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Facile synthesis of zirconia nanoparticles-decorated graphene hybrid nanosheets for an enzymeless methyl parathion sensor
Sensors and Actuators B 162 (2012) 341–347
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Facile synthesis of zirconia nanoparticles-decorated graphene hybrid nanosheets for an enzymeless methyl parathion sensor Jingming Gong ∗ , Xingju Miao, Huifang Wan, Dandan Song Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China
a r t i c l e
i n f o
Article history: Received 13 October 2011 Received in revised form 18 December 2011 Accepted 27 December 2011 Available online 2 January 2012 Keywords: Zirconia nanoparticles Graphene One-step co-electrodeposition Organophosphate pesticide Enzymeless sensor
a b s t r a c t This paper proposed a facile electrochemical approach to the synthesis of high quality zirconia nanoparticles decorated graphene nanosheets (labeled as ZrO2 NPs-GNs) onto a cathodic substrate. This facile one-step co-electrodeposition approach for the construction of GNs based hybrid is environmentally friendly, without involving the chemical reduction of GO, and therefore will not result in further contamination. The electrochemically synthesized ZrO2 NPs-GNs composite has been carefully characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and various electrochemical techniques. Such a nanostructured composite, combining the advantages of ZrO2 NPs (high recognition and enrichment capability for phosphoric moieties) together with GNs (large surface area and high conductivity), is highly efficient to capture organophosphate pesticides (OPs). Combined with the square-wave voltammetry, a highly sensitive enzymeless OPs sensor was fabricated using the prepared ZrO2 NPs-GNs composite as solid phase extraction (SPE). The detection limit for methyl parathion (MP) in aqueous solutions was determined to be of 0.6 ng mL−1 (S/N = 3). This work provides a green and facile route for the preparation of GNs-based hybrid, and also offers a new promising protocol for OPs analysis. © 2011 Elsevier B.V. All rights reserved.
1. Introduction As a “rising star” material, graphene nanosheets (GNs), a new two-dimensional (2D) structure consists of sp2 hybridized carbon atoms, have attracted tremendous attention since experimentally produced in 2004 [1–3], finding potential applications in synthesizing nanocomposites, nanoelectronics, and ultrasensitive sensors [4–6]. 2D network structure of GNs could provide advantageous workplace for the incorporation of other building units [7–9], which makes it possible for utilizing GNs-based hybrid as a multifunctional assembly. Recently, GNs-based hybrids, combining well unique properties of individual nanostructures, have been highly concerned in various applications, ranging from environmental science, energy conversion, to sensing [10–24]. Particularly, it has been demonstrated that GNs-based hybrid nanomaterials as sensing platforms, display extraordinary activity. For example, various GNs-based hybrid nanocomposite, including 2D GNs combined with 0D metal NPs, metallic oxide NPs, or cyclodextrin, etc., have been fabricated as enhanced sensing platforms for electrochemical determination of glucose [14,15], -nicotinamide adenine dinucleotide [16], cytochrome c [17], heavy metal ions [18], dopamine [19,20], and organophosphates pesticides (OPs), etc. [10,22]. Obviously, graphene-based materials posses remarkable superiorities
∗ Corresponding author. Tel.: +86 27 6786 7535; fax: +86 27 6786 7535. E-mail address: [email protected] (J. Gong). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.12.094
on sensing applications. To meet the rapidly increasing demands for on-site environmental monitoring (e.g. high sensitivity, portable instruments, and simple operations), currently, it is still a great challenge to construct a high-performance GNs-based hybrid sensing platform in a simple, green and controllable fashion. To date, GNs based hybrid are primarily prepared by the chemical reduction of exfoliated graphite oxide (GO) combined with (1) chemical precipitation [11,12], or (2) using the obtained GNs as the support for further deposition of other moieties [10,14,18,22], always involved with chemical reduction process. Nevertheless, in this laborious procedure, the excessive amounts of reducing agents employed may contaminate the resulting materials; a large amount of framework defects is inevitable to be produced, and then the produced defects would severely impair the conductivity of the reduced GNs [23]; during the chemical reduction process, the monolayer graphene are considerably easy to agglomerate [25,26]. These severely restrict the further applications of GNs. In this aspect, several research groups have made outstanding achievements [27,28]. For example, Williams et al. proposed a UVassisted photocatalytic reduction approach toward GNs [27]. Deng et al. developed a protein-induced reduction of GO approach [28]. Recently, an in situ electrochemical reduction of exfoliated GO has been put forward as a green and fast approach toward GNs [29]. Xia et al. demonstrated that this approach has several clear advantages: no toxic solvents are used and therefore will not result in contamination of the product; the high negative potential can overcome the energy barriers for the reduction of oxygen functionalities,
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Scheme 1. Schematic illustration for green synthesis of ZrO2 NPs-GNs/GCE via a one-step electrochemical approach and electrochemical sensing nitroaromatic OP compound.
leading to the exfoliated GO efficiently reduced; and the final solid film of GNs can be further used in sensor [29]. As exotic buildings unit of the assembly, nanostructured zirconia (ZrO2 ), a useful inorganic oxide has aroused interest owing to its thermal stability, chemical inertness, and lack of toxicity [30,31]. It has been reported that ZrO2 NPs have a strong affinity to phosphoric moieties and was originally reported for detection of phosphopeptide [32–34], phosphorprotein capture [35,36], and OPs [22,37]. More conveniently, metallic oxide of ZrO2 film could be prepared by electroreduction of ZrOCl2 at bare gold surface [37,38]. Inspired by this, in this work, we propose a facile one-step co-electrodeposition approach to construct zirconia nanoparticles-decorated graphene nanosheets hybrid coating on a glassy carbon electrode (labeled as ZrO2 NPs-GNs/GCE), without involving the chemical reduction of GO.
Recently, as a more convenient and cost-effective alternative, an enzymeless sensor, i.e., solid phase extraction (SPE) of OPs combined with square-wave voltammetric analysis, has shown to be an ideal and highly sensitive technology [39–42,10,37]. The resulting ZrO2 NPs-GNs composite, combining the individual properties of GNs (large surface area and high conductivity) together with ZrO2 NPs (high recognition and enrichment capability for phosphoric moieties) is believed to be an excellent approach for OPs capture. It is expected that the resulting ZrO2 NPs-GNs composite could dramatically facilitate the enrichment of OPs, effectively accelerate the electron transfer and realize their rapid, stable and sensitive square-wave voltammetric detection. This new electrochemical sensing protocol involves simple one-step electrosynthesis of a thin ZrO2 NPs-GNs film onto a GCE surface, subsequent entrapment of methyl parathion (MP as a model of OPs), and final electrochemical detection of adsorbed MP (Scheme 1). To the best of our knowledge, this is the first report on a green and facile electrochemical synthesis of ZrO2 NPs-decorated GNs hybrid and further utilizing ZrO2 NPs-GNs composite as SPE for electrochemical analysis of nitroaromatic OPs.
