Rapid and highly sensitive MnOx nanorods array platform for a glucose analysis

Sensors and Actuators B 218 (2015) 137–144

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Rapid and highly sensitive MnOx nanorods array platform for a glucose analysis Sang Ha Lee 1 , Jiao Yang 1 , You Jeong Han, Misuk Cho, Youngkwan Lee ∗ School of Chemical Engineering, Sungkyunkwan University, 440-746 Suwon, Republic of Korea

a r t i c l e

i n f o

Article history: Received 18 February 2015 Received in revised form 16 April 2015 Accepted 4 May 2015 Available online 13 May 2015 Keywords: Manganese oxide Nanorods array Pulse reverse potential Electrochemical oxidation Glucose sensor

a b s t r a c t A Mn2 O3 nanorod array has been successfully prepared by using simple pulse-reverse electrodeposition that was then followed by transformation to a higher MnOx oxidation state via electrochemical oxidation. The surface morphology and composition of the MnOx nanorods was verified via scanning electron microscopy and X-ray photoelectron spectroscopy. The MnOx nanorod array exhibits excellent electrocatalytic activity toward glucose oxidation under alkaline conditions and these properties were confirmed by using cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The structure of the MnOx nanorod array not only provides a large surface area, but also facilitates electron transfer and mass diffusion. The optimum oxidation time was determined to be 30 min, at which the MnOx glucose sensor demonstrated a sensitivity at 811.8 ␮A mM−1 , which is the highest among the various types of MnOx -based sensors that have been reported so far. Moreover, it also exhibited an excellent long-term stability and a good selectivity in the presence of interferents, such as ascorbic acid, dopamine, uric acid, urea, and aspartic acid. The practical applicability of MnOx sensor was also studied by detecting glucose in human serum, in which it demonstrated an excellent selectivity and reliability. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Simple, accurate glucose detection is extremely important for food analysis, applications in the textile industry, environmental monitoring, and medical diagnosis [1]. To date, a number of effective techniques have been developed to measure glucose, and among these, the electrochemical method has attracted much attention due to the high sensitivity of the resulting devices, ease of operation, and low cost [2,3]. Although enzyme-based glucose sensors can achieve a good selectivity and a high sensitivity [4–6], these have a disadvantage in terms of the chemical and thermal instabilities that originate from the intrinsic nature of the enzymes. As a result, recent efforts have focused on the direct determination of glucose using non-enzymatic electrodes [7]. A considerable amount of research has focused on utilizing transition metal oxide-based catalysts as alternatives to noble metals, such as Au and Pt, for use in highly efficient non-enzymatic electrocatalytic applications [8].

∗ Corresponding author. Tel.: +82 31 290 7259; fax: +82 31 299 4711. E-mail addresses: [email protected] (S.H. Lee), [email protected] (J. Yang), [email protected] (Y.J. Han), [email protected] (M. Cho), [email protected] (Y. Lee). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.snb.2015.05.005 0925-4005/© 2015 Elsevier B.V. All rights reserved.

Manganese oxide (MnOx ) is one of the most attractive transition metal oxides since it has a low toxicity, is abundant as a raw material, and can be obtained at a low cost. MnOx -based materials have been considered desirable candidates for electrochemical glucose sensing. However, most MnOx -based electrodes showed a low sensitivity and a poor selectivity in glucose sensing [5]. The reason for such inferior performance has been considered to be the limited charge percolation behavior through the material due to its poor electrical conductivity. In general, electrochemical processes occur at the interface between an electrode and an electrolyte, and therefore, an electrode with a large surface area is required to improve the ion mobility. Electrode materials with nano-scale structures have a large surface area and a high porosity, and these offer the best performance due to the improvement in the conducting pathways and increased surface interactions. Nanostructured MnOx has been prepared using a variety of synthetic techniques, including hydrothermal, sol–gel, room-temperature precipitation, solid-state reactions and electrochemical deposition [9–13]. Of these, many studies have focused on anodic or cathodic electrodeposition since it is a simple, cost-effective process [14]. Recently, pulse and pulse-reverse electrodeposition has been extensively studied since the nucleation and growth rate can be easily controlled to yield various types of nanostructures [15,16]. Although MnOx nanostructures can improve the electrochemical properties and performance, only a few studies so

