Flexible three-dimensional covalent organic frameworks for ultra-fast and selective extraction of uranium via hydrophilic engineering

Constructing asymmetric D'-A-D" imine covalent organic frameworks to optimize exciton effect and enhance photocatalytic reduction of U(VI)

Imine-based covalent organic frameworks (COFs) exhibit high exciton binding energies due to their weaker electronic delocalization. Herein, we report an asymmetric ternary imine COF (NCOF-3) bridged by D2 h and C2 h units, which is synthesized via an aldol-type condensation reaction mediated by amine monomers. Compared to symmetric NCOF-1 and NCOF-2, the asymmetric architecture of NCOF-3 maximizes interlayer interactions, leading to enhance crystallinity. Moreover, by incorporating two electron-donating units into the triazine framework, an asymmetric D'-A-D'' topological structure is formed, which facilitates the forward dissociation of photogenerated excitons, achieving a high carrier separation efficiency. Temperature-dependent fluorescence spectroscopy reveals that NCOF-3 possesses a smaller exciton binding energy (40.15 meV) compared to those of symmetric imine-based COFs. Under illumination conditions, NCOF-3 exhibits uranium removal capacity of 1084 mg g-1, significantly surpassing those of binary COFs containing imine bonds. Additionally, anti-interference experiments demonstrate that NCOF-3 maintains a uranium removal efficiency of over 90 % even in the presence of excess competing metal ions, confirming its high selectivity. Density functional theory (DFT) calculations indicate that lattice dipole asymmetry plays crucial roles in reducing the electronic bandgap of NCOF-3. This work highlights the importance of designing and fabricating imine-based COFs with dipole asymmetry of the connecting components to effectively manipulate the internal charge distribution, ultimately enhancing photocatalytic activity.

Covalent organic framework nanomaterials: Syntheses, architectures, and applications

Covalent Organic Frameworks (COFs) are characterized by high thermochemical stability, low backbone density, well-controlled physical and chemical properties, large specific surface volume and porosity, permanently open pore structure, and various synthesis strategies. These remarkable attributes confer COFs with significant potential for a myriad of applications ranging from catalysis technology, gas separation and storage, optoelectronic materials, environmental and energy sciences, and biomedical development. There are many synthetic design methods for COF materials, and dynamic covalent chemistry is the scientific basis of COF materials-oriented design, which gives the error correction ability of the covalent assembly process, and is the key to obtaining crystallization and stability at the same time. However, "crystallinity" and "stability" in the synthesis and preparation of COF materials are often like "You can't have your cake and eat it, too": on the one hand, the reversible covalent bonds used in the synthesis of highly crystalline COF framework are easy to decompose under extreme conditions, which greatly limits its application scenarios; On the other hand, although highly stable COF materials can be prepared by using irreversible covalent bonds, it is usually poor crystalline and difficult to have high performance. In addition, the strict deoxygenation operation required for synthesizing COF materials also limits its macro preparation and large-scale application. Therefore, the synthesis strategy and efficient preparation of highly stable and crystalline COF materials are a major obstacle to the practical application of this field. This paper describes the four structures of COF materials, as well as their synthesis methods, electrical energy-storing electrocatalysis, and significant environmental protection applications. The future directions, prospects, and possible barriers to the development of these materials are envisioned in.

Unlocking Enhanced Detection of Perfluoroalkanesulfonic Acids via Fluorinated Nonpolar 3D Covalent Organic Frameworks-Based Ambient Probe Nanoelectrospray Ionization Mass Spectrometry

The trace levels and severe matrix interferences greatly limited the determination of stable, persistent, and long-range-transported perfluoroalkanesulfonic acids (PFSAs) in complex environments. Here, we design and prepare the first fluorinated nonpolar 3D COF (TFPM-Pa-CF3) as an adsorbent, consisting of tetrakis(4-formylphenyl)methane (TFPM) and 2,5-diaminobenzo-trifluoride (Pa-CF3) for adsorption and extraction of PFSAs. The proposed TFPM-Pa-CF3 demonstrates excellent adsorption capacity (509.1 mg g-1) and rapid adsorption kinetics (5 min) for PFSAs attributed to the synergistic effects of F-F, hydrophobic, and electrostatic interactions. Furthermore, TFPM-Pa-CF3 is grown in situ on a stainless needle and coupled with ambient probe nanoelectrospray ionization mass spectrometry (PESI-MS) to develop a rapid and direct determination method with a low limit of detection (0.05-0.86 ng L-1) and wide linear range (1-10,000 ng L-1) for trace perfluorooctanesulfonate and its alternatives in environmental soil, algae and water. This work unlocks the efficient determination or removal of PFSAs in a complex environment, facilitating the solution of critical environmental PFSAs problems.

