Microwave-assisted synthesis of highly fluorescent nanoparticles of a melamine-based porous covalent organic framework for trace-level detection of nitroaromatic explosives

Multimodal biosensing systems based on metal nanoparticles

Biosensors are currently among the most commonly used devices for analysing biomarkers and play an important role in environmental detection, food safety, and disease diagnosis. Researchers have developed multimodal biosensors instead of single-modal biosensors to meet increasing sensitivity, accuracy, and stability requirements. Metal nanoparticles (MNPs) are beneficial for preparing core probes for multimodal biosensors because of their excellent physical and chemical properties, such as easy regulation and modification, and because they can integrate diverse sensing strategies. This review mainly summarizes the excellent physicochemical properties of MNPs applied as biosensing probes and the principles of commonly used MNP-based multimodal sensing strategies. Recent applications and possible improvements of multimodal biosensors based on MNPs are also described, among which on-site inspection and sensitive detection are particularly important. The current challenges and prospects for multimodal biosensors based on MNPs may provide readers with a new perspective on this field.

Powders to Thin Films: Advances in Conjugated Microporous Polymer Chemical Sensors

Chemical sensing of harmful species released either from natural or anthropogenic activities is critical to ensuring human safety and health. Over the last decade, conjugated microporous polymers (CMPs) have been proven to be potential sensor materials with the possibility of realizing sensing devices for practical applications. CMPs found to be unique among other porous materials such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) due to their high chemical/thermal stability, high surface area, microporosity, efficient host-guest interactions with the analyte, efficient exciton migration along the π-conjugated chains, and tailorable structure to target specific analytes. Several CMP-based optical, electrochemical, colorimetric, and ratiometric sensors with excellent selectivity and sensing performance were reported. This review comprehensively discusses the advances in CMP chemical sensors (powders and thin films) in the detection of nitroaromatic explosives, chemical warfare agents, anions, metal ions, biomolecules, iodine, and volatile organic compounds (VOCs), with simultaneous delineation of design strategy principles guiding the selectivity and sensitivity of CMP. Preceding this, various photophysical mechanisms responsible for chemical sensing are discussed in detail for convenience. Finally, future challenges to be addressed in the field of CMP chemical sensors are discussed.

Schiff-base Polymer Immobilized Ruthenium for Efficient Catalytic Cross-coupling of Secondary Alcohols with 2-amino- and γ-aminobenzyl Alcohols to Give Quinolines and Pyridines

A Schiff-base porous polymer has been impregnated with ruthenium trichloride for acceptor-free dehydrogenation coupling (ADC) of secondary alcohols with γ-amino- and 2-aminobenzyl alcohols to give pyridines and quinolines. This heterogenous catalyst exhibited high catalytic efficiency over repeated cycles with wide functional group tolerance.

Cost-effective removal of Cr(VI) ions from aqueous media using L-cysteine functionalized gold nanoparticles embedded in melamine-based covalent organic framework (Cys-AuNPs@COF)

Due to the growing concern about the environmental effects of heavy metals, researchers are developing materials that possess high absorption capacity in addition to selectivity and high absorption speed. Recently, covalent organic frameworks (COFs) have been considered as emerging and promising adsorbents for the removal of many types of pollutants. In this work, a novel and selective adsorbent (Cys-AuNPs@COF) was prepared by embedding gold nanoparticles functionalized with L-cysteine in melamine-based COF for the removal of Cr(VI) ions from wastewater. The synthesized Cys-AuNPs@COF were characterizedby Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction analysis (XRD), Field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EDX), Thermo-gravimetric analysis (TGA), and elemental mapping (EMA) analysis. The removal of Cr(VI) ions was performed using a batch mode process by taking advantage of response surface methodology (RSM) based on a central composite design (CCD) model. The maximum adsorption capacity of Cys-AuNPs@COF was 151.5 mg g-1. The experimental results followed the Langmuir model and showed pseudo-second-order kinetics. A portable, low-cost, and highly sensitive device with a smartphone colorimeter platform was developed for in situ measurement of trace amounts of chromium (VI) ions. Due to its simplicity and versatility, this method has the potential to serve as an alternative to conventional field analysis methods.

