Enzyme-Free Glucose Sensor Based on Au Nanobouquet Fabricated Indium Tin Oxide Electrode

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1 Copyright 2014 American Scientific Publishers All rights reserved Printed in the United States of America Article Journal of Nanoscience and Nanotechnology Vol. 14, , Enzyme-Free Glucose Sensor Based on Au Nanobouquet Fabricated Indium Tin Oxide Electrode Jin-Ho Lee 1 2, Waleed Ahmed El-Said 3, Byung-Keun Oh , and Jeong-Woo Choi 1 Department of Chemical and Biomolecular Engineering, Sogang University, 35 Baekbeom-Ro, Mapo-Gu, Seoul , Republic of Korea 2 Research Institute for Basic Science, Sogang University, 35 Baekbeom-Ro, Mapo-Gu, Seoul , Republic of Korea 3 Interdisciplinary Program of Integrated Biotechnology, Sogang University, 35 Baekbeom-Ro, Mapo-Gu, Seoul , Republic of Korea In this study, we demonstrated a simple, rapid and inexpensive fabrication method to develop a novel gold nanobouquet structure fabricated indium tin oxide (GNB/ITO) electrode based on electrochemical deposition of gold ions onto ITO substrate. The morphology of the fabricated electrode surface was characterized by scanning electron microscopy (SEM) to confirm the GNB formation. Enzyme-free detection of glucose using a GNB/ITO electrode was described with high sensitivity and selectivity based on cyclic voltammetry assay. The results demonstrate a linear relation within wide concentration Delivered range by (500 Publishing nm to 10 Technology mm) of glucose, to: Kyung with a Hee correlation University coefficient of The interference effect IP: of uric acid was effectively On: Mon, avoided 17 Nov for 2014 the 02:25:46 detection of glucose (1 M to 10 mm). Moreover, the developed Copyright: sensor American was applied Scientific to determine Publishers the concentration of glucose in the presence of human serum to indicate the ability of GNB/ITO electrodes in real samples. Hence, newly developed GNB/ITO electrode has potential application in enzyme-free glucose sensor with highly sensitivity and selectivity. Keywords: Enzyme Free, Glucose, Nano Pattern, Electrochemical Sensor, Nanobouquet. 1. INTRODUCTION The determination of glucose has drawn much attention in the field of biotechnology for clinical diagnosis, management of diabetes mellitus and in food industry. Therefore, tremendous efforts have been made to develop a glucose sensor with high sensitivity and selectivity. 1 2 Early studies done by Clark and Lyons that have been shown an enzymatic glucose sensor based on immobilized glucose oxidase (GOx) enzyme were drawn much attention. 3 Although, these enzyme based glucose sensors have shown good selectivity and sensitivity; however, these sensors have a lot of drawbacks such as loss of enzyme activity, chemical and thermal instabilities originated from the intrinsic nature of enzymes. 4 Previous studies reported that, the environmental conditions of enzyme based glucose sensors, including strong acidic conditions, basic conditions, or high temperatures (above 40 C) could cause fatal damage to GOx enzyme, which leads to loss of Author to whom correspondence should be addressed. sensing activity. Moreover, the activity of GOx is very sensitive to sodium dodecyl sulfate (SDS) under acidic conditions as well as to hexadecyltrimethylammonium bromide (CTAB) under basic conditions. In addition to ph, temperature, toxic chemicals for sterilization, and humidity effects of enzyme, potentially causes significant harm to the sensor activity in use as well as in storage. 5 Another limitation for the enzyme based glucose sensor is the severe interference that could be caused by endogenous electro activity in the whole blood samples. Instability of GOx upon sterilization might limit the enzymatic glucose sensors from being used for long term monitoring in humans. Therefore, the need for electrochemical enzyme free glucose sensors has received considerable interest. The main advantage of enzyme free glucose sensor includes high sensitivity and selectivity, in addition to the prevention of fouling by adsorbed intermediates and some anions, such as chloride ions. All of these issues depend on the properties of the electrode materials because the electro catalytic activity is the main factor that affects both the sensitivity and selectivity of the glucose 8432 J. Nanosci. Nanotechnol. 2014, Vol. 14, No /2014/14/8432/007 doi: /jnn

