Thiourea-based novel fluorescence chemosensor for selective detection of mercuric ion and its application in bio-imaging

Sensitive and reliable fluorescence chemosensors for the monitoring of Hg²⁺ levels are very important for the protection of environment and living systems. Herein, a simple thiourea-based irreversible fluorescence and colorimetric chemosensor L has been devised and characterised by various spectral analysis. Probe L selectively detects Hg²⁺ ion due to the binding site-signalling strategy, where the pyridine ring serves as the fluorophore unit and the thiourea moiety serves as the coordinating site. The incorporation of Hg²⁺ ions to a DMSO solution of L shows substantial alterations in the UV–Vis spectrum and fluorescence spectra. This alteration in absorption as well as fluorescence profile refers to the increase in the intra-molecular charge transfer (ICT) and chelation-induced enhanced fluorescence (CHEF) of L-Hg²⁺ complex. For the Hg²⁺ ion, the detection limit is reached up to 2.5 × 10⁻⁸ M, which is calculated from the IUPAC formula C_DL = 3σ/slope. The Job’s plot reveals a 1:1 binding stoichiometry between L and Hg²⁺. Applying Benesi-Hildebrand equation, the binding constant for the L-Hg²⁺ complex was estimated as 7.54 × 10⁶ M⁻¹. To validate the mechanism involved in the formation of L-Hg²⁺ complex, the DFT and TD-DFT calculations were performed in the gas phase. L has been used well to identify Hg²⁺ ions in soil samples over a wide pH range. The receptor L was also applied for cell imaging study.