Enzyme immunoassay using a crossed-beam thermal lens technique

An enzyme immunoassay based on the use of crossed-beam thermal lens detection is described. In this assay, poly-N-isopropylacrylamide, a water-soluble, thermally precipitating synthetic polymer, was used as a carrier to minimize non-specific binding. The enzyme substrate of the horseradish peroxidase that was employed was 3,3′,5,5′-tetramethylbenzidine. The color development of the enzyme–substrate reaction was stopped by SDS and Na₂SO₃ to achieve a stable blue solution. The background reduction and stabilization made it possible to use a crossed-beam thermal lens technique as the measurement method. This method was demonstrated to be applicable by determination of hepatitis B surface antigen in human serum. A detection limit of 0.15 ng/ml was obtained. This was more sensitive than that of the commercially available ELISA method.