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.

Simultaneous detection and removal of mercury (II) using multifunctional fluorescent materials

Environmental problems caused by mercury ions are increasing due to growing industrialization, poor enforcement, and inefficient pollutant treatment. Therefore, detecting and removing mercury from the ecological chain is of utmost significance. Currently, a wide range of small molecules and nanomaterials have made remarkable progress in the detection, detoxification, adsorption, and removal of mercury. In this review, we summarized the recent advances in the design and construction of multifunctional materials, detailed their sensing and removing mechanisms, and discussed with emphasis the advantages and disadvantages of different types of sensors. Finally, we elucidated the problems and challenges of current multifunctional materials and further pointed out the direction for the future development of related materials. This review is expected to provide a guideline for researchers to establish a robust strategy for the detection and removal of mercury ionsin the environment.

Combination of covalent organic frameworks (COFs) and polyoxometalates (POMs): the preparation strategy and potential application of COF-POM hybrids

Both covalent organic frameworks (COFs) and polyoxometalates (POMs) show excellent properties and application potential in many fields, thus receiving widespread attention. In recent years, COF-POM hybrid materials were prepared by combining COFs and POMs through physical or chemical methods. COF-POM hybrids have shown high performance in many fields, such as catalysis, sensing, energy storage, and biomedicine. In this review, we introduced the preparation strategy and application of COF-POM hybrids in detail. We believe that the combination of COFs and POMs will provide more abundant functions and broad application prospects.

Controlling the sensitivity and selectivity for the detection of nitro-based explosives by modulating the electronic substituents on the ligand of AIPE-active cyclometalated iridium(III) complexes

Nitroaromatic compounds are extremely explosive materials that pose a national security risk and raise environmental concerns. The design and development of sensitive and selective compounds for explosive materials are highly desirable. 'Aggregation-Induced Emission' (AIE) active materials are best suited for sensing purposes because of their sensitivity, fast detection time, and easy operation. By rationally incorporating substituents on the cyclometalated (C^N) ligand, four different AIE active iridium(III) based monocyclometalated complexes with the general formula [Ir(PPh3)2(H)(Cl)(C^N)] were synthesized. The phenyl ring of the phenyl pyridine cyclometalated portion of an iridium(III) complex was substituted with the right substituents to adjust the FMO levels thus, leading to appropriate alignment of the energy levels. Each of the resulting complexes displayed a significant property known as 'Aggregation-Induced Phosphorescent Emission' (AIPE). The complexes were subjected to structural characterization, electrochemical analysis, and photophysical property studies. The synthesized complexes were employed for the detection of aromatic nitro explosive compounds such as trinitrophenol (TNP) and trinitrotoluene (TNT) in the aqueous phase with a high degree of sensitivity. The sensing capabilities of each complex were assessed for these nitro explosive compounds and compared to those of the unsubstituted iridium(III) complex (M). Notably, the best limits of detection for TNP and TNT have been achieved with iridium(III) complexes [M1 (489 pM) and M3 (3.6 nM)] within the literature reported until now. For detecting picric acid with M1, FRET was found to be the potential mechanism, and for TNT, PET was found to be the cause of emission quenching by M3. Furthermore, for low-cost detection, filter paper-based sensing was also found effective for each complex. Real-field sensing of PA in soil samples was also performed.

