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Design, Preparation, Properties And Applications Of Near-Infrared Long Phosphorescent Phosphors

Posted on:2015-02-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y LiFull Text:PDF
GTID:1261330422981640Subject:Materials science
Abstract/Summary:PDF Full Text Request
Optical imaging has become an indispensable tool in cancer research, clinical translation,and medical practice over the last few decades, owing to its distinct merits of high sensitivity,portability, non-invasiveness, and time effectiveness. There has been a huge increase in thenumber of imaging technologies and their expression vectors. However, there is a noteworthymismatch between the vectors and the technologies. The requirements of most advancedprobes are not meet satisfactorily, as follows:(1) the probes in the region of biologicallytransparent window region (700-1000nm and1100-1350nm) meet the requirement of deeptissue penetration;(2) the excitation wavelength in near-infrared region rather than in Uv-vismeet the requirement of low autofluorescence;(3) lower excitation power is benefit to collectthe imaging with high resolution, weak light disturbance and high signal-to-noise ratio;(4)proving the real-time, holistic, distributed optical imaging. Among all the suggested probes,long persistent phosphors are unique and have already proven their superiority over otherlabels, since their emission lifetime is sufficiently long to permit late time-gated imaging.Persistent phosphorescence is an optical phenomenon, whereby a material is excited with highenergy radiation and the resulting luminescent emission remains visible for an appreciabletime, due to the featured stored ability of electrons. This phenomenon is used in safetysignage, dials and displays and decoration. Yet the previous work about the long persistentphosphors mainly force on the phosphors in visible region, only fewer persistent phosphors innear-infrared region are involved.In this dissertation, we introduce the research developments on long persistent phosphorsin near-infrared region, their luminescent properties, defect properties, special performanceand applications at first. Then we propose a new afterglow model by further exploring anddiscussing the luminescence properties and defects, discover the novel photo-stimulatedluminescence for the first time, experimentally demonstrate a long persistent phosphor innear-infrared and at last observe the persistent phosphorescence with the Zn imperfection asthe recombination luminescence center. The research results in the dissertation can besummarized as follows:(1) A long persistent phosphor, Zn3Ga2Ge2O10:0.5%Cr, with remarkable photoluminescence and long persistent phosphorescence is prepared via the solid statereaction. Two emission bands are observed in emission spectra, assigned to the inter-transitionof Cr3+and defects. We also confirm that five excitation bands were observed in the excitationspectra by taking the measurements of afterglow spectra and decay curves, photocurrentcurves and thermo-luminescence spectra; two of them in the visible region were assigned tothe transitions of Cr3+[4A2â†'4T2] and [4A2â†'4T1]; the excitation peak at260nm should beattributed to the band transition; the excitation peak at290nm should be attributed to thetransition of Cr3+[4A2â†'CB]; And the excitation peak at320nm should be assigned to thetransition of Cr3+[4A2â†'4T2(4P)]. We also discuss the preparation condition and point out thatthe phosphors prepared with longer sintering time, higher sintering temperature and theatmosphere in air or argon have the advanced afterglow properties.(2) Although inorganic anti-Stokes fluorescent probes have long been developed, theoperational mode of today’s most advanced examples still involves the harsh requirement ofcoherent laser excitation, which often yields unexpected light disturbance or evenphoton-induced deterioration during optical imaging. Here, we demonstrate an efficientanti-Stokes fluorescent probe with incoherent excitation. Charging of the probe can beachieved by either X-rays or ultraviolet-visible light irradiation, which enables multiplexeddetection and function integration with standard X-ray medical imaging devices. We discussincoherent anti-Stokes luminescent probing of biological tissue with Zn3Ga2Ge2O10:0.5Cr3+.We show that the high defect capacity of this material enables effective optical charging,before (ex situ) or after (in situ) injection into the analyte. We demonstrate incoherentactivation for large-area (~6cm2) as well as large-depth (~1cm) detection capability. Byregulating the energy level position of the electron reservoir and excitation parameters such asenergy, intensity and duration, it provides tunable decay kinetics. Charging of the probe canbe done by either X-rays or UV-vis light, what enables multiplexed detection and functionintegration with standard X-ray medical imaging devices.