Chapter 3

Electrocatalytic Oxidation of Glucose on Copper

Oxide Modified Copper Electrode

3. Electrocatalytic Oxidation of Glucose on Copper

Oxide Modified Copper Electrode

In order to combat the drawbacks of enzymatic glucose biosensors, it

was thought to develop a non-enzymatic sensor, which adopts the direct

electrocatalytic oxidation of glucose. Several materials such as platinum, gold,

copper, silver, bismuth and mercury which can catalyse the oxidation of

glucose in their native form or in modified forms were reported. Out of all

these materials, copper and copper oxide based materials are shown to

facilitate the inherent tendency for the oxidation of glucose. Further added

advantage is its low cost, but its toxic nature excludes its use as an invasive

sensor. This chapter describes the development, characterization and

application of copper oxide modified copper electrode for the detection of

glucose. The copper electrode has been anodized in sodium potassium tartrate

through potentiostatic and potentiodynamic techniques. The developed

electrode was characterized for its morphology, surface composition and

tested for its potential to use as an amperometric glucose sensor. Its

application was extended to test the glucose concentration in blood serum also.

3.1. Experimental

3.1.1. Development of CuO Modified Copper (CuO/Cu) Electrode

Copper sheet of 0.5 mm thickness was sheared into small strips and

selectively masked with Teflon tape to expose an area of 0.06825 cm2 which

was measured using high resolution video measuring system (ARCS, KIM

series, Taiwan). These electrodes were polished with a series of emery papers,

washed with double distilled water, rinsed with acetone and dried in nitrogen

atmosphere. The strip was anodized by CV at a scan rate of 50 mV s-1 between

-1 and +1 V in sodium potassium tartrate solutions of different concentrations

50  

(1, 0.5 and 0.25 M). Then it was repeatedly washed with water and used for

electrochemical and morphological studies.

3.1.2. Electrochemical Characterization of CuO/Cu Electrodes

Electrochemical impedance spectra of the bare copper and CuO

modified copper electrodes were carried out in 0.1 M NaOH solution at their

open circuit potentials, in the frequency range of 0.01 Hz to 100 KHz with

potential amplitude of 10 mV. The impedance spectra were plotted in the form

of complex plane diagrams.

3.1.3. Electrochemical Detection of Glucose using CuO/Cu Electrode

The CV and LSV of the bare and modified copper electrodes were

carried out in 0.1M NaOH solution in the potential window of 0 to 0.80 V at a

scan rate of 50 mV s-1. In order to study the mechanism of oxidation of

glucose, the scan rate was varied between 1-5000 mV s-1. To find the optimum

potential and concentration of NaOH for the best response, the amperometry

was carried out using the modified electrode at various potentials in a stirred

solution of 0.01 M to 1 M NaOH. 10 µL of glucose solution was injected at

regular intervals so that the resultant concentration varied from 2 µM to 20

mM. Trials produced identical results with and without nitrogen purging.

Hence, the experiments were carried out without nitrogen purging. The

interference of ascorbic acid and uric acid was studied by injecting 10 µL of

the respective solutions into the test solution. All the experiments were

conducted at room temperature and were repeated at least three times to check

the reproducibility.

3.2. Results and Discussion

3.2.1. Electrochemical Formation of CuO on Cu Electrode (CuO/Cu)

Cyclic voltammogram of copper electrode in sodium potassium tartrate

solution is shown in Figure 3.1. Two anodic peaks a and b appeared at 0.10

and 0.70 V, respectively. The peak a is sharp and well defined corresponding

to a single electron transfer for the conversion of metallic copper to cuprous

51  

ions. The second peak observed at 0.70 V corresponds to the conversion of

cuprous to cupric ion. The current intensity of these peaks was relatively high

indicating facile oxidation of copper in tartrate medium. On reversing the

scan, two cathodic peaks were observed, one at -0.15 V and the other at about

-0.30 V; the first one corresponding to the conversion of Cu(II) to Cu(I). The

second peak was composite in nature which may be due to the involvement of

more than one electron transfer such as Cu(II) to Cu and Cu(I) to Cu. The

cathodic peak current was comparatively less than that of the anodic showing

the stability of the oxide formed and its reduction is less facilitated in the

cathodic scan.

Figure 3.1. Cyclic voltammogram obtained for copper electrode in 0.5 M sodium

potassium tartrate at a scan rate of 100 mV s-1

In order to correlate the anodic and cathodic peaks, the direction of

potential was reversed immediately after peak a was observed in the anodic

direction and only one peak at c was observed which corresponds to the

reduction of the products formed during the oxidation at a. Hence the other

two peaks (b and d) must be due to the redox reaction of Cu(II) and Cu(I).

