Transport and Fate of Nanomaterials: Aggregation, reactivity, and transport

Overview: 

Protocols for nanomaterials dispersions

We have standardized and validated an approach for the preparation of titanium dioxide (TiO2) nanoparticle dispersions in relevant biological media (PBS and DMEM-FBS). This study utilizes a candidate TiO2 reference nanomaterial based on a commercially relevant powder that has been widely applied in both acute and chronic toxicity studies.  The adopted characterization, optimization and validation approaches (i.e., sonication energy calibration, optimization sequence for particle pre-dispersion, protein stabilization and medium incorporation steps) can be more generally applied to the preparation of protein-stabilized dispersions in relevant media for other metal oxide ENM powders, with the overall objective of harmonizing sample preparation practices that maximize dispersion while minimizing sources of artifacts and variability.

Aggregation and reactivity

Aggregation of photocatalytic semiconductors was determined to reduce the generation of free hydroxyl radicals in aqueous suspensions in a fashion dependent on aggregate size and structure. Static light scattering measurements were used to follow temporal changes in the fractal dimension of aggregating TiO2 and ZnO nanoparticles. At length scales comparable to nanoparticle size, the structure of aggregated TiO2 nanoparticles was independent of particle stability and the associated aggregation rate, consistent with the fused nature of TiO2 primary particles in the initial suspension. In contrast, ZnO aggregates were characterized by smaller fractal dimensions when ionic strength, and the resulting aggregation rate, were increased. The photocatalytic activity of ZnO and TiO2 in generating free hydroxyl radicals varied with aggregate structure and size, consistent with theory that predicts reduced reactivity as aggregates become larger and more dense.

We continue to evaluate reactivity as measured by microbial inactivation. An ongoing study of virus inactivation by varying degrees of hydroxylated fullerols has demonstrated the need to consider functionalization type and extent when determining the efficacy of photosensitization. While singlet oxygen production has previously been observed to vary between fullerol and fullerene, we have demonstrated that an intermediate hydroxylated fullerene lies in between the two production rates. When looking at inactivation efficiency, three distinct CT curves appear, indicating that physical association, as well as reactive oxygen species production, play a significant role. This finding is supported by hyperspectral imaging of virus-FNP association in suspension.

Work evaluating nanoparticle reactivity has also evolved to a consideration of more complex  composite nanomaterials. In particular, we have synthesized and characterized the reactivity of composite of fullerene and nano zerovalent iron (NZVI) nanoparticles. We find that the 2:1 (Fe0:C60; mass basis) is crucial for the composite to exhibit the highest hydroxyl (pH 3) and superoxide (pH 7.8) production. With these composites, gradual production of hydroxyl radicals or a sudden burst of superoxide radicals can be generated under appropriate pH or light conditions, respectively.

Theory for nanoparticle deposition

We investigate the potential energy for colloidal interaction between a fractal aggregate and a flat surface within the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory framework. Using Monte-Carlo simulation, we generated fractal aggregates of different numbers (100, 500, 1000) of primary particles formed under either Diffusion-Limited-Aggregate (DLA) or Reaction-Limited-Aggregation (RLA). We use the sum of the interaction energy between each particle and the surface (V0) to approximate the interaction energy between the aggregate the surface (Vagg). The results shows that the energy of interaction between a fractal aggregate and a surface is only multiples of that between a single primary particle and the surface at the same separation, and is significantly lower than that as estimated by treating the fractal aggregate as an equivalent sphere of the gyration radius of the aggregate. Such an observation is in fact similar to the observation that the surface roughness of a particle, or of collector surface would significantly lower the interaction energy between the particles or between a particle and a surface. The reason behind such an observation is that (1). The total interaction energy between an aggregate and a surface is primarily determined by the several particles that are closest to the collector surface; and (2). The interaction energy is positively related to particle size.

Characterization of surface hydrophobicity

Nanoparticle hydrophobicity will likely play an important role in determining the relative affinity of nanoaparticles for various environmental surfaces. The adsorption isotherms of naphthalene (dissolved in acetone) on nanoparticles surface were measured and fitted to Freundlich isotherm. The Freundlich constant KF is used as a measure of the degree of nanoparticle hydrophobicity. The larger the KF, the more hydrophobic the particle surface is. The results from naphthalene adsorption experiment agree well with previous results obtained from organic dye adsorption, contact angle and KOW measurement in that aqu-nC60 is the most hydrophobic, followed by THF-nC60, Ag-PVP, Ag-GA, Ag-CIT, Au-CIT and fullerol.