A minireview on covalent organic frameworks as stationary phases in chromatography.

Advances in the design of novel porous materials open new avenues for the development of chromatographic solid stationary phases. Covalent organic frameworks (COFs) are promising candidates in this context due to their remarkable structural versatility and exceptional chemical and textural properties. In this minireview, we summarize the main strategies followed in recent years to apply these materials as stationary phases for chromatographic separations. We also comment on the perspectives of this new research field and potential directions to expand the applicability and implementation of COF stationary phases in analytical systems.

Free PCR virus detection via few-layer bismuthene and tetrahedral DNA nanostructured assemblies.

In this work we describe a highly sensitive method based on a biocatalyzed electrochemiluminescence approach. The system combines, for the first time, the use of few-layer bismuthene (FLB) as a platform for the oriented immobilization of tetrahedral DNA nanostructures (TDNs) specifically designed and synthetized to detect a specific SARS-CoV-2 gene sequence. In one of its vertices, these TDNs contain a DNA capture probe of the open reading frame 1 ab (ORF1ab) of the virus, available for the biorecognition of the target DNA/RNA. At the other three vertices, there are thiol groups that enable the stable anchoring/binding to the FLB surface. This novel geometry/approach enables not only the binding of the TDNs to surfaces, but also the orientation of the capture probe in a direction normal to the bismuthine surface so that it is readily accessible for binding/recognition of the specific SARS-CoV-2 sequence. The analytical signal is based on the anodic electrochemiluminescence (ECL) intensity of luminol which, in turn, arises as a result of the reaction with H2O2, generated by the enzymatic reaction of glucose oxidation, catalyzed by the biocatalytic label avidin-glucose oxidase conjugate (Av-GOx), which acts as co-reactant in the electrochemiluminescent reaction. The method exhibits a limit of detection (LOD) of 4.31 aM and a wide linear range from 14.4 aM to 1.00 µM, and its applicability was confirmed by detecting SARS-CoV-2 in nasopharyngeal samples from COVID-19 patients without the need of any amplification process.

Scalable Synthesis and Electrocatalytic Performance of Highly Fluorinated Covalent Organic Frameworks for Oxygen Reduction.

In this study, we present a novel approach for the synthesis of covalent organic frameworks (COFs) that overcomes the common limitations of non-scalable solvothermal procedures. Our method allows for the room-temperature and scalable synthesis of a highly fluorinated DFTAPB-TFTA-COF, which exhibits intrinsic hydrophobicity. We used DFT-based calculations to elucidate the role of the fluorine atoms in enhancing the crystallinity of the material through corrugation effects, resulting in maximized interlayer interactions, as disclosed both from PXRD structural resolution and theoretical simulations. We further investigated the electrocatalytic properties of this material towards the oxygen reduction reaction (ORR). Our results show that the fluorinated COF produces hydrogen peroxide selectively with low overpotential (0.062 V) and high turnover frequency (0.0757 s-1 ) without the addition of any conductive additives. These values are among the best reported for non-pyrolyzed and metal-free electrocatalysts. Finally, we employed DFT-based calculations to analyse the reaction mechanism, highlighting the crucial role of the fluorine atom in the active site assembly. Our findings shed light on the potential of fluorinated COFs as promising electrocatalysts for the ORR, as well as their potential applications in other fields.

The Methylene Spacer Matters: The Structural and Luminescent Effects of Positional Isomerism of n-Methylpyridyltriazole Carboxylate Semi-Rigid Ligands in the Structure of Zn(II) Based Coordination Polymers.

Two Zn(II) coordination polymers (CPs) based on n-methylpyridyltriazole carboxylate semi-rigid organic ligands (n-MPTC), with n = 3 (L1) and 4 (L2), have been prepared at the water n-butanol interphase by reacting Zn(NO3)2·4H2O with NaL1 and NaL2. This allows us to systematically investigate the influence of the isomeric positional effect on their structures. The organic ligands were obtained by saponification from their respective ester precursors ethyl-5-methyl-1-(pyridin-3-ylmethyl)-1H-1,2,3-triazole-4-carboxylate (P1) and ethyl-5-methyl-1-(pyridin-4-ylmethyl)-1H-1,2,3-triazole-4-carboxylate (P2), resulting in their corresponding sodium salt forms, 3-MPTC, and 4-MPTC. The structure of the Zn(II) CPs determined by single-crystal X-ray diffraction reveals that both CPs have 2D supramolecular hydrogen bond networks. The 2D supramolecular network of [Zn(L1)]n (1) is built up by hydrogen bond interactions between oxygen and hydrogen atoms between neighboring n-methylpyridyltriazole molecules, whereas in [Zn(L2)·4H2O]n (2) the water molecules link 1D polymeric chains forming a 2D supramolecular aggregate. The structures of 1 and 2 clearly show that the isomeric effect in the semi-rigid ligands plays a vital role in constructing the Zn(II) coordination polymers, helped by the presence of the methylene spacer group, in the final structural conformation. The structures of 1 and 2 significantly affect their luminescent properties. Thus, while 2 shows strong emission at room temperature centered at 367 nm, the emission of 1 is quenched substantially.

