Fluorescence (cross)-Correlation Spectroscopy (FCS/FCCS)

Fluorescence correlation spectroscopy (FCS) is a correlation analysis of fluctuations in the fluorescence intensity. The analysis provides information on physical parameters of the fluorescent particles (molecules) in solution, such as concentration, average fluorescence intensity and diffusion speed. By following changes on these parameters it is possible to study binding events of the molecules or even conformational changes on them.

Fluorescence cross-correlation spectroscopy (FCCS) extends the FCS procedure in that it looks at the correlation between different colors (cross-correlation) rather than just the same color (auto-correlation). In other words, coincident green and red intensity fluctuations correlate if green and red labeled particles are moving together. As a result, FCCS provides a highly sensitive measurement of molecular interactions independent of diffusion rate.