2 Lee et al. sensor. Therefore, numerous of studies have been devoted in the investigation and preparation of glucose enzyme free sensors. 6 7 It is reported that bare metal electrodes (platinum or gold electrodes) can act as enzyme free sensors for the determination of glucose. 8 9 However, these electrodes suffer from low sensitivity, poor selectivity and poisoning by intermediates and chloride. 10 Much efforts have been focused on developing enzymefree glucose sensors based on the direct detection of the glucose redox behavior on various electrode materials including nanotubular arrayed platinum (Pt), 11 gold (Au) nanoparticles, 12 copper nanoparticles, Pt nanoparticles, nickel nanoparticles, 17 carbon nanotubes (CNTs), mesoporous Pt, 20 macroporous Pt films, 21 Pt Pb nanowire arrayed electrodes, 22 three dimensional Au films, 23 and Pt Ru nanoparticles 24 to overcome the disadvantages of the bulk electrodes. Among all these materials, Au nanostructures are attracted much attention for the use in a wide range of applications, including biosensors, chemical-sensor, optical scattering, diffraction, and other applications due to their higher conductivity, inertness, biocompatibility and large surface area. 27 The enhancement of the electrochemical conductivity of the nanostructured modified electrodes compared to that of the bare electrodes could be related to the increase of the electrode s active surface area. Conversely, the immersion of a nanomaterial Delivered in an by electrolyte Publishing could Technology nique to: is Kyung depicted Hee in University Figure 1(a). induce charge on the surface regions IP: of a material via On: anmon, 17 Nov :25:46 application of a potential across the Copyright: electrolyte material American Scientific 2.3. Electrochemical Publishers Measurements of interface. However, the development of a simple, rapid, inexpensive method for fabrication of a highly sensitive electrical nanostructured substrate is still in demand to monitor the electrochemical characteristics of glucose in a mixture with good selectivity. In this work, we present a simple, rapid, and inexpensive method for fabricating a uniform Au nanobouquet (GNB) modified ITO electrode. The highly sensitive GNB/ITO electrode was used to investigate the interdependence of the electrochemical signals on the oxidation of wide range of glucose (500 nm to 10 mm) without enzymes using the cyclic voltammetry (CV) technique. Furthermore, the CV assay was used for the simultaneous determination of glucose in the presence of high concentration (500 M) of uric acid (UA) as interference. Further, the determination of different concentrations of glucose (1 M to10mm) in the presence of human serum was used to prove the ability of GNB modified ITO electrodes for detection in real sample. These results indicate that low detection limits for glucose were obtained due to the high electro-catalytic properties of the GNB/ITO electrode. 2. EXPERIMENTAL DETAILS 2.1. Materials Glucose, UA, human serum, SDS, and gold chloride (99.9%+) were purchased from Sigma Aldrich (St. Louis, MO, USA). All other solutions were prepared with distilled Millipore (Milli Q) water. Other chemicals that were used in this study were obtained commercially at reagent grades Fabrication of Gold Nanobouquet Pattern on ITO Electrode ITO-coated glass substrates were cleaned by sonication for 15 min in 1% Triton X 100, deionized water (DIW), and ethanol. Then, they were treated with basic piranha solution (1:1:5, H 2 O 2 :NH 4 OH:H 2 O) for 30 min at 80 C. Finally, the ITO substrates were cleaned again with DIW and dried under N 2 stream to obtain a clean ITO surface. GNB was electrochemically deposited onto ITO substrates (2 cm 1 cm) using