doi: 10.1149/2.010212jes2012, Volume 159, Issue 11, Pages B825-B829.J. Electrochem. Soc. 

 Mamun Jamal, Maksudul Hasan, Alan Mathewson and Kafil M. Razeeb Electrodeon Pd Nanoparticle Modified Gold Nanowire Array

Sensor Based2O2Non-Enzymatic and Highly Sensitive H

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© 2012 The Electrochemical Society

Journal of The Electrochemical Society, 159 (11) B825-B829 (2012) B8250013-4651/2012/159(11)/B825/5/$28.00 © The Electrochemical Society

Non-Enzymatic and Highly Sensitive H2O2 Sensor Based on PdNanoparticle Modified Gold Nanowire Array ElectrodeMamun Jamal, Maksudul Hasan,∗ Alan Mathewson, and Kafil M. Razeebz

Tyndall National Institute, University College Cork, Cork, Ireland

A non-enzymatic hydrogen peroxide (H2O2) sensor based on a vertically aligned gold nanowire array electrode modified withpalladium nanoparticles (PdNP/AuNAE) has been fabricated. PdNP have been deposited onto the AuNAE surface by a hydrothermalreduction method using a borohydride technique. Scanning electron microscope inspection (SEM) has revealed granular Pd particlesdeposited on the surface of the nanowires. X-ray diffraction (XRD) peak analysis has determined an average crystallite size of 3 to5 nm for PdNP. The catalytic behavior of PdNP/AuNAE has been studied using amperometry and voltammetry which showeda linear response up to 2 mM of H2O2 with a sensitivity of 530 μA mM−1 cm−2 at 20◦C. This electrode can detect 5 μM(S/N = 3) of H2O2 at normal conditions without using any enzyme or mediator. A high selectivity toward H2O2 (up to 97%) hasbeen obtained using this electrode platform in the presence of common electrochemical interferences such as ascorbic acid, uricacid, acetaminophen and dopamine.© 2012 The Electrochemical Society. [DOI: 10.1149/2.010212jes] All rights reserved.

Manuscript submitted June 28, 2012; revised manuscript received August 17, 2012. Published September 17, 2012. This was Paper1633 presented at the Seattle, Washington, Meeting of the Society, May 6–10, 2012.

Development of a reliable and sensitive analytical methodologyfor the precise monitoring of H2O2 is of significant interest for pointof care, environmental control and industrial applications. In par-ticular, H2O2 is extensively used as an oxidizing agent in chemicaland food industries1 and it is also being used as a vital mediator infood, pharmaceutical, clinical, and environmental analysis.2,3 More-over, H2O2 is one of the important markers for oxidative stress andalso acts as a precursor in the formation of the highly reactive andpotentially harmful hydroxyl radical. H2O2 is a catalytic by-productof a large number of biochemical reactions and its accurate measure-ment is crucial for many diagnostic industries. A number of strategiessuch as spectrophotometry,3 chemiluminescence4,5 and electrochem-ical methods6–8 have been developed to detect H2O2. Among these,electrochemical methods have been found to be the most convenientfor H2O2 determination, due to their operational simplicity, low ex-pense, and suitability for real-time detection.9–15 However, the pres-ence of electrochemically active interference such as L-ascorbic acid,uric acid, and acetaminophen are also a major issues in this method16

because they distort the measurement signal. When added to hydrogenperoxide, these compounds are oxidized at the electrode resulting in alarge current response. This increase in the background current makesit difficult to accurately determine the current increase which is simplydue to the concentration of hydrogen peroxide in the sample. This hasresulted in the common use of an enzyme and electrochemical media-tor to mitigate the problem. Enzyme-mediator system shuttle electronsfrom the active site of the enzyme or biomolecule to the electrode sur-face, thus reduced the oxidation or reduction potential of the analyte ofinterest.17,18 Horseradish peroxidase (HRP) and ferrocene have beenfound to be the most common enzyme-mediator system, which is be-ing used for H2O2 detection.19 A different mode of detection is alsoused based on subtraction of the signal obtained in the presence andabsence of enzyme. This provides a specific signal for the analyte onlyand correlates directly to its concentration in solution.20 However, en-zyme or bio-molecule leaching out from the electrode surface is oneof the common problems for enzyme system and is thus renderedunsuitable for continued use. To avoid the interference from oxidizedspecies, noble metals (such as ruthenium, platinum) can be depositedon the electrode surface to decrease the working potential and to avoidthe interfering influence of electroactive impurities.21 The electrode-position of a noble metal on the electrode will allow it to catalyze thehydrogen peroxide at lower over-potential and in some cases providea porous structure consisting of aggregates of fine metal particles,resulting in an increase in surface area that can be used for the detec-tion of the hydrogen peroxide. However, detection of H2O2 without

∗Electrochemical Society Active Member.zE-mail: kafil.mahmood@tyndall.ie

using enzyme remains a challenge to the researcher, where selectivityis one of the major issues in the presence of electrochemically activeinterfering substances.