Alpha-Germanium Nanolayers for High-Performance Li-ion Batteries.

The exfoliation of tridimensional crystal structures has recently been considered a new source of bidimensional materials. The new approach offers the possibility of dramatically enlarging the library of bidimensional materials, but the number of nanolayers produced so far is still limited. Here, we report for the first time the use of a new type of material, α-germanium nanolayers (2D α-Ge). The 2D α-Ge is obtained by exfoliating crystals of α-germanium in a simple one-step procedure assisted by wet ball-milling (gram-scale fabrication). The α-germanium nanolayers have been tested as anode material for high-performance LIBs. The results show excellent performance in semi-cell configuration with a high specific capacity of 1630 mAh g-1 for mass loading of 1 mg cm-2 at 0.1 C. The semi-cell was characterized by a constant current rate of 0.5 C during 400 cycles and different scan rates (0.1 C, 0.5 C, and 1 C). Interestingly, the structural characterization, including Raman spectroscopy, XRPD, and XPS, concludes that 2D α-Ge largely retains its crystallinity after continuous cycling. These results can be used to potentially apply these novel 2D germanium nanolayers to high-performance Li-ion batteries.

Iron Oxyhydroxide-Covalent Organic Framework Nanocomposite for Efficient As(III) Removal in Water.

The presence of heavy metal ions in water is an environmental issue derived mainly from industrial and mineral contamination. Metal ions such as Cd(II), Pb(II), Hg(II), or As(III) are a significant health concern worldwide because of their high toxicity, mobility, and persistence. Covalent organic frameworks (COFs) are an emerging class of crystalline organic porous materials that exhibit very interesting properties such as chemical stability, tailored design, and low density. COFs also allow the formation of composites with remarkable features because of the synergistic combination effect of their components. These characteristics make them suitable for various applications, among which water remediation is highly relevant. Herein, we present a novel nanocomposite of iron oxyhydroxide@COF (FeOOH@Tz-COF) in which lepidocrocite (γ-FeOOH) nanorods are embedded in between the COF nanoparticles favoring As(III) remediation in water. The results show a remarkable 98.4% As(III) uptake capacity in a few minutes and impressive removal efficiency in a wide pH range (pH 5-11). The chemical stability of the material in the working pH range and the capability of capturing other toxic heavy metals such as Pb(II) and Hg(II) without interference confirm the potential of FeOOH@Tz-COF as an effective adsorbent for water remediation even under harsh conditions.

Electrochemical Double-Layer Capacitor based on Carbon@ Covalent Organic Framework Aerogels.

High energy demand results in comprehensive research of novel materials for energy sources and storage applications. Covalent organic frameworks (COFs) possess appropriate features such as long-range order, permanent porosity, tunable pore size, and ion diffusion pathways to be competitive electrode materials. Herein, we present a deep electrochemical study of two COF-aerogels shaped into flexible COF-electrodes (ECOFs) by a simple compression method to fabricate an electrochemical double-layer capacitor (EDLC). This energy storage system has considerable interest owing to its high-power density and long cycle life compared with batteries. Our result confirmed the outstanding behavior of ECOFs as EDLC devices with a capacity retention of almost 100 % after 10 000 charge/discharge cycles and, to our knowledge, the highest areal capacitance (9.55 mF cm-2 ) in aqueous electrolytes at higher scan rates (1000 mV s-1 ) for COFs. More importantly, the hierarchical porosity observed in the ECOFs increases ion transport, which permits a fast interface polarization (low τ0 values). The complete sheds light on using ECOFs as novel electrode material to fabricate EDLC devices.

