3D Hydrazone-Functionalized Covalent Organic Frameworks as pH-Triggered Rotary Switches

Hydrazone-functionalized nanoscale covalent organic frameworks as a nanocarrier for pH-responsive drug delivery enhanced anticancer activity

Nanoscale covalent organic frameworks (NCOFs) as emerging drug-delivery nanocarriers have received much attention in biomedicine in recent years. However, there are few reports on the application of pH-responsive NCOFs for drug delivery nanosystems. In this work, hydrazone-decorated NCOFs as pH-triggered molecular switches are designed for efficient cancer therapy. These functionalized NCOFs with hydrazone groups on the channel walls (named NCOFs-NHNH2) are obtained via a post-synthetic modification strategy. Subsequently, the anticancer drug doxorubicin (DOX) as the model molecule is loaded through covalent linkage to yield NCOFs-NN-DOX. Finally, soybean phospholipid (SP) is coated on the surface of HNTs-NN-DOX, named NCOFs-NN-DOX@SP, to further enhance the dispersibility, stability and biocompatibility of HNTs in physiological solution. NCOFs-NN-DOX@SP showed an excellent and intelligent sustained-release effect with an almost sixfold increase at pH = 5.2 than at pH = 7.4. In vitro cell toxicity and imaging assays of NCOFs-NN-DOX@SP exhibited an enhanced therapeutic effect on Lewis lung carcinoma (LLC) cells, demonstrating that the fabricated NCOFs have a great potential in cancer therapy. Thus, this work provides a new way toward designing stimulus-responsive functionalized NCOFs and promotes their potential application as an on-demand drug delivery system in the field of cancer treatment.

3D Covalent Organic Frameworks from Design, Synthesis to Applications in Optoelectronics

Covalent organic frameworks (COFs), a new class of crystalline materials connected by covalent bonds, have been developed rapidly in the past decades. However, the research on COFs is mainly focused on two-dimensional (2D) COFs, and the research on three-dimensional (3D) COFs is still in the initial stage. In 2D COFs, the covalent bonds exist only in the 2D flakes and can form 1D channels, which hinder the charge transport to some extent. In contrast, 3D COFs have a more complex pore structure and thus exhibit higher specific surface area and richer active sites, which greatly enhance the 3D charge carrier transport. Therefore, compared to 2D COFs, 3D COFs have stronger applicability in energy storage and conversion, sensing, and optoelectronics. In this review, it is first introduced the design principles for 3D COFs, and in particular summarize the development of conjugated building blocks in 3D COFs, with a special focus on their application in optoelectronics. Subsequently, the preparation of 3D COF powders and thin films and methods to improve the stability and functionalization of 3D COFs are summarized. Moreover, the applications of 3D COFs in electronics are outlined. Finally, conclusions and future research directions for 3D COFs are presented.

Strategies for utilizing covalent organic frameworks as host materials for the integration and delivery of bioactives

Covalent organic frameworks (COFs), a new and developing class of porous framework materials, are considered a type of promising carrier for the integration and delivery of bioactives, which have diverse fascinating merits, such as a large specific surface area, designable and specific porosity, stable and orderly framework structure, and various active sites. However, owing to the significant differences among bioactives (including drugs, proteins, nucleic acid, and exosomes), such as size, structure, and physicochemical properties, the interaction between COFs and bioactives also varies. In this review, we firstly summarize three strategies for the construction of single or hybrid COF-based matrices for the delivery of cargos, including encapsulation, covalent binding, and coordination bonding. Besides, their smart response release behaviors are also categorized. Subsequently, the applications of cargo@COF biocomposites in biomedicine are comprehensively summarized, including tumor therapy, central nervous system (CNS) modulation, biomarker analysis, bioimaging, and anti-bacterial therapy. Finally, the challenges and opportunities in this field are briefly discussed.

Donor-Acceptor Interactions Enhanced Colorimetric Sensors for Both Acid and Base Vapor Based on Two-Dimensional Covalent Organic Frameworks

Due to the high surface area and uniform porosity of covalent organic frameworks (COFs), they exhibit superior properties in capturing and detecting even trace amounts of gases in the air. However, the COFs materials that possess dual detected functionality are still less reported. Here, an imine-based COF containing thiophene as a donor and triazine as an acceptor to form spatial-distribution-defined D-A structures was prepared. D-A system between thiophene and triazine facilitates the charge transfer process during the protonation process of the imine and the triazine units. The obtained COF exhibits simultaneous sensing ability toward both acidic and alkaline vapors with obvious colorimetric sensing functionality.