Palladium (Pd) has been found to have powerful electrocatalytic ac-tivity, which is relatively inexpensive and found to have better electro-deposition characteristics.21–24 The deposition of nanoparticles of Pdon the electrode surface results in an increase in the electrode surfacearea and an increase in surface activity that can be used for the detec-tion of hydrogen peroxide.21 Furthermore, Pd based electrode exhibitsnegligible fouling to the common electrochemically active interferentssuch as ascorbic acid, uric acid, acetaminophen and dopamine and thatmay allow easy fabrication of the sensor by alleviating the requirementof enzyme and mediator addition.25 The objective of this research hasbeen to develop an enzyme free amperometric H2O2 detection systemusing a PdNP modified vertically aligned nanowire array electrodeplatform. The operational stability of the sensor, comparison withsimilar systems (which used nanowire array and other nanostructureelectrodes) as well as the effect of electrochemical interference on thedetection of H2O2 will be described.

Experimental

Synthesis of AuNAE.— Free standing AuNAE were synthesizedusing a template assisted electrodeposition technique. A thin seedlayer of Au (thickness of 300 nm) was deposited using a Temescal FC-2000 sputtered onto one side of the polycarbonate (PC) template. Thiswas an Isopore membrane filter of 47 mm diameter, with nanoporediameter of ∼0.22 μm, a thickness of ∼25–30 μm and porosity of∼13.8%). A copper wire was connected to the seed layer using silverpaste. Electro-deposition of Au was performed at a constant currentdensity of 1.0 mA cm−2 for 9000 s in a PURAMET 402, AMIDODUCO Au bath. After deposition, the PC template was etchedaway by immersing the sample in dichloromethane for 20 mins. Theresulting AuNAE was rinsed in deionised water and dried in air beforefurther use.

Synthesis of AuNAE supported PdNP.— The nanowire array wasthen prepared for deposition of the PdNP by using a NaBH4 hydrother-mal reduction method from a chemical bath containing the metal pre-cursor salt (10 mM PdCl2) in water. A piece (subset) of the AuNAEwas immersed into a 5 mL glass tube containing 1 mL aliquots of Pdmetal precursor salt at 80◦C, followed by stirring for 20 min. After-wards, a freshly prepared NaBH4 (10 mM) solution was added dropwise to the mixture and stirred slowly for 10 minutes until the reactionwas completed (color changed). The resulting hybrid PdNP/AuNAEwas then removed from the mixture and washed repeatedly with deion-ized water for the complete removal of residual Cl− ions. The filtrate

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water was analyzed with 1 M silver nitrate solution and no AgCl pre-cipitate was detected, thereby, confirming the complete removal ofCl− ions. The synthesized electrocatalyst was then dried in an oven at100◦C for 1.5 h. The amount of Pd reduced onto the surface of AuNAEwas determined using a high precision electronic microbalance (Met-tler Toledo, XS 105 Dual Range, repeatability (SD), d = 0.01 mg) andcalculated using the weight difference before and after the reductionof Pd onto the surface of AuNAE support structure. PdNP/AuNAEwas cut into 0.5 × 0.5 cm2 pieces and was cleaned in 1M nitric acidfor 1 minute, washed repeatedly with deionized water and dried in airbefore use.

Characterization.— The morphology of the modified electrodesand elemental analysis were performed using a JEOL JSM6100 SEM connected with EDX. The crystal structures of thenanowires were investigated using X-ray powder diffractometer(XRD) (Philips X’pert PW3710-MPD diffractometer using filteredCu-Kα (λ = 1.54 Å) radiation, operating at 40 kV and 35 mA. Cyclicvoltammetric and amperometric measurements were performed usinga CHI 660 (CH Instruments, Austin, TX) and Versastat 3F (AMETEKPAR, Oak Ridge, TN) potentiostat/galvanostat. For the purpose ofperformance characterization the PdNP/AuNAE, AuNAE and a flatgold electrode (Auflat) were employed as working electrodes with aAg/AgCl and a Pt sheet as the reference and the counter electrode,respectively.

