A novel, sensitive and environmentally friendly supramolecular solvent dispersive liquid-liquid microextraction method based on the solidification of a floating drop was developed for the preconcentration of trace lead (Pb) prior to its determination by graphite furnace atomic absorption spectrometry. In this method, a supramolecular solvent, a coacervate of reverse micelles formed by decanoic acid (DeA) dispersed in tetrahydrofuran (THF)-water mixture, was used to extract the metal chelate produced from the reaction of Pb(II) with diethyldithiocarbamate. After centrifugation, the coacervate phase floated on the surface of the solution, and was then solidified in an ice bath to favor phase separation. Several parameters that affect the extraction efficiency, such as the amount of DeA, the volume of THF, the pH of the sample solution, the concentration of the chelating agent, the extraction time and coexisting ions were systematically investigated. Under the optimized conditions, the linearity of the calibration curve was in the Pb concentration range of 0.1-30 ng mL À1 with a correlation coefficient (R 2 ) of 0.9995. The detection limit of this method for Pb was 27 ng L À1 with an enhancement factor of 52, and the relative standard deviations for 1.0 ng mL À1 and 20 ng mL À1 of Pb were 3.2% and 2.5%, respectively. The method was successfully applied in the determination of trace amounts of Pb in food and water samples.

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We developed a new magnetic nanoparticles sandwich-like immunoassay using protein cage nanoparticles (PCN) for signal amplification together with graphite furnace atomic absorption spectrometry (GFAAS) for quantification of organophosphorylated acetylcholinesterase adduct (OP-AChE), the biomarker of exposure to organophosphate pesticides (OPs) and nerve agents. OP-AChE adducts were firstly captured by titanium dioxide coated magnetic nanoparticles (TiO2-MNPs) from the sample matrixes through metal chelation with phospho-moieties, and then selectively recognized by anti-AChE antibody labeled on PCN which was packed with lead phosphate in its cavity (PCN-anti-AChE). The sandwich-like immunoreaction was performed among TiO2-MNPs, OP-AChE and PCN-anti-AChE to form TiO2-MNPs/OP-AChE/PCN-anti-AChE immunocomplex. The complex could be easily isolated from the sample solution with the help of magnet, and the released lead ions from PCN were detected by GFAAS for the quantification of OP-AChE. Greatly enhanced sensitivity was achieved because PCN increased the amount of metal ions in the cavity of each apoferritin. The proposed immunoassay yielded a linear response over a broad OP-AChE concentrations from 0.01 nM to 2 nM, with a detection limit of 2 pM, which has enough sensitivity for monitoring of low-dose exposure to OPs. This new method showed an acceptable stability and reproducibility and was validated with OP-AChE spiked human plasma.

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