Mimicking π Backdonation in Ce-MOFs for Solar-Driven Ammonia Synthesis

WO3/Ru@CeO2 Bilayer Gas Sensor for ppb-Level Xylene Detection Based on a Catalytic-Sensitive Synergistic Mechanism

Volatile aromatic hydrocarbons present a significant threat to both the environment and human health. However, due to the low reactivity of toxic gases containing benzene rings and insufficient selectivity of existing sensors, real-time monitoring of benzene series (BTEX) gases remains a challenge. The development of catalytically sensitive synergistic bilayer sensors offers a promising strategy to overcome this challenge. A series of Ru@CeO2 nanosheets with different Ru doping levels were synthesized by using a simple solvothermal and further calcination method. Interestingly, the incorporation of Ru effectively modulates the morphology of Ce-BDC from nanorods to porous nanosheets. The WO3/Ru@CeO2 bilayer sensor is constructed by using WO3 nanofibers as the lower sensitive layer and Ru@CeO2 as the upper catalytic layer. At the operating temperature of 160 °C, the response value (Ra/Rg) of the WO3/Ru@CeO2 bilayer sensor to 5 ppm xylene is 37.04, which is obviously better than that of the WO3 nanofiber sensor. In addition, the sensor also reacted significantly to low concentrations of xylene, as low as 1 ppb. Additionally, the combination of online mass spectrometry and density functional theory was employed to validate the enhanced sensing performance arising from the synergistic mechanism between the catalytic and sensing materials. Hence, the work presents a new material for detecting ppb level BTEX through an effective bilayer structure design and material selection.

Constructing Synergistically Catalytic Lewis Acidic-Basic Sites for Boosting Reactivity of a Flexible Coordination Polymer

Targeted construction of Lewis acidic-basic sites in the skeleton of coordination polymers (CPs) can greatly enhance their catalytic efficiency due to the synergistic effect of acidic and basic sites. However, research on validating the coexistence of Lewis acidic-basic sites for boosting the catalytic activity of CPs toward the Knoevenagel condensation (KC) reaction, widely applied in the synthesis of high-added-value intermediates and products under mild conditions, is missing so far. Based on the above consideration, we have artificially constructed Lewis acidic-basic sites and introduced vacancy in the framework of a new flexible cerium CP {Ce-CP: [Ce3+Ce4+(obb2-)3(OH)(H2O)(DMF)]∞} (DMF: N,N-dimethylformamide) via applying the functional ligand 4,4'-oxidibenzoate (obb2-) with the bridging O atom as the Lewis basic site and removing the coordinating solvent molecules and counterions to form cerium coordination unsaturated sites (Ce-CUSs) as Lewis acidic sites. Interestingly, Ce-CP exhibits reversible structural transformation associated with a desolvation and resolvation process. The Lewis acidic and basic sites in the resulting Ce-CP (LAB-Ce-CP) have been confirmed by CO2 temperature-programmed desorption (TPD) and NH3 combined with pyrrole-TPD (NH3-Py-TPD) for the first time. Benefiting from the coexistence of Lewis acidic and basic sites as well as the flexibility of the framework, LAB-Ce-CP shows high activity and excellent recyclability toward KC reactions. Moreover, we have found that (1) the activation temperature of Ce-CP plays a critical role in its porosity, exposure of Lewis acidic-basic sites, and thus reactivity; (2) the stronger electron-withdrawing ability of the substituent groups in benzaldehyde derivatives and the smaller size of the reactants lead to the higher yield of product and turnover number (TON) value when the disparity of electron-withdrawing and electron-donating abilities between the substituent groups in benzaldehyde derivatives is not significant. Hence, this work has exploited a new strategy for designing excellent heterogeneous catalysts with constructed active sites of synergistic catalysis capability toward KC reactions.

Photo-induced multiple charge transfer resonance of Ce-MOF for SERS detection of nucleic acid

BACKGROUND

Sensitive and accurate detection of important cancer markers MicroRNAs (miRNAs) is critical to prevent and treat disease. Among many detection techniques, surface-enhanced Raman scattering(SERS) has attracted much attention due to its advantages such as narrow spectral peak, low interference and non-destructive detection. Interestingly, non-noble metal SERS substrates show good prospects due to their outstanding spectral reproducibility and biocompatibility. However, the low SERS sensitivity of non-noble metal substrates also limited their detection performance. Therefore, it is crucial to design a non-noble metal substrate with high SERS sensitivity for nucleic acid detection.

