Highly K⁺ -Selective Fluorescent Probes for Lifetime Sensing of K⁺ in Living Cells

The new K⁺ -selective fluorescent probes 1 and 2 were obtained by Cu^I -catalyzed 1,3-dipolar azide alkyne cycloaddition (CuAAC) reactions of an alkyne-substituted [1,3]dioxolo[4,5-f][1,3]benzodioxole (DBD) ester fluorophore with azido-functionalized N-phenylaza-18-crown-6 ether and N-(o-isopropoxy) phenylaza-18-crown-6 ether, respectively. Probes 1 and 2 allow the detection of K⁺ in the presence of Na⁺ in water by fluorescence enhancement (2.2 for 1 at 2000 mm K⁺ and 2.5 for 2 at 160 mm K⁺ ). Fluorescence lifetime measurements in the absence and presence of K⁺ revealed bi-exponential decay kinetics with similar lifetimes, however with different proportions changing the averaged fluorescence decay times (τ_f(av) ). For 1 a decrease of τ_f(av) from 12.4 to 9.3 ns and for 2 an increase from 17.8 to 21.8 ns was observed. Variation of the substituent in ortho position of the aniline unit of the N-phenylaza-18-crown-6 host permits the modulation of the K_d value for a certain K⁺ concentration. For example, substitution of H in 1 by the isopropoxy group (2) decreased the K_d value from >300 mm to 10 mm. 2 was chosen for studying the efflux of K⁺ from human red blood cells (RBC). Upon addition of the Ca²⁺ ionophor ionomycin to a RBC suspension in a buffer containing Ca²⁺ , the fluorescence of 2 slightly rose within 10 min, however, after 120 min a significant increase was observed.