摘要
最近几十年光学成像由于其独特的高灵敏度、可移植性、非侵害性和时间有效性等优点，已经在癌症研究、临床手术、医疗研究等领域引起了极大关注，并逐渐发展成为一种独立的检测技术。伴随着高精度、高质量光学成像要求的提出，很多新的成像设备、成像方法和荧光探针也不断涌现，但是现在光学成像的发展处于一个瓶颈期，原因在于荧光探针的发展跟不上成像设备和成像技术的发展需求。现代医学光学检测要求（1）荧光探针的工作区域为生物透过窗口（700-1000nm和1100-1350nm）的近红外光区工作以便增加检测光的深组织透过率；（2）荧光探针的激发光不宜采用紫外光和可见光，以避免组织的自体荧光；（3）荧光探针的激发光强度不宜太强，以便降低背景信号，增大成像的信噪比；（4）能够提供实时、整体、分布式的观测结果。纵观所有潜在的荧光探针，只有近红外长余辉发光材料能够很好的满足上述要求。长余辉材料由于其独特的结构特点，使得这类材料能够在体内存储大量电子和能量，从而在激发停止后依然能获得较长时间的光发射。这种现象已经被广泛应用在了安全指示、仪器仪表显示、装饰装潢等领域。但是早先的长余辉发光材料是集中在可见区域的，近红外发光的长余辉材料的种类十分稀有。
本文首先系统的阐述了长余辉发光材料的历史、种类、理论模型、测试方法、应用领域以及相关的研究进展。然后我们通过对已经报道的长余辉发光材料的探索和剖析，进一步解释了其余辉发光机理；在此基础上发现了这种材料体系中存在的新光学现象；在此基础上我们开发了一种新的近红外长余辉发光材料；并且提出了一种新的具有近红外长余辉发光的激活离子；提出了一种缺陷作为复合发光中心的长余辉发光材料。具体的研究内容和结果包括以下几个方面：
（1）通过固相反应法制备了Zn3Ga2Ge2O10:Cr发光材料，并且观察到了近红外荧光和长余辉发射。确认了在荧光发射光谱中发现了两个发射峰位于近红外区域的覆盖698尖峰的宽带发射峰650-900nm，和位于可见区域的400-600nm的宽带发射峰：分别与Cr离子跃迁和缺陷复合跃迁有关。通过荧光光谱、长余辉光谱、热释光光谱、光激励发光光谱、光电流曲线等测试，确认了在荧光激发光谱中存在5个激发峰的归属：可见区域的两个激发峰归属为Cr离子4A2→4T2和4A2→4T1跃迁，紫外区域的三个激发峰中260nm属于带间跃迁，290nm属于4A2→CB激活离子到导带的跃迁，320nm属于Cr离子4A2→4T2(4P)的跃迁，并基于上述观察的现象提出了一个不同于以往的余辉发射模型。并通过工艺探索，发现样品制备时间越长、制备温度越高、制备气氛为空气或者氩气时，制备的样品余辉初始强度越大，余辉时间越长。
（2）针对现在使用的近红外光学成像方式依然是基于相干光激发的问题，本研究通过固相反应法制备了Zn3Ga2Ge2O10:Cr发光材料，并且首次发现了该体系中存在的近红外的光激励荧光和长余辉发光，并且这种光激励发光可以在近红外相干光和非相干光激发下获得。这种光激励发光具有波长选择性、光发射稳定性、可重复激发性、激发功率可调谐性、激发时间可调谐性、通过紫外光和X光再次充电等优点。生物成像演示了完美的可循环利用的成像模式：先用紫外光对缺陷充电，获得的近红外长余辉发光能够很好的用于纳米探针的前期观测（包括分布式检测、靶向检测）；经过一段时间后长余辉衰减至某一强度时，检测效果较差或需要定量观测时，用近红外LED灯作为辐照源可以重新获得发光强度较强的近红外荧光和余辉发光；最后当存储的电子耗尽后，通过X射线的辐照重新填满陷阱，又可以获得新一轮的成像观测。
（3）针对近红外长余辉发光材料基质种类少、在应用、制备、机理模型上还存在缺陷等问题，在对已有长余辉性能评价方式、设计方式掌握的基础上，本研究通过固相反应法首次发现并制备了一种新的长余辉发光材料，Cr掺杂的Zn3Ga2Sn1O8粉末发光材料，余辉时间长于300小时，并且通过荧光光谱、长余辉光谱、长余辉衰减曲线、ESR谱、正电子湮没谱、热释光光谱、光激励发光光谱等测试手段综合证明其余辉时间、缺陷浓度、缺陷深度等各个缺陷参数均优于Cr掺杂的Zn3Ga2Ge1O8粉末发光材料。此发光材料也可以在近红外非相干光激发下获得长期、可重复的近红外光光激励荧光和长余辉，深组织成像显示长余辉成像和光激励发光成像的有效结合可以发展一种长期的、实时的、可靠的、稳定的、多路复用的探测方式，从而丰富了生物光学成像的模式。
（4）针对近红外长余辉发光激活离子少的问题，本研究提出了一个合理的设计近红外长余辉发光材料的思路，并且通过固相反应法首次发现并制备了一系列的Mn4+掺杂的LaAlO3和GdAlO3发光材料，这些发光材料的发射峰位于730nm、719nm附近。通过ESR和热释光等研究了Mn4+掺杂发光材料的缺陷性质，指出带正电荷的缺陷[MnAl]和负电荷缺陷铝空位[VAl]在余辉发射中起主要作用。通过共掺Ge4+最终获得一个余辉发光长达20小时的LaAlO3:0.1%Mn4+,0.9%Ge4+长余辉发光材料和GdAlO3:0.1%Mn4+,0.9%Ge4+长余辉发光材料。这些材料注射进猪肉组织中获得的深组织成像图片，证明这种离子能够作为一种优良的深组织成像荧光探针来实现细胞、组织、活体系统的成像。
（5）针对现在的研究主要关注分立发光中心的长余辉发光，而忽视了复合发光中心长余辉发光特性的问题，本研究通过高温固相法制备了6ZnO:3GeO2: Al2O3发光材料，首次观察到了Zn缺陷作为复合中心的长余辉发光现象。在254nm激发下观察到了350nm到800nm超宽带发光，峰值位于480nm，这个发射带源于缺陷跃迁，即Ge填隙缺陷(Gei)、Zn填隙缺陷(Zni)、氧空位(VO)到锌空位(VZn)的跃迁。余辉发光为青白色，余辉时间长达2小时，但是发光峰随着时间的延长从480nm红移到550nm处。将Cr掺杂进入6ZnO:3GeO2: Al2O3发光材料中可以获得暖白光长余辉发光。
最后对本文的重点进行了讨论和总结，并对近红外长余辉发光材料的性能和应用前景进行了展望。
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+[4A2→4T2] and [4A2→4T1]; the excitation peak at260nm should beattributed to the band transition; the excitation peak at290nm should be attributed to thetransition of Cr3+[4A2→CB]; And the excitation peak at320nm should be assigned to thetransition of Cr3+[4A2→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.
引文
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