RESULTS

In this study, we found that the photo induction (30 min) of Ce-MOF has high SERS performance (abbreviated as Ce-MOF30). It is due to that photo-induction can regulate the energy level of the Ce-MOF and break the benzene-C bonds to introduce a defect energy level, which provides additional charge transfer channels. The Ce-MOF30 substrates can achieve an enhancement factor (EF) of 6.71 × 106 for Methylene blue (MB), about 7-fold higher than pristine Ce-MOF, which is the best Ce-based SERS substrate so far. Finally, a SERS detection platform was fabricated by combining the Mg2+ assisted dual signal amplification for miRNA 141 assay, and the detection limit was 1.72 fM.

SIGNIFICANCE

This work proposed a simple photo-induction strategy to enhance SERS performance, which supplied a new detection platform for detection of nucleic acid and further expand the application of non-noble metal substrate in SERS field.

Surface plasmon resonance effect cooperated with Z-scheme heterostructure for enhancing photocatalytic nitrogen reduction to ammonia

The photocatalytic efficiency can be improved by constructing a Z-scheme heterojunction, but hindered by the only half utilization efficiency of photogenerated carriers. Thus, a novel material, UiO-66-NH2@TAPB-BTCA-COP-Ag (U6N@COP-Ag), with surface plasmon resonance (SPR) effect synergistic Z-scheme heterostructure has been prepared by depositing Ag nanoparticles (Ag NPs) on TAPB-BTCA-COP (COP)-coated UiO-66-NH2. The deposited Ag NPs expand the range of light absorption and introduce more photogenerated electrons in the composite. The SPR effect of noble metal compensates for the limited utilization of the Z-scheme heterojunction photogenerated carriers and the increased density of semiconductor carriers at the reducing end, which is more conducive to the redox reaction of the catalyst. Without sacrificial agents, U6N@COP-Ag shows great photocatalytic nitrogen reduction conversion efficiency with the rate of NH4+ in ammonia water at 167.63μmol g-1h-1, which is 6.6 and 2.8 times that of the original UiO-66-NH2 and COP, respectively. In-situ XPS and Kelvin probe technology verify that UiO-66-NH2 and Ag nanoparticles provide more photogenerated electrons to COP. The cleavage and conversion of N2 to NH4+ on U6N@COP-Ag was confirmed by the enhancement of NH bonds and NH4+ characteristic absorption peaks in the in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS). This work presents a great method to improve the Z-scheme heterojunction photogenerated carrier utilization and the density of semiconductor carriers at the reducing end by the noble metal SPR effect, which is more conducive to enhance the redox reaction of the catalyst.

Ti3+-mediated MIL-125(Ti) by metal substitution for boosting photocatalytic N2 fixation

Photocatalysis, which uses sunlight, N2 and H2O to produce NH3, is a more sustainable approach to N2 fixation than the Haber-Bosch process. However, its efficiency is severely limited by the difficulty of activating NN bonds. This work presents metal (M = Cu, Fe, V)-substituted MIL-125(Ti) (MIL-(MTi)) for photocatalytic N2 fixation without using any sacrificial agents. Structural characterizations reveal that the active sites including oxygen vacancies (OV) and Ti3+ species are formed by the resulting crystal distortion due to the partial substitution of Ti4+ by other metal ions (Cu+, Fe2+, V3+) in MIL-125(Ti). MIL-(CuTi) possesses a larger number of OV and Ti3+ compared to MIL-(FeTi) and MIL-(VTi) due to the larger valence difference between Cu+ and Ti4+. These active sites not only promote the adsorption and activation of N2 and H2O, but also facilitate the photogenerated charge mobility. Photogenerated holes oxidize H2O to produce O2 and H+. Photogenerated electrons reduce N2 activated on Ti3+ sites by combining with H+ to form NH4+. Therefore, MIL-(CuTi) shows the highest NH4+ production rate 46.5 µmol·h-1·g-1, which is much higher than that (1.2 µmol·h-1·g-1) of the pristine MIL-125(Ti). This work provides a new insight into rational design for artificial N2 fixation systems by the construction of the active site.

A label-free electrochemical biosensor based on a bimetallic organic framework for the detection of carbohydrate antigen 19-9

