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Note: the numbers
in square brackets refer (and link) to the List of
Publications
Biomedical Applications of Gold Nanoparticles
The uptake of metal nanoparticles by living cells is
currently a field of great research interest with significant scope for
diagnostic and therapeutic applications due to the potential of modifying
the nanoparticles with multiple functional groups, which can provide for
specific cell targeting, diagnostic functionality and/or therapeutic
effects.
Photolytic
Release of Gold Nanoparticles into the Cell Cytoplasm
Gold nanoparticles are readily taken up by cells via endocytosis, but remain located exclusively in endosomes, which limits their use for many potential
applications, since no contact with the cytoplasm or the cell nucleus is
made. We have shown that irradiation of such endosomatic
gold nanoparticles with cw-laser light (at the
wavelength of the nanoparticles’ Plasmon resonance) leads to the partial or
complete destruction of the endosome and to the
release of the nanoparticles into the cytosol [27].
Careful choice of the irradiation conditions allows this to be achieved
without inducing cell death, providing an excellent route to deliver nanoparticles
and other agents to the cytoplasm, with the additional benefit of high
spatial and temporal control of the release.
Novel
Photochemical Approach to Cancer Therapy Using Gold Nanoparticles
At higher irradiation intensities or after prolonged
irradiation of endocytosed gold nanoparticles,
cell death occurs. This opens the direct potential of selective cancer
treatment, since preferential uptake of nanoparticles by cancer cells can
be achieved by binding of antibody conjugated nanoparticles to malignant
cells overexpressing certain biomarkers.
Previously, the mechanism of interaction of irradiated nanoparticles with
cells had been ascribed to local heating, which would affect all tissue in
the irradiated area and thus limit the specific targeting capability of
nanoparticles. However, we were able to show that irradiation of entocytosed gold nanoparticles leads to cell death
under conditions where only minor heating occurs [27].
This suggests a photochemical effect which is expected to be limited to
cells containing nanoparticles, so that much higher selectivity of cell
killing should be achievable. In fact, we have been able to show that
irradiation of gold nanoparticles with cw-laser
light leads to the formation of detectable amounts of singlet oxygen,
although the quantum yield is significantly smaller than for irradiation
with pulsed laser light [36]. We are currently
investigating this effect in more detail, including the use of gold nanorods which would allow using NIR light with much
larger penetration depth (“biological window”). In this context, we have
also carefully characterised the detection sensitivity of commonly used
singlet oxygen probes in aqueous environments [40].
Peptide
Ligand Shells on Gold Nanoparticles
Biomimetic gold nanoparticles can be prepared using short peptides as capping
layer. These constructs combine the physical characteristics of inorganic
nanoparticles, such as their optical properties, with the remarkable
biochemical properties of proteins. However, in general, better control
over the structure of such peptide capping layers, which is required to
make full use of the potential of these constructs, would be desirable.
Using FTIR spectroscopy, we were able to show that not only the primary
sequence of the peptide, but also the packing density and the curvature of
the nanoparticle surface affect the secondary
structure of such short peptides on nanoparticles. Thus, the CALNN peptide
adopts a fully disordered structure at lower packing density, but is forced
into a straight conformation at higher packing densities [37], independent of nanoparticle
size. In contrast, the CFGAILSS peptide can form parallel b-sheets, which are more prominent on
nanoparticles with 25 nm diameter than those with 5 nm diameter because of
constraints imposed on the peptide layer by the curvature of the nanoparticle surface [29]. These conclusions are supported by
MD-simulations which also give more detailed insight into the structure of
the peptide capping layer [37].
a-helical peptides, on the other hand, have been shown to keep their
structure upon binding to gold nanoparticles [33].
Photodissociation of Gold Nanoparticle
Ligand Shell
We have shown that thiol-bound peptide ligands of gold nanoparticles can
be photodissociated using short laser pulses,
which in principal should allow for the controlled release of a nanoparticle’s "payload", for example in a
cell after targeted uptake of NPs with multiple functional ligands
(antibody for targeting + drug payload). Using femtosecond
cross-correlation experiments, we found that this effect is due to the
interaction of "hot" electrons with the thiol
bond. However, the efficiency of photodissociation
is also affected by interactions within the capping layer.
Determination
of Nanoparticle Size and Ligand Shell Thickness
Using Differential Centrifugal Sedimentation
We have developed a
sensitive method, based on DCS (Differential Centrifugal Sedimentation),
which allows the rapid determination of nanoparticle
core size as well as ligand shell thickness [33],
and have used this to study the formation of protein coronas on gold
nanoparticles [39].
Synthesis
of Magnetic Cobalt Nanoparticles by Laser Irradiation
Magnetic nanoparticles have diverse applications in
biomedicine and as novel materials for engineering and devices, especially
in areas such as magnetic resonance imaging, targeted drug delivery, hyperthermia
treatment of solid tumours and cell separation. Their properties depend on
size and shape and on the nature of the ligands bound to their surface.
However, it is difficult to achieve a small nanoparticle
size with standard synthetic methods.
We have succeeded in synthesising monodisperse
cobalt nanoparticles with less than 5 nm diameters, using short laser
pulses to stimulate the rapid decomposition of cobalt carbonyl in
a solution of stabilizers [26].
By controlling the reaction conditions, i.e. ligand concentration and
wavelength of light, it is possible to control size and size distribution
of the nanoparticles. In particular, we found that light pulses at 355 nm
yielded larger nanoparticles with a broad distribution of sizes, whereas
light pulses at 266 nm resulted in smaller and monodisperse
nanoparticles, which are of particular interest for practical applications.
The different results most likely are due to the transient heating of the
solution which occurs upon irradiation at 266 nm in addition to photolysis
of cobalt carbonyl, which may cause a short burst of nanoparticle
nucleation, followed by a slower growth phase.
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