Fluorescence Turn-on Response Based On Confined Indicator Displacement In Metal-organic Frameworks

Indicator displacement assays?IDAs?are an analytical strategy that uses the signal change of an indicator caused by the indicator-receptor and analyte-receptor competitive interactions.For fluorescent IDAs,the indicator--receptor interactions suppress the fluorescence of the indicator,and the fluorescence is awakened by analyte-induced indicator displacement.IDAs,as a supramolecular sensing mode have been widely used in analytical chemistry.However,the efficiency of traditional fluorescent IDAs in homogeneous solutions is often adversely affected by low indicator-receptor or analyte-receptor affinity.Besides,the homogeneous ensembles are difficult to regenerate after use and incapable of reversible off-on switching.Metal-organic frameworks?MOFs?are crystalline porous network-like hybrid materials composed of metal ions/clusters and organic ligands.Based on their advantages such as adjustable structure,high porosity and easy modification,MOFs have become a popular research area.MOFs can be endowed with various luminescent properties from the metal ions,organic ligands,loaded guest molecules and the interactions between these components,and they can produce specific fluorescent responses to ions or small molecules.This thesis is devoted to the implementation of the IDA strategy in MOFs.Two novel were constructed,with electron-deficient cationic Zr-MOFs as receptors and an anionic dye as fluorescent indicator.Their fluorescence responses to electron-rich aliphatic amines were studied.The main contents are summarized below.1 Construction of the MOF-based IDA ensembles of Zr-DQ-ANS and Zr-MQ-ANS:Zr-bpy was obtained by solvothermal synthesis using 2,2'-bipyridine-5,5'dicarboxylic acid?H₂bpydc?and ZrCl₄,and was the precursor of post-synthesized.The Zr-bpy and N-alkylating reagents(methyl trifluoromethanesulfonate and ethylene di?trifluoromethanesulfonate?synthesize TfO-MOFs?Zr-DQ-TfO and Zr-MQ-TfO?with a modification ratio of about 40%.Then the TfO-MOFs were completely ion-exchanged with anionic dye NaANS at room temperature to obtain Zr-DQ-ANS and Zr-MQ-ANS.The structure,morphology and modification ratio were characterized by means of X-ray powder diffractometer?PXRD?,Fourier-infrared spectroscopy?FT-IR?,scanning electron microscope?SEM?,ultraviolet-visible absorption spectroscopy?UV-vis?,and proton nuclear magnetic spectroscopy?1H NMR?.2.Fluorescent turn-on response of Zr-DQ-ANS and Zr-MQ-ANS to amines and the indicator-displacement mechanismThe ANS-MOFs are rendered a fluorescence-resting state because the indicator's fluorescence is efficiently quenched by the ground-state charge-transfer?CT?complexation between the indicator and receptor.Addition of an alkylamine efficiently turns on the fluorescence because the indicator is displaced by the CT complexation between alkylamine with receptor.The turn-on response is highly specific to alkylamines.The detection limit for alkylamines,especially for butylamine?BA?is as low as subnanomolar?0.5 nM?,which is lower than the reported Zr?IV?MOF and improved by two orders of magnitude.The fluorescence in solid state can be reversibly switched on and off with high contrast.The sensitive and high-contrast response can be attributed to the space confinement effects of the porous frameworks.The confined space can significantly enhance indicator-receptor and analyte-receptor interactions,and thereby both the quenching efficiency in the off state and the displacement efficiency in the on state are amplified.