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To the Editor.—
The quantum yield of fluorescence from neutral red is determined by several factors including the solvent in which the dye is dissolved and the pH of the solution. The pH of 0.1% neutral red in both aqueous and ethanol solution is approximately 4.5. Therefore, the pH variable is eliminated and the solvent factor is pertinent.A solvent may induce radiationless (nonfluorescent) transition of an excited molecule to its ground state by enhancement of predissociative transition (dissipation of energy via breaking chemical bonds) or by interaction with the excited molecule and dissipation of its energy as heat.The fact that neutral red fluorescence is quenched in water but not in ethanol is not surprising. Many substances are known to fluoresce in one solvent but not another. However, the fluorescence of neutral red in alcohol and the lack of it in water probably is not pertinent to the
Jarratt MT, Felber T. More on Neutral Red Fluorescence. Arch Dermatol. 1975;111(5):657. doi:10.1001/archderm.1975.01630170115023