Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 67

focus on the use of nanomaterials in a range of applications which have been
realised with some consideration regarding where these might lead in the future.

2.6.2.1 Nanocomposites

A composite is a material consisting of two phases, one which serves to bind the
material together and one which serves to enhance the properties of the material.
Generally, they are thought of as structural materials, that is carbon fi bre or glass
fi bre composites, although composites are prepared for a range of other applications
and from a more diverse range of materials. Once again nature has beaten man in
the use of nanocomposites and they form the basis of a huge range of biologically
derived structural materials. Man-made materials of this type have general focus on
polymer based nanocomposites. Exfoliated clay nanomaterials as reinforcing agents
in nylon-6 based composites were initially explored as possible cam-belt casing
materials in automobiles in the 1990s (Usuki et al. , 1993). It was found that the incor-
poration of the nanomaterial resulted in signifi cant improvements in mechanical
performance and thermal stability. This has been explained by the large interfacial
areas and the shape of the nanomaterial itself. Clay nanocomposites have now found
applications in a range of cars from different manufacturers, including timing belt
and engine covers based on nylon-6 (Gao, 2004), door components and steps based
on polyolefi n clay nanocomposites (Cox et al. , 2004; Patterson, 2004) and seat backs
from polypropylene clay nanocomposite (Hussain et al. , 2006 ). Similar materials
have also found applications in packaging where the limited diffusion of oxygen
through the material results in improved food lifetimes.

2.6.2.2 Immunolabelling

A specialist market has arisen in photoluminescent nanoparticles for a range of
applications. Perhaps one of the more well exploited is that of fl uorescent tags.
Fluorescent nanoparticles based on cadmium selenide or indium phosphide have
been prepared with quantum yields of

70%. These nanoparticles may be surface
modifi ed to make them hydrophilic and then further modifi ed to add a receptor
group to the surface. Using this technology it is possible to prepare a nanoparticle
which will selectively bind to a site on a cell. Unlike common organic fl uorescent
dyes, nanoparticles may absorb light at a range of wavelengths. This means that a
range of fl uorescence processes may be observed during one excitation process.
The use of nanoparticles in these types of application allows the use of a single
excitation wavelength to initiate fl uorescence. The excitation wavelength may also
be selected in order to minimise the fl uorescence from the tissue or container. To
date the exact composition of these materials is kept as a closely guarded secret.
However, it is well known that to achieve the levels of quantum yields reported in
the literature a core-shell structure is needed (Section 2.4.3).

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2.6.2.3 Photovoltaics

The challenge in photovoltaics is to produce a cell which will harvest all the visible
light falling onto it and convert it effi ciently into electrical energy. Whilst the exact
architecture of the cells may differ from cell to cell the principles are the same