Biomedical Engineering Reference
In-Depth Information
electrochemical activity, the sidewalls of CNTs are believed to have properties similar to
those of the basal plane of highly oriented pyrolytic graphite (HOPG) while their open
ends resemble the edge planes of HOPG.
Enzymes can be simply mixed with polymer to produce CNT-based electrochemical
biosensor. Since CNT is insoluble in most solvents, a number of efforts have been made to
disperse CNTs in a variety of appropriate solvents. The reported CNT-modified elec-
trodes, which relied on casting CNTs onto a GC surface after dispersing them in bromo-
form [9] or dimethylformamide [11], are not compatible with the immobilization of
biomolecules. Therefore, some biocompatible polymers such as Nafion, titania-nafion,
Teflon, and sol-gel silica have been used for the solubilization of CNTs and the immobi-
lization of biomolecules onto the electrode surface.
Since Nafion is known to contain both hydrophobic domain composed of fluorocarbon
skeleton and hydrophilic domain composed of sulfonates, CNT can be solubilized in solu-
tions of Nafion in phosphate buffer or alcohol [8]. The use of Nafion permits easy prepa-
rations of CNT-based electrochemical biosensors with outstanding performances because
of the biocompatibility and charge discrimination ability of Nafion. For effective biosens-
ing of glucose, glucose oxidase (GOx) was incorporated together with CNT on the surface
of GC electrode.
Similar to Nafion, the composite materials composed of sol-gel-derived titania (TiO
2
)
and Nafion (1:1, v/v) can also solubilize both SWCNT and MWCNT [12]. The solubility of
CNT in the solution of titania-Nafion composite is strongly dependent upon the Nafion
content. The Nafion content in the composite ranging from 5-0.7 wt% can solubilize the
single-wall and multiwall CNT. In contrast, the composite containing 0.5 wt% Nafion can
solubilize single-wall CNT but cannot solubilize multiwall CNT. This behavior is exactly
the same as observed in pure Nafion solution. This fact indicates that the titania-Nafion
composite solution does not impair the solubility of Nafion toward CNT. Scanning
electron microscopy (SEM) was employed to gain insights into the nature of the new
CNT-titania-Nafion composites. Figure 13.2 shows the SEM (a) and transmission electron
microscope (TEM) (b) images for MWCNT-titania-Nafion composite films. The
MWCNT-titania-Nafion surface is characterized with bundles of MWCNT covering the
titania-Nafion composites. The MWCNT fibers have a diameter ranging from 30-50 nm.
TEM image of MWCNT dispersed in the composite indicates that CNTs are well distrib-
uted in the composite. SWCNT can also be well dispersed in the composite.
(a)
(b)
FIGURE 13.2
FE-SEM (a) and TEM (b) images of titania-Nafion (1:1, v/v) composite films containing MWCNT (1.0 mg/mL).
