pH-dependent selective ion exchange based on (ethylenediamintetraacetic acid-nickel)-layered double hydroxide to catalyze the polymerization of aniline for detection of Cu²⁺ and Fe³

A pH-dependent selective ion exchange coupled with catalytic polymerization of aniline has been developed for sensitive detection of copper (Cu²⁺) and ferric ions (Fe³⁺). Ethylenediamintetraacetic acid (EDTA) chelated with nickel ion (Ni²⁺) were intercalated in a layered double hydroxide via a co-precipitation reaction. The product was subsequently applied as sorbent for the enrichment of Cu²⁺ at pH 6.5 and Fe³⁺ at pH 4.5. Since both Cu²⁺ and Fe³⁺ have stronger complex formation constants with EDTA, Ni²⁺ exchanges with Cu²⁺/Fe³⁺ selectively. The resulting sorbent containing Cu²⁺/Fe³⁺ was transferred to catalyze the aniline polymerization reaction, since Cu²⁺/Fe³⁺ could be released by the sorbent effectively at different pH values and have high catalytic abilities for the polymerization reaction. The resulting polyaniline with different colors were produced at different pH values, an observation that was utilized to distinguish between the colorimetric signals of Cu²⁺ and Fe³⁺. The extraction temperature, extraction time, catalysis time and pH were optimized. The results showed that this method provided low limits of detection of 0.1 nM (6.4 ng/L) for Cu²⁺, 1 nM (56 ng/L) for Fe³⁺, wide linear ranges (0.0005-2.5 µM, and 0.005-5 µM, respectively), and good linearities (r² values of 0.9904, and 0.9965, respectively). The optimized method was applied to river water samples. Using Cu²⁺/Fe³⁺ as examples, this work provided a new and interesting approach for the convenient and efficient detection of metal ions in aqueous samples.