| Rare earth elements have special 4f electronic layer structures and rich energy levels, which make the lanthanide-doped luminescent materials have good prospects in information technology, data storage, color display and biological labels. Especially the rare earth ion doped upconversion nanomaterials have a series of excellent chemical and optical properties, such as low background autofluorescence and cell toxicity, sharp emission bands and high penetration depth in biological tissue. These characteristics make it have great application value in biological application. In this paper, we choose NaYF4 as a matrix material and two major challenges that limit the use of nanocrystals in biological applications have been successfully resolved:the traditional 980nm excitation light will lead to overheating, thereby inducing significant cell death and tissue damage; the upconversion nanocrystals suffer from low emission efficiency. The main research contents of this paper are as follows:①In this paper, the a-NaYF4:Yb3+/Er3+ nanocrystals were successfully synthesized by a simple hydrothermal method. By changing the experimental temperature, three samples were obtained. The structures and properties of as-prepared products were characterized XRD, TEM, upconversion luminescence spectra and other tests. With the increasing grain size, the color of emission gradually changed from green to red. We discussed the relationship between the nanoparticle size and upconversion luminescent color, and explained the reason by the EDS and other test methods.(2)In order to obtain a higher emission efficiency, we successfully synthesized β-NaYF4:Yb3+/Er3+ upconversion nanocrystals by the solvothermal method. The prepared samples have the characterstics of high purity, regular morphology, uniform size and good dispersion. From the photoluminescence spectra, four main emission peaks can be clearly observed at the positions of 410nm,525nm,540nm and 655nm, respectively. By comparing the emission spectra of a-and β-NaYF4 nanocrystals with the same size, we confirmed that the luminescence efficiency of β-NaYF4 nanocrystals is higher.③To avoid the overheating effect of 980nm excitation source, we introduce a Nd3+ which can effectively absorb the near infrared emission of 808nm and use Yb3+ as a bridge to transfer the excitation energy to the activator ion. The β-NaYF4:Nd3+, Yb3+, Ho3+/Tb3+ with good morphology and dispersion were successfully prepared by the method of solvent thermal and we test their luminescence properties respectively. Finally, the luminescence mechanism of the samples are analyzed and discussed.④In order to further improve the luminous efficiency, we introduce a homogeneous shell layer on the surface of β-NaYF4 nanoparticles. We have successfully synthesized the NaYF4:Nd3+/Yb3+ /Tm3+@NaYF4 core-shell structure nanomaterials by the solvent thermal method and its epitaxial growth structure were confirmed by the XRD and TEM results. By comparing the spectra of two samples before and after coating, it is proved that the core-shell structure can improve the luminescence efficiency of the nanocrystals.⑤We have designed the β-NaYF4:Nd3+/Yb3+/Ho3+@SiO2@Ag core-shell nanocomposite materials by the combination of core β-NaYF4:Nd3+/Yb3+/Ho3+nanocrystals and Ag nanoparticles. We discuss the influence of SiO2 layer thickness on upconverting optical properties. The largest enhancement of upconversion luminescence occured when 10 nm-thick SiO2 shell was used. In addition, we also found that after Ag nanoparticles coated, the luminescence lifetime of β-NaYF4@SiO2 nanoparticles decrease and the radiative decay rates increase, thus confirming the effect of metal surface plasma resonance on upconversion luminescence. We also observed the three photon absorption process of the upconversion luminescence, and discuss the related mechanism in the end.
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