octahedral Cu2O nanocomposites for non-enzymatic glucose and hydrogen peroxide amperometric sensor

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Carbon quantum dots/octahedral Cu2 O nanocomposites for non-enzymatic glucose and hydrogen peroxide amperometric sensor Yancai Li a,b,∗ , Yanmei Zhong a , Yayun Zhang a , Wen Weng a,b , Shunxing Li a,b,∗ a b

College of Chemistry & Environment, Minnan Normal University, Zhangzhou 363000, China Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology, Minnan Normal University, Zhangzhou 363000, China

a r t i c l e

i n f o

Article history: Received 1 May 2014 Received in revised form 20 July 2014 Accepted 7 September 2014 Available online xxx Keywords: Carbon quantum dots Octahedral cuprous oxide Glucose Hydrogen peroxide Non-enzymatic sensor

a b s t r a c t Non-enzymatic electrochemical sensors for the detection of glucose and hydrogen peroxide (H2 O2 ) were designed based on novel nanostructure electrocatalyst of carbon quantum dots (CQDs)/octahedral cuprous oxide (Cu2 O) nanocomposites. The CQDs/octahedral Cu2 O nanocomposites has been smoothly by a facile method with ultrasonic treatment and the morphologies of the synthesized materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) with Energy dispersive X-ray spectroscopy (EDS) and powder X-ray diffraction measurements (XRD). Compared to octahedral Cu2 O, the CQDs/octahedral Cu2 O exhibited preferable electrocatalysis to the glucose oxidation and H2 O2 reduction. Amperometric sensing of glucose was realized with a linear response range from 0.02 to 4.3 mM, a detection limit of 8.4 ␮M (S/N = 3). The interferents of ascorbic acid (AA), uric acid (UA), dopamine (DA) and sodium chloride (NaCl) was also detected using the CQDs/octahedral Cu2 O modified electrode, the results showed good selectivity for glucose detection. Besides, the nonenzymatic sensor also has good performance to the electrocatalytic reduction of H2 O2 , with a linear response range from 5 ␮M to 5.3 mM and a detection limit of 2.8 ␮M (S/N = 3). The CQDs/octahedral Cu2 O nanocomposites have good selectivity for the H2 O2 detection with the AA, NaCl and UA. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Recently, as a novel class of discovered nanocarbons, carbon quantum dots (CQDs) have attracted tremendous attention because of abundant physical or chemical properties, such as water solubility, nontoxicity, photoinduced electron transfer, redox properties, luminescence and high stability [1,2]. In particular, CQDs are usually coated with hydroxyl and carboxyl, which can greatly enhance the water solubility and biocompatibility. In addition, CQDs are biocompatible, small, and have low molecular weight and toxicity, which makes them superior to other quantum dots [3,4]. Thus, CQDs was an upsurge in many areas, such as photocatalysis [5,6], fluorescent sensor [7], peroxidase-like catalysts for the colorimetric detection of glucose [8,9] and chemical imaging [10]. However, using the CQDs in electrochemical sensors to promote the redox reaction processes and electrochemical detection of glucose and hydrogen peroxide (H2 O2 ) still remains in the early stage [11].

∗ Corresponding authors at: College of Chemistry & Environment, Minnan Normal University, Zhangzhou 363000, China. Tel.: +86 596 2591445; fax: +86 596 2520035. E-mail addresses: [email protected] (Y. Li), [email protected] (S. Li).

Cuprous oxide (Cu2 O), as a p-type semiconductor with direct band gap of 2.17 eV, possesses wide applications in various fields [12,13], such as photochemical catalysis [14], biosensing [15], gas sensor [16,17], electrochemical sensing [18–20] and solar/photovoltaic energy conversion [21]. The extensive applications of Cu2 O attributed to its unique optical and electrical properties, handy synthesis method, low cost, and low toxicity [22]. In addition, the electrochemical performance of Cu2 O also depends on its morphology. That is to say the shape effect mainly originates from the different crystal planes exposed on the nanocrystal surface. Generally, nanocrystals with high-index facets are found to demonstrate dramatic high catalytic activity [23,24]. However, nanocrystals with high-index facets commonly have a high surface energy, which will lead to possible structural instability and decrease relevant repeated utilization of their activity [25]. On the contrary, low-index planes of {1 1 1}-crystal face possess good electrochemical performance and stability in all low-index planes [26–29]. To date, diabetes is a common chronic disease, which is a world concern health problem resulting from insulin secretion delayed or released [19]. In order to best control their glucose levels and prevent complications like kidney failure or nerves and blood vessels, various effective analytical methods have been developed

http://dx.doi.org/10.1016/j.snb.2014.09.016 0925-4005/© 2014 Elsevier B.V. All rights reserved.

