Biomedical Engineering Reference
In-Depth Information
field of analytical chemistry [66,131,132] and physics [133]. The first type of CNT electrodes
were simply assembled by packing a paste of CNTs and binder materials (such as mineral
oil [135-137], bromoform [75], and liquid paraffin [139]) into a glass capillary or a Perspex
with a narrow cylindrical slot. The resulting CNT electrodes exhibited an improved
reversibility of dopamine oxidation [137] and a good reversible voltammetric response to
cytochrome C [140] and zaurin [138]. From the perspective of electrochemical biosensor
application, enzyme-modified CNTs have shown superior performance by means of effi-
cient electron transfer reactions between the underlying electrode and the redox protein,
even when the redox center is embedded deep within a glycoprotein shell, such as hemo-
globin, cytochrome C, zaurin, and horseradish peroxidase. The desired enzymes (such as
glucose oxidase and alcohol dehydrogenase) and NAD(P)H cofactors could be incorpo-
rated within the 3D CNT/Teflon matrix. The resulting composite was then packed firmly
into the electrode cavity as a sensor electrode, which performed effective low-potential
amperometric biosensing of glucose and ethanol [141]. In contrast to the methods using
CNTs as an aligned nanoelectrode array [142,143] or an electrocatalytic modifier that will
be described below, the composite device has major advantages in terms of minimization
of surface fouling and surface renewability.
12.5.1.1.2 Carbon Nanotubes Used as Catalytic Mediators
With the major barrier for solubilization of CNTs overcome, solubilizied CNTs have been
widely used as catalytic modifiers in connection with another electrode surface. A straightfor-
ward approach to handle CNT positioning consists of proton-conducting and bioreactive
polymers, such as Nafion, Teflon, or Ppy, that are used to immobilize CNTs on electrodes
[144-148]. The combination of the antifouling/discriminative properties of a Nafion film with
the efficient electrocatalytic activities of CNTs provides a useful avenue for improving the sen-
sitivity of biosensors. For example, the CNT/Nafion-modified glassy carbon electrode dra-
matically improved the detection of catecholamine neurotransmitters by means of giving an
enhanced dopamine response and discrimination against ascorbic acid [149]. Redox-polymer
hydrogels provide another candidate for constructing CNT-based amperometric enzyme
biosensors (Figure 12.16) [150]. The incorporation of SWCNTs modified with enzymes (such
as glucose oxidase and horseradish peroxidase) into redox hydrogels resulted in a two- to
threefold increase in the sensor's current output while the amount of electrochemically acces-
sible osmium redox centers increased up to 10-fold. The attractive performance of the CNT-
coated electrodes or CNT/binder composite electrodes for low potential detection of
hydrogen peroxide and NADH suggests great promise for dehydrogenase- and oxidase-based
amperometric biosensors, such as glucose, organophosphorus pesticides, and for nerve agents
[151]. The experimental approach for organophosphorus pesticides and nerve agent analysis
involves coimmobilization of CNTs and acetylcholine esterase/choline oxidase enzymes on a
screen-printed electrode. The enzyme can be anchored on the CNTs with carboxylic acid
groups that are created by electrochemical treatment. The large surface of the CNTs and the
catalytic activity of CNTs promote the redox reaction of hydrogen peroxide produced
enzymatically, which provide an ideal model for a highly sensitive analysis system based on
hydrogen peroxide detection. However, it should be noted that the metal impurities present in
the nanotubes could also act as the catalytic sites.
12.5.1.1.3 The Enhanced Catalytic Performance of Nanoparticles and Carbon Nanotube
Complexes
The catalytic features of metal NPs such as gold, Pt, silver, copper, and nickel allow the
CNT/metallic NP composites to act as excellent sensor materials [151-154]. We recently
confirmed that combining metal particles such as Pt NPs with CNT/Nafion-modified
electrodes (Figure 12.17) provides a means of further improvement in the sensitivity for
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