Microrheology with fluorescence correlation spectroscopy

Area of Science:

• Materials Science
• Physical Chemistry
• Biophysics

Background:

• Conventional rheological methods often require large sample volumes and may not capture local variations.
• Passive microrheology (MR) offers a way to probe material properties on smaller length scales.
• Fluorescence correlation spectroscopy (FCS) is a sensitive technique for studying molecular dynamics.

Purpose of the Study:

• To demonstrate the application of fluorescence correlation spectroscopy (FCS) for passive microrheological (MR) measurements.
• To validate FCS-based MR by comparing results with conventional rheology techniques.
• To explore the potential of FCS-MR for probing length-scale-dependent rheological properties in heterogeneous materials.

Main Methods:

• Utilized a commercial spectrometer for fluorescence correlation spectroscopy (FCS) measurements.
• Performed experiments on a high molecular weight poly(ethylene oxide)-water solution.
• Compared FCS-derived mean-square displacements and shear moduli with data from diffusing wave spectroscopy, quasielastic light scattering, particle tracking, rotational rheometry, and squeeze flow apparatus.

Main Results:

• Successfully detected mean-square displacements from 100 nm² to 10⁶ nm².
• Derived bulk rheological shear moduli spanning over five decades of frequency (10⁻¹ to 10⁴ rad/s).
• Achieved good agreement between FCS-MR results and conventional rheological measurements across the entire frequency range.

Conclusions:

• Fluorescence correlation spectroscopy (FCS) is a viable and powerful tool for passive microrheology (MR).
• FCS-MR provides accurate local rheological property measurements that correlate well with macroscopic behavior.
• The spatial resolution and sensitivity of FCS-MR make it particularly suitable for heterogeneous samples, including biological and medical applications.