Suppose the hardware of our proposed electronic nose is an array of two QCM-ZIF sensors as depicted Fig. . Box. 1 explains QCM-ZIF sensors and their working principle (gravimetry). We chose ZIF-8 and ZIF-71 as the recognition elements for their hydrophobicity and stability and for the availability of their experimentally-measured C 2 H 4 , CO 2 , and H 2 O adsorption isotherms near room temperature. Generally, the sensitivity and selectivity of each ZIF sensing element to/for various gases depends on the structure of the ZIF, including pore shape and size, topology, and internal surface chemistry. We describe the structures of the ZIFs below and show a cage of each ZIF in Fig. .