Results and Discussion

In this work Pd particles were prepared on the metallic (Au) sur-face by means of NaBH4 hydrothermal reduction method. Few groupshave used electrochemical deposition methods to deposit Pd26,27 andin most of the cases it forms Pd film instead of nano particles. Alsonanostructured Pd films have been deposited on the polycrystallinegold surface using a surfactant as a soft template.28,29 The advantageof using the NaBH4 hydrothermal reduction method over the electro-chemical deposition techniques is that it generates regular distribu-tion of Pd nanoparticles on the surface. Moreover, it is much easierto synthesize Pd nanoparticles on larger substrates without requiringany extra processing steps. Figure 1a–1b shows electron micrographsof an as-prepared PdNP/AuNAE electrode. The average length ofthe nanowires is approximately 6 μm with an average diameter of∼200 nm. Figure 1b shows that granular Pd nanoparticles were formedon the nanowire surface after the reduction process. From the EDX(Fig. 1a inset), it can be seen that the ratio of Pd:Au is approximately44:56 on the PdNP/AuNAE electrode. The XRD pattern of the as-synthesized AuNAE and PdNP/AuNAE is shown in Fig. 2. Sharp andwell defined peaks are observed for Pd at 2θ values of 39.2, 45.4,65.6, 78.7, and 82.7◦ corresponding to the 〈111〉, 〈200〉, 〈220〉, 〈311〉,and 〈222〉 crystal orientations of the fcc metal. The diffraction peaksfor Au and Pd planes appears at the same 2θ values assuming thatno alloy has been formed between Au and Pd.30 The average crystalsize of PdNP was determined from the full-width at half maximum(FWHM) of the (111) plane using the Debye-Scherer equation andthe crystallite sizes were found to vary between 3 to 5 nm.

Electrochemical characterization of AuNAE and PdNP/AuNAE.—Prior to the implementation of the as prepared PdNP/AuNAE asa non-enzymatic sensor, their electrochemical behavior was investi-gated using cyclic voltammetry. The voltammogram obtained for thePdNP/AuNAE in 0.5 M H2SO4 shows a profile that is different to theblank AuNAE (Fig. 3). For PdNP/AuNAE a clear double layer regionis observed between 0.09 and 0.31 V in the positive scan and between0.4 and 0.09 V in the negative scan. The oxide formation in the anodicscan started at 0.5 V and finished at 1.0 V. The anodic curve displaysan extended oxidation limit, and the cathodic curve presents a reduc-tion peak at 0.48 V, which is determined to be the reduction peak ofPd oxides. It was also proposed that the sharp peaks observed below0.1 V are due to hydrogen adsorption-desorption at the nanostructured

Figure 1. SEM images of (a) PdNP/AuNAE (inset showing correspondingEDX) (b) Higher magnification image of PdNP/AuNAE.

Figure 2. XRD of AuNAE and PdNP/AuNAE electrodes.

Journal of The Electrochemical Society, 159 (11) B825-B829 (2012) B827

Figure 3. Cyclic voltammograms of PdNP/AuNAE and AuNAE in 0.5 MH2SO4 at a scan rate of 20 mV/s. Pd loading is estimated to be 0.11 mgcm−2

in the PdNP/AuNAE electrode.

Pd surface.31 Since the adsorption process and the catalytic reactionrates are based on the electrochemically active surface area (ECSA),it is important to measure this value. The ECSA was calculated fromthe Columbic charge associated with the reduction of palladium ox-ide (peak centered at 0.48 V in Fig. 3) using the expression ECSA= (QR/QMR .WPd), where QR is the columbic charge required for thePdO reduction, QMR is the charge for the reduction of the PdO mono-layer (0.405 mCcm−2 for Pd), WPd is the Pd loading in the electrode.32

The ECSA was estimated to be approximately 30 m2g−1, which isconsiderably higher than that of Pd nanowires prepared in hexago-nal mesophases (12 m2g−1)33 and that of nanoporous Pd prepared bydealloying (23 m2g−1).34 It indicates higher catalytic activity of thePd modified AuNAE compare to the literatures. Figure 4a depicts the

Figure 4. (a) Cyclic voltammograms of PdNP/AuNAE electrode in PBS buffer(pH 7.4) containing 1 mM H2O2 at the scan rates of 20, 40, 60, 80, 100, 120and 140 mV/s; (b) Plot of current density vs. scan rate of PdNP/AuNAE.

