Facile green synthesis of graphene-Au nanorod nanoassembly for on-line extraction and sensitive stripping analysis of methyl parathion

Electrochimica Acta 146 (2014) 419–428

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Facile green synthesis of graphene-Au nanorod nanoassembly for online extraction and sensitive stripping analysis of methyl parathion Wenxin Zhu a , Wei Liu a,1, Tianbao Li b , Xiaoyue Yue a , Tao Liu a , Wentao Zhang a , Shaoxuan Yu a , Daohong Zhang a, * , Jianlong Wang a, * a b

College of Food Science and Engineering, Northwest A&F University, Yangling, 712100, Shaanxi, China College of Science, Northwest A&F University, Yangling, 712100 Shaanxi, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 27 July 2014 Received in revised form 17 September 2014 Accepted 17 September 2014 Available online 22 September 2014

This paper described a facile green electrochemical approach to synthesize graphene-AuNRs nanocomposite (GN-AuNRs) onto glassy carbon electrode (GCE) for electrocatalytic analysis of methyl parathion (MP). This electrochemical synthesis of GN-AuNRs hybrid is environmentally friendly for not involving the chemical reduction of graphene oxide (GO) and facile for just on the basis of electrostatic interaction between GO and AuNRs, as well as electrochemical reduction of GO-AuNRs to GN-AuNRs. Combined the high conductivity, large surface area, good adsorption capacity towards aromatic rings and high catalytic ability of graphene with the excellent electronic properties and adsorption capacity of AuNRs, the high sensitive methyl parathion sensor was fabricated with the GN-AuNRs nanocomposite. The limit of detection (LOD) of the proposed sensor was calculated to be 0.82 ng/mL, which was lower than many previously reported enzyme or nonenzyme-based sensors. In the meantime, the linear detection range of this sensor was from 10 to 500 ng/mL and 750 to 4000 ng/mL, which was also wider than many other enzyme or enzymeless sensors. Furthermore, the facile and green electrochemical reduction strategy provided here could also be used to construct more GN-based hybrids. And the GNbased hybrid might be a new and highly efficient SPE factor, which opens new opportunities for green, facile and sensitive analysis of nitroaromatic OPs such as MP. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: Electrochemical sensor Electrochemical reduction Graphene Au nanorod Methyl parathion

1. Introduction Organophosphate compounds, for example, parathion, methyl parathion and paraoxon, have been widely used as pesticides to control boll weevils and many biting or sucking insects of crops in agriculture for high yield and high quality in recent years [1,2]. Nevertheless, organophosphate pesticides, even trace amount of residues possess high toxicity towards humans and may cause severe pollution to our environment. Meanwhile, traditional methods for organophosphate pesticides determination, such as gas chromatography [3], high-performance liquid chromatography [4], gas chromatography-mass spectrometry [5], high performance liquid chromatography-mass spectrometry [6] and capillary electrophoresis [7], though operated with sensitivity, accuracy and high throughput, suffer from expensive analysis settings, long

* Corresponding author. Tel.: +86 29-8709-2275; Fax: +86 29-8709-2275. E-mail addresses: [email protected] (D. Zhang), [email protected] (J. Wang). 1 Both of Wenxin Zhu and Wei Liu rank the first authors. http://dx.doi.org/10.1016/j.electacta.2014.09.085 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.

analysis time, entailing well-trained personnel and inconvenience for on-site applications. Therefore, it is essential to develop lowcost, rapid, easy-handling, field deployable, reliable and sensitive methods for organophosphate pesticides residues determination in food and our environment. Electroanalysis, emerged as an alternative and viable method to cater to trace determination, has attracted much attention for its high sensitivity, low-cost, portability and rapid analysis time [8– 10]. Presently, enzyme biosensors based on the immobilization of AChE or OPH onto electrodes have always been a research hotspot due to their rapid response, small size, novelty and excellent signal amplification effect for the incorporation of nanomaterials and biomolecules [11,12]. However, the AChE and OPH can be partly devitalized in progress, which will no doubt affect the reproducibility, stability and accuracy of these biosensors. Moreover, AChEinhibition based biosensors are not specific to OPs because the AChE is also the target of carbamate pesticides [13] and OPHhydrolysis based biosensors limit their wide applications on account that OPH is still not commercially available and only can be produced in laboratories [14]. Recently, nanomaterial-catalysis based electrochemical sensors have attracted tremendous

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Scheme 1. Schematic illustration for the facile green synthesis of GN-AuNRs/GCE and its application for the extraction and electrochemical detection of MP.

