Rare-Earth Doped Fluoride Ultrasmall Nanoparticles: Synthesis And Upconversion Luminescence Properties

Upconversion luminescence nanoparticles（UCLNPs）refer to materials which emit high shortwave radiation under emission of long-wave radiation with low energy.UCLNPs have potential applications in variety fields like drug delivery,anti-counterfeiting recognition and optical implements after decades of development.With the deepening of the research,many advantages of UCLNPs have been discovered by researchers including low toxic,high penetration depth and low auto-fluorescence background.It is well known that small-sized nanomaterials can not only serve as target molecule but also clean from the body by function of human body automatically.Therefore,high efficiency ultrasmall rare-earth doping UCLNPs is still one of the hot topic.However,reports about series of high efficiency ultrasmall（sub-10 nm）rare-earth dopingβ-NaREF₄（Gd,Yb,Lu and Y）UCLNPs are rare due to restrict growth behavior of rare earth doped composite fluoride nanomaterial.Based on the above reasons,a series of uniform,monodisperse,small size,high efficiency and water-soluble UCLNPs were synthesized in the presence of high boiling solvents of oleic acid and 1-octadecene via thermal decomposition method with Er³⁺,Ho³⁺,Tm³⁺and Nd³⁺as the activator ions,Yb³⁺as the sensitizer ion andβ-NaREF₄as the host material.The main results of this work are as follows:（1）By using the combination technique of thermal decomposition and hydrothermal lanthanide trifluoroacetate precursors,uniform,monodisperse,small size and high efficiencyβ-NaGdF₄:Yb/Er NPs can be obtained under the optimal conditions of NaF/RE=12,reaction temperature of precursor=100°C,reaction temperature=160³00°C,reaction time=1 h and reaction concentration=0.067 mol/L.The size of the nanoparticles is controlled between 3 and 9 nm.Under the excitation of 980 nm LD,UC spectrum ofβ-NaGdF₄:Yb/Er at room temperature shows two emission bands,green and red,of which the radiative transitions are ²H_11/2→⁴I_15/2,⁴S_3/2→⁴I_15/25/2 and ⁴F_9/2→⁴I_15/2,respectively.（2）By tuning the volume ratio(Gd³⁺doping concentration)of the heterogeneous precursor to obtainβ-NaGdYF₄:Yb/Er,β-NaGdLuF₄:Yb/Er andβ-NaGdYbF₄:Er NPs of 7 nm⁵4 nm at 290°C.The results showed that viatheGd³⁺dopingmethod,β-NaYF₄:27%Gd/Yb/Er,β-NaLuF₄:45%Gd/Yb/Er andβ-NaYbF₄:45%Gd/Er can realize the controlled size of sub-10 nm.The whole controlled synthesis mechanism can be explained from the proper balance between the rates for nucleation and growth stages.We investigate the effect of different Gd³⁺doping concentrationstotheluminescencepropertyofheterogeneousβ-NaGdYF₄:Yb/Er,β-NaGdLuF₄:Yb/Erandβ-NaGdYbF₄:ErNPs.Comparison of three different heterogeneous NPs decay lifetime on the same condition showed that the optimal UCLNPs is the heterogeneousβ-NaYF₄:27%Gd/Yb/Er.（3）Sub-17 nmβ-NaGdF₄:Yb/RE@β-NaYF₄（RE=Er/Ho/Tm）multicolor core-shell NPs were synthesized at 290°C via thermal decomposition method.The experimental result shows that light intensity of core-shell is 50⁷0 times of the core.Three emission peak center of Ho³⁺ion are 488,543 and 648 nm corresponding to energy level transition of ⁵F₃→⁵I₈,（⁵S₂,⁵F₄）→⁵I₈ and ⁵F₅→⁵I₈.Three emission peak center of Tm³⁺ion are 453,477 and 650 nm corresponding to energy level transition of ¹D₂→³F₄,¹G₄→³H₆ and ¹G₄→³F₄.β-NaGdF₄:Yb/Er@β-NaYF₄:Yb/Er core-shell NPs have bright up-conversion luminescence by two mode（980 nm/808 nm）laser diode excitation.Light intensity of water-soluble sample with SiO₂ coating is weaker than unmodified one.These researches have laid a certain experimental foundation the preparation of more efficient rare earth doping composite fluoride.Hope it can have further application in the biological field in the future.