Ammonium fluoride-catalyzed sol-gel route applied for curcumin-based pH sensors

Curcumin has potential application as a colorimetric sensor and can be used, for example, to monitor the freshness of foods, although some of its physicochemical properties, such as solubility and stability, limit its use. In the present work, a natural curcumin extract was incorporated into organic‒inorganic hybrid silica matrices produced by the sol-gel method using ammonium fluoride as the main catalyst. A series of pH sensors based on the encapsulation of curcumin were prepared using a series of organosilanes (methytriethoxysilane, octyltriethoxysilane, octadecyltrimethoxysilane and 3-(aminopropyltriethoxysilane)) and were used in the synthesis of hybrid silicas. The effectiveness of the employed sol-gel route and the properties of the encapsulated materials were evaluated by a set of complementary analytical techniques, namely, infrared spectroscopy, ²⁹ Si solid-state nuclear magnetic resonance spectroscopy, nitrogen porosimetry, dynamic light scattering, confocal laser scanning microscopy and diffuse reflectance UV‒Vis spectroscopy. The immobilization of compounds in a solid matrix can cause a shift in the maximum absorption bands in the ultraviolet‒visible region as a result of interactions between the organic molecules and the solid support. The incorporation of curcumin into different silica matrices improved the performance of the sensors to up to 3.5 times greater than that obtained by the free compound when interacting with ammonia vapor.