Using Machine Learning and Data Mining to Leverage Community Knowledge for the Engineering of Stable Metal-Organic Frameworks

Although the tailored metal active sites and porous architectures of MOFs hold great promise for engineering challenges ranging from gas separations to catalysis, a lack of understanding of how to improve their stability limits their use in practice. To overcome this limitation, we extract thousands of published reports of the key aspects of MOF stability necessary for their practical application: the ability to withstand high temperatures without degrading and the capacity to be activated by removal of solvent molecules. From nearly 4000 manuscripts, we use natural language processing and image analysis to obtain over 2000 solvent-removal stability measures and 3000 thermal degradation temperatures. We analyze the relationships between stability properties and the chemical and geometric structures in this set to identify limits of prior heuristics derived from smaller sets of MOFs. By training predictive machine learning (ML, i.e., Gaussian process and artificial neural network) models to encode the structure-property relationships with graph- and pore-structure-based representations, we are able to make predictions of stability orders of magnitude faster than conventional physics-based modeling or experiment. Interpretation of important features in ML models provides insights that we use to identify strategies to engineer increased stability into typically unstable 3d-transition-metal-containing MOFs that are frequently targeted for catalytic applications. We expect our approach to accelerate the time to discovery of stable, practical MOF materials for a wide range of applications.

TEMPO-radical-bearing metal-organic frameworks and covalent organic frameworks for catalytic applications

It is known that 2,2,6,6-tetramethylpiperidinyl-1-oxy (or TEMPO) is a stable, radical-containing molecule, which has been utilized in various areas of organic synthesis, catalysis, polymer chemistry, electrochemical reactions, and materials chemistry. Its unique stability, attributable to its structural features, and molecular tunability allows for the modification of various materials, including the heterogenization of solid materials. Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are porous and tunable because of their ligand or linker portion, and both have been extensively studied for use in catalytic applications. Therefore, synergistically combining the chemistry of TEMPO with the properties of MOFs and COFs is a natural choice and should allow for significant advancements, including improved recyclability and selectivity. This article focuses on TEMPO-bearing MOFs and COFs for use in catalytic applications. In addition, recent strategies related to the use of these functional porous materials in catalytic reactions are also discussed.

Post-synthetic modifications (PSM) on metal-organic frameworks (MOFs) for visible-light-initiated photocatalysis

The utilization of green and sustainable solar energy via photocatalysis is regarded as a promising strategy to tackle the ever-increasing energy shortage and environmental deterioration. In addition to traditional semiconductor-based photocatalysts, metal-organic frameworks (MOFs), a class of crystalline micro-mesoporous hybrid materials constructed from metal or metal nodes interconnected with multi-dentate organic linkers, are emerging as a new type of photocatalytic material. Post-synthetic modifications (PSM) on MOFs, in which chemical transformations or exchanges are made on pre-synthesized MOF materials, are found to be a powerful strategy for fabricating photoactive MOFs based on already existing MOFs. In this frontier article, different PSM strategies for the development of photoactive MOFs, including coordination on unsaturated metal sites, metalation on open coordinated sites, covalent modifications on ligands, ligand exchange, metal exchange and cavity encapsulation, have been summarized. Our views on the challenges and the direction in developing photocatalytic MOFs by PSM are also addressed. We hope that this frontier article can provide some guidance for rational designing of highly efficient MOF-based photocatalysts via PSM strategies and to stimulate more research interest to be devoted to this promising yet largely unexplored field.

From Molecules to Frameworks to Superframework Crystals

Building chemical structures of complexity and functionality approaching the level of biological systems is an ongoing challenge. A general synthetic strategy is proposed by which progressive levels of complexity are achieved through the building block approach whereby molecularly defined constructs at one level serve as constituent units of the next level, all being linked through strong bonds-"augmented reticular chemistry". Specifically, current knowledge of linking metal complexes and organic molecules into reticular frameworks is applied here to linking the crystals of these frameworks into supercrystals (superframeworks). This strategy allows for the molecular control exercised on the molecular regime to be translated into higher augmentation levels to produce systems capable of dynamics and complex functionality far exceeding current materials.

