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Chapter 4
Copper Oxide/Copper Oxalate Modified Copper Electrode for the Direct Electrocatalytic
Oxidation of Glucose
4. Copper Oxide/Copper Oxalate Modified Copper
Electrode for the Direct Electrocatalytic
Oxidation of Glucose
The development of a copper oxide modified copper electrode for the
non-enzymatic detection of glucose was described in the previous chapter. The
sensor performed extremely well in the presence of other analytes such as uric
acid, ascorbic acid and dopamine without any interference. The modification
of copper is done by anodisation in another electrolyte, potassium oxalate, to
check the sensitivity. The modified electrode was found to show better results
than that processed in sodium potassium tartrate. The present chapter
describes the experimental conditions for the anodisation of the electrode in
potassium oxalate solution, characterization of the resulting modified
electrode and testing for the detection of glucose.
4.1. Experimental
4.1.1. Development of CuO/CuOx Modified Copper (CuO/CuOx/Cu)
Electrode
The pretreatment of electrode and the anodisation were carried out as
described in the previous chapter (section 3.1.1). The copper strip was
anodized by cycling the potential between -1 and +1 V in potassium oxalate
solutions of different concentrations (1, 0.5 and 0.25 M) at a scan rate of 50
mV s-1. Then it was repeatedly washed with water and used for
electrochemical and morphological studies. The modified electrode is denoted
as CuO/CuOx/Cu.
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4.1.2. Electrochemical Characterization of CuO/CuOx/Cu and Direct
Electrocatalytic Oxidation of Glucose.
EIS and CV studies were conducted as explained in the previous
chapter (3.1.2). The direct electrooxidation of glucose on the modified
electrode was studied as described earlier in 3.1.3.
4.1.3. Determination of Glucose in Blood Serum
Serum samples were collected from hypo, normo and hyper glycemic
volunteers. 100 µL of the serum samples were injected to 5 mL of constantly
stirred solution of 0.1 M NaOH and the amperometric response was recorded
at 0.7 V. Thus, the response was corresponding to an effective concentration
of 1/50th of the original.
4.2. Results and Discussion
4.2.1. Anodisation and Electrochemical Characterisation
CV recorded during the anodisation of copper in 0.5 M potassium
oxalate at a scan rate of 100 mV s-1 is given in Figure 4.1A. Two anodic peaks
were observed, one at -0.1 and the other at 0.70 V; the first peak is sharp
indicating a single electron transfer representing the conversion of Cu to Cu(I)
whereas the second peak, broad and composite is attributed to more than one
reaction. As explained in the previous chapter, two reactions are possible, one
the conversion Cu(I) to Cu(II) and the other the formation of Cu(II) directly
from Cu. During the subsequent cycles, a decrease in current density was
observed which is due to surface stabilization.
On reversing the scan, a nucleation loop was obtained which may be
due to the deposition of dissolved copper ions as copper oxalate. Only one
cathodic peak was appeared at -0.65 V in addition to two shoulder waves were
observed at 0.40 and -0.10 V, respectively. The current intensity of cathodic
peak was very much less compared to the anodic peaks indicating that the
reduction of the copper oxalate, which was formed during the anodic scan, is
difficult. The formation of copper oxalate is represented as,
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Cu Cu2+ + 2e- …………………………..(4.1)
Cu2+ + (C2O4)2- + H2O CuC2O4. H2O………….(4.2)
Figure 4.1. Continuous cyclic voltammograms (5 cycles) obtained for copper
electrode in (A) 0.5 M and (B) 1 M potassium oxalate solutions at a scan rate of 50
mV s-1
68
In 1 M potassium oxalate solution, the oxidation peaks were very close
to each other (-0.05 and 0.25 V) and at less positive value when compared to
that in 0.5 M potassium oxalate solution indicating that the oxidation is
favoured in 1M than in 0.5M (Figure 4.1B). Lowering of concentration from
0.5 M resulted in the shifting of oxidation potential to higher positive value.
This observation is reinforced by polarization and coulometric studies. Tafel
plots show that the equilibrium potentials are more positive at lower
concentration and are more negative at higher concentration (Figure 4.2 A). It
is clear from this study that the oxidation of copper and hence the formation of
copper oxalate/copper oxide is more facile at higher concentrations.
Figure 4.2. (A) Tafel plot at a scan rate of 10 mV s-1 at copper electrode in 0.5 M
potassium oxalate solution of various concentration and (B) coulometric curves
obtained during the potentiostatic anodisation of copper in potassium oxalate
69
Further, the coulometric curve obtained during the potentiostatic anodisation
(Figure 4.2B) confirms this observation. Passivation of electrode surface
occurs in less than 10 seconds while it requires 20 seconds in 0.5 M solution
and in 0.25 M solution even after 60 seconds the electrode is not getting
passivated.
