Indo-1 is a popular dye that is used as a ratiometric calcium indicator similar to Fura-2.

It is widely used in flow cytometry and laser scanning microscopy, due to its single excitation property.

However, its use for confocal microscopy is limited due to its photo-instability caused by photobleaching.

[1] The penta potassium salt is commercially available and preferred to the free acid because of its higher solubility in water.

[2] In intact heart muscle, Indo-1, in combination with bioluminescent protein aequorin, can be utilized as a tool to distinguish between the internal and exterior inotropic regulation processes.