Congo red is the sodium salt of 3,3'-([1,1'-biphenyl]-4,4'-diyl)bis(4-aminonaphthalene-1-sulfonic acid)(formula: C
32H
22N
6Na
2O
6S
2; molecular weight: 696.66 g/mol). It is a secondary diazo dye. Congo red is water soluble, yielding a red
colloidA colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
al solution; its solubility is better in organic solvents such as ethanol.
It has a strong, though apparently non-covalent affinity to
celluloseCellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
fibers. However, the use of congo red in the cellulose industries (
cottonCotton is a soft, fluffy staple fiber that grows in a boll, or protective capsule, around the seeds of cotton plants of the genus Gossypium. The fiber is almost pure cellulose. The botanical purpose of cotton fiber is to aid in seed dispersal....
textile,
wood pulpPulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper. Wood pulp is the most common raw material in papermaking.-History:...
&
paperPaper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
) has long been abandoned, primarily because of its tendency to change color when touched by sweaty fingers, to run, and because of its toxicity.
History
Congo red was first synthesized in 1883 by Paul Bottiger who was working then for the Friedrich
BayerBayer AG is a chemical and pharmaceutical company founded in Barmen , Germany in 1863. It is headquartered in Leverkusen, North Rhine-Westphalia, Germany and well known for its original brand of aspirin.-History:...
Company in Elberfeld, Germany. He was looking for textile dyes that did not require a
mordantA mordant is a substance used to set dyes on fabrics or tissue sections by forming a coordination complex with the dye which then attaches to the fabric or tissue. It may be used for dyeing fabrics, or for intensifying stains in cell or tissue preparations. The term mordant comes from the Latin...
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 ConferenceThe Berlin Conference of 1884–85 regulated European colonization and trade in Africa during the New Imperialism period, and coincided with Germany's sudden emergence as an imperial power...
, an important event in the
Colonisation of AfricaThe colonisation of Africa has a long history, the most famous phase being the European Scramble for Africa during the late 19th and early 20th century.- Ancient colonialism :...
. 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.
Behaviour in solution
Due to a color change from blue to red at
pHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
3.0-5.2, congo red can be used as a
pH indicatorA pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...
. Since this color change is an approximate inverse of that of
litmusLitmus or litmus test may refer to:* Litmus test, a common pH test* Litmus , a test case management tool maintained by Mozilla* "Litmus" , an episode in the first season of the television series...
, 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.
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
stackingIn chemistry, pi stacking refers to attractive, noncovalent interactions between aromatic rings. These interactions are historically thought to be important in to base stacking of DNA nucleotides, protein folding, template-directed synthesis, materials science, and molecular recognition, although...
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 (even if the "
micelleA micelle is an aggregate of surfactant molecules dispersed in a liquid colloid. A typical micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single tail regions in the micelle centre. This phase is...
" term is not totally appropriate here). 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
spectrophotometricIn chemistry, spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength...
properties. Indeed, its UV-visible absorption spectrum shows a characteristic, intense peak around 498 nm in aqueous solution, at low dye concentration. Congo red
molar extinction coefficientThe molar absorption coefficient, molar extinction coefficient, or molar absorptivity, is a measurement of how strongly a chemical species absorbs light at a given wavelength...
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
fluorescentFluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
activity when bound to amyloid fibrils, which tends to be used as a sensitive diagnosis tool for
amyloidosisIn medicine, amyloidosis refers to a variety of conditions whereby the body produces "bad proteins", denoted as amyloid proteins, which are abnormally deposited in organs and/or tissues and cause harm. A protein is described as being amyloid if, due to an alteration in its secondary structure, it...
, instead of the traditional histological
birefringenceBirefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
test.
Diagnostic use
In
biochemistryBiochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
and
histologyHistology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...
, congo red is used to
stainStaining is an auxiliary technique used in microscopy to enhance contrast in the microscopic image. Stains and dyes are frequently used in biology and medicine to highlight structures in biological tissues for viewing, often with the aid of different microscopes...
microscopic preparates, especially as a
cytoplasmThe cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
and erythrocyte stain. Apple-green
birefringenceBirefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
of Congo red stained preparates under polarized light is indicative for the presence of
amyloidAmyloids are insoluble fibrous protein aggregates sharing specific structural traits. Abnormal accumulation of amyloid in organs may lead to amyloidosis, and may play a role in various neurodegenerative diseases.-Definition:...
fibrils. Additionally, congo red is used in microbiological epidemiology to rapidly identify the presence of virulent serotype 2a
Shigella flexneriShigella flexneri is a species of Gram-negative bacteria in the genus Shigella that can cause diarrhea in humans. There are several different serogroups of Shigella; S. flexneri belongs to group B. S. flexneri infections can usually be treated with antibiotics although some strains have become...
, where the dye binds the bacterium's unique
lipopolysaccharideLipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals.-Functions:LPS is the major...
(LPS) structure.