Photographs of EGS under different pH conditions are shown in Fig. 1. There was a clear change in color from bright red to dark green with the pH value increasing from 1 to 10. Specifically, color changed from pink to red with increasing acidity, and gradually became green with a higher degree of alkalinity. The UV–vis spectra of EGS in the pH range of 1–10 were also examined. The maximum peak in the pH range of 1–5 was 528 nm and the intensity of the peak decreased with increasing pH. When pH was around 5, the peak was almost absent. Conversely, the intensity of the peak around 618 nm increased with increasing alkalinity in the pH range of 6–10. The adsorption peak around 518 nm was due to the presence of flavylium cations and the adsorption peak around 618 nm corresponded to the formation of the carbinol pseudobase