Spontaneous Formation of Vesicles by Self-Assembly of Cationic Block Copolymer in the Presence of Anionic Surfactants and Their Application in Formation of Polymer Embedded Gold Nanoparticles

Abstract

Block copolymers (BCPs), synthesized from cationic monomer (3-(methacryloylamino)propyl)trimethylammonium chloride (MAPTAC) and PEG-based monomer poly(ethylene glycol) methyl ether acrylate (PEGMEA), were found to spontaneously form water-soluble vesicles when mixed with a stoichiometric quantity of negatively charged double-tail surfactant, AOT, at room temperature. However, with single-tail anionic surfactant, i.e., SDS, these block copolymers were seen to form water-soluble micelle-like aggregates. Also, cationic random copolymers (RCPs) of similar composition synthesized from the same monomers showed formation of water-soluble micelle-like aggregates when complexed with SDS or AOT. Such self-assembled vesicle formation observed specifically for the BCP/AOT systems was attributed to a sequential arrangement of monomers in the BCP and the higher hydrophobic volume of AOT, that made the packing factor "p" assume a value that favors the formation of vesicles. TEM analysis showed that average diameters of the vesicles were in the range of ∼100 nm. Pyrene fluorescence experiments indicated a high degree of hydrophobicity of vesicle membrane made from BCP/AOT complexes which were even higher than that of the cores of the water-soluble micelle-like aggregates made from BCP/SDS, RCP/AOT, and RCP/SDS systems. Importantly, the vesicles made from these BCP/AOT stoichiometric complexes were successfully utilized to reduce HAuCl4 to gold nanoparticles. TEM analysis revealed that the gold nanoparticles so formed were successively embedded within the hydrophobic bilayer shell of the vesicles.