Since the peak d is composite in nature, it may be due to two competing or

52  

parallel processes such as the reduction of Cu(II) to Cu(I) and also the

reduction of Cu(II) to Cu. But during the subsequent repeated cycles, current

intensity decreases due to stabilization of the copper surface due to the

formation of copper oxide (Figure 3.2).

Figure 3.2. Continuous cyclic voltammograms (5 cycles) obtained for copper

electrode in 0.5 M sodium potassium tartrate at a scan rate 100 mV s-1

The effect of concentration of sodium potassium tartrate (1, 0.5 and

0.25 M) on the anodisation of copper was studied and is shown in Figure 3.3.

It is found that the oxidation peak current is maximum at 0.25 M

concentration. The peak currents obtained for 0.5 and 0.25 M were almost

same, but a slight shift in peak potential towards more anodic region when

0.25 M solution was used. The decrease in peak current due to passivation

during the continuous cycling happens with lesser number of potential cycles

in more concentrated solution as shown in Figure 3.3.

The EIS obtained for copper electrode in 0.1 M NaOH after each

potential cycle in potassium tartrate is depicted in Figure 3.4. It shows a

decrease in diameter of the semicircular portion and hence the electron

transfer resistance decreases during the repeated cycling. The semicircle

53  

portion observed at higher frequency range corresponds to the electron-

transfer-limited process and a linear segment at lower frequencies represents

the diffusion limited process.

Figure 3.3 Continuous cyclic voltammograms (5 cycles) obtained for copper

electrode in the presence of 1, 0.5 and 0.1 M sodium potassium tartrate at a scan rate

of 100 mV s-1

The diameter of the semicircle in the Nyquist plot equals the electron-

transfer resistance, Ret, which is related to the electron-transfer kinetics of the

redox probe at the electrode surface. From the figure, it is obvious that the

bare copper exhibits maximum electron transfer resistance (curve a) and it

decreases continuously in every potential cycle in sodium potassium tartrate

(curves b-f).

3.2.2. Surface Characterisation

Scanning electron micrographs of the bare and the modified electrode

are presented in Figure 3.5. It is clear that the electrode surface turns rough

after modification and micro-growth formed on the surface are visible. The

54  

effective surface area of the electrode increases tremendously due to this

micro-growth on the surface.

Figure 3.4. EIS in 0.1 M NaOH at open circuit potentials in the frequency range of 1

Hz-100 KHz with amplitude 10 mV. Curve a for bare copper and b-f for modification

by successive potential cycling in 0.5 M sodium potassium tartrate. Inset shows the

equivalent circuit

Figure 3.5. Scanning electron micrographs of bare copper electrode (A) and modified

copper electrode (B); modification was carried out by CV in 0.5 M sodium potassium

tartrate at a scan rate of 100 mVs-1 for five cycles

The EDAX spectrum obtained for the modified electrode is shown in

Figure 3.6. The elemental composition of the surface species was obtained as

Cu (79.89) and O (20.11), that is the ratio of Cu:O is 3.9726:1 which is very

close to the theoretical ratio 3.9718:1 for CuO.

55  

Figure 3.6. EDAX spectrum of the CuO modified electrode

3.2.3. Electrocatalytic Oxidation of Glucose

Trials conducted on electrocatalytic oxidation of glucose on the

modified electrode in different electrolytes such as NaOH solution, acetate and

phosphate buffer. It was found that the modified electrode was not stable in

acetate and phosphate buffer solutions. Hence, NaOH solution was chosen as

the electrolyte for the oxidation of glucose. Most of the previous reports on

direct oxidation of glucose, which involve copper or copper oxide, indicate

use of NaOH as the electrolyte [155, 156, 188, 189, 276, 277]. Figure 3.7

shows the CV obtained for a CuO/Cu electrode (prepared by five cycles of CV

in 1 M sodium potassium tartrate), in 0.1 M NaOH at a scan rate of 100 mVs-1

in the absence (a) and presence of 6 mM glucose (b). In the absence of

glucose, no characteristic peak was observed except a small plateau at 0.60 V

and in the presence of glucose a distinct peak appears at 0.60 V, which is 200

mV less positive potential than that the reported (0.80 V) at bare copper and

modified copper electrodes by Torto et al. [155]. This observed result

establishes that modification process has a definite role on the oxidation of

glucose.

