Polyelectrolyte multilayer nanoreactors for preparing silver nanoparticle composites: Controlling metal concentration and nanoparticle size

The size of in situ prepared silver nanoparticles and the overall metal concentration within poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) multilayer films are systematically controlled through multilayer processing conditions. Carboxylic acid groups in the PAH/PAA-based multilayers bind silver cations by ion exchange with the acid protons. Subsequent reduction forms metallic nanoparticles. Because PAA has a pH-dependent degree of ionization, the multilayer film can be fabricated with different concentrations of free acid groups that are available to bind silver cations depending on the multilayer assembly pH. We show that nanoparticle size and silver concentration, examined through a combination of UV-visible spectroscopy, transmission electron microscopy, and elemental analysis, can be controllably increased by reducing the assembly pH of PAH/PAA-based multilayers. Nanoparticles with diameters of 2 to 4 nm at volume fractions of 4 to 8% are obtained for multilayers assembled from PAA and PAH solution pHs of 4.5 to 2.5. Furthermore, since the metal-binding carboxylic acid groups are reprotonated upon nanoparticle formation, the synthesis methodology can be repeatedly cycled to incorporate more silver cations. Up to five cycles of silver cation exchange and reduction have been accomplished to produce nanoparticles with average diameters up to 9 nm at a volume fraction of 24%.