Covalent Organic Frameworks: Opportunities for Rational Materials Design in Cancer Therapy

Nanomedicines are extensively used in cancer therapy. Covalent organic frameworks (COFs) are crystalline organic porous materials with several benefits for cancer therapy, including porosity, design flexibility, functionalizability, and biocompatibility. This review examines the use of COFs in cancer therapy from the perspective of reticular chemistry and function-oriented materials design. First, the modification sites and functionalization methods of COFs are discussed, followed by their potential as multifunctional nanoplatforms for tumor targeting, imaging, and therapy by integrating functional components. Finally, some challenges in the clinical translation of COFs are presented with the hope of promoting the development of COF-based anticancer nanomedicines and bringing COFs closer to clinical trials.

Flexibility On-Demand: Multivariate 3D Covalent Organic Frameworks

Dynamic 3D covalent organic frameworks (dynaCOFs) have shown concerted structural transformation and responses upon adaptive guest adsorption. The multivariate (MTV) strategy incorporating multiple functionalities within a backbone is attractive for tuning the framework flexibility and dynamic responses. However, a major synthetic challenge arises from the different chemical reactivities of linkers usually resulting in phase separation. Here, we report a general synthetic protocol for making 3D MTV-COFs by balancing the linker reactivity and solvent polarity. Specifically, 15 crystalline and phase pure MTV-COF-300 isostructures are constructed by linking a tetrahedral unit with eight ditopic struts carrying various functional groups. We find that the electron-donating groups make the linker reactivity too low to allow the reaction to proceed fully, while the electron-withdrawing groups afford increased reactivity and hardly yield crystalline materials. To overcome the crystallization dilemma, the combination of polar aprotic with nonpolar solvents was used to improve the solubility of oligomers and slow the reaction kinetics in MTV-COF synthesis. We demonstrate the abilities of these MTV-COFs to tune gas dynamic behaviors and the separation of benzene and cyclohexane. These findings reveal the integration of multivariate functionalities into dynaCOFs with on-demand flexibility to achieve dynamic synergism in particular applications, outperforming their pure, monofunctional counterparts.

Polymeric nanoparticles (PNPs) for oral delivery of insulin

Successful oral insulin administration can considerably enhance the quality of life (QOL) of diabetes patients who must frequently take insulin injections. Oral insulin administration, on the other hand, is seriously hampered by gastrointestinal enzymes, wide pH range, mucus and mucosal layers, which limit insulin oral bioavailability to ≤ 2%. Therefore, a large number of technological solutions have been proposed to increase the oral bioavailability of insulin, in which polymeric nanoparticles (PNPs) are highly promising for oral insulin delivery. The recently published research articles chosen for this review are based on applications of PNPs with strong future potential in oral insulin delivery, and do not cover all related work. In this review, we will summarize the controlled release mechanisms of oral insulin delivery, latest oral insulin delivery applications of PNPs nanocarrier, challenges and prospect. This review will serve as a guide to the future investigators who wish to engineer and study PNPs as oral insulin delivery systems.

Adjusting the Stacking Model of Two-Dimensional Covalent Organic Frameworks for Volatile Acid Sensing via Spatial Effects

Covalent organic frameworks (COFs) are polymer networks with a precise structure and permanent porosity, making them an attractive platform for the detection of volatile analytes due to their chemical stability and accessible active sites. In this study, based on electron-rich N,N,N',N'-tetrakis(4-aminophenyl)-1,4-benzenediamine moiety, two 2D COFs with different topological structures and stacking models were designed by the strategy of spatial effect. The conductivity of the AB-stacked COF-NUST-20 was an order of magnitude higher than that of the AA-stacked COF-NUST-30. With the protonation of the imine bond, both COFs exhibited a strong, rapid, and reversible visible color change in response to corrosive HCl vapor. In addition, the AB-stacked COF-NUST-20, which facilitates both interlayer and intralayer charge transfer, shows better sensing performance. These findings demonstrate the usefulness of all-aromatic 2D COFs as real-time responsive chemosensors and provide insight into the design of sensing materials with high sensitivity.

