Color Generattion of Colloidal Gold

The intense red to purple color seen for the Purple of Cassius pigment is achieved by precipitating a colloidal gold solution onto a solid oxide substrate, typically tin oxide. To maximize color intensity,the gold particles need to be unagglomerated and less than 40 nm in size. The color effect is caused by a narrow adsorption band at 520 nm, referred to as the surface plasmon resonance band. If particles agglomerate together and increase above 40 nm, a blue shift occurs and light scattering starts to dominate, resulting in muddy color hues. In 1902, Gustav Mie [112], using classical electromagnetic theory, calculated from bulk properties of metallic gold the absorbance of colloid gold particles as a function of the particle size. His theoretical work also predicted that the wavelength of the plasmon band depends on the shape, surface composition, and dielectric environment of the gold particles.

These concepts are used in modern-day production and research and development to predict and control the color effects associated with Purple of Cassius. For example, most gold colloid solutions seen in the literature are as dispersions in water with fairly consistent dielectric constants. When considering glass enamels, the glass media in which the colloid is dispersed may consist of many different components, all with vastly different dielectric constants. This can lead to massive differences in hue and color strength for the final gold enamels. A practical demonstration illustrating the effect of dielectric constant of the surrounding metal oxide on color shift. Gold nanoparticles of approximately the same size (before and after heat treatment) have been deposited onto three oxides having differing dielectric values. It can clearly be seen that the oxides with low dielectric constants such as silica have red shifts whereas high dielectric materials such as titania shift the color to blue shades which is in accordance with predictions made by Mie. Another strategy for controlling the color is to alloy silver with gold. According to Mie, additions of silver will shift the plasmon band to a shorter wavelength and hence make the particles appear redder. Theory predicts that a single plasmon band with alloys having up to 25% silver will exist, but above this level two peaks appear, one caused by silver and the other gold. In practice, an excess of silver is used to redden off the gold.