56  

Figure 3.7. CV in 0.1 M NaOH solution containing 6 mM glucose at 100 mV s-1 on

electrodes CuO modified in the absence of glucose (a) and in the presence of glucose

(b)

The mechanism of electrochemical oxidation of glucose on copper and

modified electrode is entirely different from that of enzyme catalysed or

chemical oxidation. Both chemical and biochemical oxidation of glucose

proceed through a gluconic acid intermediate where as the oxidation at CuO

modified electrodes results in the oxidation of glucose to formates. Although

the exact mechanism for the oxidation of glucose at CuO modified electrodes

in alkaline medium is still not known with certainty, the most accepted one

was that suggested by Marioli and Kuwana [277]. According to them, the

oxidation was triggered by the deprotonation of glucose and isomerization to

its enediol form followed by adsorption onto the electrode surface and

oxidation by Cu(I), Cu(II) and Cu(III) states.

Kano et al. conducted a detailed study on the effect of surface species

responsible for the electrooxidation of glucose [276] and proved that CuO is

responsible for the oxidation of glucose by showing maximum response when

57  

compared to Cu2O and Cu(OH)2. According to the mechanism proposed by

them the oxidation of glucose results in formates along with 12 electrons.

Though the presence of CuO is essential for oxidation of glucose, the

oxidation involves the catalysis of higher oxidation species such as CuO+ or

CuO(OH) [276]. This was supported by the fact that these species will be

formed in alkaline medium which favour the oxidation of glucose. Again, the

possible activation of these species by the electric field also cannot be

neglected [276]. According to them, alkoxide formation between the alcoholic

oxygen and the (soluble) Cu(III) ion [379] seems to be essential for the

electron transfer from carbohydrates to Cu(III) to generate radical

intermediates and CuO [273]. The radicals would be oxidized immediately to

yield formate while Cu(II) (or CuO) can be oxidized to regenerate the

catalytically active Cu(III) species. Hence, a net two-electron transfer is

accomplished in each step of catalytic cycle. The oxidation of glucose occurs

in the potential range of 0.40 to 0.8 V where the oxidation wave for

Cu(II)/Cu(III) was reported [155, 156, 188, 189, 276, 277]. Here, the Cu(III)

species was proposed to act as an electron transfer mediator [188, 380].

Three electrodes (A, B and C) were modified under three different

concentrations of tartrate solutions and were tested for sensing glucose by

taking 6 mM glucose in 0.1 M NaOH solution at 100 mV s-1 (Figure 3.8). The

peak potentials for the oxidation of glucose were shifted to more anodic value

for electrodes modified at lower concentrations (curves b and c). Therefore,

the amperometry and other studies were carried out using the electrode

anodised from 1 M solution of sodium potassium tartrate.

LSVs carried out with increasing concentration of glucose in 0.1 M

NaOH at 100 mVs-1 are presented in Figure 3.9. In the absence of glucose

(curve a) no characteristic peak was obtained, but a small shoulder can be seen

around 0.60 V which may be due to the redox wave of Cu(II)/Cu(III). But, in

the presence of glucose a distinct peak at 0.55 V was obtained (curve b).

58  

Figure 3.8. Cyclic voltammograms in 0.1 M NaOH solution containing 6 mM

glucose at a scan rate of 100 mVs-1. The electrodes modified in: (a) 1, (b) 0.5 and (c)

0.1 M sodium potassium tartrate solution

Figure 3.9. Linear sweep voltammograms obtained for glucose at modified copper

electrode in 0.1 M NaOH. Curve a in the absence of glucose. Each addition of

glucose increases the concentration by 0.625 mM (b-k). Inset shows the calibration

plot

59  

Further additions of glucose resulted in increase of current response with a

very small shift in peak potential. Each addition corresponds to an increment

of 0.6 mM and a linear response was observed with linear regression equation

Ip (µA) = 82.4090 + 187.8836 C (mM) with an r = 0.9988.

3.2.4. Effect of Experimental Parameters on Amperometric Response

The optimum concentration of NaOH solution for the amperometric

detection of glucose at the modified electrodes was determined using different

concentrations ranging from 0.001 to 1 M and achieved the best response in

0.1 M solution (Figure 3.10). The optimum potential for the oxidation of

glucose in alkaline media was established to be 0.7 V which was also found

true by amperometry studies at different applied potentials (Figure 3.11).

Hence all amperometric analyses were carried out using 0.1 M NaOH at 0.70

V.

