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Orthogonal luminescence properties of a single rare-earth activator ion doped upconversion nanoparticles

LING Xiao MEI Qing-song

凌枭, 梅青松. 单一稀土激活剂离子的正交上转换发光特性[J]. 中国光学, 2020, 13(5): 1158-1170. doi: 10.37188/CO.2020-0020
引用本文: 凌枭, 梅青松. 单一稀土激活剂离子的正交上转换发光特性[J]. 中国光学, 2020, 13(5): 1158-1170. doi: 10.37188/CO.2020-0020
LING Xiao, MEI Qing-song. Orthogonal luminescence properties of a single rare-earth activator ion doped upconversion nanoparticles[J]. Chinese Optics, 2020, 13(5): 1158-1170. doi: 10.37188/CO.2020-0020
Citation: LING Xiao, MEI Qing-song. Orthogonal luminescence properties of a single rare-earth activator ion doped upconversion nanoparticles[J]. Chinese Optics, 2020, 13(5): 1158-1170. doi: 10.37188/CO.2020-0020

单一稀土激活剂离子的正交上转换发光特性

doi: 10.37188/CO.2020-0020
详细信息
  • 中图分类号: TP394.1; TH691.9

Orthogonal luminescence properties of a single rare-earth activator ion doped upconversion nanoparticles

Funds: Supported by National Natural Science Foundation of China (No. 21675038)
More Information
    Author Bio:

    LING Xiao (1994—), male, born in Wangjiang Country, Anhui Province. In 2013-2017, he studied in the pharmaceutical engineering major, School of Chemistry and Chemical Engineering, Hefei Normal University and obtained the bachelor's degree. Since 2017, he has been studying in the School of Food and Biological Engineering, Hefei University of Technology as a graduate student, while conducting the research on the up-conversion nanoparticles doped with rare earth. E-mail: 18755132335@163.com

    MEI Qing-song (1986—), male, doctor, associate professor and master supervisor. In 2012, he received his doctorate from the University of Science and Technology of China. He is currently engaged in the preparation of up-conversion luminescence nanomaterials and the research of their application in biomedical imaging. E-mail: qsmei@hfut.edu.cn

    Corresponding author: qsmei@hfut.edu.cn
  • 摘要: 上转换发光纳米材料由于其特有的光学性质而一直备受关注,但常见的上转换发光纳米材料多为单色发光,为了实现上转换多色正交发光,同时避免多种掺杂剂离子的合成复杂性以及相互间的干扰性,利用Er3+2H11/2,4S3/24I15/2能级跃迁产生的绿色发光和4F9/24I15/2能级跃迁产生的红色发光,设计出由Er3+单个激活剂离子掺杂的双激发上转换纳米颗粒。通过热分解法一步步合成出NaErF4:Yb(19.5%)/Tm(0.5%)@NaYF4:Yb(10%)@NaNdF4:Yb(10%)三层结构的上转换纳米颗粒。此方法合成出来的颗粒大小均一、结构稳定、分散性好。该双激发纳米颗粒能够在980 nm和808 nm的激发光下实现红色和绿色的正交发射光,且其单独发光不受影响。在980 nm激发下,红色光中650 nm处的发射峰强度大约能达到540 nm处发射峰的9.46倍;在808 nm激发下,绿色光中540 nm处发射峰强度大约能达到650 nm处发射峰强度的5.39倍。
  • 图  1  核、核壳、核壳壳3种结构纳米颗粒的XRD衍射图谱。

    Figure  1.  X-ray diffraction patterns of the core NPs, core@shell NPs and core@shell@shell NPs.

    图  2  (a) NaErF4:Yb/Tm, (b) NaErF4:Yb/Tm@NaYF4:Yb, (c) NaErF4:Yb/Tm@NaYF4:Yb@NaNdF4:Yb 3种不同结构纳米颗粒透射电镜图。

    Figure  2.  The TEM images of (a) core NaErF4:Yb/Tm NPs, (b) core@shell NaErF4:Yb/Tm@NaYF4:Yb NPs, and (c) core@shell@shell NaErF4:Yb/Tm@NaYF4:Yb@NaNdF4:Yb NPs.

    图  3  不同激发波长下的纳米颗粒荧光光谱图。插图为980 nm和808 nm激光器激发下的发光实物图。

    Figure  3.  Fluorescence spectra of NPs under different excitation wavelengths. The inset shows the corresponding fluorescence photo excited by 980 nm and 808 nm laser.

    图  4  在(a)980 nm和(b)808 nm激光激发下的纳米颗粒荧光光谱变化图。(c)不同激发波长下纳米颗粒随时间变化的荧光比率的变化图(II0分别表示在650 nm和540 nm处发光峰强度)。

    Figure  4.  The luminescence spectrum variations of NPs under (a) 980 nm excitation and (b) 808 nm excitation . (c) The luminescence ratio variations of NPs with different synthesis time at different excitation wavelengths (I and I0 represent the luminescence peak intensities at 650 nm and 540 nm, respectively).