Nanoscale Metal-Organic Frameworks: Recent developments in synthesis, modifications and bioimaging applications

Porous Metal-Organic Frameworks (MOFs) have emerged as eye-catching materials in recent years. They are widely used in numerous fields of chemistry thanks to their desirable properties. MOFs have a key role in the development of bioimaging platforms that are hopefully expected to effectually pave the way for accurate and selective detection and diagnosis of abnormalities. Recently, many types of MOFs have been employed for detection of RNA, DNA, enzyme activity and small-biomolecules, as well as for magnetic resonance imaging (MRI) and computed tomography (CT), which are valuable methods for clinical analysis. The optimal performance of the MOF in the bio-imaging field depends on the core structure, synthesis method and modifications processes. In this review, we have attempted to present crucial parameters for designing and achieving an efficient MOF as bioimaging platforms, and provide a roadmap for researchers in this field. Moreover, the influence of modifications/fractionalizations on MOFs performance has been thoroughly discussed and challenging problems have been extensively addressed. Consideration is mainly focused on the principal concepts and applications that have been achieved to modify and synthesize advanced MOFs for future applications.

Anodic electrosynthesis of MIL-53(Al)-N(CH2PO3H2)2 as a mesoporous catalyst for synthesis of novel (N-methyl-pyrrol)-pyrazolo[3,4-b]pyridines via a cooperative vinylogous anomeric based oxidation

In this paper, the MIL-53(Al)-NH2 metal-organic frameworks (MOFs) was prepared based on the anodic electrosynthesis under green conditions. The anodic electrosynthesis as an environmentally friendly procedure was performed in the aqueous solution, room temperature, atmospheric pressure, and in the short reaction time (30 min). Also, the employed procedure was accomplished without the need for the ex-situ salt and base/probase additives as cation source and ligand activating agent at the constant current mode (10.0 mA cm-2). The electrosynthesized MOFs was functionalized with phosphorus acid tags as a novel mesoporous catalyst. This mesoporous catalyst was successfully employed for synthesis of new series (N-methyl-pyrrol)-pyrazolo[3,4-b]pyridines by one-pot condensation reaction of 3-methyl-1-phenyl-1H-pyrazol-5-amine, 3-(1-methyl-1H-pyrrol-2-yl)-3-oxopropanenitrile and various aromatic aldehydes (mono, bis and tripodal). This catalyst proceeded the organic synthetic reaction via a cooperative vinylogous anomeric based oxidation mechanism with a marginal decreasing its catalytic activity after recycling and reusability.

Pre-combustion gas separation by ZIF-8-polybenzimidazole mixed matrix membranes in the form of hollow fibres-long-term experimental study

Polybenzimidazole (PBI) is a promising and suitable membrane polymer for the separation of the H₂/CO₂ pre-combustion gas mixture due to its high performance in terms of chemical and thermal stability and intrinsic H₂/CO₂ selectivity. However, there is a lack of long-term separation studies with this polymer, particularly when it is conformed as hollow fibre membrane. This work reports the continuous measurement of the H₂/CO₂ separation properties of PBI hollow fibres, prepared as mixed matrix membranes with metal-organic framework (MOF) ZIF-8 as filler. To enhance the scope of the experimental approach, ZIF-8 was synthesized from the transformation of ZIF-L upon up-scaling the MOF synthesis into a 1 kg batch. The effects of membrane healing with poly(dimethylsiloxane), to avoid cracks and non-selective gaps, and operation conditions (use of sweep gas or not) were also examined at 200°C during approximately 51 days. In these conditions, for all the membrane samples studied, the H₂ permeance was in the 22-47 GPU range corresponding to 22-32 H₂/CO₂ selectivity values. Finally, this work continues our previous report on this type of application (Etxeberria-Benavides et al. 2020 Sep. Purif. Technol. 237, 116347 (doi:10.1016/j.seppur.2019.116347)) with important novelties dealing with the use of ZIF-8 for the mixed matrix membrane coming from a green methodology, the long-term gas separation testing for more than 50 days and the study on the membrane operation under more realistic conditions (e.g. without the use of sweep gas).

Facile fabrication of BiOIO3/MIL-88B heterostructured photocatalysts for removal of pollutants under visible light irradiation

In this work, a Z-scheme heterojunction of BiOIO₃/MIL-88B was constructed via two steps solvothermal method. Various characterization techniques showed that this Z-scheme heterojunction is an effective strategy to promote spatial charge separation, and the catalytic performance was evaluated by degrading simulated organic pollutants. Herein, the BiOIO₃/MIL-88B composites exhibited an exceptional removal rate for Reactive Blue 19 and tetracycline hydrochloride (TC) under visible light irradiation, which was approximately 3.28 and 4.22 times higher than the pristine BiOIO₃, respectively. Additionally, the analysis of photocatalysis mechanism showed that the active species O₂^- and OH could strongly affect the degradation of tetracycline hydrochloride (TC) in the studied system. Furthermore, the degradation process of TC was tracked and detected by identifying intermediates produced in the reaction system. It is anticipated that this research can deepen the understanding of BiOIO₃/MIL-88B heterojunction structure to remove organic contaminants and provide a strategy for applying photocatalytic technology in the practical industry.

