OVERVIEW ON THE MAJOR RESEARCH ACTIVITIES ON CARBON NANOTUBES BEING DONE IN AMERICA, EUROPE AND ASIA - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 274

By comparison among the three synthesised samples, it emerged that
the urea-functionalised SWCNTs (U-F-SWCNTs) showed the highest stability
of the tubes' dispersion in DMF, water and aqueous urea solutions; though
urea is extremely soluble in water, it can be still drained to the van der Waals
forces within the F-SWCNT bundles, intercalate, wrap and thus separate
them, thereby creating new opportunities for biomedical applications with
nanotubes.

Pyridine, heat

F F HN-(C=X)N H 2

F H 2 N-(C=X )N H 2

-HF

(X = O, NH, S)

Figure 9.20 Functionalisation of F-SWCNTs with urea (X = O), guanidine (X = NH) and
thiourea (X = S).

The same F-SWCNTs were also treated with either branched (MW between
600 and 25,000 Da) or linear (MW = 25,000 Da) polyethyleneimine (PEI), 120
providing the covalent attachment of the polymer to the sidewalls of the
nanotubes in the form of PEI-SWCNTs. As expected, the number of polymeric
units per SWCNT was larger for low molecular weight PEI. However, above
1,800 Da, such value did not vary remarkably, as conirmed by Raman
spectroscopy. Solid-state 13 C NMR indicated the presence of carboxylate
substituents as a consequence of the reversible CO 2 adsorption to the primary
amine substituents of the PEI. The authors also showed how to accomplish
the desorption of CO 2 by heating under argon at 75°C, opening the way for
further investigations.

9.2.4.3 Opcal properes of nanomaterials

Carbon nanotubes represent a remarkable section towards advances in
analytical nanotechnology, especially on the basis of their optical properties.
SWCNTs have been recently shown to be exceptional NIR luorophores
that display high optical anisotropy 121 and an absence of luorescence
intermittency, 122 which are characteristics not present in quantum dots
or luorescent dyes and make them extremely promising for applications
as luorescent markers and biosensors. Weisman and collaborators have
been focusing on the photoluminescence properties of these carbon-based
nanomaterials, demonstrating that CNTs' structure can be modiied to detect
individual molecules. It is important to note that, as the excitation intensity