Carbohydrate antigen 19-9 (CA19-9) is an important marker for pancreatic cancer, ovarian cancer and other tumors, and its rapid and stable detection is the basis for early diagnosis and treatment. In this paper, a label-free electrochemical immunosensor for the sensitive detection of CA19-9 has been developed. First, the synthesis of two novel core-shell bimetallic nanomaterials, namely Ce-MOF-on-Fe-MOF and Fe-MOF-on-Ce-MOF, was accomplished using the MOF-on-MOF approach. The poor electrical conductivity of MOF materials was addressed by incorporating polyethylenimide (PEI) functionalized rGO with Ce-MOF-on-Fe-MOF and Fe-MOF-on-Ce-MOF nanomaterials. Simultaneously, toluidine blue (Tb) was employed as a redox probe and physically adsorbed onto the synthesized materials, resulting in the formation of two nanomaterials: rGO@Ce-MOF-on-Fe-MOF@Tb and rGO@Fe-MOF-on-Ce-MOF@Tb. The fundamental characterization reveals that the sensing performance of the rGO@Ce-MOF-on-Fe-MOF@TB-based immune sensor surpasses that of the rGO@Fe-MOF-on-Ce-MOF@TB-based immune sensor, which is attributed to the fact that, unlike the interlayer-constrained structure of Fe-MOF-on-Ce-MOF, in Ce-MOF-on-Fe-MOF, Ce-MOF penetrates into Fe-MOF to form a heterogeneous structure due to the relatively large pore size of Fe-MOF, which better combines the excellent biocompatibility and strong anchoring effect of Fe MOFs on antibodies, as well as the high electrochemical activity and conductivity of Ce-MOF, to enhance sensing performance. The proposed label-free immunosensor based on rGO@Ce-MOF-on-Fe-MOF@Tb has a wide linear range (1-100 000 mU mL-1), a low detection limit (0.34 mU mL-1), good stability, reproducibility, and repeatability, and satisfactory applicability, which provides a potential platform for clinical applications.

Efficient photocatalytic nitrogen fixation via oxygen vacancies in Zr-MOFs at ambient conditions

Photocatalytic nitrogen fixation is seen to be a potential technology for nitrogen reduction due to its eco-friendliness, low energy consumption, and environmental protection. In this study, photocatalysts with abundant oxygen vacancies (Zr-naphthalene dicarboxylic acid (Zr-NDC) and Zr-phthalic acid (Zr-BDC)) were designed using 1,4-naphthalene dicarboxylic acid (H2NDC) and 1,4-phthalic acid (H2BDC) as ligands. Since the structure of H2NDC includes one extra benzene ring than H2BDC, the charge density differential of the organic ligand is probably altered. The hypothesis is proved by density function theory (DFT) calculation. The abundant oxygen vacancies of the catalyst offer numerous active sites for nitrogen fixation. Concurrently, the process of ligand-metal charge transfer facilitates photo-electron transfer, creating an active center for nitrogen reduction. Additionally, the functionalization of ligand amplifies another pathway for charge transfer, broadening the light absorption range of Metal-organic framework (MOF) and increasing its capacity for nitrogen reduction. In contrast to H2BDC, the benzene ring added in H2NDC structure acts as an electron energy storage tank with a stronger electron density difference favorable for photogenerated electron-hole separation resulting in higher photocatalytic activity in Zr-NDC. The experimental results show that the nitrogen fixation efficiency of Zr-NDC is 163.7 µmol g-1h-1, which is significantly better than that of Zr-BDC (29.3 µmol g-1h-1). This work utilizes cost-effective and non-toxic ingredients to design highly efficient photocatalysts, thereby significantly contributing to the practical implementation of green chemistry principles.

Modelling Photocatalytic N2 Reduction to Ammonia: Where We Stand and Where We Are Going

Artificial ammonia synthesis via the Haber-Bosch process is environmentally problematic due to the high energy consumption and corresponding CO2 ${_2 }$ emissions, produced during the reaction and before hand in hydrogen production upon methane steam reforming. Photocatalytic nitrogen fixation as a greener alternative to the conventional Haber-Bosch process enables us to perform nitrogen reduction reaction (NRR) under mild conditions, harnessing light as the energy source. Herein, we systematically review first-principles calculations used to determine the electronic/optical properties of photocatalysts, N2 adsorption and to expound possible NRR mechanisms. The most commonly studied photocatalysts for nitrogen fixation are usually modified with dopants, defects, co-catalysts and Z-scheme heterojunctions to prevent charge carrier recombination, improve charge separation efficiency and adjust a band gap to for utilizing a broader light spectrum. Most studies at the atomistic level of modeling are grounded upon density functional theory (DFT) calculations, wholly foregoing excitation effects paramount in photocatalysis. Hence, there is a dire need to consider methods beyond DFT to study the excited state properties more accurately. Furthermore, a few studies have been examined, which include higher level kinetics and macroscale simulations. Ultimately, we show there is still ample room for improvement with regard to first principles calculations and their integration in multiscale models.

