Impact of Environmental Conditions (pH, Ionic Strength, and Electrolyte Type) on the Surface Charge and Aggregation of Silver Nanoparticles Suspensions

The impact of capping agents and environmental conditions (pH, ionic strength, and background electrolytes) on surface charge and aggregation potential of silver nanoparticles (AgNPs) suspensions were investigated. Capping agents are chemicals used in the synthesis of nanoparticles to prevent aggregation. The AgNPs examined in the study were as follows: (a) uncoated AgNPs (H-2-AgNPs), (b) electrostatically stabilized (citrate and NaBH4-AgNPs), (c) sterically stabilized (polyvinylpyrrolidone (PVP)-AgNPs), and (d) electrosterically stabilized (branched polyethylene imine (BPEI)-AgNPs)). The uncoated (H-2-ANPs), the citrate, and NaBH4-coated AgNPs aggregated at higher ionic strengths (100 mM NaNO3) and/or acidic pH (3.0). For these three nanomaterials, chloride (Cl-, 10 mM), as a background electrolyte, resulted in a minimal change in the hydrodynamic diameter even at low pH (3.0). This was limited by the presence of residual silver ions, which resulted in the formation of stable negatively charged AgCl colloids. Furthermore, the presence of Ca2+ (10 mM) resulted in aggregation of the three previously identified AgNPs regardless of the pH. As for PVP coated AgNPs, the ionic strength, pH and electrolyte type had no impact on the aggregation of the sterically stabilized AgNPs. The surface charge and aggregation of the BPEI coated AgNPs varied according to the solution pH.