Functional Design And Sensing Performance Of Lanthnide Metal Frameworks Based Luminescent Probe

Metal-organic Frameworks（MOFs）is a kind of emerging organic-inorganic hybrid porous materials which are self-assembled by the coordination bonds between organic ligands and metal ions（cluster）.As a novel kind of organic-inorganic hybrid porous materials,the intrinsic diversity in structure endows MOFs with many amazing performances and great application potentials in many fields,such as gas storage and separation,selective adsorption,drug loading and delivery,heterogeneous catalysis,etc.Among them,lanthanide metal organic framework materials（Ln-MOFs）which are modified by the lanthanide ions,as phosphors and luminescent probes,have been widely used in the white light LEDs,fluorescent sensing,biological imaging,and many other frontier fields because of their excellent optical properties(extremely narrow emission spectrum of Ln³⁺,high quantum yield,etc.).Both the organic ligand and Ln³⁺in Ln-MOFs could act as the fluroescent centers.Moreover,photoluminescent（PL）intensity from Ln³⁺ions can be greatly enhanced by an efficient ligand-to-Ln energy transfer（ET）process.However,the external environmental changes such as the presence of some organic small molecules and ions as well as the variation of ambient pH value and temperature could cause the shift of emission spectrum or enhancement/quenching of PL intensity for Ln-MOFs.Based on these phenomena,quantitative detection of the source of stimulation could be realized.Therefore,Ln-MOFs based luminescent probes with the specific functions can be constructed by the rational design and post-synthesis modification（PSM）to detect precisely some kind of analytes.In this paper,four kinds of probe based on Ln-MOFs had been successfully prepared by the direct doping and and post-synthesis modification,respectively.Their structures,morphologies,and performances were investigated and characterized in detail.It was found that as-preared Ln-MOFs owned the excellent PL properties and could be used as fluorescent probes to precisely detect nitrobenzene,bilirubin,glutathione,and temperature,respectively,with low detection limit,fast response and excellent anti-interference ability.The researched details are listed as follows:1.Firstly,Zr-MOFs with the same phase structure as UiO-66 was synthesized successfully through replacing partial organic ligands.At the same time,the free and active-COOH groups were introduced in Zr-MOFs.Subsequently,Ln@Zr-MOFs was synthesized in further through PSM approach,and Ln³⁺ions were imported by the reaction between free-COOH and Ln³⁺.As-prepared Ln@Zr-MOFs could give the characteristic emissions of Ln³⁺because of the“antenna effect”of ligands.Specially,the tunable full-spectrum emission was easily achieved in Ln@Zr-MOFs by controlling the Ln³⁺contents and types.In the further study,it was found that Ln@Zr-MOFs showed the nitrobenzene-dependent PL performance.Taking Eu₁₀@Zr-MOFs as an example,it has shown many appealing properties,such as a very board response window（0-180μM）,ultrafast response（1 min）,high sensitivity（LOD=1.04μM）.Meanwhile,the sensing performance of Eu₁₀@Zr-MOFs is not easily affected by other organic pollutants in the water due to its excellent anti-interference ability.The detailed studies and theoretical calculation reveal that the fluorescent quenching mechanism of Eu₁₀@Zr-MOFs caused by nitrobenzene origins from the photo-induced electron transfer between Eu₁₀@Zr-MOFs and nitrobenzene.2.UiO-66（COOH）₂:Eu³⁺was successfully synthesized via PSM approach by using Eu³⁺to modify UiO-66（COOH）₂ with the active-COOH sites.Due to the“antenna effect”of benzenetetracarboxylic acid,the UiO-66（COOH）₂:Eu³⁺could simultaneously emit both the characteristic red light of Eu³⁺and blue light from the ligands under the excitation of UV light.It can be served as a turn-off fluorescent switch for bilirubin,the biomarker of hyperbilirubinemia and jaundice hepatitis,because its red emission from Eu³⁺can be easily quenched by BR through a fluorescent resonant energy transfer（FRET）process between bilirubin and its ligands.Particularly,UiO-66（COOH）₂:Eu³⁺has shown many appealing properties,such as broad response window（0–15μM）,high sensitivity,quick response（5mins）,high sensitivity（LOD=0.45μM）,and anti-interference ability.Most importantly,a kind of portable luminescent test paper based on UiO-66（COOH）₂:Eu³⁺has been developed for directly assessing the level of BR in real human serum and further diagnosing bilirubin-related diseases via visually observing the luminescent color variation by naked eye and color-scanning application.3.A kind of 2D layered structure metal organic frameworks,Eu（DTBA）,was synthesized successfully via hydrothermal approach by selecting Eu（NO₃）₃·6H₂O and 4,4’-dithiobisbenzoic acid as start materials.Due to the“antenna effect”of 4,4’-dithiobisbenzoic acid,the Eu（DTBA）could show the characteristic red emission of Eu³⁺and exhibits a strong glutathione-dependent PL performance.Particularly,during sensing glutathione,Eu（DTBA）has shown many appealing properties,such as broad response window（0–20 mM）,high sensitivity,quick response（2 mins）,high sensitivity（LOD=0.35μM）,and excellent anti-interference ability.Glutathione is usually considered as a marker of tumor cell because it is generally overexpressed in tumor cells.Therefore,Eu（DTBA）can be used as a fluorescent probe for tumor cells.Under a fluorescence microscope,tumor cells and normal cells can be easily distinguished by simply observing cell imaging because the fluorescence of Eu（DTBA）has been quenched in tumor cells by the interaction between Eu（DTBA）and glutathione.4.A new Eu and Tb co-doped luminescent compound based on MOF-5 was synthesized successfully by modulating the content of precursor(replacing Zn²⁺by Eu³⁺and Tb³⁺)under reflux and stirring environment.By maintaining the doping content at 2.5%,the product,MOF-5:Eu_2.5/Tb_2.5-a,could emit the characteristic emissions of Eu³⁺and Tb³⁺.More importantly,MOF-5:Eu_2.5/Tb_2.5-a exhibits the strong temperature-dependent PL performance.The emission intensity of Eu³⁺increases dramatically,while that of Tb³⁺decreases as the temperature rises.This interesting phenomenon is caused by the thermally-driven phonon-assisted transfer process between Eu³⁺ions and Tb³⁺.Further studies reveal that MOF-5:Eu_2.5/Tb_2.5-a is extremely sensitive to temperature,and its relative sensitivity to temperature is 1.8%/K at 473K.In addition,MOF-5:Eu_2.5/Tb_2.5-a could detect temperature as a multi-channel readout in emission intensity ratio,decay time ratio,and CIE chromaticity coordinate.By taking advantage of this work mode,MOF-5:Eu_2.5/Tb_2.5-a exhibit a very high self-correction ability which makes it own high accuracy for temperature detecting.