Design of ratiometric emission probe with visible light excitation for determination of Ca²⁺ in living cells

An organic salt as a fluorescent probe based on intramolecular charge transfer for Ca²⁺ determination is developed. Ca²⁺ can be detected by ratiometric emission at 490 and 594 nm with an excitation wavelength of 405 nm. This probe is highly selective for Ca²⁺ over other divalent metal cations and displays a large Stokes shift of 189 nm that can avoid interference of the excitation light beam and autofluorescence of biological samples. The dissociation constant for Ca²⁺ is 2.25 ± 0.47 μM and pertinent to Ca²⁺ detection in cellular resting and dynamic states. The probe demonstrates its application in monitoring Ca²⁺ in living cells under confocal microscopic imaging.