Eco-friendly approach of bio-indigo synthesis and developing purification methods towards isolation of indigo from indirubin and bacterial fragments

Bio-dyes are natural dyes developed from renewable and sustainable sources that offer an ecologically responsible option to conventional synthetic dyes. Indigo dye is an organic compound with a distinctive blue color, and developing an ecologically suitable and efficient biocatalyst for indigo production is essential given the current need for blue colors. 2 g/L indigo was generated in a fermenter from 11.2 g/L tryptophan utilizing growth parameters of 3 vvm aeration, 200 rpm agitation, neutral pH, and 30 °C. Sustaining sufficient levels of oxygen is a critical factor in producing bio-indigo. E. coli gains the ability to withstand its toxicity in batch fermentation with large amounts of collected indigo, and the cells exhibit outstanding plasmid stability. Indigo was extracted from the E. coli mixture using solvent extraction, followed by purification via ultrasonication, centrifugation, and filtration. The purified bio-indigo was confirmed using NMR and UV–visible spectral techniques. For further confirmation, this bio-indigo dye was used to dye various fabrics and the dyeing results were compared with the samples dyed using commercial indigo. Dyeing tests on various fabrics with bio-indigo and commercial indigo, in alkaline and acidic media. The dyed samples showed similar fastness ratings in both indigo samples and color strength (K/S) analysis revealed superior results for alkaline leuco indigo dyeing, particularly on PET and nylon. Acidic leuco indigo showed poor color strength on cotton, likely due to cellulose hydroxyl group protonation. Therefore, the bio-indigo samples exhibited excellent stability, fastness, and color strength, demonstrating effective dye penetration and absorption in fabrics.