Being an effective tool for studying the interfacial properties of the
bare and modified electrodes, the electrochemical impedance spectrum
observed is shown in Figure 4.3. The diameter of the semicircular portion is
found to increase with increasing cycles of oxidation (curves a, b, c and d
corresponding to 0,2,4 and 8 potential cycles of oxidation in 0.5 M potassium
oxalate solution). Increase in the CV cycles results in the conversion of Cu to
CuOx, ie., higher extent of surface passivation. This, in turn, causes a decrease
in conductance and increase in electron transfer resistance. As is the case with
a typical Nyquist plot, this causes the diameter of the semicircle to increase
with increase in number of CV cycles.
Figure 4.3. EIS of bare copper electrode (a) and after anodisation for two cycles (b),
four cycles (c), eight cycles(d) in 0.5 M potassium oxalate. EIS in 0.1 M NaOH
solution at open circuit potential; the frequency from 100 KHz to 0.001 Hz and
amplitude 5 mV
70
4.2.2. Surface Characterization
The SEM images of the bare and modified copper electrodes are
shown in Figure 5.4. The bare electrode surface was smooth (A) whereas after
anodisation in potassium oxalate solution the electrode surface became rough
and porous (B). Further, change in the concentration of the electrolyte causes a
change in the morphology. For instance, the SEM image of the electrode
obtained from 0.5 M solution (Figure 4.4B) was distinctly different from that
obtained from 1 M solution (Figure 4.4C). The electrode oxidized in 0.5 M
potassium oxalate was more porous and rough than the electrode oxidized in
1M potassium oxalate.
Figure 4.4. Scanning electron micrograph; (A) the unmodified copper electrode, (B)
copper electrode modified by CV in 0.5 M potassium oxalate at 50 mV s-1, (C) copper
electrode modified by CV in 1 M potassium oxalate at 50 mV s-1, (D) EDS spectrum
of the modified electrode
The EDS spectrum of the modified electrode (Figure 4.4D) shows the
presence of CuO on the electrode. The quantitative analysis data show the
presence of carbon, oxygen and copper in the ratio 4.49:19.21:76.30
71
respectively. This confirms the presence of both oxalate and oxides on the
surface. Further confirmation for the presence of oxalate was obtained from
FT-IR study (Figure 4.5).
The FTIR spectrum showed two sharp peaks, one at 1525 cm-1 which
corresponds to the symmetric stretch and the other at 1700 cm-1 corresponding
to the asymmetric stretch of carboxylate group. Comparison of this spectrum
with that of pure potassium oxalate (Figure 4.5 inset) confirms the formation
of copper oxalate on the electrode surface during anodisation. Similar
observations are reported in the literature [388, 389]. Based on the above
observations it is confirmed that the modified electrode surface contains both
copper oxide and copper oxalate (hence represented as CuO/CuOx/Cu).
Figure 4.5. FTIR spectrum of the electrode modified by CV at 50 mV s-1 between -1
and 0.8 V for 5 cycles in 0.5 M potassium oxalate solution. Inset: FTIR spectrum of
pure potassium oxalate
4.2.3. Electrocatalytic Oxidation of Glucose
The CV of the modified electrode in 0.1 M NaOH solution containing
5 mM glucose solution (Figure 4.6 curve a) shows a well defined anodic peak
around 0.5V due to the oxidation of glucose and this oxidation is irreversible
72
since no peak is observed in the reverse scan. No electrochemical response
was obtained in the absence of glucose (Figure 4.6 curve b).
Figure 4.6. Cyclic voltammograms obtained at CuO/CuOx/Cu electrode at a scan rate
of 50 mV s-1 in 0.1 M NaOH solution in the presence of 5 mM glucose (a) and in the
absence of glucose (b)
The LSV study also shows the same trend, a characteristic peak at
0.5V in the presence of glucose and no peak in its absence (Figure 4.7). Since
it is well known that metallic copper also catalyses the oxidation of glucose
[156], we investigated it using bare copper electrode, without any modification
(Figure 4.7 curve c). Comparison of the two LSVs (curves b and c) points out
that oxidation takes place on both, but at a lower potential at modified
electrode with a high current response than at bare Cu.