attention, which are more robust, stable, easy to handle and operate, rapid and sensitive in contrast to above biosensors [15– 19]. Graphene, as a classic two-dimensional carbon material, has attracted considerable attention since first reported in 2004 by virtue of its high surface areas (calculated about 2630 m2/g), outstanding electrical conductivity, strong adsorption capacity and good catalytic activity [20,21]. Moreover, GN-based hybrids, as a kind of multifunctional assembly, have been also highly concerned in sensing applications nowadays [22,23]. All sorts of GN-based hybrids like GN-metal nanoparticles [24,25], GN-metal nanorods [26,27], GN-metal nanowires [28], GN-metal nanotubes [29,30], GN-metallic oxides [31,32], GN-metal hydroxides [10], GNchitosan [33], GN-b-cyclodextrin [34] and GN-nafion [35], have been widely applied in the electrochemical platform. Generally, GN-based hybrids are obtained from chemical or hydrothermal insitu synthesis, which involves toxic chemicals like hydrazine hydrate or needs several hundred degrees Celsius. Xia et al. proposed a green method of electrochemical reduction of GO to GN in 2009 [36], which opens up a novel way to synthesize GN-based hybrids. In the past several years, various GN-based electrochemical sensors on account of this principle have been fabricated, such as the GN-CNTs, GN-chitosan, GN-b-cyclodextrin and GN-ZrO2 based sensors [29,33,34,37]. These sensors are sensitive, selective, easy to operate and environmental-friendly, which not only satisfies trace determination requirements of targets, but also accords with the concept of ‘Green Chemistry’. Au nanorod is a kind of anisotropic and elongated gold nanoparticle, which has drawn widespread concern for its higher surface area-to-volume, good conductivity, strong adsorption capacity and catalytic activity. At present, Au nanorods, modified or unmodified, as a powerful and effective tool, have been utilized to detect and monitor different targets, however, to the best of our knowledge, to date, there were no reports about AuNRs used for detection of pesticide residues. In addition, the AuNRs fabricated following the seed-mediated growth method are CTAB-capped and possess positive charges while the GO or GN synthesized by

modified Hummers method are negatively charged. There were already several GN-AuNRs or GO-AuNRs based electrochemical platforms established in previous reports upon the electrostatic interaction between GN or GO and AuNRs [38–41]. The PVP or PEG was used to modify GO to protect it from aggregation [39,40]. In fact, our experiment results showed that it will not cause the aggregation of GO if you control the concentration ratio of unmodified GO and AuNRs well. Nevertheless, up to now, there were also no reports proposing the concept about synthesis of GN-AuNRs by use of electrostatic interaction between unmodified GO and AuNRs as well as electrochemical reduction of unmodified GO-AuNRs. In this work, we herein firstly developed a facile green approach to synthesize GN-AuNRs hybrids in virtue of electrostatic interaction and electrochemical reduction, and prepared a GN-AuNRs film modified glassy carbon electrode (GN-AuNRs/GCE) for nanomolar detection of MP (Scheme 1). The fabricated electrochemical sensor combined the excellent properties of graphene and AuNRs, which makes it a sensitive, selective and reliable approach for MP determination under synergic action. 2. Experiment 2.1. Reagents Graphite (CP, 99.85%, d < 30 mm), hydrogen tetrachloroaurate (III) hydrate (HAuCl4
4H2O) were purchased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). Cetyltrimethylammonium bromide (CTAB) was purchased from Sigma (St. Louis, USA). Methyl parathion, furadan and carbaryl were purchased from China National Institute of Standardization (Beijing, China). 0.1 M PBS was prepared by mixing stock solutions of NaH2PO4 and Na2HPO4. NaBH4, AgNO3 and other chemicals were commercially available, at least of analytical reagent grade and used without further purification. Ultrapure (UP) water (18.5 MV*cm), obtained from a water purification system, was used in the whole experiment. Glasswares used in all procedures

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were soaked and cleaned in a bath of freshly prepared aqua regia, then rinsed thoroughly with ultrapure water, and dried in air prior to use. All experiments were carried out at ambient temperature. 2.2. Apparatus Cyclic voltammetric (CV) and square wave voltammetric (SWV) experiments were performed on a CHI 620D electrochemical workstation (CH Instruments Co., Shanghai, China) with a conventional three-electrode system comprising a platinum wire as auxiliary electrode, a Ag/AgCl (3 M KCl) electrode as reference electrode and the modified or unmodified GCE (3 mm diameter) as working electrode. UV-vis absorption spectra were recorded by a UV-2550 spectrometer (Shimadzu, Japan) with 1 cm pathlength cells. SEM measurements were performed on a Hitachi S-4800 (Hitachi Co., Japan) while TEM measurements were performed on a Hitachi 7700 (Hitachi Co., Japan) at 80 kV. Electrochemical impedance spectroscopy (EIS) measurements were carried out on a ZAHNER ZENNIUM electrochemical workstation with a three-electrode system in 5.0 mM [Fe(CN)6]3/4 containing 0.2 M KCl at the opencircuit potential over a frequency range of 100 kHz to 10 mHz with the AC signal amplitude of 5 mV.

