## 4 Future Directions and Challenges  
Despite significant advancements, challenges remain in optimizing the selectivity, reducing response times, and extending the operational lifespan of gas sensors for real-world applications. While single metal oxides like ZnO, TiO\({}_{2}\), CuO, and Fe\({}_{2}\)O\({}_{3}\) have demonstrated substantial sensitivity to various gases, mixed metal oxides (MMOs) and doped systems (e.g., Sn-doped In\({}_{2}\)O\({}_{3}\) or Ni-doped Fe\({}_{2}\)O\({}_{3}\)) have shown promising improvements in selectivity and stability. The improved sensitivities of some gases are highlighted in Table 1, showing just a small portion of the research into metal and mixed metal oxides. This should serve as a guide for future research, showcasing the immense possibilities these systems offer in terms of gas sensing. Additionally, nanostructuring techniques, such as the formation of nanorods, nanosheets, and nanotubes, have enhanced the surface area and facilitated faster electron transport, improving response times and enabling low-temperature operation. When metal oxides are structured into nanorods, nanosheets, or nanotubes, their surface-to-volume ratio increases significantly compared to bulk materials. This allows more active sites for gas adsorption, leading to higher sensitivity and lower detection limits. Traditional metal oxide gas sensors require high operating temperatures (200-400 \({}^{\circ}\)C) to achieve adequate gas sensing performance. Nanostructures, especially when combined with catalyst doping (e.g., Pt, Pd) or UV/visible light activation, allow gas detection at lower temperatures or even room temperature. However, issues such as environmental interference, long-term stability, and high-power consumption remain significant barriers to practical deployment in complex, real-world environments. Emerging fabrication techniques, including 3D printing and atomic layer deposition, enable precise control over sensor architecture and material composition, potentially addressing issues related to response time and selectivity. Furthermore, integrating metal oxides with nanocomposites like graphene and carbon nanotubes offers additional pathways to enhance conductivity, sensitivity, and selectivity by creating high-surface-area interfaces and enhancing electron mobility. These advancements hold potential for developing low-power, highly responsive, and selective gas sensors suitable for environmental monitoring, industrial safety, and wearable devices in diverse conditions. However, implementing these advancements will face challenges such as scalability and cost, as high-precision methods like atomic layer deposition and 3D printing are difficult and expensive to scale for mass production. Material stability and durability remain concerns, as hybrid materials and composites may degrade under varying environmental conditions. Achieving reproducibility and consistent performance across production batches is also a hurdle, as minor variations can affect sensor responses. Complex fabrication protocols must be simplified for broader industrial use, and maintaining ultra-low power consumption without compromising sensitivity is challenging. Additionally, environmental and safety concerns related to nanomaterial production and disposal need to be addressed, along with navigating regulatory standards to ensure compliance for market readiness. Overcoming these issues is essential for realizing the potential of advanced gas sensors. Conceptualization, V.M., I.P., A.B. ([PERSON]), F.R.-P. and D.G. ; methodology, V.M., I.P., A.B. ([PERSON]), F.R.-P. and D.G. ; software, V.M., I.P., A.B. ([PERSON]), F.R.-P. and D.G. ; validation, V.M., I.P., A.B. ([PERSON]), F.R.-P. and D.G. ; formal analysis, I.P., A.B. ([PERSON]) and F.R.-P.; investigation, V.M., I.P., A.B. ([PERSON]), F.R.-P. and D.G. ; resources, V.M., Z.S. and A.B. ([PERSON]); data curation, V.M., I.P., A.B. ([PERSON]), F.R.-P. and D.G. ; writing--original draft preparation, V.M. and I.P. ; writing--review and editing, V.M., I.P., A.B.