Effects of mesoporous silica coating and postsynthetic treatment on the transverse relaxivity of iron oxide nanoparticles

Mesoporous silica nanoparticles have the capacity to load and deliver therapeutic cargo and incorporate imaging modalities, making them prominent candidates for theranostic devices. One of the most widespread imaging agents utilized in this and other theranostic platforms is nanoscale superparamagnetic iron oxide. Although several core-shell magnetic mesoporous silica nanoparticles presented in the literature have provided high T₂ contrast in vitro and in vivo, there is ambiguity surrounding which parameters lead to enhanced contrast. Additionally, there is a need to understand the behavior of these imaging agents over time in biologically relevant environments. Herein, we present a systematic analysis of how the transverse relaxivity (r₂) of magnetic mesoporous silica nanoparticles is influenced by nanoparticle diameter, iron oxide nanoparticle core synthesis, and use of a hydrothermal treatment. This work demonstrates that samples which did not undergo a hydrothermal treatment experienced a drop in r₂ (75% of original r₂ within 8 days of water storage), while samples with hydrothermal treatment maintained roughly the same r₂ for over 30 days in water. Our results suggest that iron oxide oxidation is the cause of r₂ loss, and this oxidation can be prevented during both synthesis and storage by use of deoxygenated conditions during nanoparticle synthesis. Hydrothermal treatment also provides colloidal stability, even in acidic and highly salted solutions, and a resistance against acid degradation of the iron oxide nanoparticle core. Results of this study show the promise of multifunctional mesoporous silica nanoparticles but will also likely inspire further investigation into multiple types of theranostic devices, taking into consideration their behavior over time and in relevant biological environments.