Synthesis of rare earth doped yttrium–vanadate nanoparticles encapsulated within apoferritin

Abstract

Luminescent europium (Eu) and dysprosium (Dy) doped yttrium–vanadate (Y–V) nanoparticles (NPs) were synthesized in the cavity of the protein, apoferritin. Y–V NPs were synthesized by incubating a solution of apoferritin with Y³⁺ and VO₃⁻ ions in the presence of ethylene diamine-N-N′-diacetic acid (EDDA). EDDA plays an important role in preventing Y–vanadate precipitation in bulk solution by chelating the Y³⁺ ions. Using high resolution electron microscopy, the obtained NPs in the apoferritin cavities were confirmed to be amorphous, and to consist of Y and V. Eu-doped Y–V (Y–V:Eu) NPs were synthesized by the same procedure as Y–V NPs, except that Eu(NO₃)₃ was added. Y–V:Eu NPs exhibited a strong absorption peak due to the O–V charge transfer transition and remarkable luminescence at 618 nm due to the ⁵D₀ → ⁷F₂ transition. The luminescence lifetime of Y:Eu and Y–V:Eu NPs measured in H₂O and D₂O solution showed reduction of non-radiative transition to the O–H vibration in Y–V:Eu NPs. Accordingly, Y–V NPs showed strong luminescence compared to Y:Eu NPs. Dy-doped Y–V NPs were also synthesized in apoferritin cavities and showed luminescence peaks at 482 nm and 572 nm, corresponding to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions. These NPs stably dispersed in water solution since their aggregation was prevented by the protein shell. NPs encapsulated in the protein are likely to be biocompatible and would have significant potential for biological imaging applications.