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稀土离子由于其特异性的4f电子组态内及4f到5d之间的电子跃迁,导致其可产生从紫外可见光区到红外光区的多种波长的光子辐射[1, 2]。特别是,稀土离子掺杂的上转换纳米粒子(UCNPs)具备将两个及两个以上的低能量近红外光子转换为一个高能量光子的特性,特别是其产生的光子发射具有谱带窄,抗漂白,低背景噪声等优点[3]。这种近红外光激发的上转换发光特性产生诸多应用,如超分辨成像,荧光标记,光动力治疗,光学防伪[4-8]。
尽管稀土离子掺杂产生的上转换发光产生诸多应用,但其相对较低的发光强度限制其进一步实际应用。如何增强上转换发光一直是上转换发光研究中的亟待解决的问题。近年来,如核壳纳米结构[9],等离子体场增强发光[10],染料敏化发光[11],实现了上转换发光强度的增强。特别是,染料敏化发光的研究不仅增强了上转换发光的强度,也拓宽了上转换发光的激发范围。相对于稀土离子弱的吸收(吸收系数0.1 到10 M−1 cm−1),改用近红外染料(吸收系数1000 到10000 M−1 cm−1)来吸收近红外光从而实现敏化增强上转换发光。然而大部分近红外染料发射(800到900 nm)和经典的Yb敏化掺杂体系的Yb的吸收(950到1000 nm)的交叠积分较小,从而限制了染料敏化上转换发光的增强。进一步,设计采用Nd,Er离子敏化体系作为染料敏化的受主[12, 13],实现更有效的染料敏化上转换发光的增强。Nd3+敏化上转换发光是近些年发展的上转换发光体系,尤其是其分区掺杂策略可以实现高效的上转换发光[14-16]。然而,目前为止,染料敏化的Nd离子掺杂体系,通常采用的是Nd敏化上转换发光的最佳结构设计。考虑到染料敏化过程中,染料会与稀土发光体系相互作用从而减弱发光[17],因此设计优化染料敏化增强的Nd掺杂的上转换发光体系,将更有效地应用于生化分析、肿瘤诊疗、发光显示等领域[18-22]。
本文设计采用Nd敏化的核/壳/壳结构作为增强染料敏化上转换发光的受主,通过采用高温热分解方法成功制备Nd敏化的核/壳/壳结构,并与染料IR-806分子耦连实现染料敏化上转换发光强度的增强。相关的结构表征证实纳米结构的成功制备。进一步通过发射光谱,荧光寿命光谱分析等研究其背后的增强机制。并且通过优化了最外层壳中Yb离子的掺杂浓度,证实无Yb掺杂情况下染料敏化上转换发光最强。
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氯化镱(YbCl3·6H2O (99.99%), 氯化钇(YCl3·6H2O), 氯化铒(ErCl3·6H2O), 氢氧化钠(NaOH, 98%), 氟化铵(NHF4, 99.99%), IR-780 iodide (99%), 4-巯基苯甲酸 (99%), 1-十八烯 (ODE), 油胺 (90%)(OM) 和油酸 (90%)(OA), 购于 Sigma-Aldrich公司。根据参考文献[14],Nd (CF3COO)3 通过 Nd2O3粉末与过量的三氟醋酸反应,然后蒸发除去过量的三氟醋酸获得。三氟醋酸镱(Yb(CF3COO)3), 三氟醋酸钇(Y(CF3COO)3), 三氟醋酸钠(CF3COONa) 购于 GFS Chemicals。二氯甲烷及三氯甲烷,二甲基甲酰胺(DMF)购买于北京化工厂,所有的化学试剂都是分析纯度。
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该核壳壳层上转换纳米结构基于已发表的化学方法[14, 15],首先合成核结构,YbCl3•6H2O (0.1 mmol), YCl3•6H2O (0.39 mmol) 和ErCl3.6H2O (0.01 mmol) 溶解在 3 mL OA, 7.5 mL ODE的三口瓶中, 并加热到 150 °C,保持 30 分钟, 之后在氩气保护下冷却至室温。然后配置NH4F (2 mmol), NaOH (1.25 mmol) 溶解在5mL甲醇中,并加入到以上稀土盐的三口瓶中并加到70 °C去除甲醇,之后加热到 300 °C 并保持1小时。然后, 加入0.25 mmol NaYF4: Yb (10%) 活性壳在ODE中(通过三氟醋酸盐法合成),加热到以上混合物中并熟化10分钟。然后加入0. 5 mmol NaYF4: Nd (20%) 活性壳(通过三氟醋酸盐法合成),并熟化10分钟。最后溶液冷却到室温,并使用乙醇离心,最终溶解在6mL三氯甲烷中。不同稀土元素掺杂的核壳壳层上转换纳米结构均采用以上类似的方法实现合成。
