• (1) An aromatic group (i.e. benzene ring),• (2) A chemical group called a chromophore, which lends a color to the aromatic group (the aromatic group with a chromophore is called a chromogen), and• (3) An auxochrome group, which allows the compound to combine either electrostatically or covalently with the target thereby staining it.
• The binding of dyes to tissues is no different to any other chemical bonding and the mechanisms rely on the same binding forces that occur in all other organic compounds.• The dye must form some type of bond or link to the cell or they will simply be rinsed out of the cell when the section is washed in another reagent.• The usual forms of bonding can be involved. Each type has its own characteristics and bond strengths.
• Most simple organic compounds such as alkanes, benzene and alcohols are colourless to the human eye but will absorb light outside the visible spectrum. Benzene, for example, absorbs strongly in the UV region of the spectrum but appears water-white to the human eye.• Benzene must be altered so that it will absorb visible light and so become a visible coloured compound that can be a useful as a dye.
• Any group that makes an organic compound coloured is called a chromophore.• Benzene can be made to absorb visible light by adding a suitable chromophore. In the example below the chromophore used is the nitro group. Adding a single nitro group gives nitrobenzene, which is a pale yellow colour; adding a second and third group intensifies the yellow colour and trinitrobenzene is a strong yellow colour.
• Common chromophore is the nitro group.• This chromophore is a nitrogen with two oxygen atoms attached. One oxygen is shown attached with a single bond, the other with a double bond. In fact, like the carbon atoms in benzene, these two oxygen atoms are attached to the nitrogen with bonds of equal strength. The extra electrons are delocalised between the three atoms.
• The quinoid ring is found in many dyes. It is a ring structure with two points at which chromophores can attach.–C=C– –C=N– –C=O––N=N– –NO2 Quinoid rings
• Trinitrobenzene, although coloured, is still not a dye, as it will not bind to tissues. Treating the section with trinitrobenzene will temporarily colour.• To turn a coloured compound into a dye requires the addition of an ionizable group that will allow binding to the tissues. Such binding groups are called auxochromes.• The addition of an ionizable OH group turns trinitrobenzene into the dye trinitrophenol, which is more commonly called picric acid in histology. Picric acid is an acid dye (the OH group is phenolic and ionizes by losing a hydrogen ion) and is very useful. –NH3 –COOH –HSO3 –OH
• Stain solutions made colorless by the reduction (removal of O2) of their chromophore;• Color is restored when oxidized by combination of the dye with tissue;• For Eg. Schiffs reagent is generally considered a leuco compound
• ACID DYE:• One in which the coloring elements(chromophore group) are located in the anionic (acid) portion of the dye; stains basic tissue elements such as cytoplasm.• For eg: Nigrosin, Congo red• BASIC DYE:• One in which the coloring elements (chromophore group) are located in the cationic (basic) portion of the compound; stains located in the cationic (basic) portion of the compound; stains acidic tissue elements such as nucleI.
• AMPHORETIC DYE:• (neutral) neither acid nor base, but can act as either, depending on the staining procedure and the pH of the solution
• Physical theories :• Simple solubility: e.g. Fat stains are effective because the stain is more soluble in fat than in 70% alcohol.• Adsorption: This is a property by which a large body attracts to itself minute particles from a surrounding medium.
• Chemical theories:• It is generally true that acid dyes stain basic elements (Cytoplasm) and basic dyes stain acidophilic material (nucleus) however this far from being complete truth, Indeed hematoxylin, which is an acid dye, does not stain the cytoplasm, but (in the presence of mordant) is one of the most widely used nuclear stains.
• INDUSTRIAL APPLICATIONS:• 1. Paints- A diversified application in the whole gamut of paints that include decorative and protective coatings, in paints that are oil and resin based, automotive finishes, emulsion paints, distempers, aqueous based paints like lime etc.• 2. Printing inks- Pigments are used in all kinds of printing inks, that includes inks for printing metal foils, lacquers etc.• 3. Colouration of Plastics ColouratIon of cement glass paints, Colours for cosmetics, Colouration for papers.
• MEDICINE:• Dyes are now an important ingredient in many of the medical tests. Many of the tests that are carried out on patients use the dye to get accurate results.• Eg. Fluorescein angiography. Fluorescein angiography derives its name from fluorescein, the dye that is used is very successfully for carrying out the tests. Angiogram is a very valuable test that gives information about the circulatory system.• KMnO4, CV are known for their medicinal properties.
• AGRICULTURE INDUSTRY:• 1. Fertlizer Industry:• Fertilizers are often added with colouring to differentiate between qualities and to avoid any type of errors in application. Fertilizers are also coloured for purely marketing purposes. Usually water based pigment is often used in the diluted form and is sprayed onto the fertilizer or sometimes incorporated into a melt.• 2. Use of colourants in crop protection:• agents of the type insecticide and fungicides helps
• Seed Dressing Industry:• The main objective of colouring the seed dressings is used for marking and a warning reference. To avoid any sort of confusion and making sure that it does not end up being used as fodder. The most important colourant used here is red with typical concentrations of 600 ppm.
• Types of ingredients used in the cosmetics:• In the category of decorative cosmetics, the majority of colors used are pigments. It is the inorganic pigments that are popular with cosmetics but are subject to purity levels of heavy metals. The range of inorganic pigments used in cosmetics is generally made up of various chemical types. A few of the popular ones is provided in the table.
Pigments Features and applications Three basic shades: black, yellow andIron Oxides red, finds use in liquid foundations, face powders, and blushers. Shades range from dull olive green, to a blue green, or bright green, finds useChromium Dioxides in most categories of cosmetic preparations but prohibited for use in lip products in the USA. Shade range from bright blue to violet,Ultramarines pink and green also, not allowed for lip products in the USA.