Covalent organic framework: A state-of-the-art review of electrochemical sensing applications

Covalent organic framework (COF), a kind of porous polymer with crystalline properties, is a periodic porous framework material with precise regulation at atomic level, which can be formed by the orderly connection of pre-designed organic construction units through covalent bonds. Compared with metal-organic frameworks, COFs exhibit unique performance, including tailor-made functions, stronger load ability, structural diversity, ordered porosity, intrinsic stability and excellent adsorption features, are more conducive to the expansion of electrochemical sensing applications and the universality of applications. In addition, COFs can accurately integrate organic structural units with atomic precision into ordered structures, so that the structural diversity and application of COFs can be greatly enriched by designing new construction units and adopting reasonable functional strategies. In this review, we mainly summarized state-of-the-art recent advances of the classification and synthesis strategy of COFs, the design of functionalized COF for electrochemical sensors and COFs-based electrochemical sensing. Then, an overview of the considerable recent advances made in applying outstanding COFs to establish electrochemical sensing platform, including electrochemical sensor based on voltammetry, amperometry, electrochemical impedance spectroscopy, electrochemiluminescence, photoelectrochemical sensor and others. Finally, we discussed the positive outlooks, critical challenges and bright directions of COFs-based electrochemical sensing in the field of disease diagnosis, environmental monitoring, food safety, drug analysis, etc.

2D Organic Materials: Status and Challenges

In the past few decades, 2D layer materials have gradually become a central focus in materials science owing to their uniquely layered structural qualities and good optoelectronic properties. However, in the development of 2D materials, several disadvantages, such as limited types of materials and the inability to synthesize large-scale materials, severely confine their application. Therefore, further exploration of new materials and preparation methods is necessary to meet technological developmental needs. Organic molecular materials have the advantage of being customizable. Therefore, if organic molecular and 2D materials are combined, the resulting 2D organic materials would have excellent optical and electrical properties. In addition, through this combination, the free design and large-scale synthesis of 2D materials can be realized in principle. Furthermore, 2D organic materials exhibit excellent properties and unique functionalities along with great potential for developing sensors, biomedicine, and electronics. In this review, 2D organic materials are divided into five categories. The preparation methods and material properties of each class of materials are also described in detail. Notably, to comprehensively understand each material's advantages, the latest research applications for each material are presented in detail and summarized. Finally, the future development and application prospects of 2D organic materials are briefly discussed.

Covalent Organic Frameworks-Based Electrochemical Sensors for Food Safety Analysis

Food safety is a key issue in promoting human health and sustaining life. Food analysis is essential to prevent food components or contaminants causing foodborne-related illnesses to consumers. Electrochemical sensors have become a desirable method for food safety analysis due to their simple, accurate and rapid response. The low sensitivity and poor selectivity of electrochemical sensors working in complex food sample matrices can be overcome by coupling them with covalent organic frameworks (COFs). COFs are a kind of novel porous organic polymer formed by light elements, such as C, H, N and B, via covalent bonds. This review focuses on the recent progress in COF-based electrochemical sensors for food safety analysis. Firstly, the synthesis methods of COFs are summarized. Then, a discussion of the strategies is given to improve the electrochemistry performance of COFs. There follows a summary of the recently developed COF-based electrochemical sensors for the determination of food contaminants, including bisphenols, antibiotics, pesticides, heavy metal ions, fungal toxin and bacterium. Finally, the challenges and the future directions in this field are discussed.

Lighting up Micro-/Nanorobots with Fluorescence

Micro-/nanorobots (MNRs) can be autonomously propelled on demand in complex biological environments and thus may bring revolutionary changes to biomedicines. Fluorescence has been widely used in real-time imaging, chemo-/biosensing, and photo-(chemo-) therapy. The integration of MNRs with fluorescence generates fluorescent MNRs with unique advantages of optical trackability, on-the-fly environmental sensitivity, and targeting chemo-/photon-induced cytotoxicity. This review provides an up-to-date overview of fluorescent MNRs. After the highlighted elucidation about MNRs of various propulsion mechanisms and the introductory information on fluorescence with emphasis on the fluorescent mechanisms and materials, we systematically illustrate the design and preparation strategies to integrate MNRs with fluorescent substances and their biomedical applications in imaging-guided drug delivery, intelligent on-the-fly sensing and photo-(chemo-) therapy. In the end, we summarize the main challenges and provide an outlook on the future directions of fluorescent MNRs. This work is expected to attract and inspire researchers from different communities to advance the creation and practical application of fluorescent MNRs on a broad horizon.

