickened at a steady rate (17.4 nm·min-1) from ∼90 nm to 10.7 μm via a growth mechanism differing from those adopted in previous electrochemical synthesis of dense MOF films with limited thickness due to the self-inhibition effect. This anodic synthesis could be further combined with a templating strategy to fabricate not only films with well-defined 2D features in sizes from micrometers to millimeters but also high aspect ratio mesostructures, such as nanorods, of Cu3(HHTP)2.

One-Step Electrodeposition of a Mesoporous Ni/Co-Imidazole-Based Bimetal-Organic Framework on Pyramid-like NiSb with Abundant Coupling Interfaces as an Ultra-Stable Heterostructural Electrocatalyst for Water Splitting

The exploration of highly efficient metal-organic framework (MOF)-based electrocatalysts is a research topic of high significance owing to their potential applications in sustainable and clean energy production. Herein, a mesoporous MOF containing Ni and Co nodes along with 2-methylimidazole (Hmim) ligands has been directly grown on the surface of the pyramid-like NiSb through a convenient cathodic electrodeposition strategy and evaluated as the catalyst for water splitting catalysis. Tailoring catalytically active sites through porous well-arranging architecture and the coupled interface offers a catalyst with exquisite performance that displays ultra-low Tafel constant of 33 and 42 mV dec^-1 toward the hydrogen evolution reaction and oxygen evolution reaction, sequentially, and also enhanced durability at high current densities over 150 h in a 1 M KOH medium. The success of the synthesized NiCo-MOF@NiSb@GB electrode is explained by the intimate contact between the NiCo-MOF and NiSb with well-tailored phase interfaces, the positive coupling effect between Ni and Co metal centers in the MOF, and the porous structure with abundant active sites toward electrocatalysis. Importantly, the present work provides a new technical reference for the electrochemical synthesis of heterostructural MOFs as a promising candidate for energy-related applications.

Cathodic deposition of MOF films: mechanism and applications

Metal-organic framework (MOF) thin films could be used for ion/molecular sieving, sensing, catalysis, and energy storage, but thus far no large-scale applications are known. One of the reasons is the lack of convenient and controllable fabrication methods. This work reviews the cathodic deposition of MOF films, which has advantages (e.g., simple operations, mild conditions, and controllable MOF film thickness/morphology) over other reported techniques. Accordingly, we discuss the mechanism of the cathodic deposition of MOF films which consists of the electrochemically triggered deprotonation of organic linkers and the formation of inorganic building blocks. Thereafter, the main applications of cathodically deposited MOF films are introduced with the aim of showing this technique's wide-ranging applications. Finally, we give the remaining issues and outlooks of the cathodic deposition of MOF films to drive its future development.

Janus Metal-Organic Framework Membranes Boosting the Osmotic Energy Harvesting

The unique ion-transport properties in nanoconfined pores enable nanofluidic devices with great potential in harvesting osmotic energy. The energy conversion performance could be significantly improved by the precise regulation of the "permeability-selectivity" trade-off and the ion concentration polarization effect. Here, we take the advantage of electrodeposition technique to fabricate a Janus metal-organic framework (J-MOF) membrane that possesses rapid ion-transport capability and impeccable ion selectivity. The asymmetric structure and asymmetric surface charge distribution of the J-MOF device can suppress the ion concentration polarization effect and enhance the ion charge separation, exhibiting an improved energy harvesting performance. An output power density of 3.44 W/m² has been achieved with the J-MOF membrane at a 1000-fold concentration gradient. This work provides a new strategy for fabricating high-performance energy-harvesting devices.

In situ fabrication of bendable epitaxial metal-organic framework films via spraying

Epitaxial metal-organic framework (MOF) films have shown huge potential for use in separation applications. Herein, bendable epitaxial MOF films are fabricated via spraying. The synthesized MOF films show excellent oil-in-water emulsion separation performance even after being bent for multiple times at high curvatures.

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.

Defect-Free Metal-Organic Framework Membrane for Precise Ion/Solvent Separation toward Highly Stable Magnesium Metal Anode

Metallic magnesium batteries are promising candidates beyond lithium-ion batteries; however, a passive interfacial layer because of the electro-reduction of solvents on Mg surfaces usually leads to ultrahigh overpotential for the reversible Mg chemistry. Inspired by the excellent separation effect of permselective metal-organic framework (MOF) at angstrom scale, a large-area and defect-free MOF membrane directly on Mg surfaces is here constructed. In this process, the electrochemical deprotonation of ligand can be facilitated to afford the self-correcting of intercrystalline voids until a seamless membrane formed, which can eliminate nonselective intercrystalline diffusion of electrolyte and realize selective Mg²⁺ transport but precisely separate the solvent molecules from the MOF channels. Compared with the continuous solvent reduction on bare Mg anode, the as-constructed MOF membrane is demonstrated to significantly stabilize the Mg electrode via suppressing the permeation of solvents, hence contributing to a low-overpotential plating/stripping in conventional electrolytes. The concept is demonstrated that membrane separation can serve as solid-electrolyte interphase, which would be widely applicable to other energy-storage systems.

