The macrocyclic ligand DOTP is used to assemble a porous, heterometallic metal-organic framework (MOF). This MOF is miniaturizable down to the nanoscale to form stable colloids, is stable in physiological saline solution and cell culture media, and is not cytotoxic. It shows interesting relaxometric properties with r(1) at high field (500 MHz) of 5 mM(-1).s(-1) and a maximum r(1) = 15 mM(-1).s(-1) at 40 MHz, which remains constant over a wide pH range and increases with temperature.