Trajectory in biological metal-organic frameworks: Biosensing and sustainable strategies-perspectives and challenges

The ligand attribute of biomolecules to form coordination bonds with metal ions led to the discovery of a novel class of materials called biomolecule-associated metal-organic frameworks (Bio-MOFs). These biomolecules coordinate in multiple ways and provide versatile applications. Far-spread bio-ligands include nucleobases, amino acids, peptides, cyclodextrins, saccharides, porphyrins/metalloporphyrin, proteins, etc. Low-toxicity, self-assembly, stability, designable and selectable porous size, the existence of rigid and flexible forms, bio-compatibility, and synergistic interactions between metal ions have led Bio-MOFs to be commercialized in industries such as sensors, food, pharma, and eco-sensing. The rapid growth and commercialization are stunted by absolute bio-compatibility issues, bulk morphology that makes it rigid to alter shape/porosity, longer reaction times, and inadequate research. This review elucidates the structural vitality, biocompatibility issues, and vital sensing applications, including challenges for incorporating bio-ligands into MOF. Critical innovations in Bio-MOFs' applicative spectrum, including sustainable food packaging, biosensing, insulin and phosphoprotein detection, gas sensing, CO2 capture, pesticide carriers, toxicant adsorptions, etc., have been elucidated. Emphasis is placed on biosensing and biomedical applications with biomimetic catalysis and sensitive sensor designing.

Efficient room-temperature phosphorescence of covalent organic frameworks through covalent halogen doping

Organic room-temperature phosphorescence, a spin-forbidden radiative process, has emerged as an interesting but rare phenomenon with multiple potential applications in optoelectronic devices, biosensing and anticounterfeiting. Covalent organic frameworks (COFs) with accessible nanoscale porosity and precisely engineered topology can offer unique benefits in the design of phosphorescent materials, but these are presently unexplored. Here, we report an approach of covalent doping, whereby a COF is synthesized by copolymerization of halogenated and unsubstituted phenyldiboronic acids, allowing for random distribution of functionalized units at varying ratios, yielding highly phosphorescent COFs. Such controlled halogen doping enhances the intersystem crossing while minimizing triplet-triplet annihilation by diluting the phosphors. The rigidity of the COF suppresses vibrational relaxation and allows a high phosphorescence quantum yield (Φ_Phos ≤ 29%) at room temperature. The permanent porosity of the COFs and the combination of the singlet and triplet emitting channels enable a highly efficient COF-based oxygen sensor, with an ultra-wide dynamic detection range (~10³-10^-5 torr of partial oxygen pressure).

Porous matrix materials in optical sensing of gaseous oxygen

The review provides comparison of porous materials that act as a matrix for luminescent oxygen indicators. These include silica-gels, sol–gel materials based on silica and organically modified silica (Ormosils), aerogels, electrospun polymeric nanofibers, metal–organic frameworks, anodized alumina, and various other microstructured sensor matrices. The influence of material structure and composition on the efficiency of oxygen quenching and dynamic response times is compared and the advantages and disadvantages of the materials are summarized to give a guide for design and practical application of sensors with desired sensitivity and response time.

Core-Shell Pluronic-Organosilica Nanoparticles with Controlled Polarity and Oxygen Permeability

Nanostructured systems constitute versatile carriers with multiple functions engineered in a nanometric space. Yet, such multimodality often requires adapting the chemistry of the nanostructure to the properties of the hosted functional molecules. Here, we show the preparation of core-shell Pluronic-organosilica "PluOS" nanoparticles with the use of a library of organosilane precursors. The precursors are obtained via a fast and quantitative click reaction, starting from cost-effective reagents such as diamines and an isocyanate silane derivative, and they condensate in building blocks characterized by a balance between hydrophobic and H-bond-rich domains. As nanoscopic probes for local polarity, oxygen permeability, and solvating properties, we use, respectively, solvatochromic, phosphorescent, and excimer-forming dyes covalently linked to the organosilica matrix during synthesis. The results obtained here clearly show that the use of these organosilane precursors allows for finely tuning polarity, oxygen permeability, and solvating properties of the resulting organosilica core, expanding the toolbox for precise engineering of the particle properties.

