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Superparamagnetic nanoparticles coated with natural polysaccharides are of great interest for biomedical applications due to their fast response to an external and high-frequency electromagnetic field and their biocompatibility. One of the greatest challenges in the development of these nanoparticles is related to the specific characteristics and properties of the polysaccharide used, which could guarantee the development of specific sizes, crystalline structure, magnetic response and its reproducibility. In this study, we present the preparation of magnetite (Fe 3 O 4) and cobalt ferrite (CoFe 2 O 4) nanoparticles by the co-precipitation method in aqueous solutions at different concentrations (1, 2, 3, 4 and 5%) of a very well-characterized agar obtained from the red marine algae Gelidium robustum. It is shown that under the same experimental conditions and using the adequate precursors, coated high-crystallinity magnetite nanoparticles are obtained. In contrast, for cobalt ferrite, our results indicated the formation of coated low-crystallinity CoFe 2 O 4 particles. Superparamagnetic nanoparticles (with crystal grain size < 8 nm) of Fe 3 O 4 agar coated at ≥ 3% agar showed high-saturation magnetization and high degree of biocompatibility and can be considered as promising candidates for biomedical applications.

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