Kinetics of Gold Particle Formation Reaction: When light falls on small metal particles, the light can excite collective coherent oscillations of surface electron-gas at the interface of the metal particle and dielectric. This collective coherent oscillation of metal surface electron-gas in metal particles is known as localized surface plasmon resonance (LSPR). LSPR falls in the visible range of electromagnetic spectrum for some metals like gold, silver and copper. Therefore, formation and development of such particles with LSPR property can be studied by using absorption spectroscopy. Metal particles can, in principle, be prepared by the reduction of their ions in suitable reaction media. For example, addition of sodium borohydride, a reducing agent to a yellow coloured aqueous gold(III) ion solution produces purple coloured dispersion of gold particles. Since silver and gold ions can be easily reduced, photo-irradiations of such ions in the presence of molecules like Triton-X 100 (TX-100), alcohols, etc. can also reduce them to metallic states. In this experiment, TX-100, a neutral surfactant is used as the reducing agent-cum-particle stabilizer. This photo-irradiation generates very small gold particles. These small gold particles can be used as seed particles and can be grown further into larger sized particles by addition of more gold ions and reducing them with the help of a weak reducing agent, such as ascorbic acid (Table 1). A weak reducing agent is necessary in this step so that nucleation (reduction) occurs only on the surface of the seed particles and seed particles grow in sizes but no new particles form via homogeneous nucleation. This ensures the formation of nearly monodispersed larger sized particle formation. Nearly spherical colloidal gold particles are purple in colour and absorb around wavelength 530 nm. Since the particle growth reaction is relatively slow, the growth reaction can be easily followed by a spectrophotometer. The growth of the particles starting from the seed can be followed spectrophotometrically due the change in absorbance during the particle growth process. Various sized gold particle samples can be prepared by varying the ratio of seed to gold ion concentrations. The development of seed-mediated larger gold particles shows sigmoidal shaped curve when increase in absorbance vs. time is plotted. This suggests an autoctalytic growth of the particles. This can be verified by an autocatalytic plot for the development of absorbance with respect to time i.e., ln a/(1-a) vs. t where a =A _t /A _∞ , with A being absorbance. Kinetics of Gold Particle-Catalysed Reduction of Eosin by Sodium Borohydride: 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 easily by following the decrease in its absorption peak intensity. Decrease in eosin absorbance was followed at 535 nm wavelength. In the absence of particles, one does not observe the sign of reduction of eosin by sodium borohydride in aqueous solution in the experimental time scale. However, the rate of reduction becomes appreciably faster in the presence of gold particles. Gold particles here act as catalysts. Depending on the gold particle size and concentration (Table 1), the rate of reduction varies, though all the sets of particles prepared here have the same amount of total gold (1×10 ^-4 M). A pseudo-first order kinetics with respect to eosin, i.e., log A _t – A _Au ) vs. time (t) could be used in this case, where A _t stands for the absorbance at any time t and A _Au that for 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.