The aggregation of 12 nm citrate-stabilized gold nanoparticles (cit-AuNPs) in the presence of four
different natural organic matter (NOM) isolates and a monovalent electrolyte (KCl) was evaluated using
time-resolved dynamic light scattering. All four NOM isolates stabilized the cit-AuNPs with respect to
aggregation. However, specific effects varied among the different NOM isolates. At pH = 6 in 80 mM
KCl, low concentrations (<0.25 mg C/L) of large molecular weight Suwannee River Humic Acid (SRHA)
was required to stabilize cit-AuNPs, while larger concentrations (>2 mg C/L) of smaller Suwannee River
Fulvic Acid (SRFA) were necessary at the same ionic strength. Suwannee River NOM (SRNOM) which
contains both SRHA and SRFA behaved in a manner intermediate between the two. Pony Lake Fulvic
Acid (PLFA), an autochthonous NOM isolate, provided substantial stability at low concentrations, yet
aggregation was induced at NOM concentrations > 2 mg C/L, a trend that is hypothesized to be the result
of favourable hydrophobic interactions between coated particles induced at increased surface coverage.
For all NOM isolates, it appears that NOM adsorption or conformational changes at the AuNP surfaces
result in significant increases in the hydrodynamic diameter that aren’t attributable to NP-NP aggregation
