A one-pot approach using recyclable template to prepare dual-responsive yolk–shell or Janus-like nanoparticles

Herein, we report a one-pot approach using polymeric micelles as the recyclable template to prepare hybrid yolk–shell or Janus-like nanoparticles (NPs). The micelles were formed by a block copolymer PEG-b-P4VP (EV) in a methanol/water (1/9, v/v) mixture with PEG as the shell and P4VP as the core. In the mixture of the micelles with AIBN (azobiisobutyronitrile), HAuCl4, NIPAM (N-isopropylacrylamide) and MBA (N,N′-methylenebisacrylamide), hybrid NPs could be obtained by simply heating the mixture at 76 °C. At this temperature, AIBN that was largely localized within the hydrophobic P4VP core decomposed into radicals, which reduced HAuCl4 to form Au NPs within the core and initiated the copolymerization of NIPAM and MBA at the core–shell interface to form a poly (NIPAM-co-MBA) network (PNM) to cover the core. The micelle template was conveniently removed from the hybrid NPs by decreasing the pH value of the medium to 3.0. At a low pH, the P4VP block chains were fully protonated; thus, EV becomes highly hydrophilic and soluble in the medium. Moreover, the micelle template dissociated and then gradually diffused into the bulk solution phase as individual EV chains. After the removal of the micelle template, Au@void@PNM yolk–shell NPs were formed. However, when the EV micelles (with the core being slightly noncovalently crosslinked) were used as the template, the same process resulted in the Janus-like NPs; moreover, the micelles were recyclable. After the yolk–shell or Janus NPs were separated by centrifugation, the micelles were recovered in the supernatant and then used again as a template for additional cycles of the yolk–shell or Janus-like NPs preparation and the same results were obtained. Both the yolk–shell and the Janus NPs were easily redispersible in water, and they were confirmed to have both thermal sensitivity and photo sensitivity. We believe that the facile and cost-effective preparation of the yolk–shell NPs (with dual sensitivities) and the Janus NPs (with a relatively high structural regularity) by localizing the chemical reactions within different nanodomains of the micelles should generate considerable interest in the fields of polymer chemistry and soft materials.