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

•Two different gold nanoparticle types (hydrophilic and hydrophobic) are impregnated into thermoresponsive polymer brushes.•The spatial structure of the polymer brush/nanoparticle hybrids is investigated using neutron reflectivity.•Hydrophilic nanoparticles penetrate the brush matrix, hydrophobic nanoparticles remain at the brush surface.•The assembly of nanoparticles determines the thermally induced color change of the polymer brush/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.

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