Solid-solid synthesis of covalent organic framework as a support for growth of controllable ultrafine Au nanoparticles

Covalent organic frameworks (COFs) are promising supports for the synthesis of noble metal nanoparticles (NM NPs) with controllable sizes and dispersities. However, it is still challenging to synthesize COFs using green and efficient routes. Herein, COFs (TpMA) were prepared by ball milling, which required less solvent and time. They were then used as a support for the growth of ultrafine Au NPs. Using the COFs as supports, five size-controlled ultrafine Au NPs (2.5 ± 0.55- 4.32 ± 1.39 nm) were synthesized (Au@TpMA). It was found that the Au NPs exhibited remarkable dispersibility owing to the support of TpMA. The reduction of 4-nitrophenol to 4-aminophenol was used as a model reaction to evaluate the performance of the Au@TpMA catalyst, which showed excellent catalytic activity for the reduction of 4-nitrophenol. The Au@TpMA catalyst exhibited good stability and recyclability, and the reduction rate was 95% at the end of six successive experiments. In addition, in the presence of the Au@TpMA catalyst, the maximum pseudo-first-order reaction rate constant of 4-nitrophenol was 0.2379 min^-1. From the results of this study, we hope that using COFs-based supports prepared by ball milling for the size-controlled synthesis of NM NPs provides a path forward for the mechanical synthesis of other COFs.

Covalent Organic Frameworks for Chemical and Biological Sensing

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with polygonal porosity and highly ordered structures. The most prominent feature of the COFs is their excellent crystallinity and highly ordered modifiable one-dimensional pores. Since the first report of them in 2005, COFs with various structures were successfully synthesized and their applications in a wide range of fields including gas storage, pollution removal, catalysis, and optoelectronics explored. In the meantime, COFs also exhibited good performance in chemical and biological sensing, because their highly ordered modifiable pores allowed the selective adsorption of the analytes, and the interaction between the analytes and the COFs’ skeletons may lead to a detectable change in the optical or electrical properties of the COFs. In this review, we firstly demonstrate the basic principles of COFs-based chemical and biological sensing, then briefly summarize the applications of COFs in sensing some substances of practical value, including some gases, ions, organic compounds, and biomolecules. Finally, we discuss the trends and the challenges of COFs-based chemical and biological sensing.

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.

Enhanced emission by stacking of crown ether side chains in a 2D covalent organic framework

Hydrazone compounds could be highly emissive in the solid state. However, most hydrazone-linking COFs are poorly luminescent. Here we report the enhancement of fluorescence in a 2D hydrazone-linking COF by side chain engineering. Stacking of the bulky and semi-rigid crown ether side chains restricts intramolecular rotation of the backbone around the hydrazone linkers, reducing the thermal dissipation and achieving 20-fold improved 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.

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.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Construction of dual-functional nitrogen-enriched fluorescent porous organic polymers for detecting m-dinitrobenzene, picric acid and capturing iodine

Two novel nitrogen-enriched porous organic polymers (POPs), HBP and TBP, were constructed via nucleophilic substitution reactions with high nitrogen contents up to 24.91% and 32.92% for sensing to nitroaromatic compounds (NACs) and adsorbing iodine. They were all systematically characterized by solid-state ¹³C NMR, FT-IR, elemental analysis, solid-state UV-Vis, and other material analysis methods. The experimental data proved that both POPs possess high chemical and thermal stability, excellent fluorescence performance, and porous properties with Brunauer-Emmett-Teller (BET) specific surface areas of 32.88 and 68.00 m² g^-1. The two POPs have dual functions of fluorescence sensing and adsorption. On the one hand, due to their excellent conjugated properties and nitrogen-enriched structures, HBP and TBP exhibited incredibly high sensitivity to m-dinitrobenzene (m-DNB) and picric acid (PA) with K_SV values of 2.57 × 10⁵ and 4.93 × 10⁴ L mol^-1 and limits of detection of 1.17 × 10^-11 and 6.08 × 10^-11 mol L^-1, respectively. On the other hand, owing to the plenty of nitrogen affinity sites, they exhibited excellent volatile iodine adsorption with 2.23 and 2.66 g g^-1, respectively.

