Pyridoxal derived red-emitting aggregates for the ratiometric sensing of pH and copper(II)

Abstract

The research on aggregation-induced emission (AIE) continue to gain a bourgeoning interest for their extensive applications in different field including sensing and biosensing. In this study, the red-emitting rod-type aggregates L (λex = 375 nm, λem = 597 nm) was prepared facilely in two steps, i.e., the vitamin B6 cofactor pyridoxal was first condensed with the p-phenylenediamine followed by the DMSO solution of the orange precipitate formed was allowed to self-aggregate by adding poor solvent. Upon self-aggregation of L, the restriction of intramolecular motion and the ESIPT phenomenon play the key role in emitting red fluorescence at 597 nm. The pH dependent emission study of the aggregates L revealed three distinct pH windows: red-emitting between pH 5.4 to 9.1, green emitting above pH 9.9 and non-fluorescent in acidic pH. The aggregates L in HEPES buffer (5% DMSO, 10 mM, pH = 7.4) showed a selective turn-off response towards Cu2+ with a good linearity between 4.8 and 14.7 μM and a detection limit of 2.7 μM. The quenching was occurred due to the non-fluorescent complex formed between L and Cu2+ in the ground state. Addition of strong chelating agent EDTA reversed the quenched fluorescence of L. To improve the practical utility, the turn-off sensor L was converted into a ratiometric sensor for Cu2+ by adding an optimized amount of fluorescein (Fl) dye. The dual emitting L@Fl showed fluorescence quenching at 597 nm whereas concomitant enhancement at 515 nm because of the energy transfer mechanism. The developed ratiometric sensor showed improved detection limit of 0.72 μM. Finally, the practical utility of both L as turn-off sensor and L@Fl as ratiometric sensor was examined by quantifying Cu2+ concentration in different real samples.