(3) There are still many limitations and adventures in the applications, preparations, andafterglow models of near-infrared long persistent phosphors. We designed and successfullyfabricated a NIR powder-form long persistent phosphor, Zn3Ga2Sn1O8:0.5%Cr3+withoutstanding persistence time over300h. To describe the nature of traps in-depth, systematic and multifarious investigations about trap types, concentrations and depths are conductedsuccessively by the measurements of electron spin resonance spectra, positron annihilationlifetime spectroscopy, and thermo-luminescence curves. Moreover, we demonstrated therevived luminescence and persistent phosphorescence under the excitation of near-infraredincoherent light. This optional multiplexed bio-detection mode can enable long-term, repeated,real-time and reliable structural imaging of deep tissues, thereby furthering the prospect ofoptical probes in clinic. Further research about nanocrystallization, functionalization, target ofLPPs, which are expected to open a possibility in the visualization of the structural andfunctional processes in cells, tissues and other complex systems, is still in progress.(4) Long persistent phosphors are unique and have already proven their superiority overother labels in in vivo bio-imaging, since their emission lifetime is sufficiently long to permitlate time-gated imaging. However, the alternative near-infrared long persistent phosphors arein the limitation. A wide variety of activation ions are used as NIR luminescent centres, butwhen it comes to persistent phosphorescence, the numbers of known activators are relativelylow. To address these issues, based on the thorough analysis for the pre-existing persistentphosphors, we propose a holistic design idea for the NIR long persistent phosphors, andsuccessfully fabricate a series of Mn4+-doped MAlO3(M=La. Gd) persistent phosphors withthe emission maximum around730nm. Further verifications and improvements for this ideaare shown by checking the emission waveband, persistent duration, defect types, and defectdepths, based on the measurements of PL spectra, PLE spectra, decay curves, TL curves, andESR spectra. At last, an improvement of persistent time over20h is demonstrated byco-doping Ge4+/Mn4+. The obtained imaging of deep tissues assures that the new luminescentindicators will open the possibility of advanced optical imaging with high resolution andweak light disturbance for understanding the structural and functional processes in cells,tissues and other complex systems.(5) Current discussions about emitting centers in long persistent phosphors are seen tofocus on the discrete luminescent centers, for instance, rare earth ions are a kind of discreteluminescent centers exactly. Except the discrete luminescent centers, it is well known thatrecombination centers are also a kind of important emitting centers, and usually defects actnot only as the traps centers, but also as the emitting centers. Yet there are few reports on the long persistent phosphorescence of defects as emitting centers. Though recombinationphoto-luminescence of Zn imperfections has been extensively investigated [4,5], persistentphosphorescence of Zn imperfections as emitting centers in long persistent phosphors wasrarely obtained. Herein, we observed a long persistent phosphorescence in blue-white visibleregion from6ZnO:3GeO2: Al2O3phosphor with Zn imperfections as emitting centers.Persistent phosphorescence could be observed beyond2h with naked eyes. Traps propertieswere also elaborated by the measurements of thermo-luminescence (TL) spectra andphoto-stimulated luminescence (PSL) decay curves. In addition, a long persistent phosphorwith warm white color could be obtained by doping Cr3+ion into6ZnO:3GeO2: Al2O3phosphor.Finally, we provide a discussion and summary of the main points of the dissertation anddescribe the application prospect of near-infrared long persistent phosphors in various fields.
Keywords/Search Tags:Long persistent phosphorescence, phosphorescence, Cr3+, Mn4+, defectsproperties, in vivo, deep tissue imaging, near-infrared, photo-stimulated luminescence, positron annihilation
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