Ultrathin two-dimensional covalent organic framework nanoprobe for interference-resistant two-photon fluorescence bioimaging

Multicolor Covalent Organic Framework-DNA Nanoprobe for Fluorescence Imaging of Biomarkers with Different Locations in Living Cells

Precisely detecting biomarkers in living systems holds tremendous promise for disease diagnosis and monitoring. Herein, we developed a covalent organic framework (COF)-based tricolor fluorescent nanoprobe for simultaneously imaging biomarkers with different spatial locations in living cells. Briefly, a TAMRA-labeled survivin mRNA antisense nucleotide and a Cy5-labeled transmembrane glycoprotein mucin 1 (MUC1) aptamer were adsorbed on a nanoscale fluorescent COF. To enhance the interactions between COF nanoparticles (NPs) and nucleic acid molecules, a freezing method was employed for improving the nucleic acid loading density and ensuring detection performance. The fluorescence signals of dyes on DNAs were first quenched by the COF NPs. Internalization and distribution of the nanoprobes can be real-time visualized by the autofluorescence of COF NPs. In living cells, recognition between MUC1 with MUC1 aptamers causes fluorescence signal recovery of Cy5, while hybridization between survivin mRNA and its antisense DNA induces the signal recovery of TAMRA. Therefore, this COF-based multicolor nanoprobe could be employed for visualizing MUC1 on the cell membrane and survivin mRNA in the cytoplasm. Cancer cell-specific diagnostic imaging and monitoring of the process of cancer cell exosomes infecting normal cells using the nanoprobe were achieved. This work not only offers a versatile nanoprobe for bioanalysis but also provides new insights for developing novel COF-based nanoprobes.

Two-dimensional guanidinium-based covalent organic nanosheets for controllable recognition and specific enrichment of global/multi-phosphopeptides

Highly specific capture of phosphopeptides, especially multi-phosphopeptides, from complex biological samples is critical for comprehensive phosphoproteomic analysis, but it still poses great challenges due to the lack of affinity material with ideal enrichment efficiency. Here, two-dimensional (2D) covalent organic framework (COFs) nanosheets was applied for selective separation of phosphopeptides for the first time. Particularly, by incorporating guanidinium units, the 2D guanidinium-based COF nanosheets (denoted as TpTG_Cl CONs) exhibited controllable and specific enrichment performance towards global/multi-phosphopeptides. TpTG_Cl CONs was easy to prepare and showed large surface area, low steric hindrance, abundant accessible interaction sites and high chemical stability. Taking these merits together, TpTG_Cl CONs exhibited excellent efficiency for phosphopeptide enrichment, such as low detection limits (0.05 fmol μL^-1 for global phosphopeptides and 0.1 fmol μL^-1 for multi-phosphopeptides), high selectivity (1:5000 of molar ratios of β-casein/BSA for both global and multi-phosphopeptides), high adsorption capacity (100 mg g^-1 for global phosphopeptides and 50 mg g^-1 for multi-phosphopeptides). Furthermore, TpTG_Cl CONs could be reused due to the high chemical stability. In addition, TpTG_Cl CONs were successfully applied to controllable and specific capture of endogenous global/multi-phosphopeptides from human serum and human saliva, indicating its good potential in rapid and sensitive detection of biomarkers from biological fluid. Finally, rat liver protein digest was used to confirm the high specificity of TpTG_Cl CONs towards multi-phosphopeptides and demonstrated its potential as an ideal enrichment probe for comprehensive phosphoproteomic analysis.

Covalent Organic Framework-Derived Carbonous Nanoprobes for Cancer Cell Imaging

Covalent organic frameworks (COFs) have emerged as promising materials for biomedical applications, but their functions remain to be explored and the potential toxicity concerns should be resolved. Herein, it is presented that carbonization significantly enhances the fluorescence quenching efficiency and aqueous stability of nanoscale COFs. The probes prepared by physisorbing dye-labeled nucleic acid recognition sequences onto the carbonized COF nanoparticles (termed C-COF) were employed for cell imaging, which could effectively light up biomarkers (survivin and TK1 mRNA) in living cells. The C-COF has enhanced photothermal conversion capacity, indicating that the probes are also promising candidates for photothermal therapy. The potential toxicity concern from the aromatic rigid building units of COFs was detoured by carbonization. Overall, carbonization is a promising strategy for developing biocompatible and multifunctional COF-derived nanoprobes for biomedical applications. This work may inspire more versatile COF-derived nanoprobes for bioanalysis and nanomedicine.