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根据文献方法[11],类似地,在氮气保护下,将有机IR-780分子 (250 mg), 4-巯基苯甲酸(115.5 mg), 和 DMF (10 mL) 溶解在50 mL三口瓶中。然后混合溶液在氮气环境下维持17小时。产物溶液用0.45 μm PTFE 过滤后,并减压蒸馏去除 DMF。然后将残余物溶解在5 mL二氯甲烷中,并再次通过 0.45 μm PTFE 过滤,并用冰乙醚实施沉淀。最后,将反应物在真空下过滤,在真空下干燥并避光保存。
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类似于我们前面的方法[8, 17],1 mL IR-806 (x mg/mL, x=0 ~ 20 mg/mL) 溶解在三氯甲烷中,并与 1 mL β-NaYF4:Yb/Er(20/2%) @ NaYF4:Yb (10%)@ NaYF4:Nd (20%)纳米粒子混合,其中(Er3+ ~1.67 mM)。整个反应混合物搅拌24小时在室温下,离心,重新分散在1 mL三氯甲烷中。IR-806 敏化的 UCNPs进行上转换光谱测试时,稀土纳米粒子浓度保持在Er3+ 离子浓度约为 16 μM的条件下进行光谱测试。
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透射电镜 (TEM)通过Tecnai G2 F20 S-Twin 电子显微镜测试在电压 200 KV测试。X-ray 衍射 (XRD) 测试通过 Rigaku D/max-2000完成,衍射半径采用 Cu Ka radiation (λ=1.5406 Å)。吸收光谱通过 Maya 2000 光谱仪测试(Ocean optics)。上转换光谱通过外在耦合的808 nm 激光在海洋光学光谱仪(Maya2000)上记录。能谱分析(EDS)通过Hitachi, S-4800 表征。上转换发光的荧光寿命测试采用500 MHz TDS 3052 作为激发光源,通过 OPO(Sunlite 8000)及示波器获得荧光寿命数据。
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采用高温热分解法,高度均匀的Nd3+敏化核/壳/壳上转换NaYF4: Yb/Er (20/2%)@ NaYF4: Yb 10 %)@ NaYF4: Nd (80 %)纳米核结构通过高温热分解法被成功制备。透射电镜照片显示上转换纳米粒子的尺寸均匀,图1统计平均粒径大小表明,核(NaYF4: Yb/Er (20/2%),记为C),核/壳(NaYF4: Yb/Er (20/2%)@ NaYF4: Yb (10 %),记为CS),核/壳/壳(NaYF4: Yb/Er (20/2%)@ NaYF4: Yb (10 %)@ NaYF4: Nd (80 %)),记为CSS)的平均尺寸分别为23.5 nm, 26.3 nm, 33.6 nm。这种逐渐增大的尺寸证实约1.4 nm的Yb过渡层,3.7 nm的Nd敏化纳米壳层逐步生长在NaYF4: Yb/Er纳米核上面。图2(a)显示我们合成的UCNPs为经典的六角相结构(JCPDS-16-0334),EDS证实Nd,Y,Yb等稀土元素有效地掺杂进纳米粒子内(图2(b))。进一步,根据文献的方法(图2(c)),我们合成了IR-806分子,由吸收图可见(图2(d)),其吸收峰位从780 nm移动到806 nm证实IR-806分子的成功合成。进一步,根据我们前面的方法,IR-806分子修饰到上转换纳米粒子表面。(图2(d))显示当IR-806分子修饰到UCNPs上之后,其吸收峰位被IR-806的吸收所掩盖,从而证实染料分子成功修饰到UCNPs上。
图 1 上转换纳米粒子的核(a),核/壳(b),核/壳/壳(c)电镜表征图及尺寸分布(d)
Figure 1. TEM images of core (a), core/shell (b), core/shell/shell (c) and their size distribution (d)
图 2 (a)上转换纳米粒子的核,核/壳,核/壳/壳XRD及标准卡片β-NaYF4 (JCPDS-16-0334, 底部),(b)上转 换CSS的EDS数据,(c)IR-806 合成过程图,(d)合成前后IR-780 和 IR-806的吸收,(e)连接IR-806之后UCNPs吸收和UCNPs本身的吸收
Figure 2. (a) XRD data of C, CS, CSS and the β-NaYF4 (JCPDS-16-0334, bottom), (b) EDS data of the CSS, (c) process of synthesis of IR-806, (d) absorption of IR-780 and IR-806, (e) absorption of UCNPs and dye UCNPs
