A FRET-based ‘off–on’ molecular switch: an effective design strategy for the selective detection of nanomolar Al3+ ions in aqueous media

Abstract

A new water-soluble rhodamine-based Al³⁺ ion-selective probe (L¹) was synthesised and characterized by physico-chemico and spectroscopic tools. In the presence of a large excess of other competing ions, L¹ specifically binds Al³⁺ ions with a concurrent visually observable change from colorless to pink in electronic spectral behavior, making it possible to detect the presence of Al³⁺ ions with the naked eye. The addition of Al³⁺ ions to a solution of L¹ in HEPES buffer (1 mM, pH 7.4, 2% EtOH) at 25 °C, results in a decrease in the weak fluorescence intensity at λ_em = 470 nm, while a new peak (at λ_em = 588 nm) increases gradually through a fluorescence resonance energy transfer process. This ratiometric enhancement helps to detect Al³⁺ ions at a very low concentration of 33 nM. The detection limit of L¹ for Al³⁺ ions was estimated to be 6.19 × 10⁻⁹ M using the 3σ method. This probe is also useful for imaging Al³⁺ ions in HeLa cells.