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with nitrogen to remove oxygen for 30 min), and then cycling the potential between 0 and -1.5 V (vs. Ag/AgCl electrode) at a scan rate of 50 mV/s for 8 consecutive cycles (as shown in Fig. S1). For comparison, the modification of GN alone onto GCE was prepared. To obtain GN/GCE, the exfoliated GO/GCE was immersed into an aqueous electrolyte of 0.1 M PBS by cycling the potential between 0 and -1.5 V (vs. Ag/AgCl electrode) at a scan rate of 50 mV/s for 8 cycles. Further, the GN-AuNRs/GCE, GN/GCE were rinsed with 0.1 M PBS solution, then dropped on about 12 mL deionized water and covered with a 1.5 mL centrifuge tube before use. 2.5. Electrochemical detection of MP based on SPE and SWV Firstly, the GN-AuNRs/GCE was immersed into a stirred 0.1 M PBS solution containing the desired concentration of MP for a given time. Secondly, after rinsing with water, the enriched MP modified GN-AuNRs/GCE (MP/GN-AuNRs/GCE) was transferred into 0.1 M PBS blank solution with a given pH for SWV measurements. The SWV was carried out with the scanning range from -0.55 V to 0.45 V. Before each electrochemical measurement, the electrolyte solution was purged with nitrogen to remove dissolved oxygen throughly for 30 min. 2.6. Desorption of MP and Regeneration of the GN-AuNRs/GCE

2.3. Preparation of GO, AuNRs and GO-AuNRs nanoassembly GO was prepared according to the Hummer's method with some modifications [42]. AuNRs were synthesized following the silver ion-assisted seed-mediated and CTAB surfactant-directed method according to the previous report [43] with necessary modifications. The specific synthesis methods of GO and AuNRs have been described in the Supplementary Data. GO-AuNRs nanocomposite was fabricated on the basis of electrostatic interaction between GO (negative charge) and AuNRs (positive charge). GO (20 mg) was dispersed in 20 mL deionized water and then sonicated for 1 h to form a homogenous dispersion. The 1.0 mg/mL GO dispersion was diluted with deionized water to form 0.5 mg/mL and 0.25 mg/mL GO dispersions. 1 mL and 0.5 mL colloid AuNRs (each for three shares) were centrifuged to discard supernatant solution thoroughly respectively, and then added with 1 mL of different concentration GO under sonication for 30 min. The resulting GO-AuNRs nanocomposite was collected by centrifugation (8000 rpm, 10 min), and re-dispersed with 1 mL deionized water respectively to form a homogenous GO-AuNRs dispersion with stirring for 30 min and ultrasonicating for 1 h. 2.4. One-step electrochemical synthesis of GN-AuNRs hybrids on the surface of GCE Prior to electrodeposition, the GCE was polished with 1.0 mm, 0.3 mm and 0.05 mm alumina powders until a mirror-like surface was obtained, then ultrasonicated successively in deionized water, ethanol and deionized water for about 60 s, respectively, in order to remove any adsorbed substances on the electrode surface. Then the electrode was applied in 5 mM [Fe(CN)6]3/4 solution with scanning range from -0.2 V to 0.8 V (vs. Ag/AgCl electrode) for several consecutive circles until a steady-state CV curve was obtained. Subsequently, the eligible GCE was rinsed with deionized water, dried with nitrogen gas for modification immediately or maintained with deionized water covering its surface for shortterm storage. Furthermore, a certain amount of GO-AuNRs dispersion was dropped on the above pretreated bare GCE using a pipette, and then dried under the stream of high purity nitrogen for further use. The GN-AuNRs/GCE was fabricated by immersing the exfoliated GO-AuNRs/GCE into an aqueous electrolyte of 0.1 M PBS(purged

