Dynamic Nuclear Polarization of Silicon Carbide Micro- and Nanoparticles

Two dominant crystalline phases of silicon carbide (SiC): alpha-SiC and beta-SiC, differing in size and chemical composition, were investigated regarding their potential for dynamic nuclear polarization (DNP). Si-29 nuclei in alpha-SiC micro- and nanoparticles with sizes ranging from 650 nm to 2.2 mu m and minimal oxidation were successfully hyperpolarized without the use of free radicals, while beta-SiC samples did not display appreciable degrees of polarization under the same polarization conditions. Long T-1 relaxation times in alpha-SiC of up to 1600 s (similar to 27 min) were recorded for the Si-29 nuclei after 1 h of polarization at a temperature of 4 K. Interestingly, these promising alpha-SiC particles allowed for direct hyperpolarization of both Si-29 and C-13 nuclei, resulting in comparably strong signal amplifications. Moreover, the T-1 relaxation time of C-13 nuclei in 750 nm-sized alpha-SiC particles was over 33 min, which far exceeds T-1 times of conventional C-13 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.