Abstract
Optical resonators play an ubiquitous role in modern optics. A particular class of optical resonators is constituted by spherical dielectric structures, where optical rays are total internal reflected. Due to minimal reflection losses and to potentially very low material absorption, these guided modes, known as whispering gallery modes, can confer the resonator an exceptionally high quality factor Q, leading to high energy density, narrow resonant-wavelength lines and a lengthy cavity ringdown. These attractive characteristics make these miniaturized optical resonators especially suited as laser cavities and resonant filters, but also as very sensitive sensors. First, a brief analysis is presented of the characteristics of microspherical resonators, of their fabrication methods, and of the light coupling techniques. Then, we attempt to overview some of the recent advances in the development of microspherical biosensors, underlining a number of important applications in the biomedical field.
Highlights
The amount of publications appeared in the last decade demonstrate the expanding utility of optical biosensors in traditional fields of life science research, drug discovery and medical applications
Several laboratory experiments have demonstrated the large potential of the WGM resonator (WGMR) based sensors in the biomedical field
The combination of WGMR and SERS led to the feasibility proof of spectroscopic detection of the fingerprint of the bound analyte
Summary
The amount of publications appeared in the last decade demonstrate the expanding utility of optical biosensors in traditional fields of life science research, drug discovery and medical applications. Evanescent wave based sensors, are one of the most outstanding optical sensor platforms These devices exploit the sensing light near their surface with a penetration depth that ranges from tens to a few hundreds of nanometers; they are capable of detecting changes induced by the binding of analytes within this length. High quality factor Q and long recirculation of light in compact WGM devices are the most important features for sensing applications, where the change in Q or resonant wavelength can be used for measuring the change of parameters in the surrounding environment or binding phenomena on the WGM resonator surface It has to be noted, that not all the published papers referring to optical sensing in biomedical field are describing real biosensors. Other health threats as food-borne pathogens require high sensitivity and specificity but are quite challenging due to larger dimensions of bacteria and more complex requirements in their detection [20,25]
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