Biomedical Engineering Reference
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X
−
H
N
O
O
O
O
OO
C
N
R
+
HN
NH
R
O
OO
OO
NH
1
+
C
N
R =
O
N
O
PF
6
−
, (1
R
,2
R
)
2a
: X
−
=
X
−
N
COO
1
: X
−
=
CIO
4
−
, (1
R
,2
R
)
CIO
4
−
, (1
S
,2
S
)
2b
: X
−
=
(a)
(b)
Scheme 5.2
The molecular structure of gelators used to prepare hollow (a) and helical
hollow transition-metal tubes (b).
using
trans
-(1
R
,2
R
)-1,2-cyclohexanedi(11-aminocarbonylundecylpyridinium) hex-
afluorophosphate (
1
in Scheme 5.2a) [23]. By replacing the counter ion PF
6
-
with
ClO
4
-
,thegelators
(Scheme 5.2a) formed chiral fibrous structures
in ethanol, and these act as template for the formation of helical hollow tubes
of transitionmetal (Ti, Ta, V) oxide (Figure 5.4a,b) [24]. In later work, helical
ribbon and double-layered nanotubes of TiO
2
were also prepared in a gel formed
by the assembly of a neutral dibenzo-30-crown-10-appended cholesterol gelator
(Scheme 5.2b) in 1-butanol [25].
Nanoribbons of copper sulfides were fabricated in an SMG with a
2a
and
2b
-glutamic
acid derivative as gelator (Figure 5.4c,d) [26]. The nanoribbons have a diameter of
30-70 nm and length of 1-10 μm. It was found that the copper ions (Cu
2
+
) were
first coupled with carboxylate anions (COO
−
) on the gelator molecules. With the
introduction of H
2
S, the preformed Cu nanoparticles on the fiber surface act as nu-
cleation centers for the growth of Cu nanoribbons. On the basis of X-ray diffraction
(XRD) and fourier transforminfrared spectroscopy (FTIR) results, a bilayermodel of
gelator assembly and templated nanoribbon formationwas proposed (Figure 5.4e,f).
Double-walled nanotubes of transition metal oxides TiO
2
,Ta
2
O
5
, and ZrO
2
,as
well as binary TiO
2
/ZrO
2
were preparedwith a cholesterol-based organogel [27]. The
gelator serves as both template and catalyst for the nanostructure formation. The
formation of double-walled nanotubes is due to the condensation of the transition
metal oxide on both outer and inner surfaces of the tubular structure of gel fibers.
Nanotubes of metal and transition metal oxide and sulfide including TiO
2
, ZrO
2
,
WO
3
, ZnO, ZnSO
4
,andBaSO
4
were fabricated using a tripodal cholamide-based
hydrogel (Scheme 5.3) by Rao's group [28]. In later work, nanotubes and nanorods
of CdS, ZnS, and CuS were also synthesized in the hydrogel based on this gelator
[29]. Necklace structures of iron oxide and copper oxide were prepared using a
structurally simple gelator 12-hydroxystearic acid and its sodium salt [30]. The
necklace feature is due to the linkage of individual nanoparticles located along
the gel fibers. In work by Dutta
et al
., TiO
2
nanoparticles were prepared in amino
acid-based SMGs with ionic liquids as solvents [31]. Ionic liquids are considered
to be alternatives to toxic and hazardous organic solvents. Their properties, such
as low vapor pressure, nonvolatility, high thermal stability, and nonflammability
l
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