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for the measurement of glucose, including electrochemical [30,31] and fluorescent techniques [32]. Among these methods, the electrochemical method is based on enzyme sensor and non-enzyme sensor. In the past decades, studies on this subject mainly focused on the use of the glucose oxidase [33–35], which has the advantages of high sensitivity and good selectivity. However, the enzyme glucose sensors often suffer from unstable response and poor reproducibility [36]. To resolve this problem, various nanomaterials with different structures have been synthesized and applied to the non-enzyme glucose sensor [19,37,38]. Due to their large surface volume ratio, excellent conductivity and extraordinary electrocatalytic activities, nanomaterials, including nano-metals, metal oxides, carbon materials and so on, have been become competitive candidates [39]. On the other hand, as a common oxidizing agent and an essential intermediate, H2 O2 widely exist in biomedical, pharmaceutical, industrial and environmental protection and enzymatic reactions [40–42]. In conclusion, the accurate, reliable and rapid measurement of glucose and H2 O2 are necessary and important. Herein, combining with the good electrochemical performance of the low-index planes of (1 1 1)-octahderal Cu2 O and the water solubility and biocompatibility of CQDs, we prepared nonenzymatic electrochemical sensors for the detection of glucose and H2 O2 based on the octahderal Cu2 O and the CQDs/octahedral Cu2 O nanocomposites. The CQDs/octahedral Cu2 O nanocomposites exhibited excellent electrocatalytic performance to glucose oxidation and H2 O2 reduction compared with the only octahedral Cu2 O. The experimental results also showed that the nanocomposites not only played an essential role in the enhanced electrocatalytic behavior, but enhanced sensitivity and stability, which can be attributed to the synergistic effect of the CQDs and the low-index planes of (1 1 1)-octahderal Cu2 O. 2. Experimental 2.1. Reagent and materials Copper (II) chloride dehydrate (CuCl2 ·2H2 O, 99.0%), sodium dodecyl sulfate (SDS, 98.5%) and hydroxylamine hydrochloride (NH2 OH·HCl, 99.99%) were purchased from Aladdin industrial. Glucose, ascorbic acid (AA) was obtained from Beijing Chemical Company (Beijing, China); uric acid (UA) was purchased from Sangon Biotech (Shanghai, China) Co. Ltd.; dopamine (DA), Nafion (5 wt%) were obtained from Sigma–Aldrich. The 0.1 M sodium hydroxide (NaOH) was employed as the supporting electrolyte. All other chemicals were of analytical grade and were used without further purification. All solutions were made up with doubledistilled water. 2.2. Apparatus Scanning electron microscopy (SEM) was provided with Hitachi S-4800; transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy (EDS) was performed on a FEI Tecnai G2F20 electron microscope and operated at 200 kV with the software package for automated electron tomography. Powder X-ray diffraction measurements (XRD) were recorded on a Panaltical X’Pert-pro MPD X-ray power diffractometer, using Cu Ka radiation. The electrochemical experiments were carried out with a CHI 660C electrochemical workstation (Shanghai Chenhua instrument Co., LTD.) with a conventional three-electrode cell. The modified or unmodified Glassy carbon electrode (GCE, 3 mm diameter) was used as the working electrode; the Pt wire and Ag/AgCl (3.0 M KCl) electrode were used as the counter and reference electrodes, respectively.

2.3. Preparation of the octahedral Cu2 O and the CQDs/octahedral Cu2 O nanocomposites The CQDs/octahedral Cu2 O nanocomposites were synthesized by a hydrothermal method and ultrasonic treatment according to anteriorly reported methods with minor modification [43]. Typically, 1 mL 0.1 M CuCl2 solution and 2 mL 1.0 M NaOH solution were added into 91 mL ultrapure water under vigorous stirring and then light blue flocculent precipitate formed immediately. Next, 0.883 g SDS powder was added with vigorous stirring of the beaker until dissolution of the powder (the solution A). And then, 6 mL 0.2 M NH2 OH·HCl was mixed with the solution A and obtained the solution B. We get the solution B slowly added into 50 mL 1.0 M NaOH and 50 mL 1.0 M glucose mixed liquor which has been ultrasonic treatment for 10 min after that the solution B was aged for 1 h, followed by keeping ultrasonic 50 min. Finally, the samples were centrifuged at 5000 rpm for 3 min (Rotina 420R centrifuge) and obtained precipitates. The brick-red precipitates (CQDs/octahedral Cu2 O) were centrifuged twice more in ethanol and were dispersed in 1 mL of ethanol for the following experiments. In addition, the octahedral Cu2 O was acquired by the solution B aged 2 h without adding 25 mL 1.0 M NaOH and 25 mL 1.0 M glucose mixture. The collection method is as well as the way to collect the CQDs/octahedral Cu2 O.

