Preparation And Luminescent Mechanism Of Dy³⁺/Cr³⁺ Co-doped Lu₃Ga₅O₁₂ Near-infrared Afterglow Nanoparticles

Near-infrared persistent luminescent nanoparticles have attracted more attention in biomedical field due to their unique advantages,such as emission wavelengths falling within biological window,fluorescence-background-free for biological tissues and large penetration depth of biological tissues.In particular,transition metal Cr³⁺ion doped zinc gallium germanate(ZGGO:Cr)near-infrared persistent luminescent nanoparticles with superlong afterglow time have shown potential applications in fluorescence-background-free bioimaging and afterglow photodynamic therapy of tumors.However,there remains unsolved issues on further understanding of their afterglow emission mechanism and further enhancement of their afterglow intensity.Therefore,it is still essential to prepare novel Cr³⁺ion-doped gallate based near-infrared persistent luminescent nanoparticles and to unravel their afterglow mechanism.In this dissertation,Lu₃Ga₅O₁₂:x%Cr³⁺and Lu₃Ga₅O₁₂:0.1%Cr³⁺,y%Dy³⁺nanoparticles with different doping concentrations were synthesized by a combustion method.Their microstructure,optical properties and afterglow mechanism were studied by X-ray diffraction,scanning electron microscope,photoluminescence,afterglow decay and thermoluminescence.The detailed results are given as follows:1.It is found that Cr³⁺replaces Ga³⁺site in Lu₃Ga₅O₁₂ host and the average particle size of these samples is about 54 nm.From excitation spectra of Lu₃Ga₅O₁₂:x%Cr³⁺nanoparticles,the excitation peaks at 256,320,470 nm and 598 nm can be ascribed to the intrinsic emissions of Cr³⁺and they are attributed to the charge transfer transition of Ga-O in the host,⁴A₂?⁴T₁(⁴P),⁴A₂?⁴T₁(⁴F)and ⁴A₂?⁴T₂(⁴F)transitions,respectively.Upon 600nm excitation,some broadband and sharp emission peaks falling within the first biological window of 650-850 nm can be observed.Among them,the emission peak at 688 nm can be attributed to the zero-phonon line related to the ²E?⁴A₂ transition of Cr³⁺.Furthermore,the emission peaks at 704 and 720 nm related to the phonon sideband appear.In the emission spectrum,the emission peaks at 675 and 680 nm belong to the anti-Stokes phonon sideband.In the 0.1%-2.0%region,with increasing Cr³⁺doping concentration,the emission peak intensity exhibits a first rising and then a decreasing trend.Especially,for Lu₃Ga₅O₁₂:1.5%Cr³⁺nanoparticles,the strongest luminescent intensity can be acquired.Meanwhile,the afterglow intensity at 703 nm is the strongest and the afterglow time exceeds 140 s.The calculated crystal field parameter Dq/B is about 4.36(>2.3).This result shows that Cr³⁺occupies a site with a strong crystal field environment.Our theoretical result is consistent with our experimental results.2.On the basis of Lu₃Ga₅O₁₂:0.1%Cr³⁺nanoparticles with the best afterglow performance,single-phased Cr³⁺and Dy³⁺doped Lu₃Ga₅O₁₂nanoparticles were prepared.It is found that,the afterglow intensity of Lu₃Ga₅O₁₂:0.1%Cr³⁺,0.5%Dy³⁺nanoparticles monitored at 25s after stopping 254 nm irradiation,is about 225%of that of Lu₃Ga₅O₁₂:0.1%Cr³⁺nanoparticles.This result shows that the afterglow performance of this sample was improved after co-doping Dy³⁺.The thermoluminescence shows that the thermoluminescenceintensity of Lu₃Ga₅O₁₂:0.1%Cr³⁺,0.5%Dy³⁺nanoparticles reaches the maximum compared to Lu₃Ga₅O₁₂:0.1%Cr³⁺nanoparticles.The above results suggest that Dy³⁺doping in Lu₃Ga₅O₁₂:Cr³⁺nanoparticles can change the electron trap concentration and trap depth,induce the creation of more effective traps,and make the afterglow intensity of Lu₃Ga₅O₁₂:Cr³⁺nanoparticles stronger.