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 (λem =540 nm, λex =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 Cu2+ 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-Cu2+ complex was applied for the cascade detection of glutathione (GSH), cysteine (Cys) and homocysteine (Hcy). The decomplexation of Cu2+ from the AIEgen L-Cu2+ by GSH, Cys and Hcy restored the quenched fluorescence emission of AIEgen L at 608 nm. With this Cu2+ 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-Cu2+ was examined by monitoring the selective analytes in real environmental and biological samples, and also applied successfully for the cell imaging applications.

A novel Schiff base derivative of pyridoxal for the optical sensing of Zn2+ and cysteine.

An easy to prepare novel vitamin B6 cofactor derivative 3-hydroxy-N'-((3 hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl)methylene)-2-naphthohydrazide (NPY) was synthesized by a one pot condensation reaction of pyridoxal with 3-hydroxy-2-naphthoic hydrazide and applied for the optical detection of Zn2+ and cysteine in the aqueous DMSO medium. The addition of Zn2+ ions leads to a selective blue-shift in the fluorescence emission spectrum of NPY from 530 nm to 475 nm, which allowed ratiometric detection of Zn2+ ions down to 8.73 × 10-7 M without any interference from other tested metal ions. This system was also successfully applied to detect intracellular Zn2+ ions in live HeLa cells. Further, when the in situ generated NPY·Zn2+ complex was interacted with various amino acids, the addition of cysteine resulted in an instantaneous colour change from light yellow to colourless and the absorbance at 435 nm of the complex was quenched selectively. Also, the fluorescence of the NPY·Zn2+ complex was quenched, which allowed the detection of cysteine down to 6.63 × 10-7 M.