2.4. Preparation of the CQDs/octahedral Cu2 O/Nafion/GCE and the octahedral Cu2 O/Nafion/GCE GCE was polished before each experiment with 1, 0.3 and 0.05 ␮m alumina powder, respectively, rinsed thoroughly with ethanol and double distilled water. The CQDs/octahedral (7.2 mg) were dissolved in a mixture of 0.1 mL Nafion and 0.9 mL doubledistilled water. Under ultrasonic mixing for a few minutes, a brick-red suspension was obtained. Then, 10 ␮L octahedral CQDs/Cu2 O mixture was coated on a cleaned GCE respectively and allowed to dry in air to fabricate the CQDs/octahedral Cu2 O/Nafion/GCE. A similar procedure was employed to fabricate the octahedral Cu2 O/Nafion/GCE.

3. Results and discussion 3.1. Characterization of the octahedral Cu2 O and the CQDs/octahedral Cu2 O nanocomposites The octahedral Cu2 O and the CQDs/octahedral Cu2 O nanocomposites were characterized by SEM, TEM and XRD. Fig. 1A and B displayed the SEM images of the octahedral Cu2 O and the CQDs/octahedral Cu2 O, which composed of uniform nanoparticle with an average size of 500 nm. Compared with the smooth surface of the octahedral Cu2 O, the surface of CQDs/octahedral Cu2 O is obviously coarse. This demonstrated that the CQDs may be primely coated on the octahedral Cu2 O surface. Fig. 1C shows TEM images of the CQDs/octahedral Cu2 O, confirmed that there are many CQDs which the diameters are less than 10 nm coated on the Cu2 O surface. The compositional analysis carried out by EDS measurements which showed the elemental presence of C (2.53%), O (6.22%) and Cu (91.28%) in the as-prepared CQDs/octahedral Cu2 O as shown in Fig. S1 (as shown in the Supplementary materials). XRD analysis is often used to determine the structure of the samples. As shown in Fig. 1D, there is a strong peak (1 1 1) of octahedral Cu2 O and CQDs/octahedral Cu2 O, which showed that the nanocrystals progressively formed with more {1 1 1} surfaces and the surface of Cu2 O coating on CQDs do not change the octahedral configurations.

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Fig. 1. SEM images of (A) octahedral Cu2 O; (B) CQDs/octahedral Cu2 O; (C) TEM images of CQDs/octahedral Cu2 O; (D) XRD pattern of octahedral Cu2 O (a) and CQDs/octahedral Cu2 O (b).

3.2. Electrochemical characterization Electrochemical impedance spectroscopy (EIS) can analyze the properties of the electrode surface during the modification process, so we determinated the EIS of the different electrodes with a frequency ranging from 0.1 to 100 kHz in 0.1 M KCl electrolyte solution containing 1 mM Fe(CN)6 4−/3− , as shown in the left of Fig. 2. The proposed equivalent circuits of the EIS data are also displayed in the right of Fig. 2, which correspond to the bare GCE (a), the octahedral Cu2 O/Nafion/GCE (b) and the CQDs/octahedral Cu2 O/Nafion/GCE(c). Rs , Rc and Rct are the solution, coating and charge transfer resistances, respectively. A constant phase element (CPE) is attributed to a charge transfer process replacing

the double layer capacity (Cdl). In equivalent circuits, CPEe and CPEc are the constant phase elements of the electrode and coating, respectively. In this circuit, the impedance of a Faradaic reaction consists of an active charge transfer resistance Rct and a specific electrochemical element of diffusion Rw (Warburg element). From the EIS curve and the fitting with the equivalent circuits, we can conclude that the octahedral Cu2 O nanoparticles and the CQDs/octahedral Cu2 O nanocomposites have been attached to the electrode surface and the electron transfer resistance of the CQDs/octahedral Cu2 O is much less than the octahedral Cu2 O. Therefore, the CQDs/octahedral Cu2 O/Nafion/GCE should have higher electrochemical activity than the bare GCE and the octahedral Cu2 O/Nafion/GCE.

Fig. 2. EIS and equivalent circuits of the EIS data of the bare GCE (a), the octahedral Cu2 O/Nafion/GCE (b) and the CQDs/octahedral Cu2 O/Nafion/GCE (c) in 0.1 M KCl electrolyte solution containing 1 mM Fe(CN)6 4−/3− and the applied ac frequency range: 0.1 Hz–100 kHz.

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Fig. 3. CVs of the bare GCE (A), the octahedral Cu2 O/Nafion/GCE (B) and the CQDs/octahedral Cu2 O/Nafion/GCE (C) in 5 mL 0.1 M NaOH in absence and presence of glucose; CVs for the above three electrodes (D) in 5 mL 0.1 M NaOH and 0.8 mM glucose, scan rate: 100 mV s−1 .

