Methods ·BSA-MnO₂@CPT (BMC) nanoprobes were prepared by biomineralization, and their physicochemical properties were characterized by transmission electron microscope (TEM) and dynamic light scattering. The magnetic resonance imaging (MRI) was used to evaluate the pH-responsive MRI T₁ activation and time-dependent activation efficacy at the cellular level, with quantitative analysis of MRI T₁ signal intensity. The reactive oxygen species (ROS) generation and glutathione (GSH) depletion by BMC nanoprobes were respectively detected by methylene blue (MB) and DTNB in vitro. The synergistic efficacy of chemotherapy and ferroptosis mediated by the nanoprobes in 4T1 breast cancer cells was evaluated using the Thiazolyl Blue Tetrazolium Bromide (MTT) assay. After co-incubation 4T1 cells with BMC, intracellular ROS levels were determined through the staining of ROS fluorescence indicator 2′,7′-dichlorodihydrofluorescein diacetate (DCFH-DA) and the level of lipid peroxide (LPO) expression was detected by using BODIPY^581/591 C11 probe. A subcutaneous xenograft tumor model of 4T1 breast cancer was established in mice, with four experimental groups: Control group (PBS group), CPT group, BSA-MnO₂(BM) group, and BMC group. The pH-responsive T₁ activation effect of the BMC nanoprobes was dynamically monitored in vivo, while the ferroptosis-based antitumor efficacy was evaluated by measuring tumor volume and ferroptosis biomarkers (LPO and ROS). Results ·TEM revealed that the prepared BMC nanoprobes exhibited a spherical morphology with an average diameter of approximately 150 nm. The MRI results demonstrated that the nanoprobes were pH-activable, exhibiting progressively enhanced T₁ signal intensity under acidic conditions, and displaying pH-dependent r₁ relaxivity enhancement. These findings validated their dual pH/time-responsive activation efficacy at the cellular level. In vitro solution-level MB and DTNB assays demonstrated that the BMC nanoprobes effectively enhanced the generation of ROS and the consumption of GSH. Fluorescence staining with DCFH-DA and BODIPY^581/591 C11 demonstrated that the combination of ferroptosis effect and chemotherapy significantly enhanced intracellular generation of ROS and LPO accumulation. The MTT assay demonstrated that the survival rate of tumor cells significantly decreased to 17% (P=0.003). In vivo MRI demonstrated that the T₁ signal was significantly enhanced and reached its peak at 4 h after tail vein injection of the BMC nanoprobes. Furthermore, in vivo antitumor therapy showed that the BMC group exhibited upregulated levels of LPO and ROS in tumor tissues, accompanied by marked tumor suppression (P=0.009). Conclusion ·The pH-responsive theranostic BMC nanoprobes enhances antitumor efficacy via the synergistic interaction of chemotherapy and ferroptosis, while enabling tumor microenvironment-activated MRI. Objective· To construct a pH-responsive manganese-based nanoprobe and explore the therapeutic efficacy of chemotherapy/ferroptosis synergistic treatment in breast cancer and the effect of pH-responsive magnetic resonance-activated imaging.