Synthesis Of Up-Conversion Luminescent Host Materials-Hexagonal Pahse NaYF₄ And NaGdF₄ Nanocrystals

With the development of biochips,up-conversion fluorescent materials have gradually become a new research focus.Until now,ytterbium and erbium co-doped hexagonal sodium yttrium fluoride(NaYF₄:Yb,Er) is among the most efficient up-conversion phosphors.Its potential applications in biological and medical fields have attracted many scientists.However,as a host material,the preparation for hexagonal NaYF₄ nanocrystals are still tough,the lowest temperature needed is about 160℃.An oil-water interface method is adopted in this paper to synthesize hexagonal NaYF₄ nanocrystals at room temperature,which is proved to be a low-cost, easy-operation method.Effects of surfactant's concentration,pH value,etc.on the NaYF₄'s crystalline types and the reaction mechanism were discussed.Besides this, other host material- NaGdF4 nanocrystals were prepared by an aqueous phase method, and some creative and significant results were obtained.(1) For NaYF₄,well-dispersed,homogeneous NaYF₄ fluorescent matrix nanocrystals about 10 nm were synthesized by the reversed micelle method with high yield.The ratio of hexagonal type to cubic type of NaYF₄ increased with the increase of sodium oleate's concentration.When the concentration was about 0.40 mol/L,the hexagonal nanocrystals are achieved.But the cubic NaYF₄ nanocrystals appeared again when the concentration exceeded 0.40 mol/L.In the range from 1 to 6,when the Y(ClO₄)₃'s pH value was about 2,the hexagonal NaYF₄ nanocrystals were achieved. But when the pH>2(or <2),the cubic NaYF₄ nanocrystals appeared and its amount increased with the increase(or decrease) of pH value.The polarities of organic solvents(cyclohexane,hexane,toluene) did not affect NaYF₄'s crystalline type obviously.Hexagonal NaYF₄ nanocrystals were not obtained in the presence of surfactant AOT or CTAB.Changes of ionic strength in water phase were favorable to get cubic NaYF₄ nanocrystals.In the range from 1:5 to 5:1,when the volume ratio of oil to water was about 3:2,the hexagonal NaYF₄ nanocrystals were achieved.But when the volume ratio>3:2(or <3:2),the cubic NaYF₄ nanocrystals appeared and its amount increased with rise(or decrease) of volume ratio.According to the ICP testing results,90%of Y still existed in the aqueous phase before NaF was added,but almost all the Y were transferred to the oil phase when NaF was added,and just 1/4 of Y could form NaYF₄ nanoparticles.Based on the FTIR testing results,the NaYF₄ nanoparticles were capped with sodium oleate.The reaction mechanism can be:the nanopartices were first formed between the micelles in the aqueous phase,then the micelle reversed and the products were transferred to oil phase.(2) NaYF₄ nanoparticles were also prepared by the oil-water interface method.In this method,the reaction occurred between yttrium oleate in cyclohexane and NaF solution,and the hexagonal NaYF₄ nanoparticles with 4-5 nm were obtained by adjusting the system's pH value.(3) For NaGdF₄,well-dispersed,homogeneous NaGdF₄ fluorescent matrix nanocrystals were synthesized in the aqueous phase.The NaGdF₄ nanocrystals are corresponding to the hexagonal crystalline structure and are in a single crystal form. The variance in pH value of Gd(ClO₄)₃ solution did not affect the size and shape of the product.The biggest particles were just obtained with the unity of Gd³⁺ to F^-([F^-≡0.17 mol/L],[Gd³⁺≡0.04 mol/L]),probably due to the weakest repulsive force between the preformed primary particles with the smallest adsorption of the excessive reactant ions,while the existence of excessive F^- or Gd³⁺ might make particles smaller. With increasing concentration of both chelators,the particle size decreased first dramatically and then gradually.However,the particles prepared in the presence of Na-Citrate were smaller than those prepared in the presence of Na-EDTA,probably due to the differences in their structures,space hindrances,and the saturation adsorption capacity.The yield reach to over 99%just after the two reactants contacted and the product did not change greatly thereafter.It was explained that the particles did not grow gradually.Seeding effects showed that the size of the products was almost invariable.Therefore the particle formation may obey the aggregation model.