A hydroxyl-rich covalent organic framework for the precisely selective fluorescence sensing of explosives with high sensitivity

Covalent organic Frameworks (COFs) have become a new platform for functional research and material design. A novel covalent organic skeleton (DHB-TFP COF) was synthesized from 2-hydroxybenzene-1,3,5-tricarbaldehyde and 3,3'-dihydroxybenzidine using Schiff base reaction. DHB-TFP COF is a highly stable porous crystalline material and exhibits exceptional thermal and chemical resistance. DHB-TFP COF exhibited a selective and sensitive "turn-off" fluorescence response to 4-NP in ethanol, and TNP not only significantly quenched the fluorescence of DHB-TFP COF but also caused the obvious red-shift. The fluorescence intensity of DHB-TFP COF exhibited a linear correlation with the concentration of 4-NP with a detection limit of 0.40 μM. Furthermore, the maximum fluorescence peak observed for DHB-TFP COF demonstrated a linear relationship with TNP concentration with a detection limit of 11.15 μM. DHB-TFP COF exhibited satisfactory recovery in the detection of 4-NP and TNP in actual water sample indicating its practical application potential. The O atoms of rich hydroxyl and N atoms of C = N present on the surface of DHB-TFP COF scaffold can establish strong hydrogen bonds with 4-NP and TNP, facilitating their mutual interaction. The spectra studies indicated that the fluorescence quenching mechanism can be attributed to the absorption competitive quenching (ACQ) and fluorescence resonance energy transfer (FRET) mechanism. This study not only proposed the approach for synthesizing novel structured organic frameworks, but also developed a highly selective and sensitive fluorescence chemical sensor for identifying and detecting 4-NP and TNP.

Fast detection of tryptamine in meat products with azide-functionalized covalent organic frameworks confined in molecularly imprinted polymers

Tryptamine is a biogenic amine that affects organoleptic quality through the generation of off-odours in foods. Herein, imine-based covalent organic frameworks (COFs) were synthesized via Schiff base reactions and postmodified with click chemistry to generate azide-functionalized COFs with tunable azide units on the walls. The combination of molecular imprinting with COFs enabled the specific recognition of the targets. The resulting optosensing system (azide-functionalized COFs@MIPs) was used as a sample-to-answer analyser for detecting tryptamine (detection time within 10 min). A linear relationship was observed for the fluorescence response to tryptamine concentrations in the range of 3-120 μg L-1, with a limit of detection of 1.74 μg L-1. The recoveries for spiked samples were satisfactory, with relative standard deviations <9.90%. The optosensing system is a potential tool for the quantitative detection of tryptamine in meat products because of its lower cost, shorter processing time, and simpler processing steps compared to conventional chromatographic techniques.

Covalent organic frameworks (COFs) as carrier for improved drug delivery and biosensing applications

Porous organic frameworks (POFs) represent a significant subclass of nanoporous materials in the field of materials science, offering exceptional characteristics for advanced applications. Covalent organic frameworks (COFs), as a novel and intriguing type of porous material, have garnered considerable attention due to their unique design capabilities, diverse nature, and wide-ranging applications. The unique structural features of COFs, such as high surface area, tuneable pore size, and chemical stability, render them highly attractive for various applications, including targeted and controlled drug release, as well as improving the sensitivity and selectivity of electrochemical biosensors. Therefore, it is crucial to comprehend the methods employed in creating COFs with specific properties that can be effectively utilized in biomedical applications. To address this indispensable fact, this review paper commences with a concise summary of the different methods and classifications utilized in synthesizing COFs. Second, it highlights the recent advancements in COFs for drug delivery, including drug carriers as well as the classification of drug delivery systems and biosensing, encompassing drugs, biomacromolecules, small biomolecules and the detection of biomarkers. While exploring the potential of COFs in the biomedical field, it is important to acknowledge the limitations that researchers may encounter, which could impact the practicality of their applications. Third, this paper concludes with a thought-provoking discussion that thoroughly addresses the challenges and opportunities associated with leveraging COFs for biomedical applications. This review paper aims to contribute to the scientific community's understanding of the immense potential of COFs in improving drug delivery systems and enhancing the performance of biosensors in biomedical applications.

Photoelectric Multi-Signal Output Sensor Based on Two-Dimensional Covalent Organic Polymer Film Modified by Novel Aggregation-Induced Emission Probes

Optical sensors, especially fluorescence sensors, have been widely used because of their advantages in sensing, such as the high sensitivity, good selectivity, no radiation source, and easy operation. Here, we report an example of fluorescence sensing based on two-dimensional (2D) covalent organic polymers and highlight that the material can achieve a fast response and multi-signal output. This 2DPTPAK+TAPB-based sensor can quickly detect aromatic hydrocarbons and Fe3+ by the fluorescence signal or electrical resistance signal.

