Design and use of a general capturing surface in optical biosensor analysis of sulphamethazine in milk

A new optical biosensor assay, based on a general capturing surface, for detection of the antimicrobial agent sulphamethazine (SMZ) was evaluated and compared with a previously described biosensor assay. At the general surface, the immobilisation is thought to be independent of type of analyte. Monoclonal antibodies against a small molecule (hapten H1) were immobilised and used to capture a conjugate between H1 and SMZ. Polyclonal SMZ antibodies were added to the milk sample and the amount of antibodies bound to the surface was in inverse proportion to the SMZ concentration in the milk sample. The detection limit of the new assay was 0.5 mu g kg(-1) and within-assay repeatability was 2.4%. This is in agreement with previous results obtained when SMZ was directly immobilised on the surface. Incurred samples from SMZ-treated cows were analysed, and non-specific binding was studied by analyses of individual cow's milk. The advantages of the new assay format include analyse-independent immobilisation and regeneration. Furthermore, the assay enables measurements with covalent interactions between analyte and detecting molecule. The main disadvantage is the requirement of a conjugate between analyte and the hapten H1. Moreover, it is likely that the antibody surface will have a shorter life span than a surface with the antimicrobial immobilised.