Natural flavonoids from tea (Camellia sinesis) leaves for optical pH sensing

This study investigates the potential of a catechin-rich extract from tea (Camellia sinensis) leaves as a natural pH-sensitive optical sensing probe. Tea flavonoids were extracted using the hot water bath extraction method and systematically studied using scanning electron microscopy, Fourier-transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, liquid chromatography mass spectrometry, UV-Visible absorption and fluorescence spectroscopy to determine their contents and compositions. The absorption spectrum of the extract revealed a characteristic peak of catechin around 273 nm, while the fluorescence spectrum demonstrated three emission bands at 318 nm, 340 nm and 442 nm, corresponding to catechin and its polymerized forms. The observed catechin-rich tea leaves extract (CTE) was studied as a pH-sensing probe, and its sensing behaviour was examined using the UV-Visible absorbance and fluorescence emission intensity measurements. A significant change in the absorbance and fluorescence intensity was observed across a wide pH range from 1.66 to 11.80. These variations in the optical properties of catechin with pH are explained based on changes in the electronic configuration of catechin molecules resulting from protonation and deprotonation, as well as subsequent polymerization. The results showed a linear correlation between absorbance and fluorescence intensity of CTE with pH, highlighting its capability as a sensitive and eco-friendly pH indicator. The study demonstrates the potential of CTE for developing a sustainable and non-toxic solution-based optical pH sensing probe.