A pH-regulated fluorescence covalent organic framework for quantitative water content detection in methanol

In this paper, we designed and synthesized a two-dimensional fluorescent covalent organic framework (TAPB-DMTP-COF) for the precise determination of H2O content in methanol. The COF was synthesized using two typical monomers by grinding method, which significantly reduced the synthesis time. By adjusting the pH value of the COF suspension to 4.0, the portion of the COF material structure is disrupted, thereby mitigating π-π stacking and resolving the aggregation-caused quenching (ACQ) effect. Consequently, the non-fluorescent TAPB-DMTP-COF exhibited blue-purple fluorescence emission in methanol. At the same time, it is observed that in the presence of H2O, there is a red shift in the maximum fluorescence emission peak of TAPB-DMTP-COF, which correlates with the H2O content within a specific range. Notably, this redshift demonstrates a linear relationship with H2O content from 4% to 80% in methanol. Our work presents novel insights for efficient analysis and detection of H2O content in methanol and could be used for H2O detection in other organic solvents.

Constructing Highly Emissive Covalent Organic Frameworks for Fe3+ Ion Detection via Wall Function

Covalent organic frameworks (COFs) represent a new type of crystalline porous polymers that possess pre-designed skeletons, uniform nanopores, and ordered π structure. These attributes make them well-suited for the design of light-emitting materials. However, the majority of COFs exhibits poor luminescence due to aggregation-caused quenching (ACQ), resulting from the strong interaction between adjacent layers. To break the limitation, the building units with three methoxy groups on the walls are used to construct TM-OMe-EBTHz-COF, which suppresses the ACQ effects to improve light-emitting activity of COF. The TM-OMe-EBTHz-COF exhibits a notable emission of yellow-green luminescence in the solid state, with a remarkably high absolute quantum yield of 21.1%. The methoxy groups and hydrazine linkage form three coordination sites, contributing to excellent performance in metal ions sensing. The TM-OMe-EBTHz-COF demonstrates high sensitivity and selectivity to Fe3+ ion. Importantly, the low detection limit is below 150 nanomolar, ranking it among the best-performing Fe3+ sensor systems.

Sensing of picric acid using an AIEE active "Turn Off" fluorescent probe derived from hydroxy naphthaldehyde and benzyloxy benzaldehyde

A novel Schiff base with AIEE characteristics has been developed and used as a chemosensor for picric acid in aqueous media. The Schiff base 1-((E)-((E)-(4-(benzyloxy) benzylidene) hydrazono) methyl) naphthalen-2-ol [BBHN] with strong fluorescence emission was obtained by the simple condensation of 1-(hydrazonomethyl)naphthan-2-ol and 4-benzoxy benzaldehyde. The characterization of BBHN was done using Fourier Transfer Infra-Red, UV-visible, Magnetic Resonance (1H and 13C) spectroscopy, and HRMS. The sensing behaviour of BBHN aggregates towards nitro explosive was then investigated. The aggregates of BBHN showed a quick, highly selective, and sensitive fluorescence 'Turn Off' response towards picric acid (PA) in an aqueous medium among various other nitroaromatics. The limit of detection was 4.04 µM with 2.03 × 106 M-1 as the quenching constant. The fluorescence "Turn Off" response in the presence of PA is mainly due to π-π interactions, and non-covalent hydrogen bonding interactions. Moreover, steady-state fluorescence lifetime measurement and Stern - Volmer plots reveal that the fluorescence quenching followed mixed quenching strategies.

Drug delivery using biocompatible covalent organic frameworks (COFs) towards a therapeutic approach

Covalent organic frameworks (COFs) are constructed exclusively with lightweight organic scaffolds, which can have a 2D or 3D architecture. The ease of synthesis, robust skeleton and tunable properties of COFs make them superior candidates among their counterparts for a wide range of uses including biomedical applications. In the biomedical field, drug delivery or photodynamic-photothermal (PDT-PTT) therapy can be individually considered a potential parameter to be investigated. Therefore, this comprehensive review is focused on drug delivery using COFs, highlighting the encapsulation and decapsulation of drugs by COF scaffolds and their delivery in biological media including live cells. Versatile COF scaffolds together with the delivery of several drug molecules are considered. We attempted to incorporate the status of drug encapsulation and decapsulation considering a wide range of recent publications.

Large-Scale Synthesis of Covalent Organic Frameworks: Challenges and Opportunities

Connecting organic building blocks by covalent bonds to design porous crystalline networks has led to covalent organic frameworks (COFs), consequently transferring the flexibility of dynamic linkages from discrete architectures to extended structures. By virtue of the library of organic building blocks and the diversity of dynamic linkages and topologies, COFs have emerged as a novel field of organic materials that propose a platform for tailor-made complex structural design. Progress over the past two decades in the design, synthesis, and functional exploration of COFs in diverse applications successively established these frameworks in materials chemistry. The large-scale synthesis of COFs with uniform structures and properties is of profound importance for commercialization and industrial applications; however, this is in its infancy at present. An innovative designing and synthetic approaches have paved novel ways to address future hurdles. This review article highlights the fundamental of COFs, including designing principles, coupling reactions, topologies, structural diversity, synthetic strategies, characterization, growth mechanism, and activation aspects of COFs. Finally, the major challenges and future trends for large-scale COF fabrication are outlined.

