Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 474

(a) (b)

FIGURE 15.11 Photographs of vials containing 1 mg ml 1 CNT dispersed in water (a) and in 0.1%
CTAB solution by ultrasonication (b). Photograph (a) was taken 0.5 h after CNT dispersed in water, pho-
tograph (b) taken 1 week after CNT dispersed in CTAB solution. (Reprinted with permission from [72].
Copyright (2004) Elsevier.)

interactions [68, 69]. Islam et al. [70] have reported that the solubilization of high
weight fraction CNTs in water is assisted by appropriate surfactants. The surfactant-
dispersed CNT fi lm electrodes have been widely applied in electrochemical and elec-
troanalytical studies. Chen et al. [71] have developed a carbon fi ber nanoelectrode
modifi ed by CNTs dispersed in a sodium dodecylsulfate (SDS) solution, which dis-
plays excellent electrochemical behaviors such as very high sensitivity toward neuro-
transmitters including dopamine, epinephrine, and norepinephrine. A stable suspension
of CNTs was also obtained by dispersing CNTs in a solution of a cationic surfactant
cetyltrimethylammonium bromide (CTAB) as demonstrated in Fig. 15.11. The direct
electron transfer of glucose oxidase (GOx) immobilized on the CNT/CTAB-modifi ed
electrode has been reported [72]. A hydrophobic surfactant such as dihexadecyl hydro-
gen phosphate (DHP) was used to prepare a CNT fi lm coated GC electrode, which
showed an enhanced-reduction peak current of metronidazole and improved the sensi-
tivity for the voltammetric determination of metronidazole [73].
The electrochemical response of analytes at the CNT-modifi ed electrodes is infl u-
enced by the surfactants which are used as dispersants. CNT-modifi ed electrodes using
cationic surfactant CTAB as a dispersant showed an improved catalytic effect for nega-
tively charged small molecular analytes, such as potassium ferricyanide and ascorbic
acid, whereas anionic surfactants such as SDS showed a better catalytic activity for
a positively charged analyte such as dopamine. This effect, which is ascribed mainly
to the electrostatic interactions, is also observed for the electrochemical response of
a negatively charged macromolecule such as DNA on the CNT (surfactant)-modifi ed
electrodes (see Fig. 15.12). An oxidation peak current near 1.0 V was observed only

at the CNT/CTAB-modifi ed electrode in the DNA solution (curve (ii) in Fig. 15.12a).
The differential pulse voltammetry of DNA at the CNT/CTAB-modifi ed electrode also
showed a sharp peak current, which is due to the oxidation of the adenine residue in
DNA (curve (ii) in Fig. 15.12b). The different effects of surfactants for CNTs to pro-
mote the electron transfer of DNA are in agreement with the electrostatic interactions