A nanoparticle comprises a metallic material at least partially covered or embedded in a hafnium oxide material whereat least 80% of the metallic material is protected from any interaction with a biological material by the hafnium oxide material. A composition containing the nanoparticle is disclosed. The nanoparticle and/or composition can be used as a diagnostic agent when the nanoparticle is exposed to radiation or can be suitable for use as a therapeutic agent in the treatment of cancer.