Fluorescence correlation spectroscopy on dielectric surfaces in total internal reflection geometries

Fluorescence correlation spectroscopy (FCS) has evolved to a valuable tool for biomolecular analysis on the single molecule level. Measurements on a single molecule level can only be performed if the measurement volume is small enough to contain on average only very few molecules. Common FCS-systems are therefore based on a confocal geometry in which a laser spot is focused into a liquid sample. This illumination concept in combination with a pinhole in the detection path leads to an observation volume in the order of one femtoliter. On the other hand, many biomolecular interactions need to be measured on surfaces. To study such interactions or the fluctuating signal of surface bound molecules itself, as for instance during single molecule enzyme catalysis, evanescent field based excitation seems advantageous as compared to confocal FCS. We discuss different schemes for evanescent field FCS and present an efficient excitation-detection scheme in an objective-based TIR-FCS configuration.