Cross Relaxation Induced Pure Red Upconversion in Activator- and Sensitizer-Rich Lanthanide Nanoparticles
Cross Relaxation Induced Pure Red Upconversion in Activator- and Sensitizer-Rich Lanthanide Nanoparticles
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DOI：
10.1021/cm5022382
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发表时间：
2014-09-23
影响因子：
8.6
通讯作者：
Tan, Timothy Thatt Yang
中科院分区：
文献类型：
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作者：
Wei, Wei;Zhang, Yan;Tan, Timothy Thatt Yang
稀土掺杂上转换纳米颗粒（UCNP）已广泛应用于光学器件、传感和治疗等领域。 1 它们的优点包括近红外 (NIR) 激发、低细胞毒性、弱自发荧光、高化学稳定性和低光漂白，这使得它们比传统有机染料或量子点更适合生物成像应用。 2 特别是，UCNP 的发射和激发波长均位于“组织光学窗口”（大约从 650 到 1200 nm）内，更适合深层组织成像。 3 然而，常用的镧系元素激活剂如Tm3+和Er3+离子含有丰富的亚稳态激发态，主要发射通常位于红光发射强度相对较低的非红光区域。 4 因此，提高红光发射强度的策略对于 UC 应用非常有用，尤其是深层组织成像。活化剂之间的交叉弛豫 (CR) 通常被认为是有害的，5 因此其调节 UC 红光发射输出的能力常常被忽视。据我们所知，UCNP 中活化剂的浓度通常限制在 2 mol% 以下，以消除 CR 引起的能量损失。 6 对重度激活剂掺杂的 UC 行为的系统研究几乎不存在。在这项工作中，我们首次报告了通过仅依赖于激活剂相互作用的 CR 效应在可见光区域接近 100% 红光发射输出的证据（图 1a、b）。这种基于 CR 效应的策略不仅成功实现了纯红色 696 或 660 nm UC 发射，而且还可以作为精确 UC 颜色调节的替代方法，并提供对 UC 机制的进一步了解。在所有主体中，与低敏化剂（20% Yb3+）和活化剂（1% Tm3+）UCNPs 相比，选择富含敏化剂的 NaYbF4 可以极大地提高 Tm3+ 掺杂 UCNPs 的 696 nm 红光发射强度。我们还表明，相同的策略可用于增强 Er3+ 掺杂 UCNP 的 660 nm 红光发射强度。这是由于 CR 和敏化剂含量增加的协同效应导致可见光区域红光发射产量的增加。此外，由于 Yb3+ 离子的高 X 射线吸收系数（例如，80 keV 时 Yb：6.91 cm2/g，I：3.51 cm2/g；100 keV 时 Yb：3.88 cm2/g，I：1.94 cm2/g），表面修饰的 NaYbF4 UCNP 优于临床碘化 CT 造影剂（碘海醇）。 keV）。 7 因此，富含活化剂和敏化剂的 UCNP 可作为 CT/荧光双模态深层组织成像的潜在多功能材料。为了最大限度地减少 CR 能量损失，选择富含敏化剂的 NaYbF4 主体，并通过成熟的共沉淀方法在高沸点溶剂中制备。 8 透射电子显微镜 (TEM) 显示了我们制备的 UCNP 的尺寸和形态（图 1c），平均对角线长度为 100 nm（支持信息图 S1）。 X 射线衍射 (XRD) 图案（图 1d）证实了它们的六方相结构（JCPDS No. 27-1427）。一般来说，高效的镧系元素UC主要限于Tm3+、Er3+和Ho3+离子作为活化剂。图9 测定了上述活化剂掺杂在NaYbF4 UCNPs中的UC发射光谱，其含量从低到高变化。随着活化剂浓度的增加，观察到 Tm3+ 掺杂的 UCNP 的主要发射从 475 nm 转移到 696 nm，而 Er3+ 掺杂的 NaYbF4 UCNP 的主要发射从 540 nm 转移到 660 nm（图 1b）。图 1a 中的照片描绘了……
Lanthanide-doped upconversion nanoparticles (UCNPs) have been widely applied in a variety of areas such as optical devices, sensing, and therapeutics. 1 Their advantages include near-infrared (NIR) excitation, low cytotoxicity, weak autofluorescence, high chemical stability, and low photobleaching, which make them more desirable than conventional organic dyes or quantum dots for bioimaging application. 2 In particular, the UCNPs, which have both emission and excitation wavelengths located within the “tissue optical window”(spanning approximately from 650 to 1200 nm), are more suitable for deep tissue imaging. 3 However, the commonly investigated lanthanide activators such as Tm3+ and Er3+ ions contain abundant metastable excited states, and the dominant emission usually lies in the nonred region with relatively low red emission intensity. 4 Hence, a strategy to boost the red emission intensity will be useful for UC applications, especially for deep tissue imaging. Cross relaxation (CR) among the activators is often perceived as deleterious, 5 and thus its ability to regulate the UC red emission output is often overlooked. To the best of our knowledge, the concentration of the activator in UCNPs is usually constrained below 2 mol% to eliminate the energy loss caused by CR. 6 A systematic investigation on UC behavior with heavy activator doping is almost nonexistent. In this work, we report, for the first time, evidence of approaching 100% red emission output in the visible region through CR effect relying solely on activator interaction (Figure 1a, b). This strategy based on CR effect is not only successful in achieving pure red 696 or 660 nm UC emission but could also act as an alternative approach for precise UC color tuning and provide further insight into the UC mechanism. Among all the hosts, the selection of sensitizer-rich NaYbF4 can vastly improve 696 nm red emission intensity for Tm3+ doped UCNPs compared with low sensitizer (20% Yb3+) and activator (1% Tm3+) UCNPs. We have also shown that the same strategy can be used to enhance the 660 nm red emission intensity for Er3+ doped UCNPs. This is attributed to the boost in red emission yield in the visible region, resulted from a collaborative effect of CR and the increase in sensitizer content. In addition, the surfacemodified NaYbF4 UCNPs are shown to be superior compared to a clinical iodinated CT contrast agent (Iohexol) due to the high X-ray absorption coefficient of Yb3+ ions (eg, Yb: 6.91 cm2/g, I: 3.51 cm2/g at 80 keV; Yb: 3.88 cm2/g, I: 1.94 cm2/g at 100 keV). 7 Therefore, the activator-and sensitizer-rich UCNPs can serve as a potential multifunctional material for CT/fluorescence dual-modal deep tissue imaging. In order to minimize CR energy loss, sensitizer-rich NaYbF4 host was selected and prepared by a well-established coprecipitation method in high-boiling-point solvents. 8 Transmission electron microscopy (TEM) shows the size and morphology of our as-prepared UCNPs (Figure 1c) with an average diagonal length of 100 nm (Supporting Information Figure S1). The X-ray diffraction (XRD) pattern (Figure 1d) confirms their hexagonal-phase structure (JCPDS No. 27-1427). In general, efficient lanthanide UC is mainly restricted to Tm3+, Er3+, and Ho3+ ions as activators. 9 The UC emission spectra of the above activators doped in NaYbF4 UCNPs with their content varying from low to high were determined. With the increase in activator concentration, it was observed that the predominant emission for Tm3+ doped UCNPs shifts from 475 to 696 nm, while for Er3+ doped NaYbF4 UCNPs, it shifts from 540 to 660 nm (Figure 1b). The photographs in Figure 1a depict …