An easily synthesizable naphthalene-based sensing platform for Al3+ and Zn2+ ions: theoretical insights and live cells imaging†

Abstract

A new naphthalene-derived Schiff base probe, 1-((2-(diphenylphosphino)ethylimino)methyl)naphthalen-2-ol (HL), has been demonstrated for fluorometric detection of Al³⁺ and Zn²⁺ ions. HL was characterized by elemental analysis, FT-IR, NMR, UV-Vis, fluorescence, and ESI-MS analyses. It exhibited high sensitivity and selectivity toward Al³⁺ and Zn²⁺ ions in a semi-aqueous medium (CH₃CN–H₂O; 4 : 1, v/v), remaining unaffected by other competing metal ions. As a ‘turn-on’ fluorogenic probe, HL displayed strong emission enhancements at 430 nm and 450 nm (λ_ex 300 nm) upon the addition of Al³⁺ and Zn²⁺ ions, respectively, with detection limits of 0.62 μM for Al³⁺ and 0.54 μM for Zn²⁺. The addition of Al³⁺ caused ca. 20 nm blue-shift in emission and absorption maxima of HL due to strong complex formation. The calculated binding constant values were found to be 1.39 × 10³ M⁻¹ and 4.78 × 10³ M⁻¹, respectively, for Al³⁺ and Zn²⁺ ions. Job's plot, NMR, ESI-MS, and density functional theory (DFT) studies supported the metal ion binding mechanism with 1 : 1 stoichiometry. Fluorescence imaging experiments further revealed HL's ability to detect intracellular Al³⁺ in live cells with very low cytotoxicity, highlighting its potential as a selective chemosensory probe.