The Research On Regulation And Optimization Of Rare-earth Ions Doped Upconversion NaYF₄ Nanocrystals

In recent years, up-conversion nanoparticles （UCNPs） have attracted great research interest for their extensive biological application, benefiting from excited by the near infrared laser, high tissue penetration depth of near-infrared excitation light, low toxicity, low auto-fluorescence background and good chemical and physical stability. However, there are restricted by the two aspects for their biological application. On the one hand, a limit to particle size for UCNPs. On the other hand, luminous efficiency for the UC process is low. Herein we use 980nm laser as an exciting resource to get the visible on convering fluorescence emission. In order to improve the luminous efficiency, we change F-ions concentration and NaOH amount, which can be proved to enhance the emission intensity.First,β-NaYF₄:Yb³⁺/Er³⁺UCNPs were synthesized by the solvothermal method, and the reaction mechanism was discussed. The effects of reaction parameters on size, phase and luminescent intensity of the UCNPs were also investigated in detail. The UCNPs synthesized by solvothermal method have prominent potentials in biological applications, due to their small size, pure phase and high luminescent intensity.Second, Yb³⁺/Er³⁺ doped β-NaYF₄ nanocrystals with different F- concentrations are synthesized, and the luminescence properties of the UCNPs have been studied under 980nm excitation for the sensitive biological applications. The upconversion luminescence spectra of NaYF₄ nanoparticles with different dopants under 980nm excitation proves that F- ion can absorb the photons effectively and can get green visible fluorescence emission. To investigate the effect of F- ion, the decay curves of the 4S3/2--4I15/2 transition at 540 nm are measured and analyzed. The NaYF₄:20%Yb³⁺/2%Er³⁺ UCNPs high emission intensity under 980nm excitation. The UC mechanism under 980nm excitation has been discussed in terms of the experimental results. It has been found that the size, morphology and luminescence intensity of the nanoparticles can be controlled by changing the concentration of fluorine ions. Low reactant concentrations always result in monodisperse, hexagonal nanocrystals, with size of NaYF₄ nanocrystals material decreases by gradually and more uniformly. While at higher con- centrations, nanocrystals grow into mixed phase. The fluorine ions concentration is helpful to promote the fluorescence emission in a certain concentration range. The fluorescent lifetime also have a significant impact with different fluorine ion concentration.Finally, Yb³⁺/Er³⁺ doped β-NaYF₄ nanocrystals with different NaOH amount are synthesized, and the luminescence properties of the UCNPs have been studied under 980nm excitation for the sensitive biological applications. It has been found that the size, morphology and luminescence intensity of the nanoparticles can be controlled by changing the amount of NaOH. Different NaOH amount always result in monodisperse, hexagonal nanocrystals, with size of NaYF₄ nanocrystals material increase by gradually. NaOH amount is helpful to promote the fluorescence emission in a certain range. Concentration quenching was caused under the higher NaOH amount. We can control the phase of UCNPs and luminescent intensity through doping with different NaOH amount.