## 1 Introduction  
The number of astronauts who stay inside a confined spacecraft for several months or years while carrying out missions is increasing. Despite the presence of air purification systems, it is known that air pollution inside the spacecraft is 2 to 5 times higher than in enclosed environments with a terrestrial atmosphere [1]. According to NASA's manned lunar exploration program, the Artemis mission, astronauts have reported that they may suffer from severe odors caused by bathroom waste throughout the duration of the mission [2]. The main sources of polluted gases inside the spacecraft include gas production from heat and friction during equipment manipulation, degassing from internal materials over time, leaks from gas pipelines, and gases generated by astronauts' physical activity, metabolism, and waste during their stay [3, 4]. According to NASA's Spacecraft Maximum Allowable Concentrations (SMAC) list, over 300 gases have been reported inside spacecraft. Of these, 21 are classified as hazardous [5]. Table 1 presents the main hazardous gases and their respective allowable concentration limits within the spacecraft. Among them, NO\({}_{2}\) is particularly toxic and dangerous [6; 7; 8; 9; 10]. It is acidic, highly reactive, has an unpleasant odor, and is continuously produced and released into the atmosphere as a result of human activities. Even at low concentrations (\(\sim\)1 ppm), NO\({}_{2}\) can cause serious damage to the respiratory system and lung tissues. It can also lead to diseases, such as emphysema and bronchitis, while exacerbating existing heart conditions. Given its toxicity and hazards, there is an urgent need for the accurate and rapid detection of low NO\({}_{2}\) levels in the environment. Several NO\({}_{2}\) sensing materials have been reported, including graphene, transition metal dichalcogenides (TMDs), metal oxide nanostructures, and others [11; 12]. Among these, single-layer two-dimensional (2D) materials exhibit the most ideal characteristics for NO\({}_{2}\) sensing [13; 14; 15]. Two-dimensional materials, such as graphene and MoS\({}_{2}\), exhibit a high surface-to-volume ratio, high mobility, and moderate bonding with target molecules, leading to fast response and recovery times. MoS\({}_{2}\) exists in two possible crystal phases, trigonal and hexagonal. The hexagonal phase is semiconducting, while the trigonal phase exhibits metallic properties. In its bulk semiconducting (hexagonal) phase, MoS\({}_{2}\) has an indirect bandgap of 1.2 eV. However, when reduced to a monolayer, MoS\({}_{2}\) transitions to a direct bandgap of 1.8 eV [16; 17; 18]. The weak van der Waals forces between the layers in the bulk form facilitate the easy exfoliation and transfer of the material. Additionally, ideal MoS\({}_{2}\) is known to be free of dangling bonds. However, at defect sites (such as vacancies, dopants, adsorbates, adatoms, and impurities) and along the edges of the 2D material, strong bonding interactions occur, enhancing its reactivity with gas molecules [19; 20; 21; 22]. Furthermore, MoS\({}_{2}\) can exhibit either n-type or p-type behavior, depending on the perturbations caused by defects. Graphene, another 2D material, has a zero bandgap [23; 24]. Similar to MoS\({}_{2}\), its electronic properties can be modified by defects, which can result in the formation of strong bonds with target molecules. Although 2D NO\({}_{2}\) sensing materials with high performance have been reported in the literature, several challenges remain [25; 26]. This study focuses on the development of an NO\({}_{2}\) sensor for spacecraft applications. In such environments, the sensor must meet several key requirements, including proper operation in oxygen-free and zero-gravity conditions, high sensitivity, fast response/recovery times, no degassing from the sensor itself, effective compensation for temperature and humidity, and UV-C radiation.