The Research Of Discoloration-Fluorescence Switch And Its Application In Sensing Based On Polyoxometalate

The color and fluorescent signal of the system can reversibly undergo"bleaching-coloration"and"fluorescense-quenching"changes under external stimuli?photo,electricity,chemical reagents,etc.?,which calls a discoloration-fluorescence switch.Based on the easy-to-detection of signal,the visible change of the signal,the easy-to-implement and controllability conditions,fast response and recyclability,it has been used in traditional and emerging fields,such as military camouflage,anti-counterfeiting,biological/chemical sensing,molecular logic gates,information storage,optoelectronic display devices and so on,which has aroused the research enthusiasm of scholars,thus promoting the development of discoloration and fluorescent materials.The realization of the discoloration switch is generally based on the nature of the chromic material itself,and the fluorescent switch can be realized by the mechanisms of forming excimer,intramolecular charge transfer,photoinduced electron transfer and fluorescence resonance energy transfer respectively;According to the external stimulus that causes"on"and"off"changes,it can be divided into different types such as photo,electric,thermal,and chemical response.Polyoxometalates?POMs?are a class of inorganic metal oxygen clusters that combine with reversible discoloration and fluorescent properties,whose crystal size is nanometer scale,and there are more than seventy metallic elements that can be used to form POMs.A variety of types,the richness of the structure can be imagined,thus determining the diverse properties of POMs,such as good thermal stability,electron and proton transport and storage capacity,magnetic properties,easy to be modified,reversible redox and discoloration properties as well as luminescent properties make it outstanding in the fields of catalysis,medicine,biology,materials,energy and so on.In our research,basing on the stable and reversible redox,discoloration and luminescent properties of POMs and basing on the films and solutons containing POMs regulated by electrochemical or chemical reagent we have developed a comprehensive range of discoloration-fluorescence switches by the mechanism of intermolecular and intramolecular fluorescence resonance energy.Among them,the discoloration-fluorescent switch based on film is not only stable and reversible,but also the optical switch of film based on POM in the blue-emitting band is realized for the first time.The research in the solution is realized by the function of the discoloration-fluorescence switch in application for chemical sensing,the details of each part of the research work are as follows:Firstly,we constructed a multifunctional blue-fluorescent composite film based on the photoluminescent protonated graphitic carbon nitride nanosheets?g-C₃N₄NS?and the reversible electrochromic silicomolybdotungstate K₈(SiMoW₁₀O₃₉)·13H₂O?SiMoW?by the layer-by-layer?LBL?assembly method for the first time.SiMoW and g-C₃N₄NS were synthesized,and their structure,morphology and property were verified by FTIR?Fourier Transform Infrared Spectroscopy?,SEM?Scanning Electron Microscope?,CV?Cyclic Voltammetry?and fluorescence spectroscopy.Then,the composite films were assembled,and the assembly process,morphology,composition and property were characterized by the means of UV-vis absorption spectroscopy,CV,amperometric and fluorescence spectroscopy,XPS?X-ray Photoelectron Spectroscopy?and AFM?Atomic Force Microscopy?.Finaly,the reversibleelectrofluorochromicpropertyofthecompositefilm[?PEI/SiMoW?₂₅/?g-C₃N₄NS/SiMoW?₃/?PEI/SiMoW?₂]was investigated by in situ spectroelectrochemical system based on the fluorescence resonance energy transfer mechanism.The results show good blue-fluorescent switching performance,stability and reversibility.This is the first time that blue luminescence switch is realized by POM.Secondly,we developed a novel way to construct the multifunctional composite films with luminescence and redox properties in this study.The luminescent molecule HOPTS?8-hydroxypyene-1,3,6-trisulfonic acid trisodium salt?was incorporated into the silica spheres to form the green emission silica materials,HOPTS@SiO₂,which were characterized by SEM?scanning electron microscope?,FTIR?fourier transform infrared spectroscopy?and luminescence spectroscopy to verify its structure,morphology and property.And then the HOPTS@SiO₂ was functionalized by APTES?3-aminopropyltriethoxysilane?to form thepositively charged surface,HOPTS@SiO₂-APTES⁺.Afterwards,the composite luminescent films containing HOPTS@SiO₂-APTES⁺and SiMoW(SiMoW₁₀O₃₉^8-)were constructed by the layer-by-layer?LBL?assembly method immediately.The films were characterized by fluorescence spectroscopy,UV-vis spectroscopy,CV?cyclic voltammetry?,XPS?X-ray photoelectron spectroscopy?and AFM?atomic force microscopy?to confirm its composition,morphology and property.Finaly,the reversible electroluminescent switch performance of the composite luminescent films was investigated by in situ spectroelectrochemical system.The results indicated that thus-prepared composite films,[?PEI/SiMoW?