Encapsulated alizarin red species: The role of the sol–gel route on the interaction with silica matrix

Solid acid–base sensors were prepared by encapsulating alizarin red pH indicator within a silica matrix using the sol–gel method with four different routes: (1) non-hydrolytic, (2) acid catalyzed, (3) basic catalyzed and (4) without catalyst hydrolytic. The silica–indicator interactions in the resulting materials were investigated by cyclic and differential pulse voltammetry. Ultraviolet–visible photoacoustic spectroscopy was also employed in the characterization. The absorption band shift (49 to 72 nm, depending on the route) between neat alizarin and alizarin encapsulated within the silica network was observed. The electrochemical behavior and the pH indicator interactions with the silica network were dependent on the nature of the employed sol–gel route. For the sensors prepared by the acid and hydrolytic (without catalyst) routes, the interactions with the silica network occurred through alizarin red hydroxyl groups. For the basic route, different cathodic and anodic peaks were observed depending on the pH, suggesting different phenomena during preparation or analysis. In the non-hydrolytic route, it is possible that the quinone form of alizarin red was consumed during the process. The voltammetric results were related to the sensor performance, whereby the acid route produced a solid sensor with the shortest response time, probably because alizarin structure was preserved after the synthetic process.