Easy Synthesis and Characterization of Holmium-Doped SPIONs

Abstract

The exceptional magnetic properties of superparamagnetic iron oxide nanoparticles (SPIONs) make them promising materials for biomedical applications like hyperthermia, drug targeting and imaging. Easy preparation of SPIONs with the controllable, well-defined properties is a key factor of their practical application. In this work, we report a simple synthesis of Ho-doped SPIONs by the co-precipitation route, with controlled size, shape and magnetic properties. To investigate the influence of the ions ratio on the nanoparticles’ properties, multiple techniques were used. Powder X-ray diffraction (PXRD) confirmed the crystallographic structure, indicating formation of an Fe₃O₄ core doped with holmium. In addition, transmission electron microscopy (TEM) confirmed the correlation of the crystallites’ shape and size with the experimental conditions, pointing to critical holmium content around 5% for the preparation of uniformly shaped grains, while larger holmium content leads to uniaxial growth with a prism shape. Studies of the magnetic behaviour of nanoparticles show that magnetization varies with changes in the initial Ho³⁺ ions percentage during precipitation, while below 5% of Ho in doped Fe₃O₄ is relatively stable and sufficient for biomedicine applications. The characterization of prepared nanoparticles suggests that co-precipitation is a simple and efficient technique for the synthesis of superparamagnetic, Ho-doped SPIONs for hyperthermia application.

Keywords: SPIONs; endoradiotherapy; holmium; rare-earth doping; superparamagnetic iron oxide nanoparticles.

Figure 1

Image of the co-precipitation of Ho-doped ferrite nanoparticles under NH₃ addition.

Figure 2

TEM images of (a) undoped iron oxide SPIONs, and doped with (b) 1%; (c) 2.5%; (d) 5%; (e) 7.5%; (f) 10% of holmium obtained by co-precipitation (scale bar: 50 nm).

Figure 3

Histogram for Fe₃O₄@2.5%Ho.

Figure 4

Size distribution by volume for Fe₃O₄@2.5%Ho unmodified (blue curve) and modified with CEPA (black curve).

Figure 5

Thermograms of the SPIONs modified with 3-phosphoropropionic acid: (a) Fe₃O₄@1%Ho@CEPA, and (b) Fe₃O₄@2.5%Ho@CEPA.

Figure 6

PXRD patterns of undoped and Ho-doped nanoferrite crystallites.

Figure 7

XPS survey spectrum of Fe₃O₄@2.5 at. % Ho nanoparticles.

Figure 8

XPS spectra of (a) Fe 2p and (b) Ho 4d region for Fe₃O₄@2.5%Ho SPIONs.

Figure 9

Magnetization isotherms for SPIONs with different Ho content measured at (a) 100 K and (b) 300 K.

Figure 10

Magnetization loops K for (a) Fe₃O₄, (b) Fe₃O₄@1% Ho registered at 100 K and 300 K from −200 Oe to 200 Oe.

Figure 11

Values of (a) saturation magnetization at 20.000 Oe and (b) coercive field as a function of holmium content in SPIONs measured at 100 K and 300 K. Error bars are smaller than the data symbols.

Figure 12

Temperature dependence of ZFC and FC magnetization at 100 Oe for nanoparticles doped with (a) 1% and (b) 2.5% of holmium.