Covalent organic framework-MnO2 nanoparticle composites for shape-selective sensing of bithiols

Covalent organic frameworks (COFs) for detecting biological macromolecules in water or biological environments are generally challenging. In this work, a composite material IEP-MnO₂ is obtained by combining manganese dioxide (MnO₂) nanocrystals and a fluorescent COF (IEP), which is synthesized by using 2,4,6-tris(4-aminophenyl)-s-triazine and 2,5-dimethoxyterephthalaldehyde. By the addition of biothiols, such as glutathione, cysteine or homocysteine with different sizes, the fluorescence emission spectra of IEP-MnO₂ changed ("turn-on" or "turn-off") via different mechanisms. The fluorescence emission of IEP-MnO₂ increased in the presence of GSH by the elimination of the FRET (Förster resonance energy transfer) effect between MnO₂ and IEP. Surprisingly, due to the formation of a hydrogen bond between Cys/Hcy and IEP, the fluorescence quenching for IEP-MnO₂ + Cys/Hcy may be explained via the photoelectron transfer (PET) process, which endows IEP-MnO₂ with specificity in distinguishing the detection of GSH and Cys/Hcy compared to other MnO₂ complex materials. Therefore, IEP-MnO₂ was used to detect GSH and Cys in human whole blood and serum, respectively. The limit of detection for GSH in whole blood and Cys in human serum was calculated to be 25.58 μM and 4.43 μM, which indicates that IEP-MnO₂ can be used to investigate some diseases related to GSH and Cys concentration. Moreover, the research expands the application of covalent organic frameworks in the fluorescence sensing field.

Covalent Triazine Framework C6N6 as an Electrochemical Sensor for Hydrogen-Containing Industrial Pollutants. A DFT Study

Industrial pollutants pose a serious threat to ecosystems. Hence, there is a need to search for new efficient sensor materials for the detection of pollutants. In the current study, we explored the electrochemical sensing potential of a C₆N₆ sheet for H-containing industrial pollutants (HCN, H₂S, NH₃ and PH₃) through DFT simulations. The adsorption of industrial pollutants over C₆N₆ occurs through physisorption, with adsorption energies ranging from -9.36 kcal/mol to -16.46 kcal/mol. The non-covalent interactions of analyte@C₆N₆ complexes are quantified by symmetry adapted perturbation theory (SAPT0), quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analyses. SAPT0 analyses show that electrostatic and dispersion forces play a dominant role in the stabilization of analytes over C₆N₆ sheets. Similarly, NCI and QTAIM analyses also verified the results of SAPT0 and interaction energy analyses. The electronic properties of analyte@C₆N₆ complexes are investigated by electron density difference (EDD), natural bond orbital analyses (NBO) and frontier molecular orbital analyses (FMO). Charge is transferred from the C₆N₆ sheet to HCN, H₂S, NH₃ and PH₃. The highest exchange of charge is noted for H₂S (-0.026 e^-). The results of FMO analyses show that the interaction of all analytes results in changes in the E_H-L gap of the C₆N₆ sheet. However, the highest decrease in the E_H-L gap (2.58 eV) is observed for the NH₃@C₆N₆ complex among all studied analyte@C₆N₆ complexes. The orbital density pattern shows that the HOMO density is completely concentrated on NH₃, while the LUMO density is centred on the C₆N₆ surface. Such a type of electronic transition results in a significant change in the E_H-L gap. Thus, it is concluded that C₆N₆ is highly selective towards NH₃ compared to the other studied analytes.

The preparation of 2,2'-bithiophene-based conjugated microporous polymers by direct arylation polymerization and their application in fluorescence sensing 2,4-dinitrophenol

In this paper, we report the synthetic strategy of direct arylation polymerization (DAP) for four 2,2'-bithiophene-based conjugated microporous polymers (the 2,2'-BTh-based CMPs) by coupling 2,2'-bithiophene with the building blocks containing bromine. Compared to conventional coupling polymerization, this synthetic scheme is simple, facile and atomically efficient owing to neither preactivating the C-H bonds in 2,2'-bithiophene using organometallic reagents nor synthesis of complex thiophene-based building blocks. The resulting 2,2'-BTh-based CMPs exhibit excellent thermal stability, high specific surface areas, and good microporosity. Their specific surface areas are higher than that of other previously reported CMPs prepared with DAP. The four 2,2'-BTh-based CMPs can be utilized for multicolor fluorescence sensing of 2,4-dinitrophenol (DNP) with the high sensitivity and selectivity. The sensitivities appear to increase with the degree of structural distortion.

