Biomedical Engineering Reference
In-Depth Information
Figure 12.7
Schematic representation of the synthesis of gold
nanoshells with superparamagnetic
γ
- Fe
2
O
3
cores.
of ammonia catalyst and water) could be employed to control the thickness of
silica shell, it proved to be more convenient and reproducible to adjust the shell
thickness by changing the concentration of the TEOS precursor. With this
method, thicknesses ranging from 2 to 100 nm could be obtained [110]. Then, in
order to produce amino-terminated silica surfaces, treatment with NH
4
OH and
3-aminopropyltrimethoxysilane was carried out. The gold nanocrystals seeds
(2-3 nm) were then attached to the amino groups by the reduction of chloroauric
acid (HAuCl
4
) with tetrakis(hydroxymethyl)phosphonium ( THPC ). The attached
gold nanoseeds were used to nucleate a gold overlayer on the silica surface so as
to form a gold nanoshell (Mnp/Si/Au). The thickness of the gold shell was tuned
by varying the concentration of the Au seeds and Mnp/Si particles, while the
concentration of the gold precursor was kept constant. In the last step, the
nanoshells were coated with PEG by treating the Mnp/Si/Au nanoparticles with
a monofunctional PEG precursor. The PEG coating, as confi rmed by Fourier
transform infrared (FT-IR) spectroscopy, afforded good temporal particle stability
in water (i.e., several weeks at 4 ° C).
The TEM images confi rmed the presence of
- Fe
2
O
3
nanoparticles encased in
silica, although most of the silica shells contained more than one magnetic core.
This could be avoided by optimizing the ratio between the concentration of iron
oxide nanoparticles and TEOS [110]. The mean diameter of the silica-coated iron
oxide nanoparticles was estimated as 66
γ
9.5 nm. After treatment with HAuCl
4
and THPC, the gold nanoseeds could be clearly observed on the silica surface.
Finally, TEM images of Mnp/Si nanoparticles coated with Au nanoshells showed
the gold coating not to be continuous, but to have a topographical roughness at
the nanometer scale. The particles had a mean diameter of 82.2
±
±
9.7 nm, and the
gold shells a thickness of approximately 8 nm.
In order to verify whether the Mnp/Si/Au nanoparticles exhibited any magnetic
properties that could be used for magnetic fi eld-guided targeting, a number of
investigations were performed. As an example, a nanoparticle dispersion in water
was prepared and a Nd
0.3 T) then used to separate the Mnp/Si/
Au. The solution, which initially was dark, became almost transparent after a
−
Fe
−
B magnet (
∼