The influence of HKUST-1 and MOF-76 hand grinding/mechanical activation on stability, particle size, textural properties and carbon dioxide sorption

In this study, we explore the mechanical treatment of two metal-organic frameworks (MOFs), HKUST-1 and MOF-76, applying various milling methods to assess their impact on stability, porosity, and CO2 adsorption capacity. The effects of different mechanical grinding techniques, such as high-energy ball milling and hand grinding, on these MOFs were compared. The impact of milling time, milling speed and ball size during high-energy ball milling was assessed via the Design of Experiments methodology, namely using a 33 Taguchi orthogonal array. The results highlight a marked improvement in CO2 adsorption capacity for HKUST-1 through hand milling, increasing from an initial 25.70 wt.% (5.84 mmol g-1) to 41.37 wt.% (9.40 mmol g-1), marking a significant 38% increase. In contrast, high-energy ball milling seems to worsen this property, diminishing the CO2 adsorption abilities of the materials. Notably, MOF-76 shows resistance to hand grinding, closely resembling the original sample's performance. Hand grinding also proved to be well reproducible. These findings clarify the complex effects of mechanical milling on MOF materials, emphasising the necessity of choosing the proper processing techniques to enhance their stability, texture, and performance in CO2 capture and storage applications.

Evaluation of Ce-MOFs as Photoanode Materials for the Water Oxidation Reaction: The Effect of Doping with [Ru(bpy)(dcbpy)(H2O)2]2+ Catalyst

Artificial photosynthesis stands out as a highly effective method for harnessing sunlight to produce clean and renewable energy. The light-absorbing properties, chemical stability, and high redox activity of Ce-based metal-organic frameworks (MOFs) make them attractive materials for visible-light-driven water splitting. Currently, Ce-based MOFs remain a relatively underexplored system for photocatalytic water oxidation in acidic media. In this study, we synthesized a Ce-MOF with different linkers (1,4-benzenedicarboxylic acid, tetrafluoroterephthalic acid, 2-nitroterephthalic acid, 2,2'-bipyridine-5,5'-dicarboxylic acid, and 4,4'-biphenyldicarboxylic acid), which exhibit light-absorbing capability. Ce-based MOFs doped with [Ru(bpy)(dcbpy)(H2O)2]2+ (MOF-1 and MOF-2) water oxidation catalyst showed an enhanced photoelectrocatalytic current of ∼10-4 A·cm-2 at pH = 1, which is comparable with the [Ru(bpy)(dcbpy)(H2O)2]2+-doped MIL-126 Fe-based MOF. We also demonstrated the long-term durability of Ru-doped Ce-MOFs for photoelectrocatalytic water oxidation under acidic conditions. The as-synthesized MOFs were analyzed with powder X-ray diffraction (PXRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), UV-visible diffuse reflectance spectroscopy, scanning electron microscopy (SEM), and electric conductivity measurements. This study contributes to the development of cost-effective materials for sustainable photocatalytic water splitting processes.

Research Progress in Composite Materials for Photocatalytic Nitrogen Fixation

Ammonia is an essential component of modern chemical products and the building unit of natural life molecules. The Haber-Bosch (H-B) process is mainly used in the ammonia synthesis process in the industry. In this process, nitrogen and hydrogen react to produce ammonia with metal catalysts under high temperatures and pressure. However, the H-B process consumes a lot of energy and simultaneously emits greenhouse gases. In the "double carbon" effect, to promote the combination of photocatalytic technology and artificial nitrogen fixation, the development of green synthetic reactions has been widely discussed. Using an inexhaustible supply of sunlight as a power source, researchers have used photocatalysts to reduce nitrogen to ammonia, which is energy-dense and easy to store and transport. This process completes the conversion from light energy to chemical energy. At the same time, it achieves zero carbon emissions, reducing energy consumption and environmental pollution in industrial ammonia synthesis from the source. The application of photocatalytic technology in the nitrogen cycle has become one of the research hotspots in the new energy field. This article provides a classification of and an introduction to nitrogen-fixing photocatalysts reported in recent years and prospects the future development trends in this field.

Rare-Earth-Modified Metal-Organic Frameworks and Derivatives for Photo/Electrocatalysis

Metal-organic frameworks (MOFs) and their derivatives have attracted much attention in the field of photo/electrocatalysis owing to their ultrahigh porosity, tunable properties, and superior coordination ability. Regulating the valence electronic structure and coordination environment of MOFs is an effective way to enhance their intrinsic catalytic performance. Rare earth (RE) elements with 4f orbital occupancy provide an opportunity to evoke electron rearrangement, accelerate charged carrier transport, and synergize the surface adsorption of catalysts. Therefore, the integration of RE with MOFs makes it possible to optimize their electronic structure and coordination environment, resulting in enhanced catalytic performance. In this review, progress in current research on the use of RE-modified MOFs and their derivatives for photo/electrocatalysis is summarized and discussed. First, the theoretical advantages of RE in MOF modification are introduced, with a focus on the roles of 4f orbital occupancy and RE ion organic coordination ligands. Then, the application of RE-modified MOFs and their derivatives in photo/electrocatalysis is systematically discussed. Finally, research challenges, future opportunities, and prospects for RE-MOFs are also discussed.

