Small particles show many interesting physical and chemical properties, which are often drastically different from their bulk counterparts. Many of the properties, such as solubility, chemical reactivity, etc., are affected by their small sizes and high surface-to-volume ratio. The large surface of the particle may be structurally and compositionally different from the bulk due to surface relaxation and reconstruction, the presence of adsorbed layers of reaction by-products and stabilizing molecules, etc. Due to the large surface area of contact and also microscopic roughness of the surface, colloidal particles are more reactive than their bulk counterparts since adsorption and reactivity are highly structure-sensitive properties. We have earlier demonstrated the surface area effects on the chemical properties of colloidal Ag particles.

Due to the finely divided states of colloidal particles, one can obtain huge surface areas for a given quantity of materials. Therefore by making very small catalyst particles, large surface areas are achieved with a small volume of material. Further such small particles have ample corners, vertices, defects, kinks, and steps. In addition to huge surface areas, different crystallographic facets and an increased number of edges, corners, defects, etc., are of critical importance in controlling the catalytic activity and selectivity. Therefore, particles of various different sizes and shapes are highly desirable as catalysts in fuel cells, waste reduction, bioprocessing, and the chemical industry. Though bulk gold is known for its inertness, small particles of gold have been observed to be catalytically active.

Eosin is an organic water-soluble bright red dye used in many applications in biological science, medical science, textile dying, ink manufacturing, etc. Its reduced form is colourless, and hence its reduction can be monitored by following the decrease in its absorption peak intensity. Eosin absorbs green light, and therefore decrease in eosin absorbance was followed at 535 nm wavelength near its absorption maximum. In the absence of catalysts, one does not observe, in the experimental time scale, the reduction of eosin in an aqueous solution by sodium borohydride. However, the rate of reduction reaction becomes appreciably faster in the presence of gold particles. Gold particles here act as catalysts. Depending on the catalyzing particle size and concentration (Table 1), the rate of reduction reaction varies through all the sets of particles prepared here have the same quantity of total gold (1×10^-4 M).

Pseudo-first order kinetics with respect to eosin, i.e., log (At - AAu) vs. time, could be used in this case, where At stands for absorbance at any time t and AAu stands for absorbance of aqueous gold particle system before the addition of dye. Thus one can determine the values of pseudo-first-order rate constants (k) and corresponding correlation coefficients from these plots.