Physicochemical and Ion-Sensing Properties of Benzofurazan-Appended Calix[4]arene in Solution and on Gold Nanoparticles: Spectroscopy, Microscopy, and DFT Computations in Support of the Species of Recognition

A calix[4]­arene conjugate (L) functionalized at the lower rim with a benzofurazan fluorophore (NBD) and at the upper rim with a thioether moiety has been synthesized and characterized by ¹H NMR, ¹³C NMR, and mass spectrometry techniques. Both the absorption and emission spectral data for L in different solvents exhibited progressive changes with an increase in polarity. Ion recognition studies were performed by absorption and fluorescence spectroscopy using 10 different metal ions. Among these, Hg²⁺ exhibited greater changes in these spectra, whereas Cu²⁺ showed only significant changes and all other ions showed no change in the spectral features. Although the Hg²⁺ has dominant influence on the spectral features and provides a detection limit of 56.0 ± 0.6 ppb, the selectivity was hampered because of the presence of the derivatizations present on both the rims of L for ion interaction in solution. Therefore, L was immobilized onto gold nanoparticles (AuNP_L’s) so that the upper rim derivatizations anchor onto the gold surface through Au–S interactions, and this leaves out only the lower rim NBD derivatization for interaction with ions selectively. The AuNP_L’s were characterized by transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses. The surface characteristics were analyzed by contact angle measurements. The AuNP_L’s exhibit greater selectivity and enhanced sensitivity for Hg²⁺ ions with a lowest detection limit of 48.0 ± 0.8 ppb. The immobilization of L onto AuNPs was reflected in the corresponding fluorescence lifetime values, and the addition of Hg²⁺ to either L or AuNP_L showed fluorescence quenching. The reversible recognition of Hg²⁺ by L was demonstrated by titrating L or AuNP_L with Hg²⁺ followed by tetra-butyl ammonium iodide for several cycles. The structural features of Hg²⁺-bound species were demonstrated by density functional theory computations and were supported by the XPS data. The Hg²⁺ induces aggregated fibrillar morphology into supramolecular L, as demonstrated by microscopy when Hg²⁺ was added either to L or to AuNP_L, supporting aggregation-caused quenching.