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far have been presented proposing their use for sensor applications. Previously MnOx combined with graphene or some other metal had been introduced, and with these, the disadvantages of using the individual substances were overcome and the sensing properties were improved [17–19]. Here, we introduce MnOx nanorod array electrodes produced through a simple electrochemical process to obtain a highly sensitive glucose sensor. The Mn2 O3 nanorod array was fabricated by using pulse reverse potential electrodeposition (PRP) and was further transformed to MnOx with a higher oxidation state by using electrochemical oxidation. Manganese oxide has several oxidation states, including MnO, Mn3 O4 , Mn2 O3 , and MnO2 . Their redox reaction and characteristics were determined according to the oxidation state of MnOx , and therefore, the sensing performance of MnOx will be greatly dependent on the oxidation state. In general, MnOx can be oxidized through calcination at a high temperature of up to 300 ◦ C. In our study, however, MnOx is electrochemically oxidized to maintain its original nanostructure. The oxidation state of the MnOx nanorods was controlled by varying the oxidation time, and the oxidation state, morphology, electrochemical properties and glucose sensing performance of the MnOx electrodes that were prepared were carefully investigated. Under an optimum oxidation time of 30 min, the resulting MnOx nanorods exhibited the best sensing performance with a sensitivity of 811.8 ␮A mM−1 over a wide linear range from 0.007 to 10.6 mM and also exhibited good selectivity in the presence of several interferent species. 2. Experimental 2.1. Experimental materials Manganese acetate, ascorbic acid, aspartic acid, dopamine, glucose, urea, and uric acid were purchased from Aldrich Chemical Co. Sulfuric acid (95%), ethanol, and acetone were purchased from Samchun Chemical Co., Korea. 2.2. Preparation of MnOx electrode The MnOx nanorod array was prepared using a two-step electrochemical process consisting of PRP electrodeposition and electrochemical oxidation. First, Mn2 O3 nanorods were prepared via PRP [20]. Before deposition, an Au plate (1 × 1 cm) was pretreated with an acidic solution and was cleaned by conducting

ultrasonication in acetone and ethanol. The electrodeposition of manganese oxide was carried out by applying a pulse reverse potential at +1 V and at −1 V with 0.5 s/0.5 s intervals for 10,000 s on a gold electrode in an aqueous solution consisting of 0.1 M manganese acetate. Then Mn2 O3 nanorods prepared via PRP were electrochemically oxidized by applying +1 V in 1 M LiClO4 electrolyte for 15, 30 or 45 min (coded as MnOx -15, MnOx -30, MnOx -45, respectively). When compared to MnOx nanorods, the MnO2 bulk film was electrochemically deposited by applying a constant potential of +1 V for 900 s. The loading mass for all electrodes was fixed at 0.4 mg cm−2 . The performance of the glucose sensor was carried out in 0.1 M NaOH solution with different concentrations of glucose. 2.3. Characterizations The morphology of manganese oxide was observed using a scanning electron microscope (SEM) (JSM7600, JEOL), and the chemical composition was analyzed via X-ray photoelectron spectroscopy (XPS) (ESCA2000, VG Microtech). All of the electrochemical measurements were performed on a VSP potentiostat (Princeton Applied Research, USA) at room temperature. A three-electrode system was employed with a modified MnOx electrode as a working electrode, a platinum plate as a counter electrode, and Ag/AgCl in a saturated KCl solution as a reference electrode. For the electrical impedance spectroscopy (EIS) measurements, a 10 mV amplitude sine wave in the frequency range of 0.1 Hz to 100 kHz was applied to the MnOx electrode, and the ZSimpWin EIS DATA analysis software (Perkin-Elmer, Version 2.00) was used to analyze the EIS data that was obtained. 3. Results and discussion The MnOx electrode nanorod array was prepared by using the two-step electrochemical process that is shown in Scheme 1. The surface morphologies of the as-prepared manganese oxides were monitored through SEM images, as shown in Fig. 1. The Mn2 O3 nanorod array prepared via PRP that is shown in Fig. 1A has an average diameter of about 20 nm. The Mn2 O3 was further electrochemically oxidized at +1 V for 15, 30, or 45 min to yield MnOx with a higher oxidation state. When the oxidation time was less than 30 min (Fig. 1B and C), there was no significant change in the morphology. However, the nanorod structure collapsed at an

Scheme 1. Fabrication process of MnOx modified electrodes.