Fully Flexible Covalent Organic Frameworks for Fluorescence Sensing 2,4,6-Trinitrophenol and p-Nitrophenol

Nitrophenols are important nitroaromatic compounds, both important environmental pollutants and dangerous explosives, posing a devastating danger and pollution threat to humans. It is vital to detect efficiently trace nitrophenols in the environment. In this contribution, a series of fully flexible cyclotriphosphazene-based COFs (FFCP COFs: HDADE, HBAPB, and HBPDA), prepared with both a flexible knot and flexible linkers of different lengths, were used for sensing 2,4,6-trinitrophenol (TNP) and p-nitrophenol (p-NP) in real time with excellent sensitivity and selectivity. The quenching constants of HDADE by TNP, HBAPB, and HBPDA by p-NP are 6.29 × 10⁴, 2.17 × 10⁵, and 2.48 × 10⁵ L·mol^-1, respectively. The LODs of TNP and p-NP are 1.19 × 10^-11, 6.91 × 10^-12, and 6.05 × 10^-12 mol·L^-1. Their sensitivities increase with the linker length, which is better than the corresponding COFs composed of rigid linkers. There is only a photoinduced electron transfer mechanism in the fluorescence quenching of HBPDA by p-NP. Meanwhile, the mechanisms of photoinduced charge transfer and resonance energy transfer exist in the fluorescence quenching of HDADE by TNP and the fluorescence quenching of HBAPB by p-NP.

Amine/hydrazone functionalized Cd(II)/Zn(II) metal-organic framework for ultrafast sensitive detection of hazardous 2,4,6-trinitrophenol in water

Amine/hydrazone functionalized dual ligand Cd(II)/Zn(II) based metal-organic frameworks (MOFs) denoted as CdMOF- and ZnMOF-NH₂, respectively were synthesized via a simple conventional high-yield reflux method using low-cost and readily available starting materials, i.e., a Schiff base linker, 4-pyridylcarboxaldehydeisonicotinoylhydrazone (L₁) and 2-aminoterephthalic acid (H₂ata) linker. Crystallographic and thermogravimetric studies confirmed the formation of MOFs with good crystallinity and thermal stability. Photoluminescence studies point out that both MOFs can be used efficiently for fast sensing of 2,4,6-trinitrophenol (TNP) in water with noticeable turn-off quenching response. Their limits of detection (LODs) for TNP were 7 ppb and 10 ppb, respectively with enhanced selectivity toward TNP (over other nitro explosives) as verified by competitive nitro explosive tests. Density functional theory calculations and spectral overlap were used to assess the sensing mechanism. These MOF-based fluorescent sensing systems for TNP are demonstrated to have easy recoverability and high sensitivity.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.

Fluorescence turn on amine detection in a cationic covalent organic framework

Ionic covalent organic frameworks (iCOFs) are new examples of porous materials and have shown great potential for various applications. When functionalized with suitable emission sites, guest uptake via the ionic moieties of iCOFs can cause a significant change in luminescence, making them excellent candidates for chemosensors. In here, we present a luminescence sensor in the form of an ionic covalent organic framework (TGH⁺•PD) composed of guanidinium and phenanthroline moieties for the detection of ammonia and primary aliphatic amines. TGH⁺•PD exhibits strong emission enhancement in the presence of selective primary amines due to the suppression of intramolecular charge transfer (ICT) with an ultra-low detection limit of 1.2 × 10^‒7 M for ammonia. The presence of ionic moieties makes TGH⁺•PD highly dispersible in water, while deprotonation of the guanidinium moiety by amines restricts its ICT process and signals their presence by enhanced fluorescence emission. The presence of ordered pore walls introduces size selectivity among analyte molecules, and the iCOF has been successfully used to monitor meat products that release biogenic amine vapors upon decomposition due to improper storage.

Ionic covalent organic frameworks (iCOFs) are new examples of porous materials and show great potential for various applications. Here, the authors demonstrate functionalization of an iCOFs with suitable emission sites and application as chemosensor for amine detection with high sensitivity which can be used to monitor meat spoilage.

Triangular Topological 2D Covalent Organic Frameworks Constructed via Symmetric or Asymmetric "Two-in-One" Type Monomers

Most of the reported covalent organic frameworks (COFs) so far are prepared from highly symmetric building blocks, which to some extent limits the expansion of COF diversity and complexity. Low-symmetric building blocks can be designed through a desymmetrized vertex strategy, which might be used to construct new topological COFs. But reports of COFs constructed by asymmetric building blocks are thus far very rare. Here, a feasible strategy to design asymmetric building blocks for COF synthesis is introduced, by simply varying the positions of functional groups in the monomer. As a proof of concept, two isomeric hexaphenylbenzene-based "two-in-one" type monomers (1,2,4-HPB-NH₂ and 1,3,5-HPB-NH₂ ) are designed and synthesized. To the authors' surprise, self-polycondensation of the asymmetric 1,2,4-HPB-NH₂ (i.e., the isomer of common C₃ -symmetric 1,3,5-HPB-NH₂ ) also affords highly crystalline COF (1,2,4-HPB-COF) similar to the symmetric 1,3,5-HPB-NH₂ counterpart with identical topological structure. The triangular porous structures of both HPB-based COFs are well resolved by powder X-ray diffraction (PXRD), theoretical simulations, nitrogen sorption, and morphologies analysis. This work demonstrates the "two-in-one" type asymmetric building blocks can also produce highly crystalline frameworks and thus provides a new structural design strategy for reticular chemistry.