Revisiting Vibrational Spectroscopy to Tackle the Chemistry of Zr6O8 Metal-Organic Framework Nodes.

The metal-organic framework MOF-808 contains Zr6O8 nodes with a high density of vacancy sites, which can incorporate carboxylate-containing functional groups to tune chemical reactivity. Although the postsynthetic methods to modify the chemistry of the Zr6O8 nodes in MOFs are well known, tackling these alterations from a structural perspective is still a challenge. We have combined infrared spectroscopy experiments and first-principles calculations to identify the presence of node vacancies accessible for chemical modifications within the MOF-808. We demonstrate the potential of our approach to assess the decoration of MOF-808 nodes with different catechol-benzoate ligands. Furthermore, we have applied advanced synchrotron characterization tools, such as pair distribution function analyses and X-ray absorption spectroscopy, to resolve the atomic structure of single metal sites incorporated into the catechol groups postsynthetically. Finally, we demonstrate the catalytic activity of these MOF-808 materials decorated with single copper sites for 1,3-dipolar cycloadditions.

Imine-Linked Covalent Organic Framework with a Naphthalene Moiety as a Sensitive Phosphate Ion Sensing.

Due to the excellent ion-sensing potential of covalent organic frameworks (COFs), the new imine-linked conjugated COF (IC-COF) is synthesized through a water-based synthesis reaction between 1,5-diaminonaphthalene and 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine to create a luminescence sensor. It is noteworthy that the green synthesized IC-COF shows excellent selectivity to phosphate ions (PO43-) with a detection limit of 0.61 µM. The recyclability performance of IC-COF is high, indicating that it can be reused without a significant reduction in performance (5.2% decline after 5 cycles). Theoretical calculations using the density functional theory are performed on the IC-COF-PO43- and IC-COF-Cu+ complexes to explore the sensing mechanism. The fluorescence quenching in the presence of PO43- ions is attributed to the difference between PO43- binding sites to the IC-COF compared to Cu+, which leads to the considerable change in the IC-COF absorption spectrum from 400 to 600 nm.

A Nanostructured Cu(II) Coordination Polymer Based on Alanine as a Trifunctional Mimic Enzyme and Efficient Composite in the Detection of Sphingobacteria.

This research raises the potential use of coordination polymers as new useful materials in two essential research fields, allowing the obtaining of a new multiartificial enzyme with the capacity to inhibit the growth of bacteria resistance. The fine selection of the ligands allows the design of a new 2D coordination polymer (CP), with the formula [Cu2(IBA)2(OH2)4]n·6nH2O, by the combination of Cu (II) as the metal center with a pseudoamino acid (H2IBA = isophthaloyl bis ß-alanine). Quantitative total X-ray fluorescence (TXRF) analyses show that the obtained CP can gradually release Cu (II) ions. Additionally, this CP can be nanoprocessed and transformed into a metal-organic gel (MOG) by using different Cu (II) salt concentrations and the application of ultrasounds. Considering its nanometric dimensions, the slow Cu (II) release and its simple processability, its performance as an artificial enzyme, and its antibacterial ability were explored. The results obtained show the first nanocoordination polymer acting as an artificial multienzyme (peroxidase, catalase, and superoxodismutase) exhibiting antibacterial activity in the presence of hydrogen peroxide, with selective behavior for three bacterium strains (S. spiritovirum, A. faecales, and B. cereus). Indeed, this CP shows a more robust inhibition capacity for Sphingobacterium. Going beyond that, as there are no comfortable and practically clinical tests capable of detecting the presence of Sphingobacteria, the compound can be easily embedded to form moldable gelatin that will facilitate the handling and low-cost commercial kits.

Ultralarge Free-Standing Imine-Based Covalent Organic Framework Membranes Fabricated via Compression.

Demand continues for processing methods to shape covalent organic frameworks (COFs) into macroscopic objects that are needed for their practical applications. Herein, a simple compression method to prepare large-scale, free-standing homogeneous and porous imine-based COF-membranes with dimensions in the centimeter range and excellent mechanical properties is reported. This method entails the compression of imine-based COF-aerogels, which undergo a morphological change from an elastic to plastic material. The COF-membranes fabricated upon compression show good performances for the separation of gas mixtures of industrial interest, N2 /CO2 and CH4 /CO2 . It is believed that the new procedure paves the way to a broader range of COF-membranes.

Inorganic Materials and Metal-Organic Frameworks: Editorial Announcement.