3.3. Electrocatalytic oxidation of glucose at the CQDs/octahedral Cu2 O/Nafion/GCE and the octahedral Cu2 O/Nafion/GCE Fig. 3A–C severally displays the cyclic voltammograms (CVs) of the bare GCE, the octahedral Cu2 O/Nafion/GCE and the CQDs/octahedral Cu2 O/Nafion/GCE in 0.1 M NaOH with and without the presence of glucose at 100 mV s−1 . Compared with the bare GCE, it can be seen that an obvious oxidation process started at circa +0.3 V and reached a peak at about +0.6 V on the both modified electrodes. The oxidation peak is due to the conversion of Cu(II) to Cu(III), which suggests strong electrocatalytic ability toward direct oxidation of glucose of the CQDs/octahedral Cu2 O and the octahedral Cu2 O. Some literature reported that the electrocatalytic activity of Cu2 O toward glucose oxidation in alkaline medium may be attributed to the involvement of Cu(II) and Cu(III) surface species [44–46]. The equation of the electrocatalytic oxidation mechanism can be expressed as follows: 2CuO + H2 O + 2e− → Cu2 O + 2OH−

CuO + OH− → CuOOH + e → Cu(OH)

− 4

(1)

orCuO + H2 O + 2OH−

−

+ e−

(2)

Cu(III) + glucose + e− → gluconolactone + Cu(II)

(3)

gluconolactone → gluconicacid

(4)

Besides, Fig. 3D shows the CVs of the bare GCE, octahedral Cu2 O/Nafion/GCE and the CQDs/octahedral Cu2 O/Nafion/GCE in 0.1 M NaOH containing with 0.8 mM glucose. Compared with the bare GCE, anodic peak current of octahedral Cu2 O/Nafion/GCE and CQDs/octahedral Cu2 O/Nafion/GCE apparently enlarged at 0.6 V. This illustrated that the CQDs/octahedral Cu2 O/Nafion/GCE and the octahedral Cu2 O/Nafion/GCE have electrocatalytic ability to glucose oxidation. Moreover, the oxidation current of the CQDs/octahedral Cu2 O/Nafion/GCE is higher than the octahedral Cu2 O/Nafion/GCE, this should be attributed to the covered CQDs which increased the specific surface area and the electron transfer ability of the Cu2 O. To sum up, the low-index planes of (1 1 1)octahderal Cu2 O present good electrocatalytic performance toward glucose and the CQDs greatly improve its performance. The effect of scan rate on the glucose oxidation was also investigated. Fig. S2A shows the CVs of the CQDs/octahedral Cu2 O/Nafion/GCE in 0.1 M NaOH containing 0.6 mM glucose at different scan rates. It is clear that both the oxidation and reduction currents increased with increasing the scan rate. Fig. S2B shows the anodic (at 0.5 V) currents linearly increase with the square root of the scan rate, Ipa (␮A) = (26.1002 ± 0.0002) − (10.9603 ± 0.0001)v1/2 (mV s−1 ), R = 0.994. This result indicating that the process of the CQDs/octahedral Cu2 O/Nafion/GCE toward glucose oxidation follows a diffusion-controlled process. Fig. 4A displays the amperometric response of the CQDs/octahedral Cu2 O/Nafion/GCE and the octahedral Cu2 O/Nafion/GCE to the successive addition of a certain amount of glucose in 5 mL 0.1 M NaOH solution at 0.6 V. The amperometric

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Fig. 4. (A) Amperometric response of the CQDs/octahedral Cu2 O/Nafion/GCE (a) and the octahedral Cu2 O/Nafion/GCE (b) upon successive additions of glucose to 5 mL 0.1 M NaOH at an applied potential of 0.6 V; (B) the corresponding calibration curves of oxidation currents vs. concentrations of glucose at the octahedral Cu2 O/Nafion/GCE (a) and the CQDs/octahedral Cu2 O/Nafion/GCE (b).

response is observed at the octahedral Cu2 O/Nafion/GCE (curve a, the response time 20 s); however, a larger and well-defined amperometric response can be observed at the CQDs/octahedral Cu2 O/Nafion/GCE (curve b, the response time 10 s). Fig. 4B shows the calibration curve of different concentrations of glucose on the two modified electrodes, respectively. From Fig. 4B, the CQDs/octahedral Cu2 O/Nafion/GCE showed a good linear response to glucose in the range from 0.02 to 4.3 mM and the low limit of detection (LOD) 8.4 ␮M, and the linear range of octahedral Cu2 O/Nafion/GCE is from 0.3 to 4.1 mM and the LOD is 128 ␮M. The linear equation can be expressed as: CQDs/octahedral Cu2 O/Nafion/GCE : I(␮A) = (0.296±0.001) + (21.072±0.002)C (␮M), R = 0.999

