Voltammetric Oxidation of Ascorbic Acid Mediated by Multi-Walled Carbon Nanotubes/Titanium Dioxide Composite Modified Glassy Carbon Electrode
A MWCNT/TiO2 composite was modified onto glassy carbon electrode and fabricated by mechanical attachment, then applied to detection of ascorbic acid using cyclic voltammetry. Electrode responses were obtained for the oxidation of 0.5 mM ascorbic acid at the glassy carbon electrode modified with MWCNT, TiO2, MWCNT/TiO2 composite and an unmodified glassy carbon electrode. A well-defined and highly resolved oxidation peak of ascorbic acid at the MWCNT/TiO2/GCE with current enhancement and peak potential shift toward the origin being observed, this indicates evidence of electrocatalytic process. In comparison with other electrodes, the observed current enhancements at the MWCNT/TiO2/GCE were 1.3 folds higher than those obtained by MWCNT/GCE and 1.5 folds by a bare glassy carbon electrode. The response peak current revealed a good linear relationship of up to 2.5 mM ascorbic acid with correlation coefficient of 0.998. A good detection limit of 4.0 μM was found using MWCNT/TiO2/GCE. Other usual parameters such as effect of pH, scan rate, temperature, supporting electrolyte and concentration of ascorbic acid were studied.
Received: October 01, 2010;
Accepted: December 17, 2010;
Published: February 24, 2011
Nanostructured materials such as carbon nanotubes (CNT), nanoparticles of metal
and metal oxide have recently received much attention from researchers. CNT
is known to exhibit excellent electrocatalytic activity, which gives rise to
its good electrochemical behavior. Thus, it has been used as an electrode modified
composite when combined with metal and metal oxide nanoparticles, such as Au
(Alexeyeva et al., 2006), Pt (Wang
et al., 2007a), Pd (Chen et al., 2009),
Ag (Xiao et al., 2007), Cu (Chai
et al., 2008), Ni (Wang et al., 2007b)
and ZnO2 (Deng et al., 2008), RuO2
(Deng et al., 2005), MnO2 (Sivakkumar
et al., 2007), SnO2 (Pang et al.,
2009), Fe2O3 (Hang et al.,
2008), TiO2 (Jiang and Zhang, 2009) or
polymeric binder composite (Tsai et al., 2004;
Wang and Musameh, 2003).
Similarly, multi-walled carbon nanotubes (MWCNT) has attracted considerable
attention in the voltammetric study since its discovery by Iijima
(1991), because of its unique structural, electronic, mechanical and electrochemical
properties (Nguyen et al., 2001; Gooding,
2005). These properties suggest that CNT causes fast electron transfer reaction
when used as an electrode modifying material (Nugent
et al., 2001). On the other hand, titanium dioxide (TiO2)
is a good semiconductor, which shows an excellent chemical reaction in the photocatalysis
(Khuanmar et al., 2007), especially oxidation
and reduction of organic or inorganic substances (Fox and
Dulay, 1993). Although TiO2 alone has poor electrochemical activity,
when combined with MWCNT, the electrocatalytic property of MWCNT is much improved.
Also, the surface chemistry has been studied at poly- and mono-crystalline
phase of TiO2 nanoparticles (Diebold, 2003),
hence when coupled with MWCNT; it provides a synergistic effect that improves
the electrochemical performance of MWCNT. A number of reports have shown that
MWCNT/TiO2 composition material has been receiving due attention
in many investigations. Various researchers have studied the preparation of
MWCNT/TiO2 either through the use of composition or mixture, using
several methods, for example, sol-gel (Gao et al.,
2009), CVD (Kuo, 2009; Wang
et al., 2008), VPT (Zhang et al., 2008),
UV (Ueda et al., 2009), EPD (Jarernboon
et al., 2008) and also simple direct mixing method (Sawatsuk
et al., 2009; Ahmmad et al., 2008). However,
to the best of our knowledge, only scant attention has been paid to MWCNT and
TiO2 composite modified electrode in the voltammetric determination
of some vitamins and amino acids.
