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

Area of Science:

• Physical Chemistry
• Colloid Science
• Spectroscopy

Background:

• Fluorescence Correlation Spectroscopy (FCS) is a powerful technique for studying molecular dynamics.
• Existing FCS models are insufficient for analyzing repulsive systems like charged colloids.
• Understanding interactions in charged colloidal systems is crucial for various applications.

Purpose of the Study:

• To expand the theoretical framework of FCS for repulsive systems.
• To differentiate and analyze self- and cross-correlation functions in these systems.
• To validate the new theoretical approach with simulations and experiments.

Main Methods:

• Theoretical development of collective correlation functions for repulsive systems.
• Decomposition of the collective correlation function into self- and cross-correlation components.
• Dual-color fluorescence cross-correlation spectroscopy for direct measurement.
• Brownian dynamics simulations and preliminary FCS experiments for validation.

Main Results:

• The collective correlation function for repulsive systems requires a model beyond non-interacting systems.
• Self-correlation functions reveal particle diffusion and concentration.
• Cross-correlation functions provide insights into inter-particle interactions and correlation volumes.
• Debye length and surface charge number of charged colloids can be determined from cross-correlation functions.

Conclusions:

• The expanded FCS theory accurately describes repulsive systems.
• Dual-color FCS effectively separates and quantifies self- and cross-correlations.
• This approach offers a robust method for characterizing charged colloidal systems.