Thiosemicarbazone-Based Bifunctional ‘Turn On’ Fluorometric Chemosensors for the Rapid Detection of Zn2+ Cation and F− Anion: Theoretical Investigation, Smartphone-Assisted Colorimetric Detection and Applications in Bioimaging of Zebrafish Larvae and Molecular Logic Gate Operation

Three thiosemicarbazone-based bifunctional chemosensors 2-(3-ethoxy-2-hydroxybenzylidene)hydrazinecarbothioamide (S1), 2-(3-ethoxy-2-hydroxybenzylidene)-N-methylhydrazinecarbothioamide (S2) and 2-(3-ethoxy-2-hydroxybenzylidene)-N-phenylhydrazinecarbothioamide (S3) have been designed and synthesized. The sensor molecules displayed highly selective and sensitive colorimetric detection towards Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Fe³⁺ cations and F⁻ anion, in addition to the remarkable selective ‘turn on’ fluorometric responses for Zn²⁺ cation and F⁻ anion. The high binding constant values are observed when S2 binds with Zn²⁺ cation (2.83 × 10⁶ M⁻¹) and S1 binds with F⁻ anion (1.16 × 10⁶ M⁻¹) with lower detection limit of 1.26 and 1.69 μM, respectively The theoretical studies endorsed the emission responses observed in the fluorescence spectral studies and the binding mode of chemosensors with sensed ions. Interestingly, the bioimaging studies with the chemosensors using DrG cells and zebrafish larvae demonstrated the detection of Zn²⁺/F⁻ ions at relatively lower concentration. Additionally, the chemosensors were successfully integrated into the molecular logic gate systems and the smartphone-assisted RGB colour values, which enabled the real-time detection of Zn²⁺/F⁻ ions in various environmental and biological samples.