The general procedure to coat collo

The general procedure to coat colloids with silicaconsist of two steps: adsorption of PVP and growth of the silicashell after transfer of the particles to ethanol. An outline of thesynthesis is shown in Figure 1.Synthesis of the Colloids. Smallgold colloids of 7 and 19 nm radius were synthesized accordingto the standard sodium citrate reduction method33,34 and werenot further purified. Silica particles (228 nm radius) with a 38nm thick gold shell were prepared as described in ref 14 andpurified by repeated sedimentation and redispersion in water.Small silver particles of 13 nm radius were synthesized by amodified polyol process:35 Silver nitrate was reduced in ethyleneglycol in the presence of PVP-10. After the synthesis, the particleswere separated from ethylene glycol by addition of acetone (500mL of acetone/75 mL of reaction mixture) and subsequentcentrifugation at 600g as described in ref 35. Next, the supernatantwas removed and the particles were redispersed inethanol, again centrifuged at 600g, and redispersed in a solutionof ammonia in ethanol (4.2 vol % ammonia (29.3 wt % NH3 inwater) in ethanol). This solution (c ) 0.31 g/L) could be directlyused in the silica-coating step (see below). Large silver colloidsof 320 nm radius were synthesized by reducing silver nitratewith ascorbic acid in the presence of the polymeric stabilizergum arabic36 and purified by repeated sedimentation andredispersion in water. Boehmite rods and gibbsite platelets wereprepared from aqueous aluminum oxide solutions by hydrothermaltreatment at 85 °C (gibbsite) and at 150 °C (boehmite)and purified by dialysis against demineralized water as describedin refs 37 and 38.Cationic polystyrene spheres were prepared by surfactantfreeemulsion polymerization as described in ref 39 and anionicsulfate stabilized polystyrene spheres as described in ref 40.