Covalent Organic Framework-Based Spherical Nucleic Acid Probe with a Bonding Defect-Amplified Modification Strategy

Developing spherical nucleic acids with new structures holds great promise for nanomedicine and bioanalytical fields. Covalent organic frameworks (COFs) are emerging promising materials with unique properties for a wide range of applications. However, devising COF-based spherical nucleic acid is challenging because methods for the preparation of functionalized COFs are still limited. We report here a bonding defect-amplified modification (BDAM) strategy for the facile preparation of functionalized COFs. Poly(acrylic acid) was employed as the defect amplifier to modify the surface of COF nanoparticles by the formation of amide bonds with amino residues, which successfully converted and amplified the residues into abundant reactive carboxyl groups. Then, amino terminal-decorated hairpin DNA was densely grafted onto the surface of COF nanoparticles (NPs) to give rise to a spherical nucleic acid probe (SNAP). A series of experiments and characterizations proved the successful preparation of the COF-based SNAP, and its application in specifically lighting up RNA biomarkers in living cells for cancer diagnostic imaging was demonstrated. Therefore, the COF-based SNAP is a promising candidate for biomedical applications and the proposed BDAM represents a useful strategy for the preparation of functionalized COFs for diverse fields.

Nucleic Acid-Gated Covalent Organic Frameworks for Cancer-Specific Imaging and Drug Release

Developing nanoplatforms that simultaneously integrate diagnostic imaging and therapy functions has been a promising but challenging task for cancer theranostics. Herein, we report the rational design of a smart nucleic acid-gated covalent organic framework (COF) nanosystem for cancer-specific imaging and microenvironment-responsive drug release. Cy5 dye-labeled single-stranded DNA (ssDNA) for mRNA recognition was adsorbed on the surface of doxorubicin (Dox)-loaded COF nanoparticles (NPs). Dox loaded in the pores of COF NPs could strengthen the interactions between ssDNA and COF and enhance the fluorescence quenching effect toward Cy5, while the densely coated ssDNA could prevent the leakage of Dox from COF NPs. The obtained nanosystem exhibited low fluorescence signal and Dox release in normal cells; however, the ssDNA could be released by the overexpressed TK1 mRNA in cancer cells to recover the intense fluorescence signal of Cy5, and the loaded Dox could be further released for chemotherapy. Therefore, cancer cell-specific diagnostic imaging and drug release were realized with the rationally developed nanosystem. This work offers a universal nanoplatform for cancer theranostics and a promising strategy for regulating the interaction between COFs and biomolecules.

Recent Progress in Nanoscale Covalent Organic Frameworks for Cancer Diagnosis and Therapy

Covalent organic frameworks (COFs) as a type of porous and crystalline covalent organic polymer are built up from covalently linked and periodically arranged organic molecules. Their precise assembly, well-defined coordination network, and tunable porosity endow COFs with diverse characteristics such as low density, high crystallinity, porous structure, and large specific-surface area, as well as versatile functions and active sites that can be tuned at molecular and atomic level. These unique properties make them excellent candidate materials for biomedical applications, such as drug delivery, diagnostic imaging, and disease therapy. To realize these functions, the components, dimensions, and guest molecule loading into COFs have a great influence on their performance in various applications. In this review, we first introduce the influence of dimensions, building blocks, and synthetic conditions on the chemical stability, pore structure, and chemical interaction with guest molecules of COFs. Next, the applications of COFs in cancer diagnosis and therapy are summarized. Finally, some challenges for COFs in cancer therapy are noted and the problems to be solved in the future are proposed.

Recent advances in covalent organic frameworks for cancer diagnosis and therapy

In recent years, the number of patients diagnosed with cancer has been soaring. Therefore, the design, development, and implementation of new approaches for the diagnosis and therapy of different types of cancers have attracted an increasing amount of attention. To date, different methods have been used for cancer diagnosis and therapy with main drawbacks in terms of severe side effects, e.g., damage to healthy cells, development of drug resistance and tumor recurrence. Therefore, there is an urgent need for the introduction and application of innovative methods. Covalent organic frameworks (COFs) are versatile materials with excellent properties in terms of biocompatibility, porous and crystalline structure, and easy functionalization. The porous structure and organic monomers in COFs allow them to load different therapeutic drugs and/or functional species efficiently. These promising properties make COFs ideal candidates for medical application, especially in cancer diagnosis and therapy. To date, many studies have focused on the design and synthesis of novel COFs while their application as diagnostic and therapeutic materials remains less understood. In this review, different synthesis and functionalization approaches of COFs were summarized. In particular, cancer diagnosis and therapy based on COFs were investigated and the advantages and limitations of each method were discussed. Most importantly, the mechanism for cancer therapy of COFs and fundamental challenges and perspectives for the application of COFs in cancer theranostics were assessed.

