The present invention relates to a water suspension of graphene oxide nanoflakes decorated by nanoparticles of metallic platinum, characterised in that it comprises: •; 100 μg/ml to 500 μg/ml of graphene oxide nanoflakes, •; 5 μg/ml to 50 μg/ml of nanoparticles of metallic platinum (Pt°), deposited on the surface of graphene oxide nanoflakes. The invention also relates to a method of preparing such suspension and its use as an antitumor agent.