Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 79

cle's basic physical characteristics (e.g. optical and magnetic properties). Possible
coatings include small molecular moieties, for example alkylthiols (Kloepfer et al. ,
2005), polymers (Harris et al. , 2003), biomolecules (Gao et al. , 2005 ; Nie et al. , 2007 ;
Rhyner et al. , 2006) and inorganic coatings (e.g. a zinc sulfi de fi lm on a cadmium
selenide (CdSe) nanoparticle) (Dabbousi et al. , 1997 ; Hines and Guyot - Sionnest,
1996). Such coatings commonly affect important nanoparticle properties, such as
solubility in water (Pellegrino et al. , 2004), resistance against chemical degradation
(e.g. passivation of a particle to prevent oxidation) (Yu et al. , 2007 ) and affi nity for
different biological tissues (Bruchez et al. , 1998 ). The specifi c chemical behaviour
of any given nanoparticle will depend strongly upon the core material composition,
the size, the shape, the coating and the aggregation state. Realistically, as a part of
nanotechnology products, nanoparticles may be embedded in a particular matrix
or on a substrate which can also infl uence nanoparticle behaviour (this matter is
outside the scope of this review).

3.3 Redox Chemistry of Nanoparticles

In nature, redox reactions are an important part of phenomena such as mineral
weathering, bacterial respiration and degradation of pollutants. Many nanoparticles
are of interest for applications due to their ability to catalyze or directly participate
in redox processes. Thus, if released into the environment, such nanoparticles may
infl uence natural redox phenomena, including those within living organisms. Here,
the varied origin and nature of redox properties in nanoparticles is discussed. (If
the reader is not already versed in the basics of electronic structure and light
absorption in inorganic nanoparticles, it is recommended that Chapter 2 of this
topic be reviewed.)

3.3.1 Photoredox Chemistry in Semiconductor Nanoparticles

When semiconductor materials absorb light of the proper energy, mobile charge
carriers (electrons and holes) can be generated. If these charge carriers reach the
surface of the semiconductor material, they may reduce or oxidize compounds
on or near the surface, depending upon the redox potentials of the compounds.
The size of the semiconductor material can affect many aspects of such redox
processes.
Generally, if a semiconductor nanoparticle is below a certain critical size (which
depends upon the parent semiconductor material), it can exhibit quantum size
effects (Alivisatos, 1996). In such cases, the wave functions of the charge carriers
extend over the entire particle. Therefore, the charge carriers will not have to diffuse
to participate in reactions at the particle surface (Hagfeldt and Gratzel, 1995).
Quantum size effects result in the shifting of band edge energies (changes in
electronic energy levels) (Alivisatos, 1996). This alters the redox potentials of
charge carriers in a given nanoparticle with respect to the bulk. Therefore, a
nanoparticle may be energetically able to participate in a particular redox reaction
that is not possible for the parent bulk material. An excellent example of this phe-