**5. Conclusions**

In conclusion, a room-temperature ethylene sensor based on a ternary rGO/WSe2/Pd heterojunction and its banana ripeness detection applications were demonstrated. The ethylene sensor shows excellent figures of merit including high sensitivity, rapid response/recovery, full repeatability, and high selectivity, making it promising for practical banana ripeness detection applications. Such excellent room-temperature ethylene sensing behaviors could be attributed to the following two aspects: (1) The negative adsorption energy of the ternary heterojunctions provides enough active sites for ethylene molecules adsorption, and (2) The electron transfer across the rGO/WSe2 and WSe2/Pd interfaces is greatly promoted through band energy alignment. Furthermore, its reliability and feasibility toward fruit quality monitoring applications were confirmed and validated through banana ripeness detection simulation experiments by comparison with traditional GC-MS technology. This work provides a feasible methodology for construction of a roomtemperature ethylene sensing material system, and also may innovatively push forward the applications of sensor technology in the intelligent agriculture field to acquire the ripeness and quality information of climacteric fruit quickly and in-time.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/foods11131879/s1, Figure S1: Schematic diagram of the gas sensing setup; Figure S2: EDS mapping results of the rGO/WSe2 heterojunction films.

**Author Contributions:** Conceptualization, funding acquisition, writing—original draft preparation, review and editing, X.L.; investigation, data curation, C.X. and J.S.; writing—review and editing, X.D.; software, visualization, Z.W. and W.H.; supervision, Y.W. and Z.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Central Public-interest Scientific Institution Basal Research Fund, China (Grand No.Y2019XK18), and the Innovation Project of Chinese Academy of Agricultural Sciences, China (Grand No.CAAS-ASTIP-2016-AII).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are available from the authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
