Mesoporous manganese silicate coated silica nanoparticles as multi-stimuli-responsive T₁-MRI contrast agents and drug delivery carriers

A novel kind of monodisperse mesoporous manganese silicate coated silica nanoparticle (MMSSN) as a highly efficient T₁-weighted MRI contrast agent (CA) and drug carrier for cancer diagnosis and chemotherapy has been constructed by a modified "SiO₂ sacrifice and in situ silicate growth" approach under a relatively low hydrothermal temperature and alkali-free condition. The mesoporous manganese silicate shell provides a large specific surface area and abundant exposed Mn paramagnetic centers to water molecules, which endows the MMSSNs with extraordinarily high longitudinal relaxivity. Meanwhile, the MMSSNs presented an efficient pH/redox-responsive T₁-MRI feature based on the significant enhancement of relaxation rate (r₁) stimulated by mild acidic environment or reducing agent (GSH) both in vitro and in vivo. Furthermore, the mesoporous structure and negatively charged pore surface of the manganese silicate shell enable the MMSSNs to attain anti-cancer drug (DOX) storage and a pH-responsive release, which is suitable for on-demand drug release for the chemotherapy of tumors. Therefore, the mesoporous manganese silicate-based nanomaterial is a promising candidate as T₁-MRI CAs and anticancer-drug delivery carriers for the theranostics of tumor in an intelligent and on-demand manner. Statement of significance MRI is one of the most frequently used imaging techniques in daily clinics for cancer diagnosis. Using contrast agents (CAs) in MRI can afford much clearer and enlarged images of detectable organs. Gadolinium (Gd³⁺)-based T₁-positive CAs are widely used but associated with the risk of nephrogenic systemic fibrosis. To achieve much safer CAs, various Mn²⁺-based T₁-positive CAs have been reported, such as MnO or core-shell MnO_x-based nanoparticles. However, the efficiency of these CAs is still lower. Herein, we report a novel kind of mesoporous manganese silicate coated silica nanoparticle as CA and anti-cancer drug carrier. Results obtained from this study, especially the pH/redox-responsive T₁-MRI feature are helpful for us to further design efficient MnSiO₃-based materials for clinical MRI applications.