were obtained from Microparticles GmbH. Deionized water (18.2 MX cm) was prepared in a Millipore Milli-Q Plus 185 purification system. Octa(3-aminopropyl)aminosilsesquioxanes (NSi8) were prepared according to a procedure previously reported to obtain a 0.47 M NSi8 solution [17]. NSi8-capped silver nanoparticles, NSi8-Ag, were produced by adding aliquots of a sodium borohydride solution (4.5 10 ยฑ2 M) to a 2.2 10 ยฑ4 M silver salt NSi8 solution under vigorous stirring until the mixture turned deep brown-yellow. The NSi8-Ag nanoparticle solution was then used without further purification. Extinction spectra were recorded with a Cary 4E UV-vis spectrophotometer in the diffuse reflectance mode. High-resolution transmission electron microscopy (HRTEM) imaging was performed using a Philips CM12 microscope operated at 120 kV and equipped with an energy dispersive X-ray analyzer.
The LbL assembly of NSi8-capped silver nanoparticles with PSS on silica or PS spheres was performed after precoating the spheres with a PAH primer layer (from a 10.7 mM PAH solution, 0.5 M NaCl, pH 6) for PS spheres or a PDDA primer layer (from a 1 wt.-% solution, 0.5 M NaCl, pH 6.5) for silica spheres. The colloids were alternately exposed to a 5.8 mM PSS solution for 10 min (pH 2, without added salt) and to a NSi8-Ag solution for 90 min. This exposure time was derived from an adsorption study of NSi8-Ag on quartz-PDDA/PSS substrate, by monitoring the intensity of the plasmon absorption as a function of dipping time; these experiments indicated that about 80 min was necessary to reach the adsorption plateau. The primer layer was adsorbed by first adding 20 lL of a 10 wt.-% PS particle (925 nm in diameter) or silica (450 nm) dispersion to 1.5 mL of the PAH or PDDA solution, respectively, shaking for 10 min, centrifuging (4200 g, 7 min for PS spheres, or 3800 g, 7 min for silica spheres), and then washing (2 mL of water). The centrifugation/washing cycles were repeated at least 4 times to remove unadsorbed polyelectrolyte. Similarly, the next PSS layer was adsorbed by redispersing the coated particles in 1.5 mL of PSS solution with the same procedure, except that the pH of the rinsing water was fixed to 2. Rinsing the PSS layer at pH 2 was found to be essential in promoting an acid-base interaction with NSi8 [22]. The resulting coated colloids were dispersed in 10 mL of the NSi8-Ag solution and shaken for 90 min, followed by centrifugation (same conditions as above, without washing). This procedure was typically repeated 4 times to reach adsorption saturation; i.e., until an excess of NSi8-Ag was detected in the supernatant. The coated particles were then extensively washed with water (3 10 mL) at pH 5. This sequence of adsorption/centrifugation/redispersion was repeated n times to alternately coat the colloids with PSS and NSi8-Ag, leading to PS-PAH/(PSS/NSi8-Ag) n or silica-PDDA/(PSS/NSi8-Ag) n . Calcination of the PS coated spheres was performed by heating (4.3 K min ยฑ1 ) at 500 C under nitrogen for 5 h and at 570 C under oxygen for another 6 h.