Preparation of ZIF-8: Solutions of zinc nitrate hexahydrate and 2-methylimidazole were prepared separately by dissolving 0.3 grams and 0.66 grams, respectively, in 14.3 mL methanol. The two solutions were mixed and stirred vigorously for 60 min. The particles were isolated by centrifugation at 10,000 x g for 10 min. Particles were washed with methanol two times. The resultant ZIF-8 particles were vacuum dried at 75˚C overnight.