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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