Chemistry Reference
In-Depth Information
The synthesis is based on a sol-gel procedure using surfactant such
as CTAB (cetyltrimethylammonium bromide) as template. The first
route consists in co-condensation under basic condition of tetraethyl
orthosilicate (TEOS) with chelating agent (such as DTPA) conjugated
with silane. The obtained solution of mesoporous nanostructure
is then mixed with GdCl
, which enables caging of Gd ions within
the chelates [34, 36]. The second route consists of condensation of
tetraethyl orthosilicate (TEOS) under basic condition. The obtained
mesoporous structures are then coated with Gd-chelates conjugated
with silane complex via a siloxane linkage [28].
As far as MRI contrast properties are concerned, the accessibility
of paramagnetic entities to water molecules is one of the most
important features to get high contrast agent relaxivities properties
[23, 24]. The large surface-to-volume ratio of mesoporous
nanomaterials appears to be an interesting property for the design
of sensitive contrast agent. In fact, Gd-containing mesoporous
nanomaterials present a remarkable relaxivity increase, compared to
free Gd-complexes or Gd entrapped in plain silica nanoparticles. This
enhancement in contrast properties is due to a concomitant effect of
the porosity of the structure, which enables free movement of water
molecules in and out of the structure, and of the interaction of Gd-
complexes with the silica structure, which impedes the rotational
movement of lanthanide complexes. This decrease in tumbling rate
of the paramagnetic unit consequently increases the relaxivities of
water molecules in the vicinity of the Gd-complex.
Five to ten fold increase in longitudinal and transverse
relaxivities was reported for mesoporous nanorods of 100 nm
with a 3/5 aspect ratio and a pore size of 2-3 nm [36], as well as
on spherical mesoporous nanoparticles of 20-50 nm with pore
size of 3.5 nm [37]. Lower enhancement on a per-Gd basis was
reported for larger nanospheres (600-700 nm with pore size of 8
nm) [37]. The difference in relaxivities between various mesoporous
nanostructures is attributed to the different accessibility of the
water molecules to the Gd-complexes, localized inside or outside
the silica pores [35]. It is hypothesized that only Gd-complexes
grafted outside the nanopores contribute to the observed relaxivity,
whereas the complexes grafted inside the nanopores do not modify
the relaxation of water molecules. This may be due to a hindered or
limited (slow) access of water molecules to the Gd-complexes inside
the pores. As the localization of the Gd-complex on the inner or outer
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