Preparation of high‐purity plant‐derived indigo via ultrasound‐assisted extraction for enhancing colour fastness of the indigo on textile substrates

Plant‐derived indigo offers the advantages over chemically synthesised indigo of biocompatibility and sustainability, but its biosynthesis is slow, because of slow formation and release of indoxyl, a precursor of indigo formation. Ultrasound treatment significantly enhanced indican hydrolysis, whereas indican underwent minimal hydrolysis during this period in conventional processes. The temperature stress experiments demonstrated that heating resulting from ultrasound treatment increased the β ‐glucosidase activity in leaves, accelerating the formation and release of indoxyl. However, excessively high temperature (80°C) inactivated β ‐glucosidase, strongly inhibiting hydrolysis of indican. Based on this finding, a constant‐temperature ultrasound treatment at 60°C that integrated the effects of temperature stress and ultrasound was proposed, which shortened the indoxyl release time from 2.5 h to 1 h, and led to an increase in the indoxyl concentration from 0.20 to 0.51 g/L. Consequently, the soaking time was 48 times shorter and the indigo yield was 2.1 times higher than those of the conventional method, respectively. Scanning electron microscope, colorimetry and colorfastness testing demonstrated that the dyeing performance of indigo produced by the new method was superior to that of the conventional method and comparable to that of synthetic indigo, suggesting the potential of the plant‐derived indigo to replace synthesised indigo.