A highly stable terbium(III) metal-organic framework MOF-76(Tb) for hydrogen storage and humidity sensing

The present study described the synthesis and characterization of MOF-76(Tb) for hydrogen storage and humidity sensing applications. The structure and morphology of as-synthesized material were studied using powder X-ray diffraction, scanning, and transmission electron microscopy. The crystal structure of MOF-76(Tb) consists of terbium(III) and benzene-1,3,5-tricarboxylate(-III) ions, one coordinated aqua ligand and one crystallization N,N´-dimethylformamide molecule. The polymeric framework of MOF-76(Tb) contains 1D sinusoidally shaped channels with sizes of 6.6 × 6.6 Å propagating along c crystallographic axis. The thermogravimetric analysis of the prepared material exhibited thermal stability up to 600 °C. At 77 K and pressure up to 20 bar; 0.6 wt.% hydrogen storage capacity for MOF-76(Tb) was observed. Finally, the humidity sensing measurements (water adsorption experiments) were performed, and the results indicate that MOF-76(Tb) is not a suitable material for moisture sensing applications.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Cationic vacancy engineering of p-TiO2 for enhanced photocatalytic nitrogen fixation

Defect engineering is one of the effective strategies to regulate and control catalyst properties. Constructing appropriate catalytically active centers effectively tunes the electronic and surface properties of the catalyst to achieve further enrichment of photogenerated electrons, enhances the electronic feedback of the catalytically active center to the anti-bonding orbitals of the nitrogen molecule, and enhances N₂ adsorption while weakening the NN bond. In this study, titanium vacancy (V_Ti)-rich undoped anatase p-TiO₂ was successfully synthesized to investigate the effect of its metal vacancies on photocatalytic nitrogen reduction reaction (NRR) performance. The cation vacancies of V_Ti-rich p-TiO₂ lead to local charge defects that enhance carrier separation and transport while trapping electrons to activate N₂, allowing effective reduction of the excited electrons to NH₃. This work provides a viable strategy for driving the efficiency of photocatalytic nitrogen fixation processes by altering the structural properties of semiconductors through cationic vacancies, offering new opportunities and challenges for the design and preparation of titanium dioxide-based materials.

Metal-Organic Framework Materials for Production and Distribution of Ammonia

The efficient production of ammonia (NH₃) from dinitrogen (N₂) and water (H₂O) using renewable energy is an important step on the roadmap to the ammonia economy. The productivity of this conversion hinges on the design and development of new active catalysts. In the wide scope of materials that have been examined as catalysts for the photo- and electro-driven reduction of N₂ to NH₃, functional metal-organic framework (MOF) catalysts exhibit unique properties and appealing features. By elucidating their structural and spectroscopic properties and linking this to the observed activity of MOF-based catalysts, valuable information can be gathered to inspire new generations of advanced catalysts to produce green NH₃. NH₃ is also a surrogate for the hydrogen (H₂) economy, and the potential application of MOFs for the practical and effective capture, safe storage, and transport of NH₃ is also discussed. This Perspective analyzes the contribution that MOFs can make toward the ammonia economy.

A facile synthesis of CeO2 from the GO@Ce-MOF precursor and its efficient performance in the oxygen evolution reaction

Electrochemical water splitting has enticed fascinating consideration as a key conduit for the advancement of renewable energy systems. Fabricating adequate electrocatalysts for water splitting is fervently preferred to curtail their overpotentials and hasten practical utilizations. In this work, a series of Ce-MOF, GO@Ce-MOF, calcinated Ce-MOF, and calcinated GO@Ce-MOF were synthesized and used as high-proficient electrocatalysts for the oxygen evolution reaction. The physicochemical characteristics of the prepared samples were measured by diverse analytical techniques including SEM, HRTEM, FTIR, BET, XPS, XRD, and EDX. All materials underwent cyclic voltammetry tests and were evaluated by electrochemical impedance spectroscopy and oxygen evolution reaction. Ce-MOF, GO@Ce-MOF, calcinated Ce-MOF, and calcinated GO@Ce-MOF have remarkable properties such as enhanced specific surface area, improved catalytic performance, and outstanding permanency in the alkaline solution (KOH). These factors upsurge ECSA and intensify the OER performance of the prepared materials. More exposed surface active-sites present in calcinated GO@Ce-MOF could be the logic for superior electrocatalytic activity. Chronoamperometry of the catalyst for 15°h divulges long-term stability of Ce-MOF during OER. Impedance measurements indicate higher conductivity of synthesized catalysts, facilitating the charge transfer reaction during electrochemical water splitting. This study will open up a new itinerary for conspiring highly ordered MOF-based surface active resources for distinct electrochemical energy applications.