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本文如图3(a)所示,我们设计的染料敏化CSS结构的上转换,相对于最早报道的经典的IR-806敏化NaYF4: Yb/Er (20/2%)纳米粒子[11],其上转换发光强度增强了约38倍,证实我们设计的这种染料敏化结构实现了上转换发光强度的增强。另外,在近红外808 nm光激发下,染料敏化CSS结构的上转换红光发射与绿光发射均表现出发光强度随激发光功率的非线性依赖特性(图3(b)),其多光子指数分别为1.67(绿光540 nm发射4S13/2→4I15/2),2.0(红光655 nm发射4F9/2→4I15/2),证实其发光特性为非线性的上转换发光。
图 3 (a)IR-806敏化CSS结构的上转换光谱及IR-806敏化核结构的上转换光谱,激发波长808 nm,(b)808 nm激发下的IR-806敏化的CSS结构的上转换发光强度随功率变化的log-log关系
Figure 3. (a) The UCL of IR-806 sensitized CSS nanostructure and IR-806 sensitized core NPs,excitation wavelength:808 nm, (b) log-log plots of the UCL intensity over laser power for the green and red emissions of the dye-sensitized CSS
我们设计的染料敏化CSS结构相对于染料敏化的核纳米粒子产生约38倍的上转换发光强度的增强,这种增强我们分析,一方面,在CSS纳米结构中最外层为Nd3+离子掺杂壳层,而图4(a)可见,Nd的吸收与IR-806染料分子的发射有大量的交叠,因而能够被IR-806分子高效敏化。另一方面,CSS结构中的纳米壳层可以有效保护发光中心。图4(b)可见,染料敏化的CSS结构的Er的发光寿命(253 μs)明显长于染料敏化核纳米粒子(146 μs)及核纳米粒子(169 μs)的Er的寿命,Er发光寿命值分别延长了1.73倍及1.50倍,证实了纳米壳层隔绝了发光中心与外部干扰环境,从而保持发光中心的产生长的发光寿命。值得注意的是,尽管我们设计的染料敏化Nd3+离子掺杂纳米结构较佳的激发波长在800 nm附近,但是在研究发光寿命时,NaYF4: Yb/Er
图 4 (a)IR-806分子的发射光谱与Nd3+的吸收交叠图,(b)核纳米粒子(黑色,NaYF4: Yb/Er (20/2%)核纳米粒子),染料敏化核纳米粒子(红色)及染料敏化的CSS纳米结构(蓝色)的Er3+ (4S3/2→4I15/2)的寿命测试在980 nm激发下,(c)CSS纳米结构及染料敏化的CSS纳米结构的Er3+ (4S3/2→4I15/2)的寿命测试在808nm激发下
Figure 4. (a) The emission spectrum of IR-806 overlapped with the absorption spectrum of Nd3+, (b) the lifetimes of Er3+ (4S3/2→4I15/2) for core nanoparticles (black), IR-806 sensitized core nanoparticles (red), and IR-806 sensitized CSS (blue) under 980 nm excitation, (c) the lifetimes of Er3+ (4S3/2→4I15/2) for CSS, and IR-806 sensitized CSS under 808 nm excitation
我们设计的这种CSS结构,相对于传统(20/2%)核纳米粒子只能被980 nm光激发产生上转换发光,因此图4(b)的测试激发波长选择为980 nm而非通常采用的808 nm。进一步图4(c)显示,对于CSS纳米结构,连接染料分子与否,其发光寿命保持不变,进一步证实纳米壳层有效地阻隔了发光中心与外界环境的相互作用,从而增强了上转换发光。
Nd3+敏化体系的不同之处在于,其最外层的纳米壳层中,我们仅仅掺杂了Nd3+离子,而没有像以往文献报道的Nd-Yb共掺杂到纳米壳层中[16]。这种结构设计是根据我们的实验结果所得。如图5所示,随着最外层Yb3+ 掺杂浓度的增加,染料敏化CSS结构的上转换发光强度反而逐渐减弱。这种掺杂Yb3+反而降低发光的现象可由最近我们发表的文章进行解释。根据我们前面的研究,染料敏化稀土上转换纳米体系,染料吸收的激发能需要逐步传递到内部的发光中心[17]。而这个激发能传递过程中,激发能迁移到表面的能量损耗非常大,掺杂Yb3+离子极易将激发能传递到表面[9, 23, 24],从而降低传递到内部的激发能能量,从而降低上转换发光。因此,如图5所示,当最外层的纳米壳层中,在不掺杂Yb3+离子的情况下,产生最强的染料敏化上转换发光。