a1mmhaucl 4 aqueous solution containing 17 g/l of SDS as a surfactant. The potential was maintained at 0.9 V (vs. Ag/AgCl). The active area for the electrochemical deposition of GNB was 1 cm 1 cm. Moreover, to remove any surfactant traces, which may be adsorbed onto the GNB surface, the substrates were rinsed with DIW and sonicated for 5 min with isopropyl alcohol. The surface morphologies of the GNB electrode were analyzed by a scanning electron microscope (SEM) (ISI DS-130C, Akashi Co., Tokyo, Japan). A schematic diagram for the formation of Au nanobouquets on ITO surface by electrochemical deposition tech- Glucose Determination All electrochemical measurements as well as the electrodes modification were performed using a potentiostat (CHI 660A, CHI, USA) controlled with general purpose electrochemical system software. An in house three electrode system comprised of GNB/ITO electrode as the working, a platinum wire as the counter, and Ag/AgCl as reference electrodes were used at a scan rate of 50 mv/s. In order to minimize the error, all the data are the mean ± standard deviation of three different experiments. All the measurement was performed in neutral ph at RT. 3. RESULTS AND DISCUSSION 3.1. Surface Morphology and Current Transient of Nanobouquet Structured Gold Film Figure 1(b) illustrates the current density versus time curve at a potential of 0.9 V (Ag/AgCl) for 30 s. The current density increased drastically during the first two milliseconds and gradually decreased to a stationary value at approximately 20 ms. This gradual decrease was due to limited AuCl 4 diffusion to the ITO surface, which most likely resulted from nucleation and growth of the Au nanostructures as indicated in the current transient profile, which demonstrates the initial nucleation and growth process during metal deposition. 28 SDS as an ionic surfactant was added to modify the interfacial properties of both the J. Nanosci. Nanotechnol. 14, ,

3 Lee et al. were observed to be in the range of 400 nm to 600 nm in diameter Electrochemical Behavior of Glucose on Bare ITO and GNB Electrodes The general oxidation pathway for the glucose can be explained: two hemiacetal types of glucose ( and glucose) are converted to each other through acid catalyzed hydrolysis via aldehyde type glucose. The ratio of two hemiacetal types of glucose would be : = 11:89, if it were not for the influence of the anomeric effect.29 A schematic of the general reaction pathway is illustrated in reaction Scheme 1 for and glucose, the hydrogen atom tethered to carbon is activated due to the stronger acidity of the hemiacetalic OH group (pka = 12 3) compare to alcoholic OH group (pka = 16). Thus, the product of electrochemical oxidation of and glucose is glucono lactone, which is the final stable product of two electron oxidation of glucose.30 Figure 2(a) shows the cyclic voltammogram behavior of the direct oxidation of 1 M of glucose at bare ITO electrode. From this result, no significant redox current peaks could be observed, which may be related to slow kinetic electron transfer at the bare ITO electrode in addition to surface fouling due to the adsorption of intermediates. On the other hand, large background was observed for to: a GNB modified ITO electrode in compare to that Delivered by Publishing Technology Kyung Hee University IP: On: Mon,of17 thenov bare2014 ITO 02:25:46 electrode (Fig. 2(a)) indicates the higher Copyright: American Scientific Publishers background charging current. This could be related to the larger surface area of the GNB modified ITO electrode. Therefore, the GNB modified ITO electrode displays an advantage for providing better electron-transfer kinetics as compared with the bare ITO electrode. The CV for the glucose (1 M) at the GNB modified ITO electrodes in the potential range from to 0.2 V (versus Ag/AgCl) at scan rate 50 mv/s (Fig. 2(a)) shows an anodic and cathodic current peak at potential 510 mv Figure 1. Fabrication of GNB modified ITO electrode surface. (A) Schematic diagram for fabrication of GNB structures modified ITO