2. Experimental 2.1. Apparatus
Fig. 1. Cyclic voltammograms of (a) exfoliated GO/GCE, (b) bare GCE, and (c) exfoliated GO/GCE in 0.1 M KCl (a), or in 5.0 mM ZrOCl2 + 0.1 M KCl (b and c) aqueous solution (the first cycle shown). Scan rate: 50 mV/s.
Electrochemical measurements were performed on a CHI 660D electrochemical workstation (CHI, USA) with a conventional threeelectrode system comprising a platinum wire as an auxiliary electrode, a saturated calomel electrode (SCE) as reference, and the modified or unmodified glassy carbon electrode (GCE) as a working electrode. The general morphology of the products was characterized by the scanning electron microscopy (SEM, JSM-5600). Tapping-mode atomic force microscopy (AFM) was conducted with a DI Nanoscope (Veeco Instruments, Inc., USA) (see Appendices for sample preparation).
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Fig. 2. Typical SEM images of (A) the as-synthesized GNs/GCE, (B) ZrO2 NPs-GNs/GCE via an electrochemical approach; typical AFM images of (C) the as-prepared graphene nanosheets (GNs), (D) ZrO2 NPs decorated graphene nanosheets (ZrO2 NPs-GNs) on mica and their corresponding section analysis. Inset of (B): corresponding magnified SEM image.
2.2. Chemicals Methyl parathion (MP) was obtained from Treechem Co (Shanghai, China). ZrOCl2 ·8H2 O was obtained from SCRC (Shanghai, China). The 0.2 M acetate buffer solutions (ABS) of different pH values were prepared for use. All other chemicals were of analytical-reagent grade and used without further purification. Doubly distilled water was used. All experiments were carried out at ambient temperature. 2.3. Preparation of the modified electrode and measurement procedure Prior to modification, the basal GCE was polished to a mirror finish using 1.0, 0.3 and 0.05 m alumina slurries. After each polishing, the electrode was sonicated in ethanol and doubly distilled water for 5 min, successively, in order to remove any adsorbed substances on the electrode surface. Graphite oxide was synthesized from graphite by a modified Hummers method [43,44]. The certain amount of GO was dispersed
into 1 mL doubly distilled water to form a homogenous exfoliated GO dispersion under mild ultrasonication for 1 h. A 10 L portion of exfoliated GO suspension was spread on a pretreated bare GCE using a pipette, and then kept at room temperature till dry (labeled as GO/GCE). ZrO2 NPs decorated GNs modified electrode (ZrO2 NPs-GNs/GCE) was prepared by immersing the exfoliated GO/GCE into an aqueous electrolyte of 5.0 mM ZrOCl2 and 0.1 M KCl, and then cycling the potential between 0 and −1.5 V (vs. SCE) at a scan rate of 50 mV/s for 8 cycles. For comparison, the modification of GNs or ZrO2 NPs alone onto GCE was prepared, respectively. To obtain GNs/GCE, according to the previous report [29], the exfoliated GO/GCE was immersed into an aqueous electrolyte of 0.1 M KCl by cycling the potential between 0 and −1.5 V (vs. SCE) at a scan rate of 50 mV/s for 8 cycles, while ZrO2 NPs modified GCE (ZrO2 NPs/GCE) was prepared by cycling the potential between 0 and −1.5 V (vs. SCE) for 8 cycles in an aqueous electrolyte of 5.0 mM ZrOCl2 and 0.1 M KCl. The as-prepared GNs/GCE, ZrO2 NPs/GCE, ZrO2 NPs-GNs/GCE were rinsed with water and dried at room temperature for further experiments, respectively. The followed measurement procedures were shown in Appendices.
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3. Results and discussion 3.1. Cyclic voltammetric responses for the formation of the modified electrode Fig. 1 illustrates the comparison between the cyclic voltammetric responses for the formation processes of thus formed GNs, ZrO2 NPs and ZrO2 NPs-GNs on the GCE. The cyclic voltammograms of an exfoliated GO/GCE in a potential range from 0.0 to −1.5 V shows a large cathodic current peak at around −1.20 V vs. SCE (shown in Fig. 1a) with a starting potential of −0.75 V vs. SCE. This large reduction current should be due to the reduction of the surface oxygen groups. In subsequent cycles, the reduction current at negative potentials decreases considerably and disappears finally. This demonstrates that the reduction of surface-oxygenated species at GO occurs quickly and irreversibly, and the exfoliated GO could indeed be reduced electrochemically at negative potentials to form GNs/GCE, agreeing well with the previous report [5]. A typical CV curve for the formation of ZrO2 /GCE is shown in Fig. 1b, recorded in an aqueous electrolyte of 5.0 mM ZrOCl2 and 0.1 M KCl. The irreversible peak around −0.85 V vs. SCE could be attributed to the redox behavior of ZrOCl2 on the bare GCE [13,14]. Interestingly, with the exfoliated GO/GCE immersed into the aqueous solution of 5.0 mM ZrOCl2 and 0.1 M KCl, the general features of the CV curves obtained (Fig. 1c) nicely combine the characteristic of the respective CV curves of Fig. 1a and b. The obvious cathodic peaks around −0.90 V and −1.27 V (vs. SCE) are believed to correspond to the complex redox behavior of the exfoliated GO and ZrOCl2 , respectively, to form the resulting ZrO2 NPs-GNs/GCE, indicating that ZrO2 decorated GNs composite coating has been successfully synthesized onto the surface of GCE via this green and facile electrochemical approach. 3.2. Characterization of the modified electrode surface GNs and ZrO2 NPs decorated GNs obtained by electrochemical reduction were analyzed by SEM and AFM analysis. As shown in Fig. 2A, typical SEM image of the as-synthesized graphene shows 2D nanosheet morphologies, exhibiting a few thin wrinkles onto the surface. With the co-electrodeposition to form ZrO2 -GNs/GCE, uniform ZrO2 NPs of ∼42 nm in average diameter formed randomly on the sheet (Fig. 2B). AFM is currently the foremost methods allowing definitive identification of single-layer crystals [5]. From AFM cross-section analysis, it can be observed that the ZrO2 NPs in the ZrO2 -GNs composite possess a thickness of about 4.0 nm. While, whether for GNs alone (Fig. 2C) or those in composite (Fig. 2D), the thickness of most GNs obtained was close to 1.0 nm, consistent with the thickness of an individual graphene layer. It further indicates that the co-electrodeposition approach is feasible to form GNs based hybrid. 3.3. Electrochemical reactivity The capability of electron transfer of different electrodes was investigated by electrochemical impedance spectra (EIS), shown in Fig. 3A. It can be seen that EIS of the bare GCE is composed of a semicircle and a straight line featuring a diffusionlimiting step of the Fe(CN)6 4−/3− processes (curve a). With the modification of the exfoliated GO onto GCE, the semicircle dramatically increases as compared to the bare GCE, suggesting that the exfoliated GO, as an insulating layer, makes the interfacial charge transfer difficult (curve b). After the exfoliated GO film is electrochemically reduced on GCE (GNs/GCE), the semicircles decrease distinctively, even smaller than bare GCE, indicating that the presence of GNs has accelerated electron transfer between the electrochemical probe of [Fe(CN)6 ]3−/4− and the electrode
Fig. 3. (A) Nyquist plots at (a) GC electrode, (b) exfoliated GO/GCE, (c) GNs/GCE, and (d) ZrO2 NPs-GNs/GCE in 10 mM Fe(CN)6 3−/4− containing 0.1 M KCl. The frequency range is from 1 Hz to 10 kHz. (B) Chronocoulometric curves at (e) GC electrode, (f) GNs/GCE, and (g) ZrO2 NPs-GNs/GCE for the reduction of 0.5 mM K3 Fe(CN)6 with 0.1 M KCl. The initial potential was 0.65 V, and the potential was stepped to −0.05 V.
