Differentially Selective Chemosensor with Fluorescence <i>Off–On</i> Responses on Cu²⁺ and Zn²⁺ Ions in Aqueous Media and Applications in Pyrophosphate Sensing, Live Cell Imaging, and Cytotoxicity

A new benzoyl hydrazone based chemosensor <b>R</b> is synthesized by Schiff base condensation of 2,6-diformyl-4-methylphenol and phenyl carbohydrazide and acts as a highly selective fluorescence sensor for Cu²⁺ and Zn²⁺ ions in aqueous media. The reaction of <b>R</b> with CuCl₂ or ZnCl₂ forms the corresponding dimeric dicopper­(II) [Cu₂(R)­(CH₃O)­(NO₃)]₂(CH₃O)₂ (<b>R-Cu</b>^<b>2+</b>) and dizinc­(II) [Zn₂(R)₂]­(NO₃)₂ (<b>R-Zn</b>^<b>2+</b>) complexes, which are characterized, as <b>R</b>, by conventional techniques including single-crystal X-ray analysis. Electronic absorption and fluorescence titration studies of <b>R</b> with different metal cations in a CH₃CN/0.02 M HEPES buffer medium (pH = 7.3) show a highly selective binding affinity only toward Cu²⁺and Zn²⁺ ions even in the presence of other commonly coexisting ions such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cd²⁺, and Hg²⁺. Quantification of the fluorescence titration analysis shows that the chemosensor <b>R</b> can indicate the presence of Cu²⁺and Zn²⁺ even at very low concentrations of 17.3 and 16.5 ppb, respectively. <b>R-Zn</b>^<b>2+</b> acts as a selective metal-based fluorescent sensor for inorganic pyrophosphate ion (PPi) even in the presence of other common anions such as F^–, Cl^–, Br^–, I^–, CH₃COO^–, CO₃^2–, HCO₃^–, N₃^–, SO₄^2–, PPi, AMP, ADP, and ATP in an aqueous medium. The propensity of <b>R</b> as a bioimaging fluorescent probe to detect Cu²⁺ and Zn²⁺ ions in human cervical HeLa cancer cell lines and their cytotoxicity against human cervical (HeLa), breast cancer (MCF7), and noncancer breast epithelial (MCF10a) cells have also been investigated. <b>R-Cu</b>^<b>2+</b> shows better cytotoxicity and sensitivity toward cancer cells over noncancer cells than <b>R</b> and <b>R-Zn</b>^<b>2+</b> under identical conditions, with the appearance of apoptotic bodies