Microrheology with fluorescence correlation spectroscopy.

We show that fluorescence correlation spectroscopy (FCS) using a commercial spectrometer can be applied to passive microrheological (MR) experiments. The method probes the local rheological properties of materials on length scales of the focus dimension of the confocal microscope. For a feasibility study, we performed measurements on a high molecular weight poly(ethylene oxide)-water solution to allow direct comparison of the results to previous studies using diffusing wave spectroscopy, quasielastic light scattering, and particle tracking methods. We were able to detect mean-square center-of-mass displacements ranging from somewhat better than [SYMBOL: SEE TEXT]2(t) approximately 100 nm2 up to above Deltar2(t) approximately 10(6) nm2. Thus, we were able to derive the bulk rheological shear moduli covering more than five decades in frequency (from omega<or=10(-1) rad/s to omega approximately 10(4) rad/s). The MR results are compared to results obtained from conventional rheological experiments on the same samples using a rotational rheometer as well as a piezo-driven squeeze flow apparatus. Good agreement between MR results probing the local rheological properties and those obtained by the conventional methods measuring the macroscopic mechanical response is found in the whole frequency range. Spatial resolution in combination with the possibility of using small tracer beads open the opportunity to probe the local, length scale-dependent rheological properties in heterogeneous samples. Small tracer concentrations and small sample sizes make FCS spectroscopy a powerful tool in particular for biological and medical applications.