Post synthetically modified IRMOF-3 for efficient recovery and selective sensing of U(vi) from aqueous medium

A simple and efficient route to develop various novel functionalized MOF materials for rapid and excellent recovery of U(vi) from aqueous medium, along with selective sensing has been demonstrated in the present study. In this connection, a set of four distinct post synthetically modified (PSM) iso-reticular metal organic frameworks were synthesized from IRMOF-3 namely, IRMOF-PC (2-pyridine carboxaldehyde), IRMOF-GA (glutaric anhydride), IRMOF-SMA (sulfamic acid), and IRMOF-DPC (diphenylphosphonic chloride) for the recovery and sensing of U(vi) from aqueous medium. The MOFs were characterized by Fourier transform infrared spectroscopy (FTIR), powder XRD, BET surface area analysis, thermogravimetric analysis (TGA), NMR (13C, 1H and 31P), Scanning Electron Microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). Among all MOFs, post synthetically modified IRMOF-SMA showed enhanced thermal stability of about 420 °C. The MOFs were investigated for U(vi) sorption studies using a batch technique. All the MOFs exhibit excellent sorption capacity towards U(vi) (>90%) and maximum uptake was observed at pH 6. Sorption capacity of MOFs have the following order; IRMOF-3-DPC (300 mg U g-1) > IRMOF-SMA (292 mg U g-1) > IRMOF-PC (289 mg U g-1) > IRMOF-GA (280 mg U g-1) > IRMOF-3 (273 mg U g-1). IRMOF-DPC shows rapid sorption of uranium within 5 min with excellent uptake of U(vi) (>99%). The desorption of U(vi) was examined with different eluents and 0.01 M HNO3 was found to be most effective. The fluorescence sensing studies of U(vi) via IRMOF-3 and its PSM MOFs revealed high sensitivity and selectivity towards U(vi) over other competing rare earth metal ions (La3+, Ce4+, Sm3+, Nd3+, Gd3+, and Eu3+), wherein IRMOF-GA displayed an impressive detection limit of 0.36 mg L-1 for U(vi).

Ultrafast and nanomolar level detection of H2S in aqueous medium using a functionalized UiO-66 metal-organic framework based fluorescent chemosensor

Here, we present a 4-nitrophenyl functionalized Zr-UiO-66 MOF (MOF = metal-organic framework) and its applications towards the selective, sensitive and rapid detection of H₂S both in the aqueous medium and vapour phase. The MOF material was synthesized using the 2-(nitrophenoxy)terepththalic acid (H₂BDC-O-Ph-NO₂) linker and ZrCl₄ salt in the presence of a benzoic acid modulator. It was carefully characterized by thermogravimetric analysis (TGA), elemental analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy and surface area analysis. Noticeable thermal stability up to a temperature of 390 °C under air and the considerable chemical stability in different liquid media (H₂O, 1 M HCl, glacial acetic acid, NaOH in the pH = 8 to 10 range) confirmed the robustness of the MOF. The BET surface area (1040 m² g^-1) indicated the porous nature of the MOF. Remarkable selectivity of the MOF towards H₂S over other potential congeners of H₂S was observed in the aqueous medium. A very high fluorescence increment (∼77 fold) was observed after adding an aqueous Na₂S solution to the MOF suspension. The MOF probe displayed the lowest limit of detection (12.58 nM) among the existing MOF-based chemosensors of H₂S. Furthermore, it exhibited a very quick (60 s) response towards H₂S detection. The MOF compound could also detect H₂S in the vapour phase as well as in real water samples. Furthermore, we developed inexpensive MOF-coated paper strips for the naked-eye sensing of H₂S. A thorough investigation was carried out in order to elucidate the fluorescence turn-on sensing mechanism.