Tuning the physicochemical properties of reticular covalent organic frameworks (COFs) for biomedical applications

Since the first report by Yaghi's group in 2005, research enthusiasm has been increasingly raised to synthesize diverse crystalline porous materials as -B-O-, -C-N-, -C-C-, and -C-O- linkage-based COFs. Recently, the biomedical applications of COFs have become more and more attractive in biomedical applications, including drug delivery, bioimaging, biosensing, antimicrobial, and therapeutic applications, as these materials bear well-defined crystalline porous structures and well-customized functionalities. However, the clinical translation of these research findings is challenging due to the formidable hindrances for in vivo use, such as low biocompatibility, poor selectivity, and long bio-persistence. Some attempts have raised a promising solution towards these obstacles by tailored engineering the functionalities of COFs. To speed up the clinical translations of COFs, a short review of principles and strategies to tune the physicochemical properties of COFs is timely and necessary. In this review, we summarized the biomedical utilities of COFs and discussed the related key physicochemical properties. To improve the performances of COFs in biomedical uses, we propose approaches for the tailored functionalization of COFs, including large-scale manufacture, standardization in nanomedicines, enhancing targeting efficacy, maintaining predesigned functions upon transformations, and manipulation of multifunctional COFs. We expect that this minireview strengthens the fundamental understandings of property-bioactivity relationships of COFs and provides insights for the rational design of their high-order reticular structures.

Magnetic COFs as satisfied support for lipase immobilization and recovery to effectively achieve the production of biodiesel by great maintenance of enzyme activity

BACKGROUND

Production of biodiesel from renewable sources such as inedible vegetable oils by enzymatic catalysis has been a hotspot but remains a challenge on the efficient use of an enzyme. COFs (Covalent Organic Frameworks) with large surface area and porosity can be applied as ideal support to avoid aggregation of lipase and methanol. However, the naturally low density limits its application. In this work, we reported a facile synthesis of core-shell magnetic COF composite (Fe₃O₄@COF-OMe) to immobilize RML (Rhizomucor miehei lipase), to achieve its utilization in biodiesel production.

RESULT

This strategy gives extrinsic magnetic property, and the magnetic COFs is much heavier and could disperse in water medium well, facilitating the attachment with the enzyme. The resultant biocomposite exhibited an excellent capacity of RML due to its high surface area and fast response to the external magnetic field, as well as good chemical stability. The core-shell magnetic COF-OMe structure not only achieved highly efficient immobilization and recovery processes but also maintained the activity of lipase to a great extent. RML@Fe₃O₄@COF-OMe performed well in practical applications, while free lipase did not. The biocomposite successfully achieved the production of biodiesel from Jatropha curcas Oil with a yield of about 70% in the optimized conditions.

CONCLUSION

Magnetic COFs (Fe₃O₄@COF-OMe) for RML immobilization greatly improved catalytic performance in template reaction and biodiesel preparation. The magneticity makes it easily recovered and separated from the system. This first successful attempt of COFs-based immobilized enzyme broadened the prospect of biodiesel production by COFs with some inspiration.

Crystalline Porous Materials for Nonlinear Optics

Crystalline porous materials have been extensively explored for wide applications in many fields including nonlinear optics (NLO) for frequency doubling, two-photon absorption/emission, optical limiting effect, photoelectric conversion, and biological imaging. The structural diversity and flexibility of the crystalline porous materials such as the metal-organic frameworks, covalent organic frameworks, and polyoxometalates provide numerous opportunities to orderly organize the dipolar chromophores and to systemically modify the type and concentration of these dipolar chromophores in the confined spaces, which are highly desirable for NLO. Here, the recent advances in the crystalline porous NLO materials are discussed. The second-order NLO of crystalline porous materials have been mainly devoted to the chiral and achiral structures, while the third-order NLO crystalline porous materials have been categorized into pure organic and hybrid organic/inorganic materials. Some representative properties and applications of these crystalline porous materials in the NLO regime are highlighted. The future perspective of challenges as well as the potential research directions of crystalline porous materials have been also proposed.

Pioneering Iodine-125-Labeled Nanoscale Covalent Organic Frameworks for Brachytherapy

Brachytherapy has been clinically used for the treatment of malignant solid tumors. However, the classic therapeutic radioactive ¹²⁵I seed must be surgically implanted directly into tumors. To avoid the surgery and prevent irrational radioactive distribution, radioiodine-loaded nanomaterials are ever-developing for brachytherapy. Hence, it is still a notable challenge to obtain an advanced material that simultaneously incorporates features of high radiolabeling rate, short labeling time, good radiolabeling stability, and long tumor retention time. Covalent organic frameworks (COFs), which are crystalline polymers with ordered pores, are widely applied in guest delivery of drugs based on their high porosity and modifiable skeleton. Herein, we developed a functionalized nanoscale PEG-COF-Ag material, which could rapidly capture radioiodine reaching a 94% radiolabeling yield in 30 s. In addition, more than 95% ¹²⁵I was maintained after 24 h in PBS (phosphate-buffered saline) as well as in serum and over 90% for nearly 1 week. PEG-COF-Ag-¹²⁵I (¹²⁵I-COF) demonstrated excellent cancer cell killing performance in vitro, and further experiments in vivo revealed a long tumor retention time and effective tumor treatment during the radiotherapy. The results indicate that radioiodine-labeled PEG-COF-Ag could be potentially applied in brachytherapy with a promising therapeutic effect.

