A sensitive and selective enzyme-free amperometric glucose biosensor using a composite from multi-walled carbon nanotubes and cobalt phthalocyanine

Abstract

In the present study, a simple and sensitive amperometric enzyme-free glucose sensor was developed at a multiwalled carbon nanotube and cobalt tetrasulfonated phthalocyanine (MWCNT–CoTsPc) modified electrode. The morphology of the fabricated composite was characterized and confirmed by transmission electron microscopy and UV-Vis spectroscopy. UV-Vis spectroscopy results confirmed that the MWCNT–CoTsPc composite was formed via the strong π–π interaction between CoTsPc and MWCNT. Compared with pristine CoTsPc, the MWCNT–CoTsPc composite modified electrode showed a higher electrocatalytic activity and lower overpotential towards the oxidation of glucose. Amperometric i–t technique was used for the determination of glucose. The response of glucose was linear over the concentration ranging from 10 μM to 6.34 mM with a sensitivity of 122.5 μA mM⁻¹ cm⁻². The response time of the sensor was determined to be 2 s with a limit of detection of 0.14 μM (S/N = 3). The fabricated sensor also exhibited a good selectivity in the presence of common interfering species. In addition, the fabricated sensor exhibited special advantages, such as low working potential, good sensitivity along with good repeatability and reproducibility, for the determination of glucose.

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1. Introduction

Over the past decade, the development of more reliable and sensitive electrochemical sensors for the determination of glucose has received considerable attention owing to its vital role in different areas, including clinical diagnostics, fuel cells and food industry.^1–7 The pioneering work of glucose biosensor was proposed by Clark in 1962, and since then the fabrication of electrochemical glucose biosensors using different modifiers has received considerable attention.⁸ Glucose oxidase (GOx) is used as the main electro-catalyst in most of the glucose biosensors due to its high selectivity towards glucose.⁹ However, GOx based glucose biosensors have shown many drawbacks such as short cell lifetime, complicated immobilization procedure, less stability at elevated temperature and pH.^10,11 To conquer these difficulties, non-enzymatic or enzyme-free glucose sensors can be used as alternative sensors. Recently, metal nanoparticles, metal oxides and metal alloys have been used extensively for the enzyme-free detection of glucose in alkaline solutions.^12–16 Although these types of modified electrodes are inexpensive, they show drawbacks such as low sensitivity and selectivity.¹⁷ Therefore, the development of a low cost, highly selective, and reliable enzyme-free glucose sensor is still being extensively researched by the electroanalytical community.

Over the past two decades, multiwalled carbon nanotubes (MWCNT) have been considered as the best electrode material in the field of sensor and biosensor application because of their unique mechanical and electronic properties.^18,19 In addition, metallophthalocyanines and metalloporphyrins, which are organo-metallic macrocycles, and in particular cobalt tetrasulfonated phthalocyanine (CoTsPc) has been well studied and has shown to be an excellent electrocatalytic materials towards several important molecules such as hydrogen peroxide, cysteine, nitrite, nitric oxide and ascorbic acid.^20,21 Moreover, CoTsPc has also been used as redox mediators for enzyme based glucose sensors.²² Pristine CoTsPc is not stable on the electrode surface due to its low conductivity along with poor electrochemical activity.²³ Carbon nanomaterials, such as single-walled (SWCNTs) or multi-walled carbon nanotubes and graphene, have been used with CoTsPc to enhance the electrochemical conductivity and electron transfer of CoTsPc; thus, it is used for many potential applications, including electrochemical sensors and biosensors.^24–29 Earlier studies have revealed that MWCNTs are an ideal carbon nanomaterial that can combine with CoTsPc to form a more stable composite electrode due to the strong π–π interaction between CoTsPc and MWCNTs.³⁰ The main aim of the present study is to utilize the special properties of the MWCNT–CoTsPc as an electro-catalyst for the oxidation of glucose.

In the present study, for the first time, MWCNT–CoTsPc composite was prepared and applied for the determination of glucose. It is found that the MWCNT–CoTsPc composite modified electrode showed an excellent electrocatalytic activity towards glucose in alkaline media compared with other modified electrodes (pristine MWCNT and pristine CoTsPc modified electrodes). The fabricated enzyme-free glucose sensor also exhibits special advantages, such as a low limit of detection (LOD), wide linear range and fast response for the determination of glucose.