After each electrochemical stripping detection of a certain concentration of MP, the MP/GN-AuNRs/GCE was applied in 0.1 M PBS solution with the scan range from 0.3 V to -1.0 V (vs. Ag/AgCl electrode) for multiturn consecutive cycles to remove the adsorbed MP, which means the regeneration of the electrode (as shown in Fig. S2). 3. Results and Discussion 3.1. Characterization of GO-AuNRs and GN-AuNRs nanocomposites The morphology of the as-synthesized GO-AuNRs was examined with TEM and UV-vis spectroscopy, while the GN-AuNRs obtained by electrochemical reduction was tested with SEM, as recorded in Fig. 1 and Fig. 2. As shown in Fig. 1A, the TEM image of GO exhibits typical wrinkle 2D nanosheet morphology and paperlike structure with single or several very thin layers. Fig. 1B shows the TEM image of the GO-AuNRs nanocomposite. It is obvious that the AuNRs with a certain aspect ratio are homodispersed on the surface of GO and the GO films are propped open further by AuNRs, which can increase the surface-to-volume ratio of GO and provide a broader platform for efficiently capturing more OPs. In Fig. 1C, the SEM image of GN exhibits a crumpled and wrinkled morphology on the surface of GCE, while Fig. 1D shows the AuNRs are in uniform dispersion on the rough surface of graphene. Meanwhile, the aspect ratio of AuNRs loaded on GN decreased apparently in contrast to that of AuNRs loaded on GO. It can be explained that as a kind of anisotropic nanoparticles, AuNRs are unstable and easy to be thickened up, even degenerated to be spherical when suffering from external stimulus, like changes of temperature and electric field. Certainly, it cannot exclude the possibility of parallax among the TEM and SEM images of AuNRs. Moreover, to confirm the electrostatic binding of GO with AuNRs, the UV–vis absorption spectrum of GO, AuNRs and GOAuNRs were tested (Fig. 2). From the UV-vis curves, it can be seen that the synthesized AuNRs (curve a) have two surface plasmon absorption bands: a strong long-wavelength band in NIR region (around 860 nm) due to the longitudinal electronic oscillation and a weak short-wavelength band in visible region (around 520 nm) due to the transverse electronic oscillation. However, the optical absorption band of GO (curve c) (around 230 nm in ultraviolet

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Fig. 1. TEM images of (A) GO and (B) GO-AuNRs nanocomposite; Typical SEM images of (C) GN/GCE and (D) GN-AuNRs/GCE.

region) cannot be observed in visible-NIR region. The absorption spectrum of GO-AuNRs (curve b) incorporates the regularity of GO and AuNRs, which further indicates the AuNRs are loaded on the surface of GO sheets. Besides, the aspect ratio of the synthesized AuNRs can be estimated by the equation [44,45]:

lmax ¼ 445:4 þ 90:6R lmax is the wavelength of longitudinal absorption band, R is the the aspect ratio of the synthesized AuNRs. Taking lmax= 860 nm into the above equation, R was calculated to be 4.57, which was

corresponded with the TEM image of the AuNRs. In addition, the molar concentration of the AuNRs can be reckoned by the latter equation [44,45]: A ¼ e  Cn Where A is the absorbance of longitudinal absorption band, Cn is the molar concentration of the AuNRs, e = (4.6  0.6)  109 M1 cm1. Substituting A = 1.05 into the equation, Cn was calculated to be 0.23 nM. 3.2. Electrochemical characterization of modified electrodes

Fig. 2. UV-vis absorption spectra of the GO-AuNRs nanocomposite.

The capability of electron transfer of different electrodes was investigated by EIS, which was performed in a supporting electrolyte containing 5 mM [Fe(CN)6]3/4 and 0.5 M KCl at the open-circuit potential over a frequency range of 100 kHz to 10 mHz with the AC signal amplitude of 5 mV. As can be seen in Fig. 3, the Nyquist plot of the EIS includes a semicircular portion and a linear portion featuring a diffusion limiting step. The semicircular portion at higher frequencies corresponds to the electrontransfer-limited process and its diameter is equal to the electron transfer resistance (Rct), which controls the electron-transfer kinetics of the redox probe at the electrode interface. Obviously, the bare GCE exhibited a big semicircle portion and the value of Rct was estimated to be 175.9 V (curve c). After the exfoliated GO was electrochemically reduced on the bare GCE (GN/GCE), the semicircle decreases dramatically compared to the bare GCE and the value of Rct was estimated to be 63.7 V (curve b), suggesting that the presence of GN has accelerated electron transfer between the electrochemical probe of [Fe(CN)6]3/4 and the surface of electrode. For the GN-AuNRs modified GCE (GN-AuNRs/GCE), the semicircle further lessens with the value of Rct reduced to be 8.6 V (curve a), which indicates that the added AuNRs facilitate the

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Fig. 3. Nyquist plots of GN-AuNRs/GCE (a), GN/GCE (b), GCE (c) in 5 mM [Fe (CN)6]3/4 containing 0.5 M KCl (the frequency range is from 100 kHz to 10 mHz).

electron transfer and the GN-AuNRs film improves obviously the diffusion of ferricyanide towards the electrode interface. 3.3. Voltammetric characteristics of adsorbed methyl parathion on modified GCE The cyclic voltammograms with two successive cycles and square wave voltammograms of GN-AuNRs/GCE (a), GN/GCE (b) in 0.1 M PBS (pH 7.0) containing 2 mg/mL MP and GN-AuNRs/GCE (c) in blank 0.1 M PBS (pH 7.0) are shown in Fig. 4.

