Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 63

nanoparticles with excellent optical properties. A range of precursor molecules
have been used including dialkyl adducts, such as dimethyl cadmium, as well as
simple carboxylates. The capping agents used range from phosphines and phos-
phine oxides to amines, thiols, pyridines and carboxylic acids.
Of particular note is the use of phosphine gas in the preparation of metal phos-
phides such as indium phosphide. Another approach to this preparation is the use
of a single molecule precursor. In this case a molecule is prepared which contains
all of the atoms needed to prepare the fi nal particle. This molecule is then dissolved
in a phosphine and added to a fl ask of hot phosphine oxide. Clearly, in many of
these cases contamination by by-products is likely and high purity hard to attain
or measure. Furthermore, whilst most of these nanoparticles are prepared in a form
which will not suspend well in aqueous media, the addition of a polymeric surfac-
tant is often enough to facilitate phase transfer from a hydrophobic environment
into the aqueous phase.

2.5.13 Polymers

There are essentially two methods for the production of polymer based nanopar-
ticles. In most cases, if a nanoparticle of a pure polymer is required it may most
easily be prepared by using a micro emulsion method. For example, a micro emul-
sion of styrene in water might be prepared by using sodium dodecyl sulfate as the
surfactant. A free radical initiator is added to the aqueous phase, for example
hydrogen peroxide or ammonium persulfate, and the reaction heated for several
hours. It is well known that this type of polymerisation tends to give excellent
conversion of the monomer to the polymer. One of the reasons for the high conver-
sion is the reaction kinetics within the micelles. Whilst transport of a radical from
the water phase into the emulsion droplets is relatively slow, the reaction within
the droplets is very rapid. This often results in an exponential increase in the viscos-
ity of the monomer/polymer phase and tends to trap unreacted polymer ends within
the particles. These unreacted polymer ends will still contain reactive radicals which
have been used in the past to reinitiate polymerisations. The fi nal nanoparticles
will have a proportion of the surfactant bound tightly to their surface because the
surfactants chain may either become entangled in the polymer chains or become
grafted onto the polymer via side reactions.
A second method for preparing polymer nanoparticles is to prepare a block
copolymer (Figure 2.19). If a polymer is prepared with two sections that are soluble
in different solvents it is possible to force them to self assemble on a molecular
scale. As already discussed, polymers are molecules with lengths on the nanometre
scale and, therefore, this molecular self assembly invariable results in a nanoparticle
being formed. One way to achieve this is to couple to short polymers together, such
as polycaprolactone and polyethylene oxide. Polycaprolactone is a water immisci-
ble biodegradable polyester whereas polyethylene oxide is a biocompatible water
miscible polyether. By preparing such a copolymer and dispersing it in water a
biodegradable polymer nanoparticle may be prepared. In general, the dimensions
of polymer nanoparticles are rarely less than 20 nm due to the large volumes which
the molecules themselves occupy.