Biomedical Engineering Reference
In-Depth Information
(a)
(b)
1.3:1 H
2
SO
4
:HNO
3
sonicated 4 hrs 40
C
°
2. Wash, filter suspend
in DMF
500 nm
200 nm
Cysteamine modified electrode
Dicyclohexylcarbodimide (DCC)
(d)
(c)
20nm
0
Microperoxidase MP-11
0
2
4
6
8
10
µ
m
FIGURE 12.19
A schematic illustration showing the steps involved in the fabrication of aligned shortened SWNT arrays for
direct electron transfer with enzymes such as microperoxidase MP-11. (From Gooding, J. J., Wibowo, R., Liu, J.,
Yang, W., Losic, D., Orbons, S., Mearns, F. J., Shapter, J. G., Hibbert, D. B. (2003). Protein Electrochemistry Using
Aligned Carbon Nanotube Arrays.
J. Am. Chem. Soc.,
125, 9006-9007.)
biomolecule components to CNTs, the capacitive charging-discharging current is still too
high because such CNT arrays behave as a highly porous macroscopic film instead of
individual nanoelectrodes.
To improve the electron transfer rate from an active open end of CNTs to the electrode
and decrease background current, each CNT should individually work as a single elec-
trode. Ideally the spacing between each CNT needs to be greater than the diameter of the
nanotube to prevent the diffusion layer from overlapping. Recently, well-insulated nano-
electrode arrays based on CNTs have been fabricated by a bottom-up approach [122,168].
In contrast to the methods to fabricate forest-like CNT arrays, controlled low-site density
aligned CNT arrays with a nanoelectrode-like electrochemical behavior show many
advantages compared to high density-packed CNT arrays (Figure 12.20). The advantages
include the increased temporal and spatial resolution, the improved signal (diffusion-
controlled current) -to-noise (background or charging current) ratios, and the ability to
perform analysis in solutions with high resistance such as natural water. The method for
fabricating insulated CNT nanoelectrode arrays by catalytic CNT growth techniques is
schematically presented in Figure 12.20, where catalyst—nickel NPs are first electrode-
posited on a metal film (chromium or Pt)-covered silicon wafer. A vertically aligned
MWCNT array is then grown on the surface by plasma-enhanced chemical vapor deposi-
tion using a DC-based hot-filament CVD system. The resulting substrate is dielectrically
encapsulated to form a conformal SiO
2
film or is spin-coated by epoxy-based polymer
[169] to fill the space between CNTs as well as to cover the substrate surface. Following the
chemical mechanical polishing, a tuneable number of MWCNTs is exposed at their end
positions to form nanoelectrode arrays. Thus, the MWCNTs can be well aligned vertically
at the substrate surface and separated from its neighbors.
