CuO nanowires based sensitive and selective non-enzymatic glucose detection

Sensors and Actuators B 191 (2014) 86–93

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

CuO nanowires based sensitive and selective non-enzymatic glucose detection Yuchan Zhang a,b , Yixin Liu b , Liang Su b , Zhonghua Zhang b , Danqun Huo a , Changjun Hou a,∗∗ , Yu Lei b,∗ a Key Laboratory of Biorheological Science and Technology (Chongqing University), Ministry of Education, Bioengineering College, Chongqing University, Chongqing 400030, China b Department of Chemical and Biomolecular Engineering, University of Connecticut, 191 Auditorium Road, Unit 3222, Storrs, CT 06269, USA

a r t i c l e

i n f o

Article history: Received 27 April 2013 Received in revised form 27 August 2013 Accepted 29 August 2013 Available online xxx Keywords: Copper oxide nanowires Glucose Electrooxidation Non-enzymatic Sensor

a b s t r a c t Copper oxide nanowires (CuO NWs) were prepared by a facile two-step procedure consisting of wetchemistry synthesis and subsequent direct calcination. The morphology, surface property, and crystal structure of the as-prepared CuO NWs were characterized by SEM, TEM, and XRD. The CuO NWs were further employed to construct a non-enzymatic glucose sensor with excellent performance toward glucose detection in 50 mM NaOH solution. The as-developed non-enzymatic glucose sensor showed a fast response time (less than 5 s) and a wide dynamic range with excellent sensitivity of 648.2 ␮A cm−2 mM−1 and 119.9 ␮A cm−2 mM−1 toward glucose detection at an applied potential of +0.55 V and +0.3 V (vs. Ag/AgCl), respectively. The Langmuir isothermal theory was employed to fit the obtained calibration curves with high correlation coefficient and the mechanisms for the glucose oxidation promoted by CuO NWs were also discussed. The good selectivity of the CuO NWs based non-enzymatic glucose sensor against electroactive compounds such as ascorbic acid, uric acid, and acetaminophen, and other sugars such as fructose and sucrose at their physiological concentrations were also demonstrated. Furthermore, good accuracy and high precision for the quantification of glucose concentration in human serum samples was attested. These good features indicate that CuO NWs have a great potential in the development of sensitive and selective non-enzymatic glucose sensor. © 2013 Elsevier B.V. All rights reserved.

1. Introduction “Silent Killer”, another name for diabetes, is a metabolic disorder and a major world health problem. As stated by International Diabetes Federation, there are over 285 million people worldwide living with diabetes in 2010 [1]. This number is projected to double in 2030. Currently, the top five nations in terms of the number of diabetic patients are China (92.4 million), India (50.8 million), US (26.8 million), Russia (9.6 million), and Brazil (7.6 million), spanning developing and developed countries and covering diverse geographic areas. Due to the extremely large financial burden caused by diabetes and its serious complications, glucose detection is becoming paramountly important in battling diabetes and reducing financial loss. Currently, self-management of blood glucose relies on glucose oxidase (GOD)-based and glucose dehydrogenase (GDH)-based electrochemical glucose detection methods. GOD is a slightly

∗ Corresponding author. Tel.: +1 860 486 4554; fax: +1 860 486 2959. ∗∗ Corresponding author. Tel.: +86 23 6511 1022; fax: +86 23 6510 2507. E-mail addresses: [email protected] (C. Hou), [email protected] (Y. Lei). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.08.096

elongated globular protein [2]. Due to its high selectivity to glucose and high activity over a broad range of pH, ionic strength, and temperature, GOD-based glucose detection allows less stringent conditions during the manufacturing process and storage [3–6]. In order to be a functional biocatalyst, GOD requires a redox cofactor – flavin adenine dinucleotide (FAD). During glucose oxidation, FAD serves as the initial electron acceptor and is reduced to FADH2 , which is regenerated by reacting with oxygen, leading to the formation of hydrogen peroxides (Glucose + GOD

