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.