Page 134 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 7
7.1 Introduction
Semiconductor nanocrystals have been widely used as luminescent materials for bio-imaging or bio-fluorescence labeling due to the quantum-size effect, where the absorbance onset and emissions shift to higher energy with decreasing size.1 However, recent studies indicate they can affect cell-membrane penetrability and engender undesirable hazardous interactions with biological systems.2 Therefore, their long term uses in bio-related areas have been limited. As a promising category of fluorescent materials, lanthanide-doped nanophosphors have attracted considerable interest for biological applications owing to their inherently low toxicity (compared to Cd and Se-based nanocrystals) and long fluorescence lifetimes. In addition, the lanthanide ions (Ln3+) have fixed emission wavelengths. Tunable emissions can be achieved by choosing proper color-center elements, endowing a great flexibility in designing and engineering products with desirable luminescent properties.3
Currently a variety of novel or well-established methods have been developed to produce lanthanide doped phosphors. In general, they can be divided into two categories: high temperature solid-state reactions and wet chemistry routes. However, solid-state reactions commonly generate highly aggregated particles (μm level) with inhomogeneous Ln3+ doping. Mechanical processes such as grinding or milling are needed to obtain fine particles, which may reduce the luminescent efficiency due to the inflicted damage on the surfaces. As to wet chemistry routes, they are mostly driven by the supersaturation of alkali precipitants. The vigorous hydrolyzing reactions and the inhomogeneous precipitant concentration while dripping throughout the synthesis can rapidly generate large amounts of sediments, leading to products of wide size distributions (from nm to μm).4,5 Moreover, the involved chemicals (e.g. stabilizers, surfactants or solvents) can occupy active centers and create traps in the phosphors, which in turn, requires complex purification procedures to get rid of possible residues.6 Therefore, by current state of techniques, it is still a challenge to produce high purity nano-sized lanthanide phosphors with homogenous Ln3+ doping in a simple, controllable and toxic chemicals free manner.
Based on the Chapter 6, herein the plasma-assisted technique was expanded to the fabrication of a series of lanthanide doped/co-doped (Ln=Eu, Tb, Dy, Tm) nanophosphors by using the same plasma setup. As a model study, Eu3+ doped yttria nanophosphor is chosen to investigate the effect of heat treatment and the dopant concentration on their photoluminescence properties. The essential advance of this study is to synthesize and engineering lanthanide-doped nanophosphors from only an aqueous solution of Y(NO3)3·6H2O and Ln(NO3)3·6H2O, without involving any hazardous chemicals and purification process. Furthermore, since Y3+ and Ln3+ are homogenously mixed at the molecular level, luminescent ions can be uniformly embedded into the host matrix.
7.2 Experiment Section
7.2.1 Experimental
In this work the Ln3+ (Ln=Eu, Tb, Tm, Dy) doped nanophosphors were obtained by the same method as the synthesis of Y2O3 nanoparticles (Chapter 6). Briefly, Y(NO3)3·6H2O and
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