Covalent organic frameworks as superior adsorbents for the removal of toxic substances

Developing new materials capable of the safe and efficient removal of toxic substances has become a research hotspot in the field of materials science, as these toxic substances pose a serious threat to human health, both directly and indirectly. Covalent organic frameworks (COFs), as an emerging class of crystalline porous materials, have advantages such as large specific surface area, tunable pore size, designable structure, and good biocompatibility, which have been proven to be a superior adsorbent design platform for toxic substances capture. This review will summarize the synthesis methods of COFs and the properties and characteristics of typical toxicants, discuss the design strategies of COF-based adsorbents for the removal of toxic substances, and highlight the recent advancements in COF-based adsorbents as robust candidates for the efficient removal of various types of toxicants, such as animal toxins, microbial toxins, phytotoxins, environmental toxins, etc. The adsorption performance and related mechanisms of COF-based adsorbents for different types of toxic substances will be discussed. The complex host-guest interactions mainly include electrostatic, π-π interactions, hydrogen bonding, hydrophobic interactions, and molecular sieving effects. In addition, the adsorption performance of various COF-based adsorbents will be compared, and strategies such as reasonable adjustment of pore size, introduction of functionalities, and preparation of composite materials can effectively improve the adsorption efficiency of toxins. Finally, we also point out the challenges and future development directions that COFs may face in the field of toxicant removal. It is expected that this review will provide valuable insights into the application of COF-based adsorbents in the removal of toxicants and the development of new materials.

Designing fluorescent covalent organic frameworks through regulation of link bond for selective detection of Al3+ and Ce3

The high thermal stability and chemical durability of amide-linked covalent organic frameworks (amide COFs) make them a promising material for a range of new applications. Nevertheless, the low reversibility of the amide bond presents a significant challenge to the direct synthesis of amide-bonded COFs. In this paper, we present a simple method for synthesizing amide COFs. The synthesis of imine-linked COFs was initially achieved through the reaction of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 2,5-dimethoxybenzene-1,4-dicarboxaldehyde. Subsequently, amide COFs were synthesized via the oxidation of the imine bond into an amide bond, utilizing ammonium persulfate as the oxidizing agent. Due to the difference of link bond, two COFs separately displayed distinct and significant fluorescence enhancement for Al3+ and Ce3+, which was highly sensitive and less affected by environmental factors. The strategy offers a novel approach to the convenient and environmentally benign synthesis of amide COFs, which may facilitate their wider applications.

Construction of a Hierarchical Core-Shell Z-Scheme Two-Dimensioanl/Two-Dimensional ZnIn2S4@TpBpy Heterostructure for Photocatalytic Reduction of U(VI)

The essential nature of the photocatalytic process is charge transfer. To optimize the spatial separation of photogenerated electron-hole (e--h+) pairs for high-performance catalytic efficiency, in this work, we have successfully prepared hierarchical core-shell two-dimensional (2D)/2D ZnIn2S4@TpBpy (ZIS@TpBpy) with well-matched Z-scheme interfacial charge transfer channels for uranium (U(VI)) photoreduction. The Z-scheme electron transfer configuration was confirmed by internal electric field (IEF) formation analysis, XPS characterization, and DMPO spin-trapping EPR spectroscopy. With large specific surface area and abundant active sites, the ZIS@TpBpy composite achieved a U(VI) extraction rate of 94.08%. In addition, the removal rate constant of ZIS@TpBpy (0.0137 min-1) was 2.05 and 4.28 times higher than those of TpBpy (0.0067 min-1) and ZnIn2S4 (0.0032 min-1), respectively. First, the combination of organic and inorganic components expanded the range of visible light absorption and utilization. Afterward, under visible-light irradiation, more photogenerated e--h+ pairs dissociated and migrated to the ZnIn2S4 surface driven by the IEF and Z-scheme heterostructure. Simultaneously, the synergistic effect between the polarization potential generated by the IEF in the ZIS@TpBpy composite and abundant active sites (N and O atoms) in the TpBpy framework further accelerated the depletion and translocation of photogenerated e--h+ pairs, which significantly improved the efficiency of photocatalytic reduction of U(VI).

