Covalent Organic Nanosheets Research Articles

Zwitterionic covalent organic nanosheets for selective analysis of domoic acid in marine environment

BackgroundDomoic acid (DA) is a neurotoxic compound causing amnesic shellfish poisoning, secreted by red algae and diatoms. As a glutamate analogue, DA accumulates in filter-feeding marine organisms, posing significant health risks to humans upon consumption. Detecting DA in marine environments remains challenging due to its low concentration and interference from complex matrices. Effective detection and removal require materials with high efficiency and selectivity, which traditional inorganic ionic materials lack due to their limited adsorption capacity and selectivity. Ionic covalent organic frameworks (iCOFs) expected to become highly efficient DA adsorbents due to tunable ionic sites. ResultsThus, a zwitterionic covalent organic nanosheet (TGDB-iCONs) was synthesized to selectively capture DA. TGDB-iCONs was prepared by one-step Schiff-base reaction of the charged monomer triaminoguanidine hydrochloride. It uniformly distributed positively charged guanidinium and negatively charged chloride ions on the surface, forming zwitterionic binding sites. The self-peeling of TGDB-iCONs facilitated the exposure of active sites and improved the adsorption efficiency. Several binding forces were generated between TGDB-iCONs and DA, including complementary electrostatic hydrophilic interactions, which were verified by density functional theory (DFT) calculation. TGDB-iCONs exhibited ultra-fast adsorption kinetics (7 min) and relatively high adsorption capacity (66.48 mg/g) for DA. Furthermore, TGDB-iCONs exhibit strong salt resistance, which is attributed to the charge “shielding” effect of the zwitterionic ions present in TGDB-iCONs. TGDB-iCONs could highly selectively enrich DA and detect trace DA from marine environment including seawater, algae and marine organisms and the limit of detection as low as 0.3 ng/kg. Significance and noveltyThis comprehensive study not only sheds light on the vast potential of ionic covalent organic frameworks nanosheets (iCONs) in supporting early warning, control, and traceability of DA, but also lays a solid foundation for future research endeavors aimed at designing and harnessing the unique properties of iCONs.

Post-Modification Approach for Self-Exfoliated Synthesis of Pyridinium Sulfobetaine Covalent Organic Frameworks for Enhanced Lithium-Ion Conductivity.

While covalent organic frameworks (COFs) have been extensively investigated in the field of organic electrolyte materials, there is potential for further enhancement of their room-temperature ionic conductivity. This study introduces a novel methodology to induce self-exfoliation in the parent COF during synthesis through a postmodification technique. This process yields covalent organic nanosheets that feature pyridinium sulfobetaine groups, referred to as PS-CON. Due to the strategic arrangement of pyridinium cations and sulfobetaine anions, the charge distribution in PS-CON varies substantially, leading to a significant enhancement in lithium-ion dissociation. The methodically organized one-dimensional pore channels, along with the linear structure of the pyridinium sulfobetaine groups, facilitate the lithium-ion transport. PS-CON demonstrated a remarkable ionic conductivity of 2.19 × 10-4 S cm-1and a low activation energy (0.26 eV) coupled with a broad electrochemical stabilization window (4.05 V). Furthermore, the symmetrical cell (Li|Li@PS-CON|Li) demonstrates stable Li plating/stripping for more than 1200 h, which highlights the vast potential of pyridinium-sulfobetaine based zwitterionic nanosheets as high-performance organic electrolytes.

Wedging crystals to fabricate crystalline framework nanosheets via mechanochemistry

Mechanochemistry studies the effect of mechanical force on chemical bonds, bringing opportunities for synthesizing alloys, ceramics, organics, polymers, and biomaterials. A vital issue of applying macro-scale mechanical force to manipulate crystal structures is finding ways to precisely adjust the force directions to break micro-scale target chemical bonds. Inspired by a common technique of driving a wedge into the wood to make wood chopping much easier, a wedging strategy of splitting three-dimensional structured crystalline frameworks and then converting them to nanosheets was proposed, where specific molecules were wedged into crystalline frameworks to drive the directional transmission of mechanical force to break chemical bonds. As a result, various crystalline framework nanosheets including metal−organic framework nanosheets, covalent organic framework nanosheets, and coordination polymer nanosheets were fabricated. This wedging crystal strategy exhibits advantages of operability, flexibility and designability, and furthermore, it is expected to expand mechanochemistry applications in material preparation.