Dear Readers, [...].

Layered Copper-Metallated Covalent Organic Frameworks for Huisgen Reactions.

Covalent organic frameworks (COFs) are porous materials formed through condensation reactions of organic molecules via the formation of dynamic covalent bonds. Among COFs, those based on imine and ß-ketoenamine linkages offer an excellent platform for binding metallic species such as copper to design efficient heterogeneous catalysts. In this work, imine- and ß-ketoenamine-based COF materials were modified with catalytic copper sites following a metallation method, which favored the formation of binding amine defects. The obtained copper-metallated COF materials were tested as heterogeneous catalysts for 1,3-dipolar cycloaddition reactions, resulting in high yields and recyclability.

Rational Design of Copper(II)-Uracil Nanoprocessed Coordination Polymers to Improve Their Cytotoxic Activity in Biological Media.

This work is focused on the rational structural design of two isostructural Cu(II) nano-coordination polymers (NCPs) with uracil-1-acetic acid (UAcOH) (CP1n) and 5-fluorouracil-1-acetic acid (CP2n). Suitable single crystals for X-ray diffraction studies of CP1 and CP2 were prepared under hydrothermal conditions, enabling their structural determination as 1D-CP ladder-like polymeric structures. The control of the synthetic parameters allows their processability into water colloids based on nanoplates (CP1n and CP2n). These NCPs are stable in water at physiological pHs for long periods. However, interestingly, CP1n is chemically altered in culture media. These transformations provoke the partial release of its building blocks and the formation of new species, such as [Cu(UAcO)2(H2O)4]·2H2O (Cu(II)-complex), and species corresponding to the partial reduction of the Cu(II) centers. The cytotoxic studies of CP1n versus human pancreatic adenocarcinoma and human uveal melanoma cells show that CP1n produces a decrease in the cell viability, while their UAcOH and Cu(II)-complex are not cytotoxic under similar conditions. The copper reduction species detected in the hydrolysis of CP1n are closely related to the formation of the reactive oxygen species (ROS) detected in the cytotoxic studies. These results prompted us to prepare CP2n that was designed to improve the cytotoxicity by the substitution of UAcO by 5-FUAcO, taking into account the anticancer activity of the 5-fluorouracil moiety. The new CP2n has a similar behavior to CP1n both in water and in biological media. However, its subtle structural differences are vital in improving its cytotoxic activity. Indeed, the release during the hydrolysis of species containing the 5-fluorouracil moiety provokes a remarkable increase in cellular toxicity and a significant increase in ROS species formation.

A Perspective on the Application of Covalent Organic Frameworks for Detection and Water Treatment.

Global population growth and water resource scarcity are significant social problems currently being studied by many researchers focusing on finding new materials for water treatment. The aim is to obtain quality water suitable for drinking and industrial consumption. In this sense, an emergent class of crystalline porous materials known as Covalent-Organic Frameworks (COFs) offers a wide range of possibilities since their structures can be designed on demand for specific applications. Indeed, in the last decade, many efforts have been made for their use in water treatment. This perspective article aims to overview the state-of-the-art COFs collecting the most recent results in the field for water detection of pollutants and water treatment. After the introduction, where we overview the classical design strategies on COF design and synthesis for obtaining chemically stable COFs, we summarize the different experimental methodologies used for COFs processing in the form of supported and free-standing membranes and colloids. Finally, we describe the use of COFs in processes involving the detection of pollutants in water and wastewater treatment, such as the capture of organic compounds, heavy metals, and dyes, the degradation of organic pollutants, as well as in desalination processes. Finally, we provide a perspective on the field and the potential technological use of these novel materials.

Synergistic Doping and Surface Decoration of Carbon Nitride Macrostructures by Single Crystal Design.

Tailored design of hybrid carbon nitride (CN) materials is quite challenging because of the drawbacks of the solid-state reaction, and the utilization of single crystals containing C-N monomers as reactants for the high-temperature reaction has been proven to imprint a given chemical composition, morphology, or electronic structure. We report the one-pot synthesis of alkali-containing CN macrostructures with ionic crystals on its surface by utilizing a tailored melamine-hydrochloride-based molecular single crystal containing NaCl and KCl as reactants. Structural and optical investigations reveal that upon calcination, molecular doping with Na+ and K+ is achieved, and additionally, the ionic species remain on the surface of the materials, resulting in an enhanced H2 evolution performance through water splitting owing to a high ionic strength of the reaction media. Additionally, the most stable configuration of the alkaline metals in the CN lattice is evaluated by DFT calculations. This work provides an approach for the rational design of CN and other related metal-free materials with controllable properties for energy-related applications and devices.

