A highly stable and effective electrochemiluminescence platform of copper oxide nanowires coupled with graphene for ultrasensitive detection of pentachlorophenol

Sensors and Actuators B 222 (2016) 747–754

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

A highly stable and effective electrochemiluminescence platform of copper oxide nanowires coupled with graphene for ultrasensitive detection of pentachlorophenol Wenqun Wu a,c , Hua Xiao a,c , Shenglian Luo a,c,∗ , Chengbin Liu a,c,∗ , Yanhong Tang b , Liming Yang a,c a State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China b College of Materials Science and Engineering, Hunan University, Changsha 410082, PR China c Hunan Provincial Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing Carbon-dioxide Emissions, Hunan University, Changsha 410082, PR China

a r t i c l e

i n f o

Article history: Received 27 June 2015 Received in revised form 20 August 2015 Accepted 1 September 2015 Available online 3 September 2015 Keywords: Stability Copper oxide nanowires Electrochemiluminescence Pentachlorophenol detection

a b s t r a c t A highly stable and effective electrochemiluminescence (ECL) sensing platform of copper oxide nanowires coupled with reduced graphene oxide (CuO NWs/rGO) is presented for ultrasensitive detection of pentachlorophenol (PCP). The CuO NWs/rGO sensing system is prepared via an electrodeposition technique followed by chemical oxidation and annealing processes. The CuO nanowire is revealed to be electroluminescent for the first time, and the rGO greatly enhances the ECL signal. In the presence of the coreactant S2 O8 2− , the CuO NWs/rGO-based ECL sensor can sensitively and selectively detect PCP with a wide linear range from 1.0 × 10−14 to 1.0 × 10−9 mol L−1 and a very low detection limit of 0.7 × 10−14 mol L−1 . The sensor shows excellent recyclability and outstanding durability evidenced by its nearly unchanged ECL signal after the sensing electrode being stored for 10 months in air at room temperature. The proposed ECL sensor could be a promising alternative method for the emergency and routine monitoring of the PCP in real environment. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Phenols and their derivatives are widely distributed throughout the environment. In particular, chlorinated phenols (CPs) are known to be a typical group of persistent organic pollutants in the environment, and several of them have been listed as priority pollutants [1,2]. Among them, pentachlorophenol (PCP) is one of the most potent carcinogens of the CPs family, and a broadspectrum biocide that has been widely used as wood preservative, pesticide, and disinfectant [3,4]. The content of PCP correlates well with the total CPs content in contaminated environmental samples, and thus, PCP is often monitored as a model compound for CPs in real environmental samples. The widespread use of PCP led it to be found not only in ecological environment throughout the world but also in human tissues [5,6]. PCP exposure was reported

∗ Corresponding authors at: State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China. E-mail addresses: [email protected] (S. Luo), chem [email protected] (C. Liu). http://dx.doi.org/10.1016/j.snb.2015.09.001 0925-4005/© 2015 Elsevier B.V. All rights reserved.

to be mutagenic and carcinogenic to human body [7], and thus, PCP was classified by IARC (International Association for Research on Cancer) as a group 2B environmental carcinogen and by the US EPA (Environmental Protection Agency) as a probable human carcinogen [3,5]. Moreover, the maximum contaminant level of PCP in drinking water was set by European Union and the US EPA (0.5 and 1 ␮g L−1 , respectively) [8], so an efficient monitoring of PCP at trace and ultratrace levels in drinking and natural waters is still a great challenge. Various analytical methods have been carried out in the last decades [9–14]. However, these methods are either complex, time-consuming sample pretreatment and sophisticated, expensive equipment, or require highly qualified personnel. Electrochemiluminescence (ECL) appears to be a newly powerful method of detection because ECL detection not only can offer inexpensive, portable instrumentation but also the ECL process shows low background noise and therefore ECL detection can potentially offer ultrasensitivity and low detection limit which mainly depend on the luminescent efficiencies of ECL materials [15,16]. In recent years, particular interest has been moved from traditional ECL materials, such as Ru(bpy)3 2+ [17] to nanocrystals (NCs) or quantum dots (QDs) (e.g., CdS NCs and Si QDs) [18,19]

