Highly selective Gd(III) fluorimetric sensor based on thiosemicarbazone Scaffold: A dual function ensemble for fluorescence and naked eye sensing of CN− and H2PO4− anions

Sensing ions by colorimetric and fluorimetric methods has grown tremendously due to the great environmental concern. This paper reports the colorimetric and fluorescent studies of cation and anion sensing by two thiosemicarbazone-supported receptors: (E)-2-((8-hydroxyquinolin-2-yl)methylene) hydrazine-1-carbothioamide (H2qctsc) (1) and H2qctsc-Gd complex (2). Under irradiation at λex = 346 nm, the fluorescence sensing behavior of H2qctsc was investigated towards various metal ions (Na+, K+, Co2+, Cu2+, Cd2+, Cr3+, Fe2+, Fe3+, Al3+, Mn2+, Pb2+, Hg2+, Ni2+, Zn2+, Nd3+, Eu3+, Gd3+, Tb3+, Ho3+and Er3+) in DMSO medium. The chemosensor H2qctsc displayed a “Turn–Off” selective recognition towards Gd3+ ions through a 3.5-fold fluorescence quenching at 476 nm with the detection limit as low as 1.42 μM. It binds with Gd3+ in a 2:1 stoichiometric ratio. Further, the (1)-Gd3+ ensemble (2) exhibited a highly selective colorimetric and fluorimetric sensing for the biologically essential anions H2PO4− and CN− detecting the ions with a detection limit of 0.94 μM and 2.19 μM respectively, amongst a wide range of anions chosen. Colorimetric is a valuable technique since visual detection can give immediate qualitative information. In the present case, a naked eye color detection, i.e., yellow → colorless (H2PO4−) and yellow → red (CN−) by the ensemble, can add value to the sensor's employability in various fields. The coordination behavior of H2qctsc was examined in a highly aqueous medium (9:1 H2O: DMSO) by adopting the potentiometric and spectrophotometric methods, which revealed the formation of a very stable complex with a high formation constant (logβ = 24.75), demonstrating it as a potent chelator for Gd3+ ion. The stoichiometry of the complex was determined by Job's plot method, which confirmed the ML2-type complex. Additionally, a solid complex 2′ abbreviated as [Gd(Hqctsc)(qctsc)] was synthesized using the conventional method and was characterized by advanced spectroscopic techniques. The spectral studies of the synthesized complex further establish its high selective recognition towards H2PO4− and CN− ions. Density functional theory (DFT) studies confirmed the structure, bonding, spectra, and sensing mechanism of H2qctsc and the Gd-complex.