electrode based on electrochemical deposition technique. (B) Current versus time profile for Au electrochemical deposition onto ITO electrode at a potential of 0.9 V (vs. Ag/AgCl) for 30 s at 25 C. (C) SEM image of GNB modified ITO electrode surface, scale bar 500 nm. particles and the electrode to control the morphology of the aggregates. The SEM image of electrodeposited GNB nanostructures on an ITO surface at 30 s is shown in Figure 1(c), which clearly demonstrates that these electrochemical deposition conditions including concentration of Au3+ (1 mm), concentration of surfactant (17 g/l), and temperature (25 C), time (30 s) and voltage ( 0.9 V, vs. Ag/AgCl) of the deposition are results in the formation of uniformed distributed Au bouquet nanostructures over a large electrode surface area. The nanobouquet structures 8434 Scheme 1. Reaction scheme for different glucose forms: Schematic diagram of the equilibrium between glucose and glucose forms in an aqueous solution and the oxidation pathway of and glucose into glucose lactone. J. Nanosci. Nanotechnol. 14, , 2014

4 Lee et al. Figure 2. (A) Electrochemical behavior of glucose at bare ITO and GNB modified ITO electrode: (a) Cyclic voltammograms of glucose on a bare ITO, (b) background signal of a GNB modified ITO electrode, and (c) CV of glucose on a GNB modified ITO electrode. (B) Electrochemical behavior of glucose at different ph range from (a) 4 to (b) 9. (C) Electrochemical behavior of glucose at different temperatures (a)10 C, (b) 24 C and (c) 37 C, respectively. and 200 mv, respectively. The separation between the potential peaks E pc E pa exceeded 59 mv, which was indicative of a distinct quasi-reversible character of the glucose at GNB/ITO electrode process. The enhancement factor for the electrochemical activity at GNB/ITO electrode is mainly due to its larger surface to volume area ratio derived by three dimensional gold nanobouquet structures. Moreover, these results might be related to moderate electrocatalytic activity of the GNB/ITO electrode, which is obtained by modifying poorly electrocatalytic electrode (ITO) with a highly electrocatalytic material Au. This might enable the electrode to obtain high signal to background ratios compared to those of Au and Pt electrodes. 31 Therefore, the GNB/ITO electrode displays advantages related to providing better electron transfer kinetics than that of bare ITO electrodes, and the catalytic properties of Au nanoparticles might advance the oxidation of glucose at the GNB/ITO electrode. These results demonstrate the sensitivity of the GNB/ITO electrode. Moreover, the effect of ph was determined using different solutions with ph in the range of 4 to 9. In the acidic ph or basic ph solutions, glucose was converted to another anomer (mannose or fructose) via the anomerization process, which shifts the oxidation peak potential of glucose (Fig. 2(b)). In addition, the effect of temperature also was determined by studying glucose oxidation behavior at different temperatures (10 C 37 C). As temperature changes, the oxidation peak potential changes its signal (Fig. 2(c)). Based on these results, we selected neutral solution and RT as the optimized conditions for further experiments Cyclic Voltammetry for Detection of Different Concentrations of Glucose on GNB Modified ITO Electrodes Figure 3(a) shows the cyclic voltammograms for different concentrations of glucose (from 500 nm to 10 mm) at the GNB/ITO electrode. Upon addition of glucose, the anodic current peak increased with increasing concentration of glucose. The lowest concentration measured in this system Delivered by Publishing Technology is 500 to: nm, Kyung which Hee isuniversity lower than that obtained by previous approaches, such as CNT composite electrodes, IP: On: Mon, 17 Nov :25: Copyright: American Scientific Publishers Au nanoparticles, 12 etc. (Table I). The anodic current peaks found to be linearly increased with the glucose concentrations. However, no change in anodic peak current was observed when the concentration of glucose was more than 10 