Design of a dual-signaling sensing system for fluorescent ratiometric detection of Al3+ ion based on the inner-filter effect

Abstract

A dual-signal sensing system based on the inner-filter effect (IFE) was demonstrated, in which the combination of two signaling mechanisms allows metal binding to turn on two fluorescence emission bands, independently. A proof-of-concept fluorescent ratiometric assay for Al³⁺ in pure aqueous solution is presented. The proposed assay is based on the Al³⁺-induced color and fluorescence changes of Alizarin red S (ARS) and IFE between ARS and meso-tetra(N-methyl-4-pyridyl)porphine tetratosylate salt (TMPyP). In the absence of Al³⁺, the absorption spectrum of the ARS in 0.2 M HAc–NaAc buffer (pH 5.5) has a strong peak at 420 nm, significantly overlapping with the excitation of TMPyP. ARS is expected to be capable of functioning as a powerful absorber to tune the emission of TMPyP on account of the spectral overlap. Binding of Al³⁺ with ARS forms a fluorometric ARS/Al³⁺ complex and shifts the maximum absorbance from 420 nm to 480 nm, which overlaps negligibly with the excitation of TMPyP and turns on the proper emission spectrum for TMPyP. Under the optimum conditions, The fluorescence intensity ratio, F₅₈₅/F₆₅₁, responds to Al³⁺ over a dynamic range of 0.1–1.5 μM, with a limit of detection of 40 nM, where F₅₈₅ and F₆₅₁ are the fluorescence intensity at 585 nm and 651 nm in the absence or presence of Al³⁺, respectively. Further application in Al³⁺-spiked water samples suggested a recovery between 95 and 108%. The fluorescence response is highly selective for Al³⁺ over other metal ions with the addition of thiourea as the masking agent.