增强上转换发光的纳米粒子新结构构建及物理性质研究
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摘要
纳米科技是近年来迅速崛起和飞速发展的前沿研究领域。纳米技术和生命科学两大学科的结合与发展使得利用生物大分子及其复合体的检测和模拟成为可能。众所周知，纳米微粒具有大的比表面积，表面原子数、表面能和表面张力随粒径下降急剧增加，从而导致其特殊的热、磁、光、敏感特性和表面稳定性。由于纳米微粒的尺寸一般比生物体内的细胞小得多，可在血液中循环，因而可以对身体各病变部位的进行特异性生物标记和检测（诊断），经过“智能”的纳米组装可实施新颖的特殊治疗，疏通脑血管中血栓，清除心脏动脉脂肪沉积物，甚至用于诱导病毒和癌细胞。纳米技术在生物医学中的应用和发展，将解决当今生物医学尚无法治愈的许多疑难病症，这将导致生物医学及人类健康保障方面发生巨大的变革。
由于稀土离子的特殊电子能级结构和4f电子屏蔽效应，因而其发光性质稳定、发射谱线接近线谱和具有较长的能级寿命等特点。稀土离子掺杂的发光上转换纳米粒子（LUN:RE~(3+)）在红外光激发下，可发生多光子吸收过程，从而将低能量的红外光转换为较高能量的近紫外光、蓝绿光和红光，此过程获得的发光即所谓的上转换发光。
正是由于上转换发光是多光子吸收过程，其发光效率比较低，对其在发光器件、生物成像和光动力治疗方面的应用构成了限制，所以长期以来如何提高其上转换发光效率和强度一直是稀土掺杂的发光材料研究领域的重大科学难题。本文针对稀土掺杂的发光材料研究领域这一重大的科学难题，在合成制备NaYF_4：Yb~(3+)，Tm~(3+)共掺的纳米粒子的研究基础之上，设计和构建发光上转换纳米粒子的新结构，以便研究解决提高上转换发光效率的重要科学问题和获得提高上转换发光效率的新途径。创新地开展了如下的主要研究内容：
1.通过高温热溶剂法研究合成NaYF_4：Yb~(3+)，Tm~(3+)共掺的纳米粒子，通过分别改变Yb~(3+)、Tm~(3+)离子浓度，研究了其优化条件及发光机理，当Tm~(3+)离子浓度从0.2%变化到2%时，发光是先提高后减弱，这是由于Tm-Tm之间的交叉弛豫导致的；Yb~(3+)离子浓度在40%时会出现浓度猝灭现象。在Yb~(3+)和Tm~(3+)在最佳浓度分别为40%和1%的条件，通过在纳米粒子外面包覆不同厚度的NaYF_4同质壳层，发现随着壳层厚度的增加，Tm~(3+)在蓝紫光区域的发光强度远远强于800nm的近红外光。通过实验我们知道，Tm~(3+)在452nm和479nm的发光分别是4光子和3光子过程，其发光强度与中间态能级的寿命呈现四次方和三次方的关系，随着壳层厚度的增加，中间态能级的寿命大大加长导致了上转换效率提高。
2.研究掺杂不同浓度的Li~+离子的NaYF_4：Yb~(3+)，Tm~(3+)纳米粒子的发光优化条件；发展了通过掺杂Li~+离子改进和提高NaYF_4：Yb~(3+)，Tm~(3+)纳米粒子发光上转换强度的新方法。揭示了Li~+离子掺入NaYF_4：Yb~(3+)，Tm~(3+)纳米粒子，当掺杂浓度低于7%时，晶格收缩，结晶度提高，上转换发光强，对应于Li~+离子替位掺杂，当达到7%时，上转换发光强度达到最大。随着Li~+离子浓度的继续提高，晶格开始逐渐膨胀直至接近无掺杂时的状态，此时对应于Li~+离子以填隙和替位二种方式共存。
3.研究裸核（core）NaYF_4：Yb~(3+)，Er~(3+)和核壳结构(core/shell)NaYF_4@NaYF_4：Yb~(3+)，Er~(3+)结构同温度的关及其对上转换发光的影响，揭示同质包覆对上转换发光影响的规律.研究发现520nm谱带主要来源于能级4S3/2的热布居。当温度从12K逐渐增加到300K时545nm谱带随着温度的升高发光先变强后减弱，这是由于热扰动和无辐射弛豫两个过程相互竞争导致的；在低温阶段，热布居占主导作用，温度升高所以发光增强，而当温度继续升高的时候，无辐射弛豫开始增强，导致发光减弱。而包壳可以减少无辐射弛豫的作用，使得发光最强的点向高温方向移动。
In recent years, the nanotechnology has been developed so fast that become thefrontier domains. The combination of nanotechnology and biology makes it possibleto use biomacromolecule and its complex to detect and simulate. It is well knownthat nanoparticals has a large ratio of surface to volume leading to the rapid increaseof the surface atoms, surface energy and surface tension with the size ofnanoparticals decreasing resulting the special properties of heat, light, magnetism,high sensitivity. The nanopartical is much smaller than the size of cells generally andcan circulate in the blood so that they can detect pathological changes and are usedas specific biomarkers. The development and applications of nanotechnology inbiomedicine will overcome complicated disorders incurable before.