    图  5  在(a)980 nm和(b)808 nm激发下的中间层和外层不同摩尔量的纳米颗粒荧光光谱变化图。(c)对应的荧光比率的变化图(II0分别表示在650 nm和540 nm处的发光峰强度)。

    Figure  5.  The luminescence spectrum variations of NPs with different molar weights in the middle and outer layers under (a) 980 nm excitation, and (b) 808 nm excitation. (c) The luminescence ratio variations of the corresponding NPs under different excitation wavelengths (I and I0 represent the luminescence intensities at 650 nm and 540 nm, respectively).

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出版历程
  • 收稿日期:  2020-02-08
  • 修回日期:  2020-04-07
  • 网络出版日期:  2020-09-16
  • 刊出日期:  2020-10-01

Orthogonal luminescence properties of a single rare-earth activator ion doped upconversion nanoparticles

doi: 10.37188/CO.2020-0020
    通讯作者: qsmei@hfut.edu.cn
  • 中图分类号: TP394.1; TH691.9

摘要: 上转换发光纳米材料由于其特有的光学性质而一直备受关注,但常见的上转换发光纳米材料多为单色发光,为了实现上转换多色正交发光,同时避免多种掺杂剂离子的合成复杂性以及相互间的干扰性,利用Er3+2H11/2,4S3/24I15/2能级跃迁产生的绿色发光和4F9/24I15/2能级跃迁产生的红色发光,设计出由Er3+单个激活剂离子掺杂的双激发上转换纳米颗粒。通过热分解法一步步合成出NaErF4:Yb(19.5%)/Tm(0.5%)@NaYF4:Yb(10%)@NaNdF4:Yb(10%)三层结构的上转换纳米颗粒。此方法合成出来的颗粒大小均一、结构稳定、分散性好。该双激发纳米颗粒能够在980 nm和808 nm的激发光下实现红色和绿色的正交发射光,且其单独发光不受影响。在980 nm激发下,红色光中650 nm处的发射峰强度大约能达到540 nm处发射峰的9.46倍;在808 nm激发下,绿色光中540 nm处发射峰强度大约能达到650 nm处发射峰强度的5.39倍。

English Abstract

凌枭, 梅青松. 单一稀土激活剂离子的正交上转换发光特性[J]. 中国光学, 2020, 13(5): 1158-1170. doi: 10.37188/CO.2020-0020
引用本文: 凌枭, 梅青松. 单一稀土激活剂离子的正交上转换发光特性[J]. 中国光学, 2020, 13(5): 1158-1170. doi: 10.37188/CO.2020-0020
LING Xiao, MEI Qing-song. Orthogonal luminescence properties of a single rare-earth activator ion doped upconversion nanoparticles[J]. Chinese Optics, 2020, 13(5): 1158-1170. doi: 10.37188/CO.2020-0020
Citation: LING Xiao, MEI Qing-song. Orthogonal luminescence properties of a single rare-earth activator ion doped upconversion nanoparticles[J]. Chinese Optics, 2020, 13(5): 1158-1170. doi: 10.37188/CO.2020-0020
    • As a means of energy transmission, light plays an important role in physical, chemical and biological applications. Due to its ultra-high spatial and temporal resolution and accurately controlled parameters such as wavelength, life, intensity and frequency, light has become an ideal external excitation signal source[1-5]. However, the visible, ultraviolet or near-ultraviolet light currently used in most of biomedical applications has a limited capacity of tissue penetration, and is strongly absorbed by many endogenous chromophores. As a resolution to this problem, the study of up-conversion nanoparticles (UCNPs) has further expanded the biomedical applications of light[6-9].

      Upconversion luminescence is an anti-stokes effect, in which the luminescent center can continuously absorb two or more low-energy excited photons, and emit the light with higher energy than the absorbed energy after returning to the ground state through a non-radiation relaxation transition. An upconversion luminescence material is usually composed of matrix material, rare-earth activator and sensitizer[10-11]. The doping of rare-earth elements can be single, double or multiple doping. The luminescence efficiency of single-doped upconversion luminescence materials is relatively low. Instead, double-doped upconversion luminescence materials are more common. In terms of double doping, one kind of rare-earth ion (usually Er3+, Ho3+ or Tm3+) acts as an activator to provide a luminescent center with rich energy levels, and the other acts as a sensitizer to absorb and transfer energy to the activator ion. The common sensitizer ion is Yb3+, whose special energy level structure can sensitize the luminescence of other rare-earth ions, so as to improve the luminescence performance of the material[12]. Upconversion luminescence materials can convert near-infrared light with strong penetration into visible or ultraviolet light, which can be better applied in living organisms[13-14].