Excellent Crystallinity and Stability Covalent-Organic Frameworks with High Emission and Anions Sensing

Covalent-organic frameworks (COFs) are a new class of porous crystalline frameworks with high π-conjugation and periodical skeletons. The highly ordered π-conjugation structures in some COFs allow exciton migration and energy transfer over the frameworks, which leads to good fluorescence probing ability. In this work, two COFs (TFHPB-TAPB-COF and TFHPB-TTA-COF) are successfully condensed via the Schiff base condensation reaction. The intramolecular hydrogen bonds between imine bonds and hydroxyl groups form the excited-state intramolecular proton transfer (ESIPT) strategy. Owing to intramolecular hydrogen bonds in the skeleton, the two COFs show high crystallinity, remarkable stability, and excellent luminescence. The COFs represent a good sensitivity and selectivity to fluoride anions via fluorescence turn-off. Other halogen anions (chloride, bromide, and iodine) and acid anions (nitrate and hydrogen carbonate) remain inactive. These results imply that only fluoride anion is capable of opening the hydrogen bond interaction and hence break the ESIPT strategy. The detection limit toward fluoride anion is down to nanomoles level, ranking the best performances among fluoride anion sensors systems.

Porous organic polymers as a platform for sensing applications

Sensing analysis is significantly important for human health and environmental safety, and has gained increasing concern. As a promising material, porous organic polymers (POPs) have drawn widespread attention due to the availability of plentiful building blocks and their tunable structures, porosity and functions. Moreover, the permanent porous nature could provide a micro-environment to interact with guest molecules, rendering POPs attractive for application in the sensing field. In this review, we give a comprehensive overview of POPs as a platform for sensing applications. POP-based sensors are mainly divided into five categories, including fluorescence turn-on sensors, fluorescence turn-off sensors, ratiometric fluorescent sensors, colorimetric sensors and chemiresistive sensors, and their various sensing applications in detecting explosives, metal ions, anions, small molecules, biological molecules, pH changes, enantiomers, latent fingerprints and thermosensation are summarized. The different structure-based POPs and their corresponding synthetic strategies as well as the related sensing mechanisms mainly including energy transfer, donor-acceptor electron transfer, absorption competition quenching and inner filter effect are also involved in the discussion. Finally, the future outlook and perspective are addressed briefly.

Application of imine covalent organic frameworks in sample pretreatment

亚胺类共价有机骨架(I-COFs)是有机单体根据席夫碱(Schiff-base)反应原理缩合形成的一类新型多孔晶体有机材料。I-COFs具有骨架密度低、比表面积大、孔隙率高、单体种类丰富、孔径尺寸可控、结构可功能化、合成方法多样和物化稳定性好等优点。近年来,I-COFs已成为材料科学领域的研究前沿,并广泛用于气体吸附、存储、催化、传感、光电材料等方面。I-COFs材料优异的物理化学性能使其非常适于用作复杂样品中痕量目标物的分离富集介质,其高比表面积、高孔隙率性能赋予了它极高的吸附负载量,这些性能使得目标分析物可被高效富集;通过控制有机单体的链段长度、几何结构、掺杂元素、取代基团等方面精确调控I-COFs的孔洞结构和功能化基团,从而实现目标痕量物质的选择性富集。目前,I-COFs材料在样品前处理领域作为新型萃取介质已引起了极大关注。该文综述了近年来I-COFs材料的主要类型、合成方法及其在固相萃取、磁性固相萃取、分散固相萃取和固相微萃取方面的研究进展,同时展望了I-COFs在样品前处理领域的发展前景。

Design of terbium (III)-functionalized covalent organic framework as a selective and sensitive turn-on fluorescent switch for ochratoxin A monitoring

For the first time, we develop a terbium (III)-functionalized covalent organic framework named Dpy-NhBt-COF@Tb³⁺, through anchoring Tb³⁺ onto a two-dimensional imine COF (Dpy-NhBt-COF), as the selective and sensitive turn-on fluorescent switch for ochratoxin A (OTA) monitoring. Of particular significance, Tb³⁺ actually plays two roles during sensing process: the specific response signal, and exclusive recognition sites for OTA, while Dpy-NhBt-COF acts as the protector for Tb³⁺. The sensing process involves the replacement of coordinated water molecules from Tb³⁺ by OTA and the energy transfer from OTA to Tb³⁺ centers, resulting in remarkable fluorescence emergence of Tb³⁺. The stabilization of Tb³⁺ via coordination with bipyridine of Dpy-NhBt-COF not only reduces the nonselective binding of naturally occurring ligands, but also avoids the non-radiative quenching caused by solvents molecules. As a sensing platform, Dpy-NhBt-COF@Tb³⁺ possesses noticeable selectivity and high sensitivity toward OTA with an ultralow detection limit of 13.5 nM and rapid response of 10 s. Taken together, our work not only demonstrates great prospect of Tb³⁺-functionalized COF for OTA detection, but also provides a potential way to explore other functionalized materials as promising sensors for other targets.

