Color Change And Fluorescence Switching Performance Regulation And Sensing Application Of The Polyoxometalates

Polyoxometalates（POM）are a kind of inorganic metal-oxygen clusters with rich composition,defined structure and tunable nanostructures,giving them excellent properties such as good proton transport and storage,reversible redox and so on.More importantly,the polyoxometalats show reversible redox properties accompanied by reversible under the action of UV/Visible light irradiation or chemical redox,but the structure have not change.Since UV-Vis absorption spectrum of the reduced polyoxometalats covers the whole visible region,the reduced polyacid can be employed as the energy receptor of the stimulus responsive fluorescence switching system.In this reserarch,the reversible discoloration and fluorescence switching properties of polyoxometalates solution and film were realized under the external stimulation of oxidant/reductant,UV/visible light and acid/base gas based on the energy and electron transfer between polyacid and luminescent rare earth ions.The bi-functional and reversible detection for chemical small molecules by UV-vis and fluorescence spectra was expanded.And details were as follows:Firstly,the reversible UV-Vis detection for hydrazine hydrate（N₂H₄·H₂O）and hydrogen peroxide（H₂O₂）in P₂Mo₁₈solution was realized based on the reversible redox and allochroic properties of Dawson type phosphomolybdate（NH₄）₆P₂Mo₁₈O₆₂(P₂Mo₁₈).When a certain amount of N₂H₄·H₂O was added to P₂Mo₁₈solution,the P₂Mo₁₈was reduced and the solution changed from light yellow to blue-green.And the absorbance in the region of 400～800 nm gradually increase as the increase of N₂H₄·H₂O concentration.On the contrary,when a certain amount of H₂O₂was added to the reduced P₂Mo₁₈solution,the reduced P₂Mo₁₈was oxidized again and the solution changed from blue-green to light yellow.And the absorbance in the visible region gradually decreased.The linear equation,linear range and detection limit of N₂H₄·H₂O and H₂O₂were obtained by drawing concentrations of N₂H₄·H₂O and H₂O₂with the absorbance at 750 nm.The response time,reversibility and anti-interference were studied by UV-Vis kinetics and spectroscopy.The detection limits for N₂H₄·H₂O and H₂O₂are 7.24×10^-5mmol/L、2.15×10^-4mmol/L.And the response time for N₂H₄·H₂O and H₂O₂detection are 44.8 min and 23.9 min,respectively.Secondly,the self-supporting flexible orange light film Sm W₁₀-agarose was constructed by solvent casting based on the functional complementarity between the excellent luminescence properties of rare-earth containing polyoxometalates Na₉Sm W₁₀O₃₆(Sm W₁₀)and the good film-forming matrix agarose.And the composition,structure and surface morphology of the films were characterized by FTIR,Raman and SEM,respectively.The transmittance and luminescent properties of the films were studied by UV-Vis spectrum and fluorescence spectrum.The thin film Sm W₁₀-agarose presented reversible acid-base response fluorescent switching properties under the stimulation of HCl and NH₃gases.The acid-base stability,response time and repeatability of the films were studied by FT-IR spectroscopy and fluorescence kinetics curves.The fluorescence spectrum detection for HCl gas was extended based on the orange luminescent thin film sensor,with a linear range of 0.39-1.6 mmol/L and a detection limit of 0.393 mmol·L^-1.Finally,a highly transparent flexible self-supporting luminescent film Eu W₁₀-Agarose was prepared by sol-gel method combined with casting technology based on functional complementarity principle,employing the red light rare-earth containing polyoxometalates Na₉Eu W₁₀O₃₆as the light response luminescent component and agarose as the film-forming matrix.The composition and structure of the films were characterized by FTIR,Raman and SEM.The effects of Na₉Eu W₁₀O₃₆content on the transmittance and fluorescence intensity of the film were investigated by UV-Vis and fluorescence spectra.After ultraviolet light irradiation for 20 min,the film changed from colorless to blue and the fluorescence intensity decreased at the same time.On the contrary,after the film was placed in room light and air for 4 days,the film changed from blue to colorless and the fluorescence recovered at the same time.The reversible fluorescence switching properties of the film were realized based on the intramolecular fluorescence resonance energy transfer between energy receptor W⁵⁺and energy donor Eu³⁺,and the energy transfer efficiency of fluorescent switching process is 0.08 s.The film color change and fluorescent switching performance were not significantly weakened after 4 cycles.