BODIPY-Equipped Benzo-Crown-Ethers as Fluorescent Sensors for pH Independent Detection of Sodium and Potassium Ions

Herein, we report on fluorescent probes which enable the selective quantification of Na+ and K+ in water by fluorescence enhancement (FE) independent of the pH value. These fluorescent probes, so called fluoroionophores, consist of benzo-crown-ether derivatives as ionophores, a benzo-15-crown-5 (cf. 5) or a benzo-18-crown-6 (cf. 6), respectively, to selectively bind either Na+ or K+ and a 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (BODIPY) fluorophore. The fluoroionophore 5 shows a FE factor of 7.3 in the presence of 1000 mM Na+ and 6 of 4.7 upon addition of 250 mM K+ in an acidic environment. The dissociation constants (K-d) for 5+Na+ is 276 mM and for 6+K+ is 18 mM enabling the fluorometric determination of biological relevant Na+ or K+ levels. The fluorescence intensities of 5 and 6 were not impacted over a broad range of proton concentrations (pH stable from 3.5 to 9.5) and by the presence of other biologically relevant cations.