Hollow Nanocapsules in Biomedical Imaging Applications - Nanotechnology for Biomedical Imaging and Diagnostics - page 87

[47, 48] to form polymersomes, it is possible to prepare vesicles with fairly high con-
trol of the size distribution and sizes ranging from tens of nanometers to tens of
micrometers. Polymersome membranes comprise a highly entangled hydrophobic
layer and two hydrophilic polymer brushes that stabilize the membrane. Both the
hydrophilic brush and the hydrophobic membrane usually have similar thickness
(Fig. 4.7a). Polymersomes form spontaneously by self-assembly of diblock or tri-
block copolymers. Typically, a diblock copolymer containing a hydrophilic and a
hydrophobic block assembles spontaneously in water to form a vesicle with 10-15 nm-
thick shells (Fig. 4.7a).
LBL assembly starts with a sacrificial core that is coated with sequential layers of
polyelectrolytes (Fig. 4.7b). After depositing the desired number of layers, the core
is dissolved in acidic medium. Repeated washings yield hollow microcapsules. These
microcapsules have been studied extensively for diverse applications, including bio-
imaging. They have been reviewed in detail in recent reviews [49-54].
Another example of formation of capsules by self-assembly of building blocks
(Fig. 4.7c) with narrow size distribution not requiring the use of templates was
recently reported [40]. Different dyes (fluoresceins, rhodamines) can be easily
entrapped inside these capsules. Another unique property of self-assembled polymer
capsules is formation of stable host-guest complexes with polyamines with recogni-
tion properties of the accessible molecular cavities exposed on the capsule periphery.
A different approach for self-assembly of hollow polymer NCs was used for choles-
terol-modified dextran (Chol-Dex) and polylactic acid (PLA) by dialysis of their
DmSO solution against water. The hollow capsules were formed initially by
anchoring of most amphiphilic polysaccharide onto the surface of swelled aggre-
gates because of the phase separation of amphiphilic Chol-Dex and hydrophobic
PLA. Followed by the deposition of PLA on the inner interface of polysaccharide
layer due to the “deswelling” of the aggregates, the remaining Chol-Dex inside the
aggregates formed the innermost hydrophilic layer. The resulting hollow capsules
may have potential application as drug and imaging agent carriers or biomembrane
models [55].
The defining functional features of a membrane are the selectivity of permeability
and the rate of mass transfer. The size-selective permeability is determined by the
size of the pores. Ideally, only molecules that are smaller than the pore size should be
able to diffuse across the polymer shell. The distinguishing characteristics of bilayer-
templated membranes are uniform molecular size pores that span the polymer mem-
branes. These pores offer very short path for molecules crossing the membrane. In
contrast, traditional polymer membranes, such as dialysis membranes, contain a
large network of interconnecting tunnels resulting in long diffusion times.
using different pore-forming templates [14], pores with different sizes were cre-
ated in vesicle-templated NCs. The size of the pores was determined by a size-selec-
tive retention assay. A mixture of colored size probes was encapsulated in vesicles
prior to the polymerization. After the formation of the polymer shell and removal of
surfactant scaffold and pore-forming templates, NCs were separated from the
released size probes by size-exclusion chromatography. The color of the capsules
with retained size probes indicated the pore size (Fig. 4.8).