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The reaction mechanism has been widely reported [10,14,15,17,24,29,31,33–35]. As shown in Fig. 4A, in the first cathodic scan of curve a or curve b, a sharp irreversible reduction peak (around -0.58 V) can be observed, which corresponds to the reduction of the nitro group (-NO2) to hydroxylamine moieties (-NHOH) via a four-electron reduction process (reaction (1)). Then it is reversibly oxidized to the nitroso group (-NO) with a oxidation peak (around 0 V) during the anodic scan (reaction (2)). In the following second cycle, the nitroso species is reversibly reduced to hydroxylamine group with another reduction peak (around -0.05 V, reaction (3)). The pair of reversible redox peaks should be attributed to a two-electron transfer redox process. The irreversible reduction peak disappears in the second cycle, which should be due to the reaction exhaustion of MP at the surface of GN/GCE or GN-AuNRs/GCE. The mechanism above accords well with the described electrochemical reactions of nitroaromatic compounds. What’s more, there is no obvious redox peak of CV observed at the GN-AuNRs/GCE in blank 0.1 M PBS solution (curve c). In contrast, an evident electrochemical response can be seen at the GN/GCE with 2 mg/mL MP (curve b) and a further remarkable increase in peak currents can be observed at the GN-AuNRs/GCE with 2 mg/mL MP (curve a), resulting from the more excellent electrocatalytic response towards MP of GN-AuNRs/GCE compared with GN/GCE. As a matter of course, it could also be attributed to the better conductivity and larger electrochemical effective surface area of GN-AuNRs/GCE. The SWV analysis has higher sensitivity than other electrochemical technologies, such as CV and differential pulse voltammetry. Fig. 4B shows the SWV of adsorbed MP on the surface of different modified GCE in 0.1 M PBS (pH 7.0). A very sharp and well defined stripping peak (curve a or b) was obtained at the potential range from -0.45 V to 0.45 V. The current of the stripping peak is

Fig. 4. (A) Cyclic voltammograms of GN-AuNRs/GCE (a), GN/GCE (b) in 0.1 M PBS (pH 7.0) containing 2 mg/mL MP with two successive scans and GN-AuNRs/GCE (c) in blank 0.1 M PBS (pH 7.0) with two successive scans. (B) Square wave voltammograms of GN-AuNRs/GCE (a), GN/GCE (b) in 0.1 M PBS (pH 7.0) containing 2 mg/mL MP and GN-AuNRs/ GCE (c) in blank 0.1 M PBS (pH 7.0).

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Fig. 5. Effects of (A) the volume of GO-AuNRs, (B) the accumulation time, (C) the concentration ratio of GN/AuNRs and (D) the pH value on the SWV response of GN-AuNRs/ GCE with 500 ng/mL MP in 0.1 M PBS.

higher than the corresponding peak current in cyclic voltammograms, which further demonstrates the higher sensitivity of SWV. Meanwhile, evidently, in comparison with MP attached onto GN/ GCE (curve b), the stripping peak current of MP captured by GNAuNRs/GCE (curve a) is enhanced. This may be attributed to the synergic action of graphene and AuNRs towards the target of MP. A control experiment (curve c) was performed under the same conditions with the GN-AuNRs/GCE in the absence of MP and no stripping peak is observed, which is in accordance with that in cyclic voltammograms. 3.4. Optimization for the detection of MP at GN-AuNRs/GCE For purpose of achieving a high performance stripping analysis of MP, the experimental parameters of electrode preparation and sample analysis, such as the volume of GN-AuNRs, accumulation time, concentration ratio of GN/AuNRs and pH value were optimized. Firstly, as the supporting skeleton, the volume of GN-AuNRs dispersed onto GCE sharply influenced the electrochemical response. Fig. 5A shows the influence of GN-AuNRs volume on the current response of MP, which is preconcentrated in 500 ng/mL MP solution for 7 min. As can be seen from it, when dispersing 7 mL GN-AuNRs onto the GCE, the cathodic peak current reaches a maximum value of 112.5 mA, which achieves a balance of the thickness and effective active area of the GN-AuNRs hybrid. Thus,

