Carbazole-quinoline based ultrasensitive fluorometric sensor for detection of Hg2+ in aqueous medium: Crystal structure, DFT and real sample application

Abstract

A novel carbazole-quinoline tagged fluorophore, (E)-9-ethyl-3-((2-(quinolin-2-yl)hydrazineylidene)methyl)-9H-carbazole (QHC), has been synthesized with an excellent yield using the simple one-pot synthesis and its molecular structure is determined by single crystal X-ray diffraction, FT-IR, NMR and mass spectroscopic techniques. QHC has been developed as an ultrasensitive fluorometric sensor for detection of Hg²⁺ in the presence of competing metal ions in aqueous medium. It exhibited remarkable sensitivity and selectivity towards Hg²⁺ with limit of detection (LOD) of 2.59 × 10⁻⁸ M and a remarkable Stern-Volmer constant (K_sv) of 1.17 × 10⁵ M⁻¹. The Job's plot displayed 1:1 stoichiometry between QHC and Hg²⁺, with a binding constant (K_a) of 1.01 × 10⁵ M⁻¹. The binding mechanism has been demonstrated by FT-IR, mass spectrometry, ¹H NMR titration and density functional theory (DFT) analysis. Upon interaction with Hg²⁺, the photo-induced electron transfer (PET) from quinoline-carbazole framework to Hg²⁺ is activated, thereby, completely quenching the fluorescence. The practical applicability of QHC was demonstrated through the development of test kits. Additionally, QHC was successfully employed to detect Hg²⁺ ions in real water samples, including lake water and Ganga river water, showing its significance in environmental monitoring.