Preparation Of Schiff Base Covalent Organic Frameworks And Studies On Their Fluorescence Detection Properties

Covalent organic framework（COFs）materials,as a new class of crystalline porous polymers,are highly ordered periodic frameworks assembled by small molecule building units through dynamic covalent bonds.Because of the designability of the frame structure,high specific surface area,adjustable topology and high stability,COFs materials have been widely studied in the storage and separation of gases,heterogeneous catalysis,photoelectric sensing and energy storage.In particular,two-dimensional COFs show great potential for fluorescence sensing due to their expandedπ-πconjugates,rigid structures and regular pore structures.This work takes several Schiff base COFs materials as the main research body,and focuses on the influencing factors in the synthesis process and their fluorescence detection performance.A brief overview of COFs materials is given.From the aspects of COFs material bonding type,preparation method,and application field,a review and analysis was conducted.This paper focuses on the application of COFs in fluorescence sensing,and summarizes the research background and current situation of COFs fluorescence sensors.Specific research contents include:1.Synthesis and functionalization of COFs in a single solvent system and their application in fluorescence sensing of antibiotics.A functionalized COFs fluorescent material（Tb@TFP-EB）of lanthanide ions was prepared for the specific detection of the antibiotic levofloxacin（LVX）.The functionalization of Tb³⁺increases the quantum yield of Tb@TFP-EB and further improves the fluorescence sensitivity and accuracy of fluorescence sensors.TFP-EB COFs play a framing role,providing adhesion site and reaction site for fluorescence reactions,while improving the stability of Tb@TFP-EB.The experimental results show that when the LVX concentration in the range of 5×10^-6-2.4×10^-4M,the fluorescence intensity of Tb@TFP-EB increases linearly with the concentration,and the detection limit reaches 1.26μM（0.47 ppm）.In addition,fluorescence differences under ultraviolet light improve the observability of the signal,making it possible to monitor and analyze the level of human metabolism of LVX using molecular logic gates,thus avoiding complex operations and the use of expensive instruments.2.The effect of composite solvent system on the structure regulation of COFs and its application in the fluorescence sensing and electrochemical sensing of metal ions.A triphenylbenzene-based COFs（PMDA-TAPB）fibers was obtained with absolute eclipsed（AA）stacking models through adjusting the proportion of mixed solvent.For their plentiful active sites and high surface area,the fluorescence and electrochemical sensing properties of PMDA-TAPB COF were developed and utilized in this research.The absorption competition quenching（ACQ）mechanism caused by Fe³⁺has been demonstrated based on experimental and theoretical calculations,and the Boolean logic gates illustrated the logical relationship between the fluorescence nature of PMDA-TAPB and system factors.In the range of 0.045-0.25 mM concentration,there is a linear relationship between the fluorescence quenching effect and the Fe³⁺concentration,the detection limit is 3.61×10^-5 M,and the fluorescence quenching efficiency achieve 96%when the Fe³⁺concentration reaches 1 mM.The PMDA-TAPB modified carbon paste electrode（PMDA-TAPB/CPE）revealed excellent electron migration and Pb²⁺chelating ability,and PMDA-TAPB/CPE can achieve electrochemical detection of Pb²⁺by differential pulse anodic dissolution voltammetry（DPASV）.3.The influence of interface design and new processing techniques on the controlled synthesis and processing applications of COFs matrix composites,as well as the application of dye adsorption and fluorescence sensing of volatile organic solvent gases.On the one hand,TFP-PPDA COFs with uniform nanofiber morphology is obtained by surfactant（CTAB）co-solvothermal method,enabling controllable synthesis of COFs.With the addition of the surfactant CTAB,TFP-PPDA COFs transform from a bulk porous structure to a fluffy structure composed of winding fibers.It is found that TFP-PPDA can be used as an adsorbent for Rh B,with a maximum adsorption capacity of 714.3 mg·g^-1 and excellent recycling capacity.On the other hand,a COFs-based composite membrane material based on the synergy effects is designed,using polyacrylamide（PAM）as the matrix and TFP-PPDA COF as reinforcement.The PAM/TFP-PPDA composite nanofibrous film（NFM）is prepared by innovative combination of electrospinning technology,which expanded the preparation method of COFs composites and developed the potential of large-scale application of COFs materials.PAM/TFP-PPDA can effectively achieve the specific detection of primary aliphatic amine,in the range of 3-40μM concentrations,there is a linear“Turn-on”relationship between PAM/TFP-PPDA fluorescence intensity and amine concentration,with the highest fluorescence enhancement reaching 80%and the detection limit being 0.025μM.At the same time,the donor-acceptor electron transfer mechanism between TFP-PPDA and amine molecules was further investigated using molecular simulation methods and density functional theory（DFT）.