A ‘gold cluster-linked immunosorbent assay’: Optical near-field biosensor chip for the detection of allergenic β-lactoglobulin in processed milk matrices

A new optical biosensor based on the resonance enhanced absorption (REA) effect is described. REA effects are observed when noble metal nanoclusters are deposited at a nanometric distance from a highly reflective mirror. The aim of our study was to adopt the REA effect for the rapid testing of proteins in a direct immunoassay format on chip and to adjust a conventional enzyme-linked immunosorbent assay (ELISA) to a cluster-linked immunosorbent assay (CLISA) by labelling the read-out antibody with monodisperse colloidal gold clusters. For generation of a strong REA signal 30 min of coating of the target protein was sufficient. To evaluate our approach we used the milk allergen β-lactoglobulin (β-LG) as analyte, and β-LG-isolations of processed milk products to prove the applicability of our method to the analysis of proteins in complex matrices at even the trace level. For validating the specificity of the CLISA biosensor we used the non-functionalised cluster reagent without antibody and a non-immunoreactive milk matrix as controls. As expected, very weak background signals were obtained with the controls, whereas the purified food samples clearly showed that β-LG was present and detectable. In conclusion, we were able to describe the successful development of a new biosensor chip for assaying proteins using the REA effect.