Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 468

carbon nanotubes from a mesoporpous silica template with embedded iron nanoparti-
cles. The direction of nanotube growth can be controlled by the orientation of the pores
from which the nanotubes grow. A similar study [44] was performed to make aligned
carbon nanotubes by the pyrolysis of hydrocarbon on a nickel catalyst embedded in a
porous silicon substrate. In these cases, porous silicons containing micro-, meso-, and
macropores were produced by electrochemically etching the crystalline silicon wafer
(as the anode) in an aqueous HF solution (using a Pt wire as the cathode). Plasma-
enhanced chemical-vapor deposition (PECVD) is another approach used to prepare
vertically aligned CNTs. Similar to chemical-vapor deposition (CVD), PECVD also
uses gaseous sources, but activation of the gas is achieved in a non-equilibrium plasma
(glow discharge). The CNTs are grown on a catalyst deposited on the substrate as in
CVD. The location and diameter of the vertically aligned CNTs is defi ned by the pat-
tern of the catalyst spots and the length is controlled by the growth rate and time. The
growth direction is controlled by the electric fi eld in the plasma sheath. Figure 15.4
shows a SEM image of vertically aligned MWNT bundles prepared with PECVD [45].
Vertically aligned CNTs produced through this approach have been used for sen-
sor designs. Lin et al. [46] have reported a glucose sensor based on vertically aligned
CNTs. Ni nanoparticles were fi rst electrodeposited on a Cr coated Si. Low site density
aligned CNT arrays were then grown from those Ni nanoparticles by PECVD. The
resulting CNT ensembles were spin coated with an epoxy-based polymer. The protrud-
ing parts of the CNTs were removed by polishing. Glucose oxidase was covalently
immobilized on the electrode by forming an amide bond between the amine residues
of the protein and carboxylic acid groups on the CNT tips. With this electrode, glucose
is indirectly sensed by measuring the catalytic reduction current of hydrogen peroxide
generated by the reaction of glucose oxidase catalyzed oxidation of glucose. The elec-
trode prepared in this way has also been used for DNA detection [45] by attaching
oligonucleotide probes to the open ends of the CNTs. This electrode can detect subat-
tomole of DNA by combining with the Ru(byp) 3 2 -mediated oxidation of guanine.
FIGURE 15.4 SEM images of vertically aligned MWNTs at (left) UV lithography and at (right) e-beam
patterned Ni spots. (Reprinted with permission from [45]. Copyright (2003) American Chemical Society.)