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Fig. 1. SEM images of (A) Mn2 O3 , (B) MnOx -15, (C) MnOx -30, (D) MnOx -45, and (E) MnO2 film.

oxidation time of 45 min (Fig. 1D). Fig. 1E shows traditional MnO2 bulk film prepared by applying a constant potential, and in this sample, aggregated small particles can be clearly observed. The oxidation state of the various MnOx was determined as a function of the oxidation time by conducting a subsequent XPS analysis. Fig. 2A shows the region for the Mn 2p spectra of the various MnOx materials, in which two peaks near 642 eV and 654 eV are attributed to the Mn 2p3/2 and Mn 2p1/2 binding energies, respectively. The Mn 2p3/2 peak for the MnO2 film prepared via CP is centered at 642.5 eV while Mn 2p3/2 for Mn2 O3 prepared via PRP is centered at 641.7 eV [21,22]. As the oxidation time increases, the positions of the Mn 2p3/2 peaks shifted slightly to higher binding energies, indicating that Mn2 O3 had been successfully oxidized. The chemical composition of the MnOx nanorods was analyzed by deconvoluting the Mn 2p3/2 peaks (Fig. 2B). The Mn 2p3/2 peak for MnOx oxidized for 15, 30 or 45 min is composed of two deconvoluted peaks centered at 642.5 and 641.7 eV, indicating that the MnOx composition is a mixture of MnO2 and Mn2 O3 . The relative composition of MnO2 and Mn2 O3 that is calculated from the area of the peaks was summarized in Fig. 2C. The MnO2 composition increased as the oxidation time increased, and the MnOx -30 composition contained 80% of MnO2 .

The electrochemical properties of the MnOx nanorod array was evaluated by performing an EIS analysis, and the values calculated are shown in Fig. 3A. The typical impedance spectrum (presented in the form of a Nyquist plot) includes a semicircular portion at a high frequency and a linear portion at a lower frequency, corresponding to the electron transfer-limited and the diffusion-limited processes, respectively. The diameter of each semicircle in the impedance spectrum is proportional to the extent of the resistance for the electron transfer, Ret [20]. The MnOx nanorods exhibited semicircles with a much smaller diameter than those of bulk MnO2 films produced through a constant potential method, indicating a lower electronic resistance that might be a result of the short electron path. The Ret of Mn2 O3 , MnOx -15 and 30 was of about 34  cm−2 . However, that of MnOx -45 increased to 48  cm−2 (Fig. 3B). The active surface areas (SE ) of the MnOx nanorod arrays were much higher than MnO2 film. However, that of MnOx -45 exhibited a value that was evidently lower due to the deformation of the nanorod array structure. Among these, the Mn2 O3 , MnOx -30, and MnO2 films were selected, and their redox behavior was observed via cyclic voltammetry in 0.1 M NaOH solution at a scan rate of 50 mV s−1 . Fig. 3C shows cyclic voltammograms (CVs) for the MnO2 (curve a), Mn2 O3 (curve b), and MnOx -30 films (curve c). Both

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Fig. 2. (A) Mn 2p spectra of MnOx , (B) Mn 2p3/2 spectra with deconvoluted peaks of MnOx -15, 30 and 45, (C) relative composition of MnOx collected from XPS analysis.

Mn2 O3 and MnOx -30 have a pair of redox peaks at 0.548/−0.144 V related to the MnOOH/MnO2 redox peak. Moreover, the redox peak currents for MnOx -30 are much larger than those for Mn2 O3 , indicating improved electrocatalytic properties for MnOx -30. In contrast, no obvious redox peaks were observed for the MnO2 film due to its relatively poor electrochemical properties. To evaluate the electrochemical kinetics of the MnOx -30 electrode, the CVs were measured in 0.1 M NaOH solution with a scan

rate ranging from 5 to 150 mV s−1 (Fig. S1 in Supporting information). The redox peak current increased as the scan rate increased, and also, the peak-to-peak separation increased due to the shift in the redox peak. Both the cathodic and the anodic peak currents demonstrate a good linear relationship with the square root of the scan rate indicating a diffusion-controlled process [24]. The electrocatalytic behavior of the MnOx electrode oxidized from Mn2 O3 toward the oxidation of glucose was studied via

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Fig. 3. (A) Nyquist plots for EIS data collected from Mn2 O3 (curve a), 15 (curve b), 30 (curve c), 45 (curve d) and MnO2 film (curve e), (B) EIS data collected from MnOx electrodes: Rs , Ret , Cdl and SE represent the solution resistance, the electron-transfer resistance, the double layer capacitance and specific surface area, respectively. (C) Cyclic voltammograms (CVs) of MnO2 film (curve a), Mn2 O3 (curve b), and MnOx -30 (curve c) in 0.1 M NaOH solution at a scan rate of 50 mV s−1 [23].