Hotpots and trends of covalent organic frameworks (COFs) in the environmental and energy field: Bibliometric analysis

Covalent organic frameworks (COFs) have attracted extensive attention due to their low density, adjustable structure, functionalization, and good stability. This paper systematically and comprehensively describes to qualitatively and quantitatively the progress, trends, and hotspots of COFs in the environmental and energy fields from the perspective of bibliometrics. Herein, based on the Web of Science database, a total of 2589 articles from 2005 to October 6, 2020, were collected. Thereafter, co-occurrence, co-citation analysis, and cluster analysis were conducted using CiteSpace and VOSviewer software. The results indicated that COFs research shows the characteristics of rapid growth. The active countries were mainly USA, Germany, Japan, China, and India. More than half of the top 20 active institutions were from China. The research hotspots in this field were systematically elaborated, including synthesis, adsorption, catalysis, membrane, sensor, and energy storage. Research has shown that various COFs are reasonably designed, synthesized, and used in different applications. For example, when COFs are used for photocatalysis, groups containing photocatalytic active sites are integrated into COFs to improve photocatalytic activity. Finally, some challenges were proposed, that are beneficial to the rapid and balanced development of the COFs field. For instance, the preparation methods still need to be further improved for mass production and there is an imbalance in environmental applications such as fewer sensor and membrane applications. We believe that this study provides a comprehensive and systematic overview of the environmental and energy applications of COFs for future investigations.

AIEgens-based fluorescent covalent organic framework in construction of chemiluminescence resonance energy transfer system for serum uric acid detection

A covalent organic framework (COF) with aggregation-induced emission (AIE) property was successfully synthesized through in situ marriage of a commonly used AIE molecule tetraphenylethylene (TPE) with Schiff base network (SNW-1) through a simple one-pot method. The TPE@SNW-1 was characterized with different techniques of Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and nitrogen adsorption/desorption experiments. The fluorescence of the TPE@SNW-1 strongly depends on the composition of tetrahydrofuran-water binary system. The AIE property of TPE@SNW-1 was directly supported with particle size distribution by dynamic light scattering technique. With the TPE@SNW-1 as an energy acceptor, a chemiluminescence resonance energy transfer (CRET) system was constructed with bis(2,4,6-trichlorophenyl) oxalate (TCPO)-hydrogen peroxide (H₂O₂) reaction as an energy donor. The chemiluminescence (CL) signal displays a good linear relationship with concentration of H₂O₂ in the 5.0-1000.0 μmol·L^-1 range, and a detection limit of 2.34 μmol L^-1. The system was further exploited to determine uric acid based on the fact that equal stoichiometric amount of H₂O₂ can be concurrently generated under the catalysis of uricase. The procedure exhibits a linear response to uric acid concentration in the range 10.0-150.0 μmol·L^-1 and a detection limit of 4.94 μmol·L^-1. The practicability of the method was demonstrated  in the determination of uric acid in human serum samples.

