Upconversion Fluorescence Properties Of NaLuF₄:Yb,Er,Gd/semiconductor/Au Nanocomposites

Among various matrix materials,Na RF₄（R=rare earth）has proved to be the most promising material due to its low phonon energy.In the past few decades,Na RF₄has been shown to have more and more potential applications in the field of biology,such as up-conversion fluorescence bioimaging in vivo and in vitro,photodynamic therapy,image-mediated drug release,etc.The selection of functional materials as shells can enrich the properties of rare earth up-conversion nanocrystals.At the same time,in order to avoid the quenching phenomenon caused by direct contact between the core and the Au shell,a semiconductor with narrow gap width and adjustable size is selected as the spacer layer in this paper,which provides a theoretical basis for the development of new luminescent materials.The near-field enhancement caused by surface plasmon resonance effect of Au nanoparticles can improve the upconversion efficiency.At present,there are few reports on the variation of luminescence intensity after Au shell coating.Therefore,the relationship between surface plasmon resonance effect and upconversion efficiency is studied in this paper.In this paper,Na Lu F₄:Yb,Er,Gd up-conversion nanocrystals with small size and strong fluorescence signal were synthesized by oil phase method.The surface of the core was coated with Ag₂S and Pb S shells,and then coated with Au shells.The relationship between the core-shell structure and the up-conversion efficiency was studied by testing and characterization:（1）Core-shell-shell Na Lu F₄:Yb,Er,Gd@Ag₂S@Au nanomaterials were constructed.The results show that the photothermal properties are improved and the upconversion efficiency is reduced after the coating of Ag₂S shell,which is due to the fact that the semiconductor shell promotes the scattering of excited light.The coating of Au shell improves the biological security and decreases the luminescence intensity continuously,because Au shell obstructs the absorption of upconversion nanonuclear,and the absorption of Au shell is also one of the reasons for the reduction of brightness.The overall nanomaterials exhibit better properties and are suitable for biological imaging and tumor therapy.（2）Core-shell-shell Na Lu F₄:Yb,Er,Gd@Pb S@Au nanomaterials were prepared.Because the narrow band-gap width of Pb S matches the band excited by the near infrared at 980 nm,and after coating the Au shell,Pb S resonates with the surface plasmons,it may enhance the upconversion efficiency and exhibit excellent properties.In this paper,it was found that the local luminescence was enhanced after the coating of Au shell.The reason is that there is a Pb S semiconductor shell between the up-conversion nanoparticle luminescent material and Au shell,which keeps a certain distance between them without direct contact,thus reducing the quenching effect.In this paper,core-shell nanomaterials with Pb S semiconductor were prepared,and Au shells were coated to improve their biosecurity,thus promoting the development of solar energy collection,security coding and bioimaging applications.