Biomedical Engineering Reference
In-Depth Information
1.2.3
Self-Assembly and Bio-Assisted Synthesis
As mentioned in the earlier section for the case of CNT growth, the assembly of
nanostructures or nanoparticles in a periodically aligned fashion or in a functionally
engineered geometry is often desired over random or disordered structures.
Approaches for ordering or assembling nanostructures in a desired geometry have
been widely investigated in the past and are still pursued in many different labora-
tories. Self-assembly provides the most convenient way for alignment, especially
considering that at such small scales, single-structure manipulation and postgrowth
rearrangement appear to be very expensive if not at all impractical. Chemistry and
molecular biomimetics are the dominant fi elds leading the design of hybrid tech-
nologies combining the tools of molecular biology and nanotechnology. Molecular
biomimetics combines the physical and biological fi elds to assemble nanomaterials
using the recognition properties of proteins.
Researchers have found ways to process nanoparticles of a wide range of mate-
rials - including organic and biological compounds, inorganic oxides, metals, and
semiconductors - using chemical self-assembly techniques. In these cases, mole-
cules are attached, patterned, or clustered on specifi c substrates or to themselves
using chemical and biomolecular recognition (e.g., the preferential docking of
DNA strands with complementary base pairs). Other techniques, like micelle,
reverse micelle, and photochemical and sonochemical synthesis, are also employed
to realize one-, two-, and three-dimensional self-assembled nanostructures. Lately,
viruses have also been used to assemble specifi c nanostructures (Mao et al.
2004
) .
A review of the path to nanotechnology through biology is provided in (Sarikaya
et al.
2003
). It was shown that proteins, through their specifi c interactions with
inorganic and other macromolecules, could be used in nanotechnology to control
structures and functions just like they do biological tissues in organisms. Taking
lessons from nature, polypeptides can be genetically engineered to specifi cally
bind to selected inorganic compounds for specifi c applications, as outlined in Fig.
8
(Sarikaya et al.
2003
) .
The use of organic molecules is also a powerful technique in the synthesis and
arrangement of semiconducting nanoparticles, for example quantum dots, used in
biological labeling and tagging. One of the major issues in the synthesis of these
isolated islands of materials is to prevent particles' agglomeration and coarsening.
Micelle processes and organic ligands capping (TOPO) are synthesis methods based
on the simple mixing of surfactant with desired materials' sources. With these meth-
ods, it is possible to synthesize and/or align a wide range of nanomaterials spanning
from magnetic nanoparticles (i.e., Fe
3
O
4
particles found in magnetotactic bacteria)
to the bright CdS quantum dots used for bioimaging. Micelles are employed as
small chemical-reacting chambers in which nanomaterials are synthesized in fi xed-
sized cells (Fig.
9
). One of the fi rst examples of nanoparticle synthesis using a
reverse-micelle process was for fabrication of FePt nanoparticles (Sun et al.
2000
)
using a combination of oleic acids and oleyl amine to stabilize the monodisperse
FePt colloids and to prevent oxidation. With this method, the authors showed that it is
also possible to control the size of the synthesized nanoparticles and their composition,
