Non-destructive analysis of indigo dye in biological and textile samples is presented. Using different but complementary techniques of vibrational spectroscopy (FT-Raman and ATR-IR) detection of indigo in various samples is performed. This work shows the possibility of both vibrational analytical methods to identify indigo directly in plant tissue, commercially available pigments and cotton and silk textiles. The experimental work is supported by quantum-chemical calculations at the B3LYP/6-311++g(d,p) level of theory. Additionally, in order to follow the process of indigo formation upon leaf drying Raman mapping of Polygonum tinctorium is demonstrated.
In the recent years, Raman and IR spectroscopies have attracted increasing attention as fast, non-invasive and widely applicable alternative analytical approaches for a variety of materials. Vibrational spectroscopy has been used in the analysis of herbal products, dyes and sensitive art objects, besides complex and aqueous biomaterials such as biopolymers or mammalian tissue. Compared to conventional analytical methods based on high-performance liquid chromatography (HPLC) or gas chromatography, which often involves extensive and time-consuming sample preparation, Raman or IR spectroscopy can avoid these procedures. The present work introduces a fast and reliable quantification method for the determination of naturally occurring indigo dye in dyer's knotweed (Polygonum tinctorium) based on Fourier transform (FT) Raman spectroscopy. The results were validated by HPLC-UV, and the merits and drawbacks of the present method are elaborated. Besides the qualitative aspects of signal assignment and comparison to appropriate attenuated total reflectance Fourier transform infrared (ATR-FT-IR) measurements, the Raman spectrum of dihydro indigo, an important intermediate in the indigo dying process, is presented for the first time and discussed with regard to its spectroscopic behaviour.
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