₅/?HOPTS@SiO₂-APTES⁺/SiMoW?₂₅/?PEI/SiMoW?₂₀],displayed good luminescence switching performance and wide voltage range as well as good stability and reversibility.Thirdly,Eu-containing polyoxometalate K₁₃Eu(SiMoW₁₀O₃₉)₂·28H₂O?Eu-SiMoW?owns the stimu-chromic and photoluminescence properties.An ingenious test of ascorbic acid?AA?and sodium nitrite?NaNO₂?was carried out based on the dual properties of Eu-SiMoW in solutions.The redox reaction of Eu-SiMoW and AA generated the blue reduced Eu-SiMoW,accompanied by fluorescence quenching;then the redox reaction of the reduced Eu-SiMoW and NaNO₂ made Eu-SiMoW back to its original pale yellow state with red luminescence.Accordingly,the content of AA(the range from 0.1 to 0.9 mmol·L^-1)and NaNO₂(the range from 0.05 to 0.4 mmol·L^-1)could be measured by the reversible change of color(the detection limit?LOD?was0.53?mol·L^-1 for AA and 1.16?mol·L^-1 for NaNO₂)and luminescence(the LOD was4.67?mol·L^-1 for AA and 5.39?mol·L^-1 for NaNO₂)of Eu-SiMoW.This simple and sensitive bi-directional detection for AA and NaNO₂ for the first time.Moreover,the fluorescence switching of Eu-SiMoW could be realized by reacting with reductant and oxidant.The detection mechanism is considered as the fluorescence resonance energy transfer process between discolor component SiMoW and luminescence component Eu in Eu-SiMoW.Fourth,inspired by the third part of the work,we consider that hydrazine is widely used in aerospace,industrial production,pharmaceutical and other industries,and has serious harm to organisms,also has strong reducing ability.But the technology used to detect hydrazin in situ is still immature and needs to be solved.So we selected Eu-PMo₁₂O₄₀ which can spontaneously reacts with hydrazine hydrate as the substrate,which shows complementary nature of luminescence and discoloration on itself.We first explored the reaction rate of Eu-PMo₁₂O₄₀0 with hydrazine hydrate by UV-vis kinetic curve.The results show that the response time can be flexibly determined according to the concentration of hydrazine hydrate;once the oxidation-reduction reaction between hydrazine hydrate and Eu-PMo₁₂O₄₀(Mo^VI)occures,a reduced state Eu-PMo₁₂O₄₀?Mo^V?was obtained whose absorption peak appears in the visible light range.Based on the fluorescence resonance energy transfer between the discoloration component[PMo₁₂O₄₀]^3-and the luminescent component Eu³⁺,the signal of fluorescence of the system is weakened and the absorbance of the system in the visible region is continuously enhanced with the increase of the concentration of hydrazine hydrate,thereby realizing the response for hydrazine hydrate by the UV-visible probe and the fluorescent probe.The linear range are all0.02-70 mmol·L^-1,and the detection limit are all 0.04?mol·L^-1.The above sensing for hydrazine hydrate is also based on the behavior of the system's discoloration-fluorescence switch“off”.The addition of oxidizing agents can also achieve the"open",but hydrazine hydrate is a highly toxic substance,and it is of little significance to regulate the switch.In addition,bsaed on above research,we realize the in-situ visual inspection of the hydrazine by the self-support sol-gel film containing Eu-PMo₁₂O₄₀.It is exposed to a vapor atmosphere of hydrazine for 3-5 minutes,and the film color will obviously change from bright yellow to blue.Therefore,we hope that the experimentally prepared film can serve as an indicator in the event of a hydrazine leak.A series of discoloration-fluorescence switches,UV-visible probes and fluorescent probes based on polyoxometalate materials have been studied by the mechanism of fluorescence resonance energy transfer,which fills the blank of the blue-emitting fluorescent switch based on thin film containing POMs;develops a preparation method for preparing material that negatively charged dye-doped silica and finally carrying a positively charged,and realizes the application of inorganic POMs-based discoloration-fluorescence switching behavior in the field of sensors.It provides reference for the application research of POMs in the fields of discoloration-fluorescence switching devices and sensors.