Fluorescence correlation spectroscopy as a tool to investigate chemical reactions in solutions and on cell surfaces.

Taking advantage of the present day possibilities for ultrasensitive detection by fluorescence, fluorescence correlation spectroscopy (FCS) has over the last ten years emerged as a potentially very powerful technique. In this article we present some results to illustrate the use of FCS for monitoring chemical kinetics on a molecular level and show how, for a wide range of chemical processes, the theoretical treatment can be strongly simplified. The experimental examples given include measurements of ion concentrations and buffer properties, electron transfer reactions, ligand-receptor interactions and diffusion of ligand-receptor complexes in cell membranes. For each of these examples the properties of the FCS technique is discussed in relation to other established techniques used for that particular application. From these examples it is found that FCS can offer important complementary information and, due to the extreme sensitivity of the technique, new information not yet explored by other methods.