Sensitive films for optical detection of ammonia and nitrogen dioxide

The sol-gel technology allows preparation of the thin silica films ranging in porosity from dense to highly porous. These films can function as matrix binding molecules of pH-sensitive indicators and can be utilized as sensitive coatings for intensity based planar evanescent wave chemical sensors. This paper is devoted to manufacturing technology and characterization of highly porous silica films deposited on soda-lime glass substrates, impregnated with bromocresole purple and cresole red. Two different methods of impregnation were used: dip coating, which entails sensitization of both silica films deposited on each side of a substrate and selective deposition of a dye solution on single side of the substrate. Manufactured sensitive films were tested toward influence of gaseous ammonia and nitrogen dioxide on their spectral characteristics of the absorption coefficient and the imaginary part of the refractive index. It was shown that bromocresole purple allows detection of both gases: NH ₃ and NO₂.