Copper shells were fabricated by a displacement method around Co nanoparticles (3.2 +/- 0.6 nm) at room temperature in a copper-citrate aqueous electrolyte. The nanoparticles were synthesized by a wet chemical approach using the surfactant sulfobetaine, dodecyldimethyl (3-sulfopropyl) ammonium hydroxide (98%) in tetrahydrofuran. X-ray absorption near-edge structure analysis confirmed that cobalt oxide was not present in the nanoparticles upon exposure to air, consistent with a shell formation. Additionally, the presence of the shell resulted in an increase of the blocking temperature of the core-shell nanoparticles, stabilizing the ferromagnetic behavior up to 235 K. (C) 2004 The Electrochemical Society.