Synthesis and characterization of a new red-emitting Ca2+ indicator, calcium ruby.

Calcium Ruby m-Cl (X = H, Y = Cl) is a visible-light excited red-emitting calcium concentration ([Ca2+]) indicator dye (579/598 nm peak excitation/emission) with a side arm for conjugation via EDC or click chemistry. Its large molar extinction and high quantum yield rank it among the brightest long-wavelength Ca2+ indicators. Calcium Ruby is a promising alternative to existing dyes for imaging [Ca2+] in multicolor fluorescence applications or in the presence of yellow-green cellular autofluorescence.