ABSTRACT
This chapter focuses on biomedical sensing at microscopic scale using spherical whispering-gallery mode (WGM) resonators. Among optical biosensors, evanescent wave-based sensors are the most outstanding sensor platforms. These transducers exploit as the sensing probe the light traveling near their surface with a penetration depth that ranges from tens to a few hundreds of nanometers. The requirements for a biosensor are selectivity, sensitivity, stability, and reversibility. Of critical importance for producing effective biosensors is the surface functionalization or the chemical modification of the transducer’s surface. Active microspheres were first proposed as sensor transducers with enhanced sensitivity by L. Yang et al. Active WGM resonators allow remote excitation and collection of the WGM resonances, avoiding coupling systems such as prisms and taper fibers. They also facilitate the use of smaller resonators, increasing the refractive index sensitivity. Highly multiplexed sensors were realized using fluorescent labeled microspheres.