Self-driven photochemical exfoliation of Fe-coordinated covalent organic frameworks for enhanced solar hydrogen production

Covalent organic frameworks (COFs) have great potential in solar hydrogen production to alleviate environmental pollution and global energy crisis. However, to achieve high H2 yield is still a challenge. Herein, we designed and synthesized two Fe3+-coordinated imine type COFs (Fe-COFs) for photocatalytic H2 production from water splitting. It was surprising to find that with the progress of H2 evolution under visible light, the bulk powders of Fe-COFs (180–250 nm) were gradually stripped into covalent organic nanosheets (Fe-CONs, 0.3–1.9 nm), which in turn efficiently catalyzed H2 production. As such, a self-driven mechanism was established in the catalytic system, and H2 evolution rate increased with the increase of cyclic runs until the thickness of the nanosheet no longer changes in the regeneration and reuse process of the Fe-CON photocatalysts. The optimal Fe-CON catalyst exhibited a record high H2 evolution rate of 67.94 mmol g−1h−1 among the currently reported COFs loaded with other non-noble metals, due to the more exposed active sites, smaller charge-transfer resistance, and stronger separation ability of the photogenerated electrons and holes of Fe-CONs. This work provides the first example for the integration of self-driven photochemical exfoliation of 2D materials and their highly efficient photocatalytic hydrogen evolution.

Cationic covalent organic framework nanosheets as the coating layer of commercial separator for high-efficiency lithium-sulfur batteries

Ion-selective separators, are crucial and in high demand for maximizing the performance of lithium-sulfur (Li–S) batteries, which can conduct lithium ions while efficiently blocking polysulfides. However, commercial separators cannot effectively block the shuttle of polysulfides after multiple cycles due to their large porosity and easy dissolution, resulting in a reduced battery life. Herein, a covalent organic framework nanosheets (CON) ion-coated separator is prepared on the commercial separator. Due to the smaller pore size of CON-TFSI compared to polysulfides, the CON-TFSI modified separator can effectively block the polysulfide from migrating across the separator. By incorporating this innovative functional layer, Li–S batteries demonstrate outstanding performance. In a Li–S battery featuring a sulfur loading of 0.6 mg/cm2, it attains an initial discharge specific capacity of up to 891.9 mA h g−1, and exhibits the capacity retention of 54.6 % after 500 cycles at a current density of 0.2 C. This work offers a fresh perspective on the advancement of high-performance membranes for Li–S batteries.

Enhanced interlayer interaction in sulfonated CONs membrane by amino-rich CONs enabling ultrafast proton transport

Covalent organic framework nanosheets (CONs) with porous crystalline features and ultrathin thickness are ideal candidates as membrane building blocks to form well-defined transfer nanochannels. The formidable challenge behind self-supporting CONs membrane lies in weak non-covalent interlayer interactions and thus loose stacking, insufficient strength and structure stabilities. Herein, we propose the fabrication of interlayer force-strengthened freestanding CONs membrane through the electrostatic attraction bridge effect of positively-charged amino-rich CONs (CON–NH2) to negatively-charged sulfonated CONs (CON–SO3H). Ultrathin and large lateral sized CON–SO3H and CON–NH2 are synthesized, followed by restacking to prepare freestanding CONs membrane with CON–SO3H as the membrane bulk. Benefiting from effective interlayer interconnection due to strong electrostatic interaction, the obtained CON–SO3H/CON–NH2 membrane displays features of ultrahigh integrity, dense stacking, eminent water/acid/base/organic solvents stabilities and mechanical strength (109 MPa). The shortened –SO3H distance contributes to construct site-continuous transfer pathways, and the deprotonated –SO3H and protonated –NH2 form acid–base pairs to decrease interfacial resistance, which impart membrane superior proton conductivity of 486 mS cm−1 (80 °C, 100% RH). This interlayer force enhancement strategy offers a promising perspective on achieving densely-stacked CONs membrane with ultrahigh mechanical property and conduction performance for fuel cell application.

