Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 267

based on the interactions of two forces: electrostatic repulsion, due to the compres-
sion of the electrical double layer of surfaces with the same charge, and van der
Waals' forces of attraction (Hunter, 2001). This theory can be applied to explain
the stability of a dispersion of charged nanoparticles in an aqueous solution. Zeta
potential measures the accumulated charge of the particle surface resulting from
its movement within a fl uid environment and gives an indication of the stability of
the particle dispersion (Hunter, 2001). For charged particles, a zeta potential of
above +30 mV or below 30 mV is generally considered as stable and unlikely to

aggregate (Malvern Instruments, 2004).
From a practical perspective, the dispersion in aqueous solutions of nanoparticles
that are supplied in powder form can be very diffi cult. Nanopowders may not fully
disperse even following sonication or addition of surfactants (Franklin et al. 2007 ).
Filtration may be required to separate the non-dispersed material prior to use.
Nanoparticle stability may be achieved by adding a surface coating referred to as
a stabiliser. The purpose of a stabiliser is to prevent aggregation by either electro-
static repulsion, in the form of a charged ligand, or steric hindrance, often in the
form of a polymer preventing the particles from coming into close contact. For
instance, citrate is used to stabilise gold and silver nanoparticles in solution.
Examples of steric stabilisation are given in chapter 4 of this topic.
Nanoparticle suspensions may be perfectly stable under optimum conditions, but
as soon as they are transferred to other aqueous environments, such as the test
solutions required to conduct ecotoxicity bioassays, this may change. Factors affect-
ing aggregation include changes in pH, ionic strength and major ion concentrations.
Particle aggregation over the course of toxicity tests appears to be commonplace.
For example, Franklin et al. (2007) found extensive aggregation of metal oxide
nanoparticles in algal growth bioassay media (Figure 7.4). Griffi tt et al. (2007) also
described the aggregation of copper nanoparticles during the execution of fi sh
toxicity tests. Aggregates may well represent the stable form of nanoparticles in
environmental systems and it is therefore appropriate to expose organisms to
nanoparticles in this form. Clearly, it is necessary to understand how aggregation
affects the bioavailability of nanoparticles. This may be linked to the physical stabil-
ity of the aggregates. In natural waters it is highly likely that nanoparticles will
interact with natural organic matter. Coatings of humic and fulvic acids on the outer
surfaces of the nanoparticles will infl uence their surface charge and ability to aggre-
gate. It appears that the interaction of certain types of manufactured nanoparticle
with natural organic matter may actually increase particle stability. Hyung et al.
(2007) showed that the addition of standard Suwannee River humic acid greatly
enhanced the dispersion of multi-walled carbon nanotubes in deionised water. The
same effects were also seen in suspensions in Suwannee River water samples. The
stabilising effect was greater than that observed with the surfactant sodium dodecy-
lsulfate, which commonly used as a dispersant for carbon nanotubes.
As noted in Figure 7.3, in aquatic systems nanoparticles may interact with natural
colloids, suspended particulate matter and natural organic matter, leading to aggre-
gation and potentially sedimentation from solution. Aggregation and sedimenta-
tion may constitute a pathway for the transport of nanoparticles from the water
column to benthic sediments. In this environment the nanoparticles may be

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