mm which could be related to the saturated GNB/ITO electrode. The calibration plot for glucose determination shows a linear relation in a wide range from 500 nm to 1 mm with a correlation coefficient of (Fig. 3(b)). These results suggest that the GNB/ITO electrode could be used to develop a highly sensitive biosensor for determination of low glucose concentrations. This indicates that the GNB/ITO electrode exhibited good electrocatalytic performance for oxidation of glucose Cyclic Voltammetry for the Detection of Glucose in a Mixture with UA on GNB Modified ITO Electrodes A major challenge in the electrochemical determination of glucose is the coexistence of interfering materials, such as uric acid (UA), which are commonly found in the human blood. The presence of UA in physiological solutions causes the greatest interference for direct electrochemical oxidation of glucose on various electrodes, especially enzyme free sensors. 24 Therefore, the ability of the GNB modified ITO electrode to monitor different concentrations of glucose in the presence of high concentration of UA was investigated. J. Nanosci. Nanotechnol. 14, ,

5 Lee et al. Figure 3. (A) Cyclic voltammograms of varying glucose concentrations (a) 500 nm, (b) 1 M, (c) 10 M, (d) 100 M, (e) 1 mm, (f) 10 mm at GNB modified ITO electrode. (B) Linear plot of anodic current peak as a function of glucose concentration ( I p X = ± X ± , R = 0 984). (C) Cyclic voltammograms of varying concentrations of UA (a) 100 M, (b) 200 M, (c) 300 M, (d) 400 M, (e) 500 M on a GNB modified ITO electrode. (D) Linear plot of anodic current peak as a function of UA concentration ( I p X = ± X ± , R = 0 996). Delivered by Publishing Technology to: Kyung Hee University Figure 3(c) shows the cyclicip: voltammetric behavior On: ofmon, anodic 17 Nov current 2014 peak 02:25:46 with an increasing concentration of UA on the GNB modified ITO electrode, Copyright: which American shows Scientific UA (Fig. Publishers 3(c)), and this increase was almost linear as an irreversible behavior with an anodic current peak at approximately 720 mv. Cyclic voltammograms for various concentrations of UA (100 M to 500 M) on the GNB modified ITO electrode demonstrate an increase of Table I. Comparison of different electrode matrix for linear range and real sample detection of enzyme free glucose sensors along with those reported in literature. Electrode matrix [Ref.] Linear range (mm) Pt-nanotube arrays Au nanoparticle Copper/MWCNT Cu nanoparticles Pt nanoparticles/cnt PtPbNP/MWCNT 16 Up to 11 Ni powder MWCNT Multiple-branching CNT forest Mesoporous Pt Macroporouse Pt film Pt-Pb nanowire array D Au film CNT supported PtRu nanoparticles Boron-doped CNT MnO 2 /MWNTs nanocomposite 34 Up to 28 GNB modified ITO electrode (Present work) Note: Real sample (Blood serum) tested. shown in Figure 3(d). The calibration plot for glucose determination shows a linear relation over range from 100 M to500 M with a correlation coefficient of (Fig. 3(d)). UA was used as an interference agent and its effect on the determination of glucose was examined and presented in Figure 4. The cyclic voltammograms for different concentrations of glucose (1 M to 10 mm) in the presence of UA (500 M) were shown in Figure 4(a). Compared to pure glucose solution, the addition of UA to glucose solution reduce the cathodic current peaks, while the oxidation current peak for glucose in a mixture was observed at nearly the same potential (approximately 510 mv). Moreover, different concentrations of glucose (1 M to 10 mm) were detected in the presence of a constant concentration of UA (500 M) at GNB/ITO electrode. The oxidation current peaks of glucose in a UA mixture showed a linear relation to the concentrations of glucose with a correlation coefficient of 0.98 (Fig. 4(b)). Moreover, it was observed that, the presence of UA (500 M) did not affect the detection of glucose at the GNB modified electrode within the concentrations ranging from 1 M to 10 mm. These results indicate that this GNB modified ITO electrode could be used for selectively detecting various concentrations of glucose in the presence of an interfering material (high concentrated UA: 500 M) J. Nanosci. Nanotechnol. 14, , 2014

