Encapsulated bromocresol purple-based sensitive materials: The role of the nature and distribution of matrix layers on ammonia sensing performance

Abstract An exploratory analysis of ammonia sensing hybrid material encapsulating Bromocresol purple were synthesized; using two types of matrices: (i) Silica produced by acid- or alkaline-catalyzed sol-gel processes was used as the inorganic matrix, and (ii) polymethacrylic acid (using methacrylic acid as the monomer) was used as the organic matrix. Eleven different sensitive structures were prepared by intercalating the pH indicator in both matrices. The resulting materials were tested for potential applications as colorimetric sensitive materials for alkaline vapors using ammonia as the probe gas. The response time and reversibility were evaluated by colorimetric analysis employing the CIELAB color space model with the illuminant D65. The color difference (∆E*ab) was measured from a Euclidian distance between the color coordinates (L*, a* and b*). Almost all the sensitive materials presented values of ∆E*ab > 5; i.e., it was possible to visually observe the difference between the two colors. The data showed that silica-based structures have faster time responses (4 min 50 s) than those of the polymer-based ones (11 min 50 s) and lower rates of reversibility, although the changes in color tone are similar (ΔE*ab, 28 and 21, respectively, for silica and polymer matrices).