MnO₂ nanoflowers as a multifunctional nano-platform for enhanced photothermal/photodynamic therapy and MR imaging

Photodynamic therapy (PDT) has been regarded as a promising strategy for tumor therapy. However, heterogeneous tumor microenvironments severely limit the efficacy of photodynamic therapy. In this work, a multifunctional theranostic platform (MnO2-SiO2-APTES&Ce6 (MSA&C)) was fabricated based on MnO2 nanoflowers, which afforded MRI-guided synergistic therapy incorporating PDT and second near-infrared window (NIR-II) photothermal therapy (PTT). Herein, MnO2 nanoflowers are first proposed as a NIR-II photothermal agent. In the MSA&C system, MnO2 nanoflowers were employed for effective photosensitizer loading, relieving tumor hypoxia, and NIR-II PTT and tumor-specific imaging. The large amount of photosensitizer, reduced tumor hypoxia, and hyperthermia all contributed to the improvement of PDT. The quantity of reactive oxygen species (ROS) generated during PDT in turn down-regulated the expression of heat shock proteins (HSP 70), thereby improving photothermal performance. Positively charged (3-aminopropyl)triethoxysilane (APTES) was used to promote cellular uptake, further improving treatment efficiency. In this system, the MSA&C nanoflowers can not only alleviate tumor hypoxia, but they also obviously induce cell apoptosis under laser irradiation through a ROS- and hyperthermia-mediated mechanism, thereby leading to remarkable tumor growth inhibition. Furthermore, the Mn2+ ions generated during treatment can be explored for MR imaging, and this could be used to finally realize MRI-guided enhanced PDT/PTT.