| Due the presence of an intermediate excited state between the absorber and emitter ions,lanthanide-doped upconversion nanoparticles（UCNPs）can efficiently convert near-infrared（NIR）photons into higher energy visible photons.UCNPs possess several qualities that make them very attractive for biological applications,e.g.,strong penetration abilities and non-autofluorescence.However,UCNPs still face two key obstacles to their widespread use for in vivo bioimaging.Firstly,UCNPs with Yb3+ as sensitizer are generally excited by a 980 nm laser.However,the use of 980 nm NIR photons has an intrinsic disadvantage due to the strong absorbance of water molecules in biological tissues at that wavelength,resulting in the risk of local temperature rise or even tissue overheating under continue irradiation.Secondly,the most widely used rare earth doped NaYF4 nanoparticles have usually multiband emissions of green（ 540 nm）and blue light（ 470 nm）,which severely limit their penetration ability towards the applications in deep tissue imaging.Most research studies on the synthesis of UCNPs are carried out to obtain a highly effective way to solve one of the problems.However,until now,no effective strategy can solve these two problems at the same time.In this thesis,on the one hand,we shifted the absorption band of Na3ZrF7 UCNPs from 980 nm to 808 nm by doping Nd3+ ions as sensitizer with the improved solvothermal method,to avoid water absorbance in tissues,but also realize the highly efficient emission of pure red light.On the other hand,we also obtained the Na3 Zr F7 UCNPs doped with Er ions,and shifted the absorption band of UCNPs to a longer band of 1480 nm.Based on the three-photon effect,the single wavelength conversion of visible red light is formed.Due to the smaller Rayleigh scattering in 1480 nm band,the proposed technique could widely be used in deeper biological imaging.The main contents and innovations of this thesis are as follows:（1）Yb/Er codoped Na3ZrF7 nanoparticles were prepared by a modified solvothermal method,which exhibited a high intensity emission spectra of almost pure red（660 nm）.Compared to the existing methods to obtain pure red light emitting UCNPs,the present method is simpler and more effective and prepared nanoparticles have more uniform morphology（tetragonal phase structure）and smaller size（11 nm）.We also showed that the amount of Na ions had influence on the emission spectra of Na3ZrF7:Yb 20%,Er 2% nanoparticles.Na3ZrF7:Yb 20%,Er 2% nanoparticles with a dosage of 2.5 mmol NaOH could contribute to the most emission intensity among all samples.In addition,the red/green emission ratio would gradually enhanced as the NaOH dosage increased.（2）In this thesis,we achieved the pure red emission（660 nm）Na3ZrF7:Er UCNPs under the three-photon excitation of 1480 nm wavelength（under the excitation of communication band）by a modified solvothermal method.Compare to other methods with three photon upconversion,the proposed synthetic method of UCNPs was more suitable for deep biological tissue imaging and therapeutic applications with the advantages of pure red light emission in the visible range.Moreover,the experimental results showed that the fluorescent intensity of Na3ZrF7:Er UCNPs decreased as Er doping concentration increased.And then,the doping concentration of Er ions can be optimized as 5%.（3）The surface of mononuclear Na3ZrF7 UCNPs was successfully coated with a layer of CaF2 shell to effectively reduce the surface defects and increase the fluorescent intensity by more than four times compared with those without CaF2 material shell.Moreover,we designed Nd doping UCNPs with three layers structure（i.e.,Na3ZrF7:Yb 20%,Er 2%@ CaF2@ NaYF4:Nd 20%）to enhance the red fluorescence emission of UCNPs under the excitation of 808 nm.（4）PEG was successfully conjugated to the surface of the Na3ZrF7@ CaF2 UCNPs by the way of ligand exchange,which made PEG-Na3ZrF7@ CaF2 UCNPs disperse well in deionized water to further meet the requirement of biomedical applications. |