Octahedral Cu2 O/Nafion/GCE : I (␮A) = (1.591±0.002) + (17.072±0.002)C (mM), R = 0.998 It is obviously come to a conclusion that not only the linear range of the CQDs/octahedral Cu2 O/Nafion/GCE toward glucose is wider than the octahedral Cu2 O/Nafion/GCE, but also the response time is shorter and the detection limit is lower. Furthermore, the LOD and sensitivities of the CQDs/octahedral Cu2 O/Nafion/GCE are relatively superior to other non-enzymatic sensors composed of octahedral Cu2 O and other composite materials, which are summed up in Table 1. The excellent electrocatalytic performance of the CQDs/octahedral Cu2 O can be ascribed to the specific surface area of the nanocomposites and the synergistic effect between the CQDs and the low-index planes of (1 1 1)-octahderal Cu2 O. Here, we also investigated the interferences from AA, UA, DA, and NaCl toward the determination of glucose, which generally exist together with glucose in human blood. As shown in Fig. 5, we

Fig. 5. Amperometric response to addition of 0.1 mM glucose in 0.1 M NaOH solution, and following the 0.1 mM interferents of DA, AA, and UA of the CQDs/octahedral Cu2 O/Nafion/GCE under 0.6 V.

investigated the amperometric response upon addition of 0.1 mM glucose, 0.1 mM AA, 0.1 mM UA, 0.1 mM DA, 0.1 mM NaCl and 0.1 mM glucose to 5 mL 0.1 M NaOH solution at 0.6 V. There is no obvious current response observed with the addition of these interfering substances, however, an obvious current response with the addition of 0.1 mM glucose was appeared. The above experimental results indicate that the CQDs/octahedral Cu2 O/Nafion/GCE have a good selectivity to glucose determination. 3.4. Application of the glucose biosensor in real samples The application of the proposed biosensor was valuated for the determination of the concentration of glucose in four human serum samples and compared to the analytical results measured by the

Table 1 Analytical performances of the CQDs/octahedral Cu2 O/Nafion/GCE compared with other non-enzymatic glucose sensors based octahedral Cu2 O and other Cu2 O composites. Electrode materials

Sensitivity (␮A ␮M−1 cm−2 )

Linear range (␮M)

Detection limit (␮M)

References

Octahedral Cu2 O CQDs/octahedral Cu2 O Cu2 O/GNs Cu2 O nanocubes Cu2 O hollow nanocubes Cu–Cu2 O hollow microsphere Porous Cu2 O microcube Cu2 O NPs

0.241 0.298 0.285 0.2 0.0525 0.0336 0.011 0.19

300–4100 20–4300 300–3300 – 1–1700 220–10890 2–350 50–1100

128 8.4 3.3 5.9 0.87 0.05 1.3 47.2

This work This work [19] [19] [49] [50] [48] [47]

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Cu2 O + H2 O2 → 2CuO + H2 O

Table 2 Determination of glucose in human serum samples with the biosensor. Sample number

Given by hospital (mM)

Measured by biosensor (mM)

Relative error (%)

1 2 3 4

3.79 4.16 4.28 4.86

3.65 4.20 417 5.03

3.7 1.0 2.6 3.5

spectrophotometric method in the hospital. To ensure the glucose concentrations located in the linear range, 1.0 mL of the real serum samples were severally added into 9.0 mL of 0.1 M PBS. As shown in Table 2, the concentration of glucose in human serum samples determinated by the glucose biosensor was close to the value measured by spectrophotometric method. The experimental results demonstrated that this biosensor has a great potential for practical applications. 3.5. Electrocatalytic reduction of H2 O2 at the CQDs/octahedral Cu2 O/Nafion/GCE and the octahedral Cu2 O/Nafion/GCE Fig. 6A–C shows the CVs of the bare GCE, the octahedral Cu2 O/Nafion/GCE and the CQDs/octahedral Cu2 O/Nafion/GCE in the absence and presence of H2 O2 in N2 -saturated 0.1 M pH 7.4 PBS at scan rate of 100 mV s−1 . There is a strong redox couple between −0.3 and −0.02 V in the both modified electrodes, the anodic and cathodic peak can be ascribed to the oxidation of Cu2 O to CuO and the reduction of CuO to Cu2 O [47,48]. The reaction equation can be expressed as: 2CuO + H2 O + 2e → Cu2 O + 2OH−

(5)

(6)

