SPR techniques for detection of biomolecules
    The opto-electronic principle of the surface plasmon resonance (SPR) provides a unique tool for the analysis of biomolecular interactions in real-time. However, in the traditional SPR sensing technique the variations of the refractive index of the sample medium induce changes in the angle of the resonance excitation, thus providing only integrated information on the mass of the material present within the adsorption layer. This information is obtained by observing the SPR curve, namely by monitoring the minimum of the angular distribution of the reflected light. If a ligate contains several molecular species, which can potentially interact with the immobilized ligand molecules, as for instance in a competition assay, information regarding the molecular identity of the interacting partners cannot be retrieved.
    To overcome the above limitation we used two novel modes in SPR measurements. The
identification of interacting biomolecules can be achieved through the covalent attachment of
small fluorophores to the various potentially interacting macromolecules. One of the techniques
developed allows the identification of the components by characteristic absorption bands of the
fluorophores which were detected with SPR by varying the excitation optical wavelength.
Another technique employs the monitoring of the fluorescence signal from the region of the
interaction which was measured simultaneously with the usual SPR response.

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