Hydrogen bonded organic framework pores differentially loading triazole for photocatalytic uranium reduction

Using temperature modulation, two distinct hydrogen bond organic frameworks HOF-C and HOF-K with different pore sizes were synthesized from the same ligands, tris(4-(4H-1,2,4-triazole-4-yl)phenyl)amine. The pore size difference prevents TRZ from entering HOF-K, while allowing TRZ to selectively insert into the larger-pored HOF-C to form HOF-C-TRZ. The donor-acceptor (D-A) structure formed in HOF-C-TRZ enhances its photoelectric response and exhibits exceptional uranium reduction under visible light irradiation. This study provides an effective strategy for pore size regulation of HOFs and opens up new ideas for the development of advanced uranium photocatalysts.

Defect Engineering Enhancing Piezoelectric Catalytic Activity of Covalent Organic Framework Matrix Composites for Uranium Removal

Piezoelectric catalysis is an emerging green strategy, but the existing piezoelectric heterostructures are not sufficient in performance for catalytic reduction of low-reduction potential uranium under harsh conditions. This study innovatively employs a defect heterogeneous engineering strategy, wherein covalent organic frameworks (COFs) are grown in situ on the surface of zinc oxide (ZnO) via Schiff base reactions, and defects are introduced into the COF shell layer via imine exchange reactions to construct D-COF@ZnO for piezoelectric catalytic uranium removal. The comprehensive study shows that defect heterogeneous engineering increases the asymmetry induced polarization of the material to promote charge redistribution, and thus significantly improves the activity of piezoelectric catalysis. In addition, defect engineering optimizes the nanosize of D-COF@ZnO to expose a richer array of active sites, resulting in ultra-fast U(VI) removal kinetics and ultra-high removal capacity. In the actual nuclear wastewater settings, D-COF@ZnO demonstrates outstanding selective removal efficacy for uranium, manifesting its considerable application potential and efficiency superiority. This strategy holds profound implications for facilitating the application of piezoelectric catalytic technology in environmental protection domains such as uranium removal, manifesting its considerable potential and value.

In Situ Electrochemical Interfacial Polymerization for Covalent Organic Frameworks with Tunable Electrochromism

A rapid in situ synthesis of electrochromic covalent organic frameworks (EC-COFs) was proposed by using green electrochemical interface polymerization of N,N,N',N'-tetrakis(4-aminophenyl)-1,4-benzenediamine (TPDA) and 2,5-dihydroxyterephthalaldehyde (DHBD). The synthetized TPDA-DHBD films exhibit stable polymorphic color variations under different applied potentials, which can be attributed to the redox state changes of bis(triphenylamine) and imine electroactive functional groups within the COFs skeleton. TPDA-DHBD represents markedly different electrochromisms from red to cyan due to the steric hindrance effect caused by the presence of UO2 2+, demonstrating the unique tunability of COFs materials. This work offers a new feasible idea for rapid EC-COFs synthesis and tunable EC-COFs realization.

Three-dimensional covalent organic framework-based artificial interphase layer endows lithium metal anodes with high stability

To gain a deeper understanding and address the scientific challenges of lithium dendrite growth, a robust solid-state electrolyte interface (SEI) with good mechanical properties and rapid ion conduction is crucial for the advancement of lithium metal batteries. Artificial SEI layers based on organic polymers, such as covalent organic frameworks (COF), have garnered widespread attention due to their flexible structural design and tunable functionality. In this work, a COF with 3D spatial geometric symmetry and a fully covalent dia topology was synthesized and used as artificial SEI layers. A combination of comprehensive DFT calculations and ex situ/in situ characterizations have unraveled the impact of interpenetrated chain segments and anchoring lithiophilic groups on the microscopic dynamics related to Li ion desolvation, charge transfer, migration pathways, and deposition morphology. The ultralow polarization voltage of 46 mV for 9400 hours with a symmetric Li|Li cell at a harsh current density of 10 mA cm-2, as well as the high Li+ utilization, low polarization voltage, and prolonged lifespan for 3D-COF-modified Li|S and Li|LFP full cells, unambiguously corroborate the interphase reliability. This work also aims to shed new light on the use of multi-dimensional porous polymer SEI layers to revive highly stable Li metal batteries.

