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.

Automated summary

In this study, fluorescent probes for the selective quantification of Na+ and K+ in water were developed. These probes showed stability over a wide range of pH values and were not affected by other biologically relevant cations.