A closed form for fluorescence correlation spectroscopy experiments in submicrometer structures.

Area of Science:

• Physical Chemistry
• Spectroscopy
• Materials Science

Background:

• Fluorescence correlation spectroscopy (FCS) measures molecular diffusion but is sensitive to boundary effects in confined volumes.
• Standard FCS models assume open detection volumes, leading to inaccuracies in microchannels or similar compartments.
• Microconfinement significantly alters diffusion dynamics, necessitating advanced analytical approaches.

Purpose of the Study:

• To develop a closed-form solution for FCS that explicitly accounts for boundary effects in microconfined spaces.
• To establish the limits of applicability for standard FCS autocorrelation function (ACF) models under confinement.
• To enable accurate measurement of diffusion coefficients and probe mobility in microstructured environments.

Main Methods:

• Derivation of a one-dimensional diffusion-constrained model for FCS.
• Generalization of the model to two- and three-dimensional constrained cases.
• Analysis of FCS experimental data using the proposed model to validate its predictions.

Main Results:

• The proposed model accurately describes the ACF of fluorescent probes confined along one direction, assuming elastic wall interactions.
• Reliable prediction of ACF shapes in micro- and submicrometric channels, enabling measurement of lateral confinement.
• Identification of conditions where confinement mimics time-dependent probe mobility, crucial for interpreting transport in submicrometric compartments.

Conclusions:

• The developed FCS model provides accurate diffusion measurements in microconfined geometries by incorporating boundary effects.
• The model is applicable to studying microstructured materials like cages and cavities.
• It offers a framework for correctly interpreting probe transport and mobility in submicrometric environments.