The surface plasmon resonance (SPR) wavelength of colloidal gold particles coated with a monoclonal antibody is red-shifted when the antibody interacts with its specific ligand. This shift results from the change in the refractive index of the particles as induced by ligand binding. This property is used to monitor in real-time the association and dissociation kinetics of the interaction in solution. The monitoring is perfomed in a clinical chemistry automated analyzer during a few minutes of incubation at 37 °C. Data treatment allows calculation of the affinity constant of the interaction. The SPR wavelength shift does not necessarily require agglutination or aggregation of the particles to occur since particles coated with one monoclonal antibody specific for a single epitope on the ligand can be used in the procedure. The affinity constants measured by this procedure correlate with those calculated from Scatchard plots or BIAcore data.
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