Great attention has been given to metal–organic frameworks (MOFs)-derived solid bases because of their attractive structure and catalytic performance in various organic reactions. The extraordinary skeleton structure of MOFs provides many possibilities for incorporation of diverse basic functionalities, which is unachievable for conventional solid bases. The past decade has witnessed remarkable advances in this vibrant research area; however, MOFs for heterogeneous basic catalysis have never been reviewed until now. Therefore, a review summarizing MOFs-derived base catalysts is highly expected. In this review, we present an overview of the recent progress in MOFs-derived solid bases covering preparation, characterization, and catalytic applications. In the preparation section, the solid bases are divided into two categories, namely, MOFs with intrinsic basicity and MOFs with modified basicity. The basicity can originate from either metal sites or organic ligands. Different approaches used for generation o...

Metal-Organic Frameworks for Heterogeneous Basic Catalysis.

Luminescent sensors based on metal-organic frameworks

Developing efficient sensor materials with superior performance for selective, fast and sensitive detection of gases and volatile organic compounds (VOCs) is essential for human health and environmental protection, through monitoring indoor and outdoor air pollutions, managing industrial processes, controlling food quality and assisting early diagnosis of diseases. Metal–organic frameworks (MOFs) are a unique type of crystalline and porous solid material constructed from metal nodes (metal ions or clusters) and functional organic ligands. They have been investigated extensively for possible use as high performance sensors for the detection of many different gases and VOCs in recent years, due to their large surface area, tunable pore size, functionalizable sites and intriguing properties, such as electrical conductivity, magnetism, ferroelectricity, luminescence and chromism. The high porosity of MOFs allows them to interact strongly with various analytes, including gases and VOCs, thus resulting in easily measurable responses to different physicochemical parameters. Although much of the recent work on MOF-based luminescent sensors have been summarized in several excellent reviews (up to 2018), a comprehensive overview of these materials for sensing gases and VOCs based on chemiresistive, magnetic, ferroelectric, and colorimertic mechanisms is missing. In this review, we highlight the most recent progress in developing MOF sensing and switching materials with an emphasis on sensing mechanisms based on electricity, magnetism, ferroelectricity and chromism. We provide a comprehensive analysis on the MOF–analyte interactions in these processes, which play a key role in the sensing performance of the MOF-based sensors and switches. We discuss in detail possible applications of MOF-based sensing and switching materials in detecting oxygen, water vapor, toxic industrial gases (such as hydrogen sulfide, ammonia, sulfur dioxide, nitrous oxide, carbon oxides and carbon disulfide) and VOCs (such as aromatic and aliphatic hydrocarbons, ketones, alcohols, aldehydes, chlorinated hydrocarbons and N,N′-dimethylformamide). Overall, this review serves as a timely source of information and provides insight for the future development of advanced MOF materials as next-generation gas and VOC sensors.

Functional metal–organic frameworks as effective sensors of gases and volatile compounds

ConspectusMetal–organic frameworks (MOFs) possess an important advantage over other candidate classes for chemosensory materials because of their exceptional structural tunability and properties. Luminescent sensing using MOFs is a simple, intuitive, and convenient method to recognize species, but the method has limitations, such as insufficient chemical selectivity and signal loss. MOFs contain versatile building blocks (linkers or ligands) with special chemical reactivity, and postsynthetic modification (PSM) provides an opportunity to exploit and expand their unique properties. The linkers in most MOFs contain aromatic subunits that can readily display luminescence after ultraviolet or visible (typically blue) excitation, and this is the main luminescent nature of most MOFs. The introduction of photoactive lanthanide ions (Ln3+) into the MOF hosts may produce new luminescent signals at different positions from that of the MOF linker, but this depends on the intramolecular energy transfer (antenna effec...

Lanthanide-Functionalized Metal–Organic Framework Hybrid Systems To Create Multiple Luminescent Centers for Chemical Sensing

Metal-organic frameworks (MOFs) with multiple emission centers are newly emerging as ratiometric sensors owing to their high sensitivity and high selectivity toward a wide range of targeted functional species. Energy transfer between the light-absorbing group and emission centers and between different emission centers is the key to rationally design and synthesize MOF-based ratiometric sensors. A good match between the energy levels of the light-absorbing groups and emission centers is the prerequisite for MOF-based sensors to exhibit multiple emissions, and a good match of the MOF-based sensors and those of the targeted species can increase the sensitivity and selectivity, but this match is highly challenging to obtain via synthesis. MOFs with multiple emission centers can be produced by functionalizing MOFs with multiple lanthanide centers, organic luminophores, dyes, carbon dots, and other such emissive groups. In this progress report, recent advances in the strategies for synthesizing MOFs with multiple emission centers and their applications for ratiometric sensing of solution conditions, including the pH value, and ion, organic molecule, and biomolecule concentrations, are summarized, as are the related sensing mechanisms.

Multicenter Metal-Organic Framework-Based Ratiometric Fluorescent Sensors.

