Synthetic Strategies to Extended Aromatic Covalent Organic Frameworks.

π­Conjugated materials are highly attractive owing to their unique optical and electronic properties. Covalent organic frameworks (COFs) offer a great opportunity for precise arrangement of building units in a π-conjugated crystalline matrix and tuning of the properties through choice of functionalities or post-synthetic modification. With this review, we aim at summarizing both the most representative as well as emerging strategies for the synthesis of π-conjugated COFs. We give examples of direct synthesis methods with large, π-extended building blocks. COFs featuring fully conjugated linkages such as vinylene, pyrazine, and azole are discussed. Then, post-synthetic modification methods that result in the extension of the COF p-system are reviewed. Throughout, mechanistic insights are presented when available. In the context of their utilization as film devices, we conduct a concise survey of the prominent COF layer deposition techniques reported and their aptness for the deposition of fused aromatic systems.

Building Blocks and COFs Formed in Concert-Three-Component Synthesis of Pyrene-Fused Azaacene Covalent Organic Framework in the Bulk and as Films.

A three-component synthesis methodology is described for the formation of covalent organic frameworks (COFs) containing extended aromatics. Notably, this approach enables synthesis of the building blocks and COF along parallel reaction landscapes, on a similar timeframe. The use of fragmental building block components, namely pyrene dione diboronic acid as aggregation-inducing COF precursor and the diamines o-phenylenediamine (Ph), 2,3-diaminonaphthalene (Naph), or (1R,2R)-(+)-1,2-diphenylethylenediamine (2Ph) as extending functionalization units in conjunction with 2,3,6,7,10,11-hexahydroxytriphenylene, resulted in the formation of the corresponding pyrene-fused azaacene, i.e., Aza-COF series with full conversion of the dione moiety, long-range order, and high surface area. In addition, the novel three-component synthesis was successfully applied to produce highly crystalline, oriented thin films of the Aza-COFs with nanostructured surfaces on various substrates. The Aza-COFs exhibit light absorption maxima in the blue spectral region, and each Aza-COF presents a distinct photoluminescence profile. Transient absorption measurements of Aza-Ph- and Aza-Naph-COFs suggest ultrafast relaxation dynamics of excited-states within these COFs.

Covalent Organic Framework Composites: Synthesis and Analytical Applications.

In the recent years, composite materials containing covalent organic frameworks (COFs) have raised increasing interest for analytical applications. To date, various synthesis techniques have emerged that allow for the preparation of crystalline and porous COF composites with various materials. Herein, we summarize the most common methods used to gain access to crystalline COF composites with magnetic nanoparticles, other oxide materials, graphene and graphene oxide, and metal nanoparticles. Additionally, some examples of stainless steel, polymer, and metal-organic framework composites are presented. Thereafter, we discuss the use of these composites for chromatographic separation, environmental remediation, and sensing.

Extraction of Ibuprofen from Natural Waters Using a Covalent Organic Framework.

Ibuprofen is one of the most widely used pharmaceuticals, and due to its inefficient removal by conventional wastewater treatment, it can be found in natural surface waters at high concentrations. Recently, we demonstrated that the TpBD-(CF3)2 covalent organic framework (COF) can adsorb ibuprofen from ultrapure water with high efficiency. Here, we investigate the performance of the COF for the extraction of ibuprofen from natural water samples from a lake, river, and estuary. In general, the complexity of the natural water matrix induced a reduction in the adsorption efficiency of ibuprofen as compared to ultrapure water. The best performance, with over 70% adsorption efficiency, was found in lake water, the sample which featured the lowest pH. According to the theoretical calculations, ibuprofen more favorably interacts with the COF pores in the protonated form, which could partially account for the enhanced adsorption efficiency found in lake water. In addition, we explored the effect of the presence of competing pharmaceuticals, namely, acetaminophen and phenobarbital, on the ibuprofen adsorption as binary mixtures. Acetaminophen and phenobarbital were adsorbed by TpBD-(CF3)2 with low efficiency and their presence led to an increase in ibuprofen adsorption in the binary mixtures. Overall, this study demonstrates that TpBD-(CF3)2 is an efficient adsorbent for the extraction of ibuprofen from natural waters as well.

