Immunoassay systems based on immunoliposomes consisting of genetically engineered single-chain antibody

The single-chain antibody against 2-phenyloxazolone was modified with lipid molecules at its amino-terminus by genetic engineering. The engineered lipid-tagged antibody molecules were incorporated into liposomes consisting of phosphatidylcholine, and the immunoassay systems were constructed by the resulting immunoliposomes. One immunoassay system was based on fluoroimmunoassay. A competitive fluoroimmunoassay for caproic acid conjugate of 2-phenyloxazolone as a model antigen was performed with the calboxyfluoresceine-encapsulated immunoliposomes. Antigen could be determined in the concentration range from 10 −7 to 10 −9 M. The other system was based on quartz crystal microbalance (QCM). The immunoliposomes were competitively reacted with analyte to the hapten-immobilized surface of a crystal plate. The frequency change was observed by injection of the mixture of the immunoliposomes and analyte in a concentration dependent manner. In this competitive QCM assay, antigen could be measured in the concentration range from 10 −5 to 10 −8 M. Furthermore, direct observation of the immunoliposomes on the hapten-coated solid-surface by atomic force microscopy (AFM) was also performed in this study.