Metal-organic Frameworks Surface Research Articles

The ortho-substituent effect regulating the separation of photogenerated carriers to efficiently photodegrade tetracycline on the surface of FeCo-based MOFs

It is feasible to improve the photodegradation efficiency of organic pollutants by metal-organic frameworks (MOF)-based semiconductors via ligand engineering. In this work, three (Fe/Co)-XBDC-based MOFs were synthesized by introducing different ortho-functional groups X (X = –H, –NO2, –NH2) next to the carboxyl group of the organic ligand (i.e., terephthalic acid). The analysis focused on the influence mechanism of the adjacent functional group effect of the ligand on the physicochemical properties of the material and the actual photodegradation activity of TC. Multiple pieces of evidences suggested that the differences in electron-induced and photocharge-transfer mechanisms of the above ortho functional groups affect the crystal morphology and photocatalytic activity of FeCo-MOF during pyrolysis. Interestingly, (Fe/Co)–NH2BDC exhibited the highest photocatalytic activity under neutral conditions. The results of density functional theory show that the introduction of a strong donor-NH2 group can enhance light absorption and act as an “electron pump” to supply electrons to the iron center, accelerating the separation and efficient transport of photogenerated carriers on the ligand-metal bridge. In conclusion, this study is a proposal for a strategy of structural regulation for the enhancement of the catalytic activity of (Fe/Co)-MOFs in the photodegradation of TC.

Conversion of PET Bottle Waste into a Terephthalic Acid-Based Metal-Organic Framework for Removing Plastic Nanoparticles from Water

Micro- and nanoparticles of plastic waste are considered emerging pollutants with significant environmental and health impacts at high concentrations or prolonged exposure time. Here we report the synthesis and characterization of a known metal-organic framework (MOF) using terephthalic acid (TPA) recovered from the hydrolysis of polyethylene terephthalate (PET) bottle waste. This approach adds value to the existing large amounts of bottle waste in the environment. Fully characterized zinc-TPA MOF (MOF-5) was used for the extraction and removal of engineered polyvinyl chloride (PVC) and polymethylmethacrylate (PMMA) nanoparticles from water with a high efficiency of 97% and 95%, respectively. Kinetic and isotherm models for the adsorption of polymer nanoparticles (PNPs) on the MOF surface were investigated to understand the mechanism. The Qmax for PVC and PMMA NPs were recorded as 56.65 mg/g and 33.32 mg/g, respectively. MOF-5 was characterized before and after adsorption of PNPs on the surface of MOF-5 using a range of techniques. After adsorption, the MOF-5 was successfully regenerated and reused for the adsorption and removal of PNPs, showing consistent results for five adsorption cycles with a removal rate of 83–85%. MOF-5 was characterized before and after adsorption of PNPs on the surface using a range of techniques. The MOF-5 with PNPs on the surface was successfully regenerated and reused for the adsorption and removal of polymer nanoparticles, showing consistent results for five extraction cycles. As a proof of concept, MOF-5 was also used to remove plastic particles from commercially available body scrub gel solutions. Such methods and materials are needed to mitigate the health hazards caused by emerging micro- and nanoplastic pollutants in the environment.

Sustainable Gold Nanoparticle (Au-NP) Growth within Interspaces of Porphyrinic Zirconium-Based Metal-Organic Frameworks: Green Synthesis of PCN-224/Au-NPs and Its Anticancer Effect on Colorectal Cancer Cells Assay.

In this work, a simple green synthesis method of the novel metal-organic framework (MOF) nanocomposite PCN-224/Au-NPs (Au-NPs = gold nanoparticles) is described. In this regard, initially, PCN-224 was synthesized. Afterward, in a single-step, one-pot procedure, under visible-light irradiation, Au-NPs were fabricated on PCN-224. The cytotoxicity effect of the synthesized PCN-224/Au-NPs nanocomposite was investigated in human colon cancer cells. Determination of the apoptosis induction was done by the Annexin- V/propidium iodide flow cytometry method. Besides, to ascertain the biocompatibility of the synthesized sample, the cytotoxicity of PCN-224/Au-NPs was evaluated on the human embryonic kidney (HEK)-293 cell line. The substantial anticancer activity with the biocompatibility of the structure, the green facile synthesis, and the MOF surface of the synthesized nanocomposite make it special for utilization in therapeutic applications.