Figure 5. (a) Electrocatalytic response of PdNP/AuNAE upon addition of30 mM H2O2. Conditions: under N2 environment, scan rate 20 mV/s in PBSbuffer, pH 7.4; (b) Schematic diagram of proposed mechanism for the oxidationof H2O2 at the surface of the Pd activated electrode.35

CVs of the PdNP/AuNAE in 0.01 M PBS solution at different scanrates. It can be seen that the peak-to-peak separation becomes widenedwith increased scan rate. In the range of 20–140 mV/s, peak currents(at −0.2 V) increase linearly with the scan rate (Fig. 4b), indicatingthat the electrochemical kinetic reaction is surface controlled.

Electrocatalytical oxidation of H2O2.— In order to address theanalytical applicability of the PdNP/AuNAE electrode, the electro-catalytic activity of these nanowire electrodes toward H2O2 was in-vestigated. Figure 5a presents the CV responses of PdNP/AuNAE in0.01 M PBS without (curve a) and with 30 mM of H2O2 (curve b).

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The electrode shows excellent catalytic activity toward H2O2 withoutusing any mediator or enzyme with maximum current at −0.2 V. Inthe presence of H2O2, the Pd surface becomes oxidized to Pd(OH)during the positive scan and this involves the chemisorptions of H2O2

molecules on its surface. It is then catalytically transformed to thereduced surface during the cathodic scan as defined by the followingequations35–37 (Fig. 5b).

2H2 O2 + 2Pd ↔ 2Pd − O H

Pd − O H + H2 O2ads ↔ Pd − O H − H2 O2ads

Pd − O H − H2 O2ads + Pd − O H − H2 O2ads

→ 2Pd − O + 3H2 O + 0.5O2

Pd − O + H2 O + 2e− → Pd + 2O H−

The status of the palladium can change rapidly between oxidizedand reduced states, and thereby the adherence of oxidized ascorbicacid or uric acid on the electrode surface can be avoided.35 Moredetails on the effect of interferences are described in section 3.4.

Amperometric sensing and analytical performance.— For the am-perometric sensing application, the PdNP/AuNAE has been exam-ined by measuring the current response for H2O2 under the opti-mal conditions. Figure 6a presents the amperometric responses forthe PdNP/AuNAE with the successive addition of 100 μM H2O2 in

Figure 6. (a) Amperometric response of PdNP/AuNAE, AuNAE and Auflatelectrode for successive additions of 100 μM H2O2 in absence of oxygen (Eapp= −0.2V vs. Ag/AgCl) (b) Corresponding concentration plot; backgroundelectrolyte 0.01 M PBS (pH 7.4), under N2 environment.

0.01 M PBS buffer at an applied potential of −0.2 V, under a N2

environment. The sensitivity to H2O2 obtained from the calibrationplot was 530 μA mM−1 cm−2. This sensitivity is significantly higherthan the majority of the reported enzyme free nanowire based sensorswhere the sensitivity varies between 0.134 and 194.6 μA mM−1 cm−2

38–42 for H2O2. In this study, we have found that the PdNP/AuNAEbased H2O2 sensor is ∼60 times more sensitive compared to the pla-nar or flat gold electrodes, and ∼3 times more sensitive compared toAuNAE electrode (Fig. 6b). This is attributed to the excellent elec-trocatalytic properties of the Pd modified electrode, as well as itsnanostructured shape and vertical orientation. The electrocatalytic re-sponse of PdNP/AuNAE shows a linearity for H2O2 detection up to2 mM, with a limit of detection of 5 μM (S/N = 3). Intra-electroderelative standard deviation (RSD) was found to be 3.8% (n = 3) andinter-electrode RSD was found to be 6.3% (n = 3) for PdNP/AuNAE.The stability of the PdNP/AuNAE was examined over a period of 4weeks, with a analysis carried out every 2 days, 5 assays each daywith 100 μM H2O2 addition. In between the testing, the electrode wasstored in air at ambient conditions. The PdNP/AuNAEs were foundto retain 95% of their initial activity after 4 weeks.

