Biomedical Engineering Reference
In-Depth Information
chains has been effective in improving their solubility without impairing their physical proper-
ties [174]. A variety of nonionic and ionic polymers were found capable of dispersing nanotubes
[175-179]. For example, a widely used perfl uorosulfonated polymer, Nafi on polymer, with a polar
side chain was found to solubilize CNTs in phosphate buffer solution or alcohol [180]. Wang et al.
reported on the ability of Nafi on to solubilize SWCNTs and MWCNTs and on the dramatically
enhanced redox activity of hydrogen peroxide at CNT/Nafi on-coated electrodes in connection
with the preparation of oxidase-based amperometric biosensors [180]. Because of their unique
properties such as ion exchange, discrimination, and biocompatibility, Nafi on fi lms have been used
extensively for the modifi cation of electrode surfaces and for the construction of amperometric
biosensors [181,182].
14.2.1.1.2 Microfabrication of Nanoelectrode Ensembles and Arrays
Electrodes modifi ed with nanotubes by drop-coating a random tangle of nanotubes onto the elec-
trode surface take advantage of the bulk properties of CNTs. However, oriented CNT arrays possess
advantage over the random tangle of CNTs. The open end of an MWCNT has a fast ETR similar
to a graphite edge-plane electrode, while the SWCNT presents a very slow ETR and low specifi c
capacitance, similar to the graphite basal plane [183]. The proper construction and orientation of
the electrode is critical for its electrochemical properties. Several approaches to the production of
aligned CNT arrays have been reported.
Liu et al. [184] used a bottom-up approach to fabricate a glucose biosensor based on CNT
nanoelectrode ensembles. Low site density-aligned CNT arrays were grown from Ni nanoparticles
(NPs) by plasma-enhanced chemical vapor deposition (CVD). A dielectric encapsulation was then
applied, leaving half of the CNTs exposed to form inlaid nanoelectrode arrays (NEAs). Such an
operation eliminates the need for permselective membrane barriers or artifi cial electron mediators,
thus greatly simplifying the sensor design and fabrication (Figure 14.5).
Gooding et al. described a more versatile approach to the production of aligned CNT arrays
by self-assembly [185] (Figure 14.6). The as-grown nanotubes were fi rst chemically cut into short
Exposed CNT tip
Epoxy passivation layer
Electrode contact
Cr
Si
(a)
EC treatment
CO
2
−
CO
2
−
CO
2
−
CO
2
−
GOD
EDC/Sulfo-NHS
(b)
HN-GOD
CO
HN-GOD
HN-GOD
HN-GOD
CO
CO
CO
FIGURE 14.5
Fabrication of a glucose biosensor based on CNT nanoelectrode ensembles: (a) electrochemi-
cal treatment of the CNT NEEs for functionalization and (b) coupling of the enzyme (GOx) to the functional-
ized CNT NEEs. (From Lin, Y.H., Lu, F., Tu, Y., and Ren, Z.F.,
Nano Lett.
,
4, 191, 2004. With permission.)