Vapor-phase zeolitic imidazolate framework-8 growth on fibrous polymer substrates

The use of metal-organic frameworks (MOFs) in practical applications is often hindered by synthesis related challenges. Conventional solution-based approaches rely on hazardous solvents and often form powders that are difficult to integrate into practical devices. On the other hand, vapor-phase approaches generally result in MOF films on silicon substrates that make it difficult to characterize the MOF surface area, which is an important quality indicator. We address these challenges by introducing a solvent-free synthesis method to form MOF–fiber composites, which can be more easily integrated into devices. Additionally, these vapor-phase-formed MOF–fiber composites are compatible with Brunauer–Emmett–Teller surface area analysis to characterize MOF quality. Atomic layer deposition is used to form a ZnO film on polypropylene, polyester, and nylon fibrous substrates, which is subsequently converted to zeolitic imidazolate framework-8 (ZIF-8) using 2-methylimidazole vapor. We describe the effects of the ZnO film thickness and MOF conversion conditions on MOF crystallinity and surface area. We report a ZIF-8 surface area of ∼1300 m2/gMOF, which is comparable to reported surface areas of ∼1250–1600 m2/gMOF from conventional synthesis techniques, demonstrating good quality of the solvent-free MOF–fiber composites. We expect these results to extend vapor-phase MOF formation to new, practical substrates for advanced sensing and catalytic applications.

Antibacterial, antibiofilm, and antioxidant activities of two novel metal-organic frameworks (MOFs) based on 4,6-diamino-2-pyrimidinethiol with Zn and Co metal ions as coordination polymers.

In the present era, the increase in free radical species (FRs) and multidrug-resistant (MDR) bacteria represents a major worldwide concern for public health. Biofilm development and the overuse and misuse of antibiotics could lead to the adaptation of bacteria to antimicrobial agents. Consequently, finding novel multifunctional species with antibacterial, antioxidant, and antibiofilm properties has become crucial in the fight against challenging bacterial infections and chronic inflammatory conditions. Metal-organic frameworks (MOFs) with zinc and cobalt metal centers are widely utilized in biological and environmental remediation owing to their versatility. In this study, multifunctional Zn-MOFs and Co-MOFs were successfully synthesized with zinc and cobalt as metal centers and 4,6-diamino-2-pyrimidinethiol as an organic linker using a hydrothermal technique. Numerous characterization techniques were used to fully examine the MOF structure, functionality, chemical makeup, crystalline structure, surface appearance, thermal behavior, and magnetic characteristics; the techniques included XPS, PXRD, FTIR, FESEM, EDX, UV-visible, BET, BJH, TGA/DTG, DSC, and magnetic susceptibility measurement. The antioxidant, antibacterial, and antibiofilm activities of the MOFs were examined, and they demonstrated potent activity in each of these aspects. The proposed mechanisms of antibacterial activity suggest that bacterial cell death results from multiple toxic effects, including electrostatic interaction and lipid peroxidation, when MOFs are attached to bacteria, leading to the formation of reactive oxygen species (ROSs). Zn-MOFs exhibit high antibacterial and antibiofilm efficacy owing to their large surface-to-volume ratio and porous nature, while Co-MOFs exhibit high antioxidant capacity owing to their redox properties.

Empowering the Water Oxidation Activity of the Bimetallic Metal-Organic Framework by Annexing Gold Nanoparticles over the Catalytic Surface.

