Fluorescent turn-off sensor for Cu2+ and Fe3+ ions in 100% aqueous media with a biocompatible polymer

A sensitive analytical method using an acridinium derivative in 100% aqueous media to detect Cu (II) and Fe (III) ions been developed. The photophysical properties of a thiophene-phenylanilide-acridinium triad (TPA) in acidic poly(methylacrylic acid) (PMAA) solution are altered in the presence of various metal ions. While dye/PMAA mixtures in acidic condition emit strongly at around 540 nm, Fe3+ and Cu2+ ions can each quench the charge shift state (CSH) emission significantly. The linear range of response to Cu2+ is 1 to 9.0 μM and to Fe3+ is 4 to 68 μM. The detection limit is 0.11 μM for Cu2+ ions and 0.60 μM for Fe3+ ions. Stern-Volmer plots showed more than one quenching mechanism for Fe3+ ions and the Benesi-Hilderbrand plot indicates a 1:1 binding ratio between dye/PMAA mixture and metal ions. While Cd2+ and Hg2+ ions interfere with the quenching, addition of iodide ions completely resolves the interference problem for both ions. The quenching by Fe3+ ions involved the inner-filter effect and partial removal of dye molecules from the hydrophobic interior of PMAA due to competitive binding. The quenching by Cu2+ ions is likely due to photoinduced electron transfer from the charge shift state of dye molecules to the partly filled d-orbitals of Cu2+ ions, leading to weaker CSH emission. Computer simulation supports the model in which dye molecules incorporate into the hydrophobic pockets formed due to the coordination of Cu2+ ions with carboxyl groups on PMAA chains.