Reversible Discoloration/Fluorescescence Switching Properties Regulation And Sensing Performance Of Rare Earth-Containing Polyoxometalates

Polyoxometalates（POMs）have diverse structures,rich composition,adjustable nanometer size,oxygen-rich polyanionic surfaces,give them proton storage,reversible redox and discoloration properties and so on.And rare-earth containing polyoxometalates（Ln-POMs）have both the structure and nature characteristics of polyoxometalates,as well as luminescence characteristics of rare earth ions.In addition,under external stimuli such as light,electricity,heat,and oxidants/reductants,Ln-POMs can exhibit reversible discoloration and fluorescence switching properties,making them a class of high-performance molecular assembly building blocks and stimulus-responsive discoloration/fluorescence switching molecules.They have potential applications in fields such as optical information storage,biological imaging,fluorescence anti-counterfeiting,biological and chemical sensing,etc.In this article,the reversible discoloration and fluorescence switching properties of rare earth polyacids have been achieved under the stimulation of ultraviolet/visible light,acid/alkali gas,oxidant/reducing agentis,based on the intramolecular or intermolecular fluorescence resonance energy transfer and electron transfer of Ln-POMs.And the fluorescence sensing function of Ln-POMs for HCl gas,vitamin C,hydrazine hydrate,and hydrogen peroxide has been expanded.The specific research content is as follows:Firstly,a flexible self-supporting orange light film Sm W₁₀/Agarose was prepared by casting technology employing rare earth polyacid Na₉Sm W₁₀O₃₆as a photochromic/fluorescent switching molecule and agarose as a film-forming substrate,based on the functional complementarity between the optical properties of Sm W₁₀and the film-forming properties of agarose.And the composition,surface morphology and functional component distribution of the thin film were characterized by XRD,FTIR,SEM,and Mapping.The effects of Sm W₁₀doping on transparency and luminescent properties of thin films were investigated through UV-Vis and fluorescence spectroscopy.Under ultraviolet light irradiation,the color of the film changes from colorless to blue accompanied with fluorescence quenching.On the contrary,under air and natural light irradiation,the blue color of the film fades accompanied with fluorescence recovery.After 5 cycles of testing,the discoloration and fluorescent switching properties of the film did not significantly decrease.And the film exhibited reversible photochromic/fluorescent switching properties.Secondly,the chemical response discoloration/fluorescence cancellation properties of Eu Si W₁₀Mo and the UV-Vis/fluorescence bi-functional detection for hydrazine hydrate were studied,based on the redox and discoloration properties of Eu Si W₁₀Mo,as well as the intramolecular fluorescence resonance energy transfer between rare earth Eu³⁺and reduced polyoxometalates Si W₁₀Mo.When an equal amount of hydrazine hydrate is added to Eu Si W₁₀Mo aqueous solution,Eu Si W₁₀Mo was reduced to heteropoly blue,and the absorbance in the visible region gradually increased,while the fluorescence intensity gradually decreased.The Eu Si W₁₀Mo solution exhibited chemical response discoloration and fluorescence cancellation properties.On this basis,the concentration of hydrazine hydrate was plotted using the logarithm of absorbance at 530 nm and fluorescence intensity at 620 nm to obtain the linear equation,linear correlation coefficient,and linear range of Eu Si W10Mo for UV visible and fluorescence dual spectral detection of hydrazine hydrate.The detection limits of the UV-Vis and fluorescence spectral detection for hydrazine hydrate were 6.69×10^-4mmol/L and 0.039 mmol/L,respectively.Thirdly,the reversible chemical response discoloration/fluorescence switching properties of Eu³⁺@P₂Mo₁₈solution and the reversible spectral detection for VC and H₂O₂have been realized,based on the functional complementary between the reversible redox and discoloration properties of Dawson type polyoxometalates P₂Mo₁₈and the luminescent properties of rare earth ions Eu³⁺.When equal amounts of reductant VC were added to the Eu³⁺@P₂Mo₁₈solution,the solution changes from light yellow to blue-green,the absorbance in the visible region gradually enhanced and the fluorescence intensity reduced.On the contrary,when an equal amount of oxidant H₂O₂was added to the reduced Eu³⁺@P₂Mo₁₈solution,the absorbance of the solution gradually decreased and the fluorescence intensity gradually increased.The Eu³⁺@P₂Mo₁₈solution exhibited reversible chemical response discoloration/fluorescence switching properties.Ploted the concentration of VC with the absorbance at 800 nm and the logarithm of fluorescence intensity at 620 nm,the linear equation and detection range for the detection of VC by UV-Vis and fluorescence spectroscopy were obtained,with detection limits of 2.43×10^-4mmol/L and 6.42×10^-3mmol/L,respectively.The selectivity and response time of the sensor were studied using spectroscopic and kinetic methods,respectively.The response time of the detection system for VC and H₂O₂detection were 45 s and 230 s,respectively.Finally,a Ln-POMs/polystyrene flexible self-supporting red emitting films was prepared by combining physical doping and interface spreading techniques employing Eu W₁₀@OTAB phase transfer complex as the luminescent component and polystyrene as film-forming substrate.The composition,optical properties of the films were characterized by XPS,FTIR,UV-Vis and fluorescence spectroscopy.The fluorescence of the film was quenched under the stimulation of HCl gas and the fluorescence of the film was restored under the stimulation of NH₃gas.After 10 cycles of stimulation,the fluorescence intensity of the film in the luminescent"on"and"off"states did not change significantly,And the film showed a reversible fluorescence switching property.The fluorescence kinetics showed that the film acid and base response time were about 180 s.The linear equation In（I）=-3.39c+8.5 was obtained by plotting the logarithm of the fluorescence intensity at 620 nm against the HCl concentration,with a detection limit of 0.089 mmol/L.