Thickness-Driven Synthesis and Applications of Covalent Organic Framework Nanosheets.

Covalent organic frameworks (COFs) are two-dimensional, crystalline porous framework materials with numerous scopes for tunability, such as porosity, functionality, stability and aspect ratio (thickness to length ratio). The manipulation of π-stacking in COFs results in truly 2D materials, namely covalent organic nanosheets (CONs), adds advantages in many applications. In this Minireview, we have discussed both top-down (COFs→CONs) and bottom-up (molecules→CONs) approaches with precise information on thickness and lateral growth. We have showcased the research progress on CONs in a few selected applications, such as batteries, catalysis, sensing and biomedical applications. This Minireview specifically highlights the reports where the authors compare the performance of CONs with COFs by demonstrating the impact of the thickness and lateral growth of the nanosheets. We have also provided the possible scope of exploration of CONs research in terms of inter-dimensional conversion, such as graphene to carbon nanotube and future technologies.

Thickness‐Driven Synthesis and Applications of Covalent Organic Framework Nanosheets

AbstractCovalent organic frameworks (COFs) are two‐dimensional, crystalline porous framework materials with numerous scopes for tunability, such as porosity, functionality, stability and aspect ratio (thickness to length ratio). The manipulation of π‐stacking in COFs results in truly 2D materials, namely covalent organic nanosheets (CONs), adds advantages in many applications. In this Minireview, we have discussed both top‐down (COFs→CONs) and bottom‐up (molecules→CONs) approaches with precise information on thickness and lateral growth. We have showcased the research progress on CONs in a few selected applications, such as batteries, catalysis, sensing and biomedical applications. This Minireview specifically highlights the reports where the authors compare the performance of CONs with COFs by demonstrating the impact of the thickness and lateral growth of the nanosheets. We have also provided the possible scope of exploration of CONs research in terms of inter‐dimensional conversion, such as graphene to carbon nanotube and future technologies.

Incorporating covalent organic framework nanosheets via solvent-exchange strategy boosted hybrid membrane dehydration performance

The uniform dispersion of fillers within the polymer matrix is crucial for the fabrication of hybrid membranes. In this study, we proposed a facile solvent-exchange strategy for developing a hybrid membrane based on a covalent organic framework (COF, TpPa-1), specifically designed to enhance filler distribution in in the membrane towards pervaporation dehydration of butanol/water mixtures. The TpPa-1 nanosheets prepared via a glycerol-mediated method and their hybrid membranes were characterized using SEM, TGA, XPS, BET, FT-IR, and XRD techniques. Molecular dynamics (MD) results revealed that the solvent-exchange strategy significantly enhanced COF nanosheets (CONs) dispersion by removing glycerol. Pervaporation dehydration performance demonstrated significant improvements in total flux and separation factor when TpPa-1 nanosheets and sodium alginate (SA) were integrated, compared to a pure SA membrane. The optimized COF-SA hybrid membrane achieved a total flux of 1753 g/m2·h and a dehydration separation factor of 4687 at 40 °C for 90 wt% butanol/water mixtures. This study not only highlights a viable approach for membrane solvent dehydration but also provides a design strategy for creating hybrid membranes with uniform filler dispersion.

