Direct dyes are dyes that adhere to their substrates (usually textiles) by non-ionic forces. The amount of this attraction is called substantivity: the higher the substantivity, the more attractive the dye is to the fiber. Direct dyes have a high affinity to cellulosic fibers, and the dyeing of cellulosic fibers should be performed in neutral or weakly basic media. Direct dyes are mainly sodium salts of aromatic compounds. They are soluble in water and their solubility increases with temperature. Direct dyes can dissociate into anions in water. According to the applications, direct dyes fall into four categories: general direct dyes, direct fast dyes, direct copper dyes, and direct diazo dyes.
Biological use: Trypan blue is a direct dye, which is used as an important stain for selective dyeing dead tissues or cells in the biosciences. Live cells or tissues with intact cell membranes are not dyed. Due to the high selectivity of the cells in the compound that passes through the membrane, trypan blue is not absorbed by living cells; however, it can traverse the membrane in dead cells. Therefore, dead cells appear a unique blue under the microscope. Since living cells are excluded from staining, this staining method is also described as a dye exclusion method. Trypan blue is commonly used for microscopy and in assessment of tissue viability of laboratory mice. It also can be used to observe fungal hyphae and stramenopiles. Trypan blue is used in ophthalmic cataract surgery to stain the anterior capsule in the presence of mature cataracts to aid visualization and then to make continuous curvilinear capsulorhexis.
Figure 1. Observation of Hyaloperonospora parasitica by using the trypan blue staining.
Industrial use: Direct dyes are mainly used to dye materials made from natural or regenerated cellulose, such as cotton, jute, viscose, or paper, without employing other mordants. The essential requirement for classification of a dye in this group is its substantivity. Absorption of cotton takes place in a neutral to soda alkaline medium and occurs on paper in a weakly acid to neutral medium.