We report a new method to prepare magnetite nanoparticles with narrow size distribution and controlled particle size. Magnetite nanoparticles were prepared in the pores of the biopolymer chitosan. To develop pores with different sizes, chitosan was gelificated with several concentrations of the crosslinker glutaraldehyde. X-ray diffraction studies showed pattern characteristic for magnetite particles with diameters varying from 4 to 12.7 nm. Mössbauer spectra at room temperature showed only paramagnetic component for the smaller particles, and magnetic and paramagnetic components for the bigger particles, indicating the transition from superparamagnetic to blocked magnetic regime as the particle size increases. The zero field cooling (ZFC) magnetization measurements showed peak temperatures varying from 55 to 151.4 K. The ZFC peak temperatures showed a trend in agreement with the size of the particles. Considering the different experimental time window between Mössbauer spectroscopy and magnetometry, the results obtained from both techniques are in agreement. © 2013 Elsevier B.V.275 7174Dos Santos, D.S., Goulet, P.J.G., Pieczonka, N.P.W., Oliverira, O.N., Aroca, R.F., Gold nanoparticle embedded, self-sustained chitosan films as substrates for surface-enhanced Raman scattering (2004) Langmuir, 20, pp. 10273-10277Reynolds, C.H., Annan, N., Beshah, K., Huber, J.H., Shaber, S.H., Lenkinski, R.E., Wortman, J.A., Gadolinium-loaded nanoparticles: New contrast agents for magnetic resonance imaging (2000) Journal of the American Chemical Society, 122, pp. 8940-8945Jain, T.K., Morales, M.A., Sahoo, S.K., Leslie-Pelecky, D.L., Labhasetwar, V., Iron oxide nanoparticles for sustained delivery of anticancer agents (2005) Molecular Pharmaceutics, 2, pp. 194-205Li, G.-Y., Jiang, Y.-R., Huang, K.-L., Ding, P., Chen, J., Preparation and properties of magnetic Fe3O 4-chitosan nanoparticles (2008) Journal of Alloys and Compounds, 466, pp. 451-456Tran, H.V., Tran, L.D., Nguyen, T.N., Preparation of chitosan/magnetite composite beads and their application for removal of Pb(II) and Ni(II) from aqueous solution (2010) Materials Science and Engineering: C, 30, pp. 304-310Liu, X., Hu, Q., Fang, Z., Zhang, X., Zhang, B., Magnetic chitosan nanocomposites: A useful recyclable tool for heavy metal ion removal (2009) Langmuir, 25, pp. 3-8Kim, D.-H., Kim, K.-N., Kim, K.-M., Lee, Y.-K., Targeting to carcinoma cells with chitosan- and starch-coated magnetic nanoparticles for magnetic hyperthermia (2009) Journal of Biomedial Materials Research: A, 88, pp. 1-11Bae, K.H., Park, M., Do, M.J., Lee, N., Ryu, J.H., Kim, G.W., Kim, C., Hyeon, T., Chitosan, oligosaccharide-stabilized ferrimagnetic iron oxide nanocubes for magnetically modulated cancer hyperthermia (2012) ACS Nano, 6, pp. 5266-5273Wang, Y., Li, B., Zhou, Y., Jia, D., In situ mineralization of magnetite nanoparticles in chitosan hydrogel (2009) Nanoscale Research Letters, 4, pp. 1041-1046Hernández, R., Zamora-Mora, V., Sibaja-Ballestero, M., Vega-Baudrit, J., López, D., Mijangos, C., Influence of iron oxide nanoparticles on the rheological properties of hybrid chitosan ferrogels (2009) Journal of Colloid and Interface Science, 339, pp. 53-59Gorski, C.A., Scherer, M., Determination of nanoparticulate magnetite stoichiometry by Mössbauer spectroscopy, acidic dissolution, and powder X-ray diffraction: A critical review (2010) American Mineralogist, 95, pp. 1017-1026Řezníček, R., Chlan, V., Štěpá nková, H., Novák, P., Maryško, M., Magnetocrystalline anisotropy of magnetite (2012) Journal of Physics: Condensed Matter, 24, pp. 0555011-0555017Allia, P., Coisson, M., Tiberto, P., Vinai, F., Knobel, M., Novak, M.A., Wallace De Castro, N., Granular Cu-Co alloys as interacting superparamagnets (2001) Physical Review B, 64, pp. 144420-144432Shtrikmann, S., Wohlfarth, E.P., The theory of the Vogel-Fulcher law of spin glasses (1981) Physics Letters A, 85, pp. 467-470García-Otero, J., Porto, M., Rivas, J., Bunde, A., Influence of dipolar interaction on magnetic properties of ultrafine ferromagnetic particles (2000) Physical Review Letters, 84, pp. 167-170Webster, A., Halling, M.D., Grant, D.M., Metal complexation of chitosan and its glutaraldehyde cross-linked derivative (2007) Carbohydrate Research, 342, pp. 1189-1201Bhatia, S.C., Ravi, N., A Mössbauer study of the interaction of chitosan and d-glucosamine with iron and its relevance to other metalloenzymes (2003) Biomacromolecules, 4, pp. 723-727Hernández, R.B., Franco, A.P., Yola, O.R., López-Delgado, A., Felcman, J., Recio, M.A.L., Mercê, A.L.R., Coordination study of chitosan and Fe3+ (2008) Journal of Molecular Structure, 877, pp. 89-99Thongngam, M., McClements, D.J., Characterization of interactions between chitosan and an anionic surfactant (2004) Journal of Agricultural and Food Chemistry, 52, pp. 987-991