A specific, ultra-sensitive and rapid naked-eye on-site recognition of CN– and Cu2+ ions in tap water using an isonicotinohydrazide-salisaldehyde hybrid probe

Abstract

Schiff base 1, namely (E)-N'-(2-hydroxy-3-methoxybenzylidene)isonicotinohydrazide, exhibits amide-iminol tautomerism in its molecular structure and functions as a colorimetric sensor for detecting cyanide (CN–) and copper ions (Cu2+) in ethanol/H2O (1:1). Compound 1 can distinguish CN– and Cu2+ ions from various competing ions based on unique color and spectral alterations. Spectral analyses reveal the prevalence of the iminol-form of 1 over the amide-form, enabling the selective detection of CN– and Cu2+ ions. When CN– is present, deprotonation occurs in both the phenolic and iminolic groups, leading to increased conjugation along molecule 1, as confirmed by 1H NMR spectroscopy. On the other hand, Cu2+ sensing involves coordination with 1 through a tridentate moiety, specifically interacting with the azomethine nitrogen atom as well as the deprotonated phenolic and iminolic groups. This mechanism allows for superior detection of Cu2+ compared to CN–. Key features of this sensor include its selectivity, sensitivity (with detection limits of 4.05 nM for CN– and 1.7 nM for Cu2+), a rapid response (less than 5 s), and the capability for on-site monitoring of CN– and Cu2+ ions in tap water using either solution or solid kits. Additionally, DFT calculations have validated the influence of Cu2+ and CN– on the system's electronic structure, as demonstrated by the results of MEP maps and changes in gap energy values.