pH‐responsive cotton fibre dyed by natural madder dye

Abstract pH‐responsive cotton can be used as a flexible sensor because it changes colour with pH. However, the safety of some synthetic pH‐responsive dyes is uncertain, so the application of pH‐responsive cotton to wounds is limited. Herein, natural madder dye was selected to explore the dyeing conditions and produce safe, pH‐responsive cotton. Based on experimental data and computational calculations, the variation in the conjugation of the molecule was responsible for the colour change of the cotton dyed with madder. When the pH of the madder solution increases, the phenolic hydroxyl group of the alizarin, which is its main component, is ionised, and the colour changes from yellow to purple. The effect of the dye concentration was studied, and the results showed that the higher the dye concentration, the more obvious the colour change of the cotton. The reversibility of cotton dyed with madder was investigated, and the results showed that there was a good reversible transformation with changing pH. The reversibility time of the colour change was less than 20 seconds.