An efficient indole-derived optical probe (InNS) has been developed for recognition of Al³⁺, Ga³⁺, In³⁺ and Fe³⁺ ions. Job's plot, NMR and DFT analyses corroborated the cation sensing mechanism. InNS could be successfully applied for Al³⁺ imaging in real-life samples such as MCF-7 live cells.

Abstract

An easily synthesizable indole-derived chromofluorogenic probe InNS has been demonstrated for recognition of trivalent metal ions (i. e., Al³⁺, Ga³⁺, In³⁺ and Fe³⁺). Both UV-Vis and emission spectral studies have been employed to assess the cation sensing ability of InNS in semi-aqueous medium. This probe exhibited a chromogenic response for these metal ions, and the related change was accompanied with the appearance of a new absorption near 376 nm. An obvious color change from pale yellow to dark yellow could also be noticed upon addition of the aforementioned metal ions to the probe's solution. Distinctively from the UV-Vis analysis, the fluorescence behavior of InNS was completely different; it displayed a ‘turn-on’ fluorescence response for only Al³⁺ among all the studied cations. The detection limit and the association constant (K _a) for Al³⁺ were determined to be 12.5 nM and 6.85×10⁶ M⁻¹, respectively. A potential 1 : 1 binding mode of Al³⁺-InNS has been established based on Job's plot, ¹H NMR and DFT analyses. The reversibility experiment was conducted using strongly chelating EDTA ion, and a corresponding logic gate has been devised. In terms of practical applications, the InNS has been utilized to detect Al³⁺ in human breast carcinoma (MCF-7) cell lines displaying promising ‘turn-on’ bioimaging experiments.