Chapter 3. Electrocatalytic Oxidation of Glucose on Copper Oxide Modified Copper Electrode

Transcription

1 Chapter 3 Electrocatalytic Oxidation of Glucose on Copper Oxide Modified Copper Electrode

2 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 Experimental 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 cm 2 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

3 (1, 0.5 and 0.25 M). Then it was repeatedly washed with water and used for electrochemical and morphological studies 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 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 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 Results and Discussion 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

4 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 V and the other at about 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

5 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

6 portion observed at higher frequency range corresponds to the electrontransfer-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 electrontransfer resistance, R et, 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) 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

7 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 :1 which is very close to the theoretical ratio :1 for CuO. 55

8 Figure 3.6. EDAX spectrum of the CuO modified electrode 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

9 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

10 compared to Cu 2 O 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

11 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 mm (b-k). Inset shows the calibration plot 59

12 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 I p (µa) = C (mm) with an r = 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 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 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

13 and this indicates that the oxidation of glucose is diffusion controlled, which is in agreement with earlier report [189] 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 Time required to obtain a stable response was less than one second, signifying a faster response than that of the reported sensors [188, ]. The sensor exhibits excellent linearity in the range of 2µM to 20 mm with the regression equation I p (µa.) = C (mm) with a correlation coefficient r = The sensitivity was found to be µa mm -1 cm -2 with a detection limit 0.1 µm. Figure 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

14 Figure 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 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

15 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 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 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 o C) 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

16 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-multiwalled 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

17 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) 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. 65