Biomedical Engineering Reference
In-Depth Information
under an energy-reducing gradient until all atoms are removed from solution.
The number of nucleation sites determines the number of particles that finally
grow in solution. When the concentration of HAuCl
4
in solution is fixed, the
concentration of the reducing agent controls the number of nuclei that forms:
the more nuclei, the smaller the AuNPs produced. In an optimized condition, all
nucleation sites will be formed instantaneously and simultaneously, resulting in
the formation of AuNPs of exactly the same size (monodisperse gold).
Colloidal AuNPs display a single absorption peak in the visible range
between 510 and 550 nm. As the particle size increases, the absorption maxi-
mum shifts to a longer wavelength. The width of the absorption spectra relates
to the size distribution range. The colors are size-dependent such that the small-
est gold colloids (2-5 nm) are yellow-orange, midrange particles (10-20 nm)
are wine-red, and larger particles (30-64 nm) are blue-green.
Various strategies are suggested to attain high quality and stability in
AuNPs. The first of such consideration is the use of thoroughly cleaned glass-
ware, 0.2-µm-filtered solutions, and, ideally, triple-distilled water or nanopure
water. These strategies suggest the adverse effect of trace contaminants on the
preparation of colloidal AuNPs.
The order of reagent addition has also been observed as a critical parameter
in controlling the quality of the gold colloid. However, the effect of adding
citrate solution to the HAuCl
4
solution or vice versa in relation to methods of
manufacturing colloidal gold suspensions has not been elucidated. In addition,
the role of mixing during the process has also not been clarified.
Researchers have not exploited the role of mixing in the formation of the
AuNP colloidal suspension. However, it must be noted that a large-scale operation
is dependent on both the chemistry of the process and the physical parameters.
Small changes in process conditions such as temperature, rate of mixing, and rate
reagent addition can adversely affect the quality of the product. The following
physical parameters affect the quality of the final gold suspension that is produced
by the reaction of aqueous HAuCl
4
with an aqueous solution of trisodium citrate.
The concentration of reactants.
l
Mixing of the reactants.
l
The order in which reactants are added.
l
Operating temperature.
l
Liquid head in the reactor.
l
The reactor's material of construction.
l
2.2.4 Synthesis of Nanorods and Nanostars
Nanorods and nanostars can be synthesized following the seed-mediated growth
method.
15,16
The protocol for the nanorods synthesis is summarized as follows:
(1)
Synthesize nanorod seeds using cetyltrimethylammonium bromide (CTAB)
aqueous solution, 5 mL of 0.20 M.
