Whispering Gallery Mode Resonators as Biosensors

The previous chapter introduces biosensor technologies according to the physical nature of their signal transduction processes. Optical biosensors are just one category that includes a wide array of novel devices. The present work deals with a technology based on whispering gallery mode (WGM) optical resonators [60, 61]. Understanding how they are fabricated and used is important for appreciating their performance and limitations. This chapter will, therefore, introduce this class of resonator sensors and describe how these devices can be used for speci c detection of biomolecules in solution. WGM optical resonators exhibit tunability and narrow resonance linewidths, and attain extraordinary optical eld intensities that originally led to their use in telecommunications as add-drop lters [62, 63], notch lters [64, 65, 66], and lasers [67]. They have since evolved into valuable tools for probing nonlinear optical phenomena [68, 69] and quantum eletrodynamical principles [70]. We are concerned here with the application of WGM optical resonators as chemical sensors, a development that is less than a decade old [5, 6]. In that short time, however, researchers have demonstrated a wide variety of sensing applications. These include, but are not limited to, biochemical assays [71], biomedical assays [72, 73, 74], and molecular biology studies [1, 33]. Their extreme sensitivity (see Chapter 5) in these arenas has generated a great deal of interest in using them to develop analytical and diagnostic instrumentation.