Characterisation of Natural Indigo and Studies on the Various Eco-Friendly Reduction Systems for its Dyeing

Abstract

The present study investigates the use of organic reducing agents, such as overripe bananas, molasses, cassia tora Linn. seeds, madder roots, dates, moringa oleifera leaves, myrobalan nuts, and tannic acid, as alternatives to the commonly used hazardous chemical reducing agent, sodium dithionite, for reducing indigo. The reduction peak potential of these organic sources is measured at specific time intervals over 18 days at room temperature using the saturated calomel electrode (SCE) as the reference electrode. In the present study, silk is dyed with indigo in the presence of organic reducing agents, as specified, and chemical reducers, such as sodium dithionite, at room temperature. However, from practical experience and previous reported work, it is observed that artisans sometimes use higher reduction temperatures (60–70 °C) to achieve a good shade depth when tamarind is used as a reducing agent during indigo dyeing. Hence, in this study, indigo does not produce good shade depth on silk when dyed at room temperature in the presence of tamarind as a reducing agent. The reported findings of the present study apply to room-temperature vats under the tested conditions. Among these reducing agents used in the study, cassia tora and madder maintain negative reduction peak potentials throughout the 18-day period. However, as the day progresses, their reducing power decreases. The same trend is observed for other reducing agents, but they do not maintain a negative reduction peak potential up to 18 days, thus, gradually losing the reducing power. The decrease in reducing power results in less conversion of leuco-indigo salt from its insoluble state, ultimately resulting in lower shade depth. The UV–visible absorption spectrum of indigo dye exhibits maxima at 286 and 603 nm, indicating an extensive conjugated system. The blue colour and absorption spectrum of indigo may be due to electronic transitions in the conjugated π-electron systems in the chromophore groups when exposed to light. Fourier transform infrared spectroscopy reveals the presence of various functional groups and bioactive compounds, including saponins and tannins, in the dye. Silk fabric treated with cassia tora seed and madder shows no fungal growth (average rating: 0). In the antibacterial assessment, silk fabric, when treated with cassia tora seeds, shows a 99.7% reduction of microorganisms against both S. aureus and K. pneumoniae, whereas madder shows 99.02 and 99% reductions in gram-positive and gram-negative bacteria, respectively. Thus, silk fabrics treated with cassia tora seed and madder exhibit excellent antimicrobial activity. Hence, to avoid interference from cassia tora seed and madder, which have inherent antifungal and antibacterial properties, these are not used as reducing agents when assessing the indigo’s antimicrobial properties. A medium (30–60%) fungal growth with an average rating of 3 is observed on untreated silk fabric, and light growth (10–30%) with an average rating of 2 is noticed for treated silk fabric with overripe banana. However, silk fabric dyed with indigo in the presence of overripe banana and sodium dithionite as a reducing agent shows no fungal growth (average rating 0), indicating the antifungal properties of indigo itself. The % reductions in S. aureus and K. pneumoniae bacterial colonies on silk fabric dyed with indigo in the presence of overripe banana and sodium dithionite are 99.77 and 99.76%, and 99.35 and 99.13%, respectively. In both cases, the excellent antibacterial properties may be attributed to the presence of tannins in indigo, which bind to proteins (such as silk) and inhibit bacterial growth. The fabric dyed with organic reducing agents exhibits very good colourfastness to washing and rubbing, comparable to that of fabric dyed with indigo in the presence of a chemical reducing agent, i.e. sodium dithionite.