Single molecule fluorescence techniques cross-correlation

The fluorescence fluctuations measured by FCS can be analyzed in several ways. The most common technique, autocorrelation analysis, provides information about characteristic diffusion time of fluorescent molecules through the observation volume. It also reports on the average number of molecules present in the observation volume, and thus the concentration of fluorescent moleculesn (14, 49, 56, 57). Other types of FCS analysis can be used to analyze molecular brightness and the oligomeric state of the fluorescent molecule. Finally, cross-correlation FCS monitors fluctuations jointly from molecules labeled with two or more different fluorophores. This technique provides a powerful approach to assay for intermolecular interactions, because molecules that are bound either directly or indirectly to one another should diffuse as a single unit (8, 59). 

Whereas FCS measures fluorescence fluctuations over time, a related technique, ICS, measures fluorescence fluctuations over space, in particular from images collected using a laser-scanning microscope (56). ICS analysis of pixels within a single image provides information about protein clustering and density. A variation of ICS known as image cross-correlation spectroscopy evaluates the interactions of molecules labeled with different fluorescent probes. ICS can also be performed on stacks of... 

---