Page 267 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

252 Principles and Methods

Figure 7.10 is the presence of well-defined facets and hexagonal shape
in 2D projection. In this case, the nC 60 is formed by solvent exchange,
where the C is initially dissolved in an organic solvent and then mixed
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with water. The images of clusters produced in this case suggest that
aggregate formation may resemble a process of crystal growth rather
than undirected particle aggregation. Similar observations have been
made for nC produced through other solvent-exchange techniques for
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dispersing these materials in water [43]. When nC 60 is formed by
extended mixing in water without the intermediary use of organic sol-
vents, the resulting aggregates are much less organized.
It is also interesting to note that modification of the nanoparticle sur-
face chemistry to enhance the solubility of the nanoparticle may not in fact
result in a molecular dispersion [48]. In other words, cluster formation may
occur even when the particle surface has been modified (e.g., functional-
ization or surfactant wrapping) to enhance stability. For instance, hydrox-
ylation of the C molecule to make fullerol does indeed increase the rate
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at which stable suspensions can be formed. Hydroxylation of the C mol-
60
ecule to form fullerol increases the affinity of the C 60 for the aqueous
phase. However, these molecules do not appear to exist as discrete entities
(Figure 7.10). Instead, as shown in the TEM of a fullerol cluster, they read-
ily form spherical clusters composed of many C OH 20–24 molecules [49]. For
60
fullerol, cluster formation is likely due to the distribution of OH groups
across the C surface resulting in heterogeneous interfacial interactions.
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The distribution of –OH groups on the C results in the formation of dis-
60
tributed hydrophobic/hydrophilic regions on the C 60 molecule [48, 50].
Agglomeration of the C OH 20–24 may occur through attractive patch-patch
60
interactions between the hydrophobic regions. Graphite similarly shows
a strong tendency to aggregate and form highly fractal aggregates in water
[51]. Cluster formation will also vary according to the functionality given
[48, 49, 52, 53].
to the nanoparticle surface as has been found for C 60
Cluster formation will thus vary depending on the type and placement of
functional groups on the nanoparticle surface.
The influence of surface modification on nanoparticle aggregation is
further illustrated in the case of maghemite (Fe O ) nanoparticles. At
3
2
a pH of 7, maghemite nanoparticles aggregate to form large clusters
(Figure 7.11a). When the pH is reduced to a value of 3, protonation of
the maghemite surface promotes nanoparticle dispersion. To reduce or
prevent this tendency of nanometric maghemite to aggregate in water at
natural pH, the nanoparticle surface may be modified by applying a coat-
ing of meso-2,3-dimercaptosuccinic acid (DMSA, C S O H ) (Figure 7.11b)
4
6
4 2
[54]. The coating procedure begins with an acidic solution containing a
molar ratio of DMSA to Fe total of 9.2 percent. Then the solution is
alkalinized as illustrated in Figure 7.11b. For pH < 8.5, the DMSA sur-
face coating does not produce sufficient surface charge to prevent the