Introduction: Superparamagnetic iron oxide nanoparticles due to their unique features, are one of the mostly used nanoparticles in the fields of biotechnology and medicine. The aim of this study was to facile biosynthesis of SPIONs by Bacillus safensis MN18 isolated from the Caspian Sea. Materials and Methods: B. safensis MN18 were grown overnight on Luria-Bertani )LB (media. Then, 50 ml of bacterial suspension was added in to 50 ml of 2.5 mM FeCl3.6H2O solution and incubated overnight at 28°C. Changing color of solution from golden yellow to dark brown was an indication for NPs biosynthesis. The biosynthesized SPIONs were purified and further characterized by UV-vis spectrophotometry, X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and vibrating sample magnetometer (VSM). Results and Discussion: Results showed that the SPIONs had a cubic structure with the size of about 56 nm, and maximum absorbance at 270 nm. FT-IR analysis revealed that there were some different functional groups on the surface of nanoparticles, also the synthesized iron oxide nanoparticles had superparamagnetic property with saturation magnetization of 7.67 emu / g. In conclusion B. safensis MN18 is an appropriate biological agent for biosynthesis of SPIONs by reduction of FeCl3.6H2O salt. These nanoparticles could be considered for several applications such as drug delivery and cell separation.

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