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

29Si silica nanoparticles (SiO2 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 29Si MR signals via enhanced nuclear spin alignment; to date, there has been limited success employing DNP for SiO2 NPs due to the lack of endogenous electronic defects that are required for the process. To create opportunities for SiO2-based 29Si MRI probes, we synthesized variously featured SiO2 NPs with selective 29Si 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 29Si 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 SiO2 NPs as hyperpolarized contrast agents for in vivo MRI.

Automated summary

29Si silica nanoparticles (SiO2 NPs) show promise as magnetic resonance imaging (MRI) probes, due to their biocompatibility and dispersibility. However, using dynamic nuclear polarization (DNP) for SiO2 NPs has been challenging due to the lack of required electronic defects. In this study, SiO2 NPs were synthesized with selective 29Si isotope enrichment and optimal DNP signal enhancement was achieved by controlling the proportion of 29Si in the shell layer. The effective hyperpolarization depth was found to be less than 10 nm, indicating improved diffusion length for the shell-enriched NPs compared to natural abundance NPs.