Design And Synthesis Of Nover Functionalized Covalent Organic Frameworks And Their Application Study In Fluorescence Detection Of Food Contaminants

Food quality safety affects people’s life quality straight,and is interrelated to human healthy and life extension.Food safety issues ensued from food contamination have attracted extensive attention around the whole international society.Through food intake and long-term bioacumulation,pollutants such as heavy metal ions,pesticide and veterinary drugs residues,fungi and mycotoxins have become invisible killers that affect human health and life.They can not only cause acute or chronic poisoning,but even more terrifying,they also have significant carcinogenicity,teratogenicity,and mutagenicity.Thus,it is of extremely urgent to realize efficient and rapid detection of food contaminants.Benefiting from characteristics of high sensitivity,superior selectivity,and easy operation of equipment,fluorescence analysis methods plays an indispensable role in analysis filed.In recent years,with the rapid development of materials science,porous organic materials play an important role in the filed of fluorescence analysis.Among them,covalent organic frameworks(COFs),a novel class of porous crystalline organic polymers formed by organic construction units through covalent bonds in characteristics of adjustable building components,easy control of functionality,design synthesis and modification diversity,show great application prospects.Based on the detection targets,this paper aims to design and synthesize novel functionalized COFs through post-synthesis modification strategy for the fast and efficient fluorescence detection of food contaminants.The main research research works are as follows:In the first part,an amino functionalized fluorescent COF(COF-NH2)was used as fluorescence sensor for the strongly selective and sensitive detection of Hg2+ in tap water.First,a fluorescent COF(COF-OH)with lots of hydroxyl groups as post-modification sites was obtained using 2,5-dihydroxyterephthalic acid dihydrazide and 1,3,5-tribenzaldehyde as building units.Subsequently,a novel type of amino functionalized fluorescent COF:COF-NH2 was successfully obtained through the substitution reaction between 2-bromoethylamine and hydroxyl groups.The uniqueness of COF-NH2 is that it achieved an excellent combination of porous network construction with functional groups through the high accessibility of the framework and the specific recognition sites in the pore channels:the π-conjugated framework as signal sensor,and the densely and uniformly distributed primary amino groups as selective and reversible recognition acceptor of Hg2+.Meanwhile,the regular pore structure is helpful for primary amino groups to recognize Hg2+and realize its real-time detection.When Hg2+is present,the fluorescence of COF-NH2 achieved effective quenches due to the photoinduced electron transfer(PET)effect between COF-NH2 and Hg2+,which realized the highly sensitive detection of Hg2+with the detection limit as low as 15.3 nM.Especially,COF-NH2 also showed excellent reusability,which still maintains excellent fluorescence property and good response toward Hg2+after five regeneration experiments under the action of sodium sulfide.In the second part,a fluorescein functionalized COF(DhBd-PT-COF-FITC)was used as the fluorescence sensor for highly sensitive and selective detection of mycotoxin—3-nitropropionci acid(3-NPA)in sugarcane.First,a new COF:DhBd-PT-COF containing lots of hydroxyl groups was obtained by using 3,3’-dihydroxybenzidine(DhBd)and 1,3,5-tris(4-formylphenyl)benzene(PT)as building units.Subsequently,a 6-isocyanatofluorescein(FITC)functionalized COF:DhBd-PT-COF-FITC was successfully obtained by the labeling reaction between the hydroxyl group and the isothiocyanate group.Through the confinement of DhBd-PT-COF framework,the aggregation of FITC is effectively reduced,and its fluorescence stability was improved.Benefiting from the inherent pH-response conformational change of FITC,DhBd-PT-COF-FITC exhibits highly sensitive detection of 3-NPA with the detection limit as low as 12.5 nM.DhBd-PT-COF-FITC shows superior selectivity toward 3-NPA than other potential interferences(mainly referring to sugar compounds)in sugarcance.In the third part,the rare-earth element terbium(Ⅲ)functionalized COF composite material(Dpy-NhBt-COF@Tb3+)was used as turn-on fluorescence sensor for highly sensitive and selective detection of ochratoxin A(OTA)in wheat flour.To anchor Tb3+ in COF,5,5’-diamino-2,2’-bipyridne(Dpy)-containing bipyridine structure with strong coordination ability toward metal ions was selected as the building unit to condensate with 2-hydroxyl-1,3,5-tribenzaldehyde(NhBt)for forming bipyridine-containing COF:Dpy-NhBt-COF.Then,Tb3+ functionalized COF(Dpy-NhBt-COF@Tb3+)was successfully obtained through the strong coordination interaction between Tb3+with bipyridine moieties in Dpy-NhBt-COF.During the detection process,Tb3+acts as the selective recognition sites and signal sensor for OTA.While OTA is presented,OTA can quickly replace water molecules to coordinate with Tb3+ and transfer energy to Tb3+ immediately,thereby generating significant fluorescence signal,which realized the highly sensitive detection of OTA with the detection limit as low as 13.5 nM.The stabilization of Tb3+ by Dpy-NhBt-COF not only reduce the non-selective binding of other natural small molecules to Tb3+,but also avoids the non-radiative quenching caused by solvent molecules coordinated with Tb3+.Compared with other mycotoxins(such as aflatoxin B1,zearalenone),Dpy-NhBt-COF@Tb3+showed extraordinary selectivity toward OTA.In the fourth part,the rare-earth element europium(Ⅲ)functionalized fluorescent COF(Dpy-PT-COF@Eu3+)was used as ratiometric fluorescence sensor for ultrasensitive and selective detection of tetracycline(TC)in milk.According to the design strategy of functionalized COFs proposed in the third section,Dpy-PT-COF with excellent fluorescence properties were obtained by using Dpy and PT as building units.Then,again relying the strong metal coordination ability of the bipyridine structure,Eu3+ was successfully loaded on Dpy-PT-COF to obtain the Eu3+functionalized fluorescent COF:Dpy-PT-COF@Eu3+.Here,Eu3+ plays as the specific recognition sites and response signal toward TC,while Dpy-PT-COF can not only stabilize Eu3+,but also act as the fluorescence reference.When TC is present,TC can quickly replace water molecules to coordinate with Eu3+ and transfer energy to it immediately,and thereby triggering significant fluorescence signal,while the fluorescence of Dpy-PT-COF is constant.Based on these,the ratiometric fluorescence detection of TC with high selectivity and ultrasensitivity(detection limit as low as 7.5 nM)was achieved.