Biomedical Engineering Reference
In-Depth Information
electrons are involved, i.e., in case of the so-called charge transfer doping.
153
Results showed that a large paramagnetism was found if a small percentage
of electrons (0.02-4%) was introduced to the C nanosystem, thus conirming
that doping of CNTs can affect their physicochemical properties.
The authors have also described an
in vitro
phagocytosis assay with
Entamoeba histolytica
to assess the biocompatibility of undoped MWCNTs
and N-doped (CN
x
) MWCNTs.
154
E. histolytica
is a protozoan parasite whose
life cycle oscillates between the infective (cyst) and the invasive (trophozoite)
stages. The latter is a mobile form.
E. histolytica
trophozoites are one of the
most actively phagocytic and proteolytic cells in nature, able to phagocyte
cells of the same size or even bigger, as well as to destroy almost every human
tissue, including bones. In most cases, they damage the intestinal epithelial
surface, causing intestinal amoebiasis or dysentery.
The nanotubes used in these experiments were pure MWCNTs and CN
x
MWCNTs, with nitrogen content of 2-4 wt%. The samples were subjected to
strong acidic conditions (H
2
SO
4
: HNO
3
in a 3:1 ratio), which are known to cut
the tubes. Results after 24 hours of incubation showed that the viability of
the trophozoites was not affected by the CN
x
MWCNTs in the culture medium.
Moreover, the mobility and shape of amoebas were not altered by the presence
of CNx MWCNTs. In contrast, undoped (pure carbon) MWCNTs severely
affected the viability of the trophozoites, with a signiicant decrease (about
50%) after 8 hours of incubation. This seems to be due to stronger van der
Waals interactions in pure MWCNTs, which promoted a larger agglomeration
of the nanotubes. In every case, the length of both types of nanotube found
inside the cells is very similar (<2 mm). These indings encourage further
study using CNx MWCNTs as drug delivery systems and cell transporters.
9.2.6.2
Electrical properes of CNTs
Spectroscopy, especially Raman scattering, has been largely used to study
defects in sp
2
carbon materials, and also doping phenomena. This is because
the so-called G band is a well-known feature that is observed in the Raman
spectra of all sp
2
carbon materials, including nanotubes, nanohorns and
nanoribbons. After doping, a new G
D
(where D stands for “defect”) peak is
observed at lower/higher frequency for N/P doping.
155
As a defect site is a
structure localised at the atomic level, it cannot be detected with accuracy;
therefore, usually near-ield Raman and near-ield photoluminescence
imaging and spectroscopy are employed to spatially resolve local defect
sites along an individual SWCNT. In fact, bright photoluminescence emission
originates from a defect site along the SWCNT. Once the defect site is located,
it is possible to monitor the G band and show that a clear change in the G line
shape is measured near the defect site, as expected.
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