pH-Responsive Iron Manganese Silicate Nanoparticles as T₁-T₂* Dual-Modal Imaging Probes for Tumor Diagnosis

Magnetic resonance imaging (MRI) probes can be concentrated in tumors through grafting targeting agents. However, the clinical application of such targeted MRI probes is largely limited because specific agents are only used to target specific characteristics of cancer cells. The development of the MRI probes that can be used regardless of tumor types or their developmental stages is highly appreciated. The acidic tumor microenvironments and acidic organelles (endosomes/lysosomes) in cancer cells are universal phenomena of solid tumors, and nanoparticles can also accumulate in tumor tissues by enhanced permeability and retention (EPR) effect. Here, we reported the synthesis of pH-responsive T1-T2* dual-modal contrast agents based on iron manganese silicate (FeMn(SiO4)) hollow nanospheres, which can release Mn(2+) ions in acidic environments, exhibiting excellent ability as agents for magnetic resonance and red fluorescence imaging. MRI for mouse models revealed that the nanoprobes could accumulate in tumors via EPR effect and then distinguish tumors from normal tissues with the synergistic effect of T1 and T2* signal only 10 min after intravenous injection. Fluorescence imaging demonstrated that the nanoprobes could be endocytosed into cancer cells and located at their lower pH compartments. Moreover, the hollow nanospheres showed no obvious toxicity and inflammation to the major organs of mice, which made them attractive diagnostic agents for different types of cancers.