(curve c). However, with the modification of ZrO2 NPs-GNs/GCE, the semicircles increase distinctively, even larger than exfoliated GO, suggesting that the introduction of semi-conductor ZrO2 NPs into the composite generate higher resistance for the redox process (curve d). It is well known that the effective surface area is a crucial factor influencing the enrichment capability of the target and the electrochemical response. Fig. 3B depicts the chronocoulometric curves at different electrodes for the reduction of 0.5 mM K3 Fe(CN)6 with 0.1 M KCl. According to equation [45], Eq. A1 (see Appendices for the detailed descriptions). From the slope of the Q − t1/2 line, the sequence of the values of A for different electrodes is ZrO2 NPs-GNs/GCE (curve g, 1.01 cm2 ) > GNs/GCE (curve f, 0.490 cm2 ) > bare GC (curve e, 0.070 cm2 ). Obviously, the introduction of ZrO2 NPs effectively decreases the aggregation of GNs, and the modification of ZrO2 NPsGNs onto GCE greatly enhances the active area of the surface, which is considerably important for sensing applications.
3.4. UV–vis spectra To prove that ZrO2 NPs-decorated graphene nanoassembly can be used for SPE of methyl parathion (a model of OPs), UV–vis spectra of MP solution before (curve a) and after SPE were shown in Fig. A-1 (the details see Appendices). The results confirmed that
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Fig. 4. Cyclic voltammograms of (a) ZrO2 NPs-GNs/GCE, (b) MP captured onto ZrO2 NPs-GNs/GCE in 0.1 M ABS (pH 5.2). Scan rate: 100 mV/s. Inset: square-wave voltammograms obtained at ZrO2 NPs-GNs/GCE (c) in the absence of MP, (d) the solution containing 0.2 g mL−1 MP without preconcentration, and (e) after SPE in 0.2 g mL−1 MP. SWV conditions: scanning potential range, −0.4 to 0.3 V; frequency, 25 Hz; potential increment, 4 mV; amplitude of the square-wave, 20 mV.
ZrO2 NPs-decorated graphene hybrid composite provides an efficient platform as the SPE of nitroaromatic OPs. 3.5. Effect of methyl parathion on response of ZrO2 NPs-GNs/GCE Fig. 4 displays the CV of MP captured on ZrO2 NPs-GNs/GCE in 0.2 M ABS (pH 5.2). During the potential range from −0.8 to 0.6 V, no obvious redox peak appeared in the absence of MP (curve a). However, a pair of well-defined redox peaks (Epa , −0.05 V; Epc , 0.05 V) and an irreversible reduction peak (Epc , −0.57 V) were observed (curve b) with the capture of MP onto ZrO2 NPs-GNs/GCE. The irreversible reduction peak corresponds to the reduction of the nitro group to the hydroxylamine group (reaction 1, Appendices). The
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reversible redox peaks are attributed to a two-electron-transfer process (reactions 2 and 3, Appendices). These profiles are consistent with those described elsewhere for nitroaromatic OPs [39–42]. Square wave voltammetry (SWV) analysis has a higher sensitivity than that of other electrochemical techniques. As shown in the inset of Fig. 4, no anodic SWV peak was observed at ZrO2 NPs-GNs/GCE in blank PBS (curve c). Evidently, a very sharp and well-defined SWV peak at a potential of about −0.015 V vs. SCE (curve e) appeared with the capture of MP into ZrO2 NPsGNs composite. Compared with the direct measurement in MP sample solution (curve d, the inset of Fig. 4), the peak current at MP captured ZrO2 NPs-GNs/GCE was greatly enhanced. This may be attributed to the enlarged surface area, the enhanced electron transfer, and strong affinity for phosphoric moieties toward the target of MP. Obviously, the SPE through the capture of MP into the composite of ZrO2 NPs-GNs, leads to the enrichment of MP onto the surface, and is therefore responsible for the significant enhancement of the current response. 3.6. Optimization for the detection of MP at ZrO2 NPs-GNs/GCE We further optimized the experimental parameters to get high-performance electrochemical analysis of OPs. First, as the supporting skeleton, the amount of exfoliated GO dispersed onto GCE sharply influenced the electrochemical response of MP. With casting the different dispersion concentration of exfoliated GO onto GCE, the final SWV response of MP at ZrO2 NP-GNs modified GCE increased at first up to 2.0 mg mL−1 and then decreased at higher concentration (Fig. 5A). Since the ensemble ZrO2 NP-GNs plays an important role in the performance of sensors, the influence of the amount of ensemble was investigated by controlling the cycles of CV scanning (in 5.0 mM ZrOCl2 and 0.1 M KCl). The potential scanning cycles would directly affect the size distribution of ZrO2 NPs. As shown in Fig. 5B, when the potential scanning cycles were increased, the peak
Fig. 5. Effects of (A) the amount of exfoliated GO precasted onto GC electrode, (B) the potential scanning cycles in 5.0 mM ZrOCl2 + 0.1 M KCl aqueous solution, (C) the pH of adsorption medium, and (D) the extraction time toward the adsorption of MP onto ZrO2 NPs-GNs/GCE. SWV conditions are the same to those in Fig. 4. Error bars indicate the standard deviation of five repeated measurements.
