A preparation method of high purity porous indigo based on the influence of proteins and saponins on its sedimentation

Indigo, as a natural dye, holds a significant position in the textile industry. However, plant indigo has the problem of low purity. In this study we explored the effects of proteins and other components in Strobilanthes cusia biomass on the sedimentation and purity of indigo. The experimental results showed that indigo had high content of saponin. Meanwhile, the coexistence of proteins and saponins in the reaction system greatly accelerated the sedimentation of indigo. Through indigo-protein-saponin addition, fluorescence quenching, and molecular docking experiments, it was suggested that the co-precipitation of complexes formed by saponins, indigo, and proteins was the main mechanism affecting indigo purity and sedimentation. Based on its sedimentation mechanism, a high purity porous indigo was prepared by removing proteins and saponins through ultrasonication combined with ethanol extraction. In the SEM images, numerous pores were found on the surface of the purified indigo, probably due to the removal of proteins and saponins. As a result, the color distribution of dyed fabrics was rendered more uniform. The findings of this study provided a theoretical basis and practical methods for improving the quality of indigo, which may play a crucial role in solving the problem of seasonal indigo production. • Saponins and proteins complex with indigo, significantly reducing product purity. • Indigo–protein–saponin co-precipitation is confirmed via multiple analytical methods. • High-purity porous indigo is fabricated by ultrasonic-assisted ethanol extraction. • The purified porous indigo shows improved uniformity and dyeing performance. • This work provides a feasible strategy for high-quality indigo production.