Tailoring Covalent Organic Frameworks To Capture Water Contaminants.

Covalent organic frameworks (COFs) are attractive materials receiving increasing interest in the literature due to their crystallinity, large surface area, and pore uniformity. Their properties can be tailored towards specific applications by judicious design of COF building blocks, giving access to tailor-made pore sizes and surfaces. In this Concept article, developments in the field of COFs that have allowed these materials to be explored for contaminant adsorption are discussed. Strategies to obtain water-stable materials with highly ordered structures and large surface areas are reviewed. Post-synthetic modification approaches, by which pore surfaces can be tuned to target specific contaminants, are described. Recent advances in COF formulations, crucial for future implementation in adsorption devices, are highlighted. At the end, future challenges which need to be addressed to allow for the deployment of COFs for the capture of water contaminants will be discussed.

Orthogonal Clickable Iron Oxide Nanoparticle Platform for Targeting, Imaging, and On-Demand Release.

A versatile iron oxide nanoparticle platform is reported that can be orthogonally functionalized to obtain highly derivatized nanomaterials required for a wide variety of applications, such as drug delivery, targeted therapy, or imaging. Facile functionalization of the nanoparticles with two ligands containing isocyanate moieties allows for high coverage of the surface with maleimide and alkyne groups. As a proof-of-principle, the nanoparticles were subsequently functionalized with a fluorophore as a drug model and with biotin as a targeting ligand towards tumor cells through Diels-Alder and azide-alkyne cycloaddition reactions, respectively. The thermoreversibility of the Diels-Alder product was exploited to induce the on-demand release of the loaded molecules by magnetic hyperthermia. Additionally, the nanoparticles were shown to target cancer cells through in vitro experiments, as analyzed by magnetic resonance imaging.

Adsorption of Pharmaceutical Pollutants from Water Using Covalent Organic Frameworks.

Capture of pharmaceutical pollutants from water was studied using a novel fluorine-bearing covalent organic framework TpBD-(CF3 )2 , which showed ibuprofen adsorption capacity of 119 mg g-1 at neutral pH. This value is further enhanced at pH 2, highlighting the potential of this class of materials to serve as adsorbents even under harsh conditions. The adsorbed pharmaceutical can be recovered from TpBD-(CF3 )2 in high yield, offering the option of recycling both the adsorbent and the pharmaceutical. The high efficiency of ibuprofen capture as compared to other less lipophilic pharmaceuticals suggests that COFs can be pre-designed for selective capture of contaminants.

Magnetite Nanoparticles for Stem Cell Labeling with High Efficiency and Long-Term in Vivo Tracking.

Superparamagnetic iron oxide nanoparticles (SPIO-PAA), ultrasmall iron oxide nanoparticles (USPIO-PAA), and glucosamine-modified iron oxide nanoparticles (USPIO-PAA-GlcN) were studied as mesenchymal stem cell (MSCs) labels for cell tracking applications by magnetic resonance imaging (MRI). Pronounced differences were found in the labeling performance of the three samples in terms of cellular dose and labeling efficiency. In combination with polylysine, SPIO-PAA showed nonhomogeneous cell internalization, while for USPIO-PAA no uptake was found. On the contrary, USPIO-PAA-GlcN featured high cellular uptake and biocompatibility, and sensitive detection in both in vitro and in vivo experiments was found by MRI, showing that glucosamine functionalization can be an efficient strategy to increase cell uptake of ultrasmall iron oxide nanoparticles by MSCs.

Extraction of photogenerated electrons and holes from a covalent organic framework integrated heterojunction.

