Design of Na -Selective Fluorescent Probes: A Systematic Study of the Na -Complex Stability and the Na /K Selectivity in Acetonitrile and Water.

There is a tremendous demand for highly Na -selective fluoroionophores to monitor the top analyte Na in life science. Here, we report a systematic route to develop highly Na /K selective fluorescent probes. Thus, we synthesized a set of fluoroionophores 1, 3, 4, 5, 8 and 9 (see Scheme ) to investigate the Na /K selectivity and Na - complex stability in CH CN and H O. These Na -probes bear different 15-crown-5 moieties to bind Na stronger than K . In the set of the diethylaminocoumarin-substituted fluoroionophores 1-5, the following trend of fluorescence quenching 1>3>2>4>5 in CH CN was observed. Therefore, the flexibility of the aza-15-crown-5 moieties in 1-4 determines the conjugation of the nitrogen lone pair with the aromatic ring. As a consequence, 1 showed in CH CN the highest Na -induced fluorescence enhancement (FE) by a factor of 46.5 and a weaker K induced FE of 3.7. The Na -complex stability of 1-4 in CH CN is enhanced in the following order of 2>4>3>1, assuming that the O-atom of the methoxy group in the ortho-position, as shown in 2, strengthened the Na -complex formation. Furthermore, we found for the N-(o-methoxyphenyl)aza-15-crown-5 substituted fluoroionophores 2, 8 and 9 in H O, an enhanced Na -complex stability in the following order 8>2>9 and an increased Na /K selectivity in the reverse order 9>2>8. Notably, the Na -induced FE of 8 (FEF=10.9), 2 (FEF=5.0) and 9 (FEF=2.0) showed a similar trend associated with a decreased K -induced FE [8 (FEF=2.7)>2 (FEF=1.5)>9 (FEF=1.1)]. Here, the Na -complex stability and Na /K selectivity is also influenced by the fluorophore moiety. Thus, fluorescent probe 8 (K =48 mm) allows high-contrast, sensitive, and selective Na measurements over extracellular K levels. A higher Na /K selectivity showed fluorescent probe 9, but also a higher K value of 223 mm. Therefore, 9 is a suitable tool to measure Na concentrations up to 300 mm at a fluorescence emission of 614 nm.