Review on fluorescence-based planar waveguide biosensors

During the last years, human diagnostics, targeting to monitor disease status with molecular specificity and on the level of a few molecules, pharmaceutical research and development, hunting for new drug targets and lead compounds, and food control, challenged for the trace analysis of genetically modified or potentially contageous, contaminated food, have driven the development of new analytical techniques, in order to achieve lower detection limits, reduced sample and reagent consumption, and increased degree of multiplexing capability. Equally, such developments have opened new fields of biomolecular analytics that could not be addressed before. Sensors based on thin-film planar waveguide technology have the potential to satisfy these challenging requirements to a large extent. In general, these versatile bioanalytical systems comprise a some hundred nanometers thin, high-index waveguiding layer deposited on a transparent substrate, and a layer containing biochemical recognition elements for specific and highly selective analyte binding and detection, immobilized on the sensing surface. The technical approaches, that have been realized so far, can be categorized in two groups, namely refractive index-based techniques and fluorescence, or more generally, luminescence-based techniques. Systems for the detection of changes of the effective refractive index on the sensing surface, due to molecular adsorption or desorption, have the advantage of label-free analyte detection. Their sensitivity, however, is directly related to the molecular mass of the analyte and often not sufficient, when low detection limits are required. Fluorescence- or luminescence-based planar waveguide techniques require an additional derivatization step, but offer the clear advantage of significantly higher sensitivity, independent from the molecular mass. In this paper, different luminescence-based sensing configurations and examples of applications in bioaffinity assays relevant for the key life science areas, demonstrating the achieved system performance, will be presented and compared with literature results from refraction-based techniques.