Core-shell reversion of hybrid polymeric micelles containing gold nanoparticles in the core

The micellization of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) in chloroform can be induced by the interaction between P4VP blocks and HAuCl4, forming micelles with PS as the shell and the P4VP/HAuCl4 complex as the core. Subsequent reduction of HAuCl4 leads to hybrid polymeric micelles (HPMs) containing gold nanoparticles (GNs) in the core. In this case, protonation of P4VP block by hydrochloride acid is necessary for the stabilization of HPMs. Further study demonstrates that switching the solvent gradually and continuously from chloroform to a methanol/chloroform (9/1, v/v) mixture, where protonated P4VP is soluble whereas PS is insoluble, leads to a core-shell reversion of the HPMs, forming vesicle-like aggregates with PS as the wall and protonated P4VP/GNs as the shell. The interaction between GNs and protonated pyridine repeating units is a necessary driving force for stable attachment of GNs to shell. When the P4VP block is deprotonated, the GNs will be released from the shell to form precipitates, whereas the vesicles are still stable in the solution. It is also found that during the core-shell reversion the aggregation of PS chains occurs at first among the shell of the HPMs, and then the dissociation of the core happens due to the continuous addition of methanol into the solution, leading to the core-shell reversion. This indicates that there is no intermediate state of the molecularly dispersed block copolymer existing between the HPMs and the RHPMs.