Chemistry Reference
In-Depth Information
A field that has grown rapidly over the past decades is the use of gold
nanoparticles in biology and life sciences. These bio-applications may be classified
into four areas: (1) labelling, (2) delivery, (3) heating and (4) sensing. For labelling
the electron-dense nature of the heavy metal particles is exploited to improve the
contrast of the transmission electron microscopy images. Their small size and the
possibility of functionalising the particles, for instance, with antibodies
(immunostaining), mean that they also provide extremely high spatial resolution
and specificity in many labelling applications. Similarly, the particles' optical
properties - strong absorption, scattering and especially plasmon resonance -
make them ideal for a large variety of light-based techniques including combined
applications such as photothermal or photo-acoustic imaging. In addition, gold
nanoparticles can be radioactively labelled by neutron activation, which allows
for very sensitive detection as an X-ray contrasting agent.
4.1 Colorimetric Analyses and Surface-Enhanced
Raman Spectroscopy
The characteristic surface plasmon resonances (SPR) of gold colloids which depend
on the size of the particle and its chemical environment have been discussed in some
detail above. This property has been exploited to develop sensors to detect a wide
range of chemicals and biomolecules. The light scattering cross section of 60 nm for
gold nanoparticles is 4-5 orders of magnitude greater than that of commonly used
fluorescence dyes such as fluorescein, and this property when combined with
dynamic light scattering techniques which are sensitive to the size of gold particles
may be used as a very sensitive technique for detecting low concentrations of DNA.
The aggregation of the gold particles is related to the concentration of DNA
[
171
]. Zare et al. have demonstrated that the colour changes associated with
spherical particles of gold may be used to provide a simple colorimetric sensor for
studying the conformational changes of the yeast iso-c-cytochrome c (Cyt c) protein
[
172
]. A fuller discussion of the biological applications of gold nanoparticles and
clusters is to be found in the chapter by Broda et al. [
173
]. Although the emphasis in
recent years has been on the biological and medical applications, gold colloids and
nanoparticles have also been used to detect inorganic metal ions and anions.
For example, the applications of fluorescent gold nanoclusters as chemical sensors
for the detection of inorganic molecules and ions (e.g. Hg
2+
,CN
) have been
reviewed [
174
].
Surface-enhanced Raman spectroscopy (SERS) has shown that molecules bound
to the surfaces of gold colloids have vibrational bands whose intensities are many
orders of magnitude stronger than those observed for the unattached molecules.
This enormous enhancement of sensitivity has been exploited to detect very low
concentrations of organic molecules and biomolecules. Therefore this technique
