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 conductometric 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 photoactivation with well controlled, uniform, and high irradiance conditions, 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. Higher irradiance values worsened the sensor response in 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 toward NO₂. The effects of humidity on the sensor response are also discussed.