**2.8 Fluorescence correlation spectroscopy (FCS)**

FCS is a confocal fluorescence where parameters of prime importance are the fluctuation of the fluorescence intensity, occurring from noise as well as biological, chemical, and physical effects on the fluorophore. Changes in chemical and physical properties, like equilibria, reactions, complexation, quenching, and like molecular motion, photophysical interactions, changes in conformation affects the overall emission [15].
