Characterization and Water-Proton Longitudinal Relaxivities of Liposome-Type Radical Nanoparticles Prepared via a Supramolecular Approach

For the construction of metal-free magnetic resonance imaging (MRI) contrast agents, radical-based nanoparticles (RNPs) are promising materials because they allow the water-proton longitudinal relaxivity (r₁) to be enhanced not only by paramagnetic resonance effects but also by prolonging the rotational correlation times (τ_R). However, the τ_R effect is limited because the radical units are often located within the central hydrophobic core of oil-in-water (o/w) emulsions, resulting in a lack of water molecules surrounding the radical units. In this study, to construct supramolecular RNPs that have high r₁ values, we designed a liposome-type RNP in which the radical units are located at positions with sufficient surrounding water molecules. Using this strategy, PRO1 with a PROXYL framework was prepared by introducing hydrophilic groups on both sides of the radical unit. The RNP composed of PRO1 formed spherical nanoparticles approximately 100 nm in size and yielded a higher r₁ value (0.26 mM^-1 s^-1) compared to those of small radical species and similar supramolecular o/w emulsion-type nanoparticles (0.17 mM^-1 s^-1 in PRO2).