Biomedical Engineering Reference
In-Depth Information
9.4.2.1 Invesgaons on molecules@CNT conjugates
Similarly to Iijima, Kataura's group is mainly focused on the in-depth
investigation of molecules encapsulated inside CNTs; the group's studies
explore the electronic properties of the newly formed nanocomposites
and their characterisation down to the atomic level. With regard to this
topic, SWCNT functionalisation via encapsulation is beneicial for tuning
the physical properties of SWCNTs without losing their unique structure,
as demonstrated by several techniques. Among the available tools, EELS is
a very useful technique to examine the chemical composition and bonding
states of matter. Its combination with TEM enables even the identiication
of the atomic components of single molecules. However, in order to realise
single-atom spectroscopy, the application of high-energy incident electrons
is essential, with consequent massive structural damage of the specimens.
Decreasing the accelerating voltage below the typical displacement energy
threshold for carbon atoms ( 80-120 kV) has been suggested as a possible
solution, 215 but with drastic loss of resolution. It was proposed the doping of
SWCNTs with various metallofullerene molecules so that single atoms could
be isolated in each fullerene cage. 216 As a proof of that, of the authors were
able to visualize erbium (Er) atoms incorporated in fullerenes encapsulated
inside SWCNTs (Er@C 82 @SWCNTs), without displaying massive irradiation
damage.
However, despite the good resolution for the lanthanoids, it was much
more dificult to detect other molecules (e.g., potassium K or calcium Ca)
which are more common in biological systems. The reason for that could be
attributable to the fact that the atom might escape from the electron probe
(especially if such probe is too narrow); therefore, the time of exposure
under the electron beam should be increased to achieve better resolution.
Alternatively, a bigger probe could result beneicial, since it could cover the
overall space occupied by the fullerene and it does not allow the encapsulated
atoms to escape during the time scale of the spectrum acquisition. In every
case, EELS was demonstrated to be a useful technique to discriminate
between single atoms of different elements. Both lanthanum (La) (atomic
number Z = 57) and Er ( Z = 68) atoms were co-doped within fullerenes inside
SWCNTs (La and Er@C 82 @SWCNTs). Although it was not possible to visualise
individual atoms, the EELS chemical maps differentiated them on the basis
of different absorption energies (99 and 168 eV for La and Er, respectively),
thus demonstrating an eficient “atom-by-atom” labelling. The situation
became much more complicated with La and Cerium (Ce) ( Z = 58), in which
atoms have a smaller atomic number difference (∆ Z = 1) and close absorption
energies (99 and 109 eV, respectively). However, chemical identiication was
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