An effective phthalazine-imidazole-based chemosensor for detecting Cu²⁺, Co²⁺ and S²⁻ via the color change

A novel and effective phthalazine-imidazole-based colorimetric chemosensor (E)-1-(2-((1H-imidazol-2-yl)methylene)hydrazinyl)phthalazine NNI was synthesized and tested to detect Cu²⁺ or Co²⁺ ions in buffer solution. NNI could probe Cu²⁺ and Co²⁺ by the color change from colorless to yellow. The complexation stoichiometries of NNI toward Cu²⁺ and Co²⁺ were, respectively, confirmed to be as 1:1 ratio and 2:1 by results of Job plot and ESI-mass. Detection limits of NNI for each metal showed 0.12 μM and 65 nM which are lower than WHO guideline (31.5 μM for Cu²⁺) and US Environmental Protection Agency (EPA) guideline (1.7 μM for Co²⁺). Quantification of NNI for each metal was successful in real water samples. The Cu²⁺-NNI complex could detect S²⁻ by demetallation with color change from yellow to colorless, and detection limit for S²⁻ was determined as 0.80 μM, which is lower than WHO guideline (14.8 μM). Cu²⁺-NNI could detect S²⁻ without interference when other anions coexisted. Detection of Cu²⁺, Co²⁺ and S²⁻ among various metal ions and anions was successfully confirmed using test papers stained with NNI and Cu²⁺-NNI. Sensing processes of Cu²⁺, Co²⁺ and S²⁻ by NNI were elucidated by UV–vis titration, ESI-mass, FT-IR, ¹H NMR titration and DFT calculations.