G Model
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Y.-L. Wu et al. / Chinese Chemical Letters xxx (2013) xxx–xxx
(EIS) measurements were carried out on an Autolab PGSTAT302N
system (Eco Chemie, Netherlands) using the above three-electrode
system.
The CNT/Au/TiO2 nanocomposites were prepared according to
the procedure in our previous report [17], ultrasonically dispersed
in 0.1% chitosan solution for 1 h to obtain the black colloidal
solution, with the optimal concentration of 0.5 mg mLÀ1. Prior to
the surface modification, the GCE was polished to mirror
(Ipa) and cathodic peak current (Ipc) at the CNT/Au/TiO2 nano-
composites modified electrode corresponded linearly on the
square root of the scan rates (from 10 mV sÀ1 to 250 mV sÀ1
)
(Fig. 3A inset, R2 = 0.997 and 0.998, respectively), verifying that the
redox reaction of K3[Fe(CN)6] at the modified electrode was a
typical diffusion-controlled process. Meanwhile, it was observed
that, with the increase of scan rates, the oxidation peak shifted to
more positive potentials, while the reduction peak shifted to more
negative potentials. According to Laviron’s theory [18], the anodic
and cathodic peak potentials are linearly dependent on the
smoothness with 0.05
then washed thoroughly with deionized water. The modified
electrode was prepared by casting 4 L of the CNT/Au/TiO2
nanocomposites on the surface of the electrode and dried at room
temperature in a desiccator. 4
L of 6 mg mLÀ1 GOx solution was
then added dropwise on the CNT/Au/TiO2 nanocomposites
modified GCE and dried at 4 8C in a refrigerator. After 2
mm alumina slurry on soft lapping pads,
m
logarithm of the scan rates (
was less than 200 mV. A plot of Ep vs. log
straight lines with a slope equal to À2.3RT/
)nF, so that the transfer coefficient (a
v
), when the peak-to-peak separation
(Fig. 3B) yields two
nF for the cathodic
) can be
n
a
m
peak, and 2.3RT/(1 À
a
m
L
estimated as 0.54 from the slope of the regression. The electron
transfer rate constant (k) was calculated as 0.92 Æ 0.79 sÀ1
according to Laviron’s equation.
solution of 2 wt.% Nafion solution was coated onto the surface of
GOx-CNT/Au/TiO2 modified electrode, the Nafion-GOx-CNT/Au/
TiO2 modified electrode was finally obtained.
Electrochemical AC impedance is one of the most appropriate
methods to characterize the heterogeneous electron-transfer
capability. From the direct measurements of the total cell
impedance in EIS, one could measure the impedance value of
the electrode surface during the process of frequency variation that
is able to offer various properties of the interface of the electrode
and solution, including the electrode impedance, the capacity of
the electric double layer, and the surface charge-transfer resistance
(Rct). In the Nyquist diagrams, the semicircle diameter of EIS and Rct
of the electrode are equal. Fig. 4 presented the EIS recorded at the
bare electrode and CNT/Au/TiO2 nanocomposites modified GCE in
5 mmol LÀ1 [Fe(CN)6]3À/4À solution containing 0.1 mol LÀ1 KCl. We
could observe a well defined quasi-semicircle portion for bare GCE
(Fig. 4 curve a). By fitting the data plot, the Rct of the bare GCE was
3. Results and discussion
The morphologies of CNT/Au/TiO2 nanocomposites were
initially characterized with the SEM. Fig. 1 illustrated the SEM
image of the CNT/Au/TiO2 nanocomposites, indicating the rough
morphologies of CNT/Au/TiO2 film with the loose and porous
nanostructures. This nano-porous interface could readily facilitate
the attachment of GOx, which was advantageous for the
construction of a robust homogeneous film for the formation of
a biosensor.
Meanwhile, the CNT/Au/TiO2 nanocomposites modified GCE
was further characterized by CV study for evaluation of the
electrochemical transducers. The cyclic voltammograms obtained
at the different electrodes immersed in 1 mmol LÀ1 K3[Fe(CN)6]
solution containing 0.1 mol LÀ1 KCl were shown in Fig. 2. The
results illustrated an apparent increase of the relevant peak
current when GCE modified with CNT/Au/TiO2 nanocomposites
compared with that of the bare GCE (Fig. 2B and C). It was evident
that CNT/Au/TiO2 nanocomposites were successfully assembled,
thus considerable enhancement of the electrochemical response of
probe molecules had been observed in the presence of CNT/Au/
TiO2 nanocomposites.
seen from the diameter of the semicircle, with a value about 200
However, as for the CNT/Au/TiO2 nanocomposites modified GCE, a
semicircle region within the range less than 100 was seen.
V.
V
Hence, the relatively high electrochemical activity could be
achieved by modification of GCE with the CNT/Au/TiO2 nano-
composites.
The glucose sensor was characterized by immobilizing glucose
oxidase (GOx) and Nafion on CNT/Au/TiO2 modified electrode
(Fig. 5). In this study, acetylferrocene was chosen as a mediator.
Fig. 6A showed cyclic voltammograms of the CNT/Au/TiO2
modified electrode in the absence (curve a) and presence (curve
b) of acetylferrocene. The bioelectrocatalytic activity of the Nafion-
GOx-CNT/Au/TiO2 modified electrode toward the oxidation of
glucose was demonstrated by the cyclic voltammetric studies in
solution with and without the presence of glucose, respectively.
Fig. 6B showed the cyclic voltammetric response of the Nafion-
GOx-CNT/Au/TiO2 modified electrode in 0.1 mol LÀ1 PBS in the
absence and presence of 0.1 mmol LÀ1 acetylferrocene, respective-
ly, at a scan rate of 1 mV sÀ1. During the relevant study, the
solutions were purged with high-purity nitrogen for 20 min prior
to experiments, and a nitrogen environment was then kept over
solutions in the cell to protect the solution from oxygen. It was
noted that the anodic currents increased drastically upon addition
of glucose (Fig. 6B, curve c). It is evident that by using the
acetylferrocene as a mediator, the GOx immobilized on CNT/Au/
TiO2 modified electrode could efficiently execute the relevant
bioelectrocatalytic activity for the oxidation of glucose. The
corresponding curve (Fig. 6C) indicated that the response current
for glucose detection at the Nafion-GOx-CNT/Au/TiO2 electrode
was from 0.1 mmol LÀ1 to 24 mmol LÀ1 glucose, and reached
saturation at about 24 mmol LÀ1. Therefore, a linear relationship
(Fig. 6C inset) was observed between current response and glucose
Fig. 3A showed the effect of the potential scan rate on the cyclic
voltammetry at the CNT/Au/TiO2 nanocomposites modified GCE.
With the increase of the scan rate, the redox potentials of
K3[Fe(CN)6] shifted slightly, and the separation between cathodic
and anodic peak potentials (DEpeak) enlarged. The anodic current
concentrations in the range from 0.1 mmol LÀ1 to 8.0 mmol LÀ1
.
The limit of detection was calculated to be 0.077 mmol LÀ1 based
on the signal-to-noise ratio of 3.
Fig. 1. SEM image of CNT/Au/TiO2 nanocomposites modified glassy carbon
electrode.
Please cite this article in press as: Y.-L. Wu, et al., Glucose biosensor based on new carbon nanotube–gold–titania nano-composites
Wu, Yue-Li
