Photodynamic cancer therapy is one of the most promising ways treating cancer.  It is based on the ability of photosensitizers (PS) to produce a reactive oxygen species in the cancer-affected tissues under photo-excitation by light of a certain wavelength.  The major drawback of this method is the low penetration of visible light into tissues.

A novel approach to visible light delivery to the PS-saturated cancerous tissue includes the application of the up-conversion mechanism: instead of low-penetrating visible radiation, one can use much better penetrating IR radiation and up-convert it in situ, i.e., at the deep layers of damaged tissue, where visible radiation cannot penetrate. Yb/Er-, Yb/Tm-, Yb/Ho-activated NaYF₄ are some of the most efficient up-converting phosphors [1]. NaYF₄ forms two types of the solid solutions: a cubic fluorite structure Na_0.5-xY_0.5+xF_2+2x and a hexagonal gagarinite structure Na_3xY_2-xF₆.

NaYF₄:Yb:Er powders can be easily synthesized by co-precipitation from aqueous solutions; and the following factors play crucial role in the preparation of products of acceptable quality: (1) initial concentrations and ratios of starting material solutions, (2) the order of their mixing, (3) concentration of polyethyleneimine (PEI) additive, and (4) pH of the initial solutions.  Namely, 25% excess of NaF is required for the preparation of single cubic phase samples, and preparation should be carried out by addition of metal nitrate solutions to 9 g/L PEI-doped 0.35 M aqueous NaF at pH = 10. According to scanning electron microscopy (SEM) data, this leads to the formation of 50-90 nm particles of cubic phase. The use of PEI concentration of 9 g/L resulted in the precipitation of 100-150 nm particle agglomerates (0.3-1 mm elongated agglomerates, SEM data) (hexagonal phase). Transmission electron microscopy (TEM) study of cubic NaYF₄:Yb:Er samples indicated that they consisted of several nm primary particles with 50 nm embryo agglomerates and 160 nm particle agglomerates. TEM data confirm the realization of the non-classical mechanism of crystal growth via the nanoparticle agglomeration. Powder samples of cubic NaYF₄:Yb:Er provided up to 2% luminescence quantum yield and can be used as a working agent in photodynamic cancer therapy.

References

[1] P.P. Fedorov, A.A. Luginina, S.V. Kuznetsov, V.V. Osiko. // J. Fluorine Chemistry. Vol.132. (2011) pp.1012-1039.

[2] S.V. Kuznetzov, A.V. Ryabova, D.S. Los`, P.P. Fedorov, V.V. Voronov, R.P. Ermakov, V.B. Loshchenov, V.V. Volkov, A.E. Baranchikov, V.V. Osiko. // Nanotechnologies in Russia. Vol.7. (2012) pp.615-628.

 This work was supported by RFBR 12-02-00851-а (Russian Federation State Contract 14.740.12.1343). TEM measurements were carried at the Center of mutual use of equipment at Prokhorov General Physics Institute.

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