Naked eye detection of anions: Dihydrazone receptor-based visual sensing

Abstract

In this study, we designed and synthesized a novel colorimetric sensor for fluoride ions by integrating a thienodicarbohydrazide framework with salicylaldehyde, resulting in a unique bis-hydrazone-phenol receptor. This sensor exhibited exceptional selectivity for fluoride and acetate ions in nonaqueous environments, with detection visible to the naked eye. Upon interaction with fluoride ions, the receptor underwent a significant red shift in its UV–Visible absorption spectrum, moving from an absorption maximum at 332 nm–380 nm, signalling the recognition event. Acetate ions induced a similar shift. The binding interactions were thoroughly investigated, with a Jobs plot revealing a 1:1 receptor-anion complex formation with fluoride ions and a 1:2 complex with acetate ions. Further validation came from ¹H NMR titration, which confirmed the presence of hydrogen bonding interactions between the receptor and the ions. The receptor demonstrated remarkable binding constants of 5.2 × 10⁴ M⁻² for acetate and 1.6 × 10⁴ M⁻¹ for fluoride ions, highlighting its strong and selective affinity. Fluorescence titrations further confirmed the receptor's potential as a chemo sensor, showing distinct emission changes in the presence of fluoride and acetate ions. This study not only underscores the receptor's capability for visual detection of fluoride ions in non-aqueous systems but also paves the way for its application in diverse analytical and environmental settings.