## 1 Background of Nanostructured Materials for Gas Sensors  
### Nanostructured Metal Oxides  
Metal oxide semiconductors are emerging as promising materials for gas sensing applications due to their high sensitivity to a variety of gases, ease of fabrication, low cost, and excellent compatibility with other components and processes [1, 2, 3, 4]. Over the years, nanostructures of ZnO, SnO\({}_{2}\), TiO\({}_{2}\), In\({}_{2}\)O\({}_{3}\), WO\({}_{3}\), CuO, CdO, Fe\({}_{2}\)O\({}_{3}\), and MoO\({}_{3}\) have been engineered in various dimensions and sensor designs. The surface properties and morphology of these metal oxides are critical to their gas sensing efficiency [5]. Depending on the desired application and available fabrication techniques, different surface morphologiesand configurations, such as single crystals, thin films, thick films, and one-dimensional (1D) nanostructures, have been realized [6]. Among these, 1D nanostructures have garnered significant attention for gas sensor development due to their high surface-to-volume ratio and excellent chemical and thermal stability across a range of operating conditions [7; 8; 9]. Due to their high sensitivity, fast response times, and durability, metal oxide-based gas sensors have found widespread applications in environmental monitoring, industrial safety, healthcare, and smart technology. In air quality monitoring, sensors for gases like NO\({}_{2}\), CO, and VOCs are integrated into urban air pollution monitoring networks, providing real-time data to regulate emissions and improve public health. In industrial safety, metal oxide sensors play a crucial role in detecting toxic or explosive gases such as H\({}_{2}\)S, CO, and CH\({}_{4}\) in chemical plants, mines, and refineries, helping to prevent hazardous leaks. In the healthcare sector, breath analysis sensors detect biomarkers such as acetone (for diabetes monitoring) and ammonia (for liver function assessment), enabling non-invasive medical diagnostics. Additionally, automotive and smart home technologies leverage these sensors for CO detection in vehicles, air purification systems, and gas leak detection in residential and commercial settings. As advancements continue, the integration of metal oxide gas sensors with Internet of Things (IoT) platforms and wireless sensor networks is further expanding their real-world applicability, enabling continuous, remote, and energy-efficient gas monitoring solutions. This review provides an in-depth overview of recent advances in the fabrication of metal oxide gas sensors based on nanostructured functional constituents. It covers various fabrication strategies, particularly those related to nanostructuring, and discusses their impact on sensing performance. This review delves into innovative techniques such as atomic layer deposition, sol-gel processes, and hydrothermal synthesis, highlighting their ability to create uniform and high-surface-area nanostructures that enhance sensor response times and sensitivity. This review will try to provide a comprehensive insight into the plethora of possible metal oxide and mixed metal oxide gas sensors, hopefully offering studies in the field a good starting point from which they can build their own research paths. The key goal is promoting better long-term stability, selectivity, and sensitivity of gas sensors. The key findings from the current research are summarized, demonstrating how the incorporation of materials such as graphene, carbon nanotubes, and other conductive nanocomposites can improve the selectivity and stability of gas sensors. This review also discusses challenges related to material integration, scalability, and power consumption, emphasizing the need for environmentally friendly and cost-effective fabrication methods. Potential future developments in the field of nanostructured metal oxide gas sensors are proposed, focusing on hybrid material systems, the use of machine learning algorithms for enhanced signal processing, and the integration of sensors into flexible and wearable technologies.