Biomedical Engineering Reference
In-Depth Information
(a)
Br
gold shell
growth
BMPA-stabilized
magnetite
nanoparticles
gold seed
nanoparticles
silica sphere
magnetic gold nanoshell
(b)
(c)
(d)
(e)
50 nm
50 nm
50 nm
50 nm
200 nm
Figure 7.4
The formation of silica - Fe
3
O
4
@Au
nanoparticles [41] . (a) 2 - Bromo - 2 -
methylpropionic acid (BMPA) - stabilized Fe
3
O
4
nanoparticles were covalently attached to the
amino-modifi ed silica spheres, which were in
turn coated with a Au layer; (b-e) TEM
images showing the different stages in the
preparation of silica - Fe
3
O
4
@Au nanoparticles:
(b) silica spheres; (c) silica spheres with
Fe
3
O
4
nanoparticles on the surface; (d) silica
spheres with Fe
3
O
4
and a gold seeds on the
outer layer; (e) the fi nal product.
groups and the amino groups. Gold seed nanoparticles of 1-3 nm were attached
to the residual amino groups of the silica spheres. Finally, a complete 15 nm-thick
gold shell with embedded Fe
3
O
4
nanoparticles was formed around the silica
spheres, resulting in what the authors referred to as a “magnetic gold nanoshell”.
Similarly structured gold nanoshells embedded with silica- stabilized magnetite
nanoparticles were fabricated by the groups of Stoeva
et al.
[42] and Salgauerino -
Maceira
et al.
[43]. More recently, Chen [44] and coworkers developed a multifunc-
tional silver nanoshell with a sandwich-like nanostructure which was composed
of a yolk-egg magnetic silica core, coated with a layer of silver, and with a thin
layer of gold sited between the core and shell. This had very high absorbance in
the NIR region, suggesting its possible application in the biomedical fi eld.
However, these syntheses yielded Au-coated nanoparticles that had an average
diameter of
30 nm [42 - 44] .
It has been shown previously that the cell uptake of nanoparticles is dependent
on the nanoparticle size, with a 50 nm diameter being optimal. Recently, the size,
charge, and concentration-dependent uptake of iron oxide particles by nonphago-
cytic cells was evaluated by Thorek and Tsourkas [45], who showed that a 107-nm
superparamagnetic iron oxide (SPIO) manifestation led to the largest T
2
signal
decrease. Thus, it is desirable to have particles with a mean size
∼
200 nm with a shell thickness of
∼
100 nm. In order
to meet these needs, a bifunctional gold-coated SPIO-silica nanoparticle was devel-
oped which had an average diameter of
<
100 nm, a strong absorption at the
NIR region, a high T
2
relaxivity value, and could be controlled by an external
magnetic fi eld [46]. The multistep synthesis of the Fe@Au nanoparticle is shown
in Figure 7.5 .
∼