Covalent organic frameworks (COFs) offer a strategy to position molecular semiconductors within a rigid network in a highly controlled and predictable manner. The π-stacked columns of layered two-dimensional COFs enable electronic interactions between the COF sheets, thereby providing a path for exciton and charge carrier migration. Frameworks comprising two electronically separated subunits can form highly defined interdigitated donor-acceptor heterojunctions, which can drive the photogeneration of free charge carriers. Here we report the first example of a photovoltaic device that utilizes exclusively a crystalline organic framework with an inherent type II heterojunction as the active layer. The newly developed triphenylene-porphyrin COF was grown as an oriented thin film with the donor and acceptor units forming one-dimensional stacks that extend along the substrate normal, thus providing an optimal geometry for charge carrier transport. As a result of the degree of morphological precision that can be achieved with COFs and the enormous diversity of functional molecular building blocks that can be used to construct the frameworks, these materials show great potential as model systems for organic heterojunctions and might ultimately provide an alternative to the current disordered bulk heterojunctions.

A carboxyl-functionalized covalent organic polymer for the efficient adsorption of saxitoxin.

Saxitoxin (STX), the most widely distributed neurotoxin in marine waters and emerging cyanotoxin of concern in freshwaters, causes paralytic shellfish poisoning in humans upon consumption of contaminated shellfish. To allow for the efficient monitoring of this biotoxin, it is of high importance to find high-affinity materials for its adsorption. Herein, we report the design and synthesis of a covalent organic polymer for the efficient adsorption of STX. Two ß-keto-enamine-based materials were prepared by self-assembly of 2,4,6-triformylphloroglucinol (Tp) with 2,5-diaminobenzoic acid (Pa-COOH) to give TpPa-COOH and with 2,5-diaminotoluene (Pa-CH3) to give TpPa-CH3. The carboxylic acid functionalized TpPa-COOH outperformed the methyl-bearing counterpart TpPa-CH3 by an order of magnitude despite the higher long-range order and surface area of the latter. The adsorption of STX by TpPa-COOH was fast with equilibrium reached within 1 h, and the Langmuir adsorption model gave a calculated maximum adsorption capacity, Qm, of 5.69 mg g-1, making this material the best reported adsorbent for this toxin. More importantly, the prepared TpPa-COOH also showed good reusability and high recovery rates for STX in natural freshwater, thereby highlighting the material as a good candidate for the extraction and pre-concentration of STX from aquatic environments.

Evaluation of Covalent Organic Frameworks for the low-cost, rapid detection of Shiga Toxin-producing Escherichia coli in ready-to-eat salads.

BACKGROUND: Ready-to-eat products, such as leafy greens, must be carefully controlled as they are directly consumed without any treatment to reduce the presence of potential pathogens. Food industries, especially those that process products with short shelf-life, demand rapid detection of foodborne pathogens such as Shiga Toxin-producing Escherichia coli (STEC). In this sense, molecular methods can fulfill both requirements of turnaround time and consumer safety. The most popular rapid methods are those based on real-time PCR (qPCR) however, vegetables contain inhibitory compounds that may inhibit the amplification reaction thus, there is a need for novel sample preparation protocols. RESULTS: In the current study, a low-cost sample treatment based on sequential filtration steps was developed. This protocol was combined with covalent organic frameworks (COFs), and compared against a chelating resin, to evaluate their performance by multiplex qPCR targeting the major virulence genes of STEC, namely stx1, stx2, and eae, along with the rfbE for the specific identification of serogroup O157 due to its particularly high incidence, and an Internal Amplification Control to assess reaction inhibition. The optimized sample treatment effectively removed vegetable qPCR inhibitory compounds, and it was possible to detect STEC in spiked ready-to-eat salad samples in one working day, roughly 5 h, with an LOD50 of 8.7 CFU/25 g with high diagnostic sensitivity and specificity. The method was also assessed in samples with cold-stressed bacteria with good results, further demonstrating its applicability. SIGNIFICANCE: It was demonstrated for the first time that COFs are suitable for DNA extraction and purification. In addition to this, due to the tunable nature of these materials, it is envisioned that future modifications in terms of pore size or combination with magnetic materials, will allow to further improve their performance. In addition to this, the rapid and low-cost sample treatment protocol developed demonstrated suitable for the rapid screening of STEC vegetable samples.

