Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 472

(a) (b)

(c)

(d)

FIGURE 15.9 Representative SEM images of [PDDA/MWNT] (a), [PDDA/MWNT]3 (b), and [PDDA/
MWNT]5 (c) assembled on a silicon wafer. The scale bar in (a)-(c) was 5 µm. (d) represents the SEM image
of [PDDA/MWNT]1 with a high amplifi cation with a scale bar of 1 µm. (Reprinted with permission from
[54]. Copyright (2004) American Chemical Society.)

NADH, epinephrine or norepinephrine has been demonstrated with enhanced currents
and reduced peak-to-peak separations in the cyclic voltammetry in comparison with
unmodifi ed electrodes. However, the dispersion of CNTs in organic solvents is not
suitable in such cases as fabricating biosensors with biomolecules and CNTs, because
the denaturalization of biomolecules caused by organic solvents may lead to a decrease
or loss of activity. Thus, the preparation of aqueous dispersions and solubilization of
CNTs have attracted much attention [61]. As described in 15.2.4, the acid treatment of
CNTs form carboxyl sites on the surface of CNTs and cause tube shortening. A typical
acid treatment procedure for CNTs is as follows [62]: a purifi ed CNT “bucky paper” is
suspended in 40 ml of a 3:1 mixture of concentrated H 2 SO 4 /HNO 3 in a 100 ml test tube
and sonicated in a water bath for 24 h at 35 to 40ºC. The resultant suspension was then
diluted with 200 ml of water, and the larger cut CNTs are collected on a 100 nm pore
fi lter membrane and washed with 10 mM NaOH solution. The cut tubes are then fur-
ther treated with a 4:1 mixture of concentrated H 2 SO 4 and 30% aqueous H 2 O 2 under