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

A pH-dependent selective ion exchange coupled with catalytic polymerization of aniline has been developed for sensitive detection of copper (Cu) and ferric ions (Fe). Ethylenediamintetraacetic acid (EDTA) chelated with nickel ion (Ni) were intercalated in a layered double hydroxide via a co-precipitation reaction. The product was subsequently applied as sorbent for the enrichment of Cu at pH 6.5 and Fe at pH 4.5. Since both Cu and Fe have stronger complex formation constants with EDTA, Ni exchanges with Cu/Fe selectively. The resulting sorbent containing Cu/Fe was transferred to catalyze the aniline polymerization reaction, since Cu/Fe 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 Cu and Fe. 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 Cu, 1?nM (56?ng/L) for Fe, wide linear ranges (0.0005-2.5?µM, and 0.005-5?µM, respectively), and good linearities (r values of 0.9904, and 0.9965, respectively). The optimized method was applied to river water samples. Using Cu/Fe as examples, this work provided a new and interesting approach for the convenient and efficient detection of metal ions in aqueous samples.