Synthesis and characterization of Fe^sub 3^O^sub 4^:Yb^sup 3+^:Er^sup 3+^ nanoparticles with magnetic and optical properties for hyperthermia applications

The synthesis and characterization of ytterbium (Yb3+) and erbium (Er3+) co–doped Fe3O4 nanoparticles (NPs) was reported for the first time in this work. A set of samples with fixed Er3+ (1 mol%) and different Yb3+concentrations (1, 10, 20 and 50 mol%) were prepared and systematically characterized to study their structural, magnetic, thermal and optical properties. The superparamagnetic behavior of Fe3O4 and Yb3+ and Er3+co–doped Fe3O4 NPs was verified by the hysteresis loop. The co–doped Fe3O4 NPs also presented an upconversion emission in the green and red region. The Fe3O4 and Fe3O4: Yb3+, Er3+ NPs showed hyperthermic behavior for an externally applied AC magnetic field, observing that the introduction of Yb3+ and Er3+ induced a temperature increase of 34.4% in optimal conditions with respect to the non–doped Fe3O4 with a reduction in saturation magnetization of 7.8%. These observations suggest that Fe3O4: Yb3+, Er3+ NPs turns out to be an important candidate in biomedical applications for simultaneous optical imaging and hyperthermic therapy.