Idiosyncatic recognition of Zn²⁺ and CN⁻ using pyrazolyl-hydroxy-coumarin scaffold and live cell imaging: depiction of luminescent Zn(<scp>ii</scp>)-metallocryptand

Abstract

Multi-responsive and selective sensor design is one of the stimulating research areas in the sensors field. We have designed a pyrazolyl-hydroxy-coumarin scaffold, 7-hydroxy-4-methyl-8-(((5-phenyl-1<i>H</i>-pyrazol-3-yl)imino)methyl)-2<i>H</i>-chromen-2-one (H₂L) and characterized it by spectroscopic techniques (¹H NMR, ¹³C NMR, ESI-MS, IR). The single crystal X-ray diffraction measurement confirms the molecular structure of the probe. It shows the selective sensing of Zn²⁺ in the presence of sixteen other cations with 'Turn On' approach through the enhancement of green florescence ((<i>λ</i>_em = 499 nm; <i>λ</i>_ex = 390 nm) in CH₃CN/H₂O (99 : 1, v/v; HEPES buffer, pH 7.5) medium with the limit of detection (LOD) of 34.76 nM. The structural depiction of the isolated Zn²⁺ complex reveals cage like metallocryptand cyclic hexamer, [Zn₆L₆] with 30.9% void of cavity along the crystallographic <i>c</i> axis of approximate dimension of 7.502 × 7.050 × 7.068 ų. The diffusion NMR study reveals only one type of complex in the solution, having 1 : 1 composition, <i>i.e.</i>, Zn²⁺ : H₂L, which affirms the isolated form of the complex. On the other hand, the receptor, H₂L, recognizes the very noxious anion CN^- out of sixteen anions. The product identification using spectroscopic techniques supports the nucleophilic addition of CN^- across the exocyclic imine (CN) bond, which shows blue emission ((<i>λ</i>_em = 447 nm; <i>λ</i>_ex = 390 nm), and the LOD was 19.91 nM. The composition of [H₂L-Zn²⁺] and [H₂L-CN^-] was established by ¹H NMR titration, Job's method, ESI-MS, and FTIR spectra. The efficacy of the probe was further studied using MTT assay in MDA-MB 231 and WI-38 cell line as well as for the intracellular imaging of Zn²⁺ and CN^- using a fluorescence microscope. Flow Cytometry was further performed for the quantitative analysis of Zn²⁺ distribution in MDA-MB 231 cells.