A Parts Per Billion (ppb) Sensor for NO2 with Microwatt (μW) Power Requirements based on Micro Light Plates.

A film of gas sensitive ZnO nanoparticles has been coupled with a low-power micro light plate (µLP) to achieve a NO2-parts-per-billion conductimetric gas sensor operating at room temperature. In this µLP configuration, an InGaN-based LED (emitting at 455 nm) is integrated at a few hundred nanometers distance from the sensor material, leading to sensor photoacti-vation with well controlled, uniform and high irradiance condi-tions, and very low electrical power needs. The response curves to different NO2 concentrations as a function of the irradiance displayed a bell-like shape. Responses of 20% to 25 ppb of NO2 were already observed at irradiances of 5 mWatts·cm-2 (applying an electrical power as low as 30 µW). In the optimum illumination conditions (around 60 mWatts · cm-2, or 200 µW of electric power), responses of 94% to 25 ppb were achieved, corresponding to a lower detection limit of 1 ppb of NO2. High-er irradiance values worsened the sensor response in all the parts-per-billion range of NO2 concentrations. The responses to other gases such as NH3, CO and CH4 were much smaller, showing a certain selectivity towards NO2. The effects of hu-midity on the sensor response are also discussed.