chronoamperometry (CA) in 0.1 M NaOH solution. Fig. 4A illustrates the amperometric responses of the Mn2 O3 (curve a), MnOx -15 (curve b), MnOx -30 (curve c), and MnOx -45 (curve d) for glucose detection at an applied potential of +0.6 V. All of the modified electrodes exhibited a remarkable increase and a fast current response as a result of the adding glucose in the electrolyte, and each calibration curve showed a linear increase in the response current with the glucose concentration (Fig. 4B). The sensing mechanism has already been previously reported in the literature [18]. With the addition of glucose, the oxidation peak current increased, indicating the electro-oxidation of glucose to glucolactone related to the

MnOOH/MnO2 redox pair in an alkaline solution. The sensitivity calculated from the slope of the calibration curve for each modified electrode was analyzed, and the sensitivity of the MnOx was observed to improve as the oxidation time increased to 30 min, time at which the MnO2 content increased with a longer oxidation time. However, the sensitivity of the MnOx -45 sample decreased even though it had a higher MnO2 content, which might be a result of the deterioration in the electrochemical properties resulting from the collapse of the nanostructure. This study therefore suggests that manganese oxide with a higher oxidation state performs well in the electrocatalytic process

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Fig. 4. (A) Amperometric responses of Mn2 O3 (a), MnOx -15 (b), MnOx -30 (c), and MnOx -45 (d) for successive addition of glucose in 0.1 M NaOH solution at +0.6 V. (B) Linear fits of response currents as a function of glucose concentrations.

toward glucose. In the MnOx electrode, the reduction of Mn(IV)O2 to Mn(III)OOH leads glucose to oxidize [18,24], and therefore, the oxidation state of metal oxide definitely influences the oxidation of glucose. The electrocatalytic behavior of the MnOx electrode toward the glucose oxidation was also investigated via cyclic voltammetry in the 0.1 M NaOH solution. The MnOx -30 CVs that were measured in various concentrations of glucose (from 0 to 10 mM) at a scan rate of 10 mV s−1 are shown in Fig. S2, and the increase in glucose concentration resulted in an increase in oxidation peak current. The analytical study of the performance of the MnOx -30 electrode was extended to detect a high glucose concentration, and the performance of this device was compared to that of a bulk MnO2 film electrode. The real-time detection of glucose was studied through the successive addition of a certain amount of glucose stock solution into 100 mL of 0.1 M NaOH solution while stirring at an applied potential of +0.6 V. Fig. 5A shows the typical amperometric responses of the MnOx -30 (curve a) and the MnO2 film electrodes (curve b) for glucose detection. A remarkable increase and a fast response in the amperometric current were observed after glucose was injected in the case of the MnOx -30 electrode. This device reached a steady state current within 5 s upon the addition of glucose. The MnOx -30 electrode showed a current increase after the addition of glucose at a concentration as low as 7 ␮M,

Fig. 5. (A) Amperometric responses and (B) calibration curve of MnOx -30 electrode (a) and MnO2 film electrode (b) for successive addition of glucose in 0.1 M NaOH solution at +0.6 V. Inset: magnified view of amperogram within 0–450 s. (C) Amperometric response of MnOx -30 electrode to glucose (1 mM) and different interferent species (0.1 mM AA, 0.1 mM DA, 0.1 mM L-AA, and 0.1 mM UA) in a stirring 0.1 M NaOH solution. The working potential was +0.6 V.

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Table 1 Comparison of sensing performance of our electrode with other published manganese oxide-based glucose sensor. Electrode a

MnO2 –GOx–Naf modified carbon fiber GOx/Naf/MnO2 /GCEb Cu/MnO2 /GCE PtAu–MnO2 /GPc Mn3 O4 /3DGFd MnO2 /MWNTs/Tae MnO2 /Au/GCE GOD/MnO2 bulk-modified CPE GOx/MnO2 /nafion/GCEf Liposomal-GOD/PAH/MnOx /ITOg GOx–mesoMnO2 –gelatin/GCE MnOx -30/Au a b c d e f g

Linear range (mM)

LOD (␮M)

Sensitivity (␮A mM−1 )

Ref.