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since the latter possess enhanced ECL efficiencies due to the quantum confinement effect and controllable surface chemistry. The NCs/QDs-based ECL studies, however, still suffer from some limitations, such as high toxicity of the most used ECL species (e.g., CdTe, CdS, and CdSe) [18,20–22], poor stability, and non-recycle of the ECL species in solutions [22–24]. Thus it is highly desirable to develop new platforms with low-toxic or non-toxic ECL species, outstanding stability and excellent recyclability for ECL detection. Clearly, CuO is a promising alternative on the basis of their excellent stability, environmental friendliness, and low cost, which has been widely used in high-performance lithium-ion batteries, photocatalysts, glucose sensors and so forth [25–28]. However, there is no report about its application in ECL. Moreover, the structure and morphology of ECL materials is one of the most important factors to determine their ECL properties. Compared to zero-dimensional (0D) nanostructures that show strong aggregation tendency, one-dimensional (1D) nanostructures can enhance the reproducibility and sensitivity of sensors because of their anisotropic shape and fully exposed surfaces for fast response to chemical environments [25,29]. Of particular interest for reproducibility, it is also necessary to immobilize ECL materials onto an appropriate substrate [30]. Graphene, with large surface area, high electrical conductivity, and excellent chemical stability, has been considered as an idea supporting substrate of catalysts for extensive applications [31,32]. Herein, we constructed reduced graphene oxide (rGO) supporting copper oxide nanowires (CuO NWs) (CuO NWs/rGO) as a novel platform for ECL sensing system. The immobilization of CuO NWs on rGO, on one hand, ensures recyclability of the ECL-based sensor; on the other hand further amplify the ECL signal of CuO NWs so as to promote the detection sensitivity. The CuO NWs/rGO hybrid was prepared through one-step electrochemical deposition of Cu nanoparticles and rGO on electrodes followed by chemical oxidation of Cu nanoparticles and annealing into CuO NWs. The proposed method to fabricate CuO NWs/rGO hybrid is simple and rapid, obtaining clean and binder-free ECL sensing electrodes. To evaluate the application of the CuO NWs/rGO-based ECL sensor, five typical phenols, including nonchlorinated phenols and chlorinated phenols, were tested. It is found that this CuO NWs/rGO-based ECL sensor shows highly selective and sensitive detection toward PCP with a detection limit as low as 0.7 × 10−14 mol L−1 , which is much lower than that of previously reported ECL sensors. Moreover, the ECL emission signal is highly stable even after the electrode was stored for 10 months in air at room temperature, which has never been demonstrated before for other ECL sensors. The CuO NWs/rGO-based ECL sensor provides a promising tool to monitor PCP in real water sample. 2. Experimental 2.1. Materials Titanium (Ti) foil (99.8%, 0.127 mm thickness) was purchased from Aldrich (Milwaukee, WI). Graphite oxide was synthesized from graphite powder by the Hummers’ method [33], and exfoliated in a 0.067 mol L−1 (pH 8.0) phosphate buffer by ultrasonication for 3 h to form a homogeneous graphene oxide (GO) dispersion with a concentration of 0.3 g L−1 . All other reagents were of analytical grade and used without further purification. Double distilled water was used throughout the experiments. 2.2. Apparatus Scanning electron microscopic (SEM) images were acquired with a JSM 6700F scanning electron microscope (JEOL, Japan).