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Fig. 6. Square-wave voltammograms of increasing MP concentrations (from bottom to top, 0, 2, 10, 30, 50, 100, 200, 300, 500, 700 and 900 ng mL−1 , respectively). The inset shows the calibration curve. Error bars indicate the standard deviation of five repeated measurements. SWV conditions are the same to those in Fig. 4.
current of MP increased at first up to 8 cycles and then decreased. It is likely related to the nanostructural platform ensembled by ZrO2 NPs–graphene onto the electrode. With CV cycles increased up to 8 cycles, the enhanced surface area from the modification of ZrO2 NPs-GNs enables more active sites available for MP capture. While the potential scanning cycles were further increased, the ZrO2 NPs would be continuously generated, even coalesced and finally aggregated, resulting in the decreased sensing performance. Thus, the condition of 8 potential cycles was optimal. The response of MP was also dependent on pH of adsorption media. The SWV peak current was enhanced with an increase of pH up to 5.2, but decreased at higher pH (Fig. 5C). Since basic media would result in the degradation of OP compounds and H+ could be involved in the entrapment or redox process of MP, a pH value of 5.2 was selected for measurements in this study. It was found that extraction time was another one of the most influential parameters in pesticide analysis. The peak currents increased rapidly with an increase of immersion time, and then tended to be stable at ∼15 min, indicating that the adsorption of MP into ZrO2 NPs-GNs reaches saturation (Fig. 5D). 3.7. Analytical performance for the detection of methyl parathion Fig. 6 displays the SWV response of adsorbed MP by the SPE process at ZrO2 NPs-GNs/GCE. Well-defined peaks, proportional to the concentration of the corresponding MP, were observed ranging from 0.002 to 0.9 g mL−1 . The linearization equations were i/A = 2.746 + 107.3c/g mL−1 , with the correlation coefficients of 0.9982 (inset of Fig. 6). A detection limit of 0.6 ng mL−1 was obtained with the calculation based on signal-noise ratio equal to 3. It is significantly lower than 13.2 ng mL−1 at a carbon-paste electrode [30], lower than those with 1.0 ng mL−1 at ZrO2 NPs, LDHs-based SPE [30–33], and also comparable with those reported using enzyme-based electrochemical sensors [39,46,47]. The stability of the ZrO2 NPs-GNs modified electrode could be maintained by being stored at 4 ◦ C in a dry condition. No obvious decrease in the response of MP was observed in the first 10-day storage. After a 30-day storage period, the sensor retained 88% of its initial current response. A series of 5 repetitive determinations of 50 ng mL−1 MP yielded reproducible peak currents with relative standard deviations of 4.2%. No obvious interferences were observed from the other electroactive nitrophenyl derivatives such as p-nitrophenol, nitrobenzene, p-nitroaniline, trinitrotoluene and other oxygencontaining inorganic ions (PO4 3− , SO4 2− , NO3 − ) with the peak
Fig. 7. Peak currents of 200 ng mL−1 MP in the absence and presence of 4 g mL−1 p-nitrophenol, 4 g mL−1 nitrobenzene, 4 g mL−1 p-nitroaniline, 4 g mL−1 trinitrotoluene, 0.1 M PO4 3− , 0.1 M SO4 2− , 0.1 M NO3 − , 200 ng mL−1 carbaryl, and 200 ng mL−1 furadan, respectively. SWV conditions are the same to those in Fig. 4.
currents of MP varied slightly (shown in Fig. 7). Such a low potential of −0.015 V applied (toward MP) could avoid the interference efficiently. As known, zirconia also has a good affinity to PO4 3− , but in this case, it does not interfere with the detection of MP. The reason may be attributed that the adsorption capability of zirconia to MP is much stronger than PO4 3− . And we further observed no obvious interferences from carbaryl and furadan, which belong to the carbamate insecticide family, further confirming the specific affinity of ZrO2 NPs to OPs. To further demonstrate the practicality of the present electrode, it was evaluated by processing real samples. We performed the recovery tests by adding different amounts of MP into real samples, including garlic, and cabbage. Results are summarized as Table A1 (Appendices). The recoveries of the spiked MP are from 96.5% to 104.4% for the real samples. The accuracy of the method was also assessed by comparing the electrochemical results with those obtained by HPLC. The original MP concentration in the cabbage sample was tested electrochemically to be 0.123 ± 0.002 mg kg−1 , and while a value of 0.109 ± 0.003 mg kg−1 was obtained by HPLC, showing a difference of 12.8%. The results indicated that the proposed method can be used for direct analysis of relevant real samples. 4. Conclusion In summary, ZrO2 NPs decorated graphene nanosheets hybrid has been successfully synthesized in a facile co-electrodeposition approach using exfoliated GO as precursor. This facile one-step electrochemical approach for the construction of GNs based hybrid is environmentally friendly. The as-prepared composite matrix, combining the advantages of GNs together with ZrO2 NPs, greatly facilitated the preconcentration of MP with the peak current response greatly enhanced. The resulting sensor showed both good reproducibility and ideal stability. This work has expanded the scope of synthesizing graphene-based hybrid and explores their sensing applications. Acknowledgments This work was supported by the Natural Science Foundation of China (Grants 20803026, 21175053), and Self-determined Research
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Biographies Jingming Gong was born on February 27, 1977. She received her PhD degree in 2004 in Chemistry from University of Science & Technology of China (USTC). Presently, she is an associate professor at Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University. Her research interests include electroanalytical chemistry, biosensor, environmental science and biomaterials. Xingju Miao was born on October 7, 1987. She received her BS degree from Leshan Normal University (China) in 2009. She is currently studying for her Master degree on the fabrication of enzymeless sensor toward the determination of pesticides in College of Chemistry, Central China Normal University. Huifang Wan was born on February 7, 1991. She is currently a senior undergraduate student in College of Chemistry, Central China Normal University. Dandan Song was born on September 25, 1957. Currently, she is a professor in College of Chemistry, Central China Normal University. She majors on the environmental analysis.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Facile synthesis of zirconia nanoparticles-decorated graphene hybrid nanosheets for an enzymeless methyl parathion sensor Jingming Gong ∗ , Xingju Miao, Huifang Wan, Dandan Song Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China
a r t i c l e
i n f o
Article history: Received 13 October 2011 Received in revised form 18 December 2011 Accepted 27 December 2011 Available online 2 January 2012 Keywords: Zirconia nanoparticles Graphene One-step co-electrodeposition Organophosphate pesticide Enzymeless sensor
a b s t r a c t This paper proposed a facile electrochemical approach to the synthesis of high quality zirconia nanoparticles decorated graphene nanosheets (labeled as ZrO2 NPs-GNs) onto a cathodic substrate. This facile one-step co-electrodeposition approach for the construction of GNs based hybrid is environmentally friendly, without involving the chemical reduction of GO, and therefore will not result in further contamination. The electrochemically synthesized ZrO2 NPs-GNs composite has been carefully characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and various electrochemical techniques. Such a nanostructured composite, combining the advantages of ZrO2 NPs (high recognition and enrichment capability for phosphoric moieties) together with GNs (large surface area and high conductivity), is highly efficient to capture organophosphate pesticides (OPs). Combined with the square-wave voltammetry, a highly sensitive enzymeless OPs sensor was fabricated using the prepared ZrO2 NPs-GNs composite as solid phase extraction (SPE). The detection limit for methyl parathion (MP) in aqueous solutions was determined to be of 0.6 ng mL−1 (S/N = 3). This work provides a green and facile route for the preparation of GNs-based hybrid, and also offers a new promising protocol for OPs analysis. © 2011 Elsevier B.V. All rights reserved.