O2 −→gluconolactone + H2 O2 ) [7]. Thus two general strategies used for the GOD-based electrochemical glucose sensing are: by measuring oxygen consumption [8–11] and by measuring the amount of hydrogen peroxide produced through the enzyme reaction (the first generation glucose biosensor) [11–15]. However, the accuracy of both methods greatly depends on dissolved oxygen concentration. Two approaches have been proposed to minimize the interference from the variation of oxygen concentration in blood. The first solution is to employ artificial mediators (e.g., ferrocene derivatives, ferricyanide, etc.) that shuttle electron between GOD and the electrode rapidly (the second generation glucose biosensor) [16–21]. The use of electron mediators can greatly reduce the applied potential and also can reduce the interference

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from the variation of oxygen concentration. However, even the best one is not completely free from oxygen effect because the electron-mediator always competes with the dissolved oxygen in the solution. Therefore, the glucose sensor suppliers commonly warn that diabetic patients undergoing oxygen therapy may yield falsely low results using GOD-based test strips. Another solution is to use GDH to replace GOD because the enzymatic reaction of GDH is independent of the dissolved oxygen. The GDH family includes GDH-pyrroquinolinequinone (PQQ) [22–24] and GDHnicotinamide-adenine dinucleotide (NAD) [25]. Unlike GDH-NAD GDH

system (glucose + NAD+ −→gluconolactone + NADH), the quinoprotein GDH recognition element uses PQQ as a cofactor (glucose + GDH

PQQ(ox)−→gluconolactone + PQQ(red)) and GDH-PQQ is a particularly efficient enzyme system with a rapid electron transfer rate and without the requirement of oxygen or NAD+ , but it is relatively expensive [7,26]. Although enzyme-based glucose biosensors possess distinct advantages in glucose detection, they typically suffer from a stability problem due to the intrinsic instability of enzymes [27]. In addition, the operation conditions (e.g., pH, temperature, oxygen dependence, humidity, etc.) [2] could greatly affect the sensing performance. Moreover, the use of enzymes remains largely expensive which impedes a broader application of enzymatic biosensors in developing countries. In this regard, the appearance of enzyme-free electrodes provides an alternative for more economical and stable glucose sensor. Non-enzymatic glucose detection is not a recent interest. Efforts to realize this idea have been made since early studies on the electrochemistry of glucose itself due to its potential application for long-term glucose monitoring [28]. Such non-enzymatic glucose detection addresses the inferior stability and oxygen dependence of common biocatalytic glucose biosensors [7]. Recently, considerable attention has been given to enzyme-free electrodes with good glucose sensitivity and selectivity. Precious metals and metal alloys (e.g., Au [29], Pt [30], Ni [31], Cu [32], Pt–Pb [33], Ni–Cu [34], Au–Pt [35] and Au–Ag [36]) have been extensively investigated as nonenzymatic glucose sensors. However, the sensing utility of these electrodes is very limited due to drawbacks such as low sensitivity, poor selectivity, high costs, and/or the potential poisoning effect of chloride ions [30,37,38]. Therefore, there are considerable demands for the development of novel, cost-effective, sensitive, selective, and reliable non-enzymatic glucose sensors. However, non-enzymatic glucose sensors lack a glucose recognition unit. Thus, they suffer from the interference from other electroactive compounds (e.g., uric acid, ascorbic acid, acetaminophen, etc.) and other sugars even at their representative physiological concentrations. Recently, our group and other groups have been extensively exploring nanostructured or bulked metal or metal oxides (e.g., metallic Cu [39], Co3 O4 nanofibers [40], CuO [41,42,43,44], Cu2 O [45], Cux O [46], NiO nanofibers [47], NiO–CdO nanofibers [48] and bulk metal oxides (RuO2 , CoO and NiO) [49]) as well as metal oxide-metal composite (e.g., NiO/Ag, NiO/Au, NiO–Pt) [47,50,51] in the construction of a variety of enzyme-free glucose sensors with enhanced performance. In this work, a facile and scalable preparation method of CuO nanowires (CuO NWs) was reported and the CuO NWs was further employed as the sensing materials to detect glucose. The CuO NWs modified glassy carbon electrode showed a specific and excellent performance with a good sensitivity, selectivity and wide dynamic range toward the quantification of glucose under an applied potential of +0.3 V and +0.55 V, respectively. The mechanisms for the glucose oxidation promoted by CuO NWs were also discussed. Moreover, the excellent performance of CuO NWs in glucose detection were achieved in 50 mM NaOH solution, which is lower than the concentration of alkaline solution (0.1 M and higher) typically used in other reports [27,40–42,47,48,50,51]. These good features

