Biomedical Engineering Reference
In-Depth Information
and then with Gd-DTPA to target them to epithelial and endothelial glycocalyceal
N-glycans and to generate contrast enhancement in MRI [49].
Lipophilic paramagnetic Gd-chelates have also been incorporated into perfl uo-
rocarbon nanoparticles, resulting in materials which have up to 55% higher relax-
ivity than the corresponding free Gd- chelate [50] . These Gd - loaded perfl uorocarbon
nanoparticles could be easily conjugated to antibodies to achieve specifi c localiza-
tion and imaging [51].
Another group of Gd-based nanoparticulate contrast agents are oxides loaded
with rare earth metals, the most commonly used examples being rare earth- doped
silica nanoparticles. For example, silica-Gd core-shell particles with a size of
71 nm were prepared by homogeneous precipitation from a water/propanol
solution of Gd(NO
3
)
3
, urea and polyvinylpyrrolidone (PVP) in the presence of a
suspension of the silica particles, followed by successive silica-coating using tet-
raethoxysilane (TEOS). These particles demonstrated relaxation enhancements
under MRI conditions [52]. Mesoporous silica nanorods have been fabricated via
surfactant-templated self-assembly under basic conditions, and subsequent mixing
with solutions of GdCl
3
salt and Dye@MSN-R produced novel fl uorescent and
paramagnetic potential contrast materials [53].
Gd-DTPA chelates were intercalated into Mg- and Al-based layered
double hydroxide (LDH) nanomaterials by anionic exchange. These novel para-
magnetic bar-like nanomaterials, which had widths of 30-60 nm and lengths
50-150 nm, demonstrated fourfold and 12 fold increases in
r
1
and
r
2
, respectively,
as compared to free Gd(DTPA) chelates in solution under the same reaction
conditions [54] .
In another study, semiconducting nanoparticles (quantum dots) or colloidal
metal (Au) nanoparticles were coated with thin silica shells, and covalently
linked to appropriate gadolinium chelates; the result was a series of nanocompos-
ites of 8-15 nm diameter, which demonstrated high relaxivities [55]. Gold nanopar-
ticles encapsulated by a multilayered organic shell composed of gadolinium
chelates, bound to each other through disulfi de bonds, have also been reported
[56, 57] .
Finally, new ultrasensitive pH - smart probes (so - called gadonanotubes ) have
been prepared by incorporating nanoscale, superparamagnetic Gd
3+
- ion clusters
within single - walled carbon nanotube s ( SWCNT s). These nanocomposites dem-
onstrated a high performance as T
1
- agents for MRI, with
r
1
180 m
M
− 1
s
− 1
, which
is about 40-fold greater than that of any current Gd
3+
ion - based clinical agent
[58, 59] .
Traditional synthetic methods for the fabrication of magnetic nanomaterials
include coprecipitation, hydrothermal and high- temperature processing, sol - gel
processing, microemulsion methods, fl ow injection syntheses, sonolysis and elec-
trospray synthesis, among others. These techniques are well established and have
been considered in other chapters of this Handbook, and also in several recent
reviews [7, 60-62]. In the following section, we present an overview of the methods
used for coating and surface functionalization, to produce stable and biocompat-
ible aqueous magnetic nanocomposite suspensions.
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