Carbazole-driven ratiometric fluorescence turn on for dual ion recognition of Zn²⁺ and Hg²⁺ by thiophene-pyridyl conjugate in HEPES buffer medium: spectroscopy, computational, microscopy and cellular studies

Carbazole-based thiophene-pyridyl conjugate (L) was synthesised and characterised. The complexation between L with Zn²⁺ and Hg²⁺ was studied in HEPES buffer medium by fluorescence, absorption and visual colour change with the detection limit of ~3.7 and ~4.8 μM, respectively. The L detects Zn²⁺ by bringing ratiometric change in the fluorescence signals at 418 and 515 nm, but in the case of Hg²⁺, the signals are observed at 418 and 365 nm, while no new band is observed at 515 nm. The structure of L has been established by single-crystal XRD and that of complexes [ZnL] and [HgL] by density functional theory calculations. TDDFT calculations were performed in order to demonstrate the electronic properties of receptors and their zinc and mercury complexes. The isolated fluorescent complexes [ZnL] and [HgL] were found to be sensitive and selective towards phosphate-bearing ions and sulfide ions, respectively, among the other anions studied. The nanostructural features such as shape and size obtained using atomic force microscopy distinguish L from its complexes formed between L and Zn²⁺ from that formed with Hg²⁺. Moreover, the utility of the conjugate L in showing the zinc recognition in live cells has also been demonstrated using RAW cells as monitored by fluorescence imaging.