Development of a novel carboxamide-based off-on switch fluorescence sensor: Hg²⁺, Zn²⁺and Cd²⁺

Here, the carboxamide ligand N-(thiazole-2-yl) picolinamide (L) was synthesized in an ionic liquid tetrabutylammonium bromide (TBAB) as the benign reaction medium. The sensitivity of L towards different metal-ions was investigated, and a comprehensive, logical optical investigation was conducted for the IIB transition metal ions, Hg²⁺, Cd²⁺ and Zn²⁺, based on off–on switch sensor protocols. In the absence of these metal ions, L showed weak emission only, but fluorescence intensity increased considerably upon their addition to a sensitivity range of 10⁻⁶ M. This phenomenon was enhanced significantly in the presence of Zn²⁺ compared to other metal ions, likely due to the coordination of metallic ions to the L, which led to the chelation-enhanced fluorescence (CHEF) effect. To prove these results, several characterization studies were conducted: a single crystal from the metal–ligand complex was prepared, followed by conduction of X-ray single crystal analysis from the Zn-complex (2) and Cd-complex (3), and conducting spectral analysis from the Zn²⁺, Hg²⁺ and Cd²⁺ complexes. These complexes and also the ligand showed very low toxicity on the PC12 cell line suggesting their promise for a wide range of future medical applications.