Selective extraction by dissolvable (nitriloacetic acid-nickel)-layered double hydroxide coupled with reaction with potassium thiocyanate for sensitive detection of iron(III).

A highly selective method has been proposed for the determination of iron cation (Fe(3+)). (Nitriloacetic acid-nickel)-layered double hydroxide ((NTA-Ni)-LDH) was successfully synthesized and used as dissolvable sorbent in dispersive solid-phase extraction to pre-concentrate and separate Fe(3+) from aqueous phase. Since Fe(3+) has a larger formation constant with NTA compared to Ni(2+), subsequently ion exchange occurred when (NTA-Ni)-LDH was added to the sample solution. The resultant (NTA-Fe)-LDH sol was isolated and transferred in an acidic medium containing potassium thiocyanate (KSCN). Since (NTA-Fe)-LDH could be dissolved in acidic conditions, Fe(3+)was released and reacted with SCN(-) to form an Fe-SCN complex. The resulting product was measured by ultraviolet-visible spectrometry for quantitative detection of Fe(3+). Extraction factors, including sample pH, reaction pH, extraction temperature, extraction time, reaction time and concentration of KSCN were optimized. This method achieved a low limit of detection of 15.2nM and a good linear range from 0.05 to 50µM (r(2)=0.9937). A nearly 18-fold enhancement of signal intensity was achieved after selective extraction. The optimized conditions were validated by applying the method to determine Fe(3+) in seawater samples.

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.

Copper is an important heavy metal in various biological processes. Many methods have been developed for detecting of copper ions (Cu(2+)) 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 (Ni(2+)) 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 Cu(2+) at pH 6. Since Cu(2+) has a stronger complex formation constant with EDTA, Ni(2+) exchanged with Cu(2+) selectively. The resulting sorbent containing Cu(2+) was transferred to catalyze the 3,3',5,5'-tetramethylbenzidine oxidation reaction, since Cu(2+) 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 Cu(2+). The extraction temperature, extraction time, and catalysis time were optimized. The results showed that this method provided a low limit of detection of 10nM, a wide linear range (0.05-100µM) and good linearity (r(2)=0.9977). The optimized conditions were applied to environmental water samples. Using Cu(2+) as an example, this work provided a new and interesting approach for the convenient and efficient detection of metal cations in aqueous samples.

Automated dispersive solid-phase extraction using dissolvable Fe3O4-layered double hydroxide core-shell microspheres as sorbent.

Automation of dispersive solid-phase extraction (d-SPE) presents significant challenges. Separation of the sorbent from the spent sample cannot be conducted without manual operations, including centrifugation, a widely used means of isolating a solid material from solution. In this work, we report an approach to d-SPE using dissolvable magnetic Fe3O4-layered double hydroxide core-shell microspheres as sorbent to enable automation of the integrative extraction and analytical processes. Through magnetic force, the sorbent, after extraction, was isolated from the sample and then dissolved by acid to release the analytes. Thus the customary analyte elution step in conventional SPE was unnecessary. The automated d-SPE step was coupled to high-performance liquid chromatography (HPLC) with photodiode array detection for determination of several pharmaceuticals and personal care products (PPCPs) [acetylsalicylic acid (ASA), 2,5-dihydroxybenzoic acid (DBA), 2-phenylphenol (PP), and fenoprofen (FP)] in aqueous samples. For the automated d-SPE process, experimental parameters such as agitation speed, temperature, time, and pH were optimized. The results showed that this method provided low limits of detection (between 0.021 and 0.042 µg/L), good linearity (r(2) ≥ 0.9956), and good repeatability of extractions (relative standard deviations ≤4.1%, n = 6). The optimized procedure was then applied to determination of PPCPs in a sewage sample and ASA and FP in drug preparations. This fully automated extraction-HPLC approach was demonstrated to be an efficient procedure for extraction and analysis of ASA, DBA, PP, and FP in these samples.