Chemistry Reference
In-Depth Information
evaporation of the metal and the pressure within the metal vapour apparatus
[
114
,
115
].
H[AuCl
4
] aqueous solutions may be reduced to colloids using radiolysis or
pulsed laser techniques [
116
-
118
]. For examples, solvated electrons may be gen-
erated by pulsed radiolysis or by 353 nm laser pulses in either pure water or
emulsions. The colloidal particles formed in this way generally have small mean
diameters and low polydispersities. Sonication may also be used to produce colloids
[
119
,
120
]. Ultrasound treatment of an aqueous solution of H[AuCl
4
] in the
presence of glucose leads to ribbons of gold with widths of 30-50 nm and length
of several micrometres. The reducing agents (hydroxyl radicals and sugar pyrolysis
radicals) are produced at the interfacial region between the collapsing cavities and
the bulk water. When glucose is replaced by cyclodextrin spherical gold particles
are obtained suggesting that glucose is essential in forming the ribbons.
2.4 Laser Ablation Techniques
Gold colloid particles may be produced by laser ablation of a gold metal plate in an
aqueous solution of sodium dodecyl sulphate as a surfactant. The absorption spectra
of the gold particles closely resemble those of gold particles prepared by more
conventional chemical procedures. The size distribution of the particles was found
to shift to a smaller mean diameter with an increase in surfactant concentration.
This behaviour was explained in terms of the dynamic formation model. The
particle abundance depends on the surfactant concentration and stable gold parti-
cles are formed as the surfactant concentration exceeds 10
5
M. The gold particles
having mean diameters larger than 5 nm were pulverised into those with diameters
of 1-5 nm using a 532-nm laser [
121
].
3 Characterisation of Colloids and Nanoparticles
The deep colours of gold sols are strongly affected by the environment, size and
physical dimensions of the metal particles and so have played an important role in
their characterisation. These relatively easy spectral measurements provide circum-
stantial evidence regarding the broad structural features and sizes of the colloidal
particles. These spectral properties arise from surface plasmons, which have been
discussed above, and are underpinned by Mie's theoretical model. Qualitatively the
reactions of colloidal solutions with salts, ligands, polymers and biological mate-
rials may be monitored by following the spectral changes. The development of
nanoscience into a predictable and reproducible discipline requires a more detailed
understanding and control of the atomic structures of the metallic cores of colloids
and nanoparticles and spectroscopic and analytical techniques which lead to the
