Biomedical Engineering Reference
In-Depth Information
terephthalate (PET) strips and dipped into a low ionic strength buffer, along
the reference electrode. The buffer acts as dielectric layer of the capacitor,
and the low ionic strength is crucial for successful detection of PSA; in fact
this device senses the additional charge induced when PSA interacts with
the antibody. For CNTs, the total device capacitance ( C t ) is given by the
electrochemical double-layer capacitance and by the quantum capacitance
( C q ), which, in the case of this 1D material, is extremely signiicant in the
determination of C t , thus making this device potentially more sensitive than
any other. CNTs were non-covalently functionalised with anti-human PSA
monoclonal antibody by simple exposure to a 5 μ g/mL buffer solution of
the antibody. The adsorption was conirmed and quantiied (about 15-20 ng
on the surface device) by Western blot analysis and ELISA. The device was
afterwards exposed to different concentrations of PSA, diluted in calf serum,
and a decrease in capacitance change was observed. The proposed device
seems to be very promising because it did not show any aspeciic recognition
of serum proteins except PSA. However, it is necessary to scale the entire
system to micro- or nano-dimensions, but it seems feasible with modern
micro- and nano-fabrication techniques.
Carcinoembryonic antigen (CEA) is another important biomarker to
be detected, its overexpression being indicative of cancer. It is involved in
cell adhesion, and generally its expression stops before birth. Even though
it is possible to reveal the presence of this marker in the bloodstream of
smokers, it can be related to cancer and is often used as a tumour marker
relapse after surgical operation. For CEA detection, Park et al . proposed an
FET produced by a patterned catalyst growth technique to synthesise CNTs
directly on the substrate. 84 They utilised a doped silicon substrate with
a thick oxide layer (500 nm), and SWCNTs were grown on this surface by
chemical vapour deposition. Afterwards, Ti/Au electrodes were obtained and
the active channel was constituted by SWCNTs. The construct was immersed
in a solution of carbonyldiimidazole (CDI)-Tween 20, which non-covalently
wrapped the nanotubes by hydrophobic interactions. The imidazoles
reacted with the amines of the anti-carcinoembryonic antigen (10 μ M, buffer
solution), and all the unreacted terminations were protected by ethanol amine.
Functionalisation with CDI-Tween 20 caused a leftward shift in the threshold
voltage, and the immobilisation of anti-CEA decreased the conductance for
CEA concentrations up to 40 ng/mL (200 nM). The sensitivity was increased
using SWCNT-FET, with a higher on-off ratio, reaching values of 54 pg/mL
(200 fM).
There are several examples in which CNTs are employed to modify
electrodes for electrochemical measurements. This use is strictly related to
CNTs' ability to amplify signals, which is a widely reported characteristic.
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