Dyes commonly used in laboratories can be classified into natural dyes and synthetic dyes. Natural dyes mainly include hematoxylin and magenta, and synthetic dyes mainly include acid magenta, congo red, methyl blue, solid green, sudan III, eosin, basic fuchsin, crystal violet, gentian violet, neutral red, safranin, methylene blue and methyl green.

1 Natural dyes

1.1 Hematoxylin

Hematoxylin is a pigment extracted from the dried branches of hematoxylon (tropical legumes) and is one of the most commonly used natural dyes. Hematoxylin can not be used directly for dyeing. It must be exposed to ventilated place to become oxidized hematoxylin, and this process is called “mature”. The “mature” process of hematoxylin takes a long time, and the longer the time, the stronger the dyeing power. The dyed material will have a coloring power only under the action of a mordant such as a metal salt, so a metal salt is required in the preparation of the hematoxylin. Commonly used mordants are aluminum sulfate, potassium alum and iron alum. Hematoxylin is a pale yellow to rust purple crystal, and it is easily soluble in alcohol, slightly soluble in water and glycerin. Hematoxylin is an excellent material for staining cell nuclei, which can differentiate different structures in cells into different colors.

1.2 Magenta

Magenta is also called carmine or kermes. Magenta is produced by extracting a dried female tropical cochineal worm red, and then removing the impurities with alum. Separate magenta cannot be used for dyeing and can only be used for dyeing after it has been dissolved in an acidic or alkaline solution. Commonly used acidic solutions are glacial acetic acid and picric acid, commonly used alkaline solutions are ammonia, borax and the like. Magenta is an excellent dye for the nucleus, and stained specimens are not easily faded. Magenta is suitable for staining of sections or tissues, and is particularly suitable for overall dyeing of small materials. The color can be maintained for several years after dyeing with a solution prepared from magenta.

2 Synthetic dyes

Synthetic dyes, namely aniline dyes or coal tar dyes, come in many varieties and are widely used. Its shortcoming is that it is easy to fade after exposure to sunlight, and syntheticdyes such as aniline blue, bright green, and methyl green are more likely to fade. However, if you pay attention to the pH during dyeing and avoid direct sunlight, the color can remain for several years.

2.1 Acid magenta

Acid magenta is an acid dye that is red powder, soluble in water and slightly soluble in alcohol (0.3%). It is a good cell staining agent and is widely used in animal cell staining. It can also be used for the staining of parenchyma cells and cellulose walls in the cortex and pith of plants. Tissue sections are immersed in acidic water prior to staining to enhance staining power.

2.2 Congo red

Congo red is an acid dye that is jujube red powder, soluble in water and alcohol, and blue in acid. It can be used as a dye and an indicator. It is often used as a liner for hematoxylin or other cellular dyes in plant staining. When Congo red is used to stain the cytoplasm, it can dye the gel or cellulose red. It is used to dye nerve shafts, elastic fibers, embryo materials, etc. in animal tissue preparation. Congo red can be double-stained with hematoxylin and can also be used for starch-like staining. Since congo red is soluble in water and alcohol, washing and dehydration must be rapid.

2.3 Methyl blue

Methyl blue is a weak acid dye that is soluble in water and alcohol. Methyl blue is widely used in the cell staining of animals and plants. It can be used in combination with eosin to stain nerve cells and is an indispensable dye in bacterial preparation. Its aqueous solution is coloring agent for protozoa. Methyl blue is extremely oxidizable, so the color cannot be preserved for a long time after being dyed with methyl blue.

2.4 Solid green

Solid green is an acid dye with a solubility of 4% in water and a solubility of 9% in alcohol. Solid green is a dye for the staining of cellulose cell tissue containing pulp. It is listed as the three most commonly used dyes in plant histology along with hematoxylin and fragrant red.

2.5 Sudan III

Sudan III is a weakly acidic dye in the form of a red powder that is readily soluble in fats and alcohols. Sudan III is a coloring agent for fat.

2.6 Eosin

There are many types of eosin dyes. The commonly used Eosin Y is an acid dye, a small crystalline with a reddish blue color or brown powder, soluble in water (solubility is 44% at 15°C) and alcohol (solubility in anhydrous alcohol is 2%). Eosin is widely used in animal cell staining and is a good cytoplasmic dye. It is often used as a lining agent for hematoxylin.

2.7 Basic fuchsin

Basic fuchsin is a basic dye that is dark red powder or crystalline, and it is soluble in water and alcohol. Basic fuchsin is widely used in biological preparation and can be used to dye collagen fibers, elastin fibers, redness-reinforcing particles and nuclear material of central nervous tissues.Basic fuchsin is commonly used to identify mycobacterium tuberculosis.

2.8 Crystal violet

Crystal violet is a basic dye with a solubility in water of 9% and a solubility in alcohol of 8.75%. Crystal violet is an excellent stain for cell, tissue and bacterial staining. It is commonly used for the staining of the nucleus. It can be used to highlight the centrosome of the chromosome and can be used for staining starch, fibrin, glial and the like. When safranin, hematoxylin can not successfully stain the nucleus, it may be possible to use crystal violet. Safranin and crystal violet can be used for the double staining of chromosomes, the chromosomes will be dyed red, and the spindle will be dyed purple, so crystal violet is also an excellent stain that can show the cell division process.

2.9 Gentian violet

Gentian violet is a basic dye and is a mixture of crystal violet and methyl violet. When necessary, gentian violet and crystal violet can be used interchangeably. The main ingredient of the purple syrup used in medicine is methyl violet, which can be replaced with gentian violet and crystal violet when needed.

2.10 Neutral red

Neutral red is a weakly basic dye in the form of red powder with a solubility of 4% in water and a solubility of 1.8% in alcohol. It appears yellow in alkaline solution, blue in strongly alkaline solution, and red in weakly acidic solution, so it can be used as an indicator. Neutral red is non-toxic and is often used for the dyeingof living body. It can be used to dye protozoa and show the contents of living cells in animal and plant tissues.

2.11 Safranin

Safranin is a basic dye that is soluble in water and alcohol. Safranin is commonly used to stain cells and animal and plant tissues. It can stain the nucleus, chromosomes, plant proteins, corked tissue, keratinized tissue, and sporangia.

2.12 Methylene blue

Methylene blue is a basic dye in the form of blue powder with a solubility in water of 9.5% and a solubility in alcohol of 6%. Methylene blue is an important dye for the nucleus, which has the advantage that the color is not too deep.

2.13 Methyl green

Methyl green is a basic dye. It is green powder with a solubility of 8% in water and a solubility of 3% in alcohol. Methyl green is the most valuable dye for cells. It is commonly used for the stainingof chromatin, and can be used together with acid magenta for the staining of xylem in plants.