Encouraged by the above performance of DDP/MOF-Fc@HA NSs, the possible mechanism of cell death was studied. Generally, GSH is overexpressed in tumor cells to compensate for redox homeostasis for tumor cell growth. Therefore, GSH depletion was initially measured with ThiolTracker as a fluorescence probe (Fig.
4A). Compared with that in the control group, there was a slight change in the fluorescence intensity in the group treated with NIR alone. However, the relative GSH level in the DDP/MOF-Fc@HA NS group decreased by 53% and 55% in the Hepa 1-6 and 4T1 cells, respectively, and the relative GSH level in the DDP/MOF-Fc@HA+NIR group decreased by 78% and 81%, respectively, in the Hepa 1-6 and 4T1 cells. The results confirmed that DDP on MOF-Fc@HA NSs can deplete intracellular GSH, and the entire process can be accelerated after NIR irradiation (Fig.
S13). In addition, the intracellular •OH and ROS levels were further measured by O28 and DCFH-DA as fluorescence probes (Fig.
4B and C and Figs.
S14 and
S15). Compared with that in the MOF-Fc@HA NS group, a stronger fluorescence signal was observed in the DDP/MOF-Fc@HA NS group, indicating that GSH depletion by DDP can significantly facilitate the generation of ROS via the Fenton catalytic reaction. Moreover, a significant increase in fluorescence intensity was observed for both MOF-Fc@HA NSs+NIR, suggesting that a temperature increase can obviously amplify the Fenton catalytic efficiency of MOF-Fc@HA NSs. After that, we further explored the expression of key ferroptosis suppressive factors, including glutathione peroxidase (GPX4), cysteine/glutamate reverse transporter protein (SLC7A11), and frataxin, by Western blot (WB) analysis [
52,
53]. Compared with those of the other groups, DDP/MOF-Fc@HA+NIR irradiation obviously suppressed the expression of GPX4, SLC7A11, and frataxin (Fig.
4D), which disrupted redox homeostasis in tumor cells and subsequently induced ferroptosis. Based on ROS generation, GSH depletion, and GPX4 suppression, the intracellular LPO level was further evaluated. Image-iT (C11-BODIPY 581/591) is an oxidation-sensitive LPO-specific fluorescent probe capable of accumulating in cellular membranes. Upon oxidation of C11-BODIPY 581/591, the maximum emission peak shifts from 590 nm (red) to 510 nm (green) and remains intrinsically lipophilic, which is favorable for membrane LPO detection. Confocal laser scanning microscope (CLSM) images revealed that in the DDP/MOF-Fc@HA and MOF-Fc@HA+NIR treatment groups, the attenuation of red fluorescence and enhancement of green fluorescence changed to a certain extent. Moreover, a very weak red fluorescence signal and bright green fluorescence were observed in the DDP/MOF-Fc@HA+NIR treatment groups (Fig.
4F). The same trend was also observed in the 4T1 cells (Fig.
S16). Then, the membrane-permeant fluorescent dye 5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylimidacarbocyanine (JC-1) was used to monitor the change in the mitochondrial membrane potential (MMP), which exhibited green fluorescence while remaining as a J-monomer in the damaged mitochondria (Fig.
4E) [
54]. For the control and NIR groups, the G/R ratios were approximately 0.04 and 0.05, respectively. After incubation with MOF-Fc@HA NSs and DDP/MOF-Fc@HA NSs, the G/R values increased to 0.65 and 1.33, respectively. Furthermore, the G/R value of DDP/MOF-Fc@HA+NIR increased to 8.4 (Fig.
S17). Additionally, direct mitochondrial staining of Hepa 1-6 hepatocellular carcinoma cells was performed, and the red fluorescence signal was negatively correlated with mitochondrial damage (Fig.
4G and Fig.
S18). The Hepa 1-6 cells treated with DDP/MOF-Fc@HA+NIR exhibited more intense red fluorescence than did the other groups. Bio-TEM revealed mitochondria in the Hepa 1-6 cells treated with DDP/MOF-Fc@HA+NIR (Fig.
S19). As shown in Fig.
4G, DDP/MOF-Fc@HA NSs caused an obvious decrease in volume, an increase in the mitochondrial membrane density, the disappearance of mitochondrial cristae, and a swelling phenotype, which are typical features of mitochondrial functional impairment through ferroptosis. The above results clearly verified that DDP/MOF-Fc@HA NSs can induce ferroptosis in tumor cells and that NIR irradiation can further improve the efficiency of ferroptosis.