Amplex Red

Amplex Red (CAS: 119171-73-2) is a highly sensitive and chemically stable fluorogenic probe widely employed for the detection of hydrogen peroxide (H₂O₂) and peroxidase activity. In its reduced form, the molecule is essentially nonfluorescent and colorless. Upon exposure to H₂O₂ in the presence of horseradish peroxidase (HRP), Amplex Red is oxidized to resorufin, a strongly fluorescent compound with a characteristic pink coloration. Resorufin can be readily detected by fluorescence measurements using excitation wavelengths in the range of approximately 530–560 nm and emission around 590 nm. Alternatively, its formation can be monitored spectrophotometrically by measuring absorbance near 560 nm.
Excessive H₂O₂ can further oxidize resorufin to the nonfluorescent product resazurin, extremely high peroxide concentrations or very strong peroxidase activity may paradoxically result in a reduction of fluorescence intensity. 
The versatility of Amplex Red extends beyond direct H₂O₂ quantification. As numerous metabolites can be linked to H₂O₂-generating enzymatic reactions, Amplex Red is a broadly applicable reagent in studies of oxidative stress, enzymology, clinical diagnostics, and environmental analysis. Typical analytes that can be quantified using the Amplex Red assay include glucose, lipids, uric acid, liver and kidney function markers, neurotransmitters, and various enzyme activities, including those associated with viral proteins. 

Reference：

[1].Zhou M, Diwu Z, Panchuk-Voloshina N, et al. A stable nonfluorescent derivative of resorufin for the fluorometric determination of trace hydrogen peroxide: Applications in detecting the activity of phagocyte NADPH oxidase and other oxidases. Analytical Biochemistry, 1997,  253:162-168.

[2].Ivanec-Goranina, R., Kulys, J. Kinetic study of peroxidase-catalyzed oxidation of 1-hydroxypyrene. Development of a nanomolar hydrogen peroxide detection system. cent.eur.j.biol. 3, 224–232 (2008). https://doi.org/10.2478/s11535-008-0021-x