Carbohydrazone/Thiocarbohydrazone-Based Dual-Responsive Probes for Highly Selective Detection of Cu²⁺/Fe³⁺ Cations and F^-/ClO₄ ^- Anions, and Their Application in Bioimaging of Live Cells and Zebrafish Larvae

Four dual-responsive probe molecules 1,5-bis(thiophene-2-carbaldehyde)carbohydrazone (R1), 1,5-bis(thiophene-2-carbaldehyde)thiocarbohydrazone (R2), 1,5-bis(indole-3-carbaldehyde)carbohydrazone (R3), and 1,5-bis(indole-3-carbaldehyde)thiocarbohydrazone (R4) were synthesized, characterized, and investigated for their sensing efficacy. The initial sensing behavior of the probes was tested by colorimetric signaling, followed by spectral and theoretical techniques, which supported the dual-sensing ability of the selected inorganic ions. The probes exhibited highly selective optical recognition for Cu²⁺/Fe³⁺ cations and F^-/ClO₄ ^- anions compared to the tested cations and anions. Interestingly, the addition of Cu²⁺ and F^- ions to the probes resulted in "turn-on" fluorescence responses. Job's plot studies showed 1:2 stoichiometry between the probe molecules and cations and 1:1 stoichiometry between the probe molecules and anions. The binding constant of the probe molecules with the sensed ions was determined by the Benesi-Hildebrand equation and was found to be between 7.08 × 10⁴ and 7.44 × 10⁶ M^-1 with a limit of detection between 0.11 and 0.80 μM, in CH₃CN:DMF (9:1, v/v). Density functional theory calculations established the nature of the interaction between the probe molecules and sensed ions. Further, the practical utility of the probes was successfully demonstrated with paper strip experiments, fluorescence imaging of Cu²⁺ ions in DrG cells and zebrafish larvae, as well as in the development of molecular logic gates.