From Micro to Nanorheology: Basic Principles of Microrheology Measurements with Fluorescence Correlation Spectroscopy

Fluorescence correlation spectroscopy (FCS) represents one of the most sensitive fluorescence techniques due to single molecule detection. Microrheology serves as a tool to study viscoelastic properties of microenvironment of observed particles. We used 100 nm latex particles to study mean square displacement (MSD) of these particles in Newtonian fluids. The comparison of MSD determined by FCS compares well with those determined by dynamic light scattering method (DLS), classical microrheology (MR) or calculated theoretical MSD. FCS can be used to determine MSD in greater extent of time than both techniques and we can determine MSD in time range where MSD cannot be determined by MR or DLS. When pure diffusion model was used, MSD as a function of time exhibited curvature from theoretical curves. The fitting procedure of FCS autocorrelation function required triplet state model.