The present invention relates to nanoparticles of a metal chosen from among platinum, bismuth or a mixture thereof, which are functionalised at their surface by functionalised polyethylene glycol, comprising in particular at least one OH, COOH, NH2 or SH functional group, and to their method of preparation, which comprises of the following steps: a) Mixing a precursor of nanoparticles with a functionalised polyethylene glycol, in particular comprising at least one OH, COOH, NH2 or SH functional group, in water; b) Exposing the mixture to ionising radiation.