The invention relates to nanoparticles made from organometallic hybrid materials made from iron carboxylates, used for example as contrast agents. The particles can also be used for the encapsulation and vectoring of molecules of interest such as active pharmaceutical agents, cosmetically interesting compounds and markers. Apart from the intrinsic properties thereof for imaging, said nanoparticles give good results in terms of capacity of loading with medicaments and in biocompatibility.