Gold colloids thiol-capped

The electronic absorption spectrum of metal nanocrystals in the visible region is dominated by the plasmon band. This absorption is due to the collective excitation of the itinerant electron gas on the particle surface and is characteristic of a nanocrystal of a given size. In metal colloids, surface plasmon excitations impart characteristic colors to the metal sols, the beautiful wine-red color of gold sols being well-known [6-8]. The dependence of the plasmon peak on the dielectric constant of the surrounding medium and the diameter of the nanocrystal was predicted theoretically by Mie and others at the turn of the last century [9-12]. The dependence of the absorption band of thiol-capped Au nanocrystals on solvent refractive index was recently verified by Templeton et al. [13]. Link et al. found that the absorption band splits into longitudinal and transverse bands in Au nanorods [6, 7]. 

FIGURE 32. Formation of thiol-capped gold colloids... 

Whetten and coworkers separated the crude thiol-protected gold particles by fractional crystallization until monodisperse fractions were obtained. Electron microscopy of these nanocrystals showed that the major components of the thiol-capped colloid have the gold core as a truncated octahedron containing, in accord with the geometrical predictions, 140, 225, 314 and 459 gold atoms487. 

Brust and coworkers studied chemical reactivity of thiol-capped gold colloids. A colloid modified with p-mercaptothiophenol is soluble in ethyl acetate. It can, however, be transferred into alkaline aqueous phase due to deprotonation of the phenolic groups. Acidification of the system effects protonation of phenolates and transfer of the colloid back to the aqueous phase483. Exposure of the above colloid to propionic anhydride leads to partial esterification. That phenolic groups, however, remain intact was attributed to steric hindrance. 

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