A highly selective indole-based sensor for Zn²⁺, Cu²⁺, and Al³⁺ ions with multifunctional applications

A wide range of chemosensors has been developed for detecting specific metal ions at trace levels, attracting considerable research interest. However, despite the significant role of indole-based molecules in the biological domain, only a few chemosensors incorporating this moiety have been reported. In this work, a novel indole-based receptor [R = (<i>Z</i>)-3-((((1<i>H</i>-indol-4-yl)methyl)imino)methyl)benzene-1,2-diol], was synthesized and characterized using single-crystal X-ray diffraction, NMR, IR, and ESI-MS techniques. Sensing studies conducted in a CH₃CN/H₂O (7 : 3, v/v) solvent system demonstrated that the receptor R exhibits selectivity towards Zn²⁺, Cu²⁺, and Al³⁺ ions, with turn-on fluorescence and UV-Vis spectral responses while showing insensitivity to other cations and anions. Binding studies revealed the formation of 1 : 2 stoichiometric complexes between R and the respective metal ions. The interaction with Zn²⁺ resulted in enhanced fluorescence emission at 497 nm, whereas Al³⁺ and Cu²⁺ ions caused significant bathochromic shifts in the absorption maxima from 290 nm to 308 nm and 318 nm, respectively. The calculated detection limits were 0.056 μM for Zn²⁺, 0.57 μM for Cu²⁺, and 0.45 μM for Al³⁺. Density functional theory (DFT) calculations confirmed that R coordinates effectively with these metal ions, stabilizing the complexes by reducing the HOMO-LUMO energy gap. Molecular docking studies further indicated strong binding affinities of R and its metal complexes to DNA and bovine serum albumin (BSA), elucidating the potential binding sites within these biomolecules. The receptor R exhibits outstanding potential for detecting Zn²⁺ ions in the <i>Caenorhabditis elegans</i> model system. Its excellent membrane permeability and biocompatible nature enable efficient intracellular uptake, ensuring accurate and reliable detection of Zn²⁺ ions in living organisms. Furthermore, the receptor was employed in designing molecular logic gates and keypad lock systems, demonstrating its utility in developing functional molecular devices.