      In recent years, a lot of work has been done to improve the brightness and efficiency of upconversion luminescence through inert shell coating, high-concentration activator doping, organic dye modification and other strategies. These strategies are intended to explore some high-quality UCNPs with unique optical properties and expand their applications[15-19]. Traditional UCNPs can only emit monochromatic light in single exciting light. However, it is urgent to develop a kind of luminescent nanoparticles with bidirectional regulatory signals when studying the synergistic effect of UCNPs. In order to achieve such bidirectional regulation, many scientists have designed the UCNPs with orthogonal emission[20-21], which, however, are often doped with two or more activator ions to achieve orthogonal emission[22-24]. Because the complex energy level structure of these activator ions may lead to their mutual interference of energy migration, the activator ions need to be separated in space when synthesizing the UCNPs. This further complicates the synthesis steps of these UCNPs.

      In this work, the green luminescence generated by the 2H11/2,4S3/24I15/2 energy level transition of the lanthanide rare-earth ion Er3+ and the red luminescence generated by the 4F9/24I15/2 energy level transition[22, 25-26] was used to realize the orthogonal luminescence of the same UCNP doped with a single activator ion, that is, the orthogonal emission of red and green light under the 980 nm and 808 nm excitation respectively. Instead of studying the orthogonal luminescence of the UCNPs doped with multiple activator ions, the UCNPs with a relatively simple core-shell-shell structure were synthesized step by step by thermal decomposition in this work, and their structural stability, synthesis effect and luminescence condition were studied and analyzed.

    • The experimental instruments used in this study include: fluorescence spectrometer (LS55 PERKINELEMR), BSA124S electronic balance (Sartorius Scientific Instrument Limited), KQ3200E ultrasonic cleaner (Kunshan Ultrasonic Instruments Co., Ltd.), DF-101S heat-collecting constant-temperature heating magnetic stirrer (Shanghai Xiangya Instrument Co., Ltd.), S10-3 constant-temperature magnetic stirrer (Shanghai Sile Instrument Co., Ltd.), TG16-WS desk-type high-speed centrifuge (Hunan Xiangyi Laboratory Instrument Development Co., Ltd.), JEM-1400flash transmission electron microscope (JEOL), and X'pert PRO MPD X-ray diffractometer (PANalytical B.V.).

      The reagents used in this study include: oleic acid (OA) (Sigma-Aldrich), ODE (Sigma-Aldrich), NaOH (SINOPHARM Shanghai Chemical Reagent Co., Ltd.), NH4F (SINOPHARM Shanghai Chemical Reagent Co., Ltd.), CH3COOH (SINOPHARM Shanghai Chemical Reagent Co., Ltd.), C6H12 (SINOPHARM Shanghai Chemical Reagent Co., Ltd.), CH3CH2OH (SINOPHARM Shanghai Chemical Reagent Co., Ltd.), Y2O3 (Shanghai Aladdin Biochemical Technology), Yb2O3 (Shanghai Aladdin Biochemical Technology), Er2O3 (Shanghai Aladdin Biochemical Technology), Nd2O3 (Shanghai Aladdin Biochemical Technology), and Tm2O3 (Shanghai Aladdin Biochemical Technology).

    • The rare-earth acetate used to prepare the UCNPs is synthesized by using the previously reported method[27]. 10 g Y2O3, 10 g Yb2O3, 10 g Er2O3, 10 g Nd2O3 and 10 g Tm2O3 is added to a single-opening flask separately. Then, 70 mL deionized water and 30 mL glacial acetic acid is added to the flask at the same time. The flask is placed in an oil bath at 120 ℃ for reaction until the reaction solution becomes transparent. Finally, the solution is cooled naturally to room temperature. The reaction liquid is distilled under reduced pressure by a rotary evaporator and then dried by an oven into solid powder, which is just rare-earth acetate.

      The preparation process of 0.1 mol/L Tm(OAc)3 aqueous solution is as follows. Weigh 346.07 mg Tm(OAc)3 with an electronic balance and transfer it to a beaker. Add 5 ml ultra-pure water to the beaker. Stir the solution with a glass rod until it is completely dissolved. Then transfer the solution from the beaker to a 10 ml volumetric flask through a glass rod, and rinse it for three times before joint adding. Finally, titrate the solution with a rubber-head dropper until it is flush with the scale line, and mix the components evenly by vibration.

    • Synthesis of core particles. Weigh 275.2 mg Y(OAc)3, 68.3 mg Yb(OAc)3 and 50 µl Tm(OAc)3 (0.1 mol/L) proportional to 1 mmol NaErF4:Yb(19.5%)/Tm(0.5%), and pour them into a two-opening flask. Add 6 mL OA and 14 mL ODE and mix and stir them. Weigh 148 mg NH4F and 100 mg NaOH proportional to 1 mmol NaErF4:Yb(19.5%)/Tm(0.5%) and pour them into a test tube. Add 6 ml methanol and dissolve both NH4F and NaOH completely with ultrasound. Heat the two-opening flask to 70~80 ℃, vacuum it to remove water, and then inject N2 into it. Heat the flask to 150 ℃ and hold for 30 min. Then stop heating and cool it to room temperature. Transfer the methanol solution with ultrasonically dissolved NH4F and NaOH to the flask. Keep the solution at about 50 ℃ and stir it for 30 min. Then heat it to about 65 ℃ for methanol removal. Finally, vacuum the flask, inject N2 into it, and heat it to 300 ℃ for 60 min. Remove the heater. Cool the flask to room temperature. Take out the particle solution, centrifuge it and rinse it with ethanol for three times. Finally, disperse the centrifuged particles into 9 ml cyclohexane for later use.

