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 noncovalent,
affinity to cellulose fibers. However, the use of Congo red in the cellulose industries
has long been abandoned, primarily because of its toxicity and tendency to run and change
color when touched by sweaty fingers.
History Congo red was first synthesized in 1883 by
Paul Bottiger, who was working then for the Friedrich Bayer Company in Elberfeld, Germany.
He was looking for textile dyes that did not require a mordant step. The company was not
interested in this bright red color, so he filed the patent under his name and sold it
to the AGFA company of Berlin. AGFA marketed the dye under the name "Congo red", a catchy
name in Germany at the time of the 1884 Berlin West Africa Conference, an important event
in the Colonisation of Africa. The dye was a major commercial success for AGFA. In the
following years, for the same reasons, other dyes were marketed using the "Congo" name:
Congo rubine, Congo corinth, brilliant Congo, Congo orange, Congo brown, and Congo blue.
Behavior in solution Due to a color change from blue to red at
pH 3.0–5.2, Congo red can be used as a pH indicator. Since this color change is an approximate
inverse of that of litmus, it can be used with litmus paper in a simple parlor trick:
add a drop or two of Congo red to both an acid solution and a base solution. Dipping
red litmus paper in the red solution will turn it blue, while dipping blue litmus paper
in the blue solution will turn it red. This property gives Congo red a metachromatic property
as a dye, both in strongly acidic solutions and with strongly acidophilic tissue.
Congo red has a propensity to aggregate in aqueous and organic solutions. The proposed
mechanisms suggest hydrophobic interactions between the aromatic rings of the dye molecules,
leading to a pi–pi stacking phenomenon. Although these aggregates are present under
various sizes and shapes, the "ribbon-like micelles" of a few molecules seem to be the
predominant form. This aggregation phenomenon is more prevalent in high Congo red concentrations,
at high salinity and/or low pH. Dyeing activity
As suggested by its intense red color, Congo red has important spectrophotometric properties.
Indeed, its UV-visible absorption spectrum shows a characteristic, intense peak around
498 nm in aqueous solution, at low dye concentration. Congo red's molar extinction coefficient is
about 45000 [l]/[mol].[cm] in these conditions. Aggregation of the dye and binding to amyloid
fibrils tends to red-shift the absorption spectrum, whereas binding to cellulose fibers
has the opposite effect. Congo red also shows a fluorescent activity when bound to amyloid
fibrils, which tends to be used as a sensitive diagnosis tool for amyloidosis, instead of
the traditional histological birefringence test.
Diagnostic use
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. Additionally,
Congo red is used in microbiological epidemiology to rapidly identify the presence of virulent
serotype 2a Shigella flexneri, where the dye binds the bacterium's unique lipopolysaccharide
structure. Furthermore, Congo red may also be used as an artificial inducer of Shigella
flexneri type III secretion needles, bringing about the secretion of various virulence proteins
such as IpaB and IpaC, which form the translocation pore within the host cell membrane, allowing
effector proteins to pass through and alter the biochemistry of the cell.
References ^ Steensma, D. P.. "Congo Red: Out of Africa?".
Archives of Pathology and Laboratory Medicine 125: 250–252. doi:10.1043/0003-9985(2001)125<0250:CR>2.0.CO;2.
PMID 11175644.