Rational design of isostructural 2D porphyrin-based covalent organic frameworks for tunable photocatalytic hydrogen evolution

Covalent organic frameworks have recently gained increasing attention in photocatalytic hydrogen generation from water. However, their structure-property-activity relationship, which should be beneficial for the structural design, is still far-away explored. Herein, we report the designed synthesis of four isostructural porphyrinic two-dimensional covalent organic frameworks (MPor-DETH-COF, M = H₂, Co, Ni, Zn) and their photocatalytic activity in hydrogen generation. Our results clearly show that all four covalent organic frameworks adopt AA stacking structures, with high crystallinity and large surface area. Interestingly, the incorporation of different transition metals into the porphyrin rings can rationally tune the photocatalytic hydrogen evolution rate of corresponding covalent organic frameworks, with the order of CoPor-DETH-COF < H₂Por-DETH-COF < NiPor-DETH-COF < ZnPor-DETH-COF. Based on the detailed experiments and calculations, this tunable performance can be mainly explained by their tailored charge-carrier dynamics via molecular engineering. This study not only represents a simple and effective way for efficient tuning of the photocatalytic hydrogen evolution activities of covalent organic frameworks at molecular level, but also provides valuable insight on the structure design of covalent organic frameworks for better photocatalysis.

Facile synthesis of 3D covalent organic frameworks via a two-in-one strategy

A "two-in-one" strategy was employed to construct 3D-COFs for the first time. Based on this strategy, a 3D-Flu-COF could be readily synthesized in various simplex organic solvents. Benefitting from the non-conjugated structure, the 3D-Flu-COF showcased excellent acidichromic sensing performance with good sensitivity, reversibility and naked eye visibility.

Covalent organic framework-engineered polydopamine nanoplatform for multimodal imaging-guided tumor photothermal-chemotherapy

A covalent organic framework-engineered polydopamine nanoplatform with improved drug loading capacity and reduced premature leakage has been developed for multimodal imaging-guided tumor-targeted photothermal-chemotherapy.

Redox-triggered switching in three-dimensional covalent organic frameworks

The tuning of molecular switches in solid state toward stimuli-responsive materials has attracted more and more attention in recent years. Herein, we report a switchable three-dimensional covalent organic framework (3D COF), which can undergo a reversible transformation through a hydroquinone/quinone redox reaction while retaining the crystallinity and porosity. Our results clearly show that the switching process gradually happened through the COF framework, with an almost quantitative conversion yield. In addition, the redox-triggered transformation will form different functional groups on the pore surface and modify the shape of pore channel, which can result in tunable gas separation property. This study strongly demonstrates 3D COFs can provide robust platforms for efficient tuning of molecular switches in solid state. More importantly, switching of these moieties in 3D COFs can remarkably modify the internal pore environment, which will thus enable the resulting materials with interesting stimuli-responsive properties.

Tuning of molecular switches in solid state toward stimuli-responsive materials attracted attention in recent years but has not yet been realized in three-dimensional (3D) covalent organic frameworks (COFs). Herein, the authors demonstrate a stable and switchable 3D COF which undergoes reversible transformation through a hydroquinone/quinone redox reaction.

Inorganic nanocrystal-dynamic porous polymer assemblies with effective energy transfer for sensitive diagnosis of urine copper

Despite their remarkable mechanical, optical, and electrical properties, inorganic particles and dynamic polymer assemblies encounter difficulties in their compatibility with regards to structural order and complexity. Here, covalent organic frameworks (COFs) constructed through reversible coupling reactions were exploited as dynamic porous polymers to prepare inorganic nanocrystal-polymer assemblies. Under an in situ growth process, carbon quantum dots (CDs) were gradually prepared in the COF cavity, with a narrow size distribution (2 ± 0.5 nm). The well-established assemblies achieve effective energy transfer from the inorganic to the organic part (efficiency > 80%), thus rendering a ∼130% increase in quantum yield compared with the pristine COF network. Notably, the hybrid material realizes a simple, selective, and sensitive diagnostic tool for urine copper, surpassing the detection limit of COF solid by 150 times. Beyond the scientific and fundamental interests, such hybrid assemblies are attractive from technological perspectives as well, for example, in energy storage, electronics, catalysis, and optics.

