State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China.
Nanoscale. 2013 Nov 21;5(22):11298-305. doi: 10.1039/c3nr03497f. Epub 2013 Oct 4.
NaGdF4 is regarded as an ideal upconversion (UC) host material for lanthanide (Ln(3+)) activators because of its unique crystal structure, high Ln(3+) solubility, low phonon energy and high photochemical stability, and Ln(3+)-doped NaGdF4 UC nanocrystals (NCs) have been widely investigated as bio-imaging and magnetic resonance imaging agents recently. To realize their practical applications, controlling the size and uniformity of the monodisperse Ln(3+)-doped NaGdF4 UC NCs is highly desired. Unlike the routine routes by finely adjusting the multiple experimental parameters, herein we provide a facile and straightforward strategy to modify the size and uniformity of NaGdF4 NCs via alkaline-earth doping for the first time. With the increase of alkaline-earth doping content, the size of NaGdF4 NCs increases gradually, while the size-uniformity is still retained. We attribute this "focusing" of size distribution to the diffusion controlled growth of NaGdF4 NCs induced by alkaline-earth doping. Importantly, adopting the Ca(2+)-doped Yb/Er:NaGdF4 NCs as cores, the complete Ca/Yb/Er:NaGdF4@NaYF4 core-shell particles with excellent size-uniformity can be easily achieved. However, when taking the Yb/Er:NaGdF4 NCs without Ca(2+) doping as cores, they could not be perfectly covered by NaYF4 shells, and the obtained products are non-uniform in size. As a result, the UC emission intensity of the complete core-shell NCs increases by about 30 times in comparison with that of the cores, owing to the effective surface passivation of the Ca(2+)-doped cores and therefore protection of Er(3+) in the cores from the non-radiative decay caused by surface defects, whereas the UC intensity of the incomplete core-shell NCs is enhanced by only 3 times.
NaGdF4 因其独特的晶体结构、高的 Ln(3+)溶解度、低的声子能量和高的光化学稳定性而被认为是镧系元素（Ln(3+)）激活剂的理想上转换（UC）主体材料，最近，Ln(3+)掺杂的 NaGdF4 UC 纳米晶体（NCs）已被广泛研究作为生物成像和磁共振成像剂。为了实现它们的实际应用，高度期望控制单分散 Ln(3+)掺杂的 NaGdF4 UC NCs 的尺寸和均匀性。与通过精细调整多个实验参数的常规路线不同，本文首次提供了一种通过碱性土掺杂来修饰 NaGdF4 NCs 尺寸和均匀性的简便策略。随着碱性土掺杂含量的增加，NaGdF4 NCs 的尺寸逐渐增大，而尺寸均匀性仍得以保留。我们将这种尺寸分布的“聚焦”归因于碱性土掺杂诱导的 NaGdF4 NCs 的扩散控制生长。重要的是，采用 Ca(2+)掺杂的 Yb/Er:NaGdF4 NCs 作为核，很容易得到具有优异尺寸均匀性的完全 Ca/Yb/Er:NaGdF4@NaYF4 核壳颗粒。然而，当采用没有 Ca(2+)掺杂的 Yb/Er:NaGdF4 NCs 作为核时，它们不能被完全覆盖 NaYF4 壳，得到的产物尺寸不均匀。结果，完整核壳 NCs 的 UC 发射强度比核的强度提高了约 30 倍，这是由于 Ca(2+)掺杂核的有效表面钝化和因此保护了核中的 Er(3+)免受表面缺陷引起的非辐射衰减，而不完全核壳 NCs 的 UC 强度仅提高了 3 倍。
