A process for making nanoparticles of biocompatible materials is described, wherein an aqueous reaction mixture comprising a metal ion, a biguanide, an oxidant, and water; optionally further comprising an alpha-amino acid or a nucleobase; is provided along with temperature conditions to directly form within the reaction mixture, a stable dispersion of metal-containing nanoparticles. Biocompatible nanoparticles comprised of cerium and a biguanide, and optionally containing an alpha-amino acid or a nucleobase, are also described. The use of metal oxide nanoparticles comprising a biguanide as a nanoparticle core/corona in the preparation of nanoscale ionic (liquid) material compositions is disclosed.