Gold nanoparticle distribution in polyelectrolyte brushes loaded at different pH conditions

Composites made of polymer brushes with inclusions of gold nanoparticles (AuNPs) combine the responsive nature of polymer brushes with the optical properties of the AuNPs, which offers the possibility to be used as colorimetric sensors. To this end, it is crucial to know how AuNPs are distributed inside the brush. Here, this distribution was elucidated by neutron reflectometry with contrast variation and a self-consistent reflectivity analysis based on the analytical parameterization of the volume fraction profiles of all chemical components. In contrast to former studies, this analysis allows the determination of the spatial distribution of components separately from each other: polyelectrolyte, AuNP, and water. Cationic poly-[2-(Methacryloyloxy) ethyl] trimethylammonium chloride (PMETAC) brushes were loaded with 5 nm AuNPs, which were coated with a pH-sensitive capping. The pH was varied during the incubation of the brush in the AuNP suspension. At a lower pH, AuNPs form aggregates in suspension and are attached to the brush periphery. They adsorb into the brush but do not fully penetrate it due to their bulkiness. At a higher pH, AuNP suspensions are electrostatically stabilized and the AuNPs penetrate the brush entirely. However, the AuNP distribution over the brush is not homogeneous but decreases gradually toward the substrate. Penetration of the AuNPs leads to a more extended conformation of the brush. According to the results of the detailed analysis of all components, an increase in water content could be excluded as a reason for brush swelling but replacement of water by the AuNP was observed.