Transmission electron microscopic (TEM) images were obtained on a JEM 3010 TEM system (JEOL, Japan). Powder X-ray diffraction (XRD) pattern was performed on a M21X X-ray diffractometer ˚ Raman spectra were measured with Cu K␣ radiation ( = 1.54 A). with an Advantage 200A Raman spectrometer with a 632.8 nm laser (DeltaNu). Electrochemical measurements were carried out on CHI 660C electrochemistry workstation (Chenhua Instrument Inc, China). 2.3. Preparation of CuO NWs/rGO hybrid Equimolar quantities of CuSO4 and disodium ethylenediamine tetraacetate (Na-EDTA) were mixed to obtain a Cu-EDTA solution. A Ti ribbon was ultrasonically cleaned in acetone and ethanol solution for 10 min, respectively. A hybrid of Cu nanoparticles and rGO (Cu NPs/rGO) was first electrodeposited on a Ti ribbon. Briefly, a dispersion containing 0.3 g L−1 GO solution and 1.0 × 10−3 mol L−1 Cu-EDTA was prepared. Then the cleaned Ti ribbon was immersed into the above dispersion under stirring and then subjected to cyclic voltammetric scanning from −1.4 V to +0.6 V at 50 mV s−1 for 10 cycles on a CHI 660C electrochemical workstation (Shanghai CH Instrument Co., China) using a conventional three-electrode system: a saturated calomel electrode (SCE) as the reference electrode, a platinum foil as the counter electrode, and the Ti ribbon (0.5 cm × 3.5 cm) as the working electrode. After the scanning, Cu NPs/rGO was electrodeposited on the Ti ribbon and then the modified electrode was thoroughly washed with double-distilled water and dried under room temperature. To obtain the CuO NWs/rGO, the Cu NPs/rGO modified Ti ribbon was immersed into 20 mL of 0.25 mol L−1 NaOH aqueous solution containing 0.01 mol L−1 K2 S2 O8 and 0.02 mol L−1 sodium dodecylsulfate at room temperature for 6 min. Then the products were heated at 120 ◦ C for 1 h and at 180 ◦ C for another 2 h in the air. After being cooled to ambient temperature, the CuO NWs/rGO was obtained. For comparison, neat CuO NWs and neat rGO on Ti ribbons were separately prepared using the same procedures and conditions. 2.4. ECL measurements The ECL of CuO NWs/rGO hybrid was measured in a 0.067 mol L−1 phosphate buffer (pH 7.0) containing 0.1 mol L−1 K2 S2 O8 on the three-electrode CHI 660C electrochemical workstation equipped with a MPI-E multifunctional chemiluminescent analyzer (Xi’an Rimax Electronics Co. Ltd., China). The photomultiplier tube voltage was biased at 800 V. During measurements, cyclic voltammetric potential from −2.0 V to +0.0 V supplied by the MPI-E electrochemical analyzer was applied to the CuO NWs/rGO working electrode (the reference electrode and the counter electrode are a SCE and a platinum foil, respectively). At the same time, the ECL emission was recorded by the MPI-E multifunctional chemiluminescence analyzer. 3. Results and discussion 3.1. Characterization The SEM image of Cu NPs/rGO displays a layered structure that Cu particles were distributed between the rGO sheets (Fig. S1, Supporting Information). Fig. 1a is a top-view SEM image of the CuO NWs/rGO hybrid, clearly showing the long crisscrossed CuO nanowires. Notably, the cross-sectional SEM image of CuO NWs/rGO reveals that besides the long nanowires, there are a large number of short nanowires growing erectly on the two sides of rGO sheets (Fig. 1a, inset). The intercalation of long and short nanowires

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Fig. 1. (a) Top-view SEM image of CuO NWs/rGO on a Ti ribbon, with its cross-sectional SEM image inset, (b) TEM image of CuO NWs/rGO, (c) TEM image of a CuO NW, and (d) HR-TEM image of the CuO NW shown in (c).

between rGO prevents the rGO sheets from conventional faceto-face tightly stacking but form an open structure with cavities between the sandwich-like CuO NWs/rGO/CuO NWs layers. In such a nanoarchitecture, rGO sheets will act as electrically conductive pathways and the void spaces between the layers will allow more CuO NWs accessible to electrolytes, ions, and molecules to favor the sensing application. TEM image discloses the presence of typical thin and transparent rGO sheets in the hybrid (Fig. 1b). Fig. 1c is the TEM image of a nanowire and its HR-TEM image presented in Fig. 1d demonstrates the fringe spacing of 0.234 nm, which is characteristic of the (111) plane of CuO [26]. The fact that the Cu NPs on the surface of rGO sheets was completely converted and crystallized into CuO via annealling in air was confirmed by XRD measurements (Fig. S2), where besides typical XRD peaks of Ti, CuO NWs/rGO modified Ti substrate only displays peaks that can be assigned to the monoclinic phase of CuO but without the peaks of Cu and Cu2 O [34]. Raman spectra of GO and the CuO NWs/rGO show D and G bands characteristic of graphene based materials (Fig. S3), and the ratio of the intensity of D to G band increases from 1.06 for the GO to 1.27 for the CuO NWs/rGO, consistent with the fact that rGO has higher reduction degree than GO. Additionally, there are two little peaks at 288 and 625 cm−1 in the spectrum of CuO NWs/rGO, and these peaks can be assigned to the Ag and Bg modes of monoclinic CuO, respectively (Fig. S3, inset) [28].

