2.3.1. Cytotoxicity Assay

Briefly, 10<sup>4</sup> cells/well were seeded in 96-well plates and cultured overnight at 37 ◦C in 5% CO2 atmosphere. The pending test cells were then further incubated with different concentrations of Cu-MOF, Cu-MOF/GOD or Cu-MOF/GOD@HA (10, 20, 30, 50, 70, 100 μg mL−1) under different conditions (GSH: 2.5, 5.0 mmol L–1, H2O2: 100 μmol L–<sup>1</sup>

or trypan blue: 10 μg mL–1) for 24 h. The cytotoxicity of Cu-MOF, Cu-MOF/GOD and Cu-MOF/GOD@HA was evaluated through an MTT assay.

#### 2.3.2. Intracellular ·OH Generation Capability of Cu-MOF/GOD@HA

For the evaluation of ·OH generation capability, MCF-7 cells were cultured with 50 μg mL–<sup>1</sup> of Cu-MOF, Cu-MOF/GOD and Cu-MOF/GOD@HA for 4 h, the cellular-ROS stress levels were then evaluated by using a ROS assay kit. After washing with PBS for twice, MCF-7 cells were further incubated with <sup>2</sup>,7-dichlorodihydrofluorescein diacetate (DCFH-DA) (10 μmol L–1) for 20 min followed by washing for 3 times with PBS. Finally, ROS associated signals in the cells were observed by fluorescence microscopy.