Fig. 6D shows the CVs of the bare GCE, octahedral Cu2 O/Nafion/GCE and the CQDs/octahedral Cu2 O/Nafion/GCE in 0.1 M pH 7.4 PBS containing 0.6 mM H2 O2 . Compared with the bare GCE, when H2 O2 was added, the reduction peak currents of the both modified electrodes increased prodigiously. The results demonstrate that the as-synthesized octahedral Cu2 O and CQDs/octahedral Cu2 O have electrocatalytic activity for the H2 O2 reduction, and the electrocatalytic ability of the CQDs/octahedral Cu2 O is much better than the octahedral Cu2 O. The influence of scan rate on the current response of H2 O2 reduction was also studied as shown in Fig. S3. The currents of the redox peaks increased with the increase of scan rate and exhibited a linear response to the square root of the scan rate, (Ipa (␮A) = (92.0735 ± 0.0002) − (21.3278 ± 0.0001)v1/2 (mV s−1 ), R = 0.994; Ipc (␮A) = (−2.9205 ± 0.0002) + (23.6830 ± 0.0003)v1/2 (mV s−1 ), R = 0.999). This result indicating that the CQDs/octahedral Cu2 O/Nafion/GCE toward H2 O2 reduction follow a diffusioncontrolled process. The amperometric response of the CQDs/octahedral Cu2 O/Nafion/GCE to successive addition of H2 O2 into the stirred PBS solution was performed with different applied potential (data not shown). From the series amperometric I–t curves, the ratio of response current to background current, namely the ratios signal to noise increases first and then decreases and the maximum ratio occurs at −0.2 V. Therefore, an applied potential of −0.2 V was chosen as the working potential for further amperometric experiments. Fig. 7A shows I–t curves of the CQDs/octahedral Cu2 O/Nafion/GCE and the octahedral Cu2 O/Nafion/GCE in 0.1 M pH 7.4 PBS for the addition of different concentration of H2 O2 .

Fig. 6. CVs for the bare GCE (A), the octahedral Cu2 O/Nafion/GCE (B) and the CQDs/octahedral Cu2 O/Nafion/GCE (C) in N2 -saturated 0.1 M pH 7.4 PBS with absence and presence of H2 O2 , scan rate: 100 mV s−1 ; CVs for the above three electrodes (D) in 5 mL 0.1 M pH 7.4 PBS with 0.6 mM H2 O2 , scan rate: 100 mV s−1 .

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Fig. 7. A steady-state current–time response of the octahedral Cu2 O/Nafion/GCE (a) and the CQDs/octahedral Cu2 O/Nafion/GCE (b) upon successive additions of different concentration H2 O2 in N2 -saturated 0.1 M pH 7.4 PBS at an applied potential of −0.2 V; (B) the corresponding calibration curves of the octahedral Cu2 O/Nafion/GCE (a) and the CQDs/octahedral Cu2 O/Nafion/GCE (b) for the H2 O2 detection. Table 3 Analytical performances of the CQDs/octahedral Cu2 O/Nafion/GCE compared with other non-enzymatic H2 O2 sensors based on octahedral Cu2 O and other Cu2 O composites. Electrode materials

Sensitivity (␮A ␮M−1 cm−2 )

Linear range (␮M)

Detection limit (␮M)

References

Octahedral Cu2 O CQDs/octahedral Cu2 O Cu2 O-rGOop Cu2 O–rGOis Cu2 O–rGOpa Cu2 O MCs The Cu2 O nanowires CuO MWCNT/PANI PAn-PAA

0.087 0.13 0.0155 0.0195 0.0207 0.0506 0.745 – 0.7484 0.4175

10–4900 5–5300 100–9800 100–7800 30–12800 1.5–150 0.25–5000 5.0–180 7–2500 40–1200

6.4 2.8 79 55.5 21.7 1.5 0.12 1.6 2 20

This work This work [12] [12] [12] [51] [52] [53] [54] [55]

It is clear that both electrodes have amperometric response to H2 O2 , but the current response of the CQDs/octahedral Cu2 O/Nafion/GCE to H2 O2 is far more tremendous than that the octahedral Cu2 O/Nafion/GCE and the response time is faster as well. This also demonstrates that rapidly and sensitively response to H2 O2 reduction of the CQDs/octahedral Cu2 O/Nafion/GCE. The both calibration curves as shown in Fig. 7B, the H2 O2 concentration showed a terrific linear relationship on both electrodes. The linear relation equation can be expressed as: CQDs/octahedral Cu2 O/Nafion/GCE : I (␮A) = (0.785 ± 0.001) − (9.021 ± 0.001)C (mM), R = 0.999

