Core/shell particles were synthesized by assembling oppositely charged ferrite (Fe3O4 or NiFe2O4) nanoparticles on the surface of monodispersed silica core particles (having size similar to 0.4 mu m) prepared by hydrolysis and condensation of tetraethylortosilicate. Optimal conditions for synthesis of silica core/nano-Fe3O4 shell particles were found at pH similar to 5.4. The obtained particles have superparamagnetic behavior above a blocking temperature of approximate to 25 K, which make them very attractive for a broad range of biomedical and bioengineering applications. Incorporation of nickel into ferrite structure could not be achieved at lower pH value, so functionalization of core particles was required. Incorporation of nickel into ferrite structure was successful at pH above 7, however at higher pH the formation rate of nickel-ferrite particles becomes very fast and the self-aggregation dominates the competing formation of the nickel-ferrite shell. Because of that the self-aggregation was prevented by surface modification of nickel-ferrite nanoparticles with citric acid before their deposition on the functionalized silica core and homogenous and continuous NiFe2O4 shell was finally obtained. (C) 2012 Elsevier Ltd. All rights reserved.