rinsing steps. The substrates then were immersed into the PAA solution for 10 min followed by identical rinsing steps. The adsorption and rinsing steps were repeated until the desired number of bilayers was obtained. To form heterostructures, multilayers of PAH/PSS were assembled on top of the previously assembled PAH/PAA; the PAH/PSS layers were assembled from solutions that additionally contained 0.1 M NaCl. Immersion times for PAH/PSS assembly were 5 min each. The PM was finally dried by a stream of air (room temperature) and stored at ambient conditions.
The Ag nanoparticle synthesis scheme is described in an earlier paper [15]. Briefly, the PM films were first immersed in a dilute aqueous Ag(ac) solution (5 mM at nominally neutral pH) for 30 h, where the acid protons of PAA carboxylic acid groups were exchanged for the silver cations. The films were then washed in water for 1 h and dried in air. The metal-containing PMs were then reduced in a H 2 atmosphere (2 atm, 85 C) for 30 h, forming zerovalent Ag nanoparticles. Additional silver was incorporated into the PM by repeating the process of ion exchange and reduction.
For cross-sectional TEM imaging, multilayer films assembled on PS substrates were cut along a direction normal to the film plane using a RMC MT-X ultramicrotome with a diamond knife (Diatome, Fort Washington, PA) at room temperature. Approximately 50 or 70 nm thick cross sections of the samples were obtained. TEM was performed on ultramicrotomed samples using a JEOL JEM-2000FX operated at 200 kV. While imaging, samples were tilted until the electron beam was oriented normal to the cross-sectional plane.
Photometric measurements were performed using a Varian Cary 500i spectrophotometer. Near-normal reflectivities (fixed 7 off normal to the plane of the film) were measured using an absolute (VW configuration) specular reflectance accessory on the spectrophotometer. Because the LbL process assembles multilayers on both sides of the substrate, the film on one side of the substrate was removed prior to characterization by wiping with a cotton swab wetted by 0.1 M HCl. Refractive indices were extracted from photometric measurements of various PAH/PAA multilayers containing Ag nanoparticles using the bivariate method [30]. Film thicknesses were extracted from cross-sectional TEM images and confirmed by profilometry (Tencor P10). The measured thicknesses were typically within 10 % between the two methods.
The films were swollen in various concentration NaCl aqueous solutions by immersion for the desired duration. Two general states of swelling were consid-eredΓthe completely wet state and the briefly dried yet hydrated state. In the wet state, the reflectance or transmittance was measured immediately without any film drying. This ensured a wetted coating of solution on the film surface during the photometric measurement. For the salt concentrations used (0.1Β± 0.5 M), the film surface was adequately hydrophilic and the evaporation of the salt solution was depressed enough to obtain a uniformly wetted coating. In the briefly dried state, the photometric measurements were taken once the film was briefly air blown dry after removal from the salt solution. The brief drying would remove any salt solution wetting the surface but leave the film in a hydrated state. The quantitative degree to which the film remained hydrated when processed in this manner was not determined.