The effect of the sol–gel route on the characteristics of acid–base sensors

The acid–base indicators alizarin red, brilliant yellow and acridine were encapsulated within a silica matrix by three different sol–gel methods: namely, (1) the non-hydrolytic and the (2) acidic and (3) basic hydrolytic routes. The sensors were characterized by several complementary techniques to investigate the effect of the nature of the sensor and of the sol–gel route on the structural and textural characteristics of the resulting sensor. The resulting acid–base sensors showed encapsulated contents ranging from 0.1 to 15.2 μmol g−1. The encapsulation yield was dependent on the nature of the sensor and on the sol–gel route. The highest content was achieved in the encapsulation of acridine by the acidic route. The morphological and textural characteristics were also shown to be dependent on the sol–gel route. The highest surface area (867 m2 g−1) and a more opened fractal structure were obtained by the acidic hydrolytic route. The sensors were evaluated in terms of the response time to ammonia gas by color changes. The response time was dependent on the nature of the indicator and on the sol–gel route. The shortest response time was observed for the system obtained by the acidic route employing the alizarin red indicator (59 s ± 11.5%).