Robust and emissive covalent organic frameworks via intramolecular hydrogen bond interaction

Covalent organic frameworks (COFs) are a new class of crystalline porous polymers with periodic structure in the skeleton and pre-designable pore structure. COFs merge excellent crystallinity, porosity, stability, and emission,...

Luminescent Covalent Organic Frameworks for Biosensing and Bioimaging Applications

Luminescent covalent organic frameworks (LCOFs) have attracted significant attention due to their tunability of structures and photophysical properties at molecular level. LCOFs are built to highly ordered and periodic 2D or 3D framework structures through covalently assembling with various luminophore building blocks. Recently, the advantages of LCOFs including predesigned properties of structure, unique photoluminescence, hypotoxicity and good biocompatibility and tumor penetration, broaden their applications in biorelated fields, such as biosensing, bioimaging, and drug delivery. A specific review that analyses the advances of LCOFs in the field of biosensing and bioimaging is thus urged to emerge. Here the construction of LCOFs is reviewed first. The synthetic chemistry of LCOFs highlights the key role of chemical linkages, which not only concrete the building blocks but also affect the optical properties and even can act as the responsive sites for potential sensing applications. How to brighten LCOFs are clarified through description of structure managements. The ability to utilize the luminescence of LCOFs for applications in biosensing and bioimaging is discussed using state-of-the-art examples of varied practical goals. A prospect finally addresses opportunities and challenges the development of LCOFs facing from chemistry, physics to the applications, according to their current progress.

Excited-state intramolecular proton transfer based covalent organic framework for fluorescence anions sensing

An azine linked covalent organic framework, ACOF, has been constructed via hydrazine hydrate and aldehyde group building unit with hydroxyl group in situ under the solvothermal condition. ACOF possesses good...

Covalent organic frameworks as multifunctional materials for chemical detection

Sensitive and selective detection of chemical and biological analytes is critical in various scientific and technological fields. As an emerging class of multifunctional materials, covalent organic frameworks (COFs) with their unique properties of chemical modularity, large surface area, high stability, low density, and tunable pore sizes and functionalities, which together define their programmable properties, show promise in advancing chemical detection. This review demonstrates the recent progress in chemical detection where COFs constitute an integral component of the achieved function. This review highlights how the unique properties of COFs can be harnessed to develop different types of chemical detection systems based on the principles of chromism, luminescence, electrical transduction, chromatography, spectrometry, and others to achieve highly sensitive and selective detection of various analytes, ranging from gases, volatiles, ions, to biomolecules. The key parameters of detection performance for target analytes are summarized, compared, and analyzed from the perspective of the detection mechanism and structure-property-performance correlations of COFs. Conclusions summarize the current accomplishments and analyze the challenges and limitations that exist for chemical detection under different mechanisms. Perspectives on how future directions of research can advance the COF-based chemical detection through innovation in novel COF design and synthesis, progress in device fabrication, and exploration of novel modes of detection are also discussed.

Synthesis of Copper Metal Organic Framework Based on Schiff Base Tricarboxylate Ligand for Highly Selective and Sensitive Detection of 2,4,6-Trinitrophenol in Aqueous Medium

By using Schiff base tricarboxylate ligand 5-(4-carboxybenzylideneamino)isophthalic acid (H₃CIP), a new imine functionalized copper metal organic framework (MOF) has been synthesized solvothermally. It was fully characterized by Fourier Transform Infrared (FTIR) Spectroscopy, Powder X-Ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and elemental mapping techniques. The as-synthesized MOF has been utilized as fluorescent probe for detection of nitro aromatic explosives (NAEs). The results show that the copper MOF can be developed into highly selective and sensitive sensor for detection of TNP in the aqueous medium via the "turn-off" quenching response. The linear fitting of the Stern-Volmer plot for TNP offered large quenching constant of 1.07 × 10⁴ M^-1 for Cu-MOF indicating the high sensitivity of the sensing process. Outstanding sensitivity of prepared material towards TNP detection was further validated by the low detection limit of 80 ppb (0.35 µM). The detailed mechanistic studies for their mode of action and density functional theory (DFT) calculations reveals that photo-induced electron transfer (PET) and fluorescence resonance energy transfer (FRET) processes, as well as electrostatic interactions (i.e. H-bonding) are the key factors for the turn-off response toward TNP by this fluorescent sensor. Thus, this new LMOF owing to their high water stability and remarkable functional features are potential candidates which can be developed into selective and sensitive TNP detection devices.

