Biomedical Engineering Reference
In-Depth Information
allowed glucose measurements possible at a very low potential where physiological levels
of acetaminophen, uric acid, and ascorbic acid produced negligible signals. In addition, the
permselectivity of Nafion offered an effective discrimination against both neutral and
anionic electroactive interferents. Higher sensitivity can be obtained with the deposition of
platinum nanoparticles onto the CNT-Nafion-GOx-electrode [20]. The deposited platinum
nanoparticles with a diameter of 2-3 nm were in electrical contact with GC electrode
through the SWCNT. Such a biosensor resulted in higher sensitivity compared with the cor-
responding biosensors based on CNT or Pt-nanoparticles alone and offered a detection limit
of 0.5
M glucose with good selectivity and a fast response time of 3 s.
Luo et al. [21] reported on the use of electrochemically deposited nanocomposite of chi-
tosan and CNT for glucose biosensor. CNT dispersed in biocompatible chitosan solution
was electrodeposited to form CNT-chitosan nanocomposite, where the thickness of the
deposited nanocomposite film was controlled through the change of the concentration of
the chitosan solution. For the construction of glucose biosensor, GOx was added into the
CNT-chitosan solution before the electrodeposition. Such electrodeposition method offers
simple and controllable method for the preparation of reproducible biosensors.
Davis et al. [22] reported on glucose biosensor based on SWCNT and ferrocene media-
tor. Prolonged incubation of SWCNT with GOx led to an effective coating of GOx onto the
sidewalls of SWCNT without gross loss of enzyme activity. Although the GOx was ran-
domly physisorbed onto the SWCNT sidewalls, the redox process of the active sites of
GOx was communicated to the SWCNT
system through the diffusive mediator of fer-
rocene monocarboxylic acid to generate a quantifiable catalytic current. The magnitude of
this catalytic current was more than one order of magnitude greater than that observed at
an activated carbon electrode.
Gao et al. [23] reported on glucose biosensors based on aligned CNT coated with
polypyrrole (ppy) conducting polymer. GOx was immobilized by electropolymerization
of pyrrole in solution containing GOx, thus resulting in the formation of an enzyme-ppy
coaxial sheath around the individually aligned CNT. Fe particles generated on the CNT
during the production process led to an attractive low-potential detection of the GOx-lib-
erated hydrogen peroxide. Such biosensor (ppy/GOx/CNT) showed the electrical contact
between the enzyme and electrode through the CNTs and redox conducting polymer, and
exhibited at least one order of magnitude greater signal than that obtained at the corre-
sponding biosensor based on gold electrode (ppy/GOx/gold).
The earlier works discussed above took advantages of the bulk properties of CNTs.
However, another important feature of CNT is its ultrasmall size, which is useful for the
production of nanolectrode. Lin et al. [24] reported on glucose biosensors based on CNT
nanoelectrode ensembles (NEEs). To make each CNT work as an individual nanoelec-
trode, the spacing between the CNTs should be sufficiently larger than the diameter of
CNT to prevent the overlap of diffusion layer with neighboring electrodes. The NEEs con-
sisting of millions of nanoelctrodes (each electrode being less than 100 nm in diameter)
embedded in a polymer epoxy on a conductive substrate takes advantages of CNT mate-
rials over conventional macroelectrodes such as increased mass transport and the
decreased influence of the solution resistance, which provides an excellent electrode in
biosensing application. The GOx was immobilized on the CNT NEE using carbodiimide
chemistry by forming amide bonds between amine residues of GOx and carboxylic acid
groups on the CNT tips. The low-potential (
0.2V vs. saturated calomel electrode (SCE))
reductive detection of the hydrogen peroxide resulted in highly selective and sensitive
amperometric measurement of the glucose, along with linearity up to 30 mM and a detec-
tion limit of 0.08 mM.
Besides glucose, several different types of electrochemical biosensors have been reported
based on CNT-modified electrode coupled with oxidase enzymes. Qiaocui et al. [25]