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进一步,我们将CSS结构中,核内的发光中心,换为Ho (NaYF4: Yb/Ho (20/1%))@ NaYF4: Yb (10%)@ NaYF4: Nd (80 %))或Tm (NaYF4: Yb/Tm (20/1%)@ NaYF4: Yb (10%)@ NaYF4: Nd (80 %)),同样实现了染料敏化上转换发光增强的现象(图6(a)和(b))。对于发光中心为Ho及Tm,同样展现出经典的发光强度随激发光功率非线性的功率依赖关系(图6(c)和(d))。对于发光中心为Ho的情况,其多光子指数分别为1.57(540 nm发射4S13/2→4I15/2),1.88(645 nm发射4F9/2→4I15/2),对于发光中心为Tm的情况,其多光子指数分别为2.82(450 nm发射1D2→3F4),1.74(470 nm发射1G4→3H6),1.80(645 nm发射1G4→3F4),1.34(695 nm发射3F2→3H6)。值得注意的是,对于Tm离子,NaYF4: Yb/Tm (20/1%)纳米核结构下,几乎没有染料敏化的上转换发光出现,这是由于Tm的800 nm发射能级(3H4→3H6)与IR-806分子的吸收交叠严重,从而猝灭了Tm的发光。而CSS结构,外面的壳层成功的阻隔了Tm向IR-806的传递,从而实现了染料敏化上转换发光。
图 6 (a)IR-806敏化Ho核结构及IR-806敏化Ho-CSS结构的上转换光谱,(b)IR-806敏化Tm核结构及IR-806敏化Tm-CSS结构的上转换光谱,(c)808 nm激发下的IR-806敏化的Ho-CSS结构的上转换发光强度随功率变化的log-log关系,(d)808 nm激发下的IR-806敏化的Tm-CSS结构的上转换发光强度随功率变化的log-log关系
Figure 6. (a) The UCL of the IR-806 sensitized Ho core nanostructure and IR-806 sensitized Ho-CSS nanostructure, (b) the UCL of the IR-806 sensitized Tm core nanostructure and IR-806 sensitized Tm-CSS nanostructure, (c) log-log plots of the UCL intensity over laser power for the green and red emissions of the dye-sensitized Ho-CSS under 808 nm excitation, (d) log-log plots of the UCL intensity over laser power for the green and red emissions of the dye-sensitized Tm-CSS under 808 nm excitation
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成功制备了高度均匀的NaYF4: Yb/Er (20/2%)@ NaYF4: Yb (10 %)@ NaYF4: Nd (80 %)上转换纳米粒子,其染料敏化上转换发光强度相对于染料敏化的NaYF4: Yb/Er (20/2%)核纳米结构上转换发光强度增强了约38倍。进一步研究表明,这种增强一方面源自最外层Nd吸收与染料IR-806分子的发射交叠大导致其能有效地吸收染料的激发能。另一方面源自纳米壳层对发光中心的保护作用,相对于染料敏化的核纳米结构其发光寿命延长了1.73倍。通过改变最外层不同Yb3+离子的掺杂浓度,证实掺杂Yb3+离子将导致染料敏化上转换发光减弱,而无掺杂Yb3+的条件下上转换发光最强。进一步,这种染料敏化的CSS结构实现了增强发光中心为Ho及Tm的染料敏化上转换发光强度的增强。
Enhanced Dye-sensitized Upconversion Luminescence Based on Nd3+-sensitized Multi-shell Nanostructures