Achievements and Perspectives in Metal–Organic Framework-Based Materials for Photocatalytic Nitrogen Reduction

Metal–organic frameworks (MOFs) are coordination polymers with high porosity that are constructed from molecular engineering. Constructing MOFs as photocatalysts for the reduction of nitrogen to ammonia is a newly emerging but fast-growing field, owing to MOFs’ large pore volumes, adjustable pore sizes, controllable structures, wide light harvesting ranges, and high densities of exposed catalytic sites. They are also growing in popularity because of the pristine MOFs that can easily be transformed into advanced composites and derivatives, with enhanced catalytic performance. In this review, we firstly summarized and compared the ammonia detection methods and the synthetic methods of MOF-based materials. Then we highlighted the recent achievements in state-of-the-art MOF-based materials for photocatalytic nitrogen fixation. Finally, the summary and perspectives of MOF-based materials for photocatalytic nitrogen fixation were presented. This review aims to provide up-to-date developments in MOF-based materials for nitrogen fixation that are beneficial to researchers who are interested or involved in this field.

Efficient ABEI-Dissolved O2-Ce(III, IV)-MOF Ternary Electrochemiluminescent System Combined with Self-Assembled Microfluidic Chips for Bioanalysis

A signal-amplified electrochemiluminescent (ECL) sensor chip was developed for sensitive analysis of procalcitonin (PCT). Herein, we first prepared a self-enhanced luminophore, which enhanced ECL responses through intramolecular reactions. Second, Au-Pd bimetallic nanocrystals and mixed-valence Ce-based metal-organic frameworks (MOFs) were introduced as co-reaction promoters to facilitate the reduction of dissolved O₂. Based on the synergistic catalysis of Au and Pd, the spontaneous cyclic reaction of Ce(III)/Ce(IV), and the high electrochemical active surface area of Ce(III, IV) MOF, a large number of superoxide anion radicals (O₂^•-) and hydroxyl radicals (OH^•) were produced. Therefore, the luminescence efficiency of N-(aminobutyl)-N-(ethylisoluminol)-dissolved O₂ (ABEI-O₂) systems were greatly improved, providing a new prospect for the application of dissolved O₂ in ECL analysis. In addition, the affinity peptide ligands were used for the directional connection of antibodies to provide protection for the bioactivity of the proposed sensor. Finally, the microfluidic technology was applied to ECL analysis to integrate the three-electrode detection system into the self-assembled microfluidic chip, which realized the automation and portability of the detection process. The developed sensor showed high sensitivity for PCT detection with a detection limit of 3.46 fg/mL, which possessed positive significance for the clinical diagnosis of sepsis.

Dehydrated UiO-66(SH)2 : The Zr-O Cluster and Its Photocatalytic Role Mimicking the Biological Nitrogen Fixation

This work reports the dehydrated Zr-based MOF UiO-66(SH)₂ as a visible-light-driven photocatalyst to mimic the biological N₂ fixation process. The ¹⁵ N₂ and other control experiments demonstrated that the new photocatalyst is highly efficient in converting N₂ to ammonia. In-situ TGA, XPS, and EXAFS as well as first-principles simulations were used to demonstrate the role of the thermal treatment and the changes of the local structures around Zr due to the dehydration. It was shown that the dehydration opened a gate for the entry of N₂ molecules into the [Zr₆ O₆ ] cluster where the strong N≡N bond was broken stepwise by μ-N-Zr type interactions driven by the photoelectrons aided by the protonation. This mechanism was discussed in comparison with the Lowe-Thorneley mechanism proposed for the MoFe nitrogenase, and with emphasis on the [Zr₆ O₆ ] cluster effect and the leading role of photoelectrons over the protonation. The results shed new light on understanding the catalytic mechanism of biological N₂ fixation and open a new way to fix N₂ under mild conditions.

A sensitive fluorescence "turn on" nanosensor for glutathione detection based on Ce-MOF and gold nanoparticles

Glutathione (GSH) as an essential biothiol maintains redox homeostasis in human body, the aberrant level of it has been related to various diseases. In this work, we constructed a facile and environment-friendly strategy by using Ce based metal-organic frameworks and gold nanoparticles (AuNPs) for detection of GSH. The fluorescence intensity of the Ce-MOF was quenched by AuNPs, which is ascribed to the existence of fluorescence resonance energy transfer (FRET) and electrostatic interaction between Ce-MOFs and AuNPS. Because of the formation of Au-SH between AuNPs and GSH, the addition GSH induced the Ce-MOF/AuNPs and prevented the occurrence of FRET and electrostatic interaction between Ce-MOFs and AuNPS, which futher recovered the fluorescence of Ce-MOF. Under the optimized conditions, this "turn-on" sensing process revealed a high selectivity toward GSH and displayed good linearity in range of 0.2-32.5 μM with low detection limit of 58 nM. In addition, the practicability of the strategy was testified through analyzing GSH in real human serum samples.

