Research Of Covalent Organic Frameworks In Fluorescence Sensing And Photodegradation Of Dyes

Covalent organic frameworks（COFs）,which are highly ordered porous conjugated polymers,have received extensive attention in the scientific community in the past 10 years.COFs combine the unique physical and chemical properties of various porous organic structures,such as high crystallinity and BET surface area,customizable pore size and shape,excellent stability in most solvents,high thermal and physical stability under harsh conditions.More importantly,COFs can also be synthesized with different building units and linkage to achieve various functions.Therefore,COFs are regarded as materials with great prospects for developing diversified scientific,technological and industrial applications in the future.In this paper,the applications of COFs are concentrated mainly on fluorescence sensing and photocatalysis.The specific research contents are as follows:1.Designing luminescent COFs with high fluorescence quantum yield is always a huge challenge.We synthesized COF Tz Dha through Schiff base reaction between 4,4′,4′’-（1,3,5-triazine-2,4,6-triyl）trianiline（Tz）and 2,5-dihydroxyterephthalaldehyde（Dha）,and post-modified COF Tz Dha-AC was grafted by refluxing in acetic anhydride.Tz Dha-AC shows higher relative fluorescence quantum yield（Φ_f）in various organic solvents and water,which is approximately 1.35 to 5.31 times to Tz Dha.What’s more surprising,Tz Dha-AC shows excellent ratiometric effect when it is used to detecte hydrazine（N₂H₄）.Detection mechanism is attributed to that hydrazine prevents the intramolecular charge transfer（ICT）from benzene ring to triazine ring of Tz Dha-AC,thus the emission blue shifted from 601 nm to 526 nm.The detection limit（DL）of N₂H₄ is calculated to be 0.038 m M in the linear range of 0-10 m M.In addition,we also use Tz Dha-AC to detecte hydrazine produced from isoniazid metabolism in mouse liver cells and find that Tz Dha-AC could be a new material employed for simple isoniazid metabolism in cell.2.Synthesizing“turn-on”COFs for detecting biological macromolecules in water or biological environment is more generally challenging.Therefore,a composite material IEP-Mn O₂ is available by combining manganese dioxide（Mn O₂）nanoparticles and fluorescent COF（IEP）which was synthesized by 2,4,6-Tris（4-aminophenyl）-s-triazine and 2,5-dimethoxyterephthalaldehyde.By the addition of biothiols,such as glutathione（GSH）,cysteine（Cys）or homocysteine（Hcy）,the fluorescence emission spectra of IEP-Mn O₂ changed（“turn-on”or“turn-off”）via different mechanisms.The fluorescence emission intensity of IEP-Mn O₂ in PBS buffer solution increased in the presence of GSH by the elimination of FRET effect between Mn O₂ and IEP.Differently,due to the formation of hydrogen bond between Cys（or Hcy）and IEP,the fluorescence of IEP-Mn O₂+Cys/Hcy quenched via photoelectron transfer（PET）process,which makes IEP-Mn O₂ behaved specificity on distinguishing detecting GSH and Cys/Hcy compared to other Mn O₂ complex materials.Therefore,IEP-Mn O₂ was used to detect GSH and Cys in human whole blood and serum,respectively.The limit of detection for GSH in whole blood and Cys in human serum were calculated to be 25.58μM and 4.43μM,which indicates that IEP-Mn O₂ can be used to investigate some diseases related to GSH and Cys concentration.3.Overdose of alkaloid drugs often causes significant damage to the human body.However,the researches about detecting alkaloids are rare and thus,it is urgent to develop a facile method to detect alkaloids.Here,four COFs Tb Azo,HTb Azo,DHTb Azo and Tp Azo with different ratios ofβ-ketoenamine/imine are synthesized When the ratio ofβ-ketoenamine/imine is 1/2,HTb Azo shows the highest fluorescence emission in water and different fluorescence responses to isoniazid and colchicine.HTb Azo was also applied in fluorescence sensing of isoniazid and colchicine in human serum,and the detection limits of isoniazid and colchicine were calculated to be 71.5μM and 4.64μM in the low concentration range,respectively.In addition,HTb Azo was also used to monitor p H in water with excellent linear relationship in the range of p H=4.00–12.00 and a very low detection limit of 0.02.Our research provides a new idea for improving the luminescence performance of COFs,and the different fluorescence responses of HTb Azo to alkaloids and p H make it a multifunctional sensor applied in real life.4.Finally,we synthesized three new chemically stable porous organic polymers（POPs）by hydrothermal method,including Tp SD with sulfone bond,Tp TD with thioether bond or Tp MD without sulfur atom for the degradation of persistent organic pollutants.We have carefully studied the catalytic mechanism of Tp SD with sulfone bond.The experimental results show that the existence of O=S=O makes Tp SD with higher conjugation degree,wider visible light absorption range,higher absorption intensity and stronger hydrophilicity than Tp MD and Tp TD.These factors lead to the highest catalytic efficiency of Tp SD among the three polymers.The photocatalytic degradation of Rhodamine B（Rh B）molecules is approximately 100%with Tp SD and its pseudo-first-order rate constant is 0.0770 min^-1,which is the highest among all reported non-metallic photocatalysts at that time.