Civil Engineering Reference
In-Depth Information
Figure 2.25
A) Digital image of hybrid nanopapers (~70 m thick) with VO
2
(V) TiO
2
(Ti) and mixtures
of both (2np). B) Optical microscope images. C) transparency and l exibility of the hybrid. Reproduced
with permission from [255].
used together a synergic ef ect is observed and an even more marked ef ect is reported
over tensile modulus increment and elongation at break reduction [255].
2.4.5 Bacterial Cellulose Hybrids with Iron Oxides (BC/Fe
x
O
y
NPs)
h e main aim of preparing BC/Fe
x
O
y
NPs hybrid materials lies on the magnetic prop-
erties imparted by the iron oxide nanoparticles. h ese hybrids are predominantly pre-
pared by
in situ
formation of the Fe
x
O
y
nanoparticles (mainly magnetite, Fe
3
O
4
[209,
257-260, 262, 265, 266] and seldom hematite, Fe
2
O
3
, [260, 266, 267]) using adequate
Fe(II)/Fe(III) precursors either with nanoi bers dispersed in aqueous solution [221],
or wet membranes [209, 257-262, 265] and dif erent pH conditions, by solvo-thermal
treatments at dif erent temperatures. h e surface functionalization of Fe
x
O
y
NPs agents
with for example hexamethylene diamine [257] or ethylene glycol [265] reduces NPs
aggregation and improves their ai nity toward BC nanoi bers surface, and increases
considerably the homogeneity and the amount of NPs deposited. Finally, the prepara-
tion of BC/Fe
x
O
y
NPs has also been studied under regeneration conditions from dis-
solved BC [263]. As an alternative to aqueous solution approaches, Katepetch
et al.
[260] proposed a method to produce BC/Fe
x
O
y
NPs using gaseous ammonia as the
in situ
precipitation media, aiming at achieving a more homogeneous dispersion of
20-39 nm magnetic NPs. As for other metal nanoparticles
ex situ
prepared magnetite
NPs can also be included into the nanoi brillar network during BC biosynthesis in agi-
tated [258] or static (with the aid of a comb-like polymer to stabilize de NPs dispersion)
[209, 261] culture conditions. Under agitated conditions this leads to Fe
3
O
4
character-
istic dark grey BC/Fe
x
O
y
NPs hybrid spheres (Figure 2.26), with up to 33 wt% of mag-
netite with average size of 15 nm spherical NPs [258]; whereas under static conditions
in the presence of the comb-like polymer the NP clusters were perfectly embedded in
the BC nanoi brilar network [209, 261]. Furthermore, if the magnetic nanoparticles
are linked to the bacteria, their location and consequently the orientation of nanoi -
brils might be controlled magnetically during biosynthesis [261]. Finally, in a dif erent
approach, freeze dried BC membranes were also used as templates for the prepara-
tion of highly porous magnetic aerogels with 40-120 nm ferromagnetic cobalt ferrite
nanoparticles [256].
h e hysteresis loops of most BC/Fe
x
O
y
NPs has been measure in several studies [209,
259, 260, 265], pointing to a wide range of saturated magnetization values depending
on the Fe
x
O
y
NPs content and in the temperature. However, it is worth mentioning that