Hierarchical porous covalent organic framework nanosheets with adjustable large mesopores

Hierarchical porous covalent organic framework nanosheets with adjustable large mesopores

A covalent organic framework nanosheet-nanochannel composite with signal amplification strategy for electrochemical enantioselective recognition

Recognition and separation of chiral isomers are of great importance in both industrial and biological applications. However, owing to identical molecular formulas and chemical properties of enantiomers, signal transduction and amplification are still two major challenges in chiral sensing. In this study, we developed an enantioselective device by integrating chiral covalent organic framework nanosheets (CONs) with nanochannels for sensitive identification and quantification of enantiomers. Using 3,4-dihydroxyphenylalanine (DOPA) as the model analyte, the as-prepared chiral nanofluidic device exhibits a remarkable chiral recognition ability to l-DOPA than d-DOPA. More importantly, due to the chelation of DOPA with Fe3+ ions, it can efficiently block the ion transport through channel and shield the channel surface charge, which will amplify the difference in the electrochemical response of l-DOPA and d-DOPA. Therefore, a sensitive chiral recognition can be achieved using the present nanofluidic device coupled using electrochemical amplification strategy. Notably, using this method, an ultra-low concentration of l-DOPA (as low as 0.21 pM) can be facilely and successfully detected with a linear range of 1 pM–10 μM. This study provides a reliable and sensitive approach for achieving highly selective detection of chiral molecules.

Fully Conjugated Covalent Organic Framework Nanosheets for Visible-Light-Driven Organic Synthesis in Water.

Covalent organic framework (COF) nanosheets have recently garnered great attention as a new class of functional materials. As one of the sustainable processes, however, the photocatalytic organic synthesis in water has not been investigated using COF nanosheets as a photocatalyst to date. Herein, we reported the synthesis of a fully conjugated COF nanosheets with carboxyl functional group through a cooperative strategy of chemical exfoliation and group transformation. The new COF nanosheets was found to be an efficient heterogeneous photocatalyst for a wide range of organic synthesis including selective oxidation of sulfides and oxidative coupling of benzylamines in water under visible-light illumination. This work contributes a new roadmap for the design and synthesis of functional COF-based nanosheets, but also further extends the application boundary of the ultrathin COF nanosheets.

Robust and highly proton conductive COF composite membranes for fuel cell and electrochemical hydrogen compression

Robust and highly conductive proton exchange membrane (PEM) is the unremitting pursuit of numerous electrochemical energy technologies such as fuel cell and the emerging electrochemical hydrogen compression. Ionic covalent organic framework nanosheets (iCONs) with long-range ordered nanochannels and abundant ionic groups show substantial potential as next-generation PEM materials. However, it is challenging to assemble iCONs into membranes with both robustness and flexibility due to the electrostatic repulsion and rigid framework. Herein, we propose a hydrogen-bond binding strategy to fabricate sulfonic iCON (SCON) composite membranes by co-assembling tannic acid (TA) nanoaggregates and SCONs. The abundant and dynamically reversible hydrogen-bond interactions introduced by TA nanoaggregates establish effective linkage between SCONs and allow some restricted movements, endowing membranes with remarkably improved mechanical properties. The stacked SCONs construct long-range ordered nanochannels with well-arranged sulfonic groups in membranes, and the hydrogen-bond networks further facilitate the proton transport through Grotthuss mechanism. Consequently, TA/SCON composite membranes displays significantly enhanced tensile strength of 101.9 MPa, good flexibility and high proton conductivity of 490.1 mS cm−1 at 80 °C and 100 % RH simultaneously. This hydrogen-bond binding strategy offers a promising approach to fabricate robust and highly conductive PEMs for various proton-conducting applications.

