Nitroxide-Modified Silica Nanoparticles: Impact of Radical Density on Relaxometric and EPR Properties

In this study, we report the synthesis and characterization of nitroxide-functionalized silica nanoparticles incorporating a TEMPO-based spin label. These nanoparticles were prepared through a reverse microemulsion method, and the nitroxide moiety was introduced via a TEMPO-modified silane, synthesized by coupling 4-amino-TEMPO with 3-(triethoxysilyl)propylsuccinic anhydride. By adjusting experimental parameters, we successfully modulated the radical surface density, obtaining values ranging from 0.36 to 2.83 radicals/nm², as determined by UV spectroscopy. Relaxometric measurements showed that both longitudinal (r₁) and transverse (r₂) relaxivities were strongly influenced by radical density, reaching maximum values of 5.42 and 11.94 s^-1·mM^-1, respectively, corresponding to enhancements of up to 489% (r₁) and 712% (r₂) compared to free 4-amino-TEMPO (at 20 MHz). Interestingly, high surface loading led to a decrease in relaxivity, highlighting the role of spin-spin interactions in modulating the relaxation process. Phantom electron paramagnetic resonance imaging (EPRI) demonstrated improved contrast and resolution for formulations with low radical densities, highlighting the importance of surface engineering to optimize the nanoparticle performance for EPRI applications.