Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 216

aspect ratio rod has a greater uptake rate than a higher aspect ratio, therefore shape
plays an important role.
Many concepts defi ning and measuring shape factors and aspect ratios that are
currently applied have been developed for larger particles and can be adopted for
NPs. However, the methods to determine the properties are different (Xu et al. ,
2003 ; Barreiros et al. , 2006; Jennings and Parslow, 1988). See also the discussion on
image analysis and fractal dimension determinations in Section 6.2.1.4.

6.2.1.3 Number and Mass Concentrations
For ordinary chemicals the conversion from a mass concentration (e.g. mg l − 1
or ppm) to a number based concentration (mol l − 1 ) is simply a factor of the molar
mass of the compound, but for particles an analogue conversion is only straight
forward for perfect spheres of known size. Since many NPs are often heterogeneous
in terms of the properties shown in Figure 6.1, this adds a certain level of uncer-
tainty. Due to the small mass of individual NPs (or high numbers per mass), is it
often stated that particle number concentration is more relevant than particle mass
concentration (Aitken et al. , 2007 ; Oberdorster et al. , 2005). Consequently, if it is
hypothesised that there is a size and shape dependence on uptake (mechanistic
explanation not yet fully understood) and that the number of a certain NP is impor-
tant, number based size distribution and shape are important metrics in character-
izing uptake. In addition, it has been claimed that it is the total particle surface area
that can be correlated with a toxic effect (Section 6.2.1.7). It should be noted,
though, that although these metrics (particle numbers, mass, diameter and area) are
geometrically dependent, for a heterogeneous system it is important to measure
them independently in order to fully characterize the exposure conditions.
Teeguarden et al. (2007) discuss that, especially for in vitro cell culture studies, it is
insuffi cient to quantify the overall physico-chemical NP characteristics. The doses
delivered to the specifi c sites of action must be considered, a challenging task to
say the least.

6.2.1.4 Dispersion State and Agglomeration

The environmental transport, biological uptake and, to some extent, reactivity of
NPs will be largely controlled by their dispersion and agglomeration behaviour.
Nanoparticles dispersed in water are a colloidal system that is affected by weak
physical forces of both attractive and repulsive character. If the particles are allowed
to come into close proximity to each other then attractive van der Waals' forces
will most likely lead to agglomeration (O'Melia, 1987). But if the NPs are stabilized
by either repulsive electrostatic forces or by means of steric stabilization, usually
by hydrophilic polymer tails on the particles (Lourenco et al. , 1996), then the system
can be kinetically stable (O'Melia, 1990). The electrostatic stabilization can occur
on particles with both a net negative or positive charge depending on the pH of
the water and the point of zero net charge of the NP. Due to protonation of surface
functional groups and sorption of natural organic matter (NOM), there is a close
coupling between the particle surface chemistry, the water chemistry and the
stability and agglomeration state.