ESIPT blocked CHEF based differential dual sensor for Zn2+ and Al3+in a pseudo-aqueous medium with intracellular bio-imaging applications and computational studies

Abstract

A novel 3-hydroxymethyl-5-methylsalicylaldehydenaphthyl-hydrazone (H₃SAL-NH) exhibits ESIPT behaviour due to proton transfer from the phenolic OH group to the azomethine N atom in the excited state. Through this ESIPT behaviour together with cis–trans isomerization of the azomethine group, the free ligand becomes very weakly fluorescent. However, in the presence of Zn²⁺ and Al³⁺ the ESIPT and isomerization are blocked due to coordination to the metal ions thereby causing turn on fluorescence for Al³⁺ and Zn²⁺. Moreover, Zn²⁺ can easily be displaced from the [H₂SAL-NH–Zn²⁺] complex by Al³⁺ thereby enhancing the differential selectivity for Al³⁺ over Zn²⁺. This probe was found to be selective for Al³⁺ over Zn²⁺ in the presence of Na₂H₂EDTA, under both intra- and extracellular conditions. The LODs for Zn²⁺ and Al³⁺ were determined by 3σ methods and were found to be 3.1 nM and 0.92 nM, respectively. Thus, the differentially selective turn-on fluorescence behaviour of H₃SAL-NH for Zn²⁺ and Al³⁺ is based on the combined blocking of ESIPT and CN isomerization, and a chelation-enhanced fluorescence (CHEF) effect. The coordination modes of the complexes were investigated through spectroscopic and computational studies. H₃SAL-NH also exhibits good photostability and very low cytotoxicity and is useful for fluorescence imaging of Zn²⁺ and Al³⁺ ions in live HepG2 cells.