The mechanism of oxidation of glucose at the modified electrode is
explained in the previous chapter. A supportive evidence for the proposed
mechanism is given below. Figure 4.8 represents the CV of the bare (a) and
modified (b) copper electrodes in 0.1M NaOH. The modified electrode shows
73
Figure 4.7. Linear sweep voltammograms obtained at CuO/CuOx/Cu electrode
(curves a and b) and bare copper electrode (c) at a scan rate of 50 mV s-1 in 0.1 M
NaOH solution; curves b and c in the presence of 5 mM glucose and curve a in the
absence of glucose
Figure 4.8 Cyclic voltammograms obtained at (a) bare Cu electrode and (b)
CuO/CuOx/Cu electrode in 0.1 M NaOH solution at a scan rate of 50 mVs-1
74
an anodic peak at 0.65V and no peak was observed for the bare electrode. The
peak observed at 0.65 V can be attributed to the formation of the Cu(III) and
the small cathodic peak at 0.62V to the reduction of Cu(III) to Cu(II) as
reported in the literature [155, 156, 188, 189, 276, 277]. The oxidation of
glucose was also seen at the same potential. It is observed that the oxidation of
glucose at the modified electrode takes place at a lower potential than that at
the bare copper electrode (Figure 4.7). From these observations, it is evident
that the oxidation of glucose is catalysed by the Cu(III) surface state which is
formed only on the oxidized electrode.
Figure 4.9. Linear sweep voltammograms obtained at CuO/CuOx/Cu electrode in 0.1
M NaOH with successive additions of 1 mM glucose solution (a-j: 0-9 mM) at a scan
rate of 50 mV s-1. Inset shows the calibration curve (peak current vs concentration)
The results of LSV given in Figure 4.9 show the effect on the anodic
peak current with change in concentration. During the successive additions of
1 mM glucose solution to 0.1 M NaOH solution at a scan rate 50 mVs-1 (a-j)
the peak current increased linearly with concentration. Curve a was in the
absence of glucose and the very first addition of 1 mM of glucose shows an
anodic wave at 0.45 V. Further additions correspond to an increment of 1 mM
75
each and a linear response was observed with the regression equation
Ip(µA) = 60.30 + 81.78 C (mM) with r = 0.9992 throughout the range of
concentration. A slight anodic shift in the peak potential was observed at
higher concentrations.
4.2.4. Effect of Experimental Parameters on Voltammetric Response
The optimum concentration of electrolyte for the oxidation of glucose
at the modified electrodes was determined from the results using different
concentrations ranging from 0.001 to 1 M to be 0.1 M. Also, the optimum
potential for the oxidation of glucose in alkaline media was established from
LSV to be 0.70 V and hence all amperometric analyses were carried out using
0.1 M NaOH at 0.70 V.
Figure 4.10. LSVs at the modified electrode in 0.1 M NaOH containing 10 mM
glucose solution with increasing scan rate. a-j: scan rates, 10 to 100 mV s-1. Inset: plot
of peak current vs square root of scan rate
The effect of scan rate on the oxidation current of glucose at the
CuO/CuOx/Cu electrode in 0.1 M NaOH solution containing 10 mM glucose
76
is depicted in Figure 4.10. The peak current increases linearly with the square
root of scan rate with the regression equation,
Ip (µA) = 99.59 + 104.39 υ1/2 (mV s-1)1/2 with r = 0.9995 and this confirms that
the oxidation of glucose is diffusion controlled, which is in agreement with the
earlier report [188].
4.2.5. Amperometric Detection of Glucose
Figure 4.11 shows the steady state response current of the modified
copper electrode in 0.1 M NaOH solution with three sets of glucose
concentrations tested by successive additions of glucose at an applied potential
of 0.7 V. Time required to obtain the stable response was less than one second,
and this is much faster than the reported values [188,381-383].
Figure 4.11. Amperometric responses of the modified electrode to glucose in a
constantly stirred solution of 0.1 M NaOH at an applied potential of 0.70V with
increment in glucose concentration, at I: 6.25 µM, II: 62.5 µM and III: 625 µM.
Insets: (A) the calibration curve and (B) enlarged view of range I
The first set of 16 additions was introduced in increments of 6.25 µM,
the next set of 12 in increments of 62.5 µM and the last set of 10 in increments
77
of 625 µM. In all the three regions the sensor show very good linearity (inset
A in 4.11) with regression equation I (µA) = 13.75 + 0.1678 C (mM);
correlation coefficient 0.9999 and standard deviation 2.3843; also in the range
of 2 µM to 15 mM with a detection limit of 0.1 µM (S/N = 3) and the
sensitivity was 1890 µA mM-1 cm-2.