2. Experimental

Materials

MWCNT, CoTsPc and D-glucose were purchased from Sigma-Aldrich and used as received. The supporting electrolyte used in all the electrochemical studies was 0.1 M NaOH solution, which was prepared using NaOH and deionised water. Prior to each experiment, all the solutions were deoxygenated with pre-purified N₂ gas for 15 min unless otherwise specified. Double distilled water with conductivity of ≥18 MΩ cm was used for all the experiments. Human blood serum sample was collected from valley biomedical, Taiwan product & services, Inc. This study was reviewed and approved by the ethics committee of Chang-Gung memorial hospital through the contract no. IRB101-5042A3.

Methods

The electrochemical measurements were carried out using a CHI 611A electrochemical work station. Electrochemical studies were performed in a conventional three electrode cell using a glassy carbon electrode (GCE) as a working electrode (working area 0.071 cm²), Ag|AgCl (saturated KCl) as a reference electrode and a Pt wire as a counter electrode. Amperometric measurements were performed with an analytical rotator AFMSRX (PINE instruments, USA) with a rotating disc electrode (RDE) having a working area of 0.24 cm². TEM images were taken with a JEOL 2000 transmission electron microscope (operating at 200 kV) equipped with an energy dispersive X-ray (EDX) analyzer. UV-Vis spectroscopy studies were performed by a U-3300 spectrophotometer. All the electrochemical experiments were performed in deoxygenated 0.1 M NaOH solution.

Preparation of MWCNT–CoTsPc composite modified electrode

For the preparation of MWCNT–CoTsPc composite, aqueous solutions of MWCNT (2 mg mL⁻¹) and CoTsPc (0.2 mg mL⁻¹) were mixed and ultrasonicated for 3 hours. The resulting residue was washed several times with deionized water to remove the excess of CoTsPc. The final composite is denoted as MWCNT–CoTsPc. Prior to modification, the GCE surface was polished with 0.05 μm alumina slurry using a Buehler polishing kit, washed with deionized water, ultrasonicated for 5 min and allowed to dry at room temperature. Then, about 5 μL of MWCNT–CoTsPc composite was drop casted on the pre-cleaned GCE surface and dried at ambient conditions. The CoTsPc and MWCNT modified GCEs were also prepared using the same procedure.

3. Results and discussion

Characterization of MWCNT–CoTsPc composite

TEM image (Fig. 1C) reveals that CoTsPc is more randomly absorbed onto/into the MWCNT networks and the width of the nanotubes before (Fig. 1A) and after the modification with CoTsPc was calculated to be 5 ± 2 and 15 ± 3 nm, respectively. Moreover, the EDX spectrum of MWCNT–CoTsPc (Fig. 1D) portrays the signals for carbon, oxygen, sulphur and cobalt with a weight percentage of 45.95%, 35.89%, 12.93% and 5.23%, respectively. The EDX spectrum of pristine MWCNT (Fig. 1B) portrays the signal only for carbon and the result confirmed the formation of MWCNT–CoTsPc composite.

Fig. 1 TEM image of pristine MWCNT (A) and MWCNT–CoTsPc (C). EDX spectra of MWCNT (B) and MWCNT–CoTsPc (D).

UV-Vis spectra are used to confirm the π–π* interaction between MWCNT and CoTsPc. Fig. 2 shows the UV-Vis spectra of CoTsPc (a) and MWCNT–CoTsPc (b). The band in the 600–710 nm range (Q-band) observed in the UV-Vis spectra is attributed to the allowed π–π* transition for phthalocyanine. The absorption maxima of pristine CoTsPc are located at 661 and 625 nm. On the other hand, the Q-band of MWCNT–CoTsPc was significantly red shifted (bathochromic shift) to 661 and 635 nm, due to the electron transfer from the MWCNT to the CoTsPc ring. The lowered band gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is also the possible reason for the red shift of the composite in the UV-Vis spectra.³¹ TEM images reveal the strong intercalation of CoTsPc onto the MWCNT through π–π interaction, resulting in the increase in the width of the MWCNT in the composite rather than the pristine MWCNT.

