Design And Spectral Properties Of Low-energy Light Excitable Near-infrared Persistent Phosphors

Persistent luminescence is a phenomenon whereby luminescence can last for minutes to hours,or even days after the stoppage of the excitation.Persistent phosphors are being widely used as emergency route signs,traffic signage,dials and displays,night-vision surveillance and bioimaging.Due to the excellent persistent luminescence performance and the great potential in bioimaging,near-infrared persistent phosphors,especially Cr3+ doped gallate-based persistent phosphors,have drawn great attentions from researchers.Traditional fluorescence imaging has become an important method in bioimaging,owning to its advantages of high-sensitivity,non-invasive,easy-to-operate,safe and low-cost.However,fluorescence imaging requires external excitation,which produces strong tissue autofluorescence.The autofluorescence increases the imaging noise,decreases the signal-to-noise ratio(S/N)and affects the quality of imaging and depth of the imaging.An ideal fluorescence probe should emit light without the need of external excitation,which can significantly decrease the tissue autofluorescence,and therefore greatly increase the S/N ratio.For persistent luminescent probe,it can emit persistent light without in situ external excitation.Therefore,persistent phosphors,particularly the phosphors capable of emitting near infrared and short-wave infrared persistent luminescence,are becoming the ideal optical probes.However,usually persistent luminescence can only be effectively excited by high-energy light(UV light and some blue light),which is harmful to normal tissues and has very short tissue penetration.Once the probes are inside a living subject,it is not possible to recharge them using UV light.Therefore,before injection into an animal,the persistent probes need to be pre-irradiated using high-energy UV light,which limits the flexibility and scopes of the applications.In addition,due to the small size of the persistent luminescent imaging probes(typically<100 nm),the longitivity of luminescence in vivo is short,about only several minutes to several hours,which is insufficient for imaging molecular targets and cell tracking,where a detection windows of days or even weeks is required.Therefore,it is urgent to develop new excitation techniques,where the persistent nanoprobes can be effectively charged by high tissue penetration,low-energy visible light or near-infrared light,or to develop new persistent phosphors that can be charged by low-energy,low-intensity general lighting sources.To address the limitations of current persistent luminescence based bioimaging,in this dissertation,we proposed and developed two low-energy light excitation techniques,including the photon-assited upconverted persistent luminescence technique using near-infrared laser diodes excitation and the upconversion charging technique using visible-light laser diodes excitation.We also developed a persistent luminescent materials that can be effectively charged by general lighting sources such as incandescent bulbs and fluorescent lamps.The main results in the dissertation are summarized below.(1)By combining the phonon-assisted anti-stokes photoluminescence with the conventional persistent luminescence,we proposed a phonon-assisted upconverted persistent luminescence(PA-UCPL)technique.In PA-UCPL,the phonon energy caused by lattice viberation acts as a part of the exciting agent to lower the required photon excitation energy to charge the electron traps in persistent phosphors.Based on the PA-UCPL,we achieved NIR persistent luminescence in Zn3Ga2GeO8:Cr3+persistent phosphors by using low-energy near-infrared laser diodes excitation.With the assistance of phonon energy,a low-energy 800 nm laser diode can produce strong 700 nm persistent luminescence in Zn3Ga2GeO8:Cr3+ phosphors at room temperature(20 ℃).The intensity of the persistent luminescence increases as the temperature increasesat 70 ℃;the intensity of persistent luminescence is about 25 times of that at 20 0C.Moreover,when the temperature is raised to 120 ℃,the persistent luminescence of Zn3Ga2GeO8:Cr3+ phosphor can even be produced by a 980 nm laser diode excitation.(2)Through the analysis of the absorption state of Cr3+ ion,we proposed a new two-photon up-conversion charging(UCC)concept to charge Cr3+-activated persistent phosphors using low-energy visible-light laser diodes..We show that when irradiated by a 635 nm laser diode for 10 s,LiGasO8:Cr3+ persistent phosphor exhibits strong persistent luminescence at 716 nm,which lasts for more than 50 h.The power of laser diode and the intensity of persistent luminescence exhibits a quadratic relationship,which clearly reveals that two 635 nm photons are absorbed in order to pump the Cr3+system to the high-energy delocalization state to charge the traps.Remarkably,the intensity of the persistent luminescence produced by the 635 nm laser diode excitation is comparable to that produced by the high-energy 335 nm UV light excitation.The efficient UCC capability by low-energy visible-light laser diodes excitation represents a breakthrough in the field of persistent luminescence.(3)By analyzing the UCC process in LiGa5O8:Cr3+,we generalize the two requirements to the UCC-enabling ions;that is,a long-lifetime metastable state and a high-energy delocalization state.According these two requirements,we propose that several other activator ions,such as rare-earth Pr3+ ions and transition metal Mn2+ and Mn4+ ions,are also capable of UCC.We fabricated MgGeO3:Pr3+,MgGeO3:Mn2+ and LaGaO3:Mn4+ persistent phosphors and observed strong UCC persistent luminescence in these phosphors.For examples,after the excitation of a 450 nm laser diode,the MgGeO3:Pr3+ phosphor shows strong UCC persistent luminescence at 625 nm When excited by a 635 nm laser diode,the MgGeO3:Mn2+ phosphors exhibits intense UCC persistent luminescence at 680 nm.The LaGaO3:Mn4+ phosphor gives UCC persistent emission at 712 nm after the excitation by a 532 nm laser diode.In these persistent phosphors,the laser output power and the intensity of persistent luminescence exhibitsa quadratic relationship,showing a two-photon absorption process.These results indicate that the UCC is a common phenomenon in persistent phosphors containing UCC-enabling activator ions.(4)CaTiO3:Pr3+ is an excellent red persistent phosphor.In this research,we-discovered that the CaTiO3:Pr3+ phosphor also emits strong near-infrared and short-wave infrared persistent luminescence.Remarkedly,we found that the persistentluminescence of CaTiO3:Pr3+ phosphor can be effectively produced by low-energy,low-intensity general lighting sources,such as incandescent bulb,fluorescent lamps,LED flashlight,LED lamps,etc.The ability of the energy traps in CaTiO3:Pr3+ being effectively charged by general lighting sources is attributed to the existence of charge transfer state in the material,i.e.,the Pr3+-O-Ti4+(?)Pr4+-O-Ti3+ charge transfer band..Moreover,the energy traps in CaTiO3:Pr3+ can also be charged through the excitationfrom the ground state to the 3PJ state or even to the 1D2 state.Using pork as the tissue model,we studied the excitability of the CaTiO3:Pr3+ phosphor using general lighting sources.Finally,based on the research in this dissertation,we outline the prospects of future work in low-energy light excitation of persistent phosphors.