1. Introduction As a “rising star” material, graphene nanosheets (GNs), a new two-dimensional (2D) structure consists of sp2 hybridized carbon atoms, have attracted tremendous attention since experimentally produced in 2004 [1–3], finding potential applications in synthesizing nanocomposites, nanoelectronics, and ultrasensitive sensors [4–6]. 2D network structure of GNs could provide advantageous workplace for the incorporation of other building units [7–9], which makes it possible for utilizing GNs-based hybrid as a multifunctional assembly. Recently, GNs-based hybrids, combining well unique properties of individual nanostructures, have been highly concerned in various applications, ranging from environmental science, energy conversion, to sensing [10–24]. Particularly, it has been demonstrated that GNs-based hybrid nanomaterials as sensing platforms, display extraordinary activity. For example, various GNs-based hybrid nanocomposite, including 2D GNs combined with 0D metal NPs, metallic oxide NPs, or cyclodextrin, etc., have been fabricated as enhanced sensing platforms for electrochemical determination of glucose [14,15], -nicotinamide adenine dinucleotide [16], cytochrome c [17], heavy metal ions [18], dopamine [19,20], and organophosphates pesticides (OPs), etc. [10,22]. Obviously, graphene-based materials posses remarkable superiorities
∗ Corresponding author. Tel.: +86 27 6786 7535; fax: +86 27 6786 7535. E-mail address: [email protected] (J. Gong). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.12.094
on sensing applications. To meet the rapidly increasing demands for on-site environmental monitoring (e.g. high sensitivity, portable instruments, and simple operations), currently, it is still a great challenge to construct a high-performance GNs-based hybrid sensing platform in a simple, green and controllable fashion. To date, GNs based hybrid are primarily prepared by the chemical reduction of exfoliated graphite oxide (GO) combined with (1) chemical precipitation [11,12], or (2) using the obtained GNs as the support for further deposition of other moieties [10,14,18,22], always involved with chemical reduction process. Nevertheless, in this laborious procedure, the excessive amounts of reducing agents employed may contaminate the resulting materials; a large amount of framework defects is inevitable to be produced, and then the produced defects would severely impair the conductivity of the reduced GNs [23]; during the chemical reduction process, the monolayer graphene are considerably easy to agglomerate [25,26]. These severely restrict the further applications of GNs. In this aspect, several research groups have made outstanding achievements [27,28]. For example, Williams et al. proposed a UVassisted photocatalytic reduction approach toward GNs [27]. Deng et al. developed a protein-induced reduction of GO approach [28]. Recently, an in situ electrochemical reduction of exfoliated GO has been put forward as a green and fast approach toward GNs [29]. Xia et al. demonstrated that this approach has several clear advantages: no toxic solvents are used and therefore will not result in contamination of the product; the high negative potential can overcome the energy barriers for the reduction of oxygen functionalities,
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Scheme 1. Schematic illustration for green synthesis of ZrO2 NPs-GNs/GCE via a one-step electrochemical approach and electrochemical sensing nitroaromatic OP compound.
leading to the exfoliated GO efficiently reduced; and the final solid film of GNs can be further used in sensor [29]. As exotic buildings unit of the assembly, nanostructured zirconia (ZrO2 ), a useful inorganic oxide has aroused interest owing to its thermal stability, chemical inertness, and lack of toxicity [30,31]. It has been reported that ZrO2 NPs have a strong affinity to phosphoric moieties and was originally reported for detection of phosphopeptide [32–34], phosphorprotein capture [35,36], and OPs [22,37]. More conveniently, metallic oxide of ZrO2 film could be prepared by electroreduction of ZrOCl2 at bare gold surface [37,38]. Inspired by this, in this work, we propose a facile one-step co-electrodeposition approach to construct zirconia nanoparticles-decorated graphene nanosheets hybrid coating on a glassy carbon electrode (labeled as ZrO2 NPs-GNs/GCE), without involving the chemical reduction of GO.
Recently, as a more convenient and cost-effective alternative, an enzymeless sensor, i.e., solid phase extraction (SPE) of OPs combined with square-wave voltammetric analysis, has shown to be an ideal and highly sensitive technology [39–42,10,37]. The resulting ZrO2 NPs-GNs composite, combining the individual properties of GNs (large surface area and high conductivity) together with ZrO2 NPs (high recognition and enrichment capability for phosphoric moieties) is believed to be an excellent approach for OPs capture. It is expected that the resulting ZrO2 NPs-GNs composite could dramatically facilitate the enrichment of OPs, effectively accelerate the electron transfer and realize their rapid, stable and sensitive square-wave voltammetric detection. This new electrochemical sensing protocol involves simple one-step electrosynthesis of a thin ZrO2 NPs-GNs film onto a GCE surface, subsequent entrapment of methyl parathion (MP as a model of OPs), and final electrochemical detection of adsorbed MP (Scheme 1). To the best of our knowledge, this is the first report on a green and facile electrochemical synthesis of ZrO2 NPs-decorated GNs hybrid and further utilizing ZrO2 NPs-GNs composite as SPE for electrochemical analysis of nitroaromatic OPs.
