To encapsulate CAT into ZIF-8 NPs, CAT and 8-arm poly(ethylene glycol) (8-arm-PEG-OH) were mixed with zinc nitrate and 2-methylimidazole (2-MIM) in an aqueous solution. The 8-arm-PEG-OH can extract Zn
2+ due to the interactions between the repeat unit of ethoxyl groups in PEG and Zn
2+, and induce the mineralization of Zn
2+ with 2-MIM and CAT [
36-
38]. Then, MPN films were easily deposited on the surface of the CAT@ZIF-8 NPs based on the high adhesion between tannic acid (TA) with substrates and the strong coordinating ability of TA with Fe
3+ [
39]. Finally, ethylenediaminetetraacetic acid disodium salt (EDTA) was used to remove the ZIF-8 NPs followed by the cooperation of Ce6 to obtain the hollow CCM capsules containing CAT and Ce6. Scanning electron microscopy (SEM) (
Fig. 2A) and transmission electron microscopy (TEM) images (Fig. S1A in Supporting information) showed that the CAT@ZIF-8 NPs have a dodecahedral morphology with a size of about 250 nm, which is similar to ZIF-8 NPs (Figs. S1B and C in Supporting information). Energy-dispersive X-ray (EDX) mapping analysis of CCM capsules showed the well-matched elements (C, O, and Fe) distribution (
Figs. 2B and
C). The patterns of ZIF-8 NPs powder X-ray diffraction (PXRD) and CAT@ZIF-8 NPs matched well with the simulated one (
Fig. 2D), which indicated that the encapsulation of CAT did not affect the crystal structure of ZIF-8 NPs. The hollow and amorphous structure of CCM capsules supported the successful removal of ZIF-8 NPs [
23,
40,
41]. As shown in
Fig. 2E, the change of zeta potential values from 22.6 mV to 17.7 mV for ZIF-8 NPs and CAT@ZIF-8 NPs indicated the successful encapsulation of CAT. After MPN films were deposited and the templates were removed, zeta potential values become ‒21.5 and ‒33.2 mV respectively. When CCM capsules were dispersed in RPMI 1640 medium, the zeta potential value changed to be −17.2 mV compared to that (‒33.2 mV) in water [
36], which could be due to the adsorption of protein corona on CCM capsules. Meanwhile, the specific absorption peaks of Ce6 (about 660 nm) and ligand-metal charge transfer band of phenol-iron coordination (500‒650 nm) can be detected in the ultraviolet-visible spectroscopy (UV–vis) [
42], indicating the successful fabrication of CCM capsules (Fig. S2 in Supporting information). In addition, the emission peak of the fluorescence spectrum around 670 nm also confirmed the successful loading of Ce6 in CCM capsules (
Fig. 2F). The loading efficiency of Ce6 in CCM capsules was about 11.7% based on the standard curve of Ce6 (Figs. S3A and B in Supporting information). The loading efficiency of CAT in CAT@ZIF-8 NPs was about 95.87% (230.1 U/mg) based on the standard curve of CAT (Fig. S4 in Supporting information). The catalytic activity of CAT was evaluated
in vitro. As shown in
Fig. 2G and Fig. S5 (Supporting information), due to the permeability of the capsules [
29,
43], obvious bubbles were generated in CCM and CAT@MPN capsule suspensions after adding of H
2O
2 solution, which was not observed in Ce6@MPN (CM) or MPN capsule suspensions. These results indicated that CAT loaded CCM capsules could effectively decompose H
2O
2 into O
2, which was expected to improve the oxygen level in hypoxic tumors microenvironment.