Colorimetric NOx sensor based on a porous glass-based NO2 sensing chip and a permanganate oxidizer

• The new NOx sensor is composed of a NO2 sensing chip and a permanganate oxidizer.
• The oxidizer based on porous glass (PG) is sufficiently active to convert NO to NO2.
• The chip, doped with Saltzman's reagent in PG, turns red upon exposure to NO2.
• The measurement of absorbance of the chip at 525 nm provides NOx concentration.
• The chip capable of detecting 0.2 ppm NO2 works as an accumulative type.

A new type of chemical sensor for gaseous nitrogen oxides (NOx, the sum of NO and NO2) has been fabricated by combining a porous glass-based NO2 sensing chip with an oxidizing agent. In this device, NO is first oxidized to NO2 over the oxidizer on a porous glass sheet (8 mm square, 1 mm thick) prepared by soaking in a sulfuric acid/potassium permanganate solution, after which the NO2 is detected by a second chip, the same size as the oxidizing chip, impregnated with Saltzman's reagent. A sensor of this type placed in a miniature aluminum holder (3 cm × 5 cm × 1.5 cm) was passively exposed to air containing ppm levels of NO or NO2. As a result, the chip changed from colorless to dark red and a plot of absorbance at 525 nm against the total exposure (defined as the sum of the product of the instantaneous gas concentration by the exposure interval) was linear. Based on the slope of the plot obtained from the NO2 exposure trials, the detection limit of the sensor was approximately 0.2 ppm NO2 assuming a 1 h exposure. It was also found that these devices exhibited equivalent sensitivity to both NO and NO2, indicating that these sensors are capable of measuring the combined NO and NO2 concentration.