@NU-1000. H4TBAPy linker and NU-1000 were synthesized according to the procedures reported in our previous study. For the synthesis of FeCN@NU-1000, 22.2 mL of 0.1 M HCl aqueous solution was prepared first in a microwave vial (Biotage® Microwave Reaction Kit, 10-20 mL) and purged with Ar gas for 15 min. Thereafter, 94 mg of potassium hexacyanoferrate(II) trihydrate was added into the solution and dissolved by sonication. Then, 60 mg of NU-1000 was added into the solution and the microwave vial was sealed with a cap with septum. Thereafter, Ar gas was continuously purged into the headspace in the vial through the cap for 5 min (with a gas outlet). After purging with Ar, the microwave vial was fully covered with aluminum foil and kept at room temperature for 3 days. The microwave vial was shaken periodically during the first two days. After 3 days of reaction, the aluminum foil covering the vial was removed; green precipitation was observed at the bottom of the microwave vial. The following washing step was conducted immediately. The solution was removed by a needle syringe through the cap, and 20 mL of water was added into the vial. The suspension was shaken and then kept in dark for overnight. This wash step was done with water for twice, followed by three times of washing with fresh acetone. After the final removal of acetone, the cap on the microwave vial was removed and the vial containing wet green solid was dried in an oven at 120 o C in air for overnight. The color of the sample turned brown after drying, and the obtained brown powder is designated as "FeCN@NU-1000" (see Figure in Supporting Information).