2. Experimental 2.1. Apparatus
Fig. 1. Cyclic voltammograms of (a) exfoliated GO/GCE, (b) bare GCE, and (c) exfoliated GO/GCE in 0.1 M KCl (a), or in 5.0 mM ZrOCl2 + 0.1 M KCl (b and c) aqueous solution (the first cycle shown). Scan rate: 50 mV/s.
Electrochemical measurements were performed on a CHI 660D electrochemical workstation (CHI, USA) with a conventional threeelectrode system comprising a platinum wire as an auxiliary electrode, a saturated calomel electrode (SCE) as reference, and the modified or unmodified glassy carbon electrode (GCE) as a working electrode. The general morphology of the products was characterized by the scanning electron microscopy (SEM, JSM-5600). Tapping-mode atomic force microscopy (AFM) was conducted with a DI Nanoscope (Veeco Instruments, Inc., USA) (see Appendices for sample preparation).
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Fig. 2. Typical SEM images of (A) the as-synthesized GNs/GCE, (B) ZrO2 NPs-GNs/GCE via an electrochemical approach; typical AFM images of (C) the as-prepared graphene nanosheets (GNs), (D) ZrO2 NPs decorated graphene nanosheets (ZrO2 NPs-GNs) on mica and their corresponding section analysis. Inset of (B): corresponding magnified SEM image.
2.2. Chemicals Methyl parathion (MP) was obtained from Treechem Co (Shanghai, China). ZrOCl2 ·8H2 O was obtained from SCRC (Shanghai, China). The 0.2 M acetate buffer solutions (ABS) of different pH values were prepared for use. All other chemicals were of analytical-reagent grade and used without further purification. Doubly distilled water was used. All experiments were carried out at ambient temperature. 2.3. Preparation of the modified electrode and measurement procedure Prior to modification, the basal GCE was polished to a mirror finish using 1.0, 0.3 and 0.05 m alumina slurries. After each polishing, the electrode was sonicated in ethanol and doubly distilled water for 5 min, successively, in order to remove any adsorbed substances on the electrode surface. Graphite oxide was synthesized from graphite by a modified Hummers method [43,44]. The certain amount of GO was dispersed
into 1 mL doubly distilled water to form a homogenous exfoliated GO dispersion under mild ultrasonication for 1 h. A 10 L portion of exfoliated GO suspension was spread on a pretreated bare GCE using a pipette, and then kept at room temperature till dry (labeled as GO/GCE). ZrO2 NPs decorated GNs modified electrode (ZrO2 NPs-GNs/GCE) was prepared by immersing the exfoliated GO/GCE into an aqueous electrolyte of 5.0 mM ZrOCl2 and 0.1 M KCl, and then cycling the potential between 0 and −1.5 V (vs. SCE) at a scan rate of 50 mV/s for 8 cycles. For comparison, the modification of GNs or ZrO2 NPs alone onto GCE was prepared, respectively. To obtain GNs/GCE, according to the previous report [29], the exfoliated GO/GCE was immersed into an aqueous electrolyte of 0.1 M KCl by cycling the potential between 0 and −1.5 V (vs. SCE) at a scan rate of 50 mV/s for 8 cycles, while ZrO2 NPs modified GCE (ZrO2 NPs/GCE) was prepared by cycling the potential between 0 and −1.5 V (vs. SCE) for 8 cycles in an aqueous electrolyte of 5.0 mM ZrOCl2 and 0.1 M KCl. The as-prepared GNs/GCE, ZrO2 NPs/GCE, ZrO2 NPs-GNs/GCE were rinsed with water and dried at room temperature for further experiments, respectively. The followed measurement procedures were shown in Appendices.
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3. Results and discussion 3.1. Cyclic voltammetric responses for the formation of the modified electrode Fig. 1 illustrates the comparison between the cyclic voltammetric responses for the formation processes of thus formed GNs, ZrO2 NPs and ZrO2 NPs-GNs on the GCE. The cyclic voltammograms of an exfoliated GO/GCE in a potential range from 0.0 to −1.5 V shows a large cathodic current peak at around −1.20 V vs. SCE (shown in Fig. 1a) with a starting potential of −0.75 V vs. SCE. This large reduction current should be due to the reduction of the surface oxygen groups. In subsequent cycles, the reduction current at negative potentials decreases considerably and disappears finally. This demonstrates that the reduction of surface-oxygenated species at GO occurs quickly and irreversibly, and the exfoliated GO could indeed be reduced electrochemically at negative potentials to form GNs/GCE, agreeing well with the previous report [5]. A typical CV curve for the formation of ZrO2 /GCE is shown in Fig. 1b, recorded in an aqueous electrolyte of 5.0 mM ZrOCl2 and 0.1 M KCl. The irreversible peak around −0.85 V vs. SCE could be attributed to the redox behavior of ZrOCl2 on the bare GCE [13,14]. Interestingly, with the exfoliated GO/GCE immersed into the aqueous solution of 5.0 mM ZrOCl2 and 0.1 M KCl, the general features of the CV curves obtained (Fig. 1c) nicely combine the characteristic of the respective CV curves of Fig. 1a and b. The obvious cathodic peaks around −0.90 V and −1.27 V (vs. SCE) are believed to correspond to the complex redox behavior of the exfoliated GO and ZrOCl2 , respectively, to form the resulting ZrO2 NPs-GNs/GCE, indicating that ZrO2 decorated GNs composite coating has been successfully synthesized onto the surface of GCE via this green and facile electrochemical approach. 3.2. Characterization of the modified electrode surface GNs and ZrO2 NPs decorated GNs obtained by electrochemical reduction were analyzed by SEM and AFM analysis. As shown in Fig. 2A, typical SEM image of the as-synthesized graphene shows 2D nanosheet morphologies, exhibiting a few thin wrinkles onto the surface. With the co-electrodeposition to form ZrO2 -GNs/GCE, uniform ZrO2 NPs of ∼42 nm in average diameter formed randomly on the sheet (Fig. 2B). AFM is currently the foremost methods allowing definitive identification of single-layer crystals [5]. From AFM cross-section analysis, it can be observed that the ZrO2 NPs in the ZrO2 -GNs composite possess a thickness of about 4.0 nm. While, whether for GNs alone (Fig. 2C) or those in composite (Fig. 2D), the thickness of most GNs obtained was close to 1.0 nm, consistent with the thickness of an individual graphene layer. It further indicates that the co-electrodeposition approach is feasible to form GNs based hybrid. 3.3. Electrochemical reactivity The capability of electron transfer of different electrodes was investigated by electrochemical impedance spectra (EIS), shown in Fig. 3A. It can be seen that EIS of the bare GCE is composed of a semicircle and a straight line featuring a diffusionlimiting step of the Fe(CN)6 4−/3− processes (curve a). With the modification of the exfoliated GO onto GCE, the semicircle dramatically increases as compared to the bare GCE, suggesting that the exfoliated GO, as an insulating layer, makes the interfacial charge transfer difficult (curve b). After the exfoliated GO film is electrochemically reduced on GCE (GNs/GCE), the semicircles decrease distinctively, even smaller than bare GCE, indicating that the presence of GNs has accelerated electron transfer between the electrochemical probe of [Fe(CN)6 ]3−/4− and the electrode
Fig. 3. (A) Nyquist plots at (a) GC electrode, (b) exfoliated GO/GCE, (c) GNs/GCE, and (d) ZrO2 NPs-GNs/GCE in 10 mM Fe(CN)6 3−/4− containing 0.1 M KCl. The frequency range is from 1 Hz to 10 kHz. (B) Chronocoulometric curves at (e) GC electrode, (f) GNs/GCE, and (g) ZrO2 NPs-GNs/GCE for the reduction of 0.5 mM K3 Fe(CN)6 with 0.1 M KCl. The initial potential was 0.65 V, and the potential was stepped to −0.05 V.
