Recent advances in nanoporous NOx gas sensors: synergizing Raman spectroscopy, IoT, and machine learning for high-performance detection.

Among various pollutants, nitrogen oxides (NOx) stand out as particularly harmful irritant gases, known to cause airway inflammation at elevated concentrations. Chemiresistive gas sensing (CGS) has revolutionized gas detection with its low power consumption, cost-effectiveness, high sensitivity, fast response, and long-term stability. Traditional materials such as metal oxides, conducting polymers, and carbon-based materials used for NOx detection often suffer from poor selectivity and require high operating temperatures, leading to high noise levels. In contrast, nanoporous materials offer superior chemiresistive NOx gas sensing performance due to their large surface area and unique structural properties. Our review focuses on the fundamental mechanisms of NOx sensing in chemiresistive sensors, comparing n-type and p-type materials. It also discusses the fabrication of flexible, wearable chemiresistive sensors while addressing challenges related to uniformity, scalability, and stability. This review primarily highlights nanoporous materials, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), porous organic frameworks (POFs), and their hybrids, which exhibit enhanced gas adsorption and tunable properties, making them highly effective for NOx detection. Furthermore, Raman spectroscopy provides molecular-level insights into surface interactions, adsorption mechanisms, and charge-transfer dynamics, optimizing sensor selectivity, sensitivity, and stability for NOx gas sensing applications. This review also explores the integration of Internet of Things (IoT) technologies and machine learning (ML) into gas sensing systems, focusing on structure optimization, material performance, and the underlying mechanisms of emerging porous materials. It emphasizes their potential for real-time monitoring and data analysis to enhance sensor performance. Finally, the review concludes with future directions, emphasizing the development of hybrid materials, advanced devices, and multifunctional sensors for industrial and environmental applications.