Manganese-containing nanoscale metal-organic frameworks (NMOFs) with controllable morphologies were synthesized using reverse-phase microemulsion techniques at room temperature and a surfactant-assisted procedure at 120 degrees C with microwave heating. The nanoparticles were characterized using a variety of methods including SEM, TEM, TGA, PXRD, and ICP-MS. Although the nanoparticles gave a modest longitudinal relaxivity (r(1)) on a per Mn basis, they provided an efficient vehicle for the delivery of large doses of Mn2+ ions which exhibited very high in vitro and in vivio r(1) values and afforded excellent MR contrast enhancement. The particle surface was also modified with a silica shell to allow covalent attachment of a cyclic RGD peptide and an organic fluorophore. The cell-targeting molecules on the Mn NMOFs enhanced their delivery to cancer cells to allow for target-specific MR imaging in vitro. The MR contrast enhancement was also demonstrated in vivo using a mouse model. Such core-shell hybrid nanostructures provide an ideal platform for targeted delivery of other imaging and therapeutic agents to diseased tissues.