the ECL intensity of the CuO NWs (Fig. 2A(c)), and particularly, more remarkable increase of the ECL signal is observed for the CuO NWs/rGO (Fig. 2A(d)), about 3 times higher than that of CuO NWs under similar conditions. Thus, rGO could not only act as an immobilization support for CuO NWs but also serve as a good ECL signal amplifier. S2 O8 2− along with rGO amplifies the ECL intensity of CuO NWs nearly 120 times, and therefore high sensitivity can be expected for the CuO NWs/rGO-based ECL sensor. As shown in Fig. 2B, no obvious ECL signal for the bare Ti ribbon, remarkable signal for the CuO NWs, and very strong signal for the CuO NWs/rGO in 0.1 mol L−1 S2 O8 2− . The cyclic voltammograms of the electrodes in S2 O8 2− were recorded simultaneously (Fig. 2B, inset), and one distinct cathodic peak, attributing to the reduction of S2 O8 2− [36], was observed for both the CuO NWs and the CuO NWs/rGO electrodes but not for the bare Ti ribbon. The more positive potential and higher current of the reduction peak on CuO NWs/rGO indicate that rGO could accelerate electron transfer between the electrode and S2 O8 2− in solution, which contributes to enhancing the ECL intensity of CuO NWs/rGO. Fig. 2C shows the ECL emission of CuO NWs/rGO with S2 O8 2− under continuous potential scanning of 35 cycles. It was found that the ECL emission remained at a constant value, suggesting excellent stability of the sensing material and high repeatability of the constructed sensor, which will ensure the accuracy in the experimental analysis.

3.2. ECL properties of CuO NWs/rGO hybrid

3.3. ECL detection of PCP

For an ECL sensor, high ECL emission intensity is essential to achieving a high sensitivity. As demonstrated in Fig. 2A(a), no obvious ECL signal was detected for the bare Ti ribbon even in the presence of coreactant S2 O8 2− that commonly has been used to amplify the ECL signal [35], while the Ti ribbons modified with CuO NWs shows an ECL peak in the absence of S2 O8 2− (Fig. 2A(b)), meaning CuO NWs is an ECL material. S2 O8 2− can significantly enhance

The performance of ECL detection greatly depends on the ECL emission intensity and thus prior to the ECL detection of PCP, some experimental conditions influencing the ECL intensity were first optimized. Usually, the pH value of solution and concentration of the coreactant S2 O8 2− play a crucial role in ECL. As shown in Fig. S4a, the strongest ECL intensity was observed at pH 7.0. It is thought that in acidic solution, the reduction of proton to hydrogen would occur

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Fig. 3. (A) Effect of PCP concentration on ECL behavior of CuO NWs/rGO in 0.1 mol L−1 S2 O8 2− (pH 7.0) at different concentrations of PCP (×10−13 mol L−1 ): (a) 0.1, (b) 0.5, (c) 1, (d) 5, (e) 10, (f) 100, (g) 1000, (h) 10,000. (B) Calibration curve for PCP detection (error bars donate S.D., n = 3).