A core-shell structured magnetic covalent organic framework as a magnetic solid-phase extraction adsorbent for phenylurea herbicides

In this work, a core-shell structured magnetic covalent organic framework named as M-TpDAB was constructed with 3,3'-diaminobenzidine (DAB) and 1,3,5-triformylphloroglucinol (Tp) as building units. M-TpDAB was characterized by infrared spectroscopy, nitrogen adsorption-desorption isotherms, powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Using the M-TpDAB as adsorbent, a simple and highly effective method was proposed for preconcentrating phenylurea herbicides before high performance liquid-phase chromatography analysis. In the optimized conditions, a good linearity was achieved within the range of 0.15-100 ng mL^-1 for water sample, 1.0-100.0 ng mL^-1 for tea drink samples. The limits of detection for the analytes were 0.05-0.15 ng mL^-1 for water sample and 0.30-0.50 ng mL^-1 for drink samples. Satisfactory recoveries of spiked target compounds were in the range of 84.6%-105% for water sample and 80.3%-102% for tea drink samples. Finally, the M-TpDAB based method was successfully used to determine phenylurea herbicides in tea drinks and water samples, demonstrating a good alternative for analyzing trace level of phenylurea herbicides in water samples.

Covalent Organic Polymers and Frameworks for Fluorescence-Based Sensors

Following the advancements and diversification in synthetic strategies for porous covalent materials in the literature, the materials science community started to investigate the performance of covalent organic polymers (COPs) and covalent organic frameworks (COFs) in applications that require large surface areas for interaction with other molecules, chemical stability, and insolubility. Sensorics is an area where COPs and COFs have demonstrated immense potential and achieved high levels of sensitivity and selectivity on account of their tunable structures. In this review, we focus on those covalent polymeric systems that use fluorescence spectroscopy as a method of detection. After briefly reviewing the physical basis of fluorescence-based sensors, we delve into various kinds of analytes that have been explored with COPs and COFs, namely, heavy metal ions, explosives, biological molecules, amines, pH, volatile organic compounds and solvents, iodine, enantiomers, gases, and anions. Throughout this work, we discuss the mechanisms involved in each sensing application and aim to quantify the potency of the discussed sensors by providing limits of detection and quenching constants when available. This review concludes with a summary of the surveyed literature and raises a few concerns that should be addressed in the future development of COP and COF fluorescence-based sensors.

Structural Characteristics and Environmental Applications of Covalent Organic Frameworks

Covalent organic frameworks (COFs) are emerging crystalline polymeric materials with highly ordered intrinsic and uniform pores. Their synthesis involves reticular chemistry, which offers the freedom of choosing building precursors from a large bank with distinct geometries and functionalities. The pore sizes of COFs, as well as their geometry and functionalities, can be pre-designed, giving them an immense opportunity in various fields. In this mini-review, we will focus on the use of COFs in the removal of environmentally hazardous metal ions and chemicals through adsorption and separation. The review will introduce basic aspects of COFs and their advantages over other purification materials. Various fabrication strategies of COFs will be introduced in relation to the separation field. Finally, the challenges of COFs and their future perspectives in this field will be briefly outlined.

Covalent Organic Frameworks for Biomedical Applications

Covalent organic frameworks (COFs) are porous organic polymeric materials that are composed of organic elements and linked together by the thermodynamically stable covalent bonds. The applications of COFs in energy sector and drug delivery are afforded because of the desirable properties of COFs, such as high stability, low density, large surface area, multidimensionality, porosity, and high-ordered crystalline structure expanded. In this review COFs are reviewed, from the perspective of different types of reported COFs, different methods for their synthesis, and their potential applications in the biomedical field. The main goal of this review is to introduce COFs as a biomaterial and to identify specific advantages of different types of COFs that can be exploited for specialized biomedical applications, such as immune engineering.

