*2.4. Theoretical Calculation*

DFT (Density Functional Theory) calculations were carried out using the Vienna Abinitio Simulation Package (VASP) with the frozen-core all-electron projector-augment-wave method. The Perdew—Burke—Ernzerhof (PBE) of Generalized Gradient Approximation (GGA) was adopted to describe the exchange, correlation potential, and structure optimization. Van der Waals interactions were considered by the DFT-D2 method of Grimme. The plane wave basis set cut-off energy was set to 500 eV, and the Monkhorst-Pack k-point sampling was set to 2 × 2 × 1. The geometry optimizations were performed until the forces on each ion was reduced below 0.01 eV/Å.

The adsorption energy (Eads) was calculated as:

$$\mathbf{E\_{ads}} = \mathbf{E\_{ads\text{color{}}}} \mathbf{s} \text{ - (E\_{ads\text{color{}}} + E\_{\text{gas}})} \tag{1}$$

where Eadsorbent was the energy of the sensitive materials, Egas represented the energy of the adsorbed ethylene molecule, and Eadsorbent+gas was the total energy of the adsorbed system. Energetically, the negative adsorption energy values are desirable for the adsorption process.

## *2.5. Banana Ripeness Detection Experiments*

The performance verification of the as-fabricated ethylene sensor was conducted by applications in banana ripeness detection. Yellowish green bananas were obtained from the local market and stored at room temperature (20 ◦C). During storage, the color of the samples changed with the increase of storage time, and the four typical color stages that are yellowish green, all yellow, yellow with brown speckles, and brown were chosen for detection of released ethylene. The bananas at the target sampling stage were put into a sealed quartz glass container. The containers were sealed for a period of time for collection of ethylene, and then the sensor was placed into the container as well to record its in-time resistance values for the detection of released ethylene. After sensing for 100 s, the sensor was taken out from the container, and placed in ambient air to recover to the initial resistance state. The above steps were repeated four times to obtain the sensing response to bananas at different ripeness stages. At the same time, the gas in the container was also extracted by a gas syringe (25 μL, VICI Precision Sampling, Inc., Baton Rouge, LA, USA) for gas analysis by gas chromatography-mass spectrometry (Agilent GC-MS 7890B-5977A, Santa Clara, CA, USA). The parameters of GC-MS were set as: 250 ◦C inlet temperature, 40 ◦C column temperature for 3 min, 40 ◦C /min speed to 100 ◦C, 1 mL/min flow rate, splitless, 230 ◦C ion source, and 150 ◦C MS quadrupole temperature. Identification of ethylene was confirmed by comparing the collected mass spectra with the spectra in the National Institute for Standards and Technology (NIST 14) data bank. The relative content of the ethylene was determined using the area normalization method. Three sampling analysis was one replicate and the average results were used.
