Biomedical Engineering Reference
In-Depth Information
Organogels were also used to prepare conducting polymer nanostructures
[53]. The associated problems are that the conductivity of the polymer struc-
tures is affected by the gelator molecules, and removal of the gel component
affects the stability of the polymer structures. To circumvent this drawback,
an organogel based on an amphiphilic sulfonic acid dopant was used to pre-
pare polyaniline conducting nanofibers [54]. The dopant molecules form an
organogel with tubular structure in polar solvents. Aniline monomers occupy
the hydrophobic nanopockets of the tubular structure. Upon chemical oxidative
polymerization, well-defined polyaniline nanofibers with controllable conductivity,
solubility, and crystallinity were produced. In other work, silver/polypyrrole com-
posite nanowires was fabricated in a small-molecule organogel [55]. In this study, a
tripodal ligand (tripyridin-3-yl-benzene-1,3,5-tricarboxylate) was synthesized. This
ligand (gelator) gelates various organic solvents in the presence of AgNO
3
to
form coordinated organogels. Upon polymerization of pyrrole monomers (added
into the solvent before gel formation), composite silver/polypyrrole nanowires
formed.
Sn nanoparticles encapsulated in carbon were prepared in gels formed by the
self-assembly of oleic acid and hexadecylamine in ethylene glycol with the addition
of NaOH [56]. To get such a composite (core-shell) structure, Sn nanoparti-
cles were synthesized in the gel through the reduction of SnCl
2
with NaBH
4
,
which was followed by calcining the gel directly under nitrogen. Such an ap-
proach can be promisingly used to fabricate other nanoparticles encapsulated in
carbon.
5.5
Controlling Crystal Growth of Pharmaceutical Substances
Controlling the growth of pharmaceutical crystals has practical significance in
terms of producing high quality pharmaceutical products. SMGs were also used as
media for the controlled growth of crystals. Growth of a range of crystals of organic
compounds including pharmaceuticals was achieved in gels formed by four bis
(urea) gelators including a metallogelator (Scheme 5.5) [8a]. These pharmaceuticals
include carbamazepine, sparfloxacin, piroxicam, theophylline, caffeine, ibuprofen,
acetaminophen (paracetamol), sulindac, and indomethacin. In many cases, crystals
were conveniently recovered using anion-triggered gel supramolecular di-assembly
(gel dissolution) (Figure 5.8), except for those that bind to anions. Controlled crystal
growth has been achieved in other gel media such as agarose, gelatin, and silica
[5b, 57]. The advantage of using SMGs is greater flexibility in solvent selection.
By designing the molecular structure of the gelator, not only water but also a
range of organic solvents can be gelated for controlled crystal growth. In addition,
the responsive character of some gels to stimuli (chemicals) offers another big
advantage. That is, the recovery of the products can be achieved under moderate
conditions without dissolving the gels at high temperatures, which is destructive
to many materials.
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