Towards All-Non-Vacuum-Processed Photovoltaic Systems: A Water-Based Screen-Printed Cu(In,Ga)Se2 Photoabsorber with a 6.6% Efficiency.

During the last few decades, major advances have been made in photovoltaic systems based on Cu(In,Ga)Se2 chalcopyrite. However, the most efficient photovoltaic cells are processed under high-energy-demanding vacuum conditions. To lower the costs and facilitate high-throughput production, printing/coating processes are proving to be effective solutions. This work combined printing, coating, and chemical bath deposition processes of photoabsorber, buffer, and transparent conductive layers for the development of solution-processed photovoltaic systems. Using a sustainable approach, all inks were formulated using water and ethanol as solvents. Screen printing of the photoabsorber on fluorine-doped tin-oxide-coated glass followed by selenization, chemical bath deposition of the cadmium sulfide buffer, and final sputtering of the intrinsic zinc oxide and aluminum-doped zinc oxide top conductive layers delivered a 6.6% maximum efficiency solar cell, a record for screen-printed Cu(In,Ga)Se2 solar cells. On the other hand, the all-non-vacuum-processed device with spray-coated intrinsic zinc-oxide- and tin-doped indium oxide top conductive layers delivered a 2.2% efficiency. The given approaches represent relevant steps towards the fabrication of sustainable and efficient Cu(In,Ga)Se2 solar cells.

Molecular recognition at the active site of factor Xa: cation-π interactions, stacking on planar peptide surfaces, and replacement of structural water.

Factor Xa, a serine protease from the blood coagulation cascade, is an ideal enzyme for molecular recognition studies, as its active site is highly shape-persistent and features distinct, concave sub-pockets. We developed a family of non-peptidic, small-molecule inhibitors with a central tricyclic core orienting a neutral heterocyclic substituent into the S1 pocket and a quaternary ammonium ion into the aromatic box in the S4 pocket. The substituents were systematically varied to investigate cation-π interactions in the S4 pocket, optimal heterocyclic stacking on the flat peptide walls lining the S1 pocket, and potential water replacements in both the S1 and the S4 pockets. Structure-activity relationships were established to reveal and quantify contributions to the binding free enthalpy, resulting from single-atom replacements or positional changes in the ligands. A series of high-affinity ligands with inhibitory constants down to K(i)=2 nM were obtained and their proposed binding geometries confirmed by X-ray co-crystal structures of protein-ligand complexes.

Covalent organic framework as adsorbent for ultrasound-assisted dispersive (micro)solid phase extraction of polycyclic synthetic fragrances from seawater followed by fluorescent determination.

In this work, a new analytical approach based on ultrasound-assisted dispersive (micro)solid phase extraction (US-D-µSPE) using TpBD-Me2 covalent organic framework (COF) as adsorbent for simple, rapid and selective fluorescent determination of two polycyclic synthetic fragrances in seawater, i.e., 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethyl-cyclopenta-(γ)-2-benzopyran (HHCB), branded galaxolide®, and 7-acetyl-1,1,3,4,4,6-hexamethyltetralin (AHTN), branded tonalide®, is proposed. Different parameters involved in both adsorption and desorption steps were optimized in order to obtain the best results. High adsorption efficiencies in the range of 91.2-97.8% were achieved for both analytes. Desorption efficiencies of ∼98% for AHTN and HHCB were obtained using methanol as solvent, rendering the material recyclable with merely minor losses in adsorption efficiency after five consecutive cycles of adsorption/desorption. Limits of detection (LODs) were 0.082 µg L-1 and 0.070 µg L-1 for AHTN and HHCB, respectively. The proposed method was successfully applied for the analysis of seawater without the need for a previous sample treatment, e.g., pH adjustment. Recoveries in the range of 90.4-101.2% with a relative standard deviation of 5.8% were obtained for both fragrances. The results proved the great capacity of TpBD-Me2 COF for the selective sorption of polycyclic fragrances in combination with fluorescent detection, being highly promising for application to environmental monitoring of other emerging organic pollutants.

