A method of sterilizing a nanoparticle preparation at a high temperature is provided. A plurality of nanoparticles are purified to form the preparation, wherein the nanoparticles comprise at least a core and a shell and the shell comprises one or more ligand species attached to the core. The nanoparticle preparation is made by a purified nanoparticle composition, a carrier fluid and an excess of the one or more ligand species not attached to the core. The ligand species attached to the core and the excess ligand species added after purification are structurally identical. The nanoparticle preparation provided by the present invention may be used as contrast agents in medical imaging techniques such as X-ray and magnetic resonance imaging.