the volume of 7 mL GN-AuNRs dispersion was used in the following experiments. Secondly, the accumulation time also has significant effects on the performance of the sensor. As shown in Fig. 5B, the responding peak current gradually enhances with increasing immersion time and reached a maximum value at 7 min. Further increasing the preconcentration time leads to no increase of the response and tend to be stable after 7 min, indicating that the adsorption of MP onto GN-AuNRs/GCE reaches saturation. The rapid and effective preconcentration of MP is ascribed to the nature of large surface area, plenty of binding sites and strong p-p interaction between the graphene and the aromatic compounds. Consequently, 7 min of the accumulation time was employed in further experiments. Besides, the concentration ratio of GN/AuNRs plays an important role likewise in the sensitivity of the sensor. The concentration of as-synthesized AuNRs was calculated previously to be 0.23 nM and for convenient presentation, we defined the concentration of the AuNRs to be 1.0 nM. As described in Fig. 5C, when the concentration of graphene varies from 0.25 to 1.0 mg/mL, the peak current enhances and reaches a summit at 0.5 mg/mL, then sharply decreases at 1.0 mg/mL, which might be due to the aggregation of graphene with high concentration and the increase of electron transfer resistance. In the meantime, with the concentration of AuNRs changing from 0.5 to 1.0 nM, the peak current response increases, in that AuNRs with high density anchored on the graphene might accelerate the electron transfer and provide more catalytic and binding sites for MP. Hence, the

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Fig. 6. (A) SWV of GN-AuNRs/GCE in 0.1 M PBS with different concentration of MP: 10, 50, 100, 250, 500, 750, 1000, 2000 and 4000 ng/mL (from a to i). SWV conditions: step potential of 4 mV, an amplitude of 25 mV, and a frequency of 5 Hz. (B) The calibration curve of GN-AuNRs/GCE to different concentration of MP from 10 to 4000 ng/mL in 0.1 M PBS (pH 7.0). Inset: the calibration curve for the proposed method to different concentration of MP from 10 to 500 ng/mL.

concentration ratio of GN/AuNRs of 0.5: 1.0 was applied in subsequent experiments. Last but not least, the pH value is a key factor for the electrochemical signal of MP as well. 0.1 M PBS as supporting electrolyte with different pH values in the range of 5–9 were compared in respect of the response for 500 ng/mL MP on GNAuNRs/GCE. As can be seen in Fig. 5D, the peak current enhances with increasing pH value and arrives at a maximum value at pH 7.0 and then sharply decreases with further increasing, indicating that the GN-AuNRs nanoassembly has the maximum adsorption and catalysis towards MP in neutral environment, which is in accordance with previous reported studies [14,16,34]. Although the lower pH value is favourable to electro-reduction according to the mechanism showed in reactions (1)–(3), the substantial reduction current of hydrogen ions in lower pH range such as 4.0 will interfere the determination of methyl parathion. Moreover, higher pH value may cause the degragation of MP. Therefore, 0.1 M PBS with pH 7.0 was used as the supporting electrolyte in the whole experiments. 3.5. Analytical performance of the sensor for determination of methyl parathion To reveal the availability of the proposed sensor, analytical features of the method such as linear range of the calibration curve, LOD, selectivity, reproducibility, stability and practicality were examined. SWV method has proved to be very sensitive for trace analysis of agrochemical residues, thus SWV was selected for trace amounts determination of MP in 0.1 M PBS (pH 7.0). Fig. 6A displays the SWV responses of MP in the concentration range of 10– 4000 ng/mL under optimized condition. As can be seen, welldefined SWV responses from adsorbed MP increase gradually with

the increase of the MP concentration. In Fig. 6B, good and expanded linear relationship between the peak current and the concentration of methyl parathion is obtained over the concentration range of 10–500 ng/mL and 750–4000 ng/mL with the regression equation of Ia = 7.77539 + 0.28764 C (R = 0.9922) and Ib = 162.8521 + 0.00987 C (R = 0.9897) (in inset of Fig. 6B). A LOD of 0.82 ng/mL was calculated on the basis of the formula IUPAC proposed [46]: LOD = 3 S0/S, where S0 (calculated to be 0.079) is the relative standard deviation (RSD) of 10 measurements taken from the signal obtained from the blank. S is the sensitivity, namely, the slope of the calibration curve. A comparison analysis on MP electrochemical sensors based on different modified electrodes reported in previous published literatures was shown in Table 1, in which it can be seen that the performance of the sensor is better than some traditional materials modified electrodes, such as LOD and linear detection range. This is mainly due to the fact that the as-prepared sensor integrates excellent electrocatalytic activity, fast electron transfer and large surface area of the GN-AuNRs nanoassembly. To test the selectivity of the proposed sensor, interferences caused by other electroactive nitrophenyl derivatives or aromatic amines such as p-nitrophenol, aniline, anisidine and p-aminothiophenol, and oxygen-containing inorganic ions (phosphate and nitrate), as well as the carbamate insecticides such as furadan and carbaryl, were chosen to appraise the selectivity of the GN-AuNRs/ GCE. Fig. 7A shows the SWV curves of GN-AuNRs/GCE with 500 ng/ mL MP under different interferents, in which we can see that there is no apparent influence towards the peak current of MP, and the anodic peaks (at 0.1–0.2 V) of aniline, anisidine and p-aminothiophenol will not affect the peak current of MP. Fig. 7B presents that these interferents have little influence on the stripping peak current of MP, which might be due to the strong affinity between

