Application of Fluorescence Spectroscopy in Indigo Reduction Process : Identification of Reduction Stages and Reduced Indigo Concentration

Abstract

In this study, we traced the stages of indigo reduction and evaluated the concentration of reduced indigo by using fluorescence spectroscopy. For the indigo reduction, the inorganic reduction system of Na₂S₂O₄ and the microbial reduction system of Dietzia sp. KDB1 strain separated from traditional fermentation bath were used. The technique of florescence excitation-emission matrix (FEEM) was applied for the tracing of indigo reduction. With the progress of indigo reduction, maximum excitation wavelength was observed at 420 nm (stage of hydroxyl group form), 440 nm (stage of mono-sodium substituted), and 460 nm (stage of di-sodium substituted) at constant maximum emission wavelength of 495 nm. The peak of KDB1 disappeared at shorter wavelength than 440 nm with the progress of reduction. When polystyrene cell was used, diagonal peak of elastic scattering also disappeared with the progress of reduction in the inorganic reduction system. This phenomenon could be explained by the absorbance increase of reduced indigo at the same wavelength region. Also, the fluorescence intensity was linearly proportional to K/S value for the reduced indigo concentration of less than 2 mM in both of the inorganic reduction system (pH 9.5–10.5, <60 °C) and the bacterial reduction system (pH 10.0, 30 °C).