Design And Synthesis Of Ratiometric Fluorescence Probe Based On Lanthanide MOFs And Its Application In Visual Sensing

Rare earth metal elements mainly contain lanthanide,scandium and yttrium,the different number and arrangement of electrons in the 4f layer lead to different energy levels and a variety of transitions pathways,then resulting in many rare earth materials with different properties.Metal organic frameworks（MOFs）containing lanthanide ions are called lanthanide organic frameworks（Ln-MOFs）.They have great attraction in building fluorescent sensors because of the long-life excited sates,large stoke shift,narrow emission band and other excellent luminescent properties.Comparing with single emission fluorescent sensor,ratiometric fluorescent sensor can improve the sensitivity and reliability of the analytical method by eliminating the influence of excitation light intensity,concentration of probe molecule,light scattering fluctuation and external environment.Furthermore,ratiometric fluorescent sensor with multiple emission centers can be constructed by effective strategies,which can effectively enrich the visual sensing range.Thus,two kinds of new ratiometric fluorescent sensors based on Ln-MOFs are designed and constructed to detect drug molecules and inorganic ions.It is noteworthy that both of the two methods can semi-quantitative detection toward the target substance by naked eyes.The main research contents are as follows:（1）Ln-MOFs with highly fluorescent was synthesized and used as a dual-mode sensor for the visual detection of berberine hydrochloride and tetracycline.Firstly,a microscale Eu-MOF was successfully prepared by using lanthanide Eu³⁺as metal node and terephthalic acid as ligand.3,5-diamino-1,2,4-triazole was modified into Eu-MOF for improving the luminescence behavior via one-pot hydrothermal method.Then it was fully characterized by Scanning electron micrograph（SEM）,Fourier transform infrared spectroscopy（FTIR）,Powder X-ray diffraction（PXRD）,Thermogravimetry（TGA）,Fluorescence and energy dispersive X-ray image（EDX mapping）.The fabricated Eu-MOF has high quantum yield of 30.99%,good thermal stability,excellent fluorescence stability and good water dispersion.Eu-MOF has different number of fluorescence emission centers under different excitation wavelengths.When the excitation wavelength is 280 nm,the strong red fluorescence from Eu³⁺can be quenched by berberine hydrochloride.When the excitation wavelength varied to 365 nm,Eu-MOF has dual-emission center from ligand and Eu³⁺based on the poor efficiency energy transfer between ligand and Eu³⁺.One of the blue emitting intensity from ligand was decreased while another red emitting intensity of Eu³⁺was enhanced owning to the improved energy transfer.Then a ratiometric fluorescence method for tetracycline detection was established based on the dual-emission Eu-MOF.Considering the distinctly different fluorescence responses of targets under different excitation wavelengths,Eu-MOF was developed as a dual-mode visualization sensor for the detection of berberine hydrochloride and tetracycline.The limits of detection（LOD）are78 n M and 17 n M,respectively.For the analysis of tetracycline,we also prepared a simple paper sensor for on-site visual detection of tetracycline by naked eyes with a fluorescence color change from blue to green.（2）Post synthesized of dual-emission center based on dye integrated Eu（BTEC）as a ratiometric fluorescence visual sensor for sensitive recognition of Cl O^-.In the previous work,we obtained a ratiometric fluorescence sensor by reducing energy transfer efficiency between ligands and Eu³⁺.However,the weak fluorescence of free ligand and poor efficiency energy transfer of Eu³⁺,resulting in a poor visualization color change and sensitivity.In order to solve these problems,in the second work,1,2,4,5-Benzenetetracarboxylic acid was used as ligand to synthesis Eu（BTEC）with strong red fluorescence via a simple room temperature agitation method.Then acriflavine（Acr）,a fluorescent dye with high quantum yield and amino group,was selected to covalently bound into carboxyl rich Eu（BTEC）by post modification.Besides,the emission center of Acr at 511 nm would not be disturbed by the long wavelength emission of Eu³⁺.Finally,dual-emission fluorescent composite（Acr@Eu（BTEC））was successfully prepared,which contain both the fluorescent emission of Acr and Eu³⁺.By adjusting the ratio of Acr and Eu（BTEC）,the fluorescence intensity change of green emission center and red emission center lead to a controllable fluorescence color change process of Acr@Eu（BTEC）suspension from red to green.In order to have an ideal visual color change,we can choose appropriate proportion of Acr@Eu（BTEC）according to the different fluorescence response to the target.In the presence of Cl O^-,the strong green fluorescence of Acr was obviously quenched.However,the red fluorescence emission of Eu（BTEC）,act as an internal reference signal,almost remained unchanged.Thus,a ratiometric fluorescence method with high sensitivity and selectivity for the detection of Cl O^-was established.The limit of detection was as low as 10.75 n M.In addition,the sensing process is accompanied by the change of fluorescent color from green to orange and then red,which can be easily distinguished by naked eyes.Anyway,this analysis method shows good performance in practical application,so it has great potential in water quality monitoring.