Functionalization of zirconium-based metal-organic frameworks for gas sensing applications

The functionalization and incorporation of noble metals in metal-organic frameworks have been widely used as efficient methods to enhance their applicability. Herein, a sulfone-functionalized Zr-MOF framework labeled Zr-BPDC-SO₂ (BPDC-SO₂ =dibenzo[b,d]-thiophene-3,7-dicarboxylate 5,5-dioxide) and its Pd-embedded composite were efficiently synthesized by adjusting their functional groups. The obtained compounds were characterized to assess their potential for gas sensing applications. X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, specific surface area measurements, and thermogravimetric analysis were employed to characterize the new sensor materials. The gas sensing properties of the novel functionalized sensor materials were systematically investigated under various temperature, concentration, and gas type conditions. Owing to the strong hydrogen bonds of the sulfonyl groups and Zr₆ clusters in the framework with the hydroxyl groups of ethanol, Zr-BPDC-SO₂ emerged as an effective sensor for ethanol detection. In addition, Pd@Zr-BPDC-SO₂ exhibited efficient hydrogen sensing performance, in terms of sensor dynamics and response. More importantly, the material showed a higher sensing response to hydrogen than to other gases, highlighting the important role of Pd in the Zr-MOF-based hydrogen sensor. The results of the sensing tests carried out in this study highlight the promising potential of the present materials for practical gas monitoring applications.

Role of Spin-Orbit Coupling in Long Range Energy Transfer in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are emerging as a promising platform for solar energy conversion applications. Their potential utilization as efficient chromophores in artificial photosynthesis is closely related to the understanding of light-harvesting and energy transfer processes that occur within these molecular scaffolds. Herein, we present the photophysical investigation of Ru(II), Ir(III), and Os(II) polypyridyl complexes incorporated into the backbone of UiO-67. In this work, we systematically study the effect of spin-orbit coupling on dipole-dipole energy transfer in MOFs using steady-state and time-resolved spectroscopic techniques. The results of our work indicate successful triplet-to-singlet energy transfer and a sizable increase in the transfer kinetics and critical distance, as direct consequences of strong spin-orbit couplings. Remarkably, the reported R0 value for OsDCBPY (R0 = 88 ± 10 Å) represents one of the largest Förster distances observed in an MOF. Collectively, this work contributes to the general knowledge of energy transfer in materials and provides groundwork for efficient utilization in artificial photosynthetic assemblies.

Coordination Polymers Based on Highly Emissive Ligands: Synthesis and Functional Properties

Coordination polymers are constructed from metal ions and bridging ligands, linking them into solid-state structures extending in one (1D), two (2D) or three dimensions (3D). Two- and three-dimensional coordination polymers with potential voids are often referred to as metal-organic frameworks (MOFs) or porous coordination polymers. Luminescence is an important property of coordination polymers, often playing a key role in their applications. Photophysical properties of the coordination polymers can be associated with intraligand, metal-centered, guest-centered, metal-to-ligand and ligand-to-metal electron transitions. In recent years, a rapid growth of publications devoted to luminescent or fluorescent coordination polymers can be observed. In this review the use of fluorescent ligands, namely, 4,4'-stilbenedicarboxylic acid, 1,3,4-oxadiazole, thiazole, 2,1,3-benzothiadiazole, terpyridine and carbazole derivatives, naphthalene diimides, 4,4',4''-nitrilotribenzoic acid, ruthenium(II) and iridium(III) complexes, boron-dipyrromethene (BODIPY) derivatives, porphyrins, for the construction of coordination polymers are surveyed. Applications of such coordination polymers based on their photophysical properties will be discussed. The review covers the literature published before April 2020.

Thiazole- and Thiadiazole-Based Metal–Organic Frameworks and Coordination Polymers for Luminescent Applications

This mini-review focuses on the 2015–2019 literature survey of thiazole- and thiadiazole-containing Metal–Organic Frameworks (MOFs) and Coordination Polymers (CPs) exploited in the applicative field of luminescent sensing.

A Multifunctional Dual-Luminescent Polyoxometalate@Metal-Organic Framework EuW10@UiO-67 Composite as Chemical Probe and Temperature Sensor

The luminescent [EuW10O36]9- polyoxometalate has been introduced into the cavities of the highly porous zirconium luminescent metal-organic framework UiO-67 via a direct synthesis approach, affording the EuW10@UiO-67 hybrid. Using a combination of techniques (TGA, BET, elemental analysis, EDX mapping,…) this new material has been fully characterized, evidencing that it contains only 0.25% in europium and that the polyoxometalate units are located inside the octahedral cavities and not at the surface of the UiO-67 crystallites. Despite the low amount of europium, it is shown that EuW10@UiO-67 acts as a solid-state luminescent sensor for the detection of amino-acids, the growth of the emission intensity globally following the growth of the amino-acid pKa. In addition, EuW10@UiO-67 acts as a sensor for the detection of metallic cations, with a high sensitivity for Fe3+. Noticeably, the recyclability of the reported material has been established. Finally, it is shown that the dual-luminescent EuW10@UiO-67 material behave as a self-calibrated-ratiometric thermometer in the physiological range.