The high sensitivity of the proposed sensor is attributed to the
synergistic effect of two significant factors, (i) the unusual electrocatalytic
activity of the Cu(III)/Cu(II) redox couple making the electrode highly
sensitive and (ii) the rough and microporous structure catalysing specifically
the oxidation of glucose. The presence of Cu(III)/Cu(II) has already been
established by CV and morphology by SEM. From the linearity of current
response it is evident that no electrode fouling occurred due to the oxidised
product of glucose on the surface even after successive additions of glucose in
increased concentrations.
4.2.6. Reproducibility and Storage Stability
The reproducibility of the sensor was studied as explained in section
3.2.7 and the results are presented in Figure 4.12 A. The variation was less
than 2%, which confirmed that the electrode modification method was highly
reproducible. The shelf life of the sensor was studied as described in 3.2.7.
The decrease in sensitivity over a period of one month was less than 2.5% of
its original value (Figure 4.12B). This study convincingly proved that the
sensor has high storage stability.
4.2.7. Effect of Interfering Species
Many earlier reports point out the interference of AA, UA and AP
which could be easily oxidized at a relatively low positive potential. The
physiological level of glucose is about 50 times higher than that of the
interfering species. Therefore the interference of these electroactive molecules
was tested by adding 0.1 mM interfering agents and 3 mM glucose solution
successively into a constantly stirred solution of 0.1 M NaOH. The response
current for these interfering species is less than 1% of that observed for
78
glucose at this applied potential. This corroborates the fact that the CuO/CuOx
modified electrodes would give higher sensitivity and selectivity for glucose
detection under physiological conditions.
Figure 4.12. A. Comparison of sensitivity of eight sensors prepared by same method
and (B) Variation in sensitivity of a sensor for a period of 30 days
79
4.2.8. Practical Applications
Figure 4.13. Steady state response current of the modified electrode to glucose
solution and serum samples tested in 0.1 M NaOH. Points a and b are 5 mM glucose
solutions; c, d and e are blood serum having glucose concentrations 2.5 mM (45
mg/dL), 14.66 mM (264 mg/dL) and 4.03 mM (72.54 mg/dL) respectively
Application of this sensor to real-time samples is an expected outcome
of this study. Figure 5.13 shows the amperometric response obtained for the
blood serum. The first two additions (a and b) are of known concentrations of
glucose to correlation. The next three (c, d and e) are additions of serum
samples. At points a and b additions of 100 µL of 5 mM glucose solution
increase the effective concentration by 0.1 mM (1.8 mg/dL) and consequently
the current by 33 µA. The results obtained with serum samples (c, d and e)
were 16.5, 96.82 and 26.6 µA. These values when compared with the values
for glucose, yield concentrations 2.5 mM (45 mg/dL), 14.66 mM (264 mg/dL)
and 4.03 mM (72.54 mg/dL) respectively. Even after several additions of very
high concentrations of glucose and blood serum, the last addition of blood
serum (e) having low glucose concentration gave the correct result, proves
beyond doubt that the sensor is not affected by other biomolecules in blood
80
serum. The glucose levels obtained in this method are well in agreement with
those from the photometric method, variation being less than 1.5%.
Figure 4.14. Comparison of sensitivity of various non-enzymatic glucose sensors. a–
Platinum nanotube arrays modified sensor[384]; b-Multi-walled carbon nanotube
modified sensor [175]; c-Mesoporous platinum sensor[173]; d–Nanoporous platinum-
lead alloy sensor [385]; e-Platinum-lead nanowire sensor [386]; f-Porous gold
sensor[382]; g-Macroporous platinum sensor [387]; h-Manganese dioxide-multi-
walled carbon nanotubes composite sensor [186]; i–Gold nanoparticles modified
sensor[304]; j-Gold nanoparticles modified sensor[303]; k-Copper oxide nanowires
modified copper sensor [189]; l–the proposed CuO modified sensor
The sensitivity of the developed sensor was compared with those
reported [173, 175, 186, 189, 303, 304, 382, 384, 385, 386, 387]. It is found
that the observed sensitivity in this study was notably higher than that of
similar non-enzymatic sensors (Figure 4.14).
81
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4.3. Conclusion
In this study, the development of a non-enzymatic sensor using single
step anodisation of copper in potassium oxalate solution was described. SEM
studies showed that the modified surface was rough and highly porous. The
response of the sensor towards glucose solution as well as glucose in blood
serum is very good. It exhibits excellent linearity. Also the high sensitivity and
selectivity which were attributed to the synergistic effect of Cu(II)/Cu(III)
redox couple and the rough, microporous structure are noteworthy. Since the
fabrication involves only a single step using commonly available materials,
the proposed sensor promises to be economically viable.