1. Fragrance and Flavor Industry
This is one of the most classic applications of indole. At high concentrations, it has a strong, unpleasant odor, but when highly diluted (<0.1%), it exhibits pleasant floral scents such as jasmine and orange blossom.
It is widely used in the formulation of daily-use fragrances with floral notes such as jasmine, lilac, gardenia, narcissus, and orchid. In food flavors, trace amounts can be used to formulate chocolate, strawberry, coffee, nut, and cheese flavors.

2. Pharmaceuticals and Biopharmaceuticals
Key intermediate: It is a raw material for the synthesis of the essential amino acid tryptophan, which can be used to produce drugs for gastric ulcers, etc.
Core structure: Many important drugs (such as the anti-inflammatory drug indomethacin and the vasodilator propranolol) and alkaloids contain the indole structure.
Biological activity research: As a metabolite and signaling molecule of the gut microbiota, it plays an important role in regulating bacterial biofilm formation and gene expression, and is a hot research topic in the development of new antibacterial and anti-tumor drugs.

3. Agricultural Chemistry
Intermediate for the synthesis of plant growth regulators, mainly used in the production of indole-3-acetic acid (IAA), indole-3-butyric acid, etc., which can effectively stimulate plant growth.

4. Chemical and Materials Industry
Dye intermediate: Used in the synthesis of sulfur dyes, indigo dyes, etc.
Resin raw material: Can be condensed with formaldehyde to produce indole-formaldehyde resin.
Organic synthesis building block: It is an important starting material for the synthesis of various fine chemicals and complex organic molecules.

5. Scientific Research and Detection
Analytical reagent: Traditionally used for the detection of gold, potassium, and nitrite.
Research tool: In the fields of biochemistry and microbiology, it is used to study bacterial quorum sensing, metabolic pathways, etc.