Since colloidal suspensions are thermodynamically unstable and tend to flocculate, the control of the aggregation of gold nanoparticles is important to modulate their applications. For biomedical applications, poor stability can lead to a total or partial loss of their nanoscale properties, alters their cellular uptake, and modifies their bioavailability and toxicity.
Colloidal stability is a result of attractive Van der Waals and repulsive electrostatic forces between particles preventing them from aggregation. The sum of these opposing forces results in a total interaction potential depending on the distance between two particles whereby the maximum is referred to as the aggregation barrier. These interactions can be influenced by environmental parameters such as pH, temperature, ionic strength, and the presence of ligands.
This experiment illustrates the high sensitivity of the coloration to compare gold nanoparticles’ stability: Individual gold nanoparticles appear red/red-purple; however, when the particles aggregate together, the plasmon resonances shift, and the color changes to blue. Upon addition of PBS to Turkevich nanoparticles, the initially red color of the gold nanoparticles solution turn to blue. Salts in PBS screen the repulsive electrostatic forces caused by the citrate layer: indeed, the positive charges of the electrolyte associate with the negative charges on the surfaces of the nanoparticles. However, TORSKAL’s gold nanoparticles showed remarkable stability in the same condition, which makes them monodisperse.