A ppb-level NO2 detection system using coloration reactions in porous glass and its humidity dependence

Coloration reactions between nitrogen dioxides (NO2) and diazo-coupling reagents have been shown to occur in the nano-pores of porous glass. We introduced sulfanilamide and N,N-dimethyl-1-naphthylamine as diazo-coupling reagents into porous glass. This organically doped porous glass sensor had one specific peak near 525 nm in the visible region and showed large changes in absorbance after exposure to NO2. We investigated the humidity dependence of its sensitivity during exposure to NO2 in an artificial environment. The sensitivity was almost constant from 35% to 70% RH. Thus, our simple NO2 detection system using the porous glass sensor, a light-emitting diode, and a photodiode was able to determine the hourly variations in NO2 concentration at the ppb level in an actual environment without being seriously affected by humidity in the range between 35% and 70% RH.