(141b) Molecular Weight Fraction of Natural Organic Matter Determines Nanoparticle Stability Against Aggregation and Attachment to Hydrated Metal-Oxide Surfaces

Nanoparticles have become of interest as a potential emerging contaminant due to their novel properties and increasing use in commerce.  Quantitative prediction of nanoparticle transport in the environment is necessary to assess exposure risks resulting from nanoparticle releases.  Adsorption of macromolecules, especially natural organic matter (NOM), has been repeatedly demonstrated to greatly affect a nanoparticle’s surface properties and its transport in the environment by imparting steric or electrosteric forces.  However, predicting the effect of adsorbed NOM on a nanoparticle’s properties is greatly complicated by the heterogeneous nature of NOM.  NOM is polydisperse in terms of molecular weight, hydrophobicity, and functional groups present.

Adsorbed NOM has been shown to either stabilize nanoparticles against aggregation, or to destabilize them.  In most studies assessing the effect of adsorbed NOM on nanoparticle transport, polydisperse NOM samples are used in whole.  These samples are rarely fully characterized, so it is not possible to determine if particular components of the NOM may have adsorbed preferentially to the nanoparticle surface to produce the observed effects.  Hence, only a qualitative explanation of the effects is currently possible.  A more complete characterization of the adsorbed NOM coating is required to determine the specific NOM properties that most influence nanoparticle behavior.

In this study, we provide molecular weight characterization of NOM by size exclusion chromatography coupled with multi-angle light scattering (SEC-MALS).  For Suwanee River NOM, two distinct molecular weight populations are observed: a smaller fraction (M_w<16 kDa) comprising ~98 wt-% of the unfractionated NOM, and a persistent large fraction (M_w~600 kDa) comprising ~2 wt-%.  Preparative fractionation of these two NOM components is achieved by ultrafiltration.  The effect of these fractions on the stability of 15 nm citrate-stabilized gold nanoparticles is studied.  The large NOM fraction is observed to stabilize the nanoparticles against aggregation in 100 mM NaCl, whereas the small NOM fraction provides no or poorer stabilization, even at similar and significantly higher mass concentrations.  A suite of characterizations is used to assess differences in the coating properties that may explain differences in stability, including adsorbed mass, chemical functionalities by ¹³C NMR and comparison of UV adsorption ratios, and charge and extended layer thickness by an electrokinetic method.  These results provide a deeper understanding of the effects of adsorbed NOM on nanoparticle transport behavior, and will be useful for interpretation of prior and future studies of NOM-coated nanoparticles.