Electrocatalytic water splitting to an anodic oxygen evolution reaction (OER) and a cathodic hydrogen evolution reaction (HER) is believed to be the most important application for sustainable hydrogen generation. Being a four-electron, four-proton transfer process, the OER plays the main obstacle for the same. Therefore, designing an effective electrocatalyst to minimize the activation energy barrier for the OER is a research topic of prime importance. The metal-organic framework (MOF) with a highly porous network is considered an appropriate candidate for the OER in alkaline conditions. Apart from several MOFs, the bimetallic one has an advantageous electrocatalytic performance due to the synergistic electronic interaction between two metal ions. However, most bimetallic MOFs have an obstacle to electrocatalytic application due to their low conductive nature, and therefore, they possess a barrier for charge transfer kinetics at the interface. Surface functionalization via various nanoparticles (NPs) is believed to be the most effective strategy for nullifying the conductive issue. In this work, we have designed a CoNi-based bimetallic MOF that was surface-functionalized by Au NPs (Au@CoNi-Bpy-BTC) for the OER under alkaline conditions. Au@CoNi-Bpy-BTC required an overpotential of just 330 mV, which is 56 mV lower as compared to the pristine MOF. Impedance analysis confirms an improved conductivity and charge transfer at the interface, where Au@CoNi-Bpy-BTC possesses a lower Rct value than CoNi-Bpy-BTC materials. Moreover, the Au-decorated MOF shows an 8.5 times increase in the TOF value compared to the pristine MOF. Therefore, this noble strategy toward the surface functionalization of MOFs via noble metal NPs is believed to be the most effective strategy for developing effective electrocatalysts for electrocatalytic application in energy-related fields. Overall, this report displays an exceptional correlation between the decorated NPs over the MOF surface, which can regulate the OER activity, as confirmed by experimental analysis.

Natural tea extract coated porous MOF nano/microparticles for highly enhanced and selective adsorption of cationic dyes from aqueous medium

A simple, sustainable method was developed for boosting the dye adsorption capability of porous metal organic frameworks (MOFs) by coating them with natural tea extract. Nano/microparticles of zinc-terephthalic acid (ZnTPA) MOFs exhibited only 40% rhodamine B (Rh B) dye adsorption. The dye adsorption capability was strongly enhanced (up to 95%) after coating with natural tea extract and highly selective cationic dye adsorption. The coating of multifunctional biomolecules onto the ZnTPA matrix was expected to facilitate strong intermolecular interaction between MOF surface and organic dyes, which contributed to strongly enhanced dye adsorption. ZnTPA/Tea MOFs not only exhibited enhanced dye adsorption but also showed high retention capability. The broader scope of tea extract coating methodology for cationic dye adsorption was also demonstrated using other MOFs matrix. The exhausted ZnTPA MOF was easily regenerated by sonication followed by re-dipping into tea extract. Tea extract coated ZnTPA MOFs was also used as column packing materials for removing cationic dyes by filtration. Thus, the present work reported a facile strategy for efficient cationic organic dyes removal from the aqueous medium.

Metal-organic frameworks: Drug delivery applications and future prospects.

Metal-organic frameworks (MOFs) have gained incredible consideration in the biomedical field due to their flexible structural configuration, tunable pore size and tailorable surface modification. These inherent characteristics of MOFs portray numerous merits as potential drug carriers, depicting improved drug loading, site-specific drug delivery, biocompatibility, biodegradability, etc. The current review article sheds light on the synthesis and use of MOFs in drug delivery applications. In the beginning, a brief overview of the key components and efficient fabrication techniques for MOF synthesis, along with its characterization methods, have been presented. The MOFs-based formulations have been critically discussed. The application of the design of experiments (DoE) approach to optimize MOFs has been elucidated. The MOFs-based formulations, especially the application of stimuli-responsive MOFs for site-specific drug delivery, have been deciphered. Along with drug release kinetic models, several administration methods for MOFs have also been enunciated. Subsequently, MOFs as future potential drug carriers have been elaborated. Recently, MOFs have emerged as versatile drug delivery carriers possessing customization potential and meeting the needs of spatio-temporal drug delivery. Researchers have devised several environment-friendly approaches for MOF construction and surface modification. Owing to stimuli-responsive potential, MOFs have demonstrated their prominent therapeutic efficacy via several routes of administration. The numerous benefits of MOFs would certainly open up a new vista for its novel drug delivery applications.