6 Lee et al. 4. CONCLUSIONS Here, a free enzymatic glucose sensor based on GNB array modified ITO electrode was developed that demonstrated several advantages, such as good analytical performance and simple preparation process. It exhibited high sensitivity with good potentiometric response, a low detection limits and a wide linear range. The GNB array was prepared by using electrochemical deposition method. Electrochemical results showed that the GNB modified ITO electrode had large active surface, a good electron transfer rate, larger current response and high electro catalytic activity for glucose oxidation in neutral solutions than a bare ITO electrode, with a detection limit of 500 nm glucose. Moreover, GNB modified ITO electrode demonstrated an efficient determination of glucose in the presence of UA (500 M) with good selectivity and with sensitivity up to 1 M. In addition, the sensor has potential applications in glucose concentrations detection in human serum samples without interferences; therefore, it could be suitable for the determination of glucose in real samples. It is expected that with such electronic and structural properties, the GNB modified ITO electrode could be a promising electrode in electro analytical and biosensing applications. High sensitivity and selectivity and surface renewal make this sensor ideal for detection of glucose in real samples. Delivered by Publishing Technology to: Kyung Hee University IP: On: Mon, Acknowledgments: 17 Nov :25:46 This work was supported by the Copyright: American Scientific Leading Publishers Foreign Research Institute Recruitment Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning(MSIP) (2013K1A4A ), by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) ( ) and by the Sogang University Research Grant of 2013 (SRF ). Figure 4. (A) Cyclic voltammograms of varying concentrations of glucose (a) 1 M, (b) 10 M, (c) 100 M, (d) 1 mm, (e) 10 mm on a GNB modified ITO electrode in presence of fixed UA concentration (500 M). (B) Linear plot of anodic current peak as a function of glucose concentration and fixed UA concentration (500 M) ( I p X = ± X ± , R = 0 98). (C) Anodic current peak corresponding to oxidation of varying concentrations of glucose (1 M to 10 mm) in both ( ) distilled water and ( ) human blood serum. Furthermore, to prove the ability of GNB modified ITO electrodes to detect glucose in real samples, different concentrations of glucose (1 M to 10 mm) were dissolved in human serum (1%). The anodic peak current corresponding to the oxidation of different concentrations of glucose and in human serum exhibited nearly identical anodic peak currents (Fig. 4(c)). These results indicated that a GNB modified ITO electrode is suitable for the determination of glucose in real samples with a detection limit of 1 M. References and Notes 1. G. S. Wilson and R. Gifford, Biosens. Bioelectron. 20, 2388 (2005). 2. J. D. Newman and A. P. F. Turner, Biosens. Bioelectron. 20, 2435 (2005). 3. J. Wang, Chem. Rev. 108, 814 (2008). 4. I. Katakis and E. Domínguez, TRAC-Trend Anal Chem. 14, 310 (1995). 5. R. Wilson and A. P. F. Turner, Biosens. Bioelectron. 7, 165 (1992). 6. E. Shoji and M. S. Freund, J. Am. Chem. Soc. 123, 3383 (2001). 7. Y. Sun, H. Buck, and T. E. Mallouk, Anal. Chem. 73, 1599 (2001). 8. M. W. Hsiao, R. R. Adzic, and E. B. Yeager, Electrochim. Acta 37, 357 (1992). 9. M. W. Hsiao, R. R. Adzic, and E. B. Yeager, J. Electrochem. Soc. 143, 759 (1996). 10. S. Park, H. Boo, and T. D. Chung, Anal. Chim. Acta 556, 46 (2006). 11. J. Yuan, K. Wang, and X. Xia, Adv. Funct. Mater. 15, 803 (2005). 12. B. K. Jena and C. R. Raj, Chem. Eur. J. 12, 2702 (2006). 13. X. Kang, Z. Mai, X. Zou, P. Cai, and J. Mo, Anal. Biochem. 363, 143 (2007). 14. Q. Xu, Y. Zhao, J. Z. Xu, and J.-J. Zhu, Sensor. Actuat. B-Chem. 114, 379 ( 2006). J. Nanosci. Nanotechnol. 14, ,

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