resorufin

Resorufin is a fluorescent redox dye and metabolite formed by the enzymatic or chemical reduction of resazurin. It is commonly used as a reporter in cell viability and metabolic assays. In typical resazurin-based assays, viable cells reduce the blue, nonfluorescent resazurin to pink, fluorescent resorufin. The fluorescence of resorufin enables quantification of cellular activity by fluorometric or spectrophotometric measurement. Its excitation maximum occurs around 570 nm and emission near 585–590 nm, yielding a bright signal under common excitation sources. Resorufin can be further reduced to hydroresorufin, which is colorless and nonfluorescent, potentially limiting signal stability and dynamic range in some settings.

Resorufin is widely used in commercial viability assays such as AlamarBlue and other resazurin-based kits, providing a convenient, non-destructive readout that is compatible with microplate readers and high-throughput screening. The method is sensitive and scalable, but the measured signal can be affected by sample turbidity, auto-fluorescence, pH, temperature, and the presence of strong reducing environments that distort the redox balance.

Chemically, resorufin belongs to the phenoxazine family of dyes and is typically handled as a solution in neutral or slightly basic media, protected from light. It is used at low micromolar concentrations and, like many dyes, should be disposed of according to laboratory safety guidelines.