Dual-responsive colorimetric and ‘turn on’ fluorometric probes for the rapid detection of Cu2+ and F ‾ ions: Crystal structure, spectral elucidation, smartphone-based colorimetric readout, and applications in bio-imaging of zebrafish larvae

In this work, we report two dual-responsive colorimetric and 'turn-on' fluorometric probe molecules 1,5-bis(4-isopropylbenzylidene)carbohydrazone (S1) and 1,5-bis(4-isopropylbenzylidene)thiocarbohydrazone (S2) for the rapid detection of biologically significant inorganic ions. The molecular structure of S1 was confirmed by single crystal X-ray technique, which crystallized in the monoclinic <i>P</i>2_1/<i>c</i> space group. The sensor molecules show dual sensing ability, which displayed highly selective optical detection toward Co²⁺ and Cu²⁺ cations and F^- anions over other tested cations (K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) and anions (F^-, Cl^-, Br^-, I^-, HSO₄^-, ClO₄^-, NO₃^- and OH^-), respectively. Astonishingly, the sensor molecules exhibit 'turn-on' fluorometric responses for Cu²⁺ and F^- ions. The binding constant of the probes with the sensed ions was determined by the Benesi-Hildebrand equation and found to be between 2.70 × 10⁵ and 1.89 × 10⁸ M^-1, and the limit of detection was found to be between 0.14 and 0.27 μM in the CH₃CN : DMF (9 : 1, v/v) solution. Job's plot analysis showed 1 : 2 stoichiometry between the probes and the cations and 1 : 1 stoichiometry between the probes and the anion. The probe molecules have been applied for on-site quantitative colorimetric analysis using smartphone-based RGB colour values. Besides, the synthesized probes have been successfully demonstrated for the fluorescence imaging of Cu²⁺/F^- ions in zebrafish gill cells (<i>Dr</i>G) and larvae.