Effect of gold nanoparticle hydrophobicity on thermally induced color change of PNIPAM brush/gold nanoparticle hybrids

This paper addresses the relation between gold nanoparticle (AuNP) distribution in thermoresponsive polymer brushes and the thermally induced color change of the polymer brush/AuNP hybrids. Therefore, we report on the effect of AuNP surface functionalization on the structure and optical response of the nanoparticles in thermoresponsive poly(N- isopropylacrylamide) (PNIPAM) brushes. Two different types of gold nanoparticles are attached to the PNIPAM brushes: (1) AuNPs that are coated with citrate anions (AuNP-citrate), and (2) AuNPs stabilized with 12-mercaptododecanoic acid ligands (AuNP-MDA). Neutron reflectivity (NR) measurements indicate that the spatial structure of the hybrid depends on the particle type. A strong increase in thickness is observed after attachment of AuNP-citrate; this is not the case for AuNP-MDA. The thermoinduced color change of the PNIPAM/AuNP hybrids depends strongly on the particle type. While a red-shift of the surface plasmon band occurs in case of AuNP-citrate, a blue-shift is found for AuNP-MDA when the temperature is increased above the PNIPAM's volume phase transition temperature. This blue-shift vanishes after a certain number of heating/cooling cycles, indicating a rearrangement of the AuNPs in the brush. The different behavior of AuNP-citrate as compared to AuNP-MDA can be explained by the hydrophobic nature of the 12-mercaptododecanoic acid ligands. (C) 2016 Elsevier Ltd. All rights reserved.