Macroscopic Ultralight Aerogel Monoliths of Imine-based Covalent Organic Frameworks.

The use of covalent organic frameworks (COFs) in practical applications demands shaping them into macroscopic objects, which remains challenging. Herein, we report a simple three-step method to produce COF aerogels, based on sol-gel transition, solvent-exchange, and supercritical CO2 drying, in which 2D imine-based COF sheets link together to form hierarchical porous structures. The resultant COF aerogel monoliths have extremely low densities (ca. 0.02 g cm-3 ), high porosity (total porosity values of ca. 99 %), and mechanically behave as elastic materials under a moderate strain (<25-35 %) but become plastic under greater strain. Moreover, these COF aerogels maintain the micro- and meso-porosity of their constituent COFs, and show excellent absorption capacity (e.g. toluene uptake: 32 g g-1 ), with high removal efficiency (ca. 99 %). The same three-step method can be used to create functional composites of these COF aerogels with nanomaterials.

Copper(i)-iodide cluster structures as functional and processable platform materials.

The combination of the copper(i)-iodide entity with organic ligands gives rise to a large variety of CuII polynuclear structures in the form of molecular complexes or extended structures. An appropriate selection of these components allows the preparation of materials showing interesting physicochemical properties and potential applications, mainly focused on organic light-emitting diodes and optical sensors. The most prominent physical feature of these materials is their emission, which can be modulated using the chemical structure and composition. This review article collates the advances in this research field, rationalizing the information into two main blocks as a function of the dimensionality of the structures: molecular complexes and extended networks. We describe the most typical ways of preparation, structures, and properties, with particular attention to the processability of the material as a fundamental aspect of the integration of the materials into real devices. Therefore, we aim to integrate the basic elements of the coordination chemistry of CuII clusters from the materials science perspective to envision this promising research field's potential technological future.

Exfoliation of Alpha-Germanium: A Covalent Diamond-Like Structure.

2D materials have opened a new field in materials science with outstanding scientific and technological impact. A largely explored route for the preparation of 2D materials is the exfoliation of layered crystals with weak forces between their layers. However, its application to covalent crystals remains elusive. Herein, a further step is taken by introducing the exfoliation of germanium, a narrow-bandgap semiconductor presenting a 3D diamond-like structure with strong covalent bonds. Pure α-germanium is exfoliated following a simple one-step procedure assisted by wet ball-milling, allowing gram-scale fabrication of high-quality layers with large lateral dimensions and nanometer thicknesses. The generated flakes are thoroughly characterized by different techniques, giving evidence that the new 2D material exhibits bandgaps that depend on both the crystallographic direction and the number of layers. Besides potential technological applications, this work is also of interest for the search of 2D materials with new properties.

Crystallization Induced Enhanced Emission in Two New Zn(II) and Cd(II) Supramolecular Coordination Complexes with the 1-(3,4-Dimethylphenyl)-5-Methyl-1H-1,2,3-Triazole-4-Carboxylate Ligand.

Two new d10 metal supramolecular metal-organic frameworks (SMOFs) with general formula [ML2(H2O)2]n (M = Zn, Cd) have been synthetized using the sodium salt of the anionic 1-(3,4-dimethylphenyl)-5-methyl-1H-1,2,3-triazole-4-carboxylate ligand (Na+L-). Both SMOFs have been structurally characterized by single-crystal X-ray diffraction analysis and IR spectroscopy. The compounds are isostructural and form supramolecular aggregates via hydrogen bonds with the presence of less common dihydrogen bonds. Interestingly, they show ionic conductivity and porosity. The luminescent properties have been also studied by means of the excitation and emission spectra. Periodic DFT and molecular TD-DFT calculations have been used to unravel the emergence of luminescence in the otherwise non-emitting 1-(3,4-dimethylphenyl)-5-methyl-1H-1,2,3-triazole-4-carboxylate ligand once incorporated in the SMOFs. Our results also illustrate the importance of considering the dielectric environment in the crystal when performing excited state calculations for isolated fragments to capture the correct electronic character of the low-lying states, a practice which is not commonly adopted in the community.