Tunable and reversible aggregation of poly(ethylene oxide-st-propylene oxide) grafted gold nanoparticles

Two amino-terminated amphiphilic copolymers, M600 and M1000, with different ethylene oxide to propylene oxide EO/PO ratios, 1/9 and 19/3, respectively, were coupled by thioctic acid, which allows an excellent affinity with gold surface. Then, amphiphilic thermally responsive gold nanoparticles (AuNPs) were prepared either by ligands exchange on precursor gold nanoparticles or by direct reduction of gold source in presence of stabilizing copolymers. The as-obtained AuNPs are monodisperse with a size varying from 2 to 17 nm depending on the synthesis used. The main parameters controlling the AuNPs assemblies were identified: the ethylene oxide to propylene oxide ratio in the polymer corona, the ionic strength of the solution, and the curvature of AuNPs. An interesting result is the possibility to tune the aggregation temperature from 8 to 15 degrees C of AuNPs coated by the same polymer only by changing the curvature of the AuNPs from 17 to 2 nm. This temperature change versus the curvature of the nanoparticle is ascribed to the decrease in hydration volume per hydrophilic group in the corona due to the change of the polymer chain conformation with changing the particle size. Moreover, one unique aggregation temperature between 12 and 60 degrees C can be also obtained by mixing copolymers with different EO/PO ratios. Then, the corona, constituted by a mixture of polymers, behaves as a corona composed by an average statistic copolymer with the intermediate composition.