Despite their remarkable mechanical, optical, and electrical properties, inorganic particles and dynamic polymer assemblies encounter difficulties in their compatibility with regards to structural order and complexity.

Research Progress in Covalent Organic Frameworks for Photoluminescent Materials

Covalent organic frameworks (COFs) are an emerging kind of crystalline porous polymers that present the precise integration of organic building blocks into extensible structures with regular pores and periodic skeletons. The diversity of organic units and covalent linkages makes COFs a rising materials platform for the design of structure and functionality. Herein, recent research progress in developing COFs for photoluminescent materials is summarised. Structural and functional design strategies are highlighted and fundamental problems that need to be solved are identified, in conjunction with potential applications from perspectives of photoluminescent materials.

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.

Nanoengineered Light-Activatable Polybubbles for On-Demand Therapeutic Delivery

Vaccine coverage is severely limited in developing countries due to inefficient protection of vaccine functionality as well as lack of patient compliance to receive the additional booster doses. Thus, there is an urgent need to design a thermostable vaccine delivery platform that also enables release of the bolus after predetermined time. Here, the formation of injectable and light-activatable polybubbles for vaccine delivery is reported. In vitro studies show that polybubbles enable delayed burst release, irrespective of cargo types, namely small molecule and antigen. The extracorporeal activation of polybubbles is achieved by incorporating near-infrared (NIR)-sensitive gold nanorods (AuNRs). Interestingly, light-activatable polybubbles can be used for on-demand burst release of cargo. In vitro, ex vivo, and in vivo studies demonstrate successful activation of AuNR-loaded polybubbles. Overall, the light-activatable polybubble technology can be used for on-demand delivery of various therapeutics including small molecule drugs, immunologically relevant protein, peptide antigens, and nucleic acids.

A COF-based nanoplatform for highly efficient cancer diagnosis, photodynamic therapy and prognosis

Covalent organic frameworks (COFs) have emerged as a kind of promising material for analytical and biomedical purposes. However, simultaneous cancer diagnosis and therapy with COFs remain a challenge. We report here a COF-based theranostic nanoplatform by integrating a dye-labeled oligonucleotide onto porphyrin-based COF nanoparticles for highly efficient cancer diagnosis and therapy. The fluorescence of the dye was effectively quenched by the COF through fluorescence resonance energy transfer (FRET). In the presence of biomarker survivin mRNA, more stable duplexes were formed and separated from the COF NPs, enabling the recovery of the fluorescence signal and selective cancer imaging. Under NIR laser irradiation, COF NPs generated abundant reactive oxygen species (ROS) to induce cancer cell apoptosis owing to their crystalline reticular structure. In vitro and in vivo experiments revealed that the nanoplatform has a high specificity and inhibition effect toward cancer cells and solid tumors. Interestingly, prognostic evaluation was also realized with COF-survivin. This work will offer new insights into COF-based probes and inspire the development of more versatile tools for biomedical applications.

MOF@COFs with Strong Multiemission for Differentiation and Ratiometric Fluorescence Detection

Aggregation-caused quenching (ACQ) is often observed in covalent organic frameworks (COFs) for their low emission. Here, we propose that limited COF layers form on UiO-66 to eliminate the ACQ by the formation of UiO@COF composites. UiO-66 is selected because this metal-organic framework (MOF) is easily prepared in nanosize with Zr⁴⁺ ion and 2-aminoterephthalic acid (BDC-NH₂). The high affinity of the Zr⁴⁺ ion to phosphate species improves sensing selectivity. The surface -NH₂ reacts with 2,4,6-triformylphloroglucinol (Tp) to integrate COF1 and COF2, which are prepared with Tp and phenylenediamine or tetraamino-tetraphenylethylene, respectively. The hydrogen bond formed between the hydroxyl group in Tp and imine nitrogen realizes excited-state intramolecular proton transfer; therefore, multiemission is observed from the enol and keto states of the COFs and UiO-66 at 360, 470, and 613 nm for UiO@COF1 and at 370, 470, and 572 nm for UiO@COF2. When phosphate ion is added in the composites, the emissions from the COFs keep stable, while that from UiO-66 is enhanced. However, adenosine-5'-triphosphate (ATP) improves the emissions from UiO-66 and COF's enol state, but that from the keto state keeps stable. The differentiation and ratiometric fluorescence detection of ATP and phosphate ion are therefore realized with the multiemission, the affinity of Zr⁴⁺ ions, and the structural selectivity of the COFs. Thus, UiO@COF is a novel strategy to integrate multiemission, affinity, and structural selectivity to improve the sensing performance for differentiation and ratiometric detection.