Octahedral Cu2 O/Nafion/GCE : I (␮A) = (−0.185 ± 0.002) − (6.132 ± 0.002)C (mM), R = 0.998 From the linear equation, the CQDs/octahedral Cu2 O/Nafion/GCE displays the linear range from 0.005 to 5.3 mM for H2 O2 , and liner range of the octahedral Cu2 O/Nafion/GCE is between 0.01 and 4.9 mM. The sensitivities were respectively 0.13 and 0.087 ␮A ␮M−1 cm−2 and the LOD were 2.8 ␮M (S/N = 3) and 6.4 ␮M (S/N = 3) for H2 O2 detection on the CQDs/octahedral Cu2 O/Nafion/GCE and octahedral Cu2 O/Nafion/GCE. It is obviously that the LOD and sensitivities of the CQDs/octahedral Cu2 O/Nafion/GCE are correspondingly superior to octahedral Cu2 O/Nafion/GCE and other non-enzymatic H2 O2 sensors, which are summed up in Table 3. Fig. S4 shows amperometric response of the CQDs/octahedral Cu2 O/Nafion/GCE to successive additions of 0.1 mM H2 O2 , 0.2 mM UA, 1.0 mM AA, 0.4 mM NaCl and 0.1 mM H2 O2 in 0.1 M pH 7.4

PBS. The results show that these interfering substances do not present distinct additional current signals, that is to say, the CQDs/octahedral Cu2 O/Nafion/GCE has a good selectivity to H2 O2 . 3.6. Stability and selectivity of the CQDs/octahedral Cu2 O/Nafion/GCE It is well known that the selectivity and stability are also two important parameters for electrochemical sensors. From Fig. 5 and Fig. S4, we can conclude that the CQDs/octahedral Cu2 O/Nafion/GCE have good selectivity toward glucose and H2 O2. The CVs of the CQDs/octahedral Cu2 O/Nafion/GCE were performed for glucose oxidation (1st, 25th, 50th) in 5 mL 0.1 M NaOH solution and for H2 O2 reduction (1st, 25th, 50th) in 0.1 M pH 7.4 PBS as show in Fig. S5. The modified electrode was drying in the air, and glucose oxidation and H2 O2 reduction every week, as show in Fig. S6. The results strongly indicate that the low-index planes of (1 1 1)-octahderal Cu2 O which coated with CQDs is very stable for glucose oxidation and H2 O2 reduction, which suggests that the CQDs/octahedral Cu2 O/Nafion/GCE possesses good stability. 4. Conclusions In this work, we have successfully synthesized the CQDs/ octahedral Cu2 O nanocomposites with a straightforward method under ambient temperature. The synthesized nanomaterials can be used to fabricate the glucose and H2 O2 sensor. Compared to the octahedral Cu2 O/Nafion/GCE, the CQDs/octahedral Cu2 O/Nafion/GCE exhibit better linear response, lower detection limit, higher selectivity and wider detection range toward the electrocatalytic oxidation of glucose and the electrocatalytic reduction of H2 O2 . This can be ascribed to the coated CQDs of octahedral

