Congo red

Congo red is a synthetic anionic azo dye used to color textiles, stain biological materials, and indicate pH through a color change.

Azo compound: An organic compound containing at least one nitrogen–nitrogen double bond linked to carbon groups. Congo red belongs to this dye-producing family because it contains two azo groups.
Textile dyeing: The process of adding color to fibers, yarns, or fabrics using dyes. Congo red was developed as a direct dye that can color cellulosic textiles.
Paul Böttiger: A German chemist credited with synthesizing Congo red while working at the Friedrich Bayer company. Böttiger developed the dye in 1883, before Bayer declined to commercialize it.
Thioflavin T: A fluorescent dye that becomes strongly fluorescent when bound to amyloid fibrils. Unlike Congo red’s characteristic polarized-light appearance, Thioflavin T is read by fluorescence.
Chromophore: The part of a molecule that absorbs particular wavelengths of light and produces color. Congo red’s extended chromophore absorbs visible light, producing its red appearance.
Histology: The study of the microscopic structure of tissues, commonly using prepared stained sections. Its selective staining makes Congo red useful for visualizing certain tissue deposits.
Friedrich Bayer & Co.: The German chemical company founded by Friedrich Bayer and Johann Friedrich Weskott in 1863. Bayer initially rejected Böttiger’s dye, which he later patented and sold elsewhere.
Amyloid beta: A peptide that can aggregate into amyloid plaques, especially in Alzheimer disease. Congo red can stain deposits containing this peptide, but does not identify its composition by itself.
Conjugated system: A molecular arrangement with alternating bonds that allows electrons to spread across several atoms. Conjugation across Congo red’s structure contributes to its strong visible-light absorption.
Amyloid: A protein aggregate with a characteristic cross-β structure that accumulates in tissues or laboratory preparations. Congo red binds amyloid deposits and produces the classic stain used to identify them.

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BenzidineBroader topic: Its manufacture historically involved benzidine, making it a notable example of a benzidine-based dye.