Metal-organic framework (MOF)-derived flower-like Ni-MOF@NiV-layered double hydroxides as peroxidase mimetics for colorimetric detection of hydroquinone

BackgroundNanozymes are one of the ideal substitutes for natural enzymes because of their excellent chemical stability and simple preparation methods. However, due to the limited catalytic ability of most reported nanozymes, constructing nanomaterials with low cost and high activity is gradually becoming an exploration focus in the field of nanozymes. Heteroatom doping of metal-organic frameworks is one of potential approaches to design nanozymes with high catalytic performance. Due to their multiple valence states properties, V-doped metal-organic framework (MOF)-derived LDH is expected to be a good enzyme-like catalyst. To our knowledge, the V-doped MOF-derived LDH as nanozyme is not explored before. ResultsWe report the in-situ synthesis of NiV-layered double hydroxides (LDHs) on nickel-based MOF, i.e. Ni-MOF@NiV-LDHs. The MOF surface is covered by 2D nanosheets. This unique structural design increases the specific surface area of the material, enables more exposure of catalytic active sites to participate in reactions and accelerates the electron transfer rate. The Ni-MOF@NiV-LDHs have high peroxidase-like activity able to catalyze TMB oxidation by H2O2 via the generation of •OH and O2•−. Relative to Ni-MOF, the Ni-MOF@NiV-LDHs shows 47-fold peroxidase-like activity rise. It had good affinity to TMB and H2O2, with the Michaelis-Menten constants of 0.12 mM and 0.007 mM, respectively. The hydroquinone (HQ) consumed the reactive oxygen species generated in the TMB + H2O2+Ni-MOF@NiV-LDHs system to inhibit the TMB oxidation. On this basis, a sensitive and rapid assay for determining HQ was developed, with a linear range of 0.50–70 μM and a LOD of 0.37 μM. SignificanceThis work provided some clues for the further development of novel nanozymes with high catalytic performance via a strategy of heteroatom doping. And the constructed colorimetric analysis method was successfully utilized for the determination of HQ in actual waters, which has the potential for practical application in the analysis of environmental pollutants.

Metal-Organic Framework-Supported Catalase Delivery for Enhanced Photodynamic Therapy via Hypoxia Mitigation.

Tumor hypoxia poses a significant challenge in photodynamic therapy (PDT), which uses molecular oxygen to produce reactive oxygen species upon light excitation of a photosensitizer. For hypoxia mitigation, an enzyme catalase (CAT) can be beneficially used to convert intracellular hydrogen peroxide to molecular oxygen, but its utility is significantly limited due to the intrinsic membrane impermeability. Herein, we present direct integration of CAT into the outer surface of unmodified metal-organic framework (MOF) nanoparticles (NPs) via supramolecular interactions for effective cellular entry of CAT and consequent enhancement of PDT. The results demonstrated that CAT-loaded MOF NPs could successfully enter hypoxic cancer cells, after which the intracellularly delivered CAT molecules became dissociated from the MOF surface to efficiently initiate the oxygen generation and PDT process along with a co-delivered photosensitizer IR780. This achievement suggests that our protein-MOF integration strategy holds great potential in biomedical studies to overcome tumor hypoxia as well as to efficiently deliver biomolecular cargos.

Self‐Assembled Nanoporous Metal–Organic Framework Monolayer Film for Osmotic Energy Harvesting

AbstractOsmotic energy represents a promising energy resource because it is sustainable and environmentally benign. Subnanoscale channels are considered as a competitive platform for generating this blue energy due to their highly selective and ultrafast ion transport. However, fabricating functional subnanochannels capable of high energy output remains challenging. Here, a heterogeneous subnanochannel membrane formed by coating a functionalized self‐assembled metal−organic framework (MOF) monolayer (SAMM) film on a porous anodic aluminum oxide membrane, is reported. The SAMM film, with a thickness of ≈160 nm, is fabricated by self‐assembly of poly(methyl methacrylate‐co‐vinylimidazole)‐modified UiO‐66‐NH2 nanoparticles at the water−air interface. In the SAMM, imidazole and NH2 groups provide abundant positive charges, while the angstrom‐scale windows act as ionic filters for selective screening of anions with different hydration diameters. As a result, the heterogeneous membrane exhibits excellent capacity for anion‐selective transport, which contributes to an optimal osmotic power of 6.76 W m−2 under a 100‐fold NaCl gradient, as well as a high Cl−/SO42− selectivity of ≈42.2. Further, the output power is increased to 10.5 W m−2 by methylating imidazole moieties on the MOF surface. This work provides a facile and modular approach to fabricate subnanochannels for enabling highly selective and efficient osmotic energy conversion.

