Lipophilic probe behavior in microemulsions evaluated by fluorescence correlation spectroscopy

Area of Science:

• Physical Chemistry
• Colloid and Surface Science
• Biophysical Chemistry

Background:

• Microemulsions (MEs) are thermodynamically stable, isotropic liquid mixtures of oil, water, and surfactant.
• Understanding the behavior of lipophilic molecules within MEs is crucial for applications in drug delivery and formulation.
• Fluorescent probes offer a sensitive method to study molecular dynamics in complex systems.

Purpose of the Study:

• To investigate the dispersion and diffusion of fluorescently labeled vitamin E (VE) in various microemulsion types.
• To compare the behavior of two distinct VE probes with differing lipophilicity and structural characteristics.
• To demonstrate the utility of fluorescence correlation spectroscopy (FCS) for analyzing lipophilic probe dynamics in microemulsions.

Main Methods:

• Preparation of fluorescently labeled vitamin E probes: VE-ATTO (lipophilic and hydrophilic parts) and VE-BODIPY (solely lipophilic part).
• Formulation of different microemulsion types: water-in-oil (W/O), oil-in-water (O/W), and bicontinuous microemulsions (BMEs).
• Application of fluorescence correlation spectroscopy (FCS) to measure the diffusion coefficients of the VE probes in the microemulsions.

Main Results:

• VE-BODIPY exhibited significantly faster diffusion compared to VE-ATTO across all tested microemulsion systems.
• VE-ATTO, with its surfactant-like properties, appeared localized at the micro-water/micro-oil interfaces.
• VE-BODIPY showed good dispersion within both the microemulsion and bulk oil phases, leading to higher mobility.
• Solvatochromism of VE-ATTO was observed in heptane, affecting its fluorescence signal.

Conclusions:

• Fluorescence correlation spectroscopy (FCS) is a powerful and rapid technique for evaluating lipophilic probe behavior in heterogeneous microemulsion systems.
• The structural characteristics of lipophilic probes, such as the presence of hydrophilic moieties, significantly influence their dispersion and diffusion dynamics within microemulsions.
• The findings provide insights into the micro-environmental partitioning and mobility of lipophilic substances in complex colloidal systems like microemulsions.