Carboxymethyl cellulose and metal-organic frameworks immobilized into polyacrylamide hydrogel for ultrahigh efficient and selective adsorption U(VI) from seawater

Metal-organic frameworks (MOF)-polymer hybrid hydrogel solves the processable forming of MOF powder and energy consumption of uranium extraction. However, the hybrid hydrogel by conventional synthesis methods inevitably lead to MOF agglomeration, poor filler-polymer interfacial compatibility and slowly adsorption. Herein, we designed that ZIF-67 was implanted into the carboxymethyl cellulose/polyacrylamide (CMC/PAM) by network-repairing strategy. The carboxyl and amino groups on the surface of CMC/PAM drive the uniform growth of ZIF-67 inside the CMC/PAM, which form an array of oriented and penetrating microchannels through coordination bonds. Our strategy eliminate the ZIF-67 agglomeration, increase the interfacial compatibility between MOF and polymer. The method also improve the free and fast diffusion of uranium in CMC/PAM/ZIF-67 hydrogel. According to the experimental, these enhancements synergistically enabled the CMC/PAM/ZIF-67 have a maximum adsorption capacity of 952 mg g-1. The adsorption process of CMC/PAM/ZIF-67 fits well with pseudo-second-order model and Langmuir isotherm. Meanwhile, the CMC/PAM/ZIF-67 maintain a high removal rate (87.3 %) and chemical stability even during ten adsorption-desorption cycles. It is worth noting that the adsorption amount of CMC/PAM/ZIF-67 in real seawater is 9.95 mg g-1 after 20 days, which is an ideal candidate adsorbent for uranium extraction from seawater.

Tailoring chelating sites in two-dimensional covalent organic framework nanosheets for enhanced uranium capture

In this study, we intricately designed and synthesized two isoreticular two-dimensional covalent organic framework nanosheets, namely TAPA-COF-1 and TAPA-COF-2, distinguished by their unique spatial arrangement of hydroxyl groups. These precisely engineered nanosheets were employed as a tailored platform for the selective capture of uranium, due to their tunable chelating sites and characteristic sheet-like morphology. Notably, TAPA-COF-1, featuring ortho-hydroxyl groups, demonstrated a significantly enhanced adsorption capacity for uranium capture originating from the additional oriented adjacent phenolic hydroxyl chelating sites in comparison to TAPA-COF-2 with para-hydroxyl groups, which was proved by theoretical calculation. The impressive features of TAPA-COF-1, including its notable selectivity, rapid adsorption kinetics, and high uptake capacity (657.2 mg g-1), endow it as a highly promising candidate for uranium capture.

A PEDOT enhanced covalent organic framework (COF) fluorescent probe for in vivo detection and imaging of Fe3

Simple and accurate in vivo monitoring of Fe3+ is essential for gaining a better understanding of its role in physiological and pathological processes. A novel fluorescent probe was synthesized via in situ solid-state polymerization of 3,4-ethylenedioxythiophene (PEDOT) in the pore channels of a covalent organic framework (COF). The PEDOT@COF fluorescent probe exhibited an absolute quantum yield (QY) 3 times higher than COF. In the presence of Fe3+ the PEDOT@COF 475 nm fluorescence emission, 365 nm excitation, is quenched within 180 s. Fluorescence quenching is linear with Fe3+ in the concentration range of 0-960 μM, with a detection limit of 0.82 μM. The fluorescence quenching mechanism was attributed to inner filter effect (IEF), photoinduced electron transfer (PET) and static quenching (SQE) between PEDOT@COF and Fe3+. A paper strip-based detector was designed to facilitate practical applicability, and the PEDOT@COF probe successfully applied to fluorescence imaging of Fe3+ levels in vivo. This work details a tool of great promise for enabling detailed investigations into the role of Fe3+ in physiological and pathological diseases.