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

Abstract

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.

Exploring the Conformational Equilibrium of Mefenamic Acid Released from Silica Aerogels via NMR Analysis

“…There are five types of Si(O 1/2 ) 4 tetrahedral units (denoted Q 0 , Q 1 , Q 2 , Q 3 , and Q 4 ) that can be observed on the 29 Si NMR spectrum [ 30 , 31 ]. The four different stereochemical units of silicon atoms are the Q 0 tetrahedron, which has no external connections and is more reactive; the Q 1 tetrahedron, which has one external connection and is less reactive than the Q 0 ; the Q 2 middle group, which has two external connections; the Q 3 branching site, which has three external connections; and the Q 4 cross-linked site, which has four external connections and is the least reactive (see Figure 2 ) [ 32 , 33 ]. The chemical shifts of the Si atoms in the different units vary due to the different types of chemical environments around the Si atoms and cover the overall range of ca.…”