The Importance of Excess Poly(N-isopropylacrylamide) for the Aggregation of Poly(N-isopropylacrylamide)-Coated Gold Nanoparticles

Thermoresponsive materials are generating significant interest on account of the sharp and tunable temperature deswelling transition of the polymer chain. Such materials have shown promise in drug delivery devices, sensing systems, and self-assembly. Incorporation of nanoparticles (NPs), typically through covalent attachment of the polymer chains to the NP surface, can add additional functionality and tunability to such hybrid materials. The versatility of these thermoresponsive polymer/nanoparticle materials has been shown previously; however, significant and important differences exist in the published literature between virtually identical materials. Here we use poly(N-isopropylacrylamide) (PNIPAm)-AuNPs as a model system to understand the aggregation behavior of thermoresponsive polymer-coated nanoparticles in pure water, made by either grafting-to or grafting-from methods. We show that, contrary to popular belief, the aggregation of PNIPAm-coated AuNPs, and likely other such materials, relies on the ...