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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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).

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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.