Luminescent europium (Eu) and dysprosium (Dy) doped Yttrium-Vanadate (Y-V) nanoparticles (NPs) were synthesized in a cavity of the protein, apoferritin. The use of inorganic nanoparticles (NPs), instead of organic dyes, is becoming popular for molecular labeling because of their strong resistance to photobleaching. Since structure of protein is strictly regulated by DNA, the size of NPs in the protein also becomes homogeneous. Moreover, apoferritin is thermally stable water soluble protein, and thus encapsulated NPs are also stably dispersed in aqueous solution. The sequence of a protein can be modified by protein engineering to enable the protein to bind to a specific molecule, e.g.an aptamer, these NPs are likely to be biocompatible and would have significant potential for biological imaging applications.
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. The average size of the obtained NPs was 6.6 ± 0.7 nm.
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. Luminescence showed maximum intensity at Eu doping ratio of 11.4%. Strong red luminescence was easily observed by eye, even in a brightly lit room. It is known that O-H vibrations play a dominant role in the non-radiative transition from excited Eu3+ ions and that transition is greatly reduced by substituting hydrogen (H) to deuterium (D). To investigate the non-radiative transition pathway of Y:Eu and Y-V:Eu NPs, the luminescence lifetime of these NPs in water (H2O) and heavy water (D2O) were compared. The lifetime difference in H2O solution and D2O solution is six times larger in Y:Eu than Y-V:Eu NPs at low Eu concentration. This means non-radiative transition due to the O-H vibration is smaller in Y-V:Eu NPs. Accordingly, Y-V NPs showed strong luminescence compared to Y:Eu NPs which we reported previously . Dy-doped Y-V (Y-V:Dy) NPs were also synthesized in apoferritin cavities and showed luminescence peaks at 482 nm and 572 nm, corresponding to 4F9/2 - 6H15/2 and 4F9/2 - 6H13/2transitions. Luminescence showed maximum intensity at Dy doping ratio of 3.5%. This solution had a yellow color under UV irradiation.
 T. Harada and H. Yoshimura, J. Appl. Phys, 114, 044309 (2013).