Fig. 2 UV-visible spectra of CoTsPc (a) and MWCNT–CoTsPc (b).

The cyclic voltammetry response of different modified electrodes towards glucose

The cyclic voltammetry response of the different modified electrodes was tested in the presence and absence of glucose in 0.1 M NaOH. Fig. 3 shows the cyclic voltammetry response obtained at the GCE modified with CoTsPc (A), MWCNT (B) and MWCNT–CoTsPc (C) in the absence (a) and presence (b) of 1 mM glucose in 0.1 M NaOH at a scan rate of 50 mV s⁻¹. As can be seen from Fig. 3C, in the absence of glucose, the MWCNT–CoTsPc modified electrode showed a pair of well-defined redox peak with a formal potential of −0.4789 V and 0.1136 V in a NaOH solution, corresponding to Co(I)/Co(II) and Co(II)/Co(III) couples in CoTsPc. However, in the presence of 1 mM glucose, a sharp anodic response was observed at an onset potential of 0.178 V along with the Co(II)/Co(III) couples at the MWCNT–CoTsPc modified electrode.³² This is due to the oxidation of glucose to gluconolactone at the surface of the modified electrode.

Fig. 3 Cyclic voltammograms obtained at the GCE modified with CoTsPc (A), MWCNT (B) and MWCNT–CoTsPc (C) in the absence (a) and presence (b) of 1 mM glucose in 0.1 M NaOH at a scan rate of 50 mV s⁻¹.

On the other hand, CoTsPc and MWCNT modified electrodes showed a broad response to 1 mM glucose with an onset potential of 0.325 and 0.412 V, respectively. The observed current response at the MWCNT–CoTsPc modified electrode for glucose is about 2.8 and 3.6 folds higher than that observed at CoTsPc and MWCNT modified electrodes, respectively. Furthermore, the onset potential of MWCNT–CoTsPc modified electrode towards glucose significantly shifted to the negative potential (0.198 V) compared with other modified electrodes. The higher electrocatalytic activity and lower overpotential of the composite towards glucose may be due to the strong intercalation of CoTsPc onto/into the MWCNT networks. The results showed that the MWCNT–CoTsPc composite modified electrode showed a more synergistic effect on the electrocatalytic oxidation to glucose.

Electro-oxidation of glucose and effect of scan rate

In order to evaluate the electrocatalytic activity of the MWCNT–CoTsPc modified GCE towards glucose, the cyclic voltammetry was carried out to investigate the electro-oxidation of glucose at the modified GCE. Fig. 4A shows the cyclic voltammetry response of MWCNT–CoTsPc modified GCE in the absence (a) and presence (b–f) of 1–6 mM glucose in 0.1 M NaOH. Upon the addition of 1 mM glucose, a sharp oxidation peak appeared, and it further increased with increasing the concentration of glucose. In the absence of glucose, the glucose oxidation peak at 0.4 V does not appear at the composite electrode. Therefore, an increase in the anodic peak current at 0.4 V is due to the presence of glucose. The results further confirmed that the MWCNT–CoTsPc modified GCE is a suitable electrode material for the determination of glucose.

Fig. 4 (A) Cyclic voltammograms obtained at the MWCNT–CoTsPc/GCE in the absence (a) and presence (b–f) of each 1 mM glucose addition in 0.1 M NaOH at a scan rate 50 mV s⁻¹. (B) The cyclic voltammetry response of MWCNT–CoTsPc/GCE in 0.1 M NaOH containing 1 mM glucose at different scan rates from 0.01 to 0.1 V s⁻¹ (a to j). (C) Linear plot for I_pa vs. scan rates.

