Microsphere-based optical system for biosensor applications

Optical microsphere resonators have been recently utilized in quantum optics, laser science, spectroscopy, and optoelectronics and attracted increasing interest due to their unique optical properties. Microspheres possess high quality factor (Q-factor) optical morphology dependent resonances, and have relatively small volumes. High-Q morphology dependent resonances are very sensitive to the refractive index change and microsphere uniformity. These tiny optical cavities, whose diameters may vary from a few to several hundred micrometers, have resonances with reported Q-factors as large as 3 x 10(9). Due to their sensitivity, morphology dependent resonances of microspheres are also considered for biosensor applications. Binding of a protein or other biomolecules can be monitored by observing the wavelength shift of morphology dependent resonances. A biosensor, based on this optical phenomenon, can even detect a single molecule, depending on the quality of the system design. In this work, elastic scattering spectra from the microspheres of different materials are experimentally obtained and morphology dependent resonances are observed. Preliminary results of unspecific binding of biomolecules onto the microspheres are presented. Furthermore, the morphology dependent resonances of the microspheres for biosensor applications are analyzed theoretically both for proteins such as bovine serum albumin.