Selective fluorescence sensing of Cu(ii) and Zn(ii) using a new Schiff base-derived model compound: naked eye detection and spectral deciphering of the mechanism of sensory action

Abstract

A new Schiff base compound 2-((benzylimino)-methyl)-naphthalen-1-ol (2BIMN1O) has been synthesized and characterized by ¹H NMR, ¹³C NMR, DEPT, FT-IR and mass spectroscopic techniques. The significantly low fluorescence yield of the compound has been rationalized in connection with photo-induced electron transfer (PET) from the imine receptor moiety to the naphthalene fluorophore unit. Subsequently, an evaluation of the transition metal ion-induced modification of the fluorophore–receptor communication reveals a promising prospect for the title compound to function as a fluorosensor for Cu²⁺ and Zn²⁺ ions selectively, through remarkable fluorescence enhancement. While perturbation of the PET process in 2BIMN1O has been argued to be the responsible mechanism behind the fluorescence enhancement, the selectivity for these two metal ions has been interpreted on the grounds of an appreciably strong binding interaction. Particularly notable aspects regarding the chemosensory activity of the compound are its ability to detect the aforesaid transition metal ions down to the level of micromolar concentration (detection limit being 0.82 and 0.35 μM respectively), along with a simple and efficient synthetic procedure. Also the spectral modulation of 2BIMN1O in the presence of the transition metal ions paves the way for the construction of a calibration curve in the context of its fluorescence signaling potential.