Fluorescent conjugated polymer containing rhodamine derivative for Förster resonance energy transfer-based detection of Al³⁺ ion

We report the synthesis of a fluorescent polymer containing the rhodamine 6G (Rh6G) derivative as a side chain. The rhodamine (Rh) moiety in the polymer had a ligand interaction with Al³⁺, which allowed the polymer to be used for detection of Al³⁺. The Rh moiety (in closed form) was non-fluorescent and colorless, whereas the open form of the Rh derivative showed strong fluorescence. Upon exposure to Al³⁺, the green-emitting polymer backbone had a spectral overlap with the absorption of the open form of Rh in the side chain, leading to an energy transfer from the polymer backbone to the Rh moiety. Upon addition of Al³⁺ to the polymer solution, the emission of the polymer backbone (green) has gradually decreased and, concomitantly, the red emission of Rh has increased via Förster resonance energy transfer (FRET). As the fluorescence of the polymer varied in the presence of Al³⁺ ions, the polymer could be used as a FRET-based sensor for detecting Al³⁺.