Record Ultralarge-Pores, Low Density Three-Dimensional Covalent Organic Framework for Controlled Drug Delivery

The unique structural characteristics of three-dimensional (3D) covalent organic frameworks (COFs) like high surface areas, interconnected pore system and readily accessible active sites render them promising platforms for a wide set of functional applications. Albeit promising, the reticular construction of 3D COFs with large pores is a very demanding task owing to the formation of interpenetrated frameworks. Herein we report the designed synthesis of a 3D non-interpenetrated stp net COF, namely TUS-64, with the largest pore size of all 3D COFs (47 Å) and record-low density (0.106 g cm^-3 ) by reticulating a 6-connected triptycene-based linker with a 4-connected porphyrin-based linker. Characterized with a highly interconnected mesoporous scaffold and good stability, TUS-64 shows efficient drug loading and controlled release for five different drugs in simulated body fluid environment, demonstrating the competency of TUS-64 as drug nanocarriers.

A biodegradable covalent organic framework for synergistic tumor therapy

Stimulus-responsive biodegradable nanocarriers with tumor-selective targeted drug delivery are critical for cancer therapy. Herein, we report for the first time a redox-responsive disulfide-linked porphyrin covalent organic framework (COF) that can be nanocrystallized by glutathione (GSH)-triggered biodegradation. After loading 5-fluorouracil (5-Fu), the generated nanoscale COF-based multifunctional nanoagent can be further effectively dissociated by endogenous GSH in tumor cells, releasing 5-Fu efficiently to achieve selective chemotherapy on tumor cells. Together with the GSH depletion-enhanced photodynamic therapy (PDT), an ideal synergistic tumor therapy for MCF-7 breast cancer via ferroptosis is achieved. In this research, the therapeutic efficacy was significantly improved in terms of enhanced combined anti-tumor efficiency and reduced side effects by responding to significant abnormalities such as high concentrations of GSH in the tumor microenvironment (TME).

Reticular Synthesis of One-Dimensional Covalent Organic Frameworks with 4-c sql Topology for Enhanced Fluorescence Emission

Covalent organic frameworks (COFs) have been extensively investigated due to their unique structure, porosity, and functionality. However, at the topological level, COFs remain as two-dimensional (2D) or three-dimensional (3D) structures, while COFs with one-dimensional (1D) topology have not been systematically explored. In this work, we proposed a synthetic strategy for the construction of 1D-COFs based on non-linear edges and suitable high-symmetry vertices. Compared with their 2D-COFs counterparts, the 1D-COFs with AIEgens located at the vertex of the frame exhibited enhanced fluorescence. The density functional theory (DFT) calculations revealed that the dimensional-induced rotation restriction (DIRR) effect could spontaneously introduce additional non-covalent interactions between the strip frames, which could substantially diminish non-radiative transitions. This work also provides protocols for the design of 1D-COFs and a guidance scheme for the synthesis of emitting COFs.

Construction of an ultra-small hydrazone-linked covalent organic polymer for selective fluorescent detection of ferric ion in aqueous solution

A novel ultra-small hydrazone-linked covalent organic polymer (UHCOP) was synthesized based on the Schiff-base reaction between 2,4,6-trihydroxy-1,3,5-benzenetricarbaldehyde and 1,4-benzenedicarbohydrazide at room temperature and utilized as a sensitive fluorescent sensor for rapid (<2 min) and selective detection of Fe³⁺ in aqueous solution. The prepared UHCOP displayed ultra-small size with the diameter of 7.98 ± 0.97 nm and gave a stable fluorescent emission at 510 nm. UHCOP exhibited good sensitivity and highly selectivity towards Fe³⁺. The coordination interaction between UHCOP and Fe³⁺ resulted in the obviously aggregation-caused quenching response of UHCOP. The linear range was from 5.0 μM to 1.4 mM (R² = 0.999) with the detection limit of 2.5 μM. Finally, UHCOP has been successfully applied in the detection of Fe³⁺ in real water samples, proving the fabricated UHCOP is promising as a sensitive fluorescent sensor for selective detection of Fe³⁺ in aqueous solution.

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.

Porous organic polymers for drug delivery: hierarchical pore structures, variable morphologies, and biological properties

Porous organic polymers have received considerable attention in recent years because of their applicability as biomaterials. In particular, their hierarchical pore structures, variable morphologies, and tunable biological properties make them suitable as drug-delivery systems. In this review, the synthetic and post forming/control methods including templated methods, template-free methods, mechanical methods, electrospun methods, and 3D printing methods for controlling the hierarchical structures and morphologies of porous organic polymers are discussed, and the different methods affecting their specific surface areas, hierarchical structures, and unique morphologies are highlighted in detail. In addition, we discuss their applications in drug encapsulation and the development of stimuli (pH, heat, light, and dual-stimuli)-responsive materials, focusing on their use for targeted drug release and as therapeutic agents. Finally, we present an outlook concerning the research directions and applications of porous polymer-based drug delivery systems.