Metal-Organic Framework Nanoparticles Induce Pyroptosis in Cells Controlled by the Extracellular pH

Ion homeostasis is essential for cellular survival, and elevated concentrations of specific ions are used to start distinct forms of programmed cell death. However, investigating the influence of certain ions on cells in a controlled way has been hampered due to the tight regulation of ion import by cells. Here, it is shown that lipid-coated iron-based metal-organic framework nanoparticles are able to deliver and release high amounts of iron ions into cells. While high concentrations of iron often trigger ferroptosis, here, the released iron induces pyroptosis, a form of cell death involving the immune system. The iron release occurs only in slightly acidic extracellular environments restricting cell death to cells in acidic microenvironments and allowing for external control. The release mechanism is based on endocytosis facilitated by the lipid-coating followed by degradation of the nanoparticle in the lysosome via cysteine-mediated reduction, which is enhanced in slightly acidic extracellular environment. Thus, a new functionality of hybrid nanoparticles is demonstrated, which uses their nanoarchitecture to facilitate controlled ion delivery into cells. Based on the selectivity for acidic microenvironments, the described nanoparticles may also be used for immunotherapy: the nanoparticles may directly affect the primary tumor and the induced pyroptosis activates the immune system.

Synthesis of an MOF-based Hg2+-fluorescent probe via stepwise post-synthetic modification in a single-crystal-to-single-crystal fashion and its application in bioimaging

Although post-synthetic modification (PSM) has been successfully applied to NMOF decoration, only a handful of PSM-based single-crystal-to-single-crystal (SCSC) examples have been reported, particularly those involving multistep MOF-based SCSC transformations. In this contribution, three new MOFs, namely, UiO-68-NCS, UiO-68-R6G and UiO-68-R6G', were prepared via the single-crystal-to-single-crystal post-synthetic modification approach. For bioimaging, nanosized UiO-68-NCS, UiO-68-R6G, and UiO-68-R6G' were also prepared. Herein, nanosized UiO-68-R6G with a rhodamine-based fluorescence switch was found to be a highly sensitive and selective fluorescent probe for the detection of Hg2+ both in vitro and in vivo.

Covalent Organic Frameworks (COFs) for Cancer Therapeutics

As newly emerged crystalline porous materials, covalent organic frameworks (COFs) possess fascinating structures and some specific features such as modularity, crystallinity, porosity, stability, versatility, and biocompatibility. Besides adsorption/separation, sensing, catalysis, and energy applications, COFs have recently shown a promise in biomedical applications. This contribution provides an overview of the recent developments of COF-based medicines in cancer therapeutics, including drug delivery, photodynamic therapy (PDT), photothermal therapy (PTT), and combined therapy. Furthermore, the major challenges and developing trends in this field are also discussed. These recent developments are summarized and discussed to help encourage further contributions in this emerging and promising field.

Construction of Magnetic Covalent Organic Frameworks with Inherent Hydrophilicity for Efficiently Enriching Endogenous Glycopeptides in Human Saliva

In this work, a magnetic covalent organic framework (COF) with inherent hydrophilicity (denoted mCTpBD) was synthesized through interface deposition of a hydrophilic COF shell on amino group-functionalized magnetite particles via the reaction between a carboxyl group-containing monomer and benzidine. Thanks to the superior hydrophilicity, appropriate porous structure, and easy magnetic separation, the resulting mCTpBD exhibited excellent performance in conveniently enriching glycopeptides from standard samples with a high sensitivity of 0.5 fmol μL^-1 and strong size-exclusion effect of up to 1:1000 (w/w). Furthermore, by using the mCTpBD adsorbent, endogenous glycopeptides in saliva of healthy people and patients with inflammatory bowel disease were successfully enriched and identified by the combined liquid chromatography-mass spectrometry/mass spectrometry technology, which indicates a promising prospective of core-shell magnetic composite microspheres with a hydrophilic COF shell in glycoproteomics research.

2D and 3D Porphyrinic Covalent Organic Frameworks: The Influence of Dimensionality on Functionality

The construction of 2D and 3D covalent organic frameworks (COFs) from functional moieties for desired properties has gained much attention. However, the influence of COFs dimensionality on their functionalities, which can further assist in COF design, has never been explored. Now, by selecting designed precursors and topology diagrams, 2D and 3D porphyrinic COFs (2D-PdPor-COF and 3D-PdPor-COF) are synthesized. By model building and Rietveld refinement of powder X-ray diffraction, 2D-PdPor-COF crystallizes as 2D sheets while 3D-PdPor-COF adopts a five-fold interpenetrated pts topology. Interestingly, compared with 2D-PdPor-COF, 3D-PdPor-COF showed interesting properties, including 1) higher CO₂ adsorption capacity; 2) better photocatalytic performance; and 3) size-selective photocatalysis. Based on this study, we believe that with the incorporation of functional moieties, the dimensionality of COFs can definitely influence their functionalities.

