Smart Breath Sentinel: A NO₂ Gas Sensor with ppt-Level Detection Lower Limit and High Signal-to-Noise Ratio Based on In(OH)₃‑α-Fe₂O₃–ZnO for an Application on Intelligent Upgrade of Ordinary Masks

Nitrogen dioxide (NO₂), the main component of pollutants in atmospheric environments, causes and exacerbates respiratory diseases, especially during outdoor sports even at the 100 ppb level. Currently, environmental gas detection still faces challenges such as high detection limits, low SNR, and low sensitivity. A NO₂ sensor based on In(OH)₃-α-Fe₂O₃–ZnO was prepared using a hydrothermal method, featuring an ultralow detection limit of 82 ppt, an exceptionally high SNR of 574,000, and an ultrahigh sensitivity of 252.25 mV/ppm (100 ppb to 1 ppm). And this sensor exhibits excellent response, selectivity, and repeatability. These excellent sensing characteristics come from the adsorbed oxygen on the surface of the material with the formation of n–n heterojunctions. Additionally, a low-power, portable, and cost-effective environmental monitoring system (named Smart Breath Sentinel (SBS)) was designed to intelligently upgrade ordinary masks. SBS enables real-time wireless environmental gas monitoring with integrated humidity compensation to ensure accurate measurements in high-humidity environments. SBS has already been tested in multiple environments, and the test results have proven the feasibility of SBS. This sensor based on In(OH)₃-α-Fe₂O₃–ZnO demonstrates significant potential for applications aimed at enhancing human safety.