The Effect Of Surface Defects On The Luminescent Properties In Nanophosphors

Persistent luminescence is a unique optical phenomenon that afterglow can remain for seconds even months after the stoppage of excitation.With their un ique luminescent properties,persistent phosphors have long been used in areas including security signs,traffic signage dials,and decoration.In the past few years,with the development of persistent luminescence nanoparticles(PLNPs),the application of persistent phosphors have been extended to biosensing,bioimaging,and cancer treatment.In biosensing and bioimaging,PLNPs with long-lived luminescence can eliminate the background fluorescence interference and light scattering interference through collecting the persistent luminescence signals after removing the external excitation.So outstanding signal-to-background ratio and exquisite sensitivity can be obtained.As for cancer treatment,particularly in photodynamic therapy(PDT),the excitation energy can be trapped by PLNPs and slowly released to activate photosensitizers after the excitation ceases.In this way,a single dose of excitation can continuously generate reactive oxygen species for cancer treatment.So the tissue damage associated with continuous irradiation in traditional PDT can be greatly minimized.It is generally accepted that persistent luminescence is originated from the crystal defects in phosphors.Defects refer to the irregular arrangement of atoms in the crystal,such as vacancies,interstitials and antisite defects.Upon excitation with high energy light sources,charge carriers(electron-hole pairs)are generated in the persistent phosphors.The charge carriers can be trapped by the crystal defects,leading to the separation of electron-hole pairs.Under thermal,optical or mechanical stimulation,the trapped charge carries can get out from the crystal defects and further recombine to generate the persistent luminescence.Researchers have shown that persistent luminescence can be enhanced with the increased amounts of crystal defects in the phosphors,while high density of crystal defects will quench the persistent luminescence due to the concentration quenching effect.Although much efforts have been made in studying the relat ionships between crystal defects and persistent luminescence,the effects of surface defects on persistent luminescence in persistent phosphors,particularly in PLNPs,remains largely unexplored.Surface defects are widely existed on PLNPs and influence th e luminescence properties of PLNPs.Studying how the surface defects regulate persistent luminescence in PLNPs can contribute to the research on persistent luminescence mechanism and provide valuable instruction for the optimization of persistent luminesce nce in PLNPs.Moreover,the biomedical applications of PLNPs including bioimaging and cancer treatment can also be greatly promoted.In this thesis,the zinc gallogermanate PLNPs with near-infrared emission were used as the model.Thermal treatment and sur face coating were employed to passivate the surface defects on ZGGO:Cr PLNPs,respectively.The effect of surface defects on persistent luminescence in PLNPs were explored.The main contents are as follows:(1)The Zn_1.2Ga_1.6Ge_0.2O₄:0.5%Cr(ZGGO:Cr)PLNPs were synthesis through hydrothermal method.Thermal treatment were employed to obtain the thermal ly treated ZGGO:Cr(T-ZGGO:Cr)PLNPs.We explored the structural and optical characteristics of ZGGO:Cr and T-ZGGO:Cr PLNPs through a series of representational methods.Results show that thermal treatment can efficiently enhance persistent luminescence intensity and prolong luminescence decay time in PLNPs,which can be attributed to the efficient surface defect passivation by thermal treatment.Then,we explored the application of PLNPs in background-free bioimaging.The T-ZGGO:Cr PLNPs display a increased signal-to-background ratio and sensitivity than ZGGO:Cr PLNPs in in vivo bioimaging.(2)The ZGGO:Cr@ZGO and ZGGO:Cr@Si O₂ PLNPs were obtained through surface coating on ZGGO:Cr PLNPs,respectively.The structur al and optical characteristics of ZGGO:Cr@ZGO and ZGGO:Cr@Si O ₂ PLNPs were explored through a series of representational methods.Surface coating efficiently enhanced persistent luminescence intensity and prolonged luminescence decay time in PLNPs,which can be attributed to the efficient surface defect passivation by surface coating.Then,we explored the application of PLNPs in background-free bioimaging.Surface coated PLNPs showed a increased signal-to-background ratio and sensitivity than ZGGO:Cr PLNPs in bioimaging.Moreover,ZGGO:Cr@ZGO PLNPs showed a great application prospects in information encryption with its bright Pers L.