Benzimidazole conjugate of 1,1'-thiobis(2-naphthol) as switch-on fluorescence receptor for Ag⁺ and the complex as secondary recognition ensemble toward Cys, Asp, and Glu in aqueous methanolic solution: synthesis, characterization, ion and amino acid recognition, computational studies, and microscopy features

Abstract

A new 1,1'-thiobis(2-naphthoxy)-based receptor molecule (L) containing a benzimidazole moiety has been synthesized and characterized by ¹H NMR, ESI-MS, and elemental analysis. The selectivity of L has been explored in aqueous methanol, resulting in selective (7.5 ± 0.5)-fold switch-on fluorescence response toward Ag⁺ among 14 different transition, alkali, and alkaline earth metal ions studied. The complexation of Ag⁺ by L has been addressed by ESI-MS, ¹H NMR, and UV-vis spectra. Microstructural features of L and its Ag⁺ complex have been measured by AFM and TEM. The morphological features of L alone and L in the presence of Ag⁺ differ dramatically both in shape and size, and the ion induces the formation of chains owing to its coordinating ability toward benzimidazole. Further, the in situ [Ag⁺-L] complex was titrated against 20 naturally occurring amino acids and found that this complex acts as a secondary recognition ensemble toward Cys, Asp, and Glu by switch-off fluorescence.