# Structure and Stability of Phospholipid-Based Microbubbles Studied Using a Spin Probe

## TL;DR

This study uses EPR spectroscopy to examine the structure and stability of phospholipid-based microbubbles over time.

## Contribution

The study introduces a novel method using a spin probe and EPR to monitor microbubble degradation in real-time.

## Key findings

- The spin probe showed slow motion and ordering in the phospholipid monolayer at room temperature.
- Over 12 hours, microbubbles degraded, forming a liquid phase with the spin probe incorporated into liposomes or micelles.
- Degradation was attributed to gas exchange and interactions with capillary walls.

## Abstract

Steady-state electron paramagnetic resonance (EPR) spectroscopy
is used to investigate the structure and stability of surfactant microbubbles
made from distearoylphosphatidylcholine (DSPC) phospholipids and a
polymeric stabilizer. The spin probe doxyl-5-stearic acid (5DSA) was
incorporated into the phospholipid monolayer at an overall concentration
of 3 × 10–7 M. The bubbles were characterized
by optical microscopy and found to range in diameter from 0.6 to 10
μm. The EPR spectrum of the spin probe at room temperature exhibited
slow motion and ordering. This behavior was simulated using the microscopic
order–macroscopic disorder (MOMD) model through the EasySpin
software package. During the course of 12 h in the EPR sample tube,
a sharper fast-motion nitroxide spectrum appeared, indicating degradation
of the bubbles over time. This is attributed to the typical process
of microbubble degradation following gas exchange and interactions
between the sample and capillary walls, leading to bubble collapse
and the formation of a liquid phase with 5DSA incorporated into liposomes,
micelles, or free molecules.