Improved liposome immunomigration strip assay for alachlor determination

The feasibility of a simple, single-use immunomigration strip assay for alachlor was previously demonstrated. In the device, capillary action caused alachlor and alachlor-tagged, dye-containing liposomes to migrate through an anti-alachlor antibody zone, on a plastic-backed nitrocellulose strip, where competitive binding occurred. Unbound liposomes continued migration to a liposome capture zone, where they were quantified by densitometry. The amount of liposome-entrapped dye measured in this zone was directly proportional to the alachlor concentration in the sample. This report describes modifications to various components of the system, leading to improvement in the sensitivity of the assay to the point where the maximum contaminant level of alachlor, 2 ppb, can be easily detected. Measurements of liposome size and antibody cross-reactivity are also presented. The new methodology involves acid treatment of the antibody and the use of preincubation of the analyte-tagged liposomes, free analyte and anti-alachlor before initiating migration. This results in strong interactions between the anti-alachlor, liposome and strip such that liposomes that have bound to antibody do not migrate. This inhibition of migration is reversed by free analyte, which competes with the liposome for the available antibody binding sites. As in the previous assay, unbound liposomes migrate to a capture zone where they can be quantified, and the color intensity of this zone is directly proportional to the amount of analyte present. This technique produces an assay capable of detecting alachlor at levels down to 1 ppb.