Exploiting sulfonated covalent organic frameworks to fabricate long-lasting stability and chlorine-resistant thin-film nanocomposite nanofiltration membrane

Incorporating hydrophilic and charged porous nanofillers to prepare high-performance thin film nanocomposite (TFN) nanofiltration (NF) membranes is an effective method to achieve efficient water treatment. In this study, we synthesize the sulfonated covalent organic framework nanosheets (S-CONs) with higher hydrophilicity and electronegativity by immobilizing sulfonic acid groups (–SO3H) on TpPa-1 nanosheets. The S-CONs are incorporated in the PA layer by interfacial polymerization (IP) reaction. The results indicated that the S-CONs could modulate the hydrophilicity, thickness, and electronegativity of TFN-NF membranes. At the optimal addition of S-CONs (0.006 g), the pure water permeance increases to 8.84 L⋅m−2⋅h−1⋅bar−1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\rm{L}}\\cdot{\\rm{m}}^{-2}\\cdot{\\rm{h}}^{-1}\\cdot{\\rm{bar}}^{-1}$$\\end{document}, which is about 1.75 times than the TFC membrane, with a high Na2SO4 rejection reaching 98.97%. The improvement of the separation performance mainly results from the reduction of PA layer thickness (from ~178.00–198.00 to ~100.00–128.00 nm) and the increase of surface electronegativity (from −20.37 to −44.41 mV at pH = 7.00). More interestingly, the amide bond formed between the S-CONs and TMC improved the chlorine resistance of the membranes. This study reveals the potential of using functionalized 2D CONs as nanofillers to modify TFC membranes for efficient nanofiltration.

Covalent organic frameworks filled Pebax hybrid membrane for efficient toluene/N2 separation

Hybrid membranes hold significant potential in the efficient recovery of aromatic volatile organic compounds (VOCs). The advanced fillers with high aromatics affinity and polymer compatibility are in urgent demand. In this study, covalent organic framework nanosheets (COFNs) were introduced into poly (ether block amide) (Pebax®2533) to fabricate hybrid membranes. The long-range ordered pore structure, tunable functional groups and organic attributes of COFNs enhanced the affinity of membrane towards aromatic VOCs, the transport of VOCs within the membrane as well as the interactions between COFNs and Pebax. The hybrid membrane filled with TpPa-COOH COFNs exhibited a noticeable improvement in both selectivity and permeability compared to pure Pebax membranes. Particularly, when the content of TpPa-COOH COFNs was at 4 wt%, the membrane demonstrated excellent separation performance for a 2.7 mol% toluene/N2 mixture, wherein the toluene permeability of membrane was 6.7 × 10−7 mol μm m−2 s−1 Pa−1 and the selectivity was 674. This study expanded the application scenarios of COFs-based hybrid membranes and provided a new approach for the design and fabrication of VOCs recovery membranes.

Self-Assembly of 2D Polyphthalocyanine in Lysosome Enables Multienzyme Activity Enhancement to Induce Tumor Ferroptosis.

Nanozymes show great potential in facilitating tumor ferroptosis by upregulation of reactive oxygen species (ROS) and downregulation of glutathione (GSH). However, mild acidity (pH 6.5-6.9) of tumor microenvironment severely restricts the activity of nanozymes. Although lysosomes as acidic organelles (pH = 3.5-5.5) are hopeful for improving enzyme-like activity, most reported nanozymes are not capable of effectively accumulating in the lysosomes. Herein, an acid-responsive self-assembly strategy based on iron phthalocyanine-rich covalent organic framework nanosheets (COFFePc NSs) is developed, which enables lysosomal targeting aggregation of COFFePc NSs due to the existence of abundant negative hydroxyl groups and rigid structure. Meanwhile, COFFePc NSs display exceptional multienzyme-mimic performance at lower pH to efficiently generate ROS to cause lysosome damage and apoptosis by synergistic photothermal effect. Subsequently, the released COFFePc with GSH oxidase-mimicking activity can consume GSH to promote ferroptosis. This is the first report of a 2D COF using its own properties to achieve lysosomal self-assembly. Overall, the work provides a new paradigm for the development of lysosome-targeted nanosystems.

