ZIF-8 was synthesized following a procedure described by Ostad et al. . A solution containing 820 mg of 2-methylimidazole (0.01 mol) in 25 mL of methanol was added into a solution of 742.5 mg of zinc nitrate hexahydrate in 25 mL of methanol at a rate of 18 mL /min under stirring at 500 rpm and 25 °C, and the stirring was continued for 24 h. The particles were then purified by 3 cycles of centrifugation (9k rcf, 15 mins, 25 °C) and ultrasonic redispersion (25 s) in methanol (30 mL), followed by drying at 55 °C for 12 h.