Crystalline Hydrogen-Bonding Networks and Mixed-Metal Framework Materials Enabled by an Electronically Differentiated Heteroditopic Isocyanide/Carboxylate Linker Group

Mixed-metal solid-state framework materials are emerging candidates for advanced applications in catalysis and chemical separations. Traditionally, the syntheses of mixed-metal framework systems rely on postsynthetic ion exchange, metalloligands, or metal-deposition techniques for the incorporation of a second metal within a framework material. However, these methods are often incompatible with the incorporation of low-valent metal centers, which preferentially bind to electronically "soft" ligands according to the tenets of hard/soft acid/base theory. Here we present the electronically differentiated isocyanide/carboxylate heteroditopic linker ligand 1,4-CNAr^Mes2C₆H₄CO₂H (TIB^Mes2H; TIB = terphenyl isocyanide benzoate; Ar^Mes2 = 2,6-(2,4,6-Me₃C₆H₂)₂C₆H₂), which is capable of selective binding of low-valent metals via the isocyano group and complexation of hard Lewis acidic metals through the carboxylate unit. This heteroditopic ligand also possesses an encumbering m-terphenyl backbone at the isocyanide function to foster coordinative unsaturation. The treatment of TIB^Mes2H with [Cu(NCMe)₄]PF₆ in a 3:1 ratio results in preferential binding of the isocyanide group to the Cu(I) center as assayed by multinuclear NMR and IR spectroscopies. IR spectroscopy also provides strong evidence for the formation of a copper(I) tris(isocyanide) complex, wherein the carboxylic acid group remains unperturbed. The addition of TIB^Mes2 to [Cu(NCMe)₄]PF₆ in a 4:1 ratio results in crystallization of the hydrogen-bonding network, [Cu(TIB^Mes2H)₄]PF₆, in which the formation of R₂²(8) hydrogen bonds results in a 7-fold interpenetrated diamondoid lattice structure. The preassembly of a copper(I) tris(isocyanide) complex using TIB^Mes2H, followed by deprotonation and the introduction of ZnCl₂, generates a novel and unusual zwitterionic solid-state phase (denoted as Cu/Zn-^ISOCN-5; ^ISOCN = isocyanide coordination network) consisting of a coordinatively unsaturated [Cu(CNR)₃]⁺ cationic secondary building unit (SBU) and an anionic, paddlewheel-type Zn(II)-based SBU of the formulation [Cl₂Zn₂(O₂CR)₃]^-. Inductively coupled plasma mass spectrometry analysis provided firm evidence for a 2:1 Zn-to-Cu ratio in the network, thereby indicating that the isocyanide and carboxylate groups selectively bind soft and hard Lewis acidic metal centers, respectively. The extended structure of Cu/Zn-^ISOCN-5 is a densely packed, noninterpenetrated AB-stacked layer network with modest surface area. However, it is thermally robust, and its formation and compositional integrity validate the use of an electronically differentiated linker for the formation of mixed-metal frameworks incorporating low-valent metal centers.

Metal-organic frameworks for advanced drug delivery

Metal-organic frameworks (MOFs), comprised of organic ligands and metal ions/metal clusters via coordinative bonds are highly porous, crystalline materials. Their tunable porosity, chemical composition, size and shape, and easy surface functionalization make this large family more and more popular for drug delivery. There is a growing interest over the last decades in the design of engineered MOFs with controlled sizes for a variety of biomedical applications. This article presents an overall review and perspectives of MOFs-based drug delivery systems (DDSs), starting with the MOFs classification adapted for DDSs based on the types of constituting metals and ligands. Then, the synthesis and characterization of MOFs for DDSs are developed, followed by the drug loading strategies, applications, biopharmaceutics and quality control. Importantly, a variety of representative applications of MOFs are detailed from a point of view of applications in pharmaceutics, diseases therapy and advanced DDSs. In particular, the biopharmaceutics and quality control of MOFs-based DDSs are summarized with critical issues to be addressed. Finally, challenges in MOFs development for DDSs are discussed, such as biostability, biosafety, biopharmaceutics and nomenclature.

