Biomedical Engineering Reference
In-Depth Information
capped by hemispherical ends, while the MWCNTs comprise several to tens of incommensurate
concentric cylinders of these graphitic shells with a layer spacing of 0.3-0.4 nm [151-153].
CNTs have excellent properties including high chemical and thermal stabilities, high elasticity,
high tensile strength, ultrasmall size, and variable conductivity [151]. It was found that CNTs
possess a high electrocatalytic effect and a fast electron transfer rate (ETR) [154-156]. MWCNTs
are regarded as metallic conductors, a highly attractive property for an electrode, while SWCNTs
can be metallic, semiconductors, or small band gap semiconductors depending on their diameter
and chirality [157-159]. The CNTs can act as electrodes, generate electrochemiluminescence in
aqueous solutions, and be derivatized with functional group that allows immobilization of biomol-
ecules. The CNTs have high surface-to-weight ratio, which is accessible to both electrochemistry
and immobilization of biomolecules. These properties, along with their favorable biocompatibility,
make them excellent intermedia for the development of effective, low-cost environmental biosen-
sors [160].
Purifi cation of as-produced CNTs is essential to applications as intermedia biosensors. One of
the most commonly used purifi cation methods involves oxidative acid treatment, such as refl ux-
ing in dilute nitric acid or refl uxing/sonication in a concentrated H
2
SO
4
/HNO
3
mixture [161]. The
acid treatment is crucial for electrochemical properties of nanotubes, because it not only makes
the nanotube stay dispersed in the solution but also leads to open-ended tubes containing either
dangling bonds in organic solvents, which undergo further chemical reaction [162] or oxygenated
functional groups such as quinones and carboxylic acids in polar solvents [161,163,164]. There
are two critical problems to introduce CNTs to electrochemical biosensors, namely, how to incor-
porate CNTs onto the surface of electrodes and how to immobilize biological molecule to the
electrode modifi ed with CNTs. The techniques to assemble CNTs into biosensors are discussed in
detail as follows.
14.2.1.1
Integration of Carbon Nanotubes [165]
14.2.1.1.1 Solution Casting Nanotubes onto Glass-Carbon Electrodes
As CNTs are insoluble in most aqueous solvents, they should be processed into a soluble product
before casting onto GCEs. Homogeneous dispersions of CNTs can be achieved by oxidative acid
treatments or
with the aid of surfactants or polymers [166]. When the CNT-casting solutions are
dropped directly onto the glassy carbon (GC) surface and allowed to dry, the electrode becomes
ready for use [167,168]. The CNT-coated GCE, prepared by modifying GCE with CNTs dispersed
in sulfuric acid [168], presented high sensitivity, low-potential, and stable amperometric sensing.
CNTs have the ability to promote NADH electron-transfer reaction and suggest great promise for
dehydrogenase-based amperometric biosensors.
SWCNTs treated with nitric acid during the purifi cation process have been reported [169]; the
carboxylic acid groups were introduced on the open ends of the SWCNTs. The nitric acid-purifi ed
SWCNT solution was cast on a GCE to form a CNT fi lm. The fi lm showed very stable electrochemi-
cal behavior and could be used to catalyze the electrochemical reaction of some biomolecules such
as dopamine, epinephrine, and AA.
In Wu's work [170], with the aid of hydrophobic surfactant dihexadecyl hydrogen phosphate
(DHP), MWCNTs were dispersed in water, achieving a MWCNT-DHP fi lm-coated GCE, exhibiting
remarkable electrocatalytic effects on the oxidation of dopamine (DA) and 5-hydroxytryptamine
(5-HT), improving their oxidation peak currents and lowering their oxidation overpotential. On
comparing with bare GCE, the modifi ed electrode shows effi cacy in detecting DA and 5-HT
simultaneously and exhibits excellent stability and reproducibility. Other surfactants, such as
N
,
N-
dimethylformamide (DMF) [171] and cetyltrimethylammonium bromide (CTAB) [172], can be
used to facilitate the dispersion of CNTs in aqueous solvents.
Comparing with common surfactants discussed above, polymers usually have more robust sur-
face adsorption because of more involved interaction sites [173]. “Wrapping” of CNT in polymeric
