About 3 × 105 cells were grown on the glasses of each well at 37°C overnight and then precultured in a hypoxic incubator for about 12 hours at 37°C. After being treated with drug materials for the indicated time under hypoxia condition, cells were washed orderly with ice-cold PBS and distilled H2O quickly, immersed into liquid nitrogen, and transferred into a freeze dryer immediately. After being dried overnight, each glass was coated with gold at 2 nm thick and then visualized.