Sequential detection of palladium and chromium oxyanion by a fluorescein based chemosensor in mixed aqueous media

A new highly selective chemosensor, based on fluorescein-allyloxy benzene conjugate 1, was developed for the sequential detection of palladium and chromium oxyanions in a mixed aqueous media, and was studied by UV-visible and fluorescence spectroscopy. The sensing of palladium ions produces a chemodosimetric and ratiometric change in the emission band of 1 from 450 to 525 nm, followed by the sensing of chromate ions by 2 that quenches the emission band at 525 nm in a buffered H2O: DMF solution (9:1, pH = 7.4). The rate constants of palladium and chromate ions were found to be 8.6 x 10⁵ M^-1, 2.1 x 10⁵ M^-1, and 5.4 x 10⁴ M^-1 respectively. The chemosensor 1 has a palladium detection limit of 49 ppb while the sequential detection limit of chromate ions (CrO₄ ^2- and Cr₂O₇ ^2-) were 127 and 259 ppb. The ratiometric change in the emission is produced due to the deallylation of 1 by palladium to produce 2 that restores the ESIPT (excited state intramolecular proton transfer) of the phenolic ring and enhances the electron transfer (ET) phenomenon from the phenolic group to fluorescein. The sequential binding of chromate ions to 2 inhibits the ESIPT and causes chelation enhanced quenching (CHEQ) of the fluorescence.