Assisted Synthesis of Coated Iron Oxide Nanoparticles for Magnetic Hyperthermia

Abstract

Magnetite nanoparticles were synthesized by the co-precipitation method with and without the assistance of an additive, namely, gelatin, agar-agar or pectin, using eco-friendly conditions and materials embodying a green synthesis process. X-ray diffraction and transmission electron microscopy were used to analyze the structure and morphology of the nanoparticles. Magnetic properties were investigated by SQUID magnetometry and ⁵⁷Fe Mössbauer spectroscopy. The results show that the presence of the additives implies a higher reproducibility of the morphological magnetic nanoparticle characteristics compared with synthesis without any additive, with small differences associated with different additives. To assess their potential for magnetic hyperthermia, water-based suspensions of these nanoparticles were prepared with and without citric acid. The stable solutions obtained were studied for their structural, magnetic and heating efficiency properties. The results indicate that the best additive for the stabilization of a water-based emulsion and better heating efficiency is pectin or a combination of pectin and agar-agar, attaining an intrinsic loss power of 3.6 nWg^-1.

Keywords: biocompatible coating; green synthesis; iron oxide nanoparticles; magnetic hyperthermia.

Figure 1

XRD patterns of iron oxide nanoparticles synthesized using a co-precipitation method. The diffractograms were normalized by the maximum of (311) diffraction in all cases, and the base line was displaced between every curve for better view.

Figure 2

(a) FTIR spectra of gelatin, agar-agar and Pectigel (pectin-containing food-grade additive) and (b) of the various synthesized powders. Some of the spectra are displaced vertically for better visualization, and the peaks considered associated with the different additives are marked by green stars (gelatin), blue circles (agar-agar) and violet diamonds/dash-dot lines in the case of Pectigel/pectin.

Figure 2

(a) FTIR spectra of gelatin, agar-agar and Pectigel (pectin-containing food-grade additive) and (b) of the various synthesized powders. Some of the spectra are displaced vertically for better visualization, and the peaks considered associated with the different additives are marked by green stars (gelatin), blue circles (agar-agar) and violet diamonds/dash-dot lines in the case of Pectigel/pectin.

Figure 3

(a) Mossbauer experimental spectra (open circles) and fitted curves (blue lines) of the as-prepared MNP M1 and M2 (without additives). Cyan and green lines correspond to the contributions of the magnetic hyperfine interactions in site 1 and site 2, respectively, associated with different oxidation states of iron. The corresponding distributions of the magnetic hyperfine fields are plotted in (b), using the same colours.

Figure 4

(a) Mossbauer experimental spectra (open circles) and fitted curves (blue lines) for samples synthetized with the aid of gelatin (M–g), Pectigel (M–p) and agar-agar (M–a) additives. Cyan and green lines correspond to the contributions of the magnetic hyperfine interactions in sites 1 and 2, respectively, associated with different oxidation states of iron. The corresponding distributions of the magnetic hyperfine fields are plotted in (b), using the same colours. For M–p, the component fitted with a quadrupole doublet is drawn with an orange line and is associated with a superparamagnetic behavior of the smaller NP.

Figure 5

(a) Mossbauer experimental spectra (open circles) and fitted curves (blue lines) for samples synthetized with the assistance of mixed media at 2:1 ratio: pectin–gelatin (M–pg) and pectin–agar-agar (M–pa). The cyan lines correspond to the contribution of magnetic hyperfine interactions and are associated to the magnetic hyperfine field distributions shown in (b). The orange lines represent the components fitted with quadrupole doublets and are related to a superparamagnetic behavior of the smaller NP.

Figure 6

Magnetization results for the MNPs synthesized without additives (a,b) or with a single additive (c,d). Left—temperature dependence after zero-field cooling (ZFC) (full symbols) and after cooling under a measuring field of 2 mT (FC) (open symbols); right—isothermal hysteresis curves at 300 K showing, in the inset, the zoomed low-field region, confirming negligible coercivity.

Figure 6

Magnetization results for the MNPs synthesized without additives (a,b) or with a single additive (c,d). Left—temperature dependence after zero-field cooling (ZFC) (full symbols) and after cooling under a measuring field of 2 mT (FC) (open symbols); right—isothermal hysteresis curves at 300 K showing, in the inset, the zoomed low-field region, confirming negligible coercivity.

Figure 7

Magnetization results for the MNP synthesized with two additives: (a) temperature dependence after zero-field cooling (ZFC) (full symbols) and after cooling under the measuring field of 2 mT (FC) (open symbols); (b) isothermal hysteresis curves at 300 K showing, in the inset, the zoomed low-field region, confirming negligible coercivity.

Figure 8

Typical TEM images of the grid-dried citric acid-based ferrofluids and corresponding size distributions for samples (a) M1_CA and (b) M2_CA.

Figure 9

Typical TEM images of the grid-dried citric acid-based ferrofluids with the corresponding size distributions for coated NPs in samples (a) M–g_CA, (b) M–p_CA, (c) M–a_CA, (d) M–pg_CA and (e) M–pa_CA.

Figure 9

Typical TEM images of the grid-dried citric acid-based ferrofluids with the corresponding size distributions for coated NPs in samples (a) M–g_CA, (b) M–p_CA, (c) M–a_CA, (d) M–pg_CA and (e) M–pa_CA.

Figure 10

Typical TEM images of the grid-dried water dispersions for (a) M–p_MQ and (b) M–pa_MQ.

Figure 11

Magnetization results for the ferrofluids: (a,c) temperature dependence after zero-field cooling (ZFC) (full symbols) and after cooling under the measuring field of 2 mT (FC) (open symbols); (b,d) isothermal magnetization curves at 250 K.

Figure 12

Temperature evolution during applied ac magnetic field for samples: (a) M–a_CA and (b) M–p_CA, showing good reproducibility of the results and the good quality of the fit.