Polyelectrolyte Multilayer Films as Templates for the In Situ Photochemical Synthesis of Silver Nanoparticles

Multilayer films formed by the sequential adsorption of polyelectrolytes (PEs) from aqueous solution were used as templates for the in situ growth of silver nanoparticles. When films consisting of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) or PAA and poly(ethyleneimine) (PEI) are immersed in AgNO3 solution, functional groups within the film bind silver cations from solution. Bound Ag ions were photochemically reduced to metal nanoparticles upon exposure to UV radiation. The number of available binding groups and binding capacity of multilayer films were shown to be strongly dependent on multilayer processing conditions, including chemical structure of PEs, multilayer assembly pH, as well as the AgNO3 solution concentration and time interval used for Ag+ binding. These parameters were studied using a combinatorial approach to multilayer analysis, which provides a means to investigate in parallel the variables that affect particle synthesis. By using UV-vis spectroscopy and scanning transmission electron microscopy (STEM), the optimal conditions were determined to generate composite films containing small (3-5 nm) isolated spherical nanoparticles that are distributed randomly throughout the PE matrix. A striking discovery was the sensitivity of Ag+ binding to the structure of deposited PEs. Films consisting of branched PEI and PAA assembled at specific pH values have displayed a remarkably high affinity for metal cations relative to PAH/PAA films assembled at the same conditions.