Dynamic Nuclear Polarization of Selectively 29 Si-Enriched Core@shell Silica Nanoparticles.

29 Si silica nanoparticles (SiO 2 NPs) are promising magnetic resonance imaging (MRI) probes that possess advantageous properties for in vivo applications, including suitable biocompatibility, tailorable properties, and high water dispersibility. Dynamic nuclear polarization (DNP) is used to enhance 29 Si MR signals via enhanced nuclear spin alignment; to date, there has been limited success employing DNP for SiO 2 NPs due to the lack of endogenous electronic defects that are required for the process. To create opportunities for SiO 2 -based 29 Si MRI probes, we synthesized variously featured SiO 2 NPs with selective 29 Si isotope enrichment on homogeneous and core@shell structures (shell thickness: 10 nm, core size: 40 nm), and identified the critical factors for optimal DNP signal enhancement as well as the effective hyperpolarization depth when using an exogenous radical. Based on the synthetic design, this critical factor is the proportion of 29 Si in the shell layer regardless of core enrichment. Furthermore, the effective depth of hyperpolarization is less than 10 nm between the surface and core, which demonstrates an approximately 40% elongated diffusion length for the shell-enriched NPs compared to the natural abundance NPs. This improved regulation of surface properties facilitates the development of isotopically enriched SiO 2 NPs as hyperpolarized contrast agents for in vivo MRI.