2.1 Preparation of indican solution from leaves of S. cusia
The fresh leaves of S. cusia were harvested from Guizhou. The extraction conditions of indican were 100 ℃ water bath heat treatment. The fresh leaves were treated with deionized water, and 10 g of it was treated with 100 mL deionized water at 100 ℃ in a water bath for 10 min. The obtained liquid was then immediately cooled rapidly in a 4 ℃ water bath for 10 min to stop the extraction reaction that was still going on. The cooled solid–liquid mixture was filtered with gauze and the leaves and indican solution were separated.