The Fe-N-C SANs were obtained after pyrolysis at 900 ℃. The transmission electron microscopy (TEM) image shows the characterization results of the Fe-N-C SANs (Figure ). Fe-N-C SANs of similar shape and size (rhombic dodecahedral) were observed in the TEM images. Figure displays a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image. Bright dots are observed in images, representing dispersed Zn and Fe atoms. This phenomenon is attributed to the Z-contrast between metal and carbon. The single-atom feature of SANs, demonstrated by the dispersed Zn and Fe in HAADF-STEM image is key to their excellent peroxidase-like activity.