Fluorescence Sensing Analysis Of Metals Ions And Pesticides Based On N,S-doped Covalent Organic Frameworks

Covalent organic frameworks（COFs）are formed from organic units connected by strong Covalent bonds.COFs is characterized by diverse structures,high porosity,high thermal and mechanical stability,controllable porosity,and good flexibility and high conjugated crystal structure.Two-dimensional COFs nanosheets are similar to graphene,but they have periodic holes and many designable functional groups.The sensor signals can be effectively transmitted and amplified due to the existence of multiple identical binding sites within an extended framework.Two-dimensional porous crystal structure can provide a large specific surface area,so that the analyte and recognition site fully interact,improve the sensitivity.The periodic pore facilitates mass transfer,produces pre-screening and pre-separation effects,and eliminates interference from potential competitors,so COFs can be used as a preconcentrator as a fluorescent probe.In this paper,fluorescent covalent organic frame materials were prepared for sensing analysis of metal ions and harmful organic solvents.The main tasks are as follows:（1）Benzotrithiophene tricarbaldehyde（BTT）and terephthalic dihydrazide（Th）as carriers,NCDs@COF_BTT-Th nanosheets were prepared by solvent thermal synthesis,and the NCDs was encapsulated in their pores.NCDs@COF_BTT-Th has a fluorescence emission peak at 547 nm under the excitation wave of 300 nm.After adding Fe³⁺ions,the fluorescence gradually decreased.In addition,there was a good linear relationship between 10.21 nM–24μM,and the detection limit was 3.40 nM.The synergistic effect of hetero atoms and Fe³⁺ions and the internal filtration effect were used to detect Fe³⁺.The material has good chemical stability and excellent selectivity,which provides more possibilities for COFs to detect metal ions.It also provides a new method to improve the fluorescence performance of COFs.（2）Two-dimensional COF_TAPT-TT synthesized by condensation of4,4’,4’’-（1,3,5-triazine-2,4,6-triyl）trianiline（TAPT）with Thieno[3,2-b]thiophene-2,5-dicarbaldehyde（TT）.The addition of N-doped carbon dots（NCDs）in the synthesis process not only enhanced the fluorescence of COF_TAPT-TT,but also showed the fluorescence of NCDs.Under the excitation of 280nm,the prepared NCDs@COF_TAPT-TT nanocomposite exhibited double emission peaks at 370 nm and 445 nm.After the addition of Cu²⁺,due to the strong coordination between NCDs@COF_TAPT-TT and Cu²⁺,the emission peak decreased at 445 nm and the fluorescence increased at 370 nm.This"on-off"fluorescent probe is used to detect Cu²⁺concentrations in water samples as low as 4.7 nM and a linear range from 14.10nM to 0.8μM and 18μM to 26μM.NCDs@COF_TAPT-TT ratio fluorescence sensor for Cu²⁺detection has high sensitivity,good selectivity and stability.（3）A fluorescent COF material(COF_ML-DHTA)was prepared by the condensation reaction of melamine（ML）and 2,5-Dihydroxyterephthalaldehyde（DHTA）,and a fluorescence sensor was constructed for the detection of methyl p-oxophosphorus pesticide.The material has a fluorescence emission peak at 368 nm under the excitation of 300 nm.Due to the strong ultraviolet absorption of methyl paraoxon at300 nm,there will be a competitive relationship with the excitation wavelength,resulting in fluorescence quenching.The fluorescence sensor based on this material showed a good linear relationship between the concentration of methyl paraoxon and I₃₆₈ nm within the concentration range of 2.07 ng mL^-1to 40μg mL^-1.The minimum detection limit can be up to 0.69 ng mL^-1.