Fig. 2. (A) ECL intensity response of (a) bare Ti ribbon, (c) CuO NWs, and (d) CuO NWs/rGO in 0.067 mol L−1 phosphate buffer (pH 7.0) containing 0.1 mol L−1 S2 O8 2− , as well as (b) CuO NWs in 0.067 mol L−1 phosphate buffer (pH 7.0) without S2 O8 2− . (B) ECL-potential curves of (a) bare Ti ribbon, (b) CuO NWs, and (c) CuO NWs/rGO in 0.067 mol L−1 phosphate buffer (pH 7.0) containing 0.1 mol L−1 S2 O8 2− . Inset: cyclic voltammograms of (a) bare Ti ribbon, (b) CuO NWs, and (c) CuO NWs/rGO. (C) ECL emission from CuO NWs/rGO with 0.1 mol L−1 S2 O8 2− under continuous 35 cycles of cyclic voltammetry.

at the applied negative potential, which might inhibit the reduction of S2 O8 2− , while in a basic solution, the intermediate SO4 •− would be scavenged by OH•− , resulting in a decrease in the ECL intensity [24]. The effect of S2 O8 2− concentration is shown in Fig. S4b, where the ECL intensity increases sharply with the concentration of S2 O8 2− in the range of 0.01 to 0.1 mol L−1 . Further increase of S2 O8 2− concentration, the ECL intensity reaches the upper detection limit of the instrument, so 0.1 mol L−1 S2 O8 2− was selected to ensure a suitable sensitivity in the sensing system. Additionally, the

loading amount of the CuO NWs on rGO sheets was another parameter that affects the ECL intensity. Fig. S4c shows the ECL intensity of CuO NWs/rGO prepared by varying the amount of precursor CuEDTA at a fixed amount of GO. Higher Cu-EDTA concentrations would result in higher amount of CuO loading on the rGO, while it is found that the CuO overload cannot efficiently enhance the ECL intensity. Thus, 1.0 × 10−3 mol L−1 Cu-EDTA was chosen for the hybrid preparation. Fig. 3 shows the application of the CuO NWs/rGO-based ECL sensor in PCP determination under the optimized conditions. The ECL intensity decreases gradually with increasing PCP concentration (Fig. 3a). Taking the relative intensity change of (I0 − I)/I0 as the function of the concentration of PCP, where I0 is the initial intensity and I0 − I is the PCP-induced decrease in ECL intensity, the (I0 − I)/I0 exhibits a linear response toward the logarithm of the concentration of PCP in the range of 1.0 × 10−14 –1.0 × 10−9 mol L−1 (Fig. 3b) with a correlation coefficient of 0.998 and a limit of detection (LOD) of 0.7 × 10−14 mol L−1 (S/N = 3). The achieved LOD is low enough to detect ultratrace PCP in water samples, and notably, it is the lowest values reported to-date for the PCP determination. Table 1 lists the best results in recent years for PCP determination using various methods including ECL sensors. It is found that ECL sensors generally have higher sensitivity than other methods, whereas our proposed CuO NWs/rGO-based ECL sensor shows the lowest LOD value among the ECL sensors. Selectivity is another important index to evaluate the performance of a sensor. Herein, nonchlorinated phenols (p-nitrophenol (p-NP) and p-dihydroxybenzene (p-DOB)), chlorinated phenols (p-chlorophenol (p-CP), 2,6-dichlorophenol (DCP)), and other

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Table 1 Comparison of methods for determination of PCP. Materials

Method

CdSex Te1−x /TiO2 NT MIPb capped Mn-dopedZnS QDs TiO2 NPs ZnSe-CTAB Cys-Ag nanoparticle GQDs-CdS NCs CuO NWs/rGO a b c d e

a

PEC RTPc ECd EC SERSe ECL ECL

Linear range (mol L−1 ) −9

−6

1.0 × 10 –0.3 × 10 0.2 × 10−6 –3.9 × 10−6 1.5 × 10−6 –0.1 × 10−3 6.0 × 10−8 –8.0 × 10−6 0.5 × 10−6 –1.0 × 10−4 38 × 10−12 –1.9 × 10−6 1.0 × 10−14 –1.0 × 10−9

Detection limit (mol L−1 )

Ref.

