Carbazole-driven ratiometric fluorescence turn on for dual ion recognition of Zn<sup>2+</sup> and Hg<sup>2+</sup> by thiophene-pyridyl conjugate in HEPES buffer medium: spectroscopy, computational, microscopy and cellular studies

<p>Carbazole-based thiophene-pyridyl conjugate (<b>L</b>) was synthesised and characterised. The complexation between <b>L</b> with Zn<sup>2+</sup> and Hg<sup>2+</sup> 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 <b>L</b> detects Zn<sup>2+</sup> by bringing ratiometric change in the fluorescence signals at 418 and 515 nm, but in the case of Hg<sup>2+</sup>, the signals are observed at 418 and 365 nm, while no new band is observed at 515 nm. The structure of <b>L</b> has been established by single-crystal XRD and that of complexes <b>[ZnL]</b> and <b>[HgL]</b> 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 <b>[ZnL]</b> and <b>[HgL]</b> 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 <b>L</b> from its complexes formed between <b>L</b> and Zn<sup>2+</sup> from that formed with Hg<sup>2+</sup>. Moreover, the utility of the conjugate <b>L</b> in showing the zinc recognition in live cells has also been demonstrated using RAW cells as monitored by fluorescence imaging.</p>