Molecular Tweezers-like Calix[4]arene Based Alkaline Earth Metal Cation (Ca2+, Sr2+, and Ba2+) Chemosensor and Its Imaging in Living Cells and Zebrafish

Although alkaline earth metal cations play an important role in our daily life, little attention has been paid to the field of fast quantitative analysis of their content due to a lack of satisfactory precision and a fast and convenient means of detection. In this study, we have designed a set of molecular tweezers based on the calix[4]arene chemosensor L, which was found to exhibit high selectivity and sensitivity toward CA²⁺, Sr²⁺, and Ba²⁺ (by UV-vis and fluorescence methods) with low detection limits of the order of 10^-7 to 10^-8 M and high association constants (of the order of 10⁶). More significantly, sensor L not only can recognize CA²⁺, Sr²⁺, and Ba²⁺ but also can further discriminate between these three cations via the differing red shifts in their UV-vis spectra (560 nm for L·Ca²⁺, 570 nm for L·Sr²⁺, and 580 nm for L·Ba²⁺ complex) which is attributed to their different atomic radii. A rare synergistic effect for the recognition mechanism has been demonstrated by ¹H NMR spectroscopic titration. Sensor L constructed a high shielding field by the cooperation of Tris with alkaline earth metal ion after complex. Additionally, the presence of acetoxymethyl group in sensor L results in enhancement of cell permeability, and as a consequence, sensor L exhibited excellent sensing and imaging (in vivo) in living cells and in zebrafish.