Preparation Of Imine-based Covalent Organic Framework Membranes And Study On Their Iodine Adsorption And Bactericidal Properties

In view of the serious impact of pathogenic infections on human production and life,we deeply appreciate the powerful killers of viruses and bacteria,which can quickly destroy our bodies as well as the surrounding living environment,which makes it crucial to develop new and efficient sterilization and antibacterial materials.Medical clinical experiments have shown that iodine is widely used as an effective sterilization and disinfection material to destroy various bacteria,fungi or viruses,but iodine particles have the disadvantages of poor water solubility and easy sublimation,making them difficult to be widely used.Researchers usually solidify iodine on water-insoluble carriers to ensure stability and rapid release of iodine for efficient sterilization or disinfection.Solid phase adsorption method has been widely studied by researchers as the main method for iodine adsorption due to its ease of operation and low cost.Studies have shown that most solid carriers due to the complex and irregular pores,will cause pore connection blockage and limited adsorption capacity during the process of iodine adsorption,and can’t achieve the expected adsorption effect.Covalent organic frameworks（COFs）have great potential for iodine adsorption due to their long-range ordered molecular structure,open pore channels,high specific surface area and designability of functional groups.A large number of studies have proved that COFs are able to immobilize iodine in large quantities and in molecular form due to theirπ-πstacking structure and adsorption sites such as imine bonds（-C=N-）.However,most of the COFs reported in the literature for iodine adsorption studies are powder materials,and their iodine adsorption capacity in organic solvents or aqueous solutions is low.In addition,there are few reports of detailed study of the iodine uptake amount and iodine release process of COFs’adsorption sites.Since the structure of COFs can be designed and the number of building units is innumerable,it is necessary to select suitable reaction monomers to prepare COFs for iodine absorption study.The methods to prepare COF membranes generally include interfacial polymerization,solvothermal synthesis and solid-phase synthesis,etc.However,it is difficult to prepare continuous and defect-free large-area membranes due to the strong substrate dependence or lack of membrane preparation techniques in these preparation methods,while screen printing technology can obtain large-area free-standing COF membranes by screen printing the reaction precursors on glass sheet carriers after heating and crystallization.In order to solve the problems of easy clogging of pore channels of porous materials and their low iodine uptake amount in solution,a variety of Schiff base-based COFs membranes based on 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde（Tp）and COFs-Cu Cl₂ membranes containing anti-chloride ions were prepared in this paper and studied for their iodine adsorption in cyclohexane solution or aqueous solution,iodine release in PBS aqueous solution and the ability to sterilize.The specific research contents are as follows:1.In order to compare the iodine uptake capacity of the Schiff base COF membranes obtained from the heated dehydration condensation of different amine-based monomers with Tp,this chapter was designed to prepare five representative free-standing COF（Tp-Azine,Tp Pa,Tp BD,Tp TD and Tp-TAPB）membranes by mechanical grinding assisted screen printing technology.The diameter of COF membranes can reach up to 4 cm,and the thickness of the membrane is 18-30μm.Due to a large number of-C-NH-sites existed in the membrane skeleton,high content of iodine can be adsorbed by COF membranes quickly in iodine/cyclohexane solution.The experimental results show that Tp-Azine membrane adsorbs the fastest rate but the lowest adsorption amount（213.5 g/mol）,while Tp BD membrane adsorbs the highest amount of I₂（640.7 g/mol）,which is due to the fact that COF membranes with larger pore size have better ability to adsorb iodine when the number of adsorption sites of COFs is the same.2.In addition to using the pore size of COF membranes to regulate their iodine uptake amount,the iodine adsorption capacity of COFs can also be improved by introducing new adsorption sites（counterions）.It has been shown that Cl^-strongly interact with I₂ to form[I₂Cl]^-ions,thereby improving the iodine uptake amount of the material.This chapter was designed to prepare free-standing membrane（Tp Pa-Cu Cl₂）with Cu Cl₂ embedded in Tp Pa backbone using screen printing technique.The resulting membranes have a high capacity for I₂ adsorption in I₂/cyclohexane solution（566.8g/mol）,corresponding to 2.2 I₂ molecules per Tp Pa-Cu Cl₂ unit cell.In other words,1.3I₂ are immobilized to 3 Cl^-ions（～60%）and 0.9 I₂ are immobilized to 3-C-NH-（～40%）,and the iodine-absorbed membrane is named Tp Pa-Cu Cl₂ COFI.When the Tp Pa-Cu Cl₂ COFI membrane was placed in aqueous PBS solution,61.1%of its loaded I₂ could be released within 10 min,mainly by I₂ on the[I₂Cl]^-ions,and the slow release of the remaining 38.9%of the loaded I₂ was the I₂ adsorbed on its-C-NH-.The iodine-absorbed Tp Pa-Cu Cl₂ COFI membrane was used for antibacterial experiments and it was found that the Tp Pa-Cu Cl₂ COFI membrane loaded with 1.5 mg I₂ could kill approximately 10⁸ CFU/m L E.coli.5 times repeatedly within 5.5 min,after which the membrane could still maintain bactericidal activity against 10⁸ CFU/m L E.coli.for the next 4 h.This indicates that the Tp Pa-Cu Cl₂ COFI membrane has greater potential for immediate and sustained bactericidal applications.3.Based on the fact that the iodine uptake ability of Cl^-is stronger than that of-C-NH-sites,three free-standing COFs-Cu Cl₂ membranes（Tp Pa-Cu Cl₂,Tp BD-Cu Cl₂and Tp TD-Cu Cl₂）with different pore sizes and containing Cu Cl₂ embedding were designed and synthesized in this section,and their ability to adsorb and release iodine was investigated in aqueous solution in order to avoid environmental pollution.In addition,the ability of COF-Cu Cl₂ membranes to kill Escherichia coli（E.coli.）and Staphylococcus aureus（S.aureus）before and after iodine uptake was also investigated,and the COFs-Cu Cl₂ membranes after iodine uptake were named as I₂@Tp Pa-Cu Cl₂,I₂@Tp BD-Cu Cl₂ and I₂@Tp TD-Cu Cl₂.Experimental results showed that The order of iodine adsorption capacity of per COFs-Cu Cl₂ unit cell in I₂/KI aqueous solution is Tp TD-Cu Cl₂>Tp BD-Cu Cl₂>Tp Pa-Cu Cl₂,which confirms that COFs with larger pore size are more capable of iodine adsorption when the number of adsorption sites in COFs is the same;and the sterilization experiments of COFs-Cu Cl₂ membranes after iodine adsorption show that I₂@Tp Pa-Cu Cl₂,I₂@Tp BD-Cu Cl₂ and I₂@Tp TD-Cu Cl₂membranes adsorbing 2 mg of I₂ could kill 10 m L of 2×10⁸ CFU/m L E.coli.or 10 m L of 3×10⁸ CFU/m L S.aureus.