New Fluorescence Analysis Methods Based On Metal-organic Frameworks

Metal-organic frameworks(MOFs)are a class of crystalline porous solid materials,in which metal ions are linked by polyfunctional organic ligands to form the porous periodic network with great specific surface area and large pore volume.As novel functional materials,MOFs exhibit a wide variety of potential applications in catalysis,gas storage and separation,luminescence,and drug delivery,owing to their specific features,such as structural diversity,flexibility and tailorability,high porosity,large surface area and extraordinary adsorption affinities.As one of the most sensitive and fast detection method,fluorescence detection has been widely applied to medicine,food and environment detection.Traditional inorganic and.organic luminescent materials have been extensively explored.A number of factors such as the homogeneity in composition,particle size might affect the luminescent properties of inorganic luminescent materials.We can adjust the particle sizes of quantum dot and carbon dot nanoparticles to change their luminescent properties.As for organic luminescent materials,we can change their luminescent properties by using a relatively larger or smaller ?-conjugated system.To assemble both inorganic and organic components into MOFs,the inorganic and the organic moieties can provide the platforms to generate luminescence.Also the organic ligands of MOFs are usually ?-conjugated systems such as benzene and naphthalene,along with the advantage of structural diversity,this new type of organic-inorganic hybrid materials is certainly very promising as a multi-functional luminescent material chemical sensor.Except for the superiority of structural diversity,MOFs have some other particular advantages comparing with other kinds of sensors.Special features of MOFs include:1)high surface areas would concentrate analytes to high levels,which enhances detective sensitivity;2)specific functional sites(open metal sites,Lewis acidic/basic sites,and tunable pore sizes)that can realize specific recognition with unprecedented selectivity through host-guest interactions or size exclusion;and 3)flexible porosity or frameworks that enable the reversible uptake and release of substrates to increase regeneration and recycling.As chemical sensors,most examples for the recognition of analytes were reported as guest-dependent,these can be divided into two categories—the shift of emission spectrum and a change in the luminescent intensity.The interaction between MOFs and the guest leads to changes in the coordination sphere and exciplex formation etc.and finally ends up with enhancement or reducing of the luminescent intensity,or even wavelength shift of the emission.Based on the superb advantages of MOFs as chemical sensors,using two kind of MOFs—MOF-253 and MIL-88,we developed new strategies for the fluorescent sensing of trace water in organic solvents,copper ions in aqueous solution and hydrogen sulfide in aqueous solution.1.We design a turn-on fluorescent probe based on MOF-253 and ferrous iron double-factor system for the detection of trace water in acetonitrile,acetone and ethyl acetate.MOF-253 itself has bright yellow fluorescence.In the organic solvent,the MOF-253 hardly reacts with Fe2+,while in the water,the nitrogen atom on the ligand H2bpydc in MOF-253 can quickly react with the Fe2+,this will result in an effective intramolecular energy transfer from the H2opydc ligand to the Fe2+ through a photo induced electron-transfer(PET)process and the fluorescence of MOF-253 will be quenched.The higher the water content in the organic solvent,the higher the degree of reaction of the MOF-253 with Fe2+ in the organic solvent,and the more the fluorescence is quenched.Our MOF-253 fluorescent sensor can realize a simple detection of trace water in organic solvent,and at the same time reduce the pollution of the organic waste during the detection process.2.Based on the work in the second chapter,using MOF-253—organic solvent double-factor system,we established a fluorescence turn-off method for the detection of copper ions in aqueous solution.In pure water,the copper ions hardly react with N on the ligand H2bpydc of MOF-253,while in organic solvent N on the ligand H2bpydc of MOF-253 can react with copper ions,and the PET process occurs,the fluorescence of MOF-253 is quenched.By adding organic reagents as auxiliary reagents to the aqueous solution to be tested,the copper ions which could not react with MOF-253 before can successfully react with the MOF-253 to produce a PET process to quench the fluorescence of the material after the addition of organic solvents.The greater the concentration of copper ions in the sample,the greater the degree of fluorescence is quenched.Based on this method,we achieved a quantitative analysis of copper ions in water,and the detection limit is 1.09 ?M.Also,we use this method for the practical sample analysis to verify its reliability.3.We illustrated a novel turn-on mechanism based on collapse of MOFs structure for luminescence MOFs sensors.Taking H2S as an analyte,we validated our mechanism using a specially selected MOFs material Fe-MIL-88-NH2 for H2S sensing.Fe-MIL-88-NH2 is an ideal model to evaluate our mechanism with fluorescent NH2-H2BDC as the ligand and paramagnetic Fe3+ ions as the vertexes.Fe3+ vertexes of Fe-MIL-88-NH2 will be seized by H2S with the addition of H2S easily and rapidly.At the same time,the MOFs collapses and fluorescent ligands are released accompanying with the appearance of fluorescence.Our Fe-MIL-88-NH2 has a rapid response and good selectivity to hydrogen sulfide,and the detection limit towards H2S is about 10 ?M,which is lower than the reported MOFs sensors for H2S designing with other turn-on mechanisms.