1.5–15 0.2–3.8 0.00025–1.02 0.1–30 0.1–8 0.01–28 0.1–20 0.111–2.776 ∼3.15 19.6–107.1 0.0009–2.73 0.007–10.6

800 25.56 0.1 20 10 – – 58 0.35 13 mM 0.18 2.7

0.001 2.7 26.96 58.54 (␮A mM−1 cm−2 ) 360 (␮A mM−1 cm−2 ) 33.19 18.9 0.309 47.4 0.267 0.76 811.8

[2] [3] [5] [17] [18] [19] [7] [25] [26] [27] [6] This study

Naf: nafion. GCE: glassy carbon electrode. GP: graphene paper. 3DGF: three-dimensional graphene foam. Ta: tantalum. GOx: glucose oxidase. GOD: glucose oxidase, PAH: poly-(allylamine hydrochloride).

Table 2 Detection of glucose in human serum samples. Sample

Concentration (mM)

Before spiking (mM)

RSD (%)

Spiked (mM)

After spiking (mM)

RSD (%)

Recovery (%)

1 2 3

4.89 2.45 1.22

4.80 2.52 1.14

2.1 2.0 7.9

5 5 5

9.62 7.36 6.21

5.9 3.3 1.7

96.4 96.8 101.4

Recover = [(Cafter − Cbefore )/Cspike ] × 100%.

while the MnO2 film only showed a response current when the glucose concentration reached 1 mM. The calibration curves for the MnOx -30 (curve a) and MnO2 film (curve b) are plotted in Fig. 5B. MnOx -30 showed a high sensitivity of 811.8 ␮A mM−1 and a broad linear range from 0.007 to 10.6 mM with detection limit of 2.7 ␮M (S/N = 3). For the MnO2 film electrode, the sensitivity was of only 34.25 ␮A mM−1 in a range from 1 to 10 mM. This indicates that the MnOx -30 nanorod array electrode not only provides a large reactive surface area, but also facilitates electron transfer and mass diffusion. As shown in Table 1, the performance of the MnOx -30 electrode is far superior to that of various manganese oxide-based enzymatic or non-enzymatic glucose sensors. Most glucose sensors based on MnOx are combined with nanostructured carbons or metals. However, a MnOx sensor that exhibits a high sensitivity and does not use any other components has never been reported in the literature, as shown in Table 1. Some oxidizable species, such as urea, uric acid (UA), aspartic acid (AP), ascorbic acid (AA), and dopamine (DP) usually co-exist with glucose in practical samples. Thus, the selectivity of the MnOx 30 electrode plays an important role in its sensing performance [24,28]. The concentration of glucose in normal human blood is nearly 20 times greater than the concentration of urea, UA, AP, AA, or DA, and the amperometric response of the MnOx -30 electrode upon addition of 1 mM glucose and 0.1 mM interferent species was examined in 0.1 M NaOH solution. The MnOx -30 electrode did not show any significant sensor response despite the presence of urea, UA, AP, AA, and DA (less than 1%) (Fig. 5C). This result indicates that the MnOx -30 sensor has a high selectivity and can be used to study the glucose levels in human blood. The practical applicability of the fabricated sensor was studied by detecting glucose in human serum. The concentration of glucose in human serum is of 4.89 mM (Sample 1, Sigma–Aldrich H4522), Samples 2 and 3 are solutions that were obtained after dilution in 0.1 M NaOH. Each sample was tested three times and its relative standard deviation (RSD) was calculated. For the spike measurement, a known amount of glucose (5 mM) was added into

the sample, and the result is shown in Table 2, the spike recoveries from these samples are found to be 96–102% with the RSD less than 8%. Normally it is suffered from the poorer selectivity of the glucose biosensors compared to enzymatic sensor, however, this prepared sensor demonstrates an excellent selectivity and reliability to detect glucose due to its high surface area and conductivity. The repeatability of the MnOx nanorod platform was confirmed with six successive detections of 1 mM glucose using the platform, which yielded a RSD of 5.52%. The reproducibility was investigated based on the CA performance of three independent MnOx -30 electrodes with an RSD of 4.31%. To ensure long-term stability, the MnOx -30 electrode was stored at room temperature, and the current response to 1 mM glucose was measured every six days. There was no remarkable decrease in the signal, and 91% of the initial response remained after one month, indicating that the asprepared electrode was sufficiently stable for glucose detection.