Advancements in two-dimensional covalent organic framework nanosheets for electrocatalytic energy conversion: current and future prospects

Humanity is confronting significant environmental issues due to rising energy demands and the unchecked use of fossil fuels. Thus, the strategic employment of sustainable and environmentally friendly energy sources is becoming increasingly vital. Additionally, addressing challenges, such as low reactivity, suboptimal energy efficiency, and restricted selectivity, requires the development of innovative catalysts. Two-dimensional (2D) covalent organic frameworks (COFs), known for their limitless structural versatility, are proving to be important materials in energy conversion applications. The exceptional properties of 2D COFs, including an organized arrangement resulting in well-defined active sites and π-π stacking interactions, enable breakthroughs in sustainable energy conversion applications. In this study, we comprehensively investigate universal synthesis methods and specific techniques, such as membrane-based deposition, liquid-phase intercalation, and polymerization. Furthermore, we demonstrate energy-conversion applications of 2D COFs as eco-friendly catalysts for electrochemical processes to promote sustainability and scalability by utilizing them in the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, and carbon dioxide reduction reaction. Additionally, we will explore methods for analyzing the physicochemical properties of precisely fabricated 2D COFs. Despite extensive research pertaining to 2D COFs, their practical industrial applications remain limited. Therefore, we propose various perspectives, including enhancing performance, improving synthesis methods, developing binder-free catalysts, expanding catalyst functionality, and advancing full-cell research, to achieve complete industrialization by leveraging their potential.

Fabrication of composite microbeads consisting of cellulose and covalent organic nanosheets via electrospray process

Fabrication of composite microbeads consisting of cellulose and covalent organic nanosheets via electrospray process

Rigidifying AIEgens in covalent organic framework nanosheets for electrochemiluminescence enhancement: TABE-PZ-CON as a novel emitter for microRNA-21 detection

Enhancing electrochemiluminescence (ECL) properties of luminophores is a hot direction in the current ECL field. Herein, we found that covalent rigidification of the aggregation-induced emission luminogens (AIEgens) TABE (TABE = tetra-(4-aldehyde-(1,1-biphenyl))ethylene) into covalent organic framework nanosheets (TABE-PZ-CON, PZ = piperazine) could result in stronger ECL emission than those of TABE aggregates and TABE monomers. We termed the interesting phenomenon “covalent rigidification-triggered electrochemiluminescence (CRT-ECL) enhancement”. The superior ECL performance of TABE-PZ-CON not only because massive TABE luminogens were covalently assembled into the rigid TABE-PZ-CON network, which limited the intramolecular motions of TABE and hampered the radiationless transition, but also because the ultrathin porous TABE-PZ-CON significantly reduced the transportation distance of ions, electrons, and coreactants, which enabled the electrochemical excitation of more TABE luminogens and thus enhanced the ECL efficiency. Bearing in mind the exceptional ECL performance of TABE-PZ-CON, it was utilized as a high-efficient ECL indicator in combination with the DNA walker and duplex-specific nuclease-assisted target recycling amplification strategies to design an “off-on” ECL biosensor for the ultrasensitive assay of microRNA-21, exhibiting a favorable response range (100 aM–1 nM) with an ultralow detection limit of 17.9 aM. Overall, this work offers a valid way to inhibit the intramolecular motions of AIEgens for ECL enhancement, which gives a new vision for building high-performance AIEgen-based ECL materials, thus offering more chances for assembling hypersensitive ECL biosensors.

Biomineralization-Inspired Synthesis of Hybrid COF Nanosheets toward Efficient Desalination Membranes.

Synthesis of covalent organic framework nanosheets (CONs) with high aspect ratio is crucial to their assembly into advanced membranes. Nonetheless, the π-π stacking between covalent organic framework (COF) layers often leads to thick CONs. Herein, inspired by biomineralization process, a series of aspect ratio CONs >15000 is synthesized by multifunctional polyelectrolytes which not only provide the nucleation sites for pre-assembly with COF monomer, but also suppress π-π interaction for anisotropic growth through protonation. The membrane assembled from CONs exhibited water permeance of 341kg m-2h-1 and salt rejection of 99.5% in desalination, outperforming ever-reported membranes. This method establishes a platform for the synthesis of crystalline nanosheets.