The effect of the scan rate of the electrocatalytic oxidation of glucose was investigated at the MWCNT–CoTsPc modified GCE. Fig. 4B depicts the cyclic voltammetry response of MWCNT–CoTsPc modified GCE in a 0.1 M NaOH solution containing 1 mM glucose at different scan rates from 0.01 to 0.1 V s⁻¹ (a–j). It can be seen that the anodic peak current response of glucose was found to be proportional to the scan rates from 0.01 to 0.1 V s⁻¹ with a correlation coefficient of 0.9944 (Fig. 4C). The result further confirms that the electro-oxidation of glucose at the surface of the modified electrode is a surface-confined electrochemical process.³³

Amperometric determination of glucose

The effect of the applied potential on the electrocatalytic oxidation of MWCNT–CoTsPc modified electrode to glucose was studied by the amperometric i–t technique. The electrocatalytic response of the MWCNT–CoTsPc modified electrode was investigated at different applied potentials (0.1–0.3 V) in a 0.1 M NaOH solution containing 100 μM glucose and the results are shown in Fig. 5A. It was found that a sharp and stable amperometric response was observed at 0.3 V and the amperometric signal decreased when the working potential was more or less than 0.3 V; hence, 0.3 V was chosen as a working potential for the amperometric experiments.

Fig. 5 (A) Amperograms obtained at the MWCNT–CoTsPc/GCE upon each addition of 100 μM glucose into the constantly stirred 0.1 M NaOH solution at different working potentials. The applied potentials are denoted as follows: 0.2 V, 0.3 V and 0.4 V. (B) An amperometric i–t response of the MWCNT–CoTsPc/GCE upon the addition of 10 μM and 100 μM glucose into 0.1 M NaOH at the applied potential of 0.30 V. (C) The corresponding calibration plot for I_pa vs. [glucose]. (D) In the same conditions, an amperometric i–t response of MWCNT–CoTsPc/GCE upon addition of 1 mM glucose (a) and 0.1 mM concentration of galactose (b), sucrose (c), fructose (d), lactose (e), ascorbic acid (f), uric acid (g) and dopamine (h).

Amperometric technique was further used for the determination of glucose at the MWCNT–CoTsPc composite modified electrode. Fig. 5B displays the typical amperometric i–t response of MWCNT–CoTsPc composite modified electrode to the successive addition of different concentration of glucose into the 0.1 M NaOH solution. The applied potential was held at 0.3 V. It can be seen from Fig. 5B that a well-defined and stable amperometric response was observed for each addition of 10 μM glucose and it increased with the increase in glucose concentration from 10 to 100 μM. The response time of the sensor was calculated as 2 s, which indicates the fast electrocatalytic oxidation of glucose at the surface of the MWCNT–CoTsPc composite electrode. Moreover, the amperometric response current was linear against the concentration of glucose in the range from 10 μM to 6.34 mM (Fig. 5C). The linear regression equation was found as I_pa (μA) = 0.0364x + 9.5473c (μM), R² = 0.9869. The sensitivity was calculated as 122.5 μA mM⁻¹ cm⁻² with a limit of detection of 0.14 μM (S/N = 3). The analytical performances (sensitivity, LOD and applied potential) of our sensors are comparable with previously reported enzyme-free glucose sensors using carbon nanotubes, metals and metal nanoparticles modified electrodes, as shown in Table 1. Table 1 confirms that the fabricated MWCNT–CoTsPc glucose sensor exhibits a good performance in more than one category. Furthermore, the fabricated electrode exhibits a good linear range and low LOD compared with other previously reported similar glucose sensors.^18,30–36 However, the sensitivity is quite low compared with metal oxides, alloys and metal hydroxides. The good analytical performance (low detection potential, wider linear response and low LOD) of the MWCNT–CoTsPc modified electrode is because of the strong π–π stacking interaction between MWCNT and phthalocyanine rings, resulting in an enhanced electrocatalytic activity towards glucose.

