Selective and pH-Independent Detection of Ba²⁺ in Water by a Benzo-21-crown-7-Functionalized BODIPY

Herein, we report on highly Ba²⁺ selective fluorescence sensing in water by a fluorescent probe consisting of a benzo-21-crown-7 as a Ba²⁺ binding unit (ionophore) and a tetramethylated BODIPY fluorophore as a fluorescence reporter. This fluorescent probe showed a Ba²⁺ induced fluorescence enhancement (FE) by a factor of 12±1 independently of the pH value and a high Ba²⁺ sensitivity with a limit of detection (LOD) of (17.2±0.3) μM. Moreover, a second fluorescent probe consisting of the same BODIPY fluorophore, but a benzo-18-crown-6 as a cation-responsive binding moiety, showed an even higher FE upon Ba²⁺ complexation by a factor of 85±3 and a lower LOD of (13±3) μM albeit a lower Ba²⁺ selectivity. The fluorescence sensing mechanism of Ba²⁺ was further investigated by time-resolved fluorescence as well as transient absorption spectroscopy (TAS) and it turned out that within these probes a blocking of a photoinduced electron transfer (PET) by Ba²⁺ is very likely responsible for the FE.