Threefold reactivity of a COF-embedded rhenium catalyst: reductive etherification, oxidative esterification or transfer hydrogenation.

The reactivity of the novel Re(I) catalyst [Re(C12Anth-py2)(CO)3Br] is modulated by its interactions with the covalent organic framework (COF) TFB-BD. The complex catalyzes either reductive etherification, oxidative esterification, or transfer hydrogenation depending on its local environment (embedded in TFB-BD, in homogeneous solution or co-incubated with TFB-BD, respectively). The results highlight that COFs can drastically modulate the reactivity of homogeneous catalysts.

Deep Eutectic Solvent Synthesis of Perovskite Electrocatalysts for Water Oxidation.

Oxide perovskites have attracted great interest as materials for energy conversion due to their stability and structural tunability. La-based perovskites of 3d-transition metals have demonstrated excellent activities as electrocatalysts in water oxidation. Herein, we report the synthesis route to La-based perovskites using an environmentally friendly deep eutectic solvent (DES) consisting of choline chloride and malonic acid. The DES route affords phase-pure crystalline materials on a gram scale and results in perovskites with high electrocatalytic activity for oxygen evolution reaction. A convenient, fast, and scalable synthesis proceeds via assisted metathesis at a lower temperature as compared to traditional solid-state methods. Among LaCoO3, LaMn0.5Ni0.5O3, and LaMnO3 perovskites prepared via the DES route, LaCoO3 was established to be the best-performing electrocatalyst for water oxidation in alkaline medium at 0.25 mg cm-2 mass loading. LaCoO3 exhibits current densities of 10, 50, and 100 mA cm-2 at respective overpotentials of approximately 390, 430, and 470 mV, respectively, and features a Tafel slope of 55.8 mV dec-1. The high activity of LaCoO3 as compared to the other prepared perovskites is attributed to the high concentration of oxygen vacancies in the LaCoO3 lattice, as observed by high-resolution transmission electron microscopy. An intrinsically high concentration of O vacancies in the LaCoO3 synthesized via the DES route is ascribed to the reducing atmosphere attained upon thermal decomposition of the DES components. These findings will contribute to the preparation of highly active perovskites for various energy applications.

Aromatic rings in chemical and biological recognition: energetics and structures.

This review describes a multidimensional treatment of molecular recognition phenomena involving aromatic rings in chemical and biological systems. It summarizes new results reported since the appearance of an earlier review in 2003 in host-guest chemistry, biological affinity assays and biostructural analysis, data base mining in the Cambridge Structural Database (CSD) and the Protein Data Bank (PDB), and advanced computational studies. Topics addressed are arene-arene, perfluoroarene-arene, S⋅⋅⋅aromatic, cation-π, and anion-π interactions, as well as hydrogen bonding to π systems. The generated knowledge benefits, in particular, structure-based hit-to-lead development and lead optimization both in the pharmaceutical and in the crop protection industry. It equally facilitates the development of new advanced materials and supramolecular systems, and should inspire further utilization of interactions with aromatic rings to control the stereochemical outcome of synthetic transformations.

Large-scale aqueous synthesis of Cu(In,Ga)Se2 nanoparticles for photocatalytic degradation of ciprofloxacin.