Introducing High Density of Very Active Sites and Stepwise Postmodification for Tailoring the Porosity of Highly Demanding Cr3+-Based Metal-Organic Frameworks

Chromium(III)-based metal-organic frameworks (Cr-MOFs) are highly robust and porous and have been very attractive in a wide range of investigations. However, the harsh direct synthetic conditions not only impede the synthesis of new Cr-MOFs but also restrict the introduction of functional groups into them. Postsynthetic modification has somewhat alleviated such difficulties; nevertheless, it still suffered from procedures that are tedious and conditions that are not mild, which often result in low concentration of the functional groups introduced. To overcome these shortcomings, here, in this paper, we supplied a new route and prepared a benzyl alcohol functionalized Cr-SXU-2 from the judiciously designed benzyl alcohol functionalized Fe-SXU-2 through solvent-assisted metal metathesis strategy. The functionalized Cr-SXU-2 shows well-preserved crystallinity, porosity, and high chemical stability. The benzyl alcohol group can be converted into a very active benzyl bromide group in an almost quantitative yield and thus for the first time produce the benzyl bromide functionalized MOF, Cr-SXU-2-Br, in which the -Br group can be exchanged by a nucleophilic group. As a proof of concept, -N₃ was introduced and transformed into other active sites via "click reaction" to further tailor the interior of Cr-SXU-2. All these functionalized Cr-MOFs showed improved adsorption performance in contrast to the nonfunctionalized one. This step-by-step postmodification process not only diversifies the functionalization of robust MOFs but also opens a new route to employ many different functional groups in the demanding highly stable Cr-MOF platforms.

NH2-UiO-66 Coated with Two-Dimensional Covalent Organic Frameworks: High Stability and Photocatalytic Activity

The poor stability and low catalytic activity of NH₂-UiO-66 in basic solutions require the reactions to be conducted in acidic solutions, which seriously hinders its potential photocatalytic application. Herein, we report that NH₂-UiO-66 coated with two-dimensional covalent organic frameworks (COFs) via imine bond connection presents not only high photocatalytic activity but also high stability and adaptability to the solution environment. The NH₂-UiO-66/COF hybrid material was fabricated through the Schiff base reaction of NH₂-UiO-66 with 4,4',4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT) and 2,4,6-triformylphloroglucinol (TP). The hybrid material showed high stability in an alkaline environment, with only 4.7% of NH₂-UiO-66 decomposed after the photocatalytic reaction. The optimum photocatalytic H₂ evolution rate was 8.44 mmol·h^-1·g^-1 when triethanolamine was used as an electron-donating agent. The results presented here illustrate the possibility for effectively improving both the photocatalytic performance and stability of NH₂-UiO-66 by coupling with COFs.

Heterogeneous postassembly modification of zirconium metal-organic cages in supramolecular frameworks

We report a heterogeneous postassembly modification (PAM) to synthesize a zirconium metal-organic cage decorated with acrylate functional groups, ZrT-1-AA, which cannot be synthesized by direct coordination-driven self-assembly owing to the reactivity and instability of the ligand. The PAM process is carried out in a single-crystal-to-single-crystal transformation under mild reaction conditions with high efficiency, which is confirmed by ESI-TOF-MS and 1H NMR. In addition, ZrT-1-AA is crosslinked into shaped materials to demonstrate its potential applications. The proposed PAM strategy sheds light on the development of Zr-MOCs decorated with reactive functional groups, whose introduction is challenging or impossible via direct self-assembly.

Breast Cancer Targeting of a Drug Delivery System through Postsynthetic Modification of Curcumin@N3-bio-MOF-100 via Click Chemistry

Metal-organic frameworks (MOFs) offer many opportunities for applications across biology and medicine. Their wide range of chemical composition makes toxicologically acceptable formulation possible, and their high level of functionality enables possible applications as delivery systems for therapeutics agents. Surface modifications have been used in drug delivery systems to minimize their interaction with the bulk, improving their specificity as targeted carriers. Herein, we discuss a strategy to achieve a tumor-targeting drug-loaded MOF using "click" chemistry to anchor functional folic acid (FA) molecules on the surface of N₃-bio-MOF-100. Using curcumin (CCM) as an anticancer drug, we observed drug loading encapsulation efficiencies (DLEs) of 24.02 and 25.64% after soaking N₃-bio-MOF-100 in CCM solutions for 1 day and 3 days, respectively. The success of postsynthetic modification of FA was confirmed by ¹H NMR spectroscopy, Fourier transform infrared spectroscopy (FTIR), and liquid chromatography-mass spectrometry (LC-MS). The stimuli-responsive drug release studies demonstrated an increase of CCM released under acidic microenvironments. Moreover, the cell viability assay was performed on the 4T1 (breast cancer) cell line in the presence of CCM@N₃-bio-MOF-100 and CCM@N₃-bio-MOF-100/FA carriers to confirm its biological compatibility. In addition, a cellular uptake study was conducted to evaluate the targeting of tumor cells.

