AbstractThe adsorption behavior of metal cations on gold nanoparticles was investigated by isothermal titration microcalorimetry (ITC) for the first time, and verified by transmission electron microscopy (TEM), and UV absorption spectroscopy. When transition metal cations were injected into the gold colloids, a binding isotherm with a sigmoidal response was obtained; and when instead of alkaline earth cations, the binding isotherm changed from the overall exothermic to endothermic nature. The binding enthalpy depends on the nature and concentration of added cations. The interactions of metal cations with the surface gold atoms and the COO−group of citrate salt that weakly bound on gold nanoparticles are the driving force for the binding; and the binding process is enthalpically driven. TEM images indicate that the addition of metal cations results in the growth and aggregation of gold nanoparticles. After the metal cations were added to the gold colloids, the intensity of the surface plasmon resonance adsorption band at 523 nm of gold nanoparticles decreased, and a new absorption at around 610 nm appeared. This is attributed to gold nanoclusters induced by the surface binding of the metal cations on gold nanoparticles, which is further confirmed by ITC result