ZIF-8 was synthesized based on the previous reports. Specifically, 297 mg (1 mM) of zinc nitrate (Zn (NO3)2•6H2O) and 656 mg (8 mM) of 2-methylimidazole were dissolved separately in 12 mL of methanol. The 2-methylimidazole solution was slowly added to the zinc nitrate solution and stirred for 6 h at room temperature (RT). The resulting white precipitate was collected by centrifugation (6000 rpm,10 min) and washed with methanol (3×30 mL) to remove excess ligands. The white residue was dried overnight in a vacuum oven at 70 ºC to obtain ZIF-8.