ANALmcA zyxwvutsr CHIMICA zyxwvutsrqpo ACIA ELSEVIER Analytica Cbimica Acta 311(1995) 309-318 Improved liposome immunomigration determination strip assay for alachlor Sui Ti A. Siebert, Stuart G. Reeves, Matthew A. Roberts, Richard A. Durst * Analytical Chemistry Laboratories, Cornell University, Geneva, NY 14456-0462, USA Received 13 September 1994; revised 24 November 1994, accepted 28 November 1994 Abstract The feasibility of a simple, single-use immunomigration strip assay for alachlor was previously demonstrated. In the device, capillary action caused ala&or 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. Keywords: Biosensors; Immunoassay; Alachlor; Liposomes; Herbicides 1. Introduction The need for rapid and inexpensive field assays in environmental screening and remediation studies has become a matter of some urgency, and the use of immunoassays, which offer high selectivity, sensitiv- ity, speed and simplicity of operation, has recently increased [ 1,2]. Immunoassays, mainly in the form of l Corresponding author. ELISAs (enzyme-linked immunosorbent assays) have been developed for alachlor, a widely used herbicide [3-71. In a previous report, however, it was sug- gested that for field assays a liposome-based tech- nique would be advantageous because of its ability to provide instantaneous visualization [8]. The exper- imental system described was a competitive im- munomigration device, where a mixture of analyte- tagged, dye-loaded liposomes and the sample analyte to be measured was allowed to migrate up a test strip. Competitive binding occurred in an antibody 0003-2670/95/$09.50 6 1995 Elsevier Science B.V. All rights reserved SSDI 0003-2670(94)00639-3