Dyes are generally defined along the lines of being coloured, aromatic compounds that can ionise. Although this definition infers that ionic interaction with oppositely charged tissue constituents is the norm, there are exceptions. Some dyes are used in the textile industry to dye cotton without using a mordant. An older name for dyeing without a mordant is direct dyeing, and the dyes are termed direct dyes or, sometimes, direct cotton dyes.
The Colour Index uses this as a classification and naming system.
Each dye is named according to the pattern:–
direct + base colour + number
These dyes are thereby specifically identified as dyes of the stated colour, and whose primary mechanism is staining without a mordant. Note that this is a functional and colour classification. It contains no chemical information, neither does it imply that dyes with similar names but unique numbers are in any way related. It should also be noted that the classification refers to the primary mechanism of staining. Other mechanisms may also be possible.
Direct dyes are usually negatively charged. In other words, the coloured part of the molecule is the anion. Although the molecular charge is often shown on a specific atom in structural formulae, it is the whole molecule that is charged.
It has been shown that direct dyes can stain cotton by hydrogen bonding. This mechanism is also the basis for their value in histological staining and explains why some are able to demonstrate amyloid very selectively. However, they are also able to stain positively charged tissue components by ionic interaction in much the same way as acid dyes. They may be viewed as a special subgroup of acid dyes, many being azo dyes.
The most commonly used direct dye is probably congo red (direct red 28) which is used for demonstrating amyloid. Erie garnet (direct red 10) can be used in a rapid frozen section staining method, and sirius red F3B (direct red 80) can be used to stain collagen ionically in a modified Van Gieson, as well as for staining amyloid by hydrogen bonding.
There are numerous other examples.