Optical studies of vaporization and stability of fluorescently labelled perfluorocarbon droplets

Droplets of liquid perfluorocarbon (PFC) are under study as the next generation of contrast agents for ultrasound (US). These droplets can be selectively vaporized into echogenic gas bubbles in situ by externally applied US, with numerous applications to diagnosis and therapy. However, little is known about the mechanisms of droplet vaporization and the stability of the bubbles so produced. Here we observe optically the vaporization of fluorescent PFC droplets and the stability of the newly created bubbles. Fluorescent markers were used to label selectively either the liquid PFC core or the shell of the droplets. It was found that, following vaporization, the fluorescent marker is quickly expelled from the core of the newly created bubble and is retained on the gas-liquid interface. At the same time, it was shown that bubbles retain the original shells encapsulating their droplet precursors. The efficiency of encapsulation was found to depend strongly on the nature of the stabilizing material itself. These results provide direct evidence of droplet encapsulation post-vaporization, and suggest that the behaviour of the vaporized droplets is strongly dependent on the choice of the stabilizing material for the emulsion.