Abstract
Transmittance in porous-glass gas sensors, which use aldol condensation of vanillin and nonanal as the detection mechanism for nonanal, decreases because of the production of carbonates by the sodium hydroxide catalyst. In this study, the reasons for the decrease in transmittance and the measures to overcome this issue were investigated. Alkali-resistant porous glass with nanoscale porosity and light transparency was employed as a reaction field in a nonanal gas sensor using ammonia-catalyzed aldol condensation. In this sensor, the gas detection mechanism involves measuring the changes in light absorption of vanillin arising from aldol condensation with nonanal. Furthermore, the problem of carbonate precipitation was solved with the use of ammonia as the catalyst, which effectively resolves the issue of reduced transmittance that occurs when a strong base, such as sodium hydroxide, is used as a catalyst. Additionally, the alkali-resistant glass exhibited solid acidity because of the incorporated SiO(2) and ZrO(2) additives, which supported approximately 50 times more ammonia on the glass surface for a longer duration than a conventional sensor. Moreover, the detection limit obtained from multiple measurements was approximately 0.66 ppm. In summary, the developed sensor exhibits a high sensitivity to minute changes in the absorbance spectrum because of the reduction in the baseline noise of the matrix transmittance.