Preparation,Characterization And Fluorescent Anti-counterfeiting Application Of Dual-mode Multilayer Core-shell Structured Rare Earth Composite Luminescent Microspheres

SiO₂ is one of the excellent core-shell materials with the advantages of simple preparation,low cost,good dispersion and light transmittance.SiO₂ core-shell nanomaterials coated with rare earth luminescent nanomaterials have potential application in the fields of display,biosensor,medical diagnosis,optical coding and so on because of their narrow spectrum,high luminescent efficiency,durability,stability and non toxicity.Double luminescent rare earth nano anti-counterfeiting ink materials with SiO₂ microspheres as core and rare earth oxide(rare earth vanadate)as interlayer and SiO₂ as shell were prepared.Different rare earth elements with down-conversion(DC)and up-conversion(UC)luminescence properties were doped into the inner or outer layer of SiO₂ core-shell structure nanomaterials to prepare dual-mode multilayer core-shell structure luminescence nanomaterials.SiO₂ shell was used to separate DC and UC luminescence layers,limit the cross relaxation between lanthanide ions,avoid the luminescence quenching of DC and UC.The morphology and composition of the synthesized samples were analyzed by scanning electron microscopy(SEM),transmission electron microscopy(TEM),high-resolution transmission electron microscopy(HRTEM),X-ray powder diffraction(XRD),thermogravimetric(TG),X-ray energy dispersive spectroscopy(EDS)and X-ray photoelectron spectroscopy(XPS).The photoluminescence properties of the samples were measured by photoluminescence spectrum(PL),decay curve and quantum yield.The anti-counterfeiting performance of the samples was studied.1.The core-shell-shell structured cubic phase SiO₂@Gd₂O₃:Eu@SiO₂microspheres with increasing luminescence properties were synthesized by coprecipitation and sol-gel method.The core-shell-shell structured luminescent microspheres with different core diameters(samples C1-C4)and different shell thickness(samples S1-S4)were obtained by adjusting the amount of tetraethyl orthosilicate(TEOS).The synthesized SiO₂@Gd₂O₃:Eu@SiO₂ microspheres had obvious core-shell-shell structure.The SiO₂ core diameters of samples C1-C4 were about 55 nm,105 nm,145 nm and 195 nm,respectively,and the SiO₂ shell thickness was about 8 nm.The SiO₂ core diameter of samples S1-S4 was about 105 nm,the SiO₂shell thickness were about 4 nm,8 nm,12 nm and 16 nm respectively,and the thickness of shell Gd₂O₃:Eu was about 3 nm.The effects of the diameter of SiO₂ core and the thickness of SiO₂ shell on the luminescence properties of Gd₂O₃:Eu were studied.When the diameter of SiO₂ core was about 105 nm,the luminescence intensity of the synthesized sample was the highest and Gd₂O₃:Eu coating was best.When the thickness of SiO₂ shell was from 4 nm to 8 nm,SiO₂ shell can enhance the luminescence properties of samples,the absolute photoluminescence quantum yield(PLQY)of the sample was from 90.4%up to 95.9%.The PLQY decreased to 85.5%when the SiO₂shell thickness increased from 8 nm to 12 nm.The anti-counterfeiting ink with double excitation wavelength and double emission luminescence was obtained by mixing the SiO₂@Gd₂O₃:Eu@SiO₂andSiO₂@AIPA-S-Si-Tb@SiO₂microspheres((HOOC)2C6H3NHCONH(CH2)3Si(OCH2CH3)3 recorded AIPA-S-Si).The effects of the mixing weight ratio of two luminous centers and the stability of luminescence dispersion on the effect of anti-counterfeiting ink were studied.2.Multilayer core-shell structured SiO₂@Gd VO4:Yb,Er@SiO₂@Gd₂O₃:Eu@SiO₂(Yb³⁺20%,Er³⁺2%)microspheres were synthesized by coprecipitation and sol-gel method.The microspheres with obvious"core-shell-shell-shell-shell"multilayer sandwich structure were composed of cubic phase Gd₂O₃ and square phase Gd VO4.Gd VO4 was coated on the surface of SiO₂ with spherical convex structure.The diameter of Gd VO4 particles was about 25 nm,the thickness of Gd₂O₃:Eu was about 6 nm,and the thickness of outer SiO₂ layer was about 8 nm.All rare earth elements had a valence of+3 and V was a valence of+5.The microspheres had good UC green light and DC redlightdual-modeluminescenceability.The SiO₂@Gd₂O₃:Eu@SiO₂@Gd VO4:Yb,Er@SiO₂ microspheres was prepared with the synthesized SiO₂@Gd₂O₃:Eu@SiO₂ microspheres(S2)as the core.The multilayer core-shell structured microspheres was composed of cubic phase Gd₂O₃ and square phase Gd VO4.The microspheres also had an obvious"core-shell-shell-shell-shell"multilayer sandwich structure.The coating thickness of Gd VO4 was about 7 nm and the thickness of the outermost SiO₂ shell was about 8 nm.The luminescence intensity was lower than that of SiO₂@Gd VO4:Yb,Er@SiO₂@Gd₂O₃:Eu@SiO₂,which showed the outer layer Gd VO4 could quench the luminescence emission of the inner layer Gd₂O₃:Eu.The UC and DC dual-mode anti-counterfeiting ink with SiO₂@Gd VO₄:Yb,Er@SiO₂@Gd₂O₃:Eu@SiO₂(Yb³⁺20%,Er³⁺2%)was also fabricated,and its application in anti-counterfeiting ink was studied.3.Themultilayercore-shellstructured SiO₂@Gd₂O₃:Yb,Er@SiO₂@Gd₂O₃:RE@SiO₂(RE=Tb³⁺or Eu³⁺)microspheres were prepared by coprecipitation and sol-gel method.The luminescent properties of Yb³⁺and Er³⁺in Gd₂O₃ matrix were studied.The prepared microspheres had obvious"core-shell-shell-shell-shell"multilayer sandwich structure,in which the diameter of SiO₂core was about 105 nm,the thickness of Gd₂O₃:Yb,Er layer was about 2 nm,and the thickness of outer SiO₂ shell was about 8 nm.All rare earth ions were+3 valence.The prepared multilayer core-shell structured microspheres had good UC orange light emission and strong green/red light emission.The fluorescent anti-counterfeiting inks prepared with these two kinds of multilayer core-shell structured microspheres showed good UC orange light and DC green/red light dual-mode fluorescent anti-counterfeiting ability.