Templating Core-Shell Particles Using Metal Ion-Chelating Biosurfactants

Designer biosurfactants can be used to stabilise and functionalise interfaces. One particularly promising use is the stabilisation of oil-in-water emulsions, enabling fine tuning physical, chemical and biological surface properties. The ability of emulsion systems to carry high payloads makes them attractive for applications in medicine, food and fragrances, and cosmetics. However, they have limited long-term stability. Here we sought to use the metal ion-chelating ability of the biosurfactant peptide, AM1, to precipitate the formation of a gold metal shell on AM1-stabilised emulsions by electroless plating. We found that replacing the commonly used zinc(II) with palladium(II) for coordination by histidine residues of adjacent AM1 peptides produced interfacial films that maintained elasticity at acidic pH. Proton NMR suggested a coordination mechanism independent of the imidazole ring of the histidines. Nevertheless. stabilisation of emulsions at low pH enabled the deposition of a gold shell, albeit by an unexpected mechanism. We propose that gold nanoparticles forming in bulk are adsorbed onto the peptide-stabilised interface, accumulating into a particulate coating. The resulting one-step method for nanoparticle precipitation and shell formation will be useful for the creation of biocompatible core-shell particles for applications where large payloads of hydrophobic active compounds require stability over long time periods.