Interferences studies.— One of the major challenges in non-enzymatic H2O2 detection system lies in the interference from elec-trochemically active substances other than analyte, which generateelectrochemical signals at the same potential as H2O2. In this work,examination of the amperometric responses of the PdNP/AuNAE atan applied potential of −0.2 V in 0.01 M PBS with subsequent ad-ditions of 100 μM H2O2, 100 μM uric acid, 100 μM ascorbic acid,140 μM acetaminophen and 1 μM dopamine (these are the averagephysiological concentration of ascorbic acid, uric acid, acetaminophenand dopamine) have been performed. From the current response inFig. 7, it can be seen that 1–3% current was obtained for uric acidand ascorbic acid and a negligible current (0–1%) obtained for ac-etaminophen and dopamine compared to that of 100 μM H2O2. Aninterference study has also been conducted for AuNAE and Auflatelectrodes where 10–15% current obtained for uric acid compared to100 μM H2O2. Furthermore, 1–3% current response was observedfor ascorbic acid, acetaminophen and dopamine for both AuNAE andAuflat electrodes. In previous work, washing steps43 have been used toeliminate electrochemical interferents. However, this was found to beless effective where the analyte concentration is low (i.e. below mMrange). Moreover, multi-step sample preparation is required, whichis not cost effective. The background subtraction method44 has beenused but found to be more suitable for an enzyme based sensors. Fromthis study it is clear that PdNP based sensor platform capable to work

Figure 7. Amperometric response of PdNP/AuNAE and AuNAE upon suc-cessive additions of 100 μM H2O2, 100 μM uric acid, 100 μM ascorbic acid,140 μM acetaminophen, 1 μM dopamine and 100 μM H2O2 at −0.2 V vs.Ag/AgCl; Eapp = −0.2V vs. Ag/AgCl, background electrolyte 0.01 M PBS(pH 7.4), under N2 environment.

Journal of The Electrochemical Society, 159 (11) B825-B829 (2012) B829

efficiently in the presence of electro-active interferences. We have alsoinvestigated the activity of the sensor in the absence of N2, where asimilar current signal for H2O2 was attained but with higher noise.The PdNP modified electrode platform can transform very rapidlybetween oxidized and reduced states, and thus prevents fouling of theelectrode surface. Furthermore, H2O2 is catalytically reduced on thePd surface at the lower potential (−0.2 V), where most of the com-mon electrochemical interferences such as ascorbic acid, uric acid,acetaminophen and dopamine do not interfere.

Conclusions

A novel hydrogen peroxide sensor based on Au nanowire arraywith a PdNP coating has been synthesized using a simple fabrica-tion route. The hybrid nano-electrocatalyst has been shown to exhibithigh electrocatalytic activity, sensitivity and selectivity toward H2O2

without using any enzyme or bio-recognition elements. The workdescribed here has demonstrated a new method for the fabricationof a highly active and stable PdNP modified AuNAE electrocatalystand its application toward H2O2 detection, which can be a potentialcost effective sensor platform capable of working in the presence ofelectrochemically active interferences.

Acknowledgments

This work was financially supported by The European Union FP7scheme (E-Brains -FP7- ICT-257488).

References

1. C. Wei, M. Yang, J. Hu, and Q. Li, Anal. Lett., 40, 3182 (2007).2. C. Y. Lin, Y. H. Lai, A. Balamurugan, R. Vittal, C. W. Lin, and K. C. Ho, Talanta,

82, 340 (2010).3. K. Sunil and B. Narayana, B. Environ. Contam. Tox., 81, 422 (2008).4. W. Chen, B. Li, C. Xu, and L. Wang, Biosens. Bioelectron., 24, 2534 (2009).5. S. Lu, J. Song, and L. Campbell-Palmer, Sci. Hortic., 120, 336 (2009).6. A. Mohammadi, A. Bayandori Moghaddam, M. Kazemzad, R. Dinarvand, and

J. Badraghi, Mat. Sci. Eng. C, 29, 1752 (2009).7. L. Zhang, Y. Zhai, N. Gao, D. Wen, and S. Dong, Electrochem. Commun., 10, 1524

(2008).8. K. J. Huang, J. Y. Sun, C. X. Jin, Q. S. Jing, and T. Zhou, Thin Solid Films, 519, 3925

(2011).