The Rare earth (RE) elements are composed of the lanthanide(Ln) series (fromlanthanum to lutetium), yttrium, and scandium.With abundant and unique energylevel structures arisingfrom4fninner shell configurations, Ln~(3+)ions can exhibitsharp fluorescent emission via intra-4f or4f–5d transitions. The luminescencefeatures of Ln~(3+)ions include narrow bandwidth, long-lived emission, large Stokesshifts, ligand-dependent luminescence sensitization which have received startlinginterest because of the continuously expanding need for luminescent materialsmeeting the stringent requirements of telecommunication, lighting, electroluminescent devices,(bio-) analytical sensors, bioimaging set-ups and solarcells. While the lower of upconversion luminescence efficiency attributed to themultiphoton absorbed process limits its applications in luminescent device,bioimaging and photodynamics therapy (PDT), so how to enhance the luminescenceefficiency and intensity is an important frontier research direction which hasattracted significant research interest. This thesis carried on the synthesis andluminescent properties of NaYF_4：Yb~(3+)，Tm~(3+)nanocrystals in order to solve theefficiency and get new approaches to enhance the luminescence efficiency:
(1) NaYF_4： Yb~(3+)， Tm~(3+)nanocrystals were synthesized by thermaldecomposition. We changed the concentrations of Yb~(3+)and Tm~(3+)ions and got theoptimization condition. Under the optimized conditions, the nanocrystals was coateda homogeneous shell of NaYF_4; the effect of different thickness on thecharacteristics of luminescence of Tm~(3+)was also investigated.
(2) NaYF_4:Yb~(3+), Tm~(3+)nanocrystals doped with Li~+ions of various concentrationswere studied in detail. The significant enhancement of the UC emission inNaYF_4:Yb~(3+), Tm~(3+)nanocrystals by introducing Li~+ions without changes in phaseand morphology were observed. In contrast to lithium-free NaYF_4:Yb~(3+), Tm~(3+), theUC emission intensitys of452nm and479nm of the NPs codiped with7ml%Li~+ions were increased by about8and5times respectively. The mechanisms for theenhancement of upconversion emission were also discussd which can be attributedto the improvement of the NPs’crystallinity and the lower local crystal fieldsymmetry around the Tm~(3+).
(3) The upconversion luminescence of core NaYF_4:Yb~(3+),Er~(3+)and core/shellconstruction NaYF_4:Yb~(3+),Er~(3+)@NaYF_4nanocrystals was studied at varioustemperatures from15K to300K. Both the luminescence intensity and peak energywere found to depend on temperature. A nonradiative process plays a key role in theupconversion luminescence, which results in the increase and then decrease in theintensity when temperature is increased.
由于稀土离子的特殊电子能级结构和4f电子屏蔽效应，因而其发光性质稳定、发射谱线接近线谱和具有较长的能级寿命等特点。稀土离子掺杂的发光上转换纳米粒子（LUN:RE~(3+)）在红外光激发下，可发生多光子吸收过程，从而将低能量的红外光转换为较高能量的近紫外光、蓝绿光和红光，此过程获得的发光即所谓的上转换发光。
正是由于上转换发光是多光子吸收过程，其发光效率比较低，对其在发光器件、生物成像和光动力治疗方面的应用构成了限制，所以长期以来如何提高其上转换发光效率和强度一直是稀土掺杂的发光材料研究领域的重大科学难题。本文针对稀土掺杂的发光材料研究领域这一重大的科学难题，在合成制备NaYF_4：Yb~(3+)，Tm~(3+)共掺的纳米粒子的研究基础之上，设计和构建发光上转换纳米粒子的新结构，以便研究解决提高上转换发光效率的重要科学问题和获得提高上转换发光效率的新途径。创新地开展了如下的主要研究内容：
1.通过高温热溶剂法研究合成NaYF_4：Yb~(3+)，Tm~(3+)共掺的纳米粒子，通过分别改变Yb~(3+)、Tm~(3+)离子浓度，研究了其优化条件及发光机理，当Tm~(3+)离子浓度从0.2%变化到2%时，发光是先提高后减弱，这是由于Tm-Tm之间的交叉弛豫导致的；Yb~(3+)离子浓度在40%时会出现浓度猝灭现象。在Yb~(3+)和Tm~(3+)在最佳浓度分别为40%和1%的条件，通过在纳米粒子外面包覆不同厚度的NaYF_4同质壳层，发现随着壳层厚度的增加，Tm~(3+)在蓝紫光区域的发光强度远远强于800nm的近红外光。通过实验我们知道，Tm~(3+)在452nm和479nm的发光分别是4光子和3光子过程，其发光强度与中间态能级的寿命呈现四次方和三次方的关系，随着壳层厚度的增加，中间态能级的寿命大大加长导致了上转换效率提高。
2.研究掺杂不同浓度的Li~+离子的NaYF_4：Yb~(3+)，Tm~(3+)纳米粒子的发光优化条件；发展了通过掺杂Li~+离子改进和提高NaYF_4：Yb~(3+)，Tm~(3+)纳米粒子发光上转换强度的新方法。揭示了Li~+离子掺入NaYF_4：Yb~(3+)，Tm~(3+)纳米粒子，当掺杂浓度低于7%时，晶格收缩，结晶度提高，上转换发光强，对应于Li~+离子替位掺杂，当达到7%时，上转换发光强度达到最大。随着Li~+离子浓度的继续提高，晶格开始逐渐膨胀直至接近无掺杂时的状态，此时对应于Li~+离子以填隙和替位二种方式共存。
