Formation and stability of gold nanoparticles in colloids prepared by citrate method

Gold nanoparticles (Au NPs) have found a variety of applications in different areas, particularly in biomedical practices. The activity of Au NPs strongly depends on the size and association of particles in colloid, that in turn are greatly affected by experimental parameters of the reaction. The obtaining of Au NPs even via classical procedure of citrate method can be a challenge. In the present work we applied different experimental approaches to affect the process of Au NPs formation in the presence of sodium citrate. Au NPs were obtained using different experimental procedures and varying the ratio of reagents, their concentrations, temperature of reaction, duration of heating, the order of introduction of reagents into the reaction mixture, pH, and so on. Comparative analyses of UV-vis spectra with DLS data by number, volume and intensity basis allowed to trace the changes in Au NPs colloid, find optimal experimental conditions and predict prolonged stability of colloids. Applying size-dependent Hamaker constant to DLVO theory explains experimental results. The formation of Au NPs strongly depends on the ratio of the functional groups of the molecule involved simultaneously in the reduction of metal ions, the binding to the surface of Au NPs and the formation of a charge for stabilization due to electrostatic repulsion. The change in the ratio of components is not enough to get a different size of Au NPs. Big concentration of the reagents mostly affects the aggregation process and colloid aging. Temperature is a critical activation factor, that should be about 100 °C, but prolonged heating causes collision induced aggregation. The initial stage of particles growth (the mechanism) can be affected with the change of pH of the system due to formation of deprotonated carboxyl groups and gold hydroxocomplexes.