为了增强 β-NaGdF4∶Yb3+,Tm3+纳米晶的上转换发光, 克服外延增长钝化壳增大尺寸的不足, 利用阳离子交换法制备核壳纳米结构, 研究了样品在980 nm激发下的上转换发光性质。首先, 利用高温热分解法制备了直径为10 nm的β-NaGdF4∶Yb3+,Tm3+ 纳米晶; 然后, 将制备的纳米晶与Gd3+在油酸-十八烯混合溶液中在300 ℃进行交换反应。实验结果表明, 随着表面Yb3+和Tm3+被Gd3+取代, 钝化壳的形成抑制了内部Yb3+的表面去激发过程, 增强了内部Yb3+ (2F5) → Tm3+ (3H5, 3F2,3 )的能量传递, 上转换发光逐渐增强。交换30 min后, Tm3+的3H4 →3H6近红外发光增强达到最大, 为对照样品的6.5倍, 而尺寸基本保持不变。在生物成像方面, 上转换纳米晶的尺寸必须与生物分子相匹配, 同时发光强度要高, 阳离子交换法既能增强近红外发光, 又能保持原来小的尺寸, 在生物成像领域具有很好的应用前景。

In order to enhance upconversion luminescence of β-NaGdF4∶Yb3+,Tm3+ nanocrystals and meanwhile overcome the particle size enlargement caused by epitaxial growth technique, NaGdF4∶Yb3+,Tm3+@NaGdF4 core-shell nanostructure was prepared using cation exchange strategy. The upconversion luminescence of the core-shell nanocrystals was investigated under 980 nm laser excitation. The oleate-capped β-NaGdF4∶Yb3+,Tm3+ nanocrystals with diameter about 10 nm were firstly prepared by thermal decomposition procedure. Then, the cation exchange reaction of the nanocrystals with Gd3+ was performed in l-octadecene and oleic acid mixture solution at 300 ℃. Experimental results show that the cation exchange strategy has significantly enhanced upconversion luminescence brightness of the nanocrystals, which attribute to the suppression of inner Yb3+ de-excitation by the cation exchange shell and the enhancement of energy transfer from Yb3+ (2F5) to Tm3+ (3H5,3F2,3 ) inside of the nanocrystals. The maximum improvement of NIR emission for Tm3+ 3H4→3H6 transition is achieved with 6.5 times than that of the contrast sample after 30 min of exchange reaction. It demonstrates that the cation exchange strategy can not only enhance NIR luminescence of the nanocrystals, but also well retain small particle size. It provides a simple and convenient way to development of high brightness upconversion nanocrystas with comparable in size to biomolecules, which has enormous applications in biomedical imaging fields.