Multiple ion (Al³⁺, Cr³⁺, Fe³⁺, and Cu²⁺) sensing using a cell-compatible rhodamine-phenolphthalein-derived Schiff-base probe

In this study, a novel rhodamine-phenolphthalein derivative (RBPF) was designed, synthesized, and characterized. RBPF exhibited the selective and sensitive colorimetric detection of Cu²⁺ and Fe³⁺ and fluorometric detection of Al³⁺ and Cr³⁺, thus functioning as a multiple-channel probe in MeOH-H₂O (v/v = 9/1, 5 µM HEPES, pH 7.2) in the presence of other cations. A strong enhancement of the absorbance at around 555 nm was observed for the four mentioned cations. In contrast, in the presence of Al³⁺ and Cr³⁺, emissions were intensified within the red region of the spectra (583 and 586 nm, respectively). The RBPF sensor was found to bind to the metal ions at a 1:2 stoichiometric ratio. The proposed mechanism for the observed sensing behavior is the opening of the spirolactam ring of the rhodamine core in the presence of the four mentioned metal ions. The detection limits for Cu²⁺, Fe³⁺, Al³⁺, and Cr³⁺ were calculated to be 1.21 µM, 1.75 µM, 2.27 µM, and 1.29 µM, respectively. To test the practical use of the probe, TLC-based paper strips were fabricated. In addition, cell-imaging analysis of Al³⁺ and Cr³⁺ ions in the A549 cancer cell line produced promising concentration and time-dependent detection results.