Single molecule fluorescence imaging of nanoconfinement in porous materials

This review covers recent progress in using single molecule fluorescence microscopy imaging to understand the nanoconfinement in porous materials. The single molecule approach unveils the static and dynamic heterogeneities from seemingly equal molecules by removing the ensemble averaging effect. Physicochemical processes including mass transport, surface adsorption/desorption, and chemical conversions within the confined space inside porous materials have been studied at nanometer spatial resolution, at the single nanopore level, with millisecond temporal resolution, and under real chemical reaction conditions. Understanding these physicochemical processes provides the ability to quantitatively measure the inhomogeneities of nanoconfinement effects from the confining properties, including morphologies, spatial arrangement, and trapping domains. Prospects and limitations of current single molecule imaging studies on nanoconfinement are also discussed.

Efficient Conversion of Biomass-Derived Levulinic Acid to γ-Valerolactone over Polyoxometalate@Zr-Based Metal-Organic Frameworks: The Synergistic Effect of Bro̷nsted and Lewis Acidic Sites

Catalytic transformation of levulinic acid (LA) to γ-valerolactone (γ-GVL) is an important route for biomass upgradation. Because both Bro̷nsted and Lewis acidic sites are required in the cascade reaction, herein we fabricate a series of H₃PW₁₂O₄₀@Zr-based metal-organic framework (HPW@MOF-808) by a facile impregnation method. The synthesized HPW@MOF-808 is active for the conversion of LA to γ-GVL using isopropanol as a hydrogen donor. Interestingly, with the increase in the HPW loading amount, the yield of γ-GVL increases first and then decreases, and 14%-HPW@MOF-808 gave the highest γ-GVL yield (86%). The excellent catalytic performance was ascribed to the synergistic effect between the accessible Lewis acidic Zr⁴⁺ sites in MOF-808 and Bro̷nsted acidic HPW sites. Based on the experimental results, a plausible reaction mechanism was proposed: the Zr⁴⁺ sites catalyze the transfer hydrogenation of carbonyl groups and the HPW clusters promote the esterification of LA with isopropanol and lactonization to afford γ-GVL. Moreover, HPW@MOF-808 is resistant to leaching and can be reused for five cycles without significant loss of its catalytic activity.

Understanding disorder and linker deficiency in porphyrinic zirconium-based metal-organic frameworks by resolving the Zr8O6 cluster conundrum in PCN-221

Porphyrin-based metal–organic frameworks (MOFs), exemplified by MOF-525, PCN-221, and PCN-224, are promising systems for catalysis, optoelectronics, and solar energy conversion. However, subtle differences between synthetic protocols for these three MOFs give rise to vast discrepancies in purported product outcomes and description of framework topologies. Here, based on a comprehensive synthetic and structural analysis spanning local and long-range length scales, we show that PCN-221 consists of Zr₆O₄(OH)₄ clusters in four distinct orientations within the unit cell, rather than Zr₈O₆ clusters as originally published, and linker vacancies at levels of around 50%, which may form in a locally correlated manner. We propose disordered PCN-224 (dPCN-224) as a unified model to understand PCN-221, MOF-525, and PCN-224 by varying the degree of orientational cluster disorder, linker conformation and vacancies, and cluster–linker binding. Our work thus introduces a new perspective on network topology and disorder in Zr-MOFs and pinpoints the structural variables that direct their functional properties.

Zirconium-based metal–organic frameworks have defective structures that are useful in catalysis and gas storage. Here, the authors study the interplay between cluster disorder and linker vacancies in PCN-221 and propose a new structure model with tilted Zr₆O₄(OH)₄ clusters rather than Zr₈O₆ clusters.