Zr-Based MOFs integrated with a chromophoric ruthenium complex for specific and reversible Hg2+ sensing

A novel metal-organic framework of RuUiO-67 was successfully fabricated and exploited as a chemical sensor for the colorimetric detection of Hg2+. The chromophoric Ru complex in RuUiO-67 was designed as a Hg2+ recognition site and a signal reporter. The elaborated probe exhibited a rapid colorimetric response, high selectivity, and sub-micromolar sensitivity for Hg2+ detection.

Mixed-Lanthanide Porous Coordination Polymers Showing Range-Tunable Ratiometric Luminescence for O2 Sensing

Luminescent porous coordination polymers (PCPs) are emerging as attractive oxygen-sensing materials, but they are mostly based on single-wavelength luminometry. Here, we report a special mixed-lanthanide strategy for self-referenced ratiometric oxygen sensing. A series of isostructural, pure-lanthanide, or mixed-lanthanide PCPs, MCF-53(Tb/Eu_x), were synthesized by solvothermal reactions. Single-crystal X-ray diffraction revealed that MCF-53(Tb/Eu_x) is composed of complicated two-dimensional coordination networks, which interdigitate to form a three-dimensional supramolecular structure retaining one-dimensional ultra-micropores. MCF-53(Tb/Eu_x) can undergo multiple single-crystal to single-crystal structural transformations upon desorption/adsorption of coordinative and lattice guest molecules, and the lanthanide metal ions are partially exposed on the pore surface at the guest-free state. Tb(III) ions are not luminescent and only act as separators between Eu(III) ions, and the Tb(III)/Eu(III) mixing ratio can tune the relative emission intensities, luminescence lifetimes of the Eu(III) phosphorescence, and the ligand fluorescence, giving rise to not only ratiometric photoluminescence oxygen sensing but also tunable emission-color-changing ranges.

The mixed-ligand strategy to assemble a microporous anionic metal–organic framework: Ln3+ post-functionalization, sensors and selective adsorption of dyes

A microporous anionic metal–organic framework has been synthesized, which can be used as potential sensor to organic molecules and selective adsorbent to dyes.

Photodynamics of Zr-based MOFs: effect of explosive nitroaromatics

The present work describes the fluorescence spectroscopy and photodynamics of two different Zr mixed-linkers MOFs (Zr-NDC/Tz and Zr-NDC/CN) and their interactions with nitroaromatics.

Photoluminescent Metal-Organic Frameworks for Gas Sensing

Luminescence of porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) is sensitive to the type and concentration of chemical species in the surrounding environment, because these materials combine the advantages of the highly regular porous structures and various luminescence mechanisms, as well as diversified host-guest interactions. In the past few years, luminescent MOFs have attracted more and more attention for chemical sensing of gas-phase analytes, including common gases and vapors of solids/liquids. While liquid-phase and gas-phase luminescence sensing by MOFs share similar mechanisms such as host-guest electron and/or energy transfer, exiplex formation, and guest-perturbing of excited-state energy level and radiation pathways, via various types of host-guest interactions, gas-phase sensing has its unique advantages and challenges, such as easy utilization of encapsulated guest luminophores and difficulty for accurate measurement of the intensity change. This review summarizes recent progresses by using luminescent MOFs as reusable sensing materials for detection of gases and vapors of solids/liquids especially for O₂, highlighting various strategies for improving the sensitivity, selectivity, stability, and accuracy, reducing the materials cost, and developing related devices.

Chemical Sensors Based on Metal-Organic Frameworks

Metal-organic frameworks (MOFs) as chemical sensors have developed rapidly in recent years. There have been many papers concerning this field and interest is still growing. The reason is that the specific merits of MOFs can be utilized to enhance sensitivity and selectivity by various energy/charge transfers occurring among different ligands, ligand, and metal centers, such as from ligands to metal centers or metal centers to ligands, as well as from MOF skeletons to guest species. This review intends to provide an update on recent progress in various applications of different MOF-based sensors on the basis of their luminescent and electrochemical responses towards small molecules, gas molecules, ions (cations and anions), pH, humidity, temperature, and biomolecules. MOF-based sensors function by utilizing different mechanisms, including luminescent responses of "turn-on" and "turn-off", as well as electrochemical responses.

Tuning the properties of the metal–organic framework UiO-67-bpy via post-synthetic N-quaternization of pyridine sites

N-Quaternization of pyridine sites in UiO-67-bpy affords a cationic MOF UiO-67-bpy-Me with faster and improved anionic dye and CO₂adsorption, and extended visible-light absorption properties, which make UiO-67-bpy-Me more efficient in photodegrading organic dyes.