Porous 2D and 3D Covalent Organic Frameworks with Dimensionality-Dependent Photocatalytic Activity in Promoting Radical Ring-Opening Polymerization

Dimensionality is a fundamental parameter to modulate the properties of solid materials by tuning electronic structures. Covalent organic frameworks (COFs) are a prominent class of porous crystalline materials, but the study of dimensional dependence on their physicochemical properties is still lacking. Herein we illustrate photocatalytic performances of N,N-diaryl dihydrophenazine (PN)-based COFs are heavily dependent on the structural dimensionality. Six isostructural imine-bonded 2D-PN COFs and one 3D-PN COF were prepared. All can be heterogeneous photocatalysts to promote radical ring-opening polymerization of vinylcyclopropanes (VCPs), which typically produces polymers with a combination of linear (l) and cyclic (c) repeat units. The 2D-PN COFs have much higher catalytic activity than the 3D-PN COF, allowing the efficient synthesis of poly(VCPs) with controlled molecular weight, low dispersity and high l/c selectivity (up to 97 %). The improved performance can be ascribed to the 2D structure which has a larger internal surface area, more catalytically active sites, higher photosensitizing ability and photoinduced electron transfer efficiency.

A new hydrazone-linked covalent organic framework for Fe(iii) detection by fluorescence and QCM technologies

A new stable hydrazone-linked covalent organic framework was synthesized, which can be used as either a fluorescent probe or a COF-based QCM sensor for the identification of metal ions, showing high selectivity and sensitivity toward Fe(iii) ions.

Self-assembled porous nanoparticles based on silicone polymers with aggregation-induced emission for highly sensitive detection of nitroaromatics

Tetraphenylbenzene functionalized polysiloxane with AIE feature can self-assemble to unique porous structure and show high performance as fluorescent sensor.

A tetraphenylethylene-based covalent organic framework for waste gas adsorption and highly selective detection of Fe3+

A tetraphenylethylene-based covalent organic framework shows an outstanding performance for waste gas adsorption and has good selectivity and detection effect for Fe3+.

Optoelectronic processes in covalent organic frameworks

Covalent organic frameworks (COFs) are crystalline porous materials constructed from molecular building blocks using diverse linkage chemistries. The image illustrates electron transfer in a COF-based donor–acceptor system. Image by Nanosystems Initiative Munich.

Crystalline C-C and C═C Bond-Linked Chiral Covalent Organic Frameworks

While crystalline covalent organic frameworks (COFs) linked by C-C bonds are highly desired in synthetic chemistry, it remains a formidable challenge to synthesize. Efforts to generate C-C single bonds in COFs via de novo synthesis usually afford amorphous structures rather than crystalline phases. We demonstrate here that C-C single bond-based COFs can be prepared by direct reduction of C═C bond-linked frameworks via crystal-to-crystal transformation. By Knoevenagel polycondensation of chiral tetrabenzaldehyde of dibinaphthyl-22-crown-6 with 1,4-phenylenediacetonitrile or 4,4'-biphenyldiacetonitrile, two olefin-linked chiral COFs with 2D layered tetragonal structure are prepared. Reduction of olefin linkages of the as-prepared CCOFs produces two C-C single bond linked frameworks, which retains high crystallinity and porosity as well as high chemical stability in both strong acids and bases. The quantitative reduction is confirmed by Fourier transform infrared and cross-polarization magic angle spinning ¹³C NMR spectroscopy. Compared to the pristine structures, the reduced CCOFs display blue-shifted emission with enhanced quantum yields and fluorescence lifetimes, while the parent CCOFs exhibit higher enantioselectivity than the reduced analogs when be used as fluorescent sensors to detect chiral amino alcohols via supramolecular interactions with the built-in crown ether moieties. This work provides an attractive strategy for making chemically stable functionalized COFs with new linkages that are otherwise hard to produce.

Transformation between 2D covalent organic frameworks with distinct pore hierarchy via exchange of building blocks with different symmetries

Transformation between 2D covalent organic frameworks (COFs) via exchange of molecular building blocks with different symmetries has been realized, which gives rise to the conversion between 2D COFs with distinct pore hierarchy. This type of monomer replacement has expanded the scope of the building-unit-exchange-based COF-to-COF transformation strategy.

Nanoscale covalent organic frameworks as theranostic platforms for oncotherapy: synthesis, functionalization, and applications

Cancer nanomedicine is one of the most promising domains that has emerged in the continuing search for cancer diagnosis and treatment. The rapid development of nanomaterials and nanotechnology provide a vast array of materials for use in cancer nanomedicine. Among the various nanomaterials, covalent organic frameworks (COFs) are becoming an attractive class of upstarts owing to their high crystallinity, structural regularity, inherent porosity, extensive functionality, design flexibility, and good biocompatibility. In this comprehensive review, recent developments and key achievements of COFs are provided, including their structural design, synthesis methods, nanocrystallization, and functionalization strategies. Subsequently, a systematic overview of the potential oncotherapy applications achieved till date in the fast-growing field of COFs is provided with the aim to inspire further contributions and developments to this nascent but promising field. Finally, development opportunities, critical challenges, and some personal perspectives for COF-based cancer therapeutics are presented.