A COF-based anti-interference nanoplatform for intracellular nucleic acid imaging

A porphyrin covalent organic framework-based nanoplatform was demonstrated. The nanoplatform could effectively avoid the interference of proteins and biothiols and realize high-fidelity mRNA imaging in living cells.

Bulk COFs and COF nanosheets for electrochemical energy storage and conversion

The current advances, structure-property relationship and future perspectives in covalent organic frameworks (COFs) and their nanosheets for electrochemical energy storage (EES) and conversion (EEC) are summarized.

In vivo therapeutic response monitoring by a self-reporting upconverting covalent organic framework nanoplatform

The real-time and in situ monitoring of reactive oxygen species (ROS) generation is critical for minimizing the nonspecific damage derived from the high doses of ROS required during the photodynamic therapy (PDT) process. However, phototherapeutic agents that can generate ROS-related imaging signals during PDT are rare, hampering the facile prediction of the future therapeutic outcome. Herein, we develop an upconverting covalent organic framework (COF) nanoplatform via a core-mediated strategy and further functionalized it with a singlet oxygen reporter for the efficient near-infrared activated and in situ self-reporting of PDT. In this work, the COF photodynamic efficacy is greatly improved (12.5 times that of irregular COFs) via tailoring the size. Furthermore, this nanoplatform is able to not only produce singlet oxygen for PDT, but it can also emit singlet oxygen-correlated luminescence, allowing the real-time and in situ monitoring of the therapeutic process for cancer cells or solid tumors in vivo via near-infrared luminescence imaging. Thus, our core-mediated synthetic and size-tailored strategy endows the upconverting COF nanoplatform with promising abilities for high-efficacy, deep-tissue, precise photodynamic treatment.

Nanoscale Covalent Organic Framework for Combinatorial Antitumor Photodynamic and Photothermal Therapy

Despite the excellent photodynamic and photothermal properties of organic molecular photosensitizers (PSs) and photothermal agents (PTAs), such as porphyrin and naphthalocyanine, their poor water solubility severely impedes their biological applications. Covalent organic frameworks (COFs), as an emerging class of organic crystalline porous materials, possess free active end groups (bonding defects) and large inner pores, which make them an ideal type of nanocarriers for loading hydrophobic organic molecular PSs and PTAs by both bonding defect functionalization (BDF) and guest encapsulation approaches to obtain multifunctional nanomedicines for PDT/PTT combination therapy. In this work, we report a nanoscale COF (NCOF) prepared via a facile synthetic approach under ambient conditions. Furthermore, a dual-modal PDT/PTT therapeutic nanoagent, VONc@COF-Por (3), is successfully fabricated by stepwise BDF and guest encapsulation processes. The covalently grafted porphyrinic PS (Por) and the noncovalently loaded naphthalocyanine PTA (VONc) are independently responsible for the PDT and PTT functionalities of the nanoagent. Upon visible (red LED) and NIR (808 nm laser) irradiation, VONc@COF-Por (3) displayed high ¹O₂ generation and photothermal conversion ability (55.9%), consequently providing an excellent combined PDT/PTT therapeutic effect on inhibiting MCF-7 tumor cell proliferation and metastasis, which was well evidenced by in vitro and in vivo experiments. We believe that the results obtained herein can significantly promote the development of NCOF-based multifunctional nanomedicines for biomedical applications.

Tumor-acidity activated surface charge conversion of two-photon fluorescent nanoprobe for enhanced cellular uptake and targeted imaging of intracellular hydrogen peroxide

