Chemistry Reference
In-Depth Information
sizes can be prepared the method is also suitable for double marking experiments.
This technique is illustrated by the localisation of polysaccharides and glyco-
proteins on yeast cell walls and erythrocyte membranes by transmission electron
microscopy and on yeast cells and intact erythrocytes by scanning electron micro-
scopy. Good spatial resolution of the marker was achieved in all cases. The method
is also suitable for marking thin sections [
179
]. Broda et al. provide an excellent
summary of recent developments in this field in their chapter [
173
].
4.3 Biochemical Applications
4.3.1 Diagnostics and Tumour Detection
As discussed above the optical properties of gold particles change upon binding to a
wide range of biomolecules and thereby allowing the detection and quantification
of analytes. The absorption spectra of gold nanoparticles change drastically when
several particles come close to each other. This may be exploited for very sensitive
DNA detection, even of a single-base mismatch. Gold nanoparticles are also used to
detect biomarkers in the diagnosis of heart diseases, cancers and infectious agents.
They are also common in lateral flow immunoassays [
180
]. New research into the
unique properties of gold nanoparticles should lead to well-established, routinely
used assays for a variety of biological applications. Another strategy for sensing
makes use of fluorescence quenching. Fluorescent molecules that are excited and in
close proximity to a gold particle can transfer their energy to the metal, resulting in
a non-radiative relaxation of the fluorophore. In several different detection
schemes, the analyte displaces the fluorescent molecules from the particle surface
or changes their conformation, so that the optical emission of those reporter
molecules is changed in the presence of the analyte. While many of the unique
optical properties of gold nanoparticles have been exploited in recent applications,
there is still plenty of room for new research. This should eventually lead to well-
established, routinely used assays for a variety of biological applications in the near
future.
4.3.2 Drug Carriers: Therapeutic Agent Delivery
Gold nanoparticles can serve as carriers for drug and gene delivery. Biologically
active molecules adsorbed on the particle surfaces can be guided inside cells and
released. DNA delivery, for instance, is the basis for gene therapy. Therapeutic
agents can also be coated onto the surface of gold nanoparticles. The large surface
area-to-volume ratio of gold nanoparticles enables their surface to be coated with
hundreds of molecules (including therapeutics, targeting agents and anti-fouling
polymers) [
181
].
