[en] Full text: Properties such as superparamagnetism, high surface area and the easy of synthesis, coating and functionalization, indicates the versatility of magnetic nanoparticles. Superparamagnetic iron oxide composites have been synthesized with a huge variety of polymers or biopolymers for several applications. Polymers coated SPION presents magnetic properties and improved dispersibility. Additionally, these particles have been studied as a promising contrast agent for resonance imaging (MRI) [1]. In this study, magnetite (Fe₃O₄) nanoparticles coated by alginate (SPION-alginate) were synthesized using an ultrasound one pot new method [2]. The structure, size, morphology were characterized by XRD, SEM, the magnetic property using a SQUID magnetometer, and the interaction between iron oxide nanoparticles and coated alginate was characterized by FTIR. From the XRD and SEM measurements we found that the particles crystallite size ranging between 8 nm to 27 nm as a function of alginate concentration and presents highest magnetization of 57 emu.g^-1 (at 6,5 x 10^-4 g.mL^-1 of alginic acid). Also a highly water dispersible and superparamagnetic composite were obtained at concentration of 0,01 g.mL-1 of alginic acid and MRI characterization shows that r1 and r2 relaxativities (on field of 0,47 T, at 25 °C) were 4,39 s^-1.mM^-1 and 63,3 s^-1.mM^-1, respectively. References [1] Dias, A.M.G.C., Hussain, A., Marcos, A. S., & Roque, A. C. A. (2011). A biotechnological perspective on the application of iron oxide magnetic colloids modified with polysaccharides. Biotechnology advances, 29(1), 142-155. [2] De Araújo, A.C.V., de Oliveira, R.J., Júnior, S. A., Rodrigues, A.R., Machado, F.L.A., Cabral, F.A.O., & de Azevedo, W. M. (2010). Synthesis, characterization and magnetic properties of polyaniline-magnetite nanocomposites. Synthetic Metals, 160(7-8), 685-690. (author)