Biomedical Engineering Reference
In-Depth Information
{CNI/PDDA} n /GC
GC
PDDA
PDDA/GC
CNI
[CNI/PDDA] 1 /GC
Repeat
FIGURE 15.8 Schematic description of layer-by-layer assembly of CNTs with PDDA on the GC
surface.
The vertically aligned monolayer of MWNTs was obtained by dipping the polyelectrolyte-
modifi ed substrate into a tetrahydrofuran suspension of the functionalized MWNTs.
15.3.1.3 Layer-by-layer fabrication of CNT electrode
The carboxylic acid group of CNTs is negatively charged, and an electrostatic assem-
bly of CNTs with a positively charged surface helps to form CNT multilayer fi lms
[53]. Zhang et al. have adopted this layer-by-layer technique and have prepared an
assembled CNT multilayer fi lm on a GC electrode surface. Figure 15.8 describes the
layer-by-layer assembly process of CNTs with poly(diallyldimethylammonium chlo-
ride) (PDDA) on the surface of a GC electrode. The electrostatic attraction between
negatively charged CNTs and positively charged PDDA molecules leads to the for-
mation of stable CNT multiple layers on the GC electrode surfaces (Fig. 15.9) by
the layer-by-layer method, which is different from other preparation methods includ-
ing cast coating, mixing the CNTs with carbon paste, and confi ning the CNTs with
polymer matrixes, in which the CNTs are mainly in the form of big bundles. CNT
multilayer fi lms assembled by the layer-by-layer method possess many striking fea-
tures such as having an adsorbed form of small CNT bundles or single tubes on the
substrate, uniformity, good stability, and remarkable electrocatalytic activity toward O 2
reduction in alkaline media [54]. Moreover, this layer-by-layer technique has prom-
ising applications in CNT-based biosensor fabrication and enzyme immobilization.
With layer-by-layer fabrication of DNA-wrapped CNTs, a DNA electrochemical sen-
sor has been developed [55]. Munge et al. have reported a novel method for dramati-
cally amplifying the electrochemical detection of proteins or DNA based on a stepwise
layer-by-layer assembly of multilayer enzyme fi lms on a CNT template [56].
15.3.1.4 CNT-coated electrodes
The coating of CNT solution or suspension on the electrode surface followed by evap-
orating the solvent is a simple but effective strategy to prepare CNT-modifi ed elec-
trodes. However, due to the strong van der Waals forces between nanotubes, CNTs
tend to aggregate, thus leading to the poor solubility of CNTs in water and other sol-
vents, which limits the preparation and application of CNT solutions. Ausman et al.
have demonstrated the ability of organic solvents to form stable CNT dispersion. CNTs
dispersed in several organic solvents such as dimethylformamide (DMF) [35, 57], ace-
tone [58], acetonitrile [59], and ethanol [60] have been coated on the electrode sur-
face. On these CNT-modifi ed electrodes, the electrocatalytic behavior of ferricyanide,
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