      Synthesis of core-shell particles. Take a new two-opening flask. Weigh 359.2 mg Y(OAc)3 and 52.5 mg Yb(OAc)3 proportional to 1.5 mmol NaYF4:Yb (10%), and pour them into the flask. Add 6 mL OA and 14 mL ODE and mix and stir them. Heat the flask to 70~80 ℃, vacuum it to remove water until no bubbles come out, and then inject N2 into it. Heat the flask to 150 ℃ and hold for 30 min. Weigh 222 mg NH4F and 150 mg NaOH proportional to 1.5 mmol NaYF4:Yb (10%) and pour them into a test tube. Add 9 mL methanol and dissolve both NH4F and NaOH completely with ultrasound. After 30 min of reaction, stop heating the flask and cool it to room temperature. Add the 9 mL cyclohexane particle solution prepared in the previous step to the flask. Transfer the methanol solution with ultrasonically dissolved NH4F and NaOH to the flask. Heat the solution to 50 ℃, hold for 30 min, and then heat it to about 65 ℃ for methanol removal. Vacuum the flask, inject N2 into it, and heat it to 300 ℃ for 2 h. Remove the heater. Cool the flask to room temperature. Take out the particle solution, centrifuge it and rinse it with ethanol for three times. Finally, disperse the centrifuged particles into 9 mL cyclohexane for later use.

      Synthesis of core-shell-shell particles. Weigh 289.2 mg Nd(OAc)3 and 35.0 mg Yb(OAc)3 proportional to 1 mmol NaNdF4:Yb (10%), and pour them into the flask. Add 6 mL OA and 14 mL ODE and mix and stir them. Heat the two-opening flask to 70~80 ℃, vacuum it to remove water until no bubbles come out, and then inject N2 into it. Heat the flask to 150 ℃ and hold for 30 min. Weigh 148 mg NH4F and 100 mg NaOH proportional to 1 mmol NaNdF4Yb (10%) and pour them into a test tube. Add 6 mL methanol and dissolve both NH4F and NaOH completely with ultrasound. After 30 min of reaction, stop heating the flask and cool it to room temperature. Add the 9 mL cyclohexane particle solution prepared in the previous step to the flask. Transfer the methanol solution with ultrasonically dissolved NH4F and NaOH to the flask. Heat the solution to 50 ℃, hold for 30 min, and then heat it to about 65 ℃ for methanol removal. Vacuum the flask, inject N2 into it, and heat it to 300 ℃ for 3 h. Remove the heater. Cool the flask to room temperature. Take out the particle solution, centrifuge it and rinse it with ethanol for three times. Finally, disperse the centrifuged particles into cyclohexane.

    • We first studied the crystalline phase of the particles obtained at each of the above three synthesis steps. From the X-ray diffraction patterns of core, core-shell and core-shell-shell particles shown in Fig. 1, it can be seen that the particles obtained in each layer have a hexagonal phase and fit very well the X-ray diffraction pattern of hexagonal crystal illustrated in standard card (JCPDS 16-0344).

      图  1  核、核壳、核壳壳3种结构纳米颗粒的XRD衍射图谱。

      Figure 1.  X-ray diffraction patterns of the core NPs, core@shell NPs and core@shell@shell NPs.

    • The morphology analysis of each layer of particles shows that the particles obtained by this method have uniform morphology and good dispersibility. The Fig. 2 shows the field emission TEM images of core, core-shell and core-shell-shell particles.

      图  2  (a) NaErF4:Yb/Tm, (b) NaErF4:Yb/Tm@NaYF4:Yb, (c) NaErF4:Yb/Tm@NaYF4:Yb@NaNdF4:Yb 3种不同结构纳米颗粒透射电镜图。

      Figure 2.  The TEM images of (a) core NaErF4:Yb/Tm NPs, (b) core@shell NaErF4:Yb/Tm@NaYF4:Yb NPs, and (c) core@shell@shell NaErF4:Yb/Tm@NaYF4:Yb@NaNdF4:Yb NPs.

      It can be seen that the first synthesis layer is composed of uniform hexagonal particles, and the second synthesis layer is formed by uniformly wrapping the first layer with an additional layer. However, after being wrapped with the last layer, the particles present a dumbbell-like structure. This is because of the lattice mismatch between NaYF4 and NaNdF4 matrices. The lattice distortion increases the interface stress, causing the outermost layer to grow not isotropically, but longitudinally along both ends of the particle. As a result, the synthesized particles present a dumbbell-like structure.

