An original technique, Spectral Self-Interference Fluorescence Microscopy (SSFM), can determine the location of fluorescent markers above a reflecting surface with sub-nanometer precision. SSFM was used to resolve the position of a fluorescent marker bound to either the top or the bottom leaflet of a lipid bilayer -- the difference in distance is only 4 nm. SSFM is a valuable tool in studying the conformation of DNA molecules immobilized on surfaces. A fluorescent label attached to a DNA molecule tethered to the surface can help elucidate its spatial orientation. This method is based on the fact that spontaneous emission of fluorophores located near a mirror is modified by the interference between direct and reflected waves, which leads to an oscillatory pattern in the emission spectrum. Spectral patterns of emission near surfaces can be precisely described with a classical model that considers the relative intensity and polarization state of direct and reflected waves depending on dipole orientation. An algorithm based on the emission model and polynomial fitting built into a software application can be used for fast and efficient analysis of self-interference spectra yielding information about the location of the emitters with very high precision.
We present a new method of fluorescence imaging, which yields nm-scale axial height determination and ~15 nm axial resolution. The method uses the unique spectral signature of the fluorescent emission intensity well above a reflecting surface to determine vertical position unambiguously. We have demonstrated axial height determination with nm sensitivity by resolving the height difference of fluorescein directly on the surface or on top of streptavidin. While different positions of fluorophores of different color are determined independently with nm precision, resolving the position of two fluorophores of the same color is a more convoluted problem due to the finite spectral emission widow of the fluorophores. Hence, for physically close (<λ/2) fluorophores, it is necessary to collect multiple spectra by independently scanning an excitation standing wave in order to deconvolute the contribution to the spectral pattern from different heights. Moving the excitation standing wave successively enhances or suppresses excitation from different parts of the height distribution, changing the spectral content. This way two fluorophores of the same color can be resolved to better than 20 nm. Design aspects of the dielectric stack for independent excitation wave scanning and limits of deconvolution for an arbitrary height distribution will be discussed.