Fluorescence correlation spectroscopy of repulsive systems: theory, simulation, and experiment.

The theoretical basis of fluorescence correlation spectroscopy (FCS) for repulsive systems, such as charged colloids or macromolecules, has been further expanded and developed. It is established that the collective correlation function can no longer be fitted using the theoretical model of non-interacting systems. Also, it is discovered that the collective correlation function can be divided into two parts: a self-part and a distinct-part, named as the self-correlation and cross-correlation function, respectively. The former indicates the self-diffusion of objects, while the latter describes mutual interactions. Dual-color fluorescence cross-correlation spectroscopy provides the direct measurements of the two parts. The particle concentration and mean squared displacement of single particles can be deduced from the self-correlation function, while the correlation volume between particles can be approximated from the cross-correlation function. In the case of charged colloids, the Debye length of the solution and particle surface charge number can be fitted from the cross-correlation function. These theoretical results are successfully proven using Brownian dynamics simulations and preliminary FCS experiments for model charged colloidal systems.