Nanoscale melamine-based porous organic frameworks as host material for efficient polysulfides chemisorption in lithium-sulfur batteries

In order to improve the electrochemical capacity of lithium-sulfur batteries (LiSBs), it is necessary to introduce the porous organic frameworks with well-defined hetero atom species in cathode. In this work, porous nanomaterials with ultra-high nitrogen containing and adjustable porosity named Schiff-based networks (SNWs) were selected as potential candidate for sulfur host in LiSBs. Two SNW samples have been constructed by reacting melamine with phenyl or biphenyl dialdehydes through microwave-assisted method, respectively. The high BET surface area provided sufficient room to impregnate sulfur and mitigated volume changes during the cycling performance. Besides, the high density and homogeneous distribution of pyridinic-N and aminic-N in SNW nanoparticles can cooperatively form lithium polysulfides (LiPSs) chemisorption via enhanced Li⁺-N interactions to effectively suppressed the 'shuttle effect'. Attributed to its structural superiorities, SNW/S cathode demonstrates excellent electrochemical performance in LiSBs. In particular, SNW-2/S cathode delivers an excellent cyclability with a specific capacity of 620 mAh · g^-1 after 500 cycles at 0.5 C, counting with a low capacity fading of 0.0508% per cycle. This work highlights the importance of rational design for effective LiPSs chemisorption and pioneers a facile strategy for developing suitable sulfur host materials towards high-performance LiSBs.

In situ controllable synthesis of Schiff base networks porous polymer coatings for open-tubular capillary electrochromatography

A uniform Schiff base network (SNW) film was synthesized in situ in a controllable way through continuous flow of reactants inside the capillary. The properties and application of the as-prepared capillary was investigated in capillary electrochromatography. The effects of reaction monomer concentration and reaction time on coating thickness were studied by SEM. The results show that the reaction condition has a significant influence on the morphology and thickness of the SNW films. The thickness of the film can be controlled by changing the concentration of reaction solution and reaction time. Capillaries coated under different conditions were employed to separate four nucleotides by capillary electrochromatography, which demonstrated significant variation of migration time, peak order, and separation efficiency. Analytes containing nitrogen heterocycle structures, such as nucleotides, methylimidazole isomers, and β-lactam antibiotics, were successfully separated with the prepared open-tubular columns. Under the selected separation conditions, theoretical plate number of four nucleotides is in a range 45,237-104,505 plates·m^-1, and the resolutions are 1.98-8.07. A resolution of 1.75 is obtained for methylimidazole isomers. The nucleotides in a real sample, chicken essence seasoning, were determined using the prepared capillary column with satisfactory recoveries in the range 95 to 105%.

Charge transfer complexes: Emerging and promising colorimetric real-time chemosensors for hazardous materials

After introducing the concept of charge transfer (CT) complex formation by Mulliken and the discovery of crystalline picrate (association of picric acid and aromatic hydrocarbons) by Fritzsches, a large interest has been drawn in this field. CT complexes have been explored and exploited for different applications for several decades. The research has been aimed mostly for discovering and characterizing new CT materials and exploring applications mainly in the field of optoelectronic properties, antimicrobial activities and DNA/protein binding properties for the last six years. However, nowadays, CT complexes are exploited for their photocatalytic activities and designing chemosensors for the colorimetric real-time detection of hazardous materials like nitro explosives, anions and toxic heavy metal ions in an aqueous medium. This review sheds light on updates on CT complexes, their types, synthesis and applications. The brief discussion on the emergence of CT complexes as highly potential chemosensors along with the explanation of sensing mechanism through article summarization is the centerpiece of this review. The final outcomes are discussed and concluded.

Microwave-Assisted Synthesis of Covalent Organic Frameworks: A Review

Covalent organic frameworks (COFs) are relatively recent materials. They have received great attention due to their interesting properties. However, the application of microwaves in their synthesis, despite its advantages such as faster and more reproducible processes, is a minority. Herein, a comprehensive compilation of the research results published in the microwave-assisted synthesis (MAS) of COFs is presented. This review includes articles of 2D and 3D COFs prepared using microwaves as source of energy. The articles have been classified depending on the functional groups including boronate ester, imines, enamines, azines, and triazines, among others. It compiles the main parameters of synthesis and characteristics of the materials together with some general issues related with COFs and microwaves. Additionally, current and future perspectives of the topic have been discussed.

