Designing a multifunctional AIE-active fluorescent Schiff base probe: sensitive heavy metal ion recognition and water-induced aggregation

Abstract

A new multifunctional Schiff base probe AnQn containing dual fluorophores (anthracene-quinoline) has been synthesised via a facile one-pot reaction. This probe demonstrated the ability to selectively discriminate group IIIA metal ions (Al³⁺ and Ga³⁺) via a dual-mode fluorescent turn-on response. Upon excitation at the same wavelength (λ_ex 320 nm), AnQn displayed two distinct emission bands for Al³⁺ (λ_em = 410 nm) and Ga³⁺ (λ_em = 487 nm) in acetonitrile solutions. The detection limits calculated for Al³⁺ and Ga³⁺ were found to be as low as 9.2 nM and 230 nM, respectively. A 1 : 1 complexation between AnQn and Al³⁺/Ga³⁺ has been established using Job's plot, FT-IR, ESI-MS, ¹H NMR and DFT analyses. The practical applicability of the probe was assessed through logic gate analysis, low-cost paper strip experiments and real water sample analysis. Intriguingly, AnQn exhibited an aggregation-induced-emission (AIE) behaviour in a semi-aqueous (CH₃CN : H₂O) medium, resulting in the formation of highly fluorescent green aggregates, which could be clearly evidenced by SEM images and time-resolved fluorescence (TRF) analyses. Interestingly, the resultant AnQn aggregates were further utilized as a viable fluorescent platform to selectively recognize heavy Pd²⁺ ions as well as highly explosive picric acid with excellent limit of detection.