Paramagnetism and improved upconversion luminescence properties of NaYF₄:Yb,Er/NaGdF₄ nanocomposites synthesized by a boiling water seed-mediated route

Abstract

In a route boiling water served as reaction medium, a stoichiometric amount of rare-earth compound and fluoride are put into this system to form α-NaYF₄:Yb,Er nuclei. Then prepared sample is heated at elevated temperature to improve the fluorescence intensity, and next a NaGdF₄ shell grows on the surface of NaYF₄ nuclei. NaYF₄:Yb,Er/NaGdF₄ core--shell structured upconversion nanoparticles (CSUCNPs) have been successfully synthesized by above route. The use of boiling water decreases the cubic-to-hexagonal phase transition temperature of NaYF₄:Yb,Er to 350°C and increases its upconversion (UC) luminescence intensity. A heterogeneous NaGdF₄ epitaxially growing on the surface of Ln³⁺-doped NaYF₄ not only improves UC luminescence, but also creates a paramagnetic shell, which can be used as contrast agents in magnetic resonance imaging (MRI). The solution of CSUCNPs shows bright green UC fluorescence under the excitation at 980 nm in a power density only about 50 mW·cm⁻². A broad spectrum with a dominant resonance at g of about 2 is observed by the electron paramagnetic resonance (EPR) spectrum of CSUCNPs. Above properties suggest that the obtained CSUCNPs could be potential candidates for dual-mode optical/magnetic bioapplications.

Keywords

boiling water / upconversion / nanoparticle / fluorescence imaging / paramagnetism

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