This cell-impermeant indo-1 pentapotassium salt is a UV light-excitable, ratiometric Ca²⁺ indicator. The emission maximum of indo-1 shifts from ~475 nm in Ca²⁺-free medium to ~400 nm when the dye is saturated with Ca²⁺. The dye is also available in the cell-permeant AM ester form—in our special packaging (I1223), in 1 mg amounts (I1203) and in 1 mM solution (I1226).