The effect of imidazole on the enhancement of gadolinium-porphyrin phosphorescence at room temperature

The mechanism for the 40-fold enhancement in Gd-HMME RTP intensity by adding imidazole and Gd³⁺ is revealed.

Metal-Organic Frameworks as Chemical Sensors

Chemical sensing is of critical importance in today's society in a variety of applications from medicine to environmental pollution control, and from food safety monitoring to the detection of illicit substances and chemical weapons. Metal–organic frameworks (MOFs) have shown tremendous promise as a new class of chemical sensor materials that could be integrated into future devices. MOFs are microporous crystalline materials with infinite, periodic structures composed of organic ‘linkers’ connected to metal ‘nodes’. Their architectures can be fine-tuned by synthetic design for task-specific purposes: as chemical sensors, MOFs can be designed to interact with specific target analytes. Interest in MOFs as chemical sensors has grown significantly over the last decade, particularly given the increasing number of examples of luminescent lanthanide-based MOFs, and MOFs that display mechanochemical responses to external stimuli. In this chapter, we discuss some of the fundamental properties required to prepare MOFs for chemical sensing. We then present an extensive review of recent research in this area, showing how MOFs have been applied in a wide range of applications, including sensing of anions, cations, small organic molecules, biomolecules, as well as changes in physical conditions such as temperature and pH.

Effective adsorption and enhanced removal of organophosphorus pesticides from aqueous solution by Zr-based MOFs of UiO-67

Though many efforts have been devoted to the adsorptive removal of hazardous materials of organophosphorus pesticides (OPs), it is still highly desirable to develop novel adsorbents with high adsorption capacities. In the current work, the removal of two representative OPs, glyphosate (GP) and glufosinate (GF), was investigated by the exceptionally stable Zr-based MOFs of UiO-67. The abundant Zr-OH groups, resulting from the missing-linker induced terminal hydroxyl groups and the inherent bridging ones in Zr-O clusters of UiO-67 particles, served as natural anchorages for efficient GP and GF capture in relation with their high affinity toward phosphoric groups in OPs. The correlation between the most significant parameters such as contact time, OPs concentration, adsorbent dose, pH, as well as ionic strength with the adsorption capacities was optimized, and the effects of these parameters on the removal efficiency of GP and GF from the polluted aqueous solution were investigated. The adsorption of GP on UiO-67 was faster than that of GF, and a pseudo-second-order rate equation effectively described the uptake kinetics. The Langmuir model exhibited a better fit to adsorption isotherm than the Freundlich model. Thanks to the strong affinity and adequate pore size, the adsorption capacities in UiO-67 approached as high as 3.18 mmol (537 mg) g(-1) for GP and 1.98 mmol (360 mg) g(-1) for GF, which were much higher than those of many other reported adsorbents. The excellent adsorption characteristics of the current adsorbents toward OPs were preserved in a wide pH window and high concentration of the background electrolytes. These prefigured the promising potentials of UiO-67 as novel adsorbent for the efficient removal of OPs from aqueous solution.

Four new lead(ii)–iridium(iii) heterobimetallic coordination frameworks: synthesis, structures, luminescence and oxygen-sensing properties

Crystalline lead(ii)–iridium(iii) heterobimetallic coordination frameworks were prepared and characterized, and one was mixed with C480 to form a ratiometric oxygen sensor for the determination of oxygen in real gas.

Gas sensing using porous materials for automotive applications

The use of zeolites and metal–organic frameworks in the sensing of gases emitted from automobile exhausts is reviewed.

Tuning oxygen-sensing behaviour of a porous coordination framework by a guest fluorophore

A new oxygen-sensing material with desirable excitation/emission characteristics was achieved by using a simple guest fluorophore.

A metal-organic framework containing unusual eight-connected Zr-oxo secondary building units and orthogonal carboxylic acids for ultra-sensitive metal detection

Two metal-organic frameworks (MOFs) with Zr-oxo secondary building units (SBUs) were prepared by using p,p'-terphenyldicarboxylate (TPDC) bridging ligands pre-functionalized with orthogonal succinic acid (MOF-1) and maleic acid groups (MOF-2). Single-crystal X-ray structure analysis of MOF-1 provides the first direct evidence for eight-connected SBUs in UiO-type MOFs. In contrast, MOF-2 contains twelve-connected SBUs as seen in the traditional UiO MOF topology. These structural assignments were confirmed by extended X-ray absorption fine structure (EXAFS) analysis. The highly porous MOF-1 is an excellent fluorescence sensor for metal ions with the detection limit of <0.5 ppb for Mn(2+) and three to four orders of magnitude greater sensitivity for metal ions than previously reported luminescent MOFs.