QSY-21

QSY-21 is a fluorescence quencher with broad absorption in the far-red and near-infrared regions, effectively quenching the fluorescence of dyes that emit in these areas. It is a carboxylic acid derivative with a strong maximum absorption at 661 nm, making it a suitable acceptor for fluorescence resonance energy transfer (FRET) applications. QSY-21 is commonly used as a quencher for Cyanine5, Cyanine5.5, AF 647, or other spectrally similar fluorescent dyes.