In this study, we prepared a known amount of MWCNT with TiO2 as
a composite using simple mixing method. The MWCNT/TiO2 composite
was modified on the surface of glassy carbon electrode (GCE) via mechanical
attachment and studied in the oxidation process of ascorbic acid in aqueous
electrolyte. Result has shown that the modified electrode has a good electrochemical
behavior in terms of stability, reproducibility and an achievable detection
limit. We also found that it exhibits a good potential as a chemically modified
electrode for electrochemical and biochemical research.
MATERIALS AND METHODS
Black powder of MWCNTs (purity >95%, diameter~20-40 nm, length~5-15 μm)
were obtained commercially (from Shenzhen Nanotech) and used without further
purification. TiO2 nanoparticles solution (TiO2 dispersed
in water by 5% weight, <100nm particle size) and L-ascorbic acid (minimum
purity of 99.7%) were purchased from Aldrich (USA). Solution containing ascorbic
acid was freshly prepared before running each experiment. Other chemicals used
during the experiment were of analytical grade reagents. All solutions were
prepared with deionized distilled water and deaerated with oxygen-free nitrogen
gas for 15 min before each measurement.
Instrumentation and apparatus: The most of the voltammetric experiments
were performed by BAS (Bioanalytical Systems, West Lafayette, Indiana, USA):
CV-50W electrochemical workstation, which connected to an external computer
was used. A conventional three-electrode cell system was employed; a platinum
wire served as a counter electrode; an Ag/AgCl (in 3 M NaCl) as a reference
electrode; a bare GCE and a CGE modified with TiO2, MWCNT, MWCNT/TiO2
composite as working electrodes. A bare working electrode (3 mm diameter)
was polished with alumina slurry, ultrasonic grinded for 1 min and rinsed with
distilled water before use. The morphology of the MWCNT/TiO2 composite
was characterized on the surface of 5 mm diameter basal plane pyrrolytic graphite
electrode (BPPGE) before and after electrolysis by scanning electron microscopy
(SEM - Model JOEL, JSM-6400 machine).
Preparation of the MWCNT/TiO2 composite: MWCNT powder and
stock solution of TiO2 (dispersed in water by 5% weight) were prepared
and ready for usage. Five milligramg of MWCNT was placed in a glass plate and
then mixed with 0.1 mL TiO2. The composite were dried at room temperature
for 30 min; it was later mechanically transferred to the surface of bare GCE.
RESULTS AND DISCUSSION
Scanning electron microscopy study: Figure 1 shows
SEM image of the MWCNT/TiO2 composite surface before (Fig.
1a, c) and after (Fig. 1b, d)
electrolysis in the presence of ascorbic acid at 5 mm diameter BPPGE. The morphology
of the MWCNT/GCE via mechanical attachment was reported (Radhi
et al., 2010). The structure of composite shaped a homogeneous film/coating
on the BPPGE surface (Fig. 1a, b). As can
be seen at magnification of 10,000 (Fig. 1c, d)
times, MWCNT was clearly shown and thin fibers which formed into bundles with
some of them joined together. The stability of the film was evident as the SEM
image remains un-scattered even after 10 potential cycling.
Enhancement study: Cyclic voltammograms were obtained for the oxidation
of 0.5 mM ascorbic acid in 0.1 M KCl aqueous solution over the potential range
of -400 mV to +1000 mV versus Ag/AgCl (in 3 M NaCl) at GCE modified with TiO2,
MWCNT, MWCNT/TiO2 composite and an unmodified (bare) GCE. The oxidation
process of ascorbic acid appears irreversible. Voltammograms were successfully
recorded on the surface of various modified and unmodified electrodes with different
responses. Result of Fig. 2 shows that the MWCNT modified
GCE (Fig. 2 (curve b)) produces a greater oxidative current
response of ascorbic acid while the TiO2 modified GCE (Fig.
2 (curve d)) appears to cause a decrease in the oxidation current of ascorbic
However, when a TiO2 was coupled to MWCNT, it further enhances the
electrocatalytic activity of MWCNT as evident in the enhanced oxidation peak
observed at +220 mV at the MWCNT/TiO2 composite modified GCE (Fig.
