Green extraction of natural indigoid from Baphicacanthus cusia (Nees) Bremek using hydrophilic and hydrophobic deep eutectic solvent technology

This study focused on the development of an alternative and more environmentally friendly extraction solvent, a deep eutectic system (DES), for extracting indigoid pigments, specifically indigo and indirubin, from Baphicacanthus cusia (BC). BC is recognized in the textile industry as a natural vat dye and in traditional Chinese medicine as "Qing-Dai". It is known for treating inflammatory diseases such as psoriasis. In this study, 46 DES systems were compared with conventional methods. The hydrophobic DES, a terpenoid and fatty acid system comprising thymol:decanoic acid (DES40), and the hydrophilic DES, a choline chloride-based system comprising choline chloride: p -toluenesulfonic acid (DES19), showed significant extraction improvements. DES40 and DES19 achieved approximately 26-fold higher indigo content compared to classical ethanol and outperformed the harsh organic solvent dichloromethane. The green extraction process was optimized using a Box–Behnken design, considering parameters such as temperature, time and co-solvent. DES19 maximized indigo and indirubin content to 270.91±14.38 and 5.70±0.11 mg/g, respectively, while DES40 yielded 108.28 ± 3.9 and 0.16 ± 0.00 mg/mg/g, respectively. Safety evaluations using a cell-based MTT model with human skin cells in keratinocytes and fibroblasts showed that both DES19 and DES40 were safe at all concentrations tested. These results indicate that a more environmentally friendly solvent technology for the extraction of indigoids from BC using the DES is an efficient and potential application in the textile and pharmaceutical industries. • Environmentally friendly solvents, 46 deep eutectic systems (DES) have been extensively prepared and investigated for indigoid extraction. • Indigoid pigments yield was significantly improved by DES. • Natural DES, thymol:decanoic acid (DES40) and the semi-natural DES, ChCl: p -toluenesulfonic acid (DES19) showed significant improvements-achieving ∼26-fold higher indigo yield than classical ethanol and were clearly superior to the harsh organic solvent, dichloromethane. • DES19 and DES40 were tested for safety on human skin cells, fibroblasts, and keratinocytes.