A minireview on catalysts for photocatalytic N2 fixation to synthesize ammonia

Ammonia (NH3) is an important feedstock in chemical industry. Nowadays NH3 is mainly produced via the industrialized Haber-Bosch process, which requires substantial energy input, since it operates at high temperatures (400-650 °C) and high pressures (20-40 Mpa). From the energy conservation point of view, it is of great significance to explore an alternative avenue to synthesize NH3, which is in line with the concept of sustainable development. Very recently, photocatalytic N2 fixation (PNF) has been discovered as a safe and green approach to synthesize NH3, as it utilizes the inexhaustible solar energy and the abundant N2 in nature to synthesize NH3 under mild conditions. A highly efficient catalyst is the core of PNF. Up to now, extensive studies have been conducted to design efficient catalysts for PNF. Summarizing the catalysts reported for PNF and unraveling their reaction mechanisms could provide guidance for the design of better catalysts. In this review, we will illustrate the development of catalysts for PNF, including semiconductors, plasmonic metal-based catalysts, iron-based catalysts, ruthenium-based catalysts and several other catalysts, point out the remaining challenges and outline the future opportunities, with the aim to contribute to the development of PNF.

Multi-stepwise electron transfer via MOF-based nanocomposites for photocatalytic ammonia synthesis

Photocatalytic ammonia (NH3) synthesis can realize the reaction of nitrogen (N2) and water by using solar energy as the sole driving force under mild conditions, however, the inferior electron transfer...

Modular Assembly of Electron Transfer Pathway in Bimetallic MOF for Photocatalytic Ammonia Synthesis

It is a long-term pursuit to implement the green and sustainable photocatalytic production of ammonia via the conversion of water and nitrogen under mild conditions. Due to the rapid recombination...

A Metal-Organic Framework as a Multiphoton Excitation Regulator for the Activation of Inert C(sp3 )-H Bonds and Oxygen

The activation and oxidization of inert C(sp³ )-H bonds into value-added chemicals affords attractively economic and ecological benefits as well as central challenge in modern chemistry. Inspired by the natural enzymatic transformation, herein, we report a new multiphoton excitation approach to activate the inert C(sp³ )-H bonds and oxygen by integrating the photoinduced electron transfer (PET), ligand-to-metal charge transfer (LMCT) and hydrogen atom transfer (HAT) events together into one metal-organic framework. The well-modified nicotinamide adenine dinucleotide (NAD⁺ ) mimics oxidized Ce^III -OEt moieties to generate Ce^IV -OEt chromophore and its reduced state mimics NAD^. via PET. The in situ formed Ce^IV -OEt moiety triggers a LMCT excitation to form the alkoxy radical EtO^. , abstracts a hydrogen atom from the C(sp³ )-H bond, accompanying the recovery of Ce^III -OEt and the formation of alkyl radicals. The formed NAD^. activates oxygen to regenerate the NAD⁺ for next recycle, wherein, the activated oxygen species interacts with the intermediates for the oxidization functionalization, paving a catalytic avenue for developing scalable and sustainable synthetic strategy.

Metal-Organic Frameworks: Molecules or Semiconductors in Photocatalysis?

In the realm of solids, metal-organic frameworks (MOFs) offer unique possibilities for the rational engineering of tailored physical properties. These derive from the modular, molecular make-up of MOFs, which allows for the selection and modification of the organic and inorganic building units that construct them. The adaptable properties make MOFs interesting materials for photocatalysis, an area of increasing significance. But the molecular and porous nature of MOFs leaves the field, in some areas, juxtapositioned between semiconductor physics and homogeneous photocatalysis. While descriptors from both fields are applied in tandem, the gap between theory and experiment has widened in some areas, and arguably needs fixing. Here we review where MOFs have been shown to be similar to conventional semiconductors in photocatalysis, and where they have been shown to be more like infinite molecules in solution. We do this from the perspective of band theory, which in the context of photocatalysis, covers both the molecular and nonmolecular principles of relevance.

Ratiometric fluorescent sensing for phosphate based on Eu/Ce/UiO-66-(COOH)2 nanoprobe

The sensing of phosphate anion (PO₄^3-) is an important subject for human health and environmental monitoring. Herein, a unique ratiometric fluorescent nanoprobe based on postsynthetic modification of metal-organic frameworks (MOF) UiO-66-(COOH)₂ with Eu³⁺ and Ce³⁺ ions toward PO₄^3- was proposed (designated as Eu/Ce/Uio-66-(COOH)₂). The Eu/Ce/Uio-66-(COOH)₂ nanoprobe exhibits three emission peaks at 377 nm, 509 nm, and 621 nm with the single excitation wavelength at 250 nm, respectively. The strong coordinating interaction between Ce³⁺ and O atoms in the PO₄^3- group can result in the fluorescence quenching at 377 nm, while the fluorescence of 621 nm almost remains unchanged. Such a useful phenomenon is exploited for the construction of a ratiometric fluorescence platform for the detection of PO₄^3-. The assay exhibited a good linear response in the 0.3-20 μM concentration range with the detection limit of 0.247 μM. In addition, this ratiometric fluorescent sensing method not only can be applied to read out PO₄^3- concentration in real water samples, but also shows higher sensitivity, easier preparation and sensing procedures than other detection strategies.