A bi-functionalized metal-organic framework based on N-methylation and Eu3+ post-synthetic modification for highly sensitive detection of 4-Aminophenol (4-AP), a biomarker for aniline in urine

4-Aminophenol (4-AP), which is a biomarker of aniline and represents the internal dose of aniline exposure in the human body, has attracted much attention for its detection in recent years. In this work, a bi-functionalized luminescent metal-organic framework (MOF), Eu@MOF-253-CH_3, is designed and prepared through encapsulating the methyl groups and the Eu³⁺ cations into MOF-253 based on post-synthetic modification strategy. This study shows that the bi-functionalized Eu@MOF-253-CH₃ can specifically recognize 4-AP upon luminescence quenching, while refraining from the interference of other coexisting species in urine. The Eu@MOF-253-CH₃ hybrid as a 4-AP sensor also displays excellent performances including high water tolerance, good pH-independent stability, fast response, great selectivity and elevated sensitivity (0.5 μg mL^-1) attributed to N-viologenized ligand. These results suggest the bi-functionalized Eu@MOF-253-CH₃ can act as a promising sensor to practically monitor 4-AP's concentrations in human urine system, and then to realize the screening and pre-diagnosis of human health. Moreover, the possible sensing mechanisms are further explored at length.

Water-stable lanthanide-organic macrocycles from a 1,2,4-triazole-based chelate for enantiomeric excess detection and pesticide sensing

Water-stable anionic Ln2L2-type (Ln = LaIII and EuIII) lanthanide-organic macrocycles have been constructed by deprotonation self-assembly of a bis-tridentate ligand consisting of two 2,6-bis-(1,2,4-triazole)-pyridine chelation arms bridged by a dibenzofuran chromophore, of which the luminescent Eu2L2 macrocycle can be used for enantiomeric excess (ee) detection toward pybox-type chiral ligands and selective colorimetric sensing of omethoate (OMA) in water.

Ultrafast removal of ppb levels of Hg(II) and volatile Hg(0) using post modified metal organic framework

A novel MOF based adsorbent was prepared by functionalization of MIL 88A with mercapto ethanol to yield MIL88A-SH and evaluated for the removal of Hg(II) in water and Hg(0) in air. The prepared MOFs were characterized by field emission scanning electron microscope (FESEM), Transmission electron microscopy (TEM), Brunauer- Emmett-Teller (BET), Fourier transform infrared spectroscopy (FTIR), and zeta X-ray photoelectron microscopy (XPS). The reaction rate was found to be very fast and within 15 min 95.5% Hg(II) was removed. The kinetics data followed pseudo second order model with rate constant values at 1.19 and 2.38 g/μg/min for MIl88A and MIL88A-SH respectively. A very high adsorption capacity in the order 1111.1 mg/g of Hg(II) was found using MIL88A-SH as adsorbent. The uptake was found to be constant in a wide range of pH from 5 to 9. Furthermore, in the presence other interfering metal ions, viz., Cu(II), As(V), Cd(II), Cr(VI), Pb(II), Zn(II), MIL88A-SH demonstrated an excellent adsorption for Hg(II). Around 45.6 mg/g of Hg(0) was found to be adsorbed by MIL88A-SH. XPS, FTIR and XRD studies suggested insitu oxidation Hg(0) to Hg(II) and complexation of Hg(II) with thiol groups during adsorption. Applicability on removal of Hg(II) at ppb levels from drinking water, fast kinetics, wide pH range, a very high sorption capacity, Hg(0) removal, selectivity and recyclability makes MIL88A-SH an efficient adsorbent to tackle mercury contamination.

Detailed total scattering analysis of disorder in ZIF-8

This work investigates the X-ray scattering signatures of disorder in the zeolitic imidazolate framework ZIF-8. Two layer disorder models are examined in reciprocal space and compared with conventional Rietveld analysis. Stacking faults along the [001] direction of the cubic lattice are in poor agreement with experimental powder diffraction data, consistent with previously reported density functional theory studies showing that these defects are energetically unfavorable compared with amorphization. Meanwhile, fluctuation of layer position along the [110] direction of the cubic lattice shows a significant agreement with experimental data. This result is interpreted analogously to an anisotropic strain mechanism, suggesting links between elastic anisotropy and crystallographic imperfections found in metal–organic framework materials. In direct space, it is demonstrated that models accounting for the static position disorder amongst the linker and metal sublattices are required to fit the experimental pair distribution function data.

Stimulus-cleavable chemistry in the field of controlled drug delivery

Stimulus-cleavable nanoscale drug delivery systems are receiving significant attention owing to their capability of achieving exquisite control over drug release via the exposure to specific stimuli. Central to the construction of such systems is the integration of cleavable linkers showing susceptibility to one stimulus or several stimuli with drugs, prodrugs or fluorogenic probes on the one hand, and nanocarriers on the other hand. This review summarises recent advances in stimulus-cleavable linkers from various research areas and the corresponding mechanisms of linker cleavage and biological applications. The feasibility of extending their applications to the majority of nanoscale drug carriers including nanomaterials, polymers and antibodies are further highlighted and discussed. This review also provides general design guidelines to incorporate stimulus-cleavable linkers into nanocarrier-based drug delivery systems, which will hopefully spark new ideas and applications.

