- Using the Indigo-Leucoindigo Reaction for Dyeing
Indigo, with molecular formula of C16H10N2O2, is a non-water-soluble non azo colorant. Indigo pigment is one of the oldest pigments known to mankind. It is widely used in food, medicine, printing, and dyeing industry.
The use of indigo as a fabric dye dates back at least 2500 BC. Some clothes worn by ancient Egyptian mummies and blue linen fabrics unearthed from Mawangdui in China are dyed with indigo. A branch of Yao nationality in China is named "blue indigo Yao" because of its unique technology of producing and using indigo dyed cloth. In the food industry, Indigo is used as food pigment in the form of sodium sulfonate or its aluminized form, which is called "brilliant blue" and brilliant blue aluminum lake in China, and mainly in the form of sodium sulfonate in the United States, which is called “Indigo".
The organic dye dark blue is made by fermentation of Polygonum, Isatis indigotica, indigo, horse blue and other plant leaves containing indole acid. It is also artificially synthesized. It is used to dye cloth. The color lasts for a long time. Commonly known as indigo, some areas are called indigo.
Indigo is generally extracted from indigofera genus plants.
- Physical and Chemical Properties of Indigo and Indigo White.
-Shape and properties: Dark blue crystalline powder, odorless.
-Solubility: Slightly soluble in water, ethanol, glycerol and propylene glycol, insoluble in oil. 0.05% aqueous solution is dark blue. 1g is soluble in about 100ml of water at 25 ℃. Its solubility in water is lower than that of other edible synthetic pigments. 0.05% aqueous solution is blue. Soluble in glycerol, propylene glycol, slightly soluble in ethanol, insoluble in oil. It is dark blue in case of concentrated sulfuric acid and blue after dilution. Its aqueous solution plus sodium hydroxide is green to yellow green. Indigo is easy to color, has a unique tone and is widely used. Poor heat resistance, light resistance, alkali resistance, oxidation resistance, salt resistance and bacterial resistance. Discoloration during reduction, such as reduction with sodium hyposulfate or glucose, becomes indigo white. Maximum absorption wavelength 610nm ± 2nm.
Indigo White (Leucoindigo):
With molecular formula of C16H12N2O2, it is soluble in ethanol, showing blue fluorescence. Also soluble in sodium hydroxide, showing the color of yellow-green. Indigo white was prepared by reducing sodium dithionite (sodium bisulfite), zinc and glucose in alkali medium, and can be made into reduction dyeing bath. It is also used for dip dyeing and printing of cotton, and its fastness is the same as that of indigo.
- Reaction Principle and Path.
Our group mainly studied two different dyeing methods: condensation dyeing and natural fermentation dyeing. These are two common indigo dyeing methods.
- Condensation dyeing.
Basic principle: After dissolving indican from plant cells, it is enzymatically hydrolyzed in water to produce indophenol and glucose. After contacting with oxygen, bimolecular condensation occurs to form indigo. Below are the three common reaction pathways.
- Hydrolysis of indican.
Equation: Indican + water→ 3-indolphenol + glucose
- Keto/enol tantomerism.
Equation: 3-indolphenol → indolone
- Oxidative condensation of indolone to indigo.
Equation: indolone + oxygen → indigo + water
- Natural formation dyeing.
With the use of dip dyeing technology, it is gradually found that when the dye solution in the container is left standing for a long time, it will be found that blue precipitates are formed at the bottom of the container, which can ferment by themselves and can be used for repeated dyeing of textiles. People have observed this phenomenon for a long time, which makes indigo production from bluegrass and natural fermentation dyeing technology appear.
Basic Principle: Indigo is reduced to form water-soluble indigo white. The textile is immersed in the solution, and then oxidized to indigo with oxygen as oxidant, so that the textile is dyed.
- Reduction of Indigo.
Equation: Indigo + reducing agent → Indigo White (Leucoindigo)
- Oxidation of Indigo White.
Equation: Indigo White + Oxygen → Indigo
Silver nanoparticles and ions are well-known for their bactericidal properties and anti-viral properties.
First, AgNPs will adhere to the cell wall and cytoplasmic membrane. This is because of the electrostatic affinity of the nanoparticles towards proteins containing sulfur (S). Then, the silver nanoparticles will act as a catalyst in the bacteria, disrupting the ATP production, mitochondria function and DNA. This will either directly or indirectly kill the bacteria.
As for viruses, though the mechanism is currently unknown, AgNP kill viruses most likely by catalyzing reactions with the virus’ protein cover. The two pictures below show the significance of the AgNP against viruses.
-  Indigo Dye, The History of Indigo, Wikipedia (n.d.)
-  Indigo, Properties, Baidu Wikipedia (n.d.)
-  Leucoindigo, Baidu Wikipedia (n.d.)
-  Tyrosine, Wikipedia, (n.d.)
-  An overview of microbial indigo-forming enzymes, Andrea N. Fabara et al., December 2019
-  Effectiveness of Silver Nanoparticles against SARS-CoV-2, ZhiHu, Marvin Zhou,