9. S. Li, L. Zhang, H. Liu, M. Pan, L. Zan, and J. Zhang, Electrochim. Acta, 55, 4403(2010).

10. Q. Rui, K. Komori, Y. Tian, H. Liu, Y. Luo, and Y. Sakai, Anal. Chim. Acta, 670, 57(2010).

11. Y. Li, J. J. Zhang, J. Xuan, L. P. Jiang, and J. J. Zhu, Electrochem. Commun., 12, 777(2010).

12. C. C. Hung, T. C. Wen, and Y. Wei, Mater. Chem. Phys., 122, 392 (2010).13. B. Haghighi, H. Hamidi, and L. Gorton, Sensor. Actuat. B-Chem., 147, 270 (2010).14. W. Ma and D. Tian, Bioelectrochemistry, 78, 106 (2010).15. S. Chakraborty and C. Retna Raj, Biosens. Bioelectron., 24, 3264 (2009).16. F. Mizutani, Y. Sato, T. Sawaguchi, S. Yabuki, and S. Iijima, Sensor. Actuat. B-Chem.,

52, 23 (1998).17. A. Lindgren, T. Ruzgas, L. Gorton, E. Csoregi, G. Bautista Ardila, I. Y. Sakharov,

and I. G. Gazaryan, Biosens. Bioelectron., 15, 491 (2000).18. J. Xu, F. Shang, J. H. T. Luong, K. M. Razeeb, and J. D. Glennon, Biosens. Bioelec-

tron., 25, 1313 (2010).19. A. Mulchandani, C.-L. Wang, and H. H. Weetall, Anal. Chem., 67, 94 (1995).20. K. Kriz, M. Anderlund, and D. Kriz, Biosens. Bioelectron., 16, 363 (2001).21. K.-S. Chang, W.-L. Hsu, H.-Y. Chen, C.-K. Chang, and C.-Y. Chen, Anal. Chim.

Acta, 481, 199 (2003).22. P. J. O’Connell, C. K. O’Sullivan, and G. G. Guilbault, Anal. Chim. Acta, 373, 261

(1998).23. F. Xu, L. Wang, M. Gao, L. Jin, and J. Jin, Talanta, 57, 365 (2002).24. H. Yamato, T. Koshiba, M. Ohwa, W. Wernet, and M. Matsumura, Synthetic Met.,

87, 231 (1997).25. R. C. Matos, M. A. Augelli, C. L. Lago, and L. Angnes, Anal. Chim. Acta, 404, 151

(2000).26. R. Hoyer, L. A. Kibler, and D. M. Kolb, Electrochim. Acta, 49, 63 (2003).27. M. Baldauf and D. M. Kolb, Electrochim. Acta, 38, 2145 (1993).28. S. Guerin and G. S. Attard, Electrochem. Commun., 3, 544 (2001).29. P. N. Bartlett, B. Gollas, S. Guerin, and J. Marwan, Phys. Chem. Chem. Phys., 4,

3835 (2002).30. J. Datta, A. Dutta, and S. Mukherjee, J. Phys. Chem. C, 115, 15324 (2011).31. L. Zhang, K. Lee, and J. Zhang, Electrochim. Acta, 52, 3088 (2007).32. R. N. Singh, A. Singh, and A. Anindita, Int. J. Hydrog. Energy, 34, 2052 (2009).33. F. Ksar, G. Surendran, L. Ramos, B. Keita, L. Nadjo, E. Prouzet, P. Beaunier,

A. Hagege, F. Audonnet, and H. Remita, Chem. Mater., 21, 1612 (2009).34. X. Wang, W. Wang, Z. Qi, C. Zhao, H. Ji, and Z. Zhang, Electrochem. Commun., 11,

1896 (2009).35. D. A. Johnston, M. F. Cardosi, and D. H. Vaughan, Electroanal., 7, 520 (1995).36. M. C. Jeong, C. H. Pyun, and I. H. Yeo, J. Electrochem. Soc., 140, 1986 (1993).37. M. Grden and A. Czerwinski, J. Solid State Electrochem., 12, 375 (2008).38. C.-L. Hsu, K.-S. Chang, and J.-C. Kuo, Food Control, 19, 223 (2008).39. X. Pang, D. He, S. Luo, and Q. Cai, Sensor. Actuat. B-Chem., 137, 134 (2009).40. Q. Kang, L. Yang, and Q. Cai, Bioelectrochemistry, 74, 62 (2008).41. X. Cui, Z. Li, Y. Yang, W. Zhang, and Q. Wang, Electroanal., 20, 970 (2008).42. M. Jamal, J. Xu, and K. M. Razeeb, Biosens. Bioelectron., 26, 1420 (2010).43. M. Jamal, M. A. Crowe, and E. Magner, Anal. Chim. Acta, 554, 79 (2005).44. M. Jamal, O. Worsfold, T. McCormac, and E. Dempsey, Biosens. Bioelectron., 24,

2926 (2009).