    • The test of luminescence spectra of a nanoparticle at different excitation wavelengths shows that the spectra signatures of the particle excited at 980 nm and 808 nm laser wavelengths are completely different. It can be seen from Fig. 3 that the intensity of red emission peak (650 nm) of the particle under 980 nm excitation is significantly higher than that of green emission peak (540 nm), while the intensity of green emission peak of the particle under 808 nm excitation is significantly higher than that of red emission peak. The colors of the cyclohexane solution of the particle in different exciting light also coincide with its luminescent spectra. The particle emits red light under 980 nm excitation, and green light under 808 nm excitation. This is because when the synthesized NaErF4:Yb/Tm@NaYF4:Yb@NaNdF4:Yb particle is excited at 980 nm, both Er and Yb ions in the core can absorb the exciting light so that all the Er ions are in the excited state. However, cross relaxation (4F7/2 + 4I11/2 →2 4F9/2) can occur easily between the Er ions doped at high concentration (80%), thus greatly increasing the odds of red light emission. In case of 808 nm excitation, most of the excitation energy is absorbed by the Nd ions in the shell due to the weak absorption by Er ions at 808 nm, and then transferred to the activator Er ions in the core through the energy of Yb ions. This long-distance energy transfer greatly reduces the number of excited Er ions and the possibility of cross relaxation, so that the luminescence of Er ions is dominated by normal green light emission.

      图  3  不同激发波长下的纳米颗粒荧光光谱图。插图为980 nm和808 nm激光器激发下的发光实物图。

      Figure 3.  Fluorescence spectra of NPs under different excitation wavelengths. The inset shows the corresponding fluorescence photo excited by 980 nm and 808 nm laser.

      We have found in the study that the synthesis time of the outermost layer of a nanoparticle has a greater influence on its luminescence performance (Fig. 4 color online). As can be seen from Fig. 4(a), the light intensity of the 650 nm main peak of the particle under 980 nm excitation does not change much after 1.5 h. As can be seen from Fig. 4(b), the light intensity of the 540 nm main peak of the particle under 808 nm excitation does not change much after 1 h. It can be seen from Fig. 4(c) that, with the increase of time, the red-green light intensity ratio under 980 nm excitation decreases slightly within 1~2.5 h, but then increases after 2.5 h. At the same time, the green-red light intensity ratio under 808 nm excitation gradually increases within 1~2.5 h, but gradually becomes stable after 2.5 h. Therefore, the synthesis time of the outermost layer is selected to be 3 h. By the end of 3 h, a UCNP can achieve significant red (980 nm excitation) and green luminescence (808 nm excitation).

      图  4  在(a)980 nm和(b)808 nm激光激发下的纳米颗粒荧光光谱变化图。(c)不同激发波长下纳米颗粒随时间变化的荧光比率的变化图(II0分别表示在650 nm和540 nm处发光峰强度)。

      Figure 4.  The luminescence spectrum variations of NPs under (a) 980 nm excitation and (b) 808 nm excitation . (c) The luminescence ratio variations of NPs with different synthesis time at different excitation wavelengths (I and I0 represent the luminescence peak intensities at 650 nm and 540 nm, respectively).

      Assuming that the structure ratio of NaErF4:Yb/Tm(19.5%/0.5%)@NaYF4:Yb(10%)@NaNdF4:Yb remains unchanged, we have studied in depth the luminescence changes of the particles doped with the sensitizer ions with different molar weights in the middle and outer layers. By changing the molar weight of crystals in the middle and outer layers, we compared the fluorescence spectra and red-green light intensity ratios of the particles under the 980 nm and 808 nm excitation respectively (I represents the peak intensity at 650 nm, and I0 represents the peak intensity at 540 nm), as shown in Fig. 5. The results show that under the 980 nm excitation, the difference factor of red peak intensities between the two doping ratios 1@1.5@1 and 1@1.5@1.5 (where the numbers represent the molar weights of the core, inner shell and outer shell, respectively) is relatively high and more than 9. Under the 808 nm excitation, the difference factor of green peak intensities between the two doping ratios 1@1@1.5 and 1@1.5@1 is more than 5. Based on the above analysis, the optimal orthogonal emission of red and green light can be achieved under the condition of 1@1.5@1, so the particles with this doping ratio are selected.

      图  5  在(a)980 nm和(b)808 nm激发下的中间层和外层不同摩尔量的纳米颗粒荧光光谱变化图。(c)对应的荧光比率的变化图(II0分别表示在650 nm和540 nm处的发光峰强度)。

      Figure 5.  The luminescence spectrum variations of NPs with different molar weights in the middle and outer layers under (a) 980 nm excitation, and (b) 808 nm excitation. (c) The luminescence ratio variations of the corresponding NPs under different excitation wavelengths (I and I0 represent the luminescence intensities at 650 nm and 540 nm, respectively).

    • In this study, a multicolor orthogonal up-conversion luminescence nanoparticle doped with a single activator ion was innovatively designed. This nanoparticle could achieve orthogonal red and green luminescence in the 980 nm and 808 nm excitation light respectively, while its separate luminescence would not be affected. Under the 980 nm excitation, the emission intensity at 650 nm red luminescence was approximately 9.46 times that at 540 nm. Under the 808 nm excitation, the emission intensity at 540 nm green luminescence was about 5.39 times that at 650 nm. This particle was prepared by thermal decomposition, a synthesis method with a simple process. The particles synthesized with this method were uniform in size, stable in structure and good in dispersibility. Through this systematic study, we have innovatively proposed and verified the transition probabilities of different levels of rare earth ions induced by different excitation light energies, so as to realize the orthogonal emission of a single rare-earth activator ion and provide a new idea for the regulation of UCNP luminance.

    • 光作为能量的一种传播方式,在物理应用、化学应用、生物应用等各个方面扮演着重要的角色。由于其具有超高的时空分辨率,且波长、寿命、强度、频率等参数能够进行精准的调控,因此成为一种理想的外部刺激信号源[1-5]。然而,目前大多数生物医学应用中用到的可见光、紫外光或近紫外光的组织穿透力有限,许多内源性的生色团对这些波段的光的吸收很强烈。上转换发光纳米颗粒(UCNPs)进一步拓宽了光的生物医学应用范畴[6-9]

      上转换发光是一种反斯托克斯效应,发光中心可以连续吸收两个或多个低能激发光子,经无辐射驰豫过渡,回到基态后,可发射出高于吸收能量的高能发射光。上转换发光材料通常由基质材料、稀土激活剂和敏化剂组成[10-11]。稀土元素的掺杂可以是单掺、双掺和多掺。单掺杂上转换发光材料发光效率相对较低,常见的是双掺杂上转换发光材料。在双掺杂的稀土离子中,一种稀土离子充当激活剂,用以提供具有丰富能级的发光中心,常用的离子为Er3+、Ho3+和Tm3+;另一种作为敏化剂,吸收能量并将其转移到激活剂离子。常见的敏化剂离子是Yb3+,其具有特殊的能级结构,可以敏化其它有关稀土离子的发光,从而改善材料的发光性能[12]。上转换发光材料可以将穿透力强的近红外光转换成可见光或者紫外光,能够更好地在生物体内进行应用[13-14]

      近年来,人们在提高上转换发光亮度和效率方面做了许多工作,包括惰性外壳涂层、高浓度激活剂掺杂、有机染料改性等。这些策略旨在探索一些具有独特光学特性的高质量UCNPs,并扩大其应用范围[15-19]。传统的UCNPs在单一激发光下仅能发出单色光,然而,当利用UCNPs研究协同作用时,迫切需要发展一种具有双向调控信号的发光纳米颗粒。为了实现这种双向的调控,许多科学家设计出了正交发射的UCNPs[20-21]。但是这些颗粒往往都是通过掺杂两个或多个激活剂离子来达到正交发射的目的[22-24],由于这些激活剂离子的能级结构复杂,可能导致能量迁移相互干扰。因此,在合成该类颗粒的时候需要将不同的激活剂离子进行空间上的分离,这就使得该类纳米颗粒的合成步骤变得更加复杂繁琐。

      在这项工作中,利用镧系稀土离子Er3+2H11/2,4S3/24I15/2能级跃迁产生的绿色发光和4F9/24I15/2能级跃迁产生的红色发光[22, 25-26],在同一UCNPs中实现由单一激活剂离子掺杂的正交发射,即在980 nm和808 nm激光激发下分别发射出红色光和绿色光。相较于多个激活剂离子掺杂的正交发射,本工作用热分解的方法逐步合成出结构较为简单的核-壳-壳三层结构的UCNPs,并研究分析了该纳米颗粒的结构稳定性、合成效果以及颗粒的发光情况。

    • 荧光光谱仪(LS55 PERKINELEMR)、电子天平(BSA124S 赛多利斯科学仪器有限公司)、超声波清洗机(KQ3200E 昆山市超声仪器有限公司)、集热式恒温加热磁力搅拌器(DF-101S 上海翔雅仪器设备有限公司)、恒温磁力搅拌器(S10-3型,上司乐仪器有限公司)、台式高速离心机(TG16-WS 湖南湘仪实验室仪器开发有限公司)、透射电子显微镜(JEM-1400flash 日本电子)、X射线衍射仪(X'Pert PRO MPD 荷兰帕纳科)。

      油酸(OA,Sigma-Aldrich公司)、十八烯(ODE,Sigma-Aldrich公司)、氢氧化钠(NaOH 国药集团上海化学试剂公司)、氟化铵(NH4F,国药集团上海化学试剂公司)、冰醋酸(CH3COOH,国药集团上海化学试剂公司)、环己烷(C6H12,国药集团上海化学试剂公司)、无水乙醇(CH3CH2OH,国药集团上海化学试剂公司)、氧化钇(Y2O3,阿拉丁生化科技有限公司)、氧化镱(Yb2O3,阿拉丁生化科技有限公司)、氧化铒(Er2O3,阿拉丁生化科技有限公司)、氧化钕(Nd2O3,阿拉丁生化科技有限公司)、氧化铥(Tm2O3,阿拉丁生化科技有限公司)。

    • 按照文献[27]报道的方法实现上转换纳米颗粒所用的稀土醋酸盐合成。在单口烧瓶中分别加入10 g Y2O3、10 g Yb2O3、10 g Er2O3、10 g Nd2O3、10 g Tm2O3,接着向每个单口瓶中同时加入70 mL去离子水和30 mL冰醋酸,再将单口瓶放在120 ℃油浴条件下反应,直到反应溶液变澄清透明,最后自然冷却至室温,反应液经旋蒸仪减压蒸馏和烘箱干燥后成固体粉末,即为稀土醋酸盐。

      配置0.1 mol/L的Tm(OAc)3水溶液:用电子天平称取346.07 mg的Tm(OAc)3放入烧杯中,加入5 mL超纯水并用玻璃棒搅拌至全部溶解,再用玻璃棒将烧杯中的溶液转移至10 mL的容量瓶中并润洗三遍一同加入,最后用胶头滴管进行滴定直至与刻度线平齐,震荡混匀。

    • 核颗粒的合成:按照1 mmol 的NaErF4:Yb(19.5%)/Tm(0.5%)称取275.2 mg的Y(OAc)3、68.3 mg Yb(OAc)3和50 µl的0.1 mol/L Tm(OAc)3,将上述试剂放入两口烧瓶,并加入6 mL油酸和14 mL十八烯,混合搅拌;按照1 mmol NaErF4:Yb(19.5%)/Tm(0.5%)的比例称取148 mg NH4F和100 mg NaOH放于试管中,并添加6 mL甲醇溶解,超声至全部溶开。将两口烧瓶加热至70~80 ℃,抽真空除水,随后通入N2,升温至150 ℃维持30 min后停止加热,冷却至室温,再将超声溶解有氟化氨和氢氧化钠的甲醇溶液加入烧瓶中,在50 ℃左右搅拌30 min后,升温至65 ℃左右除甲醇,最后抽真空通氮气并升温至300 ℃维持60 min。除去加热装置,冷却至室温,取出颗粒溶液,进行离心并用乙醇洗涤3次,最后将离心下来的颗粒用9 mL环己烷分散备用。

      核-壳颗粒的合成:取一个新的两口烧瓶,按照1.5 mmol NaYF4:Yb(10%),359.2 mg Y(OAc)3和52.5 mg Yb(OAc)3的比例称取放于烧瓶中并加入6 mL油酸和14 mL十八烯,混合搅拌,将两口烧瓶均加热至70~80 ℃,抽真空除水至无气泡,随后通入N2,升温至150 ℃维持30 min。按照1.5 mmol NaYF4:Yb(10%)的比例称取222 mg NH4F和150 mg NaOH于试管中,并添加9 mL甲醇,超声溶解至全部溶开。将反应了30 min 的烧瓶停止加热,冷却至室温,将上一步备用的9 mL颗粒环己烷溶液加入到烧瓶中,再将超声溶解有氟化氨和氢氧化钠的甲醇溶液加入到烧瓶中,将温度升至50 ℃,并维持30 min。随后升温至65 ℃左右以除甲醇,随后抽真空通氮气并升温至300 ℃维持2 h。除去加热装置,冷却至室温后取出颗粒溶液,用乙醇离心洗涤3次后将离心下来的颗粒用9 mL环己烷分散备用。

      核-壳-壳颗粒的合成:按照1 mmol NaNdF4:Yb(10%)称取289.2 mg Nd(OAc)3和35.0 mg Yb(OAc)3放于烧瓶中并加入6 mL油酸和14 mL十八烯,混合搅拌,将两口烧瓶加热至70~80 ℃,抽真空除水至无气泡,随后通入N2,升温至150 ℃维持30 min;按照1 mmol NaNdF4Yb(10%)的比例称取148 mg NH4F和100 mg NaOH于试管中,并添加6 mL甲醇溶解,超声至全部溶开。将反应了30 min 的烧瓶停止加热,冷却至室温,将上一步备用的9 mL颗粒环己烷溶液加入到烧瓶中,再将超声溶解有氟化氨和氢氧化钠的甲醇溶液加入到烧瓶中,将温度升至50 ℃,并在50 ℃维持30 min后,升温至65 ℃左右除甲醇,随后抽真空通氮气并升温至300 ℃维持3 h。除去加热装置,将烧瓶冷却至室温,取出颗粒溶液,用乙醇离心洗涤3次后将离心下来的颗粒分散在环己烷中。

    • 首先,对三步合成法中每一步得到的颗粒进行晶相研究。通过图1给出的核、核-壳和最终的核-壳-壳颗粒的X射线衍射花样,可以看出得到的每一层颗粒都是六方相颗粒,与标准卡片六方相晶体X射线衍射花样(JCPDS 16-0344)很好的吻合。

