Civil Engineering Reference
In-Depth Information
aggregates that decrease the benei ts associated to the presence of nanosized i llers. For
this reason, many dif erent methods can be considered for the combination of metal-
lic nanoparticles and cellulose-based nanoreinforcement. h e preparation of cellulose/
metal nanocomposites by the
in-situ
reduction of metal salts in cellulose aqueous sus-
pensions has been extensively investigated for the production of silver [75, 76], copper
[77] and platinum nanoparticle hybrids [78, 79]. Typically, this involves the use of a
soluble metal salt as precursor, a reducing agent and a co-stabilizer to avoid agglomera-
tion. In the presence of cellulose surfaces, the
in-situ
method can be indeed employed
without addition of an external reducing agent, because adsorption of metal ions on
the cellulose surfaces may be subsequently reduced to metal nanoparticles by organic
moieties such as terminal aldehyde or carboxylic groups [80, 81]. At the same time,
the polymer chains play an important role leading to a narrow size distribution and
well dei ned shape for the metal nanoparticles [82]. Another commonly used
in-situ
approach to prepare metal dispersion in cellulose matrices, involves the entrapment
of metal cations in the i bers followed by their reduction with an external reducing
agent. Sodium borohydride (NaBH
4
) has been extensively used to reduce metal ions
in cellulose matrices. h e particle size distribution is adjusted by varying the NaBH
4
:
metal salt molar ratio. Some reports have described the loading of silver nanoparticles
into grat ed i lter paper [83]and bacterial cellulose [84]. h e
in-situ
reduction using
UV irradiation is another simple method to produce metal nanoparticles on the sur-
face of cellulosic reinforcements. h e preparation of the nanocomposites is based on
the photo-activation of cellulose surface by photons: for cellulose/Ag nanocomposites
[85, 86] UV light intensity and time of irradiation were demonstrated to be important
parameters to control the amount of silver, its dispersion and morphology. h e elec-
trostatic assembly of nanoparticles is another useful technique for the preparation of
cellulose-metal nanoparticles hybrids, based on the sequential adsorption of oppositely
charged species on a solid substrate. h is technique of ers some advantages over other
methodologies due to the possibility of a better control of inorganic content in the i nal
nanocomposites with full control of nanoparticles size and morphology. h e use of a
positively charged polyelectrolyte as the outer layer favored electrostatic interactions
of cellulose with negatively surface charged gold nanoparticles. h is methodology has
been also applied to the fabrication of Ag/NFC composites using distinct polyelectro-
lytes as macromolecular linkers [87]. Finally, chemical modii cation of cellulose can
be performed to produce distinct types of cellulose/metal nanocomposites: chemical
surface modii cation of hydroxyl groups into aminic groups, which act as selective
coordination sites [88] and the use of thiol labelled cellulose through spontaneous che-
misorption [89] has been demonstrated, such as the fabrication of size-controlled metal
nanowires using cellulose nanocrystals as biomolecular templates has been reported
[90] .
Most papers on the subject deal with the use of silver nanoparticles used in conjunc-
tion of cellulose-based materials for antimicrobial application (as in the case of food
- packaging, water treatment, wound dressing). Some works are nonetheless based on
hybrid nanoparticles of gold (Au), platinum (Pt), copper (Cu) and cobalt (Co) in pres-
ence of cellulosic templates can be also found and interesting applications as biosen-
sors, electronic devices and actuators, fuel cells, drug delivery and conducting systems
for medical diagnosis, environmental control and food safety were investigated. In