Synthesis methods of microporous organic polymeric adsorbents: a review

MOPs can be synthesized in a large variety of ways, which affect their pores and surface area. Variation in synthesis and porosity has a significant effect on their adsorption properties.

An imine linked fluorescent covalent organic cage: the sensing of chloroform vapour and metal ions, and the detection of nitroaromatics

Fluorescent covalent organic cage molecule (F-COC) showed enhanced emission intensity in chloroform solution and polymer matrix film form in presence of chloroform vapours.

Covalent organic framework Schiff base network-1-based pipette tip solid phase extraction of sulfonamides from milk and honey

In this work, a covalent organic framework Schiff base network-1 (SNW-1), was synthesized based on the Schiff base reaction between terephthalaldehyde and melamine and characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction and nitrogen adsorption-desorption isotherm analyses. The prepared SNW-1 was employed as pipette tip solid phase extraction adsorbent for the extraction of sulfonamides (SAs) prior to high performance liquid chromatography analysis. The parameters affecting the extraction efficiency, including the salt concentration, sample pH, amount of adsorbent, and types and volume of eluent were investigated in detail. Good linearities were obtained between the peak area and SAs concentration ranging from 5 to 500 ng mL^-1 with correlation coefficients (R²) higher than 0.9998. The limits of detection and RSDs were lower than 0.25 ng mL^-1 and 1.9 %, respectively. The developed method was further applied for the determination of SAs in milk and honey samples with recoveries in the range of 85.8 % - 118.0 % and RSDs less than 9.5 %. The results demonstrate that the SNW-1 shows great potential for the enrichment of trace SAs in complex matrices.

Covalent organic framework and montomorillonite nanocomposite as advanced adsorbent: synthesis, characterization, and application in simultaneous adsorption of cationic and anionic dyes

In this work, Schiff base network-1 (SNW-1), as a new generation of covalent organic frameworks (COFs), was synthesized and modified by fabrication of a composite with clay mineral montomorillonite (Mt). It was used for simultaneous removal of anionic and cationic dyes from aqueous solutions. The fabricated composite was characterized successfully with various techniques. Tartrazine (TT) and methylene blue (MB) were selected as model anionic and cationic dyes, respectively. The effects of the percentage of each component in the composite, initial pH, and initial dye concentration were evaluated on the adsorption capacity. Adsorption reaction models and adsorption diffusion models were used to study the kinetic process of adsorption. Adsorption of both dyes reached equilibrium after 40 min. The obtained results were fitted to Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) models to predict the isotherms of adsorption. Under optimum conditions for removal of each dye with the composite, the maximum adsorption capacity of 519.2 and 602.7 mg g^-1 were obtained for TT and MB, respectively. The used SNW-1/Mt composite could be regenerated by salty methanol. The high adsorption capacity and excellent reusability make SNW-1/Mt composite attractive for the simultaneous removal of anionic and cationic dyes from aqueous solutions.

Electrochemical Sensors Based on Covalent Organic Frameworks: A Critical Review

The metal-free cousins of metal-organic frameworks, covalent organic frameworks (COFs), are a class of pre-designable crystalline polymers composed of light elements and connected by strong covalent bonds. COFs are being given more and more attention in the electrochemical sensor field due to their fascinating properties, such as highly tunable porosity, intrinsic chemical and thermal stability, structural diversity, large specific surface area, and unique adsorption characteristics. However, there are still some key issues regarding COFs that need to be urgently resolved before they can be effectively applied in electrochemical sensing. In this review, we summarized recent achievements in developing novel electrochemical sensors based on COFs, and discussed the key fundamental and challenging issues that need to be addressed, including the mechanisms underlying charge transport, methods to improve electrical conductivity, immobilization methods on different substrates, synthesis strategies for nanoscale COFs, and the application of COFs in different fields. Finally, the challenges and outlooks in this promising field are tentatively proposed.

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.