Acid–base sensor based on sol–gel encapsulation of bromothymol blue in silica: application for milk spoilage detection

Abstract

In this work, silica capsules containing bromothymol blue (BTB) were synthesized by a modified Stober sol–gel method. These silica capsules were dispersed into a high-density polyethylene (HDPE)/ethylene-vinyl acetate (EVA) polymeric matrix and then applied in milk spoilage detection tests. The effect of the KOH/BTB molar ratio used in the sol–gel synthesis on the structural, textural, and morphological characteristics of the resulting silica capsules was also investigated. Field emission gun scanning electron microscopy images of the silica capsules showed particles with spherical morphology and size ranging from 0.1 to 1 µm. All silica capsules presented a very low porosity and an encapsulated BTB content in the range of 12–37 µmol g−1. The increase of the KOH/BTB molar ratio resulted in an increase in the relative amount of six-fold siloxane rings and a decrease in the BET surface area of the silica capsules. Spoilage kinetics of milk at room and fridge temperature were much different, as evidenced by pH milk measurements. The milk storage temperature and the KOH content in the silica capsules affected the intensity and rate of blends color change. Blends (HDPE/EVA/silica capsules) with lower KOH content in the capsules showed faster kinetics and higher intensities of color change during the milk spoilage at both tested temperatures. These results showed the potential of the HDPE/EVA/silica capsules as a base material to a sensor packaging, which allows the verification of the milk quality by the end-users.