Composite nanoarchitectonics for efficient lithium storage by encapsulating black phosphorus quantum dots in cobalt/iron based Prussian blue analogues

It is the critical to efficiently using black phosphorus quantum dots (BQ) in lithium batteries to obtain high energy density with improved stability. In this work, BQ was encapsulated in the size-matched pores of metal-organic frameworks (MOF) Cobalt/Iron-based Prussian blue analogues (Co/Fe-based PBA) by a coprecipitation method to construct a series of Co/Fe-based PBA@xBQ composite materials. The composite materials added with BQ exhibit significantly improved stability, demonstrating excellent electrochemical performances in lithium batteries. The close contact of BQ with Co/Fe catalytic sites promoted the rapid transfer of lithium ions between the metal sites. The unique structure of black phosphorus allowed storing more lithium ions in the pores of Co/Fe-based PBA. Grafting the surface of MOF with BQ could reduce side reactions on the electrode surface, and their synergistic effects enhanced the lithium storage capacity. The energy density of Li||Co-Co PBA@ 8BQ and Li||Fe-Co PBA@ 10BQ batteries was 130.5 mAh·g−1 and 187.4 mAh·g−1 higher than the Li||Co/Fe-based PBA. Li||Fe-Co PBA@ 10BQ battery had a long cycle stability, with 70% capacity retention rate after 500 cycles. The new composite materials achieved the combination of BQ and PBA to obtain lithium batteries with higher capacity density and stability with more cycles.

Defect Engineered Metal–Organic Framework with Accelerated Structural Transformation for Efficient Oxygen Evolution Reaction

AbstractMetal–organic frameworks (MOFs) have been increasingly applied in oxygen evolution reaction (OER), and the surface of MOFs usually undergoes structural transformation to form metal oxyhydroxides to serve as catalytically active sites. However, the controllable regulation of the reconstruction process of MOFs remains as a great challenge. Here we report a defect engineering strategy to facilitate the structural transformation of MOFs to metal oxyhydroxides during OER with enhanced activity. Defective MOFs (denoted as NiFc′xFc1‐x) with abundant unsaturated metal sites are constructed by mixing ligands of 1,1′‐ferrocene dicarboxylic acid (Fc′) and defective ferrocene carboxylic acid (Fc). NiFc′xFc1‐x series are more prone to be transformed to metal oxyhydroxides compared with the non‐defective MOFs (NiFc′). Moreover, the as‐formed metal oxyhydroxides derived from defective MOFs contain more oxygen vacancies. NiFc′Fc grown on nickel foam exhibits excellent OER catalytic activity with an overpotential of 213 mV at the current density of 100 mA cm−2, superior to that of undefective NiFc′. Experimental results and theoretical calculations suggest that the abundant oxygen vacancies in the derived metal oxyhydroxides facilitate the adsorption of oxygen‐containing intermediates on active centers, thus significantly improving the OER activity.

Defect Engineered Metal-Organic Framework with Accelerated Structural Transformation for Efficient Oxygen Evolution Reaction.

Metal-organic frameworks (MOFs) have been increasingly applied in oxygen evolution reaction (OER), and the surface of MOFs usually undergoes structural transformation to form metal oxyhydroxides to serve as catalytically active sites. However, the controllable regulation of the reconstruction process of MOFs remains as a great challenge. Here we report a defect engineering strategy to facilitate the structural transformation of MOFs to metal oxyhydroxides during OER with enhanced activity. Defective MOFs (denoted as NiFc'x Fc1-x ) with abundant unsaturated metal sites are constructed by mixing ligands of 1,1'-ferrocene dicarboxylic acid (Fc') and defective ferrocene carboxylic acid (Fc). NiFc'x Fc1-x series are more prone to be transformed to metal oxyhydroxides compared with the non-defective MOFs (NiFc'). Moreover, the as-formed metal oxyhydroxides derived from defective MOFs contain more oxygen vacancies. NiFc'Fc grown on nickel foam exhibits excellent OER catalytic activity with an overpotential of 213 mV at the current density of 100 mA cm-2 , superior to that of undefective NiFc'. Experimental results and theoretical calculations suggest that the abundant oxygen vacancies in the derived metal oxyhydroxides facilitate the adsorption of oxygen-containing intermediates on active centers, thus significantly improving the OER activity.

