Tunable, Controlled and Reversible Assemblies of poly(ethylene oxide-st-propylene oxide) grafted Gold Nanoparticles

In recent years, assemblies of functionalised gold nanoparticles (AuNPs) have received a growing interest as a route toward hybrid materials with new functionalities. Here, we report on thermally reversible assemblies formed by mixing thermosensitive polymerprotected AuNPs and Pluronic triblock copolymers. Amino-terminated copolymers, poly(EO-st-PO), with different EO/PO ratios were coupled by thioctic acid, which allows an excellent affinity with gold surface. Then, monodisperse and thermoresponsive AuNPs, with a size varying from 2 to 17 nm, were prepared either by ligands exchange on precursor AuNPs or by direct reduction of gold salt in presence of stabilizing copolymers. The aggregation of the polymer-protected AuNPs can be tune from 8 to 15°C only by changing the AuNPs curvature from 17 to 2 nm. This temperature change versus the nanoparticle curvature is ascribed to the decrease in hydration volume per hydrophilic group in the corona, due to the change of the polymer chain conformation. Moreover, one unique aggregation temperature between 12 and 60°C can be also obtained by mixing copolymers with different hydrophilicity (Fig. 1). Then, the corona behaves as a corona composed by an average statistic copolymer with the intermediate composition. Stable hybrid colloids of defined size were prepared by mixing PEO-PPO-PEO triblock copolymers (Pluronic®) and thermoresponsive AuNPs. Indeed, copolymers allow to control the AuNPs aggregation. Moreover, the Pluronic to AuNPs ratio determines the size of the as-formed aggregates (Fig. 2). The hybrid assemblies are reversible and the aggregates remain in colloidal state for several days.