Molecular Scalpel to Chemically Cleave Metal-Organic Frameworks for Induced Phase Transition

A bottom-up chemical synthesis of metal-organic frameworks (MOFs) permits significant structural diversity because of various combinations of metal centers and different organic linkers. However, fabrication generally complies with the classic hard and soft acids and bases (HSAB) theory. This restricts direct synthesis of desired MOFs with converse Lewis type of metal ions and ligands. Here we present a top-down strategy to break this limitation via the structural cleavage of MOFs to trigger a phase transition using a novel "molecular scalpel". A conventional CuBDC MOF (BDC = 1,4-benzenedicarboxylate) prepared from a hard acid (Cu²⁺) metal and a hard base ligand was chemically cleaved by l-ascorbic acid acting as chemical scalpel to fabricate a new Cu₂BDC structure composed of a soft acid (Cu¹⁺) and a hard base (BDC). Controlled phase transition was achieved by a series of redox steps to regulate the chemical state and coordination number of Cu ions, resulting in a significant change in chemical composition and catalytic activity. Mechanistic insights into structural cleavage and rearrangement are elaborated in detail. We show this novel strategy can be extended to general Cu-based MOFs and supramolecules for nanoscopic casting of unique architectures from existing ones.

Hybrid ZIF-8-90 for Selective Solid-Phase Microextraction of Exhaled Breath from Gastric Cancer Patients

Metal-organic frameworks (MOFs) are a new kind of microporous materials whose unique properties make them promising as coatings for solid phase microextraction (SPME). However, previous MOF coatings for SPME exclusively focus on single-linker MOFs, and the selective enrichment of polar or nonpolar targets depends on the polarity of linker on the surface of MOFs, which greatly limits the application of MOF coating for SPME in real samples. Here, we report a hybrid MOF-coated stainless steel fiber for SPME of biomarkers in exhaled breath from gastric cancer patients. Zeolitic imidazolate framework-8-90 (ZIF-8-90) possesses the aldehyde groups and methyl groups in the framework as a model MOF, and eight biomarkers (ethanol, acetone, hexanal, hexanol, nonane, isoprene, heptane, and decane) were used as the target analytes. The ZIF-8-90-coated fiber shows high enrichment efficiency for hydrophilic targets and hydrophobic targets, wide linearity (three orders of magnitude), and low detection limits (0.82-2.64 μg L^-1). The ZIF-8-90-coated fiber exhibited higher enrichment performance for all the investigated analytes as a result of the synergy of methyl and aldehyde groups, the porous structure, and the suitable pore size of ZIF-8-90 (4-5 Å). The relative standard deviation (RSD) of six repetitions for extractions using the same ZIF-8-90-coated fiber ranged from 2.5 to 7.3%. The reproducibility between the three fibers prepared in parallel varied in the range of 4.8-12% (RSD). The fabricated ZIF-8-90-coated fiber lasted for at least 120 cycles of extraction/desorption/conditioning without an obvious reduction in extraction efficiency and precision. Finally, the developed ZIF-8-90-coated SPME fiber has been successfully used for the analysis of exhaled breath samples from gastric patients with satisfied recoveries (88-106%).

The Application of Nanomaterials for the Electrochemical Detection of Antibiotics: A Review

Antibiotics can accumulate through food metabolism in the human body which may have a significant effect on human safety and health. It is therefore highly beneficial to establish easy and sensitive approaches for rapid assessment of antibiotic amounts. In the development of next-generation biosensors, nanomaterials (NMs) with outstanding thermal, mechanical, optical, and electrical properties have been identified as one of the most hopeful materials for opening new gates. This study discusses the latest developments in the identification of antibiotics by nanomaterial-constructed biosensors. The construction of biosensors for electrochemical signal-transducing mechanisms has been utilized in various types of nanomaterials, including quantum dots (QDs), metal-organic frameworks (MOFs), magnetic nanoparticles (NPs), metal nanomaterials, and carbon nanomaterials. To provide an outline for future study directions, the existing problems and future opportunities in this area are also included. The current review, therefore, summarizes an in-depth assessment of the nanostructured electrochemical sensing method for residues of antibiotics in different systems.