Environmentally friendly synthesis of Cu(In,Ga)Se2 (CIGS) nanoparticles (NPs) is pivotal for producing sustainable photocatalytic compounds to be applied in the remediation of contaminants of emerging concern from water. To this end, we herein report an aqueous synthesis of CIGS NPs, followed by annealing, to give access to phase-pure CIGS crystals with chalcopyrite structure and no signs of secondary phases. Morphological and compositional characterization revealed NPs with an average size of 10-35 nm and uniform distribution of Cu, In, Ga, and Se elements. In addition, the first aqueous large-scale synthesis of CIGS NPs is developed by up-scaling the synthesis procedure, resulting in 5 g of highly crystalline nanoparticles exhibiting an ideal optical band gap of 1.14 eV. The as-synthesized NPs proved the ability to remove 71 and 83% of a contaminant of emerging concern, ciprofloxacin (CIP), under ultraviolet (UV) and visible (Vis) radiations, respectively.

Selection of Covalent Organic Framework Pore Functionalities for Differential Adsorption of Microcystin Toxin Analogues.

Microcystins (MCs), produced by Microcystis sp, are the most commonly detected cyanotoxins in freshwater, and due to their toxicity, worldwide distribution, and persistence in water, an improvement in the monitoring programs for their early detection and removal from water is necessary. To this end, we investigate the performance of three covalent organic frameworks (COFs), TpBD-(CF3)2, TpBD-(NO2)2, and TpBD-(NH2)2, for the adsorption of the most common and/or toxic MC derivatives, MC-LR, MC-RR, MC-LA, and MC-YR, from water. While MC-LR and MC-YR can be efficiently adsorbed using all three COF derivatives, high adsorption efficiencies were found for the most lipophilic toxin, MC-LA, with TpBD-(NH2)2, and the most hydrophilic one, MC-RR, with TpBD-(NO2). Theoretical calculations revealed that MC-LA and MC-RR have a tendency to be located mainly on the COF surface, interacting through hydrogen bonds with the amino and nitro functional groups of TpBD-(NH2)2 and TpBD-(NO2)2, respectively. TpBD-(NO2)2 outperforms the adsorbent materials reported for the capture of MC-RR, resulting in an increase in the maximum adsorption capacity by one order of magnitude. TpBD-(NH2)2 is reported as the first efficient adsorbent material for the capture of MC-LA. Large differences in desorption efficiencies were observed for the MCs with different COFs, highlighting the importance of COF-adsorbate interactions in the material recovery. Herein we show that efficient capture of these toxins from water can be achieved through the proper selection of the COF material. More importantly, this study demonstrates that by careful choice of COF functionalities, specific compounds can be targeted or excluded from a group of analogues, providing insight into the design of more efficient and selective adsorbent materials.

Study on the efficiency of a covalent organic framework as adsorbent for the screening of pharmaceuticals in estuary waters.

Herein, we demonstrate, for the first time, that covalent organic frameworks (COFs) can be efficient adsorbents for the screening of pharmaceuticals in real water samples, obtaining highly representative data on their occurrence and avoiding the cost of carrying high volume samples and tedious and costly clean-up and preconcentration steps. Of the 23 pharmaceuticals found present in the water samples from the Tagus river estuary using state-of-the-art solid-phase extraction (SPE), 22 were also detected (adsorbed and recovered for analysis) using a COF as the adsorbent material with adsorption efficiency of over 80% for nearly all compounds. In specific cases, acidification of the water samples was identified to lead to a dramatic loss of adsorption efficiency, underlining the effect of sample pre-treatment on the results. The COF efficiently adsorbed (>80%) 19 pharmaceuticals without acid treatment of the sample, highlighting the potential of this class of materials for representative in situ passive adsorption of pharmaceuticals, making this material suitable for being used in water monitoring programs as a simple and cost-efficient sample preparation procedure. In the case of α-hydroxyalprazolam and diclofenac, the COF outperformed the SPE procedure in the recovery efficiency. Although further efforts should be made in tailoring the desorption of the pharmaceuticals from the COF by using different solvents or solvent mixtures, we propose COFs as convenient adsorbent for broad-scope screening and as an efficient adsorbent material to target specific classes of pharmaceuticals. To the best of our knowledge, this is the first study on the use of COFs for contaminant screening in real, naturally contaminated water samples.