Gold-coated nanoparticles comprising: a) a magnetic core Of XFe2O4 wherein X is a metal selected from the group consisting of Fe, Mn, and Co; b) carbohydrates covalently coupled through a spacer to the gold-coated nanoparticle, wherein said spacer has a thiol linking it to the nanoparticle; and c) an immunoglobulin-binding protein coupled to the gold-coated nanoparticle through an amphiphilic molecule, comprising a functional group capable of coupling the protein and through a thiol group which is attached to the gold coating of the nanoparticle, where a immunoglobulin-binding protein can be coupled to further bioconjugation with antibodies. Said nanoparticles have been found to be useful as contrast agents by MRI.