**1. Introduction**

Ethylene is an important plant hormone that regulates the physiological and biochemical changes in climacteric fruits and vegetables to control their maturity, freshness, softness, and deterioration [1,2]. The released ethylene can potentially express the flavor quality of fruits coupled with sugar content [3,4]. The accumulated ethylene molecules inside a fresh fruit package could also stimulate physiological activity and consequently, accelerate fruit deterioration, which limits their storage life and leads to product losses [5]. Moreover, the wounding and spoilage of fruits also induces the biosynthesis of ethylene [6]. According to a report from the Food and Agriculture Organization (FAO), 1.3 billion tons of food loss per year was reported, which represented 33% of total food production, among which fruits and vegetables held the highest loss rate (45%) [7]. Fruit abnormalities in early stages can be discovered promptly through ethylene monitoring and thus most of these losses could be avoided. Moreover, for the timely export of fresh climacteric fruits, the production as well as respiration of ethylene should also be of concern during long-supply chains [8]. Therefore, the continuous and accurate detection/monitoring of ethylene released from fruits is vital for managing and controlling the harvesting, storage, package, transportation, and selling process of climacteric fruits, which is much more prominent especially in today's intelligent agriculture era.

**Citation:** Li, X.; Xu, C.; Du, X.; Wang, Z.; Huang, W.; Sun, J.; Wang, Y.; Li, Z. Assembled Reduced Graphene Oxide/Tungsten Diselenide/Pd Heterojunction with Matching Energy Bands for Quick Banana Ripeness Detection. *Foods* **2022**, *11*, 1879. https://doi.org/ 10.3390/foods11131879

Academic Editor: Seung-Hyun Kim

Received: 20 May 2022 Accepted: 23 June 2022 Published: 24 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

So far, various techniques including chromatography [9], spectroscopy [10,11], electrochemical sensor [12], chemical sensor [13], and fluorescence probe [14] have been reported for ethylene detection, among which chemical sensor stands out, owing to its real-time response, high sensitivity, manpower operation, and low cost. Recently, many attempts have been made to develop high-performance chemiresistive ethylene sensors, in which the gas concentration is translated into a resistive electrical signal for detection. Bulk or nanostructured metal oxide semiconductors (MOSs, WO3 [15], SnO2 [16], and ZnO [17]) have been reported to show high sensitivity to ethylene. However, in metal oxide-based ethylene sensors, high temperature (typically 170–500 ◦C) or light irradiation is usually required to activate the reaction between adsorbed oxygen and ethylene, which results in high system complexity and high power consumption and thus greatly hinders its practical applications. Therefore, how to achieve high sensitivity to ethylene without additional activation energy is of great urgency and importance for practical agricultural applications due to its merits in reducing power consumption and system complexity. For MOSs, doping with noble metals such as Au, Pt, Pd [18–20], and transition metal halides (CuCl2, NiCl2 [21]) has been demonstrated to greatly lower the operating temperature. However, how to achieve excellent ethylene sensing performances at room temperature is still challenging.

The construction of a sensing material system for room-temperature ethylene detection should be considered from the following two aspects. Firstly, the gas sensing response arises from the physical adsorption of ethylene molecules onto the sensing films, and thus the adsorption capabilities of the sensing films should be optimized to achieve high sensitivity toward ethylene. It has been theoretically proven that the negative adsorption energy of the target analyte-sensing film system is beneficial for the adsorption of target analyte molecules, which also has been proven experimentally [22,23]. Secondly, when the target analyte molecules were adsorbed onto the sensing film, the electron transfer takes place between the target analyte and sensing film [24,25]. How to translate this electron transfer process into electrical resistance change greatly depends on the energy level alignment of the sensing material system, where suitable energy alignment promotes the electron transfer and thus results in a larger sensing response.

Following this regard, the ternary reduced graphene oxide (rGO)/tungsten diselenide (WSe2)/Pd heterojunctions were designed and fabricated toward room-temperature ethylene detection. The negative adsorption energy of the ternary heterojunctions provides enough active sites for ethylene molecules adsorption, and the electron transfer across the rGO/WSe2 and WSe2/Pd interfaces through band energy alignment greatly promotes the sensing response. Compared to the solely one or two components-based heterojunctions, the ternary heterojunction-based ethylene sensor exhibits higher sensitivity and quicker p-type response to the ppm level of ethylene at room temperature, and the sensitivity to 10 ppm of ethylene was 0.001%, with the response and recovery time being 33 and 13 s. Moreover, the sensor exhibits full reversibility and excellent selectivity at room temperature. Furthermore, the application feasibility of the sensor to fruit quality monitoring was verified by comparison with routine techniques through banana ripeness detection simulation experiments. This work provides a feasible methodology for designing and fabricating a sensing material system toward room-temperature ethylene detection, pushing forward the development of modernized intelligent agriculture.
