pH-dependent intercalation of gold nanoparticles into a synthetic fluoromica modified with poly(allylamine)

The intercalation of poly(allylamine) (PAA) into a synthetic fluoromica (Na-TSM) was studied in order to understand the subsquent process of gold nanoparticle intercalation into the interlayer galleries. The composition and structure of the PAA intercalation compound (PAA/Na-TSM) was strongly influenced by the pH of its synthesis. At pH 3.0, the fully protonated form of the polymer was intercalated as a single layer (d = 1.42-1.46 nm), accompanied by 0.2-0.4 chloride ions per PAA monomer unit. The role of chloride appears to be to screen the electrostatic repulsion of polycations within the gallery. At pH 12.0, a partially protonated form of PAA intercalates as an expanded layer (d = 1.50-1.61 nm) with no chloride ions. From equilibrium PAA adsorption data, the standard free energy for PAA intercalation at pH 12.0 was -35 kJ/mol. At pH 3.0 and low PAA loading, the corresponding ΔG^o is -64 kJ/mol, consistent with the stronger interaction of protonated PAA chains with the anionic clay sheets. Au nanoparticles (<10 nm diameter) intercalated into PAA/Na-TSM composites made at pH 12.0, but not those made at pH 3.0. The most effective pH for intercalation of well-dispersed Au nanoparticles was 11.0. ζ potential measurements show that, at pH 11.0, the PAA/Na-TSM and Au colloids have nearly neutral and negative surface charges, respectively. From this result, it was concluded that electrostatic interactions as well as covalent bonding between free amine groups and intercalated Au nanoparticles are important for optimal intercalation. A red shoulder extending from 570 to 730 nm on the plasmon resonance absorption of Au/PAA/Na-TSM was observed and was attributed to close interactions between nanoparticles in adjacent galleries.