Controllable Synthesis And Luminescent Properties Of Lanthanide Doped BaLnF₅/KLnF₄（Ln=Gd,Y,Yb） Nanocrystals

The rare earth upconversion luminescent（UCL） nanomaterials are widely used in laser, three dimensional displaying, bioimaging and other fields due to their unique UCL properties. In the study of rare earth luminescent nanomaterials, MLnF（M=Na, K, Ba） system is usually regarded as excellent host material. However, Yb³⁺/Ho³⁺ co-doped MLnF（M=Na, K, Ba） nanomaterials usually present high-energy green emission, limiting their application in in vivo bioimaging. Compare with green light emission, red light possesses deeper tissue penetration and less damage to biological tissue. Therefore, the rare earth UCL nanomaterials with red or near infrared emission（650-850 nm） have potential application in deeper tissue bioimaging. Based on the above description, the purpose of this paper is to construct nanomaterials with pure red emission and high optical efficiency, and realize their applications in bioimaging. The main research works are listed as follows:（1） Yb³⁺/Ho³⁺/Ce³⁺co-doped BaLnF₅（Ln=Gd, Y） nanomaterials were successfully synthesized via a typically hydrothermal method using oleic acid as stabilizing agents. The nanomaterials were characterized by X-ray diffraction（XRD） and transmission electron microscope（TEM）. The results reveal that Yb³⁺/Ho³⁺/Ce³⁺co-doped BaLnF₅（Ln=Gd, Y） nanocrystals possess pure cubic phase and uniform morphologies. In addition, the influence Ce³⁺ doping on the UCL properties of Ba GdF₅/BaYF₅ nanocrystals was studied. It was found that the multicolor tuning from yellow to red emission as well as the pure red emission can be successfully realized by adjusting Ce³⁺ contents. These results provide a novelty route for synthesizing nanomaterials with color controlled and pure red emission, which offers a new-type of nanoprobe with pure red emission for the application of bioimaging.（2） Li+/Yb³⁺/Er³⁺ co-doped KLnF₄ nanocrystals with good monodispersity were fabricated by hydrothermal method, and the effect of the different Li+ contents on UCL properties of nanocrystals was systematically analyzed. The results of scanning electron microscope（SEM） and TEM reveal that nanocrystals structure including crystal phase and morphology cannot be changed by low contents of Li+. However, the UCL efficiency and red/green（R/G） ratio can be improved by adjusting Li+ concentrations. The result shows that the intensity of nanocrystals emission is the strongest when the concentration of Li+ is 1%. Furthermore, compared with Li+-free sample, the emitting intensity of the nanocrystals doped with 1%Li+ was increased to 12.51 times and the R/G ratio was improved to 1.76 times. This is mainly ascribed to the changes of symmetry crystal field and crystal structure caused by Li+ doping. Therefore, UCL efficiency can be improved and UCL R/G ratio is successfully enhanced by doping low concentration of Li+.（3） The oleic acid capped 20%Yb³⁺/2%Ho³⁺/30%Ce³⁺ co-doping Ba GdF₅ nanomaterials with pure red emission were converted to hydrophilic nanoprobes by HCl treatment. The hydrophilic nanoprobes were successfully applied in in vivo UCL bioimaging with high contrast and high sensitivity owing to the excellent pure red emission. The in vivo UCL bioimaging demonstrated that these nanoprobes were mainly accumulated in liver, spleen and lung. Additionally, the nanoprobes were successfully used as contrast agents in X-ray bioimaging due to the excellent X-ray absorption coefficient of Ba and rare earth elements. These results show that the synthesized nanomaterials with red emission can be served as ideal bioimaging nanoprobe due to the excellent multi-functional properties, which have important applications in the field of multi-modal imaging.