pubs.acs.org/cm Published on Web 04/30/2010 r 2010 American Chemical Society Chem. Mater. 2010, 22, 3353–3361 3353 DOI:10.1021/cm903837g CoFe 2 O 4 and CoFe 2 O 4 /SiO 2 Core/Shell Nanoparticles: Magnetic and Spectroscopic Study Carla Cannas,* ,† Anna Musinu, † Andrea Ardu, † Federica Orr u, † Davide Peddis, † Mariano Casu, † Roberta Sanna, † Fabrizio Angius, ‡ Giacomo Diaz, ‡ and Giorgio Piccaluga † † Dipartimento di Scienze Chimiche, Universit  a di Cagliari, Cittadella Universitaria, 09042, Monserrato (Cagliari), Italy, and ‡ Dipartimento di Scienze e Tecnologie Biomediche Sezione di Patologia Sperimentale via Porcell, 4 - 09100 Cagliari (CA), Italy Received December 23, 2009. Revised Manuscript Received March 23, 2010 Spherical nanoparticles of surfactant-coated CoFe 2 O 4 (core) were prepared through thermal decomposition of metal acetylacetonates in the presence of a mixture of oleic acid and oleylamine and uniformly coated with silica shell by using tetraethylorthosilicate (TEOS) and ammonia in a micellar solution (core/shell). Transmission electron microscopy (TEM) analysis of core/shell nanoparticles evidenced the high homogeneity of the coating process in producing single core/shell nanoparticles with a narrow size distribution. The combined use of spectroscopic studies (NMR and FTIR) on core and core/shell nanoparticles pointed out that the surfactants’ layer bound to the surface core nanoparticles is retained also after the silica coating process. This allows to obtaining systems with very similar magnetic behavior but weaker dipolar interparticle interactions and lower values of saturation magnetization. In view of the interest in biomedical field, the effect of the CoFe 2 O 4 nanoparticles silica coating was also studied by controlling the possible modifications in cytotoxicity by trypan blue and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) assays on human cells. Introduction The use of magnetic nanoparticles for biomedical purposes has been proposed to a large extent in recent years. 1-4 This kind of application requires that the surface of the nanoparticles be modified both for protec- tion and functionalization needs. 5,6 This is particularly important in the case of cobalt ferrites nanoparticles. CoFe 2 O 4 is in fact very attractive in the biomedical field for its high magnetic anisotropy and saturation magneti- zation which give rise to suitable magnetic behavior at room temperature, but the presence of cobalt makes it potentially toxic. 7,8 To protect magnetic nanoparticles, encapsulation both in polymeric and inorganic matrixes has been proposed, 9 but silica has been most often used. 10-12 This is why SiO 2 provides coating shells that are stable, nontoxic, hy- drophilic, and biocompatible, and in addition, it can be easily functionalized to bind biomolecules on its surface silanolic groups. However, it has been reported that the shell of SiO 2 can alter the properties of the magnetic coated core. 13 This can be ascribed to the modification of interparticles interactions, to the arising of novel properties in confined nanoparticles and, more probably, to the surface effects at the core/shell contact. In particular, this effect has been observed in Mn and Co spinel ferrite-silica nanoparti- cles. 14 In this case the modification of magnetic properties (for instance, coercivity) was greater in MnFe 2 O 4 than in CoFe 2 O 4 , presumably because the high magnetocrystal- line anisotropy of cobalt ferrite diminishes the impor- tance of the contribution of surface anisotropy to the total anisotropy and its variation upon coating. The present paper examines the effects on magnetic properties of CoFe 2 O 4 nanoparticles induced by their coating with SiO 2 . Among the many methods proposed for the nanoferrites’ preparation (precipitation from *Corresponding author. E-mail:ccannas@unica.it. (1) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D, Appl. Phys. 2003, 36, R167. (2) Pankhurst, Q. A.; Thanh, N. K. T.; Jones, S. K.; Dobson, J. J. Phys. D, Appl. Phys. 2009, 42, 224001. (3) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995. (4) Roca, A. G.; Costo, R.; Rebolledo, A. 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