Design,Preparation Of Rare Earth Upconversion Nanophosphors With Multilayer Core-Shell Structure And Application For Ion Detection

In disease diagnosis and biological detection,fluorescence imaging technology is a very efficient and accurate tool which has the ability to visualize and study tissue morphology in real time.Fluorescence bioimaging technology based on rare earth upconversion nanoparticles（UCNPs）has the advantages of weak background autofluorescence,deep tissue penetration,and low photobleaching,making UCNPs become an ideal biomarker.Due to the small absorption cross-section of lanthanides,the upconversion quantum efficiency is limited,which severely limits its application in biomedicine.In this thesis,we designed and synthesized a nanocomposite with a multilayer core-shell structure to adjust their upconversion luminescence（UCL）.Further,a UCL nanoprobe was designed and fabricated to detect sulfur ions.The contents are list as follow.（1）To enhance the low luminescent efficiency of upconversion nanophosphors,we designed and synthesized a small size Na YF₄:Yb,Er@Na Yb F₄@Na Yb F₄:Nd@Na YF₄ nanocrystal through epitaxial growth of the shell layer and appropriate doping to adjust the energy transfer,radiation and non-radiation probability,achieving the luminescence enhancement of UCNPs.We chose optically inert Na YF₄ as the host;In addition,Er³⁺with rich electronic energy levels was used as the luminescence center.Na Yb F₄ shell can prevent the cross-relaxation process from Er³⁺to Nd³⁺.After excitation at 980 nm laser,Yb³⁺energy can be transferred to Er³⁺to produce strong upconversion emission.By adjusting the concentration of Nd³⁺,nanoparticles with adjustable luminescence can be obtained.The outermost layer is an inert shell,which can effectively shield the reaction of internal ions and the external environment.The UCNPs have regular shape,uniform particle size and dispersion.The upconversion spectra results showed that the luminescence of the UCNPs had increased by more than 10 times with the coating of the shell layer.By changing the doping concentration of Nd³⁺,the emission of UCNPs had also been effectively adjusted.（2）In order to overcome the problem of poor dispersion of UCNPs with oleic acid ligands on the surface,we further designed and synthesized an amphiphilic polymer coating on the rare earth nanoparticles.Firstly,Na YF₄:Yb,Er@Na YF₄（Er-NPs）and 1-oxo-1H-phenalene-2,3-dicarbonitrile（OPD）were prepared respectively.Then,the amphiphilic polymer DSPE-m PEG 5000 was used to encapsulate Er-NPs and OPD inside through self-assembly.The hydrophobic end interacted with the surface ligands of UCNPs to form a hydrophobic layer,and the hydrophilic end was used as the shell exposed to form an oil-in-water system to obtain UCNP-OPD@PEG.The OPD in this system has only a UV absorption peak at 440 nm at room temperature.After reacting with S^2-,the new UV characteristic absorption peak overlaps with the green emission peak of UCNPs.When UCNP-OPD@PEG is excited by 980 nm,the emission of UCNPs will be quenched due to the process of fluorescence resonance energy transfer（FRET）,so the real-time detection of S^2-can be achieved through changes in UCL spectra.Electron microscopy results showed that the composite nanostructure of UCNP-OPD@PEG is successfully prepared with good dispersibility and biological stability.The upconversion emission spectra results proved that UCNP-OPD@PEG has the ability to detect S^2-specifically.Cytotoxicity and cell imaging results showed that UCNP-OPD@PEG have good biocompatibility and capacity of detection to S^2-,which provides a technical guarantee for its further application in real-time detection of sulfide ions in vivo.