Natural indigo is now being used by several fashion retail giants such as H&M on a large scale to replace synthetic indigo dye for developing safe, green, and sustainable jeans. Since natural indigo originates from natural resources, it is treble the cost of synthetic indigo dye. Clear evidence of dyeing fabrics with natural indigo is needed to prevent adulteration and protect the interests of consumers. Natural indigo is easily oxidized to indirubin during the extraction process, so the presence of indirubin and other unknown impurities is a key feature of natural indigo dye. In this regard, an in-depth understanding of similarities and differences in the molecular structures features and various vibrational spectra of indigo and indirubin molecules is critical for establishing the authenticity of natural indigo dye.
Indirubin, an isomer of indigo, which is an effective component of an anti-leukaemia agent, also has pharmacological effects such as sterilization. The difference in molecular structures of indigo and indirubin gives rise to different physical and chemical properties as well as their applications. Indigo and indirubin with the molecular formula of C16H10N2O2 have a molecular weight of 262.6 Daltons, both of which have a double indole structure. Indigo is a planar molecule with an asymmetrical trans structure and a strong conjugation effect, in which two intramolecular hydrogen bonds are formed between the adjacent carbonyl groups and the amino groups. Indirubin is an asymmetric structural isomer of indigo by condensing two indole rings. In their production processes, the conditions of reaction should be strictly controlled in order to avoid the interconversion of indigo and indirubin.
The primary use for indigo is as a dye for cotton yarn, mainly used in the production of denim cloth suitable for blue jeans; on average, a pair of blue jeans requires just 3 grams (0.11 oz) – 12 grams (0.42 oz) of dye to produce. Smaller quantities are used in the dyeing of wool and silk.
Molecular Structures and Spectral Properties of Natural Indigo and Indirubin
Indigo dye Structure
As shown in Figure 1, indigo and indirubin with the molecular formula of C16H10N2O2 have a molecular weight of 262.6 Daltons, both of which have double indole structure. Indigo is a planar molecule with an asymmetrical trans structure and a strong conjugation effect , in which two intramolecular hydrogen bonds are formed between the adjacent carbonyl groups and the amino groups. Indirubin is an asymmetric structural isomer of indigo by condensing two indole rings. In their production processes, the conditions of reaction should be strictly controlled in order to avoid the interconversion of indigo and indirubin.
indigo and indirubin from natural resources comprehensively characterized their spectral properties and analyzed the relationship between the luminescent mechanism, spectral characteristics, and molecular structures of the two molecules. Accordingly, the FTIR, Raman, UV-Visible, and fluorescence spectral properties of indigo and indirubin have been measured and calculated. It is expected that this study could provide a reference for the spectral analysis of indigo and indirubin and lay the scientific basis for the assessment of purity and identification of natural indigo dye.
What Is Indigo blue dye?
Indigo is a type of blue dye that originate from India and is generally used for colouring cotton yarn which is used for the production of denim cloth for blue jeans.
Most people are not really familiar with indigo dye or with indigo in general. However, indigo dye is a natural, organic dye that is extracted from the indigo plant. For a long time, the most common usage of this blue dye was for dying blue jeans.
Now that we’ve answered the what is indigo question, it is important to note that not all blue jeans are actually made from natural indigo dye. In fact, most of the jeans on the market today, especially the cheaper ones, are made from synthetic dyes. Indigo is also used for dyeing wool and silk. After that term passed through Italian dialect and then into English as the word “indigo”.
Indigo was a natural dye extracted from plants but now it is synthetic. Dr Munirenkatappa Sanjappa had done his doctoral thesis on the indigo plant species. He was the Deputy Director of India’s most important herbarium and shared his own story of how he had camped at 17,000 feet altitude for forty days and daily climbed to 20,000 feet to find and gather his collection.
There are 62 varieties in India and he collected 58 of them. (The main species name is Indigofera Tinctoria). What is True Indigo? Indigofera is a genus of over 70 species of plants, many of which go by the common name “indigo.” Indigofera tinctoria often called true indigo or simply just indigo is probably the most famous and widespread dye plant in the world.
It’s Indigofera tinctoria, however, that gives indigo colour, so named for the deep blue dye it produces, which has been used for thousands of years. One of those plants is Indigofera tinctoria which is native to India which was the earliest major centre for the production and processing of indigo dye. Other Asian countries like China, Japan and South-East Asian nations also used indigo for centuries but India is considered the oldest centre of indigo dyeing. Even its name points to that.
The Greek word for the dye, indikón – means Indian. Romans changed that to indicum. After that term passed through Italian dialect and then into English as the word “indigo”. The first synthetic indigo dye was made by German chemist Adolf von Baeyer in 1878 but a synthesis of indigo those first tries was impractical. Experimenting continued and the first commercially practical synthetic indigo was made in 1897. choosing naturally dyed jeans is better for the environment and safer and healthier for everyone. Plus, naturally dyed jeans tend to last longer than synthetic-dyed jeans, especially when properly cared for.
Indigo is obtained from plants that contain the indican molecule. These belong to different plant families; the most common are of the genus Indigofera, but they also belong to others including the buckwheat family, the Lonchocarpus cyanescens found in Africa, or the Indonesian Marsdenia, and of course Woad. Although the indican molecule is contained in all these plants, producing the traditional blues requires more than merely steeping fibres with the plants in a pot full of water.
Synthetic indigo, prepared in the laboratory, contains the identical molecular structure to the natural indigo, but it has a much higher percentage of indigo per weight than the natural form. It is necessary to use only 1/4 to 1/3 as much synthetic indigo as natural. The powdered form is easiest, as the lump indigo must be ground or pounded to reduce it for use.
Extracting the Colour from Indigo
Although the indigo powder is blue, the indigo molecule does not produce its blue colour until it is oxidised. The indigo blue powder must be dissolved in an alkali bath with the combined oxygen removed. This is done by adding spectrolite (thiourea dioxide) to the indigo vat during its preparation. Indigo is only soluble in an alkaline solution made by dissolving sodium carbonate (as Soda Ash or Washing Soda) or caustic soda in water. The resultant solution is yellow-green in colour.
The alkalinity of the solution is controlled by the amount of Soda Ash dissolved. This can be tested with the Universal Indicator Paper. The pH needs to be between 9-11. At pH 11 it is easiest to reduce the indigo and will be best for dyeing cellulose fibre such as cotton, linen and viscose. A level of pH 9 is gentler on silk and wool and the Soda Ash should be added in increments to test the alkalinity until this level is achieved. For your first indigo bath, it helps to work at the higher range but not above pH11 and wash the dyed fabric in vinegar as the final rinse.
The alkalinity of the solution is controlled by the amount of Soda Ash dissolved. This can be tested with the Universal Indicator Paper. The pH needs to be between 9-11. At pH 11 it is easiest to reduce Hank dying procedures.