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

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.