Biomedical Engineering Reference
In-Depth Information
400
b
a
d
e
350
300
c
250
200
150
100
50
0
0
200
400
600
800
1000
Z
re
(Ohm)
FIGURE 4.30
Nyquist plots obtained from bare electrode: MWCNT-modified electrode (a), ABA/MWCNT-modified electrode
(b), AuNPs/PABA/MWCNT-modified electrode (c), and ssDNA/AuNPs/PABA/MWCNT-modified electrode (d).
(e) The supporting electrolyte was 0.1 M phosphate-buffered solution (pH 7.0) containing 10.0 mM K
4
Fe(CN)
6
/
K
3
Fe(CN)
6
(1: 1). (From Zhang et al.,
Colloids Surf B: Biointerfaces
, 75, 179-185, 2010. With permission.) [161]
author gave the explanation that the electrostatic effect and the steric effect was due to the
polyelectrolyte character of oligonucleotides strands; thus, the ionic transportation across
the polymer-solution interface changed.
Zhang and Wang [161] fabricated a sensitive electrochemical DNA biosensor with ami-
nobenzoic acid (ABA) electropolymerized on the surface of glassy carbon electrode (GCE)
modified with MWCNTs. ABA was electropolymerized on the surface of GCE modified
with MWCNTs, and then gold nanoparticles (AuNPs) were subsequently introduced to
the surface of poly-ABA-MWCNTs by electrodeposition mode. Finally, the probe DNA was
immobilized on the surface of AuNPs through Au-S bond. The EIS measurements were
carried on with a frequency range between 1 and 10
5
Hz at the formal potential of +0.15 V,
with the amplitude of 5 mV alternative voltage, and used 10 mM K
3
[Fe(CN)
6
]/K
4
[Fe(CN)
6
]
(1:1) mixture as a redox probe.
Figure 4.30 shows the Nyquist plot of the sensing electrode response at different stages
in assembly process. When the MWCNTs were assembled on the GCE surface, the electron
transfer resistance (
R
ct
) decreased greatly (curve b) compared with that of bare GCE (curve
a) due to the promotion from the MWCNTs. When ABA was electropolymerized onto the
surface of the MWCNTs/GCE, the
R
ct
value increased. After AuNPs was introduced, the
R
ct
decreased, and when probe DNA was immobilized on the electrode surface,
R
ct
obvi-
ously increased. The author explained that DNA was negatively charged and had electro-
static repulsion toward negatively charge [Fe(CN)
6
]
3-/4-
. The experimental results proved
that different species had been immobilized on the surface of modified electrode from
change in electron transfer resistance.
Enzyme Detection
Recently, nitrogen-doped carbon nanotubes (CNx-MWCNTs) are extremely attractive as
important nanomaterials in biosensing applications. Because of the hydrophobic char-
acteristics of CNTs surface, it is necessary to functionalize CNTs with polymers, guest
molecules, or side wall substituents to improve the biocompatibility [162]. Compared with
