Triazole-Linked Quinoline Conjugate of Glucopyranose: Selectivity Comparison among Zn2+, Cd2+, and Hg2+ Based on Spectroscopy, Thermodynamics, and Microscopy, and Reversible Sensing of Zn2+ and the Structure of the Complex Using DFT

A water-soluble triazole-linked quinoline conjugate of glucopyranose ( L ) has been synthesized and characterized, and its single-crystal X-ray diffraction (XRD) structure has been established. Binding of L toward different biologically relevant metal ions has been studied using fluorescence and absorption spectroscopy in HEPES buffer at pH 7.4. The conjugate L detects Zn 2+ and Cd 2+ with 30 ± 2 and 14 ± 1-fold fluorescence enhancement, respectively, but in the case of Hg 2+ , only a fluorescence quench was observed. The stoichiometry of the complex is 1:2 metal ion to the ligand in the case of Zn 2+ and Cd 2+ resulting in [Zn( L ) 2 ] and [Cd( L ) 2 ], and it is 1:1 in the case of Hg 2+ , as confirmed from their electrospray ionization mass spectrometry (ESIMS) spectra. Zn 2+ shows greater exothermicity over Cd 2+ , whereas Hg 2+ shows endothermicity , which supports the differences in their binding strength and the nature of the corresponding complex. L exhibits rod-shaped particles and upon complexation with Zn 2+ , it exhibits sphere-like morphological features in scanning electron microscopy (SEM) images. However, clustered aggregates are observed in Cd 2+ , whereas the [Hg L ] complex exhibits small fused spherical structures, and therefore the signature of these ions is seen in microscopy images. The computational studies revealed that the syn -[Zn( L ) 2 ] complex is stabilized by 9.7 kcal mol -1 more than that in the case of anti -[Zn( L ) 2 ] owing to the formation of hydrogen bonds between the two glucosyl moieties within the syn -complex. Among the anions studied, [Zn( L ) 2 ] is sensitive and selective toward the phosphate ion (H 2 PO 4 - ) with a minimum detection limit of 16 ± 2 ppb. Similarly, the [Hg L ] can act as a secondary sensor for CN - while also exhibiting reversibility. Based on the input-output characteristics, INHIBIT logic gate was built in the case of Zn 2+ vs H 2 PO 4 - and IMPLICATION logic gate was built in the case of Hg 2+ vs CN - .