Rational Design of Advanced Metal Organic Framework‐Based Nanostructured Catalysts toward Electrocatalytic CO2 Conversion: A Mini Review

Carbon dioxide emission into the atmosphere, which originates from fossil fuels consumption, has led to a serious environmental and energy crisis. Electrochemical CO2 reduction reaction (CO2RR) into valuable chemicals such as carbon monoxide, ethanol, ethylene, and methane not only help lower the CO2 concentration in the atmosphere but is also beneficial for sustainable energy production. Therefore, numerous works are assigned to improving efficient electrocatalytic materials for the selective CO2RR. Metal–organic framework (MOF)‐derived catalytic materials mostly based on transition metals demonstrate permissible CO2RR owing to their specific composition, tunable surface nanostructure and texture, and affordable market prices. In this work, the best effort is put to describe the most basic concept of electrochemical CO2RR to provide a consensus. The effect of MOF structure and surface analysis are discussed. This is reason that MOFs as catalysts for electrochemical CO2RR offers several advantages over traditional catalysts, such as tunable surface structure, multifunctionality, selectivity control, and stability under CO2RR operation. And then, some of the most recent reported works on MOF‐based catalysts are summarized, to provide and inspire further investigations and ideas in this research field. However, despite these advantages, there are still research gaps that need to be addressed. Some of these gaps include being cost‐effective and economical process for preparing MOF‐based catalysts, long‐term stability for industrial usage, and scalability of the catalyst. Thereby, further studies are essential to fully exploit the potential of MOFs in advancing CO2RR technologies.

Oriented Antibody-Assembled Metal-Organic Frameworks for Persistent Wearable Sweat Cortisol Detection.

The level of cortisol can reflect people's psychological stress, help diagnose adrenal gland diseases, and is also related to several mental diseases. In this study, we developed a cortisol monoclonal antibody-oriented approach to modify an immunosensor for wearable label-free and persistent sweat cortisol detection. On such an antibody-oriented immunosensor, the fragment crystallizable (Fc) region is partially inserted within the metal-organic framework (MOF), and antibody-binding regions of the cortisol monoclonal antibody (Cmab) were exposed on the MOF surface via selective growth and self-assembly. Such ordered and oriented embedding of antibodies in the MOF resulted in excellent antibody activity and improved stability and antigen-binding capacity. We also engineered the full integrated system for on-body sweat cortisol biosensing performance in several volunteers, and the results indicated that this wearable sensor is suitable for practical cortisol detection with a good linear detection range from 1 pg/mL to 1 μg/mL with a lower limit of detection of 0.26 pg/mL. Moreover, the wearable sensor demonstrated good persistence in detecting cortisol, with only 4.1% decay after 9 days of storage. The present work represents a simple oriented antibody assembling approach to improve the stability of antibodies, providing an important step toward long-term continuous sweat biomarker detection.

Recent Advances in Metal–Organic Frameworks for the Surface Modification of the Zinc Metal Anode: A Review

Aqueous zinc ion batteries (AZIBs) are considered as one of the most promising energy storage technologies due to their advantages of being low in cost, high in safety, and their environmental friendliness. However, dendrite growth and parasitic side reactions on the zinc metal anode during cycling lead to a low coulombic efficiency and an unsatisfactory lifespan, which seriously hinders the further development of AZIBs. In this regard, metal–organic frameworks (MOFs) are deemed as suitable surface modification materials for the Zn anode to deal with the abovementioned problems because of their characteristics of a large specific surface area, high porosity, and excellent tunability. Considering the rapidly growing research enthusiasm for this topic in recent years, herein, we summarize the recent advances in the design, fabrication, and application of MOFs and their derivatives in the surface modification of the zinc metal anode. The relationships between nano/microstructures, synthetic methods of MOF-based materials, and the enhanced electrochemical performance of the zinc metal anode via MOF surface modification are systematically summarized and discussed. Finally, the existing problems and future development of this area are proposed.