Ratiometric fluorescence for sensitive detection of phosphate species based on mixed lanthanide metal organic framework

Phosphate (PO₄^3-) plays a major role in aquatic ecosystems and biosystems. Developing a highly sensitive and selective ratiometric fluorescence probe for detection of PO₄^3- is of great significance to the ecological environment and human health. In this work, a novel dual lanthanide metal organic framework was synthesized via hydrothermal reaction based on Tb³⁺ and Ce³⁺ as the center metal ions and terephthalic acid as the organic ligand (designated as Tb-Ce-MOFs). The fluorescence of Tb-Ce-MOFs shows emission at 375 nm. In the presence of PO₄^3-, with increased concentration of PO₄^3-, the fluorescence intensity of Tb-Ce-MOFs at 500 nm and 550 nm increased, while the intensity at 375 nm was reduced. Hence, ratiometric fluorescence detecting of PO₄^3- can be achieved by measuring the ratio of fluorescence at 550 nm (FL₅₅₀) to 375 nm (FL₃₇₅) in the fluorescent spectra of the Tb-Ce-MOFs. In this sensing approach, the Tb-Ce-MOFs probe exhibits highly sensitive and selective for detection of PO₄^3-. The limit of detection is calculated to be 28 nM and the detection range is 0.1 to 10 μM. In addition, the Tb-Ce-MOFs were used in the detection of PO₄^3- in real samples. We design and synthesize a mixed lanthanide metal organic framework fluorescence probe (Tb-Ce-MOFs) for ratiometric fluorescence for the detection of PO₄^3- based on Tb³⁺ and Ce³⁺ as the center metal ions and terephthalic acid as the organic ligand.

Recent advances in photocatalytic nitrogen fixation: from active sites to ammonia quantification methods

Photocatalytic nitrogen fixation has become a hot topic in recent years due to its mild and sustainable advantages. While modifying the photocatalyst to enhance its electron separation, light absorption and nitrogen reduction abilities, the role of the active sites in the catalytic reaction cannot be ignored because the N[triple bond, length as m-dash]N nitrogen bond is too strong to activate. This review summarizes the recent research on nitrogen fixation, focusing on the active sites for N2 on the catalyst surface, classifying common active sites, explaining the main role and additional role of the active sites in catalytic reactions, and discussing the methods to increase the number of active sites and their activation ability. Finally, the outlook for future research is presented. It is hoped this review could help researchers understand more about the activation of the nitrogen molecules and lead more efforts into research on nitrogen fixation photocatalysts.

The chemistry and applications of hafnium and cerium(iv) metal-organic frameworks

The coordination connection of organic linkers to the metal clusters leads to the formation of metal-organic frameworks (MOFs), where the metal clusters and ligands are spatially entangled in a periodic manner. The immense availability of tuneable ligands of different length and functionalities gives rise to robust molecular porosity ranging from several angstroms to nanometres. Among the large family of MOFs, hafnium (Hf) based MOFs have been demonstrated to be highly promising for practical applications due to their unique and outstanding characteristics such as chemical, thermal, and mechanical stability, and acidic nature. Since the report of UiO-66(Hf) and DUT-51(Hf) in 2012, less than 200 Hf-MOFs (ca. 50 types of structures) have been reported. Besides, tetravalent cerium [Ce(iv)] has been proven to be capable of forming similar topological MOF structures to Zr and Hf since its first discovery in 2015. So far, ca. 40 Ce(iv) MOFs with 60% having UiO-66-type structure have been reported. This review will offer a holistic summary of the chemistry, uniqueness, synthesis, and applications of Hf/Ce(iv)-MOFs with a focus on presenting the development in the Hf/Ce(iv)-clusters, topologies, ligand structures, synthetic strategies, and practical applications of Hf/Ce(iv)-MOFs. In the end, we will present the research outlook for the development of Hf/Ce(iv)-MOFs in the future, including fundamental design of Hf/Ce(iv)-clusters, defect engineering, and various applications including membrane development, diversified types of catalytic reactions, irradiation absorption in nuclear waste treatment, water production and wastewater treatment, etc. We will also present the emerging computational approaches coupled with machine-learning algorithms that can be applied in screening Hf and Ce(iv) based MOF structures and identifying the best-performing MOFs for tailor-made applications in future practice.