Rare Earth Doped Upconversion Core@Shell Nanostructure Luminescent Materials And The Applications In Temperature Sensing And Water Detection

Rare earth(RE3+)doped upconversion(UC)nanocrystals have been extensively investigated recently because of their unique optical properties,such as sharp emission bands,long luminescence lifetimes(micro-to milli-seconds),good chemical stability,high penetration depth,low auto-fluorescence,high signalto-noise ratio as well as low toxicity.In recent years,the research of UC nanocrystals mainly focuses on the synthesis of UC nanocrystals with different sizes and emission wavelengths by various methods,and then studies their applications in bioimaging,bioassay,cancer treatment and biological safety,solar cells and other applications through surface modification or biological coupling.However,there are few studies on other applications of UC nanocrystals.Hence,we designed UC nanocrystal with different components and structures,and explore the influence of different structural designs on the applications of temperature sensing and water detection,which is meaningful for development of temperature probe and water probe.The research are as following four aspects:(1)a new type of Yb/Ho/Ce: NaGdF4@Yb/Tm: NaYF4 UC nanocrystal was designed and synthesized,achieving white-light-emission and temperature sensing simultaneously under the excitation of a 980 nm laser.Ho3+ and Tm3+ ions were the origins of red,green,blue(RGB)emission.The core@shell nanoarchitecture was beneficial for spatial separation of Ho3+ and Tm3+ and thus effectively prohibited their adverse energy transfers.The addition of Ce3+ ions in the core not only realized the regulation of emission color,but also improved sensitivity of temperature sensing from 0.7% K-1 to 2.4% K-1.In addition,UC nanocrystals also have excellent signal resolution(3040 cm-1),which is more suitable for optical temperature measurement materials.(2)Yb/Er: NaGdF4@NaYF4 core@shell nanocrystals with various shell thicknesses were prepared.High-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)was adopted to evidence the formation of core@shell structures.With increase of shell thickness,Er3+ upconversion luminescence was gradually enhanced and the optimal upconversion intensity of the core@shell sample was about 78 times as high as that of the core-only one.Additionly,temperature-dependent upconversion emission spectra revealed that the shell thickness had no obvious influence on the related sensitivity.However,the thermal effect caused by the input 980 nm laser was severe for the core-only sample,whereas it was negligible for the core@shell product,which is beneficial to obtain a stronger fluorescence intensity signal and improve the accuracy of temperature detection.(3)In Er3+ doped UC nanocrystals system,when the concentration of Er3+ increases,it is easy to trigger concentration quenching.the spatial distribution of Er3+,which is a key factor that affects the concentration quenching,is an important issue that must be considered.We selected Yb: Na Er F4 as a light-emitting layer and investigated its upconversion performance and temperature sensing behaviors in two kinds of core-shell nanoarchitectures.Yb3+ and Er3+ activators were distributed in a three-dimensional sphere and two-dimensional thin layer in Yb: Na Er F4@Yb/Nd: NaYF4@NaGdF4 and NaGdF4@Yb: Na Er F4@Yb/Nd: NaYF4@NaGdF4 core-shell nanocrystals,respectively.The difference in Er3+ spatial distribution in the core-shell structure resulted in significant modification of red-to-green ratios and decay behaviors upon excitation at 376 nm,808 nm,980 nm and 1532 nm,which caused by the two-dimensional space shorten the distance between adjacent ions in the light-emitting layer and easily induces cross relaxation(CR1: Er3+: 2H9/2 + Yb3+: 2F7/2 ? Er3+: 4F9/2 + Yb3+: 2F5/2 under 376 nm light excitation,CR2: Er3+: 4F7/2 + Er3+: 4I11/2 ? Er3+: 4F9/2 + Er3+: 4F9/2 under 808/980 nm laser excitation and CR3: Er3+: 4S3/2 + 4I13/2 ? Er3+: 4I9/2 + Er3+: 4I9/2 under 1532 nm laser excitation).In addition,the spatial distribution of Er3+ was demonstrated to have no obvious effect on the relative sensitivity for temperature determination under 808 nm laser excitation.(4)a strategy to achieve an 808 nm excitable water probe based on an Nd3+-sensitized Yb/Er: NaGdF4@Yb/Nd: NaYF4 core-shell UC nanoarchitecture is presented,and the limit of detection(LOD)was determined to be as low as 0.01%(100 ppm).The core@shell nanoarchitecture makes the blue,green and red upconversion emission of Er3+ significantly enhanced.The water detection mechanism of the UC nanocrystals was investigated systematically by measuring the effect of water on the UC emission and decay behaviors of three different structures of core,core@shell and core@shell@shell nanocrystals under different excitation wavelengths,which include excitation attenuation(EA),Yb3+ excited state quenching(Yb-ESQ)and Er3+ excited state quenching(Er-ESQ).