Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 219

(Sholkowitz et al. , 1978 ; Behrens et al. , 1998 ; Wilkinson et al. , 1997 ). Similar results
have also been shown for manufactured NPs and NOM (Hyung et al. , 2007 ).

6.2.1.9 Surface Functionality

The relative importance of nanoparticle surface functionality and nanoparticle core
properties still needs to be researched but it is probably safe to say that surface
functionality is a major factor in fate, behaviour and effects. In many nanoparticle
applications the nanoparticle growth is hindered by a capping agent, which is often
a polymer. The same polymer is sometimes used to provide the NP steric stabiliza-
tion (Lourenco et al. , 1996; Heijman and Stein, 1995).
But there may also be a charge stabilization functionalization. Natural examples
have been discussed in Section 6.2.1.8. Other types of NOM, mainly polysaccha-
rides, can induce agglomeration due to a bridging fl occulation mechanism. Both
natural and synthetic organic functionalization can control the shape of NP formed
(Banfi eld and Navrotsky, 2001; Yin and Alivisatos, 2005). In addition, it is known
that quantum dot skin permeability changes with different surface functionalization
(Ryman - Rasmussen et al. , 2006). Further, a study of phytotoxicity of alumina NPs
showed that the presence of organic coatings of certain organic compounds known
to be free hydroxyl radical scavengers decreased the growth inhibition (Yang and
Watts, 2005 ).

6.2.1.10 Surface Speciation

The surfaces of metal NPs often oxidize (corrode) in aqueous solutions. Consequently,
these metal NPs may behave very similar to their pure metal oxide counterparts.
The oxidation state of the material on the surface is often termed surface specia-
tion. An example is the presence of Ag + ions on the surface of a metal Ag(0)
nanoparticle. The surface speciation is important for both reactivity (Stumm, 1993;
Waychunas, 2001) and (eco)toxicology (Chapters 7 and 9).

6.2.1.11 Dissolution Rates and Desorption of Trace Constituents

Since many of the inorganic manufactured NPs contain heavy metals that are
known to be toxic in their dissolved form, it is essential to determine the dissolution
of metals from these NPs (Limbach et al. , 2005 ; Borm et al. , 2006 ; Franklin et al. ,
2007). These include, for example, metallic, metal oxide, metal sulfi de NPs. For
example, it has been reported that a signifi cant part of the observed toxic effect of
silver NPs is due to the dissolution of silver ions from the particles (Lok et al. , 2006 ).
In addition, it has been shown that desorption of contaminant metals or doping
metals from, for example, fullerenes or carbon nanotubes is responsible for the
generation of reactive oxygen species (ROS) once the nanoparticle has entered the
cell (Limbach et al. , 2005). Free metal ions can also be quenched in the ecotoxicol-
ogy experiment by using chelating ligands, either free (e.g. EDTA for transition
metals or cystein for silver) or matrix bound (e.g. Chelex TM resin). However, the
use of chelators must be done with some caution, since these also bind essential
di- and trivalent elements and, moreover, enhance the nanoparticle dissolution
rates.