Table 1 Comparison of the analytical parameters obtained at the MWCNT–CoTsPc/GCE with previously reported enzyme-free glucose sensors

Modified electrode	Eapp^a (V)	Linear range (mM)	LOD^b (μM)	Sensitivity (μA μM^−1 cm^−2)	Ref.
a Eapp-applied potential.b LOD-limit of detection.c GOD & GOx-glucose oxidase.d LBL-layer by layer.e ABSGR-alcian blue pyridine-tetrasulfonated phthalocyanine graphene.f CoPc-cobalt phthalocyanine.g SPCE-screen printed electrode.h CoTPP-cobalt(II) phthalocyanine-cobalt(II)tetra(5-phenoxy-10,15,20 triphenylporphyrin).i FePc-iron phthalocyanine.j CoTCAPc-cobalt tetracarboxylic acid phthalocyanine.k SAM-ME-self-assembled monolayer (SAM)-2-mercaptoethanol.l FeTMPyP-iron(III) meso-tetrakis(N-methylpyridinum-4-yl) porphyrin.m RGNR-reduced graphene nanoribbons.n PG-pencil graphite.o OPPyNF-overoxidized PPy nano fiber.p LDH-layered double hydroxide.q G-graphene.
GOD^c/Nafion/(LbL)^d/ABSGR^e	−0.4	0.1–8	50	17.5	34
GR-CoPc/GOx^c	0.4	0.01–14.8	1.6	5.09	35
CoPc^f/SPCE^g	0.5	0.2–5	—	1.12	22
CoPc–(CoTPP)4^h	0.4	2–11	10	0.024	36
GOx/TiO2/FePc^i-CNTs	0.5	0.05–4	30	8.25	37
CoTCAPc^j SAM-ME–Au^k	0.6	0.1–25	8.4	0.007	38
FeTMPyP^l/RGNR^m	−0.41	0.5–10	—	—	39
PG^n/OPPyNF^o/CoPcTS	0.4	0.25–20	100	5.695	40
(Ni–Co)(OH)2	0.47	0.025–3.7	—	0.112	41
CoOOH	0.50	0.03–0.7	10.9	0.967	42
Au/LDH^p-CNTs–G^q	0.55	0.01–6.1	1.0	1.98	43
Co3O4	0.2	0.1–12	26.5	0.0458	44
MWCNT–CoTsPc	0.3	0.01–6.34	0.14	0.1225	This work

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Selectivity of the sensor

The selectivity of the sensor is necessary for the practical applications in amperometric sensors. The electroactive compounds, such as dopamine, ascorbic acid, uric acid and other carbohydrate compounds, usually co-exist with glucose in human blood and they can easily oxidize at the positive potential. In order to verify the selectivity of the fabricated MWCNT–CoTsPc glucose sensor, amperometric i–t was performed in the presence of other potentially active compounds. Fig. 5D shows the amperometric i–t response of MWCNT–CoTsPc modified electrode upon the addition of 1 mM glucose (a) and 0.1 mM of galactose (b), sucrose (c), fructose (d), lactose (e), ascorbic acid (f), uric acid (g) and dopamine (h). A well-defined and stable amperometric response was obtained for the addition of 1 mM glucose, indicating the good electrocatalytic behaviour of the fabricated sensor. However, the 200 fold excess concentration of the interfering species showed a very less response compared with the response to glucose. The result confirmed that the MWCNT–CoTsPc modified electrode is more sensitive and selective for the detection of glucose in the presence of potentially active interfering compounds.

Repeatability and reproducibility

The repeatability and reproducibility of the sensor has been assessed by CV. CVs were recorded in 0.1 M NaOH at the scan rate of 50 mV s⁻¹ in the presence of 1 mM glucose. The fabricated sensor offered a good repeatability with the relative standard deviation (RSD) of 2.39% for 5 successive measurements in a single modified electrode. In addition, the biosensor exhibited a good reproducibility with an RSD of 2.21% for 5 individual measurements carried out at five different modified electrodes.

Practical application of the sensor

In order to ensure the practical applicability of the developed electrode, the MWCNT–CoTsPc modified electrode was used for the detection of glucose in human blood serum samples. Human serum samples were collected from normal and diabetic patients. The serum samples were predetermined by a commercial Tecan Sunrise plate reader and the detected values are summarized in Table 2. We also investigated the glucose concentration in different serum samples using an amperometric i–t method. The experimental conditions are similar to those described in Sec. 3.5. The obtained results are summarized in Table 2. It can be seen from Table 2 that a good recovery (97.0–102.4%) towards glucose was obtained using the MWCNT–CoTsPc electrode and the detected values are in good agreement with the values detected by a commercial Tecan Sunrise plate reader. Hence, the MWCNT–CoTsPc electrode can be used as a more suitable material for the detection of glucose in human serum samples.

Table 2 Determination of glucose in normal and diabetic patients' blood serum samples:^a

Sample	Glucose detected by commercial detector (mM)	Glucose detected by our sensor (mM)	Recovery (%)	RSD
a 1, 2, 3-serum samples from normal patients. 4, 5, 6-serum samples from diabetic patients. RSD-relative standard deviation for three measurements.
1	5.05	4.92	97.42	3.2
2	4.62	4.58	99.13	3.4
3	5.31	5.44	102.4	3.9
4	6.85	6.72	98.01	4.1
5	7.12	7.23	101.5	4.6
6	6.61	6.46	97.7	4.8

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4. Conclusions

For the first time, an amperometric enzyme-free glucose sensor has been developed at the MWCNT–CoTsPc composite modified electrode. The developed MWCNT–CoTsPc composite electrode exhibited excellent electrooxidation behaviour towards glucose in 0.1 M NaOH. The observed sensitivity of the sensor is about 1.8 and 2.5 folds higher than that of MWCNT and CoTsPc modified electrodes, respectively. Moreover, the fabricated enzyme-free glucose sensor showed good analytical features such as wide linear response (up to 6.34 mM), low working potential (0.3 V), high sensitivity, low LOD (0.14 μM) and fast amperometric response (2 s) along with good repeatability and reproducibility. In addition, the fabricated sensor also exhibited a good selectivity towards glucose in the presence of an excess addition of potentially active interfering species.

Acknowledgements

The support of the visiting professorship to SMC at the King Saud University is gratefully acknowledged.