Figure 3.10. Effect of NaOH concentration on peak current of glucose oxidation

The effect of scan rate on the oxidation current of glucose at the CuO

electrode was examined by CV in 0.1 M NaOH solution containing 10 mM

glucose. The peak current increases linearly with the square root of scan rate

60  

and this indicates that the oxidation of glucose is diffusion controlled, which is

in agreement with earlier report [189].

3.2.5. Amperometric Detection of Glucose

Amperometric responses obtained by the successive additions of

glucose into 0.1 M NaOH solution at 0.70 V are shown Figure 3.12. Time

required to obtain a stable response was less than one second, signifying a

faster response than that of the reported sensors [188, 381-383]. The sensor

exhibits excellent linearity in the range of 2µM to 20 mM with the regression

equation Ip (µA.) = 53.8886 + 42.8954 C (mM) with a correlation coefficient

r = 0.9979. The sensitivity was found to be 761.9 µA mM-1 cm-2 with a

detection limit 0.1 µM.

Figure 3.11. Amperometric response of the modified electrode to glucose in a stirred

solution of 0.1 M NaOH at various applied potentials. Each addition of glucose

increased the concentration by 0.6 mM

61  

Figure 3.12. Amperometric response of the modified electrode to glucose in a stirring

solution of 0.1 M NaOH. Each addition of glucose increased the concentration by

0.60 mM. Inset shows the calibration curve. Applied potential was 0.70 V

The very high sensitivity of the proposed sensor may be attributed to

the synergistic effect of two significant factors, (i) the unusual electrocatalytic

activity of the Cu(II)/Cu(III) redox couple making the electrode highly

sensitive and (ii) the fact that the rough surface with high electroactive surface

area specifically catalyses the oxidation of glucose. Further, from the good

linearity of current response it is evident that no electrode fouling occurred

due to the presence of oxidised product on the surface even after successive

addition of glucose in increased concentrations.

3.2.6. Effect of Interfering Species

Ascorbic acid, uric acid, dopamine and acetaminophen in biological

sample get easily oxidized at positive potential, and consequently these

biomolecules interfere with the detection of glucose. The physiological level

of glucose is much higher than that of the interfering species. Therefore, the

interference of these electroactive molecules was tested by adding 0.1 mM

interfering agents with 3 mM glucose solution successively into a constantly

62  

stirred solution of 0.1 M NaOH (Figure 3.13). The response current for these

interfering species is less than 1.5% of that observed for glucose at this applied

potential. This corroborates the fact that both sensitivity and selectivity were

achieved for the determination of glucose at CuO modified electrode.

Figure 3.13. Effect of interfering molecules such as AA, UA, DA and AP on the

amperometric detection of glucose. a-b: glucose; b-c: AA; c-d: glucose; d-e: UA; e-f:

glucose; f-g: DA; g-h: glucose; h-i: AP and i-j: glucose

3.2.7. Reproducibility and Storage Stability

In order to evaluate the reproducibility of the sensor, eight CuO

electrodes were made and tested with 5 mM glucose solution in 0.1 M NaOH

at 0.70 V. The variation was observed to be less than 2.5 ± 0.5 % which

establishes the reproducibility of the electrode modification method. The

sensor was preserved in deaerated distilled water at room temperature

(25 ± 2 oC) when not in use. The long term storage stability of the sensor was

examined by measuring the amperometric response for glucose once in every

four days over a period of one month. The decrease in sensitivity was less than

3% of its original value. This study shows that the sensor has good

reproducibility and storage stability.

63  

3.2.8. Practical Applications

Blood serum samples were collected from a nearby clinical laboratory,

tested with the sensor developed and the results were compared with that

obtained by photometric method. The variation was small, within 2 ± 0.2%.

Since the development of the sensor involves a single process step and low

cost chemicals, this process is expected to be economically viable.

Figure 3.14 Comparison of sensitivity of various nonenzymatic

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.

64  

65  

Further, it is worth mentioning that the observed sensitivity in this

study was remarkably higher than that of similar non-enzymatic sensors

already reported (Figure 3.14).

3.3. Conclusion

Development of a non-enzymatic sensor for the determination of

glucose by a single step modification of copper in sodium potassium tartrate

solution is described in this chapter. SEM images showed that the modified

surface was rough. The electrocatalytic activity of the modified electrode was

evaluated using LSV and amperometry by injecting glucose solution into the

test solution. The response of the sensor towards glucose solution as well as

glucose in blood serum was good. The sensor has shown very good sensitivity,

selectivity, linearity, wide detection range, reproducibility and fast detection.