Dynamic Nuclear Polarization of Silicon Carbide Micro- and Nanoparticles.

Two dominant crystalline phases of silicon carbide (SiC): α-SiC and β-SiC, differing in size and chemical composition, were investigated regarding their potential for dynamic nuclear polarization (DNP). Si nuclei in α-SiC micro- and nanoparticles with sizes ranging from 650 nm to 2.2 μm and minimal oxidation were successfully hyperpolarized without the use of free radicals, while β-SiC samples did not display appreciable degrees of polarization under the same polarization conditions. Long relaxation times in α-SiC of up to 1600 s (∼27 min) were recorded for the Si nuclei after 1 h of polarization at a temperature of 4 K. Interestingly, these promising α-SiC particles allowed for direct hyperpolarization of both Si and C nuclei, resulting in comparably strong signal amplifications. Moreover, the relaxation time of C nuclei in 750 nm-sized α-SiC particles was over 33 min, which far exceeds times of conventional C DNP probes with values in the order of 1-2 min. The present work demonstrates the feasibility of DNP on SiC micro- and nanoparticles and highlights their potential as hyperpolarized magnetic resonance imaging agents.