3.研究裸核（core）NaYF_4：Yb~(3+)，Er~(3+)和核壳结构(core/shell)NaYF_4@NaYF_4：Yb~(3+)，Er~(3+)结构同温度的关及其对上转换发光的影响，揭示同质包覆对上转换发光影响的规律.研究发现520nm谱带主要来源于能级4S3/2的热布居。当温度从12K逐渐增加到300K时545nm谱带随着温度的升高发光先变强后减弱，这是由于热扰动和无辐射弛豫两个过程相互竞争导致的；在低温阶段，热布居占主导作用，温度升高所以发光增强，而当温度继续升高的时候，无辐射弛豫开始增强，导致发光减弱。而包壳可以减少无辐射弛豫的作用，使得发光最强的点向高温方向移动。
In recent years, the nanotechnology has been developed so fast that become thefrontier domains. The combination of nanotechnology and biology makes it possibleto use biomacromolecule and its complex to detect and simulate. It is well knownthat nanoparticals has a large ratio of surface to volume leading to the rapid increaseof the surface atoms, surface energy and surface tension with the size ofnanoparticals decreasing resulting the special properties of heat, light, magnetism,high sensitivity. The nanopartical is much smaller than the size of cells generally andcan circulate in the blood so that they can detect pathological changes and are usedas specific biomarkers. The development and applications of nanotechnology inbiomedicine will overcome complicated disorders incurable before.
The Rare earth (RE) elements are composed of the lanthanide(Ln) series (fromlanthanum to lutetium), yttrium, and scandium.With abundant and unique energylevel structures arisingfrom4fninner shell configurations, Ln~(3+)ions can exhibitsharp fluorescent emission via intra-4f or4f–5d transitions. The luminescencefeatures of Ln~(3+)ions include narrow bandwidth, long-lived emission, large Stokesshifts, ligand-dependent luminescence sensitization which have received startlinginterest because of the continuously expanding need for luminescent materialsmeeting the stringent requirements of telecommunication, lighting, electroluminescent devices,(bio-) analytical sensors, bioimaging set-ups and solarcells. While the lower of upconversion luminescence efficiency attributed to themultiphoton absorbed process limits its applications in luminescent device,bioimaging and photodynamics therapy (PDT), so how to enhance the luminescenceefficiency and intensity is an important frontier research direction which hasattracted significant research interest. This thesis carried on the synthesis andluminescent properties of NaYF_4：Yb~(3+)，Tm~(3+)nanocrystals in order to solve theefficiency and get new approaches to enhance the luminescence efficiency:
(1) NaYF_4： Yb~(3+)， Tm~(3+)nanocrystals were synthesized by thermaldecomposition. We changed the concentrations of Yb~(3+)and Tm~(3+)ions and got theoptimization condition. Under the optimized conditions, the nanocrystals was coateda homogeneous shell of NaYF_4; the effect of different thickness on thecharacteristics of luminescence of Tm~(3+)was also investigated.
(2) NaYF_4:Yb~(3+), Tm~(3+)nanocrystals doped with Li~+ions of various concentrationswere studied in detail. The significant enhancement of the UC emission inNaYF_4:Yb~(3+), Tm~(3+)nanocrystals by introducing Li~+ions without changes in phaseand morphology were observed. In contrast to lithium-free NaYF_4:Yb~(3+), Tm~(3+), theUC emission intensitys of452nm and479nm of the NPs codiped with7ml%Li~+ions were increased by about8and5times respectively. The mechanisms for theenhancement of upconversion emission were also discussd which can be attributedto the improvement of the NPs’crystallinity and the lower local crystal fieldsymmetry around the Tm~(3+).
(3) The upconversion luminescence of core NaYF_4:Yb~(3+),Er~(3+)and core/shellconstruction NaYF_4:Yb~(3+),Er~(3+)@NaYF_4nanocrystals was studied at varioustemperatures from15K to300K. Both the luminescence intensity and peak energywere found to depend on temperature. A nonradiative process plays a key role in theupconversion luminescence, which results in the increase and then decrease in theintensity when temperature is increased.
引文
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