BIOMEDICAL APPLICATIONS II: INFLUENCE OF CARBON NANOTUBES IN CANCER THERAPY - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 71

In this device the conducting channel is realised by a single CNT or a
network of CNTs, usually modiied by attachment of speciic antibodies.
Another important characteristic is the property of CNTs to be p - or n -type
semiconductors: SWCNTs themselves are p -type semiconductors (conduction
in negative gate voltages). In general, both p -type and n -type semiconductors
can be obtained depending on the functionalisation, and the conductance
between the transistor and the absorbed molecules (i.e., proteins) can be
measured. The sensing eficiency depends on the alignment of CNTs, because
the molecule-binding events are controlled by the orientation of the entire
system. CNTs can be aligned during the synthesis or after that (i.e., by
micropatterning), and they can be aligned as forests or arrays, perpendicular
or parallel to the substrates.
In 2005, Kerman et al. reported an example of CNT-FETs on a silicon
dioxide/silicon (SiO 2 /Si) substrate based on peptide nucleic acid (PNA)
associated with tumour necrosis factor- α gene (TNF- α ), for real-time
electrical detection of DNA hybridisation with easy discrimination of single-
nucleotide polymorphism. 79 Tumour necrosis factor belongs to the cytokine
family and is responsible for the inlammatory response and for some
anomalies in the regulation process related to cancer (i.e., overexpression),
and so it can be considered a biomarker for early cancer detection. PNA is a
synthetic oligonucleotide, in which the phosphate and deoxyribose skeleton
is replaced by a polypeptide, and it can hybridise with complementary DNA
or RNA sequences. There are some advantages in using PNAs: their backbone
is neutral and does not suffer from electrostatic repulsions, the base pairing
is not inluenced by ionic strength and they are not altered by proteases and
nuclease degradation.
Source and drain electrodes were separated by a 4 μ m gap and the junction
was constituted by a single SWCNT, while the solid state gate was replaced
by molecules able to modulate CNT conductance. The biomolecules were not
adsorbed on the CNT, but on the gold electrode (AuE) with, on the backside,
a self-assembled monolayer (SAM) of PNA fully complementary to the wild-
type TNF- α sequence. The entire system was fabricated by photolithography
and conventional lit-off technique on a p +-doped-Si substrate with a layer of
natural SiO 2 (100 nm thick). The contacts between source and drain were
Ti/Au electrodes, and they were built, together with the side gate, onto the
patterned SiO 2 substrate after the growth of CNTs. The back gate contact in
Ti/Au was formed on the Si substrate. All these parts were in contact with
a microluidic chip in polydimethylsiloxane (PDMS), which is often used for
these applications. The backside electrode was modiied by a SAM of both
3-mercaptopropionic acid, used to link PNA (30 pM) by amidic bond, and a
SAM of mercapto propanol, used as a iller to prevent non-speciic adsorption
on gold (Fig. 3.15).