Substituent effect on fluorescence signaling of the cell permeable HSO4− receptors through single point to ratiometric response in green solvent

Abstract

Two new 2-(2-aminophenyl)benzimidazole-based HSO₄⁻ ion selective receptors, 6-(4-nitrophenyl)-5,6-dihydrobenzo[4,5]imidazo[1,2-c]quinazoline (L₁H) and 6-(4-methoxyphenyl)-5,6-dihydrobenzo[4,5]imidazo[1,2-c]quinazoline (L₂H), and their 1 : 1 molecular complexes with HSO₄⁻ were prepared in a facile synthetic method and characterized by physicochemical and spectroscopic techniques along with the detailed structural analysis of L₁H by single crystal X-ray crystallography. Both receptors (L₁H and L₂H) behave as highly selective chemosensor for HSO₄⁻ ions at biological pH in ethanol–water HEPES buffer (1/5) (v/v) medium over other anions such as F⁻, CI⁻, Br⁻, I⁻, AcO⁻, H₂PO₄⁻, N₃⁻ and ClO₄⁻. Theoretical and experimental studies showed that the emission efficiency of the receptors (L₁H and L₂H) was tuned successfully through single point to ratiometric detection by employing the substituent effects. Using 3σ method the LOD for HSO₄⁻ ions were found to be 18.08 nM and 14.11 nM for L₁H and L₂H, respectively, within a very short responsive time (15–20 s) in 100 mM HEPES buffer (ethanol–water: 1/5, v/v). Comparison of the utility of the probes (L₁H and L₂H) as biomarkers for the detection of intracellular HSO₄⁻ ions concentrations under a fluorescence microscope has also been included and both probes showed no cytotoxic effect.