Use of diformyl‐triazolo Schiff base for Zn²⁺ sensing and intracellular live cell imaging

Abstract

A diformylphenol Schiff base of triazole‐amine, 4‐methyl‐2, 6‐bis‐[(1H‐[1,2,4] triazol‐3‐ylimino)‐methyl]‐phenol (PTR) has been synthesised and characterised by spectroscopic data (ESI‐MS, UV‐vis, FT‐IR, NMR spectra). The probe, PTR, emits at 610 nm upon excitation at 380 nm and the emission is strongly intensified on interacting with Zn2+ ion in DMSO‐water (99:1, v/v; HEPES buffer, pH, 7.2) solution even in the presence of 15 other cations (Cu2+, Mn2+, Co2+, Ni2+, Pd2+, Cd2+, Pb2+, Hg2+, Fe3+, Cr3+, Al3+, Na+, K+, Ba2+, Ca2+) and the emission band has been shifted to 530 nm. The limit of detection of Zn2+, 0.30 μM is much lower than WHO recommended value (76 μM). The binding constant (Kd) is 4.2585 × 104 M−1. Selective and sensitive chemosensing behaviour of PTR to Zn2+ has been explained by switching off ESIPT quenching of the keto‐enol tautomerisation of the probe along with the chelation enhancement of fluorescence (CHEF) by binding with Zn2+. The Job's plot and NMR titration have been extended to measure the 1:1 M composition [PTR + Zn2+]. The energy calculation by the DFT computation of keto and enol isomers supports easy tautomerisation and helps ESIPT quenching by proton transfer. Intracellular Zn2+ ions in living cells of HEK293 (PBS: phosphate buffer saline; pH, 7.2) have also been identified by the probe, PTR, using fluorescence microscopic imaging technique.