Biomedical Engineering Reference
In-Depth Information
CNTs and POAS-coated CNTs, but it varies for the device when exposed to HCl. In
particular, it shows that the conductance of nanotubes slightly increases, while the
POAS-coated device shows a significant increment in conductance. It is interesting to
note that the response transient of POAS-coated device is a few seconds, while a sensor
based on resistance changes of POAS exhibits a poorer response time. The circumstance
to maintain .lm resistance below 1 k
, which is significantly lower with respect to that
reported for POAS sensors (about 50 M
), makes POAS-coated CNTs film integration in
electronic circuitry easier and cheaper, since lower direct current (DC) voltages are
required to drive the sensor response. The doping process of polyanilines is always asso-
ciated with conformational modifications of the polymer chains owing to the local
distortions created by the addition of H
ions to the basic sites and usually provides sta-
ble systems. It means that the conducting polymer in the doped form can be maintained
in this state for long periods of time till the material reacts with basic reagents and
strongly changes its chemical-physical properties. In other words, the reversibility of
the process is not spontaneous.
If we define sensor sensitivity (
S
) as the ratio
S
100, where
R
A
rep-
resents the resistance in air and
R
G
the resistance in vapor, the gas sensitivity increases
from
S
[(
R
A
-
R
G
)
R
A
]
3.0-27.9%. It reveals that by selecting proper polymer functionalization, sensor
sensitivity to HCl may be improved.
In conclusion, CNTs thin films prepared by pulsed RF-PECVD demonstrated their poten-
tiality as a new class of materials for HCl detection for environmental applications.
Moreover, polymer functionalization enhances the sensitivity to these devices. The advances
made here shall pave the way for future work in developing CNTs sensor arrays for highly
sensitive and specific molecular detection and recognition in gases and in solutions.
9.4
Nanostructured Cytochrome Matrices for Liquid Sensing
All known cytochromes are heme-proteins, which are involved in the electron transfer
process. P450 is the most versatile biological catalyst ever known (67,68). Cytochrome
P450 is known to catalyze a great deal of reactions such as hydroxylation, O-dealkylation,
and N-oxidation. Recombinant cytochrome P450s were employed, since recombinant
proteins are more suitable for technological applications. In fact, they represent the homol-
ogous population of molecules with a controlled sequence, which can be ad hoc modified
by site-specific mutagenesis, and they can undergo cheap mass production and further-
more have a high level of purity.
The presence of various important substrates for cytochrome P450s shows the possibil-
ity of utilizing the interaction between these enzymes and some specific substances for
biosensor applications. In this field, the immobilization of the enzymes onto solid support
is crucial. Several kinds of deposition techniques have been used over the last few years
to produce sensitive elements based on protein molecules. In the previous studies, we
used the layer-by-layer technique to immobilize P450 fusion protein (11,14), and the
spreading technique to develop P4501A2 nanostructures (6). By comparing the results to
the ones obtained by utilizing the Langmuir-Blodgett (LB) technique, it was found that in
all cases the latter is the most suitable method for producing sensitive cytochrome
matrixes. Moreover, this type of immobilization preserves the functionality of the
molecules under the same working conditions of the biosensors (e.g., room temperature
and dry conditions). Another work was concerned with an investigation of electron trans-
fer between cytochrome P450scc (CYP11A1) and gold nanoparticles immobilized on
