A chiral fluorescent COF prepared by post-synthesis modification for optosensing of imazamox enantiomers

Silver nanoparticle-functionalized covalent organic frameworks for the inhibition of foodborne pathogenic bacteria and their application in green grape preservation

Foodborne pathogens continue to pose a significant threat to human health. This study aims to enhance the antimicrobial activity of low-dose silver nanoparticles (AgNPs) against foodborne pathogens and use the enhanced AgNPs to preserve green grapes. A chemical delivery carrier for covalent organic frameworks (COFs) impregnated with AgNPs was developed. We investigated the bacteriostatic properties (minimum bacteriostatic concentration, bacteriostatic growth curve), the mechanism of action of the bacteriostatic agent, and the performance of the bacteriostatic film. The bacteriostatic preservation rate of the AgNPs@COFs composite on green grapes was evaluated. The minimum bacteriostatic concentration of the AgNPs@COFs composite was 10 μg/mL, and the bacteriostatic rate varied between 94.01 % and 98.77 %. The developed antibacterial AgNPs@COFs composite has potential applications in food packaging and preservation.

Construction of a Defective Chiral Covalent Organic Framework for Fluorescence Recognition of Amino Acids

The design and synthesis of chiral covalent organic frameworks (COFs) with controlled defect sites are highly desirable but still remain largely unexplored. Herein, we report the synthesis of a defective chiral HD-TAPB-DMTP COF by modifying the chiral monomer helicid (HD) into the framework of an achiral imine-linked TAPB-DMTP COF using a chiral monomer exchange strategy. Upon the introduction of the chiral HD unit, the obtained defective chiral HD-TAPB-DMTP COF not only displays excellent crystallinity, large specific surface area (up to 2338 m2/g) and rich accessible chiral functional sites but also exhibits fluorescence emission, rendering it a good candidate for discrimination of amino acids. Notably, the resultant defective chiral HD-TAPB-DMTP COF can be used as a fluorescent sensor for enantioselective recognition of both tyrosine and phenylalanine enantiomers in water, showing enhanced fluorescent responses for the L conformations over those of the D conformations with the enantioselectivity factors being 1.84 and 2.02, respectively. Moreover, molecular docking simulations uncover that stronger binding affinities between chiral HD-TAPB-DMTP COF and L-tyrosine/L-phenylalanine in comparison to those with D-tyrosine/D-phenylalanine play important roles in enantioselective determination. This work provides new insights into the design and construction of highly porous defective chiral COFs for enantioselective fluorescence recognition of amino acids.

Dual-functional porphyrinic zirconium-based metal-organic framework for the fluorescent sensing of histidine enantiomers and Hg2

A novel fluorescence sensor based on a porphyrinic zirconium-based metal-organic framework, L-cysteine-modified PCN-222 (L-Cys/PCN-222), was developed to selectively recognize histidine enantiomers and sensitively detect Hg2+. The dual-functional sensor was successfully prepared via the solvent-assisted ligand incorporation method and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), 1H nuclear magnetic resonance (1H NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, circular dichroism (CD), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption-desorption analyses. L-Cys/PCN-222 not only showed a higher quenching response for L-histidine than that for D-histidine with a fast fluorescent response rate of <40 s but also exhibited low detection limits for L- and D-histidine (2.48 μmol L-1 and 3.85 μmol L-1, respectively). Moreover, L-Cys/PCN-222 was employed as a fluorescent and visual sensor for the highly sensitive detection of Hg2+ in the linear range of 10-500 μmol L-1, and the detection limit was calculated to be 2.79 μmol L-1 in surface water. The specific and selective recognition of chiral compounds and metal ions by our probe make it suitable for real field applications.