1.0 × 10−12 86 × 10−9 1.0 × 10−8 1.0 × 10−8 0.2 × 10−6 11 × 10−12 0.7 × 10−14

[38] [12] [11] [39] [14] [40] This work

PEC: photoelectrochemical. MIP: molecularly imprinted polymer. RTP: room-temperature phosphorescence. EC: electrochemical. SERS: surface-enhanced Raman spectroscopy.

Table 2 Recovery study on PCP detection in water samples. Sample

PCP added (10−12 mol L−1 )

Our method (10−12 mol L−1 )

River water

0 5.0 10.0 15.0

1.7 7.0 12.4 16.2

Tap water

0 5.0 10.0 15.0

– 4.8 ± 0.4 9.5 ± 0.2 15.2 ± 0.1

± ± ± ±

GC–MS (10−12 mol L−1 )

0.2 0.5 0.1 0.1

1.9 6.6 11.5 16.8

± ± ± ±

Recovery (%)

0.1 0.3 0.3 0.2

– 102.0 105.0 95.3

– 4.7 ± 0.1 9.7 ± 0.2 14.5 ± 0.3

– 96.0 95.0 101.3

3.4. Detection mechanism

Fig. 4. ECL responses of the CuO NWs/rGO sensing electrode to different species at the concentration of 1.0 × 10−12 mol L−1 .

There are two mechanisms dominating the ECL process: one is radical annihilation and the other is coreactant mechanism [23]. In our case, weak ECL response was observed for CuO NWs without the coreactant S2 O8 2− while the presence of S2 O8 2− remarkably increased the ECL intensity (Fig. 2A), implying that S2 O8 2− plays an important role for the ECL of CuO NWs, and the ECL process can be speculated to be dominated by the coreactant mechanism [23,24,35]. The further increase of the ECL signal by rGO, as observed for the CuO NWs/rGO, could be attributed to the increased exposed surface areas of CuO NWs due to rGO supporting (Fig. 1a) and also the accelerated electron transfer between the electrode and the S2 O8 2− in solution through the conductive rGO medium (Fig. 2B). According to the proposed model [15,23], the possible ECL detection mechanism is illustrated in Scheme 1 with the following equations: rGO

CuO + e− −→CuO common organic compounds (fulvic acid (FA) and corrosive acid (HA)) as well as several common cations, anions and heavy metals ions were individually investigated to evaluate the selectivity. As shown in Fig. 4, only PCP dramatically quenches the ECL intensity, and the investigated species show nearly no effect on the ECL signal except for the very slight ECL quenching caused by DCP due to their more stable chemical properties [37], meaning a high selectivity of the CuO NWs/rGO-based ECL sensor toward the detection of PCP. The low LOD and high selectivity of the proposed sensor guarantees the PCP determination in real environmental aqueous samples. Tap water and river water from Xiangjiang were collected, and the real water samples were then spiked with different amounts of PCP. The prepared solutions were tested using our proposed method and GC–MS, respectively. As shown in Table 2, the testing results of PCP in all the water samples by our method were in good agreement with those obtained by GC–MS, and moreover, the recovery rate for the water samples ranged from 95.0% to 105.0%, indicating that the proposed method was reliable and able to be employed in analysis of real water samples.

•−

rGO

S2 O8 2− + e− −→SO4 2− + SO4 2SO4

•−

∗+

CuO

•−

+ CuO

(1) •−

→ 2SO4 2− + CuO∗+

−

+ e → CuO + hv

CuO∗+ + PCP → CuO + TCQ

(2) (3) (4) (5)

Primarily, with the potential scanning negatively, the electrons were transferred from the substrate electrode to the CuO NWs and the S2 O8 2− through rGO sheets, resulting in negatively charged species (CuO•− ) and oxidant SO4 •− , respectively (Reactions (1) and (2)). Then, the SO4 •− injected hole into the highest occupied molecular orbital of CuO•− to produce the excited state species (CuO*+ ) that emitted light (Reactions (3) and (4)). When PCP was added, the PCP molecules would be absorbed onto the CuO NWs surface and be oxidized by (CuO*+ ) (Reaction (5)), leading to a decrease in ECL, and meanwhile, the PCP was oxidized into chloranil (TCQ), which was determined by mass spectroscopy analysis (Fig. S5), confirming the oxidization detection mechanism.

