Study Of Luminescence Properties And Anti-counterfeiting Application Of NaErF₄-based Core-shell Upconversion Nanoparticles

Rare earth-doped upconversion luminescent nanomaterials have been widely used in anti-counterfeiting,information encryption,biological imaging and other fields due to their advantages of nearly background-free luminescence,high resistance to photobleaching,narrow emission bands and so on.In the activator fully-doped nanomaterial system,efficient upconversion luminescence can be achieved by constructing a core-shell structure to suppress the concentration quenching effect.The design of shell is the key to control the luminescence properties of the activator fully-doped upconversion nanomaterials,but its modulation of the luminescence color still needs further investigation.To this end,this paper takes NaErF₄ as the research object,designs and constructs NaYF₄ inert shell,NaYbF₄ active shell and NaYF₄@NaGd F₄:Ce,Eu inert@active shell coated NaErF₄ fully-doped nanomaterials,realizing the modulation of its luminescence intensity and luminescence color.The application of the nanomaterials in multiple fluorescent anti-counterfeiting has also been explored.The paper provides a reference for the design and application of heavily-doped upconversion luminescent nanomaterials.The main result of this paper is as follows:（1）Hexagonal phase NaErF₄ upconversion nanoparticles coated with NaYF₄ inert shell were prepared.The results of steady-state and transient luminescence spectra show that with the increase of the shell thickness,the inhibition of the surface quenching effect strengthens due to the passivation of the shell,thus increasing the upconversion luminescence intensity.The competition between the nonradiative relaxation of ⁴F_7/2→²H_11/2/⁴S_3/2 and the cross-relaxation of ⁴F_7/2+⁴I_11/2→2⁴F_9/2 of Er³⁺results in a tunable red-to-green emission intensity ratio between2.45 and 13.16 under 980 nm excitation,while the competition among the nonradiative relaxation of ⁴I_9/2→⁴I_11/2,the three-photon transition of ⁴I_9/2→²H_11/2/⁴S_3/2 and the cross-relaxation of ⁴S_3/2+⁴I_9/2→2⁴F_9/2 leads to a controllable red-to-green emission intensity ratio between 0.50 and 5.65 under 1532 nm excitation.The pattern drawn with the core-shell nanoparticle as invisible ink emits multi-colored light visible to the naked eye under 980 nm and 1532 nm excitation,realizing advanced double luminescence anti-counterfeiting.（2）Hexagonal phase NaErF₄ upconversion nanoparticles coated with NaYbF₄ active shell were prepared.The active shell not only has the function of surface passivation,but also improves the ability of the nanoparticles to capture 980 nm excitation light.The energy transfer between Yb³⁺and Er³⁺and the energy migration between Yb³⁺can further control the upconversion luminescence of NaErF₄.Therefore,as the thickness of the NaYbF₄ shell increases,the upconversion luminescence intensity of NaErF₄ gradually increases,and the control range of the red-to-green emission intensity ratio under 980 nm and 1532 nm excitation is 0.14-6.30 and 0.12-0.86,respectively.The NaErF₄@NaYbF₄ upconversion nanoparticles also have good double anti-counterfeiting ability.（3）Hexagonal phase NaErF₄ nanoparticles coated with NaYF₄ inert shell and NaGd F₄:Ce,Eu active shell were prepared.The double-shell structure design modulates the upconversion luminescence properties of NaErF₄ under 980 nm and 1532 nm excitation.The energy transfer of Ce³⁺→Gd³⁺→Eu³⁺also makes the nanomaterial emit red down-shifting luminescence.The introduction of down-shifting luminescence further improves the anti-counterfeiting ability of the material.