(curve c). However, with the modification of ZrO2 NPs-GNs/GCE, the semicircles increase distinctively, even larger than exfoliated GO, suggesting that the introduction of semi-conductor ZrO2 NPs into the composite generate higher resistance for the redox process (curve d). It is well known that the effective surface area is a crucial factor influencing the enrichment capability of the target and the electrochemical response. Fig. 3B depicts the chronocoulometric curves at different electrodes for the reduction of 0.5 mM K3 Fe(CN)6 with 0.1 M KCl. According to equation [45], Eq. A1 (see Appendices for the detailed descriptions). From the slope of the Q − t1/2 line, the sequence of the values of A for different electrodes is ZrO2 NPs-GNs/GCE (curve g, 1.01 cm2 ) > GNs/GCE (curve f, 0.490 cm2 ) > bare GC (curve e, 0.070 cm2 ). Obviously, the introduction of ZrO2 NPs effectively decreases the aggregation of GNs, and the modification of ZrO2 NPsGNs onto GCE greatly enhances the active area of the surface, which is considerably important for sensing applications.
3.4. UV–vis spectra To prove that ZrO2 NPs-decorated graphene nanoassembly can be used for SPE of methyl parathion (a model of OPs), UV–vis spectra of MP solution before (curve a) and after SPE were shown in Fig. A-1 (the details see Appendices). The results confirmed that
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Fig. 4. Cyclic voltammograms of (a) ZrO2 NPs-GNs/GCE, (b) MP captured onto ZrO2 NPs-GNs/GCE in 0.1 M ABS (pH 5.2). Scan rate: 100 mV/s. Inset: square-wave voltammograms obtained at ZrO2 NPs-GNs/GCE (c) in the absence of MP, (d) the solution containing 0.2 g mL−1 MP without preconcentration, and (e) after SPE in 0.2 g mL−1 MP. SWV conditions: scanning potential range, −0.4 to 0.3 V; frequency, 25 Hz; potential increment, 4 mV; amplitude of the square-wave, 20 mV.
ZrO2 NPs-decorated graphene hybrid composite provides an efficient platform as the SPE of nitroaromatic OPs. 3.5. Effect of methyl parathion on response of ZrO2 NPs-GNs/GCE Fig. 4 displays the CV of MP captured on ZrO2 NPs-GNs/GCE in 0.2 M ABS (pH 5.2). During the potential range from −0.8 to 0.6 V, no obvious redox peak appeared in the absence of MP (curve a). However, a pair of well-defined redox peaks (Epa , −0.05 V; Epc , 0.05 V) and an irreversible reduction peak (Epc , −0.57 V) were observed (curve b) with the capture of MP onto ZrO2 NPs-GNs/GCE. The irreversible reduction peak corresponds to the reduction of the nitro group to the hydroxylamine group (reaction 1, Appendices). The
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reversible redox peaks are attributed to a two-electron-transfer process (reactions 2 and 3, Appendices). These profiles are consistent with those described elsewhere for nitroaromatic OPs [39–42]. Square wave voltammetry (SWV) analysis has a higher sensitivity than that of other electrochemical techniques. As shown in the inset of Fig. 4, no anodic SWV peak was observed at ZrO2 NPs-GNs/GCE in blank PBS (curve c). Evidently, a very sharp and well-defined SWV peak at a potential of about −0.015 V vs. SCE (curve e) appeared with the capture of MP into ZrO2 NPsGNs composite. Compared with the direct measurement in MP sample solution (curve d, the inset of Fig. 4), the peak current at MP captured ZrO2 NPs-GNs/GCE was greatly enhanced. This may be attributed to the enlarged surface area, the enhanced electron transfer, and strong affinity for phosphoric moieties toward the target of MP. Obviously, the SPE through the capture of MP into the composite of ZrO2 NPs-GNs, leads to the enrichment of MP onto the surface, and is therefore responsible for the significant enhancement of the current response. 3.6. Optimization for the detection of MP at ZrO2 NPs-GNs/GCE We further optimized the experimental parameters to get high-performance electrochemical analysis of OPs. First, as the supporting skeleton, the amount of exfoliated GO dispersed onto GCE sharply influenced the electrochemical response of MP. With casting the different dispersion concentration of exfoliated GO onto GCE, the final SWV response of MP at ZrO2 NP-GNs modified GCE increased at first up to 2.0 mg mL−1 and then decreased at higher concentration (Fig. 5A). Since the ensemble ZrO2 NP-GNs plays an important role in the performance of sensors, the influence of the amount of ensemble was investigated by controlling the cycles of CV scanning (in 5.0 mM ZrOCl2 and 0.1 M KCl). The potential scanning cycles would directly affect the size distribution of ZrO2 NPs. As shown in Fig. 5B, when the potential scanning cycles were increased, the peak
Fig. 5. Effects of (A) the amount of exfoliated GO precasted onto GC electrode, (B) the potential scanning cycles in 5.0 mM ZrOCl2 + 0.1 M KCl aqueous solution, (C) the pH of adsorption medium, and (D) the extraction time toward the adsorption of MP onto ZrO2 NPs-GNs/GCE. SWV conditions are the same to those in Fig. 4. Error bars indicate the standard deviation of five repeated measurements.
