Design of a novel photonic biosensor based on whispering-gallery modes of dielectric microspheres for high-throughput immunoassays

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Our goal is to develop a new generation of label-free biosensor using a simple, versatile, and highly sensitive optical method. During the last decade, label-free optical biosensors have become valuable tools for clinical and military use as well as drug discovery. They are important devices for a range of applications going from detection of infectious agents, toxins, proteins and DNA to investigation of whole cell behaviour (e.g. attachment, spreading and proliferation of animal cells on solid surfaces), to quantitative determination of binding affinities and interactions kinetics between molecules. Modern label-free optical biosensors, like the widely utilised surface plasmon resonance (SPR) method, are based on an evanescent light field travelling along a planar surface, probing the target material deposited on the surface. A more sensitive type of biosensor based on evanescent field coupling can be realized by exploiting narrow-linewidth photonic resonances of dielectric microspheres, the so called whispering-gallery modes (WGMs), where the field is confined close to the inner surface of the sphere and is travelling around the sphere via total internal reflection. The light can orbit many thousand times before escaping the resonator, giving rise to an effective interaction length between the evanescent light field and the target molecules many orders of magnitude longer than the physical length on the sphere surface where molecules are deposited (its SPR analogous would correspond to a planar chip of more than one meter length!). This method should show unprecedented sensitivity but to fully explore the range of possible applications a proper theoretical model is still missing. We have recently commenced the construction and experimental study of a prototype WGM biosensor for immunosensing based on antibody-antigen interaction where many microspheres coated with different antibodies can be addressed in parallel via their evanescent coupling to planar waveguide modes propagating near a substrate surface. With the present project we want to develop a theoretical modelling to assess the applicability of this optical biosensor toward sensitive detection not only of proteins but also of viruses and bacteria. By combining this rigorous theoretical study with the experiments already ongoing we aim to realize the design of an optimum label-free biosensor for high-throughput immunoassays.