Hollow Nanocapsules in Biomedical Imaging Applications - Nanotechnology for Biomedical Imaging and Diagnostics - page 82

Monomers

Polymerization

+

Scaffold removal

Bilayer-forming
molecules

Pore-forming
templates Polymer shell

Figure 4.2 Overview of vesicle-templated synthesis of NCs.

4.2 syNtHesis aNd cHaracterizatioN oF NaNocapsules

Functionally and structurally, NCs can be considered as simple models of cells and
their organelles. Not surprisingly, bioinspired synthetic strategies proved successful
for the fabrication of NCs [1-5]. Vesicle-templated assembly is based on the poly-
merization of hydrophobic monomers distributed in the hydrophobic interior of bila-
yers formed by lipids [6-8] or other surfactants (Fig. 4.2) [9-12].
In this approach, the bilayer acts as a spontaneously formed two-dimensional sol-
vent. The polymer network propagates laterally within the bilayer but does not grow
in the dimension orthogonal to the plane of the bilayer. The polymerization creates a
cross-linked network within the hydrophobic interior of the bilayer. After the poly-
merization, the lipid or surfactant scaffold can be removed to yield polymer NCs.
While outside the scope of this review, other structures, such as hollow silica micro-
spheres, can be prepared using bilayers as templates.
The permeability of the NC shell can be controlled by creating nanopores [13-
18]. To achieve this goal, pore-forming templates are loaded into the bilayer interior
together with monomers and cross-linkers (Fig. 4.3). After the polymerization, these
templates become embedded in the cross-linked polymer network within the bilayer.
These templates can be removed by chemical degradation or by washing NCs with
an organic solvent to yield uniform nanopores. This method of the pore formation
essentially constitutes molecular imprinting. When inert hydrophobic molecules are
used as pore-forming templates, such as glucose pentaesters, the chemical composi-
tion of the pore orifice is the same as the composition of the NC shell, that is, a
hydrophobic cross-linked polymer. An extension of the pore-imprinting method uses
pore-forming templates containing a polymerizable moiety connected to a bulky
group with a chemically degradable linker (Fig. 4.3). This template copolymerizes
with the monomers and cross-linkers. Cleavage of the linker and removal of the
bulky group create nanopores with functional group(s) in the pore orifice. Embedded
functional groups can be modified further using click chemistry. This method enables
controlling both size and chemical environment of nanopores, permitting control of
permeability in a broad range. Structural characterization of vesicle-templated NCs