Biomedical Engineering Reference
In-Depth Information
high chemical stability, high surface area, and unique electronic properties (
Sherijara et al.,
2003; Balasubramanian and Burghard, 2006; Merkoci, 2006
).
Lu et al. (2007)
point out that
CNTs have excellent electocatalytic activities (
Lin et al., 2005
). Furthermore, they promote
electron transfer reactions involving hydrogen peroxide (
Wang et al., 2003
), NADH
(
Musameh et al., 2002; Wang and Musameh, 2003
), cytochrome (
Wang et al., 2002a
), and
AA (
Wang et al., 2002b
).
Lu et al. (2007)
also report that there is one drawback in the use
of CNTs which is the high price of the CNTs. This can range from twenty to hundreds of
dollars per gram.
Lu et al. (2007)
report a more affordable alternative to CNTs which is exfoliated
nanoplatelets (xGNP). The platelets, according to these authors consist of sp
2
hybridized car-
bon atoms which are arranged in a sheet-like structure unlike the cylindrical geometry found
in CNTs. These authors have demonstrated the potential use of graphite nanoplatelets in
biosensors. Glucose oxidase (GO
x
) was used to develop a glucose biosensor. GO
x
catalyzes
the oxidation of glucose to gluconolactone (
Wise, 1989
):
Glucose
þ
O
2
!
gluconolactone
þ
H
2
O
2
ð
3
:
2
Þ
Glucose is made quantitative by the electrochemical detection of H
2
O
2.
Lu et al. (2007)
point out that there are several methods to immobilize enzymes for glucose
biosensor applications. A common method to prepare biosensors is to use Nafion encapsula-
tion (
Wise, 1989
).
Lu et al. (2007)
indicate that Nafion is a sulfonated tetrafluoroethylene
copolymer that has been used as a proton conductor for proton exchange membrane in fuel
cells (
Rizukawa and Sanui, 2000
), and in biosensor applications (
Fan and Harrison, 1992
).
Lu et al. (2007)
point out that the main advantages for using Nafion in biosensor applications
is its biocompatibility, excellent thermal and mechanical stability, mechanical strength, and
antifouling properties. These authors have developed a highly sensitive and quick responding
glucose biosensor using the combination of xGuP (graphite nanoplatelets), GO
x
, and Nafion.
Drzal and Fukushima (U.S. patent application
20040127621
) have previously demonstrated a
method to produce exfoliated graphite nanoplatelets from natural crystalline graphite by micro-
wave and milling.
Lu et al. (2007)
point out that a major challenge of using graphite
nanoplatelets in biosensor applications is their insolubility in most enzyme compatible solvents
owing to their large hydrophobic basal plane. These authors report that the polyelectrolytes used
earlier to solubilize CNTs can also disperse graphite nanoplatelets. These polyelectrolytes are
poly(diallydimethylammonium chloride) (PDAC), sulfonated poly(styrene) (SPS), and poly-
ethyleneimine (PEI) (
Lu et al., 2007
). They indicate that Nafion is a negatively charged poly-
electrolyte and can be used to suspend graphite nanoplatelets in water or alcohol.
Finally,
Lu et al. (2007)
have demonstrated that graphite nanoplatelets may be used as an
inexpensive alternative to CNT for the fabrication of a highly sensitive and fast responding
