Chapter 9 - Cavity ring-down biosensing

Abstract

Cavity ring-down spectroscopy (CRDS) is an established technique for gas sensing that is newly emerging in the field of optical biosensing. This approach measures changes in the rate of decay of light injected into an optical resonator and relates the change to optical loss along the length of the resonator. This principle has recently been adapted for use in liquids, providing a highly sensitive method for quantitative biosensing applications. Cavity ring-down spectroscopy, which can incorporate evanescent field sensing elements, has been demonstrated for the detection of near-infrared absorption in liquids and the scattering response of adherent cells. The relatively recent adaptation of CRDS to biosensing applications leaves significant room for development, both for improved selectivity and for expanded sensitivity. Because CRDS biosensing devices rely on evanescent wave techniques, advanced technology developed for traditional evanescent sensors can be readily adapted for CRDS instrumentation. Such advances in sample concentration and separation can be incorporated with CRDS sensors, providing new approaches for highly sensitive biosensors. Highly specific surface coatings can be combined with advances in waveguide materials and laser emitters to dramatically improve CRDS biosensing. The development of new fiber materials is likely to permit single-mode transmission at shorter visible wavelengths, where optical scattering is enhanced and electronic transitions in biological samples dominate, and at longer infrared wavelengths, where vibrational transitions are strong. Such approaches are complemented by broadband white light-scanning CRDS, bringing the high sensitivity of CRDS to the diverse range of biological sensing problems.