Chiral hydroxyl-controlled covalent organic framework-modified stationary phase for chromatographic enantioseparation

Chiral covalent organic frameworks (CCOFs) possess a superior chiral recognition environment, abundant pore configuration, and favorable physicochemical stability. In the post-synthetic chiral modification of COFs, research usually focused on increasing the density of chiral sites as much as possible, and little attention has been paid to the influence of the density of chiral sites on the spatial structure and chiral separation performance of CCOFs. In this article, 1,3,5-tris(4-aminophenyl) benzene (TPB), 2,5-dihydroxyterephthalaldehyde (DHTP), and 2,5-dimethoxyterephthalaldehyde (DMTP) served as the platform molecules to directly establish hydroxyl-controlled COFs through Schiff base condensation reactions. Then the novel chiral selectors 6-deoxy-6-[1-(2-aminoethyl)-3-(4-(4-isocyanatobenzyl)phenyl)urea]-β-cyclodextrin (UB-β-CD) were pended into the micropore structures of COFs via covalent bond for further construction the [UB-β-CD]x-TPB-DMTP COFs (x represents the density of chiral sites). The chiral sites density on [UB-β-CD]x-TPB-DMTP COFs was regulated by changing the construction proportion of DHTP to obtain a satisfactory CCOFs and significantly improve the ability of chiral separation. [UB-β-CD]x-TPB-DMTP COFs were coated on the inner wall of a capillary via a covalently bonding strategy. The prepared open tubular capillary exhibited strong and broad enantioselectivity toward a variety of chiral analytes, including sixteen racemic amino acids and six model chiral drugs. By comparing the outcomes of chromatographic separation, we observed that the density of chiral sites in CCOFs was not positively correlated with their enantiomeric separation performance. The mechanism of chiral recognition [UB-β-CD]x-TPB-DMTP COFs were further demonstrated by molecular docking simulation. This study not only introduces a new high-efficiency member of the COFs-based CSPs family but also demonstrates the enantioseparation potential of CCOFs constructed with traditional post-synthetic modification (PSM) strategy by utilizing the inherent characteristics of porous organic frameworks.

Recent Progress in Detecting Enantiomers in Food

The analysis of enantiomers in food has significant implications for food safety and human health. Conventional analytical methods employed for enantiomer analysis, such as gas chromatography and high-performance liquid chromatography, are characterized by their labor-intensive nature and lengthy analysis times. This review focuses on the development of rapid and reliable biosensors for the analysis of enantiomers in food. Electrochemical and optical biosensors are highlighted, along with their fabrication methods and materials. The determination of enantiomers in food can authenticate products and ensure their safety. Amino acids and chiral pesticides are specifically discussed as important chiral substances found in food. The use of sensors replaces expensive reagents, offers real-time analysis capabilities, and provides a low-cost screening method for enantiomers. This review contributes to the advancement of sensor-based methods in the field of food analysis and promotes food authenticity and safety.

Raising the Asymmetric Catalytic Efficiency of Chiral Covalent Organic Frameworks by Tuning the Pore Environment

Chiral covalent organic frameworks (COFs) hold considerable promise in the realm of heterogeneous asymmetric catalysis. However, fine-tuning the pore environment to enhance both the activity and stereoselectivity of chiral COFs in such applications remains a formidable challenge. In this study, we have successfully designed and synthesized a series of clover-shaped, hydrazone-linked chiral COFs, each with a varying number of accessible chiral pyrrolidine catalytic sites. Remarkably, the catalytic efficiencies of these COFs in the asymmetric aldol reaction between cyclohexanone and 4-nitrobenzaldehyde correlate well with the number of accessible pyrrolidine sites within the frameworks. The COF featuring nearly one pyrrolidine moiety at each nodal point demonstrated excellent reaction yields and enantiomeric excess (ee) values, reaching up to 97 and 83%, respectively. The findings not only underscore the profound impact of a deliberately controlled chiral pore environment on the catalytic efficiencies of COFs but also offer a new perspective for the design and synthesis of advanced chiral COFs for efficient asymmetric catalysis.

