Deprotonation of the –NH proton from a pyrrole moiety enables concentration-dependent colorimetric and fluorescence quenching of silver(i) ions

Abstract

A novel ligand (1) involving 2-amino pyridine and a heterocyclic aldehyde was designed and synthesized through a multicomponent reaction, and was found to be an effective sensor for Ag⁺ ions over other metal ions. Incremental concentrations of Ag⁺ ions lead to a large bathochromic shift (300 nm → 435 nm) in UV-visible spectroscopy, with a noticeable color change from colorless to brown and complete quenching of the intense blue fluorescence at 465 nm. Ligand (1) is exceptionally selective and sensitive (LOD: 0.59 nM) for the targeted analyte even in the presence of excess concentrations of interfering cations. Deprotonation of the –NH proton from the pyrrole ring played an important role in the selective recognition of Ag⁺ ions by forming a 2 : 1 complex (Job plot), which was corroborated through ¹H NMR titrations and theoretical DFT calculations. In addition, rapid, onsite laboratory testing strips as well as silica gel sheets revealed additional evidence for the real-time application of ligand (1). Quantitative determination of real sample analysis for ligand (1) revealed recovery percentages in the range of 92.63% to 95.41%, suggesting sensitivity and selectivity toward Ag⁺ ions in various environmental samples. Ligand (1) also favors in vivo imaging of Ag⁺ ions in live onion epidermal cells via fluorescence microscopy.