Direct Evidence of a Surface Quenching Effect on Size-Dependent Luminescence of Upconversion Nanoparticles
Direct Evidence of a Surface Quenching Effect on Size-Dependent Luminescence of Upconversion Nanoparticles
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DOI：
10.1002/anie.201003959
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Liu, Xiaogang
中科院分区：
文献类型：
--
作者：
Wang, Feng;Wang, Juan;Liu, Xiaogang
镧系元素掺杂的上转换（UC）纳米粒子在生物标记，成像和治疗方面显示出相当大的前景。[1]然而，尽管目前的合成方法允许制备超小的UC纳米颗粒，并精确控制颗粒形态和发射颜色，[2]较小的纳米颗粒是以较弱的发射为代价的，这是一个实际上不可能超越的限制。这些纳米材料中UC发光的许多基本方面仍然缺乏足够的理解。特别是，几个小组已经观察到不同的相对强度的多峰UC发射与不同的颗粒尺寸。[3]UC发光主要源于局域化镧系元素掺杂剂离子内的构型内4fn电子跃迁。由于UC基质中激子的Bohr半径较小以及镧系掺杂离子的4fn电子与基质基质之间的弱相互作用，很难用经典理论（如与半导体和金属纳米颗粒的光学性质相关的量子限制和表面等离子体共振）来解释尺寸依赖的UC发光。[4]虽然声子限制[3a-d]已被用来解释尺寸依赖的UC发光，但由于通常与极端条件下的固态样品测量相关的限制（例如，约100 ℃的低温），这一直是一个有争议的问题。10 K）和排除振动能量和粒子表面产生的光陷阱。为此，提出了表面猝灭效应[3f-i]，并与尺寸依赖的UC发光相关。然而，表面猝灭效应尚未最终建立，主要是因为缺乏直接的证据，表面猝灭诱导的发光调制不同尺寸的颗粒。本文中，我们提出了一系列Yb/Tm共掺杂的六方相NaGdF 4纳米颗粒（10，15和25 nm）的比较光谱研究，有或没有薄（约10 nm）。2.5 nm）表面保护层。我们表明，通过薄层涂层，这些纳米粒子的特征光学特性（如相对发射强度）可以保留，从而提供直接的证据来支持负责的尺寸依赖性UC发光的表面猝灭效应。选择阻碍相NaGdF 4作为模型主体系统，因为其能够提供高UC效率以及产生相对小（< 20 nm）且均匀的纳米颗粒的益处。[2a此外，以半填充4f轨道为特征的Gd 3+主体离子在发光过程中相对惰性，因此与掺杂剂离子的相互作用可忽略不计。[2a]为了在宽波长范围内提供粒子的相对发射强度之间的直接比较，选择具有梯状能级布置的Tm 3+离子作为能够产生跨越紫外（UV）到近红外（NIR）光谱区域的上转换发射峰的活化剂为了探测UC纳米颗粒的尺寸依赖性发光的表面猝灭效应，我们首先合成了NaGdF 4：Yb/Tm（25/0.3摩尔%）材料以块状形式（> 100 nm）和以不同尺寸（10、15和25 nm）的纳米颗粒形式存在。通过X射线粉末衍射确定所有样品均为六方相NaGdF 4（支持性信息，图S1）。在近红外激发下，体相NaGdF 4磷光体中的Tm 3+离子表现出特征发射（1G 4！3 H6），其强度显著高于NIR光谱区域（3 H4！3 H6;图1b）。在...
Lanthanide-doped upconversion (UC) nanoparticles have shown considerable promise in biological labeling, imaging, and therapeutics.[1] However, although current synthetic approaches allow for preparation of ultrasmall UC nanoparticles with precise control over particle morphology and emission color,[2] smaller nanoparticles come at the expense of weaker emissions, which is a constraint that is practically impossible to surpass. Many fundamental aspects of the UC luminescence in these nanomaterials still lack sufficient understanding. In particular, several groups have observed varied relative intensity of the multi-peak UC emissions with varying particle size.[3] The UC luminescence primarily originates from intra-configurational 4fn electron transitions within the localized lanthanide dopant ions. Due to a small Bohr radius of the exciton in UC hosts and weak interactions between 4fn electrons of the lanthanide dopant ions and the host matrix, the size-dependent UC luminescence can hardly be explained by classic theories, such as quantum confinement and surface plasmon resonance related to optical properties of semiconductor and metal nanoparticles.[4] Although phonon confinement [3a–d] has been used to account for the size-dependent UC luminescence, it has been a matter of much debate, owing to the constraints typically associated with solid-state sample measurements at extreme conditions (for example, low temperatures of ca. 10 K) and exclusion of vibrational energies and optical traps arising from particle surface. To this end, a surface quenching effect [3f–i] is proposed and correlated with size-dependent UC luminescence. However, the surface quenching effect has not been conclusively established, largely because of the lack of direct evidence on surface-quenching-induced luminescence modulation of different-sized particles. Herein, we present a comparative spectroscopic investigation of a series of Yb/Tm co-doped hexagonal-phase NaGdF4 nanoparticles (10, 15, and 25 nm) with or without a thin (ca. 2.5 nm) surface protection layer. We show that, through the thin layer coating, the characteristic optical features (such as relative emission intensities) of these nanoparticles can be retained, thereby providing direct evidence to support the surface quenching effect responsible for the size-dependent UC luminescence. Hexagonal-phase NaGdF4 was chosen as the model host system owing to its ability to render high UC efficiency and the benefits of producing relatively small (< 20 nm) and uniform nanoparticles.[2a, 5] Furthermore, the Gd3+ host ion that features half-filled 4f orbitals is relatively inert in the luminescence process and thus has negligible interaction with the dopant ions.[2a] To provide a direct comparison over a broad wavelength range between the relative emission intensity of the particles, the Tm3+ ion with a ladder-like arrangement of energy levels was selected as the activator capable of generating upconverted emission peaks that span from ultraviolet (UV) to near-infrared (NIR) spectral regions (Figure 1 a).To probe the surface quenching effect on size-dependent luminescence of UC nanoparticles, we first synthesized NaGdF4: Yb/Tm (25/0.3 mol%) materials in bulk form (> 100 nm) and in the form of nanoparticles of different size (10, 15, and 25nm). All samples were determined as hexagonal-phase NaGdF4 by X-ray powder diffraction (Supporting Information, Figure S1). Upon NIR excitation, Tm3+ ions in bulk NaGdF4 phosphors exhibit a characteristic emission (1G4! 3H6) in the blue spectral region with an intensity that is significantly higher than that in the NIR spectral region (3H4! 3H6; Figure 1b). In …