Dual sensing performance of a rhodamine-derived scaffold for the determination of Cu2+ and Ce4+ in aqueous media

Abstract

A rhodamine 6G-based probe bearing a carbazole unit (R6G-CB) was developed as a fluorescent chelation agent for Cu2+ and a chemodosimeter for Ce4+ in acetonitrile:aqueous (5:5, v/v) media. The R6G-CB chemosensor exhibited selectivity toward Cu2+ ions over other metal cations by showing “off-on”-type changes in its absorption and emission spectra, which were attributed to the transformation of the non-fluorescent and colorless spirolactam derivative to the ring-opened, fluorescent, pink amide. The Cu2+-induced chelation-enhanced fluorescence (CHEF) is associated with the spirolactam ring opening of the rhodamine unit. This R6G-CB chemosensor also showed a fast response (<1min) to an irreversible Ce4+-promoted oxidation reaction, responding instantaneously at room temperature with a change that is observable by the naked eye. This through-bond energy transfer (TBET)-based sensor is also highly selective and sensitive to Ce4+. Spectroscopic analysis revealed that different structural substitution patterns of the probe resulted in different sensitivities and selectivities for specific metal ions. The optical titration, Job's plot estimation, mass spectroscopy, and Fourier transform infrared (FT-IR) results indicated that 1:2 and 1:1 complexes formed between R6G-CB and Cu2+ and between R6G-CB and Ce4+, respectively. The reversibility of the R6G-CB chemosensor was verified via its spectral response to Cu2+ and EDTA disodium salt titration experiments. All of these features make these sensors advantageous for Cu2+/Ce4+ test strip applications. Due to the significant absorption intensity changes resulting from the addition of Cu2+ and Ce4+ ions, the novel system can act as a single-output combinational logic circuit with two chemical inputs. Finally, density functional theory (DFT) calculations were performed to gain insight into the structures and electronic properties of R6G-CB, R6G-CB-Cu2+ complexes and R6G-CB-Ce4+ adducts.