Unsafe food is a huge threat to human health and the economy, and detecting food spoilage early is an ongoing and imperative need. Herein, a simple and effective strategy combining a fluorescence sensor and one-to-two logic operation is designed for monitoring biogenic amines, indicators of food spoilage. Sensors (methyl red@lanthanide metal-organic frameworks (MR@EuMOFs)) are created by covalently modifying MR into NH2 -rich EuMOFs, which have a high quantum yield (48%). A double-stimuli-responsive fluorescence center is produced via energy transfer from the ligands to Eu3+ and MR. Portable sensory hydrogels are obtained by dispersing and solidifying MR@EuMOFs in water-phase sodium salt of carboxy methyl cellulose (CMC-Na). The hydrogels exhibit a color transition upon "smelling" histamine (HI) vapor. This transition and shift in the MR-based emission peak are closely related to the HI concentration. Using the HI concentration as the input signal and the two fluorescence emissions as output signals, an advanced analytical device based on a one-to-two logic gate is constructed. The four output combinations, NOT (0, 1), YES (1, 0), PASS 1 (1, 1), and PASS 0 (0, 0), allow the direct analysis of HI levels, which can be used for real-time food-freshness evaluation. The novel strategy suggested here may be a new application for a molecular logic system in the sensing field.

A Double‐Stimuli‐Responsive Fluorescent Center for Monitoring of Food Spoilage based on Dye Covalently Modified EuMOFs: From Sensory Hydrogels to Logic Devices

A series of nano-sized lanthanide metal–organic frameworks (Ln-MOFs) which are isostructural with parent MOF-76(Ln) (Ln = Eu, Tb, Sm, Dy) are synthesized under solvothermal conditions. The morphology and particle size of the Ln-MOFs demonstrated by scanning electron microscopy can be changed by increasing the quantities of sodium acetate whose capping effect provides more coordinate sites for lanthanide ions. Furthermore, we explore the adsorption properties of nanosized MOF-76(Ln) for organic dyes, indicating that nanosized MOF-76(Ln) can adsorb some cationic dyes like methylene blue, etc. In addition, MOF-76(Eu) and MOF-76(Tb) are selected for fluorescence sensing for monoaromatic hydrocarbons (BTEX). Such system serves as an acetophenone detection platform through a fluorescence quench process. This work highlights the practical application of MOF-76(Ln) as sensors, and it paves the way for detecting BTEX in solution or in vapor phase.

A lanthanide metal–organic framework (MOF-76) for adsorbing dyes and fluorescence detecting aromatic pollutants

Eu3+ functionalized Sc-MOFs: Turn-on fluorescent switch for ppb-level biomarker of plastic pollutant polystyrene in serum and urine and on-site detection by smartphone

The research on luminescent lanthanide-functionalized covalent organic framework (COF)-based hybrid materials has not been reported so far. When a straightforward functionalized modification strategy is employed, a chemical stable Eu3+-modified COF hybrid material, Eu@TpPa-1, is first developed. The functional process does not affect the size and shape of TpPa-1, but after the introduction of Eu3+, Eu@TpPa-1 as a fluorescence sensor exhibits a turn-on response toward levofloxacin. Eu3+ plays a bridge role in the whole detection system, and TpPa-1 serves as a host matrix to provide protection for the introduced Eu3+. This reusable sensor shows strong pink-white and yellow emissions toward the high and low concentration ranges of levofloxacin, respectively. It displays excellent sensitivity and fast response to levofloxacin within 1 min, while refraining from the interference of other coexisting species in serum and urine. The good selectivity and high antidisturbance lead Eu@TpPa-1 to be successfully appli...

Eu3+-Functionalized Covalent Organic Framework Hybrid Material as a Sensitive Turn-On Fluorescent Switch for Levofloxacin Monitoring in Serum and Urine

The recent discovery of lanthanide organic frameworks (Ln-MOFs) offers the potential for biomarkers or metabolites sensing. This is another field that is closely connected with life science and medicine. In this work, a stable and luminescent phosphonate Ln-MOFs MIL-91(Al:Eu) and its derivatives composite Cu2+@MIL-91(Al:Eu) were synthesized. The rebound of luminescence of Cu2+@MIL-91(Al:Eu) that origin of Eu3+ is observed in the presence of uric acid. This On-Off-On pattern is utilized for detecting uric acid, which is the final metabolite of purine. The composite reveals excellent selectivity and sensitivity for sensing uric acid. The detection of uric acid in real urine is also investigated. The effective detection of uric acid and tentative diagnosis of hyperuricuria on the basis of test paper is demonstrated.

Xiao Lian

Papers

A Double‐Stimuli‐Responsive Fluorescent Center for Monitoring of Food Spoilage based on Dye Covalently Modified EuMOFs: From Sensory Hydrogels to Logic Devices

A lanthanide metal–organic framework (MOF-76) for adsorbing dyes and fluorescence detecting aromatic pollutants

Eu3+ functionalized Sc-MOFs: Turn-on fluorescent switch for ppb-level biomarker of plastic pollutant polystyrene in serum and urine and on-site detection by smartphone

Eu3+-Functionalized Covalent Organic Framework Hybrid Material as a Sensitive Turn-On Fluorescent Switch for Levofloxacin Monitoring in Serum and Urine

Phosphonate MOFs Composite as Off-On Fluorescent Sensor for Detecting Purine Metabolite Uric Acid and Diagnosing Hyperuricuria.