Biomedical Engineering Reference
In-Depth Information
amino silanes as an initial “sticky” group for the surface from which a shell can
grow [104]. Recently, Dravid
et al.
used pure Co nanoparticles prepared by aqueous
reduction with NaBH
4
and modifi ed their surfaces with APTES as an initial
binding layer [105]. This report demonstrated that the Co core size was a function
of the initial Co/citrate ratio and, more importantly, that the SiO
2
shell thickness
was determined by the subsequent APTES : TEOS ratio [105] .
Ultimately, the goal of these encapsulating procedures is to produce nanopar-
ticles capable of simple functionalization via siloxane reagents. Undoubtedly, the
majority of these reports involve the use of trialkoxy-aminopropyl silane reagents
for the attachment of metals salts [106], biomolecules [82d, 83, 107], or fl uoro-
phores [108]. However, there have been many reports of alternative applications
of these materials. Chen and Chen attached octadecyl chains to iron oxide@SiO
2
as a method of rapidly desalinating DNA samples [109]. By employing microwave
heating, these composite particles trapped DNA at concentrations of up to 625
pmol mg
- 1
nanoparticles [109]. Xue and coworkers employed Fe
3
O
4
@SiO
2
particles
as the substructure for magnetic fl uorescent structures [110], where the magnetic
nanoparticles were fi rst encapsulated with a SiO
2
shell in a reverse micelle, after
which quantum dots (QDs) were added directly to the silanization solution together
with additional reactants to form more SiO
2
on the particle surface [110]. The
∼
80 nm-diameter composite particles retained the strong QD luminescence (blue
shift
20 nm), although the magnetic moment was reduced to 7% of the pure
Fe
3
O
4
sample [110] .
Silica shells have also been used as intermediates for subsequent encapsulation
in other materials. Chen and coworkers reported the use of SiO
2
as an intermedi-
ary for TiO
2
shells that could then be used to break down biological materials
under ultraviolet (UV) irradiation [111]. Sunkara and Misra, on the other hand,
used TiCl
4
to directly form a titanium oxide shell on the surface of NiFe
2
O
4
nanoparticles (i.e., NiFe
2
O
4
@TiO
2
) [112]. Tungsten doping of the TiO
2
shell further
enhanced the effectiveness of the photocatalyst [112]. Qian
et al.
recently prepared
Fe
2
O
3
@SiO
2
particles with an outer shell of lanthanide-doped NaYF
4
, yielding
magnetic particles with luminescent surfaces that could also be functionalized
with siloxane chemistry [113]. Finally, Chen and coworkers recently used a thin
layer of SiO
2
to coat iron oxide nanoparticles and enable subsequent deposition of
a shell of Al
2
O
3
using aluminum isopropoxide [114].
∼
14.3.5.2
Encapsulation: Metallic and Semiconductor Shells
Au-thiol monolayer chemistry is arguably the most recognizable method of func-
tionalizing nanoparticles. Signifi cant efforts have therefore been made towards
encapsulating magnetic nanoparticles in Au, or analogously in Ag. In addition to
their well- defi ned surface chemistry, noble metal shells are expected to have
surface plasmon waves with resonant frequencies in the visible to near-infrared
(NIR) region of the spectrum.
Encapsulating magnetic nanomaterials in metallic shells can be synthetically
diffi cult because of the surface energies, lattice matching, and wettability, in addi-
tion to the need to selectively deposit metal on the surface of existing particles
