UCLA

The Gd element and other rare-earth metals are notoriously difficult to use in chemical synthesis due to their high reduction potentials and aggressive reactivities with ambient oxygen, which almost always leads to the formation of oxides. The challenge in chemical synthesis limits the range of applications for rare-earth metals. The most important use of Gd at the moment is nanocrystals for technological applications. Herein, we report for the first time the successful production of size-controllable, solid core-shell oxide-free Gd metal nanocrystals. We have solved the long-standing problem of oxidation through a reduction process and appropriate capping. The manuscript describes the procedure and detailed characterizations of the process to ensure the highest quality of the produced particles. In particular, the nanocrystals displayed the largest saturation magnetization observed to date for nanocrystalline Gd metal. This value (206 emu/g at 2K) currently stands as the world record.

Another important application of Gd is contrast agent development for MRI. To this end, we have performed NMR relaxivity measurements to evaluate the performance of the nanoparticles as potential MRI contrast agents. Typically, Gd based nanoconstructs such as FDA approved Gd-DTPA chelates are T1 MRI contrast agents; however, we demonstrate, for the first time, that pure Gd nanoparticles can also be used as state-of-the art T2 contrast agents. World record high values for transverse proton relaxitivity (r2 of 232 mM-1s-1 per Gd atom and per-particle relaxivity (2.9 x 10^8 mM-1s-1 have been obtained, exceeding the current highest per-particle r2 values. These results make our Gd nanocrystals the most promising MRI contrast agents for use in biomedical applications. For the first time, this puts MRI on par with positron emission tomography in terms of sensitivity to detection of a contrast agent.

We further developed the nanoparticles and we demonstrate record high saturation magnetizations for Gd nanoparticles, namely, 226 emu/g at 5 T. This magnetization is substantially higher than anything achieved to date. We have achieved such high magnetizations in a reliable and reproducible manner by controlling the crystallinity of the grown Gd nanofilm. The crystallinity of Gd is found to play an important role in the observed magnetization values. The higher magnetization is observed for nanoparticles that have a lower content of paramagnetic face-centered cubic (fcc) phase and greater content of ferromagnetic hexagonal close-packed (hcp) phase. Control over fcc and hcp content in the lattice was achieved by adjusting the deposition rate of Gd metal during the nanofabrication process. Our results indicate the remarkable influence of nanocrystallinity on the magnetism of Gd and the ability to control it.

Our novel fabrication technique, which overcomes the problems of current synthetic approaches to rare-earth nanoparticle synthesis through the careful optimization of capping and hydrogen reduction techniques, can also be applied to other rare-earth metals and alloys. This opens the door to fundamental studies on these materials at the nanoscale. It will also enable the realization of the full potential of rare-earth metals in industry.