Novel sodium-selective fluorescent PET and optically based chemosensors: towards Na+ determination in serum

Abstract

The anthracene based fluorescent PET chemosensor 1 and the azo-dye based chemosensor 2 show high selectivity for Na⁺ over other alkali and alkaline-earth metal cations in the 12–300 mM concentration range in 50 ∶ 50 MeOH–H₂O at pH 7.4. Chemosensor 1 shows fluorescence ‘off-on’ switching upon Na⁺ complexation with λ_Fmax of 440 nm and a log β_Na of 2.5 (±0.05) and a pK_a of 5.3 with no concomitant changes in the absorption spectra. Conversely, 2 displays only a weak fluorescent emission at around 520–640 nm, and large changes in its absorption spectra upon addition of Na⁺, with a log β_Na of 1.25 (±0.05) and a pK_a of 3.9. In 100% water the sensitivity of 2 for Na⁺ was somewhat lower with a log β_Na of 0.8 (±0.05). The crystal structure of 2, and its corresponding protonated form (2·H⁺) were obtained, showing 2 in its trans conformation with the crown ether moiety at a 75° angle to the plane of the chromophore. These results, in conjunction with ¹H NMR measurements of 2, and UV–VIS measurements of the ion receptor 3, suggest that upon complexation of Na⁺, the 2′-methoxy group of the crown receptor participates in the Na⁺ complexation through chelation to the Na⁺ ion. We propose that this interaction forces the amine moiety of the crown ether to twist out of the plane of the chromophore, inducing loss of conjugation which gives rise to large Na⁺-induced spectral changes in the absorption spectra, which are most noticeable for 2.