Biology Reference
In-Depth Information
stabilization can be added to by steric factors. If a polymer layer is
adsorbed on the particle or tethered to the particle at one end, then
this will further limit the inter-particle approach [13]. Examples of
polymeric stabilizers include polyvinyl alcohol (PVA) and sodium
polyacrylate. Typically the stabilizer is present during the metal-ion
reduction,andthismeansitcanhaveaneffectonthegrowthprocess
of the particle. Strong polymer adsorption will slow the growth
rate. Stabilizers can also have a catalytic effect on the reaction
[10]. In some cases a variation in the stabilizer concentration
can change the nanoparticle shape [14]. With regard to gold
nanoparticles, the synthesis can be performed and then long-chain
molecules get attached to the gold by a thiol terminus [15-16]. The
polymer stabilizer can also be provided by performing the reaction
in a water-in-oil (w/o) microemulsion. This is done by reacting
reverse micelles containing a metal salt with reverse micelles
containing reducing agent [17-20]. Mixing the two microemulsions
causes an exchange of material between the micelles. The reaction
occurs first at the edges of the micelle (the initial locus of the
reaction) and then moves into the centre, as demonstrated by
TEM studies [21]. Nanoparticles can also be synthesized from
a single microemulsion, usually containing the metal salt, while
adding the reducing agent directly to the mixture [22-23]. The
principle of microemulsion synthesis has been extended to water-
in-supercritical CO
2
microemulsions, the rationale being that the
nanomaterialcanbesimplyrecoveredbyreducingthepressureand
releasingtheresultinggas.Silver[24]andcoppernanoparticles[25]
have been reported.
In general terms, if the rate of growth of the nanoparticles
is high relative to the rate of nucleation (i.e., the rate of new
particles forming), then the resulting materials will have a narrow
size distribution. This is highly desirable if the particles are to
be used as electrochemical labels, since the size distribution will
affect the precision of the resulting sensor. The rate of the reaction
can be influenced by the nature and concentration of the reducing
agent, with strong reducing agents favoring a faster reaction rate
and smaller nanoparticles [13]. Note however, that an
overall
fast
reaction does not necessarily imply a faster rate of growth relative
to nucleation.