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Scheme 1. Schematic illustration for the ECL detection of PCP with CuO NWs/rGO in S2 O8 2− solution.

3.5. Repeatability and long-term stability The repeatability and stability of the CuO NWs/rGO electrode were tested. The ECL responses to 1.0 × 10−12 mol L−1 PCP were evaluated under the same condition by using eleven different CuO NWs/rGO electrodes prepared under the same conditions. The relative standard deviation (RSD) was 3.09%. Fig. 5a investigates the ECL responses of the CuO NWs/rGO-based ECL sensor to 1.0 × 10−12 mol L−1 PCP during one month. The sensor was exposed to air at room temperature and its ECL intensity was tested at three-day intervals. It is shown that the ECL response to 1.0 × 10−12 mol L−1 PCP at the tenth test retained about 87% of its original value, suggesting excellent repeatability and recyclability of the sensor. Moreover, the ECL emission signal of CuO NWs/rGO is highly stable even after the electrode was stored for 10 months in air at room temperature (Fig. 5b). The outstanding long-term stability of the sensing electrode, which has never been demonstrated before in literatures, means not only excellent chemical stability of the CuO NWs/rGO hybrid material and its robust physical adhesion on the substrate electrode, but also firmly anchoring of the 1D CuO NWs on the rGO sheets, in contrast to aggregation that usually occurs for supported 0D NCs or QDs [25,26,41].

4. Conclusion

Fig. 5. (a) ECL responses of the CuO NWs/rGO sensing electrode to 1.0 × 10−12 mol L−1 PCP in 0.067 mol L−1 phosphate buffer (pH 7.0) containing 0.1 mol L−1 S2 O8 2− during 30 days (1 month) (the ECL intensity is normalized). (b) ECL emission of the CuO NWs/rGO electrode in 0.067 mol L−1 phosphate buffer (pH 7.0) containing 0.1 mol L−1 S2 O8 2− during its 300-day (10-month) storage in air at room temperature.

In conclusion, for the first time, non-toxic, chemical stable, and low cost CuO NWs was explored for ECL sensing, and meanwhile a simple, rapid and green “in-situ” strategy was developed for supporting CuO NWs on large-area and electrically conductive rGO. rGO not only served as a superior immobilization platform, but also greatly enhanced the ECL intensity of CuO NWs, and moreover, 1D nanowires can suppress aggregation as compared to the popular 0D nanocrystals or quantum dots. Consequently, the ECL sensor based on the CuO NWs/rGO hybrid showed wonderful performances for the determination of PCP in water, such as wide linear range, low detection limit, high selectivity, excellent reproducibility, and outstanding durability. Our presented ECL sensor exhibits its integrated performances superior to the published methods for

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PCP detection and would be promising for the monitoring of PCP in real environment. Acknowledgements This work was supported by the NSFC (no. 51478171, 51178173, 51238002, and 51272099), Hunan Provincial Natural Science Foundation of China (14JJ1015), and Innovation Research Team in University (IRT1238).

[22]

[23]

[24]

[25]

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.09.001.

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Biographies Wenqun Wu is presently a graduate student in State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering of Hunan University, Changsha, China. Her research interests cover chemical sensors and biosensors. Hua Xiao is presently a graduate student in State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering of Hunan University, Changsha, China. Her research interests cover chemical sensors and biosensors. Shenglian Luo is currently a professor of the State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, China. His research concentrates on environment science, and biological and chemical sensing. Chengbin Liu is currently a professor of the State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, China. His research

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concentrates on advanced functional materials and nanotechnology for environmental pollution control and new energy development. Yanhong Tang is currently an associate professor of the College of Materials Science and Engineering, Hunan University, China. Her research concentrates on advanced functional materials and nanotechnology for sensors and new energies.

Liming Yang is presently a PhD student in State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering of Hunan University, Changsha, China. His research interests cover the design and preparation of electrocatalysts and their applications in the field of energy conversion and storage, wastewater treatment.