Chemistry Reference
In-Depth Information
The interaction of gold colloids with light is strongly influenced by their envi-
ronment, size and physical dimensions. The oscillating electric field of light inter-
acts with the free electrons, causing a concerted oscillation of electron charge that is
in resonance with the frequency of visible light. These resonant oscillations are
known as surface plasmons. For small (~30 nm) monodispersed gold particles the
surface plasmon resonance (SPR) results in an absorption of light in the blue-green
portion of the spectrum (~450 nm) while red light (~700 nm) is reflected, giving the
colloid solution a rich red colour. When the particle size is increased, the wave-
length of the surface plasmon resonance shifts to longer wavelengths. Red light is
then absorbed, and blue light is reflected, yielding sols with a pale blue or purple
colour. As the particle size continues to increase toward the bulk limit, surface
plasmon resonance wavelengths move into the IR portion of the spectrum and most
visible wavelengths are reflected, giving the sols a clear or translucent colour. If
NaCl is added to a gold colloid sol, the surface charge on the gold particles becomes
neutral and they aggregate. The larger average size results in a colour change from
red to blue. The degree of aggregation may be reduced by polymers, ligands and
biological molecules with good donor atoms, e.g. N, P or S. This surface modifica-
tion enables gold nanoparticles to be used extensively in the chemical, biological,
engineering and medical applications discussed below [
7
-
9
,
11
,
28
].
The interpretation of SPR depends primarily on a paper published in 1908 by
Mie [
6
], who solved Maxwell's equations for spherical particles with the appropri-
ate boundary conditions. It attributes the surface plasmon resonance to the dipole
oscillations of the free electrons in the conduction band which occupy energy levels
directly below the Fermi level. The theory rationalises the following broad features
of SPR: (1) its position around 520 nm, (2) its decrease when the colloid core size is
reduced to 1.4-3.2 nm because of the onset of quantum size effects, (3) for particles
with 1.1-1.9 nm diameters, sharper transitions which may be attributed to discrete
“molecular transitions” are observed which is also suggestive of quantum size
effects, (4) the decrease in intensity of SPR as the size of the colloid decreases is
accompanied by a broadening of the bandwidth. The SPR maximum wavelength
and bandwidth are greatly influenced by the size and shape of the colloid particle
and the refractive index of the solvent. Not surprisingly the position of SPR also
depends on the strength of interaction between ligands and the metal atoms.
For example, sulphur ligands, which are particularly effective at bonding to surface
gold atoms, cause a large perturbation. The charge on the metal core is another
significant factor and an excess electronic charge results in shifts to higher frequen-
cies and a deficiency to lower frequencies.
For non-spherical particles multiple plasmon bands may be observed if the
asymmetry is significant. The positions and intensities of the bands depend on the
size, shape and the local dielectric environment. Gold nanorods which have a high
degree of asymmetry have attracted considerable attention in recent years and are
discussed further in Sect.
4.4
.
Cluster
is defined as a number of metal centres grouped close together which can
have direct metal-metal bonding interactions or interactions through a bridging
ligand, but are not necessarily held together by these interactions. Molecular cluster
