Lateral phase separation in lipid-coated microbubbles

Mark A. Borden, Gary V. Martinez, Josette Ricker, Nelly Tsvetkova, Marjorie Longo, Robert J. Gillies, Paul A. Dayton, Katherine W. Ferrara

Research output: Contribution to journalArticlepeer-review

100 Scopus citations

Abstract

In the design of lipid-coated microbubble ultrasound contrast agents for molecular imaging and targeted drug delivery, the surface distribution of the shell species is important because it dictates such properties as ligand location, brush coverage, and amount of drug loading. We used a combination of spectroscopy and microscopy techniques to test the prevailing notion that the main phosphatidyl choline (PC) and lipopolymer species are completely miscible within the monolayer shell. NMR spectroscopy showed that the shell composition is roughly equivalent to the bulk lipid ratio. FTIR spectroscopy showed a sharp melting peak corresponding to the main phase-transition temperature of the main PC species, with no observed pretransitions while scanning from room temperature, indicating a single PC-rich ordered phase. Electron and fluorescence microscopy showed a heterogeneous microstructure with dark (ordered) domains and bright (disordered) regions. Domain formation was thermotropic and reversible. Fluorescent labeling of the lipopolymer following shell formation showed that it partitions preferentially into the disordered interdomain regions. The ordered domains, therefore, are composed primarily of PC, and the disordered interdomain regions are enriched in lipopolymer. Phase heterogeneity was observed at all lipopolymer concentrations (0.5 to 20 mol %), and the degree of phase separation increased with lipopolymer content. The composition and temperature dependence of the microstructure indicates that phase separation is driven thermodynamically rather than being a kinetically trapped relic of the shell-formation process. The overall high variation in microstructure, including the existence of anomalous three-phase coexistence, highlights the nonequilibrium (history-dependent) nature of the monolayer shell.

Original languageEnglish (US)
Pages (from-to)4291-4297
Number of pages7
JournalLangmuir
Volume22
Issue number9
DOIs
StatePublished - Apr 25 2006

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Colloid and Surface Chemistry

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