Fig. 1. TGA of Cu NWs in the oxygen atmosphere and the corresponding firstderivative weight curve.

indicate the potential applicability of the as-prepared CuO NWs in the fabrication of non-enzymatic glucose sensor with high performance. 2. Experimental 2.1. Reagents and materials Copper nitrate (Cu(NO3 )2 ), sodium hydroxide (NaOH), sodium chloride (NaCl), acetaminophen (AP), uric acid (UA), ascorbic acid (AA), and glucose were purchased from ACROS. Ethylenediamine (>99.9%), hydrazine, Nafion 117 solution, ethanol, sucrose, and fructose were used as received from Sigma–Aldrich. Human serum samples (from female O plasma) were purchased from the hospital of Chongqing Medical University. All aqueous solutions were prepared with deionized (DI) water (18.2 M-cm, Barnstead DI water system). 2.2. Preparation of CuO NWs modified electrode A number of approaches/processes have been developed to synthesize Cu or CuO nanowires, including wet chemical method [52], thermal oxidation method [53], low-temperature solid-phase process [54], directly heating copper substrates in air [55], and thermal dehydration method [56]. Considering the size controllability, suitability for mass production and convenience of operation, the wet chemical method and thermal oxidation process were employed to synthesize CuO nanowires. Briefly, Cu NWs were synthesized following the method reported elsewhere [52,57,58]. Then, the reddish Cu NWs were directly calcined in oven at 400 ◦ C for 5 h to obtain CuO NWs. The calcination at 400 ◦ C was sufficient to convert Cu NWs to CuO NWs, supported by the results from thermogravimetric analysis (TGA) in oxygen atmosphere and the corresponding first-derivative weight curve (Fig. 1). Then the atramentous final products (CuO NWs) were suspended and stored in ethanol at a concentration of 5 mg/mL for further use. Glassy carbon electrodes (GCE, dia. 3 mm) were polished with 1 ␮m and 0.05 ␮m alumina slurries sequentially, and rinsed with DI water followed by drying at room temperature before surface modification. An appropriate volume of the as-prepared CuO NWs suspension was dropped on the surface of GCE. After drying in air, 20 ␮L of Nafion (0.1 wt%) was then cast on the top of CuO NWs layer to entrap CuO NWs. The asprepared electrode is denoted as Nafion/CuO NWs/GCE. The control

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Fig. 2. Typical SEM image of (A) CuO NWs and (B) a single CuO NW. Inset shows the EDX spectrum of CuO NWs.

electrode prepared by following a similar procedure is denoted as Nafion/GCE. 2.3. Apparatus and electrochemical measurements A JEOL 6335F field-emission scanning electron microscope (FESEM) (10 kV operating voltage) was used to observe the morphology and the size of the as-prepared samples. Energy-dispersive X-ray spectroscopy (EDX) detector in the FESEM was used for chemical composition analysis. A JEOL JEM2010 Fast transmission electron microscope (TEM) was used for morphological imaging and structure analysis. A Rigaku X-ray diffraction instrument with ˚ was used to examine the crysa Cu K␣ X-ray source ( = 1.54 A) talline phase of the as-synthesized sample. A beam voltage of 40 kV and 44 mA current were used. The data were collected in the 2 range of 10–70◦ with a scanning rate of 2◦ /min. All electrochemical experiments were performed in alkaline electrolyte on CHI 660 D electrochemical workstation (CH Instruments, USA). Ag/AgCl (3 M KCl) electrode and Pt wire were used as the reference and the counter electrodes, respectively. Cyclic voltammetry (CV) was carried out between 0 V and +0.8 V at a scan rate of 50 mV/s. All the experiments were conducted at room temperature and were repeated at least three times to verify reproducibility. 3. Results and discussion

Fig. 3. (A) The XRD pattern of CuO NWs. (B) The TEM image of CuO NW and the corresponding SAED pattern (insert).

nanowires [58], the surface of the CuO NWs obtained after calcination was very rough with many nano-featured protuberance, which can provide larger accessible surface area for the subsequent electrochemical oxidation of glucose. The EDX spectrum of CuO NWs (inset of Fig. 2A) reveals the peaks associated with copper and oxygen, indicating that the nanowires are only composed of copper and oxygen atoms. Furthermore, the calculated atomic ratio of copper to oxygen was approximately 1:1, which was in a good agreement with the stoichiometric ratio of Cu to O in CuO. XRD was further carried out to characterize the crystal structure and the phase composition of the as-prepared CuO NWs. As shown in Fig. 3A, the XRD pattern of CuO NWs samples agrees well with the standard pattern (JCPDS 45-0937). All of the reflection peaks in the XRD pattern can be indexed to tenorite CuO, while no other impurity was observed. The detailed structure of CuO NWs was further examined by TEM. The TEM image in Fig. 3B verifies the rough surface of the as-prepared CuO NW and the calculated diameter of CuO NW is also ca. 200 nm, which is in a good agreement with the SEM result. The SAED pattern (inset in Fig. 3B) exhibits “polycrystalline” ring pattern, which consists of several sets of strong single crystalline diffraction spots in the monoclinic structured CuO NWs.

3.1. Characterization of CuO NWs 3.2. Electrochemical behavior of CuO NWs Fig. 2 shows typical SEM images of CuO NWs. The as-prepared CuO NWs exhibit relatively uniform distribution with an average diameter of ca. 200 nm. Unlike the smooth surface of precursor Cu

The electrochemical behavior of CuO NWs in the absence and presence of glucose was evaluated by cyclic voltammetry (CV) in

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hydroxyl radicals with limit electrocatalytic sites on CuO NWs. In order to use electrolyte with low alkalinity while maintain high glucose electrooxidation capability, 50 mM NaOH solution was thus selected as the alkaline electrolyte for subsequent experiments. The loading of CuO NWs on glassy carbon electrode was also optimized with respect to the electrooxidation current of 0.02 mM glucose in 50 mM NaOH solution. The corresponding result is presented in Fig. 4B. As expected, the response initially increased and reached a maximum at 100 ␮g CuO NWs loading followed by a decrease. The initial increase is attributed to increased catalytic sites responsible for glucose oxidation, while the decrease at high CuO NWs loading is due to the transport resistance of glucose to CuO NWs embedded deeper in the immobilization layers. A similar trend was observed with other non-enzymatic glucose sensors [57,58]. Thus Nafion/CuO NWs/GCE with 100 ␮g CuO NWs loading was applied in subsequent experiments. Operated under the optimized conditions, Fig. 4C and its inset present CVs of the Nafion/CuO NWs/GCE and the Nafion/GCE (control) in the absence and presence of 2 mM glucose, respectively. On the Nafion/CuO NWs/GCE, the anodic peak current density dramatically increases in the presence of 2 mM glucose compared to that on the control electrode (Fig. 4C and its inset). The shoulder peak at ca. +0.55 V can be ascribed to the electrooxidation of glucose, where Cu(III) and hydroxyl radicals are probably involved [58]. On the control electrode, there was no obvious oxidation peak observed in the presence of 2 mM glucose. The good performance of the Nafion/CuO NWs/GCE may be attributed to the large surface area, high surface energy, and enhanced electron transfer of as-synthesized CuO NWs. Based on these studies, amperometric detection of glucose is conducted at +0.55 V (shoulder peak position) and +0.3 V (lower applied potential as a comparison). 3.3. Amperometric detection of glucose

Fig. 4. (A) Cyclic voltammograms of the Nafion/CuO NWs/GCE in different concentration of NaOH solutions ([glucose] = 2 mM, the CuO NWs loading = 100 ␮g, the scan rate = 50 mV/s). (B) Optimization of CuO NWs loading through amperometric response to 20 ␮M glucose injection at an applied potential of +0.55 V in 50 mM NaOH (all currents are normalized on the basis of the current at 100 ␮g CuO NWs loading). (C) Cyclic voltammograms of the Nafion/CuO NWs/GCE and Nafion/GCE (inset) in the absence and presence of 2 mM glucose in 50 mM NaOH (the CuO NWs loading = 100 ␮g, the scan rate = 50 mV/s).

alkaline electrolyte, and a Nafion modified glassy carbon electrode was also examined as a control. All CVs were carried out in the potential range from 0 V to +0.8 V which covers the glucose electrooxidation range in alkaline electrolyte [59,60]. The experimental conditions were first optimized with respect to NaOH solution concentration. As presented in Fig. 4A, the increase of NaOH solution concentration results in the increase of the anodic current of glucose electrooxidation, which can be explained by the formation of more hydroxyl radicals in high concentration of NaOH at high potential. One can also see that further increase of NaOH concentration from 50 mM to 100 mM gives only moderate increase of the current which may be attributed to the excessive amount of

Fig. 5A and B show the typical amperometric detection of the Nafion/CuO NWs/GCE to the successive addition of glucose at an applied potential of +0.55 V and +0.3 V, respectively. The corresponding calibration curves are presented in Fig. 5C and D. As electrochemical oxidation of glucose on CuO NWs is a process of adsorption/reaction/desorption process which typically follows Langmuir isothermal kinetics, the Langmuir adsorption equation was used to fit the calibration curves for the Nafion/CuO NWs/GCE. At an applied potential of +0.55 V (Fig. 4C), the regressed Langmuir adsorption equation (R2 = 0.9999) was presented as I = 7998.47 × Cglucose /(12.34 + Cglucose ), which can cover a broad dynamic range required for human blood glucose detection. Cglucose represents glucose concentration in 50 mM NaOH electrolyte. When Cglucose is negligible compared to 12.34 (e.g., Cglucose < 1 mM), a good sensitivity of 648.2 ␮A cm−2 mM−1 was obtained. A limit of detection (LOD) is 2 ␮M (S/N = 3). At an applied potential of +0.3 V (Fig. 4D), it also exhibited a good fitting curve with a regressed Langmuir adsorption equation (R2 = 0.9997) of I = 676.05 × Cglucose /(5.64 + Cglucose ), which also cover a broad dynamic range required for human blood glucose detection. When glucose concentration is low (e.g., Cglucose < 0.5 mM), the corresponding calibration curve also shows good sensitivity of 119.9 ␮A cm−2 mM−1 and a LOD of 5 ␮M (S/N = 3). Compared to the results obtained at an applied potential of +0.3 V, much better sensitivity was achieved at +0.55 V, which is also much higher than our previous reports [40,41,47,48,50,51]. 3.4. Reproducibility and selectivity The response of the CuO NWs based glucose detection is highly reproducible as demonstrated by the low relative standard deviations of 3.32% (n = 6) for 200 ␮M glucose. Additionally, there is an excellent electrode-to-electrode reproducibility as characterized

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Fig. 5. Real-time amperometric response of the Nafion/CuO NWs/GCE in 50 mM NaOH to the successive addition of glucose (triple injections per concentration) at an applied potential of (A) +0.55 V and (B) +0.3 V (vs. Ag/AgCl), respectively, and the corresponding calibration curves with Langmuir isothermal fittings (C and D).

by the very low relative standard deviations of 4.54% (n = 5) in the response of five CuO NWs modified electrodes prepared with a same protocol to 200 ␮M glucose. Selectivity is a very important but challenging aspect to nonenzymatic glucose detection since the easily oxidative species may co-exist with glucose sample. The developed sensor was further evaluated for selective detection of glucose. The selectivity study was first conducted by holding the Nafion/CuO NWs/GCE at an applied potential of +0.55 V and electroactive small molecules such as uric acid (UA), ascorbic acid (AA) and acetaminophen (AP) at their physiological concentration level were used as model compounds. As shown in Fig. 6A, 0.3 mM UA, 0.1 mM AA and 0.1 mM AP only result in insignificant responses compared to that of 5 mM glucose (in the normal range of blood glucose concentration), indicating the remarkable selectivity of the Nafion/CuO NWs/GCE for glucose detection in the presence of electroactive small molecules. As a comparison, the selectivity is also investigated at +0.3 V and the interference from 0.3 mM UA, 0.1 mM AA and 0.1 mM AP is significant and cannot be ignored (Fig. 6B), indicating that higher applied potential is more favorable for glucose oxidation than UA, AA and AP. In addition, due to the significant response to UA, AA and AP at +0.33 V, the injection of 5 mM glucose resulted in a smaller response as expected, which can be ascribed to the gradual saturation of the electrode response by UA, AA and AP. The selectivity was further evaluated against a range of sugars at an applied potential of +0.55 V and +0.3 V (vs. Ag/AgCl). As shown in Fig. 6C and D, even molecularly similar monosaccharide (e.g., 8.1 ␮M fructose

[61]) and disaccharide (e.g., 74 ␮M sucrose [62]) at their physiological concentration level in healthy people show negligible response compared to that of glucose at both applied potentials. The fact that neither fructose nor sucrose would introduce interference to the detection of glucose could be attributed to their low concentration in blood compared to glucose. Therefore, the blood fructose and sucrose is not the major concern in our sensor. Although many CuO glucose sensors have been reported [41–44], the CuO NWs used in this study could be facilely mass-produced with better size control while maintaining good performance in glucose detection. These features should make the CuO NWs an ideal material for the construction of non-enzymatic glucose sensor for applications in medical diagnostics, biological processes, and food industry. 3.5. Human serum sample detection As a real application, the CuO NWs modified glassy carbon electrodes were further employed to detect glucose in human serum samples. Human serum sample was injected into 5 mL of 50 mM NaOH and the measurement was conducted at an applied potential of +0.55 V and +0.3 V, respectively. The measured current change was correlated with the glucose concentration according to the calibration curve in Fig. 5C and D, and then compared with the values obtained using a commercial glucose meter (SAFE-ACCU Blood Glucose Monitoring System, Changsha Sinocare, Inc., P.R.C.). As shown in Table 1 and Table 2, the good accuracy (recovery) along with the high precision (low RSD) demonstrates the reliability of CuO

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Fig. 6. Glucose selectivity study against various commonly co-existing compounds at their representative physiological concentration level: (A) and (B) Amperometric response of the Nafion/CuO NWs/GCE to 0.3 mM uric acid, 0.1 mM ascorbic acid, 0.1 mM acetaminophen, and 5 mM glucose in 50 mM NaOH at an applied potential of +0.55 V and +0.3 V (vs. Ag/AgCl), respectively. (C) and (D) Amperometric response of the Nafion/CuO NWs/GCE to 8.1 ␮M fructose, 74 ␮M sucrose, and 5 mM glucose in 50 mM NaOH at an applied potential of +0.55 V and +0.3 V (vs. Ag/AgCl), respectively.

Table 1 The detection of glucose concentration in human serum samples at an applied potential of +0.55 V. Sample

Glucose concentration (mM) from our glucose sensor

RSD (N = 5) (%)

Glucose concentration (mM) from commercial glucose meter

Recovery

1 2 3 4 5

6.36 6.37 6.68 6.50 7.66

5.1 6.5 5.5 4.7 4.6

6.3 6.5 6.5 6.4 7.8

1.01 0.98 1.03 1.02 0.98

Table 2 The detection of glucose concentration in human serum samples at an applied potential of +0.3 V. Sample

Glucose concentration (mM) from our glucose sensor

RSD (N = 5) (%)

Glucose concentration (mM) from commercial glucose meter

Recovery

1 2 3 4 5

6.50 6.52 6.78 6.54 7.45

2.4 4.2 2.1 2.2 2.3

6.3 6.5 6.5 6.4 7.8

1.03 1.00 1.04 1.02 0.96

NWs based non-enzymatic glucose sensor for the detection of real samples. 4. Conclusion A new non-enzymatic electrocatalyst based on CuO NWs for glucose detection has been fabricated. CuO NWs with an average diameter of ca. 200 nm were produced by a facile two-step procedure consisting of wet-chemistry synthesis and subsequent direct calcinations. SEM and TEM were performed to investigate

the morphology of the as-prepared CuO NWs, while XRD and EDX were employed to study its crystal structure and composition. The CuO NWs modified electrode displayed excellent performance toward glucose detection in 50 mM NaOH solution. Under the optimal conditions, the developed non-enzymatic sensor can detect glucose over a wide dynamic range with excellent sensitivity at an applied potential of +0.55 V (648.2 ␮A cm−2 mM−1 ) and +0.3 V (119.9 ␮A cm−2 mM−1 ). In addition, Langmuir isothermal theory was applied to fit the calibration curves with high correlation coefficient. The applicability of the as-prepared CuO NWs for selective

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Biographies Yuchan Zhang is an assistant researcher of Life Sciences Institute at the Chongqing Medical University, China. She obtained her Ph.D. degree in 2012 at the Chongqing University in Biomedical Engineering. Her research interests lie in electrochemical sensors and colorimetric sensor arrays for biomedical application. Yixin Liu is a Ph.D. student in the Department of Chemical, Materials and Biomolecular Engineering, University of Connecticut, USA. She earned her Bachelor degree

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in 2010 from Zhejiang University, China. Her research concentrates on functional nanomaterials for chemical sensing including harsh environmental sensing. Liang Su earned a doctor degree in 2013 in Chemical and Biomolecular Engineering at the University of Connecticut, USA. His Ph.D. research concentrates on the development of novel nanocatalysts for oxygen reduction and alcohol oxidation for fuel cell application. Zhonghua Zhang is a postdoctoral fellow of Institution of Materials Sciences, University of Connecticut, USA. He obtained his Ph.D. degree in Chemistry in 2009 at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. His current research interests lie in the energy storage and conversion, especially the research and development of fuel cells and lithium ion batteries. Danqun Huo received her Ph.D. degree in 2004 from Chongqing University. Currently she is a full professor (Ph.D. supervisor) of Chongqing University. Her main research direction is biological chip and testing technology. Changjun Hou is a full professor (Ph.D. supervisor) of Chongqing University. He received his Ph.D. degree in 2004 from Chongqing University. Currently his main research focuses on nanotechnology and sensor materials, biochemical sensors, cell signal detection technology and applications, and detection and processing of biomedical information. Yu Lei is a Castleman associate professor of Chemical, Materials and Biomolecular Engineering at the University of Connecticut, USA. Dr. Lei obtained his Ph.D. degree in 2004 at the University of California-Riverside in Chemical and Environmental Engineering. His current research combines biotechnology, nanotechnology, and sensing technology, especially as applied to the development of gas sensors, electrochemical sensors, and biosensors.