Preparation Of Noval Covalent Organic Frameworks And Their Sensing Properties

Covalent organic frameworks（COFs）as an emerging material with large surface areas,highly crystallinity,tunable pores structures and chemical and thermal stabilities can be applied in separation and adsorption,catalytic reactions,photoelectricity,energy storage and fluorescence sensing.Different topology structures of the COFs linked by covalent bonds have been formed through selecting different rigid monomers.The regular pore structure of the COFs makes target molecule easier enter to interact with specific sites.The different pore size and functionalization of the COFs can provide specific selectivity for targets.In addition,2D layer-layer stack of the COFs can increase the conjugated degree,which provide the COFs stronger fluorescence emission than other porous materials.In this study,three imine-linked COFs have been prepared by selecting different unit blocks.The optical performance and the application in sensing have also been explored.（1）Water is often identified as a common impurity and contaminant in organic solvent,which usually results in the decrease of yields and activities in chemical reactions or other industrial production processes.Hence,it is important to detect and monitor the content of water in organic solvent.A highly crystalline covalent organic frameworks（COFs）formed by condensation reaction between 1,3,5-tris（4-aminophenyl）benzene and 4,4’-biphenyldicarboxaldehyde is utilized.The composition,crystallinity,BET and fluorescence property of the COFs are obtained through FT-IR,NMR,PXRD,N₂ adsorption-desorption analysis and fluorescence spectrophotometer,respectively.And the properties of the COFs as a sensing probe for water in organic solvent has also been considered.The resulting COFs exhibits brilliant fluorescence in various organic solvents such as methanol,DMF,acetonitrile and ethanol,moreover its fluorescence intensity has a significant and rapid response to the content of water in organic solvent over a broad concentration range.The broadest sensing range is achieved over 7%～70%（v/v）for water in DMF,and the lowest limit of detection is 0.042%（v/v）for water in methanol among the investigated organic solvents.The superior properties of the sensing platform expand the application ranges of the COFs and endow the resulting COFs with a great prospect in practical applications for highly efficient detecting water in organic solvent.（2）Among various transition metal ions,nickel is widely applied in various processes,such as metallurgy,catalysis,chemical textile and food production.Unproper treatment may result in exposure and cause serious pollution even damage human health.Hence,the increasing requirements to develop sensitive and selective methods for the monitoring and quantification of Ni²⁺are attracting extensive research efforts.Covalent organic frameworks（COFs）with predesigned bipyridine recognition sites were prepared by using 2,2’-bipyridine-5,5’-diamine and benzene-1,3,5-tricarbaldehyde as monomers,and the composition,crystallinity,BET and fluorescence property of the COFs is characterized by FT-IR,NMR,PXRD,N₂ adsorption-desorption analysis and fluorescence spectrophotometer,respectively.The COFs as a sensing probe for trace of Ni²⁺detection in real samples of chocolate and tea have also been considered.In order to explore the sensing performance and mechanism,corresponding small molecule model compound（2,2’-bipyridine-5,5’-diiminebenzene）using benzaldehyde and 2,2’-bipyridine-5,5’-diamine and biphenyl-linked COFs using benzidine and benzene-1,3,5-tricarbaldehyde as the monomers have been successfully synthesized.The unique structure characteristics such as large conjugated structure,high surface area,regularity pore canals and predesigned bipyridine link endow the COFs with prominent fluorescence emission,enough space and strong specificity for Ni²⁺.The fluorescence quenching degree of the COFs is four times more than that of small molecule model compound.Besides better selectivity,the COFs also displays the prominent advantages,such as the wider concentration range(4.20×10^-10～1.26×10^-6 M)and lower limit of detection(6.80×10^-11 M)compared to that of small molecule model compound(1.00×10^-7～5.00×10^-7 M,4.50×10^-8 M).Good spiked recoveries（>92.0%）in chocolate and tea support that the COFs is competent to detect Ni²⁺in real samples.All these results indicate that the COFs is potential as a fast,sensitive,and accurate fluorescence sensing platform for the selective detection of Ni²⁺in complex system.（3）Amino acid,as the essential component of human protein,has great effect to human health,which have been attracted great attention to researchers in various fields.Thus,evaluating the composition and the content of amino acid is very important for human health.In this study,a bipyridine COFs is successfully prepared by benzene-1,3,5-tricarbaldehyde and 2,2’-bipyridine-5,5’-diamine,which can coordinate well with transition metal such as Ni²⁺,Zn²⁺,Cu²⁺and Co²⁺.Four novel metal coordinated COFs have been prepared.The identification and sensing properties of the metal coordinated COFs as fluorescence array sensor for amino acid have also been considered.Owning to the different functional groups in amino acid,the coordination ability between amino acid and metal is different.Therefore,the assay-based sensor based on metal coordinated COFs can determinate and identify different kinds of amino acid.The amino acid can be quantitatively detected by linear discriminant analysis（LDA）.When the content of amino acid is low as 5×10^-6 M,the amino acid can be 100%identified based on this assay-based sensor.Furthermore,the assay-based sensor can detect cysteine without interfering by other amino acid mixture and can obtain100%identification in different content and percentage of cysteine.The successfully detection in milk also indicate practical application value of the assay-based sensor composed by metal coordinated COFs in amino acid determination.（4）Various products,including foods and pharmaceuticals,are sensitive to temperature fluctuations.Temperature monitoring during production,transportation and storage is critical for guaranteeing the quality of the products.Thus,synthesizing a facile indicator to monitor temperature conditions via color changes in real time is badly needed.Inspired by the color changes associated with thiol oxidation,a thiol-functionalized covalent organic frameworks（COFs）was synthesized using 2,5-diaminobenzene-1,4-dithioldihydrochloride and benzene-1,3,5-tricarbaldehyde as monomers via a solvothermal reaction.The composition,crystallinity,BET and adsorption property of the COFs are characterized by FT-IR,NMR,PXRD,N₂ adsorption-desorption analysis and ultraviolet spectrophotometer,respectively.The monitoring of thermal history and temperature abuse of the thiol-functional COFs has also been considered.Owing to the oxidation of thiol groups at COFs interlayer,the COFs underwent obvious color changes from bright yellow to purple after exposure to different temperatures for varying durations.The reaction kinetics are analyzed under isothermal conditions,which reveal that the order of reaction rates is k–₂₀℃<k₄℃<k₂₀℃<k₃₅℃<k₅₅℃.The activation energy（E_a）of the COFs is calculated using the Arrhenius equation as 50.71 kJ moL^–1.The COFs are capable of sensitive color changes and offer a broad temperature tracking range,thereby demonstrating their application potential for the monitoring of temperature and time exposure history during production,transportation,and storage.This excellent performance temperature and time history indicator also shows promise for expanding the application field of COFs.