Highly sensitive and ultrafast film sensor based on polyethyleneimine-capped quantum dots for trinitrophenol visual detection

Real-time detection of nitroaromatic explosives is a pressing problem for public security and environmental monitoring. Although many efforts have been devoted, sensing of explosives still suffers limitations of complicated and time-consuming sensing procedure. In this work, we develop a fluorescent film for rapid and visual detection of 2,4,6-trinitrophenol (TNP) by using polyethyleneimine-capped quantum dots (QDs-PEI) as fluorescent sensing probe and electrospun membrane as matrix. Fluorescent sensing is mainly based on effective reaction between amino groups of PEI and nitro groups, phenol hydroxyl groups of TNP. It also benefits from the rapid absorption of TNP in aqueous solution by PA6 nanofiber membranes with high hydrophilicity and porosity. As a result, visual sensing could be realized when TNP is >100 ng·mL^-1 by obvious fluorescent change with a thirty-second response time. We believe this fluorescent membrane sensor holds a promising prospect for real time and visual detection of TNP in environmental science and analytical fields.

Covalent organic frameworks as a sensing platform for water in organic solvent over a broad concentration range

A highly crystalline covalent organic frameworks (COFs) formed by condensation reaction between 1, 3, 5-tris (4-aminophenyl) benzene and 4, 4'-biphenyldicarboxaldehyde is utilized as a sensing platform for water in organic solvent over a broad concentration range. The resulting COFs exhibits brilliant fluorescence in various organic solvents such as methanol, DMF, acetonitrile and ethanol, moreover its fluorescence intensity has a significant and rapid response to the content of water in organic solvent over a broad concentration range. The broadest sensing range is achieved over 7%-70% (v/v) for water in DMF, and the lowest limit of detection is 0.042% (v/v) for water in methanol among the investigated organic solvents. The superior properties of the sensing platform expand the application ranges of COFs and endow the resulting COFs with a great prospect in practical applications for highly efficient detecting water in organic solvent.

One-Pot Fabrication of Pd Nanoparticles@Covalent-Organic-Framework-Derived Hollow Polyamine Spheres as a Synergistic Catalyst for Tandem Catalysis

Facile fabrication of nanocatalysts consisting of metal nanoparticles (NPs) anchored on a functional support is highly desirable, yet remains challenging. Covalent organic frameworks (COFs) provide an emerging materials platform for structural control and functional design. Here, a facile one-pot in situ reduction approach is demonstrated for the encapsulation of small Pd NPs into the shell of COF-derived hollow polyamine spheres (Pd@H-PPA). In the one-pot synthetic process, the nucleation and growth of Pd NPs in the cavities of the porous shell take place simultaneously with the reduction of imine linkages to secondary amine groups. Pd@H-PPA shows a significantly enhanced catalytic activity and recyclability in the tandem dehydrogenation of ammonia borane and selective hydrogenation of nitroarenes through an adsorption-activation-reaction mechanism. The strong interactions of the secondary amine linkage with borane and nitroarene molecules afford a positive synergy to promote the catalytic reaction. Moreover, the hierarchical structure of Pd@H-PPA allows the accessibility of active Pd NPs to reactants.

The effects of the crosslinking position and degree of conjugation in perylene tetraanhydride bisimide microporous polymers on fluorescence sensing performance

In this study, two fluorescence conjugated microporous polymers based on perylene tetraanhydride bisimide (DP4A0 and DP4A2) were prepared via Sonogashira-Hagihara cross-coupling polymerization for the efficient detection of o-nitrophenol (o-NP). They were well characterized via FT-IR, solid state 13C NMR, elemental analysis, and other material characterization techniques. The experiments proved that both CMPs possess high thermal and chemical stability and a porous nature with Brunauer-Emmett-Teller (BET) specific surface areas of 41.3 and 402.1 m2 g-1. Importantly, owing to signal amplification by the conjugated skeleton, DP4A0 and DP4A2 exhibit extremely high sensitivity to o-NP with K sv values of 1.83 × 104 and 1.69 × 104 L mol-1 and limits of detection of 5.73 × 10-9 and 7.36 × 10-9 mol L-1, respectively. The sensing performance of DP4A0 and DP4A2 was dependent on the position of crosslinking points and crosslinking density. Finally, super amplified quenching was considered the electron transfer mechanism and hydrogen bond interactions were also present.

Covalent Organic Frameworks: Design, Synthesis, and Functions

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Two-dimensional covalent organic frameworks with hierarchical porosity

This review highlights the state-of-the-art progress achieved in two-dimensional covalent organic frameworks (COFs) with hierarchical porosity, an emerging class of COFs constructed by integrating different types of pores into one framework.