Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 217

The degree of agglomeration, which often shifts the particle size distribution
from the nanometre to the micrometre size range probably affects biological
uptake. Agglomeration reduces the pool of particles that can be taken up by non-
specifi c diffusion (below a few nanometres) (Kloepfer et al. , 2005 ) and perhaps
endocytosis, and increases the pool of ingestible material that can be taken up
through the gut. Agglomeration also changes the diffusion rates of the particles and
hence decreases the transport rate of nanoparticle through the diffusive boundary
layer and mucus to the organism surface.
It is also important to determine agglomerate structure, in addition to agglomer-
ate size. A porous, loosely assembled agglomerate has a lower effective density and
a proportionally higher hydrodynamic friction drag than a more dense, compact
agglomerate. Agglomerate structure is often an indicator of whether the agglomer-
ate growth rate has been reaction limited (slow agglomeration) or diffusion limited
(fast agglomeration). Reaction limited agglomeration produces loose, open struc-
tured agglomerates and diffusion limited agglomeration produces spherical,
compact agglomerates. A determination of the fractal dimension is an objective
method for quantifying agglomerate structure (Rizzi et al. , 2004 ; Sterling et al. , 2005 ;
Limbach et al. , 2005 ). Baalousha et al. (2008) used a fractal dimension analysis of
TEM images to characterize iron oxide aggregates.
Agglomeration may also partly make the nanoparticle surface less available for
chemical reactions for several reasons. As dense agglomerates form, fractions of
the particles merge together and the total exposed surface area of the sample
decreases The diffusion of chemical reactants or products may be slower through
the three dimensional structure of an agglomerate, which may impede catalytic
reactions and, moreover, there may also be surface area less available for light
(photocatalysis).

6.2.1.5 Structure and Crystallinity

Even though the structure activity relationships for NPs are still poorly studied,
from both reactivity and toxicology points of view, there is evidence that crystal-
linity and crystal structure are important. For example, for titanium dioxide it is
anatase, one of the crystalline polymorph structures, that has shown to be photoac-
tive, while the others, rutile and brookite, are much less active, or not active at all
(Augustynski, 1993). To complicate the picture it has been shown that a mixed
phase of anatase and rutile is more photoactive than pure anatase, and it has been
suggested that the interfaces between the two crystallinities are important for the
formation of photoactive hotspots (Li et al. , 2007) In toxicology an example is that
crystalline quartz shows pulmonary toxic effects, while amorphous silica is much
less harmful (Castranova, 2000 ).

6.2.1.6 Chemical Composition

The chemical composition of NPs is, of course, an important property in character-
izing unknown materials or tracking the particles in certain experiments. It could
either include an elemental analysis or chemical composition (e.g. distinguishing
C 60 and C 70 fullerenes, or polystyrene and polyacrylate NPs). The elemental