Probing Biothiols Using a Red-Emitting Pyridoxal Derivative by Adopting Copper(II) Displacement Approach and Cell Imaging.

An aggregation-induced emission (AIE) active Schiff base L was obtained by reacting pyridoxal and 2-hydroxy-1-naphthaldehyde with p-phenylenediamine in two simple steps. The colorimetric, UV/VIS and fluorescence studies of L revealed that the yellow emissive L (λ =540 nm, λ =450 nm) in pure DMSO turned to a red-emissive L, when the poor solvent fraction (HEPES buffer, 10 mM, pH 7.4) was increased above 50 % in DMSO. The SEM and DLS results indicated the formation of self-aggregates of L that restricted the intramolecular motion and promoted the excited state intramolecular proton transfer (ESIPT) process. The cations sensing ability of the AIEgen L was explored in HEPES buffer (5 % DMSO, 10 mM, pH 7.4), where Cu selectively quenched the fluorescence at 608 nm due to the chelation-enhanced fluorescence quenching (CHEQ) effect with an estimated sensitivity limit of 0.9 μM. Subsequently, the in situ formed AIEgen L-Cu complex was applied for the cascade detection of glutathione (GSH), cysteine (Cys) and homocysteine (Hcy). The decomplexation of Cu from the AIEgen L-Cu by GSH, Cys and Hcy restored the quenched fluorescence emission of AIEgen L at 608 nm. With this Cu displacement approach, the concentration of Cys, Hcy and GSH can be detected down to 2.8 μM, 3.12 μM and 2.0 μM, respectively. The practical utility of AIEgen L and AIEgen L-Cu was examined by monitoring the selective analytes in real environmental and biological samples, and also applied successfully for the cell imaging applications.