Neutral dyes are chromium metal dyes. They are mainly chelated at a ratio of 1:2 by coordination metal atoms (metal chromium, cobalt, etc.) and mother dye (acid azo dye) molecules, such as neutral orange RL, neutral gray 2BL. They are slightly less soluble in water than the metal complexes and can be dyed in a neutral or slightly acidic solution. Neutral dyes are suitable for the dyeing of vinylon, nylon, silk, wool and tussah. Neutral dyes have high fastness.
Biological use: Neutral red is a neutral dye for histology staining and a synthetic phenazine with promising prospect as an electron shuttle in environmental biotechnology. It stains lysosomes in red. It is used as a general stain in histology and a counterstain in combination with other dyes. It is used for staining embryonal tissues and supravital staining of blood. It can also be used to stain Golgi apparatus in cells and Nissl particles in neurons. In microbiology, it is used in MacConkey agar to differentiate bacteria for lactose fermentation. Neutral red can be used as a stain. Living cells incorporate neutral red into their lysosomes. When cells begin to die, their ability to incorporate neutral red diminishes. Therefore, the loss of neutral red intake corresponds to the loss of cell viability. It is also used to stain cell cultures for plate titration viruses.
Figure 1. Staining wheat root with 0.05% neutral red
Industrial use: The cationic modified cotton dyed with a neutral dye enhances the dye uptake, and a satisfied dyeing effect can be obtained. Neutral dyes also dye wool. Dyeing with ultrasonic can not only shorten the dyeing time, increase the dye uptake rate, but also increase the diffusion coefficient and the dyeing rate. Neutral dyes can also dye PA/PU superfine fiber synthetic leather.
1. Trolove S, Tan Y, Reid J. Staining with 0.05% neutral red reduces nutrient uptake by wheat roots[J]. Plant Physiology and Biochemistry, 2015, 96: 401-405.