Cathodic Electrodeposition of MOF Films Using Hydrogen Peroxide

Metal-organic framework (MOF) films can be made by cathodic electrodeposition, where a Brønsted base is formed electrochemically which deprotonates the MOF linkers that are present in solution as undissociated/partially dissociated weak acids. However, the co-deposition of metal and the narrow range of possible metal nodes limit the scope of this method. In this work, we propose the use of hydrogen peroxide (hydrogen peroxide assisted cathodic deposition or HPACD), to overcome these limitations. Electrochemical measurements indicate that in DMF, hydrogen peroxide is reduced to superoxide anions that deprotonate the carboxylic ligands. This single-electron reduction happens at much higher potentials than all previous reported methods. This prevents the co-deposition of metal and extends the range of possible metal nodes. Various pure MOF films (HKUST-1, MIL-53(Fe) and MOF-5) were prepared via this approach. HPACD was also used for the preparation of patterned MOF films and of flexible Cu-BTC coated paper membranes which reject 99.1 % of Rose Bengal from water with a permeance of 8.4 L m^-2  h^-1  bar^-1 .

One-Step Electrochemical Growth of 2D/3D Zn(II)-MOF Hybrid Nanocomposites on an Electrode and Utilization of a PtNPs@2D MOF Nanocatalyst for Electrochemical Immunoassay

To date, two-dimensional (2D) and three-dimensional (3D) metal organic frameworks (MOFs) have been promising materials for applications in electrocatalysis, separation, and sensing. However, the exploration of a simple method for simultaneous fabrication of 2D/3D MOFs on a surface remains challenging. Herein, a one-step and in situ electrosynthesis strategy for fabrication of 2D Hemin-bridged MOF sheets (Hemin-MOFs) or 2D/3D Zn(II)-MOF hybrid nanocomposites on an electrode is reported. It exhibits varied morphologies at different electrodeposition times and attains a 2D/3D complex morphology by adding 1,3,5-benzenetricarboxylic acid (H₃BTC) as an organic ligand. The morphology and size of 2D Hemin-MOFs are important factors that influence their performance. Since Pt nanoparticles (PtNPs) are grown on 2D Hemin-MOF sheets, this composite can serve as the peroxidase mimics and PtNPs can act as an anchor to capture the antibody. Therefore, this hybrid nanosheet-modified electrode is used as an electrochemical sensing platform for ultrasensitive pig immunoglobulin G (IgG) and the surface-protective antigen (Spa) protein of Erysipelothrix rhusiopathiae immunodetection. Moreover, this work provides a new avenue for the electrochemical synthesis of 2D/3D MOF hybrid nanocomposites with a high surface area and biomimetic catalysts.

Electrolytic synthesis of porphyrinic Zr-metal-organic frameworks with selective crystal topologies

The thermodynamic (PCN-222) and kinetic (PCN-224) products of porphyrinic Zr-metal-organic frameworks (MOFs) were synthesized via an anodic dissolution approach for the first time. To the best of our knowledge, this is the first report of MOF polymorphs being controlled by electrolysis. The selective formation of PCN-222 requires an amorphous component to be present on the electrode during the initial reaction process.

Cathodic synthesis of a Cu-catecholate metal–organic framework

Uniform copper-catecholate metal–organic framework (Cu-CAT-1) films with tunable thickness were prepared by oxygen-assisted cathodic synthesis.

Bulk and local structures of metal-organic frameworks unravelled by high-resolution electron microscopy

The periodic bulk structures of metal-organic frameworks (MOFs) can be solved by diffraction-based techniques; however, their non-periodic local structures-such as crystal surfaces, grain boundaries, defects, and guest molecules-have long been elusive due to a lack of suitable characterization tools. Recent advances in (scanning) transmission electron microscopy ((S)TEM) has made it possible to probe the local structures of MOFs at atomic resolution. In this article, we discuss why high-resolution (S)TEM of MOFs is challenging and how the new low-dose techniques overcome this challenge, and we review various MOF structural features observed by (S)TEM and important insights gained from these observations. Our discussions focus on real-space imaging, excluding other TEM-related characterization techniques (e.g. electron diffraction and spectroscopy).

