Design And Synthesis Of Nitrogen-containing Aromatic Fluorescent Probes And Their Selective Recognition Of Metal Cadmium Ions

Heavy metal environmental pollution has attracted more and more attention because of its great harm to public health.As a highly toxic heavy metal,cadmium can cause food and water pollution,causing a variety of physiological diseases,such as pneumonia,glomerulonephritis,emphysema and even cancer.Very low concentrations of cadmium in living cells can significantly affect cell growth and DNA synthesis.Therefore,it is necessary to develop an economical and effective method for qualitative and quantitative determination of cadmium ion.Fluorescent probes are often designed for the detection of heavy metals because of their high sensitivity and strong controllability.In this paper,two kinds of fluorescent probes are designed and synthesized based on the dehydration and condensation reaction of Schiff base.The binding characteristics of the probe with cadmium ion are studied in detail by UV-vis absorption spectrum,fluorescence titration,Job curve,mass spectrometry,single crystal X-ray diffraction analysis and density functional theory calculation.The specific studies are as follows:Pyridine-2-formaldehyde and 2-phenoxy aniline as raw materials.Pyridine Schiff base compounds（2-（pyridine-2-methylene）phenoxy aniline（PMPA））with specific recognition to cadmium ions are synthesized under methanol/formic acid solvothermal conditions.The weak fluorescence emission peak of PMPA in acetonitrile solution is located at 468 nm(Φ_F=0.006,λ_ex=380 nm).The obvious change of PMPA is observed only after the addition of cadmium ion,the fluorescence emission peak is red-shifted to 506 nm,the fluorescence intensity is magnified by 27 times,and the strong blue-green fluorescence is observed by naked eye under the irradiation of 365 nm ultraviolet lamp.PMPA is hardly affected by other interfering metal ions,and the fluorescence intensity is kept at a high level in the range of p H 5 to 9.The stoichiometric ratio of PMPA with cadmium ion is 1:1,the detection limit is 0.12μM,and the binding constant K is 4.36×10⁴ M^–1.The response time of PMPA to cadmium ion is short and the reversibility is good.The results of density functional theory further confirmed that the sensing mechanism of Cd²⁺detection by PMPA is based on inhibition–CH=N isomerization and CHEF process.Quinoline Schiff base compound 4-（quinoline-2-methylene）amino anisole（QMAA）is synthesized from quinoline-2-formaldehyde and 4-aminoanisole under ethanol/acetic acid solvothermal conditions.The weak emission peak of QMAA in DMF solution is located at 453nm(Φ_F=0.007,λ_ex=360 nm).Obvious changes are observed in QMAA after the addition of cadmium ions.The maximum emission peak is red-shifted to 479 nm and showed bright blue fluorescence under the irradiation of 365 nm ultraviolet lamp.After the interaction between QMAA and Cd²⁺ions,the color of the solution changed from colorless to light pink.The recognition of Cd²⁺ions by QMAA is not affected by the interference of metal ions,anions and amino acids,and the fluorescence performance is better in the range of p H 6 to 9,which can be detected in physiological environment.The stoichiometric ratio of QMAA with Cd²⁺ion is 1:1,the detection limit is about 0.1547μM,and the binding constant K is 7.47×10⁴ M^-1.QMAA has short response time and good reversibility.The results of density functional theory study show that the interaction between probe QMAA and Cd²⁺ions can effectively reduce the HOMO–LUMO energy gap of the complex,thus stabilizing the sensing system.