Selective extraction and release using (EDTA-Ni)-layered double hydroxide coupled with catalytic oxidation of 3,3′,5,5′-tetramethylbenzidine for sensitive detection of copper ion

• (EDTA-Ni)-layered double hydroxides were synthesized as a sorbent to extract Cu2+.
• A selective extraction-release-catalysis approach has been developed for detection of Cu2+.
• Through two steps, extraction and catalysis, the detection signal was amplified.
• Ion-exchange, release and efficient catalysis ensured the high selectivity of the method.

Copper is an important heavy metal in various biological processes. Many methods have been developed for detecting of copper ions (Cu2+) in aqueous samples. However, an easy, cheap, selective and sensitive method is still desired. In this study, a selective extraction-release-catalysis approach has been developed for sensitive detection of copper ion. Ethylenediaminetetraacetic 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 in dispersive solid-phase extraction for the enrichment of Cu2+ at pH 6. Since Cu2+ has a stronger complex formation constant with EDTA, Ni2+ exchanged with Cu2+ selectively. The resulting sorbent containing Cu2+ was transferred to catalyze the 3,3′,5,5′-tetramethylbenzidine oxidation reaction, since Cu2+ could be released by the sorbent effectively and has high catalytic ability for the reaction. Blue light emitted from the oxidation product was measured by ultraviolet–visible spectrophotometry for the determination of Cu2+. The extraction temperature, extraction time, and catalysis time were optimized. The results showed that this method provided a low limit of detection of 10 nM, a wide linear range (0.05–100 μM) and good linearity (r2 = 0.9977). The optimized conditions were applied to environmental water samples. Using Cu2+ as an example, this work provided a new and interesting approach for the convenient and efficient detection of metal cations in aqueous samples.

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