Imaging transmitter release. II. A practical guide to evanescent-wave imaging.

Total internal reflection of a laser beam at an interface between two media of different refractive index sets up an evanescent wave field in the medium with lower refractive index. This near field decays over a distance of approximately lambda/5, lambda denoting the wavelength of light, and thus provides a convenient means for the confinement of fluorescence excitation to the near-interface region. Evanescent-wave excitation thereby permits, for example, the observation of individual fluorophores at the surface despite the presence of high concentrations in bulk solution. Although evanescent-wave excitation of fluorescence and the related technique of surface-plasmon resonance have a long record in the study of chemical reactions at surfaces, adsorption kinetics or spectroscopy, their potential for biomedical studies is only gradually emerging. Evanescent-wave microscopy provides high-contrast images of the near-membrane region of cells grown on a glass substrate at unprecedented resolution. At present, no commercial equipment is available for evanescent-wave microscopy. This review aims at readers who want to modify their fluorescence microscope to include an evanescent-wave illumination mode. Starting from the point that every objective exceeding a certain numerical aperture is generating evanescent waves, we demonstrate how the optical path can be modified to suppress the far-field excitation, and how one can switch easily between these types of illumination. The phenomena resulting from interactions of evanescent waves with cells are reviewed. The ways in which systematic variations of the angle of incident light can be used to obtain quantitative information on fluorophore distance, distribution and concentration are also discussed.