Novel bis(rhodamine)-based fluorescence sensor for highly selective detection of Al(III): DFT mechanistic studies and analysis in real samples

A novel bis(rhodamine)-based sensor, BRTDA, was synthesized and its molecular structure and interaction with Al³⁺ ions were investigated. The BRTDA probe, featuring two rhodamine moieties linked by a 2,5-thiophene dicarboxylic acid, was designed to exhibit enhanced fluorescence upon Al³⁺ binding. In a CH₃CN–H₂O (v/v) 2:1 solution, the probe is colorless, but the addition of Al³⁺ induces a color change to pink and a substantial increase in fluorescence. Density functional theory calculations revealed that this fluorescence enhancement is associated with a distinct change in the molecular structure of BRTDA upon Al³⁺ coordination. Specifically, the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreased markedly from 3.246 eV for free BRTDA to 2.127 eV for the BRTDA-Al³⁺ complex, which corresponds with the observed fluorescence enhancement. The sensor displays a detection limit for Al³⁺ of 76.3 nM, a linear range of 0–66.7 µM, and a 1:1 binding stoichiometry with Al³⁺. The BRTDA sensor was applied successfully to determine Al³⁺ levels in tap water and mineral water.