Biomedical Engineering Reference
In-Depth Information
which make them similar to the surfactant micelles. The individual poly-
mer molecules have a hydrophobic and a hydrophilic part and in water they
self-assemble and form vesicles with diameters in the nanometer range [36].
Unlike the surfactant micelles, due to the cross-linking of the particle pre-
cursor chains, the polymeric NPs are able to maintain stability regardless of
precursor chain concentration [36].
The amphiphilic polyurethane (APU) NPs are specifically developed for
remedial applications. Researchers have synthesized a number of APU par-
ticles using (poly)urethane acrylate anionomer (UAA) and polyethylene
glycol-modified urethane acrylate as precursor chains [36]. APU NPs can
potentially replace the traditional surfactants that are commonly used to
facilitate the remediation of hydrophobic organic contaminants (HOCs).
The HOCs readily sorb to soils or form nonaqueous phase liquids
[25,37], which makes it very difficult to remove them from the soil matrix,
using pump-and-treat remediation techniques. Because of similar rea-
sons, the conventional pump-and-treat methods have had limited success
in removing polynuclear aromatic hydrocarbons (PAHs) from the ground
[37]. Surfactants have been shown to significantly enhance the solubility of
HOCs and PAHs and improve their recovery rate. However, since they are
chemically unstable, the surfactant micelles are easily lost in the process. The
cross-linking of polymer chains within UAA particles makes them signifi-
cantly more stable than surfactant micelles, and having similar desorption
capabilities, they may replace them in the near future [36].
7.4.1.4 Ferritin-Encapsulated Metal Oxides
Proteins have attracted the attention of researchers for their ability to control
the formation of mineral structures. Cage-shaped proteins can function as
controlled environments where nanosized materials can be assembled and
encapsulated. An example of such a protein is ferritin: a protein able to store
iron. Ferritin is composed of 24 polypeptide subunits, which self-assemble
into three-dimensional, hollow complexes under certain conditions [38]. The
metal ions then become bound to sites in the central cavity. The diameter
of the assembled apoferritin (i.e., iron-free ferritin) is about 12 nm, and the
inside cavity is approximately 8 nm [25]. Iron molecules can diffuse into the
cavity through channels in the protein shell, where iron becomes mineral-
ized and converted to iron oxide (i.e., ferric oxyhydroxide) NPs [38].
The most promising potential benefit of ferritin for remediation is that it
can contribute to the photoreduction of contaminants [39]. Despite that nor-
mally Fe (III) is able to carry out significant photochemical processes, the Fe
(III)-bearing iron oxide quickly becomes photoreduced to Fe (II) and thus
deactivated [39]. The ferritin naturally converts Fe (II) to Fe (III) and the pro-
tein encapsulation of the iron oxide prevents its conversion back to Fe (II)
without inhibiting the rate of decontamination [39].
