It is well known that surface plasmon absorption (color) of An nanoparticles highly depends on its size, shape, and surrounding surface environment, providing a great opportunity for the development of An nanoparticles as a probe for chemical and biochemical sensing and as a building block for nano-optical devices. Currently, it remains incompletely understood how the surface properties of An nanoparticles ultimately define its optical properties, hindering the progress of rational design of Au nanoparticles for biochemical sensing and assembly of nano-optical devices. Ru(bpy)(3)(2+) possesses rich photochemical and electrochemical properties, offering a unique probe to study the surface environment of Au nanoparticles. In this study, we investigate the kinetics of surface plasmon absorption of Au nanoparticies (6.5, 19, 48, 97 nm) in the presence of Ru(bPY)(3)(2+) at 24.0, 33.0, and 51.0 degreesC. We find that the color-change rate of Au nanoparticles is highly sensitive to the amount of Ru(bpy)(3)(2+). In addition, the minimum (critical) concentration of Ru(bpy)(3)(2+) needed to change the color of An nanoparticles is highly dependent upon temperature and nanoparticle size. TEM images illustrate that the color-change mechanism of Au nanoparticles induced by Ru(bpy)(3)(2+) highly depends on the size of nanoparticles. Control experiments using NaCl and MgCl2 to replace Ru(bpy)(3)(2+) show no color change, suggesting that the color change of Au nanoparticles induced by Ru(bpy)(3)(2+) is not solely attributable to the salt effect. Other factors, such as adsorption of Ru(bpy)(3)(2+) on the Au nanoparticle surface, may be involved. Moreover, in the presence of the critical concentration of Ru(bpy)(3)(2+), as temperature increases from 24.0 degreesC to 51.0 degreesC, the color-change rate of 19 nm An nanoparticles increases 8-fold, whereas the color-change rate of 48 nm Au nanoparticles decreases 3-fold, showing the temperature dependence of surface plasmon absorption of An nanoparticles in the presence of surface adsorbates, Ru(bpy)(3)(2+).