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摘要: 对于稀土离子掺杂的上转换发光,由于稀土离子吸收截面小、吸收范围窄的因素,导致其发光强度受限。最近,通过在稀土上转换纳米粒子的表面连接近红外染料分子敏化发光的方式,被证实是提高上转换发光强度的有效策略。然而,仅仅将染料分子连接经典的稀土Yb掺杂纳米粒子,并不能有效利用染料分子的敏化能力。针对这一问题,我们通过高温热分解法成功制备了Nd3+敏化的核/壳/壳 (NaYF4: Yb/Er (20/2%)@ NaYF4: Yb (10 %)@ NaYF4: Nd (80 %))纳米结构,与经典的IR-806敏化的NaYF4: Yb/Er纳米结构相比,IR-806敏化的Nd3+掺杂的核/壳/壳纳米结构的上转换发光(500到700 nm)强度增强了约38倍。通过荧光光谱及荧光寿命分析证实,上转换发光强度增强源于Nd的吸收与近红外染料分子的有效交叠,以及壳层结构对发光中心的保护作用(Er3+ (4S3/2→4I15/2)的寿命延长了1.7倍)。另外,研究发现纳米壳层结构中最外层掺杂Yb3+离子将导致染料敏化发光的减弱。进一步,这种IR-806敏化的Nd掺杂的核/壳/壳纳米结构可实现增强发光中心为Ho及Tm的上转换发光,为提高染料敏化上转换发光及应用提供了新途径。Abstract: Lanthanide ion doped upconversion luminescence is limited by the small absorption cross-section and narrow absorption band of lanthanide ions, which results in weak luminescence. Recently, a dye-sensitized method has proven to be an effective strategy of increasing upconversion luminescence. However, simply attaching dye molecules to nanoparticles with classic Yb-doped nanostructures cannot effectively activate the sensitizing ability of the dye molecules. In response to this problem, we designed Nd-sensitized core/shell/shell (NaYF4: Yb/Er (20/2%)@ NaYF4: Yb (10 %)@ NaYF4: Nd (80 %)) nanostructures, which compared with the classic IR-806 sensitized NaYF4: Yb/Er nanostructure, their upconversion luminescence (500 to 700 nm) was approximately enhanced by a factor of 38. Through analysis of the nanostructure’s emission and luminescence lifetime data, the enhancement was confirmed by the effective overlap of Nd absorption with the emission of near-infrared dye molecules and the protective effects of the shell structure on the luminescent center (the lifetime of Er (4S3/2→4I15/2) was increased by 1.7 times). In addition, we found that the doping Yb3+ in the outermost layer will decrease the dye-sensitized luminescence intensity. Furthermore, this Nd-sensitized core/shell/shell also achieved enhancement in the sensitized upconversion luminescence of the luminescence centers of Ho and Tm, which establishes a foundation for enhanced dye-sensitized upconversion luminescence.