Sequential Ag+/biothiol and synchronous Ag+/Hg2+ biosensing with zwitterionic Cu2+-based metal-organic frameworks

Zwitterionic metal-organic frameworks (MOFs) of {[Cu(Cbdcp)(Dps)(H₂O)₃]·6H₂O}_n (MOF 1) and [Cu₄(Dcbb)₄(Dps)₂(H₂O)₂]_n (MOF 2) (H₃CbdcpBr = N-(4-carboxybenzyl)-(3,5-dicarboxyl)pyridinium bromide; H₂DcbbBr = 1-(3,5-dicarboxybenzyl)-4,4'-bipyridinium bromide; Dps = 4,4'-dipyridyl sulfide) quench the fluorescence of cytosine-rich DNA tagged with 5-carboxytetramethylrhodamine (TAMRA, emission at 582 nm, denoted as C-rich P-DNA-1) and yield the corresponding P-DNA-1@MOF hybrids. Exposure of these hybrids to Ag⁺ results in the release of the P-DNA-1 strands from the MOF surfaces as double-stranded, hairpin-like C-Ag^I-C (ds-DNA-1@Ag⁺) with the restoration of TAMRA fluorescence. The ds-DNA-1@Ag⁺ formed on the surface of 1 can subsequently sense biothiols cysteine (Cys), glutathione (GSH), and homocysteine (Hcy) due to the stronger affinity of mercapto groups for Ag⁺ that serves to unfold the ds-DNA-1@Ag⁺ duplex, reforming P-DNA-1, which is re-adsorbed by MOF 1 accompanied by quenching of TAMRA emission. Meanwhile, MOF 2 is also capable of co-loading a thymine-rich probe DNA tagged with 5-carboxyfluorescein (FAM, emission at 518 nm, denoted as T-rich P-DNA-2) to achieve synchronous sensing of Ag⁺ and Hg²⁺, resulting from the simultaneous yet specific ds-DNA-1@Ag⁺ and T-Hg^II-T duplex (ds-DNA-2@Hg²⁺) formation, as well as the distinctive emission wavelengths of TAMRA and FAM. Detection limits are as low as 5.3 nM (Ag⁺), 14.2 nM (Cys), 13.5 nM (GSH), and 9.1 nM (Hcy) for MOF 1, and 7.5 nM (Ag⁺) and 2.6 nM (Hg²⁺) for MOF 2, respectively. The sequential sensing of Ag⁺ and biothiols by MOF 1, and the synchronous sensing of Ag⁺ and Hg²⁺ by MOF 2 are rapid and specific, even in the presence of other mono- and divalent metal cations or other biothiols at much higher concentrations. Molecular simulation studies provide insights regarding the molecular interactions that underpin these sensing processes.

Nitrogen-Doped Graphitic Carbon-Supported Ultrafine Co Nanoparticles as an Efficient Multifunctional Electrocatalyst for HER and Rechargeable Zn-Air Batteries

The construction of high-efficiency electrocatalysts for hydrogen evolution, oxygen reduction, and oxygen evolution reactions (HER/ORR/OER) is critical for the overall water splitting system, fuel cells, and rechargeable metal-air batteries. Here, we report a viable strategy for tuning the size of a Co-based zeolitic imidazolate framework (ZIF). As a result, a nitrogen-doped graphitic carbon-supported ultrafine Co nanoparticle electrocatalyst (Co/NGC-3) with multifunctional activity was developed. Owing to the smaller ZIF-67 polyhedrons with relatively uniform distribution, more effective active sites, and a strong coupling effect of Co-pyridinic-N, the proposed Co/NGC-3 catalyst exhibited an impressive HER activity. It also showed brilliant catalytic activity in both the ORR and OER, delivering a more positive half-wave potential and a lower overpotential than that of the Pt/C catalyst, respectively. Moreover, the Co/NGC-3 involved the Zn-air battery displayed satisfactory power density, excellent energy density, and superior stability. This approach provides an efficient strategy for the preparation of high-performance multifunctional electrocatalysts for energy-related applications.

Metal-Organic Framework Crystal-Assembled Optical Sensors for Chemical Vapors: Effects of Crystal Sizes and Missing-Linker Defects on Sensing Performances

Microporous metal-organic frameworks (MOFs) are promising candidate materials for chemical sensing, but the reproducible fabrication of MOF-based sensors with optimized and stable performances remains a significant challenge. Here, we report the fabrication of MOF optical sensors with steady but tunable optical properties via assembling UiO-66 crystals with controllable sizes and missing-linker defects. The well-defined but tunable microscopic and mesoscopic structural features of MOF sensing components greatly facilitate the optimization of device performance. The UiO-66 crystal-assembled sensors display fast response (2.00 s) and short recovery (3.00 s) to ethanol vapor (one of the analytes we tested). Our systematical investigation indicates that the mesoporous features of sensing components contribute greatly to the enhanced sensitivity (by ∼24.6% to the saturated ethanol vapor), response speed (by ∼42.9%), and recovery speed (by ∼59.7%) of the crystal-assembled sensors in comparison to their dense counterpart. The building crystal sizes show a slight influence on the response speed but profound effects on the sensitivity and recovery performances of sensors. The missing-linker defects have obvious beneficial effects on the desorption kinetics of analyte and can cause a faster recovery of sensors.