Preparation And Properties Of NaYF₄:Yb,Er Upconversion Nanocrystals

Rare-earth doped upconversion nanoparticles can convert near-infrared excitation light into visible radiation through a multiphoton absorption mechanism,showing unique advantages in the biological field especially.It has advantages such as high signal-to-noise ratio,deep penetration depth,high chemical stability,good luminous stability and low toxicity.Asa commonly used preparation method of NaReF4 up-conversion nanoparticles,co-precipitation method in high boiling solvent could synthesise products with characteristics such as uniform morphology,small size and high crystallinity through a relatively simple process at low temperature.However,the preparation of ultra-small upconversion nanoparticles(<10nm)in accordance with the biological application requirements is still a challenge.Therefore,the synthesis process and mechanism of co-precipitation method need to be studied deeply.Secondly,surface hydrophilic modification is needed for the use in the biological field because of the adsorption of organic hydrophobic groups to the surface of the particles prepared by co-precipitation method.In this paper,we focus on the growth behavior of NaReF4 upconversion nanocrystals and the surface hydrophilicity modification.The specific contents are as follows:NaYF4:Yb3+,Er3+ upconversion nanoparticles were prepared by coprecipitation method.The effects of reaction temperature and reaction time on the morphology,size and crystal phase of the upconversion nanoparticles were studied firstly.At low temperature,only cubic phase nanoparticles are formed.As the temperature increases,the hexagonal phase particles begin to nucleate and grow up.Prolonged reaction time when the temperature is above 305 degrees Celsius,the particle phase gradually changes from cubic to hexagonal.Therefore the synthesis mechanism of NaYF4:Yb3+,Er3+ upconversion nanoparticles are as follow:the cubic phase particles which nucleate first continue to dissolve,then the hexagonal phase nanoparticles recrystallize.That is,the dissolution-recrystallization mechanism.At the same time,the growth behavior of nanoparticles(NaYF4,NaGdF4,NaYbF4)with different substrates were compared.It was found that the synthesis process of NaGdF4 did not appear phase transformation process.The hexagonal phase nanoparticles were formed at low temperature.Yet cubic-hexagonal phase transition appeared both in NaYbF4 and NaYF4 synthesis processes.Besides,the hexagonal phase NaYbF4 appeared after 60minutes of reaction at 305 degrees Celsius,which was slower than NaYF4.The reason is the formation of hexagonal phase NaYbF4 is lower,so the nucleation rate is also lower,resulting in late hexagonal phase nucleation and larger particle size.Besides,the surface of NaYF4:Yb3+,Er3+ was coated with silica shell by reversed-phase microemulsion method in order to improve its water solubility.The influence of the amount of raw materials,the thickness of silica shell and the dispersion of particles were studied.It was found that the concentration of surfactant and silicon source show obvious effects on the thickness of silicon shell while too high concentration cause particle aggregation.Increasing ammonia concentration,the particle dispersion is improved.With the increase of the amount of cyclohexane,the particle dispersion is gradually improved and then particle agglomeration became more serious when the amount is further increased.According to the optimum process parameters,the composite nanoparticles with better water solubility as well as weak fluorescent properties can be obtained.