The repulsive forces arising from the surface charge of gold nanoparticles create an energy barrier that individual particles must overcome to interact. This energy barrier is too strong to allow interactions between particles in the absence of electrolytes or when traces of electrolytes are present. Adding sodium chloride (NaCl) reduces the energy barrier. The gold nanoparticles can now interact and aggregate.
The aggregation, known as surface-plasmon coupling, changes the adsorption maximum of light to a higher wavelength resulting in a change in color of the solution.