Impedance-Based Detection of NO2 Using Ni-MOF-74: Influence of Competitive Gas Adsorption.

Chemically robust, low-power sensors are needed for the direct electrical detection of toxic gases. Metal-organic frameworks (MOFs) offer exceptional chemical and structural tunability to meet this challenge, though further understanding is needed regarding how coadsorbed gases influence or interfere with the electrical response. To probe the influence of competitive gases on trace NO2 detection in a simulated flue gas stream, a combined structure-property study integrating synchrotron powder diffraction and pair distribution function analyses was undertaken, to elucidate how structural changes associated with gas binding inside Ni-MOF-74 pores correlate with the electrical response from Ni-MOF-74-based sensors. Data were evaluated for 16 gas combinations of N2, NO2, SO2, CO2, and H2O at 50 °C. Fourier difference maps from a rigid-body Rietveld analysis showed that additional electron density localized around the Ni-MOF-74 lattice correlated with large decreases in Ni-MOF-74 film resistance of up to a factor of 6 × 103, observed only when NO2 was present. These changes in resistance were significantly amplified by the presence of competing gases, except for CO2. Without NO2, H2O rapidly (<120 s) produced small (1-3×) decreases in resistance, though this effect could be differentiated from the slower adsorption of NO2 by the evaluation of the MOF's capacitance. Furthermore, samples exposed to H2O displayed a significant shift in lattice parameters toward a larger lattice and more diffuse charge density in the MOF pore. Evaluating the Ni-MOF-74 impedance in real time, NO2 adsorption was associated with two electrically distinct processes, the faster of which was inhibited by competitive adsorption of CO2. Together, this work points to the unique interaction of NO2 and other specific gases (e.g., H2O, SO2) with the MOF's surface, leading to orders of magnitude decrease in MOF resistance and enhanced NO2 detection. Understanding and leveraging these coadsorbed gases will further improve the gas detection properties of MOF materials.

Quantifying Ligand Binding to the Surface of Metal-Organic Frameworks.

The binding of molecules to the exterior surface of metal-organic frameworks (MOFs) is not a well-understood phenomenon. Herein, the surface chemistry of three MOFs, UiO-66, MIL-88B-NH2, and ZIF-8, is investigated using dye-displacement experiments. MOF particle surfaces were modified with ligand-appended BODIPY dyes. The ability of the coordinated dyes to be displaced by a variety of exogenous ligands was measured by ultraviolet-visible spectroscopy. This method allowed for measurement of apparent binding constants for different ligands to the MOF surface. As might be expected, ligand affinity was dependent on the nature of the underlying metal-ligand composition of the MOF. This work provides a quantitative evaluation of ligand binding to MOF surfaces and important insights for the modulation, modification, and manipulation of MOFs.

Modulating the Conduction Band of MOFs by Introducing Tiny TiO2 Nanoparticles for Enhanced Photocatalytic Performance: Importance of the Loading Position.

The preparation of TiO2 and metal-organic framework (MOF) into composite photocatalysts has been proven to be a mature and effective strategy to achieve stronger catalytic activity. In this work, we focus on exploring the significant effects and mechanisms of the relative positions of decorated titanium oxide nanoparticles and MOFs on the final catalytic activity. We first used a simple in situ method to encapsulate tiny TiO2 nanoparticles into a Zr-MOF (PCN-222), where Zr-Ti bonds were created at the interface of the two components. Thanks to the strong interfacial interaction forces, band bending occurred in TiO2@PCN-222 and a more negative conduction band (Δ = 0.26 V) with better electron transport properties was obtained. The results of photocatalytic CO2 reduction experiments under visible light showed a 78% increase (142 μmol g-1 h-1) in the production rate of HCOO-. Surprisingly, the loading of TiO2 nanoparticles on the MOF surface (TiO2@PCN-222) resulted in a significant decrease of 56% in the catalyst yield activity due to poor adsorption and electron transfer properties. This work demonstrates the possibility of tuning the band structure and catalytic activity of MOFs with the help of changing the position of the dopant and shows the importance of the rational design of MOF-based composites.