Stability and Quality of Silver Nanoparticle Dispersions Under Natural Environmental Conditions.

This study focuses on descriptions of work conducted by the Soil & Sediment Geochemistry Team, US Army ERDC to characterize the dispersion behavior of commercially synthesized silver nanoparticles, relative to the stability and utility of this material for toxicological exposure studies.   Here, we describe simultaneous dissolution-dispersion experiments for silver nanoparticles.  Batch studies were conducted by dispersing silver nanoparticles in solutions containing different concentrations of surfactants (Brij 35), chelating agents (EDTA, oxalic acid), or humics (Aldrich HA), and different ionic strength backgrounds.  Settling studies showed that a 100x increase in solution ionic strength completely destabilized the dispersion, suggesting the particles’ surface charge density appears to be relatively diffuse.  Dynamic light scattering (DLS) measurements showed a roughly 10-fold increase in particle size when solution electrolyte concentration is increased from 1 to 100 mM NaNO₃.  Small angle x-ray scattering (SAXS) studies indicate the dispersions are polydisperse, containing two main particle size distributions around 30 and 150 Å, that appears to shift in proportion based on system ionic strength.  Surfactive, humic, and chelating agents were shown to only increase particle size, but differentially impact settling based on surface loading. Total Ag and Ag⁺ measured in solution appeared to vary depending on the type of organic molecule added to the system.  The results show that nanosilver suspension stability is largely controlled by pH-dependent surface charge.  Electrophoretic and potentiometric measurements show that the manufacturer-supplied nanosilver particles appear to be coated with an adsorbed layer of citrate, most likely a residual from the synthesis process.  For this reason, the nanosilver particles exhibit negative charge at low pH, whereas metallic silver should only exhibit positive charge from surface oxides.  The results of this study suggest creating a reproducible dispersion with nano-sized particles of silver represents a significant challenge under environmental ionic strength and pH conditions.