2 (curve a)). The oxidation peak currents of ascorbic acid at the MWCNT/GC
modified and the MWCNT/TiO2/GCE modified surfaces were obtained with
enhancements of about 1.3 folds and 1.5 folds as compared with an unmodified
GCE (Fig. 2 (curve c)). It was interesting to note that the
peak current of ascorbic acid at the MWCNT/TiO2/GCE was more resolved
than those obtained for the MWCNT/GCE modified and an unmodified electrode.
The oxidation peak potential of ascorbic acid was found to shift by -60 mV,
-26 mV toward the origin (negative) and +27 mV to positive value when the MWCNT/TiO2/GC,
MWCNT/GC and TiO2/GC modified electrodes when compared to that of
an unmodified GCE.
||SEM image of the MWCNT/TiO2 composite shows on
BPPGE surface before electrolysis (a, c) and after electrolysis (b, d) at
magnifications of 500 and 10,000 times
||Cyclic voltammogram obtained for the oxidation of 0.5 mM ascorbic
acid in 0.1 M KCl solution with a scan rate of 100 mV sec-1 at
the (a) MWCNT/TiO2/GCE; (b) MWCNT/GCE; (c) bare GCE and (d) TiO2/GCE
without MWCNT (after background subtracted)
Therefore, the MWCNT/TiO2/GCE modified electrode which appears
to be more sensitive of all the electrodes was used in the subsequent studies.
Effect of potential cycling: The potential cycling for the oxidation
of ascorbic acid in 0.1 M KCl was carried out at the MWCNT/TiO2
composite modified GCE by Cyclic voltammetry (Fig. 3). From
the first cycle of the voltammogram, the peak current of ascorbic acid decreased
slightly. Then it was established and even after 10th potential cycle, the oxidation
peak remained high. Only about 20% decrease in current was observed; reflecting
its stability. Furthermore, Faradaic activity was reproducible at the MWCNT/TiO2
composite modified solid state electrode.
Effect of pH: The next experiments were carried out to determine the
effect of pH on the voltammogram of ascorbic acid mediated by the MWCNT/TiO2
composite modified electrode (Fig. 4). It was observed
that the oxidative peak current of ascorbic acid was high, more pronounced and
almost constant under acidic condition, but slightly decreased in neutral condition.
Additionally, potential responses were similar, which means working range of
pH is wider in this condition, but fell with the increase of pH value. The peak
current decreased significantly and shifted to negative potential direction
from pH value of 6.0 onward. The capacitance current of all solutions varied
little under different pH conditions. So, it is reasonable to apply pH range
of between 2.0 and 6.0 for further experimentation.
Effect of varying scan rate: The effect of scan rate on the anodic peak
current of 0.5 mM ascorbic acid in 0.1 M KCl at the MWCNT/TiO2/GCE
was studied using different scan rates in the range of 10-1000 mV sec-1.
||Cyclic voltammogram of potential cycling for the oxidation
of ascorbic acid at the MWCNT/TiO2/GCE in 0.1 M KCl solution
with a scan rate of 100 mV sec-1 for 10 cycles
||A plot showing the dependence of pH value for the oxidation
of ascorbic acid in 0.1 M KCl mediated by the MWCNT/TiO2/GCE
at different pH solutions
It was observed that when the scan rate increased, the anodic peak potential
was shifted slightly to the positive direction and the oxidative current of
ascorbic acid increases, which was also affected by heterogeneous kinetics and
IR drop effect as shown in Fig. 5. Oxidation peak current
of ascorbic acid versus scan rate was plotted linearly and equation is shown
as y = 0.45x+0.88 with R2 = 0.992. An experimental slope of 0.45
was obtained, which is close to the theoretical value of 0.5, indicating that
the current is largely diffusion controlled.