Notes and references

1. L. Mosconi, Microchim. Acta, 2010, 168, 221–229.
2. V. Mani, R. Devasenathipathy, S.-M. Chen, J. A. Gu and S. T. Huang, Renewable Energy, 2015, 74, 867–874.
3. L. A. Terry, S. F. White and L. J. Tigwell, J. Agric. Food Chem., 2005, 53, 1309–1316.
4. B.-W. Zhu, L. Cai, X.-P. He, G.-R. Chen and Y.-T. Long, Chem Cent J, 2014, 8, 6.
5. X.-P. He, B.-W. Zhu, Y. Zang, J. Li, G.-R. Chen, H. Tiana and Y.-T. Long, Chem. Sci., 2015, 6, 1996–2001.
6. X. Sun, B. Zhu, D.-K. Ji, Q. Chen, X.-P. He, G.-R. Chen and T. D. James, ACS Appl. Mater. Interfaces, 2014, 6, 10078–10082.
7. Z. Li, S.-S. Deng, Y. Zang, Z. Gu, X.-P. He, G.-R. Chen, K. Chen, T. D. James, J. Li and Y.-T. Long, Sci. Rep., 2013, 3, 2293.
8. J. Wang, Chem. Rev., 2008, 108, 814–825.
9. N. German, A. Ramanaviciene, J. Voronovic and A. Ramanavicius, Microchim. Acta, 2010, 168, 221–229.
10. D. Rathod, C. Dickinson, D. Egan and E. Dempsey, Sens. Actuators, B, 2010, 143, 547–554.
11. S. Park, H. Boo and T. Chung, Anal. Chim. Acta, 2006, 556, 46–57.
12. M. M. Guo, Y. Xia, W. Huang and Z. Li, Electrochim. Acta, 2015, 151, 340–346.
13. L. M. Lu, H. B. Li, F. Qu, X. B. Zhang, G. L. Shen and R. Q. Yu, Biosens. Bioelectron., 2011, 26, 3500–3504.
14. S. Meher and G. Ranga Rao, Nanoscale, 2013, 5, 2089–2099.
15. Y. Mu, D. Jia, Y. He, Y. Miao and H. L. Wu, Biosens. Bioelectron., 2011, 26, 2948–2952.
16. X. Ji, A. Wang and Q. Zhao, J. Nanomater., 2014, 287303.
17. P. Zhang, Z. Li, G. Zhao and F. Feng, Microchim. Acta, 2012, 176, 411–417.
18. W. D. Zhang, J. Chen, L. C. Jiang, Y. X. Yu and J. Q. Zhang, Microchim. Acta, 2010, 168, 259–265.
19. J. Huang, Z. Dong, Y. Li, J. Li and J. Wang, et al., Sens. Actuators, B, 2013, 182, 618–624.
20. J. H. Zagal, S. Griveau, J. F. Silva, T. Nyokong and F. Bedioui, Coord. Chem. Rev., 2010, 254, 2755–2791.
21. T. Nyokong, Coord. Chem. Rev., 2007, 251, 1707–1722.
22. E. Crouch, D. C. Cowell, S. Hoskins, R. W. Pittson and J. P. Hart, Anal. Biochem., 2005, 347, 17–23.
23. L. Cui, T. Pu, Y. Liu and X. He, Electrochim. Acta, 2013, 88, 559–564.
24. F. C. Moraes, D. L. C. Golinelli, L. H. Mascaro and S. A. S. Machado, Sens. Actuators, B, 2010, 148, 492–497.
25. M. P. Siswana, K. I. Ozoemena and T. Nyokong, Electrochim. Acta, 2006, 52, 114–122.
26. F. C. Moraes, L. H. Mascaro, S. A. S. Machado and C. M. A. Brett, Talanta, 2009, 79, 1406–1411.
27. T. Mugadza and T. Nyokong, Electrochim. Acta, 2009, 54, 6347–6353.
28. T. Nyokong and F. Bediouib, J. Porphyrins Phthalocyanines, 2006, 10, 1101–1115.
29. E. Jubete, K. Zelechowska, O. A. Loaiza, P. J. Lamas and E. Ochoteco, et al., Electrochim. Acta, 2011, 56, 3988–4395.
30. B. O. Agboola, S. L. Vilakazi and K. Ozonemena, J. Solid State Electrochem., 2009, 13, 1367–1379.
31. J. H. Yang, Y. Gao, W. Zhang, P. Tang and J. Tan, et al., J. Phys. Chem. C, 2013, 117, 3785–3788.
32. J. H. Zagal, D. A. Geraldo, M. Sancy and M. A. Paez, J. Serb. Chem. Soc., 2013, 78, 2039–2052.
33. S. Ku, S. Palanisamy and S. M. Chen, J. Colloid Interface Sci., 2013, 411, 182–186.
34. Y. Zhang, Y. J. Fan, L. Cheng, L. L. Fan, Z. Y. Wang and J. P. Zhong, et al., Electrochim. Acta, 2013, 104, 178–184.
35. V. Mani, R. Devasenathipathy, S. M. Chen, S. T. Huang and V. S. Vasantha, Enzyme Microb. Technol., 2014, 66, 60–66.
36. K. I. Ozoemena and T. Nyokong, Electrochim. Acta, 2006, 51, 5131–5136.
37. H. F. Cui, K. Zhang, Y. F. Zhang, Y. L. Sun, J. Wang, W. D. Zhang and J. H. T. Luong, Biosens. Bioelectron., 2013, 46, 113–118.
38. P. N. Mashazi, K. I. Ozoemena and T. Nyokong, Electrochim. Acta, 2006, 52, 177–186.
39. S. Zhang, S. Tang, J. Lei, H. Dong and H. Ju, J. Electroanal. Chem., 2011, 656, 285–288.
40. L. Ozcan, Y. Sahin and H. Turk, Biosens. Bioelectron., 2008, 24, 512–517.
41. C.-H. Lien, J.-C. Chen, C.-C. Hu and D. S.-H. Wong, J. Taiwan Inst. Chem. Eng., 2014, 45, 846–851.
42. K. K. Lee, P. Y. Loh, C. H. Sow and W. S. Chin, Electrochem. Commun., 2012, 20, 128–132.
43. S. Fu, G. Fan, L. Yang and F. Li, Electrochim. Acta, 2015, 152, 146–154.
44. K. Khun, Z. H. Ibupoto, X. Liu, V. Beni and M. Willander, Mater. Sci. Eng., B, 2015, 194, 94–100.

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