Hollow MnOxPy and Pt/MnOxPy yolk/shell nanopartides were fabricated via the nanoscale acid-etching process from the solid MnO and Pt/MnO core/shell nanoparticles, respectively. In the synthesis, alkylphosphonic acid impurity in trioctylphosphine oxide was a key component, resulting in amorphous hollow metal phosphate nanoparticles. Hollow nanoparticles containing Mn2+ ions showed positively enhanced T-1 relaxation, in which the r(1) values of the hollow MnOxPy and Pt/MnOxPy yolk/shell were greater than that of the original MnO nanocrystals. The increased surface area of the hollow nanoparticles enhanced interaction between Mn2+ ions of the nanoparticle surface and water molecules. Cytotoxicity experiment revealed that Mn ions released from hollow walls of the MnOxPy nanoparticles were responsible for the cytotoxicity, while Pt ions from the Pt/MnOxPy yolk/shell were not released in the cells. These nanoparticles provide potential insights into an anticancer drug, enabling simultaneous T-1 magnetic resonance imaging (MRI) and therapy. (C) 2014 Elsevier Inc. All rights reserved.