Table 1 Comparison of the proposed electrode with other reported electrodes for determination of MP. Electrode

Technique

LOD (ng/mL)

Linear range (mg/mL)

Reference

AuNPs-Nafion/GCE GN-Nafion/GCE GN-ZrO2/GCE GN-Chitosan/GCE GN-CNTs-Chitisan/GCE ZrO2/GCE AChE/Au-PPy/GCE

SWV SWV SWV SWV SWV SWV CV

26.3 1.6 0.6 0.8 0.5 3.0 2.0

[47] [35] [37] [33] [29] [14] [11]

GN-AuNRs/GCE

SWV

0.13–31.56 0.02–20 0.002–0.9 0.004–0.4 0.002–0.5 0.005–0.1 0.005–0.12 0.5–4.5 0.01–0.5 0.75–4.0

0.82

Present work

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Fig. 7. (A) Square wave voltammograms and (B) bar graph of peak current of GN-AuNRs/GCE with 500 ng/mL MP in the absence and presence of 50 mg/mL p-nitrophenol, 50 mg/mL p-aminothiophenol, 50 mg/mL aniline, 50 mg/mL anisidine, 50 mg/mL furadan, 50 mg/mL carbaryl, 0.1 M phosphate and 0.1 M nitrate.

Fig. 8. The contrastive study of the stability of the sensor with different treatment.

the GN-AuNRs nanoassembly and MP (the quantification of the selectivity of the sensor towards MP is shown in Table S1). In addition, the reproducibility of the sensor was evaluated by relative standard deviation (RSD) of intra-assay and inter-assay. The inter-assay precision was estimated at five different GNAuNRs/GCE for the determination of 500 ng/mL MP, while the intra-assay precision was assessed by assaying one GN-AuNRs/GCE for 10 replicate measurements of 500 ng/mL MP under uniform conditions. The RSD values of the inter-assay and intra-assay were calculated to be 6.7% and 5.4%, respectively, indicating an

Table 2 Recovery studies of MP in tap water and kiwi fruit samples (n = 3). sample

Spiked (mg/mL)

Found (mg/mL)

Recovery (%)

Tap water

0.05 0.1 0.2 0.05 0.1 0.2

0.045 0.107 0.193 0.039 0.083 0.182

90.1 107.0 96.3 78.0 83.3 91.0

Kiwi fruit

acceptable reproducibility. What's more, the stability of the GNAuNRs modified electrode was contrastive studied by different treatment. Primarily, the decrease in the current response after several days was due to the insufficient regeneration and part abscission of graphene and AuNRs, therefore, we chose two ways (one was only to increase the CV cycle number from 50 to 100 to regenerate the sensor, the other one was to coat 4 mL 0.5% nafion solution onto the surface of the GN-AuNRs/GCE and increase the CV cycle number from 50 to 100) to explore the possibility of reducing the current loss. From Fig. 8, the current response of the GN-AuNRs/GCE decreased above 10% with 50 successive CV cycles, while the current response decreased only 6.3% with 100 successive CV cycles in contrast to the original value after 30-day storage, which was already superior to that of other studies reported [10,17,24,31]. Meanwhile, as can be seen in Fig. 8, although the GNAuNRs/GCE has less stability than the nafion/GN-AuNRs/GCE in the 15-day storage, it has higher current response than the nafion/GNAuNRs/GCE. Therefore, here, we choose to only increase the CV scan cycles from 50 to 100, which manifests an acceptable stability and high sensitivity of the sensor. In order to further demonstrate the practicality of the sensor, the recovery test was carried out by the standard samples recovery. Water samples were got from the faucet tap in the laboratory. Kiwi fruit samples were collected from the production base nearby and the pretreatment was on the basis of the Standard of Ministry of Agriculture (NY/T 761–2008). The results were summarized in Table 2. As can be seen, the recovery ranges of tap water samples are from 90.1% to 107.0% and that of kiwi fruit samples are from 78.0% to 91.0%, indicating that the proposed method outlined in this investigation is accurate and can be used for direct assay of relevant real samples. 4. Conclusion In summary, this paper has proposed a facile and rapid approach to synthesize the GN-AuNRs nanoassembly onto glassy carbon electrode by one-step electro-reduction from GO-AuNRs. Meanwhile, with the excellent conductivity, strong adsorption towards Ops and good catalytic activity of the GN-AuNRs nanocomposite, the developed novel sensor fabricated with the above as-synthesized nanoassembly for determination of OPs shows

