Host-guest association studied by fluorescence correlation spectroscopy

Fluorescence Correlation Spectroscopy (FCS) is a powerful single molecule technique for the study of the stability and the association dynamics of supramolecular systems and, in particular, of host-guest inclusion complexes. With FCS the host-guest binding equilibrium constant is determined analysing the variation in the diffusion coefficient of the fluorescent guest or host with no need for a change in the photophysical properties of the fluorescent probe. FCS gives also access to the association/dissociation rate constants of the host-guest inclusion providing that the fluorescence intensity of host or guest changes upon complexation. These rate constants can be compared with that of a diffusion-controlled process estimated from the same FCS experiment allowing for a better understanding of the association dynamics. The results show that cyclodextrin cavities act as "hard" cages which put geometric and orientational restrictions on the inclusion of a hydrophobic guest, whereas micelles behave as "soft" cages without geometrical requirements. In our contribution to this special issue we review briefly the application of FCS to the study of host-guest inclusion complexes with an emphasis on practical aspects and relevant bibliographic references.