Aggregation behaviour of gold nanoparticles in saline aqueous media

The evaluation of the effect of saline aqueous media in the aggregation of colloidal gold nanoparticles (AuNP) suspensions has been followed by means of both dynamic light scattering and UV-Vis spectroscopy techniques. A wide range of ionic strengths from 0 to 1 M has been covered for suspensions of AuNPs with a hydrodynamic radius of 6.5 nm. The growth of the clusters over time is described by a power law whose exponent varies with the ionic strength. Two main aggregation regimes are found in qualitative agreement with predictions of the DLVO theory. At low ionic strength, AuNPs form small aggregates that are long-term stable. When a critical electrolyte concentration is exceeded, big clusters are formed rapidly and then colloidal suspension becomes unstable and precipitation occurs. That so-called critical coagulation concentration was estimated by analyzing the time evolution of both the nanoparticle size (determined by DLS) and the plasmon peak (determined by UV-Vis spectroscopy), thus showing the validity of both techniques for such a purpose. The stability is also studied in terms of the zeta potential which approaches zero when salt concentration is above the critical coagulation concentration.