BIOMEDICAL APPLICATIONS II: INFLUENCE OF CARBON NANOTUBES IN CANCER THERAPY - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 49

a role as sensors. Many examples concerning CNTs will be reported in the
next paragraphs. Scanning probe microscopy, as atomic force microscopy
(AFM) and high-resolution atomic force microscopy, improved the design
of new biosensoring systems because it overcomes the restriction of optical
imaging, related to wavelength and molecular object dimensions. At the state
of the art, the design of sensors based on nanoarrays is possible by grafting
molecules in nanopatches with nanolithography. 8,9
3.2 CARBON NANOTUBES: A BRIEF OVERVIEW
It is widely reported in literature that CNTs present interesting electronic
and chemical properties, as well as mechanical strength and thermal
stability. Their high aspect ratio (length/diameter) makes CNTs extremely
important substrates to develop biosensors with high-packaged
immobilised biomolecules, and their wide surface area is highly accessible to
electrochemical manipulation.
Their dimensions and the possibility of modifying their external surface
by functionalisation with suitable chains and molecules (Fig. 3.2) or of illing
their cavities with bioactive agents render CNTs optimal as drug vectors,
but it is necessary to clarify their toxicity effects. It is possible to roughly
summarise that cytotoxicity is higher for longer CNTs, 10 but it is also related to
the presence of catalytic metals used in CNT preparation and to the solubility
of the analysed compounds. In this scenario, a preliminary puriication is
often needed and chemical functionalisation is the keystone to achieving
good solubility in biocompatible solvents and to decreasing toxicity.
Covalent and supramolecular functionalisations are currently used. The
irst affects the inherent properties of CNTs but, at the same time, permits to
link a high number of biomolecules on the nanotube surface, as is necessary
in biosensor design. Many different reactions on CNTs are reported in
literature. 11,12
The attachment of biomolecules, anticancer drugs or radionuclides to
CNTs, through covalent or non-covalent functionalisation, transforms CNTs
into vectors or electronic devices for biosensoring, leading to multifunctional
nanosystems. 13 Nevertheless, the two approaches can be adopted
simultaneously: Pantarotto et al . reported the functionalisation of CNTs by
1,3-dipolar cycloaddition combined with the attachment of plasmid DNA,
through electrostatic forces, for gene delivery in cancer therapy. 14