Congo Red (CR)

Congo red is the sodium salt of 3,3"""-([1,1"""-biphenyl]-4,4"""-diyl)bis(4-aminonaphthalene-1-sulfonic acid)(formula: C32H22N6Na2O6S2; molecular weight: 696.66 g/mol). It is a secondary diazo dye. Congo red is water soluble, yielding a red colloidal solution; its solubility is better in organic solvents such as ethanol. It has a strong, though apparently non-covalent affinity to cellulose fibers. However, the use of Congo red in the cellulose industries (cotton textile, wood pulp & paper) has long been abandoned, primarily because of its toxicity and tendency to run and change color when touched by sweaty fingers. In biochemistry and histology, Congo red is used to stain microscopic preparates, especially as a cytoplasm and erythrocyte stain. Apple-green birefringence of Congo red stained preparates under polarized light is indicative for the presence of amyloid fibrils.