Eosin
Eosin is a term referring to several fluorescent acidic compounds that bind to and form salts with basic, or eosinophilic, compounds like proteins containing amino acid residues such as arginine and lysine[2]. These compounds are used as histologic stains, particularly in the widely used Hematoxylin and Eosin (H&E) staining technique in histology. In this technique, eosin stains cytoplasmic structures, collagen, and muscle fibers, while hematoxylin stains nuclei[2].
There are two closely related compounds commonly referred to as eosin: Eosin Y and Eosin B. Eosin Y is the form of eosin used most often as a histologic stain and has a very slightly yellowish cast, while Eosin B has a very faint bluish cast[2]. Both dyes are interchangeable, and the choice between them is a matter of preference and tradition.
Eosin Y is a tetrabromo derivative of fluorescein, and Eosin B is a dibromo dinitro derivative of fluorescein[2]. Eosin Y is typically used in concentrations of 1 to 5 percent weight by volume, dissolved in water or ethanol, for staining purposes[2]. It is also used as a red dye in inks, although the molecule tends to degrade over time, causing the paint to obtain a darker brown tinge[2].
Eosin is listed as an IARC class 3 carcinogen[2]. It is available as a ready-to-use secondary stain for histology and cytology, particularly for routine hematoxylin and eosin staining and fast cytoplasmic staining of rush specimens[1]. The product is available in various quantities, with prices ranging from $43.44 to $63.25[1].
In addition to its use in histology, eosin is also used as a red dye in inks and as a photosensitizer in organic synthesis[3]. Its red to brown sodium or potassium salt is used especially as a biological stain for cytoplasmic structures[4]. Eosin Y is commonly used in the Papanicolaou stain (or Pap stain used in the Pap test) and the Romanowsky type cytologic stains[3].
Citations:
[1] https://shop.leicabiosystems.com/us/histology-consumables/routine-he-special-stains/pid-eosin-secondary-counter-stains
[2] https://en.wikipedia.org/wiki/Eosin
[3] https://en.wikipedia.org/wiki/Eosin_Y
[4] https://www.merriam-webster.com/dictionary/eosin
[5] https://www.sigmaaldrich.com/US/en/product/sial/230251