4. Conclusion We synthesized a MnOx nanorod array and evaluated its use for the glucose detection. The nanostructure was constructed through simple electrochemical deposition. The electrocatalytic performance of the MnOx nanorods was enhanced with electrochemical oxidation, and the large surface area and high conductivity of the MnOx nanorod facilitates the rapid access of the electrolyte ions and the fast electron transfer. Under an optimal oxidation time, the MnOx glucose sensor that was developed demonstrated a highest sensitivity of 811.8 ␮A mM−1 among various types of MnOx -based glucose sensors that have been previously reported. Moreover, it also exhibited excellent reliability and selectivity when it was applied for human serum analysis. Considering these advantages, the MnOx electrode fabricated herein is a promising candidate for the practical analysis of glucose through a fast, selective, and sensitive approach.

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Acknowledgements This work was supported by Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20142010102690) and the Basic Science Research Program through the National Research Foundation of Korea Grant, funded by the Ministry of Science, ICT & Future Planning (2009-0083540). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.05.005 References [1] Z. Wang, S. Liu, P. Wu, C. Cai, Detection of glucose based on direct electron transfer reaction of glucose oxidase immobilized on highly ordered polyaniline nanotubes, Anal. Chem. 81 (2009) 1638–1645. [2] S.B. Hocevar, B. Ogorevc, K. Schachl, K. Kalcher, Glucose microbiosensor based on MnO2 and glucose oxidase modified carbon fiber microelectrode, Electroanalysis 16 (2004) 1711–1716. [3] L. Zhang, S.-M. Yuan, L.-M. Yang, Z. Fang, G.C. Zhao, An enzymatic glucose biosensor based on a glassy carbon electrode modified with manganese dioxide nanowires, Microchim. Acta 180 (2013) 627–633. [4] B. He, L. Hong, J. Lu, J. Hu, Y. Yang, J. Yuan, L. Niu, A novel amperometric glucose sensor based on PtIr nanoparticles uniformly dispersed on carbon nanotubes, Electrochim. Acta 91 (2013) 353–360. [5] Z. Meng, Q. Sheng, J. Zheng, A sensitive non-enzymatic glucose sensor in alkaline media based on Cu/MnO2 -modified glassy carbon electrode, J. Iran. Chem. Soc. 9 (2012) 1007–1014. [6] J. Yu, T. Zhao, B. Zeng, Mesoporous MnO2 as enzyme immobilization host for amperometric glucose biosensor construction, Electrochem. Commun. 10 (2008) 1318–1321. [7] Y.J. Yang, S. Hu, Electrodeposited MnO2 /Au composite film with improved electrocatalytic activity for oxidation of glucose and hydrogen peroxide, Electrochim. Acta 55 (2010) 3471–3476. [8] A. Lupu, P. Lisboa, A. Valsesia, P. Colpo, F. Rossi, Hydrogen peroxide detection nanosensor array for biosensor development, Sens. Actuator B: Chem. 137 (2009) 56–61. [9] J.-H. Moon, H. Munakata, K. Kanamura, Hydrothermal synthesis of Fesubstituted manganese dioxide and its electrochemical characterization for lithium rechargeable batteries, Electrochmi. Acta 134 (2014) 92–99. [10] X. Wang, A. Yuan, Y. Wang, Supercapacitive behaviors and their temperature dependence of sol–gel synthesized nanostructured manganese dioxide in lithium hydroxide electrolyte, J. Power Sources 172 (2007) 1007–1011. [11] Q. Qu, P. Zhang, B. Wang, Y. Chen, S. Tian, Y. Wu, R. Holze, Electrochemical performance of MnO2 nanorods in neutral aqueous electrolytes as a cathode for asymmetric supercapacitors, J. Phys. Chem. C 113 (2009) 14020–14027. [12] L. Qingwen, W. Yiming, L. Quoan, pH-response of nanosized MnO prepared with solid state reaction route at room temperature, Sens. Actuator B: Chem. 59 (1999) 42–47. [13] W. Yan, T. Ayvazian, J. Kim, Y. Liu, K.C. Donavan, W. Xing, Y. Yang, J.C. Hemminger, R.M. Penner, Mesoporous manganese oxide nanowires for highcapacity, high-rate, hybrid electrical energy storage, ACS Nano 5 (2011) 8275–8287. [14] C.-C. Hu, K.-H. Chang, Y.-T. Wu, C.-Y. Hung, C.-C. Lin, Y.-T. Tsai, Pulse deposition of large area, patterned manganese oxide nanowires in variable aspect ratios without templates, Electrochem. Commun. 10 (2008) 1792–1796.

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Biographies Sang Ha Lee is a student at School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea. Jiao Yang is a student at School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea. You Jeong Han is a student at School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea. Misuk Cho is a research professor at School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea. Youngkwan Lee is a professor at School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea.