Nonenzymatic amperometric determination of glucose by CuO nanocubes–graphene nanocomposite modified electrode

Bioelectrochemistry 88 (2012) 156–163

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Nonenzymatic amperometric determination of glucose by CuO nanocubes–graphene nanocomposite modified electrode Liqiang Luo a,⁎, Limei Zhu a, b, Zhenxin Wang b,⁎⁎ a b

College of Sciences, Shanghai University, Shanghai 200444, PR China State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China

a r t i c l e

i n f o

Article history: Received 16 September 2011 Received in revised form 8 March 2012 Accepted 23 March 2012 Available online 2 April 2012 Keywords: Graphene Copper oxide nanocubes Glucose Electrodeposition Nonenzymatic sensor

a b s t r a c t Here, we report a nonenzymatic amperometric glucose sensor based on copper oxide (CuO) nanocubes– graphene nanocomposite modified glassy carbon electrode (CuO–G–GCE). In this case, the graphene sheets were cast on the GCE directly. CuO nanocubes were obtained by oxidizing electrochemically deposited Cu on the graphene. The morphology of CuO–G nanocomposite was characterized by scanning electron microscopy. The CuO–G–GCE-based sensor exhibited excellent electrocatalytic activity and high stability for glucose oxidation. Under optimized conditions, the linearity between the current response and the glucose concentration was obtained in the range of 2 μM to 4 mM with a detection limit of 0.7 μM (S/N = 3), and a high sensitivity of 1360 μA mM − 1 cm − 2. The proposed electrode showed a fast response time (less than 5 s) and a good reproducibility. The as-made sensor was applied to determine the glucose levels in clinic human serum samples with satisfactory results. In addition, the effects of common interfering species, including ascorbic acid, uric acid, dopamine and other carbohydrates, on the amperometric response of the sensor were investigated and discussed in detail. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Diabetes mellitus is a global health care problem that seriously affects normal life of hundreds of millions of people [1]. Blood glucose level is a basic issue in the diagnosis and treatment of diabetes. Therefore, there is a great demand for the development of glucose biosensor which can reliably and rapidly monitor the level of blood glucose in clinical applications [2,3]. Amperometric glucose biosensor is one of promising methodologies which can meet the requirements of clinic blood glucose level measurement [4–6]. Glucose oxidase (GOx) modified electrode is the most common class of amperometric biosensors for glucose detection because GOx enables catalytic oxidation of glucose with high sensitivity and selectivity [7–9]. However, the GOx-based biosensors have several potential drawbacks, such as instability, high cost, complicated immobilization procedure and critical operating situation [10,11]. Recently, it has been recognised that enzymeless glucose electrochemical sensors can offer a unique set of physical and practical properties that may be exploited in blood glucose detection as an alternative to the use of GOx-based biosensors [12,13]. Various metal nanomaterials have been used for developing nonenzymatic glucose electrochemical sensors [14–16]. Especially, the inexpensive metallic nanoparticles, such as Cu [17,18], Ni ⁎ Corresponding author. Tel.: + 86 21 66134734. ⁎⁎ Corresponding author. Tel.: + 86 431 85262243. E-mail addresses: [email protected] (L. Luo), [email protected] (Z. Wang). 1567-5394/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.bioelechem.2012.03.006

[19,20], and their oxides [21–24], attract more attention due to their low cost, plenty of morphologies, high specific surface area, good electrocatalytic activity and the possibility of promoting electron transfer reactions at a lower overpotential. Copper oxide (CuO), a p-type semiconductor with a narrow band gap of 1.2 eV, has been extensively studied because of its numerous applications in catalysis, field-effect transistors and biosensors [4,25,26]. A series of CuO nanomaterials-based amperometric sensors have also been fabricated and applied for sensing glucose with high sensitivity and good stability [4,21,22]. Because of its unique nanostructure and extraordinary properties (e.g. excellent electronic transport property and high electrocatalytic activity), graphene, the flat monolayer of carbon atoms arranged in a two-dimensional honey-combed lattice, has attracted tremendous attention [27,28]. This inexpensive nanomaterial has been employed for developing optoelectronic devices, electrochemical supercapacitors, field-effect transistors, gas sensors and biosensors [29–33]. In this paper, a kind of CuO nanocubes and graphene hybrid nanostructures modified glassy carbon electrode (CuO–G–GCE) has been built by electrodepositing CuO nanocubes onto graphene sheets. Comparing to the published CuO-based sensors [21,22], this electrochemical deposition method to fabricate CuO–G nanocomposite is simple and time-saving without using complicated and expensive equipment. Due to the great ability to promote electron-transfer reactions and large surface area, the CuO–G–GCE can be used for rapidly amperometric sensing of glucose with high sensitivity and excellent selectivity.

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2. Materials and methods 2.1. Chemicals and reagents D-(+)-glucose, D-(+)-galactose (Gal), D-(+)-mannose (Man) and α-D-lactose monohydrate (Lac) were purchased from Fluka (USA). L-ascorbic acid (AA), uric acid (UA), dopamine (DA) and Nafion (5 wt.% in lower aliphatic alcohol and water mixture) were obtained from Sigma (USA). Sodium citrate (Na3C6H5O7•2H2O), monopotassium phosphate (KH2PO4), sodium chloride (NaCl), copper (II) chloride dihydrate (CuCl2•2H2O), potassium ferricyanide (K3[Fe(CN)6]) and potassium ferrocyanide (K4[Fe(CN)6]•3H2O) were obtained by Beijing Chemical Plant (China). Hydrazine hydrate (NH2NH2•H2O, 80%) was obtained from Aladdin (China). Graphite oxide (GO) was obtained from Nanjing XFNano Materials Tech Co., Ltd (China). Human blood serum samples were kindly provided by a local hospital (Changchun, China). All chemical reagents were of analytical grade and used as received without further purification. Milli-Q water (18.25 MΩ.cm) was used in all experiments. 2.2. Instruments Transmission electron microscopy (TEM) images were obtained by a Hitachi H-600 (Japan, at an acceleration voltage of 100 kV) and scanning electron microscopy (SEM) images were recorded with an XL30 ESEM-FEG (Netherlands, FEI Company, at an acceleration voltage of 20.0 kV). CA-958 H rapid semi-automatic biochemical analyzer (Changchun optics-mechanics medical instrument Co., Ltd, China) was used to determine glucose content in human blood serum samples. Electrochemical measurements were performed using a CHI 660D electrochemical workstation (Shanghai CH Instrument Co., China). A conventional three-electrode system was used with a saturated calomel electrode (SCE) as the reference electrode, a platinum sheet as the counter electrode, and a modified GCE (3 mm in diameter) as the working electrode. Prior to surface coating, the GCE was polished carefully with 1.0, 0.3 and 0.05 μm alumina powder, respectively. Then, the polished GCE was cleaned sequentially with 1:1 HNO3, ethanol and water by continuous sonication, respectively. The electrode was allowed to dry at ambient temperature for further using. Electrochemical impedance spectroscopy (EIS) measurements were performed using an Autolab PGSTST 30 analyzer (Metrohm Autolab B.V., Switzerland). [Fe(CN)6] 3 −/4 − was used as the electrochemical probe. A 5 mV amplitude of sine voltage signal was applied to the three-electrode system under open circuit potential, and the frequency varied from 0.1 Hz to 100 kHz. The results were shown in the form of Nyquist plots. 2.3. Preparation of graphene and the CuO–G–GCE The graphene was prepared according to the published route involving the steps of graphite oxidation, exfoliation and chemical reduction [34,35]. The procedure was as following: the GO dispersion (0.1 mg mL − 1 dispersed in 200 mL water) was firstly exfoliated by sonicating under ambient conditions for 40 min. Then, NH2NH2•H2O (1% v/v) was added into the GO dispersion. The resulting mixture was heated to 100 °C and kept stirring for 24 h. Subsequently, the solution was filtered, and the filtrate was discharged. Finally, black hydrophobic powder graphene was obtained by drying in vacuum at 60 °C, and stored at ambient conditions. To fabricate CuO–G–GCE, 10 μL of 0.1% Nafion aqueous containing 2 mg mL− 1 graphene was dropped onto the GCE and allowed to dry in air to form graphene modified GCE (G–GCE). Then, Cu was electrodeposited on the G–GCE by maintaining potential −0.40 V for 120 s in 0.1 M KCl solution containing 0.01 M CuCl2 and the obtained electrode was named as Cu–G–GCE. After washed with water (10 mL three times) and dried with a flow of N2, the Cu–G–GCE was immersed in 0.1 M NaOH

157

and repeatedly scanned with cyclic voltammetry (CV) under the potential range from −0.50 to +0.30 V at 100 mV s− 1 for 20 cycles. During the repetitive cyclic potential scanning in the supporting solution of 0.1 M NaOH, CuO nanocubes were formed on the graphene. The mechanism of the electrodeposition can be shown below [36,37]: −

−

Cu þ OH →CuOH þ e

ð1Þ

−

−

2CuOH þ 2OH →Cu2 O þ 2H 2 O þ 2e −

−

Cu2 O þ 2OH →2CuO þ H 2 O þ 2e

ð2Þ ð3Þ

Finally, the electrode was rinsed with water (10 mL three times) and dried with a flow of N2. The as-prepared modified electrode was named as CuO–G–GCE. For comparison, the CuO modified GCE (named as CuO–GCE) was also prepared by employing similar electrodeposition procedure on GCE directly, and Nafion modified GCE (named as Nafion–GCE) was prepared by casting 10 μL of 0.1% Nafion aqueous solution onto the GCE directly. Especially, all of electrodeposition experiments were protected by saturated N2 atmosphere. 3. Results and discussion 3.1. Characterization of the prepared CuO–G nanocomposite The morphology of the as-prepared graphene and CuO–G nanocomposite were characterized by SEM and TEM as shown in Fig. 1. As can be seen in Fig. 1A, the graphene sheets were crumpled and wrinkled on the surface of the glassy carbon (GC) substrate, which provided an ideal matrix for the distribution of CuO. As can be seen in Fig. 1B, the uniform CuO nanocubes were well dispersed on the graphene sheets. The average edge length of the CuO nanocubes was about 200 nm, indicating that the graphene as a large surface area provider was fit for the formation of the modest size of CuO nanocubes. No significant aggregation of graphene was observed in the TEM image (as shown in Fig. 1C), indicating that the graphene sheets were well dispersed in Nafion aqueous solution. 3.2. Electrochemical property of the CuO–G–GCE The electrocatalytic properties of GCE, CuO–GCE, G–GCE and CuO–G–GCE were investigated by CVs in 0.1 M NaOH solution with or without 2.0 mM glucose. As shown in Fig. 2A, in 0.1 M NaOH solution, a single somewhat broad reduction peak with the potential of about +0.60 V was observed on both CuO–GCE (curve b) and CuO–G–GCE (curve d), while the corresponding oxidation peak of Cu(II)/Cu(III) within +0.40 to +0.80 V is not very clearly observed at the CuO–G–GCE. This wave might correspond to a Cu(II)/Cu(III) redox couple similar to the previous reports [22,38,39]. When 2 mM glucose was present, an obvious oxidation peak corresponding to the irreversible glucose oxidation was observed on both CuO–GCE (Fig. 2B, curve b and f) and CuO–G–GCE (Fig. 2B, curve d and h). The oxidation of glucose started at approximately + 0.25 V, with a peak at about + 0.55 V. As can be seen in Fig. 2A, in that potential range (corresponding to glucose oxidation) no similar electrochemical response was observed at four electrodes in 0.1 M NaOH, in the absence of glucose. Especially, the oxidation peak current of glucose remarkably increased at CuO–G–GCE (approximate two-fold larger than that of CuO–GCE), indicating that graphene sheets can accelerate the electron transfer rate. In order to provide more evidence, the effect of the scan rate on the performance of CuO–G–GCE was also studied. As shown in Fig. 3, in the scan rate range of 25–300 mV s − 1, the oxidation peak current increased linearly with increasing scan rate, implying that the electrochemical oxidation of glucose on CuO–G–GCE is a surfacecontrolled electrochemical process [40].

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Fig. 2. Cyclic voltammograms of GCE (a), CuO–GCE (b), G–GCE (c), and CuO–G–GCE (d) in 0.1 M NaOH solution in the absence (A) or presence (B) of 2 mM glucose at 100 mV s− 1, respectively. Inset in Fig. 2B: cyclic voltammograms of CuO–GCE in the absence (e) or presence (f) of 2 mM glucose; cyclic votammograms of CuO–G–GCE in the absence (g) or presence (h) of 2 mM glucose, respectively. Fig. 1. SEM micrographs of graphene (A) and CuO–G nanocomposite (B) on GC substrate. TEM micrograph of graphene (C).

The mechanism of electrocatalytic oxidation of glucose in alkaline electrolyte at the CuO modified electrode is generally considered that the corresponding electrocatalytic oxidation process undergoes the following steps: Firstly, CuO is electrochemically oxidized to strong oxidizing agent Cu(III) species such as CuOOH• or Cu(OH)4− [41,42]: −

−

−

CuO þ OH →CuOOH þ e orCuOþH2 Oþ2OH →CuðOH Þ4

−

−

þe

ð4Þ

And then, glucose is catalytically oxidized by the Cu(III) species and produce hydrolyzate gluconic acid [43]. −

CuðIII Þ þ Glu cose þ e →Gluconolactone þ CuðIIÞ

ð5Þ

Gluconolactone→Gluconic acidðhydrolysisÞ

ð6Þ

It is obvious that the electrocatalytic oxidation of glucose at the CuO–G–GCE involves OH − ion. So, the electrolyte concentration for glucose sensing has been optimized in detail. The capabilities of electron transfer on different electrodes were investigated by EIS. EIS can give useful information of the impedance

Fig. 3. Cyclic voltammograms of CuO–G–GCE in 0.1 M NaOH containing 2 mM glucose at different scan rates from 25 to 300 mV s− 1. Inset shows the oxidation peak current vs. scan rate.

L. Luo et al. / Bioelectrochemistry 88 (2012) 156–163

changes on the electrode surface during the modification process [44]. Fig. 4 presents typical Nyquist plots obtained on four different electrodes. The impedance-plane plots in Fig. 4 can be characterized by two distinct regions: (i) a semicircle in the higher frequency range related to the charge transfer process, (ii) an inclined line in the complex-plane impedance plot defining a warburg region of semiinfinite diffusion of species to the modified electrode. In the equivalent circuit from the inset of Fig. 4, R, Q, Rct and Rw represent the resistance of the electrolyte solution, the value of constant phase element, the charge–transfer resistance and the warburg impedance, respectively. The Rct at the electrode surface can be quantified using the diameter of the semicircle in EIS [45]. From the Nyquist plots of the GCE, Nafion– GCE, G–GCE and CuO–G–GCE as shown in Fig. 4, the Rct of the four different electrodes were estimated to be 81.8 Ω, 8.1 × 104 Ω, 103.8 Ω and 115.3 Ω, respectively. There was a very small semicircle domain on the bare GCE (curve a), implying a very low Rct to the redox-probe dissolved in the KCl solution. However, the diameter of the semicircle at high frequency increased at the surface of Nafion modified the electrode (curve d). After been modified with the G and CuO–G nanocomposite on GCE, the plots exhibited much smaller semicircle in the high frequency region and steeper straight line in the low frequency region (curve b and c). The results indicate that the doping of graphene sheets into Nafion layer can decrease the electron transfer resistance and improve the heterogeneous electron transfer process. To estimate the effective surface area of the CuO–G–GCE, 5 mM [Fe(CN)6] 3 −/4 − was used as a probe. CVs on CuO–G–GCE in a solution containing 5 mM [Fe(CN)6] 3 −/4 − and 0.1 M KCl were performed. For a reversible process under semi-infinite linear diffusion conditions at 25 °C, the effective surface area of the CuO–G–GCE was estimated according to the Randles-Sevcik equation [46]: −5

Ip ¼ 2:69  10

3=2

n

AD

1=2 1=2

v

C

159

[Fe(CN)6] 3 −/4 − (5 mM). The calculated value of A was 0.062 cm2 for the CuO–G–GCE.

ð7Þ

Where A is the effective surface area (cm 2), Ip is the peak current of the redox reaction of [Fe(CN)6] 3 −/4 − (A), n is the number of electrons transferred (1), D is the diffusion coefficient (0.76 × 10− 5 cm2 s − 1, 25 °C), v is the scan rate (V s− 1) and C is the concentration of

Fig. 4. Nyquist plots at GCE (curve a), G–GCE (curve b), CuO–G–GCE (curve c) and Nafion–GCE (curve d) in 0.1 M KCl solution containing 5 mM [Fe(CN)6]3 −/4 −. Inset in the top right corner is the equivalent circuit. R, Q, Rct and Rw represent the resistance of the electrolyte solution, the value of constant phase element, the charge–transfer resistance and the warburg impedance, respectively. The frequency range is from 0.1 Hz to 100 kHz.

Fig. 5. Effects of deposition time (A), NaOH concentration (B) and applied potential (C) on the amperometric response of the CuO–G–GCE towards 0.1 mM glucose, respectively. Error bars represent the standard deviation of 3 independent experiments.

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3.3. Optimization of the sensing conditions The effect of the Cu electrodeposition time on the G–GCE was studied in 0.1 M NaOH containing 0.1 mM glucose. As shown in Fig. 5A, the peak current response increased with the increment of Cu electrodeposition time and achieved a maximum value at 120 s. With the increasing of Cu electrodeposition time from 120 to 210 s, the current decreased gradually. Therefore, an optimized Cu electrodeposition time (120 s) was used in subsequent experiments. The pH value usually has a great influence on the electrocatalytic oxidation of glucose by CuO, and therefore the effect of NaOH concentration on the response of glucose at the CuO–G–GCE was investigated as shown in Fig. 5B. The current response of the CuO–G–GCE increased from 0.01 to 0.1 M of NaOH. Then, the current response decreased and the background current increased in higher NaOH concentrations. To obtain optimal amperometric response to glucose, the effect of different applied potentials on the response current of the sensor was also investigated (Fig. 5C). The oxidation current of glucose increased with the increasing of the potential from +0.40 to +0.55 V. However, current response decreased in the potential range from +0.55 to +0.65 V.

Therefore, a constant potential of +0.55 V was chosen for further amperometric investigations.

3.4. Amperometric response of the CuO–G–GCE Fig. 6A displays the typical current–time plot for the CuO–G–GCE upon successive addition of 0.2 mM glucose in 0.1 M NaOH solution at + 0.55 V. As can be seen from Fig. 6A, the current response of the CuO–G–GCE increased along with glucose concentration increment. The inset a of Fig. 6A shows the sensitive current response to 2 μM glucose at the proposed sensor. Also, the proposed sensor reached steady-state current within 5 s as shown in the inset b of Fig. 6A, which is much faster than the reported results of GOx-based biosensors [47]. Fig. 6B shows the calibration curve of glucose at the CuO–G–GCE. The proposed electrode gave a linear response to glucose in the range from 2 μM to 4 mM with a correlation coefficient of 0.998. The current response exhibited a linear dependence on glucose concentration which regression equation was Ip /μA = (12.68 ± 0.31) / μA + (84.29 ± 2.67) [glucose] /μA mM− 1, R2 = 0.997. The electrode has a high sensitivity of 1360 μA mM− 1 cm− 2 and a low detection limit 0.7 μM (S/N= 3) for glucose sensing. For comparison, the performances of the CuO–G–GCE and other glucose sensors reported in literature have been listed in Table 1. As can been seen in Table 1, the developed CuO–G–GCE exhibits a higher sensitivity to glucose sensing than that of Nafion/CuO/GCE (404.5 μA mM − 1 cm − 2) [21] and Cellulose/MWCNT/GOx/GCE (6.57 μA mM − 1 cm − 2) [51]. At the same time, the proposed sensor shows a range of 0.002–4 mM, which is wider than that of Nafion/CuO/GCE (0.05–2.55 mM) and Cellulose/ MWCNT/GOx/GCE (0.05–1.0 mM). The high sensitivity and wide linear range of the CuO–G–GCE could be attributed to the excellent catalytic activity and large active surface area of the CuO–G nanocomposite. 3.5. Interference study The interference from electroactive compounds normally coexisting with glucose in real samples (e.g. human blood) such as AA, UA, DA, Gal, Man, Lac, Na3C6H5O7•2H2O, KH2PO4 and NaCl, may cause accuracy problems in the glucose determination [22]. Under optimal experimental conditions, the potential interferences have been carefully examined (as shown in Fig. 7A and B). The tolerance limit was taken as the physiological level of the foreign substances, which caused an approximately ±5% relative error in the determination. The influences of the interferences on the glucose current response are shown in Table 2. The results show that the foreign substances did not interfere significantly on the glucose determination, i.e., the CuO–G–GCE presented good selectivity toward glucose detection.

3.6. Reproducibility and stability

Fig. 6. Chronoamperometric current responses of CuO–G–GCE on successive addition of 0.2 mM glucose to 0.1 M NaOH (A), Insets in Fig. 6A: the current response towards 2 μM glucose (a); The response time of CuO–G–GCE to achieve steady-state current (b). Calibration curve of concentration versus peak current of glucose, the calibration equation: Ip /μA = (12.68 ± 0.31) /μA + (84.29 ± 2.67) [glucose] /μA mM− 1 with the slope of (84.29 ± 2.67) μA mM− 1 (B). Error bars represent the standard deviation of 3 independent experiments.

The batch-to-batch reproducibility at three individually modified electrodes gave the relative standard deviation (R.S.D.) of 4.36% for amperometric response to glucose, indicating high reproducibility of the sensor. The reproducibility of a single electrode was measured with the same CuO–G–GCE by 10 times successive amperometric measurements and yielded a R.S.D. of 1.94%. The long-term stability of the sensor was also evaluated by measuring the current response to glucose within a 14-day period (as shown in Fig. 8). The results show that the CuO–G–GCE retained 85% of its initial current response to glucose after stored under chamber conditions which could be mainly ascribed to the chemical stability of CuO [57]. In addition, the steadystate response current of the CuO–G–GCE only decreased less than 5% after 3000 s continuous measurement (see the inset of Fig. 8). These results suggest that the CuO–G–GCE has a good reproducibility and high stability for sensing glucose.

L. Luo et al. / Bioelectrochemistry 88 (2012) 156–163

161

Table 1 Analytical performances of different nanomaterials-based glucose sensors. Electrodes

Applied potential (V, vs. Ag/AgCl)

Sensitivity (μA mM− 1 cm− 2)

Linear range (mM)

Detection limit (μM)

Cuo–G–GCE (this work) GOx–graphene–chitosan [48] GOx–graphene GCE [49] GOx/MCM-41/Nafion/GCE [50] Cellulose/MWCNT/GOx/GCE [51] Ti/TiO2 nanotube array/Ni composite electrode [24] Cu–MWCNTsa–GCE [17] Cu NPsb–ZnO–ITOc [23] CuO nanowire Cu electrode [22] Cu NPs/SWCNTd [52] CuO nanorods [53] Nafion/CuO/GCE [21] CuO flowers [53] Cu nanocubes/MWCNTs [54] OMCe/GCE [55] NiO/MWCNTs/GCE [56]

+ 0.59 – − 0.43 0.39 − 0.50 + 0.59 + 0.65 + 0.80 + 0.33 + 0.65 + 0.60 + 0.60 + 0.60 + 0.55 0.45 0.60

1360 37.93 110 – 6.57 200 253.0 0.16 μA μM− 1 0.49 μA μM− 1 256 μA mM− 1 371.4 404.5 709.5 1096 10.81 μA mM− 1 –

0.002–4 0.08–12 0.1–10 0.32–15.12 0.05–1.0 0.1–1.7 0.0007–3.5 0.001–1.53 0.0004–2 0.00025–0.5 0.004–8 0.05–2.55 N/a ~ 7.5 0.5–2.5 0.2–12

0.7 20 10 180 – 4 0.21 0.2 0.049 0.25 4 1.0 4 1.0 20 160

a b c d e

Multi-walled carbon nanotubes. Nanoparticles. Indium tin oxide. Single-walled carbon nanotubes. Ordered mesoporous carbon.

3.7. Application of the proposed sensor for glucose determination in human blood serum samples To evaluate the applicability of the CuO–G–GCE in real samples, it was applied to determine glucose in human blood serum samples. In this case, the serum sample was diluted 100 times with 0.1 M NaOH solution, and measured under optimal experimental conditions. Table 3 displays the determination results from four different serum samples. The results are comparable with that measured by a hospital-used instrument (CA-958 H rapid semi-automatic biochemical analyzer). 4. Conclusions In summary, a simple electrochemical deposition method has been used for fabricating CuO nanocubes–graphene nanocomposite modified GCE which can be employed for amperometric sensing glucose. The CuO-modified graphene displayed substantially higher electrocatalytic activity and faster response to glucose oxidation with a higher current response than the unmodified graphene or CuO. This CuO–G-based electrochemical sensor has a low detection limit of 0.7 μM and a very high sensitivity of 1360 μA mM − 1 cm − 2, and its response is linear up to 4.0 mM glucose concentration. When these superior performance characteristics are combined with long-term stability, good reproducibility and excellent specificity to glucose in the presence of common interferents, the CuO–G–GCE is a potential Table 2 Effects of interfering species on glucose determination by the CuO–G–GCE.

Fig. 7. Interference test of CuO–G–GCE in 0.1 M NaOH at + 0.55 V with 1 mM glucose in the presence of (A): 0.05 mM AA, UA, DA, Gal, Man and Lac and (B): 0.2 mM Na3C6H5O7•2H2O, KH2PO4 and NaCl, respectively.

Interfering species

Glucose:interfering species (mole ratio)a

Current ratio (%)

AA UA DA Gal Man Lac Na3C6H5O7·2H2O KH2PO4 NaCl

20:1

6.03

20:1

3.67

20:1

4.33

20:1

4.59

20:1

4.72

20:1

2.36

5:1

3.70

5:1

2.62

5:1

2.62

a

The normal physiological level of glucose is 3–8 mm and the interfering species (e.g. AA, UA, DA, Gal, Man and Lac) are about 0.1 mm, with a glucose: interfering species ratio of more than 30:1 [22]. therefore, the mole ratio of 20:1 was used.

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Fig. 8. Stability tests of the CuO–G–GCE. The concentration of glucose was fixed at 0.1 mM. Inset: the chronoamperogram for 1 mM glucose in 0.1 M NaOH at + 0.55 V over a long period of operational time for 3000 s.

Table 3 Determination of glucose concentration in the human blood serum sample. Sample

Measured by the developed electrode (mM)

Measured by the hospital-used instrument (mM)

R.S.D.% (n = 3)

Accuracy (%)

1

6.57

6.5

1.2

101.1

2

4.77

4.9

4.9

97.3

3

4.85

5.1

2.0

95.1

4

5.80

5.8

4.8

100.0

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163 Liqiang Luo, Associate professor of analytical chemistry in Shanghai University. He received his BS and MS degree from Henan Normal University, China, in 1994 and 1997, respectively; and PhD degree from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, China, in 2000. His research interests concern fuel cell, nanoelectrochemistry and biosensors.

Limei Zhu, received her BS degree in Qingdao University of Science and Technology in 2008, and will obtain her MS degree in analytical chemistry in 2012 from Shanghai University, China. Her current interests include the development of biosensor and modified electrode.

Zhenxin Wang, Professor in State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. He received his BS and MS degree from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. China, in 1997 and 2000, respectively. His research interests are the synthesis and modification of precious metal nanoparticles; biological chip based on nanoparticles marker; nanoparticles surface enzyme process and drug screening.