Calibration graphs: Figure 6 presents the dependence
of the voltammetric response to the MWCNT/TiO2 composite modified
GCE in the addition of different ascorbic acid concentrations ranging between
0.05-2.5 mM in 0.1 M KCl electrolyte solution. From the oxidative current against
concentration of ascorbic acid up to 2.5 mM, it showed a linear relationship
of R2 = 0.998, based on the equation y = 44.03x+0.314 with a high
sensitivity of response at 44 μA mM-1 and a good detection limit
of 4 μM for ascorbic acid detection.
||Cyclic voltammogram obtained for the oxidation of ascorbic
acid in 0.1 M KCl at the MWCNT/TiO2/GCE with different scan rates
of 10-1000 mV sec-1
||A plot showing the dependence of the oxidation current on
different concentration of ascorbic acid in 0.1 M KCl solution at the MWCNT/TiO2/GCE
with a scan rate of 100 mV sec-1
Effect of varying supporting electrolytes: Based on the pH study, different
types of 0.1 M aqueous supporting electrolytes with neutral conditions were
studied. In the presence of SO4-2 and H2PO4¯
in electrolyte, ascorbic acid peak currents were in slightly negative potential
shift as compared to others in electrolyte solution (Fig. 7).
Most of the aqueous solutions observed showed no distinct changes. Similar potential
ranges were obtained using electrolytes solutions of Kcl, NH4Cl and
KClO4. Consequently, an aqueous solution of KCl was chosen as a main
supporting electrolyte based on the highest peak shown.
||Overall graph of the oxidation current of ascorbic acid at
the MWCNT/TiO2/GCE in different supporting electrolytes at 25°C
with a scan rate of 100 mV sec-1
||Cyclic voltammograms obtained for the oxidation of ascorbic
acid using the MWCNT/TiO2/GCE immersed in 0.1 M KCl at various
temperatures between 10°C and 80°C with a scan rate of 100 mV sec-1
Electrolyte solutions of NH4Cl and K2SO4
also show good potential for usage as supporting electrolyte for oxidation of
ascorbic acid using the MWCNT/TiO2 composite modified GCE.
Effect of varying temperature study: The effect of temperature on the
oxidation process of ascorbic acid was studied. Figure 8 shows
a series of cyclic voltammograms obtained over temperature ranging from 10 to
80°C for the oxidation of ascorbic acid using the MWCNT/TiO2
composite modified GCE with a scan rate of 100 mV sec-1. The peak
currents increased significantly and the oxidation peak potential shifted to
origin when the temperature of electrolyte solution increased from 10-80°C.
||A plot of Ln current against 1/Tk-1 for the MWCNT/TiO2/GCE
in 0.1 M KCl electrolyte in the presence of 0.5 mM ascorbic acid in 0.1
It indicates a strong dependence of solid state oxidation process on temperature.
The values of the peak current and potential of varying temperature using the
MWCNT/TiO2 composite mechanically attached to a 3 mm GCE in the presence
of 0.5 mM ascorbic acid. Based on Fig. 8, a plot of the natural
logarithm of oxidation current of ascorbic acid against temperature (Fig.
9) was found fairly linear with thermodynamic expectation. This plot illustrates
that the oxidation current of ascorbic acid is significantly temperature dependent.
Conductivity and diffusivity of solid with the increase in temperature dependence,
the reason is temperature has a significant influence on the activation energy
of compounds following Arrhenius equations. Based on this Arrhenius plot, the
activation energy Ea = 8.5 kJ mol-1 was obtained. The
conductivity of the MWCNT/TiO2 composite with the increase in temperature
also plays a significant influence on the activation energy for diffusion of
the substrate of interest.
We have demonstrated the MWCNT/TiO2 composite using direct mixing
method and fabricated by mechanical attachment method on the surface of GCE.
This study shows that in the presence of TiO2, by appear to exert
negative electrocatalytic effect and require the presence of MWCNT to produce
positive catalytic effect for the oxidation of ascorbic acid. The MWCNT/TiO2
composite modified GCE response showed enhanced electrocatalytic activity
as indicated by the current enhancement and peak shift to origin when it was
compared to an unmodified GCE. The linear relationship was obtained from a plot
of calibration graph with a high sensitivity and a good detection limit. Though,
the detected percentage of elements in composite is not very homogeneous, the
preparation of composites is very simple, straight forward and easy to apply.
The stability and selectivity of the composite modified electrode are adequate
from the obtained results, thus, the electrode appears to be potentially of
great benefit in electrochemical research.
The authors wish to thank the Universiti Putra Malaysia and Ministry of Science
and Technology and Innovation Malaysia (MOSTI) for providing the research fund
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