A highly potential acyclic Schiff base fluorescent turn on sensor for Zn ions and colorimetric chemosensor for Zn, Cu and Co ions and its applicability in live cell imaging.

Herein, we report two acyclic Schiff base receptors CS-1 and CS-2 capable of being selective fluorescent turn on for Znions and colorimetric chemosensor for Zn, Cu, and Co ions by showing a colour change from colourless to yellow in 1:1 ratio of acetonitrile and HEPES buffer (1:1, v/v, pH 7.4) without the interference from other metal ions screened (Cd, Hg, Sn, Ni, Cr, Mn, Pb, Ba, Al, Ca, Mg, K and Na). The fluorescence turn on enhancement towards Zn ions is ascribed to PET blocking, suppression of -C=N- isomerisation, and the ESIPT process. The selectivity, competitivity and reversibility of the synthesised probes (CS-1 and CS-2) made them promising chemosensors for the detection of Zn, Cu, and Co ions. The density functional theory (DFT) calculations have theoretically endorsed the colorimetric changes in the examined absorption spectra and binding mode of both CS-1/CS-2 with metals ions. In addition, H NMR titrations were also consistent with the recognition mechanism of Zn ions with the CS-1/CS-2. Further, the Jobs plot analysis infers a 1:1 stoichiometric ratio for both evaluating receptors CS-1 and CS-2 with Zn, Cu and Co ions and was supported by DFT, NMR (only for Zn ions), UV-Visible, and fluorescence spectroscopic studies. Moreover, the detection limits of CS-1 and CS-2 for Zn ions were determined to be 7.69 and 5.35 nM, respectively, which is less compared to the detection limit of Cu, Co ions as well as the limit approved by the United State Environmental Protection Agency (US EPA). The probes CS-1 and CS-2 found to show high fluorescence quantum yields at pH = 7 during the titration with Zn as compared with other pHs (5-6 and 8-11). Gratifyingly, fluorescence microscopy imaging in HeLa cells revealed that the pair of receptors can be employed as an excellent fluorescent probe for the detection of Znions in living cells, indicating that this facile chemosensor has a huge potential in cellular imaging.