Synthesis of Uniformly Sized Manganese Oxide Nanocrystals with Various Sizes and Shapes and Characterization of Their T1 Magnetic Resonance Relaxivity

Abstract

We synthesized manganese oxide (MnO and Mn 3O 4) nanocrystals with various sizes and shapes by the thermal reaction of a Mn II-oleate complex through a "heat-up process". When a Mn II-oleate complex was thermally decomposed in non-coordinating hydrocarbon solvents, uniformly sized MnO nanocrystals with cubic and octahedral shapes were produced. We were able to synthesize anisotropic, multibranched MnO nanocrystals by the oriented attachment of MnO truncated-nanocube building blocks. When the Mn II-oleate complex was heated in 1-hexadecene in the presence of strongly coordinating carboxylic acid surfactants, spherical nanocrystals were generated, and their diameter was controlled in the range 3-13 nm by varying the chain length of the carboxylic acid. When oleyl alcohol was added to the Mn-oleate complex in phenyl ether, tetrahedral MnO nanocrystals were synthesized. The as-synthesized MnO nanocrystals were oxidized in air to Mn 3O 4 or MnO/Mn 3O 4 core-shell structures, which exhibited exchange coupling with shifted magnetic hysteresis loops. The effect of the size and shape of the phospholipid-capped manganese oxide nanocrystals on their applicability as T 1 contrast agents in magnetic resonance imaging (MRI) were examined. As the size of the nanocrystals decreased, their relaxivities increased, thereby generating brighter MR images. In particular, spherical 3 nm-sized Mn 3O 4 nanocrystals had a high specific relaxivity (r 1) of 2.38 mM -1 s -1, clearly demonstrating their potential for use as an efficient T 1 MRI contrast agent. Manganese oxide nanocrystals with various sizes and shapes were synthesized by thermal decomposition of a Mn-oleate complex. The effect of the size and shape of the phospholipid-capped nanocrystalson their applicability as T 1 contrast agents in magnetic resonance imaging (MRI) was examined. As their size decreased, their specific relaxivities increased, which generated brighter MR images. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.