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Key words:
- upconversion luminescence /
- dye-sensitized /
- lanthanide ion /
- nanoparticles
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图 2 (a)上转换纳米粒子的核,核/壳,核/壳/壳XRD及标准卡片β-NaYF4 (JCPDS-16-0334, 底部),(b)上转 换CSS的EDS数据,(c)IR-806 合成过程图,(d)合成前后IR-780 和 IR-806的吸收,(e)连接IR-806之后UCNPs吸收和UCNPs本身的吸收
Figure 2. (a) XRD data of C, CS, CSS and the β-NaYF4 (JCPDS-16-0334, bottom), (b) EDS data of the CSS, (c) process of synthesis of IR-806, (d) absorption of IR-780 and IR-806, (e) absorption of UCNPs and dye UCNPs
图 3 (a)IR-806敏化CSS结构的上转换光谱及IR-806敏化核结构的上转换光谱,激发波长808 nm,(b)808 nm激发下的IR-806敏化的CSS结构的上转换发光强度随功率变化的log-log关系
Figure 3. (a) The UCL of IR-806 sensitized CSS nanostructure and IR-806 sensitized core NPs,excitation wavelength:808 nm, (b) log-log plots of the UCL intensity over laser power for the green and red emissions of the dye-sensitized CSS
图 4 (a)IR-806分子的发射光谱与Nd3+的吸收交叠图,(b)核纳米粒子(黑色,NaYF4: Yb/Er (20/2%)核纳米粒子),染料敏化核纳米粒子(红色)及染料敏化的CSS纳米结构(蓝色)的Er3+ (4S3/2→4I15/2)的寿命测试在980 nm激发下,(c)CSS纳米结构及染料敏化的CSS纳米结构的Er3+ (4S3/2→4I15/2)的寿命测试在808nm激发下
Figure 4. (a) The emission spectrum of IR-806 overlapped with the absorption spectrum of Nd3+, (b) the lifetimes of Er3+ (4S3/2→4I15/2) for core nanoparticles (black), IR-806 sensitized core nanoparticles (red), and IR-806 sensitized CSS (blue) under 980 nm excitation, (c) the lifetimes of Er3+ (4S3/2→4I15/2) for CSS, and IR-806 sensitized CSS under 808 nm excitation
图 6 (a)IR-806敏化Ho核结构及IR-806敏化Ho-CSS结构的上转换光谱,(b)IR-806敏化Tm核结构及IR-806敏化Tm-CSS结构的上转换光谱,(c)808 nm激发下的IR-806敏化的Ho-CSS结构的上转换发光强度随功率变化的log-log关系,(d)808 nm激发下的IR-806敏化的Tm-CSS结构的上转换发光强度随功率变化的log-log关系
Figure 6. (a) The UCL of the IR-806 sensitized Ho core nanostructure and IR-806 sensitized Ho-CSS nanostructure, (b) the UCL of the IR-806 sensitized Tm core nanostructure and IR-806 sensitized Tm-CSS nanostructure, (c) log-log plots of the UCL intensity over laser power for the green and red emissions of the dye-sensitized Ho-CSS under 808 nm excitation, (d) log-log plots of the UCL intensity over laser power for the green and red emissions of the dye-sensitized Tm-CSS under 808 nm excitation
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