    • 对得到的每一层颗粒进行形貌分析发现,这种方法得到的颗粒形貌均一, 分散性非常好。图2给出了核、核-壳和最终的核-壳-壳颗粒的场发射透射电子显微镜图。 可以看出,第一层合成出来后是均一的六方相颗粒, 第二层是在第一层的外面均匀地包覆了一层, 最后一层包裹完之后颗粒呈现出哑铃状结构。哑铃状颗粒的出现是因为NaYF4和NaNdF4基质晶格不匹配,晶格畸变使得界面应力增大, 导致最外层没有各向同性生长, 而是沿着颗粒两端纵向生长, 最终使得合成出来的颗粒呈现哑铃状结构。

    • 对颗粒在不同激发波长下的发光光谱进行测试,发现该纳米颗粒在980 nm和808 nm波长的激光器激发下产生的光谱特征完全不一样,如图3(彩图见期刊电子版)。从图3可以看出在980 nm激光激发下其红光发射峰(650 nm)的强度明显高于绿光发射峰(540 nm),而在808 nm激光激发下颗粒的绿光发射峰显著高于红光发射峰。颗粒的环己烷溶液在不同激发光照射下的颜色也与发光光谱吻合,980 nm激光激发时颗粒发红光,而808 nm激光激发时颗粒发绿光。这是由于合成的NaErF4:Yb/Tm@NaYF4:Yb@NaNdF4:Yb颗粒在980 nm激光激发时,核里面Er和Yb离子都能吸收激发光,使所有的Er离子都处于激发态,而高浓度掺杂的Er离子(80%)之间容易发生交叉弛豫过程,4F7/2 + 4I11/2 →2 4F9/2,从而大大增强了红光发射几率。在808 nm激光激发时,由于Er离子在808 nm处吸收很弱,绝大部分激发态能量被壳层Nd离子吸收,再经过Yb离子的能量传递到核里面的激活剂Er离子,这种长距离的能量传递大大减小了激发态Er离子的数量,减小了交叉弛豫的可能性,使Er离子的发光以正常的绿光发射为主。

      在研究中发现,纳米颗粒最外层的合成时间长短对其发光性能的影响较大(图4彩图见期刊电子版),从图4(a)中可以得出,从1.5 h以后980 nm激光激发下颗粒的650 nm的主峰光强度没有发生太大的变化,图4(b)中可以得出,从1 h以后808 nm激光激发下颗粒在540 nm左右的主峰光强度没有发生太大变化,从图4(c)中可以得出随时间的增加980 nm下红绿光强度比在1~2.5 h内出现略微下降,但在2.5 h后又呈现增加的趋势,在此时808 nm激光激发下的绿红光强度比率逐渐增加,但在2.5 h后逐渐趋于稳定,因此选择最外层的合成时间为3 h,此时可以得到一个在980 nm和808 nm激发下分别有很明显的红光和绿光的UCNPs。

      本文在结构配比NaErF4:Yb/Tm(19.5%/0.5%)@NaYF4:Yb(10%)@NaNdF4:Yb不改变的前提下,深入研究了中间层和外层掺杂不同量敏化剂离子时颗粒的发光变化情况。如图5所示,在改变中间层和外层晶体摩尔量的条件下,分别研究了颗粒在980 nm激光激发下和808 nm激光激发下的光谱对比图,以及在980 nm激光和808 nm激光激发下的红光和绿光荧光强度比率变化(I代表650 nm处峰强度,I0表示540 nm处峰强度)。结果显示:在980 nm激光激发下,1@1.5@1和1@1.5@1.5(数字分别表示核、内壳和外壳的摩尔量)两个掺杂比例下红光峰差值比例相对较高且能达到9倍以上;在808 nm激发下,1@1@1.5和1@1.5@1两个掺杂比例下绿光峰差值比例能达到5倍以上。综合上述分析,在1@1.5@1的条件下能够实现最佳的红光绿光正交发射性质,因此选择该比例颗粒。

    • 本工作创新性地设计出一种单一激活剂离子掺杂的多色正交上转换发光纳米颗粒。该上转换纳米颗粒能够在980 nm和808 nm的激光激发下实现红色和绿色的正交发射,其单独发光不受影响。在980 nm激光激发下,红色发光中650 nm处发射峰强度大约能达到540 nm处发射峰的9.46倍;在808 nm激发下,绿色发光中540 nm处发射峰强度大约能达到650 nm处发射峰强度的5.39倍。该颗粒的制备采用热分解法,该方法合成步骤简单,且该方法合成出来的颗粒大小均一、结构稳定、颗粒分散性好。通过本文的系统研究,创新性地提出并验证了不同激发光能量引导稀土离子不同能级的跃迁概率,从而实现单个稀土激活剂离子正交发射,为上转换发光纳米材料发光性质的调控提供了一个新的研究思路。

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