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Cu2 O and the low-index planes of (1 1 1)-octahderal Cu2 O, which increases the electrocatalytic surface area and enhanced electrochemical stability and sensitivity. Moreover, the glucose and H2 O2 sensor based on the CQDs/octahedral Cu2 O nanocomposites possess excellent selectivity and stability. It is believe that the CQDs/octahedral Cu2 O nanocomposites would widely used in biological sensors and catalysts and the CQDs would play an important role in the next generation of non-enzyme sensor. Acknowledgements This work was supported by the National Natural Science Foundation of China (21175115), the Program for New Century Excellent Talents in University, Outstanding Youth Science Foundation of Fujian Province in China (No. 2010J06005), Natural Science Foundation of Fujian province in China (2012J05031 and 2012Y0065), and the Innovation Base Foundation for Graduate Students Education of Fujian Province. 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.2014.09.016. References [1] B. Zhu, S. Sun, Y. Wang, S. Deng, G. Qian, M. Wang, A. Hu, Preparation of carbon nanodots from single chain polymeric nanoparticles and theoretical investigation of the photoluminescence mechanism, J. Mater. Chem. C 1 (2013) 580–586. [2] H. Zhang, H. Huang, H. Ming, H. Li, L. Zhang, Y. Liu, Z. Kang, Carbon quantum dots/Ag3 PO4 complex photocatalysts with enhanced photocatalytic activity and stability under visible light, J. Mater. Chem. 22 (2012) 10501–10506. [3] S. Ray, A. Saha, N.R. Jana, R. Sarkar, Fluorescent carbon nanoparticles: synthesis, characterization, and bioimaging application, J. Phys. Chem. C 113 (2009) 18546–18551. [4] S.N. Baker, G.A. Baker, Luminescent carbon nanodots: emergent nanolights, Angew. Chem. Int. Ed. 49 (2010) 6726–6744. [5] B.Y. Yu, S.Y. Kwak, Carbon quantum dots embedded with mesoporous hematite nanospheres as efficient visible light-active photocatalysts, J. Mater. Chem. 22 (2012) 8345–8353. [6] H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C.H.A. Tsang, X. Yang, S.T. Lee, Water-soluble fluorescent carbon quantum dots and photocatalyst design, Angew. Chem. Int. Ed. 49 (2010) 4430–4434. [7] Y. Dong, R. Wang, G. Li, C. Chen, Y. Chi, G. Chen, Polyamine-functionalized carbon quantum dots as fluorescent probes for selective and sensitive detection of copper ions, Anal. Chem. 84 (2012) 6220–6224. [8] W. Shi, Q. Wang, Y. Long, Z. Cheng, S. Chen, H. Zheng, Y. Huang, Carbon nanodots as peroxidase mimetics and their applications to glucose detection, Chem. Commun. 47 (2011) 6695–6697. [9] X. Wang, K. Qu, B. Xu, J. Ren, X. Qu, Multicolor luminescent carbon nanoparticles: synthesis, supramolecular assembly with porphyrin, intrinsic peroxidase-like catalytic activity and applications, Nano Res. 4 (2011) 908–920. [10] X. Zhang, H. Ming, R. Liu, X. Han, Z. Kang, Y. Liu, Y. Zhang, Highly sensitive humidity sensing properties of carbon quantum dots films, Mater. Res. Bull. 48 (2013) 790–794. [11] A. Zhu, Q. Qu, X. Shao, B. Kong, Y. Tian, Carbon-dot-based dual-emission nanohybrid produces a ratiometric fluorescent sensor for in vivo imaging of cellular copper ions, Angew. Chem. Int. Ed. 51 (2012) 7185–7189. [12] F. Xu, M. Deng, G. Li, S. Chen, L. Wang, Electrochemical behavior of cuprous oxide-reduced graphene oxide nanocomposites and their application in nonenzymatic hydrogen peroxide sensing, Electrochim. Acta 88 (2013) 59–65. [13] Z. Yan, J. Zhao, L. Qin, F. Mu, P. Wang, X. Feng, Non-enzymatic hydrogen peroxide sensor based on a gold electrode modified with granular cuprous oxide nanowires, Microchim. Acta 180 (2012) 145–150. [14] W.C. Huang, L.M. Lyu, Y.C. Yang, M.H. Huang, Synthesis of Cu2 O nanocrystals from cubic to rhombic dodecahedral structures and their comparative photocatalytic activity, J. Am. Chem. Soc. 134 (2012) 1261–1267. [15] H. Zhu, J. Wang, G. Xu, Fast synthesis of Cu2 O hollow microspheres and their application in DNA biosensor of hepatitis B virus, Cryst. Growth Des. 9 (2008) 633–638. [16] S. Deng, V. Tjoa, H.M. Fan, H.R. Tan, D.C. Sayle, M. Olivo, S. Mhaisalkar, J. Wei, C.H. Sow, Reduced graphene oxide conjugated Cu2 O nanowire mesocrystals for high-performance NO2 gas sensor, J. Am. Chem. Soc. 134 (2012) 4905–4917. [17] F. Zhang, Y. Li, Y.e. Gu, Z. Wang, C. Wang, One-pot solvothermal synthesis of a Cu2 O/graphene nanocomposite and its application in an electrochemical sensor for dopamine, Microchim. Acta 173 (2011) 103–109.

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Biographies Yancai Li graduated in Chemistry College from Jilin University in China and obtained Doctor Degree in 2007, is associate professor of College of Chemistry and Environment in Minnan Normal University and Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology since 2012. Her main research interests are focus on electrochemical sensors and biosensor. Yanmei Zhong is currently a graduate student of College of Chemistry and Environment Science, Minnan Normal University, China. The current research interest is synthesis of nanomaterials and its application in electrochemical sensors. Yayun Zhang is currently a graduate student of College of Chemistry and Environmental Science, Minnan Normal University, China. The current research interest is synthesis of nanomaterials and its application in electrochemical sensors. Wen Weng is a professor in Department of Chemistry and Environment Science, Minnan Normal University. His research interest is the preparation of novel functional material for bioanalysis. Shunxing Li graduated in Environmental Science from Wuhan University in China and obtained PhD in 2003, is full professor of Environmental Science and Analytical Chemistry, Head of Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology at Minnan Normal University since 2006. His main research interests are focus on (a) visible light-induced photocatalysis and photoelectrocatalysis and their application on sample pretreatment for the analysis and removing of pollutants and (b) metal species and bioavailability in environment, food, and herbal plants.

Please cite this article in press as: Y. Li, et al., Carbon quantum dots/octahedral Cu2 O nanocomposites for non-enzymatic glucose and hydrogen peroxide amperometric sensor, Sens. Actuators B: Chem. (2014), http://dx.doi.org/10.1016/j.snb.2014.09.016