Three bimodal MRI/optical nanosized contrast agents with high per-nanoparticle relaxivity (up to 2523 mM-1 s-1 at 35 MHz and 932 mM-1 s-1 at 200 MHz) have been prepared connecting up to 115 tris-aqua GdIII complexes to fluorescent non-toxic InP/ZnS quantum dots. The structure of the linker has an important effect on the relaxivity of the final multimeric contrast agent. © 2013 The Royal Society of Chemistry.