Elevated levels of intracellular hydrogen peroxide (H2O2) are closely related to the development of cancers. Specific imaging of H2O2 in tumor sites would be significant not only for cancer diagnosis but also for gaining a deep understanding of the role of H2O2 in cancer. However, traditional fluorescent probes based only on responses to overexpression levels of H2O2 in cancer cells are insufficient to distinguish cancer cells from other unhealthy or healthy cells in complex biological systems. Herein, we developed a smart, two-photon fluorescent GC-NABP nanoprobe with pH-dependent surface charge conversion for tumor-targeted imaging of H2O2. The nanoprobe was constructed by the self-assembly of amphiphilic GC-NABP, which was synthesized by grafting the hydrophobic, H2O2-responsive and two-photon fluorophore, NABP, onto hydrophilic biopolymer glycol chitosan (GC). Taking advantage of pH-titratable amino groups on GC, the nanoprobe had the capability of surface charge conversion from negative at physiologic pH to positive in the acidic tumor microenvironment. The positive charge of the nanoprobe promoted electrostatic interactions with cell membranes, leading to enhanced cellular uptake in acidic environment. Upon cellular uptake, the high level of H2O2 in tumor cells triggered boronate deprotections of the nanoprobe, generating a "turn-on" fluorescence emission for H2O2 imaging. The nanoprobe exhibited good sensitivity and selectivity to H2O2 with a detection limit down to 110 nM in vitro. The results from flow cytometry and two-photon fluorescence imaging of H2O2 in living cells and tissues evidenced the enhanced cellular uptake and targeted imaging of intracellular H2O2 in acidic environment. Compared to control nanoparticles that lack pH sensitivity, our nanoprobe showed enhanced accumulation in tumor sites and was applied to targeted imaging of H2O2 in a tumor-bearing mouse model. This work demonstrates that the nanoprobe GC-NABP holds great promise for tumor-specific imaging of cellular H2O2, providing a potential tool to explore the role of H2O2 in tumor sites.

A Mitochondrial-Targeting Near-Infrared Fluorescent Probe for Visualizing and Monitoring Viscosity in Live Cells and Tissues

The intracellular viscosity is closely related to many functional disorders and diseases. Especially, abnormal mitochondrial viscosity changes are one of the distinct indications in metabolite diffusion as well as mitochondrial metabolism. In this work, we report a novel fluorescent probe (NI-VIS), which uses quinoline as an acceptor group and employs a TICT mechanism (twisted intramolecular charge transfer) to detect viscosity. NI-VIS features a good mitochondrion targeting ability and near-infrared emission. NI-VIS possesses a highly sensitive response toward viscosity changes in aqueous environments. As the viscosity of a DPBS-glycerol system increased from 1.0 to 999 cP, NI-VIS exhibited a hundred-fold enhancement in fluorescence. We demonstrated that after the treatment with ionophores, NI-VIS could identify the variation of mitochondrial viscosity in HeLa cells. The probe also recognized the decrease of mitochondria viscosity during starvation-induced mitophagy. More importantly, NI-VIS was successfully applied to visualize the viscosity variation in cirrhotic liver tissues. Our trial with zebrafish suggested this probe could map the microviscosity in vivo. These findings reveal that NI-VIS can serve as a powerful tool to monitor viscosity of biological samples and shows broad potential applications in the biomedical field.

Isostructural Three-Dimensional Covalent Organic Frameworks

Herein, we reported the designed synthesis of three isostructural three-dimensional covalent organic frameworks (3D COFs) with -H, -Me, or -F substituents, which have similar crystallinity and topology. Their crystal structures were determined by continuous rotation electron diffraction (cRED), and all three 3D COFs were found to adopt a fivefold interpenetrated pts topology. More importantly, the resolution of these cRED datasets reached up to 0.9-1.0 Å, enabling the localization of all non-hydrogen atomic positions in a COF framework directly by 3D ED techniques for the first time. In addition, the precise control of the pore environments through the use of different functional groups led to different selectivities for CO₂ over N₂ . We have thus confirmed that polycrystalline COFs can be definitely studied to the atomic level as other materials, and this study should also inspire the design and synthesis of 3D COFs with tailored pore environments for interesting applications.

BODIPY-Decorated Nanoscale Covalent Organic Frameworks for Photodynamic Therapy

Covalent organic frameworks (COFs), an emerging class of organic porous materials, have attracted intense attention due to their versatile applications. However, the deliberate fabrication of COF-based nanomaterials for nanomedical application remains challenging due to difficulty in their size- and structure-controlled synthesis and poor aqueous dispersibility. Herein, we report two boron-dipyrromethene (BODIPY)-decorated nanoscale COFs (NCOFs), which were prepared by the Schiff-base condensation of the free end -CHO (bonding defects in COFs) on the established imine-based NCOFs with the amino-substituted organic photosensitizer BODIPY via "bonding defects functionalization" approach. Thus BODIPY has been successfully nanocrystallized via the NCOF platform, and can be used for photodynamic therapy (PDT) to treat tumors. These NCOF-based PDT agents featured nanometer size (∼110 nm), low dark toxicity, and high phototoxicity as evidenced by in vitro and in vivo experiments. Moreover, the "bonding defects functionalization" approach might open up new avenues for the fabrication of additional COF-based platforms for biomedical treatment.

Recent advances in covalent organic frameworks (COFs) as a smart sensing material

Recent advances in covalent organic frameworks (COFs) as a smart sensing material are summarized and highlighted.

Post-synthetic modification of covalent organic frameworks

This review is aimed at providing an in-depth understanding of the potential of post-synthetic strategies for the modification of covalent organic frameworks.