The lateral flow immunochromatographic assay (LFIA) based on colored particles as labels of antibody is a promising tool in medical applications, food control, and environmental monitoring. In comparison with the laboratory-oriented instrumental analysis, LFIA is a simple, fast-responsive and inexpensive technique widely used in the detection of kinds of important analytes. However, the sensitivity and detection limit of LFIA, which are determined by the molar absorption coefficient of the particle and its accumulation caused by unit analyte on detection zones, remain to be improved for detecting trace analytes of being highly hazardous.
    Following our previous investigations on the preparation and bioapplication of PEG modified Fe₃O₄ (Adv. Mater., 2005, 17, 1001–1005; Adv. Mater., 2006, 18, 2553–2556; Mol. Pharmaceutics, 2009, 6, 1074–1082; J. Phys. Chem. C, 2010, 114, 21270-21276), herein we report a highly sensitive and specific magnetic lateral flow immunochromatographic assay (LFIA) using the Fe₃O₄ particle aggregates for detecting pesticide residue. The Fe₃O₄ particle aggregates were prepared by crosslinking the PEG-coated Fe₃O₄ nanoparticles bearing surface reactive carbonyl moieties with Poly-L-Lysine (PLL) upon suitable Fe₃O₄ to PLL ratio. Particle aggregates of different hydrodynamic sizes were obtained and used as labels in LFIA. Owing to the significant amplification effect, the Fe₃O₄ particle aggregates offer greatly improved visual detection limit in detecting paraoxon methyl, in addition to excellent detection specificity. More quantitative analysis through relative optical density demonstrates that the detection limit is decreased by more than 40 folds, reaching 1.7 ng/mL.The relative results were published in Analytical Chemistry (Anal. Chem., 2011, 83, 6778-6784).
    We believe the current investigations pave a novel strategy for constructing ultra-sensitive LFIA through the amplification effect endowed by the controlled particle aggregation. Moreover, the current method may provide a highly sensitive detection method for hazardous substances through magnetic signal detection on Fe₃O₄ particle aggregates. Therefore, it should have a broad readership in the communities of LFIA.
                                                                                                                                                                                                           Chunyan Liu et al.