Two surface plasmon resonance (SPR)-based biosensor assays for detection of beta-lactam antibiotics in milk are reported. The assays are based on the enzymatic activity of a carboxypeptidase converting a 3-peptide into a 2-peptide, a reaction that is inhibited in the presence of beta-lactams. Antibodies were used to measure either the amount of formed enzymatic product or the amount of remaining enzymatic substrate. Both assays detected different beta-lactams at or below European maximum residue limits (MRLs), and the detection limit for penicillin G was 1.2 microg/kg and 1.5 microg/kg for the 2- and 3-peptide assays, respectively. The precision (CV) was < 5%, both within and between assays at the penicillin G MRL (4 microg/kg). The biosensor results obtained upon analysis of incurred milk samples were compared with results obtained by liquid chromatography (HPLC), and the method agreements were, in general, good.

An optical biosensor assay for detection of b-lactam antibiotics in milk based on a microbial receptor protein was developed. The assay uses a general sensor surface previously described with a small organic molecule (H1) immobilized. A conjugate between a b-lactam (cephalosporin C) and a monoclonal H1 antibody is injected across the sensor surface before injection of the sample mixed with receptor protein. Receptor inhibited by b-lactam residues in the milk sample will not bind to the sensor surface and the reduction in response is inversely related to the b-lactam concentration of the sample. The detection limit for a number of commonly used b-lactams was below or near the respective maximum residue limit and the relative standard deviation (CV) for penicillin G in milk was 6-12% in the interval 2.0-12.5 mg kg -1 . For application in the field further optimization is needed to solve problems related to non-specific binding to the sensor surface.

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