Effect of the cross-linking density on the gold core oxidation in hybrid core@ shell Au@pNIPAM and Janus Au@p4VP systems.

Abstract

Here we report a detailed study that concerns the oxidation of spherical gold nanoparticles (AuNPs) encapsulated into thermo- and pH-responsive microgels. Initially, two types of hybrid systems structured as 50 nm spherical gold nanoparticles coated with a thermo-responsive (N-Isopropylacrylamide) or a pH-responsive (4-vinylpyridine) microgel are synthesized. Then, a gradual gold elimination is achieved introducing each nanohybrid system into a mixture of cetyltrimethylammonium bromide (CTAB) and gold chloride (HAuCl4), finally obtaining hollow colloidal structures with potential application in controlled drug release. The synthesis of Au@pNIPAM and Au@p4VP systems is performed at different crosslinking densities (5, 10 and 15%), and the decrease in the gold plasmon absorption band with the oxidation time is monitored at the two possible microgel states (swollen and collapsed). Two parameters are determined, the Au core etching rate and the molecule diffusion coefficient (AuCl4/CTAB complex). We also present in our study the incorporation of methyl methacrylate (MMA) as co-monomer during the p4VP polymerization to avoid the “Janus” structure in Au@p4VP partiles. This investigation offers the possibility to fabricate thermo- and pH-responsive hollow microgels that can be used for biomedical applications for controlled drug delivery purposes upon change in temperature or pH.