Post-synthetic modifications of covalent organic frameworks (COFs) for diverse applications

Covalent organic frameworks (COFs) are porous and crystalline organic polymers, which have found usage in various fields. These frameworks are tailorable through the introduction of diverse functionalities into the platform. Indeed, functionality plays a key role in their different applications. However, sometimes functional groups are not compatible with reaction conditions or can compete and interfere with other groups of monomers in the direct synthetic method. Also, pre-synthesis of bulky moieties in COFs can negatively affect crystal formation. To avoid these problems a post-synthetic modification (PSM) approach is a helpful tactic. Also, with the assistance of this strategy porous size can be tunable and stability can be improved without considerable effect on the crystallite. In addition, conductivity, hydrophobicity/ hydrophilicity, and chirality are among the features that can be reformed with this method. In this review, different types of PSM strategies based on recent articles have been divided into four categories: (i) post-functionalization, (ii) post-metalation, (iii) chemical locking, and (iv) host-guest post-modifications. Post-functionalization and chemical locking methods are based on covalent bond formation while in post-metalation and host-guest post-modifications, non-covalent bonds are formed. Also, the potential of these post-modified COFs in energy storage and conversion (lithium-sulfur batteries, hydrogen storage, proton-exchange membrane fuel cells, and water splitting), heterogeneous catalysts, food safety evaluation, gas separation, environmental domains (greenhouse gas capture, radioactive element uptake, and water remediation), and biological applications (drug delivery, biosensors, biomarker capture, chiral column chromatography, and solid-state smart nanochannels) have been discussed.

Chiral Materials: Progress, Applications, and Prospects

Chirality is a universal phenomenon in molecular and biological systems, denoting an asymmetric configurational property where an object cannot be superimposed onto its mirror image by any kind of translation or rotation, which is ubiquitous on the scale from neutrinos to spiral galaxies. Chirality plays a very important role in the life system. Many biological molecules in the life body show chirality, such as the "codebook" of the earth's biological diversity-DNA, nucleic acid, etc. Intriguingly, living organisms hierarchically consist of homochiral building blocks, for example, l-amino acids and d-sugars with unknown reason. When molecules with chirality interact with these chiral factors, only one conformation favors the positive development of life, that is, the chiral host environment can only selectively interact with chiral molecules of one of the conformations. The differences in chiral interactions are often manifested by chiral recognition, mutual matching, and interactions with chiral molecules, which means that the stereoselectivity of chiral molecules can produce changes in pharmacodynamics and pathology. Here, the latest investigations are summarized including the construction and applications of chiral materials based on natural small molecules as chiral source, natural biomacromolecules as chiral sources, and the material synthesized by design as a chiral source.

Chiral covalent organic framework-based open tubular capillary electrochromatography column for enantioseparation of selected amino acids and pesticides

A novel chiral covalent organic framework (CCOF) was synthesized with an imine covalent organic framework TpBD (synthesized via Schiff-base reaction between phloroglucinol (Tp) and benzidine (BD)) modified using (1S)-(+)-10-camphorsulfonyl chloride as chiral ligand by chemical bonding method for the first time, and was characterized by X-ray diffraction, Fourier-transform infrared spectra, X-ray photoelectron spectroscopy, nitrogen adsorption/desorption, thermogravimetry analysis, and zeta-potential. The results revealed that the CCOF had good crystallinity, high specific surface area and good thermal stability. Then, the CCOF was employed as stationary phase in open-tubular capillary electrochromatography (OT-CEC) column (the CCOF-bonded OT-CEC column) for enantioseparation of 21 single chiral compounds (12 natural amino acids including acidic, neutral and basic, 9 pesticides including herbicides, insecticides and fungicides) and simultaneous enantioseparation of mixture amino acids and pesticides with similar structures or properties. Under the optimized CEC conditions, all the analytes reached the baseline separation with high resolutions of 1.67-25.93 and selectivity factors of 1.06-3.49 in 8 min. Finally, the reproducibility and stability of the CCOF-bonded OT-CEC column were measured. Relative standard deviations (RSDs) of retention time and separation efficiency were 0.58-4.57% and 1.85-4.98%, and not obviously changed after 150 runs. These results demonstrate that COFs-modified OT-CEC explore a promising method to separate chiral compounds.