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some outstanding properties such as wide linear concentration range, low LOD, excellent reproducibility, long-time storage stability and satisfactory anti-interference ability. Moreover, the acceptable recovery results of tap water and kiwi fruits samples prove the good practicality of the sensor. Thus, this paper further enlarges the scope of facile green synthetic methods of GN-based hybrids and paves a promising way for green, facile and sensitive analysis of nitroaromatic OPs. Acknowledgements This work is supported by National Science & Technology Pillar Program (2012BAK17B06), National Natural Science Foundation of China (No. 31101274, No. 31201357), the Shaanxi Provincial Research Fund (2012KJXX-17, 2014KJXX-42, 2014K02-13-03, 2014K13-10), Open Fund of State Key Laboratory of Electroanalytical Chemistry (SKLEAC201301). 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.electacta.2014.09.085. References [1] D. l. Silva, C.M. Cortez, J. Cunha-Bastos, S.R.W. Louro, Methyl parathion interaction with human and bovine serum albumin, Toxicol Lett. 147 (2004) 53–61. [2] B.J. Sanghavi, G. Hirsch, S.P. Karna, A.K. Srivastava, Potentiometric stripping analysis of methyl and ethyl parathion employing carbon nanoparticles and halloysite nanoclay modified carbon paste electrode, Anal. Chim. Acta 735 (2012) 37–45. [3] B. Albero, C. Sánchez-Brunete, J.L. Tadeo, Determination of organophosphorus pesticides in fruit juices by matrix solid-phase dispersion and gas chromatography, J. Agric. Food. Chem. 51 (2003) 6915–6921. [4] C.C. Leandro, P. Hancock, R.J. Fussell, B.J. Keely, Comparison of ultraperformance liquid chromatography and high-performance liquid chromatography for the determination of priority pesticides in baby foods by tandem quadrupole mass spectrometry, J. Chromatogr. A 1103 (2006) 94–101. [5] E. Borras, P. Sanchez, A. Munoz, L.A. Tortajada-Genaro, Development of a gas chromatography-mass spectrometry method for the determination of pesticides in gaseous and particulate phases in the atmosphere, Anal. Chim. Acta 699 (2011) 57–65. [6] C. Ferrer, M.J. Gómez, J.F. García-Reyes, I. Ferrer, E.M. Thurman, A.R. FernándezAlba, Determination of pesticide residues in olives and olive oil by matrix solid-phase dispersion followed by gas chromatography/mass spectrometry and liquid chromatography/tandem mass spectrometry, J. Chromatogr. A 1069 (2005) 183–194. [7] M. Lecoeur-Lorin, R. Delepee, P. Morin, Simultaneous enantioselective determination of fenamiphos and its two metabolites in soil sample by CE, Electrophoresis 30 (2009) 2931–2939. [8] O. Shulga, J.R. Kirchhoff, An acetylcholinesterase enzyme electrode stabilized by an electrodeposited gold nanoparticle layer, Electrochem. Commun. 9 (2007) 935–940. [9] X. Yue, S. Pang, P. Han, C. Zhang, J. Wang, L. Zhang, Carbon nanotubes/carbon paper composite electrode for sensitive detection of catechol in the presence of hydroquinone, Electrochem. Commun. 34 (2013) 356–359. [10] H. Liang, X. Miao, J. Gong, One-step fabrication of layered double hydroxides/ graphene hybrid as solid-phase extraction for stripping voltammetric detection of methyl parathion, Electrochem. Commun. 20 (2012) 149–152. [11] J. Gong, L. Wang, L. Zhang, Electrochemical biosensing of methyl parathion pesticide based on acetylcholinesterase immobilized onto Au-polypyrrole interlaced network-like nanocomposite, Biosens. Bioelectron. 24 (2009) 2285–2288. [12] S. Chen, J. Huang, D. Du, J. Li, H. Tu, D. Liu, A. Zhang, Methyl parathion hydrolase based nanocomposite biosensors for highly sensitive and selective determination of methyl parathion, Biosens. Bioelectron. 26 (2011) 4320–4325. [13] Y. Zhao, W. Zhang, Y. Lin, D. Du, The vital function of Fe3O4@Au nanocomposites for hydrolase biosensor design and its application in detection of methyl parathion, Nanoscale 5 (2013) 1121–1126. [14] G. Liu, Y. Lin, Electrochemical sensor for organophosphate pesticides and nerve agents using zirconia nanoparticles as selective sorbents, Anal. Chem. 77 (2005) 5894–5901. [15] Y. Qu, H. Min, Y. Wei, F. Xiao, G. Shi, X. Li, L. Jin, Au-TiO2/Chit modified sensor for electrochemical detection of trace organophosphates insecticides, Talanta 76 (2008) 758–762.

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