Preparation of Water-stable Metal-Organic Frameworks for Vitamin Sensing

Author:Yue Dan

Supervisor:qian guo dong


Degree Year:2018





Vitamins are indispensable organic substances in human metabolism process.They play a key role in the energy conversion process of biological tissues.They can strengthen the body’s energy metabolism,improve the body’s coordination ability,prevent diseases,and maintain good health.Although the required amount of some water-soluble vitamins is extremely small,but due to the body can’t synthesis itself,and therefore must be obtained from food.Detection of vitamin in food is an important indicator for analyzing food safety and nutrient composition.Efficient detection of vitamins has important practical significance.Developing new types of vitamin probes faces tremendous opportunities and challenges for development.As a new multi-functional hybrid material,metal-organic frameworks(MOFs)have the advantages of easy synthesis,easy functional modification of the pore surface,excellent physical and chemical properties,etc.They are widely applied in gas storage and separation,catalysis,drug loading and sustained release,and chemical sensing.Luminescent MOFs,which combine the porous and luminescent properties,have rich host-guest response characteristics,and have received extensive attention in the field of fluorescence detection vitamin.However,the luminescent MOFs sensors for vitamin are relatively less,and the water stability of MOFs is also an important influencing factor in the detection process.By selecting suitable organic ligands,the water-stabile MOFs with excellent luminescent properties was designed and synthesized.The porous properties of MOFs can be used to aggregate the vitamin molecules,Base on the chemical properties of vitamins and the host-guest interaction,the detection of vitamin molecules can be achieved through fluorescence changes of MOFs.In this paper,by using organic ligand 4,4’-(1H,1’H-[2,2’-biimidazole]-1,1’-diylbis(methylene))dibenzoic acid(H2L),we designed and synthesized a Zn(Ⅱ)-based MOF ZJU-140,Zn3L3(DMF).ZJU-140 has good stability in vitamin B6 solution.The hydrogen bonding effect between the nitrogen atom in ZJU-140 and vitamin B6 was influencing the electronic transition of ligand.So that,with the increase of vitamin B6 concentration,the fluorescence intensity of ZJU-140 gradually decreased.ZJU-140 showed high sensitivity and selectivity for the detection of vitamin B6.This work can broaden the applications of luminescent MOFs in sensing vitamin B6.Based on 4,4’-(1H-pyrazole-l,3-diyl)dibenzoic acid(H2PDBA),two terbium MOFs,ZJU-138 and ZJU-139 were designed and synthesized.The struture of ZJU-138 and ZJU-139 is different,the Tb atom of ZJU-138 and ZJU-139 are nine-coordinated and enght-coordinated.When the H2O concentration was changed,ZJU-138 can transform into ZJU-139,on this basis,after the addition of sodium formate,the structural transformation process will accelerate.The emission spectrum of ZJU-138 suspension is composed of emission bands of H2PDBA,however,the the emission spectrum of ZJU-139 suspension are composed of emission bands of H2PDBA and Tb3+ions’characteristic sharp emissions.ZJU-138 can detect ascorbic acid(AA)and acetone in water with fluorescence quenching,and ZJU-139 exhibits highly selective ratiometric fluorescence for H2O determination in dioxane.The novel MOFs,ZnPDBA(H2O)(ZJU-13 7)was designed and synthesized with the organic linker H2PDBA.The layers of ZJU-137 were stacked through π…πinteractions.ZJU-137 exhibits good water and pH stability.The large overlap between absorption spectra of H2PDBA and AA,indicating that energy transfer between organic ligand and AA.ZJU-137 demonstrates high selectivity and sensitivity for the detection of AA via a fluorescence quenching.The detection limit of ZJU-137 for the sensing of AA was estimated to be 34 nM,hence,the ZJU-137 has promising application potential for determination of AA in practical applications.In order to achieve turn-on flourescent sensing of AA,we designed and synthesized the mixed valence state Ce-MOF(ZJU-136-Ce)with 1,1’:4’,1"-terphenyl-2’,4,4",5’-tetracarboxylic acid(H4TPTC).the ZJU-136-Ce can offer a larger channel for the reactions between AA and Ce4+ ions.A simple redox reaction was presented throughout the system of ZJU-136-Ce detection of AA.After the redox reaction(Ce4+→Ce3+),AA was oxidized to generate dehydroascorbic acid(DHA).The DHA will block the photoinduced electron transfer(PET)process from TPTC ligand to Ce ion,so that the fluorescence of TPTC ligand is enhanced obviously.This work successfully demonstrates that ZJU-136-Ce with desirable photophysical properties for detection of AA and promote further research on Ln-MOFs for chemical sensing.The mixed Ce/Eu metal-organic framework[ZJU-136-Ce1-xEux(x=0.24,0.36)]were designed and synthesized.Due to the specific redox reaction between the enediol group of AA and Ce4+,the fluorescence intensity of ligand increases significantly with a simultaneous fluorescence quenching of Eu ion,resulting in a ratiometric fluorescent probe toward AA.It is worth noting that the ZJU-136-Cei-xEux(x=0.24,0.36)showed good luminescence property and ratiometric fluorescence for AA determination.Furthermore,the regeneration cycle test shows that the mixed Ce/Eu metal-organic framework can be used as potential ratiometric sensor for AA determination.