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.