Synthesis And PAA Modification Of NaYF₄:Ln (Yb,Er,Tm,Eu) Luminescent Nanocrystals

Rare-earth nano-materials can be applied in biological labeling field due to their distinguished optical property from rare-earth elements and small size effect from their nano-scale. As is known to all, lanthanide(Ln) co-doped hexagonal sodium yttrium fluoride (NaYF₄:Ln) is one of the most efficient fluorescent materials. However, NaYF₄ itself is poorly water-soluble, and the frequently used surfactants during the synthetic process make it more difficult to dissolve in polar solvents, which is unfavourable for the biological application. Therefore, it is necessary to carry out the surface modification of NaYF₄. In this paper, the co-doped NaYF₄ nanoparticles were produced initially and then modified by polyacrylic acid (PAA) to improve surface properties. Additionally, due to the significant role of the outside NaYF₄ layer, the nanocomposites NaYF₄:Ln/NaYF₄/PAA with better comprehensive performance were also synthesized to eliminate the fluorescence quenching effect mainly from organic ligands. The details are showed in the following parts.First, well-dispersed hexagonal NaYF₄ fluorescent matrix nanocrystals were synthesized by reversed micelle method, and then doped with lanthanide elements. NaYF₄ nanocrystals doped with Yb, Er (Tm) are up-converting phospors while doped with Eu are down-converting. X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), fourier transform spectroscopy (FTIR), and fluorescence spectroscopy were used to characterize the luminescent nanocrystals. The results indicated that NaYF₄ nanoparticles were hexaganol crystals with an average size of 8 nm. After doping with lanthanide elements, the materials performed good fluorescent property. However, the nanoparticles were proved to be capped with sodium oleate and need to be modified.Next part is relative to the study of surface modification. Three modifying methods were used:â‘ Modifying NaYF₄:Ln by ligand exchange is a mild and simple reaction method. The synthesized NaYF₄:Ln /PAA composites were small, with an average size of 10 nm. But the dispersion and fluorescent property of the composites were in poor performance;â‘¡Then the NaYF₄:Ln nanoparticles were modified by microemulsion method. The synthesized NaYF₄:Ln/PAA composites with an average size of 15 nm were well-dispersed in aqueous such as water and alcohol. The surface modification almost had no influence on up-converting property but greatly reduced the down-converting luminescence. The reaction process is complex and high-energy-consumed;â‘¢In-situ polymerization is the third modification method. The products were well-dispersed NaYF₄:Ln/PAA nanospheres with an excellent fluorescence property. But these nanospheres were big, and the minimum was also about 50 nm. In a word, except the shortcoming of size, the composites synthesized by in-situ polymerization performed the best comprehensive property and were closest to the requirement of biological application among all the three methods.To reduce the fluorescence quenching effect caused by polymer, NaYF₄:Ln/NaYF₄ core-shell particles were finally synthesized through the reverse micelle method. NaYF₄ shell eliminated the surface defects of the internal light-emitting nuclear NaYF₄:Ln, thus perfecting its nanocrystal. Then NaYF₄:Ln/NaYF₄/PAA nanocomposites were produced by ligand exchange method. Fluorescence test results proved that the fluorescence intensity of NaYF₄:Ln/NaYF₄/PAA was higher than that of NaYF₄:Ln/PAA. In addition, the influence of rare-earth doping ratio on fluorescense property was explored in detail. For up-converting property, the change of the doping ratio between Yb³⁺ sensitizer and Er³⁺ (Tm³⁺) activator altered the color of emission light. For down-converting property, Eu³⁺ was directly stimulated. The strongest fluorescence was found by regulating the concentration of Eu³⁺...