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Fig. 6. Square-wave voltammograms of increasing MP concentrations (from bottom to top, 0, 2, 10, 30, 50, 100, 200, 300, 500, 700 and 900 ng mL−1 , respectively). The inset shows the calibration curve. Error bars indicate the standard deviation of five repeated measurements. SWV conditions are the same to those in Fig. 4.
current of MP increased at first up to 8 cycles and then decreased. It is likely related to the nanostructural platform ensembled by ZrO2 NPs–graphene onto the electrode. With CV cycles increased up to 8 cycles, the enhanced surface area from the modification of ZrO2 NPs-GNs enables more active sites available for MP capture. While the potential scanning cycles were further increased, the ZrO2 NPs would be continuously generated, even coalesced and finally aggregated, resulting in the decreased sensing performance. Thus, the condition of 8 potential cycles was optimal. The response of MP was also dependent on pH of adsorption media. The SWV peak current was enhanced with an increase of pH up to 5.2, but decreased at higher pH (Fig. 5C). Since basic media would result in the degradation of OP compounds and H+ could be involved in the entrapment or redox process of MP, a pH value of 5.2 was selected for measurements in this study. It was found that extraction time was another one of the most influential parameters in pesticide analysis. The peak currents increased rapidly with an increase of immersion time, and then tended to be stable at ∼15 min, indicating that the adsorption of MP into ZrO2 NPs-GNs reaches saturation (Fig. 5D). 3.7. Analytical performance for the detection of methyl parathion Fig. 6 displays the SWV response of adsorbed MP by the SPE process at ZrO2 NPs-GNs/GCE. Well-defined peaks, proportional to the concentration of the corresponding MP, were observed ranging from 0.002 to 0.9 g mL−1 . The linearization equations were i/A = 2.746 + 107.3c/g mL−1 , with the correlation coefficients of 0.9982 (inset of Fig. 6). A detection limit of 0.6 ng mL−1 was obtained with the calculation based on signal-noise ratio equal to 3. It is significantly lower than 13.2 ng mL−1 at a carbon-paste electrode [30], lower than those with 1.0 ng mL−1 at ZrO2 NPs, LDHs-based SPE [30–33], and also comparable with those reported using enzyme-based electrochemical sensors [39,46,47]. The stability of the ZrO2 NPs-GNs modified electrode could be maintained by being stored at 4 ◦ C in a dry condition. No obvious decrease in the response of MP was observed in the first 10-day storage. After a 30-day storage period, the sensor retained 88% of its initial current response. A series of 5 repetitive determinations of 50 ng mL−1 MP yielded reproducible peak currents with relative standard deviations of 4.2%. No obvious interferences were observed from the other electroactive nitrophenyl derivatives such as p-nitrophenol, nitrobenzene, p-nitroaniline, trinitrotoluene and other oxygencontaining inorganic ions (PO4 3− , SO4 2− , NO3 − ) with the peak
Fig. 7. Peak currents of 200 ng mL−1 MP in the absence and presence of 4 g mL−1 p-nitrophenol, 4 g mL−1 nitrobenzene, 4 g mL−1 p-nitroaniline, 4 g mL−1 trinitrotoluene, 0.1 M PO4 3− , 0.1 M SO4 2− , 0.1 M NO3 − , 200 ng mL−1 carbaryl, and 200 ng mL−1 furadan, respectively. SWV conditions are the same to those in Fig. 4.
currents of MP varied slightly (shown in Fig. 7). Such a low potential of −0.015 V applied (toward MP) could avoid the interference efficiently. As known, zirconia also has a good affinity to PO4 3− , but in this case, it does not interfere with the detection of MP. The reason may be attributed that the adsorption capability of zirconia to MP is much stronger than PO4 3− . And we further observed no obvious interferences from carbaryl and furadan, which belong to the carbamate insecticide family, further confirming the specific affinity of ZrO2 NPs to OPs. To further demonstrate the practicality of the present electrode, it was evaluated by processing real samples. We performed the recovery tests by adding different amounts of MP into real samples, including garlic, and cabbage. Results are summarized as Table A1 (Appendices). The recoveries of the spiked MP are from 96.5% to 104.4% for the real samples. The accuracy of the method was also assessed by comparing the electrochemical results with those obtained by HPLC. The original MP concentration in the cabbage sample was tested electrochemically to be 0.123 ± 0.002 mg kg−1 , and while a value of 0.109 ± 0.003 mg kg−1 was obtained by HPLC, showing a difference of 12.8%. The results indicated that the proposed method can be used for direct analysis of relevant real samples. 4. Conclusion In summary, ZrO2 NPs decorated graphene nanosheets hybrid has been successfully synthesized in a facile co-electrodeposition approach using exfoliated GO as precursor. This facile one-step electrochemical approach for the construction of GNs based hybrid is environmentally friendly. The as-prepared composite matrix, combining the advantages of GNs together with ZrO2 NPs, greatly facilitated the preconcentration of MP with the peak current response greatly enhanced. The resulting sensor showed both good reproducibility and ideal stability. This work has expanded the scope of synthesizing graphene-based hybrid and explores their sensing applications. Acknowledgments This work was supported by the Natural Science Foundation of China (Grants 20803026, 21175053), and Self-determined Research
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Biographies Jingming Gong was born on February 27, 1977. She received her PhD degree in 2004 in Chemistry from University of Science & Technology of China (USTC). Presently, she is an associate professor at Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University. Her research interests include electroanalytical chemistry, biosensor, environmental science and biomaterials. Xingju Miao was born on October 7, 1987. She received her BS degree from Leshan Normal University (China) in 2009. She is currently studying for her Master degree on the fabrication of enzymeless sensor toward the determination of pesticides in College of Chemistry, Central China Normal University. Huifang Wan was born on February 7, 1991. She is currently a senior undergraduate student in College of Chemistry, Central China Normal University. Dandan Song was born on September 25, 1957. Currently, she is a professor in College of Chemistry, Central China Normal University. She majors on the environmental analysis.