Carbohydrazone/Thiocarbohydrazone-Based\nDual-Responsive\nProbes for Highly Selective Detection of Cu<sup>2+</sup>/Fe<sup>3+</sup> Cations and F<sup>–</sup>/ClO<sub>4</sub><sup>–</sup> Anion

Four dual-responsive\nprobe molecules 1,5-bis(thiophene-2-carbaldehyde)carbohydrazone\n(R1), 1,5-bis(thiophene-2-carbaldehyde)thiocarbohydrazone (R2), 1,5-bis(indole-3-carbaldehyde)carbohydrazone\n(R3), and 1,5-bis(indole-3-carbaldehyde)thiocarbohydrazone (R4) were\nsynthesized, characterized, and investigated for their sensing efficacy.\nThe initial sensing behavior of the probes was tested by colorimetric\nsignaling, followed by spectral and theoretical techniques, which\nsupported the dual-sensing ability of the selected inorganic ions.\nThe probes exhibited highly selective optical recognition for Cu<sup>2+</sup>/Fe<sup>3+</sup> cations and F<sup>–</sup>/ClO<sub>4</sub><sup>–</sup> anions compared to the tested cations\nand anions. Interestingly, the addition of Cu<sup>2+</sup> and F<sup>–</sup> ions to the probes resulted in “turn-on”\nfluorescence responses. Job’s plot studies showed 1:2 stoichiometry\nbetween the probe molecules and cations and 1:1 stoichiometry between\nthe probe molecules and anions. The binding constant of the probe\nmolecules with the sensed ions was determined by the Benesi–Hildebrand\nequation and was found to be between 7.08 × 10<sup>4</sup> and\n7.44 × 10<sup>6</sup> M<sup>–1</sup> with a limit of detection\nbetween 0.11 and 0.80 μM, in CH<sub>3</sub>CN:DMF (9:1, v/v).\nDensity functional theory calculations established the nature of the\ninteraction between the probe molecules and sensed ions. Further,\nthe practical utility of the probes was successfully demonstrated\nwith paper strip experiments, fluorescence imaging of Cu<sup>2+</sup> ions in <i>Dr</i>G cells and zebrafish larvae, as well\nas in the development of molecular logic gates.